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Sample records for reentrant spin glass

  1. Reentrant spin glass ordering in an Fe-based bulk metallic glass

    SciTech Connect

    Luo, Qiang; Shen, Jun

    2015-02-07

    We report the results of the complex susceptibility, temperature, and field dependence of DC magnetization and the nonequilibrium dynamics of a bulk metallic glass Fe{sub 40}Co{sub 8}Cr{sub 15}Mo{sub 14}C{sub 15}B{sub 6}Er{sub 2}. Solid indication of the coexistence of reentrant spin glass (SG) and ferromagnetic orderings is determined from both DC magnetization and AC susceptibility under different DC fields. Dynamics scaling of AC susceptibility indicates critical slowing down to a reentrant SG state with a static transition temperature T{sub s} = ∼17.8 K and a dynamic exponent zv = ∼7.3. The SG nature is further corroborated from chaos and memory effects, magnetic hysteresis, and aging behavior. We discuss the results in terms of the competition among random magnetic anisotropy and exchange interactions and compare them with simulation predictions.

  2. Reentrant spin-glass state in a geometrical frustrated multiferroic system: Role of disorder

    SciTech Connect

    Chattopadhyay, S.; Giri, S.; Majumdar, S.; Ganesan, V.; Venkateshwarlu, D.

    2014-05-07

    We investigated the effect of magnetic (Mn) and nonmagnetic (Ga) doping at the Cr site of the layered geometrically frustrated antiferromagnetic compound LiCrO{sub 2}. 10% Ga doping at the Cr site does not invoke any metastability typical of a glassy magnetic state. However, similar amount of Mn doping drives the system to a spin glass (SG) state which is particularly evident from the magnetic memory and heat capacity studies. The onset of glassy state in 10% Mn doped sample is of reentrant type developing out of higher temperature antiferromagnetic state. The SG state in the Mn-doped sample shows a true reentry with the complete disappearance of the antiferromagnetic phase below the SG transition. The lack of SG state on Ga doping indicates the importance of random ferromagnetic/antiferromagnetic bonds for the glassy ground state in LiCrO{sub 2}.

  3. Scaling and memory effects in the reentrant spin glass phase of nanostructured MnxTaS2

    NASA Astrophysics Data System (ADS)

    Shand, Paul; Danker, John; Xiao, Xun; Kidd, Tim; Strauss, Laura

    2015-03-01

    We have investigated the nature of the reentrant spin glass phase of nanostructured Mn-intercalated TaS2. The sample consisted of bundles of nanoscale fibers with an average atomic concentration of intercalated Mn of 22%. The sample exhibits a ferromagnetic transition at 74 K and a transition to a cluster glass state at 40 K. The ac susceptibility measured in small dc bias fields near the cluster glass transition exhibited scaling behavior, indicating a magnetic-field dependent crossover to glassy dynamics. At temperatures below the cluster-glass transition, the nature of the dynamics was probed by ac susceptibility and zero-field cooled (ZFC) magnetization measurements. Aging and memory effects were observed, consistent with the non-equilibrium dynamics exhibited by glassy magnetic systems. In particular, we probed the ZFC magnetization memory effect as a function of cooling rate, aging time and magnetic field. The behavior is explained in terms of domain growth within the framework of droplet theory. Supported by National Science Foundation Grant No. DMR 1206530.

  4. Transversal spin freezing and re-entrant spin glass phases in chemically disordered Fe-containing perovskite multiferroics

    NASA Astrophysics Data System (ADS)

    Stephanovich, V. A.; Laguta, V. V.

    We propose experimental verification and theoretical explanation of magnetic anomalies in the complex Fe-contained double perovskite multiferroics like PbFe$_{1/2}$Nb$_{1/2}$O$_3$. The theoretical part is based on our model of coexistence of long-range magnetic order and spin glass in the above substances. In our model, the exchange interaction is anisotropic, coupling antiferromagnetically $z$ spin components of Fe$^{3+}$ ions. At the same time, the $xy$ components are coupled by much weaker exchange interaction of ferromagnetic sign. In the system with spatial disorder (half of corresponding lattice cites are occupied by spinless Nb$^{5+}$ ions) such frustrating interaction results in the fact that antiferromagnetic order is formed by $z$ projection of the spins, while their $xy$ components contribute to spin glass behaviour. Our theoretical findings are supported by the experimental evidence of coexistence of antiferromagnetic and spin glass phases in chemically disordered Fe-contained double perovskite multiferroics.

  5. Transversal spin freezing and re-entrant spin glass phases in chemically disordered Fe-containing perovskite multiferroics.

    PubMed

    Stephanovich, V A; Laguta, V V

    2016-03-14

    We propose experimental verification and theoretical explanation of magnetic anomalies in the complex Fe-containing perovskite multiferroics like PbFe1/2Nb1/2O3 and PbFe1/2Ta1/2O3. The theoretical part is based on our model of coexistence of the long-range magnetic order and spin glass in the above compounds. In our model, the exchange interaction is anisotropic, coupling antiferromagnetically z spin components of Fe(3+) ions. At the same time, the xy components are coupled by much weaker exchange interaction of ferromagnetic sign. In the system with spatial disorder (half of the corresponding lattice sites are occupied by spinless Nb(5+) ions) such frustrating interaction results in the fact that the antiferromagnetic order is formed by the z projection of the spins, while their xy components contribute to spin glass behaviour. Our theoretical findings are supported by the experimental evidence of such a coexistence of antiferromagnetic and spin glass phases in chemically disordered Fe-containing complex perovskite multiferroics. PMID:26890550

  6. Reentrant spin-glass behavior and bipolar exchange-bias effect in "Sn" substituted cobalt-orthotitanate

    NASA Astrophysics Data System (ADS)

    Nayak, S.; Joshi, D. C.; Krautz, M.; Waske, A.; Eckert, J.; Thota, S.

    2016-01-01

    We report the co-existence of longitudinal ferrimagnetic behavior with Néel temperature TN ˜ 46.1 K and reentrant transverse spin-glass state at 44.05 K in Tin (Sn) doped cobalt-orthotitanate (Co2TiO4). The ferrimagnetic ordering is resulting from different magnetic moments of Co2+ on the A-sites (3.87 μB) and B-sites (5.069 μB). The magnetic compensation temperature (TCOMP) shifts from 31.74 K to 27.1 K when 40 at. % of "Sn4+" substitutes "Ti4+" at B-sites where the bulk-magnetization of two-sublattices balance each other. For T > TN, the dc-magnetic susceptibility (χdc = M/Hdc) fits well with the Néel's expression for the two-sublattice model with antiferromagnetic molecular field constants NBB ˜ 15.44, NAB ˜ 32.01, and NAA ˜ 20.88. The frequency dependence of ac-magnetic susceptibility χac data follows the Vogel-Fulcher law, and the power-law of critical slowing-down with "zν" = 6.01 suggests the existence of spin-clusters (where "z" and "ν" being dynamic critical-exponent and correlation length of critical-exponent, respectively). This system exhibits unusual hysteresis loops with large bipolar exchange-bias effect (HEB ˜ 13.6 kOe at 7 K) after zero-field cooling process from an un-magnetized state, and a dramatic collapse of remanence (MR) and coercive field (HC) across TCOMP. The possible origins of such anomalous characteristics were discussed.

  7. Experimental evidence of the formation of a reentrant spin-glass phase in alloying two ferromagnetic CoPt3 and MnPt3 compounds

    NASA Astrophysics Data System (ADS)

    Kim, T. H.; Cadeville, M. C.; Dinia, A.; Rakoto, H.

    1996-01-01

    The magnetic properties and the magnetic phase diagram of the pseudobinary Co1-xMnxPt3 system resulting from alloying two ferromagnetic CoPt3 and MnPt3 compounds have been investigated. Structural and microstructural characterizations added to thorough preparation conditions yield well-ordered samples having the L12 structure, large ordered domains, and a long-range order parameter close to its maximum value. Magnetic properties including ac and dc susceptibilities, zero-field-cooled and field-cooled magnetizations as a function of temperature and field (0-35 T) have been investigated. The resulting magnetic phase diagram displays, below a ferromagnetic region spreading over the whole (0<=x<=1) concentration range, a reentrant spin-glass phase for 0spin-glass orderings. This phase diagram is well described in the theoretical infinite-range model of Gabay and Toulouse. The randomness of the average exchange interaction is suggested to arise from competing magnetic interactions between the positive values of J2ndCoCo and J2ndMnMn and the negative value of J2ndCoMn for 3d atoms in position of second-nearest neighbors.

  8. Spin freezing process in a reentrant ferromagnet studied by neutron depolarization analysis

    SciTech Connect

    Sato, T.; Shinohara, T.; Ogawa, T.; Takeda, M.

    2004-10-01

    The spin freezing process and the magnetic nature of reentrant spin-glass (RSG) and the ferromagnetic (FM) phases of a typical reentrant ferromagnet Ni{sub 78}Mn{sub 22} were investigated based on neutron depolarization analysis, and the results were compared with the previous Moessbauer measurements [Phys. Rev. B 64, 184432 (2001)]. The wavelength-dependent polarization, under a field cooled (FC) condition, showed the damped oscillatory behavior in both the RSG and FM phases, except in the temperature region just above the RSG temperature T{sub RSG}{approx}60 K. At a temperature of around 80 K, however, it showed a double oscillatory behavior. The field integral I, which is proportional to the mean local magnetic induction, was deduced as a function of the temperature. Two branches of temperature-dependent field integrals were found: a low-temperature I{sub low}-branch, which has a small value of I, stopped at a temperature below the Curie temperature T{sub C}{approx}160 K, and a high temperature I{sub high}-branch, which has a large value of I, appeared just below 80 K. This means that there are two kinds of magnetic environments, and they have different values of magnetization. This is consistent with the observation of the double peak spectrum of the hyperfine field in the previous Moessbauer measurements. The present neutron data and the Moessbauer data can be interpreted along a scenario of reentrant behavior, which consists of the low-temperature spin canting state and the ''melting of frustrated spins'' mechanism introduced by Saslow and Parker [Phys. Rev. Lett. 56, 1074 (1986)], except for the absence of the observation of singularity in the temperature-dependent magnetization. Based on such considerations, we constructed a comprehensive picture of the spin freezing process and the magnetic nature of the RSG and FM phases in the reentrant ferromagnet.

  9. On the dynamics of the reentrant spin-glass state of the Fe{sub 100−x}Zr{sub x} (x = 9, 10) amorphous alloys

    SciTech Connect

    Ribeiro, P. R. T.; Machado, F. L. A.; Dahlberg, E. Dan

    2015-05-07

    Low temperature magnetization (M) and ac susceptibility (χ{sub ac}) data were used to investigate the reentrant spin-glass state in lengths of melt-spun ribbons of Fe{sub 100−x}Zr{sub x} (x = 9, 10) amorphous alloys. The temperature range investigated was from 2 to 300 K, while the applied magnetic field H was varied in the range of ±85 kOe. The magnetic properties were found to be strongly influenced by the sample composition despite the fact that the amount of Fe varies by 1 at. %. For instance, the Curie temperature (T{sub C}) is reduced from 232.5 K to 213.0 K with decreasing Fe concentration, while M for the lower Fe concentration, measured at the highest applied magnetic field (H = 85 kOe) was nearly double the value for that of the higher; the coercivity in the ferromagnetic regime is reduced by a factor close to five when x is increased from 9 at. % to 10 at. %. The ac susceptibility measured for frequencies f in the range of 10–10{sup 4 }Hz showed a sharp drop in the magnitude of the in-phase contribution (χ{sub ac}{sup ′}) and a peak at the out-of-phase component (χ{sub ac}{sup ″}), which shifts to higher temperatures with increasing values of f. The Voguel-Fulcher law applied to the χ{sub ac}{sup ″} data yielded an activation energy E{sub a}/k{sub B} = 21.3 K (40.1 K), the glassy temperature T{sub G} = 15.5 K (38.2 K), and a relaxation time τ{sub 0}=9.1×10{sup −7} s (8.3×10{sup −7} s), for the sample with x = 10 (x = 9). A plot of χ{sub ac}{sup ″}vsχ{sub ac}{sup ′} for a broad range of T and f yielded a broad maximum near the glassy temperature T{sub G} for both sample concentrations.

  10. Dynamic scaling in spin glasses

    NASA Astrophysics Data System (ADS)

    Pappas, C.; Mezei, F.; Ehlers, G.; Manuel, P.; Campbell, I. A.

    2003-08-01

    We present neutron spin echo (NSE) results and a revisited analysis of historical data on spin glasses, which reveal a pure power-law time decay of the spin autocorrelation function s(Q,t)=S(Q,t)/S(Q) at the glass temperature Tg. The power law exponent is in excellent agreement with that calculated from dynamic and static critical exponents deduced from macroscopic susceptibility measurements made on a quite different time scale. This scaling relation involving exponents of different physical quantities determined by completely independent experimental methods is stringently verified experimentally in a spin glass. As spin glasses are a subgroup of the vast family of glassy systems also comprising structural glasses and other noncrystalline systems the observed strict critical scaling behavior is important. Above the phase transition the strikingly nonexponential relaxation, best fitted by the Ogielski (power-law times stretched exponential) function, appears as an intrinsic, homogeneous feature of spin glasses.

  11. Evidence for reentrant strain glass behavior in a ferroelastic-martensitic system Ti-Pd-V

    NASA Astrophysics Data System (ADS)

    Zhang, Xiaoming; Xu, Guizhou; Liu, Enke; Wang, Wenhong; Wu, Guangheng; Institute of Physics Team

    2015-03-01

    The concept of strain glass (SG) has recently received much attention because of its unique properties such as shape memory effect, superelasticity, and stress-tuned intelligent damping behavior. Recent in-situ TEM experiments have proved that, only local-symmetry change in the crystal structure has been observed during the SG transition, but the macroscopic symmetry or average structure is still keeping unchanged. In this presentation, we report the discovery of reentrant-strain-glass (RSG) behavior in a ferroelastic-martensitic system Ti50Pd50-xVx (x is from 6 to 12). Unlike the SG, with decreasing of temperature, the RSG system first undergoes a macroscopic martensitic transition and then the martensite variants further transforms to a frustrated glassy state below a critical temperature. The X-ray diffraction and high resolution TEM results further indicate the RSG no longer remains the average structure of the high-temperature parent phase, but rather low-temperature martensitic phase. This new discovery may open a new research field and may lead to new insights into a range of possible applications of this unique class of materials.

  12. Universality in bipartite mean field spin glasses

    NASA Astrophysics Data System (ADS)

    Genovese, Giuseppe

    2012-12-01

    In this work, we give a proof of universality with respect to the choice of the statistical distribution of the quenched noise, for mean field bipartite spin glasses. We use mainly techniques of spin glasses theory, as Guerra's interpolation and the cavity approach.

  13. Spin glasses in the nonextensive regime

    NASA Astrophysics Data System (ADS)

    Wittmann, Matthew; Young, A. P.

    2012-04-01

    Spin systems with long-range interactions are “nonextensive” if the strength of the interactions falls off sufficiently slowly with distance. It has been conjectured for ferromagnets and, more recently, for spin glasses that, everywhere in the nonextensive regime, the free energy is exactly equal to that for the infinite range model in which the characteristic strength of the interaction is independent of distance. In this paper we present the results of Monte Carlo simulations of the one-dimensional long-range spin glasses in the nonextensive regime. Using finite-size scaling, our results for the transition temperatures are consistent with this prediction. We also propose and provide numerical evidence for an analogous result for diluted long-range spin glasses in which the coordination number is finite, namely, that the transition temperature throughout the nonextensive regime is equal to that of the infinite-range model known as the Viana-Bray model.

  14. Correlated rattling-ion origins of dielectric properties in reentrant dipole glasses BaTiO{sub 3}-BiScO{sub 3}

    SciTech Connect

    Krayzman, Victor; Levin, Igor Woicik, Joseph C.; Bridges, Frank

    2015-11-09

    The local structure of the pseudo-cubic solid solution 0.6BaTiO{sub 3}-0.4BiScO{sub 3}, which exhibits reentrant dipole-glass behavior, has been determined using the Reverse Monte Carlo method to simultaneously fit (1) neutron and X-ray total scattering data (including the corresponding real-space pair-distribution functions), (2) Bi and Sc extended X-ray absorption fine structure, and (3) patterns of diffuse scattering in electron diffraction. These structural refinements revealed the multi-site probability density distributions for both Bi (14-sites) and Ti (8 sites), whereas Ba and Sc featured normal unimodal distributions. Bi atoms are displaced along both the 〈111〉 and 〈100〉 directions, while Ti atoms are shifted along 〈111〉. Correlated dynamic hopping of Bi and Ti over their corresponding split sites combined with chemical disorder is proposed as the origin of the strong frequency dispersion observed in dielectric measurements. The existence of split sites also explains the reentrant dipole-glass behavior reported for this system.

  15. Spin glasses and error-correcting codes

    NASA Technical Reports Server (NTRS)

    Belongie, M. L.

    1994-01-01

    In this article, we study a model for error-correcting codes that comes from spin glass theory and leads to both new codes and a new decoding technique. Using the theory of spin glasses, it has been proven that a simple construction yields a family of binary codes whose performance asymptotically approaches the Shannon bound for the Gaussian channel. The limit is approached as the number of information bits per codeword approaches infinity while the rate of the code approaches zero. Thus, the codes rapidly become impractical. We present simulation results that show the performance of a few manageable examples of these codes. In the correspondence that exists between spin glasses and error-correcting codes, the concept of a thermal average leads to a method of decoding that differs from the standard method of finding the most likely information sequence for a given received codeword. Whereas the standard method corresponds to calculating the thermal average at temperature zero, calculating the thermal average at a certain optimum temperature results instead in the sequence of most likely information bits. Since linear block codes and convolutional codes can be viewed as examples of spin glasses, this new decoding method can be used to decode these codes in a way that minimizes the bit error rate instead of the codeword error rate. We present simulation results that show a small improvement in bit error rate by using the thermal average technique.

  16. Dynamics of ferromagnetic spin glass: randomly canted ferromagnet versus skewed spin glass

    NASA Astrophysics Data System (ADS)

    Janutka, Andrzej

    2003-12-01

    A ferromagnetic spin glass (FSG) is one of the three isotropic and homogeneous phases of the long-range partially ordered magnets with spin and atomic disorder which are selected by symmetry (Andreev 1978 Sov. Phys.—JETP 47 411) (the others are genuine and antiferromagnetic spin glasses). The linear dynamical response to a magnetic field of two sub-phases of a FSG with drastically different dynamics, a randomly canted ferromagnet, in which the component spins create an acute angle with the summary magnetic moment, and a less-ordered skewed spin glass is analysed in the spin-wave approximation in the framework of phenomenological theory. The spin-wave damping coefficients and frequency shifts due to a magnon-magnon interaction are evaluated as functions of temperature and wavevector as well as the spectral-weight functions of the linear response to a magnetic field and the neutron scattering cross section which provides the possibility for experimental verification of the results. Substantial differences in the spin-wave characteristics of the FSG compared to those of the Heisenberg spin glass and the Heisenberg ferromagnet are found to be due to non-linear anisotropy effects in a FSG.

  17. Spin correlations near the ferromagnetic-spin glass crossover in Eu/sub x/Sr/sub 1-x/S

    SciTech Connect

    Maletta, H.; Aeppli, G.; Shapiro, S.M.

    1982-09-01

    Bulk susceptibility and high-resolution neutron-diffraction measurements have been performed on single crystals of Eu/sub x/Sr/sub 1-x/S for x = 0.52 and 0.54 near the ferromagnet (FM) - spin glass (SG) crossover. A reentrant phase is established, which is induced by random-field effects. As the temperature is reduced the FM correlation length xi passes through a resolution-limited maximum where a frustrated FM phase is observed. In the SG at lower temperature xi decreases again and a coexisting FM component can be excluded. 6 figures.

  18. Almost sure convergence in quantum spin glasses

    NASA Astrophysics Data System (ADS)

    Buzinski, David; Meckes, Elizabeth

    2015-12-01

    Recently, Keating, Linden, and Wells [Markov Processes Relat. Fields 21(3), 537-555 (2015)] showed that the density of states measure of a nearest-neighbor quantum spin glass model is approximately Gaussian when the number of particles is large. The density of states measure is the ensemble average of the empirical spectral measure of a random matrix; in this paper, we use concentration of measure and entropy techniques together with the result of Keating, Linden, and Wells to show that in fact the empirical spectral measure of such a random matrix is almost surely approximately Gaussian itself with no ensemble averaging. We also extend this result to a spherical quantum spin glass model and to the more general coupling geometries investigated by Erdős and Schröder [Math. Phys., Anal. Geom. 17(3-4), 441-464 (2014)].

  19. Almost sure convergence in quantum spin glasses

    SciTech Connect

    Buzinski, David Meckes, Elizabeth

    2015-12-15

    Recently, Keating, Linden, and Wells [Markov Processes Relat. Fields 21(3), 537-555 (2015)] showed that the density of states measure of a nearest-neighbor quantum spin glass model is approximately Gaussian when the number of particles is large. The density of states measure is the ensemble average of the empirical spectral measure of a random matrix; in this paper, we use concentration of measure and entropy techniques together with the result of Keating, Linden, and Wells to show that in fact the empirical spectral measure of such a random matrix is almost surely approximately Gaussian itself with no ensemble averaging. We also extend this result to a spherical quantum spin glass model and to the more general coupling geometries investigated by Erdős and Schröder [Math. Phys., Anal. Geom. 17(3-4), 441–464 (2014)].

  20. Nature of ordering in Potts spin glasses

    NASA Astrophysics Data System (ADS)

    Banavar, Jayanth R.; Cieplak, Marek

    1989-09-01

    The zero-temperature scaling approach is applied to the three-state Potts spin glass. Our results suggest that the +/-J model has a lower critical dimensionality greater than 3, whereas for the Gaussian model this dimensionality is slightly less than 3. The T=0 scaling exponents are estimated in D=2 and 3. The results are based mostly on exact transfer-matrix calculations and on a Monte Carlo quenching procedure.

  1. Spin Effects in Bose-Glass Phases

    NASA Astrophysics Data System (ADS)

    Paganelli, S.; ŁaÇki, M.; Ahufinger, V.; Zakrzewski, J.; Sanpera, A.

    2011-12-01

    We study the mechanism of formation of Bose glass (BG) phases in the spin-1 Bose Hubbard model when diagonal disorder is introduced. To this aim, we analyze first the phase diagram in the zero-hopping limit, there disorder induces superposition between Mott insulator (MI) phases with different filling numbers. Then BG appears as a compressible but still insulating phase. The phase diagram for finite hopping is also calculated with the Gutzwiller approximation. The bosons' spin degree of freedom introduces another scattering channel in the two-body interaction modifying the stability of MI regions with respect to the action of disorder. This leads to some peculiar phenomena such as the creation of BG of singlets, for very strong spin correlation, or the disappearance of BG phase in some particular cases where fluctuations are not able to mix different MI regions.

  2. Spin glass dynamics at the mesoscale

    NASA Astrophysics Data System (ADS)

    Guchhait, Samaresh; Kenning, Gregory G.; Orbach, Raymond L.; Rodriguez, Gilberto F.

    2015-01-01

    The mesoscale allows a new probe of spin glass dynamics. Because the spin glass lower critical dimension dl>2 , the growth of the correlation length ξ (t ,T ) can change the nature of the spin glass state at a crossover time tco when ξ (tco,T )=ℓ , a minimum characteristic sample length (e.g., film thickness for thin films and crystallite size for bulk samples). For thin films, and times t tco , a crossover to d =2 behavior takes place. The parallel correlation length, associated with a Tg=0 transition, increases in time from the saturated value of the perpendicular correlation length ℓ to an equilibrium value of the parallel correlation length proportional to T-ν. This results in a pancakelike correlated state, with a thickness ℓ and a temperature-dependent in-plane radius that increases with decreasing temperature. Activated dynamics is associated with these states. Measurements on Cu:Mn thin films are analyzed quantitatively within this framework. We extract a temperature-dependent activation energy from a fit to the frequency dependence of the dynamic susceptibility. The extrapolated temperature at which the activation energy would become large is close to the extrapolated glass transition temperature from ac susceptibility measurements. All known relevant experimental data are consistent with this approach. For polycrystalline materials, there is a distribution of length scales P (ℓ ) . For sufficiently broad distributions, a logarithmic time dependence is derived for the time decay of the thermoremanent magnetization MTRM(t ,T ) using an approach originally derived by Ma. Properties dependent upon an effective waiting time tweff are derived that are consistent with experiment, and further measurements are suggested.

  3. A spin model for strain glass

    SciTech Connect

    Lookman, Turab; Vasseur, Romain

    2009-01-01

    We demonstrate that a strain pseudo-spin model for martensitic alloys predicts a glass phase in the presence of disorder, consistent with recent experiments on binary and temary alloys that have established the existence of such a phase above a critical composition. We find that the glass phase, as characterized by the Edwards-Andersen order parameter, exists even in the absence of elastic long-range interactions which compete with the disorder to shift the glass transition to higher values of disorder. Our model predicts a second order phase transition between the martensite and strain glass phases as a function of the disorder. Together with the cusp in the susceptibility and the history dependence in the glass phase in zero-field-cooling and field-cooling curves, these predictions may be tested experimentally by varying the alloy composition. Our approach using mean-field analysis and Monte Carlo simulations may be generalized to the study of glassy behavior in more complex structural transformations in two and three dimensions.

  4. Quantum Optimization of Fully Connected Spin Glasses

    NASA Astrophysics Data System (ADS)

    Venturelli, Davide; Mandrà, Salvatore; Knysh, Sergey; O'Gorman, Bryan; Biswas, Rupak; Smelyanskiy, Vadim

    2015-07-01

    Many NP-hard problems can be seen as the task of finding a ground state of a disordered highly connected Ising spin glass. If solutions are sought by means of quantum annealing, it is often necessary to represent those graphs in the annealer's hardware by means of the graph-minor embedding technique, generating a final Hamiltonian consisting of coupled chains of ferromagnetically bound spins, whose binding energy is a free parameter. In order to investigate the effect of embedding on problems of interest, the fully connected Sherrington-Kirkpatrick model with random ±1 couplings is programmed on the D-Wave TwoTM annealer using up to 270 qubits interacting on a Chimera-type graph. We present the best embedding prescriptions for encoding the Sherrington-Kirkpatrick problem in the Chimera graph. The results indicate that the optimal choice of embedding parameters could be associated with the emergence of the spin-glass phase of the embedded problem, whose presence was previously uncertain. This optimal parameter setting allows the performance of the quantum annealer to compete with (and potentially outperform, in the absence of analog control errors) optimized simulated annealing algorithms.

  5. Low temperature structural transformations of dilute KTa1-xNbxO3: x = 0.018, quantum superparaelectric or reentrant glass scenario?

    NASA Astrophysics Data System (ADS)

    Trepakov, V. A.; Savinov, M. E.; Prosandeev, S. A.; Kapphan, S. E.; Jastrabik, L.; Boatner, L. A.

    2005-01-01

    Impurity induced structural transformations are studied for KTa1-xNbxO3 with x = 0.0018 (KTN-1.8) by low-frequency linear and non-linear dielectric spectroscopy techniques. Nb admixture leads to the appearance of a sharp (T) maximum accompanying the ferroelectric phase transition at TC 27 K, and a glass-like state formation at lower temperatures. The linear permittivity evidences properties inherent to systems being near the quantum-mechanical displacive limit and shows the long range polar order with macroscopic polarization formation at 27 K. This conclusion was proven by nonlinear polarization response stuidy under dc electric field action, which evidenced also the proximity of the three-critical point. Above 27 K, the nonlinearity increases at cooling in accordance with the Devonshire theory for the paraelectric phase of conventional ferroelectrics. Below 27 K, the random distribution of Nb5+ ions and related system's inhomogeneities provoke a new low temperature phase state formation, which can be treated as reentrant polar-glass.

