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Sample records for pnictide superconductor ba1-xkxfe2as2

  1. Advantageous grain boundaries in iron pnictide superconductors

    PubMed Central

    Katase, Takayoshi; Ishimaru, Yoshihiro; Tsukamoto, Akira; Hiramatsu, Hidenori; Kamiya, Toshio; Tanabe, Keiichi; Hosono, Hideo

    2011-01-01

    High critical temperature superconductors have zero power consumption and could be used to produce ideal electric power lines. The principal obstacle in fabricating superconducting wires and tapes is grain boundaries—the misalignment of crystalline orientations at grain boundaries, which is unavoidable for polycrystals, largely deteriorates critical current density. Here we report that high critical temperature iron pnictide superconductors have advantages over cuprates with respect to these grain boundary issues. The transport properties through well-defined bicrystal grain boundary junctions with various misorientation angles (θGB) were systematically investigated for cobalt-doped BaFe2As2 (BaFe2As2:Co) epitaxial films fabricated on bicrystal substrates. The critical current density through bicrystal grain boundary (JcBGB) remained high (>1 MA cm−2) and nearly constant up to a critical angle θc of ∼9°, which is substantially larger than the θc of ∼5° for YBa2Cu3O7–δ. Even at θGB>θc, the decay of JcBGB was much slower than that of YBa2Cu3O7–δ. PMID:21811238

  2. Materials and Physics in Pnictide Superconductors

    NASA Astrophysics Data System (ADS)

    Wen, Hai-Hu

    2009-03-01

    Superconductivity in the pnictides has shown itself to be very interesting and attractive. Some experimental results have revealed that the superconducting mechanism could be unconventional. In this talk I will survey our recent progress of both material synthesizing and physical properties of this rich family. We have made several major contributions to the synthesizing of new pnictide superconductors. (1) Fabrication of the hole doped RE1-xSrxFeAsO samples (RE=La and Pr); (2) Fabrication of a series of new parent compounds DvFeAsF (Dv=divalent metals: Sr, Ca, Eu etc.) and many new superconductors with Tc beyond 50 K by doping electrons into the system; (3) Invention of the new material (Sr3Sc2O5)Fe2As2 with rather large spacing distance between the FeAs planes. We have successfully grown the NdFeAsO1-xFx and Ba1-xKxFe2As2 single crystals. It is found that the anomalous electron scattering in the normal state cannot be simply attributed to the multiband effect. The influence given by the magnetic correlation may play an important role. Specific heat, lower critical field and point contact tunneling all indicate the unconventional superconductivity and multigap features, while the paring symmetry of the superconducting gap may be a non-trivial issue. In the 1111 phase, the superfluid density is rather low and contains probably a nodal feature. While in the 122 phase, both the superfluid density and the quasiparticle density of states is about 5-10 times higher than that in the 1111 phase. An s-wave component was found in the 122 phase. I will also report the measurements on anisotropy, critical current density, critical fields and vortex phase diagram. Small anisotropy, high upper critical field and fish-tail effect (in 122) were observed. All these suggest very good potential applications. In collaboration with Gang Mu, Zhaosheng Wang, Huiqian Luo, Huan Yang, Xiyu Zhu, Ying Jia, Yonglei Wang, Fei Han, Bing Zeng, Bing Shen, Cong Ren, Lei Shan.

  3. Low resistivity contact to iron-pnictide superconductors

    DOEpatents

    Tanatar, Makariy; Prozorov, Ruslan; Ni, Ni; Bud& #x27; ko, Sergey; Canfield, Paul

    2013-05-28

    Method of making a low resistivity electrical connection between an electrical conductor and an iron pnictide superconductor involves connecting the electrical conductor and superconductor using a tin or tin-based material therebetween, such as using a tin or tin-based solder. The superconductor can be based on doped AFe.sub.2As.sub.2, where A can be Ca, Sr, Ba, Eu or combinations thereof for purposes of illustration only.

  4. Interplay between interband coupling and ferromagnetism in iron pnictide superconductor/ferromagnet/iron pnictide superconductor junctions

    SciTech Connect

    Liu, S. Y.; Tao, Y. C.; Hu, J. G.

    2014-08-28

    An extended eight-component Bogoliubov-de Gennes equation is applied to study the Josephson effect between iron-based superconductors (SCs) with s{sub ±}-wave pairing symmetry, separated by an ferromagnet (FM). The feature of damped oscillations of critical Josephson current as a function of FM thickness, the split of the peaks induced by the interband coupling is much different from that for the junction with the s{sub ±}-wave SCs replaced by s{sub ++}-wave ones. In particular, a 0−π transition as a function of interband coupling strength α is found to always exhibit with the corresponding dip shifting toward the larger α due to enhancing the spin polarization in the FM, while there exits no 0−π transition for the SC with s{sub ++}-wave pairing symmetry. The two features can be used to identify the pairing symmetry in the iron pnictide SC different from the s{sub ++}-wave one in MgB{sub 2}. Experimentally, by adjusting the doping level in the s{sub ±}-wave SCs, one can vary α.

  5. Point Contact Andreev Reflection Studies on Iron Pnictide Superconductors

    NASA Astrophysics Data System (ADS)

    Zhang, Xiaohang; Saha, S. R.; Butch, N. P.; Kirshenbaum, K.; Paglione, J.; Greene, R. L.; Takeuchi, I.; Oh, Y.-S.; Liu, Y.; Yan, L. Q.; Kim, K.-H.

    2011-03-01

    We have systematically investigated the temperature, doping and the directional dependence of the gap structure for various types of single crystal iron pnictide superconductors by point contact Andreev reflection spectroscopy. Our studies were performed on highly transparent junctions evidenced by sharp and dramatic conductance enhancements at low temperatures. For the 122 family, despite some small features occasionally observed on the spectroscopy curves which may originate from the multiband superconductivity, a more conclusive characteristic of our obtained spectra is the presence of one predominant superconducting gap. By applying the BTK model, we find that the determined gap size scales well with the transition temperature, resulting in the 2 Δ / kBTC value of ~ 3.1 for both potassium doped and cobalt doped single crystals. Directional studies suggest that this gap is highly isotropic. Results on chalcogenide and nickel doped 122 superconductors will also be discussed. [X. H. Zhang et al., Phys. Rev. B 81, 024518 (2010)]. Supported by CNAM and NSF DMR-0653535.

  6. Infrared pseudogap in cuprate and pnictide high-temperature superconductors

    SciTech Connect

    Moon, S. J.; Lee, Y. S.; Schafgans, A. A.; Chubukov, A. V.; Kasahara, S.; Shibauchi, T.; Terashima, T.; Matsuda, Y.; Tanatar, M. A.; Prozorov, R.; Thaler, A.; Canfield, Paul C.; Bud'ko, Sergey L.; Sefat, A. S.; Mandrus, D.; Segawa, K.; Ando, Y.; Basov, D. N.

    2014-07-01

    We investigate infrared manifestations of the pseudogap in the prototypical cuprate and pnictide superconductors, YBa2Cu3Oy and BaFe2As2 (Ba122) systems. We find remarkable similarities between the spectroscopic features attributable to the pseudogap in these two classes of superconductors. The hallmarks of the pseudogap state in both systems include a weak absorption feature at about 500cm-1 followed by a featureless continuum between 500 and 1500cm-1 in the conductivity data and a significant suppression in the scattering rate below 700–900 cm-1. The latter result allows us to identify the energy scale associated with the pseudogap ΔPG. We find that in the Ba122-based materials the superconductivity-induced changes of the infrared spectra occur in the frequency region below 100–200 cm-1, which is much lower than the energy scale of the pseudogap. We performed theoretical analysis of the scattering rate data of the two compounds using the same model, which accounts for the effects of the pseudogap and electron-boson coupling. We find that the scattering rate suppression in Ba122-based compounds below ΔPG is solely due to the pseudogap formation, whereas the impact of the electron-boson coupling effects is limited to lower frequencies. The magnetic resonance modes used as inputs in our modeling are found to evolve with the development of the pseudogap, suggesting an intimate correlation between the pseudogap and magnetism

  7. Collective mode at Lifshitz transition in iron-pnictide superconductors

    NASA Astrophysics Data System (ADS)

    Rodriguez, J. P.

    2016-09-01

    We obtain the exact low-energy spectrum of two mobile holes in a t-J model for an isolated layer in an iron-pnictide superconductor. The minimum d xz and d yz orbitals per iron atom are included, with no hybridization between the two. After tuning the Hund coupling to a putative quantum critical point (QCP) that separates a commensurate spin-density wave from a hidden-order antiferromagnet at half filling, we find an s-wave hole-pair groundstate and a d-wave hole-pair excited state. Near the QCP, both alternate in sign between hole Fermi surface pockets at the Brillouin zone center and emergent electron Fermi surface pockets at momenta that correspond to commensurate spin-density waves (cSDW). The dependence of the energy splitting with increasing Hund coupling yields evidence for a true QCP in the thermodynamic limit near the putative one, at which the s-wave and d-wave Cooper pairs are degenerate. A collective s-to-d-wave oscillation of the macroscopic superconductor that couples to orthorhombic shear strain is also identified. Its resonant frequency is predicted to collapse to zero at the QCP in the limit of low hole concentration. This implies degeneracy of Cooper pairs with s, d and s+\\text{i}d symmetry in the corresponding quantum critical state. We argue that the critical state describes Cooper pairs in hole-doped iron superconductors at the Lifshitz transition, where electron bands first rise above the Fermi level. We thereby predict that the s-to-d-wave collective mode observed by Raman spectroscopy in Ba1-x K x Fe2As2 at optimal doping should also be observed at higher doping near the Lifshitz transition.

  8. Collective mode at Lifshitz transition in iron-pnictide superconductors

    NASA Astrophysics Data System (ADS)

    Rodriguez, J. P.

    2016-09-01

    We obtain the exact low-energy spectrum of two mobile holes in a t-J model for an isolated layer in an iron-pnictide superconductor. The minimum d xz and d yz orbitals per iron atom are included, with no hybridization between the two. After tuning the Hund coupling to a putative quantum critical point (QCP) that separates a commensurate spin-density wave from a hidden-order antiferromagnet at half filling, we find an s-wave hole-pair groundstate and a d-wave hole-pair excited state. Near the QCP, both alternate in sign between hole Fermi surface pockets at the Brillouin zone center and emergent electron Fermi surface pockets at momenta that correspond to commensurate spin-density waves (cSDW). The dependence of the energy splitting with increasing Hund coupling yields evidence for a true QCP in the thermodynamic limit near the putative one, at which the s-wave and d-wave Cooper pairs are degenerate. A collective s-to-d-wave oscillation of the macroscopic superconductor that couples to orthorhombic shear strain is also identified. Its resonant frequency is predicted to collapse to zero at the QCP in the limit of low hole concentration. This implies degeneracy of Cooper pairs with s, d and s+\\text{i}d symmetry in the corresponding quantum critical state. We argue that the critical state describes Cooper pairs in hole-doped iron superconductors at the Lifshitz transition, where electron bands first rise above the Fermi level. We thereby predict that the s-to-d-wave collective mode observed by Raman spectroscopy in Ba1‑x K x Fe2As2 at optimal doping should also be observed at higher doping near the Lifshitz transition.

  9. Collective mode at Lifshitz transition in iron-pnictide superconductors.

    PubMed

    Rodriguez, J P

    2016-09-21

    We obtain the exact low-energy spectrum of two mobile holes in a t-J model for an isolated layer in an iron-pnictide superconductor. The minimum d xz and d yz orbitals per iron atom are included, with no hybridization between the two. After tuning the Hund coupling to a putative quantum critical point (QCP) that separates a commensurate spin-density wave from a hidden-order antiferromagnet at half filling, we find an s-wave hole-pair groundstate and a d-wave hole-pair excited state. Near the QCP, both alternate in sign between hole Fermi surface pockets at the Brillouin zone center and emergent electron Fermi surface pockets at momenta that correspond to commensurate spin-density waves (cSDW). The dependence of the energy splitting with increasing Hund coupling yields evidence for a true QCP in the thermodynamic limit near the putative one, at which the s-wave and d-wave Cooper pairs are degenerate. A collective s-to-d-wave oscillation of the macroscopic superconductor that couples to orthorhombic shear strain is also identified. Its resonant frequency is predicted to collapse to zero at the QCP in the limit of low hole concentration. This implies degeneracy of Cooper pairs with s, d and [Formula: see text] symmetry in the corresponding quantum critical state. We argue that the critical state describes Cooper pairs in hole-doped iron superconductors at the Lifshitz transition, where electron bands first rise above the Fermi level. We thereby predict that the s-to-d-wave collective mode observed by Raman spectroscopy in Ba1-x K x Fe2As2 at optimal doping should also be observed at higher doping near the Lifshitz transition. PMID:27419913

  10. Competing Magnetic Fluctuations in Iron Pnictide Superconductors: Role of Ferromagnetic Spin Correlations Revealed by NMR.

    PubMed

    Wiecki, P; Roy, B; Johnston, D C; Bud'ko, S L; Canfield, P C; Furukawa, Y

    2015-09-25

    In the iron pnictide superconductors, theoretical calculations have consistently shown enhancements of the static magnetic susceptibility at both the stripe-type antiferromagnetic and in-plane ferromagnetic (FM) wave vectors. However, the possible existence of FM fluctuations has not yet been examined from a microscopic point of view. Here, using ^{75}As NMR data, we provide clear evidence for the existence of FM spin correlations in both the hole- and electron-doped BaFe_{2}As_{2} families of iron-pnictide superconductors. These FM fluctuations appear to compete with superconductivity and are thus a crucial ingredient to understanding the variability of T_{c} and the shape of the superconducting dome in these and other iron-pnictide families. PMID:26451577

  11. Competing magnetic fluctuations in iron pnictide superconductors: Role of ferromagnetic spin correlations revealed by NMR

    DOE PAGESBeta

    Wiecki, P.; Roy, B.; Johnston, D. C.; Bud’ko, S. L.; Canfield, P. C.; Furukawa, Y.

    2015-09-22

    In the iron pnictide superconductors, theoretical calculations have consistently shown enhancements of the static magnetic susceptibility at both the stripe-type antiferromagnetic and in-plane ferromagnetic (FM) wave vectors. However, the possible existence of FM fluctuations has not yet been examined from a microscopic point of view. Here, using 75As NMR data, we provide clear evidence for the existence of FM spin correlations in both the hole- and electron-doped BaFe2As2 families of iron-pnictide superconductors. Furthermore, these FM fluctuations appear to compete with superconductivity and are thus a crucial ingredient to understanding the variability of Tc and the shape of the superconducting domemore » in these and other iron-pnictide families.« less

  12. Competing magnetic fluctuations in iron pnictide superconductors: Role of ferromagnetic spin correlations revealed by NMR

    SciTech Connect

    Wiecki, P.; Roy, B.; Johnston, D. C.; Bud’ko, S. L.; Canfield, P. C.; Furukawa, Y.

    2015-09-22

    In the iron pnictide superconductors, theoretical calculations have consistently shown enhancements of the static magnetic susceptibility at both the stripe-type antiferromagnetic and in-plane ferromagnetic (FM) wave vectors. However, the possible existence of FM fluctuations has not yet been examined from a microscopic point of view. Here, using 75As NMR data, we provide clear evidence for the existence of FM spin correlations in both the hole- and electron-doped BaFe2As2 families of iron-pnictide superconductors. Furthermore, these FM fluctuations appear to compete with superconductivity and are thus a crucial ingredient to understanding the variability of Tc and the shape of the superconducting dome in these and other iron-pnictide families.

  13. Controllable 0 − π transition in iron pnictide superconductor junctions with a spacer of strong ferromagnet

    SciTech Connect

    Liu, S. Y.; Tao, Y. C. Ji, T. T.; Di, Y. S.; Hu, J. G.

    2014-03-17

    We investigate the control of 0−π transition in Josephson junctions consisting of a highly spin-polarized ferromagnet coupled to two iron pnictide superconductors (SCs). It is shown that, a 0−π transition as a function of interband coupling strength is always exhibited, which can be experimentally used to discriminate the s{sub ±}-wave pairing symmetry in the iron pnictide SCs from the s{sub ++}-wave one in MgB{sub 2}. By tuning the doping level in the s{sub ±}-wave SCs, one can vary the interband coupling strength so as to obtain the controllable 0−π transition. This device may be realized with current technologies and has practical use in Cooper pair spintronics and quantum information.

  14. Transition from slow Abrikosov to fast moving Josephson vortices in iron pnictide superconductors.

    PubMed

    Moll, Philip J W; Balicas, Luis; Geshkenbein, Vadim; Blatter, Gianni; Karpinski, Janusz; Zhigadlo, Nikolai D; Batlogg, Bertram

    2013-02-01

    Iron pnictides are layered high T(c) superconductors with moderate material anisotropy and thus Abrikosov vortices are expected in the mixed state. Yet, we have discovered a distinct change in the nature of the vortices from Abrikosov-like to Josephson-like in the pnictide superconductor SmFeAs(O,F) with T(c)~48-50 K on cooling below a temperature T*~41-42 K, despite its moderate electronic anisotropy γ~4-6. This transition is hallmarked by a sharp drop in the critical current and accordingly a jump in the flux-flow voltage in a magnetic field precisely aligned along the FeAs layers, indicative of highly mobile vortices. T* coincides well with the temperature where the coherence length ξ(c) perpendicular to the layers matches half of the FeAs-layer spacing. For fields slightly out-of-plane (> 0.1°- 0.15°) the vortices are completely immobilized as well-pinned Abrikosov segments are introduced when the vortex crosses the FeAs layers. We interpret these findings as a transition from well-pinned, slow moving Abrikosov vortices at high temperatures to weakly pinned, fast flowing Josephson vortices at low temperatures. This vortex dynamics could become technologically relevant as superconducting applications will always operate deep in the Josephson regime.

  15. Proximity of iron pnictide superconductors to a quantum tricritical point

    PubMed Central

    Giovannetti, Gianluca; Ortix, Carmine; Marsman, Martijn; Capone, Massimo; van den Brink, Jeroen; Lorenzana, José

    2011-01-01

    In several materials, unconventional superconductivity appears nearby a quantum phase transition where long-range magnetic order vanishes as a function of a control parameter like charge doping, pressure or magnetic field. The nature of the quantum phase transition is of key relevance, because continuous transitions are expected to favour superconductivity, due to strong fluctuations. Discontinuous transitions, on the other hand, are not expected to have a similar role. Here we determine the nature of the magnetic quantum phase transition, which occurs as a function of doping, in the iron-based superconductor LaFeAsO1–xFx. We use constrained density functional calculations that provide ab initio coefficients for a Landau order parameter analysis. The outcome is intriguing, as this material turns out to be remarkably close to a quantum tricritical point, where the transition changes from continuous to discontinuous, and several susceptibilities diverge simultaneously. We discuss the consequences for superconductivity and the phase diagram. PMID:21772269

  16. Direct observation of a Fermi liquid-like normal state in an iron-pnictide superconductor.

    PubMed

    Tytarenko, Alona; Huang, Yingkai; de Visser, Anne; Johnston, Steve; van Heumen, Erik

    2015-01-01

    There are two prerequisites for understanding high-temperature (high-Tc) superconductivity: identifying the pairing interaction and obtaining a correct description of the normal state from which superconductivity emerges. The nature of the normal state of iron-pnictide superconductors, and the role played by correlations arising from partially screened interactions, are still under debate. Here we show that the normal state of carefully annealed electron-doped BaFe(2-x)Co(x)As2 at low temperatures has all the hallmark properties of a local Fermi liquid, with a more incoherent state emerging at elevated temperatures, an identification made possible using bulk-sensitive optical spectroscopy with high frequency and temperature resolution. The frequency dependent scattering rate extracted from the optical conductivity deviates from the expected scaling M2 (ω, T) ∝ (ħω)(2) + (pπkBT)(2) with p ≈ 1.47 rather than p = 2, indicative of the presence of residual elastic resonant scattering. Excellent agreement between the experimental results and theoretical modeling allows us to extract the characteristic Fermi liquid scale T0 ≈ 1700 K. Our results show that the electron-doped iron-pnictides should be regarded as weakly correlated Fermi liquids with a weak mass enhancement resulting from residual electron-electron scattering from thermally excited quasi-particles. PMID:26201499

  17. Two-dimensional oxide topological insulator with iron-pnictide superconductor LiFeAs structure

    NASA Astrophysics Data System (ADS)

    Xu, Qiunan; Song, Zhida; Nie, Simin; Weng, Hongming; Fang, Zhong; Dai, Xi

    2015-11-01

    By using first-principles calculations, we propose that ZrSiO can be looked at as a three-dimensional (3D) oxide weak topological insulator (TI) and its single layer is a long-sought-after 2D oxide TI with a band gap up to 30 meV. Calculated phonon spectrum of the single layer ZrSiO indicates it is dynamically stable and the experimental achievements in growing oxides with atomic precision ensure that it can be readily synthesized. This will lead to novel devices based on TIs, the so-called "topotronic" devices, operating under room temperature and stable when exposed in the air. Thus a new field of "topotronics" will arise. Another intriguing thing is this oxide 2D TI has the similar crystal structure as the well-known iron-pnictide superconductor LiFeAs. This brings great promise in realizing the combination of superconductor and TI, paving the way to various extraordinary quantum phenomena, such as topological superconductor and Majorana modes. We further find that there are many other isostructural compounds hosting the similar electronic structure and forming a W H M family with W being Zr, Hf, or La, H being group IV or group V element, and M being group VI one.

  18. Temperature dependent structural modulation in {{Ca}}_{0.82}{{La}}_{0.18}FeAs2 pnictide superconductors

    NASA Astrophysics Data System (ADS)

    Joseph, Boby; Marini, Carlo; Demitri, Nicola; Capitani, Francesco; Bernasconi, Andrea; Zhou, Wei; Xing, Xiangzhuo; Shi, Zhixiang

    2015-09-01

    Temperature dependent synchrotron x-ray powder diffraction data on {{Ca}}0.82{{La}}0.18FeAs2 pnictide superconductor demonstrate anisotropic thermal expansion along the out-of-plane and in-plane lattice parameters. Thermal expansion along the c-axis shows a distinct change from 2.9 × 10-4 Å K-1 at ˜300 K to 1.0 × 10-4 Å K-1 at ˜110 K, whereas the in-plane axes, a and b, in the temperature range 300 to 100 K, show a constant thermal expansion that is an order of magnitude smaller (˜0.3×10-4 Å K-1). The temperature dependence of the out-of-plane lattice parameter is found to have a good correlation with the behavior of the resistivity indicating the importance of the interlayer interaction in governing the transport properties of 112 pnictide superconductors.

  19. Hydrostatic pressure: a very effective approach to significantly enhance critical current density in granular iron pnictide superconductors.

    PubMed

    Shabbir, Babar; Wang, Xiaolin; Ghorbani, S R; Shekhar, Chandra; Dou, Shixue; Srivastava, O N

    2015-01-01

    Pressure is well known to significantly raise the superconducting transition temperature, Tc, in both iron pnictides and cuprate based superconductors. Little work has been done, however, on how pressure can affect the flux pinning and critical current density in the Fe-based superconductors. Here, we propose to use hydrostatic pressure to significantly enhance flux pinning and Tc in polycrystalline pnictide bulks. We have chosen Sr4V2O6Fe2As2 polycrystalline samples as a case study. We demonstrate that the hydrostatic pressure up to 1.2 GPa can not only significantly increase Tc from 15 K (underdoped) to 22 K, but also significantly enhance the irreversibility field, Hirr, by a factor of 4 at 7 K, as well as the critical current density, Jc, by up to 30 times at both low and high fields. It was found that pressure can induce more point defects, which are mainly responsible for the Jc enhancement. Our findings provide an effective method to significantly enhance Tc, Jc, Hirr, and the upper critical field, Hc2, for other families of Fe-based superconductors in the forms of wires/tapes, films, and single crystal and polycrystalline bulks. PMID:25645351

  20. Hydrostatic pressure: A very effective approach to significantly enhance critical current density in granular iron pnictide superconductors

    PubMed Central

    Shabbir, Babar; Wang, Xiaolin; Ghorbani, S. R.; Shekhar, Chandra; Dou, Shixue; Srivastava, O. N.

    2015-01-01

    Pressure is well known to significantly raise the superconducting transition temperature, Tc, in both iron pnictides and cuprate based superconductors. Little work has been done, however, on how pressure can affect the flux pinning and critical current density in the Fe-based superconductors. Here, we propose to use hydrostatic pressure to significantly enhance flux pinning and Tc in polycrystalline pnictide bulks. We have chosen Sr4V2O6Fe2As2 polycrystalline samples as a case study. We demonstrate that the hydrostatic pressure up to 1.2 GPa can not only significantly increase Tc from 15 K (underdoped) to 22 K, but also significantly enhance the irreversibility field, Hirr, by a factor of 4 at 7 K, as well as the critical current density, Jc, by up to 30 times at both low and high fields. It was found that pressure can induce more point defects, which are mainly responsible for the Jc enhancement. Our findings provide an effective method to significantly enhance Tc, Jc, Hirr, and the upper critical field, Hc2, for other families of Fe-based superconductors in the forms of wires/tapes, films, and single crystal and polycrystalline bulks. PMID:25645351

  1. Magnetic and pairing properties of a two-orbital model for the pnictide superconductors: a quantum Monte Carlo study.

    PubMed

    Liu, Guang-Kun; Huang, Zhong-Bing; Wang, Yong-Jun

    2014-08-13

    Using the constrained-path Monte Carlo method, a two-orbital model for the pnictide superconductors is studied at half filling and in both the electron- and hole-doped cases. At half filling, a stable (π, 0)/(0, π) magnetic order is explicitly observed and the system tends to be in an orthomagnetic order rather than the striped antiferromagnetic order on increasing the Coulomb repulsion U. In the electron-doped case, the (π, 0)/(0, π) magnetic order is enhanced upon doping and suppressed eventually and a s(±) pairing state dominates all the possible nearest-neighbour-bond pairings. Whereas in the hole-doped case, the magnetic order is straightforwardly suppressed and two nearly degenerate A(1g) and B(1g) intraband pairings become the dominant ones. PMID:25029986

  2. Quantum fluctuations in iron-pnictide superconductor BaFe2(As1-xPx)2

    NASA Astrophysics Data System (ADS)

    Shu, Lei; Ding, Z. F.; Zhang, J.; Tan, C.; Huang, K.; Liu, L.; Cheung, S.; Uemura, Y. J.; Maclaughlin, D. E.; Bernal, O. O.; Ho, P.-C.; Hu, D.; Dai, P. C.

    Muon-spin-relaxation/rotation (μSR) experiments were performed on single crystals of iron-pnictide superconductors BaFe2(As1-xPx)2 (x = 0 . 28 , 0 . 30 , and 0.33). Our preliminary results reveal that the static muon relaxation rate from ZF- μSR measurements is temperature independent through Tc, suggesting that time reversal symmetry is preserved in the superconducting state. Above Tc, the field dependence of muon relaxation rate shows NFL behaviors for optimal composition x = 0 . 3 . A maximum of zero temperature penetration depth at x = 0 . 3 is also observed. This work was supported by Chinese NSF, Grant 1147060, US NSF, Grant DMR-1506677 and DMR-1105380.

  3. Various forms of coexistence of superconductivity and magnetism in iron-pnictide superconductors: a NMR study

    NASA Astrophysics Data System (ADS)

    Bobroff, Julien

    2013-03-01

    Our NMR studies of iron pnictides allowed us to discover various forms of coexistence between superconductivity and magnetism. In Co-doped BaFe2As2, superconductivity and incommensurate antiferromagnetism coexist at the atomic level in an homogeneous state. In contrast, Ru isovalent doping leads to a disorderd situation where superconducting clusters appear in an antiferromagnetic background. Finally, in the 245 iron-selenide RbFeSe, antiferromagnetism and superconductivity separate in alternate layers of nanometer thickness. But in all these componds, the superconducting state remains similar in terms of local susceptibility and carrier doping. It looks as if, for superconductivity to appear, frozen Fe magnetic moments need to be small enough or far enough in distance, whatever the cause. Y. Texier et al., PRL 108, 237002 (2012); Y. Laplace et al., PRB Rapid Com 86, 020510(R) (2012); Y. Laplace, PRB Rapid Com 80, 140501 (2009) This work benefited from the support of the ANR PNICTIDES.

  4. Fermi Surfaces of Iron-Pnictide High-Tc Superconductors from the Limit of Local Magnetic Moments

    NASA Astrophysics Data System (ADS)

    Araujo, Miguel; Sacramento, Pedro; Rodriguez, Jose

    2012-02-01

    We study a 2-orbital t-J model for an isolated square lattice of iron atoms, which stack up to form an iron-pnictide high-Tc superconductor. The two orbitals in question are the degenerate d±= 3d(x±iy)z ones, which maximize the Hund's Rule coupling. First-neighbor and second-neighbor hopping (t) and Heisenberg exchange (J) are included. A Schwinger-boson-slave-fermion mean-field analysis yields a hidden half metal state in which holes hop through a d+d- spin background without much hopping across orbitals. This state is characterized by an inner and an outer Fermi surface pocket centered at the γ point. The Fermi surface pockets resemble those predicted by band structure calculations that include all five 3d orbitals. By sweeping the Hund's coupling, we also identify a quantum-critical point (QCP) where zero-energy spin-wave excitations exist at the momenta associated with commensurate spin-density-wave (cSDW) order. These low-energy spin-waves result in nested Fermi-surface pockets centered at cSDW momenta. Exact diagonalization of one hole in the 2-orbital t-J model over a 4x4 square lattice yields low-energy spectra that are consistent with the nested Fermi surfaces that are predicted to exist at the QCP.

  5. Correlation between linear resistivity and Tc in the Bechgaard salts and the pnictide superconductor Ba(Fe1-xCox)2As2

    NASA Astrophysics Data System (ADS)

    Doiron-Leyraud, Nicolas; Auban-Senzier, Pascale; René de Cotret, Samuel; Bourbonnais, Claude; Jérome, Denis; Bechgaard, Klaus; Taillefer, Louis

    2009-12-01

    The quasi-one-dimensional organic Bechgaard salt (TMTSF)2PF6 displays spin-density-wave (SDW) order and superconductivity in close proximity in the temperature-pressure phase diagram. We have measured its normal-state electrical resistivity ρa(T) as a function of temperature and pressure, in the T→0 limit. At the critical pressure where SDW order disappears, ρa(T)∝T down to the lowest measured temperature (0.1 K). With increasing pressure, ρa(T) acquires a curvature that is well described by ρa(T)=ρ0+AT+BT2 , where the strength of the linear term, measured by the A coefficient, is found to scale with the superconducting transition temperature Tc . This correlation between A and Tc strongly suggests that scattering and pairing in (TMTSF)2PF6 have a common origin, most likely rooted in the antiferromagnetic spin fluctuations associated with SDW order. Analysis of published resistivity data on the iron-pnictide superconductor Ba(Fe1-xCox)2As2 reveals a detailed similarity with (TMTSF)2PF6 , suggesting that antiferromagnetic fluctuations play a similar role in the pnictides.

  6. Comparison of the local structures of Ca0.82La0.18FeAs2 and Ba0.64K0.36Fe2As2 pnictide superconductors using atomic pair distribution function analysis

    NASA Astrophysics Data System (ADS)

    Joseph, Boby; Iadecola, Antonella; Bernasconi, Andrea; Rispoli, Pasquale; Demitri, Nicola; Xing, Xiangzhuo; Zhou, Wei; Shi, Zhixiang

    2015-09-01

    A comparative local structure study of pnictide superconductors Ca0.82La0.18FeAs2 (112-type, Tc∼ 40 K) and Ba0.64K0.36Fe2As2 (122-type, Tc∼ 37 K), using room temperature x-ray total scattering measurements is reported. The Fe-As superconducting active layer is found to be globally similar in both the systems consisting of edge-sharing FeAs4/4 tetrahedra as in all the iron-pnictide superconductors discovered so far. Although optimally superconducting, the active layer in these compounds is found to sustain a large local inhomogeneity. These results thus imply that a nanoscopic manipulation of the Fe-As active layer, rather than its isotropic structural tuning, is the key parameter to control the superconducting properties of the iron-based systems.

  7. From d-wave to s-wave pairing in the iron-pnictide superconductor (Ba, K)Fe2As2

    SciTech Connect

    Reid, J.-Ph.; Juneau-Fecteau, A.; Gordon, R.T.; Rene de Cotret, S.; Doiron-Leyraud, N.; Luo, X.G.; Shakeripour, H.; Chang, J.; Tanatar, Makariy A.; Kim, Hyunsoo; Prozorov, Ruslan; Saito, T.; Fukazawa, H.; Kohori, Y.; Kihou, K.; Lee, C.H.; Iyo, A.; Eisaki, H.; Shen, B.; Wen, H.-W.; Taillefer, Louis

    2012-07-17

    The nature of the pairing state in iron-based superconductors is the subject of much debate. Here we argue that in one material, the stoichiometric iron pnictide KFe2As2, there is overwhelming evidence for a d-wave pairing state, characterized by symmetry-imposed vertical line nodes in the superconducting gap. This evidence is reviewed, with a focus on thermal conductivity and the strong impact of impurity scattering on the critical temperature Tc. We then compare KFe2As2 to Ba0.6K0.4Fe2As2, obtained by Ba substitution, where the pairing symmetry is s-wave and the Tc is ten times higher. The transition from d-wave to s-wave within the same crystal structure provides a rare opportunity to investigate the connection between band structure and pairing mechanism. We also compare KFe2As2 to the nodal iron-based superconductor LaFePO, for which the pairing symmetry is probably not d-wave, but more likely s-wave with accidental line nodes.

  8. Enhancement of magnetic stripe order in iron-pnictide superconductors from the interaction between conduction electrons and magnetic impurities.

    PubMed

    Gastiasoro, Maria N; Andersen, Brian M

    2014-08-01

    Recent experimental studies have revealed several unexpected properties of Mn-doped BaFe(2)As(2). These include extension of the stripelike magnetic (π,0) phase to high temperatures above a critical Mn concentration only, the presence of diffusive and weakly temperature dependent magnetic (π,π) checkerboard scattering, and an apparent absent structural distortion from tetragonal to orthorhombic symmetry. Here, we study the effects of magnetic impurities both below and above the Néel transition temperature within a real-space five-band model appropriate to the iron pnictides. We show how these experimental findings can be explained by a cooperative behavior of the magnetic impurities and the conduction electrons mediating the Ruderman-Kittel-Kasuya-Yosida interactions between them. PMID:25148344

  9. A NiCrAl pressure cell up to 4.6 GPa and its application to cuprate and pnictide superconductors

    NASA Astrophysics Data System (ADS)

    Fujiwara, Naoki; Uwatoko, Yoshiya; Matsumoto, Takehiko

    2013-06-01

    A NiCrAl-CuBe hybrid cell has been paid much attention because its maximum pressure goes beyond 3 GPa despite its large sample space. In the previous pressurizing trials for this pressure cell, we reached 4.0 GPa under a steady load of 15 ton. In the present trial, we have succeeded in reaching 4.6 GPa by using a short Teflon capsule as a pressure-mediation-liquid container. The pressure efficiency at 15 ton was 75 %. The maximum expansion of the inner diameter of the NiCrAl cylinder was 5 %, suggesting that 4.6 GPa is the upper limit of pressure. To keep high pressure above 4 GPa, a steady load control is needed: a pressure of 4.0 GPa under a steady load decreased to 3.7 GPa after the pressure cell was clamped and the steady load was released. The pressure cell is available to various experiments that need a large sample space. We have applied this pressure cell to nuclear magnetic resonance (NMR) measurements on cuprate and pnictide superconductors, such as Sr2Ca12Cu24O41, LaFeAsO1-xFx, and CaFe1-xCoxAsF. These compounds have superconducting layers, and Tcs of these compounds are enhanced by pressure application. We review what happens at optimal pressure in electric and/or magnetic properties on a microscopic level. Grant-in-Aid (Grant No. 23340101) from the Ministry of Education, Science and Culture, Japan.

  10. Correlation-induced self-doping in iron-pnictide superconductor Ba2Ti2Fe2As4O

    NASA Astrophysics Data System (ADS)

    Qian, Tian; Ma, Junzhang; Roekeghem, A. Van; Richard, Pierre; Cao, Guanghan; Biermann, Silke; Ding, Hong

    2015-03-01

    The electronic structure of the iron-based superconductor Ba2Ti2Fe2As4O (Tconset = 23.5 K) has been investigated by using angle-resolved photoemission spectroscopy and combined local density approximation and dynamical mean field theory calculations. The electronic states near the Fermi level are dominated by both the Fe 3 d and Ti 3 d orbitals, indicating that the spacer layers separating different FeAs layers are also metallic. By counting the enclosed volumes of the Fermi surface sheets, we observe a large self-doping effect, i. e. 0.25 electrons per unit cell are transferred from the FeAs layer to the Ti2As2O layer, leaving the FeAs layer in a hole-doped state. This exotic behavior is successfully reproduced by our dynamical mean field calculations, in which the self-doping effect is attributed to the electronic correlations in the 3 d shells. Our work provides an alternative route of effective doping without element substitution for iron-based superconductors.

  11. Correlation induced self-doping in the iron-pnictide superconductor Ba2 Ti2 Fe2 As4 O

    NASA Astrophysics Data System (ADS)

    Ma, J. Z.; Richard, P.; Chen, G. F.; Miao, H.; Zeng, L. K.; Roekeghem, A. Van; Biermann, S.; Xu, N.; Shi, M.; Liu, Z. H.; He, J. B.; Wang, S. C.; Cao, C.; Sun, Y. L.; Cao, G. H.; Qian, T.; Ding, H.

    The electronic structure of the intercalated iron-based superconductor Ba2Ti2Fe2As4O (Tc ~21.5 K) has been investigated by using ARPES and combined LDA + DMFT calculations. The electronic states near the Fermi level are dominated by both the Fe 3d and Ti 3d orbitals, indicating that the spacing layers separating different FeAs layers are also metallic. By counting the enclosed volumes of the Fermi surface sheets, we observe a large self-doping effect, i.e., 0.25 electrons per unit cell are transferred from the FeAs layer to the Ti2As2O layer, leaving the FeAs layer in a hole-doped state, which is in contrast with the LDA prediction of an electron-doped FeAs layer. This exotic behavior is successfully reproduced by the LDA + DMFT calculations, in which the self-doping effect is attributed to the electronic correlations in the Fe 3d shell. Our work provides an alternative route of effective doping without element substitution for iron-based superconductors. Beijing 100190, China.

  12. Mechanism for explaining differences in the order parameters of FeAs-based and FeP-based pnictide superconductors.

    PubMed

    Thomale, Ronny; Platt, Christian; Hanke, Werner; Bernevig, B Andrei

    2011-05-01

    We put forward a scenario that explains the difference between the order-parameter character in arsenide (As) and phosphorous (P) iron-based superconductors. Using functional renormalization group to analyze it in detail, we find that nodal superconductivity on the electron pockets (hole pocket gaps are always nodeless) can naturally appear when the hole pocket at (π,π) in the unfolded Brillouin zone is absent, as is the case in LaOFeP. There, electron-electron interactions render the gap on the electron pockets softly nodal (of s(±) form). When the pocket of d(xy) orbital character is present, intraorbital interactions with the d(xy) part of the electron Fermi surface drives the superconductivity nodeless.

  13. Correlation-Enhanced Odd-Parity Interorbital Singlet Pairing in the Iron-Pnictide Superconductor LiFeAs

    NASA Astrophysics Data System (ADS)

    Nourafkan, R.; Kotliar, G.; Tremblay, A.-M. S.

    2016-09-01

    The rich variety of iron-based superconductors and their complex electronic structure lead to a wide range of possibilities for gap symmetry and pairing components. Here we solve in the two-Fe Brillouin zone the full frequency-dependent linearized Eliashberg equations to investigate spin-fluctuations mediated Cooper pairing for LiFeAs. The magnetic excitations are calculated with the random phase approximation on a correlated electronic structure obtained with density functional theory and dynamical mean field theory. The interaction between electrons through Hund's coupling promotes both the intraorbital dx z (y z ) and the interorbital magnetic susceptibility. As a consequence, the leading pairing channel, conventional s+- , acquires sizable interorbital dx y-dx z (y z ) singlet pairing with odd parity under glide-plane symmetry. The combination of intra- and interorbital components makes the results consistent with available experiments on the angular dependence of the gaps observed on the different Fermi surfaces.

  14. Emergent nesting of the Fermi surface from local-moment description of iron-pnictide high-Tc superconductors

    NASA Astrophysics Data System (ADS)

    Rodriguez, Jose P.; Araujo, Miguel A. N.; Sacramento, Pedro D.

    2014-07-01

    We uncover the low-energy spectrum of a t-J model for electrons on a square lattice of spin-1 iron atoms with 3dxz and 3dyz orbital character by applying Schwinger-boson-slave-fermion mean-field theory and by exact diagonalization of one hole roaming over a 4 × 4 × 2 lattice. Hopping matrix elements are set to produce hole bands centered at zero two-dimensional (2D) momentum in the free-electron limit. Holes can propagate coherently in the t-J model below a threshold Hund coupling when long-range antiferromagnetic order across the d + = 3d(x + iy)z and d - = 3d(x - iy)z orbitals is established by magnetic frustration that is off-diagonal in the orbital indices. This leads to two hole-pocket Fermi surfaces centered at zero 2D momentum. Proximity to a commensurate spin-density wave (cSDW) that exists above the threshold Hund coupling results in emergent Fermi surface pockets about cSDW momenta at a quantum critical point (QCP). This motivates the introduction of a new Gutzwiller wavefunction for a cSDW metal state. Study of the spin-fluctuation spectrum at cSDW momenta indicates that the dispersion of the nested band of one-particle states that emerges is electron-type. Increasing Hund coupling past the QCP can push the hole-pocket Fermi surfaces centered at zero 2D momentum below the Fermi energy level, in agreement with recent determinations of the electronic structure of mono-layer iron-selenide superconductors.

  15. Magnetism and Superconductivity in Iron Pnictides

    SciTech Connect

    Singh, David J

    2012-01-01

    The discovery of high temperature superconductivity in iron pnictides and chalcogenides has resulted in surprising new insights into high temperature superconductivity and its relationship with magnetism. Here we provide an overview of some of what is known about these materials and in particular about the interplay of magnetism and superconductivity in them. Similarities and contrasts with cuprate superconductors are emphasized and the superconducting pairing is discussed within the framework of spin fluctuation induced pairing.

  16. Power-law-like correlation between condensation energy and superconducting transition temperatures in iron pnictide/chalcogenide superconductors: Beyond the BCS understanding

    NASA Astrophysics Data System (ADS)

    Xing, Jie; Li, Sheng; Zeng, Bin; Mu, Gang; Shen, Bing; Schneeloch, J.; Zhong, R. D.; Liu, T. S.; Gu, G. D.; Wen, Hai-Hu

    2014-04-01

    Superconducting condensation energy U0int has been determined by integrating the electronic entropy in various iron pnictide/chalcogenide superconducting systems. It is found that U0int∝Tcn with n =3-4, which is in sharp contrast to the simple BCS prediction U0BCS=1/2NFΔs2, with NF the quasiparticle density of states at the Fermi energy and Δs the superconducting gap. A similar correlation holds if we compute the condensation energy through U0cal=3γneffΔs2/4π2kB2, with γneff the effective normal state electronic specific heat coefficient. This indicates a general relationship γneff∝Tcm with m =1-2, which is not predicted by the BCS scheme. A picture based on quantum criticality is proposed to explain this phenomenon.

  17. Power-law-like correlation between condensation energy and superconducting transition temperatures in iron pnictide/chalcogenide superconductors: Beyond the BCS understanding

    NASA Astrophysics Data System (ADS)

    Xing, Jie; Li, Sheng; Zeng, Bin; Mu, Gang; Shen, Bing; Schneeloch, J.; Zhong, R. D.; Liu, T. S.; Gu, G. D.; Wen, Hai-Hu

    2015-03-01

    Superconducting condensation energy U0int has been determined by integrating the electronic entropy in various iron pnictide/chalcogenide superconducting systems. It is found that U0int ~Tcn with n = 3 to 4, which is in sharp contrast to the simple BCS prediction U0BCS = 1 / 2NFΔs2 , with NF the quasiparticle density of states at the Fermi energy and Δs the superconducting gap. A similar correlation holds if we compute the condensation energy through U0cal = 3γneff Δs2 / 4π2kB2 , with γneff the effective normal state electronic specific heat coefficient. This indicates a general relationship γneff ~Tcm with m = 1 to 2, which is not predicted by the BCS scheme. A picture based on quantum criticality is proposed to explain this phenomenon.

  18. Itinerant electrons, local moments, and magnetic correlations in the pnictide superconductors CeFeAsO₁₋xFxand Sr(Fe₁₋xCox)₂As₂

    DOE PAGESBeta

    Vilmercati, Paolo; Fedorov, Alexei; Bondino, Federica; Offi, Francesco; Panaccione, Giancarlo; Lacovig, Paolo; Simonelli, Laura; McGuire, Michael A.; Sefat, Athena S. M.; Mandrus, David; et al

    2012-06-15

    A direct and element-specific measurement of the local Fe spin moment has been provided by analyzing the Fe 3s core level photoemission spectra in the parent and optimally doped CeFeAsO₁₋xFx (x = 0, 0.11) and Sr(Fe₁₋xCox)2As2 (x = 0, 0.10) pnictides. The rapid time scales of the photoemission process allowed the detection of large local spin moments fluctuating on a 10⁻¹⁵ s time scale in the paramagnetic, antiferromagnetic, and superconducting phases, indicative of the occurrence of ubiquitous strong Hund's magnetic correlations. The magnitude of the spin moment is found to vary significantly among different families, 1.3μB in CeFeAsO and 2.1μBmore » in SrFe₂As₂. Surprisingly, the spin moment is found to decrease considerably in the optimally doped samples, 0.9μB in CeFeAsO₀.₈₉F₀.₁₁ and 1.3μB in Sr(Fe₀.₉Co₀.₁)₂As₂. The strong variation of the spin moment against doping and material type indicates that the spin moments and the motion of itinerant electrons are influenced reciprocally in a self-consistent fashion, reflecting the strong competition between the antiferromagnetic superexchange interaction among the spin moments and the kinetic energy gain of the itinerant electrons in the presence of a strong Hund's coupling. By describing the evolution of the magnetic correlations concomitant with the appearance of superconductivity, these results constitute a fundamental step toward attaining a correct description of the microscopic mechanisms shaping the electronic properties in the pnictides, including magnetism and high-temperature superconductivity.« less

  19. Correlation-induced self-doping in the iron-pnictide superconductor Ba2Ti2Fe2As4O.

    PubMed

    Ma, J-Z; van Roekeghem, A; Richard, P; Liu, Z-H; Miao, H; Zeng, L-K; Xu, N; Shi, M; Cao, C; He, J-B; Chen, G-F; Sun, Y-L; Cao, G-H; Wang, S-C; Biermann, S; Qian, T; Ding, H

    2014-12-31

    The electronic structure of the iron-based superconductor Ba2Ti2Fe2As4O (Tc(onset)=23.5  K) has been investigated by using angle-resolved photoemission spectroscopy and combined local density approximation and dynamical mean field theory calculations. The electronic states near the Fermi level are dominated by both the Fe 3d and Ti 3d orbitals, indicating that the spacer layers separating different FeAs layers are also metallic. By counting the enclosed volumes of the Fermi surface sheets, we observe a large self-doping effect; i.e., 0.25 electrons per unit cell are transferred from the FeAs layer to the Ti2As2O layer, leaving the FeAs layer in a hole-doped state. This exotic behavior is successfully reproduced by our dynamical mean field calculations, in which the self-doping effect is attributed to the electronic correlations in the 3d shells. Our work provides an alternative route of effective doping without element substitution for iron-based superconductors.

  20. Correlation-Induced Self-Doping in the Iron-Pnictide Superconductor Ba2Ti2Fe2As4O

    NASA Astrophysics Data System (ADS)

    Ma, J.-Z.; van Roekeghem, A.; Richard, P.; Liu, Z.-H.; Miao, H.; Zeng, L.-K.; Xu, N.; Shi, M.; Cao, C.; He, J.-B.; Chen, G.-F.; Sun, Y.-L.; Cao, G.-H.; Wang, S.-C.; Biermann, S.; Qian, T.; Ding, H.

    2014-12-01

    The electronic structure of the iron-based superconductor Ba2Ti2Fe2As4O (Tconset=23.5 K ) has been investigated by using angle-resolved photoemission spectroscopy and combined local density approximation and dynamical mean field theory calculations. The electronic states near the Fermi level are dominated by both the Fe 3 d and Ti 3 d orbitals, indicating that the spacer layers separating different FeAs layers are also metallic. By counting the enclosed volumes of the Fermi surface sheets, we observe a large self-doping effect; i.e., 0.25 electrons per unit cell are transferred from the FeAs layer to the Ti2As2O layer, leaving the FeAs layer in a hole-doped state. This exotic behavior is successfully reproduced by our dynamical mean field calculations, in which the self-doping effect is attributed to the electronic correlations in the 3 d shells. Our work provides an alternative route of effective doping without element substitution for iron-based superconductors.

  1. Electronic structure in a one-Fe Brillouin zone of the iron pnictide superconductors CsFe2As2 and RbFe2As2

    NASA Astrophysics Data System (ADS)

    Kong, S.; Liu, D. Y.; Cui, S. T.; Ju, S. L.; Wang, A. F.; Luo, X. G.; Zou, L. J.; Chen, X. H.; Zhang, G. B.; Sun, Z.

    2015-11-01

    Using angle-resolved photoemission spectroscopy, we studied the electronic structures of CsFe2As2 and RbFe2As2 . Contrary to other iron-based superconductors where the band structures are usually depicted in the two-Fe Brillouin zone (BZ), we found that the distribution of electronic spectral weight in CsFe2As2 and RbFe2As2 follows the one-Fe BZ, and that the emerging band structure is qualitatively consistent with theoretical band calculations of the one-Fe BZ except for some shadow band effect. Our data suggest that the interlayer separation is an important tuning factor for the physics of FeAs layers, the increase of which can reduce the coupling between Fe and As and lead to the emergence of the electronic structure in accord with the one-Fe symmetry of the Fe square lattice. Our finding puts strong constraints on the theoretical models constructed on the basis of the one-Fe BZ.

  2. Electronic and structural properties of low-temperature superconductors and ternary pnictides ANi2Pn2 ( A=Sr,Ba and Pn=P,As )

    NASA Astrophysics Data System (ADS)

    Shein, I. R.; Ivanovskii, A. L.

    2009-02-01

    Based on first-principles full-potential linearized augmented plane wave method (FLAPW)-generalized gradient approximation calculations, we have investigated structural and electronic properties of low-temperature superconductors SrNi2As2 (TC˜0.6K) , BaNi2As2 (TC˜0.7K) , and BaNi2P2 (TC˜3K) , as well as SrNi2P2 . Our results show that the replacement of alkaline-earth metal (Sr↔Ba) and pnictogen (P↔As) types leads to anisotropic deformations of crystal structure caused by strong anisotropy of interatomic bonds. The band structure, density of states, and Fermi-surface features for (Sr,Ba)Ni2(P,As)2 are evaluated and discussed. As distinct from (Ca,Sr,Ba)Fe2As2 —the parent phases for “122” FeAs superconductors—the Fermi level in (Sr,Ba)Ni2(P,As)2 phases is shifted to the bands with higher dispersion E(k) but lower density of states as a result of increased electron concentration. Therefore the Fermi surfaces for (Sr,Ba)Ni2(P,As)2 phases differ essentially from those of the FeAs-based materials and adopt a multisheet three-dimensional type. Our estimations show that (Sr,Ba)Ni2(P,As)2 are within the weak-coupling limit with a small average electron-phonon coupling constant λep˜0.16-0.24 . The bonding in (Sr,Ba)Ni2(P,As)2 is of a complex anisotropic character. Namely, the bonding in [NiP(As)] layers may be described as a mixture of metallic, ionic, and covalent contributions. In turn, between adjacent [NiP(As)] layers and (Sr,Ba) atomic sheets, ionic bonds emerge, whereas between adjacent [NiP(As)]/[NiP(As)] layers covalent bonds occur owing to hybridization of p states of pnictogen atoms.

  3. A Twisted Ladder: Relating the Fe Superconductors to the High Tc Cuprates

    SciTech Connect

    Berg, E.

    2010-05-26

    We construct a 2-leg ladder model of an Fe-pnictide superconductor and discuss its properties and relationship with the familiar 2-leg cuprate model. Our results suggest that the underlying pairing mechanism for the Fe-pnictide superconductors is similar to that for the cuprates.

  4. Orbital Order, Structural Transition, and Superconductivity in Iron Pnictides

    NASA Astrophysics Data System (ADS)

    Yanagi, Yuki; Yamakawa, Youichi; Adachi, Naoko; Ōno, Yoshiaki

    2010-12-01

    We investigate the 16-band d- p model for iron pnictide superconductors in the presence of the electron-phonon coupling g with the orthorhombic mode which is crucial for reproducing the recently observed ultrasonic softening. Within the RPA, we obtain the ferro-orbital order below TQ which induces the tetragonal-orthorhombic structural transition at Ts = TQ, together with the stripe-type antiferromagnetic order below TN. Near the phase transitions, the system shows the s++-wave superconductivity due to the orbital fluctuation for a large g case with TQ>TN, while the s±-wave due to the magnetic fluctuation for a small g case with TQ ≤ TN. The former case is consistent with the phase diagram of doped iron pnictides with Ts>TN.

  5. Structure and physical properties for a new layered pnictide-oxide: BaTi₂As₂O.

    PubMed

    Wang, X F; Yan, Y J; Ying, J J; Li, Q J; Zhang, M; Xu, N; Chen, X H

    2010-02-24

    We have successfully synthesized a new layered pnictide-oxide: BaTi(2)As(2)O. It shares similar characteristics with Na(2)Ti(2)Sb(2)O. The crystal has a layered structure with a tetragonal P4/nmm group (a = 4.047(3) Å, c = 7.275(4) Å). The resistivity shows an anomaly at 200 K, which should be ascribed to an SDW or structural transition. The SDW or structural transition is confirmed by magnetic susceptibility and heat capacity measurements. These behaviors are very similar to those observed in parent compounds of high-T(c) iron-based pnictide superconductors, in which the superconductivity shows up when the anomaly due to the SDW or structural transition is suppressed. Therefore, the new layered pnictide-oxide, BaTi(2)As(2)O, could be a potential parent compound for superconductivity. It is found that Li( + ) doping significantly suppresses the anomaly, but no superconductivity emerges so far.

  6. Itinerant electrons, local moments, and magnetic correlations in the pnictide superconductors CeFeAsO₁₋xFxand Sr(Fe₁₋xCox)₂As₂

    SciTech Connect

    Vilmercati, Paolo; Fedorov, Alexei; Bondino, Federica; Offi, Francesco; Panaccione, Giancarlo; Lacovig, Paolo; Simonelli, Laura; McGuire, Michael A.; Sefat, Athena S. M.; Mandrus, David; Sales, Brian C.; Egami, Takeshi; Ku, Wei; Mannella, Norman

    2012-06-15

    A direct and element-specific measurement of the local Fe spin moment has been provided by analyzing the Fe 3s core level photoemission spectra in the parent and optimally doped CeFeAsO₁₋xFx (x = 0, 0.11) and Sr(Fe₁₋xCox)2As2 (x = 0, 0.10) pnictides. The rapid time scales of the photoemission process allowed the detection of large local spin moments fluctuating on a 10⁻¹⁵ s time scale in the paramagnetic, antiferromagnetic, and superconducting phases, indicative of the occurrence of ubiquitous strong Hund's magnetic correlations. The magnitude of the spin moment is found to vary significantly among different families, 1.3μB in CeFeAsO and 2.1μB in SrFe₂As₂. Surprisingly, the spin moment is found to decrease considerably in the optimally doped samples, 0.9μB in CeFeAsO₀.₈₉F₀.₁₁ and 1.3μB in Sr(Fe₀.₉Co₀.₁)₂As₂. The strong variation of the spin moment against doping and material type indicates that the spin moments and the motion of itinerant electrons are influenced reciprocally in a self-consistent fashion, reflecting the strong competition between the antiferromagnetic superexchange interaction among the spin moments and the kinetic energy gain of the itinerant electrons in the presence of a strong Hund's coupling. By describing the evolution of the magnetic correlations concomitant with the appearance of superconductivity, these results constitute a fundamental step toward attaining a correct description of the microscopic mechanisms shaping the electronic properties in the pnictides, including magnetism and high-temperature superconductivity.

  7. Electrodynamics of superconducting pnictide superlattices

    SciTech Connect

    Perucchi, A.; Pietro, P. Di; Capitani, F.; Lupi, S.; Lee, S.; Kang, J. H.; Eom, C. B.; Jiang, J.; Weiss, J. D.; Hellstrom, E. E.; Dore, P.

    2014-06-02

    It was recently shown that superlattices where layers of the 8% Co-doped BaFe{sub 2}As{sub 2} superconducting pnictide are intercalated with non superconducting ultrathin layers of either SrTiO{sub 3} or of oxygen-rich BaFe{sub 2}As{sub 2}, can be used to control flux pinning, thereby increasing critical fields and currents, without significantly affecting the critical temperature of the pristine superconducting material. However, little is known about the electron properties of these systems. Here, we investigate the electrodynamics of these superconducting pnictide superlattices in the normal and superconducting state by using infrared reflectivity, from THz to visible range. We find that multigap structure of these superlattices is preserved, whereas some significant changes are observed in their electronic structure with respect to those of the original pnictide. Our results suggest that possible attempts to further increase the flux pinning may lead to a breakdown of the pnictide superconducting properties.

  8. Correlation between superconductivity and structural properties under high pressure of iron pnictide superconductor Ce[subscript 0.6]Y[subscript 0.4]FeAsO[subscript 0.8]F[subscript 0.2

    SciTech Connect

    Kanagaraj, M.; Arumugam, S.; Kumar, Ravhi S.; Selvan, N.R. Tamil; Muthu, S. Esakki; Prakash, J.; Thakur, Gohil S.; Yoshino, H.; Murata, K.; Matsubayashi, K.; Uwatoko, Y.; Sinogeikin, S.; Cornelius, Andrew; Ganguli, A.K.; Zhao, Yusheng

    2012-02-28

    We report here the pressure dependence of the electrical resistivity and magnetic susceptibility of polycrystalline Ce{sub 0.6}Y{sub 0.4}FeAsO{sub 0.8}F{sub 0.2} superconductor in the temperature range 4 K to 300 K up to 8 GPa. In-situ high pressure-low temperature x-ray diffraction was performed at 8 K up to 32 GPa using synchrotron x-rays with helium pressure medium. The results show that the applied pressure slightly increases the T{sub c} up to 1 GPa and then it decreases on further pressure increase. The reduction of superconducting transition temperature occurs with a transition to a collapsed tetragonal phase and may be associated with a possible valence change of Ce.

  9. Low temperature properties of pnictide CrAs single crystal

    NASA Astrophysics Data System (ADS)

    Wu, Wei; Zhang, Xiaodong; Yin, Zhihua; Zheng, Ping; Wang, Nanlin; Luo, Jianlin

    2010-07-01

    High quality single crystal CrAs was grown by Sn flux method. The results of magnetic susceptibility and electrical resistivity are reported in a temperature range of 2 to 800 K. At low temperatures, a T 2 dependence of resistivity is observed showing a Fermi-liquid behavior. The Kadowaki-Woods ratio is found to be 1×10-5 μΩ cm mol2 K2 mJ-2, which fits well to the universal value for many correlated electron systems. At about 270 K, a clear magnetic transition is observed with sharp changes of resistivity and susceptibility. Above 270 K, a linear-temperature dependence of the magnetic susceptibility is observed up to 700 K, which resembles the T-dependent magnetic susceptibility of parents of iron-pnictides superconductors.

  10. Superconductivity versus bound-state formation in a two-band superconductor with small Fermi energy: Applications to Fe pnictides/chalcogenides and doped SrTiO3

    NASA Astrophysics Data System (ADS)

    Chubukov, Andrey V.; Eremin, Ilya; Efremov, Dmitri V.

    2016-05-01

    Fe pnictides and Fe chalcogenides in which a superconducting gap has been detected on the bands that do not cross the Fermi level, and to FeSe, in which the superconducting gap is comparable to the Fermi energy. We apply the results for the model with two electron bands to Nb-doped SrTiO3 and argue that our theory explains the rapid increase of Tc when both bands start crossing the Fermi level.

  11. Contribution of interlayer hybridization to the electronic structure in iron pnictides: a study of EELS and first-principles calculations.

    PubMed

    Ma, Chao; Yang, Huaixin; Tian, Huanfang; Shi, Honglong; Wang, Zhiwei; Li, Jianqi

    2013-03-20

    Using electron energy loss spectroscopy (EELS) measurements and first-principles electronic structure calculations, the significant interlayer hybridization between the insulating layers (ReO or Ba) and the conducting FeAs layers was investigated in the layered iron pnictides, which is quite different from the case in the cuprate superconductors. This interlayer hybridization would result in an increase in the bandwidth near the Fermi level and interorbital charge transfer in the Fe 3d orbitals, which subsequently leads to a decrease in the Fe local moment and the modification of the Fermi surface topology. Therefore, a three-dimensional character of the electronic structure due to the interlayer hybridization is expected, as observed in previous experiments. These findings indicate that reduced dimensionality is no longer a necessary condition in the search for high-T(c) superconductors in iron pnictides.

  12. Disorder-induced topological change of the superconducting gap structure in iron pnictides.

    PubMed

    Mizukami, Y; Konczykowski, M; Kawamoto, Y; Kurata, S; Kasahara, S; Hashimoto, K; Mishra, V; Kreisel, A; Wang, Y; Hirschfeld, P J; Matsuda, Y; Shibauchi, T

    2014-11-28

    In superconductors with unconventional pairing mechanisms, the energy gap in the excitation spectrum often has nodes, which allow quasiparticle excitations at low energies. In many cases, such as in d-wave cuprate superconductors, the position and topology of nodes are imposed by the symmetry, and thus the presence of gapless excitations is protected against disorder. Here we report on the observation of distinct changes in the gap structure of iron-pnictide superconductors with increasing impurity scattering. By the successive introduction of nonmagnetic point defects into BaFe2(As(1-x)P(x))(2) crystals via electron irradiation, we find from the low-temperature penetration depth measurements that the nodal state changes to a nodeless state with fully gapped excitations. Moreover, under further irradiation the gapped state evolves into another gapless state, providing bulk evidence of unconventional sign-changing s-wave superconductivity. This demonstrates that the topology of the superconducting gap can be controlled by disorder, which is a strikingly unique feature of iron pnictides.

  13. Theory of Two-Magnon Raman Scattering in Iron Pnictides and Chalcogenides

    SciTech Connect

    Chen, C. C.

    2011-08-15

    Although the parent iron-based pnictides and chalcogenides are itinerant antiferromagnets, the use of local moment picture to understand their magnetic properties is still widespread. We study magnetic Raman scattering from a local moment perspective for various quantum spin models proposed for this new class of superconductors. These models vary greatly in the level of magnetic frustration and show a vastly different two-magnon Raman response. Light scattering by two-magnon excitations thus provides a robust and independent measure of the underlying spin interactions. In accord with other recent experiments, our results indicate that the amount of magnetic frustration in these systems may be small.

  14. A Minimal Two-band Model for the Superconducting Fe-pnictides

    SciTech Connect

    Raghu, S.

    2010-03-25

    Following the discovery of the Fe-pnictide superconductors, LDA band structure calculations showed that the dominant contributions to the spectral weight near the Fermi energy came from the Fe 3d orbitals. The Fermi surface is characterized by two hole surfaces around the {Lambda} point and two electron surfaces around the M point of the 2 Fe/cell Brillouin zone. Here, we describe a 2-band model that reproduces the topology of the LDA Fermi surface and exhibits both ferromagnetic and q = ({pi}, 0) spin density wave (SDW) fluctuations. We argue that this minimal model contains the essential low energy physics of these materials.

