Longitudinal spin fluctuations in nickel
Boeni, P. , Villigen ); Martinez, J.L. ); Tranquada, J.M. )
1989-10-10
The longitudinal and transverse spin fluctuations in Ni have been measured below {Tc} by means of polarized neutron scattering in the momentum range 0.06 < q < 0.18 {angstrom}{sup -1}. In transverse scans spin wave peaks at E{sub q} = Dq{sup 2} appear as expected from early measurements performed with unpolarized neutrons. The longitudinal magnetic scattering {sub {chi}L}(q, E), on the other hand, is quasielastic without any signature of inelastic peaks near E{sub q}. The q and T dependences of {sub {chi}L}(q, E) resemble the paramagnetic scattering above {Tc}, i.e., the linewidth is roughly proportional to q{sup 2.5} and the integrated intensity I(q) is proportional to (q{sup 2} + {kappa}{sub z}{sup 2}){sup -1}. 8 refs., 3 figs.
Spin-current noise from fluctuation relations
Lim, Jong Soo; Sánchez, David; López, Rosa
2013-12-04
We present fluctuation relations that connect spin-polarized current and noise in mesoscopic conductors. In linear response, these relations are equivalent to the fluctuation-dissipation theorem that relates equilibrium current-current correlations to the linear conductance. More interestingly, in the weakly nonlinear regime of transport, these relations establish a connection between the leading-order rectification spin conductance, the spin noise susceptibility and the third cumulant of spin current fluctuations at equilibrium. Our results are valid even for systems in the presence of magnetic fields and coupled to ferromagnetic electrodes.
Electric probe for spin transition and fluctuation
NASA Astrophysics Data System (ADS)
Qiu, Zhiyong; Li, Jia; Hou, Dazhi; Arenholz, Elke; N'diaye, Alpha T.; Tan, Ali; Uchida, Ken-Ichi; Sato, Koji; Tserkovnyak, Yaroslov; Qiu, Z. Q.; Saitoh, Eiji
Spin fluctuation and transition have always been one of central topics of magnetism and condense matter science. To probe them, neutron scatterings have been used as powerful tools. A part of neutrons injected into a sample is scattered by spin fluctuation inside the sample. This process transcribes the spin fluctuation onto scattering intensity, which is commonly represented by dynamical magnetic susceptibility of the sample and is maximized at magnetic phase transitions. Importantly, a neutron carries spin without electric charge, and it thus can bring spin into a sample without being disturbed by electric energy: an advantage of neutrons, although large facilities such as a nuclear reactor is necessary. Here we show that spin pumping, frequently used in nanoscale spintronic devices, provides a desktop micro probe for spin fluctuation and transition; not only a neutron beam, spin current is also a flux of spin without an electric charge and its transport reflects spin fluctuation in a sample. We demonstrate detection of anti-ferromagnetic transition in ultra-thin CoO films via frequency dependent spin-current transmission measurements.
Coherent spin control by electromagnetic vacuum fluctuations
Wang Jing; Liu Renbao; Zhu Bangfen; Sham, L. J.; Steel, D. G.
2011-05-15
In coherent control, electromagnetic vacuum fluctuations usually cause coherence loss through irreversible spontaneous emission. However, since the dissipation via emission is essentially due to correlation of the fluctuations, when emission ends in a superposition of multiple final states, correlation between different pathways may build up if the 'which way' information is not fully resolved (i.e., the emission spectrum is broader than the transition energy range). Such correlation can be exploited for spin-flip control in a {Lambda}-type three-level system, which manifests itself as an all-optical spin echo in nonlinear optics with two orders of optical fields saved as compared with stimulated Raman processes. This finding represents a class of optical nonlinearity induced by electromagnetic vacuum fluctuations.
Thermal fluctuations in artificial spin ice.
Kapaklis, Vassilios; Arnalds, Unnar B; Farhan, Alan; Chopdekar, Rajesh V; Balan, Ana; Scholl, Andreas; Heyderman, Laura J; Hjörvarsson, Björgvin
2014-07-01
Artificial spin ice systems have been proposed as a playground for the study of monopole-like magnetic excitations, similar to those observed in pyrochlore spin ice materials. Currents of magnetic monopole excitations have been observed, demonstrating the possibility for the realization of magnetic-charge-based circuitry. Artificial spin ice systems that support thermal fluctuations can serve as an ideal setting for observing dynamical effects such as monopole propagation and as a potential medium for magnetricity investigations. Here, we report on the transition from a frozen to a dynamic state in artificial spin ice with a square lattice. Magnetic imaging is used to determine the magnetic state of the islands in thermal equilibrium. The temperature-induced onset of magnetic fluctuations and excitation populations are shown to depend on the lattice spacing and related interaction strength between islands. The excitations are described by Boltzmann distributions with their factors in the frozen state relating to the blocking temperatures of the array. Our results provide insight into the design of thermal artificial spin ice arrays where the magnetic charge density and response to external fields can be studied in thermal equilibrium. PMID:24908258
Love triangles, quantum fluctuations and spin jam
NASA Astrophysics Data System (ADS)
Lee, Seung-Hun
When magnetic moments are interacting with each other in a situation resembling that of complex love triangles, called frustration, a large set of states that are energetically equivalent emerge. This leads to exotic spin states such as spin liquid and spin ice. Recently, we presented evidence for the existence of a topological glassy state, that we call spin jam, induced by quantum fluctuations. The case in point is SrCr9pGa12-9pO19 (SCGO(p)), a highly frustrated magnet, in which the magnetic Cr ions form a quasi-two-dimensional triangular system of bi-pyramids. This system has been an archetype in search for exotic spin states. Understanding the nature of the state has been a great intellectual challenge. Our new experimental data and theoretical spin jam model provide for the first time a coherent understanding of the phenomenon. Furthermore, the findings strongly support the possible existence of purely topological glassy states. Reference:
Spin fluctuations in 3d paramagnetic metals
NASA Astrophysics Data System (ADS)
Wysocki, Aleksander; Kutepov, Andrey; Antropov, Vladimir
Spin fluctuations (SFs) in 3d paramagnetic metals were investigated using the linear response formalism within the time dependent density functional theory. An efficient scheme of frequency integration using the Matsubara technique has been implemented and tested. The SFs spectrum in 3d paramagnets is analyzed in real and reciprocal spaces as a function of frequency and temperature. For all materials the SFs are characterized by the coexistence of low and high energy branches which originate from different regions of the Brillouin zone. The low-energy ones can be measured by neutron scattering experiments while the high-energy SFs appear to be more localized. Further, we studied the nature of square of fluctuating magnetic moment in these materials. This work was supported, in part, by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy (DOE), and by the Office of Basic Energy Science, Division of Materials Science and Engineering. The research was performed at Ames Laboratory, which is operated for the U.S. DOE by Iowa State University under contract # DE-AC02-07CH11358.
Mixed Valence and Spin Fluctuations in Cerium
NASA Astrophysics Data System (ADS)
Andraka, Bohdan
The pseudobinary alloys Ce(Ni(,x)Co(,1-x))(,2), (Ce(,x)La(,1-x))Ni(,2) and (Ce(,x)Y(,1-x))Ni(,2), where 0 < x < 1, have been studied. The room temperature lattice constant, the magnetic susceptibility in the 6 to 300 K temperature range, the low temperature specific heat in the 0.4 to 25 K range and the electrical resistivity in the 0.4 to 300 K range were measured. Additionally, X -ray absorption around L(,3) edge has been studied using the synchroton radiation. Both, thermodynamic and L(,3) probes yield similar results for the changes of valence of Ce across these systems. We have established that Ce is in the saturated valence state in the Ce(Ni(,x)Co(,1 -x))(,2) for x < 0.25. The valence decreases with x in the 0.25 to 1 concentration range. Similarly, the valence of Ce decreases in the (Ce, Y)Ni(,2) systems upon substitution of Y for Ce. The results obtained for the (Ce(,x)La(,1 -x))Ni(,2) system are interpreted in terms of changes of the Ce('3+) impurity levels. Spin fluctuations due to the 3d electrons play an important role in these systems.
Spin fluctuations of nonequilibrium electrons and excitons in semiconductors
NASA Astrophysics Data System (ADS)
Glazov, M. M.
2016-03-01
Effects that are related to deviations from thermodynamic equilibrium have a special place in modern physics. Among these, nonequilibrium phenomena in quantum systems attract the highest interest. The experimental technique of spin-noise spectroscopy has became quite widespread, which makes it possible to observe spin fluctuations of charge carriers in semiconductors under both equilibrium and nonequilibrium conditions. This calls for the development of a theory of spin fluctuations of electrons and electron-hole complexes for nonequilibrium conditions. In this paper, we consider a range of physical situations where a deviation from equilibrium becomes pronounced in the spin noise. A general method for the calculation of electron and exciton spin fluctuations in a nonequilibrium state is proposed. A short review of the theoretical and experimental results in this area is given.
Effect of spin fluctuations on quasiparticles in simple metals
NASA Astrophysics Data System (ADS)
Lischner, Johannes; Bazhirov, Timur; MacDonald, Allan; Cohen, Marvin; Louie, Steven
2014-03-01
We present a first-principles theory for quasiparticle excitations in condensed matter systems that includes their interaction with spin fluctuations. We apply this theory to sodium and lithium. Despite several previous studies, the importance of spin fluctuations in these materials and, in particular, their effect on the occupied band width remains unclear. We show that the coupling to spin fluctuations does not significantly change the occupied band width, but gives an important contribution to the quasiparticle lifetime. To obtain quantitative agreement with experiment for the occupied band width, we find that it is necessary to include vertex corrections beyond the random-phase approximation in the screening by charge fluctuations. S. G. L. acknowledges support by a Simons Foundation Fellowship in Theoretical Physics. This work was supported by NSF Grant No. DMR10-1006184 and by DOE Grant No. DE-AC02-05CH11231.
Thermal spin fluctuations in spinor Bose-Einstein condensates
NASA Astrophysics Data System (ADS)
Melé-Messeguer, M.; Juliá-Díaz, B.; Polls, A.; Santos, L.
2013-03-01
We study the thermal activation of spin fluctuations in dynamically stable spinor Bose-Einstein condensates. We analyze the specific cases of a nondipolar spin-1 condensate in the state m=0, where thermal activation results from spin-changing collisions, and of a chromium condensate in the maximally stretched state m=-3, where thermal spin fluctuations are due to dipole-induced spin relaxation. In both cases, we show that the low energy associated to the spinor physics may be employed for thermometry purposes down to extremely low temperatures, typically impossible to measure in Bose-Einstein condensates with the usual thermometric techniques. Moreover, the peculiar dependence of the system's entropy with the applied Zeeman energy opens a possible route for adiabatic cooling.
NASA Astrophysics Data System (ADS)
Solontsov, A.
2015-06-01
The paper critically overviews the recent developments of the theory of spatially dispersive spin fluctuations (SF) in itinerant electron magnetism with particular emphasis on spin-fluctuation coupling or spin anharmonicity. It is argued that the conventional self-consistent renormalized (SCR) theory of spin fluctuations is usually used aside of the range of its applicability actually defined by the constraint of weak spin anharmonicity based on the random phase approximation (RPA) arguments. An essential step in understanding SF in itinerant magnets beyond RPA-like arguments was made recently within the soft-mode theory of SF accounting for strong spin anharmonicity caused by zero-point SF. In the present paper we generalize it to apply for a wider range of temperatures and regimes of SF and show it to lead to qualitatively new results caused by zero-point effects.
Unconventional superconductivity from local spin fluctuations in the Kondo lattice.
Bodensiek, Oliver; Žitko, Rok; Vojta, Matthias; Jarrell, Mark; Pruschke, Thomas
2013-04-01
The explanation of heavy-fermion superconductivity is a long-standing challenge to theory. It is commonly thought to be connected to nonlocal fluctuations of either spin or charge degrees of freedom and therefore of unconventional type. Here we present results for the Kondo-lattice model, a paradigmatic model to describe heavy-fermion compounds, obtained from dynamical mean-field theory which captures local correlation effects only. Unexpectedly, we find robust s-wave superconductivity in the heavy-fermion state. We argue that this novel type of pairing is tightly connected to the formation of heavy quasiparticle bands and the presence of strong local spin fluctuations. PMID:25167017
Spin fluctuation and Fermi surface instability in ferromagnetic superconductors
NASA Astrophysics Data System (ADS)
Aoki, Dai; Gourgout, Adrien; Pourret, Alexandre; Bastien, Gaël; Knebel, Georg; Flouquet, Jacques
2014-08-01
We review the ferromagnetic superconductivity observed in the uranium based compounds, namely UGe2, URhGe and UCoGe, where the spin-triplet state is most likely realized. An unusual upper critical field Hc2, which is enhanced under a magnetic field in a certain field direction, is discussed in terms of spin fluctuations and of Fermi surface instabilities. xml:lang="fr"
Number Fluctuation Dynamics of Atomic Spin Mixing inside a Condensate
Chang, Lee; Zhai, Q.; Lu Rong; You, L.
2007-08-24
We investigate the quantum dynamics of number fluctuations inside an atomic condensate during coherent spin mixing among internal states of the ground state hyperfine manifold, by quantizing the semiclassical nonrigid pendulum model in terms of the conjugate variable pair: the relative phase and the atom number. Our result provides a theoretical basis that resolves the resolution limit, or the effective ''shot-noise'' level, for counting atoms that is needed to clearly detect quantum correlation effects in spin mixing.
Charge and spin fluctuations in the density functional theory
Gyoerffy, B.L.; Barbieri, A. . H.H. Wills Physics Lab.); Staunton, J.B. . Dept. of Physics); Shelton, W.A.; Stocks, G.M. )
1990-01-01
We introduce a conceptual framework which allow us to treat charge and spin fluctuations about the Local density Approximation (LDA) to the Density Functional Theory (DFT). We illustrate the approach by explicit study of the Disordered Local Moment (DLM) state in Fe above the Curie Temperature {Tc} and the Mott insulating state in MnO. 27 refs., 6 figs.
NASA Astrophysics Data System (ADS)
Garibay-Alonso, R.; Dorantes-Dávila, J.; Pastor, G. M.
2015-05-01
A local electronic theory of transition-metal magnetism at finite temperatures is presented, which takes into account longitudinal and transverse spin fluctuations on the same footing. The magnetic properties are determined in the framework of a rotational-invariant d -band model Hamiltonian by applying a four-field Hubbard-Stratonovich functional-integral method in the static approximation. The role of transverse spin excitations on the temperature-dependent magnetic properties is investigated by performing alloy averages in the single-site virtual crystal approximation. Bulk Fe is considered as the representative example for the applications. Results are given for the average magnetization M , for the spin-excitation energies, and for the transverse and longitudinal contributions to the local magnetic moments μl at atom l . The importance of noncollinear spin excitations is quantified by comparison with the corresponding collinear calculations. An important reduction of about 33% of the calculated Curie temperature TC is obtained, which now amounts to 1250 K and is thus relatively close to the experimental value. The longitudinal (transverse) components of μl are found to decrease (increase) as a function of temperature until the full rotational symmetry is reached at TC. This reflects the increasing importance of the transverse spin fluctuations. The origin of the temperature dependence of M and μl is analyzed in terms of the local spin-fluctuation energies.
Spin Fluctuations from Hertz to Terahertz on a Triangular Lattice.
Nambu, Yusuke; Gardner, Jason S; MacLaughlin, Douglas E; Stock, Chris; Endo, Hitoshi; Jonas, Seth; Sato, Taku J; Nakatsuji, Satoru; Broholm, Collin
2015-09-18
The temporal magnetic correlations of the triangular-lattice antiferromagnet NiGa_{2}S_{4} are examined through 13 decades (10^{-13}-1 sec) using ultrahigh-resolution inelastic neutron scattering, muon spin relaxation, and ac and nonlinear susceptibility measurements. Unlike the short-ranged spatial correlations, the temperature dependence of the temporal correlations show distinct anomalies. The spin fluctuation rate decreases precipitously upon cooling towards T^{*}=8.5 K, but fluctuations on the microsecond time scale then persist in an anomalous dynamical regime for 4 K
Phase transition in spin systems with various types of fluctuations.
Miyashita, Seiji
2010-01-01
Various types ordering processes in systems with large fluctuation are overviewed. Generally, the so-called order-disorder phase transition takes place in competition between the interaction causing the system be ordered and the entropy causing a random disturbance. Nature of the phase transition strongly depends on the type of fluctuation which is determined by the structure of the order parameter of the system. As to the critical property of phase transitions, the concept "universality of the critical phenomena" is well established. However, we still find variety of features of ordering processes. In this article, we study effects of various mechanisms which bring large fluctuation in the system, e.g., continuous symmetry of the spin in low dimensions, contradictions among interactions (frustration), randomness of the lattice, quantum fluctuations, and a long range interaction in off-lattice systems. PMID:20689226
Zero-Point Spin-Fluctuations of Single Adatoms.
Ibañez-Azpiroz, Julen; Dos Santos Dias, Manuel; Blügel, Stefan; Lounis, Samir
2016-07-13
Stabilizing the magnetic signal of single adatoms is a crucial step toward their successful usage in widespread technological applications such as high-density magnetic data storage devices. The quantum mechanical nature of these tiny objects, however, introduces intrinsic zero-point spin-fluctuations that tend to destabilize the local magnetic moment of interest by dwindling the magnetic anisotropy potential barrier even at absolute zero temperature. Here, we elucidate the origins and quantify the effect of the fundamental ingredients determining the magnitude of the fluctuations, namely, the (i) local magnetic moment, (ii) spin-orbit coupling, and (iii) electron-hole Stoner excitations. Based on a systematic first-principles study of 3d and 4d adatoms, we demonstrate that the transverse contribution of the fluctuations is comparable in size to the magnetic moment itself, leading to a remarkable ≳50% reduction of the magnetic anisotropy energy. Our analysis gives rise to a comprehensible diagram relating the fluctuation magnitude to characteristic features of adatoms, providing practical guidelines for designing magnetically stable nanomagnets with minimal quantum fluctuations. PMID:27248465
Theory of spin and charge fluctuations in the Hubbard model
Vilk, Y.M.; Chen, L.; Tremblay, A.S. )
1994-05-01
A self-consistent theory of both spin and charge fluctuations in the Hubbard model is presented. It is in quantitative agreement with Monte Carlo data at least up to intermediate coupling ([ital U][similar to]8[ital t]). It includes both short-wavelength quantum renormalization effects, and long-wavelength thermal fluctuations, which can destroy long-range order in two dimensions. The last effect leads to a small energy scale, as often observed in high-temperature superconductors. The theory is conserving, satisfies the Pauli principle, and includes three-particle correlations necessary to account for the incipient Mott transition.
Stoichiometry, spin fluctuations, and superconductivity in LaNiPO
Klimczuk, Tomasz; Mcqueen, Tyrel M; Williams, Anthony J; Huang, Qiang; Cava, Robert J
2009-01-01
Superconductivity in LaNiPO is disrupted by small ({approx}5%) amounts of non-stoichiometry on the lanthanum site, even though the electronic contribution to the heat capacity increases with increasing non-stoichiometry. All samples also exhibit specific heat anomalies consistent with the presence of ferromagnetic spin fluctuations (T{sub sf}{approx} 14K). Comparison of layered nickel phosphide and nickel borocarbide superconductors reveals different structure-property correlations in the two families.
Spin jam induced by quantum fluctuations in a frustrated magnet.
Yang, Junjie; Samarakoon, Anjana; Dissanayake, Sachith; Ueda, Hiroaki; Klich, Israel; Iida, Kazuki; Pajerowski, Daniel; Butch, Nicholas P; Huang, Q; Copley, John R D; Lee, Seung-Hun
2015-09-15
Since the discovery of spin glasses in dilute magnetic systems, their study has been largely focused on understanding randomness and defects as the driving mechanism. The same paradigm has also been applied to explain glassy states found in dense frustrated systems. Recently, however, it has been theoretically suggested that different mechanisms, such as quantum fluctuations and topological features, may induce glassy states in defect-free spin systems, far from the conventional dilute limit. Here we report experimental evidence for existence of a glassy state, which we call a spin jam, in the vicinity of the clean limit of a frustrated magnet, which is insensitive to a low concentration of defects. We have studied the effect of impurities on SrCr9pGa12-9pO19 [SCGO(p)], a highly frustrated magnet, in which the magnetic Cr(3+) (s = 3/2) ions form a quasi-2D triangular system of bipyramids. Our experimental data show that as the nonmagnetic Ga(3+) impurity concentration is changed, there are two distinct phases of glassiness: an exotic glassy state, which we call a spin jam, for the high magnetic concentration region (p > 0.8) and a cluster spin glass for lower magnetic concentration (p < 0.8). This observation indicates that a spin jam is a unique vantage point from which the class of glassy states of dense frustrated magnets can be understood. PMID:26324917
Spin jam induced by quantum fluctuations in a frustrated magnet
Yang, Junjie; Samarakoon, Anjana; Dissanayake, Sachith; Ueda, Hiroaki; Klich, Israel; Iida, Kazuki; Pajerowski, Daniel; Butch, Nicholas P.; Huang, Q.; Copley, John R. D.; Lee, Seung-Hun
2015-01-01
Since the discovery of spin glasses in dilute magnetic systems, their study has been largely focused on understanding randomness and defects as the driving mechanism. The same paradigm has also been applied to explain glassy states found in dense frustrated systems. Recently, however, it has been theoretically suggested that different mechanisms, such as quantum fluctuations and topological features, may induce glassy states in defect-free spin systems, far from the conventional dilute limit. Here we report experimental evidence for existence of a glassy state, which we call a spin jam, in the vicinity of the clean limit of a frustrated magnet, which is insensitive to a low concentration of defects. We have studied the effect of impurities on SrCr9pGa12-9pO19 [SCGO(p)], a highly frustrated magnet, in which the magnetic Cr3+ (s = 3/2) ions form a quasi-2D triangular system of bipyramids. Our experimental data show that as the nonmagnetic Ga3+ impurity concentration is changed, there are two distinct phases of glassiness: an exotic glassy state, which we call a spin jam, for the high magnetic concentration region (p>0.8) and a cluster spin glass for lower magnetic concentration (p<0.8). This observation indicates that a spin jam is a unique vantage point from which the class of glassy states of dense frustrated magnets can be understood. PMID:26324917
Problem of Phase Transition in Spin-fluctuation Theory
NASA Astrophysics Data System (ADS)
Melnikov, N. B.; Paradezhenko, G. V.
A first-order phase transition is a characteristic feature of the Gaussian approximation in spin-fluctuation theory. We describe a method for taking into account the fourth-order terms of the free energy expansion using partial averaging. In the example of the Ising model, we show that renormalization of the magnetic susceptibility leads to the second-order phase transition, which is experimentally observed in metals. Near the phase transition, we use the parameter substitution method to compute temperature dependencies. We perform a qualitative analysis and explain the mechanism of the renormalization.
Mott physics and spin fluctuations: A functional viewpoint
NASA Astrophysics Data System (ADS)
Ayral, Thomas; Parcollet, Olivier
2016-06-01
We present a formalism for strongly correlated systems with fermions coupled to bosonic modes. We construct the three-particle irreducible functional K by successive Legendre transformations of the free energy of the system. We derive a closed set of equations for the fermionic and bosonic self-energies for a given K . We then introduce a local approximation for K , which extends the idea of dynamical mean-field theory (DMFT) approaches from two- to three-particle irreducibility. This approximation entails the locality of the three-leg electron-boson vertex Λ (i ω ,i Ω ) , which is self-consistently computed using a quantum impurity model with dynamical charge and spin interactions. This local vertex is used to construct frequency- and momentum-dependent electronic self-energies and polarizations. By construction, the method interpolates between the spin-fluctuation or G W approximations at weak coupling and the atomic limit at strong coupling. We apply it to the Hubbard model on two-dimensional square and triangular lattices. We complement the results of [T. Ayral and O. Parcollet, Phys. Rev. B 92, 115109 (2015), 10.1103/PhysRevB.92.115109] by (i) showing that, at half-filling, as DMFT, the method describes the Fermi-liquid metallic state and the Mott insulator, separated by a first-order interaction-driven Mott transition at low temperatures, (ii) investigating the influence of frustration, and (iii) discussing the influence of the bosonic decoupling channel.
High-energy spin-density-wave correlated fluctuations in paramagnetic Cr + 5 at. % V
Werner, S.A.; Fawcett, E.; Elmiger, M.W.; Shirane, G.
1992-11-01
Measurements of the magnetic fluctuations, termed spin-density-wave (SDW) paramagnons, in the nearly antiferromagnetic alloy Cr + 5 at.%V are extended up in energy to about 80 MeV. These fluctuating spin-spin correlations occur at incommensurate positions, corresponding to the SDW wavevector Q. Their characteristic energy is at least an order of magnitude larger than that of the magnetic fluctuations seen in the paramagnetic phase of pure Cr, but their intensity is more than two orders of magnitude smaller. We find that the dynamic susceptibility decreases by about 50% between temperature T = 10K and 300K.
High-energy spin-density-wave correlated fluctuations in paramagnetic Cr + 5 at. % V
Werner, S.A. . Dept. of Physics); Fawcett, E. . Dept. of Physics); Elmiger, M.W.; Shirane, G. )
1992-01-01
Measurements of the magnetic fluctuations, termed spin-density-wave (SDW) paramagnons, in the nearly antiferromagnetic alloy Cr + 5 at.%V are extended up in energy to about 80 MeV. These fluctuating spin-spin correlations occur at incommensurate positions, corresponding to the SDW wavevector Q. Their characteristic energy is at least an order of magnitude larger than that of the magnetic fluctuations seen in the paramagnetic phase of pure Cr, but their intensity is more than two orders of magnitude smaller. We find that the dynamic susceptibility decreases by about 50% between temperature T = 10K and 300K.
NASA Astrophysics Data System (ADS)
Lischner, Johannes; Bazhirov, Timur; MacDonald, Allan H.; Cohen, Marvin L.; Louie, Steven G.
2015-01-01
We present first-principles calculations of the coupling of quasiparticles to spin fluctuations in iron selenide and discuss which types of superconducting instabilities this coupling gives rise to. We find that strong antiferromagnetic stripe-phase spin fluctuations lead to large coupling constants for superconducting gaps with s± symmetry, but these coupling constants are significantly reduced by other spin fluctuations with small wave vectors. An accurate description of this competition and an inclusion of band-structure and Stoner parameter renormalization effects lead to a value of the coupling constant for an s±-symmetric gap which can produce a superconducting transition temperature consistent with experimental measurements.
NASA Astrophysics Data System (ADS)
Melnikov, N. B.; Reser, B. I.; Paradezhenko, G. V.
2016-08-01
To study the spin-density correlations in the ferromagnetic metals above the Curie temperature, we relate the spin correlator and neutron scattering cross-section. In the dynamic spin-fluctuation theory, we obtain explicit expressions for the effective and local magnetic moments and spatial spin-density correlator. Our theoretical results are demonstrated by the example of bcc Fe. The effective and local moments are found in good agreement with results of polarized neutron scattering experiment over a wide temperature range. The calculated short-range order is small (up to 4 Å) and slowly decreases with temperature.
Hanle effect in (In,Ga)As quantum dots: Role of nuclear spin fluctuations
NASA Astrophysics Data System (ADS)
Kuznetsova, M. S.; Flisinski, K.; Gerlovin, I. Ya.; Ignatiev, I. V.; Kavokin, K. V.; Verbin, S. Yu.; Yakovlev, D. R.; Reuter, D.; Wieck, A. D.; Bayer, M.
2013-06-01
The role of nuclear spin fluctuations in the dynamic polarization of nuclear spins by electrons is investigated in (In,Ga)As/GaAs quantum dots. The photoluminescence polarization under circularly polarized optical pumping in transverse magnetic fields (Hanle effect) is studied. A weak additional magnetic field parallel to the optical axis is used to control the efficiency of nuclear spin cooling and the sign of nuclear spin temperature. The shape of the Hanle curve is drastically modified when changing this control field, as observed earlier in bulk semiconductors and quantum wells. However, the standard nuclear spin cooling theory, operating with the mean nuclear magnetic field (Overhauser field), fails to describe the experimental Hanle curves in a certain range of control fields. This controversy is resolved by taking into account the nuclear spin fluctuations owed to the finite number of nuclei in the quantum dot. We propose a model considering cooling of the nuclear spin system by electron spins experiencing fast vector precession in the random Overhauser fields of nuclear spin fluctuations. The model allows us to accurately describe the measured Hanle curves and to evaluate the parameters of the electron-nuclear spin system of the studied quantum dots.
Quantized massive collective modes and massive spin fluctuations in high-Tc cuprates
NASA Astrophysics Data System (ADS)
Kanazawa, I.; Sasaki, T.
2015-10-01
We have analyzed angle-resolved photoemission spectra of the single- and double-layered Bi-family high-Tc superconductors by using quantized massive gauge fields, which might contain effects of spin fluctuations, charge fluctuations, and phonons. It is suggested strongly that the quantized massive gauge fields might be mediating Cooper pairing in high-Tc cuprates.
Generalized correlation functions for conductance fluctuations and the mesoscopic spin Hall effect
NASA Astrophysics Data System (ADS)
Ramos, J. G. G. S.; Barbosa, A. L. R.; Bazeia, D.; Hussein, M. S.; Lewenkopf, C. H.
2012-12-01
We study the spin Hall conductance fluctuations in ballistic mesoscopic systems. We obtain universal expressions for the spin and charge current fluctuations, cast in terms of current-current autocorrelation functions. We show that the latter are conveniently parametrized as deformed Lorentzian shape lines, functions of an external applied magnetic field and the Fermi energy. We find that the charge current fluctuations show quite unique statistical features at the symplectic-unitary crossover regime. Our findings are based on an evaluation of the generalized transmission coefficients correlation functions within the stub model and are amenable to experimental test.
Kim, Y H; Kaur, N; Atkins, B M; Dalal, N S; Takano, Y
2009-12-11
At a quantum critical point (QCP)--a zero-temperature singularity in which a line of continuous phase transition terminates--quantum fluctuations diverge in space and time, leading to exotic phenomena that can be observed at nonzero temperatures. Using a quantum antiferromagnet, we present calorimetric evidence that nuclear spins frozen in a high-temperature nonequilibrium state by temperature quenching are annealed by quantum fluctuations near the QCP. This phenomenon, with readily detectable heat release from the nuclear spins as they are annealed, serves as an excellent marker of a quantum critical region around the QCP and provides a probe of the dynamics of the divergent quantum fluctuations. PMID:20366226
Stripe Antiferromagnetic Spin Fluctuations in SrCo2As2
Jayasekara, W.; Lee, Y; Pandey, Abishek; Tucker, G. S.; Sapkota, A; Lamsal, Jagat; Calder, Stuart A; Abernathy, Douglas L; Niedziela, Jennifer L; Harmon, B N; Kreyssig, A.; Vaknin, D; Johnston, D C; Goldman, A. I.; McQueeney, R. J.
2013-01-01
Inelastic neutron scattering measurements of paramagnetic SrCo2As2 at T = 5 K reveal antifer- romagnetic (AFM) spin fluctuations that are peaked at a wavevector of QAFM = (1/2, 1/2, 1) and possess a large energy scale. These stripe spin fluctuations are similar to those found in AFe2As2 compounds, where spin-density wave AFM is driven by Fermi surface nesting between electron and hole pockets separated by QAFM. SrCo2As2 has a more complex Fermi surface and band structure calculations indicate a potential instability towards either a ferromagnetic or stripe AFM ground state. The results suggest that stripe AFM magnetism is a general feature of both iron and cobalt- based arsenides and the search for spin fluctuation-induced unconventional superconductivity should be expanded to include cobalt-based compounds.
Stripe antiferromagnetic spin fluctuations in SrCo2As2.