  6. Finite-connectivity spin-glass phase diagrams and low-density parity check codes

    NASA Astrophysics Data System (ADS)

    Migliorini, Gabriele; Saad, David

    2006-02-01

    We obtain phase diagrams of regular and irregular finite-connectivity spin glasses. Contact is first established between properties of the phase diagram and the performance of low-density parity check (LDPC) codes within the replica symmetric (RS) ansatz. We then study the location of the dynamical and critical transition points of these systems within the one step replica symmetry breaking theory (RSB), extending similar calculations that have been performed in the past for the Bethe spin-glass problem. We observe that the location of the dynamical transition line does change within the RSB theory, in comparison with the results obtained in the RS case. For LDPC decoding of messages transmitted over the binary erasure channel we find, at zero temperature and rate R=1/4 , an RS critical transition point at pc≃0.67 while the critical RSB transition point is located at pc≃0.7450±0.0050 , to be compared with the corresponding Shannon bound 1-R . For the binary symmetric channel we show that the low temperature reentrant behavior of the dynamical transition line, observed within the RS ansatz, changes its location when the RSB ansatz is employed; the dynamical transition point occurs at higher values of the channel noise. Possible practical implications to improve the performance of the state-of-the-art error correcting codes are discussed.

  7. Heineman Prize Talk: Spin Glasses Between Mathematics and Physics

    NASA Astrophysics Data System (ADS)

    Parisi, Giorgio

    2005-03-01

    The amount of work that has been done theoretical on spin glasses is quite large and I will concentrate on some of the main results. I will start from the physical explanation of the very slow response of many disordered systems (e.g. glasses and spin glasses). I will introduce a soluble model for spin glasses, the Sherrington-Kirkpatrick model, that generalizes the local Edwards- Anderson model in infinite dimensions. The model, where each spin is connected to all other spins, should be solved by using the appropriate mean field approximation. I will show how it can be heuristically solved using algebraic or probabilistic methods. I will briefly describe the recent rigorous mathematical proof that shows that this solution is correct. Finally I will describe the results and the problems that are present in extending the mean field theory to finite dimensions.

  8. Spin glasses: redux: an updated experimental/materials survey

    NASA Astrophysics Data System (ADS)

    Mydosh, J. A.

    2015-05-01

    This article reviews the 40+ year old spin-glass field and one of its earliest model interpretations as a spin density wave. Our description is from an experimental phenomenological point of view with emphasis on new spin glass materials and their relation to topical problems and strongly correlated materials in condensed matter physics. We first simply define a spin glass (SG), give its basic ingredients and explain how the spin glasses enter into the statistical mechanics of classical phase transitions. We then consider the four basic experimental properties to solidly characterize canonical spin glass behavior and introduce the early theories and models. Here the spin density wave (SDW) concept is used to explain the difference between a short-range SDW, i.e. a SG and, in contrast, a long-range SDW, i.e. a conventional magnetic phase transition. We continue with the present state of SG, its massive computer simulations and recent proposals of chiral glasses and quantum SG. We then collect and mention the various SG ‘spin-off’s'. A major section uncovers the fashionable unconventional materials that display SG-like freezing and glassy ground states, such as (high temperature) superconductors, heavy fermions, intermetallics and Heuslers, pyrochlor and spinels, oxides and chalogenides and exotics, e.g. quasicrystals. Some conclusions and future directions complete the review.

  9. Thermodynamic glass transition in a spin glass without time-reversal symmetry

    PubMed Central

    Baños, Raquel Alvarez; Cruz, Andres; Fernandez, Luis Antonio; Gil-Narvion, Jose Miguel; Gordillo-Guerrero, Antonio; Guidetti, Marco; Iñiguez, David; Maiorano, Andrea; Marinari, Enzo; Martin-Mayor, Victor; Monforte-Garcia, Jorge; Muñoz Sudupe, Antonio; Navarro, Denis; Parisi, Giorgio; Perez-Gaviro, Sergio; Ruiz-Lorenzo, Juan Jesus; Schifano, Sebastiano Fabio; Seoane, Beatriz; Tarancon, Alfonso; Tellez, Pedro; Tripiccione, Raffaele; Yllanes, David

    2012-01-01

    Spin glasses are a longstanding model for the sluggish dynamics that appear at the glass transition. However, spin glasses differ from structural glasses in a crucial feature: they enjoy a time reversal symmetry. This symmetry can be broken by applying an external magnetic field, but embarrassingly little is known about the critical behavior of a spin glass in a field. In this context, the space dimension is crucial. Simulations are easier to interpret in a large number of dimensions, but one must work below the upper critical dimension (i.e., in d < 6) in order for results to have relevance for experiments. Here we show conclusive evidence for the presence of a phase transition in a four-dimensional spin glass in a field. Two ingredients were crucial for this achievement: massive numerical simulations were carried out on the Janus special-purpose computer, and a new and powerful finite-size scaling method. PMID:22493229

  10. Novel disordering mechanisms in dipolar spin glasses and ferromagnets

    NASA Astrophysics Data System (ADS)

    Schechter, Moshe; Andersen, Juan Carlos; Katzgraber, Helmut

    2012-02-01

    At and below the critical dimension the disordering of an ordered phase by a random field occurs via a collective effect of large domains at infinitesimal random field [Imry & Ma, Phys. Rev. Lett. 35, 1399 (1975)]. At larger space dimensions the disordering requires a large random field, of the order of the interaction energy. In a random field, the lower critical dimension is 2 for Ising ferromagnets, whereas it is infinity for spin glasses. We have generalized the Imry-Ma argument for ferromagnets with competing interactions and an underlying spin-glass phase, and for dilute dipolar spin glasses. For dilute dipolar spin glasses we have found [EPL 88, 66002 (2009)] that the broad distribution of random fields dictates more efficient disordering of the glass phase, and domain sizes which depend explicitly on the concentration, i.e., do not obey simple scaling. Here we show that as a result of a competing spin-glass phase, the disordering of the ferromagnet occurs at a finite random field, which is yet much smaller than the interactions. Our results are verified numerically, explain the recently-observed peculiar linear dependence of Tc on the random field strength [Nature 448, 567 (2007)], and predict a zero-temperature random-field driven transition between a ferromagnetic and a quasi spin glass phase.

  11. Non-perturbative effects in spin glasses.

    PubMed

    Castellana, Michele; Parisi, Giorgio

    2015-01-01

    We present a numerical study of an Ising spin glass with hierarchical interactions--the hierarchical Edwards-Anderson model with an external magnetic field (HEA). We study the model with Monte Carlo (MC) simulations in the mean-field (MF) and non-mean-field (NMF) regions corresponding to d ≥ 4 and d < 4 for the d-dimensional ferromagnetic Ising model respectively. We compare the MC results with those of a renormalization-group (RG) study where the critical fixed point is treated as a perturbation of the MF one, along the same lines as in the -expansion for the Ising model. The MC and the RG method agree in the MF region, predicting the existence of a transition and compatible values of the critical exponents. Conversely, the two approaches markedly disagree in the NMF case, where the MC data indicates a transition, while the RG analysis predicts that no perturbative critical fixed point exists. Also, the MC estimate of the critical exponent ν in the NMF region is about twice as large as its classical value, even if the analog of the system dimension is within only ~2% from its upper-critical-dimension value. Taken together, these results indicate that the transition in the NMF region is governed by strong non-perturbative effects. PMID:25733337

  12. Bond chaos in spin glasses revealed through thermal boundary conditions

    NASA Astrophysics Data System (ADS)

    Wang, Wenlong; Machta, Jonathan; Katzgraber, Helmut G.

    2016-06-01

    Spin glasses have competing interactions that lead to a rough energy landscape which is highly susceptible to small perturbations. These chaotic effects strongly affect numerical simulations and, as such, gaining a deeper understanding of chaos in spin glasses is of much importance. The use of thermal boundary conditions is an effective approach to study chaotic phenomena. Here we generalize population annealing Monte Carlo, combined with thermal boundary conditions, to study bond chaos due to small perturbations in the spin-spin couplings of the three-dimensional Edwards-Anderson Ising spin glass. We show that bond and temperature-induced chaos share the same scaling exponents and that bond chaos is stronger than temperature chaos.

  13. Efficient Cluster Algorithm for Spin Glasses in Any Space Dimension.

    PubMed

    Zhu, Zheng; Ochoa, Andrew J; Katzgraber, Helmut G

    2015-08-14

    Spin systems with frustration and disorder are notoriously difficult to study, both analytically and numerically. While the simulation of ferromagnetic statistical mechanical models benefits greatly from cluster algorithms, these accelerated dynamics methods remain elusive for generic spin-glass-like systems. Here, we present a cluster algorithm for Ising spin glasses that works in any space dimension and speeds up thermalization by at least one order of magnitude at temperatures where thermalization is typically difficult. Our isoenergetic cluster moves are based on the Houdayer cluster algorithm for two-dimensional spin glasses and lead to a speedup over conventional state-of-the-art methods that increases with the system size. We illustrate the benefits of the isoenergetic cluster moves in two and three space dimensions, as well as the nonplanar chimera topology found in the D-Wave Inc. quantum annealing machine. PMID:26317743

  14. Efficient Cluster Algorithm for Spin Glasses in Any Space Dimension

    NASA Astrophysics Data System (ADS)

    Zhu, Zheng; Ochoa, Andrew J.; Katzgraber, Helmut G.

    2015-08-01

    Spin systems with frustration and disorder are notoriously difficult to study, both analytically and numerically. While the simulation of ferromagnetic statistical mechanical models benefits greatly from cluster algorithms, these accelerated dynamics methods remain elusive for generic spin-glass-like systems. Here, we present a cluster algorithm for Ising spin glasses that works in any space dimension and speeds up thermalization by at least one order of magnitude at temperatures where thermalization is typically difficult. Our isoenergetic cluster moves are based on the Houdayer cluster algorithm for two-dimensional spin glasses and lead to a speedup over conventional state-of-the-art methods that increases with the system size. We illustrate the benefits of the isoenergetic cluster moves in two and three space dimensions, as well as the nonplanar chimera topology found in the D-Wave Inc. quantum annealing machine.

  15. Finite block pseudo-spin approach of proton glass

    NASA Astrophysics Data System (ADS)

    Lee, Kwang-Sei; Koo, Je Huan; Lee, Cheol Eui

    2016-08-01

    We herein propose an alternative phenomenology to explain the phase of proton glass by reference to finite block spin theory in magnetism, in which the phase may be considered as being a short-range ferroelectric ordering of block pseudo-spins comprised of random pseudo-spins that have a majority of individual pseudo-spins in a given sense. By making use of the Curie law of block pseudo-spins, we obtained the dielectric susceptibility for the lower and higher temperature approximations of the Brillouin function. The experimental results for the susceptibility in hydrogen-bonded mixed crystals of ferroelectric RbH2P(As)O4 and antiferroelectric NH4H2P(As)O4 were thus fitted fairly well at low temperatures in the proton glass phase whereas some deviation from our formulation was seen at high temperatures in the paraelectric phase.

  16. Strong Suppression of the Spin Hall Effect in the Spin Glass State

    NASA Astrophysics Data System (ADS)

    Niimi, Y.; Kimata, M.; Omori, Y.; Gu, B.; Ziman, T.; Maekawa, S.; Fert, A.; Otani, Y.

    2015-11-01

    We have measured spin Hall effects in spin glass metals, CuMnBi alloys, with the spin absorption method in the lateral spin valve structure. Far above the spin glass temperature Tg where the magnetic moments of Mn impurities are randomly frozen, the spin Hall angle of a CuMnBi ternary alloy is as large as that of a CuBi binary alloy. Surprisingly, however, it starts to decrease at about 4 Tg and becomes as little as 7 times smaller at 0.5 Tg. A similar tendency was also observed in anomalous Hall effects in the ternary alloys. We propose an explanation in terms of a simple model considering the relative dynamics between the localized moment and the conduction electron spin.

  17. Deviations from Curie-Weiss behavior in spin-glasses

    NASA Astrophysics Data System (ADS)

    Rao, K. V.; Fähnle, M.; Figueroa, E.; Beckman, O.; Hedman, L.

    1983-03-01

    The magnetic susceptibility in the spin-glass concentration regime of amorphous alloys (FexNi1-x)75P16B6Al3 with x=0.04, 0.06, and 0.08 has been measured using a superconducting quantum interference detector magnetometer with an external field of 5 Oe for temperatures 5<=T<=60.5 K, and a Faraday balance with an external field of 5 kOe for 4.3<=T<=300 K. We observe a high-temperature (T>20Tf) Curie-Weiss law with positive values for the paramagnetic Curie temperature, indicating predominantly ferromagnetic exchange couplings. At temperatures approaching the spin-glass freezing temperature, however, there are strong deviations from Curie-Weiss behavior. A simple theory is developed which allows a determination of the angular spin correlations in the spin-glass ground state from these deviations. The number of such correlated spins (1.5 to 4.4) determined from our data compares well with computer-simulation studies by Walker and Walstedt as well as data obtained by other authors for Au-Fe, Cu-Mn, and Co- and Mn-aluminosilicate spin-glasses.

  18. Improved fair sampling of ground states in Ising spin glasses

    NASA Astrophysics Data System (ADS)

    Katzgraber, Helmut G.; Zhu, Zheng; Ochoa, Andrew J.

    2015-03-01

    Verifying that an optimization approach can sample all solutions that minimize a Hamiltonian is a stringent test for any newly-developed algorithm. While most solvers easily compute the minimum of a cost function for small to moderate input sizes, equiprobable sampling of all ground-state configurations (within Poissonian fluctuations) is much harder to obtain. Most notably, methods such as transverse-field quantum annealing fail in passing this test for certain highly-degenerate problems. Here we present an attempt to sample ground states for Ising spin glasses based on a combination of low-temperature parallel tempering Monte Carlo combined with the cluster algorithm by Houdayer. Because the latter is rejection free and obeys details balance, the ground-state manifold is efficiently sampled. We illustrate the approach for Ising spin glasses on the D-Wave Two quantum annealer topology, known as the Chimera graph, as well as two-dimensional Ising spin glasses.

  19. Aging dynamics of quantum spin glasses of rotors

    NASA Astrophysics Data System (ADS)

    Kennett, Malcolm P.; Chamon, Claudio; Ye, Jinwu

    2001-12-01

    We study the long time dynamics of quantum spin glasses of rotors using the nonequilibrium Schwinger-Keldysh formalism. These models are known to have a quantum phase transition from a paramagnetic to a spin-glass phase, which we approach by looking at the divergence of the spin-relaxation rate at the transition point. In the aging regime, we determine the dynamical equations governing the time evolution of the spin response and correlation functions, and show that all terms in the equations that arise solely from quantum effects are irrelevant at long times under time reparametrization group (RPG) transformations. At long times, quantum effects enter only through the renormalization of the parameters in the dynamical equations for the classical counterpart of the rotor model. Consequently, quantum effects only modify the out-of-equilibrium fluctuation-dissipation relation (OEFDR), i.e. the ratio X between the temperature and the effective temperature, but not the form of the classical OEFDR.

  20. 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.

  1. Non-compact local excitations in spin-glasses

    NASA Astrophysics Data System (ADS)

    Lamarcq, J.; Bouchaud, J.-P.; Martin, O. C.; Mézard, M.

    2002-05-01

    We study numerically the local low-energy excitations in the 3d Edwards-Anderson model for spin-glasses. Given the ground state, we determine the lowest-lying connected cluster of flipped spins with a fixed volume containing one given spin. These excitations are not compact, having a fractal dimension close to two, suggesting an analogy with lattice animals. Also, their energy does not grow with their size; the associated exponent is slightly negative whereas the one for compact clusters is positive. These findings call for a modification of the basic hypotheses underlying the droplet model.

  2. Non-self-averaging in Ising spin glasses and hyperuniversality

    NASA Astrophysics Data System (ADS)

    Lundow, P. H.; Campbell, I. A.

    2016-01-01

    Ising spin glasses with bimodal and Gaussian near-neighbor interaction distributions are studied through numerical simulations. The non-self-averaging (normalized intersample variance) parameter U22(T ,L ) for the spin glass susceptibility [and for higher moments Un n(T ,L ) ] is reported for dimensions 2 ,3 ,4 ,5 , and 7. In each dimension d the non-self-averaging parameters in the paramagnetic regime vary with the sample size L and the correlation length ξ (T ,L ) as Un n(β ,L ) =[Kdξ (T ,L ) /L ] d and so follow a renormalization group law due to Aharony and Harris [Phys. Rev. Lett. 77, 3700 (1996), 10.1103/PhysRevLett.77.3700]. Empirically, it is found that the Kd values are independent of d to within the statistics. The maximum values [Unn(T,L ) ] max are almost independent of L in each dimension, and remarkably the estimated thermodynamic limit critical [Unn(T,L ) ] max peak values are also practically dimension-independent to within the statistics and so are "hyperuniversal." These results show that the form of the spin-spin correlation function distribution at criticality in the large L limit is independent of dimension within the ISG family. Inspection of published non-self-averaging data for three-dimensional Heisenberg and X Y spin glasses the light of the Ising spin glass non-self-averaging results show behavior which appears to be compatible with that expected on a chiral-driven ordering interpretation but incompatible with a spin-driven ordering scenario.

  3. Compressible Sherrington-Kirkpatrick spin-glass model

    NASA Astrophysics Data System (ADS)

    Liarte, Danilo B.; Salinas, Silvio R.; Yokoi, Carlos S. O.

    2009-05-01

    We introduce a Sherrington-Kirkpatrick spin-glass model with the addition of elastic degrees of freedom. The problem is formulated in terms of an effective four-spin Hamiltonian in the pressure ensemble, which can be treated by the replica method. In the replica-symmetric approximation, we analyze the pressure-temperature phase diagram, and obtain expressions for the critical boundaries between the disordered and the ordered (spin-glass and ferromagnetic) phases. The second-order para-ferromagnetic border ends at a tricritical point, beyond which the transition becomes discontinuous. We use these results to make contact with the temperature-concentration phase diagrams of mixtures of hydrogen-bonded crystals.

  4. Effect of glass structure on spin Hamiltonian parameters: Cu doped tellurite glasses

    SciTech Connect

    Ramamoorthy, Raj Kumar; Bhatnagar, Anil K.

    2015-06-24

    Cu-doped glasses with compositions [(70TeO{sub 2}−(30−x)ZnO−xPbO){sub 0.98}− (CuO){sub 0.02}] (x = 5, 10, 15, 20) were prepared using the melt quenching technique and characterized by EPR. Cu{sup 2+} ions are found to be in distorted oxygen octahedral cage and their corresponding spin Hamiltonian (splitting) parameters are deduced for all glasses as a function of increasing PbO. Finally, effect of the matrix on spin Hamiltonian parameters of Cu{sup 2+} ions are correlated with the help of EPR and earlier Raman analysis.

  5. Avalanches and hysteresis in frustrated superconductors and XY spin glasses.

    PubMed

    Sharma, Auditya; Andreanov, Alexei; Müller, Markus

    2014-10-01

    We study avalanches along the hysteresis loop of long-range interacting spin glasses with continuous XY symmetry, which serves as a toy model of granular superconductors with long-range and frustrated Josephson couplings. We identify sudden jumps in the T=0 configurations of the XY phases as an external field is increased. They are initiated by the softest mode of the inverse susceptibility matrix becoming unstable, which induces an avalanche of phase updates (or spin alignments). We analyze the statistics of these events and study the correlation between the nonlinear avalanches and the soft mode that initiates them. We find that the avalanches follow the directions of a small fraction of the softest modes of the inverse susceptibility matrix, similarly as was found in avalanches in jammed systems. In contrast to the similar Ising spin glass (Sherrington-Kirkpatrick) studied previously, we find that avalanches are not distributed with a scale-free power law but rather have a typical size which scales with the system size. We also observe that the Hessians of the spin-glass minima are not part of standard random matrix ensembles as the lowest eigenvector has a fractal support. PMID:25375434

  6. Chaotic nature of the spin-glass phase

    NASA Technical Reports Server (NTRS)

    Bray, A. J.; Moore, M. A.

    1987-01-01

    The microscopic structure of the ordered phase of spin glasses is investigated theoretically in the framework of the T = 0 fixed-point model (McMillan, 1984; Fisher and Huse, 1986; and Bray and Moore, 1986). The sensitivity of the ground state to changes in the interaction strengths at T = 0 is explored, and it is found that for sufficiently large length scales the ground state is unstable against arbitrarily weak perturbations to the bonds. Explicit results are derived for d = 1, and the implications for d = 2 and d = 3 are considered in detail. It is concluded that there is no hidden order pattern for spin glasses at all T less than T(C), the ordered-phase spin correlations being chaotic functions of spin separation at fixed temperature or of temperature (for a given pair of spins) at scale lengths L greater than (T delta T) exp -1/zeta, where zeta = d(s)/2 - y, d(s) is the interfacial fractal dimension, and -y is the thermal eigenvalue at T = 0.

  7. Local excitations of a spin glass in a magnetic field

    NASA Astrophysics Data System (ADS)

    Lamarcq, J.; Bouchaud, J.-P.; Martin, O. C.

    2003-07-01

    We study the minimum energy clusters (MEC) above the ground state for the 3-d Edwards-Anderson Ising spin glass in a magnetic field. For fields B below 0.4, we find that the field has almost no effect on the excitations that we can probe, of volume V⩽64. As found previously for B=0, their energies decrease with V, and their magnetization remains very small (even slightly negative). For larger fields, both the MEC energy and magnetization grow with V, as expected in a paramagnetic phase. However, all results appear to scale as BV (instead of B(V) as expected from droplet arguments), suggesting that the spin glass phase is destroyed by any small field. Finally, the geometry of the MEC is completely insensitive to the field, giving further credence that they are lattice animals, in the presence or the absence of a field.

  8. Universality in p-spin glasses with correlated disorder

    NASA Astrophysics Data System (ADS)

    Bonzom, Valentin; Gurau, Razvan; Smerlak, Matteo

    2013-02-01

    We introduce a new method, based on the recently developed random tensor theory, for studying the p-spin glass model with non-Gaussian, correlated disorder. Using a suitable generalization of Gurau’s theorem on the universality of the large N limit of the p-unitary ensemble of random tensors, we exhibit an infinite family of such non-Gaussian distributions which leads to the same low temperature phase as the Gaussian distribution. While this result is easy to show (and well known) for uncorrelated disorder, its robustness with respect to strong quenched correlations is surprising. We show in detail how the critical temperature is renormalized by these correlations. We close with speculation on possible applications of random tensor theory to finite-range spin glass models.

  9. Structural approaches to spin glasses and optimization problems

    NASA Astrophysics Data System (ADS)

    de Sanctis, Luca

    We introduce the concept of Random Multi-Overlap Structure (RaMOSt) as a generalization of the one introduced by M. Aizenman et al. for non-diluted spin glasses. We use this concept to find generalized bounds for the free energy of the Viana-Bray model of diluted spin glasses and to formulate and prove the Extended Variational Principle that implicitly provides the free energy of the model. Then we exhibit a theorem for the limiting RaMOSt, analogous to the one found by F. Guerra for the Sherrington-Kirkpatrick model, that describes some stability properties of the model. We also show how our technique can be used to prove the existence of the thermodynamic limit of the free energy. We then propose an ultrametric breaking of replica symmetry for diluted spin glasses in the framework of Random Multi-Overlap Structures (RaMOSt). Such a proposal is closer to the Parisi theory for non-diluted spin glasses than the theory based on the iterative approach. Our approach allows to formulate an ansatz in which the Broken Replica Symmetry trial function depends on a set of numbers, over which one has to take the infimum (as opposed to a nested chain of probabilty distributions). Our scheme suggests that the order parameter is determined by the probability distribution of the multi-overlap in a similar sense as in the non-diluted case, and it is not necessarily a functional. Such results are then extended to the K-SAT and p-XOR-SAT optimization problems, and to the spherical mean field spin glass. The ultrametric structure exhibits a factorization property similar to the one of the optimal structures for the Viana-Bray model. The present work paves the way to a revisited Parisi theory for diluted spin systems. Moreover, it emphasizes some structural analogies among different models, which also seem to be plausible for models that still escape good mathematical control. This structural analysis seems quite promising both mathematically and physically.

  10. Optimal protein-folding codes from spin-glass theory.

    PubMed Central

    Goldstein, R A; Luthey-Schulten, Z A; Wolynes, P G

    1992-01-01

    Protein-folding codes embodied in sequence-dependent energy functions can be optimized using spin-glass theory. Optimal folding codes for associative-memory Hamiltonians based on aligned sequences are deduced. A screening method based on these codes correctly recognizes protein structures in the "twilight zone" of sequence identity in the overwhelming majority of cases. Simulated annealing for the optimally encoded Hamiltonian generally leads to qualitatively correct structures. Images PMID:1594594

  11. The glass crossover from mean-field Spin-Glasses to supercooled liquids

    NASA Astrophysics Data System (ADS)

    Rizzo, Tommaso

    2016-03-01

    Stochastic-Beta-Relaxation provides a characterisation of the glass crossover in discontinuous Spin-Glasses and supercoooled liquid. Notably it can be derived through a rigorous computation from a dynamical Landau theory. In this paper, I will discuss the precise meaning of this connection in a language that does not require familiarity with statistical field theory. I will discuss finite-size corrections in mean-field Spin-Glass models and loop corrections in finite-dimensional models that are both described by the dynamical Landau theory considered. Then I will argue that the same Landau theory can be associated to supercooled liquid described by Mode-Coupling Theory invoking a physical principle of time-scale invariance.

  12. Magnetic ordering in the ilmenite-hematite solid solution: A computational study of the low-temperature spin glass region

    NASA Astrophysics Data System (ADS)

    Harrison, Richard J.

    2009-02-01

    Magnetic ordering in the ilmenite-hematite solid solution (Fe2-xTixO3) has been investigated using Monte Carlo simulations, with particular emphasis on the low-temperature spin glass region of the phase diagram. Complex magnetic behavior is observed due to the presence of two competing magnetic order parameters: a "hematite-like" ordering with a two-layer repeat (Q2) and an "ilmenite-like" ordering with a four-layer repeat (Q4). The susceptibility and degree of magnetic order were calculated from the Fourier transform of the layer-averaged spin distribution, allowing long-range and short-range contributions from Q2 and Q4 to be analyzed separately. For x < 0.8, the ferrimagnetic (FM) phase remains stable down to 0 K. For x ≥ 0.8 a heterogeneous FM phase (HFM) followed by a modulated FM phase (MFM) develops. There is an increasing contribution from Q4 with increasing x, and a pronounced cusp in both Q2 and Q4 susceptibilities develops at 30 K. The superposition of Q2 and Q4 leads to frustrated layers containing dynamically disordered spins. Freezing of this spin disorder below 30 K is responsible for the cusp in susceptibility, which can be classified as a reentrant spin glass (RSG) transition. A gradual loss of long-range FM order occurs as the percolation threshold is approached, resulting in a conventional spin glass (CSG) with no long-range order below 30 K for x ≥ 0.92. For x > 0.95, a transition to an antiferromagnetic (AF) phase occurs at 40-55 K, followed by an RSG transition at 20-30 K. Changes to the phase diagram caused by chemical clustering are determined using a preannealing algorithm. Clustering expands the AF field to x > 0.9 and the HFM field to x ≥ 0.55. The topology of the simulated phase diagram compares favorably with experiments but suggests that the nature of some phase boundaries should be reexamined from both experimental and computational perspectives.