  15. Magneto-structural correlations in rare-earth cobalt pnictides

    NASA Astrophysics Data System (ADS)

    Thompson, Corey Mitchell

    Magnetic materials are used in many applications such as credit cards, hard drives, electric motors, sensors, etc. Although a vast range of magnetic solids is available for these purposes, our ability to improve their efficiency and discover new materials remains paramount to the sustainable progress and economic profitability in many technological areas. The search for magnetic solids with improved performance requires fundamental understanding of correlations between the structural, electronic, and magnetic properties of existing materials, as well as active exploratory synthesis that targets the development of new magnets. Some of the strongest permanent magnets, Nd 2Fe14B, SmCo5, and Sm2Co17, combine transition and rare-earth metals, benefiting from the strong exchange between the 4f and 3d magnetic sublattices. Although these materials have been studied in great detail, the development of novel magnets requires thorough investigation of other 3d-4 f intermetallics, in order to gain further insights into correlations between their crystal structures and magnetic properties. Among many types of intermetallic materials, ternary pnictides RCo 2Pn2 (R = La, Ce, Pr, Nd; Pn = P, As) are of interest because, despite their simple crystal structures, they contain two magnetic sublattices, exchange interactions between which may lead to rich and unprecedented magnetic behavior. Nevertheless, magnetism of these materials was studied only to a limited extent, especially as compared to the extensive studies of their silicide and germanide analogues. The ThCr2Si2 structure type, to which these ternary pnictides belong, is one of the most ubiquitous atomic arrangements encountered among intermetallic compounds. It accounts for over 1000 known intermetallics and has received increased attention due to the recently discovered FeAs-based superconductors. This dissertation is devoted to the investigation of

  16. Scaling between magnetic and lattice fluctuations in iron pnictide superconductors.

    PubMed

    Fernandes, Rafael M; Böhmer, Anna E; Meingast, Christoph; Schmalian, Jörg

    2013-09-27

    The phase diagram of the iron arsenides is dominated by a magnetic and a structural phase transition, which need to be suppressed in order for superconductivity to appear. The proximity between the two transition temperature lines indicates correlation between these two phases, whose nature remains unsettled. Here, we find a scaling relation between nuclear magnetic resonance and shear modulus data in the tetragonal phase of electron-doped Ba(Fe1-xCox)2As2 compounds. Because the former probes the strength of magnetic fluctuations while the latter is sensitive to orthorhombic fluctuations, our results provide strong evidence for a magnetically driven structural transition. PMID:24116808

  17. Possible suggestions for order parameter phase-sensitive experiments in the superconducting iron pnictides

    NASA Astrophysics Data System (ADS)

    Parker, David

    2010-03-01

    The iron pnictide superconductors have undergone intensive study since the original discovery by Kamihara et al early in 2008, with maximum Tc's exceeding 50 K. Despite this, the most basic questions such as pairing symmetry and mechanism have not been definitively settled. For the cuprates, the SQUID loop and tricrystal phase-sensitive experiments were instrumental in finally determining the d-wave gap symmetry; similar experiments were designed and implemented for triplet p-wave superconductivity. However, the main challenge in pnictides is to distinguish between two superconducting states, the sign-changing ``s±" and single-sign ``s++ states, which belong to the same point symmetry class. This means that while designing a Josephson loop one needs to invent a recipe to filter out different types of carriers at the two different contacts. By definition this is a quantitative rather than qualitative effect, and involves the relative amplitude of the order parameter, density-of-states and Fermi velocity, and the character of the electronic wavefunctions. Presently proposed methods either attempt to determine an optimal angle (i.e., non-90^o) for a SQUID junction, use different barrier characteristics in different directions, or exploit `sandwich' junctions employing two or more superconductors. In this talk, I discuss several recent proposals for phase-sensitive experiments which could help resolve the pairing symmetry controversy, as well as experimental work in this area.

  18. 2011 Aspen Winter Conference on Contrasting Superconductivity of Pnictides and Cuprates

    SciTech Connect

    Johnson, P.; Schmalian, J.; Canfield, P.; Chakravarty, S.

    2011-05-02

    Our quest for materials with better properties is closely integral to the fabric of our society. Currently the development of materials that will allow for improved generation, transport, and storage of energy is at the forefront of our research in condensed matter physics and materials science. Among these materials, compounds that exhibit correlated electron states and emergent phenomena such as superconductivity have great promise, but also difficulties that need to be overcome: problems associated with our need to reliably find, understand, improve and control these promising materials. At the same time, the field of correlated electrons represents the frontier of our understanding of the electronic properties of solids. It contains deep open scientific issues within the broad area of quantum phenomena in matter. The aim of this workshop is to explore and understand the physics of recently discovered Fe-based high-temperature superconductors and contrast and compare them with the cuprates. The superconductivity in iron pnictides, with transition temperatures in excess of 55 K, was discovered in early 2008. The impact of this discovery is comparable to cuprates discovered in 1986. At the same time a number of recent experimental developments in cuprates may lead to a shift in our thinking with regards to these materials. There is therefore much to be learned by devoting a conference in which both classes of superconductors are discussed, especially at this nascent stage of the pnictides.

  19. Correlations and effects of pressure in Fe-pnictides

    NASA Astrophysics Data System (ADS)

    Valenti, Roser

    2014-03-01

    In this talk we will explore the effects of correlations and pressure in Fe-based superconductors by considering a combination of density functional theory calculations and dynamical mean field theory and compare our results with recent ARPES and de Haas van Alphen experiments. We will discuss the importance of orbital-selective correlations in the 111 (LiFeAs, LiFeP) and 122 families (BaFe2As2,CaFe2As2, KFe2As2) and indicate how the topology of the Fermi surface, specially in KFe2As2, is influenced by these effects. In this context, we will show why MgFeGe, an isostructural and isoelectronic system to LiFeAs, doesn't superconduct. In the case of the 122 systems, we will predict and analyze changes in the electronic and magnetic properties under hydrostatic, tensile and compressive pressure and will discuss our results in relation to (i) superconductivity, (ii) magnetism and (iii) the mechanisms involved in the detwinning process of an orthorhombic iron-pnictide crystal a. Funding has been provided by the German Science Foundation (DFG).

  20. Weak-coupling superconductivity in a strongly correlated iron pnictide.

    PubMed

    Charnukha, A; Post, K W; Thirupathaiah, S; Pröpper, D; Wurmehl, S; Roslova, M; Morozov, I; Büchner, B; Yaresko, A N; Boris, A V; Borisenko, S V; Basov, D N

    2016-01-05

    Iron-based superconductors have been found to exhibit an intimate interplay of orbital, spin, and lattice degrees of freedom, dramatically affecting their low-energy electronic properties, including superconductivity. Albeit the precise pairing mechanism remains unidentified, several candidate interactions have been suggested to mediate the superconducting pairing, both in the orbital and in the spin channel. Here, we employ optical spectroscopy (OS), angle-resolved photoemission spectroscopy (ARPES), ab initio band-structure, and Eliashberg calculations to show that nearly optimally doped NaFe0.978Co0.022As exhibits some of the strongest orbitally selective electronic correlations in the family of iron pnictides. Unexpectedly, we find that the mass enhancement of itinerant charge carriers in the strongly correlated band is dramatically reduced near the Γ point and attribute this effect to orbital mixing induced by pronounced spin-orbit coupling. Embracing the true band structure allows us to describe all low-energy electronic properties obtained in our experiments with remarkable consistency and demonstrate that superconductivity in this material is rather weak and mediated by spin fluctuations.

  1. Coexistence of superconductivity, ferromagnetism and antiferromagnetism in iron pnictides

    NASA Astrophysics Data System (ADS)

    Gill, Raminder

    2016-05-01

    Coexistence of Superconductivity and magnetism have always been the fascinating area of interest for condensed-matter physicists. A variety of systems, such as cuprates, heavy fermions, and iron pnictides showed superconductivity in a narrow region near the border to antiferromagnetism (AFM)as a function of pressure or doping. However, the coexistence of superconductivity and ferromagnetism (FM) has seen in URhGe, UGe2, ErRh4B4 and many compounds. Here, we present a third situation where superconductivity coexists with FM and AFM. The recent experimental finding of interplay of ferromagnetism,antiferromagnetism and superconductivity in EuFe2(As1-xPx)2 impressed us to discuss this problem in detail. Ironpnictides are high Tc magnetic superconductors and could be very useful in finding many new superconductorswith high Tc probably near to room temperature. In this paper, we have theoretically calculated the superconducting order parameter of EuFe2(As1-xPx)2 where magnetic ordering is due to Eu+ moments and superconductivity is due to Fe3+ moments. The Eu ions order antiferromagnetically for x≤0.13, while a crossover is observed for x≥0.22 whereupon the Eu ions order ferromagnetically.

  2. Correlation effects in the iron pnictides

    SciTech Connect

    Zhu, Jian-xin; Si, Qimiao; Abrahams, Elihu; Dai, Jianhui

    2009-01-01

    One of the central questions about the iron pnictides concerns the extent to which their electrons are strongly correlated. Here we address this issue through the phenomenology of the charge transport and dynamics, single-electron excitation spectrum, and magnetic ordering and dynamics. We outline the evidence that the parent compounds, while metallic, have electron interactions that are sufficiently strong to produce incipient Mott physics. In other words, in terms of the strength of electron correlations compared to the kinetic energy, the iron pnictides are closer to intermediately-coupled systems lying at the boundary between itinerancy and localization, such as V{sub 2}O{sub 3} a or Se-doped NiS{sub 2} , rather than to simple antiferromagnetic metals like Cr. This level of electronic correlations produces a new small parameter for controlled theoretical analyses, namely the fraction of the single-electron spectral weight that lies in the coherent part. Using this expansion parameter, we construct the effective low-energy Hamiltonian and discuss its implications for the magnetic order and magnetic quantum criticality. Finally, this approach sharpens the notion of magnetic frustration for such a metallic system, and brings about a multi band matrix t-J{sub 1}-J{sub 2} model for the carrier-doped iron pnictides.

  3. Fabrication and characterization of iron pnictide wires and bulk materials through the powder-in-tube method

    NASA Astrophysics Data System (ADS)

    Ma, Yanwei; Gao, Zhaoshun; Qi, Yanpeng; Zhang, Xianping; Wang, Lei; Zhang, Zhiyu; Wang, Dongliang

    2009-05-01

    The recent discovery of superconductivity in the iron-based superconductors with very high upper critical fields presents a new possibility for practical applications, but fabricating fine-wire is a challenge because of mechanically hard and brittle powders and the toxicity and volatility of arsenic. In this paper, we report the synthesis and the physical characterization of iron pnictide wires and bulks prepared by the powder-in-tube method (PIT). A new class of high- Tc iron pnictide composite wires, such as LaFeAsO 1-xF x, SmFeAsO 1-xF x and Sr 1-xK xFeAs, has been fabricated by the in situ PIT technique using Fe, Ta and Nb tubes. Microscopy and X-ray analysis show that the superconducting core is continuous, and retains phase composition after wire drawing and heat treatment. Furthermore, the wires exhibit a very weak Jc-field dependence behavior even at high temperatures. The upper critical field Hc2(0) value can exceed 100 T, surpassing those of MgB 2 and all the low temperature superconductors and indicating a strong potential for applications requiring very high field. These results demonstrate the feasibility of producing superconducting pnictide composite wire. We also applied the one-step PIT method to synthesize the iron-based bulks, due to its convenience and safety. In fact, by using this technique, we have successfully discovered superconductivity at 35 K and 15 K in Eu 0.7Na 0.3Fe 2As 2 and SmCoFeAsO compounds, respectively. These clearly suggest that the one-step PIT technique is unique and versatile and hence can be tailored easily for other rare earth derivatives of novel iron-based superconductors.

  4. Proximity Effect at Graphene - High Tc Superconductor Junctions

    NASA Astrophysics Data System (ADS)

    Wang, Da; Shih, En-Min; Arefe, Ghidewon; Kim, Youngduck; Edelberg, Drew; Andrade, Erick; Wang, Dennis; Hone, James; Dean, Cory; Pasupathy, Abhay; Department of Physics, Columbia University, New York, NY 10027, USA Collaboration

    The proximity effect is a well-known mesoscopic phenomenon where Cooper pairs from a superconductor (S) enter into a normal metal (N) that is well coupled to it. Since graphene was discovered a decade ago, the proximity effect at superconductor-graphene junctions has been extensively studied and interesting phenomena such as specular Andreev reflection and ballistic transport at graphene Josephson junctions have been observed. However, superconductors used in these experiments to date are of conventional low Tc, such as aluminum(Tc=1.2K), NbSe2(Tc=7K), and MoRe(Tc=8K). Understanding how the proximity effect works between high-Tc superconductors (pnictides and cuprates) and the Dirac Fermions of graphene remains largely unexplored. The chief technical challenge here is to create high-quality junctions between high-Tc superconductors and graphene. In this work, we will introduce a home-made setup that allows us to exfoliate, transfer and encapsulate superconductor-graphene junctions in a well controlled inert atmosphere. Transport measurements of the proximity effect at graphene-iron pnictide(FeSe, FeTeSe) and graphene-cuprate(BSCCO) junctions will be described.

  5. Of Substitution and Doping: Spatial and Electronic Structure in Fe Pnictides

    NASA Astrophysics Data System (ADS)

    Merz, Michael; Schweiss, Peter; Nagel, Peter; Huang, Meng-Jie; Eder, Robert; Wolf, Thomas; von Löhneysen, Hilbert; Schuppler, Stefan

    2016-04-01

    A highly intriguing aspect in iron-pnictide superconductors is the composition-dependent electronic structure, in particular the question if and how charge carriers are introduced to the system upon substitution of Ba by alkali metals or of Fe by other transition metals, TM. We report on a systematic study of spatial structure and electronic states by x-ray diffraction and x-ray absorption on a large number of compositions in the (Ba,K)(Fe,TM)2As2 family. The coherent combination of detailed structural information with an in-depth analysis of the electronic structure allows us to sensitively disentangle (charge-carrier) "doping" effects from "substitutional" effects. Results include a doping character that is site-decoupled, as well as TM 3d energy-level schemes that exhibit non-standard level sequences and even t2-e level crossings. Our study indicates that doping per se seems to play a lesser role than expected for pnictide superconductivity and magnetism.

  6. Magnetic and Ising quantum phase transitions in a model for isoelectronically tuned iron pnictides

    NASA Astrophysics Data System (ADS)

    Wu, Jianda; Si, Qimiao; Abrahams, Elihu

    2016-03-01

    Considerations of the observed bad-metal behavior in Fe-based superconductors led to an early proposal for quantum criticality induced by isoelectronic P for As doping in iron arsenides, which has since been experimentally confirmed. We study here an effective model for the isoelectronically tuned pnictides using a large-N approach. The model contains antiferromagnetic and Ising-nematic order parameters appropriate for J1-J2 exchange-coupled local moments on an Fe square lattice, and a damping caused by coupling to itinerant electrons. The zero-temperature magnetic and Ising transitions are concurrent and essentially continuous. The order-parameter jumps are very small, and are further reduced by the interplane coupling; consequently, quantum criticality occurs over a wide dynamical range. Our results reconcile recent seemingly contradictory experimental observations concerning the quantum phase transition in the P-doped iron arsenides.

  7. Chemistry and Electronic Structure of Iron-Based Superconductors

    SciTech Connect

    Safa-Sefat, Athena; Singh, David J

    2011-01-01

    The solid state provides a richly varied fabric for intertwining chemical bonding, electronic structure, and magnetism. The discovery of superconductivity in iron pnictides and chalcogenides has revealed new aspects of this interplay, especially involving magnetism and superconductivity. Moreover, it has challenged prior thinking about high-temperature superconductivity by providing a set of materials that differ in many crucial aspects from the previously known cuprate superconductors. Here we review some of what is known about the superconductivity and its interplay with magnetism, chemistry, and electronic structure in Fe-based superconductors.

  8. Numerical Studies of Doped Iron Pnictides

    NASA Astrophysics Data System (ADS)

    Bishop, Christopher; Liang, Shuhua; Moreo, Adriana; Dagotto, Elbio

    The phase diagram of electron-doped pnictides is studied varying the temperature, electronic density, and isotropic disorder strength and dilution via numerical studies of a three-orbital spin-fermion model with lattice degrees of freedom. Doping introduces disorder but in theoretical studies the effect of the randomly located dopants is difficult to address. Numerically the effects of electronic doping, regulated by a chemical potential, and impurity disorder at randomly selected sites can be independently controlled. It was found that the reduction with doping of the Neel and the structural transition temperatures, and the stabilization of a nematic state, is mainly controlled by the magnetic dilution due to the disorder. Fermi surface changes due to doping affect only slightly both critical temperatures. Our findings are compatible with neutron scattering and STM results, unveiling a patchy network of locally magnetically ordered anisotropic clusters, despite the isotropic disorder. The fragile tendency to nematicity intrinsic of translational invariant electronic systems needs to be supplemented by disorder and dilution to stabilize the robust nematic phase experimentally found in electron-doped 122 pnictides. National Science Foundation Grant No. DMR-1404375.

  9. Scanning Tunneling Microscopy and Spectroscopy of Iron-Based Superconductors

    NASA Astrophysics Data System (ADS)

    Hoffman, Jennifer

    2010-03-01

    Two decades after the discovery of high-Tc superconductivity in the cuprates, superconductivity was discovered up to 55, in a second family of materials: the iron-pnictides. This recent discovery has generated tremendous excitement for several reasons. First, there is hope that the iron-pnictides will finally provide the foil necessary to understand the enormous yet puzzling body of research on the cuprates. Second, reports of low anisotropy and strong vortex pinning in these new materials have spurred optimism that the iron-pnictides may finally lead to the widespread technological applications which have been elusive for cuprates. In this talk, I will present the first scanning tunneling spectroscopic imaging study of a single crystal iron-pnictide superconductor in high magnetic fields. We study optimally doped BaCo0.2Fe1.8As2 with Tc= 25.3,, finding a ˜6, superconducting gap with nanoscale inhomogeneity, which leads to an average reduced gap of 2δ/kBTc˜5.7. We further observe a static disordered vortex lattice at 9,, and demonstrate that vortices are pinned in the bulk of this material, a promising observation for practical application.

  10. Importance of effective dimensionality in manganese pnictides

    NASA Astrophysics Data System (ADS)

    Zingl, Manuel; Assmann, Elias; Seth, Priyanka; Krivenko, Igor; Aichhorn, Markus

    2016-07-01

    In this paper we investigate the two manganese pnictides BaMn2As2 and LaMnAsO, using fully charge self-consistent density functional plus dynamical mean-field theory calculations. These systems have a nominally half-filled d shell, and as a consequence, electronic correlations are strong, placing these compounds at the verge of a metal-insulator transition. Although their crystal structure is composed of similar building blocks, our analysis shows that the two materials exhibit a very different effective dimensionality, LaMnAsO being a quasi-two-dimensional material in contrast to the much more three-dimensional BaMn2As2 . We demonstrate that the experimentally observed differences in the Néel temperature, the band gap, and the optical properties of the manganese compounds under consideration can be traced back to exactly this effective dimensionality. Our calculations show excellent agreement with measured optical spectra.

  11. Organic Superconductors

    SciTech Connect

    Charles Mielke

    2009-02-27

    Intense magnetic fields are an essential tool for understanding layered superconductors. Fundamental electronic properties of organic superconductors are revealed in intense (60 tesla) magnetic fields. Properties such as the topology of the Fermi surface and the nature of the superconducting order parameter are revealed. With modest maximum critical temperatures~13K the charge transfer salt organic superconductors prove to be incredibly valuable materials as their electronically clean nature and layered (highly anisotropic) structures yield insights to the high temperature superconductors. Observation of de Haas-van Alphen and Shubnikov-de Haas quantum oscillatory phenomena, magnetic field induced superconductivity and re-entrant superconductivity are some of the physical phenomena observed in the charge transfer organic superconductors. In this talk, I will discuss the nature of organic superconductors and give an overview of the generation of intense magnetic fields; from the 60 tesla millisecond duration to the extreme 1000 tesla microsecond pulsed magnetic fields.

  12. Spin-Lattice Coupling and Superconductivity in Fe Pnictides

    DOE PAGESBeta

    Egami, T.; Fine, B. V.; Parshall, D.; Subedi, A.; Singh, D. J.

    2010-01-01

    We consider strong spin-lattice and spin-phonon coupling in iron pnictides and discuss its implications on superconductivity. Strong magneto-volume effect in iron compounds has long been known as the Invar effect. Fe pnictides also exhibit this effect, reflected in particular on the dependence of the magnetic moment on the atomic volume of Fe defined by the positions of the nearest neighbor atoms. Through the phenomenological Landau theory, developed on the basis of the calculations by the density functional theory (DFT) and the experimental results, we quantify the strength of the spin-lattice interaction as it relates to the Stoner criterion for themore » onset of magnetism. We suggest that the coupling between electrons and phonons through the spin channel may be sufficiently strong to be an important part of the superconductivity mechanism in Fe pnictides.« less

  13. A universal explanation of tunneling conductance in exotic superconductors

    NASA Astrophysics Data System (ADS)

    Hong, Jongbae; Abergel, D. S. L.

    2016-08-01

    A longstanding mystery in understanding cuprate superconductors is the inconsistency between the experimental data measured by scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES). In particular, the gap between prominent side peaks observed in STS is much bigger than the superconducting gap observed by ARPES measurements. Here, we reconcile the two experimental techniques by generalising a theory which was previously applied to zero-dimensional mesoscopic Kondo systems to strongly correlated two-dimensional (2D) exotic superconductors. We show that the side peaks observed in tunneling conductance measurements in all these materials have a universal origin: They are formed by coherence-mediated tunneling under bias and do not directly reflect the underlying density of states (DOS) of the sample. We obtain theoretical predictions of the tunneling conductance and the density of states of the sample simultaneously and show that for cuprate and pnictide superconductors, the extracted sample DOS is consistent with the superconducting gap measured by ARPES.

  14. Anomalous superfluid density in quantum critical superconductors

    PubMed Central

    Hashimoto, Kenichiro; Mizukami, Yuta; Katsumata, Ryo; Shishido, Hiroaki; Yamashita, Minoru; Ikeda, Hiroaki; Matsuda, Yuji; Schlueter, John A.; Fletcher, Jonathan D.; Carrington, Antony; Gnida, Daniel; Kaczorowski, Dariusz; Shibauchi, Takasada

    2013-01-01

    When a second-order magnetic phase transition is tuned to zero temperature by a nonthermal parameter, quantum fluctuations are critically enhanced, often leading to the emergence of unconventional superconductivity. In these “quantum critical” superconductors it has been widely reported that the normal-state properties above the superconducting transition temperature Tc often exhibit anomalous non-Fermi liquid behaviors and enhanced electron correlations. However, the effect of these strong critical fluctuations on the superconducting condensate below Tc is less well established. Here we report measurements of the magnetic penetration depth in heavy-fermion, iron-pnictide, and organic superconductors located close to antiferromagnetic quantum critical points, showing that the superfluid density in these nodal superconductors universally exhibits, unlike the expected T-linear dependence, an anomalous 3/2 power-law temperature dependence over a wide temperature range. We propose that this noninteger power law can be explained if a strong renormalization of effective Fermi velocity due to quantum fluctuations occurs only for momenta k close to the nodes in the superconducting energy gap Δ(k). We suggest that such “nodal criticality” may have an impact on low-energy properties of quantum critical superconductors. PMID:23404698

  15. Evidence for weak electronic correlations in Fe-pnictides

    SciTech Connect

    Yang, W.L.

    2010-04-29

    Using x-ray absorption and resonant inelastic x-ray scattering, charge dynamics at and near the Fe L edges is investigated in Fe pnictide materials, and contrasted to that measured in other Fe compounds. It is shown that the XAS and RIXS spectra for 122 and 1111 Fe pnictides are each qualitatively similar to Fe metal. Cluster diagonalization, multiplet, and density-functional calculations show that Coulomb correlations are much smaller than in the cuprates, highlighting the role of Fe metallicity and strong covalency in these materials. Best agreement with experiment is obtained using Hubbard parameters U {approx}< 2eV and J {approx} 0.8eV.

  16. Evidence for weak electronic correlations in Fe-Pnictides

    SciTech Connect

    Yang, W. L.; Sorini, A. P.; Chen, C-C.; Moritz, B.; Lee, W.-S.; Vernay, F.; Olalde-Velasco, P.; Denlinger, J. D.; Delley, B.; Chu, J.-H.; Analytis, J.G.; Fisher, I. R.; Ren, Z. A.; Yang, J.; Lu, W.; Zhao, Z. X.; van den Brink, J.; Hussain, Z.; Shen, Z.-X.; Devereaux, T. P.

    2009-06-11

    Using x-ray absorption and resonant inelastic x-ray scattering, charge dynamics at and near the Fe L edges is investigated in Fe pnictide materials, and contrasted tothat measured in other Fe compounds. It is shown that the XAS and RIXS spectra for 122 and 1111 Fe pnictides are each qualitatively similar to Fe metal. Cluster diagonalization, multiplet, and density-functional calculations show that Coulomb correlations are much smaller than in the cuprates, highlighting the role of Fe metallicity and strong covalency in these materials. Best agreement with experiment is obtained using Hubbard parameters U<~;; 2eV and J ~;; 0.8eV.

  17. Percolative theories of strongly disordered ceramic high-temperature superconductors

    PubMed Central

    Phillips, J. C.

    2010-01-01

    Optimally doped ceramic superconductors (cuprates, pnictides, etc.) exhibit transition temperatures T c much larger than strongly coupled metallic superconductors like Pb (T c = 7.2 K, E g/kT c = 4.5) and exhibit many universal features that appear to contradict the Bardeen, Cooper, and Schrieffer theory of superconductivity based on attractive electron-phonon pairing interactions. These complex materials are strongly disordered and contain several competing nanophases that cannot be described effectively by parameterized Hamiltonian models, yet their phase diagrams also exhibit many universal features in both the normal and superconductive states. Here we review the rapidly growing body of experimental results that suggest that these anomalously universal features are the result of marginal stabilities of the ceramic electronic and lattice structures. These dual marginal stabilities favor both electronic percolation of a dopant network and rigidity percolation of the deformed lattice network. This “double percolation” model has previously explained many features of the normal-state transport properties of these materials and is the only theory that has successfully predicted strict lowest upper bounds for T c in the cuprate and pnictide families. Here it is extended to include Coulomb correlations and percolative band narrowing, as well as an angular energy gap equation, which rationalizes angularly averaged gap/T c ratios, and shows that these are similar to those of conventional strongly coupled superconductors. PMID:20080578

  18. Correlation, magnetization and conduction in iron pnictides and iron chalcogenides

    NASA Astrophysics Data System (ADS)

    Yin, Zhiping; Haule, Kristjan; Kotliar, Gabriel

    2011-03-01

    By combining density functional theory (DFT) and dynamical mean field theory (DMFT), we study the electronic properties of iron pnictides and iron chalcogenides in both the paramagnetic and magnetic states. With ab initio derived realistic Coulomb interaction U and Hund's exchange coupling J, we find detailed agreements bewtween our calculations and many experimental observations in these compounds, including ARPES, magnetic properties, optical conductivity and anisotropy, and so on, WITHOUT any adjustment such as shifting of atomic positions, Fermi level and bands and renormalizations of bands which are commonly needed in DFT calculations in order to compare with experiments. Our theory explains the origin of the different magnetizations in FeTe and other iron pnictides and provides a unique physical picture. We find that in the magnetic phase of the iron pnictides, both the spin and the orbital polarization are strongly energy dependent. The spin polarization becomes weaker around Fermi level when the orbital polarization is stronger and vice verse at high energies. We stress on the role of the Hund's J rather than the Coulomb U and show how the iron pnictides and iron chalcogenides differ from other compounds.

  19. The electronic phase diagram of the LaO(1-x)F(x)FeAs superconductor.

    PubMed

    Luetkens, H; Klauss, H-H; Kraken, M; Litterst, F J; Dellmann, T; Klingeler, R; Hess, C; Khasanov, R; Amato, A; Baines, C; Kosmala, M; Schumann, O J; Braden, M; Hamann-Borrero, J; Leps, N; Kondrat, A; Behr, G; Werner, J; Büchner, B

    2009-04-01

    The competition of magnetic order and superconductivity is a key element in the physics of all unconventional superconductors, for example in high-transition-temperature cuprates, heavy fermions and organic superconductors. Here superconductivity is often found close to a quantum critical point where long-range antiferromagnetic order is gradually suppressed as a function of a control parameter, for example charge-carrier doping or pressure. It is believed that dynamic spin fluctuations associated with this quantum critical behaviour are crucial for the mechanism of superconductivity. Recently, high-temperature superconductivity has been discovered in iron pnictides, providing a new class of unconventional superconductors. Similar to other unconventional superconductors, the parent compounds of the pnictides show a magnetic ground state and superconductivity is induced on charge-carrier doping. In this Letter the structural and electronic phase diagram is investigated by means of X-ray scattering, muon spin relaxation and Mössbauer spectroscopy on the series LaO(1-x)F(x)FeAs. We find a discontinuous first-order-like change of the Néel temperature, the superconducting transition temperature and the respective order parameters. Our results strongly question the relevance of quantum critical behaviour in iron pnictides and prove a strong coupling of the structural orthorhombic distortion and the magnetic order both disappearing at the phase boundary to the superconducting state.

  20. Electronic and magnetic phase diagram in KxFe2-ySe2 superconductors

    PubMed Central

    Yan, Y. J.; Zhang, M.; Wang, A. F.; Ying, J. J.; Li, Z. Y.; Qin, W.; Luo, X. G.; Li, J. Q.; Hu, Jiangping; Chen, X. H.

    2012-01-01

    The correlation and competition between antiferromagnetism and superconductivity are one of the most fundamental issues in high temperature superconductors. Superconductivity in high temperature cuprate superconductors arises from suppressing an antiferromagnetic (AFM) Mott insulator1 while in iron-pnictide superconductors arises from AFM semimetals and can coexist with AFM orders23456789. This difference raises many intriguing debates on the relation between the two classes of high temperature superconductors. Recently, superconductivity at 32 K has been reported in iron-chalcogenide superconductors AxFe2−ySe2 (A = K, Rb, and Cs)101112. They have the same structure as that of iron-pnictide 122-system131415. Here, we report electronic and magnetic phase diagram of KxFe2−ySe2 system as a function of Fe valence. We find a superconducting phase sandwiched between two AFM insulating phases. The two insulating phases are characterized by two distinct superstructures caused by Fe vacancy orders with modulation wave vectors of q1 = (1/5, 3/5, 0) and q2 = (1/4, 3/4, 0), respectively. PMID:22355726

  1. Three- to two-dimensional transition of the electronic structure in CaFe2As2: a parent compound for an iron arsenic high-temperature superconductor.

    PubMed

    Liu, Chang; Kondo, Takeshi; Ni, Ni; Palczewski, A D; Bostwick, A; Samolyuk, G D; Khasanov, R; Shi, M; Rotenberg, E; Bud'ko, S L; Canfield, P C; Kaminski, A

    2009-04-24

    We use angle-resolved photoemission spectroscopy (ARPES) to study the electronic properties of CaFe2As2-parent compound of a pnictide superconductor. We find that the structural and magnetic transition is accompanied by a three- to two-dimensional (3D-2D) crossover in the electronic structure. Above the transition temperature (T_{s}) Fermi surfaces around Gamma and X points are cylindrical and quasi 2D. Below T_{s}, the Gamma pocket forms a 3D ellipsoid, while the X pocket remains quasi 2D. This finding strongly suggests that low dimensionality plays an important role in understanding the superconducting mechanism in pnictides. PMID:19518747

  2. Non-Fermi liquid behavior and non-universal superconducting gap structure in Fe-pnictides

    NASA Astrophysics Data System (ADS)

    Matsuda, Yuji

    2010-03-01

    The discovery of Fe-pnictide superconductors with Tc exceeding 55 K raises fundamental questions about origin of high-Tc superconductivity. Here we report the systematic studies of the normal-state charge transport, Fermi surface structure and superconducting gap structure in high-quality single crystals of BaFe2(As1-xPx)2 (0 <=x <=0.71), ranging from the SDW state to overdoped Fermi liquid state. Near the SDW boundary, the transport coefficients, including resistivity, Hall coefficient and magnetoresistance, exhibit striking deviations from the Fermi liquid properties [1]. The Fermi surface structure determined by the dHvA effect shows that in the superconducting dome the volume of the electron and hole sheets shrink linearly and the effective masses become strongly enhanced with decreasing x [2]. It is likely that these trends originate from the many-body interaction which gives rise to superconductivity. The penetration depth, thermal conductivity and NMR data for BaFe2(As0.67P0.33)2 (Tc=30 K) provide unambiguous evidence for line nodes in the superconducting gap function [3], in sharp contrast to the other Fe-based compounds with fully gapped structure. This indicates that the gap structure of Fe-based high-Tc superconductors is not universal.[1] S. Kasahara et al., arXiv:0905.4427 [2] H. Shishido et al., arXiv:0910.3634 [3] K. Hashimoto et al., arXiv:0907.4399 [4] K. Hashimoto et al., Phys. Rev. Lett. 102, 017002 (2009), ibid 102, 207001 (2009).

  3. Coexistence of ferromagnetism and superconductivity in iron based pnictides: a time resolved magnetooptical study

    PubMed Central

    Pogrebna, A.; Mertelj, T.; Vujičić, N.; Cao, G.; Xu, Z. A.; Mihailovic, D.

    2015-01-01

    Ferromagnetism and superconductivity are antagonistic phenomena. Their coexistence implies either a modulated ferromagnetic order parameter on a lengthscale shorter than the superconducting coherence length or a weak exchange coupling between the itinerant superconducting electrons and the localized ordered spins. In some iron based pnictide superconductors the coexistence of ferromagnetism and superconductivity has been clearly demonstrated. The nature of the coexistence, however, remains elusive since no clear understanding of the spin structure in the superconducting state has been reached and the reports on the coupling strength are controversial. We show, by a direct optical pump-probe experiment, that the coupling is weak, since the transfer of the excess energy from the itinerant electrons to ordered localized spins is much slower than the electron-phonon relaxation, implying the coexistence without the short-lengthscale ferromagnetic order parameter modulation. Remarkably, the polarization analysis of the coherently excited spin wave response points towards a simple ferromagnetic ordering of spins with two distinct types of ferromagnetic domains. PMID:25583548

  4. Coexistence of ferromagnetism and superconductivity in iron based pnictides: a time resolved magnetooptical study.

    PubMed

    Pogrebna, A; Mertelj, T; Vujičić, N; Cao, G; Xu, Z A; Mihailovic, D

    2015-01-13

    Ferromagnetism and superconductivity are antagonistic phenomena. Their coexistence implies either a modulated ferromagnetic order parameter on a lengthscale shorter than the superconducting coherence length or a weak exchange coupling between the itinerant superconducting electrons and the localized ordered spins. In some iron based pnictide superconductors the coexistence of ferromagnetism and superconductivity has been clearly demonstrated. The nature of the coexistence, however, remains elusive since no clear understanding of the spin structure in the superconducting state has been reached and the reports on the coupling strength are controversial. We show, by a direct optical pump-probe experiment, that the coupling is weak, since the transfer of the excess energy from the itinerant electrons to ordered localized spins is much slower than the electron-phonon relaxation, implying the coexistence without the short-lengthscale ferromagnetic order parameter modulation. Remarkably, the polarization analysis of the coherently excited spin wave response points towards a simple ferromagnetic ordering of spins with two distinct types of ferromagnetic domains.

  5. Spin-orbital interplay and topology in the nematic phase of iron pnictides

    NASA Astrophysics Data System (ADS)

    Fanfarillo, Laura; Cortijo, Alberto; Valenzuela, Belén

    2015-06-01

    The origin of the nematic state is an important puzzle to be solved in iron pnictides. Iron superconductors are multiorbital systems and these orbitals play an important role at low energy. The singular C4 symmetry of dz x and dy z orbitals has a profound influence at the Fermi surface since the Γ pocket has vortex structure in the orbital space and the X /Y electron pockets have y z /z x components, respectively. We propose a low-energy theory for the spin-nematic model derived from a multiorbital Hamiltonian. In the standard spin-nematic scenario, the ellipticity of the electron pockets is a necessary condition for nematicity. In the present model, nematicity is essentially due to the singular C4 symmetry of y z and z x orbitals. By analyzing the (π ,0 ) spin susceptibility in the nematic phase, we find a spontaneous generation of orbital splitting, extending previous calculations in the magnetic phase. We also find that the (π ,0 ) spin susceptibility has an intrinsic anisotropic momentum dependence due to the nontrivial topology of the Γ pocket.

  6. Coexistence of ferromagnetism and superconductivity in iron based pnictides: a time resolved magnetooptical study.

    PubMed

    Pogrebna, A; Mertelj, T; Vujičić, N; Cao, G; Xu, Z A; Mihailovic, D

    2015-01-01

    Ferromagnetism and superconductivity are antagonistic phenomena. Their coexistence implies either a modulated ferromagnetic order parameter on a lengthscale shorter than the superconducting coherence length or a weak exchange coupling between the itinerant superconducting electrons and the localized ordered spins. In some iron based pnictide superconductors the coexistence of ferromagnetism and superconductivity has been clearly demonstrated. The nature of the coexistence, however, remains elusive since no clear understanding of the spin structure in the superconducting state has been reached and the reports on the coupling strength are controversial. We show, by a direct optical pump-probe experiment, that the coupling is weak, since the transfer of the excess energy from the itinerant electrons to ordered localized spins is much slower than the electron-phonon relaxation, implying the coexistence without the short-lengthscale ferromagnetic order parameter modulation. Remarkably, the polarization analysis of the coherently excited spin wave response points towards a simple ferromagnetic ordering of spins with two distinct types of ferromagnetic domains. PMID:25583548

  7. Pairing Strengths for a Two Orbital Model of the Fe-pnictides

    SciTech Connect

    Qi, Xiao-Liang; Raghu, S.; Liu, Chao-Xing; Scalapino, D.J.; Zhang, Shou-Cheng; /Stanford U., Phys. Dept.

    2010-03-25

    Using an RPA approximation, we have calculated the strengths of the singlet and triplet pairing interactions which arise from the exchange of spin and orbital fluctuations for a 2-orbital model of the Fe-pnictide superconductors. When the system is doped with F, the electron pockets become dominant and we find that the strongest pairing occurs in the singlet d-wave pairing and the triplet p-wave pairing channels, which compete closely. The pairing structure in the singlet d-wave channel corresponds to a superposition of near neighbor intra-orbital singlets with a minus sign phase difference between the d{sub xz} and d{sub yz} pairs. The leading pairing configuration in the triplet channel also involves a nearest neighbor intra-orbital pairing. We find that the strengths of both the singlet and triplet pairing grow, with the singlet pairing growing faster, as the onsite Coulomb interaction approaches the value where the S = 1 particle-hole susceptibility diverges.

  8. Superconductor Composite

    DOEpatents

    Dorris, Stephen E.; Burlone, Dominick A.; Morgan; Carol W.

    1999-02-02

    A superconducting conductor fabricated from a plurality of wires, e.g., fine silver wires, coated with a superconducting powder. A process of applying superconducting powders to such wires, to the resulting coated wires and superconductors produced therefrom.

  9. Anomalous critical fields in quantum critical superconductors.

    PubMed

    Putzke, C; Walmsley, P; Fletcher, J D; Malone, L; Vignolles, D; Proust, C; Badoux, S; See, P; Beere, H E; Ritchie, D A; Kasahara, S; Mizukami, Y; Shibauchi, T; Matsuda, Y; Carrington, A

    2014-01-01

    Fluctuations around an antiferromagnetic quantum critical point (QCP) are believed to lead to unconventional superconductivity and in some cases to high-temperature superconductivity. However, the exact mechanism by which this occurs remains poorly understood. The iron-pnictide superconductor BaFe2(As(1-x)P(x))2 is perhaps the clearest example to date of a high-temperature quantum critical superconductor, and so it is a particularly suitable system to study how the quantum critical fluctuations affect the superconducting state. Here we show that the proximity of the QCP yields unexpected anomalies in the superconducting critical fields. We find that both the lower and upper critical fields do not follow the behaviour, predicted by conventional theory, resulting from the observed mass enhancement near the QCP. Our results imply that the energy of superconducting vortices is enhanced, possibly due to a microscopic mixing of antiferromagnetism and superconductivity, suggesting that a highly unusual vortex state is realized in quantum critical superconductors. PMID:25477044

  10. Synthesizing new, high-temperature superconductors

    NASA Astrophysics Data System (ADS)

    Weaver, Claire; Aronson, Meigan

    2015-03-01

    Currently, there is no accepted theory behind type-II, high-temperature superconductors, but there is a distinct relationship between anti-ferromagnetism and superconductivity. Our research focuses on synthesizing new superconducting materials by observing the link between atomic structure and magnetic moments of anti-ferromagnetic compounds and attempting to reproduce the molecular physics of these known materials in new compounds. Consider the square-planar arrangement of the transition metal Fe in the Fe-pnictide superconductors of the ZrCuSiAs ``11 11'' and the ThCr2Si2 ``122'' structure types. We believe that the physics behind this superconductor, where Fe has d6 valence electrons, contributes to the superconducting state, not the presence of Fe itself. For this reason, we are synthesizing materials containing neighboring transition metals, like Mn and Co, combined with other elements in similar crystal lattice arrangements, having ionization properties that hopefully impose d6 valence electrons on the transition metals. This project was supported in part by the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists (WDTS) under the Science Undergraduate Laboratory Internships Program (SULI).

  11. Anomalous critical fields in quantum critical superconductors

    PubMed Central

    Putzke, C.; Walmsley, P.; Fletcher, J. D.; Malone, L.; Vignolles, D.; Proust, C.; Badoux, S.; See, P.; Beere, H. E.; Ritchie, D. A.; Kasahara, S.; Mizukami, Y.; Shibauchi, T.; Matsuda, Y.; Carrington, A.

    2014-01-01

    Fluctuations around an antiferromagnetic quantum critical point (QCP) are believed to lead to unconventional superconductivity and in some cases to high-temperature superconductivity. However, the exact mechanism by which this occurs remains poorly understood. The iron-pnictide superconductor BaFe2(As1−xPx)2 is perhaps the clearest example to date of a high-temperature quantum critical superconductor, and so it is a particularly suitable system to study how the quantum critical fluctuations affect the superconducting state. Here we show that the proximity of the QCP yields unexpected anomalies in the superconducting critical fields. We find that both the lower and upper critical fields do not follow the behaviour, predicted by conventional theory, resulting from the observed mass enhancement near the QCP. Our results imply that the energy of superconducting vortices is enhanced, possibly due to a microscopic mixing of antiferromagnetism and superconductivity, suggesting that a highly unusual vortex state is realized in quantum critical superconductors. PMID:25477044

  12. Geometric Frustration in the Mixed Layer Pnictide Oxides

    SciTech Connect

    Enjalran, M.; Scalettar, R.T.; Kauzlarich, S.M.

    2000-06-06

    The authors present results from a Monte Carlo investigation of a simple bilayer model with geometrically frustrated interactions similar to those found in the mixed layer pnictide oxides (Sr{sub 2}Mn{sub 3}Pn{sub 2}O{sub 2}, Pn = As, Sb). The model is composed of two inequivalent square lattices with nearest-neighbor intra- and interlayer interactions. They find a ground state composed of two independent Neel ordered layers when the interlayer exchange is an order of magnitude weaker than the intralayer exchange, as suggested by experiment. Evidence for local orthogonal order between the layers is found, but it occurs in regions of parameter space which are not experimentally realized. Qualitatively similar results were observed in models with a larger number of layers. They conclude that frustration caused by nearest-neighbor interactions in the mixed layer pnictide oxides is not sufficient to explain the long-range orthogonal order that is observed experimentally.

  13. Metallicity of Ca2Cu6P5 with single and double copper-pnictide layers

    DOE PAGESBeta

    Li, Li; Parker, David; Chi, Miaofang; Tsoi, Georgiy M.; Vohra, Yogesh K.; Sefat, Athena S.

    2016-02-16

    Here, we report thermodynamic and transport properties, and also theoretical calculations, for Cu-based compound Ca2Cu6P5 and compare with CaCu2-dP2. Both materials have layers of edge-sharing copper pnictide tetrahedral CuP4, similar to Fe–As and Fe–Se layers (with FeAs4, FeSe4) in the iron-based superconductors. Despite the presence of this similar transition-metal pnictide layer, we find that both Ca2Cu6P5 and CaCu2-δP2 have temperature-independent magnetic susceptibility and show metallic behavior with no evidence of either magnetic ordering or superconductivity down to 1.8 K CaCu2-δP2 is slightly off-stoichiometric, with δ = 0.14. Theoretical calculations suggest that unlike Fe 3d-based magnetic materials with a large densitymore » of states (DOS) at the Fermi surface, Cu have comparatively low DOS, with the majority of the 3d spectral weight located well below Fermi level. The room-temperature resistivity value of Ca2Cu6P5 is only 9 μΩ-cm, due to a substantial plasma frequency and an inferred electron-phonon coupling λ of 0.073 (significantly smaller than that of metallic Cu). Also, microscopy result shows that Cu–Cu distance along the c-axis within the double layers can be very short (2.5 Å), even shorter than metallic elemental copper bond (2.56 Å). The value of dρ/dT for CaCu2-δP2 at 300 K is approximately three times larger than in Ca2Cu6P5, which suggests the likelihood of stronger electron-phonon coupling. This study shows that the details of Cu-P layers and bonding are important for their transport characteristics. In addition, it emphasizes the remarkable character of the DOS of ‘122’ iron-based materials, despite much structural similarities.« less

  14. Dual role of d electrons in iron pnictides

    NASA Astrophysics Data System (ADS)

    Gor'kov, Lev P.; Teitel'baum, Gregory B.

    2013-01-01

    Recent Fe x-ray emission spectroscopy experiments [H. Gretarsson , Phys. Rev. B1098-012110.1103/PhysRevB.84.100509 84, 100509 (2011)] unveiled sizable local moments in iron pnictides in the room-temperature paramagnetic state. In an effort to further clarify the notion of coexisting magnetic moments and itinerant carriers in iron pnictides we focus on the interactions between the two subsystems. At a moderate on-site Coulomb repulsion the intra-atomic Hund's interaction leads to the formation of nonzero (“bare”) local moments on the Fe sites. We show that the Kondo-like exchange with the itinerant electrons may significantly renormalize the “bare” value of the moments manifested in different experiments. In turn, the itinerant carriers scatter on the renormalized moments that remain disordered in the paramagnetic phase. On the one hand, the scattering mechanism is responsible for high values of resistivity of the stoichiometric pnictides at the temperature of their transition into the antiferromagnetic phase, and on the other hand, it washes out the fine details of the Fermi surfaces. The results are rigorous and were obtained without use of any Born-type approximation. It also turned out that the value of the local moment and the inverse free time for scattering of carriers on the moments tend in the limit of the strong Kondo exchange to the finite universal values. The independence of the results on the on-site Coulomb repulsion is then illustrated in the frameworks of a simplistic model. It is shown that the spin density wave transition is driven by the Ruderman-Kittel-Kasuya-Yosida interactions between the renormalized moments via exchange by the electron-hole pairs. The applicability of the Boltzmann approach to transport in multiband pnictides is discussed briefly.

  15. Possible surface nematic order in iron pnictides

    NASA Astrophysics Data System (ADS)

    Song, Kok Wee; Koshelev, Alexei

    Nematic fluctuations play important role in the physics of the iron-based superconductors. Indications for weak precursor nematic transition has been found in the compound BaAs2-xPxFe2 [1 ]. However, high-resolution specific-heat measurements did not reveal any bulk transition [2 ]. To resolve this controversy, we consider the possibility of the surface nematic transition preceding the bulk transition. We consider the simplest model of two interacting quasi-two-dimensional electronic bands and explore the free-surface effects on the nematic order. We found that three-dimensional effects suppress the bulk nematic order and therefore this order is enhanced near the surface. [1 ]Kasahara, S., et al. ''Electronic nematicity above the structural and superconducting transition in Ba(As1-xPxFe)2.'' Nature 486.7403 (2012): 382-385. [2 ]Luo, X., et al. ''Antiferromagnetic and nematic phase transitions in Ba(As1-xPxFe)2 studied by ac microcalorimetry and SQUID magnetometry.'' Physical Review B 91.9 (2015): 094512. This work was supported by the Center for Emergent Superconductivity, an Energy FrontierResearch Center funded by the US DOE, Office of Science, under Award No. DEAC0298CH1088.

  16. Superconductor cable

    DOEpatents

    Allais, Arnaud; Schmidt, Frank; Marzahn, Erik

    2010-05-04

    A superconductor cable is described, having a superconductive flexible cable core (1) , which is laid in a cryostat (2, 3, 4), in which the cable core (1) runs in the cryostat (2, 3, 4) in the form of a wave or helix at room temperature.

  17. Microstructure refinement and enhanced critical current density in binary doped SmFeAsO superconductor

    SciTech Connect

    Anooja, J. B.; Aswathy, P. M.; Varghese, Neson; Syamaprasad, U.; Aloysius, R. P.

    2014-04-24

    The iron-pnictide Sm{sub 1−x}Ca{sub x}FeAsO{sub 1−2x}F{sub 2x} superconductor was prepared and the combined effect of electron and hole doping was studied in detail. It is observed that the binary doping using CaF{sub 2} improves the microstructure tremendously with a preferred orientation of the (00l) planes. Moreover, a maximum T{sub C} of 53.8 K and a transport J{sub C} of 880 A/cm{sup 2} (12 K), which is double to that of the F-doped sample, are achieved. The dopant CaF{sub 2} seems to be a potential candidate for solving the grain-connectivity concerns in iron-pnictides paving the way towards conductor development.

  18. Microstructure refinement and enhanced critical current density in binary doped SmFeAsO superconductor

    NASA Astrophysics Data System (ADS)

    Anooja, J. B.; Aswathy, P. M.; Varghese, Neson; Aloysius, R. P.; Syamaprasad, U.

    2014-04-01

    The iron-pnictide Sm1-xCaxFeAsO1-2xF2x superconductor was prepared and the combined effect of electron and hole doping was studied in detail. It is observed that the binary doping using CaF2 improves the microstructure tremendously with a preferred orientation of the (00l) planes. Moreover, a maximum TC of 53.8 K and a transport JC of 880 A/cm2 (12 K), which is double to that of the F-doped sample, are achieved. The dopant CaF2 seems to be a potential candidate for solving the grain-connectivity concerns in iron-pnictides paving the way towards conductor development.

  19. Critical Activation Parameters for LaFeAsO-BASED Superconductors

    NASA Astrophysics Data System (ADS)

    Chu, Zotin Kwang-Hua

    The occurrence of high-Tc superconductivity in the iron pnictides shares a similar amorphous characteristic with that of high-Tc superconducting cuprates. Here we show that nearly frictionless (electric-field-driven) transport of condensed electrons in amorphous superconductors could happen after using the Eyring's transition-rate approach which has been successfully adopted to study the critical transport of other superconductors as well as supersolid helium in very low temperature environment. The critical temperatures related to the nearly frictionless transport of electrons were found to be directly relevant to the superconducting temperature of high-temperature superconductors (like La[O1-xFx]FeAs (x = 0.11-0.12)) after selecting specific activation energies and activation volumes.

  20. Anomalous thermodynamic power laws in nodal superconductors

    NASA Astrophysics Data System (ADS)

    Quintanilla, Jorge; Mazidian, Bayan; Annett, James F.; Hillier, Adrian D.

    2013-03-01

    Unconventional superconductors are frequently identified by the observation of power law behaviour on low temperature thermodynamic properties such as specific heat. These power laws generally derive from the linear spectrum near points or lines of zeros, or nodes, in the superconducting energy gap on the Fermi surface. Here we show that, in addition to the usual point and line nodes, a much wider class of different nodal types can occur. Some of these new types of nodes typically occur when there are transitions between different types of gap node topology, for example when point or line nodes first appear as a function of some physical parameter. We derive anomalous, non-integer thermodynamic power laws associated with these new nodal types and predict their occurrence in iron pnictide superconductors and in the noncentrosymmetric system Li2Pd3-xPtxB. This works was supported by EPSRC and STFC (U.K.) J.Q. gratefully acknowledges funding from HEFCE and STFC through the South-East Physics network (SEPnet).

  1. Unconventional Hall effect in pnictides from interband interactions.

    PubMed

    Fanfarillo, L; Cappelluti, E; Castellani, C; Benfatto, L

    2012-08-31

    We calculate the Hall transport in a multiband system with a dominant interband interaction between carriers having electron and hole character. We show that this situation gives rise to an unconventional scenario, beyond the Boltzmann theory, where the quasiparticle currents dressed by vertex corrections acquire the character of the majority carriers. This leads to a larger (positive or negative) Hall coefficient than what may be expected on the basis of the carrier balance, with a marked temperature dependence. Our results explain the puzzling measurements in pnictides and provide a more general framework for transport properties in multiband materials.

  2. Superconductor cable

    DOEpatents

    Smith, Jr., Darrell F.; Lake, Bill L.; Ballinger, Ronald G.

    1988-01-01

    A superconducting cable comprising an in-situ-formed type II superconductor, e.g. Nb.sub.3 Sn, in association with a stabilizing conductor both in heat transfer relationship with at least one passage adapted to carry liquified gaseous refrigerant. The conductor and said at least one passage are enclosed by a sheath comprising an alloy consisting essentially of about 49% nickel, about 4% chromium, about 3% niobium, about 1.4% titanium, about 1% aluminum, balance essentially iron.

  3. Spatial competition of the ground states in 1111 iron pnictides

    NASA Astrophysics Data System (ADS)

    Lang, G.; Veyrat, L.; Gräfe, U.; Hammerath, F.; Paar, D.; Behr, G.; Wurmehl, S.; Grafe, H.-J.

    2016-07-01

    Using nuclear quadrupole resonance, the phase diagram of 1111 R FeAsO1 -xFx (R =La , Ce, Sm) iron pnictides is constructed as a function of the local charge distribution in the paramagnetic state, which features low-doping-like (LD-like) and high-doping-like (HD-like) regions. Compounds based on magnetic rare earths (Ce, Sm) display a unified behavior, and comparison with La-based compounds reveals the detrimental role of static iron 3 d magnetism on superconductivity, as well as a qualitatively different evolution of the latter at high doping. It is found that the LD-like regions fully account for the orthorhombicity of the system, and are thus the origin of any static iron magnetism. Orthorhombicity and static magnetism are not hindered by superconductivity but limited by dilution effects, in agreement with two-dimensional (2D) (respectively three-dimensional) nearest-neighbor square lattice site percolation when the rare earth is nonmagnetic (respectively magnetic). The LD-like regions are not intrinsically supportive of superconductivity, contrary to the HD-like regions, as evidenced by the well-defined Uemura relation between the superconducting transition temperature and the superfluid density when accounting for the proximity effect. This leads us to propose a complete description of the interplay of ground states in 1111 pnictides, where nanoscopic regions compete to establish the ground state through suppression of superconductivity by static magnetism, and extension of superconductivity by proximity effect.

  4. Soft-mode transitions of alkaline-earth 122 pnictides

    NASA Astrophysics Data System (ADS)

    Widom, Michael; Quader, Khandker

    A -122 pnictides (A=Ca, Sr, Ba) exhibit three pressure-driven transitions: a first order enthalpic transition at PH from the striped AFM orthorhombic (OR) to a tetragonal (T) or a collapsed tetragonal (cT) phase; a transition at PM >PH from the metastable AFM OR to a T or cT phase; a Lifshitz transition at PL that causes T to collapse to a cT phase. Transitions at PH and PL were previously examined through total energy and band structure calculations. Here we address the transition at PM, beyond which the metastable AFM OR state ceases to exist. We show this transition occurs through a loss of elastic stability caused by softening of a shear mode associated with stretching along the c-axis. Simultaneously, magnetism and orthorhombicity approach limiting values with an approximately square-root singularity. Together these suggest a strong magneto-elastic coupling that may be relevant to a further understanding of the A-122-pnictides under pressure. This work was supported in part by the DOE under Grant DE-SC0014506.

  5. Half-metallic zinc-blende pnictides in real environments

    NASA Astrophysics Data System (ADS)

    Shi, Li-Jie; Liu, Bang-Gui

    2005-02-01

    The structural stability of half-metallic zinc-blende pnictides and the robustness of their half-metallic ferromagnetism in the presence of tetragonal and orthorhombic crystalline deformations are studied using a full-potential linear augmented plane wave method within the density-functional theory. The total energies of zinc-blende MnAs, CrAs, and CrSb are proved to increase with deformation increase, in contrast to those of other zinc-blende half-metallic pnictides, and therefore these three are stable against the deformations but the others are not. This is consistent with the experimental fact that only these three have been fabricated. On the other hand, the half-metallic ferromagnetism of the latter two is proved to be robust enough to survive large crystal deformations. This implies that half-metallic ferromagnetism may be achieved experimentally even in substantially deformed zinc-blende ultrathin films or layers of CrAs and CrSb in real environments.

  6. ARPES view of orbitally resolved quasiparticle lifetimes in iron pnictides

    NASA Astrophysics Data System (ADS)

    Brouet, Véronique; LeBoeuf, David; Lin, Ping-Hui; Mansart, Joseph; Taleb-Ibrahimi, Amina; Le Fèvre, Patrick; Bertran, François; Forget, Anne; Colson, Dorothée

    2016-02-01

    We study with angle-resolved photoemission spectroscopy (ARPES) the renormalization and quasiparticle lifetimes of the dx y and dx z/dy z orbitals in two iron pnictides, LiFeAs and Ba (Fe0.92Co0.08 )2As2 (Co8). We find that both quantities depend on orbital character rather than on the position on the Fermi surface (for example, hole or electron pocket). In LiFeAs, the renormalizations are larger for dx y, while they are similar for both types of orbitals in Co8. The most salient feature, which proved robust against all the ARPES caveats we could think of, is that the lifetimes for dx y exhibit a markedly different behavior than those for dx z/dy z. They have smaller values near EF and exhibit larger ω and temperature dependences. While the behavior of dx y is compatible with a Fermi-liquid description, that is not the case for dx z/dy z. This situation should have important consequences for the physics of iron pnictides, which have not been considered up to now. More generally, it raises interesting questions about how a Fermi-liquid regime can be established in a multiband system with small effective bandwidths.

  7. Measurements of the anisotropic in-plane resistivity of underdoped FeAs-based pnictide superconductors.

    PubMed

    Ying, J J; Wang, X F; Wu, T; Xiang, Z J; Liu, R H; Yan, Y J; Wang, A F; Zhang, M; Ye, G J; Cheng, P; Hu, J P; Chen, X H

    2011-08-01

    We systematically investigated the in-plane resistivity anisotropy of electron-underdoped EuFe(2-x)Co(x)As(2) and BaFe(2-x)Co(x)As(2) and hole-underdoped Ba(1-x)K(x)Fe(2)As(2). Large in-plane resistivity anisotropy was found in the former samples, while tiny in-plane resistivity anisotropy was detected in the latter ones. When it is detected, the anisotropy starts above the structural transition temperature and increases smoothly through it. As the temperature is lowered further, the anisotropy takes a dramatic enhancement through the magnetic transition temperature. We found that the anisotropy is universally tied to the presence of T-linear behavior of resistivity. Our results demonstrate that the nematic state is caused by electronic degrees of freedom, and the microscopic orbital involvement in the magnetically ordered state must be fundamentally different between the hole- and electron-doped materials.

  8. Role of Hydrogen in the Electronic Properties of H-rich Pnictide Superconductors

    NASA Astrophysics Data System (ADS)

    Huang, Yina; Yu, Xianglong; Liu, Dayong; Zou, Liangjian

    The electronic and magnetic properties of the parent material CaFeAsH and its La/Co-doped compounds are investigated using first-principles calculations based on the generalized gradient approximation (GGA). We predict that the ground state of CaFeAsH is a spin-density-wave (SDW)-type striped antiferromagnet driven by Fermi surface nesting. We find a sandglass-type hole pocket near the Γpoint in CaFeAsH that is not present in CaFeAsF. In comparison with CaFeAsF, the sandglass-shaped pocket, mainly contributed from Fe dxz + dyz orbitals, arises from the weak oxidization of CaH layers and the hybridization enhancement between FeAs layers. In contrast, the electronic properties of electron doped Ca0.75La0.25FeAsH and CaFe0.75Co0.25AsH indicate that La or Co doping almost does not affect the sandglass-type Fermi surface, while the suppression of Fermi surface nesting in Ca0.75La0.25FeAsH is weaker than that in CaFe0.75Co0.25AsH. This features may contribute to the higher Tc in La-substituted CaFeAsH.

  9. Quantum criticality in the 122 iron pnictide superconductors emerging from orbital-selective Mottness

    NASA Astrophysics Data System (ADS)

    Das, S. D.; Laad, M. S.; Craco, L.; Gillett, J.; Tripathi, V.; Sebastian, S. E.

    2015-10-01

    The twin issues of the nature of the "normal" state and competing order(s) in the iron arsenides are central to understanding their unconventional, high-Tc superconductivity. We use a combination of transport anisotropy measurements on detwinned Sr (Fe1-xCox) 2As2 single crystals and local density approximation plus dynamical mean field theory (LDA + DMFT) calculations to revisit these issues. The peculiar resistivity anisotropy and its evolution with x are naturally interpreted in terms of an underlying orbital-selective Mott transition (OSMT) that gaps out the dx z or dy z states. Further, we use a Landau-Ginzburg approach using LDA + DMFT input to rationalize a wide range of anomalies seen up to optimal doping, providing strong evidence for secondary electronic nematic order. These findings suggest that strong dynamical fluctuations linked to a marginal quantum-critical point associated with this OSMT and a secondary electronic nematic order constitute an intrinsically electronic pairing mechanism for superconductivity in Fe arsenides.

  10. Unusual phase transition in a natural heterostructure of iron pnictides and vanadium oxides

    NASA Astrophysics Data System (ADS)

    Ok, Jong Mok; Baek, S.-H.; Eom, Man Jin; Hoch, C.; Kremer, R. K.; Kim, Dong-Hwan; Chang, Chun-Fu; Ko, Kyung-Tae; Park, Sang-Youn; Ji, Sung Dae; Büchner, B.; Park, Jae-Hoon; Shim, J. H.; Mazin, I. I.; Kim, Jun Sung

    We report the unusual phase transition in Sr2VO3FeAs single crystal, where the Mott-insulating vanadium oxides and the high-Tc superconducting iron pnictides form a natural heterostructure. Clear evidence of the phase transition at T0 = 155 K was observed in the iron pnictide layer, not in the vanadium oxide layer, using bulk and NMR measurements. Neither magnetic ordering with sufficient spin moment nor symmetry change in the crystal structure has been detected at T0. At Tmag ~ 45 K, far below T0, magnetic transition occurs in the iron pnictide layer, while the vanadium oxide layer remains nonmagnetic at low temperatures. The complex evolution of various phases in Sr2VO3FeAs is drastically distinct from the phase transitions found in other iron pnictides or vanadium oxides, highlighting the importance of the additional interlayer coupling between the layers. Equal contribution, corresponding author.

  11. A New ZrCuSiAs-Type Superconductor: ThFeAsN.

    PubMed

    Wang, Cao; Wang, Zhi-Cheng; Mei, Yu-Xue; Li, Yu-Ke; Li, Lin; Tang, Zhang-Tu; Liu, Yi; Zhang, Pan; Zhai, Hui-Fei; Xu, Zhu-An; Cao, Guang-Han

    2016-02-24

    We report the first nitrogen-containing iron-pnictide superconductor ThFeAsN, which is synthesized by a solid-state reaction in an evacuated container. The compound crystallizes in a ZrCuSiAs-type structure with the space group P4/nmm and lattice parameters a = 4.0367(1) Å and c = 8.5262(2) Å at 300 K. The electrical resistivity and dc magnetic susceptibility measurements indicate superconductivity at 30 K for the nominally undoped ThFeAsN.