Jayasekara, W; Lee, Y; Pandey, Abhishek; Tucker, G S; Sapkota, A; Lamsal, J; Calder, S; Abernathy, D L; Niedziela, J L; Harmon, B N; Kreyssig, A; Vaknin, D; Johnston, D C; Goldman, A I; McQueeney, R J
2013-10-11
Inelastic neutron scattering measurements of paramagnetic SrCo2As2 at T=5 K reveal antiferromagnetic (AFM) spin fluctuations that are peaked at a wave vector of Q(AFM)=(1/2,1/2,1) and possess a large energy scale. These stripe spin fluctuations are similar to those found in AFe2As2 compounds, where spin-density wave AFM is driven by Fermi surface nesting between electron and hole pockets separated by Q(AFM). SrCo2As2 has a more complex Fermi surface and band-structure calculations indicate a potential instability toward either a ferromagnetic or stripe AFM ground state. The results suggest that stripe AFM magnetism is a general feature of both iron and cobalt-based arsenides and the search for spin fluctuation-induced unconventional superconductivity should be expanded to include cobalt-based compounds. PMID:24160618
Ning, F. L.; Ahilan, K.; Imai, T.; Sefat, A. S.; McGuire, Michael A; Sales, Brian C; Mandrus, David; Cheng, P.; Shen, B.; Wen, H.-H.
2010-01-01
We report the first NMR investigation of spin dynamics in the overdoped nonsuperconducting regime of Ba(Fe{sub 1-x}Co{sub x}){sub 2}As{sub 2} up to x=0.26. We demonstrate that the absence of interband transitions with large momentum transfer Q{sub AF}-({pi}/a,0) between the hole and electron Fermi surfaces results in complete suppression of antiferromagnetic spin fluctuations for x {ge} 0.15. Our experimental results provide direct evidence for a correlation between T{sub c} and the strength of Q{sub AF} antiferromagnetic spin fluctuations.
Rice, William D; Liu, Wenyong; Baker, Thomas A; Sinitsyn, Nikolai A; Klimov, Victor I; Crooker, Scott A
2016-02-01
Strong quantum confinement in semiconductors can compress the wavefunctions of band electrons and holes to nanometre-scale volumes, significantly enhancing interactions between themselves and individual dopants. In magnetically doped semiconductors, where paramagnetic dopants (such as Mn(2+), Co(2+) and so on) couple to band carriers via strong sp-d spin exchange, giant magneto-optical effects can therefore be realized in confined geometries using few or even single impurity spins. Importantly, however, thermodynamic spin fluctuations become increasingly relevant in this few-spin limit. In nanoscale volumes, the statistical fluctuations of N spins are expected to generate giant effective magnetic fields Beff, which should dramatically impact carrier spin dynamics, even in the absence of any applied field. Here we directly and unambiguously reveal the large Beff that exist in Mn(2+)-doped CdSe colloidal nanocrystals using ultrafast optical spectroscopy. At zero applied magnetic field, extremely rapid (300-600 GHz) spin precession of photoinjected electrons is observed, indicating Beff ∼ 15 -30 T for electrons. Precession frequencies exceed 2 THz in applied magnetic fields. These signals arise from electron precession about the random fields due to statistically incomplete cancellation of the embedded Mn(2+) moments, thereby revealing the initial coherent dynamics of magnetic polaron formation, and highlighting the importance of magnetization fluctuations on carrier spin dynamics in nanomaterials. PMID:26595331
Zero-Temperature Fluctuations in Short-Range Spin Glasses
NASA Astrophysics Data System (ADS)
Arguin, L.-P.; Newman, C. M.; Stein, D. L.; Wehr, J.
2016-06-01
We consider the energy difference restricted to a finite volume for certain pairs of incongruent ground states (if they exist) in the d-dimensional Edwards-Anderson Ising spin glass at zero temperature. We prove that the variance of this quantity with respect to the couplings grows proportionally to the volume in any d ≥ 2. An essential aspect of our result is the use of the excitation metastate. As an illustration of potential applications, we use this result to restrict the possible structure of spin glass ground states in two dimensions.
NASA Astrophysics Data System (ADS)
Lischner, Johannes; Bazhirov, Timur; MacDonald, Allan H.; Cohen, Marvin L.; Louie, Steven G.
2015-03-01
We present first-principles calculations of the coupling of quasiparticles to spin fluctuations in iron selenide and discuss which types of superconducting instabilities this coupling gives rise to. We find that strong antiferromagnetic stripe-phase spin fluctuations lead to large coupling constants for superconducting gaps with s +/- -symmetry, but these coupling constants are significantly reduced by other spin fluctuations with small wave vectors. An accurate description of this competition and an inclusion of band structure and Stoner parameter renormalization effects lead to a value of the coupling constant for an s +/- symmetric gap which can produce a superconducting transition temperature consistent with experimental measurements. This work was supported by NSF Grant No. DMR10-1006184 and by DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by the DOE at NERSC.
Quantum spin fluctuations in quasi-one-dimensional chlorine-bridged platinum complexes
NASA Astrophysics Data System (ADS)
Wei, Xing; Donohoe, Robert J.; Wang, Wen Z.; Bishop, Alan R.; Gammel, Jan T.
1997-12-01
We report experimental and theoretical studies of spin dynamic process in the quasi-one-dimensional chlorine- bridged platinum complex, [PtII(en)2][PtIV(en)2Cl2](ClO4)4, where en equals ethylenediamine, C2N2H8. The process manifests itself in collapsing of the hyperfine and superhyperfine structures in the electron spin resonance spectrum and non-statistical distribution of spectral weight of the Pt isotopes. More surprisingly, it is activated only at temperatures below 6 K. We interpret the phenomenon in terms of quantum tunneling of the electronic spin in a strong electron-electron and electron-phonon coupling regime. This is modeled using a non-adiabatic many-body approach, in which polarons and solitons represent local spin-Peierls regions in a strongly disproportional charge- density-wave background and display intriguing spin-charge separation in the form of pinned charge and tunneling spin fluctuations.
Superconducting mechanism due to the orbital and spin fluctuations in Fe-based superconductors
NASA Astrophysics Data System (ADS)
Kontani, Hiroshi; Yamakawa, Youichi; Onari, Seiichiro
The rich variety of the phase diagrams in Fe-based superconductors, such as the nonmagnetic/magnetic nematic phase in FeSe/LaFeAsO, is not able to be explained by the mean-field level approximations. Recently, we explained the phase diagrams of FeSe and LaFeAsO in term of the orbital +spin fluctuation theory, by including the Aslamazov-Larkin vertex correction (AL-VC). The nematic orbital order without magnetization in FeSe is well explained. In the present study, we analyze the superconducting states in FeSe and LaFeAsO, by applying the orbital +spin fluctuation theory. Rich variety of the superconducting gap structures are induced by the strong orbital and spin fluctuations driven by the AL-VC (=orbital-spin interplay). We find that the pairing interaction due to the orbital fluctuations is strongly enlarged by the AL-VC for the anomalous self-energy, so the Migdal theory is seriously violated in the orbital-fluctuation pairing mechanism.
NASA Astrophysics Data System (ADS)
Thomaz, M. T.; Corrêa Silva, E. V.
2016-03-01
We derive the exact Helmholtz free energy (HFE) of the standard and staggered one-dimensional Blume-Emery-Griffiths (BEG) model in the presence of an external longitudinal magnetic field. We discuss in detail the thermodynamic behavior of the ferromagnetic version of the model, which exhibits magnetic field-dependent plateaux in the z-component of its magnetization at low temperatures. We also study the behavior of its specific heat and entropy, both per site, at finite temperature. The degeneracy of the ground state, at T=0, along the lines that separate distinct phases in the phase diagram of the ferromagnetic BEG model is calculated, extending the study of the phase diagram of the spin-1 antiferromagnetic (AF) Ising model in S.M. de Souza and M.T. Thomaz, J. Magn. and Magn. Mater. 354 (2014) 205 [5]. We explore the implications of the equality of phase diagrams, at T=0, of the ferromagnetic BEG model with K/|J| = - 2 and of the spin-1 AF Ising model for D/|J| > 1/2.
The slave-fermion approach of spin fluctuations in ferromagnet metals
NASA Astrophysics Data System (ADS)
Hu, C. D.
2015-11-01
In this work we propose a method to treat the spin fluctuations in itinerant ferromagnets. It is able to do calculation with a convergent series. The slave fermion method is applied to separate the charge (denoted by fermions) and spin (denoted by bosons) degrees of freedom. The spin operators are then replaced by the Schwinger boson fields. This way, the interaction term in the model can be reduced to a very simple form and can be teated without difficulty. Finally the equations of motion are derived in order to obtain the forms of Green's functions of fermions and bosons. The result is applied to the calculation of resistivity as a function temperature.
Fluctuations of the heat exchanged between two quantum spin chains
NASA Astrophysics Data System (ADS)
Landi, Gabriel T.; Karevski, Dragi
2016-03-01
The statistics of the heat exchanged between two quantum XX spin chains prepared at different temperatures is studied within the assumption of weak coupling. This provides simple formulas for the average heat and its corresponding characteristic function, from which the probability distribution may be computed numerically. These formulas are valid for arbitrary sizes and therefore allow us to analyze the role of the thermodynamic limit in this nonequilibrium setting. It is found that all thermodynamic quantities are extremely sensitive to the quantum phase transition of the XX chain.
Influence of spinons fluctuations near the spin liquid Mott transition
NASA Astrophysics Data System (ADS)
Lee, Tsung-Han; Florens, Serge; Dobrosavljevic, Vladimir
We investigate the metal to Mott-insulator transition (MIT) in the Hubbard-Heisenberg model using the slave-rotor technique, which allows to combine for the first time the dynamical mean field theory (DMFT) with the Resonating Valence Bond (RVB) approach. In the spin-liquid phase at large Coulomb repulsion, the system shows a RVB transition from a trivial paramagnetic Mott insulator towards a low temperature insulating state with long lived spinons, as seen by the emergence of a linear specific heat. This quenching of the entropy in the spin liquid phase provides strong modifications in the shape of the standard DMFT phase diagram for the MIT occurring at intermediate values of the Coulomb repulsion. We find that the RVB transition happens concomitantly with the first order MIT lines at low temperature. This implies that the Mott insulator always accommodates a spinon Fermi surface, even in the coexistence regime of the MIT, and that the metallic state always stays a Fermi-liquid as it rejects the presence of free spinons, due to their strong scattering onto the holons.
NASA Astrophysics Data System (ADS)
Schütt, Michael; Fernandes, Rafael M.
2015-03-01
The existence of strong in-plane electronic anisotropies in underdoped cuprates has been reported by a variety of experimental probes, such as transport measurements, scanning tunneling microscopy, and x-ray and neutron scattering. Understanding the origin of these anisotropies and their interplay is fundamental to elucidate the role played by electronic nematicity in the phase diagram of the cuprates. Here we employ a Boltzmann equation approach to investigate the resistivity anisotropy due to scattering by anisotropic spin and charge fluctuations. We show that while spin fluctuations give rise to larger resistivity along the a direction, charge fluctuations promote larger resistivity along the b direction. Because anisotropic charge and spin fluctuations compete, these behaviors give rise to a particular dependence of the resistivity anisotropy with doping, which is consistent with transport experiments performed in YBa2Cu3O7. We discuss the important role played by the CuO chains in YBCO to select the observed type of nematic domains, and propose transport measurements in strained HgBa2CuO4 and Bi2Sr2CaCu2O8 to shed light on the interplay between anisotropic fluctuations and anisotropic resistivity. MS acknowledges the support from the Humboldt Foundation. RMF is supported by the Department of Energy under Award Number DE-SC0012336.
Quantifying fluctuations of resting state networks using arterial spin labeling perfusion MRI.
Dai, Weiying; Varma, Gopal; Scheidegger, Rachel; Alsop, David C
2016-03-01
Blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) has been widely used to investigate spontaneous low-frequency signal fluctuations across brain resting state networks. However, BOLD only provides relative measures of signal fluctuations. Arterial Spin Labeling (ASL) MRI holds great potential for quantitative measurements of resting state network fluctuations. This study systematically quantified signal fluctuations of the large-scale resting state networks using ASL data from 20 healthy volunteers by separating them from global signal fluctuations and fluctuations caused by residual noise. Global ASL signal fluctuation was 7.59% ± 1.47% relative to the ASL baseline perfusion. Fluctuations of seven detected resting state networks vary from 2.96% ± 0.93% to 6.71% ± 2.35%. Fluctuations of networks and residual noise were 6.05% ± 1.18% and 6.78% ± 1.16% using 4-mm resolution ASL data applied with Gaussian smoothing kernel of 6mm. However, network fluctuations were reduced by 7.77% ± 1.56% while residual noise fluctuation was markedly reduced by 39.75% ± 2.90% when smoothing kernel of 12 mm was applied to the ASL data. Therefore, global and network fluctuations are the dominant structured noise sources in ASL data. Quantitative measurements of resting state networks may enable improved noise reduction and provide insights into the function of healthy and diseased brain. PMID:26661226
The role of spin fluctuations in the conductivity of CrO2
NASA Astrophysics Data System (ADS)
Heffernan, Kate; Talbayev, D.; Zhang, Xueyu; Gupta, A.
2015-03-01
Chromium dioxide is a half-metallic ferromagnet with TC = 390K. Below TC, the conductivity of CrO2 grows by two orders of magnitude and is temperature independent below about 30 K. It is believed that electron scattering by spin fluctuations is responsible for the strong temperature dependence of the conductivity. We performed time-resolved THz spectroscopy (TRTS) and time-resolved magneto-optical Kerr effect (TRMOKE) to study the role of spin fluctuations in electron conduction. A thin film CrO2 sample was excited by an optical pump pulse. The induced conductivity changes were measured by TRTS and the induced spin response by TRMOKE. A fast and a slow component were observed in both responses. The fast component dominates the TRTS response, while the slow dominates the TRMOKE which we attribute to the spin demagnetization in CrO2. Since the slow component contributes only a small fraction of the total conductivity change in TRTS, we conclude that spin fluctuations may not play the dominant role in the pump-induced conductivity change. We also observed that the film transmits less THz light after the pump excitation, which corresponds to it becoming more conductive. We will discuss the relationship of our observations to the electronic and optical properties of CrO2. The work at Tulane was supported by the Louisiana Board of Regents through the Board of Regents Support Fund Contract Number LEQSF(2012-15)-RD-A-23.
NASA Astrophysics Data System (ADS)
Lischner, Johannes; Bazhirov, Timur; MacDonald, Allan H.; Cohen, Marvin L.; Louie, Steven G.
2014-02-01
We present first-principles calculations for quasiparticle excitations in sodium and lithium, including the effects of charge and spin fluctuations. We employ the Overhauser-Kukkonen form for the electron self-energy arising from spin fluctuations and demonstrate that the coupling of electrons to spin fluctuations gives an important contribution to the quasiparticle lifetime but does not significantly reduce the occupied bandwidth. Including correlation effects beyond the random-phase approximation in the screening from charge fluctuations yields good agreement with experiment.
NASA Astrophysics Data System (ADS)
Chekhovich, E. A.; Hopkinson, M.; Skolnick, M. S.; Tartakovskii, A. I.
2015-02-01
Interaction with nuclear spins leads to decoherence and information loss in solid-state electron-spin qubits. One particular, ineradicable source of electron decoherence arises from decoherence of the nuclear spin bath, driven by nuclear-nuclear dipolar interactions. Owing to its many-body nature nuclear decoherence is difficult to predict, especially for an important class of strained nanostructures where nuclear quadrupolar effects have a significant but largely unknown impact. Here, we report direct measurement of nuclear spin bath coherence in individual self-assembled InGaAs/GaAs quantum dots: spin-echo coherence times in the range 1.2-4.5 ms are found. Based on these values, we demonstrate that strain-induced quadrupolar interactions make nuclear spin fluctuations much slower compared with lattice-matched GaAs/AlGaAs structures. Our findings demonstrate that quadrupolar effects can potentially be used to engineer optically active III-V semiconductor spin-qubits with a nearly noise-free nuclear spin bath, previously achievable only in nuclear spin-0 semiconductors, where qubit network interconnection and scaling are challenging.
Chekhovich, E A; Hopkinson, M; Skolnick, M S; Tartakovskii, A I
2015-01-01
Interaction with nuclear spins leads to decoherence and information loss in solid-state electron-spin qubits. One particular, ineradicable source of electron decoherence arises from decoherence of the nuclear spin bath, driven by nuclear-nuclear dipolar interactions. Owing to its many-body nature nuclear decoherence is difficult to predict, especially for an important class of strained nanostructures where nuclear quadrupolar effects have a significant but largely unknown impact. Here, we report direct measurement of nuclear spin bath coherence in individual self-assembled InGaAs/GaAs quantum dots: spin-echo coherence times in the range 1.2-4.5 ms are found. Based on these values, we demonstrate that strain-induced quadrupolar interactions make nuclear spin fluctuations much slower compared with lattice-matched GaAs/AlGaAs structures. Our findings demonstrate that quadrupolar effects can potentially be used to engineer optically active III-V semiconductor spin-qubits with a nearly noise-free nuclear spin bath, previously achievable only in nuclear spin-0 semiconductors, where qubit network interconnection and scaling are challenging. PMID:25704639
Chekhovich, E.A.; Hopkinson, M.; Skolnick, M.S.; Tartakovskii, A.I.
2015-01-01
Interaction with nuclear spins leads to decoherence and information loss in solid-state electron-spin qubits. One particular, ineradicable source of electron decoherence arises from decoherence of the nuclear spin bath, driven by nuclear–nuclear dipolar interactions. Owing to its many-body nature nuclear decoherence is difficult to predict, especially for an important class of strained nanostructures where nuclear quadrupolar effects have a significant but largely unknown impact. Here, we report direct measurement of nuclear spin bath coherence in individual self-assembled InGaAs/GaAs quantum dots: spin-echo coherence times in the range 1.2–4.5 ms are found. Based on these values, we demonstrate that strain-induced quadrupolar interactions make nuclear spin fluctuations much slower compared with lattice-matched GaAs/AlGaAs structures. Our findings demonstrate that quadrupolar effects can potentially be used to engineer optically active III-V semiconductor spin-qubits with a nearly noise-free nuclear spin bath, previously achievable only in nuclear spin-0 semiconductors, where qubit network interconnection and scaling are challenging. PMID:25704639
NASA Astrophysics Data System (ADS)
Arai, Hayato; Usui, Hidetomo; Suzuki, Katsuhiro; Fuseya, Yuki; Kuroki, Kazuhiko
2015-04-01
Motivated by recent experiments on isovalent-doped 1111 iron-based superconductors LaFeAs1-xPxO1-yFy and the theoretical study that followed, we investigate, within the five-orbital model, the correlation between spin fluctuations and the superconducting transition temperature, which exhibits a double-dome feature upon varying the Fe-As-Fe bond angle. Around the first dome with higher Tc, the low-energy spin fluctuation and Tc are not tightly correlated because the finite-energy spin fluctuation also contributes to superconductivity. On the other hand, the strength of the low-energy spin fluctuation originating from the dx z /y z orbital is correlated with Tc in the second dome with lower Tc. These calculation results are consistent with a recent NMR study, and hence strongly suggest that the pairing in iron-based superconductors is predominantly caused by multiorbital spin fluctuations.
Role of spin fluctuations in the conductivity of CrO2
NASA Astrophysics Data System (ADS)
Heffernan, Kate H.; Yu, Shukai; Deckoff-Jones, Skylar; Zhang, Xueyu; Gupta, Arunava; Talbayev, D.
2016-04-01
We present a time-resolved terahertz spectroscopic study of the half-metallic ferromagnet CrO2. The ultrafast conductivity dynamics excited by an optical pump displays very short (several picoseconds) and a very long (several hundred picoseconds) characteristic time scales. We attribute the former to the electron-phonon relaxation and the latter to the spin-lattice relaxation. We use this distinction to quantify the relative contribution of the scattering by spin fluctuations to the resistivity of CrO2: We find that they contribute less than one half of all scattering events below room temperature. This contribution rises to ˜70 % as the temperature approaches TC=390 K. The small effect of spin fluctuations on the resistivity is unexpected in light of the proposed double-exchange nature of the electronic and magnetic properties of CrO2.
Spin Fluctuation in YBaFe4O7+δ with Geometrically Frustrated Pyrochlore Lattice of Fe Spins
NASA Astrophysics Data System (ADS)
Kamazawa, Kazuya; Ishikado, Motoyuki; Ohira-Kawamura, Seiko; Kakurai, Kazuhisa; Nakajima, Kenji; Kawakita, Yukinobu; Yamada, Kazuyoshi; Arai, Masatoshi; Sato, Masatoshi
2015-10-01
Magnetic excitation spectra were measured by inelastic neutron scattering on a powder sample of YBaFe4O7+δ having a geometrically frustrated pyrochlore lattice of Fe spins. The observed scattering intensity map constructed in the energy ω-wave vector Q space has a streaklike magnetic contribution extending to a rather high-ω region at the Q position of Qp ˜ 1.25 Å-1, where the width κ and position Qp of the streak are found to be insensitive to both the temperature T and ω. These results indicate the existence of the short-time spin correlation of Fe hexagons in the pyrochlore lattice, and can be understood by considering the spin fluctuation arising from the purely frustrating nature of three-dimensional classical insulating systems.
Thermal and quantal isospin and spin fluctuations in heavy ion reactions
Moretto, L.G.
1980-01-01
The isobaric charge distributions are discussed in terms of quantal and classical isospin fluctuations. The roles of mass asymmetry and of the higher giant isovector modes are treated within the framework of a cylinder model that is worked out exactly. Spin fluctuations are considered first in terms of quantal fluctuations in a cylinder model and second in terms of thermal fluctuations in a two-sphere model. The results are applied to the calculation of in- and out-of-plane angular distributions for sequential fission, alpha and gamma decay. Analytical expressions are obtained for the angular distributions. The theoretical predictions are compared with experimental results for sequential fission, alpha, and gamma angular distributions. 23 figures.
NASA Astrophysics Data System (ADS)
Asfaw, Abraham; Tyryshkin, Alexei; Lyon, Stephen
Global magnetic field fluctuations present significant challenges to pulsed electron spin resonance experiments on systems with long spin coherence times. We will discuss results from experiments in which we follow instantaneous changes in magnetic field by locking to the free induction decay of a proton NMR signal using a phase-locked loop. We extend conventional field-frequency locking techniques used in NMR to follow slow magnetic field drifts by using a modified Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence in which the phase of the pi-pulses follows the phase of the proton spins at all times. Hence, we retain the ability of the CPMG pulse sequence to refocus local magnetic field inhomogeneities without refocusing global magnetic field fluctuations. In contrast with conventional field-frequency locking techniques, our experiments demonstrate the potential of this method to dynamically track global magnetic field fluctuations on timescales of about 2 seconds and with rates faster than a kHz. This frequency range covers the dominant noise frequencies in our electron spin resonance experiments as previously reported.
Stripe Antiferromagnetic Spin Fluctuations in SrCo_{2}As_{2}
Jayasekara, Wageesha; Lee, Young-Jin; Pandey, Abhishek; Tucker, Gregory; Sapkota, Aashish; Lamsal, J.; Calder, S.; Abernathy, D. L.; Niedziela, J. L.; Harmon, Bruce; Kreyssig, Andreas; Vaknin, David; Johnston, David; Goldman, A. I.; McQueeney, R. J.
2013-10-01
Inelastic neutron scattering measurements of paramagnetic SrCo_{2}As_{2} at T = 5 K reveal antiferromagnetic (AFM) spin fluctuations that are peaked at a wave vector of Q_{AFM} = (1/2, 1/2, 1) and possess a large energy scale. These stripe spin fluctuations are similar to those found in AFe_{2}As_{2} compounds, where spin-density wave AFM is driven by Fermi surface nesting between electron and hole pockets separated by Q_{AFM}. SrCo_{2}As_{2} has a more complex Fermi surface and band-structure calculations indicate a potential instability toward either a ferromagnetic or stripe AFM ground state. The results suggest that stripe AFM magnetism is a general feature of both iron and cobalt-based arsenides and the search for spin fluctuation-induced unconventional superconductivity should be expanded to include cobalt-based compounds.
Possible enhancements of AFM spin-fluctuations in high-TC cuprates
NASA Astrophysics Data System (ADS)
Jarlborg, Thomas
2009-03-01
Ab-initio band calculations for high-TC cuprates, together with modelling based of a free electron like band, show a strong interaction between anti-ferromagnetic (AFM) spin waves and periodic lattice distortions as for phonons, even though this type of spin-phonon coupling (SPC) is underestimated in calculations using the local density approximation. The SPC has a direct influence on the properties of the HTC cuprates and it can explain many observations. The strongest effects are seen for modulated waves in the CuO bond direction, and a band gap is formed near the X,Y points, but unusal band dispersion (like ``waterfalls'') might also be induced below the Fermi energy (EF) in the diagonal direction. The band results are used to propose different ways of increasing AFM spin-fluctuations locally, and to have a higher density-of-states (DOS) at EF. Static potential modulations, via periodic distribution of dopants or lattice distortions, can be tuned to increase the DOS. This opens for possibilities to enhance coupling for spin fluctuations (λsf) and superconductivity. The exchange enhancement is in general increased near a surface, which suggests a tendency towards static spin configurations. The sensivity of the band results to corrections of the local density potential are discussed.
Zhuravlev, I. A.; Antropov, V. P.; Belashchenko, K. D.
2015-11-16
The origins of the anomalous temperature dependence of magnetocrystalline anisotropy in (Fe1–xCox)2B alloys are elucidated using first-principles calculations within the disordered local moment model. Excellent agreement with experimental data is obtained. The anomalies are associated with the changes in band occupations due to Stoner-like band shifts and with the selective suppression of spin-orbit “hot spots” by thermal spin fluctuations. Under certain conditions, the anisotropy can increase, rather than decrease, with decreasing magnetization. These peculiar electronic mechanisms are in stark contrast to the assumptions of the existing models.
Zhuravlev, I. A.; Antropov, V. P.; Belashchenko, K. D.
2015-11-16
The origins of the anomalous temperature dependence of magnetocrystalline anisotropy in (Fe_{1–x}Co_{x})_{2}B alloys are elucidated using first-principles calculations within the disordered local moment model. Excellent agreement with experimental data is obtained. The anomalies are associated with the changes in band occupations due to Stoner-like band shifts and with the selective suppression of spin-orbit “hot spots” by thermal spin fluctuations. Under certain conditions, the anisotropy can increase, rather than decrease, with decreasing magnetization. These peculiar electronic mechanisms are in stark contrast to the assumptions of the existing models.
Effect of thermal fluctuations in spin-torque driven magnetization dynamics
NASA Astrophysics Data System (ADS)
Bonin, R.; Bertotti, G.; Serpico, C.; Mayergoyz, I. D.; d'Aquino, M.
2007-09-01
Nanomagnets with uniaxial symmetry driven by an external field and spin-polarized currents are considered. Anisotropy, applied field, and spin polarization are all aligned along the symmetry axis. Thermal fluctuations are described by adding a Gaussian white noise stochastic term to the Landau-Lifshitz-Gilbert equation for the deterministic dynamics. The corresponding Fokker-Planck equation is derived. It is shown that deterministic dynamics, thermal relaxation, and transition rate between stable states are governed by an effective potential including the effect of current injection.
Spin-orbit fluctuations in frustrated heavy-fermion metal LiV(2)O(4).
Tomiyasu, K; Iwasa, K; Ueda, H; Niitaka, S; Takagi, H; Ohira-Kawamura, S; Kikuchi, T; Inamura, Y; Nakajima, K; Yamada, K
2014-12-01
Spin fluctuations were studied over a wide momentum (ℏQ) and energy (E) space in the frustrated d-electron heavy-fermion metal LiV_{2}O_{4} by time-of-flight inelastic neutron scattering. We observed the overall Q-E evolutions near the characteristic Q=0.6 Å^{-1} peak and found another weak broad magnetic peak around 2.4 Å^{-1}. The data are described by a simple response function, a partially delocalized magnetic form factor, and antiferromagnetic short-range spatial correlations, indicating that heavy-fermion formation is attributable to spin-orbit fluctuations with orbital hybridization. PMID:25526141
Structural and magnetic field effects on spin fluctuations in Sr3Ru2O7
NASA Astrophysics Data System (ADS)
Mukherjee, Shantanu; Lee, Wei-Cheng
2016-08-01
We investigate the evolution of magnetic excitations in Sr3Ru2O7 in the paramagnetic metallic phase using a three-band tight-binding model. The effect of Mn or Ti dopant ions on the Sr3Ru2O7 band structure has been included by taking into account the dopant-induced suppression of the oxygen octahedral rotation in the tight-binding band structure. We find that the low-energy spin fluctuations are dominated by three wave vectors around q ⃗=( (0 ,0 ) ,(π /2 ,π /2 ) ) and (π ,0 ) , which compete with each other. As the octahedral rotation is suppressed with increasing doping, the three wave vectors evolve differently. In particular, the undoped compound has dominant wave vectors at q ⃗=( (0 ,0 ) ,(π /2 ,π /2 ) ) , but doping Sr3Ru2O7 leads to a significant enhancement in the spin susceptibility at the q ⃗=(π ,0 ) wave vector, bringing the system closer to a magnetic instability. All the features calculated from our model are in agreement with neutron scattering experiments. We have also studied the effect of a c -axis Zeeman field on the low-energy spin fluctuations. We find that an increasing magnetic field suppresses the antiferromagnetic (AFM) fluctuations and leads to stronger competition between the AFM and ferromagnetic spin fluctuations. The magnetic field dependence observed in our calculations therefore supports the scenario that the observed nematic phase in the metamagnetic region in Sr3Ru2O7 is intimately related to the presence of a competing ferromagnetic instability.
Effect of spin fluctuations on the resistivity of LaCrGe3
NASA Astrophysics Data System (ADS)
Singh, Durgesh; Gangrade, Mohan; Ganesan, V.
2016-05-01
Resistivity of LaCrGe3 at low temperatures and high magnetic fields is reported for fields upto 12T. Spin fluctuations play an important role in this compound whose TC is 90K. The normal state above TC is anomalous in the sense that a T1/2 term is to be added to the normal phonon contribution [ρ=ρ0+aT+bT1/2] to get a good fit, whose origin is debatable. Magneto resistance (MR) vs. applied field H in PM region confirms the presence of strong spin fluctuations in this material. Effect of magnetic field on resistivity shows marked deviation below 170K. Suppression of resistivity in field up to 12T near TC is observed. A negative magnetoresistance (MR) is seen and is consistent with the ferromagnetic behavior. The resistivity data fitted below 80K could be fitted with an equation ρ(H,T) = ρ0(H) + B(H)*Tn where n varies between 2.3 - 2.4, closed to n=2, signifying the presence of possible spin fluctuation.
Low-energy paramagnetic spin fluctuations in the weak itinerant ferromagnet MnSi
NASA Astrophysics Data System (ADS)
Ishikawa, Y.; Noda, Y.; Fincher, C.; Shirane, G.
1982-01-01
Low-energy paramagnetic excitations in the weak itinerant ferromagnet (WIF) MnSi have been studied by neutron scattering. The observed spectrum has a Lorentzian form (ΓΓ2+ω2) and is clearly separated from excitations in the Stoner continuum. The generalized susceptibility, χ(q), has been obtained by integrating the scattering intensity over energy. It is found that χ(q) depends upon the wave vector q as χ(q)-1=κ2(T)+q2 for q<=0.125(2πa) with κ2(T)=κ20(T-Tc). After extrapolating these results to q=0, it is found that χ(q=0) follows the Curie-Weiss law, suggesting that the observed spin fluctuations correspond to the Moriya-Kawabata (MK) spin fluctuations responsible for the Curie-Weiss dependence of the static susceptibility of a WIF. The linewidth Γ is found to be proportional to qχ(q) as predicted by the MK theory, in contrast with the q2χ(q) relation expected in a Heisenberg system. These results provide the first direct experimental evidence for the existence of MK spin fluctuations in a WIF above Tc.