  13. Collective nature of the reentrant integer quantum Hall states in the second Landau level.

    PubMed

    Deng, N; Kumar, A; Manfra, M J; Pfeiffer, L N; West, K W; Csáthy, G A

    2012-02-24

    We report an unexpected sharp peak in the temperature dependence of the magnetoresistance of the reentrant integer quantum Hall states in the second Landau level. This peak defines the onset temperature of these states. We find that in different spin branches the onset temperatures of the reentrant states scale with the Coulomb energy. This scaling provides direct evidence that Coulomb interactions play an important role in the formation of these reentrant states evincing their collective nature. PMID:22463555

  14. Collective Nature of the Reentrant Integer Quantum Hall States in the Second Landau Level

    NASA Astrophysics Data System (ADS)

    Deng, N.; Kumar, A.; Manfra, M. J.; Pfeiffer, L. N.; West, K. W.; Csáthy, G. A.

    2012-02-01

    We report an unexpected sharp peak in the temperature dependence of the magnetoresistance of the reentrant integer quantum Hall states in the second Landau level. This peak defines the onset temperature of these states. We find that in different spin branches the onset temperatures of the reentrant states scale with the Coulomb energy. This scaling provides direct evidence that Coulomb interactions play an important role in the formation of these reentrant states evincing their collective nature.

  15. Spin-polarized nitroxide radicals in organic glasses.

    SciTech Connect

    Tarasov, V. F.; Shkrob, I. A.; Trifunac, A. D.; Chemistry

    2002-01-01

    Nonequilibrium spin polarization formed in a stable nitroxide radical, 2,2,6,6-tetramethyl-1-piperidinyloxy (Tempo) due to the occurrence of Chemically Induced Dynamic Electron Polarization (CIDEP) in photoexcited molecular complexes of this radical with 1,4-benzoquinone, 1,4-naphthaquinone, 9,10-anthraquinone, and their derivatives is observed. These complexes occur spontaneously in low-temperature organic glasses (20-70 K) upon freezing the concentrated liquid solutions. The emissive net polarization in the nitroxide radical is observed 0.1-10 {mu}s after the photoexcitation of the p-quinone moiety. No degradation of the polarized magnetic resonance signal from Tempo after >104 excitation cycles was observed. This spin polarization is shown to be mainly due to a polarization transfer from the lowest triplet state of the p-quinone. This transfer is driven by the electron spin exchange interaction between the nitroxide radical and the triplet p-quinone; it occurs simultaneously with a spin-selective electronic relaxation of the photoexcited complex. The resulting mechanism combines the features of the electron spin polarization transfer (ESPT) and radical-triplet pair mechanisms (RTPM) in liquid. A theoretical model of such a mechanism is suggested.

  16. Spin-glass-like behaviour in IrSr{sub 2}RECu{sub 2}O{sub 8} (RE=Sm and Eu)

    SciTech Connect

    Santos-Garcia, A.J. dos Duijn, J. van; Alario-Franco, M.A.

    2008-12-15

    We report the results of magnetic and specific heat measurements on the 1212-type compounds IrSr{sub 2}RECu{sub 2}O{sub 8} with RE=Sm and Eu, prepared by high-pressure and high-temperature synthesis. The magnetic susceptibility of these compounds shows a large difference in the temperature dependence of the magnetization measured under zero-field-cooled and field-cooled conditions below 87 and 71 K, respectively, and upon further cooling below {approx}10 K substantial maxima are observed too. Further AC susceptibility measurements support a glassy behaviour in lower magnetic transitions whereas the specific heat measurements do not show the typical long-range ordering commonly displayed in ferro, ferri or antiferromagnetic transitions. Hysteresis loops suggest the presence of magnetic clusters in the otherwise paramagnetic zone, indicating that these compounds probably display a reentrant spin-glass transition. Results are presented and discussed. - Graphical abstract: IrSr{sub 2}RECu{sub 2}O{sub 8} with RE=Sm and Eu were prepared by high-pressure and high-temperature synthesis. Both samples adopt a M-1212-type perovskite structure and a microdomain texturing of the long c-axis is observed by TEM. A very interesting magnetic behaviour is observed in these materials. A 'cluster by cluster freezing' model is proposed, instead of the classical individual spin freezing one to explain the spin-glass-like behaviour that seems to coexist with weak ferromagnetism in both compounds.

  17. Some Spin Glass Ideas Applied to the Clique Problem

    NASA Astrophysics Data System (ADS)

    Iovanella, Antonio; Scoppola, Benedetto; Scoppola, Elisabetta

    2007-03-01

    In this paper we introduce a new algorithm to study some NP-complete problems. This algorithm is a Markov Chain Monte Carlo (MCMC) inspired by the cavity method developed in the study of spin glass. We will focus on the maximum clique problem and we will compare this new algorithm with several standard algorithms on some DIMACS benchmark graphs and on random graphs. The performances of the new algorithm are quite surprising. Our effort in this paper is to be clear as well to those readers who are not in the field.

  18. Spin Glass a Bridge Between Quantum Computation and Statistical Mechanics

    NASA Astrophysics Data System (ADS)

    Ohzeki, Masayuki

    2013-09-01

    In this chapter, we show two fascinating topics lying between quantum information processing and statistical mechanics. First, we introduce an elaborated technique, the surface code, to prepare the particular quantum state with robustness against decoherence. Interestingly, the theoretical limitation of the surface code, accuracy threshold, to restore the quantum state has a close connection with the problem on the phase transition in a special model known as spin glasses, which is one of the most active researches in statistical mechanics. The phase transition in spin glasses is an intractable problem, since we must strive many-body system with complicated interactions with change of their signs depending on the distance between spins. Fortunately, recent progress in spin-glass theory enables us to predict the precise location of the critical point, at which the phase transition occurs. It means that statistical mechanics is available for revealing one of the most interesting parts in quantum information processing. We show how to import the special tool in statistical mechanics into the problem on the accuracy threshold in quantum computation. Second, we show another interesting technique to employ quantum nature, quantum annealing. The purpose of quantum annealing is to search for the most favored solution of a multivariable function, namely optimization problem. The most typical instance is the traveling salesman problem to find the minimum tour while visiting all the cities. In quantum annealing, we introduce quantum fluctuation to drive a particular system with the artificial Hamiltonian, in which the ground state represents the optimal solution of the specific problem we desire to solve. Induction of the quantum fluctuation gives rise to the quantum tunneling effect, which allows nontrivial hopping from state to state. We then sketch a strategy to control the quantum fluctuation efficiently reaching the ground state. Such a generic framework is called

  19. Microwires fabricated by glass-coated melt spinning

    SciTech Connect

    Zhao, Y. Y.; Li, H.; Hao, H. Y.; Li, M.; Zhang, Y.; Liaw, P. K.

    2013-07-15

    The glass-coated melt spinning method offers a route for the manufacture of metal filaments with a few micrometers in diameter in a single operation directly from the melt. Cobalt-based amorphous wires, Cu-15.0 atomic percent (at. %) Sn shape-memory wires, and Ni{sub 2}MnGa (atomic percent) ferromagnetic wires were successfully produced by this method. The cobalt-based amorphous wire is flexible, and Cu-15.0 at. % Sn shape-memory wires have the tensile elongation of 14%. However, because of chemical reaction with glass and oxidation, it is hard to make Cu–Al–Ni shape-memory wires and Ni–Nb–Sn amorphous wires. Conditions for preparing these materials were summarized, and the differences of the solidification processes among glass-coated amorphous cobalt-based wires, Cu-15.0 at. % Sn shape-memory wires, and Ni{sub 2}MnGa wires were analyzed and discussed.

  20. Mean-field theory of spin-glasses with finite coordination number

    NASA Technical Reports Server (NTRS)

    Kanter, I.; Sompolinsky, H.

    1987-01-01

    The mean-field theory of dilute spin-glasses is studied in the limit where the average coordination number is finite. The zero-temperature phase diagram is calculated and the relationship between the spin-glass phase and the percolation transition is discussed. The present formalism is applicable also to graph optimization problems.

  1. Spin glass model for dynamics of cell reprogramming

    NASA Astrophysics Data System (ADS)

    Pusuluri, Sai Teja; Lang, Alex H.; Mehta, Pankaj; Castillo, Horacio E.

    2015-03-01

    Recent experiments show that differentiated cells can be reprogrammed to become pluripotent stem cells. The possible cell fates can be modeled as attractors in a dynamical system, the ``epigenetic landscape.'' Both cellular differentiation and reprogramming can be described in the landscape picture as motion from one attractor to another attractor. We perform Monte Carlo simulations in a simple model of the landscape. This model is based on spin glass theory and it can be used to construct a simulated epigenetic landscape starting from the experimental genomic data. We re-analyse data from several cell reprogramming experiments and compare with our simulation results. We find that the model can reproduce some of the main features of the dynamics of cell reprogramming.

  2. Local excitations in mean-field spin glasses

    NASA Astrophysics Data System (ADS)

    Krzakala, F.; Parisi, G.

    2004-06-01

    We address the question of geometrical as well as energetic properties of local excitations in mean-field Ising spin glasses. We study analytically the Random Energy Model and numerically a dilute mean-field model, first on tree-like graphs, equivalent to a replica-symmetric computation, and then directly on finite-connectivity random lattices. In the first model, characterized by a discontinuous replica symmetry breaking, we found that the energy of finite-volume excitation is infinite, whereas in the dilute mean-field model, described by a continuous replica symmetry breaking, it slowly decreases with sizes and saturates at a finite value, in contrast with what would be naively expected. The geometrical properties of these excitations are similar to those of lattice animals or branched polymers. We discuss the meaning of these results in terms of replica symmetry breaking and also possible relevance in finite-dimensional systems.

  3. Superexchange and spin-glass formation in semimagnetic semiconductors

    NASA Astrophysics Data System (ADS)

    Rusin, Tomasz M.

    1996-05-01

    The Mn-Mn superexchange interaction in semimagnetic semiconductors A1-xMnxB (where A=Zn, Cd and B=S, Se, Te) is studied within the three-level model of the band structure. We focus on the dependence of the interaction on the interion distance Jdd(r)=J0f(r). In the present work, the function f(r) is obtained analytically. This, only weakly material-dependent function is found to decrease with Mn-Mn distance much slower than its Gaussian approximation derived previously. The exact form of the decay of the superexchange can be approximated by a power law J0r-8.5. This is close to an experimental result, J0r-6.8, determined on the basis of the spin-glass transition temperature on the composition.

  4. The decoupling of the glass transitions in the two-component p-spin spherical model

    NASA Astrophysics Data System (ADS)

    Ikeda, Harukuni; Ikeda, Atsushi

    2016-07-01

    Binary mixtures of large and small particles with a disparate size ratio exhibit a rich phenomenology at their glass transition points. In order to gain insights on such systems, we introduce and study a two-component version of the p-spin spherical spin glass model. We employ the replica method to calculate the free energy and the phase diagram. We show that when the strengths of the interactions of each component are not widely separated, the model has only one glass phase characterized by the conventional one-step replica symmetry breaking. However when the strengths of the interactions are well separated, the model has three glass phases depending on the temperature and component ratio. One is the ‘single’ glass phase in which only the spins of one component are frozen while the spins of the other component remain mobile. This phase is characterized by the one-step replica symmetry breaking. The second is the ‘double’ glass phase obtained by cooling the single glass phase further, in which the spins of the remaining mobile component are also frozen. This phase is characterized by the two-step replica symmetry breaking. The third is also the ‘double’ glass phase, which, however, is formed by the simultaneous freezing of the spins of both components at the same temperatures and is characterized by the one-step replica symmetry breaking. We discuss the implications of these results for the glass transitions of binary mixtures.

  5. Magnetostriction of rare-earth random magnetic anisotropy spin glasses

    SciTech Connect

    del Moral, A.; Arnaudas, J.I.

    1989-05-01

    A model of magnetostriction for single-ion random magnetic anisotropy (RMA) spin glasses (SG) is developed, the calculation being based on the replica technique. An overall uniform strain is assumed and coupled to the local easy axis (or easy plane) by an adequate projection. The obtained bulk magnetostriction becomes proportional to the average quadrupolar moment, which depends upon the assumed ferromagnetic uniform exchange, J/sub 0/, and the RMA crystal field (CEF), D/sub 0/, strengths. Magnetostriction measurements parallel (lambda/sub X/) and perpendicular (lambda/sub perpendicular/) to the applied magnetic field (up to 7 T) have been performed between 4.2 and 150 K (much larger than the SG temperature, T/sub SG/) for the amorphous spin glasses R/sub 40/Y/sub 23/Cu/sub 37/ (R = Tb, Dy, Ho, and Er). Anisotropic magnetostriction is a forced effect, with no sign of saturation, and is quite large well above T/sub SG/. The developed model fits quantitatively and remarkably well the temperature variation of the anisotropic magnetostriction, lambda/sub t/ = lambda/sub X/-lambda/sub perpendicular/, in the case of Tb, Dy, and Ho alloys. The values obtained from the fit for D/sub 0/, respectively, are +3.0, +1.25, and +0.6 K. For Er, D/sub 0/ becomes -0.37 K. The signs of D/sub 0/ are in agreement with having local axial anisotropy for the Tb, Dy, and Ho compounds, and planar for the Er one, in good agreement with the signs of the ..cap alpha../sub J/ Stevens quadrupolar parameter.

  6. Memory effects in Heisenberg spin glasses: Spontaneous restoration of the original spin configuration rather than preservation in a frozen state

    SciTech Connect

    Mamiya, Hiroaki; Nimori, Shigeki

    2012-04-01

    Universality of spin configuration restoration is studied in Heisenberg spin glasses: a dilute magnetic semiconductor Cd{sub 55}Mn{sub 45}Te as well as a dilute magnetic alloy Cu{sub 97}Mn{sub 3}. Reversions of the relaxations of magnetization were observed in both systems undergoing positive/negative temperature cycling under a constant magnetic field. Because the magnetization mirrors evolution of the spin configuration, these reversions indicate that the spin configurations are spontaneously restored to the initially stabilized states when the temperature is returned to the original. Whereas such spin restoration does not occur if the spin glasses are simply frozen, it is possible in the alternative ghost domain scenario of the droplet picture. This finding thus provides fresh insight into the nature of glassy systems.

  7. Reentrant Wetting of Network Fluids

    NASA Astrophysics Data System (ADS)

    Bernardino, N. R.; Telo da Gama, M. M.

    2012-09-01

    We use a simple mesoscopic Landau-Safran theory of network fluids to show that a reentrant phase diagram, in the “empty liquid” regime, leads to nonmonotonic surface tension and reentrant wetting, as previously reported for binary mixtures. One of the wetting transitions is of the usual kind, but the low temperature transition may allow the display of the full range of fluctuation regimes predicted by renormalization group theory.

  8. Inverse spin glass and related maximum entropy problems.

    PubMed

    Castellana, Michele; Bialek, William

    2014-09-12

    If we have a system of binary variables and we measure the pairwise correlations among these variables, then the least structured or maximum entropy model for their joint distribution is an Ising model with pairwise interactions among the spins. Here we consider inhomogeneous systems in which we constrain, for example, not the full matrix of correlations, but only the distribution from which these correlations are drawn. In this sense, what we have constructed is an inverse spin glass: rather than choosing coupling constants at random from a distribution and calculating correlations, we choose the correlations from a distribution and infer the coupling constants. We argue that such models generate a block structure in the space of couplings, which provides an explicit solution of the inverse problem. This allows us to generate a phase diagram in the space of (measurable) moments of the distribution of correlations. We expect that these ideas will be most useful in building models for systems that are nonequilibrium statistical mechanics problems, such as networks of real neurons. PMID:25260004

  9. Numerical simulation of spin-glass transition phenomena (invited)

    NASA Astrophysics Data System (ADS)

    Walstedt, R. E.; Walker, L. R.

    1982-11-01

    Results are presented from a continuing investigation of spin-glass transition effects by numerical simulation. The model is one of classical unit vectors randomly distributed on an fcc lattice, with RKKY exchange coupling and nearest-neighbor dipolar coupling in an approximate representation of dilute Mn in Cu. Results for system energy and for dipolar energy as a function of temperature show no resolvable features at the freezing temperature TG* in accord with the high-precision specific heat data of Fogle, Boyer, Phillips, and Van Curen. Additional details of shattered susceptibility behavior in the vicinity of the ground-state transition are presented. Further data on freezing temperatures as a function of dipolar coupling strength D show a trend toward macroscopic behavior (i.e., TG* independent of D) with a sample of 4928 spins. Finally, a moderately sharp onset of freezing transverse to an applied magnetic field is found, as predicted by the mean-field calculations of Toulouse and Gabay. As expected, this transition is unrelated to transverse susceptibilities.

  10. Classical mutual information in mean-field spin glass models

    NASA Astrophysics Data System (ADS)

    Alba, Vincenzo; Inglis, Stephen; Pollet, Lode

    2016-03-01

    We investigate the classical Rényi entropy Sn and the associated mutual information In in the Sherrington-Kirkpatrick (S-K) model, which is the paradigm model of mean-field spin glasses. Using classical Monte Carlo simulations and analytical tools we investigate the S-K model in the n -sheet booklet. This is achieved by gluing together n independent copies of the model, and it is the main ingredient for constructing the Rényi entanglement-related quantities. We find a glassy phase at low temperatures, whereas at high temperatures the model exhibits paramagnetic behavior, consistent with the regular S-K model. The temperature of the paramagnetic-glassy transition depends nontrivially on the geometry of the booklet. At high temperatures we provide the exact solution of the model by exploiting the replica symmetry. This is the permutation symmetry among the fictitious replicas that are used to perform disorder averages (via the replica trick). In the glassy phase the replica symmetry has to be broken. Using a generalization of the Parisi solution, we provide analytical results for Sn and In and for standard thermodynamic quantities. Both Sn and In exhibit a volume law in the whole phase diagram. We characterize the behavior of the corresponding densities, Sn/N and In/N , in the thermodynamic limit. Interestingly, at the critical point the mutual information does not exhibit any crossing for different system sizes, in contrast with local spin models.

  11. Efficient cluster Monte Carlo algorithm for Ising spin glasses in more than two space dimensions

    NASA Astrophysics Data System (ADS)

    Ochoa, Andrew J.; Zhu, Zheng; Katzgraber, Helmut G.

    2015-03-01

    A cluster algorithm that speeds up slow dynamics in simulations of nonplanar Ising spin glasses away from criticality is urgently needed. In theory, the cluster algorithm proposed by Houdayer poses no advantage over local moves in systems with a percolation threshold below 50%, such as cubic lattices. However, we show that the frustration present in Ising spin glasses prevents the growth of system-spanning clusters at temperatures roughly below the characteristic energy scale J of the problem. Adding Houdayer cluster moves to simulations of Ising spin glasses for T ~ J produces a speedup that grows with the system size over conventional local moves. We show results for the nonplanar quasi-two-dimensional Chimera graph of the D-Wave Two quantum annealer, as well as conventional three-dimensional Ising spin glasses, where in both cases the addition of cluster moves speeds up thermalization visibly in the physically-interesting low temperature regime.

  12. Evidence for a re-entrant character of magnetism of σ-phase Fe-Mo alloys: Non-linear susceptibilities

    NASA Astrophysics Data System (ADS)

    Dubiel, S. M.

    2016-06-01

    Non-linear ac magnetic susceptibility terms viz. quadratic χ2 and cubic χ3 were measured versus temperature and frequency for a series of the σ-phase Fe100-xMox (47≤x≤53) compounds. Clear evidence was found that the ground magnetic state of the samples is mixed i.e. constituted by a mixture of two phases: a spin glass (SG) and ferromagnetic (FM) clusters, hence the magnetism of the investigated samples can be regarded as re-entrant. Based on the present data, previously reported magnetic phase diagram has been upgraded [Przewoznik and Dubiel (2015) [12

  13. Mean field spin glasses treated with PDE techniques

    NASA Astrophysics Data System (ADS)

    Barra, Adriano; Del Ferraro, Gino; Tantari, Daniele

    2013-07-01

    Following an original idea of Guerra, in these notes we analyze the Sherrington-Kirkpatrick model from different perspectives, all sharing the underlying approach which consists in linking the resolution of the statistical mechanics of the model (e.g. solving for the free energy) to well-known partial differential equation (PDE) problems (in suitable spaces). The plan is then to solve the related PDE using techniques involved in their native field and lastly bringing back the solution in the proper statistical mechanics framework. Within this strand, after a streamlined test-case on the Curie-Weiss model to highlight the methods more than the physics behind, we solve the SK both at the replica symmetric and at the 1-RSB level, obtaining the correct expression for the free energy via an analogy to a Fourier equation and for the self-consistencies with an analogy to a Burger equation, whose shock wave develops exactly at critical noise level (triggering the phase transition). Our approach, beyond acting as a new alternative method (with respect to the standard routes) for tackling the complexity of spin glasses, links symmetries in PDE theory with constraints in statistical mechanics and, as a novel result from the theoretical physics perspective, we obtain a new class of polynomial identities (namely of Aizenman-Contucci type, but merged within the Guerra's broken replica measures), whose interest lies in understanding, via the recent Panchenko breakthroughs, how to force the overlap organization to the ultrametric tree predicted by Parisi.

  14. Practical engineering of hard spin-glass instances

    NASA Astrophysics Data System (ADS)

    Marshall, Jeffrey; Martin-Mayor, Victor; Hen, Itay

    2016-07-01

    Recent technological developments in the field of experimental quantum annealing have made prototypical annealing optimizers with hundreds of qubits commercially available. The experimental demonstration of a quantum speedup for optimization problems has since then become a coveted, albeit elusive goal. Recent studies have shown that the so far inconclusive results, regarding a quantum enhancement, may have been partly due to the benchmark problems used being unsuitable. In particular, these problems had inherently too simple a structure, allowing for both traditional resources and quantum annealers to solve them with no special efforts. The need therefore has arisen for the generation of harder benchmarks which would hopefully possess the discriminative power to separate classical scaling of performance with size from quantum. We introduce here a practical technique for the engineering of extremely hard spin-glass Ising-type problem instances that does not require "cherry picking" from large ensembles of randomly generated instances. We accomplish this by treating the generation of hard optimization problems itself as an optimization problem, for which we offer a heuristic algorithm that solves it. We demonstrate the genuine thermal hardness of our generated instances by examining them thermodynamically and analyzing their energy landscapes, as well as by testing the performance of various state-of-the-art algorithms on them. We argue that a proper characterization of the generated instances offers a practical, efficient way to properly benchmark experimental quantum annealers, as well as any other optimization algorithm.

  15. Atomic Fermi-Bose Mixtures in Inhomogeneous and Random Lattices: From Fermi Glass to Quantum Spin Glass and Quantum Percolation

    NASA Astrophysics Data System (ADS)

    Sanpera, A.; Kantian, A.; Sanchez-Palencia, L.; Zakrzewski, J.; Lewenstein, M.

    2004-07-01

    We investigate strongly interacting atomic Fermi-Bose mixtures in inhomogeneous and random optical lattices. We derive an effective Hamiltonian for the system and discuss its low temperature physics. We demonstrate the possibility of controlling the interactions at local level in inhomogeneous but regular lattices. Such a control leads to the achievement of Fermi glass, quantum Fermi spin-glass, and quantum percolation regimes involving bare and/or composite fermions in random lattices.

  16. Phase separation, clustering, and fractal characteristics in glass: A magic-angle-spinning NMR spin-lattice relaxation study

    NASA Astrophysics Data System (ADS)

    Sen, S.; Stebbins, J. F.

    1994-07-01

    A comparative study of the 29Si spin-lattice relaxation behavior (induced by trace amounts of paramagnetic dopants in the glass) in phase-separated Li2Si4O9 and monophasic Li2Si2O5 and Na2Si2O5 glasses has been made in order to understand the nature of clustering and the resulting intermediate-range ordering. Optically clear tetrasilicate and disilicate glasses were prepared with 500 to 2000 ppm of Gd2O3, a paramagnetic dopant. The constituent structural units (Q3 and Q4 species) in all tetrasilicate glasses show strong differential relaxation following a power-law behavior. This is due to preferential partitioning of Gd3+ into the lower silica (Q3-rich) regions of these glasses, indicating the presence of Q species clusters too small to produce optical opalescence (a few nm to perhaps tens of nm). Preliminary results on 6Li spin-lattice relaxation in these glasses support this hypothesis. Differential relaxation becomes more pronounced on annealing due to growth of such clusters. No such differential relaxation was observed in the monophase disilicate glasses. For spin-lattice relaxation induced by direct dipolar coupling to paramagnetic ions, the recovery of magnetization is proportional to time as M(t)~tα where α is a function of the dimensionality D of mass distribution of the constituent Q species around the Gd3+ paramagnetic centers in the glass. For tetrasilicate glasses D~=2.62+/-0.22 and the system behaves as a mass fractal up to a length scale of 2 to 3 nm. D is thus equal to, within error, the theoretical value of 2.6 for an infinite percolation cluster of one type of Q species in another. For disilicate glasses, D~=3.06+/-0.18 which indicates a three-dimensional (and thus nonfractal) mass distribution of the constituent Q species over the same length scale.

  17. Time Dependence of the freezing temperature for thin film spin glasses

    NASA Astrophysics Data System (ADS)

    Orbach, Raymond

    There have been many measurements of the dependence of the ``freezing temperature'', Tf, on the thickness o of thin film spin glasses. Tf decreases with decreasing o, but never vanishes. This contribution suggests that the dependence of Tf on o is a time dependent relationship. Because the lower critical dimension of a spin glass, dl ~ 2 . 5 , when the spin glass correlation length ξ (t , T) grows to o, the spin glass dimensionality crosses over from d = 3 to d = 2 . What remains are spin glass correlations for length scales <= o . The time dependence of the magnetization dynamics are then activated, with activation energy equal to a largest barrier Δmax (o) , and an associated activation time τ. For measurements at time scales such that ξ (t , T) < o , the effective dimension d = 3 , and the characteristic cusp and knee of a spin glass is observed. For experimental time scales greater than τ, with ξ (t , T) ~ o , the zero-field cooled magnetization has grown to the field-cooled value of the magnetization, leading to the identification of Tf. Quantitative agreement with experiment is exhibited. Supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering under Award DE-SC0013599.

  18. The magnetic susceptibility on the transverse antiferromagnetic Ising model: Analysis of the reentrant behavior

    NASA Astrophysics Data System (ADS)

    Neto, Minos A.; de Sousa, J. Ricardo; Padilha, Igor T.; Rodriguez Salmon, Octavio D.; Roberto Viana, J.; Dinóla Neto, F.

    2016-06-01

    We study the three-dimensional antiferromagnetic Ising model in both uniform longitudinal (H) and transverse (Ω) magnetic fields by using the effective-field theory (EFT) with finite cluster N = 1 spin (EFT-1). We analyzed the behavior of the magnetic susceptibility to investigate the reentrant phenomena that we have seen in the same phase diagram previously obtained in other papers. Our results shows the presence of two divergences in the susceptibility that indicates the existence of a reentrant behavior.