  12. Superconductor cable

    DOEpatents

    Allais, Arnaud; Schmidt, Frank (Langenhagen, DE

    2009-12-15

    A superconductor cable includes a superconductive cable core (1) and a cryostat (2) enclosing the same. The cable core (1) has a superconductive conductor (3), an insulation (4) surrounding the same and a shielding (5) surrounding the insulation (4). A layer (3b) of a dielectric or semiconducting material is applied to a central element (3a) formed from a normally conducting material as a strand or tube and a layer (3c) of at least one wire or strip of superconductive material is placed helically on top. The central element (3a) and the layer (3c) are connected to each other in an electrically conducting manner at the ends of the cable core (1).

  13. A universal explanation of tunneling conductance in exotic superconductors

    PubMed Central

    Hong, Jongbae; Abergel, D. S. L.

    2016-01-01

    A longstanding mystery in understanding cuprate superconductors is the inconsistency between the experimental data measured by scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES). In particular, the gap between prominent side peaks observed in STS is much bigger than the superconducting gap observed by ARPES measurements. Here, we reconcile the two experimental techniques by generalising a theory which was previously applied to zero-dimensional mesoscopic Kondo systems to strongly correlated two-dimensional (2D) exotic superconductors. We show that the side peaks observed in tunneling conductance measurements in all these materials have a universal origin: They are formed by coherence-mediated tunneling under bias and do not directly reflect the underlying density of states (DOS) of the sample. We obtain theoretical predictions of the tunneling conductance and the density of states of the sample simultaneously and show that for cuprate and pnictide superconductors, the extracted sample DOS is consistent with the superconducting gap measured by ARPES. PMID:27511315

  14. EDITORIAL: Focus on Iron-Based Superconductors FOCUS ON IRON-BASED SUPERCONDUCTORS

    NASA Astrophysics Data System (ADS)

    Hosono, Hideo; Ren, Zhi-An

    2009-02-01

    pace of research within the last year, iron-based superconductors have revealed several unique properties such as a high upper critical field and a robustness to impurities. Participation of five 3d-orbitals in the Fermi levels also means that the electronic structure is complex compared with the cuprates. So, we now have a new family of superconductors and it is worth stressing that we have only just begun looking at the many varieties of candidate materials containing an iron square lattice. At this time we do not know whether a material with a critical temperature greater than 100 K exists, or if completely new properties are to be found. However, as a research community we should go ahead with hope and 'strike while the iron is hot'—this saying is always true! This focus issue of New Journal of Physics was put together to provide a broad-based, free-to-read snapshot of the current state of research in this rapidly emerging field. The papers included cover many aspects related to material exploration, physical analysis, and the theory of these materials, and, as editors, we thank the authors for their fine contributions, and the many referees for their considerable efforts that have ensured fast publication. As an aside, the first special issue on this SUBject was published in November 2008 in the Journal of the Physical Society of Japan (vol 77, supplement c) as the proceedings of the International Symposium on Iron-Pnictide Superconductors held in Tokyo on 29-30 June 2008. We would like to encourage the community to read both issues. On a final note we would like to acknowledge the staff of New Journal of Physics for all of their efficient work in bringing this collection to fruition. Focus on Iron-Based Superconductors Contents Microwave response of superconducting pnictides: extended s+/- scenario O V Dolgov, A A Golubov and D Parker Orbital and spin effects for the upper critical field in As-deficient disordered Fe pnictide superconductors G Fuchs, S

  15. EDITORIAL: Focus on Superconductors with Exotic Symmetries FOCUS ON SUPERCONDUCTORS WITH EXOTIC SYMMETRIES

    NASA Astrophysics Data System (ADS)

    Rice, T. Maurice; Sigrist, Manfred; Maeno, Yoshiteru

    2009-05-01

    Superconductors can usefully be divided into two classes, those that are well described by the classic Bardeen-Cooper-Schrieffer (BCS) theory and its extensions and those which require a different microscopic description. The BCS theory of superconductivity solved the long standing mystery of this spectacular phenomenon and described all superconductors that were known when it was formulated in the 1950s. The key ingredient is an attractive interaction generated by the exchange of phonons between electrons which overcomes a Coulomb repulsion weakened by screening, to give a net attractive force on the low energy scale. In this case the simplest s-wave pairing always maximises the energy gain. There were speculations a little later that other types of electron pairing could be possible, but it took a quarter of a century until the first signs of superconductors with different and exotic pairing appeared. In the intervening thirty years many superconductors with exotic pairing have been and continue to be discovered and the study of their superconductivity has grown into a major subfield of condensed matter physics today. The importance of these exotic superconductors with unconventional symmetry is that their pairing is of electronic origin. As a result they are freed from the restrictions of low transition temperatures that go along with the phonon driven conventional superconductors. However in two of the main classes of the exotic superconductors, namely heavy fermion and organic superconductors, the intrinsic energy scales are very small leading to low temperature scales. The third class contains the small number of superconducting transition metal compounds with exotic pairing symmetry. The most studied of these are the high-Tc cuprates, the newly discovered iron pnictides and strontium ruthenate which is closely related to superfluid 3He. Although the basic electronic structure of these materials is well understood, the origin of the pairing is more complex

  16. Intra-unit-cell nematic charge order in the titanium-oxypnictide family of superconductors.

    PubMed

    Frandsen, Benjamin A; Bozin, Emil S; Hu, Hefei; Zhu, Yimei; Nozaki, Yasumasa; Kageyama, Hiroshi; Uemura, Yasutomo J; Yin, Wei-Guo; Billinge, Simon J L

    2014-01-01

    Understanding the role played by broken-symmetry states such as charge, spin and orbital orders in the mechanism of emergent properties, such as high-temperature superconductivity, is a major current topic in materials research. That the order may be within one unit cell, such as nematic, was only recently considered theoretically, but its observation in the iron-pnictide and doped cuprate superconductors places it at the forefront of current research. Here, we show that the recently discovered BaTi2Sb2O superconductor and its parent compound BaTi2As2O form a symmetry-breaking nematic ground state that can be naturally explained as an intra-unit-cell nematic charge order with d-wave symmetry, pointing to the ubiquity of the phenomenon. These findings, together with the key structural features in these materials being intermediate between the cuprate and iron-pnictide high-temperature superconducting materials, render the titanium oxypnictides an important new material system to understand the nature of nematic order and its relationship to superconductivity. PMID:25482113

  17. Universal non-Landau, self-organized, lattice disordering percolative dopant network sub-Tc phase transition in ceramic superconductors

    PubMed Central

    Phillips, J. C.

    2009-01-01

    Ceramic superconductors (cuprates, pnictides, etc.) exhibit universal features in both Tcmax and in their planar lattice disordering measured by EXAFS, as reflected by three phase transitions. The two highest temperature transitions are known to be associated with formation of Jahn–Teller pseudogaps and superconductive gaps, with corresponding Landau order parameters, but no new gap is associated with the third transition below Tc, and its origin is mysterious. It is argued that the third subTc transition is a dopant glass transition, which is remarkably similar to topological transitions previously observed in chalcogenide and oxide alloy network glasses (like window glass). PMID:19805211

  18. Absence of superconductivity in fluorine-doped neptunium pnictide NpFeAsO

    NASA Astrophysics Data System (ADS)

    Walters, A. C.; Walker, H. C.; Springell, R.; Krisch, M.; Bosak, A.; Hill, A. H.; Zvorişte-Walters, C. E.; Colineau, E.; Griveau, J.-C.; Bouëxière, D.; Eloirdi, R.; Caciuffo, R.; Klimczuk, T.

    2015-08-01

    X-ray diffraction, specific heat, magnetic susceptibility and inelastic x-ray scattering measurements on the transurarium oxypnictides NpFeAsO and NpFeAsO0.85F0.15 are presented. No superconductivity down to 2 K was observed upon fluorine doping, contrary to the structurally analogous rare-earth pnictides. No modification of the phonon density of states was observed upon doping with fluorine. We discuss our results in light of the latest experimental and theoretical studies on the role of phonons in the superconducting pnictide compounds.

  19. Absence of superconductivity in fluorine-doped neptunium pnictide NpFeAsO.

    PubMed

    Walters, A C; Walker, H C; Springell, R; Krisch, M; Bosak, A; Hill, A H; Zvorişte-Walters, C E; Colineau, E; Griveau, J-C; Bouëxière, D; Eloirdi, R; Caciuffo, R; Klimczuk, T

    2015-08-19

    X-ray diffraction, specific heat, magnetic susceptibility and inelastic x-ray scattering measurements on the transurarium oxypnictides NpFeAsO and NpFeAsO0.85F0.15 are presented. No superconductivity down to 2 K was observed upon fluorine doping, contrary to the structurally analogous rare-earth pnictides. No modification of the phonon density of states was observed upon doping with fluorine. We discuss our results in light of the latest experimental and theoretical studies on the role of phonons in the superconducting pnictide compounds.

  20. Scaling between magnetic field and temperature in the high-temperature superconductor BaFe2(As1-xPx)2

    NASA Astrophysics Data System (ADS)

    Hayes, Ian M.; McDonald, Ross D.; Breznay, Nicholas P.; Helm, Toni; Moll, Philip J. W.; Wartenbe, Mark; Shekhter, Arkady; Analytis, James G.

    2016-10-01

    Many exotic metallic systems have a resistivity that varies linearly with temperature, and the physics behind this is thought to be connected to high-temperature superconductivity in the cuprates and iron pnictides. Although this phenomenon has attracted considerable attention, it is unclear how the relevant physics manifests in other transport properties, for example their response to an applied magnetic field. We report measurements of the high-field magnetoresistance of the iron pnictide superconductor BaFe2(As1-xPx)2 and find that it obeys an unusual scaling relationship between applied magnetic field and temperature, with a conversion factor given simply by the ratio of the Bohr magneton and the Boltzmann constant. This suggests that magnetic fields probe the same physics that gives rise to the T-linear resistivity, providing a new experimental clue to this long-standing puzzle.

  1. Itinerant scenario for Fe pnictides: Comparison with quantum Monte Carlo

    NASA Astrophysics Data System (ADS)

    Chubukov, Andrey V.; Xing, Rui-Qi

    2016-04-01

    Recent applications of quantum Monte Carlo (QMC) technique to Fe-based superconductors opened a way to directly verify the applicability of the itinerant scenario for these systems. Fe-based superconductors undergo various instabilities upon lowering temperature (magnetism, superconductivity, nematicity/orbital order), and one can check whether the hierarchy of instabilities obtained within the itinerant approach is the same as in unbiased QMC simulations. In a recent paper [arXiv:1512.08523] the authors considered the simplest two-band model with interaction tailored to favor orbital order. The type of the orbital order found in QMC is different from the one found in earlier itinerant analysis. We report the results of our calculations within the itinerant scenario and argue that they are in perfect agreement with QMC.

  2. EDITORIAL: Focus on Iron-Based Superconductors FOCUS ON IRON-BASED SUPERCONDUCTORS

    NASA Astrophysics Data System (ADS)

    Hosono, Hideo; Ren, Zhi-An

    2009-02-01

    pace of research within the last year, iron-based superconductors have revealed several unique properties such as a high upper critical field and a robustness to impurities. Participation of five 3d-orbitals in the Fermi levels also means that the electronic structure is complex compared with the cuprates. So, we now have a new family of superconductors and it is worth stressing that we have only just begun looking at the many varieties of candidate materials containing an iron square lattice. At this time we do not know whether a material with a critical temperature greater than 100 K exists, or if completely new properties are to be found. However, as a research community we should go ahead with hope and 'strike while the iron is hot'—this saying is always true! This focus issue of New Journal of Physics was put together to provide a broad-based, free-to-read snapshot of the current state of research in this rapidly emerging field. The papers included cover many aspects related to material exploration, physical analysis, and the theory of these materials, and, as editors, we thank the authors for their fine contributions, and the many referees for their considerable efforts that have ensured fast publication. As an aside, the first special issue on this SUBject was published in November 2008 in the Journal of the Physical Society of Japan (vol 77, supplement c) as the proceedings of the International Symposium on Iron-Pnictide Superconductors held in Tokyo on 29-30 June 2008. We would like to encourage the community to read both issues. On a final note we would like to acknowledge the staff of New Journal of Physics for all of their efficient work in bringing this collection to fruition. Focus on Iron-Based Superconductors Contents Microwave response of superconducting pnictides: extended s+/- scenario O V Dolgov, A A Golubov and D Parker Orbital and spin effects for the upper critical field in As-deficient disordered Fe pnictide superconductors G Fuchs, S

  3. Possible role of bonding angle and orbital mixing in iron pnictide superconductivity: Comparative electronic structure studies of LiFeAs and Sr2VO3FeAs

    NASA Astrophysics Data System (ADS)

    Kim, Y. K.; Koh, Y. Y.; Kyung, W. S.; Han, G. R.; Lee, B.; Kim, Kee Hoon; Ok, J. M.; Kim, Jun Sung; Arita, M.; Shimada, K.; Namatame, H.; Taniguchi, M.; Mo, S.-K.; Kim, C.

    2015-07-01

    A well-known universal feature among iron pnictide superconductors is the correlation between the As-Fe-As bonding angle and the superconducting transition temperature. However, the origin of such a correlation has not been clearly understood despite its potential importance in understanding the mechanism of superconductivity. Here, we present comparative electronic structure studies of LiFeAs and Sr2VO3FeAs , two representative systems without any dopant that can show bonding angle dependence of the electronic structure. Captured distinct features of the higher Tc compound Sr2VO3FeAs such as an unusual kz modulation and anomalous polarization dependence suggest that the difference between the two systems is in the interorbital coupling strength. This could be the essential element of the bonding angle dependence that allows an enhanced pairing instability and Tc.

  4. Fine uniform filament superconductors

    DOEpatents

    Riley, Jr., Gilbert N.; Li, Qi; Roberts, Peter R.; Antaya, Peter D.; Seuntjens, Jeffrey M.; Hancock, Steven; DeMoranville, Kenneth L.; Christopherson, Craig J.; Garrant, Jennifer H.; Craven, Christopher A.

    2002-01-01

    A multifilamentary superconductor composite having a high fill factor is formed from a plurality of stacked monofilament precursor elements, each of which includes a low density superconductor precursor monofilament. The precursor elements all have substantially the same dimensions and characteristics, and are stacked in a rectilinear configuration and consolidated to provide a multifilamentary precursor composite. The composite is thereafter thermomechanically processed to provide a superconductor composite in which each monofilament is less than about 50 microns thick.

  5. Superconductor rotor cooling system

    DOEpatents

    Gamble, Bruce B.; Sidi-Yekhlef, Ahmed; Schwall, Robert E.; Driscoll, David I.; Shoykhet, Boris A.

    2002-01-01

    A system for cooling a superconductor device includes a cryocooler located in a stationary reference frame and a closed circulation system external to the cryocooler. The closed circulation system interfaces the stationary reference frame with a rotating reference frame in which the superconductor device is located. A method of cooling a superconductor device includes locating a cryocooler in a stationary reference frame, and transferring heat from a superconductor device located in a rotating reference frame to the cryocooler through a closed circulation system external to the cryocooler. The closed circulation system interfaces the stationary reference frame with the rotating reference frame.

  6. Photothermal measurements of superconductors

    SciTech Connect

    Kino, G.S.; Studenmund, W.R.; Fishman, I.M.

    1996-12-31

    A photothermal technique has been used to measure diffusion and critical temperature in high temperature superconductors. The technique is particularly suitable for determining material quality and inhomogeneity.

  7. Superconductor rotor cooling system

    DOEpatents

    Gamble, Bruce B.; Sidi-Yekhlef, Ahmed; Schwall, Robert E.; Driscoll, David I.; Shoykhet, Boris A.

    2004-11-02

    A system for cooling a superconductor device includes a cryocooler located in a stationary reference frame and a closed circulation system external to the cryocooler. The closed circulation system interfaces the stationary reference frame with a rotating reference frame in which the superconductor device is located. A method of cooling a superconductor device includes locating a cryocooler in a stationary reference frame, and transferring heat from a superconductor device located in a rotating reference frame to the cryocooler through a closed circulation system external to the cryocooler. The closed circulation system interfaces the stationary reference frame with the rotating reference frame.

  8. Divergent nematic susceptibility of optimally doped Fe-based superconductors

    NASA Astrophysics Data System (ADS)

    Chu, Jiun-Haw; Kuo, Hsueh-Hui; Fisher, Ian

    2015-03-01

    By performing differential elastoresistivity measurements on a wider range of iron based superconductors, including electron doped (Ba(Fe1-xCox)2As2, Ba(Fe1-xNix)2As2),holedoped(Ba1-xKxFe2As2), isovalent substituted pnictides (BaFe2(As1-xPx)2) and chalcogenides (FeTe1-xSex), we show that a divergent nematic susceptibility in the B2g symmetry channel appears to be a generic feature of optimally doped compositions. For the specific case of optimally ``doped'' BaFe2(As1-xPx)2, the nematic susceptibility can be well fitted by a Curie-Weiss temperature dependence with critical temperature close to zero, consistent with expectations of quantum critical behavior in the absence of disorder. However for all the other optimal doped iron based superconductors, the nematic susceptibility exhibits a downward deviation from Curie-Weiss behavior, suggestive of an important role played by disorder.

  9. Universal self-field critical current for thin-film superconductors.

    PubMed

    Talantsev, E F; Tallon, J L

    2015-01-01

    For any practical superconductor the magnitude of the critical current density, Jc, is crucially important. It sets the upper limit for current in the conductor. Usually Jc falls rapidly with increasing external magnetic field, but even in zero external field the current flowing in the conductor generates a self-field that limits Jc. Here we show for thin films of thickness less than the London penetration depth, λ, this limiting Jc adopts a universal value for all superconductors-metals, oxides, cuprates, pnictides, borocarbides and heavy Fermions. For type-I superconductors, it is Hc/λ where Hc is the thermodynamic critical field. But surprisingly for type-II superconductors, we find the self-field Jc is Hc1/λ where Hc1 is the lower critical field. Jc is thus fundamentally determined and this provides a simple means to extract absolute values of λ(T) and, from its temperature dependence, the symmetry and magnitude of the superconducting gap. PMID:26240014

  10. Local quantum criticality of an iron-pnictide tetrahedron.

    PubMed

    Ong, T Tzen; Coleman, Piers

    2012-03-01

    Motivated by the close correlation between transition temperature (T(c)) and the tetrahedral bond angle of the As-Fe-As layer observed in the iron-based superconductors, we study the interplay between spin and orbital physics of an isolated iron-arsenide tetrahedron embedded in a metallic environment. Whereas the spin-Kondo effect is suppressed to low temperatures by Hund's coupling, the orbital degrees of freedom are expected to quantum mechanically quench at high temperatures, giving rise to an overscreened, non-Fermi liquid ground state. Translated into a dense environment, this critical state may play an important role in the superconductivity of these materials.

  11. Hubbard interactions in iron-based pnictides and chalcogenides: Slater parametrization, screening channels, and frequency dependence

    NASA Astrophysics Data System (ADS)

    van Roekeghem, Ambroise; Vaugier, Loïg; Jiang, Hong; Biermann, Silke

    2016-09-01

    We calculate the strength of the frequency-dependent on-site electronic interactions in the iron pnictides LaFeAsO, BaFe2As2 , BaRu2As2 , and LiFeAs and the chalcogenide FeSe from first principles within the constrained random phase approximation. We discuss the accuracy of an atomiclike parametrization of the two-index density-density interaction matrices based on the calculation of an optimal set of three independent Slater integrals, assuming that the angular part of the Fe d localized orbitals can be described within spherical harmonics as for isolated Fe atoms. We show that its quality depends on the ligand-metal bonding character rather than on the dimensionality of the lattice: it is excellent for ionic-like Fe-Se (FeSe) chalcogenides and a more severe approximation for more covalent Fe-As (LaFeAsO, BaFe2As2 ) pnictides. We furthermore analyze the relative importance of different screening channels, with similar conclusions for the different pnictides but a somewhat different picture for the benchmark oxide SrVO3: the ligand channel does not appear to be dominant in the pnictides, while oxygen screening is the most important process in the oxide. Finally, we analyze the frequency dependence of the interaction. In contrast to simple oxides, in iron pnictides its functional form cannot be simply modeled by a single plasmon, and the actual density of modes enters the construction of an effective Hamiltonian determining the low-energy properties.

  12. Fabrication of high temperature superconductors

    DOEpatents

    Balachandran, Uthamalingam; Dorris, Stephen E.; Ma, Beihai; Li, Meiya

    2003-06-17

    A method of forming a biaxially aligned superconductor on a non-biaxially aligned substrate substantially chemically inert to the biaxially aligned superconductor comprising is disclosed. A non-biaxially aligned substrate chemically inert to the superconductor is provided and a biaxially aligned superconductor material is deposited directly on the non-biaxially aligned substrate. A method forming a plume of superconductor material and contacting the plume and the non-biaxially aligned substrate at an angle greater than 0.degree. and less than 90.degree. to deposit a biaxially aligned superconductor on the non-biaxially aligned substrate is also disclosed. Various superconductors and substrates are illustrated.

  13. ARPES of single layer iron pnictide on STO

    NASA Astrophysics Data System (ADS)

    Shen, Zhi-Xun

    Quantum systems in confined geometries have been a very rich ground for discoveries. In this talk, I will discuss recent progresses in uncovering novel physics at ultra-thin limit, with focus on mono-unit-cell (UC) superconductor FeSe grown on SrTiO3, where the Cooper pairing temperature is reported to have dramatically enhanced from its bulk value of 8K to ~60K. Of interest are the cause of the enhanced pairing strength, and the nature of the superconducting state. We show angle-resolved photoemission spectroscopy (ARPES) data that provide clear evidence for strong cross-interface electron-phonon coupling in single UC limit, suggesting that pairing is significantly enhanced by the strong interface mode coupling. We will also show other results on the nature of the superconducting state in this system..

  14. Large area bulk superconductors

    DOEpatents

    Miller, Dean J.; Field, Michael B.

    2002-01-01

    A bulk superconductor having a thickness of not less than about 100 microns is carried by a polycrystalline textured substrate having misorientation angles at the surface thereof not greater than about 15.degree.; the bulk superconductor may have a thickness of not less than about 100 microns and a surface area of not less than about 50 cm.sup.2. The textured substrate may have a thickness not less than about 10 microns and misorientation angles at the surface thereof not greater than about 15.degree.. Also disclosed is a process of manufacturing the bulk superconductor and the polycrystalline biaxially textured substrate material.

  15. Superconductor as movie star

    SciTech Connect

    Pool, R.

    1993-12-03

    Japanese researchers have succeeded in producing a movie of changes in the magnetic flux lattice of a high-Tc superconductor as it is warmed. They used a technique called electron holography, in which electrons are passed through a superconductor, and flux lines are visualized as interference patterns induced by the electrons as they undergo a phase change as they pass to one side or another of the flux lines. The technique will have application in designing superconductors so that they do not lose their superconductivity when exposed to magnetic fields.

  16. The magnetic moment enigma in Fe-based high temperature superconductors.

    PubMed

    Mannella, Norman

    2014-11-26

    The determination of the most appropriate starting point for the theoretical description of Fe-based materials hosting high-temperature superconductivity remains among the most important unsolved problem in this relatively new field. Most of the work to date has focused on the pnictides, with LaFeAsO, BaFe(2)As(2) and LiFeAs being representative parent compounds of three families known as 1111, 122 and 111, respectively. This topical review examines recent progress in this area, with particular emphasis on the implication of experimental data which have provided evidence for the presence of electron itinerancy and the detection of local spin moments. In light of the results presented, the necessity of a theoretical framework contemplating the presence and the interplay between itinerant electrons and large spin moments is discussed. It is argued that the physics at the heart of the macroscopic properties of pnictides Fe-based high-temperature superconductors appears to be far more complex and interesting than initially predicted. PMID:25352180

  17. Continuous lengths of oxide superconductors

    DOEpatents

    Kroeger, Donald M.; List, III, Frederick A.

    2000-01-01

    A layered oxide superconductor prepared by depositing a superconductor precursor powder on a continuous length of a first substrate ribbon. A continuous length of a second substrate ribbon is overlaid on the first substrate ribbon. Sufficient pressure is applied to form a bound layered superconductor precursor powder between the first substrate ribbon and the second substrate ribbon. The layered superconductor precursor is then heat treated to establish the oxide superconducting phase. The layered oxide superconductor has a smooth interface between the substrate and the oxide superconductor.

  18. Doping dependence of spin excitations and its correlations with high-temperature superconductivity in iron pnictides.

    PubMed

    Wang, Meng; Zhang, Chenglin; Lu, Xingye; Tan, Guotai; Luo, Huiqian; Song, Yu; Wang, Miaoyin; Zhang, Xiaotian; Goremychkin, E A; Perring, T G; Maier, T A; Yin, Zhiping; Haule, Kristjan; Kotliar, Gabriel; Dai, Pengcheng

    2013-01-01

    High-temperature superconductivity in iron pnictides occurs when electrons and holes are doped into their antiferromagnetic parent compounds. Since spin excitations may be responsible for electron pairing and superconductivity, it is important to determine their electron/hole-doping evolution and connection with superconductivity. Here we use inelastic neutron scattering to show that while electron doping to the antiferromagnetic BaFe₂As₂ parent compound modifies the low-energy spin excitations and their correlation with superconductivity (<50 meV) without affecting the high-energy spin excitations (>100 meV), hole-doping suppresses the high-energy spin excitations and shifts the magnetic spectral weight to low-energies. In addition, our absolute spin susceptibility measurements for the optimally hole-doped iron pnictide reveal that the change in magnetic exchange energy below and above T(c) can account for the superconducting condensation energy. These results suggest that high-T(c) superconductivity in iron pnictides is associated with both the presence of high-energy spin excitations and a coupling between low-energy spin excitations and itinerant electrons.

  19. Nematic quantum critical point without magnetism in FeSe1-xSx superconductors.

    PubMed

    Hosoi, Suguru; Matsuura, Kohei; Ishida, Kousuke; Wang, Hao; Mizukami, Yuta; Watashige, Tatsuya; Kasahara, Shigeru; Matsuda, Yuji; Shibauchi, Takasada

    2016-07-19

    In most unconventional superconductors, the importance of antiferromagnetic fluctuations is widely acknowledged. In addition, cuprate and iron-pnictide high-temperature superconductors often exhibit unidirectional (nematic) electronic correlations, including stripe and orbital orders, whose fluctuations may also play a key role for electron pairing. In these materials, however, such nematic correlations are intertwined with antiferromagnetic or charge orders, preventing the identification of the essential role of nematic fluctuations. This calls for new materials having only nematicity without competing or coexisting orders. Here we report systematic elastoresistance measurements in FeSe1-xSx superconductors, which, unlike other iron-based families, exhibit an electronic nematic order without accompanying antiferromagnetic order. We find that the nematic transition temperature decreases with sulfur content x; whereas, the nematic fluctuations are strongly enhanced. Near [Formula: see text], the nematic susceptibility diverges toward absolute zero, revealing a nematic quantum critical point. The obtained phase diagram for the nematic and superconducting states highlights FeSe1-xSx as a unique nonmagnetic system suitable for studying the impact of nematicity on superconductivity. PMID:27382157

  20. Nematic quantum critical point without magnetism in FeSe1-xSx superconductors.

    PubMed

    Hosoi, Suguru; Matsuura, Kohei; Ishida, Kousuke; Wang, Hao; Mizukami, Yuta; Watashige, Tatsuya; Kasahara, Shigeru; Matsuda, Yuji; Shibauchi, Takasada

    2016-07-19

    In most unconventional superconductors, the importance of antiferromagnetic fluctuations is widely acknowledged. In addition, cuprate and iron-pnictide high-temperature superconductors often exhibit unidirectional (nematic) electronic correlations, including stripe and orbital orders, whose fluctuations may also play a key role for electron pairing. In these materials, however, such nematic correlations are intertwined with antiferromagnetic or charge orders, preventing the identification of the essential role of nematic fluctuations. This calls for new materials having only nematicity without competing or coexisting orders. Here we report systematic elastoresistance measurements in FeSe1-xSx superconductors, which, unlike other iron-based families, exhibit an electronic nematic order without accompanying antiferromagnetic order. We find that the nematic transition temperature decreases with sulfur content x; whereas, the nematic fluctuations are strongly enhanced. Near [Formula: see text], the nematic susceptibility diverges toward absolute zero, revealing a nematic quantum critical point. The obtained phase diagram for the nematic and superconducting states highlights FeSe1-xSx as a unique nonmagnetic system suitable for studying the impact of nematicity on superconductivity.

  1. Universal self-field critical current for thin-film superconductors

    PubMed Central

    Talantsev, E. F.; Tallon, J. L.

    2015-01-01

    For any practical superconductor the magnitude of the critical current density, Jc, is crucially important. It sets the upper limit for current in the conductor. Usually Jc falls rapidly with increasing external magnetic field, but even in zero external field the current flowing in the conductor generates a self-field that limits Jc. Here we show for thin films of thickness less than the London penetration depth, λ, this limiting Jc adopts a universal value for all superconductors—metals, oxides, cuprates, pnictides, borocarbides and heavy Fermions. For type-I superconductors, it is Hc/λ where Hc is the thermodynamic critical field. But surprisingly for type-II superconductors, we find the self-field Jc is Hc1/λ where Hc1 is the lower critical field. Jc is thus fundamentally determined and this provides a simple means to extract absolute values of λ(T) and, from its temperature dependence, the symmetry and magnitude of the superconducting gap. PMID:26240014

  2. Nematic quantum critical point without magnetism in FeSe1‑xSx superconductors

    NASA Astrophysics Data System (ADS)

    Hosoi, Suguru; Matsuura, Kohei; Ishida, Kousuke; Wang, Hao; Mizukami, Yuta; Watashige, Tatsuya; Kasahara, Shigeru; Matsuda, Yuji; Shibauchi, Takasada

    2016-07-01

    In most unconventional superconductors, the importance of antiferromagnetic fluctuations is widely acknowledged. In addition, cuprate and iron-pnictide high-temperature superconductors often exhibit unidirectional (nematic) electronic correlations, including stripe and orbital orders, whose fluctuations may also play a key role for electron pairing. In these materials, however, such nematic correlations are intertwined with antiferromagnetic or charge orders, preventing the identification of the essential role of nematic fluctuations. This calls for new materials having only nematicity without competing or coexisting orders. Here we report systematic elastoresistance measurements in FeSe1‑xSx superconductors, which, unlike other iron-based families, exhibit an electronic nematic order without accompanying antiferromagnetic order. We find that the nematic transition temperature decreases with sulfur content x; whereas, the nematic fluctuations are strongly enhanced. Near x0.17x≈0.17, the nematic susceptibility diverges toward absolute zero, revealing a nematic quantum critical point. The obtained phase diagram for the nematic and superconducting states highlights FeSe1‑xSx as a unique nonmagnetic system suitable for studying the impact of nematicity on superconductivity.

  3. Manipulation of gap nodes by uniaxial strain in iron-based superconductors.

    PubMed

    Kang, Jian; Kemper, Alexander F; Fernandes, Rafael M

    2014-11-21

    In the iron pnictides and chalcogenides, multiple orbitals participate in the superconducting state, enabling different gap structures to be realized in distinct materials. Here we argue that the spectral weights of these orbitals can, in principle, be controlled by a tetragonal symmetry-breaking uniaxial strain, due to the enhanced nematic susceptibility of many iron-based superconductors. By investigating multiorbital microscopic models in the presence of orbital order, we show that not only Tc can be enhanced, but pairs of accidental gap nodes can be annihilated and created in the Fermi surface by an increasing external strain. We explain our results as a mixture of nearly degenerate superconducting states promoted by strain, and show that the annihilation and creation of nodes can be detected experimentally via anisotropic penetration depth measurements. Our results provide a promising framework to externally control the superconducting properties of iron-based materials. PMID:25479515

  4. Angle-resolved spectroscopy study of Ni-based superconductor SrNi2As2

    NASA Astrophysics Data System (ADS)

    Zeng, L.-K.; Richard, P.; van Roekeghem, A.; Yin, J.-X.; Wu, S.-F.; Chen, Z. G.; Wang, N. L.; Biermann, S.; Qian, T.; Ding, H.

    2016-07-01

    We performed an angle-resolved photoemission spectroscopy study of the Ni-based superconductor SrNi2As2 . Electron and hole Fermi surface pockets are observed, but their different shapes and sizes lead to very poor nesting conditions. The experimental electronic band structure of SrNi2As2 is in good agreement with first-principles calculations after a slight renormalization (by a factor 1.1), confirming the picture of Hund's exchange-dominated electronic correlations decreasing with increasing filling of the 3 d shell in the Fe-, Co-, and Ni-based compounds. These findings emphasize the importance of Hund's coupling and 3 d -orbital filling as key tuning parameters of electronic correlations in transition-metal pnictides.

  5. Point-contact spectroscopic studies on normal and superconducting AFe2As2-type iron pnictide single crystals

    SciTech Connect

    Lu, Xin; Park, W. K.; Yuan, H. Q.; Chen, G. F,; Luo, G. L.; Wang, N. L.; Sefat, A. S.; McGuire, Michael A; Jin, R.; Sales, Brian C; Mandrus, David; Gillett, J; Sebastian, Suchitra; Greene, L. H.

    2010-01-01

    Point-contact Andreev reflection spectroscopy is applied to investigate the gap structure in iron pnictide single-crystal superconductors of the AFe{sub 2}As{sub 2} (A = Ba, Sr) family (Fe-122). The observed point-contact junction conductance curves, G(V), can be divided into two categories: one where Andreev reflection is present for both (Ba{sub 0.6}K{sub 0.4})Fe{sub 2}As{sub 2} and Ba(Fe{sub 0.9}Co{sub 0.1}){sub 2}As{sub 2}, and the other with a V{sup 2/3} background conductance universally observed, extending even up to 100 meV for Sr{sub 0.6}Na{sub 0.4}Fe{sub 2}As{sub 2} and Sr(Fe{sub 0.9}Co{sub 0.1}){sub 2}As{sub 2}. The latter is also observed in point-contact junctions on the nonsuperconducting parent compound BaFe{sub 2}As{sub 2} and superconducting (Ba{sub 0.6}K{sub 0.4})Fe{sub 2}As{sub 2} crystals. Mesoscopic phase-separated coexistence of magnetic and superconducting orders is considered to explain distinct behaviors in the superconducting samples. For Ba{sub 0.6}K{sub 0.4}Fe{sub 2}As{sub 2}, double peaks due to Andreev reflection with a strongly sloping background are frequently observed for point contacts on freshly cleaved c-axis surfaces. If normalized using a background baseline and analyzed using the Blonder-Tinkham-Klapwijk model, the data show a gap size of {approx} 3.0-4.0 meV with 2{Delta}{sub 0}/k{sub B}T{sub c} {approx} 2.0-2.6, consistent with the smaller gap size reported for the LnFeAsO family (Fe-1111). For the Ba(Fe{sub 0.9}Co{sub 0.1}){sub 2}As{sub 2}, the G(V) curves typically display a zero-bias conductance peak.

  6. Point-contact spectroscopic studies on normal and superconducting AFe2As2-type iron-pnictide single crystals

    SciTech Connect

    Lu, Xin; Park, W.K.; Yuan, H.Q.; Chen, G.F.; Luo, G.L.; Wang, N.L.; Sefat, A.S.; McGuire, M.A.; Jin, R.; Sales, B.C.; Mandrus, D.; Gillett, J.; Sebastian, Suchitra E.; Greene, L.H.

    2010-04-23

    Point-contact Andreev reflection spectroscopy is applied to investigate the gap structure in iron pnictide single-crystal superconductors of the AFe{sub 2}As{sub 2} (A = Ba, Sr) family ('Fe-122'). The observed point-contact junction conductance curves, G(V), can be divided into two categories: one where Andreev reflection is present for both (Ba{sub 0.6}K{sub 0.4})Fe{sub 2}As{sub 2} and Ba(Fe{sub 0.9}Co{sub 0.1}){sub 2}As{sub 2}, and the other with a V{sup 2/3} background conductance universally observed, extending even up to 100 meV for Sr{sub 0.6}Na{sub 0.4}Fe{sub 2}As{sub 2} and Sr(Fe{sub 0.9}Co{sub 0.1}){sub 2}As{sub 2}. The latter is also observed in point-contact junctions on the nonsuperconducting parent compound BaFe{sub 2}As{sub 2} and superconducting (Ba{sub 0.6}K{sub 0.4})Fe{sub 2}As{sub 2} crystals. Mesoscopic phase-separated coexistence of magnetic and superconducting orders is considered to explain distinct behaviors in the superconducting samples. For Ba{sub 0.6}K{sub 0.4}Fe{sub 2}As{sub 2}, double peaks due to Andreev reflection with a strongly sloping background are frequently observed for point contacts on freshly cleaved c-axis surfaces. If normalized using a background baseline and analyzed using the Blonder–Tinkham–Klapwijk model, the data show a gap size of ~ 3.0–4.0 meV with 2Δ{sub 0}/k{sub B}T{sub c} ~ 2.0–2.6, consistent with the smaller gap size reported for the LnFeAsO family ('Fe-1111'). For the Ba(Fe{sub 0.9}Co{sub 0.1}){sub 2}As{sub 2}, the G(V) curves typically display a zero-bias conductance peak.

  7. Instabilities of High Temperature Superconductors

    PubMed Central

    Matthias, B. T.; Corenzwit, E.; Cooper, A. S.; Longinotti, L. D.

    1971-01-01

    We have observed the transition temperature of both the cubic and tetragonal phases of several high-temperature β-W superconductors. The instability of the cubic lattice appears to be characteristic of high-temperature superconductors. PMID:16591897

  8. Structural symmetries of the 112-type iron-based superconductor (Ca1-xLax)FeAs2 studied using nonlinear and ultrafast optics

    NASA Astrophysics Data System (ADS)

    Harter, John; Chu, Hao; Jiang, Shan; Ni, Ni; Hsieh, David

    The crystal structure of the newly discovered 112-type iron-based superconductors contains symmetry-breaking arsenic chains, avoiding the need for local probes or uniaxial strain in order to study the ubiquitous electronic nematic state that exists in the vicinity of magnetic order in the iron pnictides. In addition, the 112-type materials are the first known high-temperature superconductors without a center of inversion, with interesting ramifications for Cooper pairing in the superconducting state. We present details of the structure of 112-type (Ca1-xLax)FeAs2 using rotational anisotropy second harmonic generation and pump-probe transient reflectivity experiments. These all-optical techniques are complimentary to conventional diffraction measurements and enable a precise determination of crystallographic symmetries. Our measurements highlight the novel structural properties of the 112-type materials.

  9. Granular Superconductors and Gravity

    NASA Technical Reports Server (NTRS)

    Noever, David; Koczor, Ron

    1999-01-01

    As a Bose condensate, superconductors provide novel conditions for revisiting previously proposed couplings between electromagnetism and gravity. Strong variations in Cooper pair density, large conductivity and low magnetic permeability define superconductive and degenerate condensates without the traditional density limits imposed by the Fermi energy (approx. 10(exp -6) g cu cm). Recent experiments have reported anomalous weight loss for a test mass suspended above a rotating Type II, YBCO superconductor, with a relatively high percentage change (0.05-2.1%) independent of the test mass' chemical composition and diamagnetic properties. A variation of 5 parts per 104 was reported above a stationary (non-rotating) superconductor. In experiments using a sensitive gravimeter, bulk YBCO superconductors were stably levitated in a DC magnetic field and exposed without levitation to low-field strength AC magnetic fields. Changes in observed gravity signals were measured to be less than 2 parts in 108 of the normal gravitational acceleration. Given the high sensitivity of the test, future work will examine variants on the basic magnetic behavior of granular superconductors, with particular focus on quantifying their proposed importance to gravity.

  10. Two types of superconducting domes in unconventional superconductors

    NASA Astrophysics Data System (ADS)

    Das, Tanmoy; Panagopoulos, Christos

    In this talk, we present a comprehensive analysis of the SC properties and phase diagrams across several families of unconventional superconductors within the copper-oxides, heavy-fermions, organics, and the recently discovered iron-pnictides, iron-chalcogenides, and oxybismuthides. We find that there are two types of SC domes present in all families of SC materials, arising sometimes as completely isolated, or merged into one, or in some materials only any one of them appears. One of the SC dome appearing at or near a possible QCP usually possesses a lower transition temperature (Tc) . The other SC dome appearing at a different value of the tuning parameter around a non-Fermi liquid (NFL) state often has higher Tc. Both SC domes are not necessarily linked to each other, and so does the QCP and NFL state. In materials, where both domes are present, they can be isolated by multiple tuning (such as such as disorder, or pressure, or magnetic field in addition to doping, and vice versa), giving a unique opportunity to decouple the relationship between QCP, NFL, and their role on superconductivity. The systematic study the NFL state might be a generic route to higher-Tc superconductivity.

  11. What drives nematic order in iron-based superconductors?

    NASA Astrophysics Data System (ADS)

    Fernandes, R. M.; Chubukov, A. V.; Schmalian, J.

    2014-02-01

    Although the existence of nematic order in iron-based superconductors is now a well-established experimental fact, its origin remains controversial. Nematic order breaks the discrete lattice rotational symmetry by making the x and y directions in the iron plane non-equivalent. This can happen because of a regular structural transition or as the result of an electronically driven instability -- in particular, orbital order or spin-driven Ising-nematic order. The latter is a magnetic state that breaks rotational symmetry but preserves time-reversal symmetry. Symmetry dictates that the development of one of these orders immediately induces the other two, making the origin of nematicity a physics realization of the `chicken and egg problem’. In this Review, we argue that the evidence strongly points to an electronic mechanism of nematicity, placing nematic order in the class of correlation-driven electronic instabilities, like superconductivity and density-wave transitions. We discuss different microscopic models for nematicity and link them to the properties of the magnetic and superconducting states, providing a unified perspective on the phase diagram of the iron pnictides.

  12. Lightning in superconductors

    PubMed Central

    Vestgården, J. I.; Shantsev, D. V.; Galperin, Y. M.; Johansen, T. H.

    2012-01-01

    Crucially important for application of type-II superconductor films is the stability of the vortex matter – magnetic flux lines penetrating the material. If some vortices get detached from pinning centres, the energy dissipated by their motion will facilitate further depinning, and may trigger a massive electromagnetic breakdown. Up to now, the time-resolved behaviour of these ultra-fast events was essentially unknown. We report numerical simulation results revealing the detailed dynamics during breakdown as within nanoseconds it develops branching structures in the electromagnetic fields and temperature, with striking resemblance of atmospheric lightning. During a dendritic avalanche the superconductor is locally heated above its critical temperature, while electrical fields rise to several kV/m as the front propagates at instant speeds near up to 100 km/s. The numerical approach provides an efficient framework for understanding the ultra-fast coupled non-local dynamics of electromagnetic fields and dissipation in superconductor films. PMID:23185691

  13. Fundamental studies of superconductors using scanning magnetic imaging

    NASA Astrophysics Data System (ADS)

    Kirtley, J. R.

    2010-12-01

    cuprates places limits on spin-charge separation in these materials. Studies of spontaneous generation of fluxoids upon cooling rings through the superconducting transition provide clues to dynamical processes relevant to the early development of the universe, while studies of vortex motion in cuprate grain boundaries allow the measurement of current-voltage characteristics at the femtovolt scale for these technologically important defects. Scanning SQUID susceptometry allows the measurement of superconducting fluctuations on samples comparable in size to the coherence length, revealing stripes in susceptibility believed to be associated with enhanced superfluid density on the twin boundaries in the pnictide superconductor Co doped Ba-122, and indicating the presence of spin-like excitations, which may be a source of noise in superconducting devices, in a wide variety of materials. Scanning magnetic microscopies allow the absolute value of penetration depths to be measured locally over a wide temperature range, providing clues to the symmetry of the order parameter in unconventional superconductors. Finally, MFM tips can be used to manipulate vortices, providing information on flux trapping in superconductors.

  14. Influence of the electronic structure on the transport properties of some iron pnictides

    NASA Astrophysics Data System (ADS)

    Rullier-Albenque, Florence

    2016-01-01

    An important feature of the iron-based pnictides is their multi-band electronic structure with both electron and hole bands at the Fermi level. The size of these pockets can be changed by different types of substitution, resulting in a variety of original magnetic and electronic properties. The contributions of both types of carriers will thus have important consequences on the evolution of the transport properties versus temperature and doping. It has been pointed out that Hund's rule interaction plays a prominent role in the physics of these compounds by allowing a strong orbital differentiation between the 3d Fe orbitals. As a result, a description in terms of more or less correlated electrons was proposed and may have important consequences on the scattering lifetimes of the different carriers. Finally, the presence of very flat bands at the Fermi level may induce a semiconductor-like behavior, with a change in carrier concentration with temperature. In this paper, we will review the evolution of transport properties with chemical doping/substitution in iron pnictides. We will more particularly focus on the 122 family (Ba(Sr,Ca)Fe2As2) and the 111 LiFeAs compound for which sizeable single crystals required for transport measurements are available. The combined resistivity, Hall effect and magnetoresistance data will be analyzed in association with electronic structure calculations, angle-resolved photoemission measurements and quantum oscillations. In spite of the strong interplay between antiferromagnetism and superconductivity in most part of their phase diagram, direct signatures of spin fluctuations are difficult to identify in the transport properties of iron pnictides. We will show that measurements of the longitudinal magnetoresistance provide a powerful tool for studying the coupling between the charge carriers and the spin degrees of freedom.

  15. Method for preparing superconductors

    DOEpatents

    Dahlgren, Shelley D.

    1976-01-01

    A superconductor having an equiaxed fine grain beta-tungsten crystalline structure found to have improved high field critical current densities is prepared by sputter-depositing superconductive material onto a substrate cooled to below 200.degree. C. and heat-treating the deposited material.

  16. Quantum phenomena in superconductors

    SciTech Connect

    Clarke, J.

    1987-08-01

    This paper contains remarks by the author on aspects of macroscopic quantum phenomena in superconductors. Some topics discussed are: Superconducting low-inductance undulatory galvanometer (SLUGS), charge imbalance, cylindrical dc superconducting quantum interference device (SQUIDS), Geophysics, noise theory, magnetic resonance with SQUIDS, and macroscopic quantum tunneling. 23 refs., 4 figs. (LSP)

  17. High-temperature resistivity in the iron pnictides and the electron-doped cuprates

    NASA Astrophysics Data System (ADS)

    Bach, P. L.; Saha, S. R.; Kirshenbaum, K.; Paglione, J.; Greene, R. L.

    2011-06-01

    We measured the high-temperature (up to 800 K) resistivity of several dopings of SrFe2-x(Ni,Co)xAs2 (Sr-122) and compared the results with similar measurements on electron-doped cuprates. We find that the Sr-122 pnictide resistivity saturates above 500 K at around 400-700 μΩcm, consistent with the Mott-Ioffe-Regel (MIR) limit and in contrast with the MIR-violating behavior of the hole-doped cuprates and our measurements on electron-doped cuprates. This supports the view that electronic correlations in the ferropnictides may be weaker than in the cuprates.

  18. SIS (Superconductor-Insulator-Superconductor) mixer research

    NASA Astrophysics Data System (ADS)

    Feldman, M. J.

    1987-03-01

    Theoretical and experimental research has been conducted to elucidate the basic physics behind the properties of superconductor-insulator- superconductor (SIS) tunnel junction receiving devices. The quantum theory of mixing was employed to calculate the saturation behavior of the SIS mixer and the SIS direct detector. The direct detector was found to saturate at far higher powers than previously believed, permitting practical application of this device. Experiments verified the saturation theory. The mixer saturation expression led to the formulation of frequency scaling regions. The origin of quantum noise sources is completely minimized. The mixer quantum noise limit was shown to be reached in only two special cases. Computer calculations determined that the behavior of SIS receivers divides into two regimes, low and high frequencies, the cross-over frequency depending upon junction quality. The properties of these two regimes were delineated. All previous SIS receivers have operated in the low frequency regime. Plasma-etched niobium nitride edge junctions have been fabricated using a novel barrier formation process. These junctions have excellent current-voltage characteristics.

  19. Electronic structure of NaFeAs superconductor: LDA+DMFT calculations compared to the ARPES experiment

    NASA Astrophysics Data System (ADS)

    Nekrasov, I. A.; Pavlov, N. S.; Sadovskii, M. V.

    2015-07-01

    We present the results of extended theoretical LDA+DMFT calculations for a new iron-pnictide high temperature superconductor NaFeAs compared with the recent high quality angle-resolved photoemission (ARPES) experiments on this system (see arXiv:1409.1537). The universal manifestation of correlation effects in iron-pnictides is narrowing of conducting bands near the Fermi level. Our calculations demonstrate that for NaFeAs the effective mass is renormalized on average by a factor of the order of 3, in good agreement with ARPES data. This is essentially due to correlation effects on Fe-3 d orbitals only and no additional interactions with any kind of Boson modes, as suggested in the work mentioned, are necessary to describe the experiment. In addition, we show that ARPES data taken at about 160 eV beam energy most probably corresponds to k z = π Brillouin zone boundary, while ARPES data measured at about 80 eV beam energy rather represents k z = 0. Contributions of different Fe-3 d orbitals into spectral function map are also discussed.

  20. Platform for engineering topological superconductors: Superlattices on Rashba superconductors

    NASA Astrophysics Data System (ADS)

    Lu, Yao; He, Wen-Yu; Xu, Dong-Hui; Lin, Nian; Law, K. T.

    2016-07-01

    The search for topological superconductors which support Majorana fermion excitations has been an important topic in condensed matter physics. In this work, we propose an experimental scheme for engineering topological superconductors. In this scheme, by manipulating the superlattice structure of organic molecules placed on top of a superconductor with Rashba spin-orbit coupling, topological superconducting phases can be achieved without or with little fine tuning of the chemical potential. Moreover, superconductors with different Chern numbers can be obtained by changing the superlattice structure of the organic molecules.

  1. Spin reorientation driven by the interplay between spin-orbit coupling and Hund's rule coupling in iron pnictides

    NASA Astrophysics Data System (ADS)

    Christensen, Morten H.; Kang, Jian; Andersen, Brian M.; Eremin, Ilya; Fernandes, Rafael M.

    2015-12-01

    In most magnetically-ordered iron pnictides, the magnetic moments lie in the FeAs planes, parallel to the modulation direction of the spin stripes. However, recent experiments in hole-doped iron pnictides have observed a reorientation of the magnetic moments from in-plane to out-of-plane. Interestingly, this reorientation is accompanied by a change in the magnetic ground state from a stripe antiferromagnet to a tetragonal nonuniform magnetic configuration. Motivated by these recent observations, here we investigate the origin of the spin anisotropy in iron pnictides using an itinerant microscopic electronic model that respects all the symmetry properties of a single FeAs plane. We find that the interplay between the spin-orbit coupling and the Hund's rule coupling can account for the observed spin anisotropies, including the spin reorientation in hole-doped pnictides, without the need to invoke orbital or nematic order. Our calculations also reveal an asymmetry between the magnetic ground states of electron- and hole-doped compounds, with only the latter displaying tetragonal magnetic states.

  2. Strain induced modification in phonon dispersion curves of monolayer boron pnictides

    SciTech Connect

    Jha, Prafulla K. E-mail: prafullaj@yahoo.com; Soni, Himadri R.

    2014-01-14

    In the frame work of density functional theory, the biaxial strain induced phonon dispersion curves of monolayer boron pnictides (BX, X = N, P, As, and Sb) have been investigated. The electron-ion interactions have been modelled using ultrasoft pseudopotentials while exchange-correlation energies have been approximated by the method of local density approximation in the parameterization of Perdew-Zunger. The longitudinal and transverse acoustic phonon modes of boron pnictide sheets show linear dependency on wave vector k{sup →} while out of plane mode varies as k{sup 2}. The in-plane longitudinal and out of plane transverse optical modes in boron nitride displaying significant dispersion similar to graphene. We have analyzed the biaxial strain dependent behaviour of out of plane acoustic phonon mode which is linked to ripple for four BX sheets using a model equation with shell elasticity theory. The strain induces the hardening of this mode with tendency to become more linear with increase in strain percentage. The strain induced hardening of out of plane acoustic phonon mode indicates the absence of rippling in these compounds. Our band structure calculations for both unstrained and strained 2D h-BX are consistent with previous calculations.

  3. Impact of uniaxial pressure on structural and magnetic phase transitions in electron-doped iron pnictides

    NASA Astrophysics Data System (ADS)

    Lu, Xingye; Tseng, Kuo-Feng; Keller, T.; Zhang, Wenliang; Hu, Ding; Song, Yu; Man, Haoran; Park, J. T.; Luo, Huiqian; Li, Shiliang; Nevidomskyy, Andriy H.; Dai, Pengcheng

    2016-04-01

    We use neutron resonance spin echo and Larmor diffraction to study the effect of uniaxial pressure on the tetragonal-to-orthorhombic structural (Ts) and antiferromagnetic (AF) phase transitions in iron pnictides BaFe2 -xNixAs2 (x =0 ,0.03 ,0.12 ),SrFe1.97Ni0.03As2, and BaFe2(As0.7P0.3)2. In antiferromagnetically ordered BaFe2 -xNixAs2 and SrFe1.97Ni0.03As2 with TN and Ts (TN≤Ts ), a uniaxial pressure necessary to detwin the sample also increases TN, smears out the structural transition, and induces an orthorhombic lattice distortion at all temperatures. By comparing temperature and doping dependence of the pressure induced lattice parameter changes with the elastoresistance and nematic susceptibility obtained from transport and ultrasonic measurements, we conclude that the in-plane resistivity anisotropy found in the paramagnetic state of electron underdoped iron pnictides depends sensitively on the nature of the magnetic phase transition and a strong coupling between the uniaxial pressure induced lattice distortion and electronic nematic susceptibility.

  4. Strain induced modification in phonon dispersion curves of monolayer boron pnictides

    NASA Astrophysics Data System (ADS)

    Jha, Prafulla K.; Soni, Himadri R.

    2014-01-01

    In the frame work of density functional theory, the biaxial strain induced phonon dispersion curves of monolayer boron pnictides (BX, X = N, P, As, and Sb) have been investigated. The electron-ion interactions have been modelled using ultrasoft pseudopotentials while exchange-correlation energies have been approximated by the method of local density approximation in the parameterization of Perdew-Zunger. The longitudinal and transverse acoustic phonon modes of boron pnictide sheets show linear dependency on wave vector k→ while out of plane mode varies as k2. The in-plane longitudinal and out of plane transverse optical modes in boron nitride displaying significant dispersion similar to graphene. We have analyzed the biaxial strain dependent behaviour of out of plane acoustic phonon mode which is linked to ripple for four BX sheets using a model equation with shell elasticity theory. The strain induces the hardening of this mode with tendency to become more linear with increase in strain percentage. The strain induced hardening of out of plane acoustic phonon mode indicates the absence of rippling in these compounds. Our band structure calculations for both unstrained and strained 2D h-BX are consistent with previous calculations.

  5. The half metallic state of transition metal pnictides in Wurtzite structure

    NASA Astrophysics Data System (ADS)

    Miao, M. S.; Lambrecht, Walter R. L.

    2004-03-01

    We perform full potential linear muffin tin orbital (FP-LMTO) calculations for a series transition metal pnictides compounds, including CrAs, CrSb, MnAs, MnSb, VAs and VSb etc., in various four and six fold coordinated structures. The volume expansion can stabilize both the Zinc Blende (ZB) and the Wurtzite (WZ) structures. The energy differences between the ZB and WZ structure are very small and for several compound, such as CrSb, VAs, etc., the WZ structure is more stable than the ZB structure. As in ZB structure, the transition metal pnictides are also half metallic in the WZ structure. The density of states at the Fermi level for majority spin, the band gap for minority spin and the valence band maximum (VBM) for minority spin relative to the Fermi level are very close for two different structures, indicating that the half metallic properties are mainly determined by the local tetrahedron environments which is similar for ZB and WZ structures. While the volume increases, minority band gap increases and the relative Fermi level position to the VBM of minority spin decreases. This is different to the semiconductor band gap dependence on the volume and is caused by the exchange interaction between the majority and the minority spins that will increase with the expansion of the volume. The same effect also causes a slight increase of the DOS for majority spin at Fermi level.

  6. Ambient-pressure organic superconductor

    DOEpatents

    Williams, Jack M.; Wang, Hsien-Hau; Beno, Mark A.

    1986-01-01

    A new class of organic superconductors having the formula (ET).sub.2 MX.sub.2 wherein ET represents bis(ethylenedithio)-tetrathiafulvalene, M is a metal such as Au, Ag, In, Tl, Rb, Pd and the like and X is a halide. The superconductor (ET).sub.2 AuI.sub.2 exhibits a transition temperature of 5 K which is high for organic superconductors.

  7. Superconductor stability 90: A review

    SciTech Connect

    Dresner, L.

    1990-01-01

    This paper reviews some recent developments in the field of stability of superconductors. The main topics dealt with are hydrodynamic phenomena in cable-in-conduit superconductors, namely, multiple stability, quench pressure, thermal expulsion, and thermal hydraulic quenchback, traveling normal zones in large, composite conductors, such as those intended for SMES, and the stability of vapor-cooled leads made of high-temperature superconductors. 31 refs., 5 figs.

  8. A unifying phase diagram with correlation-driven superconductor-to-insulator transition for the 122 series of iron chalcogenides

    NASA Astrophysics Data System (ADS)

    Niu, X. H.; Chen, S. D.; Jiang, J.; Ye, Z. R.; Yu, T. L.; Xu, D. F.; Xu, M.; Feng, Y.; Yan, Y. J.; Xie, B. P.; Zhao, J.; Gu, D. C.; Sun, L. L.; Mao, Qianhui; Wang, Hangdong; Fang, Minghu; Zhang, C. J.; Hu, J. P.; Sun, Z.; Feng, D. L.

    2016-02-01

    The 122 series of iron chalcogenide superconductors, for example KxFe2 -ySe2 , only possesses electron Fermi pockets. Their distinctive electronic structure challenges the picture built upon iron pnictide superconductors, where both electron and hole Fermi pockets coexist. However, partly due to the intrinsic phase separation in this family of compounds, many aspects of their behavior remain elusive. In particular, the evolution of the 122 series of iron chalcogenides with chemical substitution still lacks a microscopic and unified interpretation. Using angle-resolved photoemission spectroscopy, we studied a major fraction of 122 iron chalcogenides, including the isovalently "doped" KxFe2 -ySe2 -zSz,RbxFe2 -ySe2 -zTez , and (Tl,K) xFe2 -ySe2 -zSz . We found that the bandwidths of the low energy Fe 3 d bands in these materials depend on doping; and more crucially, as the bandwidth decreases, the ground state evolves from a metal to a superconductor, and eventually to an insulator, yet the Fermi surface in the metallic phases is unaffected by the isovalent dopants. Moreover, the correlation-driven insulator found here with small band filling may be a novel insulating phase. Our study shows that almost all the known 122-series iron chalcogenides can be understood via one unifying phase diagram which implies that moderate correlation strength is beneficial for the superconductivity.

  9. Orbital-dependent electron correlation effects in iron-based superconductors

    NASA Astrophysics Data System (ADS)

    Yi, Ming

    The iron chalcogenide superconductors constitute arguably one of the most intriguing families of the iron-based high temperature superconductors given their ability to superconduct at comparable temperatures as the iron pnictides, despite the lack of similarities in their magnetic structures and Fermi surface topologies. In particular, the lack of hole Fermi pockets at the Brillouin zone center posts a challenge to the previous proposal of spin fluctuation mediated pairing via Fermi surface nesting. In this talk, using angle-resolved photoemission spectroscopy measurements, I will present evidence that show that instead of Fermi surface topology, strong electron correlation observed in electron bandwidth is an important ingredient for superconductivity in the iron chalcogenides. Specifically, I will show i) there exists universal strong orbital-selective renormalization effects and proximity to an orbital-selective Mott phase in Fe1+yTe1-xSex, AxFe2-ySe2, and monolayer FeSe film on SrTiO3, and ii) in RbxFe2(Se1-zSz)2 , where sulfur substitution for selenium continuously suppresses superconductivity down to zero, little change occurs in the Fermi surface topology while a substantial reduction of electron correlation is observed in an expansion of the overall bandwidth, implying that electron correlation is one of the key tuning parameters for superconductivity in these materials.

  10. Robustness of quantum critical pairing against disorder in iron-based superconductors

    NASA Astrophysics Data System (ADS)

    Kang, Jian; Fernandes, Rafael

    Several experiments in iron pnictides and cuprates reveal a superconducting (SC) state remarkably robust against non-magnetic disorder -- at least when compared to the simple extension of the Abrikosov-Gor'kov formalism to dirty unconventional superconductors. Motivated by the fact that most of these SC states appear in proximity to a magnetic instability, here we study the impact of non-magnetic disorder on the SC state promoted by quantum critical magnetic fluctuations. We go beyond the weak coupling approach by applying a variational formalism of the Eliashberg equations of the spin-fermion model, taking into account the effects of disorder on both fermionic and bosonic degrees of freedom. We find that the reduced fermionic coherent spectral weight near the magnetic quantum critical point strongly decreases the suppression rate of Tc by weak disorder, as compared to the Abrikosov-Gor'kov universal value. Furthermore, because the bosons promoting the Cooper pairs emerge as collective modes of the fermions, they are also impacted by disorder, giving rise to an additional reduction of the suppression rate of Tc by weak disorder. Our results qualitatively agree with experiments, shedding new light on why unconventional superconductors are robust against disorder. This work is supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award Number DE-SC0012336.

  11. Probing Topological Superconductors

    NASA Astrophysics Data System (ADS)

    Schmeltzer, David

    2015-03-01

    The presence of attractive interaction on the surface of a 3D topological insulator which is characterized by spinors carrying a Berry phase of π gives rise to superconductivity that support space time half vortices (Majorana zero modes). We construct the effective dual action for the superconductor with the vortices, and show that the 2 n Majorana fermions are localized and can be replaced with n spinless fermions. The effect of the Majorana zero modes can be observed trough the the Andreev cross reflection when metallic leads are attached to the superconductor. The presence of the Majorana fermions can be detected with transverse sound waves. We have computed the effect of elastic strain fields and obtain an anomalous response indicating the presence of the Majorana fermions.

  12. Analytic holographic superconductor

    NASA Astrophysics Data System (ADS)

    Herzog, Christopher P.

    2010-06-01

    We investigate a holographic superconductor that admits an analytic treatment near the phase transition. In the dual 3+1-dimensional field theory, the phase transition occurs when a scalar operator of scaling dimension two gets a vacuum expectation value. We calculate current-current correlation functions along with the speed of second sound near the critical temperature. We also make some remarks about critical exponents. An analytic treatment is possible because an underlying Heun equation describing the zero mode of the phase transition has a polynomial solution. Amusingly, the treatment here may generalize for an order parameter with any integer spin, and we propose a Lagrangian for a spin-two holographic superconductor.

  13. Vortex cutting in superconductors

    NASA Astrophysics Data System (ADS)

    Glatz, A.; Vlasko-Vlasov, V. K.; Kwok, W. K.; Crabtree, G. W.

    2016-08-01

    Vortex cutting and reconnection is an intriguing and still-unsolved problem central to many areas of classical and quantum physics, including hydrodynamics, astrophysics, and superconductivity. Here, we describe a comprehensive investigation of the crossing of magnetic vortices in superconductors using time dependent Ginsburg-Landau modeling. Within a macroscopic volume, we simulate initial magnetization of an anisotropic high temperature superconductor followed by subsequent remagnetization with perpendicular magnetic fields, creating the crossing of the initial and newly generated vortices. The time resolved evolution of vortex lines as they approach each other, contort, locally conjoin, and detach, elucidates the fine details of the vortex-crossing scenario under practical situations with many interacting vortices in the presence of weak pinning. Our simulations also reveal left-handed helical vortex instabilities that accompany the remagnetization process and participate in the vortex crossing events.