Thermal magnetization fluctuations in CoFe spin-valve devices (invited)
NASA Astrophysics Data System (ADS)
Smith, Neil; Synogatch, Valeri; Mauri, Danielle; Katine, J. A.; Cyrille, Marie-Claire
2002-05-01
Thermally induced magnetization fluctuations in the Co86Fe14 free (sense) layer of micron-sized, photolithographically defined giant magetoresistive spin-valve devices are measured electrically, by passing a dc current through the devices and measuring the current-dependent part of the voltage noise power spectrum. Using fluctuation-dissipation relations, the effective Gilbert damping parameter α for 1.2, 1.8, and 2.4 nm thick free layers is estimated from either the low-frequency white-noise tail, or independently from the observed thermally excited ferromagnetic resonance peaks in the noise power spectrum, as a function of applied field. The geometry, field, and frequency dependence of the measured noise are found to be reasonably consistent with fluctuation-dissipation predictions based on a quasianalytical eigenmode model to describe the spatial dependence for the magnetization fluctuations. The extracted effective damping constant α≈0.06 found for the 1.2 nm free layer was close to 3× larger than that measured in either the 1.8 or 2.4 films, which has potentially serious implications for the future scaling down of spin-valve read heads.
Quantum Spin Fluctuations in Quasi-One-Dimensional Chlorine-Bridged Platinum Complexes
Wei, X.; Donohoe, R. J.; Wang, W. Z.; Bishop, A. R.; Gammel, J. T.
1997-01-01
We report experimental and theoretical studies of spin dynamic process in the quasi-one-dimensional chlorine-bridged platinum complex, [Pt{sup II}(en){sub 2}][Pt{sup IV}(en){sub 2}Cl{sub 2}](ClO{sub 4}){sub 4}, where en = ethylenediamine, C{sub 2}N{sub 2}H{sub 8}. The process manifests itself in collapsing of the hyperfine and superhyperfine structures in the electron spin resonance (ESR) spectrum and non-statistical distribution of spectral weight of the Pt isotopes. More surprisingly, it is activated only at temperatures below 6 K. We interpret the phenomenon in terms of quantum tunneling of the electronic spin in a strong electron-electron and electron-phonon coupling regime. This is modeled using a non-adiabatic many-body approach, in which polarons and solitons represent local spin-Peierls regions in a strongly disproportional charge-density-wave background and display intriguing spin-charge separation in the form of pinned charge and tunneling spin fluctuations. 24 refs., 5 figs., 1 tab.
Strong interplay between stripe spin fluctuations, nematicity and superconductivity in FeSe.
Wang, Qisi; Shen, Yao; Pan, Bingying; Hao, Yiqing; Ma, Mingwei; Zhou, Fang; Steffens, P; Schmalzl, K; Forrest, T R; Abdel-Hafiez, M; Chen, Xiaojia; Chareev, D A; Vasiliev, A N; Bourges, P; Sidis, Y; Cao, Huibo; Zhao, Jun
2016-02-01
In iron-based superconductors the interactions driving the nematic order (that breaks four-fold rotational symmetry in the iron plane) may also mediate the Cooper pairing. The experimental determination of these interactions, which are believed to depend on the orbital or the spin degrees of freedom, is challenging because nematic order occurs at, or slightly above, the ordering temperature of a stripe magnetic phase. Here, we study FeSe (ref. )-which exhibits a nematic (orthorhombic) phase transition at Ts = 90 K without antiferromagnetic ordering-by neutron scattering, finding substantial stripe spin fluctuations coupled with the nematicity that are enhanced abruptly on cooling through Ts. A sharp spin resonance develops in the superconducting state, whose energy (∼4 meV) is consistent with an electron-boson coupling mode revealed by scanning tunnelling spectroscopy. The magnetic spectral weight in FeSe is found to be comparable to that of the iron arsenides. Our results support recent theoretical proposals that both nematicity and superconductivity are driven by spin fluctuations. PMID:26641018
Transformation between spin-Peierls and incommensurate fluctuating phases of Sc-doped TiOCl
NASA Astrophysics Data System (ADS)
Zhang, Jian; Wölfel, Alexander; Bykov, Maxim; Schönleber, Andreas; van Smaalen, Sander; Kremer, Reinhard K.; Williamson, Hailey L.
2014-07-01
Single crystals of ScxTi1-xOCl(x=0.005) have been grown by the vapor phase transport technique. Specific heat measurements prove the absence of phase transitions for 4-200 K. Instead, an excess entropy is observed over a range of temperatures that encompasses the incommensurate phase transition at 90 K and the spin-Peierls transition at 67 K of pure TiOCl. Temperature-dependent x-ray diffraction on ScxTi1-xOCl gives broadened diffraction maxima at incommensurate positions between Tc1=61.5(3) and ˜90 K, and at commensurate positions below 61.5 K. These results are interpreted as due to the presence of an incommensurate phase without long-range order at intermediate temperatures, and of a highly disturbed commensurate phase without long-range order at low temperatures. The commensurate phase is attributed to a fluctuating spin-Peierls state on an orthorhombic lattice. The monoclinic symmetry and local structure of the fluctuations are equal to the symmetry and structure of the ordered spin-Peierls state of TiOCl. A novel feature of ScxTi1-xOCl(x =0.005) is a transformation from one fluctuating phase (the incommensurate phase at intermediate temperatures) to another fluctuating phase (the spin-Peierls-like phase). This transformation is not a phase transition occurring at a critical temperature, but it proceeds gradually over a temperature range of ˜10 K wide. The destruction of long-range order requires much lower levels of doping in TiOCl than in other low-dimensional electronic crystals, like the canonical spin-Peierls compound CuGeO3. An explanation for the higher sensitivity to doping has not been found, but it is noticed that it may be the result of an increased two-dimensional character of the doped magnetic system. The observed fluctuating states with long correlation lengths are reminiscent of Kosterlitz-Thouless-type phases in two-dimensional systems.
Magnetic ground state and spin fluctuations in MnGe chiral magnet as studied by muon spin rotation
NASA Astrophysics Data System (ADS)
Martin, N.; Deutsch, M.; Bert, F.; Andreica, D.; Amato, A.; Bonfà, P.; De Renzi, R.; Rößler, U. K.; Bonville, P.; Fomicheva, L. N.; Tsvyashchenko, A. V.; Mirebeau, I.
2016-05-01
We have studied by muon spin resonance (μ SR ) the helical ground state and fluctuating chiral phase recently observed in the MnGe chiral magnet. At low temperature, the muon polarization shows double-period oscillations at short-time scales. Their analysis, akin to that recently developed for MnSi [A. Amato et al., Phys. Rev. B 89, 184425 (2014), 10.1103/PhysRevB.89.184425], provides an estimation of the field distribution induced by the Mn helical order at the muon site. The refined muon position agrees nicely with ab initio calculations. With increasing temperature, an inhomogeneous fluctuating chiral phase sets in, characterized by two well-separated frequency ranges which coexist in the sample. Rapid and slow fluctuations, respectively, associated with short-range and long-range ordered helices, coexist in a large temperature range below TN=170 K. We discuss the results with respect to MnSi, taking the short helical period, metastable quenched state, and peculiar band structure of MnGe into account.
Non-equilibrium spin-boson model: counting statistics and the heat exchange fluctuation theorem.
Nicolin, Lena; Segal, Dvira
2011-10-28
We focus on the non-equilibrium two-bath spin-boson model, a toy model for examining quantum thermal transport in many-body open systems. Describing the dynamics within the noninteracting-blip approximation equations, applicable, e.g., in the strong system-bath coupling limit and/or at high temperatures, we derive expressions for the cumulant generating function in both the Markovian and non-Markovian limits by energy-resolving the quantum master equation of the subsystem. For a Markovian bath, we readily demonstrate the validity of a steady-state heat exchange fluctuation theorem. In the non-Markovian limit a "weaker" symmetry relation generally holds, a general outcome of microreversibility. We discuss the reduction of this symmetry relation to the universal steady-state fluctuation theorem. Using the cumulant generating function, an analytic expression for the heat current is obtained. Our results establish the validity of the steady-state heat exchange fluctuation theorem in quantum systems with strong system-bath interactions. From the practical point of view, this study provides tools for exploring transport characteristics of the two-bath spin-boson model, a prototype for a nonlinear thermal conductor. PMID:22047227
Effect of pairing fluctuations on the spin resonance in Fe-based superconductors
NASA Astrophysics Data System (ADS)
Hinojosa, Alberto; Chubukov, Andrey V.; Wölfle, Peter
2014-09-01
The spin resonance observed in the inelastic neutron scattering data on Fe-based superconductors has played a prominent role in the quest for determining the symmetry of the order parameter in these compounds. Most theoretical studies of the resonance employ an RPA-type approach in the particle-hole channel and associate the resonance in the spin susceptibility χS(q,ω) at momentum Q =(π,π) with the spin-exciton of an s+- superconductor, pulled below 2Δ by residual attraction associated with the sign change of the gap between Fermi points connected by Q. Here we explore the effect of fluctuations in the particle-particle channel on the spin resonance. Particle-particle and particle-hole channels are coupled in a superconductor and to what extent the spin resonance can be viewed as a particle-hole exciton needs to be addressed. In the case of purely repulsive interactions, we find that the particle-particle channel at total momentum Q (the π channel) contributes little to the resonance. However, if the interband density-density interaction is attractive and the π resonance is possible on its own, along with spin-exciton, we find a much stronger shift of the resonance frequency from the position of the would-be spin-exciton resonance. We also show that the expected double-peak structure in this situation does not appear because of the strong coupling between particle-hole and particle-particle channels, and ImχS(Q ,ω) displays only a single peak.
Spin fluctuations and frustrated magnetism in multiferroic FeVO4
NASA Astrophysics Data System (ADS)
Zhang, J.; Ma, L.; Dai, J.; Zhang, Y. P.; He, Zhangzhen; Normand, B.; Yu, Weiqiang
2014-05-01
We report 51V nuclear magnetic resonance (NMR) studies on single crystals of the multiferroic material FeVO4. The high-temperature Knight shift shows Curie-Weiss behavior 51K=a/(T+θ), with a large Weiss constant θ ≈ 116 K. However, the 51V spectrum shows no ordering near these temperatures, splitting instead into two peaks below 65 K, which suggests only short-ranged magnetic order on the NMR time scale. Two magnetic transitions are identified from peaks in the spin-lattice relaxation rate 1/51T1 at temperatures TN1≈ 19 K and TN2≈ 13 K, which are lower than the estimates obtained from polycrystalline samples. In the low-temperature incommensurate spiral state, the maximum ordered moment is estimated as 1.95μB/Fe, or 1/3 of the local moment. Strong low-energy spin fluctuations are also indicated by the unconventional power-law temperature dependence 1/51T1∝T2. The large Weiss constant, short-range magnetic correlations far above TN1, small ordered moment, significant low-energy spin fluctuations, and incommensurate ordered phases all provide explicit evidence for strong magnetic frustration in FeVO4.
Spin fluctuations of BaFe2(As,P)2 studied by neutron scattering
NASA Astrophysics Data System (ADS)
Lee, Chul-Ho; Steffens, P.; Qureshi, N.; Kihou, K.; Nakajima, M.; Iyo, A.; Eisaki, H.; Braden, M.
2013-03-01
Superconductivity can be induced in parent compounds of iron-based superconductors by several methods: carrier doping, external pressure and chemical pressure. To understand their superconducting mechanism, clarifying what is a common property for achieving high-Tc superconductivity is crucial. To date, studies on spin fluctuations have been mainly performed on carrier doped samples. On the other hand, there are only a few studies on chemical pressurized samples examined by powder samples. In this work, thus, we studied spin fluctuations of P doped BaFe2(As,P)2>(Tc = 29.5K) using single crystal samples. Inelastic neutron scattering measurements were conducted using triple axis spectrometer IN8 of ILL. As results, well-defined commensurate peaks have been observed at (0.5,0.5, L), which is consistent with the nesting vector of the Fermi surface. Energy spectrums at T = Tc show L dependence, suggesting a three dimensional character remains even in superconducting BaFe2(As,P)2. Clear spin gap has been observed below Tc, whose gap structure depends on L. Details will be discussed at the conference.
NASA Astrophysics Data System (ADS)
Wiecki, P.; Roy, B.; Johnston, D. C.; Bud'ko, S. L.; Canfield, P. C.; Furukawa, Y.
2015-09-01
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. 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.
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 ^{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. Furthermore, 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.
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
Magnetic ordering in tetragonal FeS: Evidence for strong itinerant spin fluctuations
Kwon, K.D.; Refson, K.; Bone, S.; Qiao, R.; Yang, W.; Liu, Z.; Sposito, G.
2010-11-01
Mackinawite is a naturally occurring layer-type FeS mineral important in biogeochemical cycles and, more recently, in the development of microbial fuel cells. Conflicting results have been published as to the magnetic properties of this mineral, with Moessbauer spectroscopy indicating no magnetic ordering down to 4.2 K but density functional theory (DFT) predicting an antiferromagnetic ground state, similar to the Fe-based high-temperature superconductors with which it is isostructural and for which it is known that magnetism is suppressed by strong itinerant spin fluctuations. We investigated this latter possibility for mackinawite using photoemission spectroscopy, near-edge x-ray absorption fine structure spectroscopy, and DFT computations. Our Fe 3{sub s} core-level photoemission spectrum of mackinawite showed a clear exchange-energy splitting (2.9 eV) consistent with a 1 {micro}{sub B} magnetic moment on the Fe ions, while the Fe L-edge x-ray absorption spectrum indicated rather delocalized Fe 3{sub d} electrons in mackinawite similar to those in Fe metal. Our DFT computations demonstrated that the ground state of mackinawite is single-stripe antiferromagnetic, with an Fe magnetic moment (2.7 {micro}{sub B}) that is significantly larger than the experimental estimate and has a strong dependence on the S height and lattice parameters. All of these trends signal the existence of strong itinerant spin fluctuations. If spin fluctuations prove to be mediators of electron pairing, we conjecture that mackinawite may be one of the simplest Fe-based superconductors.
Free-Spinning-Tunnel Investigation of a 1/24-Scale Model of the Grumman AF-2S, -2W Airplane
NASA Technical Reports Server (NTRS)
Klinar, Walter J.; Wilson, Jack H.
1950-01-01
An investigation of the spin and recovery characteristics of a 1/24-scale model of the Grumman AF-2S, -2W airplane was conducted in the Langley 20-foot free-spinning tunnel. The effects of controls on the erect and inverted spin and recovery characteristics for a range of possible loadings of the.airplane were determined. The effect of a revised-tail installation (small dual fins added to the stabilizer of the original tail and the vertical-tail height of the original tail increased) and the effect of various ventral-fin and antispin-fillet installations were determined. The investigation also included spin-recovery parachute tests.
Dynamic and static fluctuations in polymer gels studied by neutron spin-echo
NASA Astrophysics Data System (ADS)
Kanaya, T.; Takahashi, N.; Nishida, K.; Seto, H.; Nagao, M.; Takeba, Y.
2006-11-01
We report neutron spin-echo measurements on three types of poly(vinyl alcohol) (PVA) gels. The first is PVA gel in a mixture of dimethyl sulfoxide (DMSO) and water with volume ratio 60/40, the second is PVA gel in an aqueous borax solution and the third is chemically cross-linked PVA gel. The observed normalized intermediate scattering functions I( Q, t)/ I( Q,0) were very different among them. The I( Q, t)/ I( Q,0) of the first and third gels showed a non-decaying component in addition to a decaying component, but the second one did not have the non-decaying one. This clearly indicates that the fluctuations in the first and third PVA gels consist of static and dynamic fluctuations whereas the second PVA gel does include only the dynamic fluctuations. The dynamic and static fluctuations of the PVA gels were analyzed in terms of a restricted motion in the gel network and the Zimm motion, respectively.
Simulation of Statistical Fluctuations in the Spin Precession Measurements at RHIC
Poblaguev, A. A.
2014-02-25
Measurements of the driven spin coherent precession S_{x}(t)=S_{x}^{(0)} - S_{x}^{(1)} sin(ωt+φ_{0}) were initiated in RHIC Run13. The expected value of the precession amplitude S_{x}^{(1)} ~ 2 x 10^{-4} is about the statistical error in a single measurement and data fit gives a biased estimate of the S_{x}^{(1)}. For a proper statistical interpretation of the results of the several measurements, statistical fluctuations were studied using Monte-Carlo simulation. Preliminary results of the spin precession measurements in RHIC Run13 are presented.
Spin fluctuations in La2-xSrxCuO4: NMR versus inelastic neutron scattering
NASA Astrophysics Data System (ADS)
Barzykin, V.; Pines, D.; Thelen, D.
1994-12-01
We use a one-component description to analyze the current experimental situation for the low-frequency magnetic properties of La1.85Sr0.15CuO4 as determined by NMR and neutron-scattering experiments. We show that the measured 17O spin-lattice relaxation rate is in sharp conflict with the incommensurate-magnetic-structure interpretation of neutron-scattering experiments, but is quantitatively explained if the local-spin-fluctuation spectrum (measured by NMR) possesses a commensurate peak. We conclude that the formation of domains, as suggested by Slichter and Phillips, represents the best (and, quite possibly, only) way of reconciling NMR and neutron-scattering experiments on La1.85Sr0.15CuO4.
NASA Astrophysics Data System (ADS)
Tajima, H.; Hanai, R.; Ohashi, Y.
2016-05-01
We theoretically investigate magnetic properties of a unitary Fermi gas in a harmonic trap. Including strong pairing fluctuations within the framework of an extended T-matrix approximation, as well as effects of a trap potential within the local density approximation, we calculate the local spin susceptibility χ (T,r) above the superfluid phase transition temperature T_c. We show that the formation of preformed singlet Cooper pairs anomalously suppresses χ (T,r) in the trap center near T_c. We also point out that, in the unitarity limit, the spin-gap temperature in a uniform Fermi gas can be evaluated from the observation of the spatial variation of χ (T,r). Since a real ultracold Fermi gas is always in a trap potential, our results would be useful for the study of how this spatial inhomogeneity affects thermodynamic properties of an ultracold Fermi gas in the BCS-BEC crossover region.
Link between spin fluctuations and Cooper pairing in copper oxide superconductors
NASA Astrophysics Data System (ADS)
Jin, Kui
2012-02-01
Although it is generally accepted that superconductivity is unconventional in the high-Tc cuprates, the relative importance of phenomena such as spin and charge (strip) order, superconductivity fluctuations, proximity to Mott insulator, a pseudogap phase and quantum criticality are still a matter of debate. In electron-doped cuprates, the absence of ``anomalousphase in the underdoped region of the phase diagram and weaker electron correlations suggest that Mott physics and other unidentified competing orders are less relevant and that antiferromagnetic (AFM) spin fluctuations are the dominant feature. In this talk, I will report results of low temperature magnetotransport experiments in optimal to overdoped (non-superconducting) thin films of the electron-doped cuprate La2-xCexCuO4 (LCCO). We find that a linear-in-T scattering rate is correlated with the superconductivity (Tc). Our results show that an envelope of such scattering surrounds the superconducting phase, surviving to 20 mK (the limit of our experiments) when superconductivity is suppressed by magnetic fields [1]. Comparison with similar behavior found in organic superconductors [2] strongly suggests that the linear-in-T resistivity in the electron-doped cuprates is caused by spin-fluctuation scattering. Because linear-in-T scattering has also been linked to T% c in some hole-doped cuprates [2], our results suggest a fundamental connection between AFM spin fluctuations and the pairing mechanism of high temperature superconductivity in all cuprates. In addition, I will discuss how quantum criticality plays a significant role in shaping the anomalous properties of the electron-doped cuprate phase diagram. We identify quantum critical scaling in LCCO with a line of quantum critical points that surrounds the superconducting phase as a function of magnetic field and charge doping [3]. [4pt] [1] K. Jin, N.P. Butch, K. Kirshenbaum, J. Paglione, and R.L. Greene, Nature 476, 73 (2011).[0pt] [2] L. Taillefer, Annu
Yamakawa, Youichi; Kontani, Hiroshi
2015-06-26
We present a microscopic derivation of the nematic charge-density wave (CDW) formation in cuprate superconductors based on the three-orbital d-p Hubbard model by introducing the vertex correction (VC) into the charge susceptibility. The CDW instability at q=(Δ(FS),0), (0,Δ(FS)) appears when the spin fluctuations are strong, due to the strong charge-spin interference represented by the VC. Here, Δ(FS) is the wave number between the neighboring hot spots. The obtained spin-fluctuation-driven CDW is expressed as the "intra-unit-cell orbital order" accompanied by the charge transfer between the neighboring atomic orbitals, which is actually observed by the scanning tunneling microscope measurements. We predict that the cuprate CDW and the nematic orbital order in Fe-based superconductors are closely related spin-fluctuation-driven phenomena. PMID:26197139
Ti3CrCu4: A possible 2-D ferromagnetic spin fluctuating system
NASA Astrophysics Data System (ADS)
Dhar, S. K.; Provino, A.; Manfrinetti, P.; Kulkarni, R.; Goyal, Neeraj; Paudyal, D.
2016-05-01
Ti3CrCu4 is a new ternary compound which crystallizes in the tetragonal Ti3Pd5 structure type. The Cr atoms form square nets in the a-b plane (a = 3.124 Å) which are separated by an unusually large distance c = 11.228 Å along the tetragonal axis, thus forming a -2-D Cr-sublattice. The paramagnetic susceptibility is characterized by a low effective moment, μeff = 1.1 μB, a low paramagnetic Curie temperature θP (below 7 K) and a temperature independent χ0 = 6.7 x 10-4 emu/mol. The magnetization at 1.8 K increases rapidly with field nearly saturating to 0.2 μB/f.u. The zero field heat capacity C/T shows an upturn below 7 K (˜190 mJ/mol K2 at ˜0.1K) which is suppressed in applied magnetic fields and interpreted as suggesting the presence of spin fluctuations. The resistivity at low temperatures shows non-Fermi liquid behavior. Overall, the experimental data thus reveal an unusual magnetic state in Ti3CrCu4, which likely has its origin in the layered nature of the Cr sub-lattice and ferromagnetic spin fluctuations. Density functional theoretical calculations reveal a sharp Cr density of states peak just above the Fermi level, indicating the propensity of Ti3CrCu4 to become magnetic.
Low-energy spin fluctuations in the metallic spinel compound LiV2O4
NASA Astrophysics Data System (ADS)
Yushankhai, V. Yu.; Thalmeier, P.; Takimoto, T.
2009-08-01
In the family of transition metal oxides the spinel compound LiV2O4 is a rare metallic system showing heavy fermion behavior. In particular, an anomalously large specific heat coefficient γ = C/T and strongly enhanced magnetic susceptibility χs were detected in the low temperature limit, T<30 K. Recently we have proposed a model which allowed us to relate such an anomalous behavior of LiV2O4 to the proximity of the underlying 3d-electron system to a magnetic instability at T→0. The emergence of a rather peculiar paramagnetic ground state with largely degenerate lowenergy "critical" antiferromagnetic fluctuations in LiV2O4 is the combined effect of strong electron correlations and the geometrical frustration of V-ion pyrochlore lattice forming the metallic system in this compound. A self-consistent renormalization theory was developed to describe effects of strong coupling between spin fluctuation modes and their evolution with varying temperature and external pressure. The theory was shown to provide a firm basis for understanding many peculiar properties of spin dynamics obtained in the inelastic neutron scattering and NMR measurements on LiV2O4.
NMR Study of Superconductivity and Spin Fluctuations in Intercalated Iron Selenides AyFe2-xSe2
NASA Astrophysics Data System (ADS)
Yu, Weiqiang
2012-02-01
The role of spin fluctuations in superconductivity is an essential topic in both cuprate and Fe-based superconductors. NMR works in several Fe-based superconductors proposed that the low-energy antiferromagnetic spin fluctuations (AFSF) is a possible pairing glue for superconductivity. However, studies on other systems such as KFe2As2 and Li1-xFeAs does not support a strong correlation between low-energy spin fluctuations and superconductivity. In the newly discovered AyFe2-xSe2 superconductors with Tc˜ 32 K, our NMR study identifies unambiguously a paramagnetic superconducting phase, which is phase separated from the block antiferromagnetic state. The low-energy AFSF is not seen at all, even though the Tc is high. The AyFe2-xSe2 are singlet superconductors evidenced from the NMR Knight shift K; However, the absence of the coherence peak in the spin-lattice relaxation rate 1/T1 suggests an unconventional behavior of superconductivity. In fact, we found that both the K and the 1/T1T increase dramatically with temperature and follow a a+bT^2 form from Tc up to 300 K. Such behavior is strong evidence for spin fluctuations with a high-energy, local nature in 3D systems, and inconsistent with a band-gap effect. Furthermore, K and 1/T1T saturate above 400 K, indicating an energy scale of 35 meV, which is distinct from the low-energy spin fluctuations. The above temperature enhanced spin fluctuations seem to be universal in Fe-based superconductors. [4pt] References: W. Yu et al., Phys. Rev. Lett. 106, 197001 (2011); Long Ma et al., Phys. Rev. B 83, 174510 (2011); L. Ma et al., arXiv:1103.4960.
Theory of spin-fluctuation induced superconductivity in iron-based superconductors
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
21 cm signal from cosmic dawn: imprints of spin temperature fluctuations and peculiar velocities
NASA Astrophysics Data System (ADS)
Ghara, Raghunath; Choudhury, T. Roy; Datta, Kanan K.
2015-02-01
The 21 cm brightness temperature δTb fluctuations from reionization promise to provide information on the physical processes during that epoch. We present a formalism for generating the δTb distribution using dark matter simulations and a 1D radiative transfer code. Our analysis is able to account for the spin temperature TS fluctuations arising from inhomogeneous X-ray heating and Lyα coupling during cosmic dawn. The δTb power spectrum amplitude at large scales (k ˜ 0.1 Mpc-1) is maximum when ˜10 per cent of the gas (by volume) is heated above the cosmic microwave background temperature. The power spectrum shows a `bump'-like feature during cosmic dawn and its location measures the typical sizes of heated regions. We find that the effect of peculiar velocities on the power spectrum is negligible at large scales for most part of the reionization history. During early stages (when the volume averaged ionization fraction ≲ 0.2) this is because the signal is dominated by fluctuations in TS. For reionization models that are solely driven by stars within high-mass (≳ 109 M⊙) haloes, the peculiar velocity effects are prominent only at smaller scales (k ≳ 0.4 Mpc-1) where patchiness in the neutral hydrogen density dominates the signal. The conclusions are unaffected by changes in the amplitude or steepness in the X-ray spectra of the sources.
NASA Astrophysics Data System (ADS)
Álvarez, Gonzalo A.; Ajoy, Ashok; Peng, Xinhua; Suter, Dieter
2010-10-01
Avoiding the loss of coherence of quantum mechanical states is an important prerequisite for quantum information processing. Dynamical decoupling (DD) is one of the most effective experimental methods for maintaining coherence, especially when one can access only the qubit system and not its environment (bath). It involves the application of pulses to the system whose net effect is a reversal of the system-environment interaction. In any real system, however, the environment is not static, and therefore the reversal of the system-environment interaction becomes imperfect if the spacing between refocusing pulses becomes comparable to or longer than the correlation time of the environment. The efficiency of the refocusing improves therefore if the spacing between the pulses is reduced. Here, we quantify the efficiency of different DD sequences in preserving different quantum states. We use C13 nuclear spins as qubits and an environment of H1 nuclear spins as the environment, which couples to the qubit via magnetic dipole-dipole couplings. Strong dipole-dipole couplings between the proton spins result in a rapidly fluctuating environment with a correlation time of the order of 100 μs. Our experimental results show that short delays between the pulses yield better performance if they are compared with the bath correlation time. However, as the pulse spacing becomes shorter than the bath correlation time, an optimum is reached. For even shorter delays, the pulse imperfections dominate over the decoherence losses and cause the quantum state to decay.
Quantum critical point and spin fluctuations in lower-mantle ferropericlase
Lyubutin, Igor S.; Struzhkin, Viktor V.; Mironovich, A. A.; Gavriliuk, Alexander G.; Naumov, Pavel G.; Lin, Jung-Fu; Ovchinnikov, Sergey G.; Sinogeikin, Stanislav; Chow, Paul; Xiao, Yuming; Hemley, Russell J.
2013-01-01
Ferropericlase [(Mg,Fe)O] is one of the most abundant minerals of the earth’s lower mantle. The high-spin (HS) to low-spin (LS) transition in the Fe2+ ions may dramatically alter the physical and chemical properties of (Mg,Fe)O in the deep mantle. To understand the effects of compression on the ground electronic state of iron, electronic and magnetic states of Fe2+ in (Mg0.75Fe0.25)O have been investigated using transmission and synchrotron Mössbauer spectroscopy at high pressures and low temperatures (down to 5 K). Our results show that the ground electronic state of Fe2+ at the critical pressure Pc of the spin transition close to T = 0 is governed by a quantum critical point (T = 0, P = Pc) at which the energy required for the fluctuation between HS and LS states is zero. Analysis of the data gives Pc = 55 GPa. Thermal excitation within the HS or LS states (T > 0 K) is expected to strongly influence the magnetic as well as physical properties of ferropericlase. Multielectron theoretical calculations show that the existence of the quantum critical point at temperatures approaching zero affects not only physical properties of ferropericlase at low temperatures but also its properties at P-T of the earth’s lower mantle. PMID:23589892
Modeling Spin Fluctuations and Magnetic Excitations from Time-Dependent Density Functional Theory
NASA Astrophysics Data System (ADS)
Gorni, Tommaso; Timrov, Iurii; Dal Corso, Andrea; Baroni, Stefano
Harnessing spin fluctuations and magnetic excitations in materials is key in many fields of technology, spanning from memory devices to information transfer and processing, to name but a few. A proper understanding of the interplay between collective and single-particle spin excitations is still lacking, and it is expected that first-principle simulations based on TDDFT may shed light on this interplay, as well as on the role of important effects such as relativistic ones and related magnetic anisotropies. All the numerical approaches proposed so far to tackle this problem are based on the computationally demanding solution of the Sternheimer equations for the response orbitals or the even more demanding solution of coupled Dyson equations for the spin and charge susceptibilities. The Liouville-Lanczos approach to TDDFT has already proven to be a valuable alternative, the most striking of its features being the avoidance of sums over unoccupied single-particle states and the frequency-independence of the main numerical bottleneck. In this work we present an extension of this methodology to magnetic systems and its implementation in the
Strong coupling critique of spin fluctuation driven charge order in underdoped cuprates
NASA Astrophysics Data System (ADS)
Mishra, Vivek; Norman, M. R.
2015-08-01
Charge order has emerged as a generic feature of doped cuprates, leading to important questions about its origin and its relation to superconductivity. Recent experiments on two classes of hole doped cuprates indicate a novel d -wave symmetry for the order. These were motivated by earlier spin fluctuation theoretical studies based on an expansion about hot spots in the Brillouin zone that indicated such an order would be competitive with d -wave superconductivity. Here, we reexamine this problem by solving strong coupling equations in the full Brillouin zone for experimentally relevant parameters. We find that bond-oriented order, as seen experimentally, is strongly suppressed. We also include coupling to B1 g phonons and do not see any qualitative change. Our results argue against an itinerant model for the charge order, implying instead that such order is likely due to Coulombic phase separation of the doped holes.