  19. Observation of Temperature Chaos in Mesoscopic Spin Glasses

    NASA Astrophysics Data System (ADS)

    Guchhait, Samaresh

    Temperature Chaos (TC) results from a change in temperature for spin glasses (SG), polymers, and other glassy materials. When the temperature is changed, TC means that the new state has no memory of the preparation of the initial state. TC was predicted long ago [PRL 48, 767 (1982)]. However, ``An experimental measurement of TC is still missing'' [EPL 103, 67003 (2013)]. One reason for this is the question of length scale. In the thermodynamic limit, even an infinitesimal temperature change, ΔT , will create a chaotic condition. However, by working at the mesoscale, one can establish a length scale sufficiently small to exhibit reversible behavior before crossing over to chaotic behavior as the temperature change increases. Observation of TC is possible because, on reasonable laboratory time scales, the SG correlation length can grow to the size of the thickness of the film, L. The lower critical dimension for a SG is ~ 2 . 5 , so that the thin film SG crosses over to a glass temperature Tg = 0 . However, there remains quasi-equilibrium SG states with length scales < L . After crossover, a small ΔT will generate a TC coherence length which, if greater than L, will leave the system in a reversible state. However, when ΔT is sufficiently large, such that the TC coherence length is less than L, and chaos will ensue. I will discuss our recent results of temperature cycling on 15.5 nm SG films of amorphous Ge:Mn. By use of end of aging and temperature cycling, both the reversible region and the chaotic region are observed. Remarkably, the transition from a reversible to chaotic behavior is abrupt, and not smooth as a function of ΔT . This is in contrast to previous work using polycrystalline materials where the distribution of length scales smoothed out the transition to chaos. Using the calculated TC critical exponent, the range of ΔT for reversible behavior is calculated and is in very good agreement with the measured range. This work was supported by the U

  20. Bimodal and Gaussian Ising spin glasses in dimension two

    NASA Astrophysics Data System (ADS)

    Lundow, P. H.; Campbell, I. A.

    2016-02-01

    An analysis is given of numerical simulation data to size L =128 on the archetype square lattice Ising spin glasses (ISGs) with bimodal (±J ) and Gaussian interaction distributions. It is well established that the ordering temperature of both models is zero. The Gaussian model has a nondegenerate ground state and thus a critical exponent η ≡0 , and a continuous distribution of energy levels. For the bimodal model, above a size-dependent crossover temperature T*(L ) there is a regime of effectively continuous energy levels; below T*(L ) there is a distinct regime dominated by the highly degenerate ground state plus an energy gap to the excited states. T*(L ) tends to zero at very large L , leaving only the effectively continuous regime in the thermodynamic limit. The simulation data on both models are analyzed with the conventional scaling variable t =T and with a scaling variable τb=T2/(1 +T2) suitable for zero-temperature transition ISGs, together with appropriate scaling expressions. The data for the temperature dependence of the reduced susceptibility χ (τb,L ) and second moment correlation length ξ (τb,L ) in the thermodynamic limit regime are extrapolated to the τb=0 critical limit. The Gaussian critical exponent estimates from the simulations, η =0 and ν =3.55 (5 ) , are in full agreement with the well-established values in the literature. The bimodal critical exponents, estimated from the thermodynamic limit regime analyses using the same extrapolation protocols as for the Gaussian model, are η =0.20 (2 ) and ν =4.8 (3 ) , distinctly different from the Gaussian critical exponents.

  1. Bimodal and Gaussian Ising spin glasses in dimension two.

    PubMed

    Lundow, P H; Campbell, I A

    2016-02-01

    An analysis is given of numerical simulation data to size L=128 on the archetype square lattice Ising spin glasses (ISGs) with bimodal (±J) and Gaussian interaction distributions. It is well established that the ordering temperature of both models is zero. The Gaussian model has a nondegenerate ground state and thus a critical exponent η≡0, and a continuous distribution of energy levels. For the bimodal model, above a size-dependent crossover temperature T(*)(L) there is a regime of effectively continuous energy levels; below T(*)(L) there is a distinct regime dominated by the highly degenerate ground state plus an energy gap to the excited states. T(*)(L) tends to zero at very large L, leaving only the effectively continuous regime in the thermodynamic limit. The simulation data on both models are analyzed with the conventional scaling variable t=T and with a scaling variable τ(b)=T(2)/(1+T(2)) suitable for zero-temperature transition ISGs, together with appropriate scaling expressions. The data for the temperature dependence of the reduced susceptibility χ(τ(b),L) and second moment correlation length ξ(τ(b),L) in the thermodynamic limit regime are extrapolated to the τ(b)=0 critical limit. The Gaussian critical exponent estimates from the simulations, η=0 and ν=3.55(5), are in full agreement with the well-established values in the literature. The bimodal critical exponents, estimated from the thermodynamic limit regime analyses using the same extrapolation protocols as for the Gaussian model, are η=0.20(2) and ν=4.8(3), distinctly different from the Gaussian critical exponents. PMID:26986300

  2. Evidence of spin glass dynamics in dilute LiHoxY1-xF4.

    PubMed

    Quilliam, J A; Meng, S; Mugford, C G A; Kycia, J B

    2008-10-31

    ac susceptibility measurements are presented on the dilute, dipolar coupled, Ising magnet LiHoxY1-xF4 for a concentration x=0.045. The frequency and temperature dependences of the susceptibility show characteristic glassy relaxation. The absorption spectrum is found to broaden with decreasing temperature suggesting that the material is behaving as a spin glass and not as an exotic spin liquid as was previously observed. A dynamical scaling analysis suggests a spin glass transition temperature of 43+/-2 mK with an exponent znu=7.8+/-0.2. PMID:18999860

  3. Evidence of Spin Glass Dynamics in Dilute LiHoxY1-xF4

    NASA Astrophysics Data System (ADS)

    Quilliam, J. A.; Meng, S.; Mugford, C. G. A.; Kycia, J. B.

    2008-10-01

    ac susceptibility measurements are presented on the dilute, dipolar coupled, Ising magnet LiHoxY1-xF4 for a concentration x=0.045. The frequency and temperature dependences of the susceptibility show characteristic glassy relaxation. The absorption spectrum is found to broaden with decreasing temperature suggesting that the material is behaving as a spin glass and not as an exotic spin liquid as was previously observed. A dynamical scaling analysis suggests a spin glass transition temperature of 43±2mK with an exponent zν=7.8±0.2.

  4. Efficient rotamer elimination applied to protein side-chains and related spin glasses.

    PubMed Central

    Goldstein, R F

    1994-01-01

    Folded proteins and spin glasses share various properties, such as seemingly random interactions between residues (spins), and one might presume that some generic behaviors of spin glasses would also be exhibited in a general way by proteins. But a comparison here shows that the side-chain conformation systems of apo-myoglobin and lysozyme are qualitatively different from specific closely related spin glass systems. This difference is manifest in the number of rotamers that can be identified as definitely not contributing to the global energy minimum. This identification is effected by using a significantly enhanced version of the Dead End Elimination theorem (Desmet, J., M. De Maeyer, B. Hazes, and I. Lasters. 1992. The dead-end elimination theorem and its use in protein side-chain positioning. Nature. 356:539-542), which is much more effective and efficient in eliminating rotamers. In several cases (for proteins, although not for spin glasses) this improved Dead End Elimination theorem succeeded in identifying the absolute global minimum of rotamer conformations, with no statistical uncertainty. The difference between protein and spin glass is due to correlations between the interactions of one residue pair with another pair, and probably will play an important role in the thermodynamic behavior of the protein system. PMID:8061189

  5. Evolution from a ferromagnetic to a spin-glass regime in the spinel-type Cu(Cr{sub 1-x}Ti{sub x}){sub 2}S{sub 4}

    SciTech Connect

    Kariya, Fumihiro; Ebisu, Shuji; Nagata, Shoichi

    2009-03-15

    Successive changes from ferromagnetic, re-entrant mixed, to spin-glass regime have been manifestly found with increasing Ti-composition x in the quaternary spinel-type Cu(Cr{sub 1-x}Ti{sub x}){sub 2}S{sub 4} system. The Curie temperature T{sub c} decreases steeply with increasing x and this transition becomes ill-defined around x=0.47. Two distinct transitions appear below T{sub c} over the range x=0.40-0.47. Coexistence of the ferromagnetism and spin-glass order would be observed below the Gabay and Toulouse transition (T{sub GT}), owing to freezing of the transverse-spin components without changing of the ferromagnetic order parameter. Finally, at a yet lower temperature de Almeida-Thouless transition (T{sub AT}), the longitudinal-spin component freezes randomly at which an irreversibility arises between zero-field-cooled (ZFC) and field-cooled (FC) magnetizations. Over the range of 0.47{<=}x{<=}0.85, a cusp of the ZFC magnetization is seen at T{sub g} like conventional spin-glass. Specimens with x{>=}0.90 remain paramagnetic down to 2.0 K. A magnetic phase diagram between T versus x has been obtained experimentally. The values of the multicritical point in 100 Oe is detected to be x=0.47 and T=7.40K. The low-field magnetization and the phase diagram are satisfactorily explained by the theory of Gabay and Toulouse on the basis of Heisenberg isotropic vector spin model rather than the Ising spin model. - An enlargement of the magnetic phase diagram for Cu(Cr{sub 1-x}Ti{sub x}){sub 2}S{sub 4} at a constant field of 100 Oe over the range of 0.40{<=}x{<=}0.70. The curves give a guide to the eye: para, paramagnetic; ferro, ferromagnetic; SGI, spin-glass I (GT-phase), and SGII: spin-glass II (AT-phase). Three characteristic temperatures of T{sub c},T{sub GT}, and T{sub AT} merge into 7.40 K at x=0.47 in H=100 Oe. This is a multicritical point.

  6. Proposal of a checking parameter in the simulated annealing method applied to the spin glass model

    NASA Astrophysics Data System (ADS)

    Yamaguchi, Chiaki

    2016-02-01

    We propose a checking parameter utilizing the breaking of the Jarzynski equality in the simulated annealing method using the Monte Carlo method. This parameter is based on the Jarzynski equality. By using this parameter, to detect that the system is in global minima of the free energy under gradual temperature reduction is possible. Thus, by using this parameter, one is able to investigate the efficiency of annealing schedules. We apply this parameter to the ± J Ising spin glass model. The application to the Gaussian Ising spin glass model is also mentioned. We discuss that the breaking of the Jarzynski equality is induced by the system being trapped in local minima of the free energy. By performing Monte Carlo simulations of the ± J Ising spin glass model and a glassy spin model proposed by Newman and Moore, we show the efficiency of the use of this parameter.

  7. Disorder, cluster spin glass, and hourglass spectra in striped magnetic insulators.

    PubMed

    Andrade, Eric C; Vojta, Matthias

    2012-10-01

    Hourglass-shaped magnetic excitation spectra have been detected in a variety of doped transition-metal oxides with stripelike charge order. Compared to the predictions of spin-wave theory for perfect stripes, these spectra display a different intensity distribution and anomalous broadening. Here we show, based on a comprehensive modeling for La5/3Sr1/3CoO4, how quenched disorder in the charge sector causes frustration, and consequently cluster-glass behavior at low temperatures, in the spin sector. This spin-glass physics, which is insensitive to the detailed nature of the charge disorder, but sensitive to the relative strength of the magnetic interstripe coupling, ultimately determines the distribution of magnetic spectral weight: The excitation spectrum, calculated using spin waves in finite disordered systems, is found to match in detail the observed hour-glass spectrum. PMID:23083273

  8. Classical spin glass system in external field with taking into account relaxation effects

    SciTech Connect

    Gevorkyan, A. S. Abajyan, H. G.

    2013-08-15

    We study statistical properties of disordered spin systems under the influence of an external field with taking into account relaxation effects. For description of system the spatial 1D Heisenberg spin-glass Hamiltonian is used. In addition, we suppose that interactions occur between nearest-neighboring spins and they are random. Exact solutions which define angular configuration of the spin in nodes were obtained from the equations of stationary points of Hamiltonian and the corresponding conditions for the energy local minimum. On the basis of these recurrent solutions an effective parallel algorithm is developed for simulation of stabile spin-chains of an arbitrary length. It is shown that by way of an independent order of N{sup 2} numerical simulations (where N is number of spin in each chain) it is possible to generate ensemble of spin-chains, which is completely ergodic which is equivalent to full self-averaging of spin-chains' vector polarization. Distributions of different parameters (energy, average polarization by coordinates, and spin-spin interaction constant) of unperturbed system are calculated. In particular, analytically is proved and numerically is shown, that for the Heisenberg nearest-neighboring Hamiltonian model, the distribution of spin-spin interaction constants as opposed to widely used Gauss-Edwards-Anderson distribution satisfies Levy alpha-stable distribution law. This distribution is nonanalytic function and does not have variance. In the work we have in detail studied critical properties of an ensemble depending on value of external field parameters (from amplitude and frequency) and have shown that even at weak external fields the spin-glass systemis strongly frustrated. It is shown that frustrations have fractal behavior, they are selfsimilar and do not disappear at scale decreasing of area. By the numerical computation is shown that the average polarization of spin-glass on a different coordinates can have values which can lead to

  9. Magnetic nature of the austenite-martensite phase transition and spin glass behaviour in nanostructured Mn2Ni1.6Sn0.4 melt-spun ribbons

    NASA Astrophysics Data System (ADS)

    Singh, Nidhi; Borgohain, Barsha; Srivastava, A. K.; Dhar, Ajay; Singh, H. K.

    2016-03-01

    Nanocrystalline ribbons of inverse Heusler alloy Mn2Ni1.6Sn0.4 have been synthesised by melt spinning of the arc-melted bulk precursor. The single-phase ribbons crystallize into a cubic structure and exhibit very fine crystallite size of <2 nm. Temperature-dependent magnetization ( M- T) measurements reveal ferromagnetic-austenite (FM-A)-antiferromagnetic-martensite (AFM-M) phase transition that begins at M S ≈ 249 K and finishes at M f ≈ 224 K. During warming, the reverse AFM-M to FM-A transitions begins at A s ≈ 240 K and finishes at A f ≈ 261 K. A re-entrant FM transition is observed in the M-phase at T_{{CM}} ≈ 145 K. These transitions are also confirmed by temperature-dependent resistivity ( ρ- T) measurements. The hysteretic behaviour of M- T and ρ- T in the temperature regime spanned by the A-M transition is a manifestation of the first-order phase transition. M- T and ρ- T data also provide unambiguous evidence in favour of spin glass at T < T_{{CM}}. The scaling of the glass freezing temperature ( T f) with frequency, extracted from the frequency-dependent AC susceptibility measurements, confirms the existence of canonical spin glass at T < T_{{CM}} ≈ 145 K. The occurrence of canonical spin glass has been explained in terms of the nanostructuring modified interactions between the coexisting FM and AFM correlations in the martensitic phase.

  10. Reaching the ground state of a quantum spin glass using a zero-temperature quantum Monte Carlo method

    NASA Astrophysics Data System (ADS)

    Das, Arnab; Chakrabarti, Bikas K.

    2008-12-01

    Here we discuss the annealing behavior of an infinite-range ±J Ising spin glass in the presence of a transverse field using a zero-temperature quantum Monte Carlo method. Within the simulation scheme, we demonstrate that quantum annealing not only helps finding the ground state of a classical spin glass, but can also help simulating the ground state of a quantum spin glass, in particular, when the transverse field is low, much more efficiently.

  11. Reaching the ground state of a quantum spin glass using a zero-temperature quantum Monte Carlo method.

    PubMed

    Das, Arnab; Chakrabarti, Bikas K

    2008-12-01

    Here we discuss the annealing behavior of an infinite-range +/-J Ising spin glass in the presence of a transverse field using a zero-temperature quantum Monte Carlo method. Within the simulation scheme, we demonstrate that quantum annealing not only helps finding the ground state of a classical spin glass, but can also help simulating the ground state of a quantum spin glass, in particular, when the transverse field is low, much more efficiently. PMID:19256816

  12. A fast vectorized program for the CDC cyber 205 to simulate the ising spin glass in three dimensions

    NASA Astrophysics Data System (ADS)

    Bhanot, Gyan; Salvador, Román; Duke, Dennis; Moriarty, K. J. M.

    1988-06-01

    We describe a computer program that performs the Metropolis algorithm for the three-dimensional ( J = ±1) Ising spin glass problem at a peak speed of 80 million spin updates per second on a 2-pipe CDC CYBER 205.

  13. Magnetoelectric relaxor and reentrant behaviours in multiferroic Pb(Fe2/3W1/3)O3 crystal

    PubMed Central

    Chen, Ling; Bokov, Alexei A.; Zhu, Weimin; Wu, Hua; Zhuang, Jian; Zhang, Nan; Tailor, Hamel N.; Ren, Wei; Ye, Zuo-Guang

    2016-01-01

    Significant quenched disorder in crystal structure can break ferroic (magnetic or electric) long-range order, resulting in the development of ferroic glassy states at low temperatures such as magnetic spin glasses, electric dipolar glasses, relaxor ferroelectrics, etc. These states have been widely studied due to novel physical phenomena they reveal. Much less known are the effects of quenched disorder in multiferroics, i.e. the materials where magnetic and electric correlations coexist. Here we report an unusual behaviour in complex perovskite Pb(Fe2/3W1/3)O3 (PFW) crystals: the coexistence of electric relaxor, magnetic relaxor and antiferromagnetic (AFM) states. The most striking finding is the transformation of the AFM phase into a new reentrant-type magnetic glassy phase below Tg ≅ 10 K. We show that the behaviour at this transformation contrasts the typical behaviour of canonical spin glasses and is similar to the behaviour of relaxor ferroelectrics. Magnetoelectric effect is also observed in the AFM phase in the temperature range of the transition into electric relaxor phase at Tf ≅ 200. The mechanism of magnetic relaxor behaviour is supposed to arise from the frustrated interactions among the spins located at the AFM domain walls. Our results should inspire further studies of multirelaxor behaviour in other multiferroic systems. PMID:26936414

  14. Magnetoelectric relaxor and reentrant behaviours in multiferroic Pb(Fe2/3W1/3)O3 crystal.

    PubMed

    Chen, Ling; Bokov, Alexei A; Zhu, Weimin; Wu, Hua; Zhuang, Jian; Zhang, Nan; Tailor, Hamel N; Ren, Wei; Ye, Zuo-Guang

    2016-01-01

    Significant quenched disorder in crystal structure can break ferroic (magnetic or electric) long-range order, resulting in the development of ferroic glassy states at low temperatures such as magnetic spin glasses, electric dipolar glasses, relaxor ferroelectrics, etc. These states have been widely studied due to novel physical phenomena they reveal. Much less known are the effects of quenched disorder in multiferroics, i.e. the materials where magnetic and electric correlations coexist. Here we report an unusual behaviour in complex perovskite Pb(Fe2/3W1/3)O3 (PFW) crystals: the coexistence of electric relaxor, magnetic relaxor and antiferromagnetic (AFM) states. The most striking finding is the transformation of the AFM phase into a new reentrant-type magnetic glassy phase below Tg ≅ 10 K. We show that the behaviour at this transformation contrasts the typical behaviour of canonical spin glasses and is similar to the behaviour of relaxor ferroelectrics. Magnetoelectric effect is also observed in the AFM phase in the temperature range of the transition into electric relaxor phase at Tf ≅ 200. The mechanism of magnetic relaxor behaviour is supposed to arise from the frustrated interactions among the spins located at the AFM domain walls. Our results should inspire further studies of multirelaxor behaviour in other multiferroic systems. PMID:26936414

  15. Magnetoelectric relaxor and reentrant behaviours in multiferroic Pb(Fe2/3W1/3)O3 crystal

    NASA Astrophysics Data System (ADS)

    Chen, Ling; Bokov, Alexei A.; Zhu, Weimin; Wu, Hua; Zhuang, Jian; Zhang, Nan; Tailor, Hamel N.; Ren, Wei; Ye, Zuo-Guang

    2016-03-01

    Significant quenched disorder in crystal structure can break ferroic (magnetic or electric) long-range order, resulting in the development of ferroic glassy states at low temperatures such as magnetic spin glasses, electric dipolar glasses, relaxor ferroelectrics, etc. These states have been widely studied due to novel physical phenomena they reveal. Much less known are the effects of quenched disorder in multiferroics, i.e. the materials where magnetic and electric correlations coexist. Here we report an unusual behaviour in complex perovskite Pb(Fe2/3W1/3)O3 (PFW) crystals: the coexistence of electric relaxor, magnetic relaxor and antiferromagnetic (AFM) states. The most striking finding is the transformation of the AFM phase into a new reentrant-type magnetic glassy phase below Tg ≅ 10 K. We show that the behaviour at this transformation contrasts the typical behaviour of canonical spin glasses and is similar to the behaviour of relaxor ferroelectrics. Magnetoelectric effect is also observed in the AFM phase in the temperature range of the transition into electric relaxor phase at Tf ≅ 200. The mechanism of magnetic relaxor behaviour is supposed to arise from the frustrated interactions among the spins located at the AFM domain walls. Our results should inspire further studies of multirelaxor behaviour in other multiferroic systems.

  16. CALL FOR PAPERS: Special issue on Spin Glasses

    NASA Astrophysics Data System (ADS)

    Coolen, Ton; Nishimori, Hidetoshi; Sourlas, Nicolas; Wong, Michael

    2007-10-01

    This is a call for contributions to a special issue of Journal of Physics A: Mathematical and Theoretical dedicated to the subject of the conference `Viewing The World Through Spin Glasses', in honour of David Sherrington on the occasion of his 65th birthday, 31 August-1 September 2007 (http://www.nottingham.ac.uk/\\verb.~.ppzjpg/DS2007/). Invited speakers and participants at that meeting and other researchers working in the field are invited to submit a research paper to this issue. The Editorial Board has invited Ton Coolen, Hidetoshi Nishimori, Nicolas Sourlas and Michael Wong to serve as Guest Editors for the special issue. Their criteria for acceptance of contributions are as follows: •The subject of the paper should relate to the subject of the conference (see the website of the conference http://www.nottingham.ac.uk/\\verb.~.ppzjpg/DS2007/). •Contributions will be refereed and processed according to the usual procedure of the journal. •Conference papers may be based on already published work but should either contain significant additional new results and/or insights or give a survey of the present state of the art, a critical assessment of the present understanding of a topic, and a discussion of open problems. •Papers submitted by non-participants should be original and contain substantial new results. The guidelines for the preparation of contributions are the following: •The DEADLINE for submission of contributions is 1 December 2007. This deadline will allow the special issue to appear in July 2008. •There is a nominal page limit of 16 printed pages per contribution. For papers exceeding this limit, the Guest Editors reserve the right to request a reduction in length. •Further advice on publishing your work in Journal of Physics A: Mathematical and Theoretical may be found at www.iop.org/Journals/jphysa. •Contributions to the special issue should, if possible, be submitted electronically by web upload at www.iop.org/Journals/jphysa or by

  17. Radiation Dose from Reentrant Electrons

    NASA Technical Reports Server (NTRS)

    Badhwar, G.D.; Cleghorn, T. E.; Watts, J.

    2003-01-01

    In estimating the crew exposures during an EVA, the contribution of reentrant electrons has always been neglected. Although the flux of these electrons is small compared to the flux of trapped electrons, their energy spectrum extends to several GeV compared to about 7 MeV for trapped electrons. This is also true of splash electrons. Using the measured reentrant electron energy spectra, it is shown that the dose contribution of these electrons to the blood forming organs (BFO) is more than 10 times greater than that from the trapped electrons. The calculations also show that the dose-depth response is a very slowly changing function of depth, and thus adding reasonable amounts of additional shielding would not significantly lower the dose to BFO.

  18. Exact algorithm for sampling the two-dimensional Ising spin glass.

    PubMed

    Thomas, Creighton K; Middleton, A Alan

    2009-10-01

    A sampling algorithm is presented that generates spin-glass configurations of the two-dimensional Edwards-Anderson Ising spin glass at finite temperature with probabilities proportional to their Boltzmann weights. Such an algorithm overcomes the slow dynamics of direct simulation and can be used to study long-range correlation functions and coarse-grained dynamics. The algorithm uses a correspondence between spin configurations on a regular lattice and dimer (edge) coverings of a related graph: Wilson's algorithm [D. B. Wilson, Proceedings of the Eighth Symposium on Discrete Algorithms (SIAM, Philadelphia, 1997), p 258] for sampling dimer coverings on a planar lattice is adapted to generate samplings for the dimer problem corresponding to both planar and toroidal spin-glass samples. This algorithm is recursive: it computes probabilities for spins along a "separator" that divides the sample in half. Given the spins on the separator, sample configurations for the two separated halves are generated by further division and assignment. The algorithm is simplified by using Pfaffian elimination rather than Gaussian elimination for sampling dimer configurations. For n spins and given floating point precision, the algorithm has an asymptotic run-time of O(n(3/2)); it is found that the required precision scales as inverse temperature and grows only slowly with system size. Sample applications and benchmarking results are presented for samples of size up to n=128(2), with fixed and periodic boundary conditions. PMID:19905483

  19. Direct observation of a resolvable spin separation in the spin Hall effect of light at an air-glass interface

    SciTech Connect

    Ren, Jin-Li; Wang, Bo; Xiao, Yun-Feng; Gong, Qihuang; Li, Yan

    2015-09-14

    We theoretically and experimentally demonstrate that it is possible to directly observe the resolvable spin separation in the spin Hall effect of light at an air-glass interface by choosing optimal parameters. When a P-polarized light with a beam waist of 10 μm is incident around Brewster's angle, the two spin components of the reflected beam can be completely separated by eliminating the influence of the in-plane wavevector spread. This not only obviously reveals the strong impacts of the polarization state, the incident angle, the beam waist, and the in-plane wavevector spread, but also intuitively visualizes the observation of the spin Hall effect of light.

  20. Evidence of exchange bias effect originating from the interaction between antiferromagnetic core and spin glass shell

    SciTech Connect

    Zhang, X. K. Yuan, J. J.; Yu, H. J.; Zhu, X. R.; Xie, Y. M.; Tang, S. L.; Xu, L. Q.

    2014-07-14

    Spin glass behavior and exchange bias effect have been observed in antiferromagnetic SrMn{sub 3}O{sub 6−x} nanoribbons synthesized via a self-sacrificing template process. The magnetic field dependence of thermoremanent magnetization and isothermal remanent magnetization shows that the sample is good correspondence to spin glass and diluted antiferromagnetic system for the applied field H < 2 T and H > 2 T, respectively. By detailed analysis of training effect using Binek's model, we argue that the observed exchange bias effect in SrMn{sub 3}O{sub 6−x} nanoribbons arises entirely from an interface exchange coupling between the antiferromagnetic core and spin glass shell. The present study is useful for understanding the nature of shell layer and the origin of exchange bias effect in other antiferromagnetic nanosystems as well.

  1. Evidence of exchange bias effect originating from the interaction between antiferromagnetic core and spin glass shell

    NASA Astrophysics Data System (ADS)

    Zhang, X. K.; Tang, S. L.; Xu, L. Q.; Yuan, J. J.; Yu, H. J.; Zhu, X. R.; Xie, Y. M.

    2014-07-01

    Spin glass behavior and exchange bias effect have been observed in antiferromagnetic SrMn3O6-x nanoribbons synthesized via a self-sacrificing template process. The magnetic field dependence of thermoremanent magnetization and isothermal remanent magnetization shows that the sample is good correspondence to spin glass and diluted antiferromagnetic system for the applied field H < 2 T and H > 2 T, respectively. By detailed analysis of training effect using Binek's model, we argue that the observed exchange bias effect in SrMn3O6-x nanoribbons arises entirely from an interface exchange coupling between the antiferromagnetic core and spin glass shell. The present study is useful for understanding the nature of shell layer and the origin of exchange bias effect in other antiferromagnetic nanosystems as well.