  14. Material Specific Rational Design of A1B2C3O7 High-Tc Superconductors without Copper [A, B, C = Cations

    NASA Astrophysics Data System (ADS)

    Isikaku-Ironkwe, O'paul; Schaffer, Michael J.

    Soon after the discovery of YBa2Cu3O7 with Tc = 93K, a similar structured system with Ag replacing Cu was discovered with a Tc = 50K. Also, the discovery of Ba0 . 6 K0 . 4 BiO3 with Tc = 30K indicated that Cu was not indispensable for high temperature superconductivity (HTSC). Latter, the discoveries of the Pnictide and Chalcogenide high-Tc superconductors confirmed those earlier experimental indications. Using our recently developed Material Specific Characterization Dataset (MSCD) model for analysis and design of superconductors, we have computed many designs that satisfy the MSCD characteristics of YBa2Cu3O7 as a design model. Our design recognizes the valence state characteristics that make YBa2Cu3O6 a semiconductor, while YBa2Cu3O7is a superconductor. Here we present ten material specific rational design examples of potential A1B2C3O7 HTSCs without Cu, using the YBa2Cu3O7 design model. This MSCD design model opens the possibility for search and discovery of high-Tc oxide superconductor systems without copper.

  15. Processing of Superconductor-Normal-Superconductor Josephson Edge Junctions

    NASA Technical Reports Server (NTRS)

    Kleinsasser, A. W.; Barner, J. B.

    1997-01-01

    The electrical behavior of epitaxial superconductor-normal-superconductor (SNS) Josephson edge junctions is strongly affected by processing conditions. Ex-situ processes, utilizing photoresist and polyimide/photoresist mask layers, are employed for ion milling edges for junctions with Yttrium-Barium-Copper-Oxide (YBCO) electrodes and primarily Co-doped YBCO interlayers.

  16. Time-reversal symmetry breaking superconductivity in the coexistence phase with magnetism in Fe pnictides.

    PubMed

    Hinojosa, Alberto; Fernandes, Rafael M; Chubukov, Andrey V

    2014-10-17

    We argue that superconductivity in the coexistence region with spin-density-wave (SDW) order in weakly doped Fe pnictides erdiffers qualitatively from the ordinary s(+-) state outside the coexistence region as it develops an additional gap component which is a mixture of intrapocket singlet (s(++)) and interpocket spin-triplet pairings (the t state). The coupling constant for the t channel is proportional to the SDW order and involves interactions that do not contribute to superconductivity outside of the SDW region. We argue that the s(+-)- and t-type superconducting orders coexist at low temperatures, and the relative phase between the two is, in general, different from 0 or π, manifesting explicitly the breaking of the time-reversal symmetry promoted by long-range SDW order. We argue that time reversal may get broken even before true superconductivity develops.

  17. Synthesis of chalcogenide and pnictide crystals in salt melts using a steady-state temperature gradient

    NASA Astrophysics Data System (ADS)

    Chareev, D. A.; Volkova, O. S.; Geringer, N. V.; Koshelev, A. V.; Nekrasov, A. N.; Osadchii, V. O.; Osadchii, E. G.; Filimonova, O. N.

    2016-07-01

    Some examples of growing crystals of metals, alloys, chalcogenides, and pnictides in melts of halides of alkali metals and aluminum at a steady-state temperature gradient are described. Transport media are chosen to be salt melts of eutectic composition with the participation of LiCl, NaCl, KCl, RbCl, CsCl, AlCl3, AlBr3, KBr, and KI in a temperature range of 850-150°C. Some crystals have been synthesized only using a conducting contour. This technique of crystal growth is similar to the electrochemical method. In some cases, to exclude mutual influence, some elements have been isolated and forced to migrate to the crystal growth region through independent channels. As a result, crystals of desired quality have been obtained using no special equipment and with sizes sufficient for study under laboratory conditions.

  18. Topological insulators and superconductors

    NASA Astrophysics Data System (ADS)

    Teo, Jeffrey C. Y.

    We study theoretical properties of robust low energy electronic excitations associated with topological insulators and superconductors. The bulk materials are described by non-interacting single particle band Hamiltonians with a finite excitation gap. Their topological phases are classifed according to symmetries and dimensions, characterized by discrete bulk invariants, and correspond to topologically protected gapless excitations bounded to boundaries, interfaces or other kinds of defects. In particular, we study the metallic surface states of the three dimensional topological insulator Bi1-- xSbx, critical edge transport behavior of quantum spin Hall insulators (QSHI) using point contact geometry, Majorana bound states in three dimensions and their resemblance to Ising statistics, and various gapless modes accompanying topological defects in insulators and superconductors. We illustrate the topological phase of Bi1-- xSbx by calculating its surface energy spectrum numerically from a previously proposed tight binding model. An odd number of surface Dirac cones occupy the surface Brillouin zone and exhibit the strong topological nature of the material. We investigate the critical conductance behavior of a point contact in QSHI using a spinful Luttinger liquid description along the edges. For weak interactions, a novel intermediate fixed point controls the pinch-off transition, and the universal crossover scaling function of conductance is extracted from the solvable limits for the Luttinger parameter g = 1 -- epsilon, g = 1/2 + epsilon, and g = 1/ 3 . Majorana fermions are studied as zero energy quasiparticle excitations associated with pointlike topological defects in 3D superconductors. The low energy modes are described phenomenologically in a Dirac-type Bogoliubov de Gennes (BdG) framework, and the Majorana bound states are shown to exhibit Ising non-Abelian statistics despite living in (3 + 1) dimensions. In particular, novel braidless operations are shown to

  19. Conventional magnetic superconductors

    DOE PAGESBeta

    Wolowiec, C. T.; White, B. D.; Maple, M. B.

    2015-07-01

    We discuss several classes of conventional magnetic superconductors including the ternary rhodium borides and molybdenum chalcogenides (or Chevrel phases), and the quaternary nickel-borocarbides. These materials exhibit some exotic phenomena related to the interplay between superconductivity and long-range magnetic order including: the coexistence of superconductivity and antiferromagnetic order; reentrant and double reentrant superconductivity, magnetic field induced superconductivity, and the formation of a sinusoidally-modulated magnetic state that coexists with superconductivity. We introduce the article with a discussion of the binary and pseudobinary superconducting materials containing magnetic impurities which at best exhibit short-range “glassy” magnetic order. Early experiments on these materials led tomore » the idea of a magnetic exchange interaction between the localized spins of magnetic impurity ions and the spins of the conduction electrons which plays an important role in understanding conventional magnetic superconductors. Furthermore, these advances provide a natural foundation for investigating unconventional superconductivity in heavy-fermion compounds, cuprates, and other classes of materials in which superconductivity coexists with, or is in proximity to, a magnetically-ordered phase.« less

  20. Hybrid superconductor magnet bearings

    SciTech Connect

    Chu, W.

    1995-04-01

    Hybrid superconductor magnet bearings (HSMB`s) utilize high temperature superconductors (HTS`s) together with permanent magnets to form a frictionless interface between relatively rotating parts. They are low mass, stable, and do not incur expenditure of energy during normal operation. There is no direct physical contact between rotor and stator, and hence there is no wear and tear. However, just as any other applications of HTS`s, it requires a very cold temperature to function. Whereas this might be perceived as a disadvantage on earth, it is of no great concern in space or on the moon. To astronomers, the moon is an excellent site for an observatory, but the cold and dusty vacuum environment on the moon precludes the use of mechanical bearings on the telescope mounts. Furthermore, drive mechanisms with very fine steps, and hence bearings with extremely low friction are needed to track a star from the moon, because the moon rotates very slowly. All aspects considered, the HSMB is about the only candidate that fits in naturally. Here, the authors present a design for one such bearing, capable of supporting a telescope that weighs about 3 lbs on Earth.

  1. Hybrid superconductor magnet bearings

    NASA Technical Reports Server (NTRS)

    Chu, Wei-Kan

    1995-01-01

    Hybrid superconductor magnet bearings (HSMB's) utilize high temperature superconductors (HTS's) together with permanent magnets to form a frictionless interface between relatively rotating parts. They are low mass, stable, and do not incur expenditure of energy during normal operation. There is no direct physical contact between rotor and stator, and hence there is no wear and tear. However, just as any other applications of HTS's, it requires a very cold temperature to function. Whereas this might be perceived as a disadvantage on earth, it is of no great concern in space or on the moon. To astronomers, the moon is an excellent site for an observatory, but the cold and dusty vacuum environment on the moon precludes the use of mechanical bearings on the telescope mounts. Furthermore, drive mechanisms with very fine steps, and hence bearings with extremely low friction are needed to track a star from the moon, because the moon rotates very slowly. All aspects considered, the HSMB is about the only candidate that fits in naturally. Here, we present a design for one such bearing, capable of supporting a telescope that weighs about 3 lbs on Earth.

  2. Conventional magnetic superconductors

    SciTech Connect

    Wolowiec, C. T.; White, B. D.; Maple, M. B.

    2015-07-01

    We discuss several classes of conventional magnetic superconductors including the ternary rhodium borides and molybdenum chalcogenides (or Chevrel phases), and the quaternary nickel-borocarbides. These materials exhibit some exotic phenomena related to the interplay between superconductivity and long-range magnetic order including: the coexistence of superconductivity and antiferromagnetic order; reentrant and double reentrant superconductivity, magnetic field induced superconductivity, and the formation of a sinusoidally-modulated magnetic state that coexists with superconductivity. We introduce the article with a discussion of the binary and pseudobinary superconducting materials containing magnetic impurities which at best exhibit short-range “glassy” magnetic order. Early experiments on these materials led to the idea of a magnetic exchange interaction between the localized spins of magnetic impurity ions and the spins of the conduction electrons which plays an important role in understanding conventional magnetic superconductors. Furthermore, these advances provide a natural foundation for investigating unconventional superconductivity in heavy-fermion compounds, cuprates, and other classes of materials in which superconductivity coexists with, or is in proximity to, a magnetically-ordered phase.

  3. The non-magnetic collapsed tetragonal phase of CaFe2As2 and superconductivity in the iron pnictides

    NASA Astrophysics Data System (ADS)

    Soh, J. H.; Tucker, G. S.; Pratt, D. K.; Abernathy, D. L.; Stone, M. B.; Ran, S.; Bud'Ko, S. L.; Canfield, P. C.; Kreyssig, A.; McQueeney, R. J.; Goldman, A. I.

    2014-03-01

    The relationship between antiferromagnetic spin fluctuations and superconductivity has become a central topic of research in studies of superconductivity in the iron pnictides. We present unambiguous evidence of the absence of magnetic fluctuations in the non-superconducting collapsed tetragonal phase of CaFe2As2 via inelastic neutron scattering time-of-flight data, which is consistent with the view that spin fluctuations are a necessary ingredient for unconventional superconductivity in the iron pnictides. We demonstrate that the collapsed tetragonal phase of CaFe2As2 is non-magnetic, and discuss this result in light of recent reports of high-temperature superconductivity in the collapsed tetragonal phase of closely related compounds. Work at the Ames Laboratory was supported by the Department of Energy, Basic Energy Sciences. Work at ORNL's Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences.

  4. Materials design for new superconductors

    NASA Astrophysics Data System (ADS)

    Norman, M. R.

    2016-07-01

    Since the announcement in 2011 of the Materials Genome Initiative by the Obama administration, much attention has been given to the subject of materials design to accelerate the discovery of new materials that could have technological implications. Although having its biggest impact for more applied materials like batteries, there is increasing interest in applying these ideas to predict new superconductors. This is obviously a challenge, given that superconductivity is a many body phenomenon, with whole classes of known superconductors lacking a quantitative theory. Given this caveat, various efforts to formulate materials design principles for superconductors are reviewed here, with a focus on surveying the periodic table in an attempt to identify cuprate analogues.

  5. Electron spin susceptibility of superconductors

    SciTech Connect

    Levitov, L.S.; Nazarov, Y.V.; Eliashberg, G.M.

    1985-03-10

    The effect of spin polarization due to the Meissner currents on the electron spin susceptibility of a superconductor is studied. This effect accounts for a susceptibility considerably stronger than that of a normal metal. The spin distribution is discussed.

  6. Preparation of superconductor precursor powders

    DOEpatents

    Bhattacharya, Raghunath

    1998-01-01

    A process for the preparation of a precursor metallic powder composition for use in the subsequent formation of a superconductor. The process comprises the steps of providing an electrodeposition bath comprising an electrolyte medium and a cathode substrate electrode, and providing to the bath one or more soluble salts of one or more respective metals which are capable of exhibiting superconductor properties upon subsequent appropriate treatment. The bath is continually energized to cause the metallic and/or reduced particles formed at the electrode to drop as a powder from the electrode into the bath, and this powder, which is a precursor powder for superconductor production, is recovered from the bath for subsequent treatment. The process permits direct inclusion of all metals in the preparation of the precursor powder, and yields an amorphous product mixed on an atomic scale to thereby impart inherent high reactivity. Superconductors which can be formed from the precursor powder include pellet and powder-in-tube products.

  7. Holographic correspondence in topological superconductors

    NASA Astrophysics Data System (ADS)

    Palumbo, Giandomenico; Pachos, Jiannis K.

    2016-09-01

    We analytically derive a compatible family of effective field theories that uniquely describe topological superconductors in 3D, their 2D boundary and their 1D defect lines. We start by deriving the topological field theory of a 3D topological superconductor in class DIII, which is consistent with its symmetries. Then we identify the effective theory of a 2D topological superconductor in class D living on the gapped boundary of the 3D system. By employing the holographic correspondence we derive the effective chiral conformal field theory that describes the gapless modes living on the defect lines or effective boundary of the class D topological superconductor. We demonstrate that the chiral central charge is given in terms of the 3D winding number of the bulk which by its turn is equal to the Chern number of its gapped boundary.

  8. Manufacturing a Superconductor in School.

    ERIC Educational Resources Information Center

    Barrow, John

    1989-01-01

    Described is the manufacture of a superconductor from a commercially available kit using equipment usually available in schools or easily obtainable. The construction is described in detail including equipment, materials, safety procedures, tolerances, and manufacture. (Author/CW)

  9. Multistrand superconductor cable

    DOEpatents

    Borden, A.R.

    1984-03-08

    Improved multistrand Rutherford-type superconductor cable is produced by using strands which are preformed, prior to being wound into the cable, so that each strand has a variable cross section, with successive portions having a substantially round cross section, a transitional oval cross section, a rectangular cross section, a transitional oval cross section, a round cross section and so forth, in repetitive cycles along the length of the strand. The cable is wound and flattened so that the portions of rectangular cross section extend across the two flat sides of the cable at the strand angle. The portions of round cross section are bent at the edges of the flattened cable, so as to extend between the two flat sides. The rectangular portions of the strands slide easil

  10. Aperiodic Weak Topological Superconductors

    NASA Astrophysics Data System (ADS)

    Fulga, I. C.; Pikulin, D. I.; Loring, T. A.

    2016-06-01

    Weak topological phases are usually described in terms of protection by the lattice translation symmetry. Their characterization explicitly relies on periodicity since weak invariants are expressed in terms of the momentum-space torus. We prove the compatibility of weak topological superconductors with aperiodic systems, such as quasicrystals. We go beyond usual descriptions of weak topological phases and introduce a novel, real-space formulation of the weak invariant, based on the Clifford pseudospectrum. A nontrivial value of this index implies a nontrivial bulk phase, which is robust against disorder and hosts localized zero-energy modes at the edge. Our recipe for determining the weak invariant is directly applicable to any finite-sized system, including disordered lattice models. This direct method enables a quantitative analysis of the level of disorder the topological protection can withstand.

  11. Stability of holographic superconductors

    SciTech Connect

    Kanno, Sugumi; Soda, Jiro

    2010-10-15

    We study the dynamical stability of holographic superconductors. We first classify perturbations around black hole background solutions into vector and scalar sectors by means of a 2-dimensional rotational symmetry. We prove the stability of the vector sector by explicitly constructing the positive definite Hamiltonian. To reveal a mechanism for the stabilization of a superconducting phase, we construct a quadratic action for the scalar sector. From the action, we see the stability of black holes near a critical point is determined by the equation of motion for a charged scalar field. We show the effective mass of the charged scalar field in hairy black holes is always above the Breitenlohner-Freedman bound near the critical point due to the backreaction of a gauge field. It implies the stability of the superconducting phase. We also argue that the stability continues away from the critical point.

  12. Aperiodic Weak Topological Superconductors.

    PubMed

    Fulga, I C; Pikulin, D I; Loring, T A

    2016-06-24

    Weak topological phases are usually described in terms of protection by the lattice translation symmetry. Their characterization explicitly relies on periodicity since weak invariants are expressed in terms of the momentum-space torus. We prove the compatibility of weak topological superconductors with aperiodic systems, such as quasicrystals. We go beyond usual descriptions of weak topological phases and introduce a novel, real-space formulation of the weak invariant, based on the Clifford pseudospectrum. A nontrivial value of this index implies a nontrivial bulk phase, which is robust against disorder and hosts localized zero-energy modes at the edge. Our recipe for determining the weak invariant is directly applicable to any finite-sized system, including disordered lattice models. This direct method enables a quantitative analysis of the level of disorder the topological protection can withstand. PMID:27391744

  13. Intrinsic Josephson junctions in the iron-based multi-band superconductor (V2Sr4O6)Fe2As2

    NASA Astrophysics Data System (ADS)

    Moll, Philip J. W.; Zhu, Xiyu; Cheng, Peng; Wen, Hai-Hu; Batlogg, Bertram

    2014-09-01

    In layered superconductors, Josephson junctions may be formed within the unit cell as a result of sufficiently low inter-layer coupling. These intrinsic Josephson junction (iJJ) systems have attracted considerable interest for their application potential in quantum computing as well as efficient sources of THz radiation, closing the famous `THz gap'. So far, iJJ have been demonstrated in single-band, copper-based high-Tc superconductors, mainly in Bi-Sr-Ca-Cu-O (refs , , ). Here we report clear experimental evidence for iJJ behaviour in the iron-based superconductor (V2Sr4O6)Fe2As2. The intrinsic junctions are identified by periodic oscillations of the flux-flow voltage on increasing a well-aligned in-plane magnetic field. The periodicity is explained by commensurability effects between the Josephson vortex lattice and the crystal structure, which is a hallmark signature of Josephson vortices confined into iJJ stacks. This finding adds the pnictide (V2Sr4O6)Fe2As2 to the copper-based iJJ materials of interest for Josephson junction applications. In particular, novel devices based on multi-band Josephson coupling may be realized.

  14. Studies of anisotropy of iron based superconductors

    SciTech Connect

    Murphy, Jason

    2013-01-01

    To study the electronic anisotropy in iron based superconductors, the temperature dependent London penetration depth, Δλ (T), have been measured in several compounds, along with the angular dependent upper critical field, Hc2(T). Study was undertaken on single crystals of Ba(Fe1-xCox)2As2 with x=0.108 and x=0.127, in the overdoped range of the doping phase diagram, characterized by notable modulation of the superconducting gap. Heavy ion irradiation with matching field doses of 6 T and 6.5 T respectively, were used to create columnar defects and to study their effect on the temperature Δλ (T). The variation of the low-temperature penetration depth in both pristine and irradiated samples was fitted with a power-law function Δλ (T) = ATn. Irradiation increases the magnitude of the pre-factor A and decreases the exponent n, similar to the effect on the optimally doped samples. This finding supports the universal s± scenario for the whole doping range. Knowing that the s± gap symmetry exists across the superconducting dome for the electron doped systems, we next looked at λ (T), in optimally - doped, SrFe2(As1-xPx)2, x =0.35. Both, as-grown (Tc ~ 25 K) and annealed (Tc ~ 35 K) single crystals of SrFe2(As1-xPx)2 were measured. Annealing decreases the absolute value of the London penetration depth from λ(0) = 300 ± 10 nm in as-grown samples to λ (0) = 275±10 nm. At low temperatures, λ (T) ~ T indicates a superconducting gap with line nodes. Analysis of the full-temperature range superfluid density is consistent with the line nodes, but differs from the simple single-gap d-wave. The observed behavior is very similar to that of BaFe2(As1-xPx)2, showing that isovalently substituted pnictides are inherently different from

  15. Disorder-Sensitive Superconductivity and Bonding Network in the Iron-Silicide Superconductor Lu2Fe3Si5

    NASA Astrophysics Data System (ADS)

    Watanabe, Tadataka; Okuyama, Hiroaki; Takase, Kouichi; Takano, Yoshiki; Yoshida, Fumiko; Moriyoshi, Chikako; Kuroiwa, Yoshihiro

    2010-03-01

    Iron silicide superconductor Lu2Fe3Si5 exhibits relatively high Tc = 6.0 K among Fe-based substances. Recent specific heat, penetration depth, and thermal conductivity measurements have provided evidences for the multigap superconductivity. We have studied non-magnetic and magnetic impurity effects on superconductivity in Lu2Fe3Si5 by investigating Tc variations in non-magnetic (Lu1-xScx)2Fe3Si5, (Lu1-xYx)2Fe3Si5 and magnetic (Lu1-xDyx)2Fe3Si5. Small amount of non-magnetic impurities (Sc and Y) on the Lu-site rapidly depresses Tc in accordance with the increase in the residual resistivity. Such a disorder-sensitive superconductivity strongly suggests the sign reversal of the superconducting order parameter. Lu2Fe3Si5 has a complicated crystal structure compared to other multigap superconductors such as MgB2 and iron pnictides. Thus it is important to map out the accurate bonding network in the crystal structure for the better understanding of the electronic structure. We have observed the charge density distribution of Lu2Fe3Si5 by analyzing the synchrotron radiation powder diffraction data using the maximum entropy method/Rietveld method.

  16. Current controlled variable resistors through superconductors

    SciTech Connect

    Lakshmi, M. V. S.; Ramkumar, K.; Satyam, M.

    1989-07-01

    This note attempts to describe a current controlled variable resistor conceived based on the property of critical current associated with a superconductor. This idea has been tested using high-temperature superconductor films.

  17. Magnetostatics of superconductors without an inversion center

    SciTech Connect

    Levitov, L.S.; Nazarov, Y.V.; Eliashberg, G.M.

    1985-05-10

    The penetration of a magnetic field into a London superconductor without an inversion center is analyzed. The magnetization produced in the Meissner layer corresponds to a magnetic-induction jump at the superconductor surface.

  18. Modelling of bulk superconductor magnetization

    NASA Astrophysics Data System (ADS)

    Ainslie, M. D.; Fujishiro, H.

    2015-05-01

    This paper presents a topical review of the current state of the art in modelling the magnetization of bulk superconductors, including both (RE)BCO (where RE = rare earth or Y) and MgB2 materials. Such modelling is a powerful tool to understand the physical mechanisms of their magnetization, to assist in interpretation of experimental results, and to predict the performance of practical bulk superconductor-based devices, which is particularly important as many superconducting applications head towards the commercialization stage of their development in the coming years. In addition to the analytical and numerical techniques currently used by researchers for modelling such materials, the commonly used practical techniques to magnetize bulk superconductors are summarized with a particular focus on pulsed field magnetization (PFM), which is promising as a compact, mobile and relatively inexpensive magnetizing technique. A number of numerical models developed to analyse the issues related to PFM and optimise the technique are described in detail, including understanding the dynamics of the magnetic flux penetration and the influence of material inhomogeneities, thermal properties, pulse duration, magnitude and shape, and the shape of the magnetization coil(s). The effect of externally applied magnetic fields in different configurations on the attenuation of the trapped field is also discussed. A number of novel and hybrid bulk superconductor structures are described, including improved thermal conductivity structures and ferromagnet-superconductor structures, which have been designed to overcome some of the issues related to bulk superconductors and their magnetization and enhance the intrinsic properties of bulk superconductors acting as trapped field magnets. Finally, the use of hollow bulk cylinders/tubes for shielding is analysed.

  19. Process for fabricating continuous lengths of superconductor

    DOEpatents

    Kroeger, Donald M.; List, III, Frederick A.

    1998-01-01

    A process for manufacturing a superconductor. The process is accomplished by depositing a superconductor precursor powder on a continuous length of a first substrate ribbon, overlaying a continuous length of a second substrate ribbon on said first substrate ribbon, and applying sufficient pressure to form a bound layered superconductor precursor between said first substrate ribbon and said second substrates ribbon. The layered superconductor precursor is then heat treated to form a super conductor layer.

  20. Ferromagnet / superconductor oxide superlattices

    NASA Astrophysics Data System (ADS)

    Santamaria, Jacobo

    2006-03-01

    The growth of heterostructures combining oxide materials is a new strategy to design novel artificial multifunctional materials with interesting behaviors ruled by the interface. With the (re)discovery of colossal magnetoresistance (CMR) materials, there has been renewed interest in heterostructures involving oxide superconductors and CMR ferromagnets where ferromagnetism (F) and superconductivity (S) compete within nanometric distances from the interface. In F/S/F structures involving oxides, interfaces are especially complex and various factors like interface disorder and roughness, epitaxial strain, polarity mismatch etc., are responsible for depressed magnetic and superconducting properties at the interface over nanometer length scales. In this talk I will focus in F/S/F structures made of YBa2Cu3O7 (YBCO) and La0.7Ca0.3MnO3 (LCMO). The high degree of spin polarization of the LCMO conduction band, together with the d-wave superconductivity of the YBCO make this F/S system an adequate candidate for the search of novel spin dependent effects in transport. We show that superconductivity at the interface is depressed by various factors like charge transfer, spin injection or ferromagnetic superconducting proximity effect. I will present experiments to examine the characteristic distances of the various mechanisms of superconductivity depression. In particular, I will discuss that the critical temperature of the superconductor depends on the relative orientation of the magnetization of the F layers, giving rise to a new giant magnetoresistance effect which might be of interest for spintronic applications. Work done in collaboration with V. Peña^1, Z. Sefrioui^1, J. Garcia-Barriocanal^1, C. Visani^1, D. Arias^1, C. Leon^1 , N. Nemes^2, M. Garcia Hernandez^2, S. G. E. te Velthuis^3, A. Hoffmann^3, M. Varela^4, S. J. Pennycook^4. Work supported by MCYT MAT 2005-06024, CAM GR- MAT-0771/2004, UCM PR3/04-12399 Work at Argonne supported by the Department of Energy, Basic

  1. Lifshitz and other transitions in alkaline-earth 122 pnictides under pressure

    NASA Astrophysics Data System (ADS)

    Quader, Khandker; Widom, Michael

    2014-10-01

    We carry out T =0 first-principles total energy calculations in the entire set of alkaline 122-pnictides (A Fe2As2 ; A = alkaline-earth element Ca, Sr, Ba, Ra) as a function of hydrostatic pressure. We find multiple distinct transitions to occur, namely an enthalpic transition in which the zero-pressure striped antiferromagnetic orthorhombic (OR-AFM) phase becomes thermodynamically less stable than a competing tetragonal (T) phase, a magnetic transition in which the OR-AFM phase loses its magnetism and orthorhombicity, and a lattice parameter anomaly in which the tetragonal c-axis collapses and a collapsed tetragonal (cT) phase becomes stable. Our results for energy band dispersions and spectra, lattice parameters, enthalpies, magnetism, and elastic constants over a wide range of hydrostatic pressure provide a coherent understanding of these experimentally observed transitions. In particular, the T-cT transition and anomalies in lattice parameters and elastic properties, observed at finite temperatures, are interpreted as arising from proximity to T =0 Lifshitz transitions, wherein pressure causes nontrivial changes in the Fermi surface topology in these materials.

  2. Quasiparticle scattering interference in iron pnictides: A probe of the origin of nematicity

    NASA Astrophysics Data System (ADS)

    Zhang, Hai-Yang; Li, Jian-Xin

    2016-08-01

    In this paper, we investigate the quasiparticle scattering interference (QPI) in the nematic phase of iron pnictides, based on the magnetic and orbital scenarios of nematicity, respectively. In the spin density wave (SDW) state, the QPI pattern exhibits a dimer structure in the energy region of the SDW gap, with its orientation along the ferromagnetic direction of the SDW order. When the energy is increased to be near the Fermi level, it exhibits two sets of dimers along the same direction. The dimer structure of the QPI patterns persists in the magnetically driven nematic phase, although the two dimers tend to merge together with energies closing to the Fermi level. While in the orbital scenario, the QPI patterns exhibit a dimer structure in a wide energy region. It undergoes a π /2 rotation with the increasing of energy, which is associated with the inequivalent energies of the two Dirac nodes induced by the orbital order. These distinct features may be used to probe or distinguish two kinds of scenarios of the nematicity.

  3. Half-Metallic p-Electron Ferromagnetism in Ca and Sr Pnictides

    NASA Astrophysics Data System (ADS)

    Geshi, Masaaki; Kusakabe, Koichi; Nagara, Hitose; Suzuki, Naoshi

    2007-07-01

    We investigated the magnetism in Ca and Sr pnictides by using the first-principles calculations. These compounds are half-metallic and ferromagnetic (FM) when they assume the zinc-blende structure at the equilibrium lattice constant. Ferromagnetism is induced by the spin polarization of the p-orbitals of the pnictogen atoms; Ca and Sr atoms have no magnetic moments, which is different from that of CrAs or CrSb with a zinc-blende structure. To confirm the mechanism of the ferromagnetism, we have calculated a hypothetical crystal — fcc-As with two additional electrons — and have shown that fcc-As has the same magnetic moment as CaAs with a zinc-blende structure. This means that the role of Ca or Sr atoms is to provide electrons with As atoms at the fcc site and to sustain the distances between the As atoms and crystal symmetry. The FM exchange interactions between the pnictogen atoms are considered to exist in these lattices, which is briefly discussed.

  4. Half-metallic ferromagnetism in transition metal pnictides and chalcogenides with wurtzite structure

    NASA Astrophysics Data System (ADS)

    Xie, Wen-Hui; Liu, Bang-Gui; Pettifor, D. G.

    2003-10-01

    Using an accurate full-potential density-functional method we explore systematically all the 3d transition metal pnictides and chalcogenides with wurtzite structure in order to find half-metallic ferromagnets which can be fabricated as thin films with thickness large enough for real spintronic applications. Nine of the wurtzite phases, MnSb, CrAs, CrSb, VAs, VSb, CrSe, CrTe, VSe, and VTe, are found to be robust half-metallic ferromagnets. They have very large half-metallic gaps (0.23 0.97 eV) and quite large bulk moduli (42 71 GPa). Most of them are quite low (down to 0.31 eV per formula unit) in total energy with respect to the corresponding ground-state phases, and therefore at least some of them would be grown epitaxially on appropriate substrates in the form of films thick enough. Since being compatible with the III-V and II-VI semiconductors, these half-metallic ferromagnetic phases, when realized experimentally, would be useful in spintronic and other applications.

  5. Ising-nematic order in the bilinear-biquadratic model for the iron pnictides

    NASA Astrophysics Data System (ADS)

    Bilbao Ergueta, Patricia; Nevidomskyy, Andriy H.

    2015-10-01

    Motivated by the recent inelastic neutron scattering (INS) measurements in the iron pnictides which show a strong anisotropy of spin excitations even above the magnetic transition temperature TN, we study the spin dynamics within the frustrated Heisenberg model with biquadratic spin-spin exchange interactions. Using the Dyson-Maleev (DM) representation, which proves appropriate for all temperature regimes, we find that the spin-spin dynamical structure factors are in excellent agreement with experiment, exhibiting breaking of the C4 symmetry even into the paramagnetic region TN

  6. Theoretical study of orbital ordering induced structural phase transition in iron pnictides

    NASA Astrophysics Data System (ADS)

    Jena, Sushree Sangita; Panda, S. K.; Rout, G. C.

    2016-05-01

    We attribute the structural phase transition (SPT) in the parent compounds of the iron pnictides to orbital ordering. Due to anisotropy of the dxz and dyz orbitals in the xy plane, orbital ordering makes the orthorhombic structure more favorable and thus inducing the SPT. We consider a one band model Hamiltonian consisting of first and second-nearest-neighbor hopping of the electrons. We introduce Jahn-Tellar (JT) distortion in the system arising due to the orbital ordering present in this system. We calculate the electron Green's function by using Zuvareb's Green's function technique and hence calculate an expression for the temperature dependent lattice strain which is computed numerically and self-consistently. The temperature dependent electron specific heat is calculated by minimizing the free energy of the system. The lattice strain is studied by varying the JT coupling and elastic constant of the system. The structural anomaly is studied through the electron occupation number and the specific heat by varying the physical parameters like JT coupling, lattice constant, chemical potential and hopping integrals of the system.

  7. Phase transitions and anomalous compressibility in 1-2-2 Iron-based superconductors

    NASA Astrophysics Data System (ADS)

    Uhoya, Walter O.

    Interplay of pressure and chemistry play an important role in discovery of novel properties such as high temperature superconductivity (high-Tc). In 2008, Hosono et al discovered superconductivity at 26 K in iron-based layered LaFeAsO (1-x)Fx. This observation was a surprise since iron based compounds are generally known to be magnetic and non-superconducting. This was quickly followed by the discovery of high temperature superconductivity in a second class of Fe-based layered pnictides AFe2As2 (1-2-2) (A= Ba, Sr, Ca, Eu). The undoped (parent) 1-2-2 compounds are non-superconducting at ambient pressure but become superconducting when their structures are tuned by the application of high pressure or through chemical doping. Application of high pressures is advantageous over chemical doping in that it provides a clean method to tune the electronic properties that determine the superconducting and magnetic states of the novel materials. Interplay of pressure induced structural transitions, magnetic and superconducting properties of Fe-based materials is not well understood and this may form a foundation for testing present theories, discovering materials with higher-Tc for wide industrial applications close to room temperature, and may lead to better theories for solving the long standing problem of high-Tc superconductivity. In my work, I have used designer diamond anvil cell (DAC) to study the electrical transport properties of 1-2-2 parents of iron based superconductors under extreme conditions of pressure and low temperatures. I have used high pressure and low temperature time of flight neutron diffraction technique at the Spallation Neutron Source and synchrotron x-ray diffraction techniques at the Advance Photon Source to determine and refine the crystallographic parameters of the 1-2-2 materials under extreme conditions, and to relate their transport and structural properties under high pressures. My works have resulted into a discovery of anomalous

  8. Multistrand superconductor cable

    DOEpatents

    Borden, Albert R.

    1985-01-01

    Improved multistrand Rutherford-type superconductor cable is produced by using strands which are preformed, prior to being wound into the cable, so that each strand has a variable cross section, with successive portions having a substantially round cross section, a transitional oval cross section, a rectangular cross section, a transitional oval cross section, a round cross section and so forth, in repetitive cycles along the length of the strand. The cable is wound and flattened so that the portions of rectangular cross section extend across the two flat sides of the cable at the strand angle. The portions of round cross section are bent at the edges of the flattened cable, so as to extend between the two flat sides. The rectangular portions of the strands slide easily over one another, so as to facilitate flexing and bending of the cable, while also minimizing the possibility of causing damage to the strands by such flexing or bending. Moreover, the improved cable substantially maintains its compactness and cross-sectional shape when the cable is flexed or bent.

  9. Tunneling in Superconductors

    NASA Astrophysics Data System (ADS)

    Giaever, Ivar

    2002-03-01

    It has been said that Thomas Edison's greatest invention was that of the "Research Laboratory" as a social institution. My greatest discovery was when I learned at 29 years of age that it was possible to work in such an institution and get paid for doing research. I had become interested in physics, gotten a job at General Electric Research Laboratory and found a great mentor in John C. Fischer, who besides instructing me in physics told me that sooner or later we all would become historians of science. I guess for me that time is now, because I have been asked to tell you about my second greatest discovery: Tunneling in superconductors. My great fortune was to be at the right place at the right time, where I had access to outstanding and helpful (not necessary an oxymoron) physicists. Hopefully I will be able to convey to you some of the fun and excitement of that area in this recollection. If you become real interested you may find a written version in my Nobel Prize talk: "Electron Tunneling and Superconductivity" Les Prix Nobel en 1973 or Science 183, 1253-1258 1974 or Reviews of Modern Physics 46 (2), 245-250 1974

  10. First principles analysis of lattice dynamics for Fe-based superconductors and entropically-stabilized phases

    SciTech Connect

    Hahn, Steven

    2012-01-01

    Modern calculations are becoming an essential, complementary tool to inelastic x-ray scattering studies, where x-rays are scattered inelastically to resolve meV phonons. Calculations of the inelastic structure factor for any value of Q assist in both planning the experiment and analyzing the results. Moreover, differences between the measured data and theoretical calculations help identify important new physics driving the properties of novel correlated systems. We have used such calculations to better and more e ciently measure the phonon dispersion and elastic constants of several iron pnictide superconductors. This dissertation describes calculations and measurements at room temperature in the tetragonal phase of CaFe{sub 2}As{sub 2} and LaFeAsO. In both cases, spin-polarized calculations imposing the antiferromagnetic order present in the low-temperature orthorhombic phase dramatically improves the agreement between theory and experiment. This is discussed in terms of the strong antiferromagnetic correlations that are known to persist in the tetragonal phase. In addition, we discuss a relatively new approach called self-consistent ab initio lattice dynamics (SCAILD), which goes beyond the harmonic approximation to include phonon-phonon interactions and produce a temperature-dependent phonon dispersion. We used this technique to study the HCP to BCC transition in beryllium.

  11. Studying the morphology of the magnetic C4 phase in the 122 superconductors

    NASA Astrophysics Data System (ADS)

    Taddei, Keith; Allred, Jared; Bugaris, Daniel; Krogstad, Matthew; Lapidus, Saul; Stadel, Ryan; Chung, Duck; Claus, Helmut; Kanatzidis, Mercouri; Brown, Dennis; Rosenkranz, Stephan; Osborn, Raymond; Chmaissem, Omar

    The iron based superconductors continue to prove an exciting system for the study of superconductivity: the recent discovery of a reentrant tetragonal phase with SDW magnetic ordering has opened new avenues to study the competition between microscopically coexistent superconductivity and magnetism. This intriguing new phase is not only an exceedingly rare example of a magnetic structure with two ordering vectors, and consequently a confirmation of itinerate magnetism, but has also allowed for the determination of spin fluctuations as the driving mechanism behind the phase evolution in these materials. Evidence has been mounting of the universality of C4 in the hole doped iron pnictides providing a useful playground for the comparison of how this phase behaves as it is stabilized out of different parent compounds and through different dopant atoms. Here all members of the hole doped family which show the C4 phase will be compared and the parameters which appear to tune the phase's extent in temperature and phase space will be discussed.

  12. Odd-frequency Cooper pairs in two-band superconductors and their magnetic response

    NASA Astrophysics Data System (ADS)

    Asano, Yasuhiro; Sasaki, Akihiro

    2015-12-01

    We discuss the appearance of odd-frequency Cooper pairs in two-band superconductors by solving the Gor'kov equation analytically. We introduce the equal-time s -wave pair potentials as realized in MgB2 and iron pnictides. Although the order parameter symmetry is conventional, the band degree of freedom enriches the symmetry variety of pairing correlations. The hybridization and the asymmetry between the two conduction bands induce odd-frequency pairs as a subdominant pairing correlation in the uniform ground state. To study the magnetic response of odd-frequency Cooper pairs, we analyze the Meissner kernel represented by the Gor'kov Green function. In contrast to the even-frequency pairs linked to the pair potential, the induced odd-frequency Cooper pairs indicate a paramagnetic property. We also discuss the relation between the amplitude of the odd-frequency pairing correlation and the stability of superconducting states in terms of the self-consistent equation for the pair potential.

  13. Fermi surface nesting induced strong pairing in iron-based superconductors

    PubMed Central

    Terashima, K.; Sekiba, Y.; Bowen, J. H.; Nakayama, K.; Kawahara, T.; Sato, T.; Richard, P.; Xu, Y.-M.; Li, L. J.; Cao, G. H.; Xu, Z.-A.; Ding, H.; Takahashi, T.

    2009-01-01

    The discovery of high-temperature superconductivity in iron pnictides raised the possibility of an unconventional superconducting mechanism in multiband materials. The observation of Fermi-surface (FS)-dependent nodeless superconducting gaps suggested that inter-FS interactions may play a crucial role in superconducting pairing. In the optimally hole-doped Ba0.6K0.4Fe2As2, the pairing strength is enhanced simultaneously (2Δ/Tc≈7) on the nearly nested FS pockets, i.e., the inner hole-like (α) FS and the 2 hybridized electron-like FSs, whereas the pairing remains weak (2Δ/Tc≈3.6) in the poorly nested outer hole-like (β) FS. Here, we report that in the electron-doped BaFe1.85Co0.15As2, the FS nesting condition switches from the α to the β FS due to the opposite size changes for hole- and electron-like FSs upon electron doping. The strong pairing strength (2Δ/Tc≈6) is also found to switch to the nested β FS, indicating an intimate connection between FS nesting and superconducting pairing, and strongly supporting the inter-FS pairing mechanism in the iron-based superconductors. PMID:19359490

  14. Electronic structure of ion arsenic high temperature superconductors studied by angle resolved photoemission spectroscopy

    SciTech Connect

    Liu, Chang

    2011-01-01

    The main purpose of the present thesis is to present our ARPES results on the iron arsenic superconductors. As revealed by a series of ARPES measurements on both the AEFe2As2 and the RFeAs(O,F) families (parent compound and carrier-doped systems), the electronic structures of the pnictides are complicated, three dimensional, and closely linked to their superconducting behavior (13; 14; 15; 16; 17; 18; 19). Parent compounds of these materials exhibit the basic hole-electron pocket dual plus an apparent Fermi surface reconstruction caused by long range antiferromagnetism (13; 15). When carriers are introduced, the chemical potential shifts in accordance with the Luttinger theorem and the rigid band shifting picture (13). Importantly, both the appearance and disappearance of the superconducting dome at low and high doping levels have intimate relation with topological changes at the Fermi surfaces, resulting in a specific Fermi topology being favored by superconductivity (15; 16). On the low doping side, superconductivity emerges in the phase diagram once the antiferromagnetic reconstruction disappears below the Fermi level, returning the Fermi surface to its paramagnetic-like appearance. On the high doping side, superconductivity disappears around a doping level at which the central hole pocket vanishes due to increasing electron concentration. Such phenomena are evidence for the governing role the electronic structure plays in their superconducting behavior.

  15. Point contact Andreev reflection spectroscopic (PCARS) studies on 122-type iron-based superconductors

    NASA Astrophysics Data System (ADS)

    Lu, Xin; Park, W. K.; Greene, L. H.; Yuan, H. Q.; Chen, G. F.; Luo, G. L.; Wang, N. L.; Sefat, A. S.; McGuire, M. A.; Jin, R.; Sales, B. C.; Mandrus, D.; Gillett, J.; Sebastian, S. E.

    2010-03-01

    PCARS is applied to investigate the superconducting gap in iron pnictide single crystal superconductors of the AFe2As2 (A=Ba, Sr) family with two categories of G(V) curves observed [1]: one where Andreev reflection (AR) is present for (Ba0.6K0.4)Fe2As2 and Ba(Fe0.9Co0.1)2As2, and the other without AR but a V^2/3 shape for Sr0.6Na0.4Fe2As2 and Sr(Fe0.9Co0.1)2As2. The latter is also observed in the nonsuperconducting parent compound BaFe2As2. Mesoscopic phase-separated coexistence of magnetic and superconducting orders is considered to explain distinct behaviors. A gap size ˜3.0-4.0 meV with 2δ0/kBTc˜2.0-2.6 is observed for PCARS on Ba0.6K0.4Fe2As2. For the Ba(Fe0.9Co0.1)2As2, G(V) curves typically display a zero-bias conductance peak, sometimes with a V-shape background. [1] Xin Lu et al., arXiv:0910.4230

  16. Who is in charge of the nematic order in iron-based superconductors?

    NASA Astrophysics Data System (ADS)

    Chubukov, Andrey

    2015-03-01

    Although the existence of nematic order in iron-based superconductors is now a well-established experimental fact, its origin remains controversial. Nematic order breaks the discrete lattice rotational symmetry by making the x- and y-directions in the iron plane non-equivalent. This can happen because of a regular structural transition or due to a electronically-driven instability - in particular, orbital order and spin-driven Ising-nematic order. The latter is a magnetic state that breaks rotational symmetry but preserves time-reversal symmetry. Symmetry dictates that the development of one of these orders immediately induces the other two, making the origin of nematicity a physics realization of the ``chicken and egg problem.'' will argue that the evidence strongly points to an electronic mechanism of nematicity, placing nematic order in the class of correlation-driven electronic instabilities, like superconductivity and density-wave transitions. I will discuss different microscopic models for nematicity and link them to the properties of the magnetic and superconducting states, providing a unified perspective on the phase diagram of the iron pnictides. (Based on R.M. Fernandes, A.V. Chubukov, and J. Schmalian, Nature Physics 10, 97 (2014).)

  17. Resolving thermoelectric "paradox" in superconductors.

    PubMed

    Shelly, Connor D; Matrozova, Ekaterina A; Petrashov, Victor T

    2016-02-01

    For almost a century, thermoelectricity in superconductors has been one of the most intriguing topics in physics. During its early stages in the 1920s, the mere existence of thermoelectric effects in superconductors was questioned. In 1944, it was demonstrated that the effects may occur in inhomogeneous superconductors. Theoretical breakthrough followed in the 1970s, when the generation of a measurable thermoelectric magnetic flux in superconducting loops was predicted; however, a major crisis developed when experiments showed puzzling discrepancies with the theory. Moreover, different experiments were inconsistent with each other. This led to a stalemate in bringing theory and experiment into agreement. With this work, we resolve this stalemate, thus solving this long-standing "paradox," and open prospects for exploration of novel thermoelectric phenomena predicted recently. PMID:26933688

  18. Materials design for new superconductors.

    PubMed

    Norman, M R

    2016-07-01

    Since the announcement in 2011 of the Materials Genome Initiative by the Obama administration, much attention has been given to the subject of materials design to accelerate the discovery of new materials that could have technological implications. Although having its biggest impact for more applied materials like batteries, there is increasing interest in applying these ideas to predict new superconductors. This is obviously a challenge, given that superconductivity is a many body phenomenon, with whole classes of known superconductors lacking a quantitative theory. Given this caveat, various efforts to formulate materials design principles for superconductors are reviewed here, with a focus on surveying the periodic table in an attempt to identify cuprate analogues. PMID:27214291

  19. Heat transport in nonuniform superconductors

    NASA Astrophysics Data System (ADS)

    Richard, Caroline; Vorontsov, Anton B.

    2016-08-01

    We calculate electronic energy transport in inhomogeneous superconductors using a fully self-consistent nonequilibrium quasiclassical Keldysh approach. We develop a general theory and apply it to a superconductor with an order parameter that forms domain walls of the type encountered in the Fulde-Ferrell-Larkin-Ovchinnikov state. The heat transport in the presence of a domain wall is inherently anisotropic and nonlocal. The bound states in the nonuniform region play a crucial role and control heat transport in several ways: (i) they modify the spectrum of quasiparticle states and result in Andreev reflection processes and (ii) they hybridize with the impurity band and produce a local transport environment with properties very different from those in a uniform superconductor. As a result of this interplay, heat transport becomes highly sensitive to temperature, magnetic field, and disorder. For strongly scattering impurities, we find that the transport across domain walls at low temperatures is considerably more efficient than in the uniform superconducting state.

  20. Thin film superconductor magnetic bearings

    DOEpatents

    Weinberger, Bernard R.

    1995-12-26

    A superconductor magnetic bearing includes a shaft (10) that is subject to a load (L) and rotatable around an axis of rotation, a magnet (12) mounted to the shaft, and a stator (14) in proximity to the shaft. The stator (14) has a superconductor thin film assembly (16) positioned to interact with the magnet (12) to produce a levitation force on the shaft (10) that supports the load (L). The thin film assembly (16) includes at least two superconductor thin films (18) and at least one substrate (20). Each thin film (18) is positioned on a substrate (20) and all the thin films are positioned such that an applied magnetic field from the magnet (12) passes through all the thin films. A similar bearing in which the thin film assembly (16) is mounted on the shaft (10) and the magnet (12) is part of the stator (14) also can be constructed.

  1. Topological surface states in nodal superconductors.

    PubMed

    Schnyder, Andreas P; Brydon, Philip M R

    2015-06-24

    Topological superconductors have become a subject of intense research due to their potential use for technical applications in device fabrication and quantum information. Besides fully gapped superconductors, unconventional superconductors with point or line nodes in their order parameter can also exhibit nontrivial topological characteristics. This article reviews recent progress in the theoretical understanding of nodal topological superconductors, with a focus on Weyl and noncentrosymmetric superconductors and their protected surface states. Using selected examples, we review the bulk topological properties of these systems, study different types of topological surface states, and examine their unusual properties. Furthermore, we survey some candidate materials for topological superconductivity and discuss different experimental signatures of topological surface states.

  2. Apparatus for fabricating continuous lengths of superconductor

    DOEpatents

    Kroeger, Donald M.; List, III, Frederick A.

    2002-01-01

    A process and apparatus for manufacturing a superconductor. The process is accomplished by depositing a superconductor precursor powder on a continuous length of a first substrate ribbon, overlaying a continuous length of a second substrate ribbon on said first substrate ribbon, and applying sufficient pressure to form a bound layered superconductor comprising a layer of said superconducting precursor powder between said first substrate ribbon and said second substrates ribbon. The layered superconductor is then heat treated to establish the superconducting phase of said superconductor precursor powder.

  3. Apparatus for fabricating continuous lengths of superconductor

    DOEpatents

    Kroeger, Donald M.; List, III, Frederick A.

    2001-01-01

    A process and apparatus for manufacturing a superconductor. The process is accomplished by depositing a superconductor precursor powder on a continuous length of a first substrate ribbon, overlaying a continuous length of a second substrate ribbon on said first substrate ribbon, and applying sufficient pressure to form a bound layered superconductor comprising a layer of said superconducting precursor powder between said first substrate ribbon and said second substrates ribbon. The layered superconductor is then heat treated to establish the superconducting phase of said superconductor precursor powder.

  4. Hydrostatic High-Pressure Studies to 25 GPA on the Model Superconducting Pnictide LaRu2P2

    NASA Astrophysics Data System (ADS)

    Lim, Jinhyuk; Forouzani, Neda; Schilling, James; Fotovat, Roxanna; Zheng, Chong; Hoffmann, Roald

    2014-03-01

    Prior to the discovery of the Fe-pnictides in 2008, the ruthenium phosphide LaRu2P2 possessed the highest value of the su- perconducting transition temperature, Tc ~ 4 K, in the entire pnictide family. Recently, there has been renewed interest in this compound in an effort to better understand why the Fe-pnictides have much higher values of Tc. In related phosphides superconductivity appears to only be present if the separation be- tween the phosphor ions dp-p in neigh- boring Ru2P2 planes is greater than the critical value 2.8 Å, too great for a P-P covalent bond to be formed. For example, in superconducting LaRu2P2, the value of dp-p is 3.0 Å. To test these ideas directly, we have carried out hydro- static high-pressure studies on single-crystalline LaRu2P2 in a diamond-anvil cell using He pressure medium to pres- sures as high as 25 GPa and temperatures as low as 1.5 K. We find that Tc initially increases under pressure, but suddenly disappears above 2.1 GPa. Since dp-p decreases under pressure, the sudden disappearance of superconductivity is likely due to the formation of a covalent P-P bond between adjacent Ru2P2 planes and a possible structural phase transition. Work at Washington University is supported by the NSF through Grant No. DMR-1104742 and by the Carnegie/DOE through NNSA/DOE Grant No. DE-FC52-08NA28554.

  5. High temperature superconductor current leads

    DOEpatents

    Hull, J.R.; Poeppel, R.B.

    1995-06-20

    An electrical lead is disclosed having one end for connection to an apparatus in a cryogenic environment and the other end for connection to an apparatus outside the cryogenic environment. The electrical lead includes a high temperature superconductor wire and an electrically conductive material distributed therein, where the conductive material is present at the one end of the lead at a concentration in the range of from 0 to about 3% by volume, and at the other end of the lead at a concentration of less than about 20% by volume. Various embodiments are shown for groups of high temperature superconductor wires and sheaths. 9 figs.

  6. High temperature superconductor current leads

    DOEpatents

    Hull, John R.; Poeppel, Roger B.

    1995-01-01

    An electrical lead having one end for connection to an apparatus in a cryogenic environment and the other end for connection to an apparatus outside the cryogenic environment. The electrical lead includes a high temperature superconductor wire and an electrically conductive material distributed therein, where the conductive material is present at the one end of the lead at a concentration in the range of from 0 to about 3% by volume, and at the other end of the lead at a concentration of less than about 20% by volume. Various embodiments are shown for groups of high temperature superconductor wires and sheaths.

  7. Multifilamentary niobium tin superconductor tape

    NASA Technical Reports Server (NTRS)

    Brisbin, P. H.; Coles, W. D.

    1975-01-01

    In the method proposed for fabricating multifilamentary Nb3Sn tape, filamentary superconducting paths are produced in standard commercial superconductor tape by chemical milling of separator slots through the Nb3Sn layer. The multifilament configuration features a matrix of ten 1.2 mm wide parallel helical superconducting paths along the length of the tape. The paths are spaced 0.4 mm apart. Tapes tested as small pancake coils demonstrated the integrity and continuity of the matrix, and showed that critical current was sustained in direct proportion to retained superconductor.

  8. Investigation of resistive losses in type II superconductors

    NASA Astrophysics Data System (ADS)

    Benapfl, Brendan W.

    For low-TC materials, the superconducting transition temperature (TC) is depressed by the application of a magnetic field. In contrast, one of the remarkable features of cuprate high-TC materials is that the superconducting transition is broadened by the application of a magnetic field. Tinkham presented a model for the field-dependent resistive transition of high-T C materials, arising from "phase slippage at a complicated network of channels." Coffey & Clem did not include this field-broadening effect in their sophisticated model for the field and temperature dependence of the surface resistance in type-II superconductors. From the model by Lee & Stroud, treating Josephson Junction-coupled superconducting segments, it is concluded that doped, layered superconductors are certain to have a field-broadened superconducting transition. This effect can be identified by measurements of the resistivity as a function of temperature, magnetic field strength, angle of field with respect to the crystal axis as well as with respect to an induced current density. The iron pnictide materials such as Ba0.6K0.4Fe2As2 (BaK122) have chemical layers with different compositions, differentiating them from elemental type-II superconductors such as niobium, and also from cuprates, by the absence of copper. Experimental data on BaK122 indicate a field-broadened transition in conjunction with a field-depressed superconducting transition temperature. In this work, techniques associated with Electron Spin Resonance (ESR) spectroscopy were used to measure the temperature and field-induced changes in the surface resistance of single-crystal BaK122 samples. In addition, polycrystalline foils of niobium and a NbTi (70/30) alloy were measured using the same techniques to provide comparison. Measurements were taken as a function of applied magnetic field, temperature, rf field intensity, and angle of the applied field with respect to the rf-induced current. BaK122 sample field-dependent surface

  9. Campbell penetration depth in Fe-based superconductors

    SciTech Connect

    Prommapan, Plegchart

    2011-01-01

    A 'true' critical current density, jc, as opposite to commonly measured relaxed persistent (Bean) current, jB, was extracted from the Campbell penetration depth, Λc(T,H) measured in single crystals of LiFeAs, and optimally electron-doped Ba(Fe0.954Ni0.046)2As2 (FeNi122). In LiFeAs, the effective pinning potential is nonparabolic, which follows from the magnetic field - dependent Labusch parameter α. At the equilibrium (upon field - cooling), α(H) is non-monotonic, but it is monotonic at a finite gradient of the vortex density. This behavior leads to a faster magnetic relaxation at the lower fields and provides a natural dynamic explanation for the fishtail (second peak) effect. We also find the evidence for strong pinning at the lower fields.The inferred field dependence of the pinning potential is consistent with the evolution from strong pinning, through collective pinning, and eventually to a disordered vortex lattice. The value of jc(2 K) ≅ 1.22 x 106 A/cm2 provide an upper estimate of the current carrying capability of LiFeAs. Overall, vortex behavior of almost isotropic, fully-gapped LiFeAs is very similar to highly anisotropic d-wave cuprate superconductors, the similarity that requires further studies in order to understand unconventional superconductivity in cuprates and pnictides. In addition to LiFeAs, we also report the magnetic penetration depth in BaFe2As2 based superconductors including irradiation of FeNi122. In unirradiated FeNi122, the maximum critical current value is, jc(2K) ≅ 3.3 x 106 A/cm2. The magnetic-dependent feature was observed near the transition temperature in FeTe0.53Se0.47 and irradiated FeNi122. Because of this feature, further studies are required in order to properly calibrate the Campbell penetration depth. Finally, we detected the crossing between

  10. The Superconductors That Magnets Really Want: What Stands in the Way? (Superconductors for Accelerator Use: What Next and How Close is the Ideal Conductor?)

    ScienceCinema

    Larbalestier, David [National High Magnetic Field Laboratory] [Florida State University, Tallahassee, Florida, United States

    2016-07-12

    There are over 5000 superconducting materials but only about 5 have ever been useful for applications in magnets, while HEP, which has been so vital for the development of superconducting magnet technology has made virtually every magnet out of just one, the simple bcc alloy Nb-Ti with Tc of 9 K and upper critical field ~ 14T (at 2K). Significant demonstrations of the capability of the brittle intermetallic Nb3Sn have shown that fields of more than 15 T can be generated in dipole form. But Nb-Ti and Nb3Sn are staid, conventional superconductors, far from the cutting edge of superconducting science research where cuprates like YBa2Cu3O7-x and Bi2Sr2CaCu2Ox remain at the scientific forefront and in 2008 were joined by the recently discovered Fe-As pnictide superconductors. What could it mean to have materials for magnets with 10 times the Tc of Nb-Ti (90-120 K) and 3 or more times the critical field (100-240 T)? One enormous barrier is that higher Tc so far always means more complexity and a more localized superconducting interaction which is sensitive to local loss of superconductivity. The issue that has made the cuprate high temperature superconductors so hard to apply is that grain boundaries which form a 3D network in any practical wire form, easily acquire degraded superconducting properties. But conductors can now be made with extreme texture so that grain boundaries are minimized. Moreover almost practical conductors of Bi2Sr2CaCu2Ox and YBa2Cu3O7-x are now are in production and in late 2008 we were, at the Magnet Lab, able to make small solenoids operating at high current density in fields of 32 and almost 34 T respectively. Within the HEP community, there is enthusiasm to embrace HTS conductors for new very high field machines that could, like the Muon Collider, use fields of 30-50 T. In my talk I would like to explore the underlying science controlling such potential applications.

  11. Effect of Orbital Nematicity on Superconductivity in the Iron Pnictides and Chalcogenides

    NASA Astrophysics Data System (ADS)

    Nevidomskyy, Andriy; Yu, Rong

    Orbital ordering leading to the observed nematic phase in the iron-based superconductors has been firmly established in a variety of experiments. It is therefore important to investigate the effect of the orbital order on the superconductivity. To this end, we have performed strong-coupling calculation within the slave-boson approach to the multiorbital t-J1-J2 models for the iron-based superconductors. We report the phase diagram as a function of both electron/hole doping and the orbital ordering strength. We find that the amplitude of the otherwise dominant A1 g (s +/-) pairing channel diminishes as the strength of orbital ordering is increased, yielding to the B1g (dx2 -y2) pairing channel. This effect is especially pronounced in the electron-doped case, with the d-wave pairing stabilized by the realistic values of the orbital splitting ~ 50 meV. While the d-wave pairing has not been conclusively observed in the iron-based superconductors, the competition between the s- and d-wave pairing found in the calculations may have ramifications for FeSe, KFe2As2 and KxFe2-ySe2.

  12. Search for pressure-induced superconductivity in CeFeAsO and CeFePO iron pnictides

    SciTech Connect

    Zocco, D. A.; Baumbach, R. E.; Hamlin, J. J.; Janoschek, M.; Lum, I. K.; McGuire, Michael A; Safa-Sefat, Athena; Sales, Brian C; Jin, Rongying; Mandrus, David; Jeffries, J. R.; Weir, S. T.; Vohra, Y. K.; Maple, M. B.

    2011-01-01

    The CeFeAsO and CeFePO iron pnictide compounds were studied via electrical transport measurements under high pressure. In CeFeAsO polycrystals, the magnetic phases involving the Fe and Ce ions coexist for hydrostatically applied pressures up to 15 GPa, and with no signs of pressure-induced superconductivity up to 50 GPa for the less hydrostatic pressure techniques. For the CeFePO single crystals, pressure further stabilizes the Kondo screening of the Ce 4f-electron magnetic moments.

  13. In-plane and c-axis optical spectroscopy study on 122 Fe-pnictides

    NASA Astrophysics Data System (ADS)

    Wang, Nan Lin

    2011-03-01

    I present the in-plane and the c-axis optical spectroscopy investigations on 122 Fe-pnictides. For the parent compound BaFe 2 As 2 , the in-plane measurement revealed two different energy gaps in the SDW state, whereas for the c-axis polarized measurement only the energy gap at smaller energy scale could be clearly observed. We suggest different driving mechanisms for the formation of the two energy gaps. The large energy gap is caused by the nesting between disconnected 2D cylinder-like electron and hole Fermi surfaces. It is the main driving force for the SDW instability. The small energy gap is the one formed on the 3D Fermi surface due to the presence of reduced magnetic Brillouin zone which crosses the 3D Fermi surface. It is the consequence of the establishment of the magnetic order. For the doped superconducting 122 samples, the in-plane optical measurement revealed a formation of full superconducting energy gap, whereas the c-axis optical measurement indicated a large residual quasiparticle population down to very low temperature. Those quasiparticles contribute specifically to the c-axis transport. We suggest that there exist horizontal nodes in the superconducting gap in regions of the 3D Fermi surface that contribute dominantly to the c-axis optical conductivity. Work done with Z. G. Chen, W. Z. Hu, B. Cheng, G. Li, J. Dong, T. Dong, R. H. Yuan, P. Zheng, G. F. Chen, J. L. Luo, Z. Fang, X. Dai, C. L. Zhang and P. Dai.

  14. Skewness and Kurtosis of the Switching Current Distribution in Superconductor-Graphene-Superconductor Junctions and Superconductor-Nanowire-Superconductor Devices

    NASA Astrophysics Data System (ADS)

    Murphy, Andrew; Aref, Thomas; Coskun, Ulas; Weinberg, Phillip; Levchenko, Alex; Vakaryuk, Victor; Bezryadin, Alexey

    2013-03-01

    We study statistical properties of the switching current in superconductor-graphene-superconductor proximity junctions and superconductor-nanowire-superconductor devices. The fluctuations of the switching current are related to Little's phase slips, generated by thermal and quantum fluctuations of the superconducting order parameter. The study focuses on higher moments of the statistical probability distributions of the switching current. Namely we study the skewness, which defines the asymmetry of the distribution, and kurtosis, which is a measure of the ``peakedness.'' The skewness is defined as sk= m3 /m23 / 2 where m2 is the second moment of the distribution, called the variance, and m3 is the third moment. Kurtosis is defined as kur= m4 /m22 , where m4 is the fourth moment of the distribution. It is known that for Gaussian distributions sk=0 and kur=3. On our devices we find, in most cases, sk ~ -1 and kur ~ 5. These results are in agreement with numerical simulations as well as an analytic model. Finally we present preliminary experimental results for a two-nanowire device. We have found that the standard deviation, skewness and kurtosis of the switching current distributions in these devices vary periodically with magnetic field.

  15. Preparation of superconductor precursor powders

    DOEpatents

    Bhattacharya, R.