Spectroscopic Evidence for Strong Quantum Spin Fluctuations with Itinerant Character in YFe2Ge2
Sirica, N.; Bondino, F.; Nappini, S.; Piz, I.; Poudel, L.; Christianson, Andrew D.; Mandrus, D.; Singh, David J; Mannella, Norman
2015-03-04
We report x-ray absorption and photoemission spectroscopy of the electronic structure in the normal state of metallic YFe2Ge2. The data reveal evidence for large fluctuating spin moments on the Fe sites, as indicated by exchange multiplets appearing in the Fe 3s core-level photoemission spectra, even though the compound does not show magnetic order. The magnitude of the multiplet splitting is comparable to that observed in the normal state of the Fe-pnictide superconductors. This shows a connection between YFe2Ge2 and the Fe-based superconductors even though it contains neither pnictogens nor chalcogens. Finally, the implication is that the chemical range of compoundsmore » showing at least one of the characteristic magnetic signatures of the Fe-based superconductors is broader than previously thought.« less
Pressure dependence of critical temperature of bulk FeSe from spin fluctuation theory
NASA Astrophysics Data System (ADS)
Hirschfeld, Peter; Kreisel, Andreas; Wang, Yan; Tomic, Milan; Jeschke, Harald; Jacko, Anthony; Valenti, Roser; Maier, Thomas; Scalapino, Douglas
2013-03-01
The critical temperature of the 8K superconductor FeSe is extremely sensitive to pressure, rising to a maximum of 40K at about 10GPa. We test the ability of the current generation of fluctuation exchange pairing theories to account for this effect, by downfolding the density functional theory electronic structure for each pressure to a tight binding model. The Fermi surface found in such a procedure is then used with fixed Hubbard parameters to determine the pairing strength using the random phase approximation for the spin singlet pairing vertex. We find that the evolution of the Fermi surface captured by such an approach is alone not sufficient to explain the observed pressure dependence, and discuss alternative approaches. PJH, YW, AK were supported by DOE DE-FG02-05ER46236, the financial support of MT, HJ, and RV from the DFG Schwerpunktprogramm 1458 is kindly acknowledged.
Spin fluctuations above 100 K in stoichiometric LiCoO2
NASA Astrophysics Data System (ADS)
Mukai, K.; Aoki, Y.; Andreica, D.; Amato, A.; Watanabe, I.; Giblin, S. R.; Sugiyama, J.
2014-12-01
Although stoichiometric lithium cobalt dioxide LiCoO2 (ST-LCO) is the most common positive electrode material for Li-ion batteries, the magnetic nature of ST-LCO is still not fully understood. Therefore, we measured susceptibility (χ), electron paramagnetic resonance (EPR), and μSR for ST-LCO, particularly above 100 K. The temperature dependence of χ shows a Pauli paramagnetic behavior, supporting the previous conclusion that Co3+ ions are in a low-spin state with S = 0 (t62g). However, the EPR and μSR measurements reveal a "dynamical" magnetic phase in ST-LCO above 100 K. Because the volume fraction of this magnetic phase reaches about 50% at 300 K, the appearance of the magnetic phase is not caused by impurities and/or muonium formation but is an intrinsic characteristic of ST-LCO. By considering the time windows of the three measurement techniques used in this study, we conclude that the origin of the dynamical magnetism is most likely spin fluctuations of the Co ions.
NASA Astrophysics Data System (ADS)
Kuroki, Kazuhiko
2015-03-01
In the itinerant spin picture of the iron-based superconductors, the nesting between electron and hole Fermi surfaces is usually considered to be the origin of the spin fluctuation and thus the pairing glue. However, there have appeared some experimental observations suggesting absence of Fermi surface nesting. For instance, in the 1111 materials Ln FeAsO1-xHx (Ln =La,Sm, etc.), electron doping rate x reaches up to 50%, which in a rigid band picture would wipe out the hole Fermi surfaces. Still, superconductivity not only survives, but is even enhanced in the largely doped regime, in contradiction to the expectation from the bad nesting. Another example is KxFe2-ySe2, where the ARPES experiments show the absence of hole Fermi surfaces. In the present talk, we first focus on Ln FeAsO1-xHx, where the band structure is actually not rigid against doping, and the hole Fermi surface originating from the dxy orbital remains nearly unchanged. The origin of this can be traced back to real space, where the nearest neighbor hopping t1 within the dxy orbital is found to be strongly suppressed with doping. Although the nesting itself is degraded, the spin fluctuation in the largely electron doped regime is enhanced due to t2 >t1 , where t2 is the 2nd neighbor diagonal hopping. This re-enhances s +/- pairing superconductivity, and explains the double dome x-Tc phase diagram of LaFeAsO1-xHx. From this viewpoint, it is also interesting to look into the relation between t1 and t2 in other materials. For instance, our first principles estimation for KFe2Se2givest1 = - 0 . 008 eV and t2 = 0 . 056 eV, and from this strong reduction of t1, both electron and hole Fermi surfaces are expected to be present around the Γ point, in contradiction to previous experimental observations. Results of a recent ARPES experiment will be discussed from this viewpoint.
Identical spin fluctuations in Cu- and Co-doped BaFe2As2 independent of electron doping
NASA Astrophysics Data System (ADS)
Grafe, H.-J.; Gräfe, U.; Dioguardi, A. P.; Curro, N. J.; Aswartham, S.; Wurmehl, S.; Büchner, B.
2014-09-01
We present As75 nuclear magnetic resonance measurements on single crystals of BaFe2As2, BaFe1.8Co0.2As2, and BaFe1.82Cu0.18As2. While only Co doping induces bulk superconductivity on a broad doping range, the spin fluctuations probed by the nuclear spin-lattice relaxation rate (T1T )-1 are identical for both dopings down to Tc. Below this temperature, (T1T)-1 of the Cu-doped sample continues to rise, proving that (a) there is a quantum critical point below the superconducting dome, and (b) adding electrons does not affect the spin fluctuations. Consequently, we analyze the Knight shift data in terms of a two-component scenario, with one hyperfine coupling to an itinerant degree of freedom and the other to Fe moments.
NASA Astrophysics Data System (ADS)
Kanaya, T.; Takahashi, N.; Nishida, K.; Seto, H.; Nagao, M.; Takeda, T.
2005-01-01
We report neutron spin-echo measurements on two types of poly(vinyl alcohol) (PVA) gels. The first is PVA gel in a mixture of dimethyl sulfoxide (DMSO) and water with volume ratio 60/40 , and the second is PVA gel in an aqueous borax solution. The observed normalized intermediate scattering functions I(Q,t)/I(Q,0) are very different between them. The former I(Q,t)/I(Q,0) shows a nondecaying component in addition to a fast decay, but the latter does not have the nondecaying one. This clearly indicates that the fluctuations in the former PVA gel consist of static and dynamic fluctuations whereas the latter PVA gel does include only the dynamic fluctuations. The dynamic fluctuations of the former and latter gels have been analyzed in terms of a restricted motion in the network and Zimm motion, respectively, and the origins of these motions will be discussed.
Kanaya, T.; Takahashi, N.; Nishida, K.; Seto, H.; Nagao, M.; Takeda, T.
2005-01-01
We report neutron spin-echo measurements on two types of poly(vinyl alcohol) (PVA) gels. The first is PVA gel in a mixture of dimethyl sulfoxide (DMSO) and water with volume ratio 60/40, and the second is PVA gel in an aqueous borax solution. The observed normalized intermediate scattering functions I(Q,t)/I(Q,0) are very different between them. The former I(Q,t)/I(Q,0) shows a nondecaying component in addition to a fast decay, but the latter does not have the nondecaying one. This clearly indicates that the fluctuations in the former PVA gel consist of static and dynamic fluctuations whereas the latter PVA gel does include only the dynamic fluctuations. The dynamic fluctuations of the former and latter gels have been analyzed in terms of a restricted motion in the network and Zimm motion, respectively, and the origins of these motions will be discussed.
NASA Astrophysics Data System (ADS)
Tajima, Hiroyuki; Hanai, Ryo; Ohashi, Yoji
2015-03-01
We theoretically discuss the spin susceptibility χ and effects of strong-coupling corrections in the BCS-BEC crossover regime of an ultracold Fermi gas. Using an extended T-matrix approximation, we calculate χ over the entire BCS-BEC crossover region, showing that this magnetic quantity is very sensitive to pairing fluctuations in both the normal and the superfluid phase. In the normal state, it is suppressed by preformed singlet Cooper pairs near Tc, being similar to the spin-gap phenomenon in high-Tc cuprates. Below Tc, on the other hand, pairing fluctuations enhance χ, in the sense that the suppression of this quantity by the superfluid order is weakened due to partial dissociation of Cooper pairs. From these, we determine the region where pairing fluctuations strongly affect spin excitations in the phase diagram of a Fermi gas with respect to the temperature and the strength of a pairing interaction. We also compare our results with the recent experiments on a 6Li Fermi gas. Our results indicate that the spin susceptibility is a useful observable in understanding strong-coupling properties of an ultracold Fermi gas in the BCS-BEC crossover region. H. T. was supported by Graduate School Doctoral Student Aid Program from Keio University.
Hydration-induced anisotropic spin fluctuations in NaxCoO2 · 1.3H2O superconductor
NASA Astrophysics Data System (ADS)
Matano, K.; Lin, C. T.; Zheng, Guo-qing
2008-12-01
We report 59Co NMR studies in single crystals of the cobalt oxide superconductor Na0.42CoO2·1.3H2O (Tc=4.25 K) and its parent compound Na0.42CoO2. We find that both the magnitude and the temperature (T) dependence of the Knight shifts are identical in the two compounds above Tc. The spin-lattice relaxation rate (1/T1) is also identical above T0~60 K for both compounds. Below T0, the unhydrated sample is found to be a non-correlated metal that well conforms to the Fermi liquid theory, while spin fluctuations develop in the superconductor. These results indicate that water intercalation does not change the density of states at the Fermi level or the carrier density but its primary role is to bring about spin fluctuations. Our result shows that, in the hydrated superconducting compound, the electron correlation is anisotropic. Namely, the spin fluctuation around the finite wave vector is much stronger along the a-axis direction than that along the c-axis direction.
Competition between spin fluctuations in Ca2-xSrxRuO4 around x=0.5
NASA Astrophysics Data System (ADS)
Arakawa, Naoya; Ogata, Masao
2013-05-01
We study the static susceptibilities for charge and spin sectors in paramagnetic states for Ca2-xSrxRuO4 in 0.5≤x≤2 within random phase approximation on the basis of an effective Ru t2g orbital Hubbard model. We find that several modes of spin fluctuation around q=(0,0) and q˜(0.797π,0) are strongly enhanced for the model of x=0.5. This enhancement arises from the increase of the corresponding susceptibilities for the dxy orbital due to the rotation-induced modifications of the electronic structure for this orbital (i.e., the flattening of the bandwidth and the increase of the density of states near the Fermi level). We also find that the ferromagnetic spin fluctuation becomes stronger for a special model than for the model of x=0.5, while the competition between the modes of spin fluctuation at q=(0,0) and around q˜(π,0) is weaker for the special model; in this special model, the van Hove singularity (vHs) for the dxy orbital is located on the Fermi level. These results indicate that the location of the vHs for the dxy orbital, which is controlled by substitution of Ca for Sr, is a parameter to control this competition. We propose that the spin fluctuations for the dxy orbital around q=(0,0) and q˜(π,0) play an important role in the electronic states around x=0.5 other than the criticality approaching the usual Mott transition where all electrons are localized.
Linearized Jastrow-style fluctuations on spin-projected Hartree-Fock
Henderson, Thomas M.; Scuseria, Gustavo E.
2013-12-21
The accurate and efficient description of strong electronic correlations remains an important objective in electronic structure theory. Projected Hartree-Fock theory, where symmetries of the Hamiltonian are deliberately broken and projectively restored, all with a mean-field computational scaling, shows considerable promise in this regard. However, the method is neither size extensive nor size consistent; in other words, the correlation energy per particle beyond broken-symmetry mean field vanishes in the thermodynamic limit, and the dissociation limit of a molecule is not the sum of the fragment energies. These two problems are closely related. Recently, Neuscamman [Phys. Rev. Lett. 109, 203001 (2012)] has proposed a method to cure the lack of size consistency in the context of the antisymmetrized geminal power wave function (equivalent to number-projected Hartree-Fock-Bogoliubov) by using a Jastrow-type correlator in Hilbert space. Here, we apply the basic idea in the context of projected Hartree-Fock theory, linearizing the correlator for computational simplicity but extending it to include spin fluctuations. Results are presented for the Hubbard Hamiltonian and for some simple molecular systems.
NASA Astrophysics Data System (ADS)
Matin, Md.; Sharath Chandra, L. S.; Pandey, Sudhir K.; Chattopadhyay, Maulindu Kumar; Roy, Sindhunil Barman
2014-06-01
We report a study of the normal and superconducting state properties of the Ti x V1- x alloys for x = 0.4, 0.6, 0.7 and 0.8 with the help of dc magnetization, electrical resistivity and heat capacity measurements along with the electronic structure calculation. The superconducting transition temperature T c of these alloys is higher than that of elemental Ti and is also higher than elemental V for x ≤ 0.7. The roles of electron density of states, electron-phonon coupling and spin fluctuations in the normal and superconducting state properties of these alloys have been investigated in detail. The experimentally observed value of T c is found to be considerably lower than that estimated on the basis of electron density of states and electron-phonon coupling in the x = 0.4, 0.6 and 0.7 alloys. There is some evidence as well for the preformed Cooper pair in all these Ti-V alloys in the temperature regime well above T c . Similar to x = 0.6 [Md. Matin, L.S. Sharath Chandra, R.K. Meena, M.K. Chattopadhyay, A.K. Sinha, M.N. Singh, S.B. Roy, Physica B 436, 20 (2014)], the normal state properties of the x = 0.4 alloy showed the signature of the presence of spin fluctuations. The difference between the experimentally observed T c and that estimated by considering electron density of states and electron-phonon coupling in the x = 0.4, 0.6 and 0.7 alloys is attributed to the possible influence of these spin fluctuations. We show that the non-monotonous variation of T c as a function of x in the Ti x V1- x alloys is due to the combined effects of the electron-phonon coupling and the spin fluctuations.
Critical spin fluctuations and the origin of nematic order in Ba(Fe1-xCox)2As2
NASA Astrophysics Data System (ADS)
Kretzschmar, F.; Böhm, T.; Karahasanović, U.; Muschler, B.; Baum, A.; Jost, D.; Schmalian, J.; Caprara, S.; Grilli, M.; di Castro, C.; Analytis, J. G.; Chu, J.-H.; Fisher, I. R.; Hackl, R.
2016-06-01
Nematic fluctuations and order play a prominent role in material classes such as the cuprates, some ruthenates or the iron-based compounds and may be interrelated with superconductivity. In iron-based compounds signatures of nematicity have been observed in a variety of experiments. However, the fundamental question as to the relevance of the related spin, charge or orbital fluctuations remains open. Here, we use inelastic light (Raman) scattering and study Ba(Fe1-xCox)2As2 (0 <= x <= 0.085) for getting direct access to nematicity and the underlying critical fluctuations with finite characteristic wavelengths. We show that the response from fluctuations appears only in B1g (x2 - y2) symmetry (1 Fe unit cell). The scattering amplitude increases towards the structural transition at Ts but vanishes only below the magnetic ordering transition at TSDW < Ts, suggesting a magnetic origin of the fluctuations. The theoretical analysis explains the selection rules and the temperature dependence of the fluctuation response. These results make magnetism the favourite candidate for driving the series of transitions.
Spin Fluctuations in (cerium, YTTRIUM)COBALT-2 and Related Systems.
NASA Astrophysics Data System (ADS)
Timlin, John
The pseudobinary alloy systems (Y_ {rm x}Zr_{1 -rm x})Co_2, (Y _{rm x}Ce _{1-rm x})Co_2 , and (Ce_{rm x} Zr_{1-rm x})Co _2, for 0 < x < 1, have been studied. The temperature dependence of the electrical resistivity, magnetic susceptibility and specific heat have been measured for these systems. The temperature ranges were: for the electrical resistivity 1.5 to 300 K, for the magnetic susceptibility 6 to 300 K and for the specific heat 1.5 to 25 K. All three measurements show a rapid falloff of enhancements due to d-electron spin fluctuations as yttrium is replaced by zirconium in the (Y,Zr)Co_2 system. The variation of both the magnetic susceptibility and the specific heat as one substitutes cerium for yttrium in the (Y,Ce)Co _2 system is strikingly similar to that observed for (Y,Zr)Co_2. However, the resistivity of the (Y,Ce)Co_2 system is markedly different from that observed in (Y,Zr)Co_2 . Measurements done on the (Ce,Zr)Co_2 system confirm both the similarities between CeCo _2 and ZrCo_2 seen in the magnetic susceptibility and specific heat and the difference seen in the resistivity. Of greatest interest to this study is the evolution of the curvature of the temperature dependent magnetic susceptibility, which evolves from an upward bending form in YCo_2 to a downward bending form in both CeCo_2 and ZrCo_2 as predicted by theory for strongly enhanced paramagnets with a suitable density of states. This is the first controlled alloy study which shows such an evolution.
NASA Astrophysics Data System (ADS)
Montes, H.; Monkenbusch, M.; Willner, L.; Rathgeber, S.; Fetters, L.; Richter, D.
1999-05-01
Diblock copolymers in the melt exhibit order-disorder phase transitions (ODT), which are accompanied by strong concentration fluctuations. These transitions are generally described in terms of the random phase approximation (RPA) of Leibler and Fredrickson, which is able to explain small angle scattering results in the neighborhood of the ODT, in particular around the correlation peak at q*. The RPA theory has been extended to include dynamical phenomena, predicting the short time relaxation of the dynamic structure factor in polymeric multicomponent systems. We report small angle neutron scattering and neutron spin echo experiments on polyethylene-block-polyethylethylene (PE-PEE) and poly(ethylene-propylene)-block-polyethylethylene (PEP-PEE) copolymers with molecular weights of 16.500 and 68.000 g/mol, which explore the structure and dynamics of these block copolymers. Studying melts with different hydrogen/deuterium labeling it was possible to observe experimentally the different relaxation modes of such systems separately. In particular the collective relaxation behavior as well as the single chain motion were accessed. The experimental results were quantitatively compared with the RPA predictions, which were based solely on the dynamical properties of the corresponding homopolymers and the static structure factors. The collective dynamics exhibits an unanticipated fast relaxation mode. This mode is most visible at low wave numbers (q⩾q*) but extends to length scales considerably shorter than the radius of gyration. Furthermore, the dynamical RPA yields expressions for the mobilities of chain segments in the block copolymer melt. These combination rules are at variance with the experimental findings for the single chain dynamics, while they hold for the collective response.
Superconductivity and spin fluctuations in the actinoid-platinum metal borides {Th ,U } Pt3B
NASA Astrophysics Data System (ADS)
Bauer, E.; Royanian, E.; Michor, H.; Sologub, O.; Scheidt, E.-W.; Gonçalves, A. P.; Bursik, J.; Wolf, W.; Reith, D.; Blaas-Schenner, C.; Moser, R.; Podloucky, R.; Rogl, P.
2015-07-01
Investigating the phase relations of the system {Th ,U } -Pt-B at 900 °C the formation of two compounds has been observed: cubic ThPt3B with P m 3 ¯m structure as a representative of the perovskites, and tetragonal UPt3B with P 4 m m structure being isotypic to the noncentrosymmetric structure of CePt3B . The crystal structures of the two compounds are defined by combined x-ray diffraction and transmission electron microscopy. Characterization of physical properties for ThPt3B reveals a superconducting transition at 0.75 K and an upper critical field at T =0 exceeding 0.4 T. For nonsuperconducting UPt3B a metallic resistivity behavior was found in the entire temperature range; at very low temperatures spin fluctuations become evident and the resistivity ρ (T ) follows non-Fermi liquid characteristics, ρ =ρ0+A T n with n =1.6 . Density functional theory (DFT) calculations were performed for both compounds for both types of structures. They predict that the experimentally claimed cubic structure of ThPt3B is thermodynamically not stable in comparison to a tetragonal phase, with a very large enthalpy difference of 25 kJ/mol, which cannot be explained by the formation energy of B vacancies. However, the presence of random boron vacancies possibly stabilizes the cubic structure via a local strain compensation mechanism during the growth of the crystal. For UPt3B the DFT results agree well with the experimental findings.
Progressive slowing down of spin fluctuations in underdoped LaFeAsO1-xFx
NASA Astrophysics Data System (ADS)
Hammerath, F.; Gräfe, U.; Kühne, T.; Kühne, H.; Kuhns, P. L.; Reyes, A. P.; Lang, G.; Wurmehl, S.; Büchner, B.; Carretta, P.; Grafe, H.-J.
2013-09-01
The evolution of low-energy spin dynamics in the iron-based superconductor LaFeAsO1-xFx was studied over a broad doping, temperature, and magnetic field range (x= 0-0.15, T≤ 480 K, μ0H≤ 30 T) by means of 75As nuclear magnetic resonance. An enhanced spin-lattice relaxation rate divided by temperature (T1T)-1 in underdoped superconducting samples (x= 0.045, 0.05, and 0.075) suggests the presence of antiferromagnetic spin fluctuations, which are strongly reduced in optimally doped (x=0.10) and completely absent in overdoped (x=0.15) samples. In contrast to previous analysis, Curie-Weiss fits are shown to be insufficient to describe the data over the whole temperature range. Instead, a Bloembergen-Purcell-Pound (BPP) model is used to describe the occurrence of a peak in (T1T)-1 clearly above the superconducting transition, reflecting a progressive slowing down of the spin fluctuations down to the superconducting phase transition.
Spin fluctuations in the exotic metallic state of Sr2RuO4 studied with β -NMR
NASA Astrophysics Data System (ADS)
Cortie, D. L.; Buck, T.; Dehn, M. H.; Kiefl, R. F.; Levy, C. D. P.; McFadden, R. M. L.; Morris, G. D.; Pearson, M. R.; Salman, Z.; Maeno, Y.; MacFarlane, W. A.
2015-06-01
A β -NMR study was performed on a Sr2RuO4 crystal in the metallic state using a beam of spin-polarized +8Li implanted at a mean depth of 90 nm. The +8Li spin-lattice relaxation rate is strongly influenced by the onset of incommensurate spin fluctuations. The nuclear relaxation rate can be explained using previously published bulk 17O NMR and inelastic neutron spectroscopy measurements of the dynamic magnetic susceptibility to model the hyperfine coupling. A well-resolved quadrupolar-split NMR for +8Li implies a static stopping position in an interstitial site. The +8Li Knight shift is highly sensitive to the anisotropic static susceptibility.
Spin fluctuations and excitations in a 2D xy-ferromagnet: CoCl/sub 2/ in graphite
Wiesler, D.G.; Zabel, H.
1989-01-01
We have investigated by neutron scattering the spin fluctuations and excitations in the stage 2 CoCl/sub 2/ -- graphite intercalation compound. This compound has easy-plane anisotropy and sufficiently weak interplanar interaction to qualify as a test material for Kosterlitz-Thouless-Berezinsky type phase transitions. We have carried out quasi-elastic scattering measurements to determine the temperature variation of the spin correlation length /xi/ above the two dimensional ordering transition. We have also probed the dependence on wave vector and temperature of the inelastic scattering cross section, consisting of both a central peak, associated with vortex diffusion, and spin wave, which become strongly damped above the transition temperature. 15 refs., 5 figs.
Low-frequency spin fluctuations in the superconducting La{sub 2-x}Sr{sub x}CuO{sub 4}
Yamada, K.; Lee, C.H.; Endoh, Y.; Shirane, G.; Birgeneau, R.J.; Kastner, M.A.
1997-02-01
Recent progress in low-energy neutron scattering study on the spatially modulated or so-called incommensurate spin fluctuations is reviewed. Well-defined incommensurate spin fluctuations are observed beyond x approx 0.05 and up to the highest Sr-concentration x approx. 0.25 in the present study. The incommensurability delta(x) is saturated at around 1/8 in the overdoped region and linearly scaled with the upper limit of Tc(x) between the underdoped and optimally doped region. A characteristic energy E* below which the dynamical magnetic susceptibility dramatically diminishes in the superconducting state can be determined to be around 7 meV in the energy spectrum of the spin fluctuations for x=0.15 and x=0.18 crystals. The disorder effect induces a low-frequency component of the incommensurate spin fluctuations below E*.
Birgeneau, R.J.; Belk, N.; Kastner, M.A.; Keimer, B. . Dept. of Physics); Endoh, Y. . Dept. of Physics); Erwin, R.W. ); Shirane, G. )
1991-01-01
We review the results of neutron scattering studies of the static and dynamic spin fluctuations crystals of La{sub 2-x}Sr{sub x}CuO{sub 4+{delta}} in the doping regime intermediate between the Neel and superconducting regions. In this regime the in-plane resistance is linear in temperature down to {approximately}80 K with a crossover due to logarithmic conductance effects at lower temperatures. The static spin correlations are well-described by a simple model in which the inverse correlation length {kappa}(x,T) ={kappa}(x,0) + {kappa}(0,T). The most dramatic new result is the discovery by Keimer et al. that the dynamic spin fluctuations exhibit a temperature dependence which is a simple function of {omega}/T for temperatures 10 K{le}T{le}500 K for a wide range of energies. This scaling leads to a natural explanation of a variety of normal state properties of the copper oxides. 21 refs., 4 figs.
Quantum lattice fluctuations in a frustrated Heisenberg spin-Peierls chain
NASA Astrophysics Data System (ADS)
Weiße, A.; Wellein, G.; Fehske, H.
1999-09-01
As a simple model for spin-Peierls systems we study a frustrated Heisenberg chain coupled to optical phonons. In view of the anorganic spin-Peierls compound CuGeO3 we consider two different mechanisms of spin-phonon coupling. Combining variational concepts in the adiabatic regime and perturbation theory in the antiadiabatic regime we derive effective spin Hamiltonians which cover the dynamical effect of phonons in an approximate way. Ground-state phase diagrams of these models are determined, and the effect of frustration is discussed. Comparing the properties of the ground state and low-lying excitations with exact diagonalization data for the full quantum spin-phonon models, good agreement is found especially in the antiadiabatic regime.
NASA Astrophysics Data System (ADS)
Yang, J.; Tang, Z. T.; Cao, G. H.; Zheng, Guo-qing
2015-10-01
We report 75As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies on the superconductor Rb2Cr3As3 with a quasi-one-dimensional crystal structure. Below T ˜100 K , the spin-lattice relaxation rate (1 /T1 ) divided by temperature, 1 /T1T , increases upon cooling down to Tc=4.8 K , showing a Curie-Weiss-like temperature dependence. The Knight shift also increases with decreasing temperature. These results suggest ferromagnetic spin fluctuation. In the superconducting state, 1 /T1 decreases rapidly below Tc without a Hebel-Slichter peak, and follows a T5 variation below T ˜3 K , which points to unconventional superconductivity with point nodes in the gap function.
NASA Astrophysics Data System (ADS)
Baños, R. A.; Cruz, A.; Fernandez, L. A.; Gil-Narvion, J. M.; Gordillo-Guerrero, A.; Guidetti, M.; Iñiguez, D.; Maiorano, A.; Mantovani, F.; Marinari, E.; Martin-Mayor, V.; Monforte-Garcia, J.; Muñoz Sudupe, A.; Navarro, D.; Parisi, G.; Perez-Gaviro, S.; Ricci-Tersenghi, F.; Ruiz-Lorenzo, J. J.; Schifano, S. F.; Seoane, B.; Tarancón, A.; Tripiccione, R.; Yllanes, D.
2011-11-01
We study the sample-to-sample fluctuations of the overlap probability densities from large-scale equilibrium simulations of the three-dimensional Edwards-Anderson spin glass below the critical temperature. Ultrametricity, stochastic stability, and overlap equivalence impose constraints on the moments of the overlap probability densities that can be tested against numerical data. We found small deviations from the Ghirlanda-Guerra predictions, which get smaller as system size increases. We also focus on the shape of the overlap distribution, comparing the numerical data to a mean-field-like prediction in which finite-size effects are taken into account by substituting delta functions with broad peaks.
Gerlovin, I. Ya.; Cherbunin, R. V.; Ignatiev, I. V.; Kuznetsova, M. S.; Verbin, S. Yu.; Flisinski, K.; Bayer, M.; Reuter, D.; Wieck, A. D.; Yakovlev, D. R.
2013-12-04
The degree of circular polarization of photoluminescence of (In,Ga)As quantum dots as a function of magnetic field applied perpendicular to the optical axis (Hanle effect) is experimentally studied. The measurements have been performed at various regimes of the optical excitation modulation. The analysis of experimental data has been performed in the framework of a vector model of regular nuclear spin polarization and its fluctuations. The analysis allowed us to evaluate the magnitude of nuclear polarization and its dynamics at the experimental conditions used.
Spin-orbital fluctuations in the paramagnetic Mott insulator (V1-xCrx)2O3
NASA Astrophysics Data System (ADS)
Leiner, Jonathan; Stone, Matthew; Lumsden, Mark; Bao, Wei; Broholm, Collin
2015-03-01
The phase diagram of rhombohedral V2O3 features several distinct strongly correlated phases as a function of doping, pressure and temperature. When doped with chromium for 180 K
Anomalous effect of disorder on spin fluctuations in non-centrosymmetric superconductors
NASA Astrophysics Data System (ADS)
Edelstein, Victor M.
2008-09-01
The spin susceptibility tensor χsij(T) of an impure superconductor (SC) with broken mirror symmetry has been evaluated and a great effect of impurity scattering has been shown. As opposed to conventional singlet superconductors, where the ordinary impurity scattering is known to have no effect on χs(T) , the spin susceptibility of a polar symmetry superconductor with s -wave pairing can be isotropic and equal to its value in the normal state in the “dirty” limit Tcτ≪1 , while the superconductor stays in a full-gap state. The effect is bound up with spin-flip transitions which accompany the electron scattering in conductors with the band spin-orbit coupling.
Phuc, Nguyen Thanh; Kawaguchi, Yuki; Ueda, Masahito
2011-10-15
We investigate the effects of thermal and quantum fluctuations on the phase diagram of a spin-1 {sup 87}Rb Bose-Einstein condensate (BEC) under the quadratic Zeeman effect. Due to the large ratio of spin-independent to spin-dependent interactions of {sup 87}Rb atoms, the effect of noncondensed atoms on the condensate is much more significant than that in scalar BECs. We find that the condensate and spontaneous magnetization emerge at different temperatures when the ground state is in the broken-axisymmetry phase. In this phase, a magnetized condensate induces spin coherence of noncondensed atoms in different magnetic sublevels, resulting in temperature-dependent magnetization of the noncondensate. We also examine the effect of quantum fluctuations on the order parameter at absolute zero and find that the ground-state phase diagram is significantly altered by quantum depletion.
NASA Astrophysics Data System (ADS)
Fujihala, M.; Zheng, X. G.; Oohara, Y.; Morodomi, H.; Kawae, T.; Matsuo, Akira; Kindo, Koichi
2012-01-01
Spin fluctuations and spin-liquid behaviors of frustrated kagome antiferromagnets have received intense recent attention. Although most severe frustration was predicted for an Ising kagome antiferromagnet, a real material system of undistorted kagome lattice has not been found so far. Here we report the frustrated magnetism of a new Ising kagome antiferromagnet, MgCo3(OH)6Cl2, which can be viewed as a Co version of the intensively researched quantum kagome antiferromagnet of Herbertsmithite ZnCu3(OH)6Cl2. Experiments of magnetization, heat capacity, μSR, and neutron scattering demonstrated a partially frozen state with persistent spin fluctuations below around T = 2.7 K. The present study has provided a real material system to study the Ising spin behaviors on undistorted kagome lattice.
Linscheid, A; Maiti, S; Wang, Y; Johnston, S; Hirschfeld, P J
2016-08-12
We investigate superconductivity in a two-band system with an electronlike and a holelike band, where one of the bands is away from the Fermi level (or "incipient"). We argue that the incipient band contributes significantly to spin-fluctuation pairing in the strong coupling limit where the system is close to a magnetic instability and can lead to a large T_{c}. In this case, T_{c} is limited by a competition between the frequency range of the coupling (set by an isolated paramagnon) and the coupling strength itself, such that a domelike T_{c} dependence on the incipient band position is obtained. The coupling of electrons to phonons is found to further enhance T_{c}. The results are discussed in the context of experiments on monolayers and intercalates of FeSe. PMID:27563992
NASA Astrophysics Data System (ADS)
Linscheid, A.; Maiti, S.; Wang, Y.; Johnston, S.; Hirschfeld, P. J.