  2. Quantum annealing for the number-partitioning problem using a tunable spin glass of ions

    PubMed Central

    Graß, Tobias; Raventós, David; Juliá-Díaz, Bruno; Gogolin, Christian; Lewenstein, Maciej

    2016-01-01

    Exploiting quantum properties to outperform classical ways of information processing is an outstanding goal of modern physics. A promising route is quantum simulation, which aims at implementing relevant and computationally hard problems in controllable quantum systems. Here we demonstrate that in a trapped ion setup, with present day technology, it is possible to realize a spin model of the Mattis-type that exhibits spin glass phases. Our method produces the glassy behaviour without the need for any disorder potential, just by controlling the detuning of the spin-phonon coupling. Applying a transverse field, the system can be used to benchmark quantum annealing strategies which aim at reaching the ground state of the spin glass starting from the paramagnetic phase. In the vicinity of a phonon resonance, the problem maps onto number partitioning, and instances which are difficult to address classically can be implemented. PMID:27230802

  3. Quantum annealing for the number-partitioning problem using a tunable spin glass of ions.

    PubMed

    Graß, Tobias; Raventós, David; Juliá-Díaz, Bruno; Gogolin, Christian; Lewenstein, Maciej

    2016-01-01

    Exploiting quantum properties to outperform classical ways of information processing is an outstanding goal of modern physics. A promising route is quantum simulation, which aims at implementing relevant and computationally hard problems in controllable quantum systems. Here we demonstrate that in a trapped ion setup, with present day technology, it is possible to realize a spin model of the Mattis-type that exhibits spin glass phases. Our method produces the glassy behaviour without the need for any disorder potential, just by controlling the detuning of the spin-phonon coupling. Applying a transverse field, the system can be used to benchmark quantum annealing strategies which aim at reaching the ground state of the spin glass starting from the paramagnetic phase. In the vicinity of a phonon resonance, the problem maps onto number partitioning, and instances which are difficult to address classically can be implemented. PMID:27230802

  4. Quantum annealing for the number-partitioning problem using a tunable spin glass of ions

    NASA Astrophysics Data System (ADS)

    Graß, Tobias; Raventós, David; Juliá-Díaz, Bruno; Gogolin, Christian; Lewenstein, Maciej

    2016-05-01

    Exploiting quantum properties to outperform classical ways of information processing is an outstanding goal of modern physics. A promising route is quantum simulation, which aims at implementing relevant and computationally hard problems in controllable quantum systems. Here we demonstrate that in a trapped ion setup, with present day technology, it is possible to realize a spin model of the Mattis-type that exhibits spin glass phases. Our method produces the glassy behaviour without the need for any disorder potential, just by controlling the detuning of the spin-phonon coupling. Applying a transverse field, the system can be used to benchmark quantum annealing strategies which aim at reaching the ground state of the spin glass starting from the paramagnetic phase. In the vicinity of a phonon resonance, the problem maps onto number partitioning, and instances which are difficult to address classically can be implemented.

  5. Study of a microcanonical algorithm on the ± J spin glass model in d =3

    NASA Astrophysics Data System (ADS)

    Ruiz-Lorenzo, J. J.; Ullod, C. L.

    2000-03-01

    We consider a microcanonical local algorithm to be applied on the ± J spin glass model. We have compared the results coming from a microcanonical Monte Carlo simulation with those from a canonical one: Thermalization times, spin glass susceptibilities and Binder parameters. For a fixed lattice size we found different results between the two thermodynamic ensembles, which tend to vanish at bigger volumes. Moreover, microcanonical thermalization times are longer than the canonical ones. Finally we have checked that one of the Guerra relations is satisfied with good precision for the two largest lattices.

  6. Unconventional critical activated scaling of two-dimensional quantum spin glasses

    NASA Astrophysics Data System (ADS)

    Matoz-Fernandez, D. A.; Romá, F.

    2016-07-01

    We study the critical behavior of two-dimensional short-range quantum spin glasses by numerical simulations. Using a parallel tempering algorithm, we calculate the Binder cumulant for the Ising spin glass in a transverse magnetic field with two different short-range bond distributions, the bimodal and the Gaussian ones. Through an exhaustive finite-size analysis, we show that the cumulant probably follows an unconventional activated scaling, which we interpret as new evidence supporting the hypothesis that the quantum critical behavior is governed by an infinite randomness fixed point.

  7. Symmetry, complexity and multicritical point of the two-dimensional spin glass

    NASA Astrophysics Data System (ADS)

    Maillard, Jean-Marie; Nemoto, Koji; Nishimori, Hidetoshi

    2003-09-01

    We analyse models of spin glasses on the two-dimensional square lattice by exploiting symmetry arguments. The replicated partition functions of the Ising and related spin glasses are shown to have many remarkable symmetry properties as functions of the edge Boltzmann factors. It is shown that the applications of homogeneous and Hadamard inverses to the edge Boltzmann matrix indicate reduced complexities when the elements of the matrix satisfy certain conditions, suggesting that the system has special simplicities under such conditions. Using these duality and symmetry arguments we present a conjecture on the exact location of the multicritical point in the phase diagram.

  8. Chaos and stiffness exponents for short-range Gaussian Ising spin glasses

    NASA Astrophysics Data System (ADS)

    Almeida, Sebastião T. O.; Curado, Evaldo M. F.; Nobre, Fernando D.

    2013-06-01

    Two important exponents in spin-glass theory, namely, the chaos (ζ) and stiffness (y) exponents, are studied for Ising spin glasses with nearest-neighbor Gaussian interactions on different approaches to Bravais lattices. We consider hierarchical lattices of the Migdal-Kadanoff type (both diamond and tress families), with varying fractal dimensions, as well as two lattices of the Wheatstone-bridge family, more specifically, those with fractal dimensions D ≈ 2.32 and D ≈ 3.58. Whenever it is possible to compare, our estimates agree with those obtained from extensive numerical simulations on Bravais lattices, suggesting the present results represent good approximations for these exponents.

  9. Monolithically integrated active waveguides and lasers using rare-earth doped spin-on glass

    SciTech Connect

    Ashby, C.I.H.; Sullivan, C.T.; Vawter, G.A.

    1996-09-01

    This LDRD program No. 3505.230 explored a new approach to monolithic integration of active waveguides and rare-earth solid state lasers directly onto III-V substrates. It involved selectively incorporating rare-earth ions into spin-on glasses (SOGs) that could be solvent cast and then patterned with conventional microelectronic processing. The patterned, rare-earth spin-on glasses (RESOGs) were to be photopumped by laser diodes prefabricated on the wafer and would serve as directly integrated active waveguides and/or rare-earth solid state lasers.

  10. Observation of spin glass transition in spinel LiCoMnO4

    NASA Astrophysics Data System (ADS)

    Chen, Hong; Yang, Xu; Zhang, Pei-Song; Liang, Lei; Hong, Yuan-Ze; Wei, Ying-Jin; Chen, Gang; Du, Fei; Wang, Chun-Zhong

    2015-12-01

    Spinel LiCoMnO4 is prepared by solid-state reaction and its magnetic properties are comprehensively studied by direct current (DC) and alternating current (AC) susceptibilities, isothermal remanent magnetizations, and magnetic hysteresis. Fitting to the Curie-Weiss law by using high-temperature zero-field-cooled susceptibility confirms a low-spin state of Co3+ with S = 0. Both the fitting parameters first increase and then tend to be saturated at high magnetic fields through using isothermal remanent magnetizations, which suggests a spin glass transition at low temperature. AC susceptibility study also supports this conclusion since the frequency dependence of peak position and intensity follows the tendency of a spin glass transition. The origin of the spin-glass transition in LiCoMnO4 might be attributed to a spatial segregation between non-magnetic Co3+ regions and spin glass ordered regions of Mn4+ ions. Project supported by the National Key Basic Research Program of China (Grant No. 2015CB251103), the Development Program of Science and Technology of Jilin Province, China (Grant No. 20140101093JC), and the Program of Science and Technology of Jilin City, China (Grant No. 201434006).

  11. Effect of magnetoelastic coupling on spin-glass behavior in Heisenberg pyrochlore antiferromagnets with bond disorder

    NASA Astrophysics Data System (ADS)

    Shinaoka, Hiroshi; Tomita, Yusuke; Motome, Yukitoshi

    2014-10-01

    Motivated by puzzling aspects of spin-glass behavior reported in frustrated magnetic materials, we theoretically investigate effects of magnetoelastic coupling in geometrically frustrated classical spin models. In particular, we consider bond-disordered Heisenberg antiferromagnets on a pyrochlore lattice coupled to local lattice distortions. By integrating out the lattice degree of freedom, we derive an effective spin-only model, the bilinear-biquadratic model with bond disorder. The effective model is analyzed by classical Monte Carlo simulations using an extended loop algorithm. First, we discuss the phase diagrams in detail by showing the comprehensive Monte Carlo data for thermodynamic and magnetic properties. We show that the spin-glass transition temperature Tf is largely enhanced by the spin-lattice coupling b in the weakly disordered regime. By considering the limit of strong spin-lattice coupling, this enhancement is ascribed to the suppression of thermal fluctuations in semidiscrete degenerate manifold formed in the presence of the spin-lattice coupling. We also find that, by increasing the strength of disorder Δ, the system shows a concomitant transition of the nematic order and spin glass at a temperature determined by b, being almost independent of Δ. This is due to the fact that the spin-glass transition is triggered by the spin collinearity developed by the nematic order. Although further-neighbor exchange interactions originating in the cooperative lattice distortions result in spin-lattice order in the weakly disordered regime, the concomitant transition remains robust with Tf almost independent of Δ. We find that the magnetic susceptibility shows hysteresis between the field-cooled and zero-field-cooled data below Tf, and that the nonlinear susceptibility shows a negative divergence at the transition. These features are common to conventional spin-glass systems. Meanwhile, we find that the specific heat exhibits a broad peak at Tf, and that the

  12. Intuitive understanding of T → 0 behavior of 2d spin glasses via renormalization group analysis

    NASA Astrophysics Data System (ADS)

    Hartmann, A. K.

    2012-07-01

    Commentary on 'The nature of the different zero-temperature phases in discrete two-dimensional spin glasses: entropy, universality, chaos and cascades in the renormalization group flow', by Thomas Jörg and Florent Krzakala, 2012 J. Stat. Mech. L01001.

  13. Monolithically integrated solid state laser and waveguide using spin-on glass

    DOEpatents

    Ashby, Carol I. H.; Hohimer, John P.; Neal, Daniel R.; Vawter, G. Allen

    1995-01-01

    A monolithically integrated photonic circuit combining a semiconductor source of excitation light with an optically active waveguide formed on the substrate. The optically active waveguide is preferably formed of a spin-on glass to which are added optically active materials which can enable lasing action, optical amplification, optical loss, or frequency conversion in the waveguide, depending upon the added material.

  14. Aging at the spin-glass/ferromagnet transition: Monte Carlo simulations using graphics processing units

    NASA Astrophysics Data System (ADS)

    Manssen, Markus; Hartmann, Alexander K.

    2015-05-01

    We study the nonequilibrium aging behavior of the ±J Edwards-Anderson model in three dimensions for samples of size up to N =1283 and for up to 108 Monte Carlo sweeps. In particular we are interested in the change of the aging when crossing from the spin-glass phase to the ferromagnetic phase. The necessary long simulation times are reached by employing a CUDA-based GPU implementation, which allows for single-spin flip times as small as 8 ps. We measure typical spin-glass correlation functions in space and time to determine the growing length scale and extract the constituting exponents. We observe a clear signature of the disorder-driven equilibrium transition in the nonequilibrium behavior.

  15. Diffusivity Maximum in a Reentrant Nematic Phase

    PubMed Central

    Stieger, Tillmann; Mazza, Marco G.; Schoen, Martin

    2012-01-01

    We report molecular dynamics simulations of confined liquid crystals using the Gay–Berne–Kihara model. Upon isobaric cooling, the standard sequence of isotropic–nematic–smectic A phase transitions is found. Upon further cooling a reentrant nematic phase occurs. We investigate the temperature dependence of the self-diffusion coefficient of the fluid in the nematic, smectic and reentrant nematic phases. We find a maximum in diffusivity upon isobaric cooling. Diffusion increases dramatically in the reentrant phase due to the high orientational molecular order. As the temperature is lowered, the diffusion coefficient follows an Arrhenius behavior. The activation energy of the reentrant phase is found in reasonable agreement with the reported experimental data. We discuss how repulsive interactions may be the underlying mechanism that could explain the occurrence of reentrant nematic behavior for polar and non-polar molecules. PMID:22837730

  16. Diffusivity maximum in a reentrant nematic phase.

    PubMed

    Stieger, Tillmann; Mazza, Marco G; Schoen, Martin

    2012-01-01

    We report molecular dynamics simulations of confined liquid crystals using the Gay-Berne-Kihara model. Upon isobaric cooling, the standard sequence of isotropic-nematic-smectic A phase transitions is found. Upon further cooling a reentrant nematic phase occurs. We investigate the temperature dependence of the self-diffusion coefficient of the fluid in the nematic, smectic and reentrant nematic phases. We find a maximum in diffusivity upon isobaric cooling. Diffusion increases dramatically in the reentrant phase due to the high orientational molecular order. As the temperature is lowered, the diffusion coefficient follows an Arrhenius behavior. The activation energy of the reentrant phase is found in reasonable agreement with the reported experimental data. We discuss how repulsive interactions may be the underlying mechanism that could explain the occurrence of reentrant nematic behavior for polar and non-polar molecules. PMID:22837730

  17. Effect of modularity on the Glauber dynamics of the dilute spin glass model

    NASA Astrophysics Data System (ADS)

    Park, Jeong-Man

    2014-11-01

    We study the Glauber dynamics of the dilute, infinite-ranged spin glass model, the so-called dilute Sherrington-Kirkpatrick (dSK) model. The dSK model has sparse couplings and can be classified by the modularity ( M) of the coupling matrix. We investigate the effect of the modularity on the relaxation dynamics starting from a random initial state. By using the Glauber dynamics and the replica method, we derive the relaxation dynamics equations for the magnetization ( m) and the energy per spin ( r), in addition to the equation for the spin glass order parameter ( q αβ ). In the replica symmetric (RS) analysis, we find that there are two solutions for the RS spin glass order parameter ( q): q = 0which is stable for r < 1/2 and q = (-1+4 r 2)/(32 r 4) which is stable for r > 1/2 in the non-modular system and q = 0 which is stable for r < 1/ and q = (-1+8 r 2)/(128 r 4) which is stable for r > 1/ in the completely modular system. By substituting the proper q values into the equations for r, we find that the relaxation dynamics of r depends on the modularity, M. These results suggest that, in the context of evolutionary theory, the modularity may emerge spontaneously in the point-mutation-only framework (Glauber dynamics) under a changing environment.

  18. Spin-glass freezing in a Ni-vermiculite intercalation compound

    NASA Astrophysics Data System (ADS)

    Marcos, C.; Argüelles, A.; Khainakov, S. A.; Rodríguez Fernández, J.; Blanco, J. A.

    2012-08-01

    We report on the magnetic properties of a Ni2+-vermiculite intercalation compound from Santa Olalla, Huelva (Spain). This modified vermiculite was studied by means of DC and AC magnetic measurements. The existence of two maxima in magnetic susceptibility below 10 K was interpreted in terms of the Cole-Cole formalism as being due to spin-glass freezing in this material. The temperature, frequency and external magnetic field dependences of these anomalies located at temperatures around 2-3 K and 8-10 K in the imaginary part of the magnetic susceptibility, χ″, seem to suggest the existence of spin-relaxation phenomena between the magnetic moments of the Ni2+ ions. A dynamic study of the relaxation processes associated with these phenomena considering the Cole-Cole formalism allows us to interpret the anomaly found at 2-3 K according to a law of activated dynamics, obtaining values for the critical exponent, ψν < 1, characteristic of a d = 2 spin-glass-like system, while the maximum observed in χ″ at 8-10 K can be described by means of a law of standard dynamics with a value of the exponent z of around 5, representative of a d = 3 spin-glass-like system.

  19. Exact ground states of large two-dimensional planar Ising spin glasses

    NASA Astrophysics Data System (ADS)

    Pardella, G.; Liers, F.

    2008-11-01

    Studying spin-glass physics through analyzing their ground-state properties has a long history. Although there exist polynomial-time algorithms for the two-dimensional planar case, where the problem of finding ground states is transformed to a minimum-weight perfect matching problem, the reachable system sizes have been limited both by the needed CPU time and by memory requirements. In this work, we present an algorithm for the calculation of exact ground states for two-dimensional Ising spin glasses with free boundary conditions in at least one direction. The algorithmic foundations of the method date back to the work of Kasteleyn from the 1960s for computing the complete partition function of the Ising model. Using Kasteleyn cities, we calculate exact ground states for huge two-dimensional planar Ising spin-glass lattices (up to 30002 spins) within reasonable time. According to our knowledge, these are the largest sizes currently available. Kasteleyn cities were recently also used by Thomas and Middleton in the context of extended ground states on the torus. Moreover, they show that the method can also be used for computing ground states of planar graphs. Furthermore, we point out that the correctness of heuristically computed ground states can easily be verified. Finally, we evaluate the solution quality of heuristic variants of the L. Bieche approach.

  20. Cluster spin glass behavior in geometrically frustrated Zn3V3O8.

    PubMed

    Chakrabarty, T; Mahajan, A V; Kundu, S

    2014-10-01

    We report the bulk magnetic properties of a yet unexplored vanadium-based multi-valenced spinel system, Zn3V3O8. A Curie-Weiss fit of our dc magnetic susceptibility χ(T) data in the temperature region of 140-300 K yields a Curie constant C = 0.75 cm(3)K mole(-1) V(-1), θCW = -370 K. We have observed a splitting between the zero field cooled (ZFC) and field cooled (FC) susceptibility curves below a temperature Tirr of about 6.3 K. The value of the 'frustration parameter' (|θcw|/T(N) ~ 100) suggests that the system is strongly frustrated. From the ac susceptibility measurements we find a logarithmic variation of freezing temperature (Tf) with frequency ν attesting to the formation of a spin glass below Tf. However, the value of the characteristic frequency obtained from the Vogel-Fulcher fit suggests that the ground state is closer to a cluster glass rather than a conventional spin glass. We explored further consequences of the spin glass behavior and observed aging phenomena and memory effect (both in ZFC and FC). We found that a positive temperature cycle erases the memory, as predicted by the hierarchical model. From the heat capacity CP data, a hump-like anomaly was observed in CP/T at about 3.75 K. Below this temperature the magnetic heat capacity shows a nearly linear dependence with T which is consistent with the formation of a spin glass state below Tf in Zn(3)V(3)O(8). PMID:25210922

  1. Typical versus averaged overlap distribution in spin glasses: Evidence for droplet scaling theory

    NASA Astrophysics Data System (ADS)

    Monthus, Cécile; Garel, Thomas

    2013-10-01

    We consider the statistical properties over disordered samples (J) of the overlap distribution PJ(q) which plays the role of an order parameter in spin glasses. We show that near zero temperature (i) the typical overlap distribution is exponentially small in the central region of -1spins (in order to consider also fully connected models in which the notion of length does not exist); (ii) the rescaled variable v=-[lnPJ(q)]/Nθ remains an O(1) random positive variable describing sample-to-sample fluctuations; (iii) the averaged distribution PJ(q)¯ is nontypical and dominated by rare anomalous samples. Similar statements hold for the cumulative overlap distribution IJ(q0)≡∫0q0dqPJ(q). These results are derived explicitly for the spherical mean-field model with θ=1/3, ϕ(q)=1-q2, and the random variable v corresponds to the rescaled difference between the two largest eigenvalues of Gaussian orthogonal ensemble random matrices. Then we compare numerically the typical and averaged overlap distributions for the long-ranged one-dimensional Ising spin glass with random couplings decaying as J(r)∝r-σ for various values of the exponent σ, corresponding to various droplet exponents θ(σ), and for the mean-field Sherrington-Kirkpatrick model (corresponding formally to the σ=0 limit of the previous model). Our conclusion is that future studies on spin glasses should measure the typical values of the overlap distribution Ptyp(q) or of the cumulative overlap distribution Ityp(q0)=elnIJ(q0)¯ to obtain clearer conclusions on the nature of the spin-glass phase.

  2. Surface spin-glass in cobalt ferrite nanoparticles dispersed in silica matrix

    NASA Astrophysics Data System (ADS)

    Zeb, F.; Sarwer, W.; Nadeem, K.; Kamran, M.; Mumtaz, M.; Krenn, H.; Letofsky-Papst, I.

    2016-06-01

    Surface effects in cobalt ferrite (CoFe2O4) nanoparticles dispersed in a silica (SiO2) matrix were studied by using AC and DC magnetization. Nanoparticles with different concentration of SiO2 were synthesized by using sol-gel method. Average crystallite size lies in the range 25-34 nm for different SiO2 concentration. TEM image showed that particles are spherical and elongated in shape. Nanoparticles with higher concentration of SiO2 exhibit two peaks in the out-of-phase ac-susceptibility. First peak lies in the high temperature regime and corresponds to average blocking temperature of the nanoparticles. Second peak lies in the low temperature regime and is attributed to surface spin-glass freezing in these nanoparticles. Low temperature peak showed SiO2 concentration dependence and was vanished for large uncoated nanoparticles. The frequency dependence of the AC-susceptibility of low temperature peak was fitted with dynamic scaling law which ensures the presence of spin-glass behavior. With increasing applied DC field, the low temperature peak showed less shift as compared to blocking peak, broaden, and decreased in magnitude which also signifies its identity as spin-glass peak for smaller nanoparticles. M-H loops showed the presence of more surface disorder in nanoparticles dispersed in 60% SiO2 matrix. All these measurements revealed that surface effects become strengthen with increasing SiO2 matrix concentration and surface spins freeze in to spin-glass state at low temperatures.

  3. Spin-polarized lithium diffusion in a glass hot-vapor cell

    NASA Astrophysics Data System (ADS)

    Ishikawa, Kiyoshi

    2016-08-01

    We report diffusion coefficients of optically pumped lithium atoms in helium buffer gas. The free-induction decay and the spin-echo signals of ground-state atoms were optically detected in an external magnetic field with the addition of field gradient. Lithium hot vapor was produced in a borosilicate-glass cell at a temperature between 290 and 360°C. The simple setup using the glass cells enabled lithium atomic spectroscopy in a similar way to other alkali-metal atoms and study of the collisional properties of lithium atoms in a hot-vapor phase.

  4. Magnetic surfactants as molecular based-magnets with spin glass-like properties

    NASA Astrophysics Data System (ADS)

    Brown, Paul; Smith, Gregory N.; Padrón Hernández, Eduardo; James, Craig; Eastoe, Julian; Nunes, Wallace C.; Settens, Charles M.; Hatton, T. Alan; Baker, Peter J.

    2016-05-01

    This paper reports the use of muon spin relaxation spectroscopy to study how the aggregation behavior of magnetic surfactants containing lanthanide counterions may be exploited to create spin glass-like materials. Surfactants provide a unique approach to building in randomness, frustration and competing interactions into magnetic materials without requiring a lattice of ordered magnetic species or intervening ligands and elements. We demonstrate that this magnetic behavior may also be manipulated via formation of micelles rather than simple dilution, as well as via design of surfactant molecular architecture. This somewhat unexpected result indicates the potential of using novel magnetic surfactants for the generation and tuning of molecular magnets.

  5. Magnetic surfactants as molecular based-magnets with spin glass-like properties.

    PubMed

    Brown, Paul; Smith, Gregory N; Hernández, Eduardo Padrón; James, Craig; Eastoe, Julian; Nunes, Wallace C; Settens, Charles M; Hatton, T Alan; Baker, Peter J

    2016-05-01

    This paper reports the use of muon spin relaxation spectroscopy to study how the aggregation behavior of magnetic surfactants containing lanthanide counterions may be exploited to create spin glass-like materials. Surfactants provide a unique approach to building in randomness, frustration and competing interactions into magnetic materials without requiring a lattice of ordered magnetic species or intervening ligands and elements. We demonstrate that this magnetic behavior may also be manipulated via formation of micelles rather than simple dilution, as well as via design of surfactant molecular architecture. This somewhat unexpected result indicates the potential of using novel magnetic surfactants for the generation and tuning of molecular magnets. PMID:27028571

  6. Metal-ion spin-on glasses: Novel materials for active waveguides

    SciTech Connect

    Ashby, C.I.H.; Sullivan, C.T.; Vawter, G.A.; Hohimer, J.P.; Hadley, G.R.; Neal, D.R.

    1993-12-31

    Monolithic integration of a rare-earth-ion-based active waveguide on the same wafer as its diode pump laser would permit compact packaging of the technology demonstrated in fiber lasers and amplifiers. This new monolithic technology would offer the potential for developing compact infrared and visible (up-conversion) lasers, amplifiers, and other photonic integrated circuit components. One approach that we are investigating for such monolithic integration uses a high concentration of one or more rare-earth ions incorporated into polysiloxane spin-on glasses that are solvent-cast onto III-V semiconductor wafers. This ``fiber on a chip`` technology substitute a relatively high-ion-concentration, short-length metal-ion spin-on glass (MISOG) waveguide for the low-ion-concentration, long-length fiber. Progress to data on developing MISOG waveguide materials and technology is discussed.

  7. Temperature chaos in a Ge:Mn thin-film spin glass

    NASA Astrophysics Data System (ADS)

    Guchhait, Samaresh; Orbach, Raymond L.

    2015-12-01

    Temperature changes in thin-film Ge:Mn spin-glass dynamics are presented that exhibit temperature chaos (TC) when the spin-glass correlation length ξ (t ,T ) grows to its thickness L . For small L ≈15.5 nm, the transition to chaos takes place over a temperature range Δ T sufficiently large to exhibit both reversible and chaotic behavior. The value of Δ T can be related to the critical exponent for TC, ζ . Experimentally, ζ is found to be ≈1.06 , in the range of recent simulations. The presence of a specific length scale L allows the transition to chaos to be examined over measurable laboratory temperature changes. The transition is found to be abrupt. Bulk materials, with a distribution of crystallite sizes, will smear out the transition, resulting in a very slow crossover. The abruptness of the transition and its nature are compared with recent theoretical calculations.

  8. A spin glass approach to the directed feedback vertex set problem

    NASA Astrophysics Data System (ADS)

    Zhou, Hai-Jun

    2016-07-01

    A directed graph (digraph) is formed by vertices and arcs (directed edges) from one vertex to another. A feedback vertex set (FVS) is a set of vertices that contains at least one vertex of every directed cycle in this digraph. The directed feedback vertex set problem aims at constructing a FVS of minimum cardinality. This is a fundamental cycle-constrained hard combinatorial optimization problem with wide practical applications. In this paper we construct a spin glass model for the directed FVS problem by converting the global cycle constraints into local arc constraints, and study this model through the replica-symmetric (RS) mean field theory of statistical physics. We then implement a belief propagation-guided decimation (BPD) algorithm for single digraph instances. The BPD algorithm slightly outperforms the simulated annealing algorithm on large random graph instances. The RS mean field results and algorithmic results can be further improved by working on a more restrictive (and more difficult) spin glass model.

  9. Laser photoablation of spin-on-glass and poly(ethyl cyanoacrylate) photoresist

    NASA Astrophysics Data System (ADS)

    Hogan, M.; Magan, J. D.; Blau, W.; Lunney, J. G.; Woods, J.