    1998-08-04

    A process for the preparation of a precursor metallic powder composition for use in the subsequent formation of a superconductor. The process comprises the steps of providing an electrodeposition bath comprising an electrolyte medium and a cathode substrate electrode, and providing to the bath one or more soluble salts of one or more respective metals which are capable of exhibiting superconductor properties upon subsequent appropriate treatment. The bath is continually energized to cause the metallic and/or reduced particles formed at the electrode to drop as a powder from the electrode into the bath, and this powder, which is a precursor powder for superconductor production, is recovered from the bath for subsequent treatment. The process permits direct inclusion of all metals in the preparation of the precursor powder, and yields an amorphous product mixed on an atomic scale to thereby impart inherent high reactivity. Superconductors which can be formed from the precursor powder include pellet and powder-in-tube products. 7 figs.

  16. Preparation of superconductor precursor powders

    DOEpatents

    Bhattacharya, Raghunath; Blaugher, Richard D.

    1995-01-01

    A process for the preparation of a precursor metallic powder composition for use in the subsequent formation of a superconductor. The process comprises the steps of providing an electrodeposition bath comprising an electrolyte medium and a cathode substrate electrode, and providing to the bath one or more soluble salts of one or more respective metals, such as nitrate salts of thallium, barium, calcium, and copper, which are capable of exhibiting superconductor properties upon subsequent appropriate treatment. The bath is continually energized to cause the metallic particles formed at the electrode to drop as a powder from the electrode into the bath, and this powder, which is a precursor powder for superconductor production, is recovered from the bath for subsequent treatment. The process permits direct inclusion of thallium in the preparation of the precursor powder, and yields an amorphous product mixed on an atomic scale to thereby impart inherent high reactivity. Superconductors which can be formed from the precursor powder include pellet and powder-in-tube products.

  17. Fluctuation conductivity and possible pseudogap state in FeAs-based superconductor EuFeAsO0.85F0.15

    NASA Astrophysics Data System (ADS)

    Solovjov, A. L.; Omelchenko, L. V.; Terekhov, A. V.; Rogacki, K.; Vovk, R. V.; Khlybov, E. P.; Chroneos, A.

    2016-07-01

    The study of excess conductivity σ \\prime (T) in the textured polycrystalline FeAs-based superconductor EuFeAsO0.85F0.15 ({T}{{c}}=11 {{K}}) prepared by the solid state synthesis is reported for the first time. The σ \\prime (T) analysis has been performed within the local pair (LP) model based on the assumption of the LPs formation in cuprate high-T c superconductors (cuprates) below the pseudogap (PG) temperature {T}* \\gg {T}{{c}}. Similarly to the cuprates, near {T}{{c}} σ \\prime (T) is adequately described by the 3D term of the Aslamasov-Larkin (AL) theory but the range of the 3D-AL fluctuations, {{Δ }}{T}3{{D}}, is relatively short. Above the crossover temperature {T}0≈ 11.7 {{K}} σ \\prime (T) is described by the 2D Maki-Thompson (MT) fluctuation term of the Hikami-Larkin theory. But enhanced 2D-MT fluctuation contribution being typical for the magnetic superconductors is observed. Within the LP model the PG parameter, {{{Δ }}}* (T), was determined for the first time. It is shown that {{{Δ }}}* (T) demonstrates the narrow maximum at {T}s≈ 160 {{K}} followed by the descending linear length down to {T}{SDW}={T}{NFe}≈ 133 {{K}}. Observed small {{Δ }}{T}3{{D}}, enlarged 2D σ \\prime (T) and linear {{{Δ }}}* (T) are considered to be the evidence of the enhanced magnetic interaction in EuFeAsO0.85F0.15. Importantly, the slop of the linear {{{Δ }}}* (T) and its length are found to be the same as it is revealed for SmFeAsO0.85. The results suggest both the similarity of the magnetic interaction processes in different Fe-pnictides and applicability of the LP model to the σ \\prime (T) analysis even in magnetic superconductors.

  18. Fluctuation conductivity and possible pseudogap state in FeAs-based superconductor EuFeAsO0.85F0.15

    NASA Astrophysics Data System (ADS)

    Solovjov, A. L.; Omelchenko, L. V.; Terekhov, A. V.; Rogacki, K.; Vovk, R. V.; Khlybov, E. P.; Chroneos, A.

    2016-07-01

    The study of excess conductivity σ \\prime (T) in the textured polycrystalline FeAs-based superconductor EuFeAsO0.85F0.15 ({T}{{c}}=11 {{K}}) prepared by the solid state synthesis is reported for the first time. The σ \\prime (T) analysis has been performed within the local pair (LP) model based on the assumption of the LPs formation in cuprate high-T c superconductors (cuprates) below the pseudogap (PG) temperature {T}* \\gg {T}{{c}}. Similarly to the cuprates, near {T}{{c}} σ \\prime (T) is adequately described by the 3D term of the Aslamasov–Larkin (AL) theory but the range of the 3D-AL fluctuations, {{Δ }}{T}3{{D}}, is relatively short. Above the crossover temperature {T}0≈ 11.7 {{K}} σ \\prime (T) is described by the 2D Maki–Thompson (MT) fluctuation term of the Hikami–Larkin theory. But enhanced 2D-MT fluctuation contribution being typical for the magnetic superconductors is observed. Within the LP model the PG parameter, {{{Δ }}}* (T), was determined for the first time. It is shown that {{{Δ }}}* (T) demonstrates the narrow maximum at {T}s≈ 160 {{K}} followed by the descending linear length down to {T}{SDW}={T}{NFe}≈ 133 {{K}}. Observed small {{Δ }}{T}3{{D}}, enlarged 2D σ \\prime (T) and linear {{{Δ }}}* (T) are considered to be the evidence of the enhanced magnetic interaction in EuFeAsO0.85F0.15. Importantly, the slop of the linear {{{Δ }}}* (T) and its length are found to be the same as it is revealed for SmFeAsO0.85. The results suggest both the similarity of the magnetic interaction processes in different Fe-pnictides and applicability of the LP model to the σ \\prime (T) analysis even in magnetic superconductors.

  19. Fluctuoscopy of Superconductors

    NASA Astrophysics Data System (ADS)

    Varlamov, Andrey

    2012-02-01

    The study of superconducting fluctuations (SF) is a subject of fundamental and practical importance. Since the moment of discovery SF became a noticeable part of research in the field of superconductivity (SC) and a variety of fluctuation effects have been detected. The interest to SF in SC was regenerated by the discovery of HTS, where, due to extremely short coherence length and low effective dimensionality of the electron system, SF manifest themselves in a wide range of temperatures. The characteristic feature of SF is their strong dependence on temperature and magnetic field. This allows to separate SFs from other contributions and to use them as a tool for characterization of SC systems (``fluctuoscopy'') for example to extract the values of Tc, Hc2(T) and phase-breaking time from experimental data. We present the complete results for fluctuation magneto-conductivity (FMC) and Nernst signal (FNS) of impure 2D superconductor in the whole phase diagram above the transition line Hc2(T), including the domain of quantum fluctuations. Along some line H0(T), in agreement with experimental findings, FMC becomes zero and beyond it remains small and negative. The corresponding surface in coordinates (T,H) becomes in particular non-trivial at low temperatures and close to Hc2(0), where it is trough-shaped. The observation of large FNS in HTS and conventional SC above Tc(H), has attracted much attention recently. The idea to attribute it to the entropy transport by analogy to vortices was proposed. On the other hand this giant effect, close to Tc(0), was explained in terms of SF. Our general results allow to successfully fit the available experimental data in a wide range of magnetic fields and temperatures, to extract the value of the ``ghost'' field and other parameters of SC. We offer also a qualitative consideration, which gives a natural explanation for the giant value of FNS attributing it to a strong dependence of the fluctuation Cooper pair (FCP) chemical

  20. Structural transition, ferro-orbital order and its fluctuation-mediated s-wave superconductivity in iron pnictides

    NASA Astrophysics Data System (ADS)

    Ōno, Yoshiaki; Yanagi, Yuki; Adachi, Naoko; Yamakawa, Youichi

    2012-04-01

    We investigate the electronic states and the superconductivity in the two-dimensional 16-band d-p model extracted from a tight-binding fit to the band structure of iron pnictides, in the presence of both the Coulomb interaction between Fe d-electrons and the electron-lattice coupling g with the orthorhombic mode which is crucial for reproducing the recently observed ultrasonic softening of the elastic constant C66. Due to the cooperative effects of these interactions, the ferro-orbital order with different occupations of d and d orbitals occurs and induces the tetragonal-orthorhombic structural transition at Ts, together with the stripe-type antiferromagnetic (AFM) order below TN. For a large g case, we obtain the phase diagram consistent with the doped iron pnictides with Ts>TN for x>0, where the s-wave superconductivity is mediated by the ferro-orbital fluctuation which is largely enhanced near the ferro-orbital QCP at xc with Ts→0. On the other hand, for a small g case, the simultaneous phase transition occurs at Ts=TN even for x>0, where the s±-wave superconductivity is mediated by the AFM fluctuation. Both the s-wave states with full superconducting gaps are consistent with most of the experiments but only the former is considered to account for the small Tc-suppression against nonmagnetic impurities.

  1. Resonance tunneling of cooper pairs in a superconductor-polymer-superconductor josephson junction

    SciTech Connect

    Ionov, A. I.

    2013-05-15

    It is shown that the superconducting current flowing though a polymer in a superconductor-polymer-superconductor Josephson structure is due to resonant tunneling of Cooper pairs. The critical current and the thickness of the polymer in which the superconducting current is observed depend on the coherence length of a Cooper pair in the superconductor contacting the polymer.

  2. Passivation Of High-Temperature Superconductors

    NASA Technical Reports Server (NTRS)

    Vasquez, Richard P.

    1991-01-01

    Surfaces of high-temperature superconductors passivated with native iodides, sulfides, or sulfates formed by chemical treatments after superconductors grown. Passivating compounds nearly insoluble in and unreactive with water and protect underlying superconductors from effects of moisture. Layers of cuprous iodide and of barium sulfate grown. Other candidate passivating surface films: iodides and sulfides of bismuth, strontium, and thallium. Other proposed techniques for formation of passivating layers include deposition and gas-phase reaction.

  3. High T(c) electron doped Ca10(Pt3As8)(Fe2As2)5 and Ca10(Pt4As8)(Fe2As2)5 superconductors with skutterudite intermediary layers.

    PubMed

    Ni, Ni; Allred, Jared M; Chan, Benny C; Cava, Robert Joseph

    2011-11-01

    It has been argued that the very high transition temperatures of the highest T(c) cuprate superconductors are facilitated by enhanced CuO(2) plane coupling through heavy metal oxide intermediary layers. Whether enhanced coupling through intermediary layers can also influence T(c) in the new high T(c) iron arsenide superconductors has never been tested due the lack of appropriate systems for study. Here we report the crystal structures and properties of two iron arsenide superconductors, Ca(10)(Pt(3)As(8))(Fe(2)As(2))(5) (the "10-3-8 phase") and Ca(10)(Pt(4)As(8))(Fe(2)As(2))(5) (the "10-4-8 phase"). Based on -Ca-(Pt(n)As(8))-Ca-Fe(2)As(2)- layer stacking, these are very similar compounds for which the most important differences lie in the structural and electronic characteristics of the intermediary platinum arsenide layers. Electron doping through partial substitution of Pt for Fe in the FeAs layers leads to T(c) of 11 K in the 10-3-8 phase and 26 K in the 10-4-8 phase. The often-cited empirical rule in the arsenide superconductor literature relating T(c) to As-Fe-As bond angles does not explain the observed differences in T(c) of the two phases; rather, comparison suggests the presence of stronger FeAs interlayer coupling in the 10-4-8 phase arising from the two-channel interlayer interactions and the metallic nature of its intermediary Pt(4)As(8) layer. The interlayer coupling is thus revealed as important in enhancing T(c) in the iron pnictide superconductors.

  4. Recent progress on carbon-based superconductors

    NASA Astrophysics Data System (ADS)

    Kubozono, Yoshihiro; Eguchi, Ritsuko; Goto, Hidenori; Hamao, Shino; Kambe, Takashi; Terao, Takahiro; Nishiyama, Saki; Zheng, Lu; Miao, Xiao; Okamoto, Hideki

    2016-08-01

    This article reviews new superconducting phases of carbon-based materials. During the past decade, new carbon-based superconductors have been extensively developed through the use of intercalation chemistry, electrostatic carrier doping, and surface-proving techniques. The superconducting transition temperature T c of these materials has been rapidly elevated, and the variety of superconductors has been increased. This review fully introduces graphite, graphene, and hydrocarbon superconductors and future perspectives of high-T c superconductors based on these materials, including present problems. Carbon-based superconductors show various types of interesting behavior, such as a positive pressure dependence of T c. At present, experimental information on superconductors is still insufficient, and theoretical treatment is also incomplete. In particular, experimental results are still lacking for graphene and hydrocarbon superconductors. Therefore, it is very important to review experimental results in detail and introduce theoretical approaches, for the sake of advances in condensed matter physics. Furthermore, the recent experimental results on hydrocarbon superconductors obtained by our group are also included in this article. Consequently, this review article may provide a hint to designing new carbon-based superconductors exhibiting higher T c and interesting physical features.

  5. Recent progress on carbon-based superconductors.

    PubMed

    Kubozono, Yoshihiro; Eguchi, Ritsuko; Goto, Hidenori; Hamao, Shino; Kambe, Takashi; Terao, Takahiro; Nishiyama, Saki; Zheng, Lu; Miao, Xiao; Okamoto, Hideki

    2016-08-24

    This article reviews new superconducting phases of carbon-based materials. During the past decade, new carbon-based superconductors have been extensively developed through the use of intercalation chemistry, electrostatic carrier doping, and surface-proving techniques. The superconducting transition temperature T c of these materials has been rapidly elevated, and the variety of superconductors has been increased. This review fully introduces graphite, graphene, and hydrocarbon superconductors and future perspectives of high-T c superconductors based on these materials, including present problems. Carbon-based superconductors show various types of interesting behavior, such as a positive pressure dependence of T c. At present, experimental information on superconductors is still insufficient, and theoretical treatment is also incomplete. In particular, experimental results are still lacking for graphene and hydrocarbon superconductors. Therefore, it is very important to review experimental results in detail and introduce theoretical approaches, for the sake of advances in condensed matter physics. Furthermore, the recent experimental results on hydrocarbon superconductors obtained by our group are also included in this article. Consequently, this review article may provide a hint to designing new carbon-based superconductors exhibiting higher T c and interesting physical features. PMID:27351938

  6. Robust upward dispersion of the neutron spin resonance in the heavy fermion superconductor Ce1−xYbxCoIn5

    PubMed Central

    Song, Yu; Van Dyke, John; Lum, I. K.; White, B. D.; Jang, Sooyoung; Yazici, Duygu; Shu, L.; Schneidewind, A.; Čermák, Petr; Qiu, Y.; Maple, M. B.; Morr, Dirk K.; Dai, Pengcheng

    2016-01-01

    The neutron spin resonance is a collective magnetic excitation that appears in the unconventional copper oxide, iron pnictide and heavy fermion superconductors. Although the resonance is commonly associated with a spin-exciton due to the d(s±)-wave symmetry of the superconducting order parameter, it has also been proposed to be a magnon-like excitation appearing in the superconducting state. Here we use inelastic neutron scattering to demonstrate that the resonance in the heavy fermion superconductor Ce1−xYbxCoIn5 with x=0, 0.05 and 0.3 has a ring-like upward dispersion that is robust against Yb-doping. By comparing our experimental data with a random phase approximation calculation using the electronic structure and the momentum dependence of the -wave superconducting gap determined from scanning tunnelling microscopy (STM) for CeCoIn5, we conclude that the robust upward-dispersing resonance mode in Ce1−xYbxCoIn5 is inconsistent with the downward dispersion predicted within the spin-exciton scenario. PMID:27677397

  7. Distinguishing between s +i d and s +i s pairing symmetries in multiband superconductors through spontaneous magnetization pattern induced by a defect

    NASA Astrophysics Data System (ADS)

    Lin, Shi-Zeng; Maiti, Saurabh; Chubukov, Andrey

    2016-08-01

    The symmetry of the pairing state in iron pnictide superconductor Ba1 -xKxFe2As2 is still controversial. At optimal doping (x ≈0.4 ), it is very likely s wave, but for x =1 there are experimental and theoretical arguments for both s wave and d wave. Depending on the choice for x =1 , intermediate s +i s and s +i d states have been proposed for intermediate doping 0.4 superconductors.

  8. A study of temperature dependent local atomic displacements in a Ba(Fe(1-x)Co(x))2As2 superconductor.

    PubMed

    Hacisalihoglu, M Y; Paris, E; Joseph, B; Simonelli, L; Sato, T J; Mizokawa, T; Saini, N L

    2016-04-01

    We have studied the local structure of a Ba(Fe(1-x)Co(x))2As2 superconductor using temperature dependent extended X-ray absorption fine structure (EXAFS) measurements. Polarized EXAFS at the Fe K-edge on an optimally doped (x = 0.06) single crystal has permitted us to determine atomic displacements across the superconducting transition temperature (T(c)). The Fe-As bondlength hardly shows any change with temperature; however, the Fe-Fe sublattice reveals a sharp anomaly across T(c), indicated by a significant drop in mean square relative displacements, similar to the one known for cuprates and A15-type superconductors. We have also found a large atomic disorder around the substituted Co, revealed by polarized Co K-edge EXAFS measurements. The Co-Fe/Co bonds are more flexible than the Fe-Fe bonds with the As-height in Co-containing tetrahedra being larger than the one in FeAs4. The results suggest that the local Fe-Fe bondlength fluctuations and the atomic disorder in this sub-lattice should have some important role in the superconductivity of Ba(Fe(1-x)Co(x))2As2 pnictides.

  9. Room-Temperature Ba(Fe1-x Cox)2 As2 is not Tetragonal: Direct Observation of Magnetoelastic Interactions in Pnictide Superconductors.

    PubMed

    Cantoni, Claudia; McGuire, Michael A; Saparov, Bayrammurad; May, Andrew F; Keiber, Trevor; Bridges, Frank; Sefat, Athena S; Sales, Brian C

    2015-05-01

    Lattice distortions corresponding to Ba displacements with respect to the FeAs sublattice are revealed to break the room-temperature tetragonal symmetry in Ba(Fe1-x Cox)2 As2. The displacements yield twin domains of the size of ≈10 nm. The domain size correlates with the magnitude of the local Fe magnetic moment and its non-monotonic dependence on Co concentration.

  10. Room-Temperature Ba(Fe1-x Cox)2 As2 is not Tetragonal: Direct Observation of Magnetoelastic Interactions in Pnictide Superconductors.

    PubMed

    Cantoni, Claudia; McGuire, Michael A; Saparov, Bayrammurad; May, Andrew F; Keiber, Trevor; Bridges, Frank; Sefat, Athena S; Sales, Brian C

    2015-05-01

    Lattice distortions corresponding to Ba displacements with respect to the FeAs sublattice are revealed to break the room-temperature tetragonal symmetry in Ba(Fe1-x Cox)2 As2. The displacements yield twin domains of the size of ≈10 nm. The domain size correlates with the magnitude of the local Fe magnetic moment and its non-monotonic dependence on Co concentration. PMID:25809406

  11. Studies on Effects of Impurity Doping and NMR Measurements of La 1111 and/or Nd 1111 Fe-Pnictide Superconductors

    NASA Astrophysics Data System (ADS)

    Sato, Masatoshi; Kobayashi, Yoshiaki; Lee, Sang Chul; Takahashi, Hidefumi; Satomi, Erika; Miura, Yoko

    2010-01-01

    Measurements of the electrical resistivity ρ, Hall coefficient RH, thermoelectric power S, and the electronic specific heat coefficient γ have been carried out for samples of LnFe1-yMyAsO1-xFx (Ln=La, Nd; M=Co, Mn; x=0.11) obtained by doping M atoms into the superconducting LnFeAsO1-xFx (Ln 1111) system. The NMR longitudinal relaxation rate 1/T1 has also been measured for samples of LaFe1-yCoyAsO1-xFx with various x values. Co atoms doped into the superconducting LnFeAsO1-xFx are nonmagnetic, and the rate of Tc-suppression |dTc/dx| by Co atoms has been found to be too small to be explained by the pair-breaking effect expected for the S± superconducting symmetry proposed as the most probable symmetry for the system. This result throws a serious doubt on whether the symmetry is realized in the system. Instead of the pair breaking, two mechanisms of Tc suppression by the doped impurities have been found: One is the electron localization, which appears when the sheet resistance R\\square exceeds h/4e2=6.45 kΩ, and the other is the disappearance (or reduction in the area) of the hole Fermi surfaces around the Γ point in the reciprocal space. The latter mechanism has been observed when the number of electrons increases with increasing Co doping level and the system changes from an “anomalous metal” to an ordinary one. Regarding the two distinct T dependences of the NMR longitudinal relaxation rate 1/T1 of LaFeAsO1-xFx, (1/T1\\propto T6 reported by our group in the T region from Tc to ˜0.4 Tc for samples with the highest Tc values with varying x, and 1/T1\\propto T2.5--3.0 observed by many groups in almost the entire T region studied below Tc), we discuss the origin of such a difference, and show that, at least, the T2.5--3.0-like dependence of 1/T1 cannot be considered as the experimental evidence for the S± symmetry of Δ.

  12. Flux dynamics and avalanches in the 122 pnictide superconductor Ba0.65Na0.35Fe2As2.

    PubMed

    Pramanik, A K; Aswartham, S; Wolter, A U B; Wurmehl, S; Kataev, V; Büchner, B

    2013-12-11

    In this work we present the results of the bulk magnetization measurements in a superconducting state of single crystals of Ba0.65Na0.35Fe2As2. The isothermal magnetic field (H ∥ c axis) dependent magnetization (M) loops exhibit a second peak (SP) or 'fishtail effect', as well as remarkable flux jumps at low temperatures. The critical current density Jc obtained from the M(H) loops is rather high, of the order of 10(6) A cm(-2). The analysis of the temperature- and field-dependent Jc implies that high Jc is mainly due to collective (weak) pinning of vortices by dense microscopic point defects with some contribution from a strong pinning mechanism. Pronounced magnetic instabilities in terms of flux jumps depend strongly on temperature as well as on the field sweep rate. The field for the first flux jump as calculated from an adiabatic model, however, is much lower than the experimentally observed values, and this enhanced stability is attributed to a flux creep phenomenon. The analysis of field-dependent magnetic relaxation data additionally supports a collective pinning model. The data further suggest that SP in M(H) is likely related to the crossover in creep dynamics from an elastic to a plastic mechanism. We have constructed the vortex phase diagram on the field-temperature plane.

  13. Local measurement of the superfluid density in the pnictide superconductor Ba(Fe1-xCox)2As2 across the superconducting dome

    SciTech Connect

    Not Available

    2011-08-12

    We measure the penetration depth {lambda}{sub ab}(T) in Ba(Fe{sub 1-x}Co{sub x}){sub 2}As{sub 2} using local techniques that do not average over the sample. The superfluid density {rho}{sub s}(T) {triple_bond} 1/{lambda}{sub ab}(T){sup 2} has three main features. First, {rho}{sub s}(T = 0) falls sharply on the underdoped side of the dome. Second, {lambda}{sub ab}(T) is flat at low T at optimal doping, indicating fully gapped superconductivity, but varies more strongly in underdoped and overdoped samples, consistent with either a power law or a small second gap. Third, {rho}{sub s}(T) varies steeply near T{sub c} for optimal and underdoping. These observations are consistent with an interplay between magnetic and superconducting phases.

  14. Local Measurement of the Penetration Depth in the Pnictide Superconductor Ba(Fe_0.95 Co_0.05)_2 As_2

    SciTech Connect

    Matsushita, Y.

    2010-01-11

    We use magnetic force microscopy (MFM) to measure the local penetration depth {lambda} in Ba(Fe{sub 0.95}Co{sub 0.05}){sub 2}As{sub 2} single crystals and use scanning SQUID susceptometry to measure its temperature variation down to 0.4 K. We observe that superfluid density {rho}{sub s} over the full temperature range is well described by a clean two-band fully gapped model. We demonstrate that MFM can measure the important and hard-to-determine absolute value of {lambda}, as well as obtain its temperature dependence and spatial homogeneity. We find {rho}{sub s} to be uniform on the submicron scale despite the highly disordered vortex pinning.

  15. NMR study of the AF-SC-SC-AF phased transition in a pnictide superconductor LaFeAsO1-xHx

    NASA Astrophysics Data System (ADS)

    Fujiwara, Naoki; Sakurai, Ryosuke; Iimura, Soushi; Matsuishi, Satoru; Hosono, Hideo; Yamakawa, Youichi; Kontani, Hiroshi

    2014-03-01

    We have performed 75As and 1H NMR measurements in LaFeAsO1xHx, an isomorphic compound of LaFeAsO1xFx. LaFeAsO1xHx is an electron doped system, and O2- can be replaced with H- up to x = 0.5. LaFeAsO1xHx is known for having double superconducting (SC) domes on H doping. Recently, we discovered that a new antiferromagnetic (AF) phase follows the double SC domes on further H doping, forming a symmetric AF-SC-SC-AF phase alignment in the electronic phase diagram Unlike the AF ordering in the lightly H-doped regime, the AF ordering in the highly H-doped regime is attributed to the nesting between electron pockets. In the conference, we will show the data of both NMR spectra and the relaxation rate 1/T1 in the whole doping region. We will discuss the difference of electronic states between the lightly H-doped AF-SC phases and highly H-doped SC-AF phases. This work is supported by a Grant-in-Aid (Grant No. KAKENHI 23340101) from the Ministry of Education, Science, and Culture, Japan.

  16. Paramagnetically induced gapful topological superconductors

    NASA Astrophysics Data System (ADS)

    Daido, Akito; Yanase, Youichi

    2016-08-01

    We propose a generic scenario for realizing gapful topological superconductors (TSCs) from gapless spin-singlet superconductors (SCs). Noncentrosymmetric nodal SCs in two dimensions are shown to be gapful under a Zeeman field, as a result of the cooperation of inversion-symmetry breaking and time-reversal-symmetry breaking. In particular, non-s -wave SCs acquire a large excitation gap. Such paramagnetically induced gapful SCs may be classified into TSCs in the symmetry class D specified by the Chern number. We show nontrivial Chern numbers over a wide parameter range for spin-singlet SCs. A variety of the paramagnetically induced gapful TSCs are demonstrated, including D +p -wave TSC, extended S +p -wave TSC, p +D +f -wave TSC, and s +P -wave TSC. Natural extension toward three-dimensional Weyl SCs is also discussed.

  17. Oxygen diffusion in cuprate superconductors

    SciTech Connect

    Routbort, J.L.; Rothman, S.J.

    1995-01-01

    Superconducting properties of the cuprate superconductors depend on the oxygen content of the material; the diffusion of oxygen is thus an important process in the fabrication and application of these materials. This article reviews studies of the diffusion of oxygen in La{sub 2}{sub {minus}}{sub {times}}Sr{sub {times}}CuO{sub 4}, YBa{sub 2}Cu{sub 3}O{sub 7}{sub {minus}}{delta}, YBa{sub 2}Cu{sub 4}O{sub 8}, and the Bi{sub 2}Sr{sub 2}Ca{sub n}{sub {minus}}{sub 1}Cu{sub n}O{sub 2}{sub +}{sub 4} (n = 1, and 2) superconductors, and attempt to elucidate the atomic mechanisms responsible.

  18. Topological properties of ferromagnetic superconductors

    DOE PAGESBeta

    Cheung, Alfred K. C.; Raghu, S.

    2016-04-27

    Here, a variety of heavy fermion superconductors, such as UCoGe, UGe2, and URhGe exhibit a striking coexistence of bulk ferromagnetism and superconductivity. In the first two materials, the magnetic moment decreases with pressure, and vanishes at a ferromagnetic quantum critical point (qcp). Remarkably, the superconductivity in UCoGe varies smoothly with pressure across the qcp and exists in both the ferromagnetic and paramagnetic regimes. We argue that in UCoGe, spin-orbit interactions stabilize a time-reversal invariant odd-parity superconductor in the high pressure paramagnetic regime. Based on a simple phenomenological model, we predict that the transition from the paramagnetic normal state to themore » phase where superconductivity and ferromagnetism coexist is a first-order transition.« less

  19. Theory of the striped superconductor

    NASA Astrophysics Data System (ADS)

    Berg, Erez; Fradkin, Eduardo; Kivelson, Steven A.

    2009-02-01

    We define a distinct phase of matter, a pair-density wave (PDW), in which the superconducting order parameter ϕ(r⃗,r⃗') varies periodically as a function of position such that when averaged over the center of mass position, (r⃗+r⃗')/2 , all components of ϕ vanish identically. Specifically, we study the simplest unidirectional PDW, the “striped superconductor,” which we argue may be at the heart of a number of spectacular experimental anomalies that have been observed in the failed high-temperature superconductor La2-xBaxCuO4 . We present a solvable microscopic model with strong electron-electron interactions which supports a PDW ground state. We also discuss, at the level of Landau theory, the nature of the coupling between the PDW and other order parameters and the origins and some consequences of the unusual sensitivity of this state to quenched disorder.

  20. Holographic superconductors with Weyl corrections

    NASA Astrophysics Data System (ADS)

    Momeni, Davood; Raza, Muhammad; Myrzakulov, Ratbay

    2016-10-01

    A quick review on the analytical aspects of holographic superconductors (HSCs) with Weyl corrections has been presented. Mainly, we focus on matching method and variational approaches. Different types of such HSC have been investigated — s-wave, p-wave and Stúckelberg ones. We also review the fundamental construction of a p-wave type, in which the non-Abelian gauge field is coupled to the Weyl tensor. The results are compared from numerics to analytical results.

  1. Recent Topics of Organic Superconductors

    NASA Astrophysics Data System (ADS)

    Ardavan, Arzhang; Brown, Stuart; Kagoshima, Seiichi; Kanoda, Kazushi; Kuroki, Kazuhiko; Mori, Hatsumi; Ogata, Masao; Uji, Shinya; Wosnitza, Jochen

    2012-01-01

    Recent developments in research into superconductivity in organic materials are reviewed. In the epoch-defining quasi-one-dimensional TMTSF superconductors with Tc ˜ 1 K, Tc decreases monotonically with increasing pressure, as do signatures of spin fluctuations in the normal state, providing good evidence for magnetically-mediated pairing. Upper critical fields exceed the Zeeman-limiting field by several times, suggesting triplet pairing or a transition to an inhomogeneous superconducting state at high magnetic fields, while triplet pairing is ruled out at low fields by NMR Knight-shift measurements. Evidence for a spatially inhomogeneous superconducting state, Fulde--Ferrel--Larkin--Ovchinnikov state, which has long been sought in various superconducting systems, is now captured by thermodynamic and transport measurements for clean and highly two-dimensional BEDT-TTF and BETS superconductors. Some of the layered superconductors also serve as model systems for Mott physics on anisotropic triangular lattice. For example, the Nernst effect and the pseudo-gap behavior in NMR relaxation are enhanced near to the Mott transition. In the case of increasing spin frustration, the superconducting transition temperature is depressed, and antiferromagnetic ordering is eliminated altogether in the adjacent Mott insulating phase. There is an increasing number of materials exhibiting superconductivity in competition or cooperation with charge order. Theoretical studies shed light on the role of spin and/or charge fluctuations for superconductivity appearing under conditions close to those of correlation-induced insulating phases in the diversity of organic materials.

  2. Edge instabilities of topological superconductors

    NASA Astrophysics Data System (ADS)

    Hofmann, Johannes S.; Assaad, Fakher F.; Schnyder, Andreas P.

    2016-05-01

    Nodal topological superconductors display zero-energy Majorana flat bands at generic edges. The flatness of these edge bands, which is protected by time-reversal and translation symmetry, gives rise to an extensive ground-state degeneracy. Therefore, even arbitrarily weak interactions lead to an instability of the flat-band edge states towards time-reversal and translation-symmetry-broken phases, which lift the ground-state degeneracy. We examine the instabilities of the flat-band edge states of dx y-wave superconductors by performing a mean-field analysis in the Majorana basis of the edge states. The leading instabilities are Majorana mass terms, which correspond to coherent superpositions of particle-particle and particle-hole channels in the fermionic language. We find that attractive interactions induce three different mass terms. One is a coherent superposition of imaginary s -wave pairing and current order, and another combines a charge-density-wave and finite-momentum singlet pairing. Repulsive interactions, on the other hand, lead to ferromagnetism together with spin-triplet pairing at the edge. Our quantum Monte Carlo simulations confirm these findings and demonstrate that these instabilities occur even in the presence of strong quantum fluctuations. We discuss the implications of our results for experiments on cuprate high-temperature superconductors.

  3. Ultrafast structural dynamics of the orthorhombic distortion in the Fe-pnictide parent compound BaFe2As2

    PubMed Central

    Rettig, L.; Mariager, S. O.; Ferrer, A.; Grübel, S.; Johnson, J. A.; Rittmann, J.; Wolf, T.; Johnson, S. L.; Ingold, G.; Beaud, P.; Staub, U.

    2016-01-01

    Using femtosecond time-resolved hard x-ray diffraction, we investigate the structural dynamics of the orthorhombic distortion in the Fe-pnictide parent compound BaFe2As2. The orthorhombic distortion analyzed by the transient splitting of the (1 0 3) Bragg reflection is suppressed on an initial timescale of 35 ps, which is much slower than the suppression of magnetic and nematic order. This observation demonstrates a transient state with persistent structural distortion and suppressed magnetic/nematic order which are strongly linked in thermal equilibrium. We suggest a way of quantifying the coupling between structural and nematic degrees of freedom based on the dynamics of the respective order parameters. PMID:27158636

  4. General principles of the synthesis of chalcogenides and pnictides in salt melts using a steady-state temperature gradient

    NASA Astrophysics Data System (ADS)

    Chareev, D. A.

    2016-05-01

    The possibilities of growing crystals of metals, alloys, chalcogenides, and pnictides in halide melts using a steady-state temperature gradient are analyzed. Halides of alkali metals and aluminum can be used as transport media. The choice is determined by the melting temperature of salt mixtures. A conducting contour can also be applied to increase transport efficiency. This technique of crystal growth is similar to the electrochemical method. To eliminate interference during migration, some elements can be isolated and forced to migrate through independent channels to the crystal formation region. The technique considered here makes it possible to grow crystals of necessary quality without special equipment; the small crystal sizes are sufficient for laboratory study.

  5. Enhancement of the London Penetration Depth in Pnictides at the Onset of Spin-Density-Wave Order under Superconducting Dome

    NASA Astrophysics Data System (ADS)

    Levchenko, A.; Vavilov, M. G.; Khodas, M.; Chubukov, A. V.

    2013-04-01

    Recent measurements of the doping dependence of the London penetration depth λ(x) at low T in clean samples of isovalent BaFe2(As1-xPx)2 at T≪Tc [Hashimoto et al., Science 336, 1554 (2012)SCIEAS0036-8075] revealed a peak in λ(x) near optimal doping x=0.3. The observation of the peak at T≪Tc, points to the existence of a quantum critical point beneath the superconducting dome. We associate such a quantum critical point with the onset of a spin-density-wave order and show that the renormalization of λ(x) by critical magnetic fluctuations gives rise to the observed feature. We argue that the case of pnictides is conceptually different from a one-component Galilean invariant Fermi liquid, for which correlation effects do not cause the renormalization of the London penetration depth at T=0.

  6. Enhancement of mechanical properties of 123 superconductors

    DOEpatents

    Balachandran, U.

    1995-04-25

    A composition and method are disclosed of preparing YBa{sub 2}Cu{sub 3}O{sub 7{minus}x} superconductor. Addition of tin oxide containing compounds to YBCO superconductors results in substantial improvement of fracture toughness and other mechanical properties without affect on T{sub c}. About 5-20% additions give rise to substantially improved mechanical properties.

  7. Enhancement of mechanical properties of 123 superconductors

    DOEpatents

    Balachandran, Uthamalingam

    1995-01-01

    A composition and method of preparing YBa.sub.2 Cu.sub.3 O.sub.7-x superconductor. Addition of tin oxide containing compounds to YBCO superconductors results in substantial improvement of fracture toughness and other mechanical properties without affect on T.sub.c. About 5-20% additions give rise to substantially improved mechanical properties.

  8. High temperature superconductors: A technological revolution

    NASA Technical Reports Server (NTRS)

    1990-01-01

    The objectives are to demonstrate the Meissner effect through magnetic levitation, to demonstrate one application of the Meissner effect, the low friction magnetic rotation bearing, and to demonstrate magnetic flux penetration and the Type II nature of ceramic superconductors via the stacking of the superconductor disks. Experimental equipment and procedures are described.

  9. Axial bearings using superconductors and permanent magnets

    SciTech Connect

    Marion-Pera, M.C.; Yonnet, J.P.

    1995-05-01

    Contactless bearings are one of the applications of high temperature superconductors. Different structures of permanent magnets and superconductors are modeled assuming a total Meissner effect. Axial force of a few hundred Newtons and stiffness of around 100 N/mm can be achieved. Consequences of real superconducting material behavior are discussed.

  10. Isotropic in-plane quenched disorder and dilution induce a robust nematic state in electron-doped pnictides

    NASA Astrophysics Data System (ADS)

    Liang, Shuhua; Bishop, Christopher B.; Moreo, Adriana; Dagotto, Elbio

    2015-09-01

    The phase diagram of electron-doped pnictides is studied varying the temperature, electronic density, and isotropic in-plane quenched disorder strength and dilution by means of computational techniques applied to a three-orbital (x z ,y z ,x y ) spin-fermion model with lattice degrees of freedom. In experiments, chemical doping introduces disorder but in theoretical studies the relationship between electronic doping and the randomly located dopants, with their associated quenched disorder, is difficult to address. In this publication, the use of computational techniques allows us to study independently the effects of electronic doping, regulated by a global chemical potential, and impurity disorder at randomly selected sites. Surprisingly, our Monte Carlo simulations reveal that the fast reduction with doping of the Néel TN and the structural TS transition temperatures, and the concomitant stabilization of a robust nematic state, is primarily controlled in our model by the magnetic dilution associated with the in-plane isotropic disorder introduced by Fe substitution. In the doping range studied, changes in the Fermi surface produced by electron doping affect only slightly both critical temperatures. Our results also suggest that the specific material-dependent phase diagrams experimentally observed could be explained as a consequence of the variation in disorder profiles introduced by the different dopants. Our findings are also compatible with neutron scattering and scanning tunneling microscopy, unveiling a patchy network of locally magnetically ordered clusters with anisotropic shapes, even though the quenched disorder is locally isotropic. This study reveals a remarkable and unexpected degree of complexity in pnictides: the fragile tendency to nematicity intrinsic of translational invariant electronic systems needs to be supplemented by quenched disorder and dilution to stabilize the robust nematic phase experimentally found in electron-doped 122 compounds.

  11. Isotropic in-plane quenched disorder and dilution induce a robust nematic state in electron-doped pnictides

    SciTech Connect

    Liang, Shuhua; Bishop, Christopher B.; Moreo, Adriana; Dagotto, Elbio

    2015-09-21

    The phase diagram of electron-doped pnictides is studied varying the temperature, electronic density, and isotropic in-plane quenched disorder strength and dilution by means of computational techniques applied to a three-orbital (xz,yz,xy) spin-fermion model with lattice degrees of freedom. In experiments, chemical doping introduces disorder but in theoretical studies the relationship between electronic doping and the randomly located dopants, with their associated quenched disorder, is difficult to address. Moreover, in this publication, the use of computational techniques allows us to study independently the effects of electronic doping, regulated by a global chemical potential, and impurity disorder at randomly selected sites. Surprisingly, our Monte Carlo simulations reveal that the fast reduction with doping of the N eel TN and the structural TS transition temperatures, and the concomitant stabilization of a robust nematic state, is primarily controlled in our model by the magnetic dilution associated with the in-plane isotropic disorder introduced by Fe substitution. In the doping range studied, changes in the Fermi surface produced by electron doping affect only slightly both critical temperatures. Our results also suggest that the specific material-dependent phase diagrams experimentally observed could be explained as a consequence of the variation in disorder profiles introduced by the different dopants. Finally, our findings are also compatible with neutron scattering and scanning tunneling microscopy, unveiling a patchy network of locally magnetically ordered clusters with anisotropic shapes, even though the quenched disorder is locally isotropic. Our study reveals a remarkable and unexpected degree of complexity in pnictides: the fragile tendency to nematicity intrinsic of translational invariant electronic systems needs to be supplemented by quenched disorder and dilution to stabilize the robust nematic phase experimentally found

  12. Isotropic in-plane quenched disorder and dilution induce a robust nematic state in electron-doped pnictides

    DOE PAGESBeta

    Liang, Shuhua; Bishop, Christopher B.; Moreo, Adriana; Dagotto, Elbio

    2015-09-21

    The phase diagram of electron-doped pnictides is studied varying the temperature, electronic density, and isotropic in-plane quenched disorder strength and dilution by means of computational techniques applied to a three-orbital (xz,yz,xy) spin-fermion model with lattice degrees of freedom. In experiments, chemical doping introduces disorder but in theoretical studies the relationship between electronic doping and the randomly located dopants, with their associated quenched disorder, is difficult to address. Moreover, in this publication, the use of computational techniques allows us to study independently the effects of electronic doping, regulated by a global chemical potential, and impurity disorder at randomly selected sites. Surprisingly,more » our Monte Carlo simulations reveal that the fast reduction with doping of the N eel TN and the structural TS transition temperatures, and the concomitant stabilization of a robust nematic state, is primarily controlled in our model by the magnetic dilution associated with the in-plane isotropic disorder introduced by Fe substitution. In the doping range studied, changes in the Fermi surface produced by electron doping affect only slightly both critical temperatures. Our results also suggest that the specific material-dependent phase diagrams experimentally observed could be explained as a consequence of the variation in disorder profiles introduced by the different dopants. Finally, our findings are also compatible with neutron scattering and scanning tunneling microscopy, unveiling a patchy network of locally magnetically ordered clusters with anisotropic shapes, even though the quenched disorder is locally isotropic. Our study reveals a remarkable and unexpected degree of complexity in pnictides: the fragile tendency to nematicity intrinsic of translational invariant electronic systems needs to be supplemented by quenched disorder and dilution to stabilize the robust nematic phase experimentally found in electron

  13. Topological surface states in nodal superconductors.

    PubMed

    Schnyder, Andreas P; Brydon, Philip M R

    2015-06-24

    Topological superconductors have become a subject of intense research due to their potential use for technical applications in device fabrication and quantum information. Besides fully gapped superconductors, unconventional superconductors with point or line nodes in their order parameter can also exhibit nontrivial topological characteristics. This article reviews recent progress in the theoretical understanding of nodal topological superconductors, with a focus on Weyl and noncentrosymmetric superconductors and their protected surface states. Using selected examples, we review the bulk topological properties of these systems, study different types of topological surface states, and examine their unusual properties. Furthermore, we survey some candidate materials for topological superconductivity and discuss different experimental signatures of topological surface states. PMID:26000466

  14. Magnetization of anisotropic Type II superconductors

    SciTech Connect

    Mints, R.G.

    1989-04-10

    Peculiarities of magnetization of anisotropic type II superconductors are of considerable interest in view of the discovery of high-T/sub c/ superconductors characterized by strongly asymmetric layered structure. Specifics of the penetration of magnetic flux into an anisotropic type II superconductor were discussed in the literature. This analysis gave the distribution of induction in an isolated vortex, its energy, and critical magnetic field H/sub c1/. However, the magnetization curve of anisotropic superconductors was not considered. This paper deals with the magnetic moment of uniaxial London superconductor in the interval H/sub c1/ /le/ H/sub 0/ << H/sub c2/, where H/sub 0/ is the external magnetic field strength.

  15. The amazing properties of crystalline color superconductors

    NASA Astrophysics Data System (ADS)

    Mannarelli, Massimo

    2014-07-01

    This paper is a brief journey into the amazing realm of crystalline color superconductors. Starting from a qualitative description of superfluids, superconductors and supersolids, we show how inhomogeneous phases may arise when the system is under stress. These basic concepts are then extended to quark matter, in which a richer variety of phases can be realized. The most interesting properties of the crystalline color superconductors are presented. This brief journey ends with a discussion of crystalline color superconductors in compact stars and related astrophysical observables. We aim at providing a pedagogical introduction for nonexpert in the field to a few interesting properties of crystalline color superconductors, without discussing the methods and the technicalities. Thus, the results are presented without a proof. However, we try to give a qualitatively clear description of the main concepts, using standard quantum field theory and analogies with condensed matter systems.

  16. Majorana Fermions and Topology in Superconductors

    NASA Astrophysics Data System (ADS)

    Sato, Masatoshi; Fujimoto, Satoshi

    2016-07-01

    Topological superconductors are novel classes of quantum condensed phases, characterized by topologically nontrivial structures of Cooper pairing states. On the surfaces of samples and in vortex cores of topological superconductors, Majorana fermions, which are particles identified with their own anti-particles, appear as Bogoliubov quasiparticles. The existence and stability of Majorana fermions are ensured by bulk topological invariants constrained by the symmetries of the systems. Majorana fermions in topological superconductors obey a new type of quantum statistics referred to as non-Abelian statistics, which is distinct from bose and fermi statistics, and can be utilized for application to topological quantum computation. Also, Majorana fermions give rise to various exotic phenomena such as "fractionalization", non-local correlation, and "teleportation". A pedagogical review of these subjects is presented. We also discuss interaction effects on topological classification of superconductors, and the basic properties of Weyl superconductors.

  17. LDA'+DMFT investigation of electronic structure of K1 - x Fe2 - y Se2 superconductor

    NASA Astrophysics Data System (ADS)

    Nekrasov, I. A.; Pavlov, N. S.; Sadovskii, M. V.

    2013-03-01

    We investigate electronic structure of the new iron chalcogenide high temperature superconductor K1- x Fe2- y Se2 (hole doped case with x = 0.24, y = 0.28) in the normal phase using the novel LDA'+DMFT computational approach. We show that this iron chalcogenide is more correlated in a sense of bandwidth renormalization (energy scale compression by factor about 5 in the interval ±1.5 eV), than typical iron pnictides (compression factor about 2), though the Coulomb interaction strength is almost the same in both families. Our results for spectral densities are in general agreement with recent ARPES data on this system. It is found that all Fe-3 d( t 2 g ) bands crossing the Fermi level have equal renormalization, in contrast to some previous interpretations. Electronic states at the Fermi level are of predominantly xy symmetry. Also we show that LDA'+DMFT results are in better agreement with experimental spectral function maps, than the results of conventional LDA+DMFT. Finally we make predictions for photoemission spectra lineshape for K0.76Fe1.72Se2.

  18. 42214 layered Fe-based superconductors: An ab initio study of their structural, magnetic, and electronic properties

    NASA Astrophysics Data System (ADS)

    Bucci, F.; Sanna, A.; Continenza, A.; Katrych, S.; Karpinski, J.; Gross, E. K. U.; Profeta, G.

    2016-01-01

    As a follow-up to the discovery of a new family of Fe-based superconductors, namely, the RE4Fe2As2Te1 -xO4 (42214) (RE = Pr, Sm, and Gd), we present a detailed ab initio study of these compounds highlighting the role of rare-earth (RE) atoms, external pressure, and Te content on their physical properties. Modifications of the structural, magnetic, and electronic properties of the pure (e.g., x =0.0 ) 42214 compounds and their possible correlations with the observed superconducting properties are calculated and discussed. The careful analysis of the results obtained shows that (i) changing the RE atoms allows one to tune the internal pressure acting on the As height with respect to the Fe planes; (ii) similarly to other Fe pnictides, the 42214 pure compounds show an antiferromagnetic-stripe magnetic ground state phase joined by an orthorhombic distortion (not experimentally found yet); (iii) smaller RE atoms increase the magnetic instability of the compounds possibly favoring the onset of the superconducting state; (iv) external pressure induces the vanishing of the magnetic order with a transition to the tetragonal phase and can be a possible experimental route towards higher superconducting critical temperature (Tc) ; and (v) Te vacancies act on the structural parameters, changing the As height and affecting the stability of the magnetic phase.

  19. Phase transitions in a frustrated biquadratic Heisenberg model with coupled orbital degrees of freedom for iron-based superconductors

    NASA Astrophysics Data System (ADS)

    Zhuo, W. Z.; Qin, M. H.; Dong, S.; Li, X. G.; Liu, J.-M.

    2016-03-01

    In this paper, we study a biquadratic Heisenberg model with coupled orbital degrees of freedom by using a Monte Carlo simulation to investigate the phase transitions in iron-based superconductors. The antiferroquadrupolar state, which may be related to the magnetism of FeSe [R. Yu and Q. Si, Phys. Rev. Lett. 115, 116401 (2015), 10.1103/PhysRevLett.115.116401], is stabilized by the anisotropic biquadratic interaction induced by a ferro-orbital-ordered state. It is revealed that the orbital and nematic transitions occur at the same temperature for all the cases, supporting the mechanism of the orbital-driven nematicity as revealed in most recent experiments [S. H. Baek, D. V. Efremov, J. M. Ok, J. S. Kim, J. van den Brink, and B. Büchner, Nat. Mater. 14, 210 (2015), 10.1038/nmat4138]. In addition, it is suggested that the orbital interaction may lead to the separation of the structural and magnetic phase transitions, as observed in many families of iron pnictides.

  20. Oxide superconductors under magnetic field

    NASA Technical Reports Server (NTRS)

    Kitazawa, K.

    1991-01-01

    One of the current most serious problems for the oxide superconductors from the standpoint of practical application is the various novel features derived mainly from their extremely short coherence. In particular, the coherence length so far observed in the cuprate superconductors is in the range of 0.1 nm perpendicular to the CuO2 plane. This seems to be creating most of the difficulties in the device fabrication and in the performance under the magnetic field. Some of the superconducting properties under the magnetic field will be discussed in terms of the short coherence length. A model will be presented based on the gradual strengthening of the pinning force with decrease in temperature and the weak coupling at the grain boundaries. Secondly, the broadening of the superconducting transition under the magnetic field is discussed. This is observed significantly only when the field is applied perpendicular to the basal plane and the relative orientation of the current to the field is insignificant in determining the extent of broadening. Besides, the change in the strength of the pinning force does not affect the width of the broadening. From these observations discussions will be made on a model based on the giant fluctuation. Based on this model, it is predicted that the coherence length along the c-axis will be the single most important material parameter to determine the performance of the superconductor under a strong magnetic field. It seems that BYCO is superior in this regard to Bi- or Tl-systems as far as the performance at 77 K is considered, although another material with the coherence length slightly longer along the c-axis is still highly desired.

  1. Microgravity Processing of Oxide Superconductors

    NASA Technical Reports Server (NTRS)

    Hofmeister, William H.; Bayuzick, Robert J.; Vlasse, Marcus; McCallum, William; Peters, Palmer (Technical Monitor)

    2000-01-01

    The primary goal is to understand the microstructures which develop under the nonequilibrium solidification conditions achieved by melt processing in copper oxide superconductor systems. More specifically, to define the liquidus at the Y- 1:2:3 composition, the Nd-1:2:3 composition, and several intermediate partial substitution points between pure Y-1:2:3 and Nd-1:2:3. A secondary goal has been to understand resultant solidification morphologies and pathways under a variety of experimental conditions and to use this knowledge to better characterize solidification phenomena in these systems.

  2. Passivation of high temperature superconductors

    NASA Technical Reports Server (NTRS)

    Vasquez, Richard P. (Inventor)

    1991-01-01

    The surface of high temperature superconductors such as YBa2Cu3O(7-x) are passivated by reacting the native Y, Ba and Cu metal ions with an anion such as sulfate or oxalate to form a surface film that is impervious to water and has a solubility in water of no more than 10(exp -3) M. The passivating treatment is preferably conducted by immersing the surface in dilute aqueous acid solution since more soluble species dissolve into the solution. The treatment does not degrade the superconducting properties of the bulk material.

  3. Vortex ice in nanostructured superconductors

    SciTech Connect

    Reichhardt, Charles; Reichhardt, Cynthia J; Libal, Andras J

    2008-01-01

    We demonstrate using numerical simulations of nanostructured superconductors that it is possible to realize vortex ice states that are analogous to square and kagome ice. The system can be brought into a state that obeys either global or local ice rules by applying an external current according to an annealing protocol. We explore the breakdown of the ice rules due to disorder in the nanostructure array and show that in square ice, topological defects appear along grain boundaries, while in kagome ice, individual defects appear. We argue that the vortex system offers significant advantages over other artificial ice systems.

  4. Superconductor lunar telescopes --Abstract only

    NASA Technical Reports Server (NTRS)

    Chen, P. C.; Pitts, R.; Shore, S.; Oliversen, R.; Stolarik, J.; Segal, K.; Hojaji, H.

    1994-01-01

    We propose a new type of telescope designed specifically for the lunar environment of high vacuum and low temperature. Large area UV-Visible-IR telescope arrays can be built with ultra-light-weight replica optics. High T(sub c) superconductors provide support, steering, and positioning. Advantages of this approach are light-weight payload compatible with existing launch vehicles, configurable large area optical arrays, no excavation or heavy construction, and frictionless electronically controlled mechanisms. We have built a prototype and will be demonstarting some of its working characteristics.

  5. NaFe0.56Cu0.44As : A Pnictide Insulating Phase Induced by On-Site Coulomb Interaction

    NASA Astrophysics Data System (ADS)

    Matt, C. E.; Xu, N.; Lv, Baiqing; Ma, Junzhang; Bisti, F.; Park, J.; Shang, T.; Cao, Chongde; Song, Yu; Nevidomskyy, Andriy H.; Dai, Pengcheng; Patthey, L.; Plumb, N. C.; Radovic, M.; Mesot, J.; Shi, M.

    2016-08-01

    In the studies of iron pnictides, a key question is whether their bad-metal state from which the superconductivity emerges lies in close proximity with a magnetically ordered insulating phase. Recently, it was found that at low temperatures, the heavily Cu-doped NaFe1 -xCuxAs (x >0.3 ) iron pnictide is an insulator with long-range antiferromagnetic order, similar to the parent compound of cuprates but distinct from all other iron pnictides. Using angle-resolved photoemission spectroscopy, we determined the momentum-resolved electronic structure of NaFe1 -xCuxAs (x =0.44 ) and identified that its ground state is a narrow-gap insulator. Combining the experimental results with density functional theory (DFT) and DFT +U calculations, our analysis reveals that the on-site Coulombic (Hubbard) and Hund's coupling energies play crucial roles in the formation of the band gap about the chemical potential. We propose that at finite temperatures, charge carriers are thermally excited from the Cu-As-like valence band into the conduction band, which is of Fe 3 d -like character. With increasing temperature, the number of electrons in the conduction band becomes larger and the hopping energy between Fe sites increases, and finally the long-range antiferromagnetic order is destroyed at T >TN . Our study provides a basis for investigating the evolution of the electronic structure of a Mott insulator transforming into a bad metallic phase and eventually forming a superconducting state in iron pnictides.

  6. NaFe_{0.56}Cu_{0.44}As: A Pnictide Insulating Phase Induced by On-Site Coulomb Interaction.

    PubMed

    Matt, C E; Xu, N; Lv, Baiqing; Ma, Junzhang; Bisti, F; Park, J; Shang, T; Cao, Chongde; Song, Yu; Nevidomskyy, Andriy H; Dai, Pengcheng; Patthey, L; Plumb, N C; Radovic, M; Mesot, J; Shi, M

    2016-08-26

    In the studies of iron pnictides, a key question is whether their bad-metal state from which the superconductivity emerges lies in close proximity with a magnetically ordered insulating phase. Recently, it was found that at low temperatures, the heavily Cu-doped NaFe_{1-x}Cu_{x}As (x>0.3) iron pnictide is an insulator with long-range antiferromagnetic order, similar to the parent compound of cuprates but distinct from all other iron pnictides. Using angle-resolved photoemission spectroscopy, we determined the momentum-resolved electronic structure of NaFe_{1-x}Cu_{x}As (x=0.44) and identified that its ground state is a narrow-gap insulator. Combining the experimental results with density functional theory (DFT) and DFT+U calculations, our analysis reveals that the on-site Coulombic (Hubbard) and Hund's coupling energies play crucial roles in the formation of the band gap about the chemical potential. We propose that at finite temperatures, charge carriers are thermally excited from the Cu-As-like valence band into the conduction band, which is of Fe 3d-like character. With increasing temperature, the number of electrons in the conduction band becomes larger and the hopping energy between Fe sites increases, and finally the long-range antiferromagnetic order is destroyed at T>T_{N}. Our study provides a basis for investigating the evolution of the electronic structure of a Mott insulator transforming into a bad metallic phase and eventually forming a superconducting state in iron pnictides. PMID:27610876

  7. Ultrafast momentum-dependent quasiparticle dynamics in high-Tc superconductors

    NASA Astrophysics Data System (ADS)

    Bovensiepen, Uwe

    2013-03-01

    Femtosecond time- and angle-resolved photoelectron spectroscopy trARPES facilitates insight into electronic relaxation and electronic structure of non-equilibrium states of matter. Hot electrons and holes relax in metals on ultrafast time scales due to the screened Coulomb interaction. In superconductors the relaxation rates of quasiparticles at energies close to the superconducting gap edge are reduced because of the loss of quasiparticle states near EF. Since in the superconducting state the relaxation of optically excited carriers proceeds partly by Cooper pair reformation, the study of the quasiparticle dynamics bears the potential to analyze the interaction responsible for Cooper pair formation. Results of trARPES will be discussed for optimally doped Bi2Sr2CaCu2O8+δ in the superconducting state and on EuFe2As2 in the antiferromagnetic state. In the cuprate system we find a predominant excitation of quasiparticles at momenta near the antinode. We show furthermore, that at excitation densities of several 10 μJ/cm2 quasiparticle relaxation is dominated by Cooper pair reformation, which again proceeds near the antinode. In the Fe-pnictide material we monitor a difference in the relaxation rate for electrons and holes near the Fermi momentum, which disappears above the Neel temperature. We conclude that this anisotropic relaxation of electrons and holes is a consequence of the optical modification of the antiferromagnetic order. Analysis of energy transfer from electrons to phonons allows to determine the momentum averaged electron-phonon coupling constant λ. We find values below 0.25 for Bi2Sr2CaCu2O8+δ and below 0.15 for EuFe2As2. We acknowledge funding through the Deutsche Forschungsgemeinschaft through BO 1823/2, SPP 1458 and the Alexander von Humboldt foundation.

  8. Inhomogeneous disorder Dirac Fermions: from heavy fermion superconductors to graphene. Final report

    SciTech Connect

    Vekhter, Ilya

    2013-08-11

    This is the final report on the award designed to foster a partnership between Louisiana State University and Los Alamos National Laboratory (LANL) in conducting fundamental research in support of energy needs. The general focus of the research effort was on developing a better understanding of materials with new functionalities. We investigated two distinct and very promising classes of new materials, which serve as a testing ground for many of the novel phenomena in condensed matter physics: the heavy fermion 115 series, where the interplay of strong interactions between the electrons leads to a rich variety of competing phases and anomalous properties, and newly discovered pnictide superconductors. The former focus was planned; the latter emerged during the collaborative effort with LANL. Our objective was to determine the origin, and to establish a functional effective theory description of the phases in these systems, and transitions between them. We report on the main accomplishments under the award that serves to clarify the nature of superconductivity in both families of materials. In particular, we collaborated with experimentalists to predict and analyze the magnetic field and temperature dependence of the bulk thermodynamic and transport properties and to determine the gap shape in CeCoIn₅ and in Ba(Fe1-xCox)₂As₂, investigated the Kondo temperature in the presence of spin-orbit coupling in the conduction band, and provided theoretical guidance for local probes such as scanning tunneling spectroscopy of vortex cores and impurity resonances, and magnetic force microscopy of the superconducting states.

  9. Superconductor bearings, flywheels and transportation

    NASA Astrophysics Data System (ADS)

    Werfel, F. N.; Floegel-Delor, U.; Rothfeld, R.; Riedel, T.; Goebel, B.; Wippich, D.; Schirrmeister, P.

    2012-01-01

    This paper describes the present status of high temperature superconductors (HTS) and of bulk superconducting magnet devices, their use in bearings, in flywheel energy storage systems (FESS) and linear transport magnetic levitation (Maglev) systems. We report and review the concepts of multi-seeded REBCO bulk superconductor fabrication. The multi-grain bulks increase the averaged trapped magnetic flux density up to 40% compared to single-grain assembly in large-scale applications. HTS magnetic bearings with permanent magnet (PM) excitation were studied and scaled up to maximum forces of 10 kN axially and 4.5 kN radially. We examine the technology of the high-gradient magnetic bearing concept and verify it experimentally. A large HTS bearing is tested for stabilizing a 600 kg rotor of a 5 kWh/250 kW flywheel system. The flywheel rotor tests show the requirement for additional damping. Our compact flywheel system is compared with similar HTS-FESS projects. A small-scale compact YBCO bearing with in situ Stirling cryocooler is constructed and investigated for mobile applications. Next we show a successfully developed modular linear Maglev system for magnetic train operation. Each module levitates 0.25t at 10 mm distance during one-day operation without refilling LN2. More than 30 vacuum cryostats containing multi-seeded YBCO blocks are fabricated and are tested now in Germany, China and Brazil.

  10. Method to improve superconductor cable

    DOEpatents

    Borden, A.R.

    1984-03-08

    A method is disclosed of making a stranded superconductor cable having improved flexing and bending characteristics. In such method, a plurality of superconductor strands are helically wound around a cylindrical portion of a mandrel which tapers along a transitional portion to a flat end portion. The helically wound strands form a multistrand hollow cable which is partially flattened by pressure rollers as the cable travels along the transitional portion. The partially flattened cable is impacted with repeated hammer blows as the hollow cable travels along the flat end portion. The hammer blows flatten both the internal and the external surfaces of the strands. The cable is fully flattened and compacted by two sets of pressure rollers which engage the flat sides and the edges of the cable after it has traveled away from the flat end portion of the mandrel. The flattened internal surfaces slide easily over one another when the cable is flexed or bent so that there is very little possibility that the cable will be damaged by the necessary flexing and bending required to wind the cable into magnet coils.

  11. Electronic transport in unconventional superconductors

    SciTech Connect

    Graf, M.J.

    1998-12-31

    The author investigates the electron transport coefficients in unconventional superconductors at low temperatures, where charge and heat transport are dominated by electron scattering from random lattice defects. He discusses the features of the pairing symmetry, Fermi surface, and excitation spectrum which are reflected in the low temperature heat transport. For temperatures {kappa}{sub B}T {approx_lt} {gamma} {much_lt} {Delta}{sub 0}, where {gamma} is the bandwidth of impurity induced Andreev states, certain eigenvalues become universal, i.e., independent of the impurity concentration and phase shift. Deep in the superconducting phase ({kappa}{sub B}T {approx_lt} {gamma}) the Wiedemann-Franz law, with Sommerfeld`s value of the Lorenz number, is recovered. He compares the results for theoretical models of unconventional superconductivity in high-{Tc} and heavy fermion superconductors with experiment. The findings show that impurities are a sensitive probe of the low-energy excitation spectrum, and that the zero-temperature limit of the transport coefficients provides an important test of the order parameter symmetry.

  12. Modified entropic gravitation in superconductors

    NASA Astrophysics Data System (ADS)

    de Matos, Clovis Jacinto

    2012-01-01

    Verlinde recently developed a theoretical account of gravitation in terms of an entropic force. The central element in Verlinde’s derivation is information and its relation with entropy through the holographic principle. The application of this approach to the case of superconductors requires to take into account that information associated with superconductor’s quantum vacuum energy is not stored on Planck size surface elements, but in four volume cells with Planck-Einstein size. This has profound consequences on the type of gravitational force generated by the quantum vacuum condensate in superconductors, which is closely related with the cosmological repulsive acceleration responsible for the accelerated expansion of the Universe. Remarkably this new gravitational type force depends on the level of breaking of the weak equivalence principle for cooper pairs in a given superconducting material, which was previously derived by the author starting from similar principles. It is also shown that this new gravitational force can be interpreted as a surface force. The experimental detection of this new repulsive gravitational-type force appears to be challenging.