2016-08-01
We investigate superconductivity in a two-band system with an electronlike and a holelike band, where one of the bands is away from the Fermi level (or "incipient"). We argue that the incipient band contributes significantly to spin-fluctuation pairing in the strong coupling limit where the system is close to a magnetic instability and can lead to a large Tc. In this case, Tc is limited by a competition between the frequency range of the coupling (set by an isolated paramagnon) and the coupling strength itself, such that a domelike Tc dependence on the incipient band position is obtained. The coupling of electrons to phonons is found to further enhance Tc. The results are discussed in the context of experiments on monolayers and intercalates of FeSe.
NMR study of spin fluctuations and superconductivity in LaFeAsO1-xHx
NASA Astrophysics Data System (ADS)
Fujiwara, Naoki; Sakurai, Ryosuke; Iimura, Soushi; Matsuishi, Satoru; Hosono, Hideo; Yamakawa, Yoichi; Kontani, Hiroshi
2013-03-01
We have performed NMR measurements in LaFeAsO1-xHx, an isomorphic compound of LaFeAsO1-xFx. LaFeAsO1-xHx is most recently known for having double superconducting (SC) domes on H doping. LaFeAsO1-xHx is an electron- doped system, and protons act as H-1 as well as F-1. The first SC dome is very similar between F and H doping, suggesting that H doping supplies the same amount of electrons as F doping. Interestingly, an excess amount of H up to x=0.5 can be replaced with O2-. In the H-overdoped regime (x > 0 . 2), LaFeAsO1-xHx undergoes the second superconducting state. We measured the relaxation rate of LaFeAsO1-xHx for x=0.2 and 0.4, and fond an anomalous electronic state; spin fluctuations measured from 1 /T1 T is enhanced with increasing the doping level from x = 0 . 2 to 0.4. The enhancement of spin fluctuations with increasing carrier doping is a new phenomenon that has not observed in LaFeAsO1-xFx in which the upper limit of the doping level is at most x = 0 . 2 . We will discuss the phenomenon in relation to superconductivity. Grant (KAKENHI 23340101) from the Ministry of Education, Sports and Science, Japan
NASA Astrophysics Data System (ADS)
Shimizu, Yusei; Braithwaite, Daniel; Salce, Bernard; Combier, Tristan; Aoki, Dai; Hering, Eduardo N.; Ramos, Scheilla M.; Flouquet, Jacques
2015-03-01
Resistivity measurements were performed for the itinerant Ising-type ferromagnet URhAl at temperatures down to 40 mK under high pressure up to 7.5 GPa, using single crystals. We found that the critical pressure of the Curie temperature exists at around Pc˜ 5.2 GPa. Near Pc, the A coefficient of the A T2 Fermi-liquid resistivity term below T* is largely enhanced with a maximum around 5.2-5.5 GPa. Above Pc, the exponent of the resistivity ρ (T ) deviates from 2. At Pc, it is close to n =5 /3 , which is expected by the theory of three-dimensional ferromagnetic spin fluctuations for a second-order quantum-critical point (QCP). However, TC(P ) disappears as a first-order phase transition, and the critical behavior of resistivity in URhAl cannot be explained by the theory of a second-order QCP. The first-order nature of the phase transition is weak, and the electron system in URhAl is still dominated by the spin fluctuation at low temperature. With increasing pressure, the non-Fermi-liquid behavior is observed in higher fields. Magnetic field studies point out a ferromagnetic wing structure with a tricritical point (TCP) at ˜4.8 -4.9 GPa in URhAl. One open possibility is that the switch from the ferromagnetic to the paramagnetic states does not occur simply but an intermediate state arises below the TCP as suggested theoretically recently. Quite generally, if a drastic Fermi-surface change occurs through Pc, the nature of the interaction itself may change and lead to the observed unconventional behavior.
Co-existence of spin fluctuation and superconductivity in electron doped cuprate Pr1-xLaCexCuO4
NASA Astrophysics Data System (ADS)
Song, Dongjoon; Park, S. R.; Kim, Chul; Choi, S. K.; Jung, W. S.; Koh, Y. Y.; Kim, Y. K.; Eisaki, H.; Yoshida, Y.; Kim, C.
2012-02-01
Even though spin fluctuation has been proposed to be as the pairing glue in the cuprate high temperature superconductivity, there is lack of a clear evidence for its coupling to electron. One of the reasons is that, for hole doped cuprates, both anti-ferromagnetism (AFM) and recently proposed charge ordering effects due to Fermi surface nesting occur in the same region of the momentum space (anti-nodal region). On the other hand, electron doped cuprates are known to have the pseudo gap effect at hot spots from AFM band renormalization. This fact makes it advantageous to investigate electron doped cuprates for the spin fluctuation issue. We performed ARPES studies on superconducting electron doped cuprates PLCCO (x=0.1, 0.15, 0.18) to investigate the relation between the spin fluctuation and superconductivity. We observe pseudo gap for all the dopings, which indicates that the short range AFM ordering survives far away from the AFM phase boundary. This coincidence of the short range AFM and superconductivity even in the over doped state may support the spin fluctuation scenarios at least in the electron doped side.
Hamed, F.; Razavi, F.S.; Zaleski, H.; Bose, S.K. )
1991-02-01
The superconducting transition temperature {ital T}{sub {ital c}} of metallic glasses Fe{sub 20}Zr{sub 80}, Fe{sub 25}Zr{sub 75}, and Cu{sub 25}Zr{sub 75} were measured under quasihydrostatic pressure up to 10 GPa. The volume (pressure) dependence of the electron-phonon coupling parameter, {lambda}{sub {ital e}-ph}, for Cu{sub 25}Zr{sub 75} was calculated using the McMillan equation. Using this volume dependence of {lambda}{sub {ital e}-ph} and the modified McMillan equation, which incorporates spin fluctuations, we determined the volume dependence of the spin-fluctuation parameter, {lambda}{sub SF}, in Ni{sub 25}Zr{sub 75} and Fe{sub 100{minus}{ital x}}Zr{sub {ital x}} ({ital x}=80, 75). It was found that with increasing pressure spin fluctuations are suppressed at a faster rate in Fe{sub 100{minus}{ital x}}Zr{sub {ital x}} as the Fe concentration is increased. The rate of suppression of spin fluctuations with pressure was also found to be higher in the Fe-Zr glasses than in Ni-Zr glasses of similar composition.
Numerical study of spin relaxation by thermal fluctuation: Effect of shape anisotropy
Lee, K. J.; Park, N. Y.; Lee, T. D.
2001-06-01
Effects of the shape anisotropy on the thermally activated spin relaxation have been investigated using the stochastic Landau{endash}Lifshitz{endash}Gilbert equation. The relaxation times of a noninteracting particle and a thin film were compared with each other. In a noninteracting particle, the relaxation time largely increased with the shape anisotropy when the damping constant was smaller than a certain critical value. In this study, the critical damping constant was 0.02. However, the effect of the shape anisotropy on the energy barrier was negligible in a thin film. All of these results can be explained from the effect of magnetostatic interaction that is enhanced by precession motion at low damping constant. {copyright} 2001 American Institute of Physics.
Influence of thermal fluctuations on the emission linewidth in MgO-based spin transfer oscillators
NASA Astrophysics Data System (ADS)
Sierra, J. F.; Quinsat, M.; Garcia-Sanchez, F.; Ebels, U.; Joumard, I.; Jenkins, A. S.; Dieny, B.; Cyrille, M.-C.; Zeltser, A.; Katine, J. A.
2012-08-01
The temperature dependence of the microwave emission linewidth Δf, the amplitude-phase coupling parameter ν, and the amplitude relaxation rate Γp were investigated experimentally for tunnel junction spin-transfer-oscillators. A linear increase of Δf and unexpectedly of Γp with temperature is observed, giving a ratio 2πΔf/Γp close to one. Analytical evaluation of the phase variance confirms that for this ratio the temperature dependence of Δf is linear and that in this temperature range Δf is enhanced by the amplitude-phase coupling. This is not changed when taking the temperature dependence of Γp into account, the origin of which remains to be elucidated.
Inelastic neutron scattering in valence fluctuation compounds
Jon M Lawrence
2011-02-15
The valence fluctuation compounds are rare earth intermetallics where hybridization of the nearly-localized 4f electrons with the conduction electrons leads to incorporation of the 4f's into the itinerant states. This hybridization slows down the conduction electrons and hence gives them a heavy effective mass, justifying application of the term 'heavy Fermion' (HF) to these materials. During the project period, we grew large single crystals of several such compounds and measured their properties using both standard thermodynamic probes and state-of-the-art inelastic neutron scattering. We obtained three main results. For the intermediate valence compounds CePd{sub 3} and YbAl{sub 3}, we showed that the scattering of neutrons by the fluctuations of the 4f magnetic moment does not have the momentum dependence expected for the itinerant heavy mass state; rather, the scattering is more typical of a localized spin fluctuation. We believe that incoherent scattering localizes the excitation. For the heavy Fermion compound Ce(Ni{sub 0.935}Pd{sub 0.065}){sub 2}Ge{sub 2}, which sits at a T = 0 critical point for transformation into an antiferromagnetic (AF) phase, we showed that the scattering from the AF fluctuations does not exhibit any of the divergences that are expected at a phase transition. We speculate that alloy disorder profoundly suppresses the growth of the fluctuating AF regions, leading to short range clusters rather than regions of infinite size. Finally, we explored the applicability of key concepts used to describe the behavior of rare earth heavy Fermions to uranium based HF compounds where the 5f electrons are itinerant as opposed to localized. We found that scaling laws relating the spin fluctuation energy measured in neutron scattering to the low temperature specific heat and susceptibility are valid for the uranium compounds, once corrections are made for AF fluctuations; however, the degeneracy of the high temperature moment is smaller than expected
Critical Casimir force and its fluctuations in lattice spin models: exact and Monte Carlo results.
Dantchev, Daniel; Krech, Michael
2004-04-01
We present general arguments and construct a stress tensor operator for finite lattice spin models. The average value of this operator gives the Casimir force of the system close to the bulk critical temperature T(c). We verify our arguments via exact results for the force in the two-dimensional Ising model, d -dimensional Gaussian, and mean spherical model with 2
Spin fluctuations and superconductivity in a 3D tight-binding model for BaFe2As2
Graser, Siegfried; Kemper, Alexander F; Maier, Thomas A; Cheng, Hai-Ping; Hirschfeld, Peter; Scalapino, Douglas
2010-01-01
Despite the wealth of experimental data on the Fe-pnictide compounds of the KFe2As2 type, K=Ba, Ca, or Sr, the main theoretical work based on multiorbital tight-binding models has been restricted so far to the study of the related 1111 compounds. This can be ascribed to the more three-dimensional electronic structure found by ab initio calculations for the 122 materials, making this system less amenable to model development. In addition, the more complicated Brillouin zone BZ of the body-centered tetragonal symmetry does not allow a straightforward unfolding of the electronic band structure into an effective 1Fe/unit cell BZ. Here we present an effective five-orbital tight-binding fit of the full density functional theory band structure for BaFe2As2 including the kz dispersions. We compare the five-orbital spin fluctuation model to one previously studied for LaOFeAs and calculate the random-phase approximation enhanced susceptibility. Using the fluctuation ex- change approximation to determine the leading pairing instability, we then examine the differences between a strictly two-dimensional model calculation over a single kz cut of the BZ and a completely three-dimensional approach. We find pairing states quite similar to the 1111 materials, with generic quasi-isotropic pairing on the hole sheets and nodal states on the electron sheets at kz=0, which however are gapped as the system is hole doped. On the other hand, a substantial kz dependence of the order parameter remains, with most of the pairing strength deriving from processes near kz=?. These states exhibit a tendency for an enhanced anisotropy on the hole sheets and a reduced anisotropy on the electron sheets near the top of the BZ.
NASA Astrophysics Data System (ADS)
Mazza, G.; Grilli, M.; Di Castro, C.; Caprara, S.
2013-01-01
In high temperature superconductors we provide evidence of spin and mixed phonon-charge collective modes as mediators of the effective electron-electron interaction and suggestive of a charge and spin density wave instability competing with superconductivity. Indeed, we show that the so-called kinks and waterfalls observed in angle-resolved photoemission spectra of La2-xSrxCuO4, a prototypical high-Tc superconducting cuprate, are due to the coupling of quasiparticles with two distinct nearly critical collective modes with finite characteristic wave vectors, typical of charge and spin fluctuations. The simultaneous presence of these two modes reconciles the long standing dichotomy whether kinks are due to phonons or spin waves.
NASA Astrophysics Data System (ADS)
Dhaka, R. S.; Das, Tanmoy; Plumb, N. C.; Ristic, Z.; Kong, W.; Matt, C. E.; Xu, N.; Dolui, Kapildeb; Razzoli, E.; Medarde, M.; Patthey, L.; Shi, M.; Radović, M.; Mesot, Joël
2015-07-01
We employed in situ pulsed laser deposition (PLD) and angle-resolved photoemission spectroscopy (ARPES) to investigate the mechanism of the metal-insulator transition (MIT) in NdNiO3 (NNO) thin films, grown on NdGaO3(110) and LaAlO3(100) substrates. In the metallic phase, we observe three-dimensional hole and electron Fermi surface (FS) pockets formed from strongly renormalized bands with well-defined quasiparticles. Upon cooling across the MIT in NNO/NGO sample, the quasiparticles lose coherence via a spectral weight transfer from near the Fermi level to localized states forming at higher binding energies. In the case of NNO/LAO, the bands are apparently shifted upward with an additional holelike pocket forming at the corner of the Brillouin zone. We find that the renormalization effects are strongly anisotropic and are stronger in NNO/NGO than NNO/LAO. Our study reveals that substrate-induced strain tunes the crystal field splitting, which changes the FS properties, nesting conditions, and spin-fluctuation strength, and thereby controls the MIT via the formation of an electronic order parameter with QAF˜(1 /4 ,1 /4 ,1 /4 ±δ ) .
NASA Technical Reports Server (NTRS)
Bowman, James S., Jr.
1957-01-01
An investigation has been completed in the Langley 20-foot free-spinning tunnel on a l/18-scale model of the Ryan X-13 airplane to determine its spin, recovery, and tumbling characteristics, and to determine the minimum altitude from which a belly landing could be made in case of power failure in hovering flight. Model spin tests were conducted with and without simulated engine rotation. Tests without simulated engine rotation indicated two types of spins: one, a slightly oscillatory flat spin; and the other, a violently oscillatory spin. Tests with simulated engine rotation indicated that spins to the left were fast rotating and steep and those to the right were slow rotating and flat. The optimum technique for recovery is reversal of the rudder to against the spin and simultaneous movement of the ailerons to full with the spin followed by movement of the elevators to neutral after the spin rotation ceases. Tumbling tests made on the model indicated that although the Ryan X-13 airplane will not tumble in the ordinary sense (end-over-end pitching motion), it may instead tend to enter a wild gyrating'motion. Tests made to simulate power failure in hovering flight by dropping the model indicated that the model entered what appeared to be a right spin. An attempt should be made to stop this motion immediately by moving the rudder to oppose the rotation (left pedal), moving the ailerons to with the spin (stick right), and moving the stick forward after the spin rotation ceases to obtain flying speed for pullout. The minimum altitude required for a belly landing in case of power failure in hovering flight was indicated to be about 4,200 feet.
Charge-spin-orbital fluctuations in mixed valence spinels: Comparative study of AlV2O4 and LiV2O4
NASA Astrophysics Data System (ADS)
Uehara, Amane; Shinaoka, Hiroshi; Motome, Yukitoshi
2015-11-01
Mixed valence spinels provide a fertile playground for the interplay between charge, spin, and orbital degrees of freedom in strongly correlated electrons on a geometrically frustrated lattice. Among them, AlV2O4 and LiV2O4 exhibit contrasting and puzzling behavior: self-organization of seven-site clusters and heavy fermion behavior. We theoretically perform a comparative study of charge-spin-orbital fluctuations in these two compounds, on the basis of the multiband Hubbard models constructed by using the maximally localized Wannier functions obtained from the ab initio band calculations. Performing the eigenmode analysis of the generalized susceptibility, we find that, in AlV2O4 , the relevant fluctuation appears in the charge sector in σ -bonding type orbitals. In contrast, in LiV2O4 , optical-type spin fluctuations in the a1 g orbital are enhanced at an incommensurate wave number at low temperature. Implications from the comparative study are discussed for the contrasting behavior, including the metal-insulator transition under pressure in LiV2O4 .
NASA Astrophysics Data System (ADS)
Christianson, A. D.
2012-02-01
The origin of superconductivity in the Fe-based superconductors, like that in other unconventional superconductors, remains shrouded in mystery. How the pairing bosons emerge either due to or in spite of the strong magnetic interactions found in the Fe-based superconductors is one of the most thoroughly investigated questions in the field. A prominent example of the interplay of superconductivity and magnetism is the dramatic shift of spectral weight from the low energy spin excitations to an energy which is related to the superconducting gap resulting in a peak in the spin excitation spectrum localized in both momentum and energy which occurs at the onset of superconductivity. The appearance of the new peak in the spin excitation spectrum below the superconducting transition temperature is referred to as s spin resonance and is most commonly interpreted as indicating a sign change of the superconducting order parameter on different portions of the Fermi surface and thus is consistent with an extended s-wave or s± pairing symmetry in many Fe-based superconductors. We will review the observations and implications of the spin resonance across the Fe-based superconductors. In particular we will examine the relationship between the resonance energy and the superconducting transition temperature as a function of chemical doping and pressure. While the spin resonance provides important information about pairing symmetry, there does not appear to be sufficient spectral to explain the pairing strength. Thus the remainder of the spin excitation spectrum must be examined to determine if spin fluctuations are ultimately responsible for pairing in the Fe-based materials. Consequently, we will discuss in detail the way in which the spin excitations evolve from the nonsuperconducting compounds to their superconducting relatives as a function of chemical doping.
NASA Astrophysics Data System (ADS)
Hrovat, Matevž Majcen; Jeglič, Peter; Klanjšek, Martin; Hatakeda, Takehiro; Noji, Takashi; Tanabe, Yoichi; Urata, Takahiro; Huynh, Khuong K.; Koike, Yoji; Tanigaki, Katsumi; Arčon, Denis
2015-09-01
The superconducting critical temperature, Tc, of FeSe can be dramatically enhanced by intercalation of a molecular spacer layer. Here we report on a 77Se,7Li , and 1H nuclear magnetic resonance (NMR) study of the powdered hyper-interlayer-expanded Lix(C2H8N2) yFe2 -zSe2 with a nearly optimal Tc=45 K. The absence of any shift in the 7Li and 1H NMR spectra indicates a complete decoupling of interlayer units from the conduction electrons in FeSe layers, whereas nearly temperature-independent 7Li and 1H spin-lattice relaxation rates are consistent with the non-negligible concentration of Fe impurities present in the insulating interlayer space. On the other hand, the strong temperature dependence of 77Se NMR shift and spin-lattice relaxation rate, 1 /77T1 , is attributed to the holelike bands close to the Fermi energy. 1 /77T1 shows no additional anisotropy that would account for the onset of electronic nematic order down to Tc. Similarly, no enhancement in 1 /77T1 due to the spin fluctuations could be found in the normal state. Yet, a characteristic power-law dependence 1 /77T1∝T4.5 still complies with the Cooper pairing mediated by spin fluctuations.
Spatial modulation of low-frequency spin fluctuations in hole-doped La{sub 2}CuO{sub 4}
Yamada, K.; Lee, C.H.; Wada, J.; Kurahashi, K.; Kimura, H.; Endoh, Y.; Hosoya, S.; Shirane, G.; Birgeneau, R.J.; Kastner, M.A.
1996-12-01
Systematic neutron scattering measurements have been performed on the Sr-doped La(2-x)Sr(x) CuO4 to study the doping dependence of spatially modulated dynamical spin correlations or so-called incommensurate spin fluctuations. The modulated spin correlations appears beyond x approx. 0.05 which is close to the lower boundary of the superconducting phase. First evidence was observed for the linear relation between the degree of spatial modulation or the incommensurability delta(x) and the maximum Tc at x. We present a universal curve for delta(x) by adding data from other La2CuO4 systems such as oxygen-doped superconductors, oxygen-reduced or Zn-substituted non-superconductors and La(1.6-x)Nd(0.4)Sr(x)CuO4.
Zhang, Qiang; Fernandes, Rafael M.; Lamsal, Jagat; Yan, Jiaqiang; Chi, Songxue; Tucker, Gregory S.; Pratt, Daniel K.; Lynn, Jeffrey W.; McCallum, R. W.; Canfield, Paul C.; et al
2015-02-04
Inelastic neutron scattering is employed to investigate the impact of electronic nematic order on the magnetic spectra of LaFeAsO and Ba(Fe0.953Co0.047)2As2. These materials are ideal to study the paramagnetic-nematic state, since the nematic order, signaled by the tetragonal-to-orthorhombic transition at TS, sets in well above the stripe antiferromagnetic ordering at TN. We find that the temperature-dependent dynamic susceptibility displays an anomaly at TS followed by a sharp enhancement in the spin-spin correlation length, revealing a strong feedback effect of nematic order on the low-energy magnetic spectrum. As a result, our findings can be consistently described by a model that attributesmore » the structural or nematic transition to magnetic fluctuations, and unveils the key role played by nematic order in promoting the long-range stripe antiferromagnetic order in iron pnictides.« less
NASA Technical Reports Server (NTRS)
Healy, Frederick M.
1958-01-01
Incipient spin characteristics have been investigated on a l/35-scale dynamic model of the Convair F-10% airplane. The model was launched by a catapult apparatus into free flight with various control settings, and the motions obtained were photographed. The model was ballasted for the combat loading. All tests were made with the speed brakes and landing gear retracted, and engine effects were not simulated. The results of the investigation indicated that the model would enter motions apparently simulating entry phases of spins when the elevators were deflected full up. Deflecting the rudder had little effect on the direction of the motion obtained, but when ailerons were deflected the model always rotated in a direction opposite to the aileron setting (that is, the model entered a right spin with the stick to the left). The ailerons were very influential in initiating spin entry, and the pilot should avoid, as far as possible, the use of ailerons in low-speed flight.
Evenson, W.E. . Dept. of Physics and Astronomy); McKale, A.G.; Su, H.T.; Gardner, J.A. . Dept. of Physics)
1990-01-01
We report numerical computations of the PAC perturbation factor G{sub 2}(t) for spin 5/2 nuclei subject to a static EFG symmetric about the z-axis and an additional axially-symmetric EFG hose symmetry axis fluctuates randomly among the x,y,z directions. For sufficiently large fluctuation rates, the numerical results are described by the expression for the static interaction alone with the addition of relaxation terms. Results of applying this model to {sup 111}Cd TDPAC measurements on tetragonal ZrO{sub 2} are described briefly. The model allows one to evaluate the probability that oxygen vacancies are trapped, the energy of association of vacancy-metal pairs, and the vacancy activation energy of motion. 4 refs., 3 figs.
NASA Astrophysics Data System (ADS)
Wu, D.; Barišić, N.; Dressel, M.; Cao, G. H.; Xu, Z.-A.; Schachinger, E.; Carbotte, J. P.
2010-10-01
The temperature and frequency dependences of the optical conductivity of Co-doped BaFe2As2 are analyzed and the electron-boson spectral density α2F(ω) are extracted using Eliashberg’s formalism. For the normal state at T=30K there is a relatively sharp and large peak around 10 meV and a secondary smaller and broader peak centered around 50 meV with the spectrum extending to high energies beyond the maximum phonon energy. The electron-boson mass enhancement parameter is 4.4, a value more consistent with spin-fluctuation scattering rather than with phonons. In addition the spectrum is found to evolve with temperature toward a less structured background at higher energies as in the spin susceptibility.
Antiferromagnetic Spin Fluctuations and Pseudogap Behavior in Ca(Fe1-xCox)2 As2 Studied by 5As NMR
NASA Astrophysics Data System (ADS)
Cui, Jinfang; Ran, Sheng; Bud'Ko, Sergey; Canfield, Paul; Furukawa, Yuji
2015-03-01
75 As NMR measurements of single-crystalline Ca(Fe1-xCox)2 As2 have been carried out for four different doping concentration crystals (x = 0.023, 0.028, 0.033, 0.059) annealed at 350°C. Co-doped CaFe2As2 is a compound in 122 family of iron-pnictide superconductors with three principle phases exhibited: paramagnetic (PM), antiferromagnetic (AFM) and superconducting (SC) states. The magnetic phase transition to AFM state occurs at TN = 180K at x =0 and is suppressed to TN = 53K for x =0.028, which is accompanied by a structural phase transition from tetragonal to orthorhombic phases. 75As NMR was used to study the low energy spin dynamics via Knight shift (K) and spin-lattice relaxation rate (1/T1) measurements. From our analysis of the temperature dependence of both K and (T1T)-1 in x =0.028 (TN = 53K), 0.033 (Tc = 9K) and 0.059 (Tc = 10K), we found a gradual decrease of AFM spin fluctuations below T* = 88K for x =0.028, 72K for x =0.033 and 41K for x =0.059, respectively, indicating the possible pseudogap behavior in spin excitation spectrum in the system. Supported by USDOE under the Contract No. DE-AC02-07CH11358.
Gupta, Sachin E-mail: suresh@phy.iitb.ac.in; Suresh, K. G. E-mail: suresh@phy.iitb.ac.in; Das, A.; Nigam, A. K.; Hoser, A.
2015-06-01
Polycrystalline NdCuSi is found to show co-existence of antiferromagnetic (AFM) and ferromagnetic (FM) phases at low temperatures, as revealed by neutron diffraction data. The coexistence is attributed to the competing exchange interactions and crystal field effect. The compound shows a large, low-field magnetoresistance (MR) of ∼ − 32% at 20 kOe below T{sub N} (3.1 K), which becomes ∼ − 36% at 50 kOe. The MR value at 50 kOe is found to be the highest among the RTX compounds. Magnetocaloric effect (MCE) is also found to show a large value of ∼11 J/kg K close to T{sub N}. Resistivity data show the presence of spin fluctuations, which get suppressed by the applied field. Large MR and MCE in this compound arise due to the coexistence of the two phases. The field dependencies of MR and MCE show quadratic behavior, confirming the presence of spin fluctuations.
NASA Astrophysics Data System (ADS)
Khmelevskyi, Sergii
2016-07-01
The V3Al alloy with D O3 crystal structure belongs to the family of the very few metallic materials that exhibit a magnetically ordered state with a high ordering temperature (˜600 K) and consist only of nonmagnetic elements. We show that, similarly to the ferromagnetism in the fcc Ni (with ordering temperature at about 630 K), the antiferromagnetism in V3Al has itinerant character, and the high value of the Néel temperature is the result of the strong longitudinal spin fluctuations in the paramagnetic state. In order to develop an ab initio-based theory of the magnetic ordering at finite temperatures, we employ an effective magnetic Heisenberg-like Hamiltonian with varying values of the on-site magnetic moments. Using a set of approximations we map this model onto the results of the first-principle-based disordered local moment formalism and the magnetoforce theorem applied in the framework of the Korringa-Kohn-Rostoker method. Our high-temperature approach is shown to describe the experimental Néel temperature of V3Al very well and thus underlines the importance of the longitudinal spin-fluctuation mechanism of formation of the vanadium magnetic moment at high temperatures.
Spin Fluctuation Effect on Electrical Resistivity of La0.8Ca0.2MnO3 Manganite Nanoparticles
NASA Astrophysics Data System (ADS)
Choudhary, K. K.
2015-04-01
The electrical resistivity ρ(T) of La0.8C0.2MnO3 manganite nanoparticles (particle size 18 nm and 70 nm) significantly depends on temperature and size of nanoparticles. ρ(T) of 70 nm La0.8C0.2MnO3 manganite exhibits metallic phase in low temperature regime (T < 250 K), develops a maxima near 250 K and decrease with T at high temperatures (250 K < T < 300 K). However, the ρ(T) of 18 nm La0.8C0.2MnO3 manganite shows insulating phase in overall temperature regime, where resistivity decrease with temperature. The resistivity in metallic phase is theoretically analyzed by considering the strong spin fluctuations effect which is modelled using Drude-Lorentz type function. In addition to the spin fluctuation-induced contribution the electron-phonon and electron-electron ρe-e(T) = BT2 contributions are also incorporated for complete understanding of experimental data. The contributions to the resistivity by inherent acoustic phonons (ρac) as well as high frequency optical phonons (ρop) were estimated using Bloch-Gruneisen [BG] model of resistivity. It is observed that the resistivity contribution due to electron-electron interaction shows typical quadratic temperature dependence. Resistivity in Semiconducting/insulating phase is discussed with small polaron conduction (SPC) model. Finally the theoretically calculated resistivity compared with experimental data which found consistent in wide range of temperature.
Zhang, Qiang; Fernandes, Rafael M.; Lamsal, Jagat; Yan, Jiaqiang; Chi, Songxue; Tucker, Gregory S.; Pratt, Daniel K.; Lynn, Jeffrey W.; McCallum, R. W.; Canfield, Paul C.; Lograsso, Thomas A.; Goldman, Alan I.; Vaknin, David; McQueeney, Robert J.
2015-02-04
Inelastic neutron scattering is employed to investigate the impact of electronic nematic order on the magnetic spectra of LaFeAsO and Ba(Fe_{0.953}Co_{0.047})_{2}As_{2}. These materials are ideal to study the paramagnetic-nematic state, since the nematic order, signaled by the tetragonal-to-orthorhombic transition at T_{S}, sets in well above the stripe antiferromagnetic ordering at T_{N}. We find that the temperature-dependent dynamic susceptibility displays an anomaly at T_{S} followed by a sharp enhancement in the spin-spin correlation length, revealing a strong feedback effect of nematic order on the low-energy magnetic spectrum. As a result, our findings can be consistently described by a model that attributes the structural or nematic transition to magnetic fluctuations, and unveils the key role played by nematic order in promoting the long-range stripe antiferromagnetic order in iron pnictides.
Moretto, L.G.
1980-08-01
The relevance of higher multipoles of giant isovector modes in the charge distribution of deep inelastic fragments is discussed and found to depend strongly on mass asymmetry. The sources of angular momentum fluctuations are investigated. Quantal effects are considered as well as effects arising from non-equilibrium and equilibrium statistical fluctuations. A model based upon equilibrium statistical mechanics is considered in detail, and used to predict both 2nd moments of the angular momentum distributions and the angular momentum misalignment. Analytical expressions are derived to calculate the angular distributions of sequentially emitted particles, fission fragments, as well as gamma rays in terms of the angular momentum misalignment. Recent data on the angular distributions of sequential alphas, fission and gamma rays are analyzed in terms of the model. 29 figures, 1 table.
Majumder, M; Ghoshray, A; Khuntia, P; Mazumdar, C; Poddar, A; Baenitz, M; Ghoshray, K
2016-09-01
Magnetization, resistivity and (11)B, (59)Co NMR measurements have been performed on the Pauli paramagnet [Formula: see text], and the superconductors [Formula: see text] ([Formula: see text] K) and [Formula: see text] ([Formula: see text] K). The site selective NMR experiment reveals the multiband nature of the Fermi surface in these systems. The temperature independent Knight shift and 1/T 1 T clearly indicate the absence of correlated low energy magnetic spin-fluctuations in the normal state, which is in contrast to other Fe-based pnictides. The density of states (DOS) of Co 3d electrons has been enhanced in superconducting [Formula: see text] and [Formula: see text] with respect to the non superconducting reference compound [Formula: see text]. The occurrence of superconductivity is related to the DOS enhancement. PMID:27355521
Sirica, N.; Bondino, F.; Nappini, S.; Piz, I.; Poudel, L.; Christianson, Andrew D.; Mandrus, D.; Singh, David J; Mannella, Norman
2015-03-04
We report x-ray absorption and photoemission spectroscopy of the electronic structure in the normal state of metallic YFe_{2}Ge_{2}. The data reveal evidence for large fluctuating spin moments on the Fe sites, as indicated by exchange multiplets appearing in the Fe 3s core-level photoemission spectra, even though the compound does not show magnetic order. The magnitude of the multiplet splitting is comparable to that observed in the normal state of the Fe-pnictide superconductors. This shows a connection between YFe_{2}Ge_{2} and the Fe-based superconductors even though it contains neither pnictogens nor chalcogens. Finally, the implication is that the chemical range of compounds showing at least one of the characteristic magnetic signatures of the Fe-based superconductors is broader than previously thought.