    The laser photoablation characteristics, at 193 and 248 nm, of a spin-on-glass (Allied Accuglass 204) in the pre-cured state, and a novel poly(ethyl cyanoacrylate) photoresist material have been studied using a low power He-Ne laser to monitor interferometrically the ablation. The etching behaviour of the photoresist material is compared with the photoablation model of Sutcliffe and Srinavasan. The dry microlithographic potential of both materials was investigated.

  10. Spin glass in semiconducting KFe1.05Ag0.88Te2 single crystals

    DOE PAGESBeta

    Ryu, H.; Lei, H.; Klobes, B.; Warren, J. B.; Hermann, R. P.; Petrovic, C.

    2015-05-26

    We report discovery of KFe1.05Ag0.88Te2 single crystals with semiconducting spin glass ground state. Composition and structure analysis suggest nearly stoichiometric I4/mmm space group but allow for the existence of vacancies, absent in long range semiconducting antiferromagnet KFe1.05Ag0.88Te2. The subtle change in stoichometry in Fe/Ag sublattice changes magnetic ground state but not conductivity, giving further insight into the semiconducting gap mechanism.

  11. Monolithically integrated solid state laser and waveguide using spin-on glass

    DOEpatents

    Ashby, C.I.H.; Hohimer, J.P.; Neal, D.R.; Vawter, G.A.

    1995-10-31

    A monolithically integrated photonic circuit is disclosed combining a semiconductor source of excitation light with an optically active waveguide formed on the substrate. The optically active waveguide is preferably formed of a spin-on glass to which are added optically active materials which can enable lasing action, optical amplification, optical loss, or frequency conversion in the waveguide, depending upon the added material. 4 figs.

  12. About a solvable mean field model of a Gaussian spin glass

    NASA Astrophysics Data System (ADS)

    Barra, Adriano; Genovese, Giuseppe; Guerra, Francesco; Tantari, Daniele

    2014-04-01

    In a series of papers, we have studied a modified Hopfield model of a neural network, with learned words characterized by a Gaussian distribution. The model can be represented as a bipartite spin glass, with one party described by dichotomic Ising spins, and the other party by continuous spin variables, with an a priori Gaussian distribution. By application of standard interpolation methods, we have found it useful to compare the neural network model (bipartite) from one side, with two spin glass models, each monopartite, from the other side. Of these, the first is the usual Sherrington-Kirkpatrick model, the second is a spin glass model, with continuous spins and inbuilt highly nonlinear smooth cut-off interactions. This model is an invaluable laboratory for testing all techniques which have been useful in the study of spin glasses. The purpose of this paper is to give a synthetic description of the most peculiar aspects, by stressing the necessary novelties in the treatment. In particular, it will be shown that the control of the infinite volume limit, according to the well-known Guerra-Toninelli strategy, requires in addition one to consider the involvement of the cut-off interaction in the interpolation procedure. Moreover, the control of the ergodic region, the annealed case, cannot be directly achieved through the standard application of the Borel-Cantelli lemma, but requires previous modification of the interaction. This remark could find useful application in other cases. The replica symmetric expression for the free energy can be easily reached through a suitable version of the doubly stochastic interpolation technique. However, this model shares the unique property that the fully broken replica symmetry ansatz can be explicitly calculated. A very simple sum rule connects the general expression of the fully broken free energy trial function with the replica symmetric one. The definite sign of the error term shows that the replica solution is optimal. Then

  13. Monte Carlo and mean field slow cooling simulations for spin glasses: relation to NP-completeness

    NASA Astrophysics Data System (ADS)

    Grest, Gary S.; Soukoulis, C. M.; Levin, K.; Randelman, R. E.

    We discuss the dependence of the ground state energy as the Monte Carlo (MC) cooling rate in a variety of spin glass and other optimization problems. Our work is motivated by recent interest in the concept of simulated annealing and the need to find efficient numerical optimization schemes. We find that the ground state energy for six spin-glass models and the optimal tour length for the N-city traveling salesman problem are a sensitive function of the cooling rate. For the one dimensional nearest neighbor Gaussian and two two dimensional spin-glass models, the nearest neighbor ± J and Gaussian models, we find E(r) = E0 + C1 rx, while for the three dimensional t J model, a two layer t J model, the infinite range model and the N-city traveling salesman problem, the dependence on r is slower, E(r) = Eo - C2 (Inr)-1.Here C1 and C2 are constants.We believe that this difference is related to the fact that finding the ground state energy for the latter four problems is an intractable problem, i.e., it is Np-complete. We assert that a logarithmic cooling rate dependence for finding the ground state is a necessary consequence of Np-completeness. Non Np-complete problems may depend on the cooling rate either as a power-law or logarithmically. However, we have found no example for the latter.

  14. Observation of spin-glass behavior in nickel adsorbed few layer graphene

    SciTech Connect

    Mitra, Sreemanta; Mondal, Oindrila; Banerjee, Sourish; Chakravorty, Dipankar

    2013-01-14

    Nickel-adsorbed graphene was prepared by first synthesizing graphite oxide (GO) by modified Hummers' method and then reducing a solution containing both GO and Ni{sup 2+}. Energy dispersive X-ray spectroscopy analysis showed 31 at. % nickel was present. Magnetization measurements under both dc and ac magnetic fields were carried out in the temperature range 2 K to 300 K. The zero field cooled and field cooled magnetization data showed a pronounced irreversibility at a temperature around 20 K. The analysis of the ac susceptibility data was carried out by both Vogel-Fulcher as well as power law. From dynamic scaling analysis, the microscopic flipping time {tau}{sub 0}{approx}10{sup -13}s and critical exponent z{nu}=5.9{+-}0.1 were found, indicating the presence of conventional spin glass in the system. The spin glass transition temperature was estimated as 19.5 K. Decay of thermoremanent magnetization was explained by stretched exponential function with a value of the exponent as 0.6. From the results, it is concluded that nickel adsorbed graphene behaves like a spin-glass.

  15. Finite-size corrections for ground states of Edwards-Anderson spin glasses

    NASA Astrophysics Data System (ADS)

    Boettcher, Stefan; Falkner, Stefan

    2012-05-01

    Extensive computations of ground-state energies of the Edwards-Anderson spin glass on bond-diluted, hypercubic lattices are conducted in dimensions d=3, ..., 7. Results are presented for bond densities exactly at the percolation threshold, p=pc, and deep within the glassy regime, p>pc, where finding ground states is one of the hardest combinatorial optimization problems. Finite-size corrections of the form 1/Nω are shown to be consistent throughout with the prediction ω=1-y/d, where y refers to the "stiffness" exponent that controls the formation of domain wall excitations at low temperatures. At p=pc, an extrapolation for d→∞ appears to match our mean-field results for these corrections. In the glassy phase, however, ω does not approach its anticipated mean-field value of 2/3, obtained from simulations of the Sherrington-Kirkpatrick spin glass on an N-clique graph. Instead, the value of ω reached at the upper critical dimension matches another type of mean-field spin glass models, namely those on sparse random networks of regular degree called Bethe lattices.

  16. Faraday rotation measurements of time dependent magnetic phenomena in insulating spin glasses (invited)

    NASA Astrophysics Data System (ADS)

    Ferré, J.; Rajchenbach, J.; Maletta, H.

    1981-03-01

    We report here on a detailed study of the relaxation of the magnetization in an external field and of the remanent magnetization for the insulating spin glass: Eu0.4Sr0.6S. The Faraday rotation has allowed us to extend previous experiments near and just above the spin glass temperature Tfo = 1.55 K on a large time scale (10-6spin glass. The unusual field dependence of the TRM, always found in SG systems, i.e., the maximum in TRM versus H, results from an increase of the demagnetization rate with H, which can compensate the large initial magnetization at high fields. The isothermal remanent magnetization is analyzed from our knowledge of the time and field dependence of the in-field magnetization. The Tf(ν) variation and the TRM (H) behavior cannot be explained by a model of non interacting clusters and suggest a cooperative behavior, as for ferromagnets.

  17. Exchange bias and coercivity for ferromagnets coupled to the domain state and spin glass state

    NASA Astrophysics Data System (ADS)

    Zhan, Xiaozhi; Mao, Zhongquan; Chen, Xi

    2016-05-01

    The exchange bias (EB) effect for systems with a ferromagnetic (FM) layer coupled to bond-diluted pinning layers has been investigated by Monte Carlo simulations. Two bond dilution concentrations are chosen to obtain two kinds of pinning layers: the antiferromagnetic domain state (DS) and the spin glass (SG) state. It is found that when coupled to the more disordered SG state, the ferromagnet shows enhanced EB with higher coercivity due to larger amounts of both frozen and reversible spins at the pinning interface. Spin configurations of the FM/DS interface layer reveal that reversible spins are mostly found in domain boundaries and small domains, while most spins in large domains maintain antiferromagnetic coupling and contribute to the EB effect. The coercivity is linear to the amount of interface reversible spins, but with different slopes in the temperature ranges above or below the blocking temperature t B. This bimodal temperature-dependent coercivity indicates a sudden change in macroscopic interface coupling at the temperature t B.

  18. Structure of Finite-RSB Asymptotic Gibbs Measures in the Diluted Spin Glass Models

    NASA Astrophysics Data System (ADS)

    Panchenko, Dmitry

    2016-01-01

    We suggest a possible approach to proving the Mézard-Parisi formula for the free energy in the diluted spin glass models, such as diluted K-spin or random K-sat model at any positive temperature. In the main contribution of the paper, we show that a certain small modification of the Hamiltonian in any of these models forces all finite-RSB asymptotic Gibbs measures in the sense of the overlaps to satisfy the Mézard-Parisi ansatz for the distribution of spins. Unfortunately, what is still missing is a description of the general full-RSB asymptotic Gibbs measures. If one could show that the general case can be approximated by finite-RSB case in the right sense then one could a posteriori remove the small modification of the Hamiltonian to recover the Mézard-Parisi formula for the original model.

  19. Spin glass state and enhanced spiral phase in doped delafossite oxide CuCr O2

    NASA Astrophysics Data System (ADS)

    Yan, Z. R.; Qin, M. H.; Dong, S.; Zeng, M.; Lu, X. B.; Gao, X. S.; Liu, J.-M.

    2016-07-01

    In this paper, we study the doping effects on the magnetic states of CuCr O2 based on the classical frustrated spin model [13aa Lin et al., Phys. Rev. B 89, 220405(R) (2014), 10.1103/PhysRevB.89.220405]. Several experimental observations can be well reproduced by the Monte Carlo simulations of the modified spin models. Our paper suggests that the disorder induced by V/Al doping cooperating with the frustration in the system may contribute to the emergence of the spin glass state. Furthermore, the hole doping by M g2 + substituting C r3 + enhances the quantum fluctuations and bond disorder which modulate the biquadratic exchanges and in turn results in the promotion of the spiral phase, consistent with the experimental report.

  20. Spin Correlations and Excitations in the Quasi-2D Triangular Bilayer Spin Glass LuCoGaO4

    NASA Astrophysics Data System (ADS)

    Fritsch, K.; Granroth, G. E.; Savici, A. T.; Noad, H. M. L.; Dabkowska, H. A.; Gaulin, B. D.

    2012-02-01

    LuCoGaO4 is a layered magnetic-bilayer material wherein Co2+ magnetic moments and nonmagnetic Ga3+ ions are randomly distributed on planar triangular bilayers. This makes it an ideal case to study the interplay between geometric frustration, site disorder and low dimensionality and its influence on the magnetic ground of the system. This novel material has been grown for the first time in single crystal form at McMaster University. We have performed magnetization measurements, revealing a previously identified spin glass transition near Tf˜19K, and a Curie Weiss temperature of Tcw˜-96K, consistent with antiferromagnetic interactions[1]. We discuss time-of-flight neutron scattering measurements using SEQUOIA at SNS which elucidate the evolution of the static and dynamic spin correlations in LuCoGaO4 over a range of temperatures from T<< Tf to T>Tcw. We observe quasielastic scattering at (1/3,1/3,L) positions in reciprocal space and rods of scattering along the c*-direction, consistent with short range antiferromagnetic correlations within decoupled bilayers, and which comfirm the 2-dimensional character of this system. Inelastic scattering measurements show a gapped ˜ 12 meV spin excitation which softens and broadens in energy, filling in the gap on a temperature scale of ˜ Tcw/2. [1] Cava et al., J. Solid State Chem. 140, 337 (1998).

  1. Nanocomposites of silver nanoparticles embedded in glass nanofibres obtained by laser spinning

    NASA Astrophysics Data System (ADS)

    Cabal, Belén; Quintero, Félix; Díaz, Luís Antonio; Rojo, Fernando; Dieste, Oliver; Pou, Juan; Torrecillas, Ramón; Moya, José Serafín

    2013-04-01

    Nanocomposites made of non-woven glass fibres with diameters ranging from tens of nanometers up to several micrometers, containing silver nanoparticles, were successfully fabricated by the laser spinning technique. Pellets of a soda-lime silicate glass containing silver nanoparticles with varying concentrations (5 and 10 wt%) were used as a precursor. The process followed to obtain the silver nanofibres did not agglomerate significantly the metallic nanoparticles, and the average particle size is still lower than 50 nm. This is the first time that glass nanofibres containing silver nanoparticles have been obtained following a process different from electrospinning of a sol-gel, thus avoiding the limitations of this method and opening a new route to composite nanomaterials. Antibacterial efficiency of the nanosilver glass fibres, tested against one of the most common Gram negative bacteria, was greater than 99.99% compared to the glass fibres free of silver. The silver nanoparticles are well-dispersed not only on the surface but are also embedded into the uniform nanofibres, which leads to a long lasting durable antimicrobial effect. All these novel characteristics will potentially open up a whole new range of applications.

  2. Nanocomposites of silver nanoparticles embedded in glass nanofibres obtained by laser spinning.

    PubMed

    Cabal, Belén; Quintero, Félix; Díaz, Luís Antonio; Rojo, Fernando; Dieste, Oliver; Pou, Juan; Torrecillas, Ramón; Moya, José Serafín

    2013-05-01

    Nanocomposites made of non-woven glass fibres with diameters ranging from tens of nanometers up to several micrometers, containing silver nanoparticles, were successfully fabricated by the laser spinning technique. Pellets of a soda-lime silicate glass containing silver nanoparticles with varying concentrations (5 and 10 wt%) were used as a precursor. The process followed to obtain the silver nanofibres did not agglomerate significantly the metallic nanoparticles, and the average particle size is still lower than 50 nm. This is the first time that glass nanofibres containing silver nanoparticles have been obtained following a process different from electrospinning of a sol-gel, thus avoiding the limitations of this method and opening a new route to composite nanomaterials. Antibacterial efficiency of the nanosilver glass fibres, tested against one of the most common gram negative bacteria, was greater than 99.99% compared to the glass fibres free of silver. The silver nanoparticles are well-dispersed not only on the surface but are also embedded into the uniform nanofibres, which leads to a long lasting durable antimicrobial effect. All these novel characteristics will potentially open up a whole new range of applications. PMID:23535995

  3. Critical properties of short-range Ising spin glasses on a Wheatstone-bridge hierarchical lattice

    NASA Astrophysics Data System (ADS)

    Almeida, Sebastião T. O.; Nobre, Fernando D.

    2015-08-01

    An Ising spin-glass model with nearest-neighbor interactions, following a symmetric probability distribution, is investigated on a hierarchical lattice of the Wheatstone-bridge family characterized by a fractal dimension D ≈3.58 . The interaction distribution considered is a stretched exponential, which has been shown recently to be very close to the fixed-point coupling distribution, and such a model has been considered lately as a good approach for Ising spin glasses on a cubic lattice. An exact recursion procedure is implemented for calculating site magnetizations, mi=T , as well as correlations between pairs of nearest-neighbor spins, T (<>T denote thermal averages), for a given set of interaction couplings on this lattice. From these local magnetizations and correlations, one can compute important physical quantities, such as the Edwards-Anderson order parameter, the internal energy, and the specific heat. Considering extrapolations to the thermodynamic limit for the order parameter, such as a finite-size scaling approach, it is possible to obtain directly the critical temperature and critical exponents. The transition between the spin-glass and paramagnetic phases is analyzed, and the associated critical exponents β and ν are estimated as β =0.82 (5 ) and ν =2.50 (4 ) , which are in good agreement with the most recent results from extensive numerical simulations on a cubic lattice. Since these critical exponents were obtained from a fixed-point distribution, they are universal, i.e., valid for any coupling distribution considered.

  4. From spin induced ferroelectricity to dipolar glasses: Spinel chromites and mixed delafossites

    SciTech Connect

    Maignan, A.

    2012-11-15

    Magnetoelectric multiferroics showing coupling between polarization and magnetic order are attracting much attention. For instance, they could be used in memory devices. Metal-transition oxides are provided several examples of inorganic magnetoelectric multiferroics. In the present short review, spinel and delafossite chromites are described. For the former, an electric polarization is evidenced in the ferrimagnetic state for ACr{sub 2}O{sub 4} polycrystalline samples (A=Ni, Fe, Co). The presence of a Jahn-Teller cation such as Ni{sup 2+} at the A site is shown to yield larger polarization values. In the delafossites, substitution by V{sup 3+} at the Cr or Fe site in CuCrO{sub 2} (CuFeO{sub 2}) suppresses the complex antiferromagnetic structure at the benefit of a spin glass state. The presence of cation disorder, probed by transmission electron microscopy, favors relaxor-like ferroelectricity. The results on the ferroelectricity of ferrimagnets and insulating spin glasses demonstrate that, in this research field, transition-metal oxides are worth to be studied. - Graphical abstract: Electric polarization as a function of temperature is measured up to T{sub C} in three chromite ferrimagnetic spinels. Largest values are reached for spinels with Jahn-Teller cations at the A site (Ni or Fe). Highlights: Black-Right-Pointing-Pointer Electric polarization is evidenced in the ferrimagnetic state of the chromite spinels. Black-Right-Pointing-Pointer Jahn-Teller cations at the A site of these spinels lead to larger polarization values. Black-Right-Pointing-Pointer Vanadium substituted at the Cr (or Fe) site of delafossites changes the antiferromagnetic state to spin glass. Black-Right-Pointing-Pointer Electric polarization is not the result of magnetic ordering but magnetic disordering in Cr or Fe delafossites. Black-Right-Pointing-Pointer Relaxor-type ferroelectricity or spin induced ferroelectricity can be observed in the delafossites.

  5. Critical properties of short-range Ising spin glasses on a Wheatstone-bridge hierarchical lattice.

    PubMed

    Almeida, Sebastião T O; Nobre, Fernando D

    2015-08-01

    An Ising spin-glass model with nearest-neighbor interactions, following a symmetric probability distribution, is investigated on a hierarchical lattice of the Wheatstone-bridge family characterized by a fractal dimension D≈3.58. The interaction distribution considered is a stretched exponential, which has been shown recently to be very close to the fixed-point coupling distribution, and such a model has been considered lately as a good approach for Ising spin glasses on a cubic lattice. An exact recursion procedure is implemented for calculating site magnetizations, mi=〈Si〉T, as well as correlations between pairs of nearest-neighbor spins, 〈SiSj〉T (〈〉T denote thermal averages), for a given set of interaction couplings on this lattice. From these local magnetizations and correlations, one can compute important physical quantities, such as the Edwards-Anderson order parameter, the internal energy, and the specific heat. Considering extrapolations to the thermodynamic limit for the order parameter, such as a finite-size scaling approach, it is possible to obtain directly the critical temperature and critical exponents. The transition between the spin-glass and paramagnetic phases is analyzed, and the associated critical exponents β and ν are estimated as β=0.82(5) and ν=2.50(4), which are in good agreement with the most recent results from extensive numerical simulations on a cubic lattice. Since these critical exponents were obtained from a fixed-point distribution, they are universal, i.e., valid for any coupling distribution considered. PMID:26382339

  6. Reentrant Processing in Intuitive Perception

    PubMed Central

    Luu, Phan; Geyer, Alexandra; Fidopiastis, Cali; Campbell, Gwendolyn; Wheeler, Tracey; Cohn, Joseph; Tucker, Don M.

    2010-01-01

    The process of perception requires not only the brain's receipt of sensory data but also the meaningful organization of that data in relation to the perceptual experience held in memory. Although it typically results in a conscious percept, the process of perception is not fully conscious. Research on the neural substrates of human visual perception has suggested that regions of limbic cortex, including the medial orbital frontal cortex (mOFC), may contribute to intuitive judgments about perceptual events, such as guessing whether an object might be present in a briefly presented fragmented drawing. Examining dense array measures of cortical electrical activity during a modified Waterloo Gestalt Closure Task, results show, as expected, that activity in medial orbital frontal electrical responses (about 250 ms) was associated with intuitive judgments. Activity in the right temporal-parietal-occipital (TPO) region was found to predict mOFC (∼150 ms) activity and, in turn, was subsequently influenced by the mOFC at a later time (∼300 ms). The initial perception of gist or meaning of a visual stimulus in limbic networks may thus yield reentrant input to the visual areas to influence continued development of the percept. Before perception is completed, the initial representation of gist may support intuitive judgments about the ongoing perceptual process. PMID:20209101

  7. Spin glass instead of superconductivity in Ba(Fe1-xCrx/2Nix/2)2As2

    NASA Astrophysics Data System (ADS)

    Xu, Sheng-Gao; Sun, Yun-Lei; Jiang, Shuai; Xing, Hui; Jiao, Lin; Yuan, Hui-Qiu; Feng, Chun-Mu; Xu, Zhu-An; Cao, Guang-Han

    2012-12-01

    We have studied an “isoelectronic” Fe-site doping with Cr and Ni in Ba(Fe1-xCrx/2Nix/2)2As2 system. With increasing x, the antiferromagnetic SDW in the parent compound is suppressed quickly. Spin glass state emerges in the range of 0.1 <= x < 0.2. The spin glass state evolves into cluster glass with further doping, and finally becomes ferromagnetism at x = 1.0. No superconductivity was observed down to 0.5 K. The electronic phase diagram is established, and the underlying physics is discussed.

  8. Hydration-induced spin-glass state in a frustrated Na-Mn-O triangular lattice

    NASA Astrophysics Data System (ADS)

    Bakaimi, Ioanna; Brescia, Rosaria; Brown, Craig M.; Tsirlin, Alexander A.; Green, Mark A.; Lappas, Alexandros

    2016-05-01

    Birnessite compounds are stable across a wide range of compositions that produces a remarkable diversity in their physical, electrochemical, and functional properties. These are hydrated analogs of the magnetically frustrated, mixed-valent manganese oxide structures, with general formula, N axMn O2 . Here we demonstrate that the direct hydration of layered rock-salt type α-NaMn O2 , with the geometrically frustrated triangular lattice topology, yields the birnessite type oxide, N a0.36Mn O2.0.2 H2O , transforming its magnetic properties. This compound has a much-expanded interlayer spacing compared to its parent α-NaMn O2 compound. We show that while the parent α-NaMn O2 possesses a Néel temperature of 45 K as a result of broken symmetry in the M n3 + sublattice, the hydrated derivative undergoes collective spin freezing at 29 K within the M n3 +/M n4 + sublattice. Scaling-law analysis of the frequency dispersion of the ac susceptibility, as well as the temperature-dependent, low-field dc magnetization confirm a cooperative spin-glass state of strongly interacting spins. This is supported by complementary spectroscopic analysis [high-angle annular dark-field scanning transmission electron miscroscopy (TEM), energy-dispersive x-ray spectroscopy, and electron energy-loss spectroscopy] as well as by a structural investigation (high-resolution TEM, x-ray, and neutron powder diffraction) that yield insights into the chemical and atomic structure modifications. We conclude that the spin-glass state in birnessite is driven by the spin frustration imposed by the underlying triangular lattice topology that is further enhanced by the in-plane bond-disorder generated by the mixed-valent character of manganese in the layers.

  9. Spin-exchange interaction between transition metals and metalloids in soft-ferromagnetic metallic glasses.

    PubMed

    Das, Santanu; Choudhary, Kamal; Chernatynskiy, Aleksandr; Choi Yim, Haein; Bandyopadhyay, Asis K; Mukherjee, Sundeep

    2016-06-01

    High-performance magnetic materials have immense industrial and scientific importance in wide-ranging electronic, electromechanical, and medical device technologies. Metallic glasses with a fully amorphous structure are particularly suited for advanced soft-magnetic applications. However, fundamental scientific understanding is lacking for the spin-exchange interaction between metal and metalloid atoms, which typically constitute a metallic glass. Using an integrated experimental and molecular dynamics approach, we demonstrate the mechanism of electron interaction between transition metals and metalloids. Spin-exchange interactions were investigated for a Fe-Co metallic glass system of composition [(Co1-x Fe x )0.75B0.2Si0.05]96Cr4. The saturation magnetization increased with higher Fe concentration, but the trend significantly deviated from simple rule of mixtures. Ab initio molecular dynamics simulation was used to identify the ferromagnetic/anti-ferromagnetic interaction between the transition metals and metalloids. The overlapping band-structure and density of states represent 'Stoner type' magnetization for the amorphous alloys in contrast to 'Heisenberg type' in crystalline iron. The enhancement of magnetization by increasing iron was attributed to the interaction between Fe 3d and B 2p bands, which was further validated by valence-band study. PMID:27143686

  10. Spin-exchange interaction between transition metals and metalloids in soft-ferromagnetic metallic glasses

    NASA Astrophysics Data System (ADS)

    Das, Santanu; Choudhary, Kamal; Chernatynskiy, Aleksandr; Choi Yim, Haein; Bandyopadhyay, Asis K.; Mukherjee, Sundeep

    2016-06-01

    High-performance magnetic materials have immense industrial and scientific importance in wide-ranging electronic, electromechanical, and medical device technologies. Metallic glasses with a fully amorphous structure are particularly suited for advanced soft-magnetic applications. However, fundamental scientific understanding is lacking for the spin-exchange interaction between metal and metalloid atoms, which typically constitute a metallic glass. Using an integrated experimental and molecular dynamics approach, we demonstrate the mechanism of electron interaction between transition metals and metalloids. Spin-exchange interactions were investigated for a Fe–Co metallic glass system of composition [(Co1‑x Fe x )0.75B0.2Si0.05]96Cr4. The saturation magnetization increased with higher Fe concentration, but the trend significantly deviated from simple rule of mixtures. Ab initio molecular dynamics simulation was used to identify the ferromagnetic/anti-ferromagnetic interaction between the transition metals and metalloids. The overlapping band-structure and density of states represent ‘Stoner type’ magnetization for the amorphous alloys in contrast to ‘Heisenberg type’ in crystalline iron. The enhancement of magnetization by increasing iron was attributed to the interaction between Fe 3d and B 2p bands, which was further validated by valence-band study.

  11. Spin-glass behaviors in carrier polarity controlled Fe3-xTixO4 semiconductor thin films

    NASA Astrophysics Data System (ADS)

    Yamahara, H.; Seki, M.; Adachi, M.; Takahashi, M.; Nasu, H.; Horiba, K.; Kumigashira, H.; Tabata, H.

    2015-08-01

    Carrier-type control of spin-glass (cluster spin-glass) is studied in order to engineer basic magnetic semiconductor elements using the memory functions of spin-glass. A key of carrier-polarity control in magnetite is the valence engineering between Fe(II) and Fe(III) that is achieved by Ti(IV) substitution. Single phases of (001)-oriented Fe3-xTixO4 thin films have been obtained on spinel MgAl2O4 substrates by pulsed laser deposition. Thermoelectric power measurements reveal that Ti-rich films (x = 0.8) show p-type conduction, while Ti-poor films (x = 0.6-0.75) show n-type conduction. The systematic Fe(III) reduction to Fe(II) followed by Ti(IV) substitution in the octahedral sublattice is confirmed by the X-ray absorption spectra. All of the Fe3-xTixO4 films (x = 0.6-0.8) exhibit ferrimagnetism above room temperature. Next, the spin-glass behaviors of Ti-rich Fe2.2Ti0.8O4 film are studied, since this magnetically diluted system is expected to exhibit the spin-glass behaviors. The DC magnetization and AC susceptibility measurements for the Ti-rich Fe2.2Ti0.8O4 film reveal the presence of the spin glass phase. Thermal- and magnetic-field-history memory effects are observed and are attributed to the long time-decay nature of remanent magnetization. The detailed analysis of the time-dependent thermoremanent magnetization reveals the presence of the cluster spin glass state.