  13. Aeronautical applications of high-temperature superconductors

    NASA Technical Reports Server (NTRS)

    Turney, George E.; Luidens, Roger W.; Uherka, Kenneth; Hull, John

    1989-01-01

    The successful development of high-temperature superconductors (HTS) could have a major impact on future aeronautical propulsion and aeronautical flight vehicle systems. A preliminary examination of the potential application of HTS for aeronautics indicates that significant benefits may be realized through the development and implementation of these newly discovered materials. Applications of high-temperature superconductors (currently substantiated at 95 K) were envisioned for several classes of aeronautical systems, including subsonic and supersonic transports, hypersonic aircraft, V/STOL aircraft, rotorcraft, and solar, microwave and laser powered aircraft. Introduced and described are the particular applications and potential benefits of high-temperature superconductors as related to aeronautics and/or aeronautical systems.

  14. Synthesis of highly phase pure BSCCO superconductors

    DOEpatents

    Dorris, S.E.; Poeppel, R.B.; Prorok, B.C.; Lanagan, M.T.; Maroni, V.A.

    1995-11-21

    An article and method of manufacture (Bi, Pb)-Sr-Ca-Cu-O superconductor are disclosed. The superconductor is manufactured by preparing a first powdered mixture of bismuth oxide, lead oxide, strontium carbonate, calcium carbonate and copper oxide. A second powdered mixture is then prepared of strontium carbonate, calcium carbonate and copper oxide. The mixtures are calcined separately with the two mixtures then combined. The resulting combined mixture is then subjected to a powder in tube deformation and thermal processing to produce a substantially phase pure (Bi, Pb)-Sr-Ca-Cu-O superconductor. 5 figs.

  15. Synthesis of highly phase pure BSCCO superconductors

    DOEpatents

    Dorris, Stephen E.; Poeppel, Roger B.; Prorok, Barton C.; Lanagan, Michael T.; Maroni, Victor A.

    1995-01-01

    An article and method of manufacture of (Bi, Pb)-Sr-Ca-Cu-O superconductor. The superconductor is manufactured by preparing a first powdered mixture of bismuth oxide, lead oxide, strontium carbonate, calcium carbonate and copper oxide. A second powdered mixture is then prepared of strontium carbonate, calcium carbonate and copper oxide. The mixtures are calcined separately with the two mixtures then combined. The resulting combined mixture is then subjected to a powder in tube deformation and thermal processing to produce a substantially phase pure (Bi, Pb)-Sr-Ca-Cu-O superconductor.

  16. Plasmonic properties of superconductor-insulator-superconductor waveguide

    NASA Astrophysics Data System (ADS)

    Ma, Youqiao; Eldlio, Mohamed; Maeda, Hiroshi; Zhou, Jun; Cada, Michael

    2016-07-01

    The simultaneous realization of low propagation loss and subwavelength mode localization remains one of the critical challenges in plasmonics. Aiming to simultaneously realize low propagation loss and subwavelength mode localization in plasmonics, we introduce a class of low-loss and deeply confined guiding schemes utilizing an alternative plasmonic material, i.e., a superconductor (SC). The optical properties of a SC-insulator-SC (SCISC) waveguide are analyzed both at terahertz (THz) and telecommunication (TC) frequencies. The SCISC waveguide features a deep-subwavelength confinement with a mode length as small as λ/6000 (λ/18) for THz (TC) frequency, while the propagation length can be extended up to 400 mm (1 mm).

  17. Majorana fermions in nanowires without gating superconductors

    NASA Astrophysics Data System (ADS)

    Lin, Chien-Hung; Hui, Hoi Yin; Sau, Jay; Das Sarma, Sankar

    2011-03-01

    Majorana fermions have been proposed to be realizable at the end of the semiconductor nanowire on top of an s-wave superconductor [1,2]. These proposals require gating the nanowire directly in contact with a superconductor which may be difficult in experiments. We analyze [1,2] in configurations where the wire is only gated away from the superconductor. We show that some signatures of the Majorana mode remain but the Majorana mode is not localized and hence not suitable for quantum computation. Therefore we propose an 1D periodic heterostructure which can support localized Majorana modes at the end of the wire without gating on the superconductor. This work is supported by DARPA-QuEST, JQI-NSF-PFC, and LPS-NSA.

  18. Nonlocal spin correlations mediated by a superconductor

    NASA Astrophysics Data System (ADS)

    Noh, Taewan; Houzet, Manuel; Meyer, Julia S.; Chandrasekhar, Venkat

    2013-06-01

    Nonlocal charge correlations induced in two normal metals contacted separately to a superconductor have been studied intensively in the past few years. Here we investigate nonlocal correlations induced by the transfer of pure spin currents through a superconductor on a scale comparable to the superconducting coherence length. As with charge currents, two processes contribute to the nonlocal spin signal: crossed Andreev reflection (CAR), where an electron with spin-up injected from one normal metal into the superconductor results in a hole with spin-down being injected into the second normal metal, and elastic cotunneling (EC), where the electron with spin-up injected from the first normal metal results in an electron with spin-up being injected into the second normal metal. Unlike charge currents, however, the spin currents associated with CAR and EC add due to the fact that the bulk superconductor cannot sustain a net spin current.

  19. Resistance domain in type II superconductors

    SciTech Connect

    Gurevich, A.V.; Mints, R.G.

    1980-01-05

    We show that traveling domains with a finite resistance can exist in type II superconductors in the presence of a transport current. An experiment in which this effect generates an alternating electric field and current is proposed.

  20. Josephson current between p-wave superconductors

    NASA Astrophysics Data System (ADS)

    Yokoyama, Takehito; Tanaka, Yukio; Golubov, Alexander; Asano, Yasuhiro

    2006-10-01

    Josephson current in p-wave superconductor/diffusive normal metal (DN)/p-wave superconductor junctions is calculated by solving the Usadel equation under the Nazarov's boundary condition extended to unconventional superconductors by changing the heights of the insulating barriers at the interfaces, the magnitudes of the resistance in DN, and the angles between the normal to the interface and the lobe directions of p-wave pair potentials. It is shown that the magnitude of the Josephson current strongly depends on the lobe directions of the p-wave pair potentials and the resulting magnitude of the Josephson current is large compared to that in the s-wave superconducting junctions due to the formation of the resonant states peculiar to p-wave superconductors.

  1. New twisted intermetallic compound superconductor: A concept

    NASA Technical Reports Server (NTRS)

    Coles, W. D.; Brown, G. V.; Laurence, J. C.

    1972-01-01

    Method for processing Nb3Sn and other intermetallic compound superconductors produces a twisted, stabilized wire or tube which can be used to wind electromagnetics, armatures, rotors, and field windings for motors and generators as well as other magnetic devices.

  2. Experimental studies of weakly coupled superconductors (Review)

    NASA Astrophysics Data System (ADS)

    Dmitrenko, I. M.

    2004-07-01

    A review is given of the main experimental results obtained in research on weakly coupled superconductors after 1964 at the Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine, Kharkov (ILTPE).

  3. A road to reality with topological superconductors

    NASA Astrophysics Data System (ADS)

    Beenakker, Carlo; Kouwenhoven, Leo

    2016-07-01

    Topological matter can host low-energy quasiparticles, which, in a superconductor, are Majorana fermions described by a real wavefunction. The absence of complex phases provides protection for quantum computations based on topological superconductivity.

  4. Heat conduction in unconventional superconductors

    NASA Astrophysics Data System (ADS)

    Lussier, Benoit

    Thermal conductivity is an excellent probe of quasiparticle excitations in superconductors both in the normal and superconducting state. We have applied this technique to the study of two unconventional superconductors, namely the heavy fermion superconductor UPtsb3 and the high-Tsb{c} cuprate YBasb2Cusb3Osb{7-delta}. In the case of UPtsb3, after reviewing previous low temperature thermal conductivity measurements, we show that, for our high quality single crystals, the thermal conductivity is totally dominated by electrons and therefore provides a direct probe of the superconducting gap structure. We demonstrate that our measurements of the anisotropy of heat conduction between b-axis and c-axis in this hexagonal crystal provide strong constraints with respect to the possible gap structures inferred by group theoretical arguments. By comparing our results with recent theoretical calculations, we show that a hybrid II gap structure provides good agreement between theory and experiments favoring an order parameter of Esb{2u} (strong spin-orbit coupling) or Asb{2u} (weak spin-orbit coupling) symmetry. For YBasb2Cusb3Osb{7-delta}, the thermal conductivity typically consists of both a phononic and an electronic contribution. After reviewing low temperature thermal conductivity measurements that address this question, we demonstrate the presence of electronic quasiparticles even at temperatures of {˜}Tsb{c}/1000, a clear indication of an unconventional gap structure. We then proceed to discuss zinc doping studies in YBasb2Cusb3Osb{7-delta} and show that we find a universal residual linear term at T=0 of a magnitude very close in value to that predicted by recent theories. These results validate the approach of resonant impurity scattering in the high-Tsb{c}, and our excellent agreement with theory reinforces the view that the gap structure in YBasb2Cusb3Osb{7-delta} is of dsb{xsp2-ysp2} symmetry. Finally, we present neutron scattering results in UPtsb3. In this chapter

  5. Simultaneous constraint and phase conversion processing of oxide superconductors

    DOEpatents

    Li, Qi; Thompson, Elliott D.; Riley, Jr., Gilbert N.; Hellstrom, Eric E.; Larbalestier, David C.; DeMoranville, Kenneth L.; Parrell, Jeffrey A.; Reeves, Jodi L.

    2003-04-29

    A method of making an oxide superconductor article includes subjecting an oxide superconductor precursor to a texturing operation to orient grains of the oxide superconductor precursor to obtain a highly textured precursor; and converting the textured oxide superconducting precursor into an oxide superconductor, while simultaneously applying a force to the precursor which at least matches the expansion force experienced by the precursor during phase conversion to the oxide superconductor. The density and the degree of texture of the oxide superconductor precursor are retained during phase conversion. The constraining force may be applied isostatically.

  6. Electrical resistivity of composite superconductors

    NASA Technical Reports Server (NTRS)

    Davis, J. H.; Lee, J. A.

    1983-01-01

    In addition to its superconducting properties, a superconductor is usually characterized by poor thermal conductivity and relatively high electrical resistivity in the normal state. To remedy this situation a study of superconducting properties of Cu-rich CU-Nb wires prepared by directionally solidified and cold-rolled technique was conducted. Some of the specimens were prepared by melting, directional solidification and diffusing in Tin. A total of 12 wire specimens was tested. Each specimen was analyzed by plotting experimental data into the following curves: the graph of the residual resistivity as a function of the specimen current at 4.3 K; and the graph of the electrical resistivity as a function of the temperature at a constant current.

  7. Quantum rotor in nanostructured superconductors

    PubMed Central

    Lin, Shi-Hsin; Milošević, M. V.; Covaci, L.; Jankó, B.; Peeters, F. M.

    2014-01-01

    Despite its apparent simplicity, the idealized model of a particle constrained to move on a circle has intriguing dynamic properties and immediate experimental relevance. While a rotor is rather easy to set up classically, the quantum regime is harder to realize and investigate. Here we demonstrate that the quantum dynamics of quasiparticles in certain classes of nanostructured superconductors can be mapped onto a quantum rotor. Furthermore, we provide a straightforward experimental procedure to convert this nanoscale superconducting rotor into a regular or inverted quantum pendulum with tunable gravitational field, inertia, and drive. We detail how these novel states can be detected via scanning tunneling spectroscopy. The proposed experiments will provide insights into quantum dynamics and quantum chaos. PMID:24686241

  8. Fluctuation phenomena in layered superconductors

    SciTech Connect

    Klemm, R.A.

    1996-10-01

    Gaussian fluctuations in layered superconductors have been the subject of study for many years. Although the FD was studied in detail long ago, the FC (fluctuation conductivity) was studied only recently, since the MT and DOS diagrams were previously neglected. Recent comparisons with experiment on YBCO have shown that the DOS diagrams are important and can lead to qualitatively different behaviors for the FC parallel and perpendicular to the layers. In both cases, Gaussian fluctuations fit the data above {Tc} very well, even for YBCO. To date, nearly all calculations of fluctuation quantities were for B{parallel}{cflx c}. Nevertheless, it should be possible to treat an arbitrary B, but the evaluation of the required matrix elements for the fluctuation quantities will be more complicated.

  9. Topological insulators and superconductors from string theory

    SciTech Connect

    Ryu, Shinsei; Takayanagi, Tadashi

    2010-10-15

    Topological insulators and superconductors in different spatial dimensions and with different discrete symmetries have been fully classified recently, revealing a periodic structure for the pattern of possible types of topological insulators and superconductors, both in terms of spatial dimensions and in terms of symmetry classes. It was proposed that K theory is behind the periodicity. On the other hand, D-branes, a solitonic object in string theory, are also known to be classified by K theory. In this paper, by inspecting low-energy effective field theories realized by two parallel D-branes, we establish a one-to-one correspondence between the K-theory classification of topological insulators/superconductors and D-brane charges. In addition, the string theory realization of topological insulators and superconductors comes naturally with gauge interactions, and the Wess-Zumino term of the D-branes gives rise to a gauge field theory of topological nature, such as ones with the Chern-Simons term or the {theta} term in various dimensions. This sheds light on topological insulators and superconductors beyond noninteracting systems, and the underlying topological field theory description thereof. In particular, our string theory realization includes the honeycomb lattice Kitaev model in two spatial dimensions, and its higher-dimensional extensions. Increasing the number of D-branes naturally leads to a realization of topological insulators and superconductors in terms of holography (AdS/CFT).

  10. Search for Majorana fermions in topological superconductors.

    SciTech Connect

    Pan, Wei; Shi, Xiaoyan; Hawkins, Samuel D.; Klem, John Frederick

    2014-10-01

    The goal of this project is to search for Majorana fermions (a new quantum particle) in a topological superconductor (a new quantum matter achieved in a topological insulator proximitized by an s-wave superconductor). Majorana fermions (MFs) are electron-like particles that are their own anti-particles. MFs are shown to obey non-Abelian statistics and, thus, can be harnessed to make a fault-resistant topological quantum computer. With the arrival of topological insulators, novel schemes to create MFs have been proposed in hybrid systems by combining a topological insulator with a conventional superconductor. In this LDRD project, we will follow the theoretical proposals to search for MFs in one-dimensional (1D) topological superconductors. 1D topological superconductor will be created inside of a quantum point contact (with the metal pinch-off gates made of conventional s-wave superconductors such as niobium) in a two-dimensional topological insulator (such as inverted type-II InAs/GaSb heterostructure).

  11. Superconductors

    DOEpatents

    Newkirk, Lawrence R.; Valencia, Flavio A.

    1977-02-01

    The structural quality of niobium germanide as a high-transition-temperature superconducting material is substantially improved by the presence of about 5 at. % oxygen. Niobium germanide having this oxygen content may readily be prepared as a bulk coating bonded to a metallic substrate by chemical vapor deposition techniques.

  12. Electron doping evolution of structural and antiferromagnetic phase transitions in NaFe1 -xCoxAs iron pnictides

    NASA Astrophysics Data System (ADS)

    Tan, Guotai; Song, Yu; Zhang, Chenglin; Lin, Lifang; Xu, Zhuang; Hou, Tingting; Tian, Wei; Cao, Huibo; Li, Shiliang; Feng, Shiping; Dai, Pengcheng

    2016-07-01

    We use transport and neutron diffraction to study the electronic phase diagram of NaFe1 -xCoxAs . In the undoped state, NaFeAs exhibits a tetragonal-to-orthorhombic structural transition below Ts followed by a collinear antiferromagnetic (AF) order below TN. Upon codoping to form NaFe1 -xCoxAs ,Ts and TN are gradually suppressed, leading to optimal superconductivity near Co-doping x =0.025 . While transport experiments on these materials reveal an anomalous behavior suggesting the presence of a quantum critical point (QCP) near optimal superconductivity, our neutron diffraction results indicate that commensurate AF order becomes transversely incommensurate with TN>Tc before vanishing abruptly at optimal superconductivity. These results are remarkably similar to electron-doping and isovalent-doping evolution of the AF order in BaFe2 -xNixAs2 and BaFe2(As1 -xPx)2 , thus suggesting a universal behavior in the suppression of the magnetic order in iron pnictides as superconductivity is induced.

  13. Magnetic lattice dynamics of the oxygen-free FeAs pnictides: how sensitive are phonons to magnetic ordering?

    PubMed

    Zbiri, Mohamed; Mittal, Ranjan; Rols, Stéphane; Su, Yixi; Xiao, Yinguo; Schober, Helmut; Chaplot, Samrath L; Johnson, Mark R; Chatterji, Tapan; Inoue, Yasunori; Matsuishi, Satoru; Hosono, Hideo; Brueckel, Thomas

    2010-08-11

    To shed light on the role of magnetism on the superconducting mechanism of the oxygen-free FeAs pnictides, we investigate the effect of magnetic ordering on phonon dynamics in the low-temperature orthorhombic parent compounds, which present a spin density wave. The study covers both the 122 (AFe(2)As(2); A = Ca, Sr, Ba) and 1111 (AFeAsF; A = Ca, Sr) phases. We extend our recent work on the Ca (122 and 1111) and Ba (122) cases by treating, computationally and experimentally, the 122 and 1111 Sr compounds. The effect of magnetic ordering is investigated through detailed non-magnetic and magnetic lattice dynamical calculations. The comparison of the experimental and calculated phonon spectra shows that the magnetic interactions/ordering have to be included in order to reproduce well the measured density of states. This highlights a spin-correlated phonon behavior which is more pronounced than the apparently weak electron-phonon coupling estimated in these materials. Furthermore, there is no noticeable difference between phonon spectra of the 122 Ba and Sr, whereas there are substantial differences when comparing these to CaFe(2)As(2) originating from different aspects of structure and bonding.

  14. Enhancement of the London penetration depth in pnictides at the onset of SDW order under superconducting dome

    NASA Astrophysics Data System (ADS)

    Levchenko, Alex; Vavilov, Maxim; Kuzmanovski, Dushko; Khodas, Maxim; Chubukov, Andrey

    2013-03-01

    Recent measurements of the doping dependence of the London penetration depth λ (x) in clean samples of isovalent BaFe2(As1-xPx)2 at T <pnictides is conceptually different from a one-component Galilean invariant Fermi liquid, for which correlation effects do not cause the renormalization of the London penetration depth at T = 0 .

  15. Performance of ceramic superconductors in magnetic bearings

    NASA Technical Reports Server (NTRS)

    Kirtley, James L., Jr.; Downer, James R.

    1993-01-01

    Magnetic bearings are large-scale applications of magnet technology, quite similar in certain ways to synchronous machinery. They require substantial flux density over relatively large volumes of space. Large flux density is required to have satisfactory force density. Satisfactory dynamic response requires that magnetic circuit permeances not be too large, implying large air gaps. Superconductors, which offer large magnetomotive forces and high flux density in low permeance circuits, appear to be desirable in these situations. Flux densities substantially in excess of those possible with iron can be produced, and no ferromagnetic material is required. Thus the inductance of active coils can be made low, indicating good dynamic response of the bearing system. The principal difficulty in using superconductors is, of course, the deep cryogenic temperatures at which they must operate. Because of the difficulties in working with liquid helium, the possibility of superconductors which can be operated in liquid nitrogen is thought to extend the number and range of applications of superconductivity. Critical temperatures of about 98 degrees Kelvin were demonstrated in a class of materials which are, in fact, ceramics. Quite a bit of public attention was attracted to these new materials. There is a difficulty with the ceramic superconducting materials which were developed to date. Current densities sufficient for use in large-scale applications have not been demonstrated. In order to be useful, superconductors must be capable of carrying substantial currents in the presence of large magnetic fields. The possible use of ceramic superconductors in magnetic bearings is investigated and discussed and requirements that must be achieved by superconductors operating at liquid nitrogen temperatures to make their use comparable with niobium-titanium superconductors operating at liquid helium temperatures are identified.

  16. Sealed glass coating of high temperature ceramic superconductors

    DOEpatents

    Wu, Weite; Chu, Cha Y.; Goretta, Kenneth C.; Routbort, Jules L.

    1995-01-01

    A method and article of manufacture of a lead oxide based glass coating on a high temperature superconductor. The method includes preparing a dispersion of glass powders in a solution, applying the dispersion to the superconductor, drying the dispersion before applying another coating and heating the glass powder dispersion at temperatures below oxygen diffusion onset and above the glass melting point to form a continuous glass coating on the superconductor to establish compressive stresses which enhance the fracture strength of the superconductor.

  17. Bernoulli effect and contact potential difference in superconductors

    SciTech Connect

    Omel'yanchuk, A.N.; Beloborod'ko, S.I.

    1983-10-01

    An expression is derived for the Bernoulli potential that arises in superconductors with an inhomogeneous current distribution. The expression is valid for arbitrary temperatures and superfluid velocities. In the superconductor--dielectric--superconductor system we consider the Bernoulli effect, which manifests itself in a contact potential difference between the superconductors. The potential difference is determined by the currents flowing through one plate of the contact and can be measured with a voltmeter in the quasi-stationary regime.

  18. Rotating superconductor magnet for producing rotating lobed magnetic field lines

    DOEpatents

    Hilal, Sadek K.; Sampson, William B.; Leonard, Edward F.

    1978-01-01

    This invention provides a rotating superconductor magnet for producing a rotating lobed magnetic field, comprising a cryostat; a superconducting magnet in the cryostat having a collar for producing a lobed magnetic field having oppositely directed adjacent field lines; rotatable support means for selectively rotating the superconductor magnet; and means for energizing the superconductor magnet.

  19. The color of polarization in cuprate superconductors

    NASA Technical Reports Server (NTRS)

    Hoff, H. A.; Osofsky, M. S.; Lechter, W. L.; Pande, C. S.

    1991-01-01

    A technique for the identification of individual anisotropic grains in a heterogeneous and opaque material involves the observation of grain color in reflected light through crossed polarizers (color of polarization). Such colors are generally characteristic of particular phases. When grains of many members of the class of hole carrier cuprate superconductors are so viewed at room temperature with a 'daylight' source, a characteristic color of polarization is observed. This color was studied in many of these cuprate superconductors and a strong correlation was found between color and the existence of superconductivity. Two members were also examined of the electron cuprate superconductors and it was found that they possess the same color of polarization as the hole carrier cuprate superconductors so far examined. The commonality of the characteristic color regardless of charge carrier indicates that the presence of this color is independent of carrier type. The correlation of this color with the existence of superconductivity in the cuprate superconductors suggests that the origin of the color relates to the origin of superconductivity. Photometric techniques are also discussed.

  20. Quantum interference in an interfacial superconductor

    NASA Astrophysics Data System (ADS)

    Goswami, Srijit; Mulazimoglu, Emre; Monteiro, Ana M. R. V. L.; Wölbing, Roman; Koelle, Dieter; Kleiner, Reinhold; Blanter, Ya. M.; Vandersypen, Lieven M. K.; Caviglia, Andrea D.

    2016-10-01

    The two-dimensional superconductor that forms at the interface between the complex oxides lanthanum aluminate (LAO) and strontium titanate (STO) has several intriguing properties that set it apart from conventional superconductors. Most notably, an electric field can be used to tune its critical temperature (Tc; ref. 7), revealing a dome-shaped phase diagram reminiscent of high-Tc superconductors. So far, experiments with oxide interfaces have measured quantities that probe only the magnitude of the superconducting order parameter and are not sensitive to its phase. Here, we perform phase-sensitive measurements by realizing the first superconducting quantum interference devices (SQUIDs) at the LAO/STO interface. Furthermore, we develop a new paradigm for the creation of superconducting circuit elements, where local gates enable the in situ creation and control of Josephson junctions. These gate-defined SQUIDs are unique in that the entire device is made from a single superconductor with purely electrostatic interfaces between the superconducting reservoir and the weak link. We complement our experiments with numerical simulations and show that the low superfluid density of this interfacial superconductor results in a large, gate-controllable kinetic inductance of the SQUID. Our observation of robust quantum interference opens up a new pathway to understanding the nature of superconductivity at oxide interfaces.

  1. Classification of Topological Insulators and Superconductors

    NASA Astrophysics Data System (ADS)

    Schnyder, Andreas P.; Ryu, Shinsei; Furusaki, Akira; Ludwig, Andreas W. W.

    2009-05-01

    An exhaustive classification scheme of topological insulators and superconductors is presented. The key property of topological insulators (superconductors) is the appearance of gapless degrees of freedom at the interface/boundary between a topologically trivial and a topologically non-trivial state. Our approach consists in reducing the problem of classifying topological insulators (superconductors) in d spatial dimensions to the problem of Anderson localization at a (d-1) dimensional boundary of the system. We find that in each spatial dimension there are precisely five distinct classes of topological insulators (superconductors). The different topological sectors within a given topological insulator (superconductor) can be labeled by an integer winding number or a Z2 quantity. One of the five topological insulators is the "quantum spin Hall" (or: Z2 topological) insulator in d = 2, and its generalization in d = 3 dimensions. For each dimension d, the five topological insulators correspond to a certain subset of five of the ten generic symmetry classes of Hamiltonians introduced more than a decade ago by Altland and Zirnbauer in the context of disordered systems (which generalizes the three well known "Wigner and Dyson" symmetry classes).

  2. Ternary CaCu{sub 4}P{sub 2}-type pnictides AAg{sub 4}Pn{sub 2} (A=Sr, Eu; Pn=As, Sb)

    SciTech Connect

    Stoyko, Stanislav S.; Khatun, Mansura; Scott Mullen, C.; Mar, Arthur

    2012-08-15

    Four ternary pnictides AAg{sub 4}Pn{sub 2} (A=Sr, Eu; Pn=As, Sb) were prepared by reactions of the elements at 850 Degree-Sign C and their crystal structures were determined from single-crystal X-ray diffraction studies. These silver-containing pnictides AAg{sub 4}Pn{sub 2} adopt the trigonal CaCu{sub 4}P{sub 2}-type structure (Pearson symbol hR21, space group R3-bar m, Z=3; a=4.5555(6) A, c=24.041(3) A for SrAg{sub 4}As{sub 2}; a=4.5352(2) A, c=23.7221(11) A for EuAg{sub 4}As{sub 2}; a=4.7404(4) A, c=25.029(2) A for SrAg{sub 4}Sb{sub 2}; a=4.7239(3) A, c=24.689(2) A for EuAg{sub 4}Sb{sub 2}), which can be derived from the trigonal CaAl{sub 2}Si{sub 2}-type structure of the isoelectronic zinc-containing pnictides AZn{sub 2}Pn{sub 2} by insertion of additional Ag atoms into trigonal planar sites within [M{sub 2}Pn{sub 2}]{sup 2-} slabs built up of edge-sharing tetrahedra. Band structure calculations on SrAg{sub 4}As{sub 2} and SrAg{sub 4}Sb{sub 2} revealed that these charge-balanced Zintl phases actually exhibit no gap at the Fermi level and are predicted to be semimetals. - Graphical abstract: SrAg{sub 4}As{sub 2} and related pnictides adopt a CaCu{sub 4}P{sub 2}-type structure in which additional Ag atoms enter trigonal planar sites within slabs built from edge-sharing tetrahedra. Highlights: Black-Right-Pointing-Pointer AAg{sub 4}Pn{sub 2} are the first Ag-containing members of the CaCu{sub 4}P{sub 2}-type structure. Black-Right-Pointing-Pointer Ag atoms are stuffed in trigonal planar sites within CaAl{sub 2}Si{sub 2}-type slabs. Black-Right-Pointing-Pointer Ag-Ag bonding develops through attractive d{sup 10}-d{sup 10} interactions.

  3. Optical devices based on dye-coated superconductor junctions: An example of a composite molecule-superconductor device

    SciTech Connect

    Zhao, J.; Jurbergs, D.; Yamazi, B.; McDevitt, J.T.

    1992-03-25

    High-temperature superconductors provide new opportunities as materials used in the construction of hybrid molecule-superconductor components. Here, the authors describe fabrication methods for and operation of optical sensors based on molecular dye-coated superconductor junctions. Devices prepared from yttrium barium cuprates and using octaethylporphyrin, phthalocyanine, and rhodamine 6G as dyes have been prepared. 9 refs., 1 fig.

  4. Bulk Superconductors in Mobile Application

    NASA Astrophysics Data System (ADS)

    Werfel, F. N.; Delor, U. Floegel-; Rothfeld, R.; Riedel, T.; Wippich, D.; Goebel, B.; Schirrmeister, P.

    We investigate and review concepts of multi - seeded REBCO bulk superconductors in mobile application. ATZ's compact HTS bulk magnets can trap routinely 1 T@77 K. Except of magnetization, flux creep and hysteresis, industrial - like properties as compactness, power density, and robustness are of major device interest if mobility and light-weight construction is in focus. For mobile application in levitated trains or demonstrator magnets we examine the performance of on-board cryogenics either by LN2 or cryo-cooler application. The mechanical, electric and thermodynamical requirements of compact vacuum cryostats for Maglev train operation were studied systematically. More than 30 units are manufactured and tested. The attractive load to weight ratio is more than 10 and favours group module device constructions up to 5 t load on permanent magnet (PM) track. A transportable and compact YBCO bulk magnet cooled with in-situ 4 Watt Stirling cryo-cooler for 50 - 80 K operation is investigated. Low cooling power and effective HTS cold mass drives the system construction to a minimum - thermal loss and light-weight design.

  5. Deviations from mean-field behavior in disordered nanoscale superconductor normal-metal superconductor arrays

    NASA Astrophysics Data System (ADS)

    Kouh, Taejoon; Valles, J. M.

    2003-04-01

    We have fabricated quasi-two-dimensional disordered arrays of nanoscale Pb grains coupled by an overlayer of Ag grains. Their temperature-dependent resistive transitions follow predictions for an array of mesoscopic superconductor normal-metal superconductor junctions. The decrease of their transition temperatures with Ag overlayer thickness systematically deviates from the Cooper limit theory of the proximity effect as the Pb grain size decreases. The deviations occur when the estimated number of Cooper pairs per grain is <1 and suggest the approach to a superconductor-to-metal transition.

  6. Theory of spin-fluctuation induced superconductivity in iron-based superconductors

    SciTech Connect

    Zhang, Junhua

    2011-01-01

    In this dissertation we focus on the investigation of the pairing mechanism in the recently discovered high-temperature superconductor, iron pnictides. Due to the proximity to magnetic instability of the system, we considered short-range spin fluctuations as the major mediating source to induce superconductivity. Our calculation supports the magnetic fluctuations as a strong candidate that drives Cooper-pair formation in this material. We find the corresponding order parameter to be of the so-called ss-wave type and show its evolution with temperature as well as the capability of supporting high transition temperature up to several tens of Kelvin. On the other hand, our itinerant model calculation shows pronounced spin correlation at the observed antiferromagnetic ordering wave vector, indicating the underlying electronic structure in favor of antiferromagnetic state. Therefore, the electronic degrees of freedom could participate both in the magnetic and in the superconducting properties. Our work shows that the interplay between magnetism and superconductivity plays an important role to the understanding of the rich physics in this material. The magnetic-excitation spectrum carries important information on the nature of magnetism and the characteristics of superconductivity. We analyze the spin excitation spectrum in the normal and superconducting states of iron pnictides in the magnetic scenario. As a consequence of the sign-reversed gap structure obtained in the above, a spin resonance mode appears below the superconducting transition temperature. The calculated resonance energy, scaled with the gap magnitude and the magnetic correlation length, agrees well with the inelastic neutron scattering (INS) measurements. More interestingly, we find a common feature of those short-range spin fluctuations that are capable of inducing a fully gapped ss state is the momentum anisotropy with elongated span along the direction transverse to the antiferromagnetic momentum

  7. Charge and spin transport in mesoscopic superconductors

    PubMed Central

    Wolf, M J; Hübler, F; Kolenda, S

    2014-01-01

    Summary Background: Non-equilibrium charge transport in superconductors has been investigated intensely in the 1970s and 1980s, mostly in the vicinity of the critical temperature. Much less attention has been paid to low temperatures and the role of the quasiparticle spin. Results: We report here on nonlocal transport in superconductor hybrid structures at very low temperatures. By comparing the nonlocal conductance obtained by using ferromagnetic and normal-metal detectors, we discriminate charge and spin degrees of freedom. We observe spin injection and long-range transport of pure, chargeless spin currents in the regime of large Zeeman splitting. We elucidate charge and spin transport by comparison to theoretical models. Conclusion: The observed long-range chargeless spin transport opens a new path to manipulate and utilize the quasiparticle spin in superconductor nanostructures. PMID:24605283

  8. Cavity quantum electrodynamics with mesoscopic topological superconductors

    NASA Astrophysics Data System (ADS)

    Dmytruk, Olesia; Trif, Mircea; Simon, Pascal

    2015-12-01

    We study one-dimensional p -wave superconductors capacitively coupled to a microwave stripline cavity. By probing the light exiting from the cavity, one can reveal the electronic susceptibility of the p -wave superconductor. We analyze two superconducting systems: the prototypical Kitaev chain and a topological semiconducting wire. For both systems, we show that the photonic measurements, via the electronic susceptibility, allow us to determine the topological phase-transition point, the emergence of the Majorana fermions, and the parity of their ground state. We show that all of these effects, which are absent in effective theories that take into account the coupling of light to Majorana fermions only, are due to the interplay between the Majorana fermions and the bulk states of the superconductors.

  9. Giant paramagnetic Meissner effect in multiband superconductors.

    PubMed

    da Silva, R M; Milošević, M V; Shanenko, A A; Peeters, F M; Aguiar, J Albino

    2015-01-01

    Superconductors, ideally diamagnetic when in the Meissner state, can also exhibit paramagnetic behavior due to trapped magnetic flux. In the absence of pinning such paramagnetic response is weak, and ceases with increasing sample thickness. Here we show that in multiband superconductors paramagnetic response can be observed even in slab geometries, and can be far larger than any previous estimate - even multiply larger than the diamagnetic Meissner response for the same applied magnetic field. We link the appearance of this giant paramagnetic response to the broad crossover between conventional Type-I and Type-II superconductors, where Abrikosov vortices interact non-monotonically and multibody effects become important, causing unique flux configurations and their locking in the presence of surfaces. PMID:26244936

  10. Practical superconductor development for electrical power applications

    SciTech Connect

    Goretta, K.C.

    1991-10-01

    Development of useful high-critical-temperature (high-{Tc}) superconductors requires synthesis of superconducting compounds; fabrication of wires, tapes, and films from these compounds; production of composite structures that incorporate stabilizers or insulators; and design and testing of efficient components. This report describes technical progress of research and development efforts aimed at producing superconducting components based on the Y-Ba-Cu, Bi-Sr-Ca-Cu, Bi-Pb-Sr-Ca-Cu, and Tl-Ba-Ca-Cu oxides systems. Topics discussed are synthesis and heat treatment of high-{Tc} superconductors, formation of monolithic and composite wires and tapes, superconductor/metal connectors, characterization of structures and superconducting and mechanical properties, and fabrication and properties of thin films. Collaborations with industry and academia are also documented. 10 figs.

  11. Anomalous Hall effect in Weyl superconductors

    NASA Astrophysics Data System (ADS)

    Bednik, G.; Zyuzin, A. A.; Burkov, A. A.

    2016-08-01

    We present a theory of the anomalous Hall effect in a topological Weyl superconductor with broken time reversal symmetry. Specifically, we consider a ferromagnetic Weyl metal with two Weyl nodes of opposite chirality near the Fermi energy. In the presence of inversion symmetry, such a metal experiences a weak-coupling Bardeen-Cooper-Schrieffer instability, with pairing of parity-related eigenstates. Due to the nonzero topological charge, carried by the Weyl nodes, such a superconductor is necessarily topologically nontrivial, with Majorana surface states coexisting with the Fermi arcs of the normal Weyl metal. We demonstrate that, surprisingly, the anomalous Hall conductivity of such a superconducting Weyl metal coincides with that of a nonsuperconducting one, under certain conditions, in spite of the nonconservation of charge in a superconductor. We relate this to the existence of an extra (nearly) conserved quantity in a Weyl metal, the chiral charge.

  12. Resolving thermoelectric “paradox” in superconductors

    PubMed Central

    Shelly, Connor D.; Matrozova, Ekaterina A.; Petrashov, Victor T.

    2016-01-01

    For almost a century, thermoelectricity in superconductors has been one of the most intriguing topics in physics. During its early stages in the 1920s, the mere existence of thermoelectric effects in superconductors was questioned. In 1944, it was demonstrated that the effects may occur in inhomogeneous superconductors. Theoretical breakthrough followed in the 1970s, when the generation of a measurable thermoelectric magnetic flux in superconducting loops was predicted; however, a major crisis developed when experiments showed puzzling discrepancies with the theory. Moreover, different experiments were inconsistent with each other. This led to a stalemate in bringing theory and experiment into agreement. With this work, we resolve this stalemate, thus solving this long-standing “paradox,” and open prospects for exploration of novel thermoelectric phenomena predicted recently. PMID:26933688

  13. Fracture toughness for copper oxide superconductors

    DOEpatents

    Goretta, Kenneth C.; Kullberg, Marc L.

    1993-01-01

    An oxide-based strengthening and toughening agent, such as tetragonal Zro.sub.2 particles, has been added to copper oxide superconductors, such as superconducting YBa.sub.2 Cu.sub.3 O.sub.x (123) to improve its fracture toughness (K.sub.IC). A sol-gel coating which is non-reactive with the superconductor, such as Y.sub.2 BaCuO.sub.5 (211) on the ZrO.sub.2 particles minimized the deleterious reactions between the superconductor and the toughening agent dispersed therethrough. Addition of 20 mole percent ZrO.sub.2 coated with 211 yielded a 123 composite with a K.sub.IC of 4.5 MPa(m).sup.0.5.

  14. Shock compaction of high- Tc superconductors

    SciTech Connect

    Weir, S.T.; Nellis, W.J.; McCandless, P.C.; Brocious, W.F. ); Seaman, C.L.; Early, E.A.; Maple, M.B. . Dept. of Physics); Kramer, M.J. ); Syono, Y.; Kikuchi, M. )

    1990-09-01

    We present the results of shock compaction experiments on high-{Tc} superconductors and describe the way in which shock consolidation addresses critical problems concerning the fabrication of high J{sub c} bulk superconductors. In particular, shock compaction experiments on YBa{sub 2}Cu{sub 3}O{sub 7} show that shock-induced defects can greatly increase intragranular critical current densities. The fabrication of crystallographically aligned Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} samples by shock-compaction is also described. These experiments demonstrate the potential of the shock consolidation method as a means for fabricating bulk high-{Tc} superconductors having high critical current densities.

  15. Giant paramagnetic Meissner effect in multiband superconductors

    PubMed Central

    da Silva, R. M.; Milošević, M. V.; Shanenko, A. A.; Peeters, F. M.; Aguiar, J. Albino

    2015-01-01

    Superconductors, ideally diamagnetic when in the Meissner state, can also exhibit paramagnetic behavior due to trapped magnetic flux. In the absence of pinning such paramagnetic response is weak, and ceases with increasing sample thickness. Here we show that in multiband superconductors paramagnetic response can be observed even in slab geometries, and can be far larger than any previous estimate - even multiply larger than the diamagnetic Meissner response for the same applied magnetic field. We link the appearance of this giant paramagnetic response to the broad crossover between conventional Type-I and Type-II superconductors, where Abrikosov vortices interact non-monotonically and multibody effects become important, causing unique flux configurations and their locking in the presence of surfaces. PMID:26244936

  16. Fracture toughness for copper oxide superconductors

    DOEpatents

    Goretta, K.C.; Kullberg, M.L.

    1993-04-13

    An oxide-based strengthening and toughening agent, such as tetragonal ZrO[sub 2] particles, has been added to copper oxide superconductors, such as superconducting YBa[sub 2]Cu[sub 3]O[sub x] (123) to improve its fracture toughness (K[sub IC]). A sol-gel coating which is non-reactive with the superconductor, such as Y[sub 2]BaCuO[sub 5] (211) on the ZrO[sub 2] particles minimized the deleterious reactions between the superconductor and the toughening agent dispersed therethrough. Addition of 20 mole percent ZrO[sub 2] coated with 211 yielded a 123 composite with a K[sub IC] of 4.5 MPa(m)[sup 0.5].

  17. High-temperature superconductor antenna investigations

    NASA Technical Reports Server (NTRS)

    Karasack, Vincent G.

    1990-01-01

    The use of superconductors to increase antenna radiation efficiency and gain is examined. Although the gain of all normal-metal antennas can be increased through the use of superconductors, some structures have greater potential for practical improvement than others. Some structures suffer a great degradation in bandwidth when replaced with superconductors, while for others the improvement in efficiency is trivial due to the minimal contribution of the conductor loss mechanism to the total losses, or the already high efficiency of the structure. The following antennas and related structures are discussed: electrically small antennas, impedance matching of antennas, microstrip antennas, microwave and millimeter-wave antenna arrays, and superdirective arrays. The greatest potential practical improvements occur for large microwave and millimeter-wave arrays and the impedance matching of antennas.

  18. Transverse acousto-electric effect in superconductors

    NASA Astrophysics Data System (ADS)

    Lipavský, P.; Koláček, J.; Lin, P.-J.

    2016-06-01

    We formulate a theory based on the time-dependent Ginzburg-Landau (TDGL) theory and Newtonian vortex dynamics to study the transverse acousto-electric response of a type-II superconductor with Abrikosov vortex lattice. When exposed to a transverse acoustic wave, Cooper pairs emerge from the moving atomic lattice and moving electrons. As in the Tolman-Stewart effect in a normal metal, an electromagnetic field is radiated from the superconductor. We adapt the equilibrium-based TDGL theory to this non-equilibrium system by using a floating condensation kernel. Due to the interaction between normal and superconducting components, the radiated electric field as a function of magnetic field attains a maximum value occurring below the upper critical magnetic field. This local increase in electric field has weak temperature dependence and is suppressed by the presence of impurities in the superconductor.

  19. Practical superconductor development for electrical power applications

    SciTech Connect

    Goretta, K.C.

    1992-10-01

    Development of useful high-critical-temperature (high-[Tc]) superconductors requires synthesis of superconducting compounds; fabrication of wires, tapes, and films from these compounds; production of composite structures that incorporate stabilizers or insulators; and design and testing of efficient components. This report describes the technical progress of research and development efforts aimed at producing superconducting components that are based on the Y-Ba-Cu, Bi-Sr-Ca-Cu, Bi-Pb-Sr-Ca-Cu, and (TI,Pb)-(Ba,Sr)-Ca-Cu oxide systems. Topics discussed are synthesis and heat treatment of high-[Tc] superconductors, formation of monolithic and composite wires and tapes, superconductor/metal connectors, characterization of structures and superconducting and mechanical properties, fabrication and properties of thin films, and development of prototype components. Collaborations with industry and academia are documented.

  20. Thermomagnetic phenomena in the mixed state of high temperature superconductors

    NASA Technical Reports Server (NTRS)

    Meilikhov, E. Z.

    1995-01-01

    Galvano- and thermomagnetic-phenomena in high temperature superconductors, based on kinetic coefficients, are discussed, along with a connection between the electric field and the heat flow in superconductor mixed state. The relationship that determines the transport coefficients of high temperature superconductors in the mixed state based on Seebeck and Nernst effects is developed. It is shown that this relationship is true for a whole transition region of the resistive mixed state of a superconductor. Peltier, Ettingshausen and Righi-Leduc effects associated with heat conductivity as related to high temperature superconductors are also addressed.

  1. Surface texturing of superconductors by controlled oxygen pressure

    DOEpatents

    Chen, N.; Goretta, K.C.; Dorris, S.E.

    1999-01-05

    A method of manufacture of a textured layer of a high temperature superconductor on a substrate is disclosed. The method involves providing an untextured high temperature superconductor material having a characteristic ambient pressure peritectic melting point, heating the superconductor to a temperature below the peritectic temperature, establishing a reduced pO{sub 2} atmosphere below ambient pressure causing reduction of the peritectic melting point to a reduced temperature which causes melting from an exposed surface of the superconductor and raising pressure of the reduced pO{sub 2} atmosphere to cause solidification of the molten superconductor in a textured surface layer. 8 figs.

  2. Topological state engineering by potential impurities on chiral superconductors

    NASA Astrophysics Data System (ADS)

    Kaladzhyan, Vardan; Röntynen, Joel; Simon, Pascal; Ojanen, Teemu

    2016-08-01

    In this work we consider the influence of potential impurities deposited on top of two-dimensional chiral superconductors. As discovered recently, magnetic impurity lattices on an s -wave superconductor may give rise to a rich topological phase diagram. We show that a similar mechanism takes place in chiral superconductors decorated by nonmagnetic impurities, thus avoiding the delicate issue of magnetic ordering of adatoms. We illustrate the method by presenting the theory of potential impurity lattices embedded on chiral p -wave superconductors. While a prerequisite for the topological state engineering is a chiral superconductor, the proposed procedure results in vistas of nontrivial descendant phases with different Chern numbers.

  3. Surface texturing of superconductors by controlled oxygen pressure

    DOEpatents

    Chen, Nan; Goretta, Kenneth C.; Dorris, Stephen E.

    1999-01-01

    A method of manufacture of a textured layer of a high temperature superconductor on a substrate. The method involves providing an untextured high temperature superconductor material having a characteristic ambient pressure peritectic melting point, heating the superconductor to a temperature below the peritectic temperature, establishing a reduced pO.sub.2 atmosphere below ambient pressure causing reduction of the peritectic melting point to a reduced temperature which causes melting from an exposed surface of the superconductor and raising pressure of the reduced pO.sub.2 atmosphere to cause solidification of the molten superconductor in a textured surface layer.

  4. Aluminum-stabilized NB3SN superconductor

    DOEpatents

    Scanlan, Ronald M.

    1988-01-01

    An aluminum-stabilized Nb.sub.3 Sn superconductor and process for producing same, utilizing ultrapure aluminum. Ductile components are co-drawn with aluminum to produce a conductor suitable for winding magnets. After winding, the conductor is heated to convert it to the brittle Nb.sub.3 Sn superconductor phase, using a temperature high enough to perform the transformation but still below the melting point of the aluminum. This results in reaction of substantially all of the niobium, while providing stabilization and react-in-place features which are beneficial in the fabrication of magnets utilizing superconducting materials.

  5. New superconductor stands up to magnetic fields

    SciTech Connect

    Service, R.F.

    1995-05-05

    For high-temperature superconductors (HTS), magnetic fields have been the equivalent of kryptonite. HTS materials are capable of carrying huge electrical currents without resistance, but when they are put in powerful magnetic fields their current-carrying ability plummets. At a Materials Research Society meeting, researchers from Los Alamos National Laboratory reported making a flexible superconducting tape that stands up to high magnetic fields at 77K. However, it is not clear it will stand up to industrial levels. This article discusses this and other research from Oak Ridge, as yet unpublished, in this area of superconductors.

  6. Network models for 2D disordered superconductors

    NASA Astrophysics Data System (ADS)

    Kagalovsky, Victor; Horovitz, Baruch; Avishai, Yshai

    2004-04-01

    We study new random matrix symmetry classes which arise in models of non-interacting quasi-particles in disordered superconductors. Within the Altland-Zirnbauer classification scheme these are class C (with time-reversal symmetry broken and spin-rotaion invariance intact), and class D where both symmetries are broken. Preliminary studies of the two remaining symmetry classes CI and DIII are briefly mentioned. New results are presented pertaining to the 3d realization of class C, which, physically, corresponds to a layered superconductor.

  7. Passivation of thin film oxide superconductors

    SciTech Connect

    Josefowicz, J.Y.; Rensch, D.B.; Nieh, K.W.

    1992-05-19

    This patent describes a passivation coating for oxide superconductors. It comprises a first layer of a Group II oxide encapsulating the oxide superconductor, the Group II having a substantially amorphous structure and having a thickness ranging from about 500 {Angstrom} to 2 {mu}m; and a second layer of a polymer covering the Group II oxide, the polymer comprising a composition selected from the group consisting of polyimide, polybenzyl methacrylate, polybutyl methacrylate, polybutyl styrene, polybutadiene, styrenes, polyamide resins, polyacrylics, polyacrylamides, polystyrenes, polyethylene, polyisoprene, polymethyl pentenes, polymethyl methacrylates, and polyvinyls.

  8. Building blocks for correlated superconductors and magnets

    NASA Astrophysics Data System (ADS)

    Sarrao, J. L.; Ronning, F.; Bauer, E. D.; Batista, C. D.; Zhu, J.-X.; Thompson, J. D.

    2015-04-01

    Recent efforts at Los Alamos to discover strongly correlated superconductors and hard ferromagnets are reviewed. While serendipity remains a principal engine of materials discovery, design principles and structural building blocks are beginning to emerge that hold potential for predictive discovery. Successes over the last decade with the so-called "115" strongly correlated superconductors are summarized, and more recent efforts to translate these insights and principles to novel hard magnets are discussed. While true "materials by design" remains a distant aspiration, progress is being made in coupling empirical design principles to electronic structure simulation to accelerate and guide materials design and synthesis.

  9. High temperature crystalline superconductors from crystallized glasses

    DOEpatents

    Shi, Donglu

    1992-01-01

    A method of preparing a high temperature superconductor from an amorphous phase. The method involves preparing a starting material of a composition of Bi.sub.2 Sr.sub.2 Ca.sub.3 Cu.sub.4 Ox or Bi.sub.2 Sr.sub.2 Ca.sub.4 Cu.sub.5 Ox, forming an amorphous phase of the composition and heat treating the amorphous phase for particular time and temperature ranges to achieve a single phase high temperature superconductor.

  10. Electrical connection structure for a superconductor element

    SciTech Connect

    Lallouet, Nicolas; Maguire, James

    2010-05-04

    The invention relates to an electrical connection structure for a superconductor element cooled by a cryogenic fluid and connected to an electrical bushing, which bushing passes successively through an enclosure at an intermediate temperature between ambient temperature and the temperature of the cryogenic fluid, and an enclosure at ambient temperature, said bushing projecting outside the ambient temperature enclosure. According to the invention, said intermediate enclosure is filled at least in part with a solid material of low thermal conductivity, such as a polyurethane foam or a cellular glass foam. The invention is applicable to connecting a superconductor cable at cryogenic temperature to a device for equipment at ambient temperature.

  11. Building blocks for correlated superconductors and magnets

    SciTech Connect

    Sarrao, J. L.; Ronning, F.; Bauer, E. D.; Batista, C. D.; Zhu, J. -X.; Thompson, J. D.

    2015-04-01

    Recent efforts at Los Alamos to discover strongly correlated superconductors and hard ferromagnets are reviewed. While serendipity remains a principal engine of materials discovery, design principles and structural building blocks are beginning to emerge that hold potential for predictive discovery. Successes over the last decade with the so-called “115” strongly correlated superconductors are summarized, and more recent efforts to translate these insights and principles to novel hard magnets are discussed. While true “materials by design” remains a distant aspiration, progress is being made in coupling empirical design principles to electronic structure simulation to accelerate and guide materials design and synthesis.

  12. Giant supercurrent states in a superconductor-InAs/GaSb-superconductor junction

    SciTech Connect

    Shi, Xiaoyan Pan, W.; Hawkins, S. D.; Klem, J. F.; Yu, Wenlong; Jiang, Zhigang; Andrei Bernevig, B.

    2015-10-07

    Superconductivity in topological materials has attracted a great deal of interest in both electron physics and material sciences since the theoretical predictions that Majorana fermions can be realized in topological superconductors. Topological superconductivity could be realized in a type II, band-inverted, InAs/GaSb quantum well if it is in proximity to a conventional superconductor. Here, we report observations of the proximity effect induced giant supercurrent states in an InAs/GaSb bilayer system that is sandwiched between two superconducting tantalum electrodes to form a superconductor-InAs/GaSb-superconductor junction. Electron transport results show that the supercurrent states can be preserved in a surprisingly large temperature-magnetic field (T – H) parameter space. In addition, the evolution of differential resistance in T and H reveals an interesting superconducting gap structure.

  13. Josephson effect in low-capacitance superconductor--normal-metal--superconductor systems

    SciTech Connect

    Bauernschmitt, R.; Siewert, J.; Nazarov, Y.V.; Odintsov, A.A. )

    1994-02-01

    The transport properties of a small superconductor--normal-metal--superconducting tunnel junction can be controlled by a gate electrode coupled capacitively to the central island. We evaluate the critical Josephson current [ital I][sub [ital c

  14. Dynamics of interfaces in superconductors

    SciTech Connect

    Dorsey, A.T. )

    1994-08-01

    The dynamics of an interface between the normal and superconducting phases under nonstationary external conditions is studied within the framework of the time-dependent Ginzburg-Landau equations of superconductivity, modified to include thermal fluctuations. An equation of motion for the interface is derived in two steps. First, the method of matched asymptotic expansions is used to derive a diffusion equation for the magnetic field in the normal phase, with nonlinear boundary conditions at the interface. These boundary conditions are a continuity equation which relates the gradient of the field at the interface to the normal velocity of the interface and a modified Gibbs-Thomson boundary condition for the field at the interface. Second, the boundary integral method is used to integrate out the magnetic field in favor of an equation of motion for the interface. This equation of motion, which is highly nonlinear and nonlocal, exhibits a diffusive instability (the Mullins-Sekerka instability) when the superconducting phase expands into the normal phase (i.e., when the external field is reduced below the critical field). In the limit of infinite diffusion constant the equation of motion becomes local in time and can be derived variationally from a static energy functional which includes the bulk-free energy difference between the two phases, the interfacial energy, and a long range self-interaction of the interface of the Biot-Savart form. In this limit the dynamics is identical to the interfacial dynamics of ferrofluid domains recently proposed by S.A. Langer et al. As shown by these authors, the Biot-Savart interaction leads to mechanical instabilities of the interface, resulting in highly branched labyrinthine patterns. The application of these ideas to the study of labyrinthine patterns in the intermediate state of type-I superconductors is briefly discussed. 29 refs.

  15. Microgravity Processing of Oxide Superconductors

    NASA Technical Reports Server (NTRS)

    Olive, James R.; Hofmeister, William H.; Bayuzick, Robert J.; Vlasse, Marcus

    1999-01-01

    Considerable effort has been concentrated on the synthesis and characterization of high T(sub c) oxide superconducting materials. The YBaCuO system has received the most intense study, as this material has shown promise for the application of both thin film and bulk materials. There are many problems with the application of bulk materials- weak links, poor connectivity, small coherence length, oxygen content and control, environmental reactivity, phase stability, incongruent melting behavior, grain boundary contamination, brittle mechanical behavior, and flux creep. The extent to which these problems are intrinsic or associated with processing is the subject of controversy. This study seeks to understand solidification processing of these materials, and to use this knowledge for alternative processing strategies, which, at the very least, will improve the understanding of bulk material properties and deficiencies. In general, the phase diagram studies of the YBaCuO system have concentrated on solid state reactions and on the Y2BaCuO(x) + liquid yields YBa2Cu3O(7-delta) peritectic reaction. Little information is available on the complete melting relations, undercooling, and solidification behavior of these materials. In addition, rare earth substitutions such as Nd and Gd affect the liquidus and phase relations. These materials have promising applications, but lack of information on the high temperature phase relations has hampered research. In general, the understanding of undercooling and solidification of high temperature oxide systems lags behind the science of these phenomena in metallic systems. Therefore, this research investigates the fundamental melting relations, undercooling, and solidification behavior of oxide superconductors with an emphasis on improving ground based synthesis of these materials.

  16. Recrystallization of high temperature superconductors

    SciTech Connect

    Kouzoudis, D.

    1996-05-09

    Currently one of the most widely used high {Tc} superconductors is the Bi-based compounds Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub z} and Bi{sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub z} (known as BSCCO 2212 and 2223 compounds) with {Tc} values of about 85 K and 110 K respectively. Lengths of high performance conductors ranging from 100 to 1000 m long are routinely fabricated and some test magnets have been wound. An additional difficulty here is that although Bi-2212 and Bi-2223 phases exist over a wide range of stoichiometries, neither has been prepared in phase-pure form. So far the most successful method of constructing reliable and robust wires or tapes is the so called powder-in-tube (PIT) technique [1, 2, 3, 4, 5, 6, 7] in which oxide powder of the appropriate stoichiometry and phase content is placed inside a metal tube, deformed into the desired geometry (round wire or flat tape), and annealed to produce the desired superconducting properties. Intermediate anneals are often incorporated between successive deformation steps. Silver is the metal used in this process because it is the most compatible with the reacting phase. In all of the commercial processes for BSCCO, Ag seems to play a special catalytic role promoting the growth of high performance aligned grains that grow in the first few micrometers near the Ag/BSCCO interface. Adjacent to the Ag, the grain alignment is more perfect and the current density is higher than in the center of the tape. It is known that Ag lowers the melting point of several of the phases but the detailed mechanism for growth of these high performance grains is not clearly understood. The purpose of this work is to study the nucleation and growth of the high performance material at this interface.

  17. Disorder-Driven Superconductor-Insulator Transition in d-Wave Superconductors

    NASA Astrophysics Data System (ADS)

    Song, Yun; He, Long

    2014-03-01

    We study the superconductor-insulator transition (SIT) in d-wave superconductors. By means of the kernel polynomial method, the Bogoliubov-de Gennes equations are solved self-consistently, making it possible to observe fully the nanoscale spatial fluctuations of the superconducting order parameters. It is shown that Anderson localization can not entirely inhibit the occurrence of the local superconductivity in strongly-disordered d-wave superconductors. Separated by an insulating ``sea'' completely, a few isolated superconducting ``islands'' with significant enhancement of the local superconducting order parameters can survive across the SIT. The disorder-driven SIT, therefore, is a transition from a d-wave superconductor to a boson insulator which consists of localized Cooper pairs. Unlike an s-wave superconductor which presents a robust single-particle gap across the SIT, the optical conductivity of a d-wave superconductor reveals a gapless insulating phase, where the SIT can be detected by observing the disappearance of the Drude weight with the increasing disorder. The National Basic Research Program of China (Grant Nos. 2011CBA00108).

  18. Investigation of TiO{sub x} barriers for their use in hybrid Josephson and tunneling junctions based on pnictide thin films

    SciTech Connect

    Döring, S. Monecke, M.; Schmidt, S.; Schmidl, F.; Tympel, V.; Seidel, P.; Engelmann, J.; Kurth, F.; Iida, K.; Holzapfel, B.; Haindl, S.; Mönch, I.

    2014-02-28

    We tested oxidized titanium layers as barriers for hybrid Josephson junctions with high I{sub c}R{sub n}-products and for the preparation of junctions for tunneling spectroscopy. For that we firstly prepared junctions with conventional superconductor electrodes, such as lead and niobium, respectively. By tuning the barrier thickness, we were able to change the junction's behavior from a Josephson junction to tunnel-like behavior applicable for quasi-particle spectroscopy. Subsequently, we transferred the technology to junctions using Co-doped BaFe{sub 2}As{sub 2} thin films prepared by pulsed laser deposition as base electrode and evaporated Pb as counter electrode. For barriers with a thickness of 1.5 nm, we observe clear Josephson effects with I{sub c}R{sub n}≈90 μV at 4.2 K. These junctions behave SNS'-like (SNS: superconductor-normal conductor-superconductor) and are dominated by Andreev reflection transport mechanism. For junctions with barrier thickness of 2.0 nm and higher, no Josephson but SIS'- (SIS: superconductor-insulator-superconductor) or SINS'-like (SINS: superconductor-normal conductor-insulator-superconductor) behavior with a tunnel-like conductance spectrum was observed.

  19. Stripe phases in high-temperature superconductors

    PubMed Central

    Emery, V. J.; Kivelson, S. A.; Tranquada, J. M.

    1999-01-01

    Stripe phases are predicted and observed to occur in a class of strongly correlated materials describable as doped antiferromagnets, of which the copper-oxide superconductors are the most prominent representatives. The existence of stripe correlations necessitates the development of new principles for describing charge transport and especially superconductivity in these materials. PMID:10430848

  20. Enhancing critical current density of cuprate superconductors

    DOEpatents

    Chaudhari, Praveen

    2015-06-16

    The present invention concerns the enhancement of critical current densities in cuprate superconductors. Such enhancement of critical current densities include using wave function symmetry and restricting movement of Abrikosov (A) vortices, Josephson (J) vortices, or Abrikosov-Josephson (A-J) vortices by using the half integer vortices associated with d-wave symmetry present in the grain boundary.

  1. Monopoles and fractional vortices in chiral superconductors

    PubMed Central

    Volovik, G. E.

    2000-01-01

    I discuss two exotic objects that must be experimentally identified in chiral superfluids and superconductors. These are (i) the vortex with a fractional quantum number (N = 1/2 in chiral superfluids, and N = 1/2 and N = 1/4 in chiral superconductors), which plays the part of the Alice string in relativistic theories and (ii) the hedgehog in the ^l field, which is the counterpart of the Dirac magnetic monopole. These objects of different dimensions are topologically connected. They form the combined object that is called a nexus in relativistic theories. In chiral superconductors, the nexus has magnetic charge emanating radially from the hedgehog, whereas the half-quantum vortices play the part of the Dirac string. Each half-quantum vortex supplies the fractional magnetic flux to the hedgehog, representing 1/4 of the “conventional” Dirac string. I discuss the topological interaction of the superconductor's nexus with the ‘t Hooft–Polyakov magnetic monopole, which can exist in Grand Unified Theories. The monopole and the hedgehog with the same magnetic charge are topologically confined by a piece of the Abrikosov vortex. Such confinement makes the nexus a natural trap for the magnetic monopole. Other properties of half-quantum vortices and monopoles are discussed as well, including fermion zero modes. PMID:10716980

  2. Noncontact Measurement Of Critical Current In Superconductor

    NASA Technical Reports Server (NTRS)

    Israelsson, Ulf E.; Strayer, Donald M.

    1992-01-01

    Critical current measured indirectly via flux-compression technique. Magnetic flux compressed into gap between superconductive hollow cylinder and superconductive rod when rod inserted in hole in cylinder. Hall-effect probe measures flux density before and after compression. Method does not involve any electrical contact with superconductor. Therefore, does not cause resistive heating and consequent premature loss of superconductivity.

  3. Study of the Multiorbital Hubbard Model for the Fe-Superconductors Beyond Weak Coupling

    NASA Astrophysics Data System (ADS)

    Dagotto, Elbio

    2012-02-01

    A variety of experimental and theoretical investigations indicate that the pnictides and chalcogenides are materials with on-site Hubbard repulsion intermediate between weak coupling, where simple nesting ideas apply, and strong coupling where the spins are localized. For this reason, it is desirable to broaden the range of couplings theoretically studied as well as the many-body models and techniques employed. In this talk, an extensive analysis of model Hamiltonians for the Fe-based superconductors is presented. The multiorbital Hubbard models with two, three, and five orbitals are studied, via the Hartree-Fock approximation and exact diagonalization techniques. The main topics to be discussed are: magnetic ordering tendencies [1], range of realistic Hubbard repulsion and Hund couplings [2], orbital-weight redistribution at the Fermi surface and comparison with photoemission data [3], low-temperature transport properties [4], and competing pairing channels [5]. The possible magnetic states of the √5x√5 Fe-vacancy arrangement will also be presented [6]. The experimental reports of local moments at room temperature leads to our most recent efforts employing a three-orbital spin-fermion model, analyzed via Monte Carlo simulations, to study the temperature dependence of the (anisotropic) conductance [7]. It is concluded that considerable progress has been made in the understanding of these materials in spite of their difficult range of intermediate couplings. However, the existence of several open problems will also be discussed.[4pt] [1] R. Yu et al., Phys. Rev. B 79, 104510 (2009); A. Moreo et al., Phys. Rev. B 79, 134502 (2009).[0pt] [2] Q. Luo et al., Phys. Rev. B 82, 104508 (2010); A. Nicholson et al., Phys. Rev. B 84, 094519 (2011).[0pt] [3] M. Daghofer et al., Phys. Rev. B 81, 180514(R) (2010).[0pt] [4] X. Zhang and E. Dagotto, Phys. Rev. B 84, 132505 (2011). See also Q. Luo et al., Phys. Rev. B 83, 174513 (2011).[0pt] [5] A. Nicholson et al., Phys. Rev

  4. Nambu-Goldstone-Leggett modes in multi-condensate superconductors

    NASA Astrophysics Data System (ADS)

    Yanagisawa, Takashi

    2015-12-01

    Multi-gap superconductors exhibit interesting properties. In an N-gap superconductor, we have in general U(1)N phase invariance. This multiple-phase invariance is partially or totally spontaneously broken in a superconductor. The Nambu-Goldstone modes, as well as Higgs modes, are important and will play an important role in multi-condensate superconductors. The additional phase invariance leads to a new quantum phase, with help of frustrated Josephson effects, such as the time-reversal symmetry breaking, the emergence of massless modes and fractionally quantized-flux vortices. There is a possibility that half-flux vortices exist in two-component superconductors in a magnetic field. The half-quantum flux vortex can be interpreted as a monopole, and two half-flux vortices form a bound state connected by a domain wall. There is an interesting analogy between quarks and fractionally quantized-flux vortices in superconductors.