Baker, Michael L; Lancaster, Tom; Chiesa, Alessandro; Amoretti, Giuseppe; Baker, Peter J; Barker, Claire; Blundell, Stephen J; Carretta, Stefano; Collison, David; Güdel, Hans U; Guidi, Tatiana; McInnes, Eric J L; Möller, Johannes S; Mutka, Hannu; Ollivier, Jacques; Pratt, Francis L; Santini, Paolo; Tuna, Floriana; Tregenna-Piggott, Philip L W; Vitorica-Yrezabal, Iñigo J; Timco, Grigore A; Winpenny, Richard E P
2016-01-26
The spin dynamics of Cr8 Mn, a nine-membered antiferromagnetic (AF) molecular nanomagnet, are investigated. Cr8 Mn is a rare example of a large odd-membered AF ring, and has an odd-number of 3d-electrons present. Odd-membered AF rings are unusual and of interest due to the presence of competing exchange interactions that result in frustrated-spin ground states. The chemical synthesis and structures of two Cr8 Mn variants that differ only in their crystal packing are reported. Evidence of spin frustration is investigated by inelastic neutron scattering (INS) and muon spin relaxation spectroscopy (μSR). From INS studies we accurately determine an appropriate microscopic spin Hamiltonian and we show that μSR is sensitive to the ground-spin-state crossing from S=1/2 to S=3/2 in Cr8 Mn. The estimated width of the muon asymmetry resonance is consistent with the presence of an avoided crossing. The investigation of the internal spin structure of the ground state, through the analysis of spin-pair correlations and scalar-spin chirality, shows a non-collinear spin structure that fluctuates between non-planar states of opposite chiralities. PMID:26748964
Disappearance of static magnetic order and evolution of spin fluctuations in Fe_{1+δ}Se_{x}Te_{1-x}
Xu, Zhijun; Wen, Jinsheng; Xu, Guangyong; Jie, Qing; Lin, Zhiwei; Li, Qiang; Chi, Songxue; Singh, D. K.; Gu, Genda; Tranquada, John M.
2010-09-29
We report neutron-scattering studies on static magnetic orders and spin excitations in the Fe-based chalcogenide system Fev Se_{1+δ} Te_{1-x} with different Fe and Se compositions. Short-range static magnetic order with an in-plane wave vector near the (0.5,0) (using the two-Fe unit cell), together with strong low-energy magnetic excitations is found in all nonsuperconducting samples for Se doping up to 45%. When the static order disappears and bulk superconductivity emerges, the spectral weight of the magnetic excitations shifts to the region of reciprocal space near the in-plane wave vector (0.5, 0.5), corresponding to “collinear” spin correlations. Our results suggest that there is a strong correlation between superconductivity and the character of the magnetic order/fluctuations in this system. Excess Fe appears to be important for stabilizing the magnetic order that competes with superconductivity.
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.
2013-11-01
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 nonsuperconducting 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 nonmagnetic, and discuss this result in light of recent reports of high-temperature superconductivity in the collapsed tetragonal phase of closely related compounds.
NASA Astrophysics Data System (ADS)
Takeda, Hikaru; Kato, Yusuke; Yoshimura, Masahiro; Shimizu, Yasuhiro; Itoh, Masayuki; Niitaka, Seiji; Takagi, Hidenori
2015-07-01
7Li and 51V NMR measurements up to 9.8 GPa have been made to elucidate local magnetic properties of a heavy-fermion spinel oxide LiV2O4 which undergoes a metal-insulator transition above ˜7 GPa. The temperature T and pressure P dependences of the 7Li and 51V Knight shifts and the nuclear spin-lattice relaxation rates 1 /T1 show that in the metallic phase, there is a crossover from a high-T region with weak ferromagnetic fluctuations to a low-T one with antiferromagnetic (AFM) fluctuations. The AFM fluctuations are enhanced below 20 K and 1.5 GPa, where a heavy Fermi-liquid state with the modified Korringa relation is formed. The evolution of the magnetic fluctuations is discussed from the aspect of the competition among several magnetic interactions. Above PMI˜6.7 GPa, we find the coexistence of metallic and insulating phases due to the first-order metal-insulator transition. The 7Li and 51V NMR spectra coming from the insulating phase have T -independent small Knight shifts and 7(1 /T1 ) with the thermally activated T dependence, indicating the formation of a spin-singlet cluster. We propose a model of a spin-singlet tetramer as discussed in geometrically frustrated materials.
NASA Astrophysics Data System (ADS)
Majumder, M.; Ghoshray, A.; Khuntia, P.; Mazumdar, C.; Poddar, A.; Baenitz, M.; Ghoshray, K.
2016-09-01
Magnetization, resistivity and 11B, 59Co NMR measurements have been performed on the Pauli paramagnet \\text{LaC}{{\\text{o}}2}{{\\text{B}}2} , and the superconductors \\text{L}{{\\text{a}}0.9}{{\\text{Y}}0.1}\\text{C}{{\\text{o}}2}{{\\text{B}}2} ({{T}\\text{c}}≈ 4.2 K) and \\text{La}{{≤ft(\\text{C}{{\\text{o}}0.7}\\text{F}{{\\text{e}}0.3}\\right)}2}{{\\text{B}}2} ({{T}\\text{c}}≈ 5.8 K). The site selective NMR experiment reveals the multiband nature of the Fermi surface in these systems. The temperature independent Knight shift and 1/T 1 T clearly indicate the absence of correlated low energy magnetic spin-fluctuations in the normal state, which is in contrast to other Fe-based pnictides. The density of states (DOS) of Co 3d electrons has been enhanced in superconducting \\text{L}{{\\text{a}}0.9}{{\\text{Y}}0.1}\\text{C}{{\\text{o}}2}{{\\text{B}}2} and \\text{La}{{≤ft(\\text{C}{{\\text{o}}0.7}\\text{F}{{\\text{e}}0.3}\\right)}2}{{\\text{B}}2} with respect to the non superconducting reference compound \\text{LaC}{{\\text{o}}2}{{\\text{B}}2} . The occurrence of superconductivity is related to the DOS enhancement.
NASA Astrophysics Data System (ADS)
Usui, Hidetomo; Suzuki, Katsuhiro; Kuroki, Kazuhiko
2015-03-01
Recently, 1111 iron based superconductors with isovalent doping have been experimentally investigated in LnFeAsxP1-xO1-yFy (Ln =La, Nd, Pr). Interestingly, it was found that Tc takes its local maximum in the intermediate regime of arsenic/phosphorous ratio, which indicates that the superconductivity is locally optimized at a certain Fe-Pn-Fe (Pn =Pnictogen) bond angle larger than 109 deg. Given this background, we study the correlation between the local lattice structure, the orbital character of the Fermi surface, and Tc in 1111 system with isovalent doping. We calculate the band structure of LnFeAsxP1-xO1-yFy and construct effective five orbital models. To our surprise, it is found that superconductivity is indeed locally optimized in the intermediate arsenic doping regime. The origin of this local optimization is traced back to the gradual variation of the orbital character and the density of states of the hole Fermi surfaces around the Γ point, which is controlled by the bond angle. The consistency with the experiment strongly indicates the importance of the spin fluctuation played in this series of superconductors.
Lancaster, Tom; Chiesa, Alessandro; Amoretti, Giuseppe; Baker, Peter J.; Barker, Claire; Carretta, Stefano; Collison, David; Güdel, Hans U.; Guidi, Tatiana; McInnes, Eric J. L.; Möller, Johannes S.; Mutka, Hannu; Ollivier, Jacques; Pratt, Francis L.; Santini, Paolo; Tuna, Floriana; Tregenna‐Piggott, Philip L. W.; Vitorica‐Yrezabal, Iñigo J.; Timco, Grigore A.
2016-01-01
Abstract The spin dynamics of Cr8Mn, a nine‐membered antiferromagnetic (AF) molecular nanomagnet, are investigated. Cr8Mn is a rare example of a large odd‐membered AF ring, and has an odd‐number of 3d‐electrons present. Odd‐membered AF rings are unusual and of interest due to the presence of competing exchange interactions that result in frustrated‐spin ground states. The chemical synthesis and structures of two Cr8Mn variants that differ only in their crystal packing are reported. Evidence of spin frustration is investigated by inelastic neutron scattering (INS) and muon spin relaxation spectroscopy (μSR). From INS studies we accurately determine an appropriate microscopic spin Hamiltonian and we show that μSR is sensitive to the ground‐spin‐state crossing from S=1/2 to S=3/2 in Cr8Mn. The estimated width of the muon asymmetry resonance is consistent with the presence of an avoided crossing. The investigation of the internal spin structure of the ground state, through the analysis of spin‐pair correlations and scalar‐spin chirality, shows a non‐collinear spin structure that fluctuates between non‐planar states of opposite chiralities. PMID:26748964
NASA Astrophysics Data System (ADS)
Liu, Hong; Jiang, Lanfang; Wang, Gengjie; Wang, Li
2008-12-01
Adaptive front lighting system (i.e., AFS) is the development trend of lighting system of motor vehicles. AFS means that headlamp can adjust beam direction to get best illumination according to road condition and its bodywork. The paper discusses the AFS key techniques: establishing calculation formulae of vehicle body state concerned road condition and steering state. Because of sensor technology limitations, it only can deal with inclination and turn of vehicle body state by means of sensor's signals. This paper studies the relationship between inclination and turn of the body and lamp lighting on the base of relative standards, and gives out the calculation formulae for the body and lamp lighting adjustment, also discusses its dynamical properties. The study is basic work for lighting adjustment automatically.
NASA Astrophysics Data System (ADS)
Yoshitake, Junki; Nasu, Joji; Motome, Yukitoshi
The quantum spin liquid, which does not show any long-range ordering down to the lowest temperature, has attracted broad interest as a new quantum state of matter. Since the ground state of the Kitaev model was shown to be a quantum spin liquid in two dimensions, there has been an explosion in both theoretical and experimental studies. Nevertheless, dynamical properties at finite temperatures remain a challenge, despite the relevance to analysis of recent experiments for Ir and Ru compounds. In this contribution, we address this problem by using the cluster dynamical mean-field approximation, which we newly develop on the basis of the Majorana fermion representation. Using the continuous-time quantum Monte Carlo method for the impurity solver, we calculate the magnetic susceptibility, dynamical spin structure factor, and relaxation time in the nuclear magnetic resonance. We find that these quantities show peculiar temperature dependences in the paramagnetic state when approaching the quantum spin liquid by decreasing temperature, which reflects the fractionalization of quantum spins. We will discuss the results while changing the anisotropy and sign (ferro/antiferro) of the exchange interactions, in comparison with experiments.
Ghosh, Sayandip; Raghuvanshi, Nimisha; Mohapatra, Shubhajyoti; Kumar, Ashish; Singh, Avinash
2016-09-14
Effective spin couplings and spin fluctuation induced quantum corrections to sublattice magnetization are obtained in the [Formula: see text] AF state of a realistic three-orbital interacting electron model involving xz, yz and xy Fe 3d orbitals, providing insight into the multi-orbital quantum antiferromagnetism in iron pnictides. The xy orbital is found to be mainly responsible for the generation of strong ferromagnetic spin coupling in the b direction, which is critically important to fully account for the spin wave dispersion as measured in inelastic neutron scattering experiments. The ferromagnetic spin coupling is strongly suppressed as the xy band approaches half filling, and is ascribed to particle-hole exchange in the partially filled xy band. The strongest AF spin coupling in the a direction is found to be in the orbital off-diagonal sector involving the xz and xy orbitals. First order quantum corrections to sublattice magnetization are evaluated for the three orbitals, and yield a significant [Formula: see text] average reduction from the Hartree-Fock value. PMID:27406889
NASA Astrophysics Data System (ADS)
Ghosh, Sayandip; Raghuvanshi, Nimisha; Mohapatra, Shubhajyoti; Kumar, Ashish; Singh, Avinash
2016-09-01
Effective spin couplings and spin fluctuation induced quantum corrections to sublattice magnetization are obtained in the (π,0) AF state of a realistic three-orbital interacting electron model involving xz, yz and xy Fe 3d orbitals, providing insight into the multi-orbital quantum antiferromagnetism in iron pnictides. The xy orbital is found to be mainly responsible for the generation of strong ferromagnetic spin coupling in the b direction, which is critically important to fully account for the spin wave dispersion as measured in inelastic neutron scattering experiments. The ferromagnetic spin coupling is strongly suppressed as the xy band approaches half filling, and is ascribed to particle-hole exchange in the partially filled xy band. The strongest AF spin coupling in the a direction is found to be in the orbital off-diagonal sector involving the xz and xy orbitals. First order quantum corrections to sublattice magnetization are evaluated for the three orbitals, and yield a significant 37% average reduction from the Hartree–Fock value.
Coupled Quantum Fluctuations and Quantum Annealing
NASA Astrophysics Data System (ADS)
Hormozi, Layla; Kerman, Jamie
We study the relative effectiveness of coupled quantum fluctuations, compared to single spin fluctuations, in the performance of quantum annealing. We focus on problem Hamiltonians resembling the the Sherrington-Kirkpatrick model of Ising spin glass and compare the effectiveness of different types of fluctuations by numerically calculating the relative success probabilities and residual energies in fully-connected spin systems. We find that for a small class of instances coupled fluctuations can provide improvement over single spin fluctuations and analyze the properties of the corresponding class. Disclaimer: This research was funded by ODNI, IARPA via MIT Lincoln Laboratory under Air Force Contract No. FA8721-05-C-0002. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of ODNI, IARPA, or the US Government.
NASA Astrophysics Data System (ADS)
Baek, S.-H.; Klingeler, R.; Neef, C.; Koo, C.; Büchner, B.; Grafe, H.-J.
2014-04-01
We report P31 and Li7 nuclear magnetic resonance (NMR) studies in new non-olivine LiZnPO4-type LiCoPO4tetra microcrystals, where the Co2+ ions are tetrahedrally coordinated. Olivine LiCoPO4, which was directly transformed from LiCoPO4tetra by an annealing process, was also studied and compared. The uniform bulk magnetic susceptibility and the P31 Knight shift obey the Curie-Weiss law for both materials with a high spin Co2+ (3d7, S =3/2), but the Weiss temperature Θ and the effective magnetic moment μeff are considerably smaller in LiCoPO4tetra. The spin-lattice relaxation rate T1-1 reveals a quite different nature of the spin dynamics in the paramagnetic state of both materials. Our NMR results imply that strong geometrical spin frustration occurs in tetrahedrally coordinated LiCoPO4, which may lead to the incommensurate magnetic ordering.
Critical behavior of a triangular lattice Ising AF/FM bilayer
NASA Astrophysics Data System (ADS)
Žukovič, M.; Bobák, A.
2016-03-01
We study a bilayer Ising spin system consisting of antiferromagnetic (AF) and ferromagnetic (FM) triangular planes, coupled by ferromagnetic exchange interaction, by standard Monte Carlo and parallel tempering methods. The AF/FM bilayer is found to display the critical behavior completely different from both the single FM and AF constituents as well as the FM/FM and AF/AF bilayers. Namely, by finite-size scaling (FSS) analysis we identify at the same temperature a standard Ising transition from the paramagnetic to FM state in the FM plane that induces a ferrimagnetic state with a finite net magnetic moment in the AF plane. At lower temperatures there is another phase transition, that takes place only in the AF plane, to different ferrimagnetic state with spins on two sublattices pointing parallel and on one sublattice antiparallel to the spins on the FM plane. FSS indicates that the corresponding critical exponents are close to the two-dimensional three-state ferromagnetic Potts model values.
NASA Astrophysics Data System (ADS)
Kumar, Manoranjan; Parvej, Aslam; Thomas, Simil; Ramasesha, S.; Soos, Z. G.
2016-02-01
An efficient density matrix renormalization group (DMRG) algorithm is presented and applied to Y junctions, systems with three arms of n sites that meet at a central site. The accuracy is comparable to DMRG of chains. As in chains, new sites are always bonded to the most recently added sites and the superblock Hamiltonian contains only new or once renormalized operators. Junctions of up to N =3 n +1 ≈500 sites are studied with antiferromagnetic (AF) Heisenberg exchange J between nearest-neighbor spins S or electron transfer t between nearest neighbors in half-filled Hubbard models. Exchange or electron transfer is exclusively between sites in two sublattices with NA≠NB . The ground state (GS) and spin densities ρr=
McDonald, R. J.; Pacheco, A. J.; Wozniak, G. J.; Bolotin, H. H.; Moretto, L. G.; Schuk, C.; Shih, S.; Diamond, R. M.; Stephens, F. S.
1981-05-01
Both the magnitude and alignment of the transferred angular momentum in the reaction {sup 165}Ho + {sup 165}Ho have been measured as a function of Q-value via continuum {gamma}-ray multiplicity and anisotropy techniques. The spin transfer and the continuum {gamma}-ray anisotropy increase throughout the quasielastic region. The spin transfer as a function of Q-value saturates at ~35{bar h}/fragment, the anisotropy peaks at a value of ~2 and then decreases to near unity for the largest Q-values. The observed anisotropies are in good agreement with predictions from an equilibrium statistical model in which thermal excitation of angular-momentum-bearing collective modes and neutron evaporation give rise to in-plane components of the angular momentum.
NASA Astrophysics Data System (ADS)
Taddei, Keith M.
With nearly innumerable applications, superconductivity stands as a holy grail in the research of quantum phenomena. Understanding the mechanism that begets the fabled pairing of electrons which leads to zero resistance is the most significant undertaking in order to bring to fruition all of superconductivity's splendor. Yet the interaction which couples electrons in the most promising family of superconductors known as unconventional superconductors, which show the highest Tc's and largest upper critical fields remains a mystery. Intense study over the past several decades on the cuprate superconductors has allowed for the identification of several candidate mechanisms --- cardinal of which is magnetic fluctuations --- however as of yet the question still remains. Recently, the discovery of the iron-based superconductors has provided another fruitful avenue through which this mechanism can be probed. Excitingly in these materials superconductivity not only arises near a magnetic instability - a situation which is expected to be particularly suited for engendering superconductivity should magnetic fluctuations be the pairing mechanism - but also exhibit the microscopic co-existence of the two presumably adversarial phenomena. In the work presented here the powerful techniques of neutron and x-ray diffraction will be used to study two particularly interesting members of this family: the intercalated iron-selenide CsxFe 2--xSe2 and two members of the iron-arsenide 122 family (BaFe2(As1--xPx)2 and Sr1--xNaxFe2As 2). Though isostructural at high temperatures, these two materials behave remarkably differently and the idiosyncratic manifestations of superconductivity and ordered magnetism in either give clues as to how the latter might stabilize the former. The iron-selenides will be shown to exhibit a complex phase space with phase separation leading to stabilization of magnetism and superconductivity in separate phases. The structure, behavior and complex vacancy
Enhancement of Thermally Injected Spin Current through an Antiferromagnetic Insulator.
Lin, Weiwei; Chen, Kai; Zhang, Shufeng; Chien, C L
2016-05-01
We report a large enhancement of thermally injected spin current in normal metal (NM)/antiferromagnet (AF)/yttrium iron garnet (YIG), where a thin AF insulating layer of NiO or CoO can enhance the spin current from YIG to a NM by up to a factor of 10. The spin current enhancement in NM/AF/YIG, with a pronounced maximum near the Néel temperature of the thin AF layer, has been found to scale linearly with the spin-mixing conductance at the NM/YIG interface for NM=3d, 4d, and 5d metals. Calculations of spin current enhancement and spin mixing conductance are qualitatively consistent with the experimental results. PMID:27203336
Enhancement of Thermally Injected Spin Current through an Antiferromagnetic Insulator
NASA Astrophysics Data System (ADS)
Lin, Weiwei; Chen, Kai; Zhang, Shufeng; Chien, C. L.
2016-05-01
We report a large enhancement of thermally injected spin current in normal metal (NM)/antiferromagnet (AF)/yttrium iron garnet (YIG), where a thin AF insulating layer of NiO or CoO can enhance the spin current from YIG to a NM by up to a factor of 10. The spin current enhancement in NM /AF /YIG , with a pronounced maximum near the Néel temperature of the thin AF layer, has been found to scale linearly with the spin-mixing conductance at the NM /YIG interface for NM =3 d , 4 d , and 5 d metals. Calculations of spin current enhancement and spin mixing conductance are qualitatively consistent with the experimental results.
NASA Astrophysics Data System (ADS)
Maes, Christian; Salazar, Alberto
2014-01-01
In contrast with the understanding of fluctuation symmetries for entropy production, similar ideas applied to the time-symmetric fluctuation sector have been less explored. Here we give detailed derivations of time-symmetric fluctuation symmetries in boundary-driven particle systems such as the open Kawasaki lattice gas and the zero-range model. As a measure of time-symmetric dynamical activity over time T we count the difference (Nℓ - Nr)/T between the number of particle jumps in or out at the left edge and those at the right edge of the system. We show that this quantity satisfies a fluctuation symmetry from which we derive a new Green-Kubo-type relation. It will follow then that the system is more active at the edge connected to the particle reservoir with the largest chemical potential. We also apply these exact relations derived for stochastic particle models to a deterministic case, the spinning Lorentz gas, where the symmetry relation for the activity is checked numerically.
Quantum critical fluctuations in the heavy fermion compound Ce(Ni0.935Pd0.065)2Ge2
Wang, C. H.; Poudel, L.; Taylor, Alice E.; Lawrence, J M.; Christianson, Andrew D.; Chang, S.; Rodriguez-Rivera, J. A.; Lynn, J. W.; Podlesnyak, Andrey A.; Ehlers, G.; et al
2014-12-03
Electric resistivity, specific heat, magnetic susceptibility, and inelastic neutron scattering experiments were performed on a single crystal of the heavy fermion compound Ce(Ni0.935Pd0.065)2Ge2 in order to research the spin fluctuations near an antiferromagnetic (AF) quantum critical point (QCP). The resistivity and the specific heat coefficient for T ≤ 1 K exhibit the power law behavior expected for a 3D itinerant AF QCP (ρ(T) ~ T3/2 and γ(T) ~ γ0 - bT1/2). However, for 2 ≤ T ≤ 10 K, the susceptibility and specific heat vary as log T and the resistivity varies linearly with temperature. In addition, despite the factmore » that the resistivity and specific heat exhibit the non-Fermi liquid behavior expected at a QCP, the correlation length, correlation time, and staggered susceptibility of the spin fluctuations remain finite at low temperature. In conclusion, we suggest that these deviations from the divergent behavior expected for a QCP may result from alloy disorder.« less
Atrial Fibrillation (AF or AFib)
... Pressure High Blood Pressure Tools & Resources Stroke More Atrial Fibrillation (AF or AFib) Updated:Feb 10,2016 What ... to the Terms and Conditions and Privacy Policy Atrial Fibrillation • Introduction • What is Atrial Fibrillation? • Why AFib Matters • ...
Exchange anisotropy and spin-wave damping in CoFe/IrMn bilayers
NASA Astrophysics Data System (ADS)
Rezende, S. M.; Lucena, M. A.; Azevedo, A.; de Aguiar, F. M.; Fermin, J. R.; Parkin, S. S. P.
2003-05-01
The magnetic properties and the spin-wave damping in FM/AF bilayers of CoFe/IrMn have been investigated with magneto-optic Kerr effect magnetometry, ferromagnetic resonance (FMR), and Brillouin light scattering (BLS). As observed in other systems, the values of the interlayer exchange field obtained with different techniques are discrepant, but they all exhibit the expected t-1 dependence with the FM film thickness t. On the other hand the spin-wave relaxation rates measured by BLS and FMR are fit with a t-2 dependence plus a constant term. This is interpreted as the sum of two independent contributions, an intrinsic mechanism dominated by Gilbert damping and an extrinsic mechanism dominated by two-magnon scattering due to fluctuations of the interlayer exchange coupling caused by interface roughness.
Pathogenesis of AF: Impact on intracardiac signals
Shah, Ashok J; Dubois, Rémi; Miyazaki, Shinsuke; Jadidi, Amir S; Scherr, Daniel; Wilton, Stephen B; Roten, Laurent; Pascale, Patrizio; Pedersen, Michala; Derval, Nicolas; Knecht, Sebastien; Sacher, Frederic; Jais, Pierre; Narayan, Sanjiv; Hocini, Mélèze; Haïssaguerre, Michel
2014-01-01
Atrial fibrillation (AF) is the most common cardiac arrhythmia, and is responsible for the highest number of rhythm-related disorders and cardioembolic strokes worldwide. Intracardiac signal analysis during the onset of paroxysmal AF led to the discovery of pulmonary vein as a triggering source of AF, which has led to the development of pulmonary vein ablation—an established curative therapy for drug-resistant AF. Complex, multicomponent and rapid electrical activity widely involving the atrial substrate characterizes persistent/permanent AF. Widespread nature of the problem and complexity of signals in persistent AF reduce the success rate of ablation therapy. Although signal processing applied to extraction of relevant features from these complex electrograms has helped to improve the efficacy of ablation therapy in persistent/permanent AF, improved understanding of complex signals should help to identify sources of AF and further increase the success rate of ablation therapy. PMID:22255589
Faraday rotation echo spectroscopy and detection of quantum fluctuations
Chen, Shao-Wen; Liu, Ren-Bao
2014-01-01
Central spin decoherence is useful for detecting many-body physics in environments and moreover, the spin echo control can remove the effects of static thermal fluctuations so that the quantum fluctuations are revealed. The central spin decoherence approach, however, is feasible only in some special configurations and often requires uniform coupling between the central spin and individual spins in the baths, which are very challenging in experiments. Here, by making analogue between central spin decoherence and depolarization of photons, we propose a scheme of Faraday rotation echo spectroscopy (FRES) for studying quantum fluctuations in interacting spin systems. The echo control of the photon polarization is realized by flipping the polarization with a birefringence crystal. The FRES, similar to spin echo in magnetic resonance spectroscopy, can suppress the effects of the static magnetic fluctuations and therefore reveal dynamical magnetic fluctuations. We apply the scheme to a rare-earth compound LiHoF4 and calculate the echo signal, which is related to the quantum fluctuations of the system. We observe enhanced signals at the phase boundary. The FRES should be useful for studying quantum fluctuations in a broad range of spin systems, including cold atoms, quantum dots, solid-state impurities, and transparent magnetic materials. PMID:24733086
NASA Astrophysics Data System (ADS)
Mukuda, H.; Engetsu, F.; Yamamoto, K.; Lai, K. T.; Yashima, M.; Kitaoka, Y.; Takemori, A.; Miyasaka, S.; Tajima, S.
2014-02-01
Systematic 31P-NMR studies on LaFe(As1-xPx)(O1-yFy) with y =0.05 and 0.1 have revealed that the antiferromagnetic spin fluctuations (AFMSFs) at low energies are markedly enhanced around x =0.6 and 0.4, respectively, and as a result, Tc exhibits respective peaks at 24 and 27 K against the P substitution for As. This result demonstrates that the AFMSFs are responsible for the increase in Tc for LaFe(As1-xPx)(O1-yFy) as a primary mediator of the Cooper pairing. From a systematic comparison of AFMSFs with a series of (La1-zYz)FeAsOδ compounds in which Tc reaches 50 K for z =0.95, we remark that a moderate development of AFMSFs causes Tc to increase up to 50 K under the condition that the local lattice parameters of the FeAs tetrahedron approach those of the regular tetrahedron. We propose that Tc of Fe-pnictides exceeding 50 K is maximized under an intimate collaboration of the AFMSFs and other factors originating from the optimization of the local structure.
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.
Magnons, Spin Current and Spin Seebeck Effect
NASA Astrophysics Data System (ADS)
Maekawa, Sadamichi
2012-02-01
When metals and semiconductors are placed in a temperature gradient, the electric voltage is generated. This mechanism to convert heat into electricity, the so-called Seebeck effect, has attracted much attention recently as the mechanism for utilizing wasted heat energy. [1]. Ferromagnetic insulators are good conductors of spin current, i.e., the flow of electron spins [2]. When they are placed in a temperature gradient, generated are magnons, spin current and the spin voltage [3], i.e., spin accumulation. Once the spin voltage is converted into the electric voltage by inverse spin Hall effect in attached metal films such as Pt, the electric voltage is obtained from heat energy [4-5]. This is called the spin Seebeck effect. Here, we present the linear-response theory of spin Seebeck effect based on the fluctuation-dissipation theorem [6-8] and discuss a variety of the devices. [4pt] [1] S. Maekawa et al, Physics of Transition Metal Oxides (Springer, 2004). [0pt] [2] S. Maekawa: Nature Materials 8, 777 (2009). [0pt] [3] Concept in Spin Electronics, eds. S. Maekawa (Oxford University Press, 2006). [0pt] [4] K. Uchida et al., Nature 455, 778 (2008). [0pt] [5] K. Uchida et al., Nature Materials 9, 894 (2010) [0pt] [6] H. Adachi et al., APL 97, 252506 (2010) and Phys. Rev. B 83, 094410 (2011). [0pt] [7] J. Ohe et al., Phys. Rev. B (2011) [0pt] [8] K. Uchida et al., Appl. Phys. Lett. 97, 104419 (2010).
Embossed Teflon AF Laminate Membrane Microfluidic Diaphragm Valves
NASA Technical Reports Server (NTRS)
Willis, Peter; Hunt, Brian; White,Victor; Grunthaner, Frank
2008-01-01
A microfluidic system has been designed to survive spaceflight and to function autonomously on the Martian surface. It manipulates microscopic quantities of liquid water and performs chemical analyses on these samples to assay for the presence of molecules associated with past or present living processes. This technology lies at the core of the Urey Instrument, which is scheduled for inclusion on the Pasteur Payload of the ESA ExoMars rover mission in 2013. Fabrication processes have been developed to make the microfabricated Teflon-AF microfluidic diaphragm pumps capable of surviving extreme temperature excursions before and after exposure to liquid water. Two glass wafers are etched with features and a continuous Teflon membrane is sandwiched between them (see figure). Single valves are constructed using this geometry. The microfabricated devices are then post processed by heating the assembled device while applying pneumatic pressure to force the Teflon diaphragm against the valve seat while it is softened. After cooling the device, the embossed membrane retains this new shape. This solves previous problems with bubble introduction into the fluid flow where deformations of the membrane at the valve seat occurred during device bonding at elevated temperatures (100-150 C). The use of laminated membranes containing commercial Teflon AF 2400 sheet sandwiched between spun Teflon AF 1600 layers performed best, and were less gas permeable than Teflon AF 1600 membranes on their own. Spinning Teflon AF 1600 solution (6 percent in FLOURINERT(Registered TradeMark) FC40 solvent, 3M Company) at 500 rpm for 1.5 seconds, followed by 1,000 rpm for 3 seconds onto Borofloat glass wafers, results in a 10-micron-thick film of extremely smooth Teflon AF. This spinning process is repeated several times on flat, blank, glass wafers in order to gradually build a thick, smooth membrane. After running this process at least five times, the wafer and Teflon coating are heated under vacuum
Quantum fluctuations stabilize skyrmion textures
NASA Astrophysics Data System (ADS)
Roldán-Molina, A.; Santander, M. J.; Nunez, A. S.; Fernández-Rossier, J.