  12. Probing quantum spin glass like system with a double quantum dot

    NASA Astrophysics Data System (ADS)

    Koh, C. Y.; Kwek, L. C.

    2016-06-01

    We study the ground state properties of a 4-qubit spin glass like (SGL) chain with probes at the end of the chain and compare our results with the non-spin glass like (NSGL) case. The SGL is modeled as a spin chain with nearest-neighbor couplings, taking on normal variates with mean J and variance Δ2. The entanglement between the probes is used to detect any discontinuity in the ground state energy spectrum. For the NSGL case, it was found that the concurrence of the probes exhibits sharp transitions whenever there are abrupt changes in the energy spectrum. In particular, for the 4-qubit case, there is a sudden change in the ground state energy at an external magnetic field B of around 0.66 (resulting in a drop in concurrence of the probes) and 1.7 (manifest as a spike). The latter spike persists for finite temperature case. For the SGL sample with sufficiently large Δ, however, the spike is absent. Thus, an absence in the spike could act as a possible signature of the presence of SGL effects. Moreover, the sudden drop in concurrence at B ≈ 0.66 does not disappear but gets smeared with increasing Δ. However, this drop can be accentuated with a smaller probe coupling. The finite temperature case is also briefly discussed.

  13. Spin-glass-like freezing of inner and outer surface layers in hollow γ-Fe2O3 nanoparticles

    PubMed Central

    Khurshid, Hafsa; Lampen-Kelley, Paula; Iglesias, Òscar; Alonso, Javier; Phan, Manh-Huong; Sun, Cheng-Jun; Saboungi, Marie-Louise; Srikanth, Hariharan

    2015-01-01

    Disorder among surface spins is a dominant factor in the magnetic response of magnetic nanoparticle systems. In this work, we examine time-dependent magnetization in high-quality, monodisperse hollow maghemite nanoparticles (NPs) with a 14.8 ± 0.5 nm outer diameter and enhanced surface-to-volume ratio. The nanoparticle ensemble exhibits spin-glass-like signatures in dc magnetic aging and memory protocols and ac magnetic susceptibility. The dynamics of the system slow near 50 K, and become frozen on experimental time scales below 20 K. Remanence curves indicate the development of magnetic irreversibility concurrent with the freezing of the spin dynamics. A strong exchange-bias effect and its training behavior point to highly frustrated surface spins that rearrange much more slowly than interior spins. Monte Carlo simulations of a hollow particle corroborate strongly disordered surface layers with complex energy landscapes that underlie both glass-like dynamics and magnetic irreversibility. Calculated hysteresis loops reveal that magnetic behavior is not identical at the inner and outer surfaces, with spins at the outer surface layer of the 15 nm hollow particles exhibiting a higher degree of frustration. Our combined experimental and simulated results shed light on the origin of spin-glass-like phenomena and the important role played by the surface spins in magnetic hollow nanostructures. PMID:26503506

  14. Spin-glass-like freezing of inner and outer surface layers in hollow γ-Fe2O3 nanoparticles

    NASA Astrophysics Data System (ADS)

    Khurshid, Hafsa; Lampen-Kelley, Paula; Iglesias, Òscar; Alonso, Javier; Phan, Manh-Huong; Sun, Cheng-Jun; Saboungi, Marie-Louise; Srikanth, Hariharan

    2015-10-01

    Disorder among surface spins is a dominant factor in the magnetic response of magnetic nanoparticle systems. In this work, we examine time-dependent magnetization in high-quality, monodisperse hollow maghemite nanoparticles (NPs) with a 14.8 ± 0.5 nm outer diameter and enhanced surface-to-volume ratio. The nanoparticle ensemble exhibits spin-glass-like signatures in dc magnetic aging and memory protocols and ac magnetic susceptibility. The dynamics of the system slow near 50 K, and become frozen on experimental time scales below 20 K. Remanence curves indicate the development of magnetic irreversibility concurrent with the freezing of the spin dynamics. A strong exchange-bias effect and its training behavior point to highly frustrated surface spins that rearrange much more slowly than interior spins. Monte Carlo simulations of a hollow particle corroborate strongly disordered surface layers with complex energy landscapes that underlie both glass-like dynamics and magnetic irreversibility. Calculated hysteresis loops reveal that magnetic behavior is not identical at the inner and outer surfaces, with spins at the outer surface layer of the 15 nm hollow particles exhibiting a higher degree of frustration. Our combined experimental and simulated results shed light on the origin of spin-glass-like phenomena and the important role played by the surface spins in magnetic hollow nanostructures.

  15. Spin-glass-like freezing of inner and outer surface layers in hollow γ-Fe2O3 nanoparticles

    DOE PAGESBeta

    Khurshid, Hafsa; Lampen-Kelley, Paula; Iglesias, Òscar; Alonso, Javier; Phan, Manh-Huong; Sun, Cheng-Jun; Saboungi, Marie-Louise; Srikanth, Hariharan

    2015-10-27

    Disorder among surface spins largely dominates the magnetic response of ultrafine magnetic particle systems. In this work, we examine time-dependent magnetization in high-quality, monodisperse hollow maghemite nanoparticles (NPs) with a 14.8±0.5 nm outer diameter and enhanced surface-to-volume ratio. The nanoparticle ensemble exhibits spin-glass-like signatures in dc magnetic aging and memory protocols and ac magnetic susceptibility. The dynamics of the system slow near 50 K, and becomes frozen on experimental time scales below 20 K. Remanence curves indicate the development of magnetic irreversibility concurrent with the freezing of the spin dynamics. A strong exchange-bias effect and its training behavior point tomore » highly frustrated surface spins that rearrange much more slowly than interior spins with bulk coordination. Monte Carlo simulations of a hollow particle reproducing the experimental morphology corroborate strongly disordered surface layers with complex energy landscapes that underlie both glass-like dynamics and magnetic irreversibility. Calculated hysteresis loops reveal that magnetic behavior is not identical at the inner and outer surfaces, with spins at the outer surface layer of the 15 nm hollow particles exhibiting a higher degree of frustration. Lastly, our combined experimental and simulated results shed light on the origin of spin-glass-like phenomena and the important role played by the surface spins in magnetic hollow nanostructures.« less

  16. Spin-glass-like freezing of inner and outer surface layers in hollow γ-Fe2O3 nanoparticles.

    PubMed

    Khurshid, Hafsa; Lampen-Kelley, Paula; Iglesias, Òscar; Alonso, Javier; Phan, Manh-Huong; Sun, Cheng-Jun; Saboungi, Marie-Louise; Srikanth, Hariharan

    2015-01-01

    Disorder among surface spins is a dominant factor in the magnetic response of magnetic nanoparticle systems. In this work, we examine time-dependent magnetization in high-quality, monodisperse hollow maghemite nanoparticles (NPs) with a 14.8 ± 0.5 nm outer diameter and enhanced surface-to-volume ratio. The nanoparticle ensemble exhibits spin-glass-like signatures in dc magnetic aging and memory protocols and ac magnetic susceptibility. The dynamics of the system slow near 50 K, and become frozen on experimental time scales below 20 K. Remanence curves indicate the development of magnetic irreversibility concurrent with the freezing of the spin dynamics. A strong exchange-bias effect and its training behavior point to highly frustrated surface spins that rearrange much more slowly than interior spins. Monte Carlo simulations of a hollow particle corroborate strongly disordered surface layers with complex energy landscapes that underlie both glass-like dynamics and magnetic irreversibility. Calculated hysteresis loops reveal that magnetic behavior is not identical at the inner and outer surfaces, with spins at the outer surface layer of the 15 nm hollow particles exhibiting a higher degree of frustration. Our combined experimental and simulated results shed light on the origin of spin-glass-like phenomena and the important role played by the surface spins in magnetic hollow nanostructures. PMID:26503506

  17. Applying Tabu Search to the Two-Dimensional Ising Spin Glass

    NASA Astrophysics Data System (ADS)

    Laguna, Manuel; Laguna, Pablo

    A variety of problems in statistical physics, such as Ising-like systems, can be modeled as integer programs. Physicists have relied mostly on Monte Carlo methods to find approximate solutions to these computationally difficult problems. In some cases, optimal solutions to relatively small problems have been found using standard optimization techniques, e.g., cutting plane and branch-and-bound algorithms. Motivated by the success of tabu search (TS) in finding optimal or near-optimal solutions to combinatorial optimization problems in a number of different settings, we study the application of this methodology to Ising-like systems. Particularly, we develop a TS method to find ground states of two-dimensional spin glasses. Our method performs a search at different levels of resolution in the spin lattice, and it is designed to obtain optimal or near-optimal solutions to problem instances with several different characteristics. Results are reported for computational experiments with up to 64×64 lattices.

  18. Difference between nuclear spin relaxation and ionic conductivity relaxation in superionic glasses

    NASA Astrophysics Data System (ADS)

    Ngai, K. L.

    1993-04-01

    Tatsumisago, Angell, and Martin [J. Chem. Phys. 97, 6968 (1992)] have compared conductivity relaxation data and 7Li nuclear spin lattice relaxation (SLR) data measured on a lithium chloroborate glass and found pronounced differences in the most probable relaxation times. The electrical conductivity relaxation (ECR) time, τ*σ, at some temperature occurs on a time scale shorter by some two orders of magnitude than the 7Li spin lattice relaxation correlation time, τ*s, and has a significantly lower activation energy. SLR and ECR monitor the motions of ions through different dynamic variables and correlation functions. Using this fact and the coupling model, I am able to explain quantitatively all aspects of the difference between SLR and ECR, and to establish relations between their different relaxation characteristics. The large difference between the observed activation energies of SLR and ECR alone should have implications on the validity of any proposed theory of the dynamics of ionic transport.

  19. Untangling complex networks: risk minimization in financial markets through accessible spin glass ground states

    PubMed Central

    Lisewski, Andreas Martin; Lichtarge, Olivier

    2010-01-01

    Recurrent international financial crises inflict significant damage to societies and stress the need for mechanisms or strategies to control risk and tamper market uncertainties. Unfortunately, the complex network of market interactions often confounds rational approaches to optimize financial risks. Here we show that investors can overcome this complexity and globally minimize risk in portfolio models for any given expected return, provided the relative margin requirement remains below a critical, empirically measurable value. In practice, for markets with centrally regulated margin requirements, a rational stabilization strategy would be keeping margins small enough. This result follows from ground states of the random field spin glass Ising model that can be calculated exactly through convex optimization when relative spin coupling is limited by the norm of the network's Laplacian matrix. In that regime, this novel approach is robust to noise in empirical data and may be also broadly relevant to complex networks with frustrated interactions that are studied throughout scientific fields. PMID:20625477

  20. Untangling complex networks: Risk minimization in financial markets through accessible spin glass ground states

    NASA Astrophysics Data System (ADS)

    Lisewski, Andreas Martin; Lichtarge, Olivier

    2010-08-01

    Recurrent international financial crises inflict significant damage to societies and stress the need for mechanisms or strategies to control risk and tamper market uncertainties. Unfortunately, the complex network of market interactions often confounds rational approaches to optimize financial risks. Here we show that investors can overcome this complexity and globally minimize risk in portfolio models for any given expected return, provided the margin requirement remains below a critical, empirically measurable value. In practice, for markets with centrally regulated margin requirements, a rational stabilization strategy would be keeping margins small enough. This result follows from ground states of the random field spin glass Ising model that can be calculated exactly through convex optimization when relative spin coupling is limited by the norm of the network’s Laplacian matrix. In that regime, this novel approach is robust to noise in empirical data and may be also broadly relevant to complex networks with frustrated interactions that are studied throughout scientific fields.

  1. Soft Modes, Localization, and Two-Level Systems in Spin Glasses.

    PubMed

    Baity-Jesi, M; Martín-Mayor, V; Parisi, G; Perez-Gaviro, S

    2015-12-31

    In the three-dimensional Heisenberg spin glass in a random field, we study the properties of the inherent structures that are obtained by an instantaneous cooling from infinite temperature. For a not too large field the density of states g(ω) develops localized soft plastic modes and reaches zero as ω(4) (for large fields a gap appears). When we perturb the system adding a force along the softest mode, one reaches very similar minima of the energy, separated by small barriers, that appear to be good candidates for classical two-level systems. PMID:26765021

  2. Statics and Dynamics of a Two-Dimensional Ising Spin-Glass Model

    NASA Astrophysics Data System (ADS)

    Young, A. P.

    1983-03-01

    The temperature and field dependence of spatial correlations and relaxation times are investigated in detail by Monte Carlo simulations for a two-dimensional Ising spin-glass model. There is no transition, but, in zero field, barrier heights and correlation range increase smoothly at low temperatures. This increase does not seem to be fast enough to explain experiments. In a field, barrier heights and the correlation length tend to a finite limit as T-->0. Points in the h-T plane with constant relaxation time satisfy T(h)-T(0)~h23 at moderately low temperatures.

  3. Nonuniqueness of H23 and H2 field-temperature transition lines in spin-glasses

    NASA Astrophysics Data System (ADS)

    Wenger, L. E.; Mydosh, J. A.

    1984-04-01

    Including the magnetic field dependence on the superparamagnetic relaxation time τ, "transition" lines in the H-T plane are obtained for constant τ. These lines follow the relation TH-T0~Hν where ν~23 except for H-->0 which shows a crossover to ν=2. Thus a power law similar to that derived from mean-field models of spin-glasses is obtained, based strictly on a superparamagnetic relaxation-time approach. This questions the conclusion that experimental observations of H-T lines are solely the result of a mean-field phase transition.

  4. Thermodynamic Identities and Symmetry Breaking in Short-Range Spin Glasses

    NASA Astrophysics Data System (ADS)

    Arguin, L.-P.; Newman, C. M.; Stein, D. L.

    2015-10-01

    We present a technique to generate relations connecting pure state weights, overlaps, and correlation functions in short-range spin glasses. These are obtained directly from the unperturbed Hamiltonian and hold for general coupling distributions. All are satisfied in phases with simple thermodynamic structure, such as the droplet-scaling and chaotic pairs pictures. If instead nontrivial mixed-state pictures hold, the relations suggest that replica symmetry is broken as described by a Derrida-Ruelle cascade, with pure state weights distributed as a Poisson-Dirichlet process.

  5. Extended variational principle for the Sherrington-Kirkpatrick spin-glass model

    NASA Astrophysics Data System (ADS)

    Aizenman, Michael; Sims, Robert; Starr, Shannon L.

    2003-12-01

    The recent proof by Guerra that the Parisi ansatz provides a lower bound on the free energy of the Sherrington-Kirkpatrick (SK) spin-glass model could have been taken as offering some support to the validity of the purported solution. In this work we present a broader variational principle, in which the lower bound as well as the actual value are expressed through an optimization procedure for which ultrametric/hierarchal structures form only a subset of the variational class. The validity of Parisi’s ansatz for the SK model is still in question. The new variational principle may be of help in critical review of the issue.

  6. A Self-Averaging ``Order Parameter'' for the Sherrington-Kirkpatrick Spin Glass Model

    NASA Astrophysics Data System (ADS)

    Wreszinski, Walter F.; Bolina, Oscar

    2004-09-01

    Following an idea of van Enter and Griffiths, we define a self-averaging parameter for the Sherrington-Kirkpatrick (SK) spin glass which is a self-averaging version of the order parameter introduced by Aizenman, Lebowitz and Ruelle. It is strictly positive at low temperature and zero at sufficiently high temperature. The proof is based on the recent construction of the thermodynamic limit of the free energy by Guerra and Toninelli. We also discuss how our definition compares with various existing definitions of order-parameter like quantities.

  7. Evidence for nonuniversal scaling in dimension-four Ising spin glasses

    NASA Astrophysics Data System (ADS)

    Lundow, P. H.; Campbell, I. A.

    2015-04-01

    The critical behavior of the Binder cumulant for Ising spin glasses in dimension four is studied through simulation measurements. Data for the bimodal interaction model are compared with those for the Laplacian interaction model. Special attention is paid to scaling corrections. The limiting infinite size value at criticality for this dimensionless variable is a parameter characteristic of a universality class. This critical limit is estimated to be equal to 0.523(3) in the bimodal model and to 0.473(3) in the Laplacian model.

  8. Universal critical behavior of the two-dimensional Ising spin glass

    NASA Astrophysics Data System (ADS)

    Fernandez, L. A.; Marinari, E.; Martin-Mayor, V.; Parisi, G.; Ruiz-Lorenzo, J. J.

    2016-07-01

    We use finite size scaling to study Ising spin glasses in two spatial dimensions. The issue of universality is addressed by comparing discrete and continuous probability distributions for the quenched random couplings. The sophisticated temperature dependency of the scaling fields is identified as the major obstacle that has impeded a complete analysis. Once temperature is relinquished in favor of the correlation length as the basic variable, we obtain a reliable estimation of the anomalous dimension and of the thermal critical exponent. Universality among binary and Gaussian couplings is confirmed to a high numerical accuracy.

  9. Discovery of room-temperature spin-glass behaviors in two-dimensional oriented attached single crystals

    NASA Astrophysics Data System (ADS)

    Ma, Ji; Chen, Kezheng

    2016-05-01

    In this study, room-temperature spin-glass behaviors were observed in flake-like oriented attached hematite (α-Fe2O3) and iron phosphate hydroxide hydrate (Fe5(PO4)4(OH)3·2H2O) single crystals. Remarkably, their coercivity (HC) values were found to be almost invariable at various given temperatures from 5 to 300 K. The spin topographic map in these flakes was assumed as superparamagnetic (SPM) "islands" isolated by spin glass (SG)-like "bridges". A spin-glass model was then proposed to demonstrate the spin frustration within these "bridges", which were formed by the staggered atomic planes in the uneven surfaces belonging to different attached nanoparticles. Under the spatial limitation and coupling shield of these "bridges", the SPM "islands" were found to be collectively frozen to form a superspin glass (SSG) state below 80 K in weak applied magnetic fields; whereas, when strong magnetic fields were applied, the magnetic coupling of these "islands" would become superferromagnetic (SFM) through tunneling superexchange, so that, these SFM spins could antiferromagnetically couple with the SG-like "bridges" to yield pronounced exchange bias (EB) effect.

  10. Design of half-reentrant SRF cavities

    NASA Astrophysics Data System (ADS)

    Meidlinger, M.; Grimm, T. L.; Hartung, W.

    2006-07-01

    The shape of a TeSLA inner cell can be improved to lower the peak surface magnetic field at the expense of a higher peak surface electric field by making the cell reentrant. Such a single-cell cavity was designed and tested at Cornell, setting a world record accelerating gradient [V. Shemelin et al., An optimized shape cavity for TESLA: concept and fabrication, 11th Workshop on RF Superconductivity, Travemünde, Germany, September 8-12, 2003; R. Geng, H. Padamsee, Reentrant cavity and first test result, Pushing the Limits of RF Superconductivity Workshop, Argonne National Laboratory, September 22-24, 2004]. However, the disadvantage to a cavity is that liquids become trapped in the reentrant portion when it is vertically hung during high pressure rinsing. While this was overcome for Cornell’s single-cell cavity by flipping it several times between high pressure rinse cycles, this may not be feasible for a multi-cell cavity. One solution to this problem is to make the cavity reentrant on only one side, leaving the opposite wall angle at six degrees for fluid drainage. This idea was first presented in 2004 [T.L. Grimm et al., IEEE Transactions on Applied Superconductivity 15(6) (2005) 2393]. Preliminary designs of two new half-reentrant (HR) inner cells have since been completed, one at a high cell-to-cell coupling of 2.1% (high- kcc HR) and the other at 1.5% (low- kcc HR). The parameters of a HR cavity are comparable to a fully reentrant cavity, with the added benefit that a HR cavity can be easily cleaned with current technology.

  11. Reentrant transition in coupled noisy oscillators.

    PubMed

    Kobayashi, Yasuaki; Kori, Hiroshi

    2015-01-01

    We report on a synchronization-breaking instability observed in a noisy oscillator unidirectionally coupled to a pacemaker. Using a phase oscillator model, we find that, as the coupling strength is increased, the noisy oscillator lags behind the pacemaker more frequently and the phase slip rate increases, which may not be observed in averaged phase models such as the Kuramoto model. Investigation of the corresponding Fokker-Planck equation enables us to obtain the reentrant transition line between the synchronized state and the phase slip state. We verify our theory using the Brusselator model, suggesting that this reentrant transition can be found in a wide range of limit cycle oscillators. PMID:25679676

  12. Why the dipolar response in dielectrics and spin-glasses is unavoidably universal

    NASA Astrophysics Data System (ADS)

    Cuervo-Reyes, Eduardo

    2016-07-01

    Materials response to electric or magnetic fields is often dominated by the dynamics of dipoles in the system. This is for instance the case of polar dielectrics and many transition metal compounds. An essential and not yet well understood fact is that, despite their structural diversity, dielectric solids exhibit a striking universality of frequency and time responses, sharing many aspects with the behaviour of spin-glasses. In this article I propose a stochastic approach to dipole dynamics within which the “universal frequency response” derives naturally with Debye’s relaxation mechanism as a special case. This formulation reveals constraints to the form of the relaxation functions, which are essential for a consistent representation of the dynamical slowing-down at the spin-glass transition. Relaxation functions with algebraic-, and exponential-tails, as well as damped oscillations, are shown to have a unified representation in which the stable limit of the distribution of waiting-times between dipole flips determines the present type of dynamics.

  13. Why the dipolar response in dielectrics and spin-glasses is unavoidably universal

    PubMed Central

    Cuervo-Reyes, Eduardo

    2016-01-01

    Materials response to electric or magnetic fields is often dominated by the dynamics of dipoles in the system. This is for instance the case of polar dielectrics and many transition metal compounds. An essential and not yet well understood fact is that, despite their structural diversity, dielectric solids exhibit a striking universality of frequency and time responses, sharing many aspects with the behaviour of spin-glasses. In this article I propose a stochastic approach to dipole dynamics within which the “universal frequency response” derives naturally with Debye’s relaxation mechanism as a special case. This formulation reveals constraints to the form of the relaxation functions, which are essential for a consistent representation of the dynamical slowing-down at the spin-glass transition. Relaxation functions with algebraic-, and exponential-tails, as well as damped oscillations, are shown to have a unified representation in which the stable limit of the distribution of waiting-times between dipole flips determines the present type of dynamics. PMID:27366866

  14. Why the dipolar response in dielectrics and spin-glasses is unavoidably universal.

    PubMed

    Cuervo-Reyes, Eduardo

    2016-01-01

    Materials response to electric or magnetic fields is often dominated by the dynamics of dipoles in the system. This is for instance the case of polar dielectrics and many transition metal compounds. An essential and not yet well understood fact is that, despite their structural diversity, dielectric solids exhibit a striking universality of frequency and time responses, sharing many aspects with the behaviour of spin-glasses. In this article I propose a stochastic approach to dipole dynamics within which the "universal frequency response" derives naturally with Debye's relaxation mechanism as a special case. This formulation reveals constraints to the form of the relaxation functions, which are essential for a consistent representation of the dynamical slowing-down at the spin-glass transition. Relaxation functions with algebraic-, and exponential-tails, as well as damped oscillations, are shown to have a unified representation in which the stable limit of the distribution of waiting-times between dipole flips determines the present type of dynamics. PMID:27366866

  15. Spin-glass phase transitions and minimum energy of the random feedback vertex set problem.

    PubMed

    Qin, Shao-Meng; Zeng, Ying; Zhou, Hai-Jun

    2016-08-01

    A feedback vertex set (FVS) of an undirected graph contains vertices from every cycle of this graph. Constructing a FVS of sufficiently small cardinality is very difficult in the worst cases, but for random graphs this problem can be efficiently solved by converting it into an appropriate spin-glass model [H.-J. Zhou, Eur. Phys. J. B 86, 455 (2013)EPJBFY1434-602810.1140/epjb/e2013-40690-1]. In the present work we study the spin-glass phase transitions and the minimum energy density of the random FVS problem by the first-step replica-symmetry-breaking (1RSB) mean-field theory. For both regular random graphs and Erdös-Rényi graphs, we determine the inverse temperature β_{l} at which the replica-symmetric mean-field theory loses its local stability, the inverse temperature β_{d} of the dynamical (clustering) phase transition, and the inverse temperature β_{s} of the static (condensation) phase transition. These critical inverse temperatures all change with the mean vertex degree in a nonmonotonic way, and β_{d} is distinct from β_{s} for regular random graphs of vertex degrees K>60, while β_{d} are identical to β_{s} for Erdös-Rényi graphs at least up to mean vertex degree c=512. We then derive the zero-temperature limit of the 1RSB theory and use it to compute the minimum FVS cardinality. PMID:27627285

  16. Spin-cluster glass state in U(Ga0.95Mn0.05)3

    NASA Astrophysics Data System (ADS)

    Dong-Hua, Xie; Wen, Zhang; Yi, Liu; Wei, Feng; Yun, Zhang; Shi-Yong, Tan; Xie-Gang, Zhu; Qiu-Yun, Chen; Qin, Liu; Bing-Kai, Yuan; Xin-Chun, Lai

    2016-04-01

    We report the study of a low temperature cluster glass state in 5% Mn-doped UGa3 heavy fermion compound. This compound transforms from a paramagnetic state to a spin-cluster glass state, which is confirmed by measuring the dc susceptibility and magnetization. The ac susceptibility exhibits a frequency-dependent peak around T f, which provides direct evidence of the cluster glass state. By analyzing the field-dependent magnetization and frequency-dependent ac susceptibility in detail, we deduce that this compound forms a spin-cluster glass state below T f. Project supported by the Natural Science Foundation of China Academy of Engineering Physic (Grant No. 2014A0301013) and the National Natural Science Foundation of China (Grant Nos. 11304291 and 11504342).

  17. Scaling behavior of domain walls at the T = 0 ferromagnet to spin-glass transition

    NASA Astrophysics Data System (ADS)

    Melchert, O.; Hartmann, A. K.

    2009-01-01

    Using mappings to combinatorial optimization problems, one can often study physical systems better by means of sophisticated algorithms from computer science. Here, we study the geometric properties of domain-wall excitations in a two-dimensional random-bond Ising spin system, where each realization of the disorder consists of a random fraction ρ of ferromagnetic bonds and a fraction (1-ρ) of bonds drawn from a Gaussian distribution with zero mean and unit width. We formulate an auxiliary graph theoretical problem in which domain walls are given by undirected shortest paths with possibly negative distances [O. Melchert and A.K. Hartmann Phys. Rev. B 76, 174411 (2007)]. Due to the details of the mapping, standard shortest path algorithms (e.g. the Dijkstra algorithm) cannot be applied. To solve such shortest path problems it requires minimum weight perfect matching algorithms. We first locate the critical point ρc, where the ferromagnet (large ρ) to spin-glass transition occurs. For certain values of ρ close to the critical point we investigate the stiffness exponent θ and the fractal dimension df that describe the scaling of the average domain-wall energy and length, respectively. Performing a finite-size scaling analysis we find that both exponents remain constant in the spin-glass phase, i.e. θ~-0.28 and df~1.275. This is consistent with conformal field theory, where it seems to be possible [C. Amoruso et al. Phys. Rev. Lett. 97, 267202 (2006)] to relate the exponents via df-1 = 3/[4(3+θ)].