  5. Sealed glass coating of high temperature ceramic superconductors

    DOEpatents

    Wu, W.; Chu, C.Y.; Goretta, K.C.; Routbort, J.L.

    1995-05-02

    A method and article of manufacture of a lead oxide based glass coating on a high temperature superconductor is disclosed. The method includes preparing a dispersion of glass powders in a solution, applying the dispersion to the superconductor, drying the dispersion before applying another coating and heating the glass powder dispersion at temperatures below oxygen diffusion onset and above the glass melting point to form a continuous glass coating on the superconductor to establish compressive stresses which enhance the fracture strength of the superconductor. 8 figs.

  6. Method and apparatus to trigger superconductors in current limiting devices

    DOEpatents

    Yuan, Xing; Hazelton, Drew Willard; Walker, Michael Stephen

    2004-10-26

    A method and apparatus for magnetically triggering a superconductor in a superconducting fault current limiter to transition from a superconducting state to a resistive state. The triggering is achieved by employing current-carrying trigger coil or foil on either or both the inner diameter and outer diameter of a superconductor. The current-carrying coil or foil generates a magnetic field with sufficient strength and the superconductor is disposed within essentially uniform magnetic field region. For superconductor in a tubular-configured form, an additional magnetic field can be generated by placing current-carrying wire or foil inside the tube and along the center axial line.

  7. Diffusive electronic transport in superconductor-semiconductor-superconductor junctions of Al or Nb on δ-doped GaAs

    NASA Astrophysics Data System (ADS)

    Kutchinsky, J.; Taboryski, R. J.; Clausen, T.; Sørensen, C. B.; Lindelof, P. E.; Hansen, J. Bindslev; Jacobsen, C. Schelde; Skov, J. L.

    1996-02-01

    We report measurements on planar superconductor-semiconductor-superconductor (S-Sm-S) junctions consisting of a n++ modulation doped conduction layer in MBE grown GaAs with superconducting contacts of Al or Nb. At distances between the two superconducting banks below ≈3.5μm we observe a coupling between the two superconductors, due to multiple Andreev reflections at the S-Sm interfaces.

  8. Parallel field penetration in a layered superconductor

    NASA Astrophysics Data System (ADS)

    Buzdin, A.; Feinberg, D.

    1992-05-01

    The Bean-Livingston entrance field Hs for vortex penetration at the surface is calculated for layered superconductors with Josephson interlayer coupling and field parallel to the layers. Two regimes must be distinguished: close to Tc, one can use the anisotropic London theory, so Hs is of the order of Hc, the thermodynamic critical field, and is the same as for a field normal to the layers. On the opposite, when the transverse coherence length ξ c is smaller than the interlayer distance d, Hs becomes smaller than Hc and is of the order of ( {ξ c}/{d})H c. Contrary to the entrance field for pure Josephson vortices (fluxons) in junctions, this field is still much larger than the first critical field Hc1∥. This behaviour is a consequence of the specific structure of the vortex core in a layered superconductor and results in a crossover from a linear to a square root temperature dependence as the temperature is lowered.

  9. Energy efficiency of adiabatic superconductor logic

    NASA Astrophysics Data System (ADS)

    Takeuchi, Naoki; Yamanashi, Yuki; Yoshikawa, Nobuyuki

    2015-01-01

    Adiabatic superconductor logic (ASL), including adiabatic quantum-flux-parametron (AQFP) logic, exhibits high energy efficiency because its bit energy can be decreased below the thermal energy through adiabatic switching operations. In the present paper, we present the general scaling laws of ASL and compare the energy efficiency of ASL with those of other energy-efficient logics. Also, we discuss the minimum energy-delay product (EDP) of ASL at finite temperature. Our study shows that there is a maximum temperature at which the EDP can reach the quantum limit given by ħ/2, which is dependent on the superconductor material and the Josephson junction quality, and that it is reasonable to operate ASL at cryogenic temperatures in order to achieve an EDP that approaches ħ/2.

  10. Iron-Based Superconductors as topological matter

    NASA Astrophysics Data System (ADS)

    Hu, Jiangping

    We show the existence of non-trivial topological properties in Iron-based superconductors. Several examples are provided, including (1) the single layer FeSe grown on SrTiO3 substrate, in which an topological insulator phase exists due to the band inversion at M point; (2) CaFeAs2, a staggered intercalation compound that integrates both quantum spin hall and superconductivity in which the nontrivial topology stems from the chain-like As layers away from FeAs layers; (3) the Fe(Te,Se) thin films in which the nontrivial Z2 topological invariance originates from the parity exchange at Γ point that is controlled by the Te(Se) height; (4 nontrivial topology that is driven by the nematic order in FeSe. These results lay ground for integrating high Tc superconductivity with topological properties to realize new emergent phenomena, such as majorana particles, in iron-based high temperature superconductors

  11. Topological properties in Iron-Based Superconductors

    NASA Astrophysics Data System (ADS)

    Hu, Jiangping; Hao, Ningning; Wu, X. X.

    2015-03-01

    We show the existence of non-trivial topological properties in Iron-based superconductors. Several examples are provided, including (1) the single layer FeSe grown on SrTiO3 substrate, in which an topological insulator phase exists due to the band inversion at M point; (2) CaFeAs2, a staggered intercalation compound that integrates both quantum spin hall and superconductivity in which the nontrivial topology stems from the chain-like As layers away from FeAs layers; (3) the Fe(Te,Se) thin films in which the nontrivial Z2 topological invariance originates from the parity exchange at ? point that is controlled by the Te(Se) height. These results lay ground for integrating high Tc superconductivity with topological properties to realize new emergent phenomena, such as majorana particles, in iron-based high temperature superconductors. The work is supported by NSFC and the Ministry of Science and Technology of China.

  12. A Fifth Force: Generalized through Superconductors

    NASA Technical Reports Server (NTRS)

    Robertson, Glen A.

    1999-01-01

    The connection between the Biefield-Brown Effect, the recent repeat of the 1902 Trouton-Noble (TN) experiments, and the gravity shielding experiments was explored. This connection is visualized through high capacitive electron concentrations. From this connection, a theory is proposed that connects mass energy to gravity and a fifth force. The theory called the Gravi-Atomic Energy theory presents two new terms: Gravi-atomic energy and quantum vacuum pressure (QVP). Gravi-atomic energy is defined as the radiated mass energy, which acts on vacuum energy to create a QVP about a mass, resulting in gravity and the fifth force. The QVP emission from a superconductor was discussed followed by the description of a test for QVP from a superconductor using a Cavendish balance.

  13. Vortex phase separation in mesoscopic superconductors

    PubMed Central

    Iaroshenko, O.; Rybalko, V.; Vinokur, V. M.; Berlyand, L.

    2013-01-01

    We demonstrate that in mesoscopic type II superconductors with the lateral size commensurate with London penetration depth, the ground state of vortices pinned by homogeneously distributed columnar defects can form a hierarchical nested domain structure. Each domain is characterized by an average number of vortices trapped at a single pinning site within a given domain. Our study marks a radical departure from the current understanding of the ground state in disordered macroscopic systems and provides an insight into the interplay between disorder, vortex-vortex interaction, and confinement within finite system size. The observed vortex phase segregation implies the existence of the soliton solution for the vortex density in the finite superconductors and establishes a new class of nonlinear systems that exhibit the soliton phenomenon.

  14. Persistent superconductor currents in holographic lattices.

    PubMed

    Iizuka, Norihiro; Ishibashi, Akihiro; Maeda, Kengo

    2014-07-01

    We consider a persistent superconductor current along the direction with no translational symmetry in a holographic gravity model. Incorporating a lattice structure into the model, we numerically construct novel solutions of hairy charged stationary black branes with momentum or rotation along the latticed direction. The lattice structure prevents the horizon from rotating, and the total momentum is only carried by matter fields outside the black brane horizon. This is consistent with the black hole rigidity theorem, and it suggests that in dual field theory with lattices, superconductor currents are made up of "composite" fields, rather than "fractionalized" degrees of freedom. We also show that our solutions are consistent with the superfluid hydrodynamics. PMID:25032917

  15. Method for preparing superconductors ceramic composition

    SciTech Connect

    Akinc, M.; Celikkaya, A.

    1990-03-06

    This patent describes a process of forming a superconductor ceramic of the formula: YBa{sub 2}Cu{sub 3}O{sub {ital x}} wherein x is from about 6.5 to bout 7.2. It comprises: heating a solid phase copper salt selected from the group of copper acetate and copper nitrate and solid phase barium hydroxide to a temperature high enough to form a meltphase; and adding to the melt a salt, hydroxide or oxide as a source of yttrium with stirring to provide a substantially homogeneous mixture, the quantities of each of said copper salt, barium hydroxide and yttrium source being sufficient to yield a Y:Ba:Cu ratio of about 1:2:3; and calcining the substantially homogeneous mixture of temperatures of from about 750{degrees} C. to about 1000{degrees} C. to form said superconductor ceramic.

  16. Symmetry and topology of noncentrosymmetric superconductors

    SciTech Connect

    Samokhin, K.V.

    2015-08-15

    We present a detailed analysis of the pairing symmetry and the order parameter topology in superconductors without centre of inversion. Strong spin–orbit coupling of electrons with the crystal lattice leads to a large splitting of the Bloch bands, which makes it necessary to use a multiband description of superconductivity. We identify stable superconducting states and derive the Bogoliubov–de Gennes Hamiltonian, which determines the spectrum of fermionic quasiparticles. To develop a topological classification of the superconducting states we introduce two different types of topological invariants, the Chern numbers and the Maurer–Cartan invariants, and apply them to three-dimensional noncentrosymmetric superconductors, both with and without time reversal symmetry breaking.

  17. Electrical bushing for a superconductor element

    DOEpatents

    Mirebeau, Pierre; Lallouet, Nicolas; Delplace, Sebastien; Lapierre, Regis

    2010-05-04

    The invention relates to an electrical bushing serving to make a connection at ambient temperature to a superconductor element situated in an enclosure at cryogenic temperature. The electrical bushing passes successively through an enclosure at intermediate temperature between ambient temperature and cryogenic temperature, and an enclosure at ambient temperature, and it comprises a central electrical conductor surrounded by an electrically insulating sheath. According to the invention, an electrically conductive screen connected to ground potential surrounds the insulating sheath over a section that extends from the end of the bushing that is in contact with the enclosure at cryogenic temperature at least as far as the junction between the enclosure at intermediate temperature and the enclosure at ambient temperature. The invention is more particularly applicable to making a connection to a superconductor cable.

  18. Method for fabrication of high temperature superconductors

    DOEpatents

    Balachandran, Uthamalingam; Ma, Beihai; Miller, Dean

    2009-07-14

    A layered article of manufacture and a method of manufacturing same is disclosed. A substrate has a biaxially textured MgO crystalline layer having the c-axes thereof inclined with respect to the plane of the substrate deposited thereon. A layer of one or more of YSZ or Y.sub.2O.sub.3 and then a layer of CeO.sub.2 is deposited on the MgO. A crystalline superconductor layer with the c-axes thereof normal to the plane of the substrate is deposited on the CeO.sub.2 layer. Deposition of the MgO layer on the substrate is by the inclined substrate deposition method developed at Argonne National Laboratory. Preferably, the MgO has the c-axes thereof inclined with respect to the normal to the substrate in the range of from about 10.degree. to about 40.degree. and YBCO superconductors are used.

  19. Magnetic chains on a triplet superconductor.

    PubMed

    Sacramento, P D

    2015-11-11

    The topological state of a two-dimensional triplet superconductor may be changed by an appropriate addition of magnetic impurities. A ferromagnetic magnetic chain at the surface of a superconductor with spin-orbit coupling may eliminate the edge states of a finite system giving rise to localized zero modes at the edges of the chain. The coexistence/competition between the two types of zero modes is considered. The reduction of the system to an effective 1d system gives partial information on the topological properties but the study of the two sets of zero modes requires a two-dimensional treatment. Increasing the impurity density from a magnetic chain to magnetic islands leads to a finite Chern number. At half-filling small concentrations are enough to induce chiral modes.

  20. Recent development on methods for superconductor joining

    SciTech Connect

    Blaugher, R.D.

    1982-01-01

    Blaugher reviews two recent joint development programs for superconductor joining. The force-cooled steel-enclosed westinghouse LCP Nb/sub 3/Sn conductor and the problem it presented of connecting lengths of full-size conductor and individual strands during wire manufacture were responsible for the first program, pursued both by Westinghouse and Airco, who focused on three main approaches: butt-resistance welding, lap joining, and cold welding. Results of the research into each of these approaches is given; butt-resistance welding was the method decided on. The second program was directed at the copper-stabilized Nb-Ti multifilamentary superconductor for the Westinghouse/EPRI 3000 MVA generator. Again, three main approaches were considered: butt-resistance, ultrasonic welding, and lap/mechanical joining. Experimentation with each approach led to the development of an ultrasonic welding method which offered high mechanical strength as well as acceptable electrical properties at low temperatures.

  1. Superconductor Digital Electronics: -- Current Status, Future Prospects

    NASA Astrophysics Data System (ADS)

    Mukhanov, Oleg

    2011-03-01

    Two major applications of superconductor electronics: communications and supercomputing will be presented. These areas hold a significant promise of a large impact on electronics state-of-the-art for the defense and commercial markets stemming from the fundamental advantages of superconductivity: simultaneous high speed and low power, lossless interconnect, natural quantization, and high sensitivity. The availability of relatively small cryocoolers lowered the foremost market barrier for cryogenically-cooled superconductor electronic systems. These fundamental advantages enabled a novel Digital-RF architecture - a disruptive technological approach changing wireless communications, radar, and surveillance system architectures dramatically. Practical results were achieved for Digital-RF systems in which wide-band, multi-band radio frequency signals are directly digitized and digital domain is expanded throughout the entire system. Digital-RF systems combine digital and mixed signal integrated circuits based on Rapid Single Flux Quantum (RSFQ) technology, superconductor analog filter circuits, and semiconductor post-processing circuits. The demonstrated cryocooled Digital-RF systems are the world's first and fastest directly digitizing receivers operating with live satellite signals, enabling multi-net data links, and performing signal acquisition from HF to L-band with 30 GHz clock frequencies. In supercomputing, superconductivity leads to the highest energy efficiencies per operation. Superconductor technology based on manipulation and ballistic transfer of magnetic flux quanta provides a superior low-power alternative to CMOS and other charge-transfer based device technologies. The fundamental energy consumption in SFQ circuits defined by flux quanta energy 2 x 10-19 J. Recently, a novel energy-efficient zero-static-power SFQ technology, eSFQ/ERSFQ was invented, which retains all advantages of standard RSFQ circuits: high-speed, dc power, internal memory. The

  2. High temperature superconductors applications in telecommunications

    NASA Technical Reports Server (NTRS)

    Kumar, A. Anil; Li, Jiang; Zhang, Ming Fang

    1995-01-01

    The purpose of this paper is twofold: (1) to discuss high temperature superconductors with specific reference to their employment in telecommunications applications; and (2) to discuss a few of the limitations of the normally employed two-fluid model. While the debate on the actual usage of high temperature superconductors in the design of electronic and telecommunications devices - obvious advantages versus practical difficulties - needs to be settled in the near future, it is of great interest to investigate the parameters and the assumptions that will be employed in such designs. This paper deals with the issue of providing the microwave design engineer with performance data for such superconducting waveguides. The values of conductivity and surface resistance, which are the primary determining factors of a waveguide performance, are computed based on the two-fluid model. A comparison between two models - a theoretical one in terms of microscopic parameters (termed Model A) and an experimental fit in terms of macroscopic parameters (termed Model B) - shows the limitations and the resulting ambiguities of the two-fluid model at high frequencies and at temperatures close to the transition temperature. The validity of the two-fluid model is then discussed. Our preliminary results show that the electrical transport description in the normal and superconducting phases as they are formulated in the two-fluid model needs to be modified to incorporate the new and special features of high temperature superconductors. Parameters describing the waveguide performance - conductivity, surface resistance and attenuation constant - will be computed. Potential applications in communications networks and large scale integrated circuits will be discussed. Some of the ongoing work will be reported. In particular, a brief proposal is made to investigate of the effects of electromagnetic interference and the concomitant notion of electromagnetic compatibility (EMI/EMC) of high T

  3. High temperature superconductors applications in telecommunications

    SciTech Connect

    Kumar, A.A.; Li, J.; Zhang, M.F.

    1994-12-31

    The purpose of this paper is twofold: to discuss high temperature superconductors with specific reference to their employment in telecommunications applications; and to discuss a few of the limitations of the normally employed two-fluid model. While the debate on the actual usage of high temperature superconductors in the design of electronic and telecommunications devices-obvious advantages versus practical difficulties-needs to be settled in the near future, it is of great interest to investigate the parameters and the assumptions that will be employed in such designs. This paper deals with the issue of providing the microwave design engineer with performance data for such superconducting waveguides. The values of conductivity and surface resistance, which are the primary determining factors of a waveguide performance, are computed based on the two-fluid model. A comparison between two models-a theoretical one in terms of microscopic parameters (termed Model A) and an experimental fit in terms of macroscopic parameters (termed Model B)-shows the limitations and the resulting ambiguities of the two-fluid model at high frequencies and at temperatures close to the transition temperature. The validity of the two-fluid model is then discussed. Our preliminary results show that the electrical transport description in the normal and superconducting phases as they are formulated in the two-fluid model needs to be modified to incorporate the new and special features of high temperature superconductors. Parameters describing the waveguide performance-conductivity, surface resistance and attenuation constant-will be computed. Potential applications in communications networks and large scale integrated circuits will be discussed. Some of the ongoing work will be reported. In particular, a brief proposal is made to investigate of the effects of electromagnetic interference and the concomitant notion of electromagnetic compatibility (EMI/EMC) of high T{sub c} superconductors.

  4. Flywheel energy storage with superconductor magnetic bearings

    DOEpatents

    Weinberger, Bernard R.; Lynds, Jr., Lahmer; Hull, John R.

    1993-01-01

    A flywheel having superconductor bearings has a lower drag to lift ratio that translates to an improvement of a factor of ten in the rotational decay rate. The lower drag results from the lower dissipation of melt-processed YBCO, improved uniformity of the permanent magnet portion of the bearings, operation in a different range of vacuum pressure from that taught by the art, and greater separation distance from the rotating members of conductive materials.

  5. Partial Meissner effect in superconductors with twins

    SciTech Connect

    Gurevich, A.L.; Mints, R.G. )

    1988-11-01

    Superconductivity of twinning planes, arising at critical temperature T/sub c/ exceeding the bulk point T/sub co/, may prove to be important for the understanding of the properties of high-temperature superconductors with well-developed twinning structure. This paper shows that regardless of a specific mechanism of superconductivity, twins at T/sub c/>T/sub co/ act as seeds for the growth of metastable superconducting domains whose order parameter phases differ by {pi}.

  6. Low angle resistivity anomaly in layered superconductors

    SciTech Connect

    Koshelev, A.E. |; Vinokur, V.M.

    1993-01-01

    The pinning effect of vortex lines by the layered structure (intrinsic pinning) on resistivity of high-T{sub c}, superconductors in the mixed state is investigated by means of perturbation theory. A sharp drop in the resistivity at small angles for which vortex lines are almost aligned with the ab-planes is shown to occur even in a high-temperature region where the pinning potential is reduced by thermal fluctuations.

  7. High temperature superconductors for computer interconnect applications

    SciTech Connect

    Nilsson, B.J.L.

    1994-12-31

    High temperature superconductors, because of their extremely low loss at high frequencies and their high current handling capability, have the potential for use in computer interconnect boards. They offer the potential advantages of high interconnect density, reduced interconnect delays, and higher data rate. Because silicon CMOS circuits dramatically improve in performance at low temperatures, cooled computers may become attractive in the future to capture both the improved interconnect and circuit benefits.

  8. Spray-Deposited Superconductor/Polymer Coatings

    NASA Technical Reports Server (NTRS)

    Wise, Stephanie A.; Tran, Sang Q.; Hooker, Matthew W.

    1993-01-01

    Coatings that exhibit the Meissner effect formed at relatively low temperature. High-temperature-superconductor/polymer coatings that exhibit Meissner effect deposited onto components in variety of shapes and materials. Simple, readily available equipment needed in coating process, mean coatings produced economically. Coatings used to keep magnetic fields away from electronic circuits in such cryogenic applications as magnetic resonance imaging and detection of infrared, and in magnetic suspensions to provide levitation and/or damping of vibrations.

  9. Semiconductor/High-Tc-Superconductor Hybrid ICs

    NASA Technical Reports Server (NTRS)

    Burns, Michael J.

    1995-01-01

    Hybrid integrated circuits (ICs) containing both Si-based semiconducting and YBa(2)Cu(3)O(7-x) superconducting circuit elements on sapphire substrates developed. Help to prevent diffusion of Cu from superconductors into semiconductors. These hybrid ICs combine superconducting and semiconducting features unavailable in superconducting or semiconducting circuitry alone. For example, complementary metal oxide/semiconductor (CMOS) readout and memory devices integrated with fast-switching Josephson-junction super-conducting logic devices and zero-resistance interconnections.

  10. The f-spin physics of rare-earth iron pnictides: influence of d-electron antiferromagnetic order on heavy fermion phase diagram

    SciTech Connect

    Zhu, Jian-xin; Dai, Jianhui; Si, Qimiao

    2009-01-01

    Some of the high {Tc} iron pnictides contain rare-earth elements, raising the question of how the existence and tunability of a d-electron antiferromagnetic order influences the heavy fermion behavior of the f-moments. With CeOFeP and CeOFeAs in mind as prototypes, we derive an extended Anderson lattice model appropriate for these quaternary systems. We show that the Kondo screening of the f-moments are efficiently suppressed by the d-electron ordering. We also argue that, inside the d-electron ordered state (as in CeOFeAs), the f-moments provide a rare realization of a quantum frustrated magnet with competing J{sub 1}-J{sub 2}-J{sub 3} interactions in an effective square lattice. Implications ofr the heavy fermion physics in broader contexts are also discussed.

  11. Effects of biaxial strain on stability and half-metallicity of Cr and Mn pnictides and chalcogenides in the zinc-blende structure

    NASA Astrophysics Data System (ADS)

    Miao, M. S.; Lambrecht, Walter R. L.

    2005-08-01

    The effects of biaxial strain, imposed by epitaxial growth conditions, on the half-metallicity properties of Cr and Mn pnictides and chalcogenides were investigated using local spin-density-functional calculations. The minority band gaps were found to decrease significantly under the biaxial strain, whereas the spin-flip gaps changed only slightly. The calculations show that under epitaxial conditions for any choice of substrate, CrSe, MnAs, MnSe, and MnTe cannot be half metallic; CrAs and CrTe are barely half metallic as their SF gap is close to zero; and only CrSb and MnSb remain distinct half metals with spin-flip gaps of 0.9 and 0.3eV , respectively.

  12. Avoided valence transition in a plutonium superconductor.

    PubMed

    Ramshaw, B J; Shekhter, Arkady; McDonald, Ross D; Betts, Jon B; Mitchell, J N; Tobash, P H; Mielke, C H; Bauer, E D; Migliori, Albert

    2015-03-17

    The d and f electrons in correlated metals are often neither fully localized around their host nuclei nor fully itinerant. This localized/itinerant duality underlies the correlated electronic states of the high-Tc cuprate superconductors and the heavy-fermion intermetallics and is nowhere more apparent than in the 5f valence electrons of plutonium. Here, we report the full set of symmetry-resolved elastic moduli of PuCoGa5--the highest Tc superconductor of the heavy fermions (Tc = 18.5 K)--and find that the bulk modulus softens anomalously over a wide range in temperature above Tc. The elastic symmetry channel in which this softening occurs is characteristic of a valence instability--therefore, we identify the elastic softening with fluctuations of the plutonium 5f mixed-valence state. These valence fluctuations disappear when the superconducting gap opens at Tc, suggesting that electrons near the Fermi surface play an essential role in the mixed-valence physics of this system and that PuCoGa5 avoids a valence transition by entering the superconducting state. The lack of magnetism in PuCoGa5 has made it difficult to reconcile with most other heavy-fermion superconductors, where superconductivity is generally believed to be mediated by magnetic fluctuations. Our observations suggest that valence fluctuations play a critical role in the unusually high Tc of PuCoGa5.

  13. Nanomaterials for superconductors from the energy prospective

    SciTech Connect

    Cantoni, Claudia; Goyal, Amit

    2010-01-01

    High Temperature Superconductors (HTS) wires or coated conductors (CC) are expected to revolutionize the transmission of electricity enabling the present electric grid to meet the world s growing energy needs. Although superconducting wires can carry 150 times more power than copper wires of the same cross section, further performance improvements are necessary for the superconducting technology to become cost-competitive. This objective can be achieved by introducing and controlling nano-sized defects and non-superconducting phases within the superconducting film s matrix. Such nanostructures, when carefully engineered, significantly increase the loss-free current sustained by the superconductor through a mechanism known as flux pinning. This chapter is a review of the various types of nanostructures that are artificially introduced in superconducting films to enhance the superconductor s performance. Different approaches, materials, and techniques are discussed and the most recent results in this field compared. The last section of this chapter discusses an additional example of nanotechnology employment in superconducting wires. This nanotechnology can be regarded as an atomic surface treatment designed to enable the right crystallographic orientation of the superconducting film deposited on the metal template.

  14. Topology of nonsymmorphic crystalline insulators and superconductors

    NASA Astrophysics Data System (ADS)

    Shiozaki, Ken; Sato, Masatoshi; Gomi, Kiyonori

    2016-05-01

    Topological classification in our previous paper [K. Shiozaki and M. Sato, Phys. Rev. B 90, 165114 (2014), 10.1103/PhysRevB.90.165114] is extended to nonsymmorphic crystalline insulators and superconductors. Using the twisted equivariant K theory, we complete the classification of topological crystalline insulators and superconductors in the presence of additional order-two nonsymmorphic space-group symmetries. The order-two nonsymmorphic space groups include half-lattice translation with Z2 flip, glide, twofold screw, and their magnetic space groups. We find that the topological periodic table shows modulo-2 periodicity in the number of flipped coordinates under the order-two nonsymmorphic space group. It is pointed out that the nonsymmorphic space groups allow Z2 topological phases even in the absence of time-reversal and/or particle-hole symmetries. Furthermore, the coexistence of the nonsymmorphic space group with time-reversal and/or particle-hole symmetries provides novel Z4 topological phases, which have not been realized in ordinary topological insulators and superconductors. We present model Hamiltonians of these new topological phases and analytic expressions of the Z2 and Z4 topological invariants. The half-lattice translation with Z2 spin flip and glide symmetry are compatible with the existence of boundaries, leading to topological surface gapless modes protected by the order-two nonsymmorphic symmetries. We also discuss unique features of these gapless surface modes.

  15. Avoided valence transition in a plutonium superconductor

    PubMed Central

    Ramshaw, B. J.; Shekhter, Arkady; McDonald, Ross D.; Betts, Jon B.; Mitchell, J. N.; Tobash, P. H.; Mielke, C. H.; Bauer, E. D.; Migliori, Albert

    2015-01-01

    The d and f electrons in correlated metals are often neither fully localized around their host nuclei nor fully itinerant. This localized/itinerant duality underlies the correlated electronic states of the high-Tc cuprate superconductors and the heavy-fermion intermetallics and is nowhere more apparent than in the 5f valence electrons of plutonium. Here, we report the full set of symmetry-resolved elastic moduli of PuCoGa5—the highest Tc superconductor of the heavy fermions (Tc = 18.5 K)—and find that the bulk modulus softens anomalously over a wide range in temperature above Tc. The elastic symmetry channel in which this softening occurs is characteristic of a valence instability—therefore, we identify the elastic softening with fluctuations of the plutonium 5f mixed-valence state. These valence fluctuations disappear when the superconducting gap opens at Tc, suggesting that electrons near the Fermi surface play an essential role in the mixed-valence physics of this system and that PuCoGa5 avoids a valence transition by entering the superconducting state. The lack of magnetism in PuCoGa5 has made it difficult to reconcile with most other heavy-fermion superconductors, where superconductivity is generally believed to be mediated by magnetic fluctuations. Our observations suggest that valence fluctuations play a critical role in the unusually high Tc of PuCoGa5. PMID:25737548

  16. Charge of a quasiparticle in a superconductor.

    PubMed

    Ronen, Yuval; Cohen, Yonatan; Kang, Jung-Hyun; Haim, Arbel; Rieder, Maria-Theresa; Heiblum, Moty; Mahalu, Diana; Shtrikman, Hadas

    2016-02-16

    Nonlinear charge transport in superconductor-insulator-superconductor (SIS) Josephson junctions has a unique signature in the shuttled charge quantum between the two superconductors. In the zero-bias limit Cooper pairs, each with twice the electron charge, carry the Josephson current. An applied bias VSD leads to multiple Andreev reflections (MAR), which in the limit of weak tunneling probability should lead to integer multiples of the electron charge ne traversing the junction, with n integer larger than 2Δ/eVSD and Δ the superconducting order parameter. Exceptionally, just above the gap eVSD ≥ 2Δ, with Andreev reflections suppressed, one would expect the current to be carried by partitioned quasiparticles, each with energy-dependent charge, being a superposition of an electron and a hole. Using shot-noise measurements in an SIS junction induced in an InAs nanowire (with noise proportional to the partitioned charge), we first observed quantization of the partitioned charge q = e*/e = n, with n = 1-4, thus reaffirming the validity of our charge interpretation. Concentrating next on the bias region eVSD ~ 2Δ, we found a reproducible and clear dip in the extracted charge to q ~ 0.6, which, after excluding other possibilities, we attribute to the partitioned quasiparticle charge. Such dip is supported by numerical simulations of our SIS structure. PMID:26831071

  17. Chemical stability of high-temperature superconductors

    NASA Technical Reports Server (NTRS)

    Bansal, Narottam P.

    1992-01-01

    A review of the available studies on the chemical stability of the high temperature superconductors (HTS) in various environments was made. The La(1.8)Ba(0.2)CuO4 HTS is unstable in the presence of H2O, CO2, and CO. The YBa2Cu3O(7-x) superconductor is highly susceptible to degradation in different environments, especially water. The La(2-x)Ba(x)CuO4 and Bi-Sr-Ca-Cu-O HTS are relatively less reactive than the YBa2Cu3O(7-x). Processing of YBa2Cu3O(7-x) HTS in purified oxygen, rather than in air, using high purity noncarbon containing starting materials is recommended. Exposure of this HTS to the ambient atmosphere should also be avoided at all stages during processing and storage. Devices and components made out of these oxide superconductors would have to be protected with an impermeable coating of a polymer, glass, or metal to avoid deterioration during use.

  18. electric dipole superconductor in bilayer exciton system

    NASA Astrophysics Data System (ADS)

    Sun, Qing-Feng; Jiang, Qing-Dong; Bao, Zhi-Qiang; Xie, X. C.

    Recently, it was reported that the bilayer exciton systems could exhibit many new phenomena, including the large bilayer counterflow conductivity, the Coulomb drag, etc. These phenomena imply the formation of exciton condensate superfluid state. On the other hand, it is now well known that the superconductor is the condensate superfluid state of the Cooper pairs, which can be viewed as electric monopoles. In other words, the superconductor state is the electric monopole condensate superfluid state. Thus, one may wonder whether there exists electric dipole superfluid state. In this talk, we point out that the exciton in a bilayer system can be considered as a charge neutral electric dipole. And we derive the London-type and Ginzburg-Landau-type equations of electric dipole superconductivity. From these equations, we discover the Meissner-type effect (against spatial variation of magnetic fields), and the dipole current Josephson effect. The frequency in the AC Josephson effect of the dipole current is equal to that in the normal (monopole) superconductor. These results can provide direct evidence for the formation of exciton superfluid state in the bilayer systems and pave new ways to obtain the electric dipole current. We gratefully acknowledge the financial support by NBRP of China (2012CB921303 and 2015CB921102) and NSF-China under Grants Nos. 11274364 and 11574007.

  19. Characterization of bulk superconductors through EBSD methods

    NASA Astrophysics Data System (ADS)

    Koblischka, M. R.; Koblischka-Veneva, A.

    2003-10-01

    The application of electron backscatter diffraction (EBSD) technique to bulk high- Tc superconductors is presented and reviewed. Due to the ceramic nature and the complex crystallographic unit cells of the perovskite-type high- Tc superconductors, the EBSD analysis is not yet as common as it deserves. We have successfully performed EBSD analysis on a variety of high- Tc compounds and samples including polycrystalline YBCO (pure and doped by alkali metals), melt-textured YBCO, thin and thick films of YBCO; the “green phase” Y 2BaCuO 5, thin film and melt-textured NdBa 2Cu 3O x and Bi-2212 single crystals and tapes. It is shown that the surface preparation of the samples is crucial due to the small information depth (up to 100 nm) of the EBSD technique. High quality Kikuchi patterns are the requirement in order to enable the automated EBSD mapping, which yields phase distributions, individual grain orientations and the misorientation angle distribution. The results can be presented in form of mappings, as charts, and as pole figures. These informations are required for a better understanding of the growth mechanism(s) of bulk high- Tc superconductors intended for applications.

  20. Electronic Raman Response in Disordered Unconventional Superconductors

    NASA Astrophysics Data System (ADS)

    Devereaux, Thomas

    1998-03-01

    Due to the strong symmetry dependence of the observed Raman spectra in all high Tc compounds, i.e., the characteristic features of light scattering for different incident and scattered polarization orientations, electronic Raman scattering in unconventional superconductors has grown to be of considerable interest in light of identifying the symmetry of the energy gap and ultimately, the mechanism of superconductivity in high temperature superconductors. I review the theory of the effects of impurities on the Raman response of unconventional superconductors with a view towards understanding the role polarization plays in determining the order parameter symmetry in the cuprates.(T. P. Devereaux, Phys. Rev. Lett. 74), 4313 (1995) (T. P. Devereaux and A. P. Kampf, Int. Journ. Mod. Phys. B 11), 2093 (1997) Signatures of the effects of disorder on the low frequency and low temperature behavior of the Raman spectra obtained for different polarizations will be discussed in relation to the magnitude and phase of the energy gap. Extensions of the theory to include anisotropic impurity scattering and inelastic spin fluctuation scattering will be discussed. Good agreement of the theory with the data on optimally- and overdoped-samples will be shown, although discrepancies remain.

  1. Peak Effect in High-Tc Superconductors

    NASA Astrophysics Data System (ADS)

    Ling, Xinsheng

    1996-03-01

    Like many low-Tc superconductors, high-quality YBCO single crystals are found(X.S. Ling and J.I. Budnick, in Magnetic Susceptibility of Superconductors and Other Spin Systems), edited by R.A. Hein, T.L. Francavilla, and D.H. Liebenberg (Plenum Press, New York, 1991), p.377. to exhibit a striking peak effect. In a magnetic field, the temperature dependence of the critical current has a pronounced peak below T_c(H). Pippard(A.B. Pippard, Phil. Mag. 19), 217 (1969)., and subsequently Larkin and Ovchinnikov(A.I. Larkin and Yu.N. Ovchinnikov, J. Low Temp. Phys. 34), 409 (1979)., attributed the onset of the peak effect to a softening of the vortex lattice. In this talk, the experimental discovery^1 of the peak effect in high-Tc superconductors will be described, followed by a brief historical perspective of the understanding of this phenomenon and a discussion of a new model(X.S. Ling, C. Tang, S. Bhattacharya, and P.M. Chaikin, cond-mat/9504109, (NEC Preprint 1995).) for the peak effect. In this model, the peak effect is an interesting manifestation of the vortex-lattice melting in the presence of weak random pinning potentials. The rise of critical current with increasing temperature is a signature of the ``melting'' of the Larkin domains. This work is done in collaboration with Joe Budnick, Chao Tang, Shobo Bhattacharya, Paul Chaikin, and Boyd Veal.

  2. Charge of a quasiparticle in a superconductor.

    PubMed

    Ronen, Yuval; Cohen, Yonatan; Kang, Jung-Hyun; Haim, Arbel; Rieder, Maria-Theresa; Heiblum, Moty; Mahalu, Diana; Shtrikman, Hadas

    2016-02-16

    Nonlinear charge transport in superconductor-insulator-superconductor (SIS) Josephson junctions has a unique signature in the shuttled charge quantum between the two superconductors. In the zero-bias limit Cooper pairs, each with twice the electron charge, carry the Josephson current. An applied bias VSD leads to multiple Andreev reflections (MAR), which in the limit of weak tunneling probability should lead to integer multiples of the electron charge ne traversing the junction, with n integer larger than 2Δ/eVSD and Δ the superconducting order parameter. Exceptionally, just above the gap eVSD ≥ 2Δ, with Andreev reflections suppressed, one would expect the current to be carried by partitioned quasiparticles, each with energy-dependent charge, being a superposition of an electron and a hole. Using shot-noise measurements in an SIS junction induced in an InAs nanowire (with noise proportional to the partitioned charge), we first observed quantization of the partitioned charge q = e*/e = n, with n = 1-4, thus reaffirming the validity of our charge interpretation. Concentrating next on the bias region eVSD ~ 2Δ, we found a reproducible and clear dip in the extracted charge to q ~ 0.6, which, after excluding other possibilities, we attribute to the partitioned quasiparticle charge. Such dip is supported by numerical simulations of our SIS structure.

  3. McMillan-Rowell like oscillations in a superconductor-InAs/GaSb-superconductor junction

    SciTech Connect

    Shi, Xiaoyan Yu, Wenlong; Hawkins, S. D.; Klem, J. F.; Pan, W.

    2015-08-03

    We have fabricated a superconductor (Ta)-InAs/GaSb bilayer-superconductor (Ta) junction device that has a long mean free path and can preserve the wavelike properties of particles (electrons and holes) inside the junction. Differential conductance measurements were carried out at low temperatures in this device, and McMillan-Rowell like oscillations (MROs) were observed. Surprisingly, a much larger Fermi velocity, compared to that from Shubnikov-de Haas oscillations, was obtained from the frequency of MROs. Possible mechanisms are discussed for this discrepancy.

  4. McMillan-Rowell Like Oscillations in a Superconductor-InAs/GaSb-Superconductor Junction

    SciTech Connect

    Shi, Xiaoyan; Yu, Wenlong; Hawkins, Samuel D.; Klem, John F.; Pan, Wei

    2015-08-04

    We fabricated a superconductor (Ta)-InAs/GaSb bilayer-superconductor (Ta) junction device that has a long mean free path and can preserve the wavelike properties of particles (electrons and holes) inside the junction. Differential conductance measurements were also carried out at low temperatures in this device, and McMillan-Rowell like oscillations (MROs) were observed. A much larger Fermi velocity, compared to that from Shubnikov-de Haas oscillations, was obtained from the frequency of MROs. Possible mechanisms are discussed for this discrepancy.

  5. Fabrication of high-quality superconductor-insulator-superconductor junctions on thin SiN membranes

    NASA Technical Reports Server (NTRS)

    Garcia, Edouard; Jacobson, Brian R.; Hu, Qing

    1993-01-01

    We have successfully fabricated high-quality and high-current density superconductor-insulator-superconductor (SIS) junctions on freestanding thin silicon nitride (SIN) membranes. These devices can be used in a novel millimeter-wave and THz receiver system which is made using micromachining. The SIS junctions with planar antennas were fabricated first on a silicon wafer covered with a SiN membrane, the Si wafer underneath was then etched away using an anisotropic KOH etchant. The current-voltage characteristics of the SIS junctions remained unchanged after the whole process, and the junctions and the membrane survived thermal cycling.

  6. Characterization of composite high temperature superconductors for magnetic bearing applications

    SciTech Connect

    Weinberger, B.R.; Lynds, L.; VanValzah, J.; Eaton, H. . Research Center); Hull, J.R.; Mulcahy, T.M.; Basinger, S.A. )

    1990-01-01

    A study of high temperature superconductor composites for use in magnetic bearings applications is presented. Fabrication and characterization techniques are described. Magnetometry and mechanical force measurements are correlated with a particular emphasis on the role of superconductor particle size. Results are discussed in terms of fundamental limits of Meissner effect levitation. 21 refs., 11 figs., 1 tab.

  7. Method of manufacturing a high temperature superconductor with improved transport properties

    DOEpatents

    Balachandran, Uthamalingam; Siegel, Richard W.; Askew, Thomas R.

    2001-01-01

    A method of preparing a high temperature superconductor. A method of preparing a superconductor includes providing a powdered high temperature superconductor and a nanophase paramagnetic material. These components are combined to form a solid compacted mass with the paramagnetic material disposed on the grain boundaries of the polycrystaline high temperature superconductor.

  8. A Double-Decker Levitation Experiment Using a Sandwich of Superconductors.

    ERIC Educational Resources Information Center

    Jacob, Anthony T.; And Others

    1988-01-01

    Shows that the mutual repulsion that enables a superconductor to levitate a magnet and a magnet to levitate a superconductor can be combined into a single demonstration. Uses an overhead projector, two pellets of "1-2-3" superconductor, Nd-Fe-B magnets, liquid nitrogen, and paraffin. Offers superconductor preparation, hazards, and disposal…

  9. Method and apparatus for dissipating remanent fields and preserving diamagnetism of ceramic superconductors

    SciTech Connect

    Youngdahl, C.A.

    1993-12-31

    A method for dissipating a remanent field, created when a magnetic field is brought into contact with a superconductor, while preserving the diamagnetism of a superconductor comprises the steps of (1) providing a ceramic superconductor; (2) continuously or intermittently generating an AC current to the ceramic superconductor; and (3) gradually decreasing the AC current until the undesired remanent field is dissipated.

  10. System and method for quench protection of a superconductor

    DOEpatents

    Huang, Xianrui; Sivasubramaniam, Kiruba Haran; Bray, James William; Ryan, David Thomas

    2008-03-11

    A system and method for protecting a superconductor from a quench condition. A quench protection system is provided to protect the superconductor from damage due to a quench condition. The quench protection system comprises a voltage detector operable to detect voltage across the superconductor. The system also comprises a frequency filter coupled to the voltage detector. The frequency filter is operable to couple voltage signals to a control circuit that are representative of a rise in superconductor voltage caused by a quench condition and to block voltage signals that are not. The system is operable to detect whether a quench condition exists in the superconductor based on the voltage signal received via the frequency filter and to initiate a protective action in response.

  11. Do organic and other exotic superconductors fail universal scaling relations?

    PubMed Central

    Dordevic, S. V.; Basov, D. N.; Homes, C. C.

    2013-01-01

    Universal scaling relations are of tremendous importance in science, as they reveal fundamental laws of nature. Several such scaling relations have recently been proposed for superconductors; however, they are not really universal in the sense that some important families of superconductors appear to fail the scaling relations, or obey the scaling with different scaling pre-factors. In particular, a large group of materials called organic (or molecular) superconductors are a notable example. Here, we show that such apparent violations are largely due to the fact that the required experimental parameters were collected on different samples, with different experimental techniques. When experimental data is taken on the same sample, using a single experimental technique, organic superconductors, as well as all other studied superconductors, do in fact follow universal scaling relations.

  12. Precursor composites for oxygen dispersion hardened silver sheathed superconductor composites

    DOEpatents

    Podtburg, Eric R.

    1999-01-01

    An oxide superconductor composite having improved texture and durability. The oxide superconductor composite includes an oxide superconductor phase substantially surrounded with/by a noble metal matrix, the noble metal matrix comprising a metal oxide in an amount effective to form metal oxide domains that increase hardness of the composite. The composite is characterized by a degree of texture at least 10% greater than a comparable oxide superconductor composite lacking metal oxide domains. An oxide superconducting composite may be prepared by oxidizing the precursor composite under conditions effective to form solute metal oxide domains within the silver matrix and to form a precursor oxide in the precursor alloy phase; subjecting the oxidized composite to a softening anneal under conditions effective to relieve stress within the noble metal phase; and converting the oxide precursor into an oxide superconductor.

  13. Precursor composites for oxygen dispersion hardened silver sheathed superconductor composites

    DOEpatents

    Podtburg, E.R.

    1999-06-22

    An oxide superconductor composite having improved texture and durability is disclosed. The oxide superconductor composite includes an oxide superconductor phase substantially surrounded with/by a noble metal matrix, the noble metal matrix comprising a metal oxide in an amount effective to form metal oxide domains that increase hardness of the composite. The composite is characterized by a degree of texture at least 10% greater than a comparable oxide superconductor composite lacking metal oxide domains. An oxide superconducting composite may be prepared by oxidizing the precursor composite under conditions effective to form solute metal oxide domains within the silver matrix and to form a precursor oxide in the precursor alloy phase; subjecting the oxidized composite to a softening anneal under conditions effective to relieve stress within the noble metal phase; and converting the oxide precursor into an oxide superconductor. 1 fig.

  14. Status of high temperature superconductor development for accelerator magnets

    NASA Technical Reports Server (NTRS)

    Hirabayashi, H.

    1995-01-01

    High temperature superconductors are still under development for various applications. As far as conductors for magnets are concerned, the development has just been started. Small coils wound by silver sheathed Bi-2212 and Bi-2223 oxide conductors have been reported by a few authors. Essential properties of high T(sub c) superconductors like pinning force, coherent length, intergrain coupling, weak link, thermal property, AC loss and mechanical strength are still not sufficiently understandable. In this talk, a review is given with comparison between the present achievement and the final requirement for high T(sub c) superconductors, which could be particularly used in accelerator magnets. Discussions on how to develop high T(sub c) superconductors for accelerator magnets are included with key parameters of essential properties. A proposal of how to make a prototype accelerator magnet with high T(sub c) superconductors with prospect for future development is also given.

  15. Interface superconductor with gap behaviour like a high-temperature superconductor.

    PubMed

    Richter, C; Boschker, H; Dietsche, W; Fillis-Tsirakis, E; Jany, R; Loder, F; Kourkoutis, L F; Muller, D A; Kirtley, J R; Schneider, C W; Mannhart, J

    2013-10-24

    The physics of the superconducting state in two-dimensional (2D) electron systems is relevant to understanding the high-transition-temperature copper oxide superconductors and for the development of future superconductors based on interface electron systems. But it is not yet understood how fundamental superconducting parameters, such as the spectral density of states, change when these superconducting electron systems are depleted of charge carriers. Here we use tunnel spectroscopy with planar junctions to measure the behaviour of the electronic spectral density of states as a function of carrier density, clarifying this issue experimentally. We chose the conducting LaAlO3-SrTiO3 interface as the 2D superconductor, because this electron system can be tuned continuously with an electric gate field. We observed an energy gap of the order of 40 microelectronvolts in the density of states, whose shape is well described by the Bardeen-Cooper-Schrieffer superconducting gap function. In contrast to the dome-shaped dependence of the critical temperature, the gap increases with charge carrier depletion in both the underdoped region and the overdoped region. These results are analogous to the pseudogap behaviour of the high-transition-temperature copper oxide superconductors and imply that the smooth continuation of the superconducting gap into pseudogap-like behaviour could be a general property of 2D superconductivity. PMID:24097347

  16. Multiple Antiferromagnetic Spin Fluctuations and Novel Evolution of Tc in Iron-Based Superconductors LaFe(As1‑xPx)(O1‑yFy) Revealed by 31P-NMR Studies

    NASA Astrophysics Data System (ADS)

    Shiota, Takayoshi; Mukuda, Hidekazu; Uekubo, Masahiro; Engetsu, Fuko; Yashima, Mitsuharu; Kitaoka, Yoshio; Lai, Kwing To; Usui, Hidetomo; Kuroki, Kazuhiko; Miyasaka, Shigeki; Tajima, Setsuko

    2016-05-01

    We report on 31P-NMR studies of LaFe(As1‑xPx)(O1‑yFy) over wide compositions for 0 ≤ x ≤ 1 and 0 ≤ y ≤ 0.14, which provide clear evidence that antiferromagnetic spin fluctuations (AFMSFs) are one of the indispensable elements for enhancing Tc. Systematic 31P-NMR measurements revealed two types of AFMSFs in the temperature evolution, that is, one is the AFMSFs that develop rapidly down to Tc with low-energy characteristics, and the other, with relatively higher energy than the former, develops gradually upon cooling from high temperature. The low-energy AFMSFs in low y (electron doping) over a wide x (pnictogen height suppression) range are associated with the two orbitals of dxz/yz, whereas the higher-energy ones for a wide y region around low x originate from the three orbitals of dxy and dxz/yz. We remark that the nonmonotonic variation of Tc as a function of x and y in LaFe(As1‑xPx)(O1‑yFy) is attributed to these multiple AFMSFs originating from degenerated multiple 3d orbitals inherent to Fe-pnictide superconductors.

  17. Exploring intertwined orders in cuprate superconductors

    DOE PAGESBeta

    Tranquada, John M.

    2014-11-22

    In this study, the concept of intertwined orders has been introduced to describe the cooperative relationship between antiferromagnetic spin correlations and electron (or hole) pair correlations that develop in copper-oxide superconductors. This contrasts with systems in which, for example, charge-density-wave (CDW) order competes for Fermi surface area with superconductivity. La2-xBaxCuO4 with x = 0.125 provides an example in which the ordering of spin stripes coincides with the onset of two-dimensional superconducting correlations. The apparent frustration of the interlayer Josephson coupling has motivated the concept of the pair-density-wave superconductor, a state that theoretical calculations show to be energetically competitive with themore » uniform d-wave superconductor. Even at x = 0.095, where there is robust superconductivity below 32 K in zero field, the coexistence of strong, low-energy, incommensurate spin excitations implies a spatially modulated and intertwined pair wave function. Recent observations of CDW order in YBa2Cu3O6+x and other cuprate families have raised interesting questions regarding the general role of charge modulations and the relation to superconductivity. While there are differences in the doping dependence of the modulation wave vectors in YBa2Cu3O6+x and La2-xBaxCuO4, the maximum ordering strength is peaked at the hole concentration of 1/8 in both cases. There are also possible connections with the quantum oscillations that have been detected about the same hole concentration but at high magnetic fields. Resolving these relationships remains a research challenge.« less

  18. Exploring intertwined orders in cuprate superconductors

    SciTech Connect

    Tranquada, John M.

    2014-11-22

    In this study, the concept of intertwined orders has been introduced to describe the cooperative relationship between antiferromagnetic spin correlations and electron (or hole) pair correlations that develop in copper-oxide superconductors. This contrasts with systems in which, for example, charge-density-wave (CDW) order competes for Fermi surface area with superconductivity. La2-xBaxCuO4 with x = 0.125 provides an example in which the ordering of spin stripes coincides with the onset of two-dimensional superconducting correlations. The apparent frustration of the interlayer Josephson coupling has motivated the concept of the pair-density-wave superconductor, a state that theoretical calculations show to be energetically competitive with the uniform d-wave superconductor. Even at x = 0.095, where there is robust superconductivity below 32 K in zero field, the coexistence of strong, low-energy, incommensurate spin excitations implies a spatially modulated and intertwined pair wave function. Recent observations of CDW order in YBa2Cu3O6+x and other cuprate families have raised interesting questions regarding the general role of charge modulations and the relation to superconductivity. While there are differences in the doping dependence of the modulation wave vectors in YBa2Cu3O6+x and La2-xBaxCuO4, the maximum ordering strength is peaked at the hole concentration of 1/8 in both cases. There are also possible connections with the quantum oscillations that have been detected about the same hole concentration but at high magnetic fields. Resolving these relationships remains a research challenge.

  19. Exploring intertwined orders in cuprate superconductors

    NASA Astrophysics Data System (ADS)

    Tranquada, John M.

    2015-03-01

    The concept of intertwined orders has been introduced to describe the cooperative relationship between antiferromagnetic spin correlations and electron (or hole) pair correlations that develop in copper-oxide superconductors. This contrasts with systems in which, for example, charge-density-wave (CDW) order competes for Fermi surface area with superconductivity. La2-xBaxCuO4 with x=0.125 provides an example in which the ordering of spin stripes coincides with the onset of two-dimensional superconducting correlations. The apparent frustration of the interlayer Josephson coupling has motivated the concept of the pair-density-wave superconductor, a state that theoretical calculations show to be energetically competitive with the uniform d-wave superconductor. Even at x=0.095, where there is robust superconductivity below 32 K in zero field, the coexistence of strong, low-energy, incommensurate spin excitations implies a spatially modulated and intertwined pair wave function. Recent observations of CDW order in YBa2Cu3O6+x and other cuprate families have raised interesting questions regarding the general role of charge modulations and the relation to superconductivity. While there are differences in the doping dependence of the modulation wave vectors in YBa2Cu3O6+x and La2-xBaxCuO4, the maximum ordering strength is peaked at the hole concentration of 1/8 in both cases. There are also possible connections with the quantum oscillations that have been detected about the same hole concentration but at high magnetic fields. Resolving these relationships remains a research challenge.

  20. Modeling forces in high-temperature superconductors

    SciTech Connect

    Turner, L. R.; Foster, M. W.

    1997-11-18

    We have developed a simple model that uses computed shielding currents to determine the forces acting on a high-temperature superconductor (HTS). The model has been applied to measurements of the force between HTS and permanent magnets (PM). Results show the expected hysteretic variation of force as the HTS moves first toward and then away from a permanent magnet, including the reversal of the sign of the force. Optimization of the shielding currents is carried out through a simulated annealing algorithm in a C++ program that repeatedly calls a commercial electromagnetic software code. Agreement with measured forces is encouraging.

  1. Discovery of a Superhard Iron Tetraboride Superconductor

    NASA Astrophysics Data System (ADS)

    Gou, Huiyang; Dubrovinskaia, Natalia; Bykova, Elena; Tsirlin, Alexander A.; Kasinathan, Deepa; Schnelle, Walter; Richter, Asta; Merlini, Marco; Hanfland, Michael; Abakumov, Artem M.; Batuk, Dmitry; Van Tendeloo, Gustaaf; Nakajima, Yoichi; Kolmogorov, Aleksey N.; Dubrovinsky, Leonid

    2013-10-01

    Single crystals of novel orthorhombic (space group Pnnm) iron tetraboride FeB4 were synthesized at pressures above 8 GPa and high temperatures. Magnetic susceptibility and heat capacity measurements demonstrate bulk superconductivity below 2.9 K. The putative isotope effect on the superconducting critical temperature and the analysis of specific heat data indicate that the superconductivity in FeB4 is likely phonon mediated, which is rare for Fe-based superconductors. The discovered iron tetraboride is highly incompressible and has the nanoindentation hardness of 62(5) GPa; thus, it opens a new class of highly desirable materials combining advanced mechanical properties and superconductivity.

  2. Magnetic refrigeration using flux compression in superconductors

    NASA Technical Reports Server (NTRS)

    Israelsson, U. E.; Strayer, D. M.; Jackson, H. W.; Petrac, D.

    1990-01-01

    The feasibility of using flux compression in high-temperature superconductors to produce the large time-varying magnetic fields required in a field cycled magnetic refrigerator operating between 20 K and 4 K is presently investigated. This paper describes the refrigerator concept and lists limitations and advantages in comparison with conventional refrigeration techniques. The maximum fields obtainable by flux compression in high-temperature supercoductor materials, as presently prepared, are too low to serve in such a refrigerator. However, reports exist of critical current values that are near usable levels for flux pumps in refrigerator applications.

  3. Dirac and Weyl superconductors in three dimensions.

    PubMed

    Yang, Shengyuan A; Pan, Hui; Zhang, Fan

    2014-07-25

    We introduce the concept of three-dimensional Dirac (Weyl) superconductors (SC), which have protected bulk fourfold (twofold) nodal points and surface Majorana arcs at zero energy. We provide a sufficient criterion for realizing them in centrosymmetric SCs with odd-parity pairing and mirror symmetry. Pairs of Dirac nodes appear in a mirror-invariant plane when the mirror winding number is nontrivial. Breaking mirror symmetry may gap Dirac nodes producing a topological SC. Each Dirac node evolves to a nodal ring when inversion-gauge symmetry is broken, whereas it splits into a pair of Weyl nodes when, and only when, time-reversal symmetry is broken. PMID:25105637

  4. Oxygen diffusion in high- Tc superconductors

    SciTech Connect

    Rothman, S.J.; Routbort, J.L.

    1992-07-01

    The cuprate superconductors are fascinating not only because of their technical promise, but also because of their structures, especially the anisotropy of the crystal lattice. There are some structural similarities among these compounds, but also significant differences. Measurements of the oxygen tracer diffusion coefficients have been carried out as a function of temperature, oxygen partial pressure, crystal orientation, and doping in the La-Sr-Cu-0, Y-Ba-Cu-0, and Bi-Sr-Ca-Cu-0 systems. These measurements have revealed a variety of defect mechanisms operating in these compounds; the exact nature of the mechanism depends on the details of the structure.

  5. Oxygen diffusion in high-{Tc} superconductors

    SciTech Connect

    Rothman, S.J.; Routbort, J.L.

    1992-07-01

    The cuprate superconductors are fascinating not only because of their technical promise, but also because of their structures, especially the anisotropy of the crystal lattice. There are some structural similarities among these compounds, but also significant differences. Measurements of the oxygen tracer diffusion coefficients have been carried out as a function of temperature, oxygen partial pressure, crystal orientation, and doping in the La-Sr-Cu-0, Y-Ba-Cu-0, and Bi-Sr-Ca-Cu-0 systems. These measurements have revealed a variety of defect mechanisms operating in these compounds; the exact nature of the mechanism depends on the details of the structure.

  6. Discovery of a superhard iron tetraboride superconductor.

    PubMed

    Gou, Huiyang; Dubrovinskaia, Natalia; Bykova, Elena; Tsirlin, Alexander A; Kasinathan, Deepa; Schnelle, Walter; Richter, Asta; Merlini, Marco; Hanfland, Michael; Abakumov, Artem M; Batuk, Dmitry; Van Tendeloo, Gustaaf; Nakajima, Yoichi; Kolmogorov, Aleksey N; Dubrovinsky, Leonid

    2013-10-11

    Single crystals of novel orthorhombic (space group Pnnm) iron tetraboride FeB4 were synthesized at pressures above 8 GPa and high temperatures. Magnetic susceptibility and heat capacity measurements demonstrate bulk superconductivity below 2.9 K. The putative isotope effect on the superconducting critical temperature and the analysis of specific heat data indicate that the superconductivity in FeB4 is likely phonon mediated, which is rare for Fe-based superconductors. The discovered iron tetraboride is highly incompressible and has the nanoindentation hardness of 62(5) GPa; thus, it opens a new class of highly desirable materials combining advanced mechanical properties and superconductivity.

  7. Paramagnetic excited vortex states in superconductors

    NASA Astrophysics Data System (ADS)

    Gomes, Rodolpho Ribeiro; Doria, Mauro M.; Romaguera, Antonio R. de C.

    2016-06-01

    We consider excited vortex states, which are vortex states left inside a superconductor once the external applied magnetic field is switched off and whose energy is lower than of the normal state. We show that this state is paramagnetic and develop here a general method to obtain its Gibbs free energy through conformal mapping. The solution for any number of vortices in any cross-section geometry can be read off from the Schwarz-Christoffel mapping. The method is based on the first-order equations used by Abrikosov to discover vortices.

  8. The classification of topological insulators and superconductors

    NASA Astrophysics Data System (ADS)

    Chiu, Ching-Kai; Stone, Michael; Hughes, Taylor

    2011-03-01

    We use the process of band crossings during quantum phase transitions to explain the periodic table of topological insulators and superconductors. This is achieved by showing how irreducible representations of the real and complex Clifford algebras are related to the 10 Altland-Zirnbauer symmetry classes of Hamiltonian matrices which are associated with time reversal, particle-hole, and chiral symmetries. The representation theory not only reveals why a unique topological invariant (0 ,Z2 , Z) exists for each specific symmetry class and dimension, but also shows the interplay between quantum phase transitions, topologically protected boundary modes, and topological invariants.

  9. Localization of resistive domains in inhomogeneous superconductors

    SciTech Connect

    Gurevich, A.V.; Mints, R.G.

    1981-01-01

    The properties of resistive domains due to the Joule heating in inhomogeneous superconductors with transport currents are studied. The equilibrium of a domain at an inhomogeneity of arbitrary type and with dimensions much smaller than the dimensions of the domain is investigated. It is shown that resistive domains can become localized at inhomogeneities. The temperature distribution in a domain and the current--voltage characteristic of the domain are determined. The stability of localized domains is discussed. It is shown that such domains give rise to a hysteresis in the destruction (recovery) of the superconductivity by the transport current.

  10. Superconductor coil geometry and ac losses

    NASA Technical Reports Server (NTRS)

    Pierce, T. V., Jr.; Zapata, R. N.

    1976-01-01

    An empirical relation is presented which allows simple computation of volume-averaged winding fields from central fields for coils of small rectangular cross sections. This relation suggests that, in certain applications, ac-loss minimization can be accomplished by use of low winding densities, provided that hysteresis losses are independent of winding density. The ac-loss measurements on coils wound of twisted multifilamentary composite superconductors show no significant dependence on ac losses on winding density, thus permitting the use of winding density as an independent design parameter in loss minimization.

  11. Potential aerospace applications of high temperature superconductors

    NASA Technical Reports Server (NTRS)

    Selim, Raouf

    1994-01-01

    The recent discovery of High Temperature Superconductors (HTS) with superconducting transition temperature, T(sub c), above the boiling point of liquid nitrogen has opened the door for using these materials in new and practical applications. These materials have zero resistance to electric current, have the capability of carrying large currents and as such have the potential to be used in high magnetic field applications. One of the space applications that can use superconductors is electromagnetic launch of payloads to low-earth-orbit. An electromagnetic gun-type launcher can be used in small payload systems that are launched at very high velocity, while sled-type magnetically levitated launcher can be used to launch larger payloads at smaller velocities. Both types of launchers are being studied by NASA and the aerospace industry. The use of superconductors will be essential in any of these types of launchers in order to produce the large magnetic fields required to obtain large thrust forces. Low Temperature Superconductor (LTS) technology is mature enough and can be easily integrated in such systems. As for the HTS, many leading companies are currently producing HTS coils and magnets that potentially can be mass-produced for these launchers. It seems that designing and building a small-scale electromagnetic launcher is the next logical step toward seriously considering this method for launching payloads into low-earth-orbit. A second potential application is the use of HTS to build sensitive portable devices for the use in Non Destructive Evaluation (NDE). Superconducting Quantum Interference Devices (SQUID's) are the most sensitive instruments for measuring changes in magnetic flux. By using HTS in SQUID's, one will be able to design a portable unit that uses liquid nitrogen or a cryocooler pump to explore the use of gradiometers or magnetometers to detect deep cracks or corrosion in structures. A third use is the replacement of Infra-Red (IR) sensor leads on

  12. NEW APPROACHES: High temperature superconductors take off

    NASA Astrophysics Data System (ADS)

    Gough, Colin

    1998-01-01

    This article describes the progress made towards real engineering applications of high temperature superconductors (HTS) in the ten years following the Nobel Prize winning discovery by Bednorz and Müller in August 1986. Examples include HTS wires and tapes for more efficient and powerful electric motors and for increasing the electrical power into the heart of modern cities, HTS permanent magnets for levitation, microwave filters for cellular telephone networks, SQUIDs (superconducting quantum interference devices) to monitor foetal heart and brain signals, and a new generation of superfast logic devices based on the flux quantum.