2015-12-01
We study the quantum spin waves associated to skyrmion textures. We show that the zero-point energy associated to the quantum spin fluctuations of a noncollinear spin texture produce Casimir-like magnetic fields. We study the effect of these Casimir fields on the topologically protected noncollinear spin textures known as skyrmions. In a Heisenberg model with Dzyalonshinkii-Moriya interactions, chosen so the classical ground state displays skyrmion textures, we calculate the spin-wave spectrum, using the Holstein-Primakoff approximation, and the associated zero-point energy, to the lowest order in the spin-wave expansion. Our calculations are done both for the single-skyrmion case, for which we obtain a discrete set of skyrmion bound states, as well as for the skyrmion crystal, for which the resulting spectrum gives the spin-wave bands. In both cases, our calculations show that the Casimir magnetic field contributes up to 10% of the total Zeeman energy necessary to delete the skyrmion texture with an applied field.
Drift transport of helical spin coherence with tailored spin-orbit interactions
NASA Astrophysics Data System (ADS)
Kunihashi, Y.; Sanada, H.; Gotoh, H.; Onomitsu, K.; Kohda, M.; Nitta, J.; Sogawa, T.
2016-03-01
Most future information processing techniques using electron spins in non-magnetic semiconductors will require both the manipulation and transfer of spins without their coherence being lost. The spin-orbit effective magnetic field induced by drifting electrons enables us to rotate the electron spins in the absence of an external magnetic field. However, the fluctuations in the effective magnetic field originating from the random scattering of electrons also cause undesirable spin decoherence, which limits the length scale of the spin transport. Here we demonstrate the drift transport of electron spins adjusted to a robust spin structure, namely a persistent spin helix. We find that the persistent spin helix enhances the spatial coherence of drifting spins, resulting in maximized spin decay length near the persistent spin helix condition. Within the enhanced distance of the spin transport, the transport path of electron spins can be modulated by employing time-varying in-plane voltages.
Spin Transport in Insulators Mediated by Magnetic Correlations Probed by Y3Fe5O12-based Spin Pumping
NASA Astrophysics Data System (ADS)
Du, Chunhui; Wang, Hailong; Hammel, P. Chris; Yang, Fengyuan
2015-03-01
Spin currents carried by mobile charges in ferromagnetic (FM) and nonmagnetic (NM) materials have been the central focus of spintronics, while spin transport in insulators is largely unexplored. FMR spin pumping has awakened intense interest in magnon-mediated spin currents in both conducting and insulating FMs and in antiferromagnets (AF). Building on the large spin pumping signals enabled by our Y3Fe5O12 (YIG) films, we report a systematic study of spin transport in six series of Pt/insulator/YIG trilayers where the insulators include one diamagnet, one paramagnet and four AFs. We observe remarkably robust spin transport in the AFs and a distinct linear relationship between the spin decay length in the insulator and the damping enhancement in the YIG, suggesting the critical role of magnetic correlations in AF insulators for spin transport. Strikingly, the insertion of a thin NiO layer between YIG and Pt significantly enhances the spin currents driven into Pt, suggesting exceptionally high spin transfer efficiency in YIG/NiO/Pt structures.
Effect of X irradiation on optical properties of Teflon-AF
NASA Astrophysics Data System (ADS)
Jahan, M. S.; Ermer, D. R.; Cooke, D. W.
1993-03-01
Radiation effects in optical-grade amorphous fluoropolymer, Teflon-AF, is investigated by UV-visible absorption and electron spin resonance (ESR) measurements. When irradiated with low-energy (40 kVp) X-rays at room temperature in air, Teflon-AF is found to develop a broad, structureless UV-absorption band in the wavelength interval 200-350 nm. While the UV absorption increases as a function of X-ray dose, with relative rates of approx 2 × 10 -5 Gy -1 (1 × 10 -5 Gy -1) in Teflon-AF 1600 (Teflon-AF 2400), its optical transparency for a given dose of 67.5 kGy, however, remains unaffected. Additional measurements conducted using electron spin resonance (ESR) technique reveal that the observed UV absorption is caused by the X-ray induced peroxy radical (POO .˙). The results also suggest that the inclusion of dioxole monomer in the PTFE chain not only improves the optical clarity of Teflon-AF, as reported, but also increases its radiation tolerance. During a post-irradiation storage in air at RT for about 30 days the peroxy radical is observed to decay, with a concomitant decrease in UV absorption. A tentative model is proposed to explain the radiation damage and recovery mechanisms.
Quantum critical fluctuations in the heavy fermion compound Ce(Ni_{0.935}Pd_{0.065})_{2}Ge_{2}
Wang, C. H.; Poudel, L.; Taylor, Alice E.; Lawrence, J M.; Christianson, Andrew D.; Chang, S.; Rodriguez-Rivera, J. A.; Lynn, J. W.; Podlesnyak, Andrey A.; Ehlers, G.; Baumbach, R. E.; Bauer, E. D.; Gofryk, Krzysztof; Ronning, F.; Mcclellan, K. J.; Thompson, J. D.
2014-12-03
Electric resistivity, specific heat, magnetic susceptibility, and inelastic neutron scattering experiments were performed on a single crystal of the heavy fermion compound Ce(Ni_{0.935}Pd_{0.065})_{2}Ge_{2} in order to research the spin fluctuations near an antiferromagnetic (AF) quantum critical point (QCP). The resistivity and the specific heat coefficient for T ≤ 1 K exhibit the power law behavior expected for a 3D itinerant AF QCP (ρ(T) ~ T^{3/2} and γ(T) ~ γ_{0} - bT^{1/2}). However, for 2 ≤ T ≤ 10 K, the susceptibility and specific heat vary as log T and the resistivity varies linearly with temperature. In addition, despite the fact that the resistivity and specific heat exhibit the non-Fermi liquid behavior expected at a QCP, the correlation length, correlation time, and staggered susceptibility of the spin fluctuations remain finite at low temperature. In conclusion, we suggest that these deviations from the divergent behavior expected for a QCP may result from alloy disorder.
Quantum critical fluctuations in the heavy fermion compound Ce(Ni_{0.935} Pd_{0.065})_{2}Ge_{2}
Wang, C. H.; Poudel, L.; Taylor, A. E.; Lawrence, J. M.; Christianson, A. D.; Chang, S.; Rodriguez-Rivera, J. A.; Lynn, J. W.; Podlesnyak, A. A.; Ehlers, G.; Baumbach, R. E.; Bauer, E. D.; Gofryk, K.; Ronning, F.; McClellan, K. J.; Thompson, J. D.
2015-01-14
Electric resistivity, specific heat, magnetic susceptibility, and inelastic neutron scattering experi- ments were performed on a single crystal of the heavy fermion compound Ce(Ni_{0.935} Pd_{0.065})_{2}Ge_{2} in order to study the spin fluctuations near an antiferromagnetic (AF) quantum critical point (QCP). The resistivity and the specific heat coefficient for T ≤ 1 K exhibit the power law behavior expected for a 3D itinerant AF QCP (ρ(T) ~ T^{3/2} and γ(T) ~ γ_{0} - bT^{1/2}). However, for 2 ≤ T ≤ 10 K, the susceptibility and specific heat vary as log T and the resistivity varies linearly with temperature. Furthermore, despite the fact that the resistivity and specific heat exhibit the non-Fermi liquid behavior expected at a QCP, the correlation length, correlation time, and staggered susceptibility of the spin fluctuations remain finite at low temperature. We suggest that these deviations from the divergent behavior expected for a QCP may result from alloy disorder.
Impact of magnetic fluctuations on lattice excitations in fcc nickel
NASA Astrophysics Data System (ADS)
Körmann, Fritz; Ma, Pui-Wai; Dudarev, Sergei L.; Neugebauer, Jörg
2016-02-01
The spin-space averaging formalism is applied to compute atomic forces and phonon spectra for magnetically excited states of fcc nickel. Transverse and longitudinal magnetic fluctuations are taken into account by a combination of magnetic special quasi random structures and constrained spin-density-functional theory. It turns out that for fcc Ni interatomic force constants and phonon spectra are almost unaffected by both kinds of spin fluctuations. Given the computational expense to simulate coupled magnetic and atomic fluctuations, this insight facilitates computational modeling of magnetic alloys such as Ni-based superalloys.
NASA Astrophysics Data System (ADS)
Liu, Hong; Jiang, Lanfang; Zhao, Qin; Wang, Li
2009-11-01
A swerving mathematical model was established after stating the shortage of the present AFS swerving algorithm. The conception of 'expected lighting distance' was extended to 'expected lighting bound' and approximate treatment of geometry of light beam falling to ground of headlamp was processed. The expected lighting bound was ascertained and the lighting range of turning angle of headlamp was calculated. The calculation formula of turning angle of headlamp was worked out. It was indicated that the turning angle of inside and outside of headlamp calculated by revised algorithm was reasonable by comparing calculation. Finally the control strategy about the turning angle of inside and outside headlamp when turning was worked out. It is of practical significance in promoting the active safety, reducing the traffic accidents caused by insufficient angle and range of irradiation of headlamp.
Fluctuations of healthy and unhealthy heartbeat intervals
NASA Astrophysics Data System (ADS)
Lan, Boon Leong; Toda, Mikito
2013-04-01
We show that the RR-interval fluctuations, defined as the difference between successive natural-logarithm of the RR interval, for healthy, congestive-heart-failure (CHF) and atrial-fibrillation (AF) subjects are well modeled by non-Gaussian stable distributions. Our results suggest that healthy or unhealthy RR-interval fluctuation can generally be modeled as a sum of a large number of independent physiological effects which are identically distributed with infinite variance. Furthermore, we show for the first time that one indicator —the scale parameter of the stable distribution— is sufficient to robustly distinguish the three groups of subjects. The scale parameters for healthy subjects are smaller than those for AF subjects but larger than those for CHF subjects —this ordering suggests that the scale parameter could be used to objectively quantify the severity of CHF and AF over time and also serve as an early warning signal for a healthy person when it approaches either boundary of the healthy range.
Eaton AF5000+Genesis Communication Driver
Energy Science and Technology Software Center (ESTSC)
1995-05-25
Communication driver allows the Genesis Control Series software to interact with Eaton AF5000+ frequency drives via RS-232 communications. All Eaton AF5000+ parameters that support communications are supported by the Genesis driver. Multidrop addressing to multiple units is available with the Genesis communication driver.
Uniaxial spin-transfer torque in an exchange-biased spin valve.
Xu, Jian-qing; Jin, Guojun
2011-07-27
We study the effects of uniaxial spin-transfer torque (USTT) on the ferromagnetic (F) as well as antiferromagnetic (AF) layers in an exchange-biased (EB) spin valve. By analytically treating the free-energy functional of the F/AF bilayer and numerically solving the Landau-Lifshitz-Gilbert equation for magnetic moments, we can reproduce and explain two existing experimental facts relevant to USTT: one is that the EB field can be reversed by both positive and negative pulsed currents, and the other is that the critical current to excite the F moments is greatly increased in the presence of an AF layer and independent of external fields. We also derive the angular dependence of the critical currents to excite AF and F moments, which suggests a possible way to quantitatively determine USTT in experiments. PMID:21727307
Spin-flip noise due to nonequilibrium spin accumulation
NASA Astrophysics Data System (ADS)
Liu, Liang; Niu, Jiasen; Guo, Huiqiang; Wei, Jian; Li, D. L.; Feng, J. F.; Han, X. F.; Coey, J. M. D.; Zhang, X.-G.
2016-05-01
When current flows through a magnetic tunnel junction (MTJ), there is spin accumulation at the electrode-barrier interfaces if the magnetic moments of the two ferromagnetic electrodes are not aligned. Here we report that such nonequilibrium spin accumulation generates its own characteristic low frequency noise (LFN). Past work viewed the LFN in MTJs as an equilibrium effect arising from resistance fluctuations (SR) which a passively applied current (I ) converts to measurable voltage fluctuations (SV=I2SR ). We treat the LFN associated with spin accumulation as a nonequilibrium effect, and find that the noise power can be fitted in terms of the spin-polarized current by SIf =a I coth(I/b )-a b , resembling the form of the shot noise for a tunnel junction, but with current now taking the role of the bias voltage, and spin-flip probability taking the role of tunneling probability.
State diagram of an orthogonal spin transfer spin valve device
Ye, Li; Wolf, Georg; Pinna, Daniele; Chaves-O'Flynn, Gabriel D.; Kent, Andrew D.
2015-05-21
We present the switching characteristics of a spin-transfer device that incorporates a perpendicularly magnetized spin-polarizing layer with an in-plane magnetized free and fixed magnetic layer, known as an orthogonal spin transfer spin valve device. This device shows clear switching between parallel (P) and antiparallel (AP) resistance states and the reverse transition (AP → P) for both current polarities. Further, hysteretic transitions are shown to occur into a state with a resistance intermediate between that of the P and AP states, again for both current polarities. These unusual spin-transfer switching characteristics can be explained within a simple macrospin model that incorporates thermal fluctuations and considers a spin-polarized current that is tilted with respect to the free layer's plane, due to the presence of the spin-transfer torque from the polarizing layer.
Spin-polaron theory of high-{Tc} superconductivity: I, spin polarons and high-{Tc} pairing
Wood, R.F.
1993-06-01
The concept of a spin polaron is introduced and contrasted with the more familiar ionic polaron picture. A brief review of aspects of ionic bipolaronic superconductivity is given with particular emphasis on the real-space pairing and true Bose condensation characteristics. The formation energy of spin polarons is then calculated in analogy with ionic polarons. The spin-flip energy of a Cu spin in an antiferromagnetically aligned CuO{sub 2} plane is discussed. It is shown that the introduction of holes into the CuO{sub 2} planes will always lead to the destruction of long-range AF ordering due to the formation of spin polarons. The pairing of two spin polarons can be expected because of the reestablishment of local (short-range) AF ordering; the magnitude of the pairing energy is estimated using a simplified model. The paper closes with a brief discussion of the formal theory of spin polarons.
An Exact SU(2) Symmetry and Persistent Spin Helix ina Spin-orbit Coupled System
Bernevig, B.A.; Orenstein, J.; Zhang, Shou-Cheng; /Stanford U., Phys. Dept.
2007-01-22
Spin-orbit coupled systems generally break the spin rotation symmetry. However, for a model with equal Rashba and Dresselhauss coupling constant (the ReD model), and for the [110] Dresselhauss model, a new type of SU(2) spin rotation symmetry is discovered. This symmetry is robust against spin-independent disorder and interactions, and is generated by operators whose wavevector depends on the coupling strength. It renders the spin lifetime infinite at this wavevector, giving rise to a Persistent Spin Helix (PSH). We obtain the spin fluctuation dynamics at, and away, from the symmetry point, and suggest experiments to observe the PSH.
An Exact SU(2) Symmetry and Persistent Spin Helix in a Spin-Orbit Coupled System
Bernevig, Andrei
2010-02-10
Spin-orbit coupled systems generally break the spin rotation symmetry. However, for a model with equal Rashba and Dresselhauss coupling constant (the ReD model), and for the [110] Dresselhauss model, a new type of SU(2) spin rotation symmetry is discovered. This symmetry is robust against spin-independent disorder and interactions, and is generated by operators whose wavevector depends on the coupling strength. It renders the spin lifetime infinite at this wavevector, giving rise to a Persistent Spin Helix (PSH). We obtain the spin fluctuation dynamics at, and away, from the symmetry point, and suggest experiments to observe the PSH.
STBC AF relay for unmanned aircraft system
NASA Astrophysics Data System (ADS)
Adachi, Fumiyuki; Miyazaki, Hiroyuki; Endo, Chikara
2015-01-01
If a large scale disaster similar to the Great East Japan Earthquake 2011 happens, some areas may be isolated from the communications network. Recently, unmanned aircraft system (UAS) based wireless relay communication has been attracting much attention since it is able to quickly re-establish the connection between isolated areas and the network. However, the channel between ground station (GS) and unmanned aircraft (UA) is unreliable due to UA's swing motion and as consequence, the relay communication quality degrades. In this paper, we introduce space-time block coded (STBC) amplify-and-forward (AF) relay for UAS based wireless relay communication to improve relay communication quality. A group of UAs forms single frequency network (SFN) to perform STBC-AF cooperative relay. In STBC-AF relay, only conjugate operation, block exchange and amplifying are required at UAs. Therefore, STBC-AF relay improves the relay communication quality while alleviating the complexity problem at UAs. It is shown by computer simulation that STBC-AF relay can achieve better throughput performance than conventional AF relay.
Antiferromagnetic spin structure and lithium ion diffusion in Li2MnO3 probed by μ+SR
NASA Astrophysics Data System (ADS)
Sugiyama, Jun; Mukai, Kazuhiko; Nozaki, Hiroshi; Harada, Masashi; Månsson, Martin; Kamazawa, Kazuya; Andreica, Daniel; Amato, Alex; Hillier, Adrian D.
2013-01-01
In order to elucidate the antiferromagnetic (AF) spin structure below TN˜35 K and to clarify the diffusive behavior of Li+ ions in the layered compound Li2MnO3, we have performed a muon-spin rotation and relaxation (μ+SR) experiment using a powder sample in the temperature range between 2 and 500 K. Below TN, the zero-field (ZF-) μ+SR spectrum showed a clear oscillation that consists of two muon-spin precession signals with different frequencies. Combining with the dipole field calculations, it was found that the most probable spin structure for Li2MnO3 is the Cx-type AF order in which Mn moments align parallel or antiparallel to the a axis in the [Li1/3Mn2/3]O2 layer, and a ferromagnetic chain along the a axis aligns antiferromagnetically along both the b and c axes. The ordered Mn moment was estimated as 2.62μB at 2 K. In the paramagnetic state, ZF- and longitudinal-field μ+SR spectra exhibited a dynamic nuclear field relaxation. From the temperature dependence of the field distribution width, the Li+ ions were found to diffuse mainly along the c axis through the Li ion in the [Li1/3Mn2/3]O2 layer. Also, based on the field fluctuation rate, a self-diffusion coefficient of Li+ ions (DLi) at 300 K was estimated as 4.7(4)×10-11 cm2/s with the thermal activation energy Ea=0.156(3) eV.
Universal Conductance Fluctuation in Two-Dimensional Topological Insulators.
Choe, Duk-Hyun; Chang, K J
2015-01-01
Despite considerable interest in two-dimensional (2D) topological insulators (TIs), a fundamental question still remains open how mesoscopic conductance fluctuations in 2D TIs are affected by spin-orbit interaction (SOI). Here, we investigate the effect of SOI on the universal conductance fluctuation (UCF) in disordered 2D TIs. Although 2D TI exhibits UCF like any metallic systems, the amplitude of these fluctuations is distinguished from that of conventional spin-orbit coupled 2D materials. Especially, in 2D systems with mirror symmetry, spin-flip scattering is forbidden even in the presence of strong intrinsic SOI, hence increasing the amplitude of the UCF by a factor of √2 compared with extrinsic SOI that breaks mirror symmetry. We propose an easy way to experimentally observe the existence of such spin-flip scattering in 2D materials. Our findings provide a key to understanding the emergence of a new universal behavior in 2D TIs. PMID:26055574
Korean Atrial Fibrillation (AF) Network: Genetic Variants for AF Do Not Predict Ablation Success
Choi, Eue-Keun; Park, Jae Hyung; Lee, Ji-Young; Nam, Chung Mo; Hwang, Min Ki; Uhm, Jae-Sun; Joung, Boyoung; Ko, Young-Guk; Lee, Moon-Hyoung; Lubitz, Steven A; Ellinor, Patrick T; Pak, Hui-Nam
2015-01-01
Background Genomewide association studies have identified several loci associated with atrial fibrillation (AF) and have been reportedly associated with response to catheter ablation for AF in patients of European ancestry; however, associations between top susceptibility loci and AF recurrence after ablation have not been examined in Asian populations. We examined whether the top single nucleotide polymorphisms (SNPs) at chromosomes 4q25 (PITX2), 16q22 (ZFHX3), and 1q21 (KCNN3) were associated with AF in a Korean population and whether these SNPs were associated with clinical outcomes after catheter ablation for AF. Methods and Results We determined the association between 4 SNPs and AF in 1068 AF patients who underwent catheter ablation (74.6% male, aged 57.5±10.9 years, 67.9% paroxysmal AF) and 1068 age- and sex-matched controls. The SNPs at the PITX2 and ZFHX3 loci, but not the KCNN3 locus, were significantly associated with AF (PITX2/rs6843082_G: odds ratio 3.41, 95% CI 2.55 to 4.55, P=1.32×10−16; PITX2/rs2200733_T: odds ratio 2.05, 95% CI 1.66 to 2.53, P=2.20×10−11; ZFHX3/rs2106261_A: odds ratio 2.33, 95% CI 1.87 to 2.91, P=3.75×10−14; KCNN3/rs13376333_T: odds ratio 1.74, 95% CI 0.93 to 3.25, P=0.085). Among those patients who underwent catheter ablation for AF, none of the top AF-associated SNPs were associated with long-term clinical recurrence of AF after catheter ablation. Conclusions SNPs at the PITX2 and ZFHX3 loci were strongly associated with AF in Korean patients. In contrast to prior reports, none of the 4 top AF-susceptibility SNPs predicted clinical recurrence after catheter ablation. PMID:26272656
Vaporization of Kitaev Spin Liquids
NASA Astrophysics Data System (ADS)
Nasu, Joji; Udagawa, Masafumi; Motome, Yukitoshi
2014-11-01
The quantum spin liquid is an exotic quantum state of matter in magnets. This state is a spin analog of liquid helium that does not solidify down to the lowest temperature due to strong quantum fluctuations. In conventional fluids, the liquid and gas possess the same symmetry and adiabatically connect to each other by bypassing the critical end point. We find that the situation is qualitatively different in quantum spin liquids realized in a three-dimensional Kitaev model; both gapless and gapped quantum spin liquid phases at low temperatures are always distinguished from the high-temperature paramagnet (spin gas) by a phase transition. The results challenge the common belief that the absence of thermodynamic singularity down to the lowest temperature is a symptom of a quantum spin liquid.
Spin liquid phases of large spin Mott insulating ultracold atoms
NASA Astrophysics Data System (ADS)
Rutkowski, Todd C.; Lawler, Michael J.
2015-03-01
Understanding exotic forms of magnetism, primarily those driven by large spin fluctuations such as the quantum spin liquid state, is a major goal of condensed matter physics. But, the relatively small number of viable candidate materials poses a difficulty. We believe this problem can be solved by Mott insulating ultracold atoms with large spin moments that interact via whole-atom exchange. The large spin fluctuations of this exchange could stabilize exotic physics similar to condensed matter systems, all in an extremely tunable environment. We have approached the problem by performing a mean field theory for spin-f bosons in an optical lattice which is exact in the large-f limit. This setting is similar to that of SU(N) magnetism proposed for alkali-earth atoms but without the SU(N) symmetry. We find that states with long-range order, such as the spin nematic phase of f = 1 Na atoms, become highly entangled spin-liquid-like states for f = 3 Cr atoms. This is evidence that the magnetic phase diagram for Mott insulating atoms at larger spins generically contains exotic forms of magnetism.
Efficient suppression of Overhauser field fluctuations with DNP
NASA Astrophysics Data System (ADS)
McNeil, Robert; Botzem, Tim; Tenberg, Stefanie; Rubbert, Sebastian; Bluhm, Hendrik
2015-03-01
In certain spin-qubit schemes the Overhauser field is a tuned control parameter and in many spin qubits this fluctuating nuclear field is a significant factor limiting coherence. Nuclear spins can be driven via dynamic nuclear polarisation (DNP) to a chosen field and selective feedback applied narrowing the distribution of nuclear Overhauser field fluctuations. The achievable narrowing of the Overhauser field is related to the maximum pump rate and previous experiments on gated GaAs quantum dots were limited by the pump rate of the pumping mechanism used. We present a method to reduce nuclear fluctuations by increasing the max achievable pump rate. Sequentially applying two ac electric fields with frequencies slightly detuned from the desired Larmor frequency results in a pump curve with a stable fixed point. In the absence of spin-orbit interaction, driving electron spin flips via electric dipole spin resonance (EDSR) will also drive nuclear spin flips and this scheme is expected to result in stronger pumping and efficient suppression of the Overhauser field fluctuations. We will present experimental evidence of this driven nuclear polarization including tracking of EDSR resonances.
Spin-current probe for phase transition in an insulator.
Qiu, Zhiyong; Li, Jia; Hou, Dazhi; Arenholz, Elke; N'Diaye, Alpha T; Tan, Ali; Uchida, Ken-Ichi; Sato, Koji; Okamoto, Satoshi; Tserkovnyak, Yaroslav; Qiu, Z Q; Saitoh, Eiji
2016-01-01
Spin fluctuation and transition have always been one of the central topics of magnetism and condensed matter science. Experimentally, the spin fluctuation is found transcribed onto scattering intensity in the neutron-scattering process, which is represented by dynamical magnetic susceptibility and maximized at phase transitions. Importantly, a neutron carries spin without electric charge, and therefore it can bring spin into a sample without being disturbed by electric energy. However, large facilities such as a nuclear reactor are necessary. Here we show that spin pumping, frequently used in nanoscale spintronic devices, provides a desktop microprobe for spin transition; spin current is a flux of spin without an electric charge and its transport reflects spin excitation. We demonstrate detection of antiferromagnetic transition in ultra-thin CoO films via frequency-dependent spin-current transmission measurements, which provides a versatile probe for phase transition in an electric manner in minute devices. PMID:27573443
Spin-current probe for phase transition in an insulator
Qiu, Zhiyong; Li, Jia; Hou, Dazhi; Arenholz, Elke; N’Diaye, Alpha T.; Tan, Ali; Uchida, Ken-ichi; Sato, Koji; Okamoto, Satoshi; Tserkovnyak, Yaroslav; et al
2016-08-30
Spin fluctuation and transition have always been one of the central topics of magnetism and condensed matter science. Experimentally, the spin fluctuation is found transcribed onto scattering intensity in the neutron-scattering process, which is represented by dynamical magnetic susceptibility and maximized at phase transitions. Importantly, a neutron carries spin without electric charge, and therefore it can bring spin into a sample without being disturbed by electric energy. However, large facilities such as a nuclear reactor are necessary. Here we present that spin pumping, frequently used in nanoscale spintronic devices, provides a desktop microprobe for spin transition; spin current is amore » flux of spin without an electric charge and its transport reflects spin excitation. Additionally, we demonstrate detection of antiferromagnetic transition in ultra-thin CoO films via frequency-dependent spin-current transmission measurements, which provides a versatile probe for phase transition in an electric manner in minute devices.« less
Intrinsic topological superfluidity - fluctuations and response
NASA Astrophysics Data System (ADS)
Levin, K.; Wu, Chien-Te; Anderson, Brandon; Boyack, Rufus
Recent interest in topological superconductivity is based primarily on exploiting proximity effects to obtain this important phase. However, in cold gases it is possible to contemplate ``intrinsic'' topological superfluidity produced with a synthetic spin-orbit coupling and Zeeman field. It is important for such future experiments to establish how low in temperature one needs to go to reach the ordered phase. Similarly, it will be helpful to have a probe of the normal (pseudogap) phase to determine if the ultimate superfluid order will be topological or trivial. In this talk, we address these issues by considering fluctuation effects in such a superfluid, and calculate the critical transition temperature and response functions. We see qualitative signatures of topological superfluidity in spin and charge response functions. We also explore the suppression of superfluidity due to fluctuations, and importantly find that the temperature scales necessary to reach topological superfluidity are reasonably accessible
NASA Astrophysics Data System (ADS)
Baek, S.-H.; Choi, K.-Y.; Berger, H.; Büchner, B.; Grafe, H.-J.
2012-11-01
We present high-field magnetization and 79Br nuclear quadrupole resonance (NQR) and 125Te nuclear magnetic resonance (NMR) studies in the weakly coupled Cu2+ (S=1/2) tetrahedral system Cu2Te2O5Br2. The field-induced level crossing effects were observed by the magnetization measurements in a long-ranged magnetically ordered state which was confirmed by a strong divergence of the spin-lattice relaxation rate T1-1 at T0=13.5 K. In the paramagnetic state, T1-1 reveals an effective singlet-triplet spin gap much larger than that observed by static bulk measurements. Our results imply that the inter- and the intratetrahedral interactions compete, but at the same time they cooperate strengthening effectively the local intratetrahedral exchange couplings. We discuss that the unusual feature originates from the frustrated intertetrahedral interactions.
Degradation of AF1Q by chaperone-mediated autophagy
Li, Peng; Ji, Min; Lu, Fei; Zhang, Jingru; Li, Huanjie; Cui, Taixing; Li Wang, Xing; Tang, Dongqi; Ji, Chunyan
2014-09-10
AF1Q, a mixed lineage leukemia gene fusion partner, is identified as a poor prognostic biomarker for pediatric acute myeloid leukemia (AML), adult AML with normal cytogenetic and adult myelodysplastic syndrome. AF1Q is highly regulated during hematopoietic progenitor differentiation and development but its regulatory mechanism has not been defined clearly. In the present study, we used pharmacological and genetic approaches to influence chaperone-mediated autophagy (CMA) and explored the degradation mechanism of AF1Q. Pharmacological inhibitors of lysosomal degradation, such as chloroquine, increased AF1Q levels, whereas activators of CMA, including 6-aminonicotinamide and nutrient starvation, decreased AF1Q levels. AF1Q interacts with HSPA8 and LAMP-2A, which are core components of the CMA machinery. Knockdown of HSPA8 or LAMP-2A increased AF1Q protein levels, whereas overexpression showed the opposite effect. Using an amino acid deletion AF1Q mutation plasmid, we identified that AF1Q had a KFERQ-like motif which was recognized by HSPA8 for CMA-dependent proteolysis. In conclusion, we demonstrate for the first time that AF1Q can be degraded in lysosomes by CMA. - Highlights: • Chaperone-mediated autophagy (CMA) is involved in the degradation of AF1Q. • Macroautophagy does not contribute to the AF1Q degradation. • AF1Q has a KFERQ-like motif that is recognized by CMA core components.
AFS Estuaries Section - A Successful Partnership
The Estuaries Section of the American Fisheries Society offers travel awards to students in support of their attendance and presentations at the AFS meeting. Since 2007, the Southern Association of Marine Laboratories has partnered with the Estuaries Section to sponsor two stude...
Charge dynamics and spin order in doped Hubbard models
Kampf, A.P.; Brenig, W.
1994-04-01
Hole motion in an antiferromagnetic (AF) environment is accompanied by the emission of spin wave excitations. Spin-wave shakeoffs are responsible for incoherent contributions to the dynamics of propagating holes. Using a spin-density-wave polaron scheme the authors calculate the optical conductivity {sigma}({omega}) and show that the incoherent part of the hole spectrum contributes to the low-frequency part of {sigma}({omega}). Separately, the authors discuss the possible formation of spiral spin patterns upon doping of the half-filled one-band Hubbard model. In particular, the authors consider the influence of band structure effects arising from nearest- and next-nearest-neighbor hopping processes on a square lattice. Differences in the ground state spin patterns for hole and electron doping are obtained offering a possible explanation for the persistence of AF order in low electron-doped cuprate superconductors.
Steady-state spin squeezing generation in diamond nanostructures
NASA Astrophysics Data System (ADS)
Ma, Yong-Hong; Zhang, Xue-Feng
2014-04-01
As one kind of many body entangled states, spin squeezed states can be used to implement the high precise measurement beyond the standard quantum limit. Inspired by the novel spin squeezing scheme based on phonon-induced spin-spin interactions [S. D. Bennett et al., Phys. Rev. Lett. 110, 156402 (2013), 10.1103/PhysRevLett.110.156402], we reexamine the steady-state behaviors for the spin ensemble in diamond nanostructures by exerting a controllable microwave field. By using the phase-space approach we calculate analytically fluctuations of collective spin operators. We find that there is bistability and spin squeezing for the steady-state spin ensemble, despite the mechanical damping considered. Moreover, our work shows that bistability and spin squeezing can be controlled by microwave field and Zeeman splitting. The present scheme can be used to increase the stability of spin clocks, magnetometers, and other measurements based on spin-spin interaction in diamond nanostructures.
Spin slush in an extended spin ice model
Rau, Jeffrey G.; Gingras, Michel J. P.