  13. High-Tc superconductor coplanar waveguide filter

    NASA Technical Reports Server (NTRS)

    Chew, Wilbert; Bajuk, Louis J.; Cooley, Thomas W.; Foote, Marc C.; Hunt, Brian D.; Rascoe, Daniel L.; Riley, A. L.

    1991-01-01

    Coplanar waveguide (CPW) low-pass filters made of YBa2Cu3O(7-delta) (YBCO) on LaAlO3 substrates, with dimensions suited for integrated circuits, were fabricated and packaged. A complete filter gives a true idea of the advantages and difficulties in replacing thin-film metal with a high-temperature superconductor in a practical circuit. Measured insertion losses in liquid nitrogen were superior to the loss of a similar thin-film copper filter throughout the 0- to 9.5-GHz passband. These results demonstrate the performance of fully patterned YBCO in a practical CPW structure after sealing in a hermetic package.

  14. Superconductivity in new iron pnictide oxide Fe2As2Sr4(Mg,Ti)2O6

    NASA Astrophysics Data System (ADS)

    Sato, Shinya; Ogino, Hiraku; Kishio, Kohji; Shimoyama, Jun-Ichi

    2010-03-01

    A new iron arsenide oxide Fe2As2Sr4MgTiO6, which is isostructural with the iron-based superconductor Fe2Pn2Sr4M2O6^[1,2], has been successfully synthesized by the solid-state reaction in quartz ampoules. Fe2As2Sr4MgTiO6 has antifluorite-type iron arsenide layer and K2NiF4-type oxide layer, while the M-site is composed of a combination of divalent (Mg^2+) and tetravalent (Ti^4+) cations as in the case of a double perovskite La(Mg,Ti)O3. This fact indicates chemical flexibility of the perovskite-related layer in this system. This compound showed bulk superconductivity with Tc of ˜20 K by partial substitution of Co for Fe. Moreover, high Tc above 35 K was recorded by samples starting from Co-free and Ti-rich compositions, Fe2As2Sr4(Mg1-xTix)2O6 (x =0.7˜0.8). [1] H. Ogino et al., Supercond. Sci. Technol. 22 (2009) 075008. [2] X. Zhu et al., Phys. Rev. B 79 (2009) 220512(R).

  15. Doping effect of Cu and Ni impurities on the Fe-based superconductor Ba0.6K0.4Fe2As2

    NASA Astrophysics Data System (ADS)

    Cheng, Peng; Shen, Bing; Han, Fei; Wen, Hai-Hu

    2013-11-01

    Copper and nickel impurities have been doped into the iron pnictide superconductor Ba0.6K0.4Fe2As2. Resistivity measurements reveal that Cu and Ni impurities suppress the superconducting transition temperature Tc with rates of \\Delta T_c/\\text{Cu-}1%= -3.5\\ \\text{K} and \\Delta T_c/\\text{Ni-}1% = -2.9\\ \\text{K} , respectively. The temperature dependence of the Hall coefficient RH of these two series of samples shows that both Cu doping and Ni doping can introduce electrons into Ba0.6K0.4Fe2As2. With more doping, the sign of RH gradually changes from positive to negative, and the changing rate of Cu-doped samples is much faster than that of Ni-doped ones. Combining this with the results of first-principles calculations published previously and the nonmonotonic evolution of the Hall coefficient in the low-temperature region, we argue that when more Cu impurities are introduced into Ba0.6K0.4Fe2As2, the removal of Fermi spectral weight in the hole-like Fermi surfaces is much stronger than that in the electron-like Fermi surfaces, which is equivalent to a significant electron doping effect. DC magnetization and the lattice constants analysis reveal that static magnetic moments and notable lattice compression have formed in Cu-doped samples. It seems that superconductivity can be suppressed by the impurities disregarding whether they are magnetic or nonmagnetic in nature. This gives strong support to a pairing gap with a sign reversal, like S^+/- . However, the relatively slow suppression rates of Tc show the robustness of superconductivity of Ba0.6K0.4Fe2As2 against impurities, implying that multi-pairing channels may exist in the system.

  16. Electronic structure and superconductivity of FeSe-related superconductors.

    PubMed

    Liu, Xu; Zhao, Lin; He, Shaolong; He, Junfeng; Liu, Defa; Mou, Daixiang; Shen, Bing; Hu, Yong; Huang, Jianwei; Zhou, X J

    2015-05-13

    FeSe superconductors and their related systems have attracted much attention in the study of iron-based superconductors owing to their simple crystal structure and peculiar electronic and physical properties. The bulk FeSe superconductor has a superconducting transition temperature (Tc) of ~8 K and it can be dramatically enhanced to 37 K at high pressure. On the other hand, its cousin system, FeTe, possesses a unique antiferromagnetic ground state but is non-superconducting. Substitution of Se with Te in the FeSe superconductor results in an enhancement of Tc up to 14.5 K and superconductivity can persist over a large composition range in the Fe(Se,Te) system. Intercalation of the FeSe superconductor leads to the discovery of the AxFe2-ySe2 (A = K, Cs and Tl) system that exhibits a Tc higher than 30 K and a unique electronic structure of the superconducting phase. A recent report of possible high temperature superconductivity in single-layer FeSe/SrTiO3 films with a Tc above 65 K has generated much excitement in the community. This pioneering work opens a door for interface superconductivity to explore for high Tc superconductors. The distinct electronic structure and superconducting gap, layer-dependent behavior and insulator-superconductor transition of the FeSe/SrTiO3 films provide critical information in understanding the superconductivity mechanism of iron-based superconductors. In this paper, we present a brief review of the investigation of the electronic structure and superconductivity of the FeSe superconductor and related systems, with a particular focus on the FeSe films.

  17. Electronic structure and superconductivity of FeSe-related superconductors.

    PubMed

    Liu, Xu; Zhao, Lin; He, Shaolong; He, Junfeng; Liu, Defa; Mou, Daixiang; Shen, Bing; Hu, Yong; Huang, Jianwei; Zhou, X J

    2015-05-13

    FeSe superconductors and their related systems have attracted much attention in the study of iron-based superconductors owing to their simple crystal structure and peculiar electronic and physical properties. The bulk FeSe superconductor has a superconducting transition temperature (Tc) of ~8 K and it can be dramatically enhanced to 37 K at high pressure. On the other hand, its cousin system, FeTe, possesses a unique antiferromagnetic ground state but is non-superconducting. Substitution of Se with Te in the FeSe superconductor results in an enhancement of Tc up to 14.5 K and superconductivity can persist over a large composition range in the Fe(Se,Te) system. Intercalation of the FeSe superconductor leads to the discovery of the AxFe2-ySe2 (A = K, Cs and Tl) system that exhibits a Tc higher than 30 K and a unique electronic structure of the superconducting phase. A recent report of possible high temperature superconductivity in single-layer FeSe/SrTiO3 films with a Tc above 65 K has generated much excitement in the community. This pioneering work opens a door for interface superconductivity to explore for high Tc superconductors. The distinct electronic structure and superconducting gap, layer-dependent behavior and insulator-superconductor transition of the FeSe/SrTiO3 films provide critical information in understanding the superconductivity mechanism of iron-based superconductors. In this paper, we present a brief review of the investigation of the electronic structure and superconductivity of the FeSe superconductor and related systems, with a particular focus on the FeSe films. PMID:25879999

  18. Neutron scattering of iron-based superconductors

    SciTech Connect

    Shamoto, S; Wakimoto, S; Kodama, K.; Ishikado, Motoyuki; Christianson, Andrew D; Lumsden, Mark D; Kajimoto, Ryoichi; Nakamura, Mitsutaka; Inamura, Yasuhiro; Arai, Masatoshi; Kakurai, K.; Esaka, Fumitaka; Iyo, Akira; Kito, Hijiri; Eisaki, Hiroshi

    2011-01-01

    Low-energy spin excitations have been studied on polycrystalline LaFeAsO{sub 1-x}F{sub x} samples by inelastic neutron scattering. The Q-integrated dynamical spin susceptibility {chi}{double_prime}({omega}) of the superconducting samples is found to be comparable to that of the magnetically ordered parent sample. On the other hand, {chi}{double_prime}({omega}) almost vanishes at x = 0.158, where the superconducting transition temperature T{sub c} is suppressed to 7 K. In addition, {chi}{double_prime}({omega}) in optimally doped LaFeAsO{sub 0.918}F{sub 0.082} with T{sub c} = 29 K exhibits a spin resonance mode. The peak energy, E{sub res}, when scaled by k{sub B}T{sub c} is similar to the value of about 4.7 reported in other high-T{sub c} iron-based superconductors. This result suggests that there is intimate relationship between the dynamical spin susceptibility and high-T{sub c} superconductivity in iron-based superconductors, and is consistent with a nesting condition between Fermi surfaces at the {Gamma} and M points.

  19. Method for fabrication of high temperature superconductors

    DOEpatents

    Balachandran, Uthamalingam; Ma, Beihai; Miller, Dean

    2006-03-14

    A layered article of manufacture and a method of manufacturing same is disclosed. A substrate has a biaxially textured MgO crystalline layer having the c-axes thereof inclined with respect to the plane of the substrate deposited thereon. A layer of one or more of YSZ or Y2O3 and then a layer of CeO2 is deposited on the MgO. A crystalline superconductor layer with the c-axes thereof normal to the plane of the substrate is deposited on the CeO2 layer. Deposition of the MgO layer on the substrate is by the inclined substrate deposition method developed at Argonne National Laboratory. Preferably, the MgO has the c-axes thereof inclined with respect to the normal to the substrate in the range of from about 10.degree. to about 40.degree. and YBCO superconductors are used.

  20. Superfluid density through 2D superconductor junctions

    NASA Astrophysics Data System (ADS)

    Nam, Hyoungdo; Shih, Chih-Kang

    As S. Qin et al. reported, two monolayer (2 ML) lead film on a silicon (111) substrate has one of two different atomic structures on the silicon substrate: the unstrained 1x1 and the psedumorphically strained √3x √3 (i.e. the same lattice constant as the Si √3x √3 lattice). Most interestingly, although these two different regions show the same quantum well state features, they have different Tc's (5 K and 4 K). These two different regions of 2 ML film naturally form superconductor-superconductor (SS or SS') junctions along silicon step edges. Physical connection of the junction is only 1 ML thickness because of the step height difference of substrate. We will present this study of SS (or SS') junction system using scanning tunneling microscopy/spectroscopy and in-situ double-coil mutual inductance measurement. The transition of superconducting gaps across either SS or SS' junctions should show how to locally affect each other. Double coil measurement show a global Tc close to the lower Tc region with sizable superfluid density. We will discuss the phase rigidity and its relationship to the superfluid density in this ultra-thin Pb film that is only 2 ML thick.

  1. Charge of a quasiparticle in a superconductor

    PubMed Central

    Ronen, Yuval; Cohen, Yonatan; Kang, Jung-Hyun; Haim, Arbel; Rieder, Maria-Theresa; Heiblum, Moty; Mahalu, Diana; Shtrikman, Hadas

    2016-01-01

    Nonlinear charge transport in superconductor–insulator–superconductor (SIS) Josephson junctions has a unique signature in the shuttled charge quantum between the two superconductors. In the zero-bias limit Cooper pairs, each with twice the electron charge, carry the Josephson current. An applied bias VSD leads to multiple Andreev reflections (MAR), which in the limit of weak tunneling probability should lead to integer multiples of the electron charge ne traversing the junction, with n integer larger than 2Δ/eVSD and Δ the superconducting order parameter. Exceptionally, just above the gap eVSD ≥ 2Δ, with Andreev reflections suppressed, one would expect the current to be carried by partitioned quasiparticles, each with energy-dependent charge, being a superposition of an electron and a hole. Using shot-noise measurements in an SIS junction induced in an InAs nanowire (with noise proportional to the partitioned charge), we first observed quantization of the partitioned charge q = e*/e=n, with n = 1–4, thus reaffirming the validity of our charge interpretation. Concentrating next on the bias region eVSD∼2Δ, we found a reproducible and clear dip in the extracted charge to q ∼0.6, which, after excluding other possibilities, we attribute to the partitioned quasiparticle charge. Such dip is supported by numerical simulations of our SIS structure. PMID:26831071

  2. Percolation effect in thick film superconductors

    SciTech Connect

    Sali, R.; Harsanyi, G.

    1994-12-31

    A thick film superconductor paste has been developed to study the properties of granulated superconductor materials, to observe the percolation effect and to confirm the theory of the conducting mechanism in the superconducting thick films. This paste was also applied to make a superconducting planar transformer. Due to high T{sub c} and advantageous current density properties the base of the paste was chosen to be of Bi(Pb)SrCaCuO system. For contacts a conventional Ag/Pt paste was used. The critical temperature of the samples were between 110 K and 115 K depending on the printed layer thickness. The critical current density at the boiling temperature of the liquid He- was between 200-300 A/cm{sup 2}. The R(T) and V(I) functions were measured with different parameters. The results of the measurements have confirmed the theory of conducting mechanism in the material. The percolation structure model has been built and described. As an application, a superconducting planar thick film transformer was planned and produced. Ten windings of the transformer were printed on one side of the alumina substrate and one winding was printed on the other side. The coupling between the two sides was possible through the substrate. The samples did not need special drying and firing parameters. After the preparation, the properties of the transformer were measured. The efficiency and the losses were determined. Finally, some fundamental advantages and problems of the process were discussed.

  3. Thermal fluctuations in superconductor/ferromagnet nanostripes

    NASA Astrophysics Data System (ADS)

    Nasti, U.; Parlato, L.; Ejrnaes, M.; Cristiano, R.; Taino, T.; Myoren, H.; Sobolewski, Roman; Pepe, G.

    2015-07-01

    Thermal fluctuations in hybrid superconductor/ferromagnetic NbN /NiCu bilayers, as well as in pure superconducting NbN, two-dimensional (2D), nanostripes, have been investigated in order to understand the origin of dark counts in superconducting nanostripes when operated as single-photon detectors in the temperature range from 4.2 to 8 K . In 2D superconductors, the dynamics of vortex motion play a significant role in the formation of a transient normal state, leading to dark-count events in current-biased nanostripes. By introducing a weak ferromagnetic overlayer on top of pure NbN, we managed to control the vortex dynamics, which subsequently enabled us to differentiate between several proposed theoretical models. In particular, a 6 -nm-thick NiCu film grown on top of 8 -nm-thick NbN nanostripes led to an enhanced critical current density in the resulting nanostructure, as well as significantly lowered fluctuation rates, as compared to pure NbN structures, leading to reduced dark counts. The enhancement of pinning in NbN /NiCu bilayers provided evidence that thermal excitations of single vortices (vortex hopping) near the edge of a 2D nanostripe were the dominant mechanism of the observed dark-count transients. On the other hand, in pure NbN the leading source of thermal fluctuations was the current-assisted thermal unbinding of vortex-antivortex pairs.

  4. Study of the glass formation of high temperature superconductors

    NASA Technical Reports Server (NTRS)

    Ethridge, Edwin C.; Kaukler, William F.; Rolin, Terry

    1992-01-01

    A number of compositions of ceramic oxide high T(sub c) superconductors were elevated for their glass formation ability by means of rapid thermal analysis during quenching, optical, and electron microscopy of the quenched samples, and with subsequent DSC measurements. Correlations between experimental measurements and the methodical composition changes identified the formulations of superconductors that can easily form glass. The superconducting material was first formed as a glass; then, with subsequent devitrification, it was formed into a bulk crystalline superconductor by a series of processing methods.

  5. Thermodynamics of the d-wave pairing in organic superconductors

    NASA Astrophysics Data System (ADS)

    Kruchinin, S.; Zolotovsky, A.; Yamashita, S.; Nakazawa, Y.

    2016-04-01

    Organic superconductors with κ-type structure are most frequently identified as nodal gap superconductors from the experimental observation of a power-law behavior in the low-temperature thermodynamic properties such as specific heat capacity. We perform series of theoretical calculations of specific heat capacity of three typical organic complexes with different transition temperatures by using Bogolyubov-de Gennes equations. The good agreement between the experimental data and the calculations demonstrates that the d-wave pairing is certainly realized in these superconductors.

  6. Spectroscopy of magnetic excitations in magnetic superconductors using vortex motion.

    PubMed

    Bulaevskii, L N; Hruska, M; Maley, M P

    2005-11-11

    In magnetic superconductors a moving vortex lattice is accompanied by an ac magnetic field which leads to the generation of spin waves. At resonance conditions the dynamics of vortices in magnetic superconductors changes drastically, resulting in strong peaks in the dc I-V characteristics at voltages at which the washboard frequency of the vortex lattice matches the spin wave frequency omegaS(g), where g are the reciprocal vortex lattice vectors. We show that if the washboard frequency lies above the magnetic gap, measurement of the I-V characteristics provides a new method to obtain information on the spectrum of magnetic excitations in borocarbides and cuprate layered magnetic superconductors.

  7. Flux-flow resistivity of three high-temperature superconductors

    SciTech Connect

    Cha, Y.S.; Evans, D.J.; Hull, J.R.; Seol, S.Y.

    1996-10-01

    Results of experiments on flux-flow resistivity (the relationship of voltage to current) of three high-temperature superconductors are described. The superconductors are a melt-cast BSCCO 2212 rod, a single filament BSCCO powder-in-tube (PIT) tape, and a multifilament PIT tape. The flux-flow resistivity of these superconductors was measured at three temperatures: 77 K (saturated liquid nitrogen), 87 K (saturated liquid argon), and 67 K (subcooled liquid nitrogen). Implications of the present results for practical applications are discussed.

  8. Rotordynamic Characterization of a Hybrid Superconductor Magnet Bearing

    NASA Technical Reports Server (NTRS)

    Ma, Ki B.; Xia, Zule H.; Cooley, Rodger; Fowler, Clay; Chu, Wei-Kan

    1996-01-01

    A hybrid superconductor magnet bearing uses magnetic forces between permanent magnets to provide lift and the flux pinning force between permanent magnets and superconductors to stabilize against instabilities intrinsic to the magnetic force between magnets. We have constructed a prototype kinetic energy storage system, using a hybrid superconductor magnet bearing to support a 42 lb. flywheel at the center. With five sensors on the periphery of the flywheel, we have monitored the position and attitude of the flywheel during its spin down. The results indicate low values of stiffnesses for the bearing. The implications of this and other consequences will be discussed.

  9. The use of high temperature superconductors to levitate lunar telescope

    NASA Technical Reports Server (NTRS)

    Brown, Beth A.

    1992-01-01

    The objective of this paper was to assist in the construction of a lunar telescope mirror model by conducting research on composite materials and other lightweight, rigid materials, and by determining how much weight can be levitated by available superconductors. It is believed that with the construction of four magnets suspended over four bulk superconductors (or vice versa), there should be no problems lifting a model mirror and stabilizing it at different positions. It may be necessary to increase the size and quality of the superconductors and/or magnets in order to achieve this.

  10. Effects of electron irradiation on high temperature superconductors and contacts to high temperature superconductors

    NASA Technical Reports Server (NTRS)

    Caton, Randall

    1988-01-01

    The discovery of a new class of ceramic superconductors with transition temperatures above liquid nitrogen has opened the doors for exciting space applications. Energy storage, pointing maneuvers, magnetic shielding, and sensitive detection of electromagnetic radiation are some of the longer term possible applications. One near term application involves low electrical resistance, high thermal resistance connections between a detector operating at approximatly or equal to 4 and the electronics operating at approximatly or equal to 77 K. The new high temperature superconductors could accomplish this providing the necessary electrical connections to the ground plane while isolating the system thermally, thus prolonging the life of the mission. With such possibilities it is clearly of value to study the effects of radiation that would be experienced during a typical space mission. In this work we focused specifically on the effects of the electron radiation environment.

  11. Shot noise of large charge quanta in superconductor/semiconductor/superconductor junctions

    NASA Astrophysics Data System (ADS)

    Camino, F. E.; Kuznetsov, V. V.; Mendez, E. E.; Schäpers, Th.; Guzenko, V. A.; Hardtdegen, H.

    2005-01-01

    We have found experimentally that the noise of ballistic electron transport in a superconductor/semiconductor/superconductor junction is enhanced relative to the value given by the general relation, SV=2eIR2coth(eV/2kT) , for two voltage regions in which this expression reduces to its thermal and shot noise limits. The noise enhancement is explained by the presence of large charge quanta, with an effective charge q*=(1+2Δ/eV)e , that generate a noise spectrum SV=2q*IR2 , as predicted by Averin and Imam [Phys. Rev. Lett. 76, 3814 (1996)]. These charge quanta result from multiple Andreev reflections at each junction interface, which are also responsible for the subharmonic gap structure observed in the voltage dependence of the junction’s conductance.

  12. Compact terahertz passive spectrometer with wideband superconductor-insulator-superconductor mixer

    NASA Astrophysics Data System (ADS)

    Kikuchi, K.; Kohjiro, S.; Yamada, T.; Shimizu, N.; Wakatsuki, A.

    2012-02-01

    We developed a compact terahertz (THz) spectrometer with a superconductor-insulator-superconductor (SIS) mixer, aiming to realize a portable and highly sensitive spectrometer to detect dangerous gases at disaster sites. The receiver cryostat which incorporates the SIS mixer and a small cryocooler except for a helium compressor has a weight of 27 kg and dimensions of 200 mm × 270 mm × 690 mm. In spite of the small cooling capacity of the cryocooler, the SIS mixer is successfully cooled lower than 4 K, and the temperature variation is suppressed for the sensitive measurement. By adopting a frequency sweeping system using photonic local oscillator, we demonstrated a spectroscopic measurement of CH3CN gas in 0.2-0.5 THz range.

  13. Compact terahertz passive spectrometer with wideband superconductor-insulator-superconductor mixer.

    PubMed

    Kikuchi, K; Kohjiro, S; Yamada, T; Shimizu, N; Wakatsuki, A

    2012-02-01

    We developed a compact terahertz (THz) spectrometer with a superconductor-insulator-superconductor (SIS) mixer, aiming to realize a portable and highly sensitive spectrometer to detect dangerous gases at disaster sites. The receiver cryostat which incorporates the SIS mixer and a small cryocooler except for a helium compressor has a weight of 27 kg and dimensions of 200 mm × 270 mm × 690 mm. In spite of the small cooling capacity of the cryocooler, the SIS mixer is successfully cooled lower than 4 K, and the temperature variation is suppressed for the sensitive measurement. By adopting a frequency sweeping system using photonic local oscillator, we demonstrated a spectroscopic measurement of CH(3)CN gas in 0.2-0.5 THz range.

  14. Low-voltage current noise in long quantum superconductor/insulator/normal-metal/insulator/superconductor junctions.

    SciTech Connect

    Kopnin, N. B.; Galperin, Y. M.; Vinokur, V.; Materials Science Division; Helsinki Univ. Tech.; L.D. Landau Inst. for Theoretical Physics; Univ. Oslo; A.F. Ioffe Physico-Tech. Inst. of Russian Academy of Sciences

    2007-01-01

    The current noise in long superconductor/insulator/normal-metal/insulator/superconductor junctions at low temperatures is sensitive to the population of the subgap states, which is far from equilibrium even at low bias voltages. A nonequilibrium distribution is established due to an interplay between voltage-driven interlevel Landau-Zener transitions and intralevel inelastic relaxation. The Fano factor (the ratio of the zero-frequency noise to the dc current) is enhanced drastically, being proportional to the number of times which a particle flies along the Andreev trajectory before it escapes from the level due to inelastic scattering. For weak Landau-Zener transitions, the enhancement is even larger due to a smaller dc current.

  15. Observation of giant supercurrent states in a superconductor-InAs/GaSb-superconductor junction

    NASA Astrophysics Data System (ADS)

    Shi, Xiaoyan; Yu, Wenlong; Jiang, Zhigang; Bernevig, B. Andrei; Pan, W.; Hawkins, S. D.; Klem, J. F.

    2015-03-01

    We report observations of the proximity effect induced giant supercurrent states in an InAs/GaSb bilayer system that is sandwiched between two superconducting tantalum electrodes to form a superconductor-InAs/GaSb-superconductor junction. Electron transport results show that the supercurrent states can be preserved in a surprisingly large temperature-magnetic field (T-H) parameter space. In addition, the evolution of differential resistance in T and H reveals an interesting superconducting gap structure. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

  16. Doping dependence of correlation effects in K1 - x Fe2 - y Se2 superconductors: LDA' + DMFT investigation

    NASA Astrophysics Data System (ADS)

    Nekrasov, I. A.; Pavlov, N. S.; Sadovskii, M. V.

    2013-11-01

    We present a detailed LDA' + DMFT investigation of the doping dependence of correlation effects in the novel K1 - x Fe2 - y Se2 superconductor. Calculations are performed at four different hole doping levels, starting from a hypothetical stoichiometric composition with the total number of electrons equal to 29 per unit cell through 28 and 27.2 electrons toward the case of 26.52, which corresponds to the chemical composition K0.76Fe1.72Se2 studied in recent ARPES experiments. In the general case, the increase in hole doping leads to quasiparticle bands in a wide energy window ±2 eV around the Fermi level becoming more broadened by lifetime effects, while correlation-induced compression of Fe-3 d LDA' bandwidths stays almost the same, of the order of 1.3 for all hole concentrations. However, close to the Fermi level, the situation is more complicated. In the energy interval from -1.0 eV to 0.4 eV, the bare Fe-3 d LDA' bands are compressed by significantly larger renormalization factors up to 5 with increased hole doping, while the value of Coulomb interaction remains the same. This fact manifests the increase in correlation effects with hole doping in the K1 - x Fe2 - y Se2 system. Moreover, in contrast to typical pnictides, K1 - x Fe2 - y Se2 does not have well-defined quasiparticle bands on the Fermi levels, but has a "pseudogap"-like dark region instead. We also find that with the growth of hole doping, Fe-3 d orbitals of various symmetries are affected by correlations differently in different parts of the Brillouin zone. To illustrate this, we determine the quasiparticle mass renormalization factors and energy shifts that transform the bare Fe-3 d LDA' bands of various symmetries into LDA' + DMFT quasiparticle bands. These renormalization factors effectively mimic more complicated energy-dependent self-energy effects and can be used to analyze the available ARPES data.

  17. Large Tunable Thermophase in Superconductor – Quantum Dot – Superconductor Josephson Junctions

    PubMed Central

    Kleeorin, Yaakov; Meir, Yigal; Giazotto, Francesco; Dubi, Yonatan

    2016-01-01

    In spite of extended efforts, detecting thermoelectric effects in superconductors has proven to be a challenging task, due to the inherent superconducting particle-hole symmetry. Here we present a theoretical study of an experimentally attainable Superconductor – Quantum Dot – Superconductor (SC-QD-SC) Josephson Junction. Using Keldysh Green’s functions we derive the exact thermo-phase and thermal response of the junction, and demonstrate that such a junction has highly tunable thermoelectric properties and a significant thermal response. The origin of these effects is the QD energy level placed between the SCs, which breaks particle-hole symmetry in a gradual manner, allowing, in the presence of a temperature gradient, for gate controlled appearance of a superconducting thermo-phase. This thermo-phase increases up to a maximal value of ±π/2 after which thermovoltage is expected to develop. Our calculations are performed in realistic parameter regimes, and we suggest an experimental setup which could be used to verify our predictions. PMID:27734919

  18. Large Tunable Thermophase in Superconductor – Quantum Dot – Superconductor Josephson Junctions

    NASA Astrophysics Data System (ADS)

    Kleeorin, Yaakov; Meir, Yigal; Giazotto, Francesco; Dubi, Yonatan

    2016-10-01

    In spite of extended efforts, detecting thermoelectric effects in superconductors has proven to be a challenging task, due to the inherent superconducting particle-hole symmetry. Here we present a theoretical study of an experimentally attainable Superconductor – Quantum Dot – Superconductor (SC-QD-SC) Josephson Junction. Using Keldysh Green’s functions we derive the exact thermo-phase and thermal response of the junction, and demonstrate that such a junction has highly tunable thermoelectric properties and a significant thermal response. The origin of these effects is the QD energy level placed between the SCs, which breaks particle-hole symmetry in a gradual manner, allowing, in the presence of a temperature gradient, for gate controlled appearance of a superconducting thermo-phase. This thermo-phase increases up to a maximal value of ±π/2 after which thermovoltage is expected to develop. Our calculations are performed in realistic parameter regimes, and we suggest an experimental setup which could be used to verify our predictions.

  19. Factors affecting characterization of bulk high-temperature superconductors

    SciTech Connect

    Hull, J.R.

    1997-11-01

    Three major factors affect the characterization of bulk high-temperature superconductors in terms of their levitation properties during interaction with permanent magnets. First, the appropriate parameter for the permanent magnet is internal magnetization, not the value of the magnetic field measured at the magnet`s surface. Second, although levitation force grows with superconductor thickness and surface area, for a given permanent magnet size, comparison of levitation force between samples is meaningful when minimum values are assigned to the superconductor size parameters. Finally, the effect of force creep must be considered when time-averaging the force measurements. In addition to levitational force, the coefficient of friction of a levitated rotating permanent magnet may be used to characterize the superconductor.

  20. Superconducting properties of copper oxide high-temperature superconductors

    PubMed Central

    Chen, Guanhua; Langlois, Jean-Marc; Guo, Yuejin; Goddard, William A.

    1989-01-01

    The equations for the magnon pairing theory of high-temperature copper-oxide-based superconductors are solved and used to calculate several properties, leading to results for specific heat and critical magnetic fields consistent with experimental results. In addition, the theory suggests an explanation of why there are two sets of transition temperatures (Tc ≈ 90 K and Tc ≈ 55 K) for the Y1Ba2Cu3O6+x class of superconductors. It also provides an explanation of why La2-xSrxCuO4 is a superconductor for only a small range of x (and suggests an experiment to independently test the theory). These results provide support for the magnon pairing theory of high-temperature superconductors. On the basis of the theory, some suggestions are made for improving these materials. PMID:16594038

  1. Theoretical modeling of critical temperature increase in metamaterial superconductors

    NASA Astrophysics Data System (ADS)

    Smolyaninov, Igor I.; Smolyaninova, Vera N.

    2016-05-01

    Recent experiments have demonstrated that the metamaterial approach is capable of a drastic increase of the critical temperature Tc of epsilon near zero (ENZ) metamaterial superconductors. For example, tripling of the critical temperature has been observed in Al -A l2O3 ENZ core-shell metamaterials. Here, we perform theoretical modeling of Tc increase in metamaterial superconductors based on the Maxwell-Garnett approximation of their dielectric response function. Good agreement is demonstrated between theoretical modeling and experimental results in both aluminum- and tin-based metamaterials. Taking advantage of the demonstrated success of this model, the critical temperature of hypothetic niobium-, Mg B2- , and H2S -based metamaterial superconductors is evaluated. The Mg B2 -based metamaterial superconductors are projected to reach the liquid nitrogen temperature range. In the case of a H2S -based metamaterial Tc appears to reach ˜250 K.

  2. Proceedings of the physics and chemistry of oxide superconductors

    SciTech Connect

    Iye, Y.; Yasuoka, H.

    1992-01-01

    This book contains the proceedings of the physics and chemistry of oxide superconductors. Topics covered include: materials; electronic states; spectroscopy; lattice properties; pressure effects; carrier dynamics; doping effects; spin dynamics; magnetism; strong correlation, and superconducting properties.

  3. Impurity-induced bound states in superconductors with topological order.

    PubMed

    Wang, Fei; Liu, Qin; Ma, Tianxing; Jiang, Xunya

    2012-11-14

    The study of classical spins in topological insulators (Liu and Ma 2009 Phys. Rev. B 80 115216) is generalized to topological superconductors. Based on the characteristic features of the so-called F-function, the Bogoliubov-de Gennes Hamiltonian for superconductors is classified to positive, negative, and zero 'gap' categories for topologically trivial and nontrivial phases of a topological superconductor as well as a BCS superconductor, respectively. It is found that the F-function determines directly the presence or absence of localized excited states, induced by bulk classical spins and nonmagnetic impurities, in the superconducting gap and their persistence with respect to impurity strength. Our results provide an alternative way to identify topologically insulating and superconducting phases in experiments without resorting to the surface properties.

  4. Nonlinear electrodynamics of vortex matter in hard superconductors (Review)

    NASA Astrophysics Data System (ADS)

    Voloshin, I. F.; Fisher, L. M.; Yampol'Skiĭ, V. A.

    2010-01-01

    Nontrivial electrodynamic properties of vortex matter, which are due to a specific nonlinearity of the material equations, in hard superconductors are discussed: collapse of the transport current and the static magnetization of superconductors by an external orthogonal ac magnetic field; appearance of jumps in the time dependence of the electric field at the boundary of a sample as a result of nonlinear interaction of waves having different frequencies; specific staged penetration of an electromagnetic field into anisotropic superconductors. Nonlocal effects resulting in a large modification of nonlinear phenomena are also examined. Special attention is given to a discussion of the unique phenomenon of macroturbulent instability, associated with the flow of Abrikosov vortices, in hard superconductors. Most results presented are based on original experimental and theoretical investigations performed with the participation of the present authors.

  5. High-temperature superconductor applications development at Argonne National Laboratory

    SciTech Connect

    Hull, J.R.; Poeppel, R.B.

    1992-02-09

    Developments at Argonne National Laboratory of near and intermediate term applications using high-temperature superconductors are discussed. Near-term applications of liquid-nitrogen depth sensors, current leads, and magnetic bearings are discussed in detail.

  6. The Discovery of a Class of High-Temperature Superconductors.

    ERIC Educational Resources Information Center

    Muller, K. Alex; Bednorz, J. Georg

    1987-01-01

    Describes the new class of oxide superconductors, the importance of these materials, and the concepts that led to its discovery. Summarizes the discovery itself and its early confirmation. Discusses the observation of a superconductive glass state in percolative samples. (TW)

  7. Antiferromagnetic topological superconductor and electrically controllable Majorana fermions.

    PubMed

    Ezawa, Motohiko

    2015-02-01

    We investigate the realization of a topological superconductor in a generic bucked honeycomb system equipped with four types of mass-generating terms, where the superconductor gap is introduced by attaching the honeycomb system to an s-wave superconductor. Constructing the topological phase diagram, we show that Majorana modes are formed in the phase boundary. In particular, we analyze the honeycomb system with antiferromagnetic order in the presence of perpendicular electric field E(z). It becomes topological for |E(z)|>E(z)(cr) and trivial for |E(z)|superconductor by controlling applied electric field. One Majorana zero-energy bound state appears at the phase boundary. We can arbitrarily control the position of the Majorana fermion by moving the spot of applied electric field, which will be made possible by a scanning tunneling microscope probe.

  8. Scaling rules for critical current density in anisotropic biaxial superconductors

    NASA Astrophysics Data System (ADS)

    Li, Yingxu; Kang, Guozheng; Gao, Yuanwen

    2016-06-01

    Recent researches highlight the additional anisotropic crystallographic axis within the superconducting plane of high temperature superconductors (HTS), demonstrating the superconducting anisotropy of HTS is better understood in the biaxial frame than the previous uniaxial coordinates within the superconducting layer. To quantitatively evaluate the anisotropy of flux pinning and critical current density in HTS, we extend the scaling rule for single-vortex collective pinning in uniaxial superconductors to account for flux-bundle collective pinning in biaxial superconductors. The scaling results show that in a system of random uncorrected point defects, the field dependence of the critical current density is described by a unified function with the scaled magnetic field of the isotropic superconductor. The obtained angular dependence of the critical current density depicts the main features of experimental observations, considering possible corrections due to the strong-pinning interaction.

  9. Effect of interlayer coupling on the coexistence of antiferromagnetism and superconductivity in Fe pnictide superconductors: A study of Ca0.74 (1 )La0.26 (1 )(Fe1 -xCox)As2 single crystals

    NASA Astrophysics Data System (ADS)

    Jiang, Shan; Liu, Lian; Schütt, Michael; Hallas, Alannah M.; Shen, Bing; Tian, Wei; Emmanouilidou, Eve; Shi, Aoshuang; Luke, Graeme M.; Uemura, Yasutomo J.; Fernandes, Rafael M.; Ni, Ni

    2016-05-01

    We report the transport, thermodynamic, muon spin relaxation, and neutron study of the Ca0.74 (1 )La0.26 (1 ) (Fe1 -xCox )As2 single crystals, mapping out the temperature-doping level phase diagram. Upon Co substitution on the Fe site, the structural and magnetic phase transitions in this 112 compound are suppressed and superconductivity up to 20 K occurs. Our measurements of the superconducting and magnetic volume fractions show that these two phases coexist microscopically in the underdoped region, in contrast to the related Ca10(Pt3As8 )((Fe1 -xPtx )2As2 )5 (10-3-8) compound, where coexistence is absent. Supported by model calculations, we discuss the differences in the phase diagrams of the 112 and 10-3-8 compounds in terms of the FeAs interlayer coupling, whose strength is affected by the character of the spacer layer, which is metallic in the 112 compound and insulating in the 10-3-8 compound.

  10. Blowpipe Mineralogy for Physics/Environment: Highest-Possible-Tc SuperConductor (Beyond: (but via!!!) MgB2 , Cuprates, Pnictides) Quest; BOTH PERMANENT FOREVER Carb-IDES SOLID-State Sequestration AND Drought(s)-Elimination

    NASA Astrophysics Data System (ADS)

    Segler, Kurt; Williams, Wendell; Siegel, Edward

    2013-03-01

    Detailed are old blowpipe new applications: charcoal-block reduction of borates to yield (''N-NW'' of MgB2) Overhauser-[PR 35,1,411(1987); Intl.J.Mod.Phys.1, 2 & 4, 927(1987)]-''land'' predicted high-EST-POSSIBLE Tc SC

  11. Blowpipe Mineralogy for Physics/Environment: Highest-Possible-Tc SuperConductor (Beyond: (but via!!!) MgB2, Cuprates, Pnictides) Quest; BOTH PERMANENT FOREVER Carb-IDES SOLID-State Sequestration AND Drought(s)-Elimination

    NASA Astrophysics Data System (ADS)

    Segler, Kurt; Williams, Wendell; Siegel, Edward

    2011-03-01

    Detailed are old blowpipe new applications: charcoal-block reduction of borates to yield ("N-NW" of MgB2) Overhauser-[PR 35,1,411(1987); Intl.J.Mod.Phys.1, 2 & 4, 927(1987)]-"land" predicted high-EST-POSSIBLE Tc SC "LiD2"; very-early: Siegel[Phys.Stat.Sol.(a)11,45(1972);Semiconductors.and Insulators 5: 39,47,62(1979)] carb-IDES SOLID-state phase-TRANSITIONED CHEMICALLY-REDOX"-REACTED STABLE PERMANENT LONG-term NOT "CO2" BUT C-sequestration: PROFITABLE "Grab and Sell" TRUMPS "cap and trade"!!!; Mott alloying/vertical metal-insulator transitions in "borax-(GLASS)-beads"; and very-earlySiegel [{3rd Intl.Conf.Alt.Energy }(1980)-vol.5/p.459!!!] "FLYING-WATER" Hindenberg-effect (H2-UP;H2O-DOWN) via Hydrogen-maximal-Archimedes-buoyancy "chemical-rain-in-pipelines", only via Siegel proprietary "magnetic-hydrogen-valve"(MHV): Renewables-Hydrogen-Water flexible versatile agile scaleable retrofitable integrated operating-system for PERMANENT drought(s)-elimination FOREVER!!!

  12. Characteristics of an electromagnetic levitation system using a bulk superconductor

    SciTech Connect

    Senba, A.; Kitahara, H.; Ohsaki, H.; Masada, E.

    1996-09-01

    It is beneficial to apply a high-Tc bulk superconductor as a large flux source to an electromagnetic levitation system, which needs large amounts of levitation force. The authors made an attractive-type electromagnetic levitation system using a hybrid magnet that mainly consisted of bulk superconductor and control coils to confirm the principle of the levitation, and obtained characteristics of its system by both experiment and numerical analysis with magnetic circuit calculation. This is applicable to maglev transportation systems.

  13. Inhomogeneities in single crystals of cuprate oxide superconductors

    NASA Technical Reports Server (NTRS)

    Moorjani, K.; Bohandy, J.; Kim, B. F.; Adrian, F. J.

    1991-01-01

    The next stage in the evolution of experimental research on the high temperature superconductors will require high quality single crystals and epitaxially grown crystalline films. However, inhomogeneities and other defects are not uncommon in single crystals of cuprate oxide superconductors, so a corollary requirement will be a reliable method for judging the quality of these materials. The application of magnetically modulated resistance methods in this task is briefly described and illustrated.

  14. On the electrodynamics of Josephson effect in anisotropic superconductors

    SciTech Connect

    Mints, R.G.

    1989-01-01

    Specificities of Josephson effect electrodynamics in anisotropic superconductors are of considerable interest for the study of high temperature superconductors with strongly anisotropic layered structure. In this paper the authors give the calculation for the tunnel Josephson contact of an isolated vortex, the law of dispersion of its low-amplitude oscillations, the critical field H/sub cl/ for the penetration of magnetic flux, and the maximum current across a rectangular contact.

  15. Magneto-optical imaging of transport current densities in superconductors

    SciTech Connect

    Crabtree, G.W.; Welp, U.; Gunter, D.O.; Zhong, W.; Balachandran, U.; Haldar, P.; Sokolowski, R.S.; Vlasko-Vlasov, V.K.; Nikitenko, V.I.

    1995-12-31

    Direct imaging of the paths of transport currents in superconductors creates many new possibilities for exploring the basic features of vortex pinning mechanisms and for improving the performance of superconducting materials. A technique for imaging the path and magnitude of the transport current density flowing in superconductors is described. Results are given for a 37-filament BSCCO 2223 powder-in-tube wire, showing a highly inhomogeneous current path within the filaments.

  16. Stability of magnetic tip/superconductor levitation systems

    NASA Astrophysics Data System (ADS)

    K. Alqadi, M.

    2015-11-01

    The vertical stability of a magnetic tip over a superconducting material is investigated by using the critical state and the frozen image models. The analytical expressions of the stiffness and the vibration frequency about the equilibrium position are derived in term of the geometrical parameters of the magnet/superconductor system. It is found that the stability of the system depends on the shape of the superconductor as well as its thickness.

  17. Meissner effect in diffusive normal metal/superconductor junctions

    NASA Astrophysics Data System (ADS)

    Yokoyama, Takehito; Tanaka, Yukio; Golubov, Alexander; Inoue, Jun-ichiro; Asano, Yasuhiro

    2005-10-01

    Meissner effect in the diffusive normal metal/insulator/s-wave superconductor junctions is studied in the presence of the magnetic impurities for various situations, where we have used the Usadel equation with Nazarov’s generalized boundary condition. It is shown that the susceptibility of the diffusive normal metal for s-wave superconductor is almost independent of the height of the insulating barrier at the interface.

  18. Tunneling Measurements of Charge Imbalance of Non-Equilibrium Superconductors

    NASA Astrophysics Data System (ADS)

    Yagi, R.; Utsunomiya, K.; Tsuboi, K.; Kubota, T.; Terao, Y.; Ikebuchi, Y.

    2008-10-01

    We have observed excess current due to charge imbalance in the voltage-current characteristics of a superconductor-insulator-normal (SIN) tunnel junction connected to a non-equilibrium superconductor. It was found that that the excess current was unchanged against the bias voltage as expected from the theory of charge imbalance. The estimated excess current approximately agreed with the estimation from one-dimensional diffusion model of charge imbalance transport.

  19. System and method for quench and over-current protection of superconductor

    DOEpatents

    Huang, Xianrui; Laskaris, Evangelos Trifon; Sivasubramaniam, Kiruba Haran; Bray, James William; Ryan, David Thomas; Fogarty, James Michael; Steinbach, Albert Eugene

    2005-05-31

    A system and method for protecting a superconductor. The system may comprise a current sensor operable to detect a current flowing through the superconductor. The system may comprise a coolant temperature sensor operable to detect the temperature of a cryogenic coolant used to cool the superconductor to a superconductive state. The control circuit is operable to estimate the superconductor temperature based on the current flow and the coolant temperature. The system may also be operable to compare the estimated superconductor temperature to at least one threshold temperature and to initiate a corrective action when the superconductor temperature exceeds the at least one threshold temperature.

  20. Correction of magnetization sextupole and decapole in a 5 centimeter bore SSC dipole using passive superconductor

    SciTech Connect

    Green, M.A.

    1991-05-01

    Higher multipoles due to magnetization of the superconductor in four and five centimeter bore Superconducting Super Collider (SSC) superconducting dipole magnets have been observed. The use of passive superconductor to correct out the magnetization sextupole has been demonstrated on two dipoles built by the Lawrence Berkeley Laboratory (LBL). This reports shows how passive correction can be applied to the five centimeter SSC dipoles to remove sextupole and decapole caused by magnetization of the dipole superconductor. Two passive superconductor corrector options will be presented. The change in magnetization sextupole and decapole due to flux creep decay of the superconductor during injection can be partially compensated for using the passive superconductor. 9 refs; 5 figs.

  1. Emergence of high-mobility minority holes in the electrical transport of the Ba (Fe1 -xMnxAs )2 iron pnictides

    NASA Astrophysics Data System (ADS)

    Urata, T.; Tanabe, Y.; Huynh, K. K.; Heguri, S.; Oguro, H.; Watanabe, K.; Tanigaki, K.

    2015-05-01

    In Fe pnictide (Pn) superconducting materials, neither Mn nor Cr doping to the Fe site induces superconductivity, even though hole carriers are generated. This is in strong contrast with the superconductivity appearing when holes are introduced by alkali-metal substitution on the insulating blocking layers. We investigate in detail the effects of Mn doping on magnetotransport properties in Ba (Fe1 -xMnxAs )2 for elucidating the intrinsic reason. The negative Hall coefficient for x =0 estimated in the low magnetic field (B ) regime gradually increases as x increases, and its sign changes to a positive one at x =0.020 . Hall resistivities as well as simultaneous interpretation using the magnetoconductivity tensor including both longitudinal and transverse transport components clarify that minority holes with high mobility are generated by the Mn doping via spin-density wave transition at low temperatures, while original majority electrons and holes residing in the paraboliclike Fermi surfaces of the semimetallic Ba (FeAs )2 are negligibly affected. Present results indicate that the mechanism of hole doping in Ba (Fe1 -xMnxAs )2 is greatly different from that of the other superconducting FePn family.

  2. Magnetic entropy change plateau in a geometrically frustrated layered system: FeCrAs-like iron-pnictide structure as a magnetocaloric prototype.

    PubMed

    Florez, J M; Vargas, P; Garcia, C; Ross, C A

    2013-06-01

    Monte Carlo modeling suggests that the magnetothermal features of the Fe2P-structured FeCrAs-like compound offer a promising route for the design of magnetocaloric materials. The prototype structure is modeled as antiferromagnetically coupled layered Heisenberg systems mimicking the distorted Kagome/triangular stacked architecture of FeCrAs iron-pnictide. The magnetic entropy change ΔSm(T) presents a plateau-like behavior which can be tailored by tuning either the JCr-Fe/JCr-Cr exchange energy ratio or the magnetic field. The plateau is defined by cooperative spin ordering within a ferrimagnetic region which exists between two critical temperatures separating at the lower bound (Tac) a canted antiferromagnetic phase and at the upper bound (Tdc) the thermally disordered phase. The refrigerant capacity and adiabatic change of temperature are A(H)(Tdc - Tac) and A(H)Tp/Cm respectively, with Tac < Tp < Tdc, A(H) an increasing positive function of the field defining the height of the plateau and Cm the magnetic specific heat, whose critical behavior is related to the T(a,d)(c) values.

  3. Improved half-metallic ferromagnetism of transition-metal pnictides and chalcogenides calculated with a modified Becke-Johnson exchange potential

    NASA Astrophysics Data System (ADS)

    Guo, San-Dong; Liu, Bang-Gui

    2011-02-01

    We use a density-functional-theory (DFT) approach with a modified Becke-Johnson exchange plus local density approximation (LDA) correlation potential (mBJLDA) (semi-local, orbital-independent, producing accurate semiconductor gaps, see Tran F. and Blaha P., Phys. Rev. Lett., 102 (2009) 226401) to investigate the electronic structures of zincblende transition-metal (TM) pnictides and chalcogenides akin to semiconductors. Our results show that this potential does not yield visible changes in wide TM d-t2g bands near the Fermi level, but makes the occupied minority-spin p-bands lower by 0.25-0.35 eV and the empty (or nearly empty) minority-spin eg bands across the Fermi level higher by 0.33-0.73 eV. Consequently, mBJLDA, having no atom-dependent parameters, makes zincblende MnAs become a truly half-metallic (HM) ferromagnet with a HM gap (the key parameter) 0.318 eV, being consistent with experiment. For zincblende MnSb, CrAs, CrSb, CrSe, or CrTe, the HM gap is enhanced by 19-56% compared to LDA and generalized gradient approximation results. The improved HM ferromagnetism can be understood in terms of the mBJLDA-enhanced spin exchange splitting.

  4. Current-carrying capacity of composite superconductors

    SciTech Connect

    Andrianov, V.; Baeu, V.; Ivanov, S.; Mints, R.; Rakhmanov, A.

    1983-05-01

    The maximum transport current I /SUB m/ of the composite superconductors is investigated both theoretically and experimentally. It is shown that the high values of the transport current observed in these materials is due to the non-linear part of the current-voltage characteristic in the range of low electric fields (E less than or equal to 10/sup -6/ Vcm/sup -1/). The conductors of rather different structure with Nb-Ti superconducting filaments were tested in a wide range of the external parameters. It is shown that in the external magnetic fields B /SUB a/ greater than or equal to 1T the ratio I /SUB m/ /I /SUB c/ (where I /SUB c/ is the critical current) is the universal function of the single dimensionless parameter which depends on the sample properties and the external conditions. The theory and experiment are in a good agreement.

  5. Quench properties of high current superconductors

    SciTech Connect

    Garber, M; Sampson, W B

    1980-01-01

    A technique has been developed which allows the simultaneous determination of most of the important parameters of a high current superconductor. The critical current, propagation velocity, normal state resistivity, magnetoresistance, and enthalpy are determined as a function of current and applied field. The measurements are made on non-inductive samples which simulate conditions in full scale magnets. For wide, braided conductors the propagation velocity was found to vary approximately quadratically with current in the 2 to 5 kA region. A number of conductors have been tested including some Nb/sub 3/Sn braids which have critical currents in excess of 10 kA at 5 T, 4.2 K.

  6. Gravimeter using high-temperature superconductor bearing.

    SciTech Connect

    Hull, J. R.

    1998-09-11

    We have developed a sensitive gravimeter concept that uses an extremely low-friction bearing based on a permanent magnet (PM) levitated over a high-temperature superconductor (HTS). A mass is attached to the PM by means of a cantilevered beam, and the combination of PM and HTS forms a bearing platform that has low resistance to rotational motion but high resistance to horizontal, vertical, or tilting motion. The combination acts as a low-loss torsional pendulum that can be operated in any orientation. Gravity acts on the cantilevered beam and attached mass, accelerating them. Variations in gravity can be detected by time-of-flight acceleration, or by a control coil or electrode that would keep the mass stationary. Calculations suggest that the HTS gravimeter would be as sensitive as present-day superconducting gravimeters that need cooling to liquid helium temperatures, but the HTS gravimeter needs cooling only to liquid nitrogen temperatures.

  7. Noncommutative effects of spacetime on holographic superconductors

    NASA Astrophysics Data System (ADS)

    Ghorai, Debabrata; Gangopadhyay, Sunandan

    2016-07-01

    The Sturm-Liouville eigenvalue method is employed to analytically investigate the properties of holographic superconductors in higher dimensions in the framework of Born-Infeld electrodynamics incorporating the effects of noncommutative spacetime. In the background of pure Einstein gravity in noncommutative spacetime, we obtain the relation between the critical temperature and the charge density. We also obtain the value of the condensation operator and the critical exponent. Our findings suggest that the higher value of noncommutative parameter and Born-Infeld parameter make the condensate harder to form. We also observe that the noncommutative structure of spacetime makes the critical temperature depend on the mass of the black hole and higher value of black hole mass is favourable for the formation of the condensate.

  8. Fabrication and wire extrusion of ceramic superconductors

    SciTech Connect

    Poeppel, R.B.; Balachandran, U.; Singh, J.P.; Dusek, J.T.; Picciolo, J.J.; Dorris, S.E.; Lanagan, M.T.; Goretta, K.C.; Youngdahl, C.A.; Hull, J.R.

    1991-05-01

    Many applications of high-temperature superconductors (HTSs) will depend on the ability to fabricate these materials into long lengths with suitable electrical and mechanical properties maintained over the entire length. The program described in this paper is focused on improvement of the relevant material properties of HTSs and on development of fabrication methods that can be transferred to industry for production of commercial conductors. Our research has resulted in advances in fabrication methods that improve the performance of long lengths of polycrystalline HTS wires and tapes. We have examined the Y-Ba-Cu-O (YBCO), Bi-Sr-Ca-Cu-O (BSCCO), and Tl- Ba-Ca-Cu-O (TBCCO) classes of HTSs. Significant results from our research and work by contemporaries are reported in the various sections of the paper. 28 refs.

  9. Dynamics of Josephson pancakes in layered superconductors

    SciTech Connect

    Mints, R.G.; Snapiro, I.B.

    1994-03-01

    We consider a pointlike vortex in a layered superconductor with linear defects in the superconducting layers. We treat these defects as Josephson junctions with high critical current density. We consider the electrodynamics of these junctions within the framework of nonlocal Josephson electrodynamics. We show that Josephson current through a linear defect in a superconducting layer results in a pointlike vortex with a superconducting core residing in this layer (Josephson pancake). We find the mobility of a Josephson pancake. We consider a small amplitude wave in a Josephson junction with nonlocal electrodynamics. We treat a bending wave for an infinite stack of Josephson pancakes. We find the dispersion relation for these waves. We show that their velocities tend to a certain finite limit when the wavelength tends to infinity.

  10. Holographic entanglement entropy in imbalanced superconductors

    NASA Astrophysics Data System (ADS)

    Dutta, Arghya; Modak, Sujoy Kumar

    2014-01-01

    We study the behavior of holographic entanglement entropy (HEE) for imbalanced holographic superconductors. We employ a numerical approach to consider the robust case of fully back-reacted gravity system. The hairy black hole solution is found by using our numerical scheme. Then it is used to compute the HEE for the superconducting case. The cases we study show that in presence of a mismatch between two chemical potentials, below the critical temperature, superconducting phase has a lower HEE in comparison to the AdS-Reissner-Nordström black hole phase. Interestingly, the effects of chemical imbalance are different in the contexts of black hole and superconducting phases. For black hole, HEE increases with increasing imbalance parameter while it behaves oppositely for the superconducting phase. The implications of these results are discussed.

  11. ``Soft'' Anharmonic Vortex Glass in Ferromagnetic Superconductors

    NASA Astrophysics Data System (ADS)

    Radzihovsky, Leo; Ettouhami, A. M.; Saunders, Karl; Toner, John

    2002-03-01

    Ferromagnetic order in superconductors can induce a spontaneous vortex (SV) state. For external field H=0, rotational symmetry guarantees a vanishing tilt modulus of the SV solid, leading to drastically different behavior than that of a conventional, external-field-induced vortex solid. We show that quenched disorder and anharmoinc effects lead to elastic moduli that are wave-vector dependent out to arbitrarily long length scales, and to non-Hookean elasticity. The latter implies that for weak external fields H, the magnetic induction scales universally like B(H) ~ B(0)+ cH^α, with α ≈ 0.72. For weak disorder, we predict the SV solid is a topologically ordered glass, in the ``columnar elastic glass'' universality class.

  12. Holographic p-wave superconductor with disorder

    NASA Astrophysics Data System (ADS)

    Areán, D.; Farahi, A.; Pando Zayas, L. A.; Salazar Landea, I.; Scardicchio, A.

    2015-07-01

    We implement the effects of disorder on a holographic p-wave superconductor by introducing a random chemical potential which defines the local energy of the charge carriers. Since there are various possibilities for the orientation of the vector order parameter, we explore the behavior of the condensate in the parallel and perpendicular directions to the introduced disorder. We clarify the nature of various branches representing competing solutions and construct the disordered phase diagram. We find that moderate disorder enhances superconductivity as determined by the value of the condensate. Though we mostly focus on uncorrelated noise, we also consider a disorder characterized by its spectral properties and study in detail its influence on the spectral properties of the condensate and charge density. We find fairly universal responses of the resulting power spectra characterized by linear functions of the disorder power spectrum.

  13. Vertical Transport in Ferroelectric/Superconductor Heterostructures

    NASA Astrophysics Data System (ADS)

    Begon-Lours, Laura; Trastoy, Juan; Bernard, Rozenn; Jacquet, Eric; Carretero, Cecile; Bouzehouane, Karim; Fusil, Stephane; Garcia, Vincent; Xavier, Stephane; Girod, Stephanie; Deranlot, Cyrile; Bibes, Manuel; Barthelemy, Agnes; Villegas, Javier E.

    2015-03-01

    We study electric field-effects in superconducting films by measuring vertical transport in ferroelectric/superconductor heterostructures. These are based on ultrathin (4 to 8 nm thick) BiFeO3-Mn grown on YBa2Cu3O7 by pulsed laser deposition. Nanoscale contacts are defined on the BiFeO3 via a series of nanofabrication steps which include e-beam lithography, metal deposition (Nb or Co capped with Pt) and lift-off. Conductive-tip atomic force microscopy and piezoresponse force microscopy are used to characterize the transport across the ferroelectric barrier as a function of its polarization (up/down). The observed electro-resistance, measured at various temperatures, allows studying the different electric-field screening in the normal and superconducting states. Work supported by DIM Oxymore.

  14. Vortex flow for a holographic superconductor

    SciTech Connect

    Maeda, Kengo; Okamura, Takashi

    2011-03-15

    We investigate energy dissipation associated with the motion of the scalar condensate in a holographic superconductor model constructed from the charged scalar field coupled to the Maxwell field. Upon application of constant magnetic and electric fields, we analytically construct the vortex-flow solution and find the vortex-flow resistance near the second-order phase transition where the scalar condensate begins. The characteristic feature of the nonequilibrium state agrees with the one predicted by the time-dependent Ginzburg-Landau (TDGL) theory. We evaluate the kinetic coefficient in the TDGL equation along the line of the second-order phase transition. At zero magnetic field, the other coefficients in the TDGL equation are also evaluated just below the critical temperature.

  15. Equivalence of topological insulators and superconductors

    NASA Astrophysics Data System (ADS)

    Ortiz, Gerardo; Cobanera, Emilio

    Systems of free fermions are classified by symmetry, space dimensionality, and topological properties described by K-homology. We show that by taking a many-body/Fock space viewpoint it becomes possible to establish equivalences of topological insulators and superconductors in terms of duality transformations. These mappings connect topologically inequivalent systems of fermions, jumping across entries in existent classification tables, because of the phenomenon of symmetry transmutation by which a symmetry and its dual partner have identical algebraic properties but very different physical interpretations and electromagnetic response. Since our analysis extends to interacting fermion systems we also briefly discuss some such applications. To illustrate main concepts we will present dual superconducting partners of paradigmatic models, such as the Haldane Chern insulator as well as a quantum spin Hall effect graphene model.

  16. High temperature superconductors for magnetic suspension applications

    NASA Technical Reports Server (NTRS)

    Mcmichael, C. K.; Cooley, R. S.; Chen, Q. Y.; Ma, K. B.; Lamb, M. A.; Meng, R. L.; Chu, C. W.; Chu, W. K.

    1994-01-01

    High temperature superconductors (HTS) hold the promise for applications in magnetic levitation bearings, vibration damping, and torque coupling. Traditional magnetic suspension systems require active feedback and vibration controls in which power consumption and low frequency vibration are among the major engineering concerns. HTS materials have been demonstrated to be an enabling approach towards such problems due to their flux trapping properties. In our laboratory at TCSUH, we have been conducting a series of experiments to explore various mechanical applications using HTS. We have constructed a 30 lb. model flywheel levitated by a hybrid superconducting magnetic bearing (HSMB). We are also developing a levitated and vibration-dampled platform for high precision instrumentation. These applications would be ideal for space usages where ambient temperature is adequate for HTS to operate properly under greatly reduced cryogenic requirements. We will give a general overview of these potential applications and discuss the operating principles of the HTS devices we have developed.

  17. Study of high temperature oxide superconductors

    NASA Astrophysics Data System (ADS)

    Wu, M. K.; Ashburn, J. R.; Higgins, C. A.; Carswell, W. E.; Loo, B. H.; Burns, D. H.; Ibrahim, A.; Rolin, T.; Peters, P. N.; Sisk, R. C.

    1988-01-01

    Experimental studies are reported whose results indicate that proper processing procedures are critical to the formation of high-temperature copper oxide superconductors. Superconducting 123 films can be fabricated using the green 211 phase as a substrate. The transition temperature and transition width characteristics of these films are better than those obtained when other oxide compounds are used as substrates. A compact or single-crystal 211 phase will be desirable as a substrate for high-quality thin films. A new high T(c) copper oxide compound with nonrare earth elements was prepared using high-temperature processing. A YBa2Cu3O7-Ag composite with improved electrical conductivity was also prepared.

  18. High temperature superconductor materials and applications

    NASA Technical Reports Server (NTRS)

    Doane, George B., III. (Editor); Banks, Curtis; Golben, John

    1991-01-01

    One of the areas concerned itself with the investigation of the phenomena involved in formulating and making in the laboratory new and better superconductor material with enhanced values of critical current and temperature. Of special interest were the chemistry, physical processes, and environment required to attain these enhanced desirable characteristics. The other area concerned itself with producing high temperature superconducting thin films by pulsed laser deposition techniques. Such films are potentially very useful in the detection of very low power signals. To perform this research high vacuum is required. In the course of this effort, older vacuum chambers were maintained and used. In addition, a new facility is being brought on line. This latter activity has been replete with the usual problems of bringing a new facility into service. Some of the problems are covered in the main body of this report.

  19. Hole-doped cuprate high temperature superconductors

    NASA Astrophysics Data System (ADS)

    Chu, C. W.; Deng, L. Z.; Lv, B.

    2015-07-01

    Hole-doped cuprate high temperature superconductors have ushered in the modern era of high temperature superconductivity (HTS) and have continued to be at center stage in the field. Extensive studies have been made, many compounds discovered, voluminous data compiled, numerous models proposed, many review articles written, and various prototype devices made and tested with better performance than their nonsuperconducting counterparts. The field is indeed vast. We have therefore decided to focus on the major cuprate materials systems that have laid the foundation of HTS science and technology and present several simple scaling laws that show the systematic and universal simplicity amid the complexity of these material systems, while referring readers interested in the HTS physics and devices to the review articles. Developments in the field are mostly presented in chronological order, sometimes with anecdotes, in an attempt to share some of the moments of excitement and despair in the history of HTS with readers, especially the younger ones.

  20. Thermodynamic Properties in Triangular-Lattice Superconductors

    NASA Astrophysics Data System (ADS)

    Ma, Xixiao; Qin, Ling; Zhao, Huaisong; Lan, Yu; Feng, Shiping

    2016-06-01

    The study of superconductivity arising from doping a Mott insulator has become a central issue in the area of superconductivity. Within the framework of the kinetic-energy-driven superconducting (SC) mechanism, we discuss the thermodynamic properties in the triangular-lattice cobaltate superconductors. It is shown that a sharp peak in the specific heat appears at the SC transition temperature T_c, and then the specific heat varies exponentially as a function of temperature for temperatures T