2016-01-01
We present a new classical spin liquid on the pyrochlore lattice by extending spin ice with further neighbour interactions. We find that this disorder-free spin model exhibits a form of dynamical heterogeneity with extremely slow relaxation for some spins, while others fluctuate quickly down to zero temperature. We thus call this state spin slush, in analogy to the heterogeneous mixture of solid and liquid water. This behaviour is driven by the structure of the ground-state manifold which extends the celebrated two-in/two-out ice states to include branching structures built from three-in/one-out, three-out/one-in and all-in/all-out tetrahedra defects. Distinctive liquid-like patterns in the magnetic correlations serve as a signature of this intermediate range order. Possible applications to materials as well the effects of quantum tunnelling are discussed. PMID:27470199
Spin slush in an extended spin ice model
NASA Astrophysics Data System (ADS)
Rau, Jeffrey G.; Gingras, Michel J. P.
2016-07-01
We present a new classical spin liquid on the pyrochlore lattice by extending spin ice with further neighbour interactions. We find that this disorder-free spin model exhibits a form of dynamical heterogeneity with extremely slow relaxation for some spins, while others fluctuate quickly down to zero temperature. We thus call this state spin slush, in analogy to the heterogeneous mixture of solid and liquid water. This behaviour is driven by the structure of the ground-state manifold which extends the celebrated two-in/two-out ice states to include branching structures built from three-in/one-out, three-out/one-in and all-in/all-out tetrahedra defects. Distinctive liquid-like patterns in the magnetic correlations serve as a signature of this intermediate range order. Possible applications to materials as well the effects of quantum tunnelling are discussed.
Spin slush in an extended spin ice model.
Rau, Jeffrey G; Gingras, Michel J P
2016-01-01
We present a new classical spin liquid on the pyrochlore lattice by extending spin ice with further neighbour interactions. We find that this disorder-free spin model exhibits a form of dynamical heterogeneity with extremely slow relaxation for some spins, while others fluctuate quickly down to zero temperature. We thus call this state spin slush, in analogy to the heterogeneous mixture of solid and liquid water. This behaviour is driven by the structure of the ground-state manifold which extends the celebrated two-in/two-out ice states to include branching structures built from three-in/one-out, three-out/one-in and all-in/all-out tetrahedra defects. Distinctive liquid-like patterns in the magnetic correlations serve as a signature of this intermediate range order. Possible applications to materials as well the effects of quantum tunnelling are discussed. PMID:27470199
Kontani, Hiroshi
2002-12-01
In high-T(c) cuprates, the Nernst coefficient (nu) as well as the magnetoresistance (Deltarho/rho) increases drastically below the pseudogap temperature, T(*), which attracts much attention as a key phenomenon in the pseudogap region. We study these transport phenomena in terms of the fluctuation-exchange+T-matrix approximation. In this present theory, the d-wave superconducting (SC) fluctuations, which are mediated by antiferromagnetic (AF) correlations, become dominant below T(*). We especially investigate the vertex corrections both for the charge current and the heat one to keep the conservation laws. As a result, the mysterious behaviors of nu and Deltarho/rho are naturally explained as the reflection of the enhancement of the SC fluctuation, without assuming thermally excited vortices. The present result suggests that the pseudogap phenomena are well described in terms of the Fermi liquid with AF and SC fluctuations. PMID:12485031
Cross-correlation spin noise spectroscopy of heterogeneous interacting spin systems
Roy, Dibyendu; Yang, Luyi; Crooker, Scott A.; Sinitsyn, Nikolai A.
2015-04-30
Interacting multi-component spin systems are ubiquitous in nature and in the laboratory. As such, investigations of inter-species spin interactions are of vital importance. Traditionally, they are studied by experimental methods that are necessarily perturbative: e.g., by intentionally polarizing or depolarizing one spin species while detecting the response of the other(s). Here, we describe and demonstrate an alternative approach based on multi-probe spin noise spectroscopy, which can reveal inter-species spin interactions - under conditions of strict thermal equilibrium - by detecting and cross-correlating the stochastic fluctuation signals exhibited by each of the constituent spin species. Specifically, we consider a two-component spin ensemble that interacts via exchange coupling, and we determine cross-correlations between their intrinsic spin fluctuations. The model is experimentally confirmed using “two-color” optical spin noise spectroscopy on a mixture of interacting Rb and Cs vapors. Noise correlations directly reveal the presence of inter-species spin exchange, without ever perturbing the system away from thermal equilibrium. These non-invasive and noise-based techniques should be generally applicable to any heterogeneous spin system in which the fluctuations of the constituent components are detectable.
Cross-correlation spin noise spectroscopy of heterogeneous interacting spin systems
NASA Astrophysics Data System (ADS)
Roy, Dibyendu; Yang, Luyi; Crooker, Scott A.; Sinitsyn, Nikolai A.
2015-04-01
Interacting multi-component spin systems are ubiquitous in nature and in the laboratory. As such, investigations of inter-species spin interactions are of vital importance. Traditionally, they are studied by experimental methods that are necessarily perturbative: e.g., by intentionally polarizing or depolarizing one spin species while detecting the response of the other(s). Here, we describe and demonstrate an alternative approach based on multi-probe spin noise spectroscopy, which can reveal inter-species spin interactions - under conditions of strict thermal equilibrium - by detecting and cross-correlating the stochastic fluctuation signals exhibited by each of the constituent spin species. Specifically, we consider a two-component spin ensemble that interacts via exchange coupling, and we determine cross-correlations between their intrinsic spin fluctuations. The model is experimentally confirmed using ``two-color'' optical spin noise spectroscopy on a mixture of interacting Rb and Cs vapors. Noise correlations directly reveal the presence of inter-species spin exchange, without ever perturbing the system away from thermal equilibrium. These non-invasive and noise-based techniques should be generally applicable to any heterogeneous spin system in which the fluctuations of the constituent components are detectable.
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Geometrical spin symmetry and spin
Pestov, I. B.
2011-07-15
Unification of General Theory of Relativity and Quantum Mechanics leads to General Quantum Mechanics which includes into itself spindynamics as a theory of spin phenomena. The key concepts of spindynamics are geometrical spin symmetry and the spin field (space of defining representation of spin symmetry). The essence of spin is the bipolar structure of geometrical spin symmetry induced by the gravitational potential. The bipolar structure provides a natural derivation of the equations of spindynamics. Spindynamics involves all phenomena connected with spin and provides new understanding of the strong interaction.
Magnetic monopoles in quantum spin ice
NASA Astrophysics Data System (ADS)
Petrova, Olga; Moessner, Roderich; Sondhi, Shivaji
Typical spin ice materials can be modeled using classical Ising spins. The geometric frustration of the pyrochlore lattice causes the spins to satisfy ice rules, whereas a violation of the ice constraint constitutes an excitation. Flipping adjacent spins fractionalizes the excitation into two monopoles. Long range dipolar spin couplings result in Coulombic interactions between charges, while the leading effect of quantum fluctuations is to provide the monopoles with kinetic energy. We study the effect of adding quantum dynamics to spin ice, a well-known classical spin liquid, with a particular view of how to best detect its presence in experiment. For the weakly diluted quantum spin ice, we find a particularly crisp phenomenon, namely, the emergence of hydrogenic excited states in which a magnetic monopole is bound to a vacancy at various distances.
Proteins, fluctuations and complexity
Frauenfelder, Hans; Chen, Guo; Fenimore, Paul W
2008-01-01
Glasses, supercooled liquids, and proteins share common properties, in particular the existence of two different types of fluctuations, {alpha} and {beta}. While the effect of the {alpha} fluctuations on proteins has been known for a few years, the effect of {beta} fluctuations has not been understood. By comparing neutron scattering data on the protein myoglobin with the {beta} fluctuations in the hydration shell measured by dielectric spectroscopy we show that the internal protein motions are slaved to these fluctuations. We also show that there is no 'dynamic transition' in proteins near 200 K. The rapid increase in the mean square displacement with temperature in many neutron scattering experiments is quantitatively predicted by the {beta} fluctuations in the hydration shell.
Thermal field fluctuations in a magnetic tip / implications for magnetic resonance force microscopy
NASA Astrophysics Data System (ADS)
Hannay, J. D.; Chantrell, R. W.; Rugar, D.
2000-05-01
Thermally excited magnetic fluctuations are fundamental to the behavior of small ferromagnetic particles and have practical consequences for the proposed detection of individual spins by magnetic resonance force microscopy (MRFM). In particular, fluctuating fields from a nearby magnetic tip can increase the relaxation rate of spins in a sample if there is significant spectral density of field fluctuation at the Larmor frequency of the target spin. As an initial step toward understanding this issue, magnetic field fluctuations have been simulated which emanate from a magnetic tip with dimensions 60 nm×60 nm×2 μm. It was found that the fluctuations in a cobalt magnetic tip were too strong for MRFM experiments aimed at detecting individual electron spins. However, the results obtained for a PrFeB tip fell within the tolerance required.
Protecting a Solid-State Spin from Decoherence Using Dressed Spin States
NASA Astrophysics Data System (ADS)
Golter, D. Andrew; Baldwin, Thomas K.; Wang, Hailin
2014-12-01
We report experimental studies of dressing an electron spin in diamond with resonant and continuous microwave fields to protect the electron spin from magnetic fluctuations induced by the nuclear spin bath. We use optical coherent population trapping (CPT) to probe the energy level structure, optically induced spin transitions, and spin decoherence rates of the dressed spin states. Dressing an electron spin with resonant microwaves at a coupling rate near 1 MHz leads to a 50 times reduction in the linewidth of the spin transition underlying the CPT process, limited by transit-time broadening. Compared with dynamical decoupling, where effects of the bath are averaged out at specific times, the dressed spin state provides a continuous protection from decoherence.
Feedback control of nuclear spin bath for a single hole spin in a quantum dot
NASA Astrophysics Data System (ADS)
Pang, Hongliang; Gong, Zhirui; Yao, Wang
2014-03-01
In a semiconductor quantum dot, the nuclear spin bath plays an important role as the ultimate environment of an electron or hole spin at low temperature. Through dynamic nuclear spin polarization driven by an oscillating electric field, we show that feedback controls can be implemented on the nuclear spin bath of a single hole spin. The feedback controls utilize the anisotropic hyperfine interaction between the hole spin and the nuclear spins. The negative feedback can suppress the statistical fluctuations of the nuclear hyperfine field and lead to longer coherence time of the hole spin. Positive feedback can possibly lead to cat like state of nuclear spin bath. The efficiency of the controls schemes is investigated under different parameters and control strategies. The work is supported by the Croucher Foundation under the Croucher Innovation Award, and the Research Grant Council of Hong Kong (HKU706309P, HKU8/CRF/11G).
Electrical control of single spin dynamics
NASA Astrophysics Data System (ADS)
Petta, Jason
2012-02-01
Over ten years ago, Daniel Loss and David DiVincenzo proposed using the spin of a single electron as a quantum bit. At the time of the proposal, it was not possible to trap a single electron in a device and measure its spin, let alone demonstrate control of quantum coherence. In this talk I will describe recent progress in the field, focusing on two new methods for single spin control that have been developed by my group at Princeton. The first method is based on quantum interference and implements spin-interferometry on a chip. The second method utilizes the strong spin-orbit coupling of InAs. By shifting the orbital position of the electronic wavefunction at gigahertz frequencies, we can control the orientation of a single electron spin and measure the full g-tensor, which exhibits a large anisotropy due to spin-orbit interactions. Both methods for single spin control are orders of magnitude faster than conventional electron spin resonance and allow investigations of single spin coherence in the presence of fluctuating nuclear and spin-orbit fields. I will also describe recent efforts to transfer these methods to silicon quantum dots, where the effects of fluctuating nuclear fields are much smaller.
Hirano, Masanori; Yamada, Yuji; Saito, Taku; Nagashima, Ryo; Konishi, Takehisa; Toriyama, Tatsuya; Ohta, Yukinori; Fukazawa, Hideto; Kohori, Yoh; Furukawa, Yuji; Kihou, Kunihiro; Lee, Chul-Ho; Eisaki, Hiroshi
2012-04-12
We have performed 75As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on single-crystalline Ba1-xKxFe2As2 for x = 0.27–1. 75As nuclear quadruple resonance frequency (νQ) increases linearly with increasing x. The Knight shift K in the normal state shows Pauli paramagnetic behavior with a weak temperature T dependence. K increases gradually with increasing x. By contrast, the nuclear spin–lattice relaxation rate 1/T1 in the normal state has a strong T dependence, which indicates the existence of large antiferomagnetic (AF) spin fluctuations for all x's. The T dependence of 1/T1 shows a gaplike behavior below approximately 100 K for 0.6 < x < 0.9. This behaviors is well explained by the change in the band structure with the expansion of hole Fermi surfaces and the shrinkage and disappearance of electron Fermi surfaces at the Brillouin zone (BZ) with increasing x. The anisotropy of 1/T1, represented by the ratio of 1/T1ab to 1/T1c, is always larger than 1 for all x's, which indicates that stripe-type AF fluctuations are dominant in this system. The K in the superconducting (SC) state decreases, which corresponds to the appearance of spin-singlet superconductivity. The T dependence of 1/T1 in the SC state indicates a multiple-SC-gap feature. A simple two-gap model analysis shows that the larger superconducting gap gradually decreases with increasing x from 0.27 to 1 and a smaller gap decreases rapidly and nearly vanishes for x > 0.6 where electron pockets in BZ disappear.
Frustration and Fluctuations in Systems with Quenched Disorder
NASA Astrophysics Data System (ADS)
Stein, D. L.
As Phil Anderson noted long ago, frustration can be generally defined by measuring the fluctuations in the coupling energy across a plane boundary between two large blocks of material. Since that time, a number of groups have studied the free energy fluctuations between (putative) distinct spin glass thermodynamic states. While upper bounds on such fluctuations have been obtained, useful lower bounds have been more difficult to derive. I present a history of these efforts, and briefly discuss recent work showing that free energy fluctuations between certain classes of distinct thermodynamic states (if they exist) scale as the square root of the volume. The perspective offered here is that the power and generality of the Anderson conception of frustration suggests a potential approach toward resolving some longstanding and central issues in spin glass physics.
Microfluidic Pumps Containing Teflon [Trademark] AF Diaphragms
NASA Technical Reports Server (NTRS)
Willis, Peter; White, Victor; Grunthaner, Frank; Ikeda, Mike; Mathies, Richard A.
2009-01-01
Microfluidic pumps and valves based on pneumatically actuated diaphragms made of Teflon AF polymers are being developed for incorporation into laboratory-on-a-chip devices that must perform well over temperature ranges wider than those of prior diaphragm-based microfluidic pumps and valves. Other potential applications include implanted biomedical microfluidic devices, wherein the biocompatability of Teflon AF polymers would be highly advantageous. These pumps and valves have been demonstrated to function stably after cycling through temperatures from -125 to 120 C. These pumps and valves are intended to be successors to similar prior pumps and valves containing diaphragms made of polydimethylsiloxane (PDMS) [commonly known as silicone rubber]. The PDMS-containing valves ae designed to function stably only within the temperature range from 5 to 80 C. Undesirably, PDMS membranes are somwehat porous and retain water. PDMS is especially unsuitable for use at temperatures below 0 C because the formation of ice crystals increases porosity and introduces microshear.
Cross-correlation spin noise spectroscopy of heterogeneous interacting spin systems
Roy, Dibyendu; Yang, Luyi; Crooker, Scott A.; Sinitsyn, Nikolai A.
2015-04-30
Interacting multi-component spin systems are ubiquitous in nature and in the laboratory. As such, investigations of inter-species spin interactions are of vital importance. Traditionally, they are studied by experimental methods that are necessarily perturbative: e.g., by intentionally polarizing or depolarizing one spin species while detecting the response of the other(s). Here, we describe and demonstrate an alternative approach based on multi-probe spin noise spectroscopy, which can reveal inter-species spin interactions - under conditions of strict thermal equilibrium - by detecting and cross-correlating the stochastic fluctuation signals exhibited by each of the constituent spin species. Specifically, we consider a two-component spinmore » ensemble that interacts via exchange coupling, and we determine cross-correlations between their intrinsic spin fluctuations. The model is experimentally confirmed using “two-color” optical spin noise spectroscopy on a mixture of interacting Rb and Cs vapors. Noise correlations directly reveal the presence of inter-species spin exchange, without ever perturbing the system away from thermal equilibrium. These non-invasive and noise-based techniques should be generally applicable to any heterogeneous spin system in which the fluctuations of the constituent components are detectable.« less
The Spin-flop Transition in Antiferromagnetic Superlattices
NASA Astrophysics Data System (ADS)
Te Velthuis, S. G. E.; Jiang, J. S.; Bader, S. D.; Felcher, G. P.
2002-03-01
An antiferromagnetically (AF) coupled Fe/Cr(211) superlattice with uniaxial magnetic anisotropy has been used to study the spin-flop transition in an AF with a finite number of layers. It has been predicted that, at a field a lower than the bulk spin-flop field, a domain wall is created at the surface and rapidly propagates toward the center of the sublattice^1. We present extensive polarized neutron reflectivity measurements that give the evolution of the magnetic configuration during the spin-flop transition and prove directly the existence of such a state, in which the superlattice splits in two anti-phase, AF domains. Magneto-optic Kerr measurements with the field tilted from the easy axis show that the spin-flop is stable over a finite angular region. In contrast to the situation for a bulk AF, the first-order nature of the spin-flop transition is preserved off-axis, but we report that the detailed character of the transition is altered. ^1R.W. Wang, D.L. Mills, Eric E. Fullerton, J.E. Mattson, and S.D. Bader, Phys. Rev. Lett. 72 (1994) 920.
Ciszewski, Pawel; Tyczka, Joanna; Nadolski, Jacek; Roszak, Magdalena; Dyszkiewicz, Wojciech
2013-01-01
OBJECTIVES The following study presents a special independent atrial fibrillation (AF) risk factor—preoperative fluctuation of heart rate variability (HRV), as well as other perioperative AF risk factors in patients qualified for pneumonectomy and undergoing pneumonectomy or lobectomy for lung cancer. METHODS The prospective study was performed in patients who had undergone anatomical resection for non-small-cell lung cancer. A total of 117 patients (92 men and 25 women) qualified for statistical research. In order to determine the risk factors, all patients were divided into two groups: Group A—98 patients without AF and Group B—19 patients with AF during the perioperative time. A number of different risk factors of AF have been analysed and further divided into preoperative, operative and postoperative. RESULTS Postoperative AF occurred in 19 patients (16%), all of them were male. The patients with higher short-term HRV parameters (SD1, RMSSD), slower mean heart rate and those with a lower fluctuation of HRV-related parameters (HRV Afternoon, Night, Day (A/N/D)) before the operation, were more prone to AF. Postoperative risk of AF was higher in patients with a higher number of ventricular ectopic beats before the operation, a higher number of supraventricular and ventricular ectopic beats and a higher maximal heart rate after the operation. Statistical analysis revealed that male gender and the extent of pulmonary resection, particularly left pneumonectomy, constituted significant risk factors. AF was more often observed in patients who had ASA physical status score of III, in comparison with ASAI and ASAII patients. CONCLUSIONS Along with other concomitant AF risk factors presented in this work, the evaluation of the fluctuation tendencies of HRV parameters should be taken into consideration before any major lung resection. The balance disturbance between the sympathetic and parasympathetic nervous systems is responsible for AF. PMID:23832838
Why Cr needs a spin-density wave to become antiferromagnetic.
NASA Astrophysics Data System (ADS)
Marcus, P. M.; Moruzzi, V. L.; Qiu, S. L.
1998-03-01
First-principles, total-energy calculations on bcc Cr show that at the volume of the energy minimum Cr is nonmagnetic, i.e., the type-I antiferromagnetic (AF) phase does not exist for bcc Cr. However 0.3% expansion of the lattice constant gives a 2nd-order phase transition to the AF phase with a rapidly growing magnetic moment at the cost of a small amount of strain energy.( Marcus, Qiu, Moruzzi, submitted to Phys. Rev. B.) The AF spin-density wave (AF-SDW) modulates the moment of the AF phase over 20 lattice constants to reduce the energy, hence compensate the strain energy. (K. Hirai, J. Phys. Soc. Jpn. 66), 560 (1997). Further arguments to support this lattice-expansion theory of the AF-SDW come from a bulk modulus in agreement with experiment and from the effects of increasing the electron density, which agree with alloy behavior.
Fluctuations in nuclear fragmentation
Aranda, A.; Dorso, C.O.; Furci, V.; Lopez, J.A.
1995-12-01
Heavy ion collisions can be used to study the thermodynamics of hot and dense nuclear matter only if the initial mass and energy fluctuations that lead to fragmentation are of thermal origin and survive the disassembly process. If this is the case, the observed fragment multiplicity should be directly related to those initial fluctuations and to the conditions of temperature and density causing them. The feasibility of this scenario is demonstrated with a molecular dynamics study of the evolution of mass and energy fluctuations, and fluctuations of the phase-space density. First, it is verified that the fluctuations leading to fragmentation are indeed early ones. Second, it is determined that different initial conditions of density and temperature can indeed produce varying final fragment multiplicities. The {rho}-{ital T} plane is mapped to the fragment multiplicity with good precision. This mapping should be easily reproducible with existing experimental data.
Hadronic Correlations and Fluctuations
Koch, Volker
2008-10-09
We will provide a review of some of the physics which can be addressed by studying fluctuations and correlations in heavy ion collisions. We will discuss Lattice QCD results on fluctuations and correlations and will put them into context with observables which have been measured in heavy-ion collisions. Special attention will be given to the QCD critical point and the first order co-existence region, and we will discuss how the measurement of fluctuations and correlations can help in an experimental search for non-trivial structures in the QCD phase diagram.
NASA Astrophysics Data System (ADS)
Pérez-Espigares, Carlos; Redig, Frank; Giardinà, Cristian
2015-08-01
For non-equilibrium systems of interacting particles and for interacting diffusions in d-dimensions, a novel fluctuation relation is derived. The theorem establishes a quantitative relation between the probabilities of observing two current values in different spatial directions. The result is a consequence of spatial symmetries of the microscopic dynamics, generalizing in this way the Gallavotti-Cohen fluctuation theorem related to the time-reversal symmetry. This new perspective opens up the possibility of direct experimental measurements of fluctuation relations of vectorial observables.
Precrystallisation fluctuation phenomena in homopolymer melts.
NASA Astrophysics Data System (ADS)
Ryan, A. J.; Fairclough, J. P. A.; Terrill, N. J.; Young, R. J.; Towns-Andrews, E.; Komanschek, B. U.
1997-03-01
In order to separate nucleation from growth, two types of experiments have been performed on polypropylene, polyethylene, polyethylene oxide and polyethylene terepthalate. Rapid crystallisations were studied by melt extrusion of a tape. The extrusion of tape is a steady-state process where the distance down the spin-line is directly proportional to the crystallisation time. This allowed long data collection times (minutes) for very short crystallisation times. Prior to the development of crystallinity, well resolved, oriented small-angle patterns could be collected with length scales (50-200Åand intensities that grew down the spin-line. The orientation of the patterns was caused by the coupling of the density fluctuations with the elongational flow-field. Slow crystallisations with long induction times have been studied by simultaneous SAXS and WAXS. Clear development of small angle scattering, due to the density fluctuations, with a characteristic length scale of ≈ 100Åwas observed prior to the presence of crystals identified by wide-angle scattering. The growth of these fluctuations is analysed in terms of Cahn Hilliard kinetics. We could estimate the both the dominant length scale and the effective diffusion coefficient and by conducting experiments at a series of temperature we could find the stability limit.
Fluctuations In Electrohydrodynamic Instability
NASA Astrophysics Data System (ADS)
Bianco, Francesco; Lucchesi, Mauro; Capaccioli, Simone; Fronzoni, Leone; Allegrini, Paolo
2005-11-01
Electrohydrodynamic Convection in Liquid Crystals (EHC) is a good system for the experimental study of spatio-temporal chaos. Particularly interesting is the behavior of the Nematic in presence of weak turbulence where ordered and disordered states are mixed. In this case, the fluctuations of velocity and electric current, for instance, are typical fluctuations of a system far from equilibrium. Recently some authors have analyzed the amplitude of the fluctuations as function of the applied electric field and they present interesting interpretations provided by some theories. Although important results have been obtained by these authors, many aspects of the dynamical behavior have to be further analyzed as the role of some localized coherences inside the turbulence regions. The direct optical observation allows us to make a correspondence between fluctuations and patterns, providing important information for a theoretical interpretation.
Fluctuations in Photosynthesis
NASA Astrophysics Data System (ADS)
Khajeh, Ramin; Nishikida, Dean; Haberstroh, John; Geissler, Phillip L.
2015-03-01
The dynamics of the energy gap fluctuations of chromophores in Fenna-Matthews-Olson (FMO) complex can lead to an understanding of the underlying mechanism which is responsible for an efficient exciton energy transfer in such photosynthetic structures. Using Molecular Dynamics simulation results, we investigate trajectory statistics of energy gap fluctuations in chromophores using methods of propagators and Fourier coefficient distributions and examine possible anharmonic signatures in their behavior. Berkeley Lab - Material Science Division.
Direct measurement of antiferromagnetic domain fluctuations.
Shpyrko, O G; Isaacs, E D; Logan, J M; Feng, Yejun; Aeppli, G; Jaramillo, R; Kim, H C; Rosenbaum, T F; Zschack, P; Sprung, M; Narayanan, S; Sandy, A R
2007-05-01
Measurements of magnetic noise emanating from ferromagnets owing to domain motion were first carried out nearly 100 years ago, and have underpinned much science and technology. Antiferromagnets, which carry no net external magnetic dipole moment, yet have a periodic arrangement of the electron spins extending over macroscopic distances, should also display magnetic noise. However, this must be sampled at spatial wavelengths of the order of several interatomic spacings, rather than the macroscopic scales characteristic of ferromagnets. Here we present a direct measurement of the fluctuations in the nanometre-scale superstructure of spin- and charge-density waves associated with antiferromagnetism in elemental chromium. The technique used is X-ray photon correlation spectroscopy, where coherent X-ray diffraction produces a speckle pattern that serves as a 'fingerprint' of a particular magnetic domain configuration. The temporal evolution of the patterns corresponds to domain walls advancing and retreating over micrometre distances. This work demonstrates a useful measurement tool for antiferromagnetic domain wall engineering, but also reveals a fundamental finding about spin dynamics in the simplest antiferromagnet: although the domain wall motion is thermally activated at temperatures above 100 K, it is not so at lower temperatures, and indeed has a rate that saturates at a finite value-consistent with quantum fluctuations-on cooling below 40 K. PMID:17476263
Scaling metabolic rate fluctuations.
Labra, Fabio A; Marquet, Pablo A; Bozinovic, Francisco
2007-06-26
Complex ecological and economic systems show fluctuations in macroscopic quantities such as exchange rates, size of companies or populations that follow non-Gaussian tent-shaped probability distributions of growth rates with power-law decay, which suggests that fluctuations in complex systems may be governed by universal mechanisms, independent of particular details and idiosyncrasies. We propose here that metabolic rate within individual organisms may be considered as an example of an emergent property of a complex system and test the hypothesis that the probability distribution of fluctuations in the metabolic rate of individuals has a "universal" form regardless of body size or taxonomic affiliation. We examined data from 71 individuals belonging to 25 vertebrate species (birds, mammals, and lizards). We report three main results. First, for all these individuals and species, the distribution of metabolic rate fluctuations follows a tent-shaped distribution with power-law decay. Second, the standard deviation of metabolic rate fluctuations decays as a power-law function of both average metabolic rate and body mass, with exponents -0.352 and -1/4 respectively. Finally, we find that the distributions of metabolic rate fluctuations for different organisms can all be rescaled to a single parent distribution, supporting the existence of general principles underlying the structure and functioning of individual organisms. PMID:17578913
Helical Spin Order from Topological Dirac and Weyl Semimetals
NASA Astrophysics Data System (ADS)
Sun, Xiao-Qi; Zhang, Shou-Cheng; Wang, Zhong
2015-08-01
We study dynamical mass generation and the resultant helical spin orders in topological Dirac and Weyl semimetals, including the edge states of quantum spin Hall insulators, the surface states of weak topological insulators, and the bulk materials of Weyl semimetals. In particular, the helical spin textures of Weyl semimetals manifest the spin-momentum locking of Weyl fermions in a visible manner. The spin-wave fluctuations of the helical order carry electric charge density; therefore, the spin textures can be electrically controlled in a simple and predictable manner.
Drift transport of helical spin coherence with tailored spin–orbit interactions
Kunihashi, Y.; Sanada, H.; Gotoh, H.; Onomitsu, K.; Kohda, M.; Nitta, J.; Sogawa, T.
2016-01-01
Most future information processing techniques using electron spins in non-magnetic semiconductors will require both the manipulation and transfer of spins without their coherence being lost. The spin–orbit effective magnetic field induced by drifting electrons enables us to rotate the electron spins in the absence of an external magnetic field. However, the fluctuations in the effective magnetic field originating from the random scattering of electrons also cause undesirable spin decoherence, which limits the length scale of the spin transport. Here we demonstrate the drift transport of electron spins adjusted to a robust spin structure, namely a persistent spin helix. We find that the persistent spin helix enhances the spatial coherence of drifting spins, resulting in maximized spin decay length near the persistent spin helix condition. Within the enhanced distance of the spin transport, the transport path of electron spins can be modulated by employing time-varying in-plane voltages. PMID:26952129
Merodio, P.; Ghosh, A.; Lemonias, C.; Gautier, E.; Ebels, U.; Chshiev, M.; Béa, H. E-mail: helene.bea@cea.fr; Baltz, V. E-mail: helene.bea@cea.fr
2014-01-20
Spintronics relies on the spin dependent transport properties of ferromagnets (Fs). Although antiferromagnets (AFs) are used for their magnetic properties only, some fundamental F-spintronics phenomena like spin transfer torque, domain wall motion, and tunnel anisotropic magnetoresistance also occur with AFs, thus making AF-spintronics attractive. Here, room temperature critical depths and absorption mechanisms of spin currents in Ir{sub 20}Mn{sub 80} and Fe{sub 50}Mn{sub 50} are determined by F-resonance and spin pumping. In particular, we find room temperature critical depths originating from different absorption mechanisms: dephasing for Ir{sub 20}Mn{sub 80} and spin flipping for Fe{sub 50}Mn{sub 50}.
Quadrupolar Spin Orders in FeSe
NASA Astrophysics Data System (ADS)
Wang, Zhentao; Nevidomskyy, Andriy
Motivated by the absence of long-range magnetic order and the strong spin fluctuations observed in the Fe-based superconductor FeSe, we study spin-1 model on a square lattice up to next-nearest neighbor Heisenberg and biquadratic spin exchanges. The zero-temperature variational phase diagram gives the conventional antiferromagnetic order and also more exotic quadrupolar spin phases. These quadrupolar phases do not host long-range magnetic order and preserve time-reversal symmetry, but break the spin SU(2) symmetry. In particular, we observe a robust ferroquadrupolar order (FQ) in immediate proximity to the columnar AFM phase. We envision that FeSe may be positioned within the FQ phase close to the phase boundary. Using the flavor-wave technique, we calculate the structure factor inside the FQ phase and find a Goldstone mode emerging from Q = (0 , 0) , which however bears zero spectral weight at ω = 0 due to time reversal symmetry. At the same time, we observe strong spin fluctuations near (π , 0) / (0 , π) , which agrees with the recent neutron scattering experiments. Further, we calculate the higher order interactions between the (π , 0) and (0 , π) spin fluctuations inside the FQ phase, which may shed light on the C4 symmetry breaking in the nematic phase of FeSe.
Fluctuations in the shape transitions of hot nuclei
NASA Astrophysics Data System (ADS)
Pacheco, J. M.; Yannouleas, C.; Broglia, R. A.
1988-07-01
The effect of quantal and thermal quadrupole shape fluctuations in the giant dipole response function of hot nuclei at high spin is studied within the Landau theory of phase transitions. The effects are found to be important in the relation of the nuclear shape to the experimental findings and in the identification of shape phase transitions.