  18. Search for the Heisenberg spin glass on rewired square lattices with antiferromagnetic interaction

    NASA Astrophysics Data System (ADS)

    Surungan, Tasrief; Bansawang B., J.; Tahir, Dahlang

    2016-03-01

    Spin glass (SG) is a typical magnetic system with frozen random spin orientation at low temperatures. The system exhibits rich physical properties, such as infinite number of ground states, memory effect, and aging phenomena. There are two main ingredients considered to be pivotal for the existence of SG behavior, namely, frustration and randomness. For the canonical SG system, frustration is led by the presence of competing interaction between ferromagnetic (FM) and antiferromagnetic (AF) couplings. Previously, Bartolozzi et al. [Phys. Rev. B73, 224419 (2006)], reported the SG properties of the AF Ising spins on scale free network (SFN). It is a new type of SG, different from the canonical one which requires the presence of both FM and AF couplings. In this new system, frustration is purely caused by the topological factor and its randomness is related to the irregular connectvity. Recently, Surungan et. al. [Journal of Physics: Conference Series, 640, 012001 (2015)] reported SG bahavior of AF Heisenberg model on SFN. We further investigate this type of system by studying an AF Heisenberg model on rewired square lattices. We used Replica Exchange algorithm of Monte Carlo Method and calculated the SG order parameter to search for the existence of SG phase.

  19. Multicritical point of Ising spin glasses on triangular and honeycomb lattices

    NASA Astrophysics Data System (ADS)

    de Queiroz, S. L. A.

    2006-02-01

    The behavior of two-dimensional Ising spin glasses at the multicritical point on triangular and honeycomb lattices is investigated with the help of finite-size scaling and conformal-invariance concepts. We use transfer-matrix methods on long strips to calculate domain-wall energies, uniform susceptibilities, and spin-spin correlation functions. Accurate estimates are provided for the location of the multicritical point on both lattices, which lend strong support to a conjecture recently advanced by Takeda, Sasamoto, and Nishimori. Correlation functions are shown to obey rather strict conformal-invariance requirements, once suitable adaptations are made to account for geometric aspects of the transfer-matrix description of triangular and honeycomb lattices. The universality class of critical behavior upon crossing the ferro-para-magnetic phase boundary is probed, with the following estimates for the associated critical indices: ν=1.49(2) , γ=2.71(4) , η1=0.183(3) , which are distinctly different from the percolation values.

  20. Electron Spin Relaxation Rates for Semiquinones between 25 and 295 K in Glass-Forming Solvents

    PubMed Central

    Kathirvelu, Velavan; Sato, Hideo; Eaton, Sandra S.; Eaton, Gareth R.

    2009-01-01

    Electron spin lattice relaxation rates for five semiquinones (2,5-di-t-butyl-1,4-benzosemiquinone, 2,5-di-t-amyl-1,4-benzosemiquinone, 2,5-di-phenyl-1,4-benzosemiquinone, 2,6-di-t-butyl-1,4-benzosemiquinone, tetrahydroxy-1,4-benzosemiquione) were studied by long-pulse saturation recovery EPR in 1:4 glycerol:ethanol, 1:1 glycerol:ethanol, and triethanolamine between 25 and 295 K. Although the dominant process changes with temperature, relaxation rates vary smoothly with temperature, even near the glass transition temperatures, and could be modeled as the sum of contributions that have the temperature dependence that is predicted for the direct, Raman, local mode and tumbling dependent processes. At 85 K, which is in a temperature range where the Raman process dominates, relaxation rates along the gxx (g~2.006) and gyy (g~2.005) axes are about 2.7 to 1.5 times faster than along the gzz axis (g = 2.0023). In highly viscous triethanolamine, contributions from tumbling-dependent processes are negligible. At temperatures above 100 K relaxation rates in triethanolamine are unchanged between X-band (9.5 GHz) and Q-band (34 GHz), so the process that dominates in this temperature interval was assigned as a local mode rather than a thermally-activated process. Because the largest proton hyperfine couplings are only 2.2 G, spin rotation makes a larger contribution than tumbling-dependent modulation of hyperfine anisotropy. Since g anisotropy is small, tumbling dependent modulation of g anisotropy make a smaller contribution than spin rotation at X-band. Although there was negligible impact of methyl rotation on T1, rotation of t-butyl or t-amyl methyl groups enhances spin echo dephasing between 85 and 150 K. PMID:19223213

  1. Zero-temperature quantum annealing bottlenecks in the spin-glass phase

    NASA Astrophysics Data System (ADS)

    Knysh, Sergey

    2016-08-01

    A promising approach to solving hard binary optimization problems is quantum adiabatic annealing in a transverse magnetic field. An instantaneous ground state--initially a symmetric superposition of all possible assignments of N qubits--is closely tracked as it becomes more and more localized near the global minimum of the classical energy. Regions where the energy gap to excited states is small (for instance at the phase transition) are the algorithm's bottlenecks. Here I show how for large problems the complexity becomes dominated by O(log N) bottlenecks inside the spin-glass phase, where the gap scales as a stretched exponential. For smaller N, only the gap at the critical point is relevant, where it scales polynomially, as long as the phase transition is second order. This phenomenon is demonstrated rigorously for the two-pattern Gaussian Hopfield model. Qualitative comparison with the Sherrington-Kirkpatrick model leads to similar conclusions.

  2. Zero-temperature quantum annealing bottlenecks in the spin-glass phase

    PubMed Central

    Knysh, Sergey

    2016-01-01

    A promising approach to solving hard binary optimization problems is quantum adiabatic annealing in a transverse magnetic field. An instantaneous ground state—initially a symmetric superposition of all possible assignments of N qubits—is closely tracked as it becomes more and more localized near the global minimum of the classical energy. Regions where the energy gap to excited states is small (for instance at the phase transition) are the algorithm's bottlenecks. Here I show how for large problems the complexity becomes dominated by O(log N) bottlenecks inside the spin-glass phase, where the gap scales as a stretched exponential. For smaller N, only the gap at the critical point is relevant, where it scales polynomially, as long as the phase transition is second order. This phenomenon is demonstrated rigorously for the two-pattern Gaussian Hopfield model. Qualitative comparison with the Sherrington-Kirkpatrick model leads to similar conclusions. PMID:27491338

  3. A remark on the infinite-volume Gibbs measures of spin glasses

    NASA Astrophysics Data System (ADS)

    Arguin, Louis-Pierre

    2008-12-01

    In this note, we point out that infinite-volume Gibbs measures of spin glass models on the hypercube can be identified as random probability measures on the unit ball of a Hilbert space. This simple observation follows from a result of Dovbysh and Sudakov on weakly exchangeable random matrices. Limiting Gibbs measures can then be studied as single well-defined objects. This approach naturally extends the space of random overlap structures as defined by Aizenman et al. We discuss the Ruelle probability cascades and the stochastic stability within this framework. As an application, we use an idea of Parisi and Talagrand to prove that if a sequence of finite-volume Gibbs measures satisfies the Ghirlanda-Guerra identities, then the infinite-volume measure must be singular as a measure on a Hilbert space.

  4. Zero-temperature quantum annealing bottlenecks in the spin-glass phase.

    PubMed

    Knysh, Sergey

    2016-01-01

    A promising approach to solving hard binary optimization problems is quantum adiabatic annealing in a transverse magnetic field. An instantaneous ground state-initially a symmetric superposition of all possible assignments of N qubits-is closely tracked as it becomes more and more localized near the global minimum of the classical energy. Regions where the energy gap to excited states is small (for instance at the phase transition) are the algorithm's bottlenecks. Here I show how for large problems the complexity becomes dominated by O(log N) bottlenecks inside the spin-glass phase, where the gap scales as a stretched exponential. For smaller N, only the gap at the critical point is relevant, where it scales polynomially, as long as the phase transition is second order. This phenomenon is demonstrated rigorously for the two-pattern Gaussian Hopfield model. Qualitative comparison with the Sherrington-Kirkpatrick model leads to similar conclusions. PMID:27491338

  5. Enhanced light extraction from UV LEDs using spin-on glass microlenses

    NASA Astrophysics Data System (ADS)

    Liu, Chi-Min; Su, Guo-Dung J.

    2016-05-01

    In this paper, we present a cost-effective method for fabricating spin-on glass (SOG) microlens arrays (MLAs) on ultra-violet light-emitting diodes. The SOG MLA is formed using thermal reflow molds and multiple replication processes, which can reduce the cost of the solution process. In this paper, we fabricate SOG MLA of different sizes, where the diameter of each microlens is approximately 50, 100, 150, and 200 μm. In each case, the light extraction efficiency is improved by 21.86%, 14.01%, 10.35%, and 7.31%, respectively. We also discuss the effects of different-shaped SOG microlenses, namely circular, square, and hexagonal. The light extraction efficiency is improved by 7.31%, 9.60%, and 13.80% for the circular, square, and hexagonal SOG MLAs, respectively. By applying an optimized lens pattern, an increase in light extraction efficiency of 21.86% is achieved.

  6. Spin-glass transition in Ni carbide single crystal nanoparticles with Ni3C - type structure

    NASA Astrophysics Data System (ADS)

    Fujieda, S.; Kuboniwa, T.; Shinoda, K.; Suzuki, S.; Echigoya, J.

    2016-05-01

    Hexagonal shaped nanoparticles about 60 nm in size were successfully synthesized in tetraethylene glycol solution containing polyvinylpyrrolidone. By the analysis of the electron diffraction pattern, these were identified as a single crystal of Ni carbide with Ni3C - type structure. Their magnetization curve at 5 K was not completely saturated under a magnetic field of 5 T. The thermomagnetization curves after zero-field cooling and after field cooling exhibited the magnetic cooling effect at low temperatures. Furthermore, the 2nd order nonlinear term of AC magnetic susceptibility exhibited a negative divergence at about 17 K. It is concluded that Ni carbide single crystal nanoparticles with the Ni3C - type structure exhibit spin-glass transition at low temperatures.

  7. Fixed-point distributions of short-range Ising spin glasses on hierarchical lattices

    NASA Astrophysics Data System (ADS)

    Almeida, Sebastião T. O.; Nobre, Fernando D.

    2015-03-01

    Fixed-point distributions for the couplings of Ising spin glasses with nearest-neighbor interactions on hierarchical lattices are investigated numerically. Hierarchical lattices within the Migdal-Kadanoff family with fractal dimensions in the range 2.58 ≤D ≤7 , as well as a lattice of the Wheatstone-Bridge family with fractal dimension D ≈3.58 are considered. Three initial distributions for the couplings are analyzed, namely, the Gaussian, bimodal, and uniform ones. In all cases, after a few iterations of the renormalization-group procedure, the associated probability distributions approached universal fixed shapes. For hierarchical lattices of the Migdal-Kadanoff family, the fixed-point distributions were well fitted either by stretched exponentials, or by q -Gaussian distributions; both fittings recover the expected Gaussian limit as D →∞ . In the case of the Wheatstone-Bridge lattice, the best fit was found by means of a stretched-exponential distribution.

  8. Spin glass-like behavior, giant magnetocaloric and giant magnetoresistance effect in PrPb manganites

    NASA Astrophysics Data System (ADS)

    Chau, N.; Hanh, D. T.; Tho, N. D.; Luong, N. H.

    2006-08-01

    The Pr 1-xPb xMnO 3 ( x=0.1-0.5) perovskites have been fabricated by solid-state reaction. The X-ray diffraction patterns show that the samples are of single phase with orthorhombic structure. The field-cooled (FC) and zero-field-cooled (ZFC) thermomagnetic curves measured at low field and low temperatures exhibit the spin glass-like state. The Curie temperature of samples increased with increase in Pb content. The maximum magnetic entropy change |Δ Sm| max reaches the giant values of 3.91 and 3.68 J/kg K for quite low magnetic field change of 1.35 T for the samples x=0.1 and 0.4, respectively. The resistance measurements show that there is insulator-metal phase transition on the R( T) curves for samples with x⩾0.3. The giant magnetoresistance effect is also observed for all samples studied.

  9. Spin-on-glass coatings for the generation of super-polishedsubstrates for extreme ultraviolet optics

    SciTech Connect

    Salmassi, Farhad; Naulleau, Patrick P.; Gullikson, Eric M.

    2005-01-01

    Substrates intended for use as extreme ultraviolet (EUV) optics have extremely stringent requirements in terms of finish. These requirements can dramatically increase the cost and fabrication time, especially when non-conventional shapes, such as toroids, are required. Here we present a spin-on-glass resist process capable of generating super-polished parts from inexpensive substrates. The method has been used to render diamond-turned substrates compatible for use as EUV optics. Toroidal diamond-turned optics with starting rms roughness in the 3.3 to 3.7 nm range have been smoothed to the 0.4 to 0.6 nm range. EUV reflectometry characterization of these optics has demonstrated reflectivities of approximately 63%.

  10. Conformal Invariance and Stochastic Loewner Evolution Processes in Two-Dimensional Ising Spin Glasses

    SciTech Connect

    Amoruso, C.; Moore, M. A.; Hartmann, A. K.; Hastings, M. B.

    2006-12-31

    We present numerical evidence that the techniques of conformal field theory might be applicable to two-dimensional Ising spin glasses with Gaussian bond distributions. It is shown that certain domain wall distributions in one geometry can be related to that in a second geometry by a conformal transformation. We also present direct evidence that the domain walls are stochastic Loewner (SLE) processes with {kappa}{approx_equal}2.1. An argument is given that their fractal dimension d{sub f} is related to their interface energy exponent {theta} by d{sub f}-1=3/[4(3+{theta})], which is consistent with the commonly quoted values d{sub f}{approx_equal}1.27 and {theta}{approx_equal}-0.28.

  11. Saturation of Ruderman-Kittel-Kasuya-Yosida interaction damping in high-resistivity spin glasses

    NASA Astrophysics Data System (ADS)

    Larsen, Ulf

    1986-04-01

    Saturation of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction damping at large resistivity or short mean free path in metallic alloys, as predicted in the Kaneyoshi model, is shown to account for the recently observed spin-glass freezing temperature, T0, in such systems as AuFe quench-condensed films and ternary XYcZx alloys, where X=Au,Ag,..., is a nonmagnetic metal host, Y=Fe,Mn,..., is the dilute magnetic species of concentration c, and Z=Cu,...,Ti,Sb,..., is a nonmagnetic impurity of concentration x. Some deeper aspects of the c dependence of the characteristic RKKY interaction energy scale are discussed, with emphasis on the necessary distinction between quenched and ergodic situations in the randomly dilute alloys. A consequent logarithmic correction to the c-scaling laws (at the marginal dimensionalities d=p=3, where d is the electronic dimension of RKKY interaction varying as R-d, and p is the space dimension of the magnetic structure), in the form of T0~c(-0.577-lnc)1/2, is shown to be due to broken dilatation invariance, by finite atomic size. The finite mean free path in real systems also breaks this invariance by providing a length scale. However, at the damping saturation limit a pseudo-c-scaling T0~c reappears, as was found in the amorphous spin-glass LaAuGd. This, and related predictions of the ``typical environment'' approach to the quenched-random-averaging problem agree remarkably well with the new data that have recently become available.

  12. Statistical Mechanics in Np-Complete Optimization Problems and Simulations of Spin Glass on Graphs with Finite Coordination Number.

    NASA Astrophysics Data System (ADS)

    Lai, Pik-Yin

    Methods of statistical mechanics are applied to two important NP-complete combinatorial optimization problems. The first is the chromatic number problem which seeks the minimal number of colors necessary to color a graph such that no two sites connected by an edge have the same color. The second is partitioning of a graph into q equal subgraphs so as to minimize inter-subgraph connections. Both models are mapped into a frustrated Potts model which is related to the q-state Potts spin glass. For the first problem, we obtain very good agreement with numerical simulations and theoretical bounds using the annealed approximation. The quenched model is also discussed. For the second problem we obtain analytic and numerical results by evaluating the ground state energy of the q = 3 and 4 Potts spin glass using Parisi's replica symmetry breaking. We also performed some numerical simulations to test the theoretical result and obtained very good agreement. In the second part of the thesis, we simulate the Ising spin-glass model on a random lattice with a finite (average) coordination number and also on the Bethe lattice with various different boundary conditions. In particular, we calculate the overlap function P(q) for two independent samples. For the random lattice, the results are consistent with a spin-glass transition above which P(q) converges to a Dirac delta -function for large N (number of sites) and below which P(q) has in addition a long tail similar to previous results obtained for the infinite ranged model. For the Bethe lattice, we obtain results in the interior by discarding the two outer shells of the Cayley tree when calculating the thermal averages. For fixed (uncorrelated) boundary conditions, P(q) seems to converge to a delta -function even below the spin-glass transition whereas for a "closed" lattice (correlated boundary conditions) P(q) has a long tail similar to its behavior in the random lattice case.

  13. Characterization of nano-sized iron particle layers spin coated on glass substrate

    NASA Astrophysics Data System (ADS)

    Dehipawala, Sunil; Samarasekara, Pubudu; Dahanayake, Rasika; Tremberger, George; Cheung, Tak D.; Gafney, Harry D.

    2015-08-01

    Nanometer scale iron particles have a variety of technological applications. They are vastly utilized in optical and microwave devices. Thin films with varying compositions of iron (III) nitrate and ethylene glycol were deposited on glass substrate using a spin coating technique. The thicknesses of the films were controlled by the spin rate. Precursor films on the substrate were then annealed to different temperatures ranging from 200°C to 600°C for 1-3 hours in air. The microstructures of iron particles in films prepared under different conditions were investigated using X-ray Absorption spectroscopy and Mossbauer spectroscopy. The main absorption edge peak position and pre-edge energy position were identical in samples with different numbers of layers, but prepared under similar conditions. This indicates that there was no change in the charge state of the iron regardless of the number of layers. However the intensity of the pre-edge feature decreases as the number of layers increases, which shows a decrease of Fe-O compounds as the number of layers increases. Mossbauer spectrum of these iron particles contains only quadrupole doublets. The absence of six-linespectrum confirms the nano-size nature of the particles.

  14. Real beards and real networks: a spin-glass model for interacting individuals

    NASA Astrophysics Data System (ADS)

    O'Neale, Dion

    ''I want to be different, just like all the other different people'' sang the band King Missile. Whether they are the Beatniks of the 1950s, the punks of the 1970s, or the hipsters of today, non-conformists often tend to look the same, seemingly at odds with their goal of non-conformity. The spin-glass model, originally developed to describe the interaction of magnetic spins, and since applied to situations as diverse as the electrical activity of networks of neurons, to trades on a financial market, has recently been used in social science to study populations of interacting individuals comprised of a mix of both conformists and anti-conformists - or hipsters. Including delay effects for the interactions between individuals has been shown to give a system with non-trivial dynamics with a phase transition from stable behaviour to periodic switching between two states (let's call them bushy bearded and clean shaven). Analytic solutions to such a model are possible, but only for particular assumptions about the interaction and delay matrices. In this work we will show what happens when the interactions in the model are based on real-world networks with ''small-world'' effects and clustering.

  15. Seeking Quantum Speedup Through Spin Glasses: The Good, the Bad, and the Ugly*

    NASA Astrophysics Data System (ADS)

    Katzgraber, Helmut G.; Hamze, Firas; Zhu, Zheng; Ochoa, Andrew J.; Munoz-Bauza, H.

    2015-07-01

    There has been considerable progress in the design and construction of quantum annealing devices. However, a conclusive detection of quantum speedup over traditional silicon-based machines remains elusive, despite multiple careful studies. In this work we outline strategies to design hard tunable benchmark instances based on insights from the study of spin glasses—the archetypal random benchmark problem for novel algorithms and optimization devices. We propose to complement head-to-head scaling studies that compare quantum annealing machines to state-of-the-art classical codes with an approach that compares the performance of different algorithms and/or computing architectures on different classes of computationally hard tunable spin-glass instances. The advantage of such an approach lies in having to compare only the performance hit felt by a given algorithm and/or architecture when the instance complexity is increased. Furthermore, we propose a methodology that might not directly translate into the detection of quantum speedup but might elucidate whether quantum annealing has a "quantum advantage" over corresponding classical algorithms, such as simulated annealing. Our results on a 496-qubit D-Wave Two quantum annealing device are compared to recently used state-of-the-art thermal simulated annealing codes.

  16. Magnetic anisotropy and spin-glass behavior in single crystalline U2PdSi3

    NASA Astrophysics Data System (ADS)

    Li, D. X.; Kimura, A.; Haga, Y.; Nimori, S.; Shikama, T.

    2011-02-01

    We present the magnetic and transport properties of single crystalline U2PdSi3 measured with the magnetic field (H) (or measuring current, I) applied along two typical crystallographic directions, i.e. H\\perp c -axis and H\\parallel c -axis (or I\\perp c -axis and I\\parallel c -axis). For both directions, a spin-glass state is confirmed to form at low temperature with the same spin freezing temperature Tf (= 11.5 K), initial frequency shift δTf (= 0.023) and activation energy Ea/kB (= 90.15 K) in zero dc field. Strong anisotropy in magnetic and transport behavior is found to be a significant feature of U2PdSi3. The unusual ferromagnetic-like anomaly in ac susceptibility and dc magnetization curves around Tm = 71 K is observed in the case of H\\parallel c -axis but not in the cases of H \\perp c -axis. The characteristic temperature Tir, below which evident irreversible magnetism originated from random spin freezing can be observed, shows much stronger field dependence for H \\perp c -axis than for H\\parallel c -axis. Moreover, an unusual finding is that the electrical resistivity measurements indicate the formation of magnetic Brillouin-zone boundary gaps and much larger magnetic scattering for I \\perp c -axis, while the coherent-Kondo-effect-like behavior is obvious for I\\parallel c -axis. We also emphasize that no resistivity minimum can be detected down to 2.5 K for either direction. The observed magnetic and transport behaviors are compared with those in polycrystalline U2PdSi3 and other 2:1:3 intermetallic compounds.

  17. Microwave Reentrant Cavities for Quantum Devices

    NASA Astrophysics Data System (ADS)

    Carvalho, Natalia C.; Bourhill, Jeremy; Creedon, Daniel; Goryachev, Maxim; Galliou, Serge; Tobar, Michael

    A microwave reentrant cavity is a device able to provide a very sensitive high-Q microwave mode. Its design can be highly advantageous for electromechanical devices and quantum measurements. In this sense, a tuneable device based on a narrow-gap superconducting reentrant cavity is under development. The resonant frequency is able to be fine-tuned over a range larger than 500 MHz at 10 mK with an electrical Q-factor of 105. Such a cavity could possibly accommodate a transmon qubit to control and manipulate its quantum state. We are also working on the investigation of bulk acoustic wave (BAW) resonators in microwave reentrant cavities. BAW resonators offer a promising way to process quantum information through the coupling between microwaves and acoustic phonons. Thus, we are developing a device able to excite phonons through non-linearities and the piezoelectricity of the plano-convex quartz crystal. We will detail our experiments that work towards cooling gram scale phonon resonances to the quantum ground state. Funded by ARC Grant No. CE110001013 (Australia) and National Counsel of Technological and Scientific Development (Brazil).

  18. Zero-point entropy of the spinel spin glasses CuGa2O4 and CuAl2O4

    NASA Astrophysics Data System (ADS)

    Fenner, L. A.; Wills, A. S.; Bramwell, S. T.; Dahlberg, M.; Schiffer, P.

    2009-01-01

    The zero-point entropy of a spin glass is a difficult property to experimentally determine and interpret. Spin glass theory provides various predictions, including unphysical ones, for the value of the zero-point entropy, however experimental results have been lacking. We have investigated the magnetic properties and zero-point entropy of two spinel Cu2+ based spin glasses, CuGa2O4 and CuAl2O4. Dc- and ac-susceptibility and specific heat measurements show many characteristic spin glass features for both materials. The spin glass freezing temperature is determined to be Tf = 2.89 ± 0.05 K for CuGa2O4 and Tf = 2.30 ± 0.05 K for CuAl2O4. By integrating the specific heat data we have found that CuGa2O4 and CuAl2O4 have zero-point entropies of S0 = 4.96 JK-1mol-1 and S0 = 4.76 JK-1mol-1 respectively. These values are closest to the prediction for a Sherrington-Kirkpatrick XY spin glass, however they are notably higher than all of the theoretical predictions. This indicates that CuGa2O4 and CuAl2O4 have a greater degeneracy in their ground states than any of the spin glass models.

  19. Spin-Glass Behavior in a Giant Unit Cell Compound Tb117Fe52Ge113.8(1)

    SciTech Connect

    Liu, Jing; Xie, Weiwei; Gschneidner, Karl A; Miller, Gordon J; Pecharsky, Vitalij K

    2014-10-15

    In this paper we demonstrate evidence of a cluster spin glass in Tb117Fe52Ge113.8(1) (a compound with a giant cubic unit cell) via ac and dc magnetic susceptibility, magnetization, magnetic relaxation and heat capacity measurements. The results clearly show that Tb117Fe52Ge113.8(1) undergoes a spin glass phase transition at the freezing temperature, ~38 K. The good fit of the frequency dependence of the freezing temperature to the critical slowing down model and Vogel-Fulcher law strongly suggest the formation of cluster glass in the Tb117Fe52Ge113.8(1) system. The heat capacity data exhibit no evidence for long-range magnetic order, and yield a large value of Sommerfeld coefficient. The spin glass behavior of Tb117Fe52Ge113.8(1) may be understood by assuming the presence of competing interactions among multiple non-equivalent Tb sites present in the highly complex unit cell.

  20. Thermally driven asymmetric responses of grains versus spin-glass related distributions of blocking temperature in exchange biased Co/IrMn bilayers

    SciTech Connect

    Baltz, V.

    2013-02-11

    Controlling ferromagnetic/antiferromagnetic blocking temperatures in exchange biased based devices appears crucial for applications. The blocking temperature is ascribed to the ability of both antiferromagnetic grains and interfacial spin-glass-like phases to withstand ferromagnetic magnetization reversal. To better understand the respective contributions of grains versus spin-glass, blocking temperature distributions were measured after various thermal treatments for cobalt/iridium-manganese bilayers. The high-temperature contribution linked to antiferromagnetic grains shifts towards lower temperatures above a threshold thermal annealing. In contrast, the occurrence and evolution of training effects for the low-temperature contribution only agree with its inferred interfacial spin-glass-like origin.

  1. Sustained phase separation and spin glass in Co-doped KxFe2-ySe2 single crystals

    DOE PAGESBeta

    Ryu, Hyejin; Wang, Kefeng; Opacic, M.; Lazarevic, N.; Warren, J. B.; Popovic, Z. V.; Bozin, Emil S.; Petrovic, C.

    2015-11-19

    We describe Co substitution effects in KxFe2-y-zCozSe2 (0.06 ≤ z ≤ 1.73) single crystal alloys. By 3.5% of Co doping superconductivity is suppressed whereas phase separation of semiconducting K2Fe4Se5 and superconducting/metallic KxFe2Se2 is still present. We show that the arrangement and distribution of superconducting phase (stripe phase) is connected with the arrangement of K, Fe and Co atoms. Semiconducting spin glass is found in proximity to superconducting state, persisting for large Co concentrations. At high Co concentrations ferromagnetic metallic state emerges above the spin glass. This is coincident with changes of the unit cell, arrangement and connectivity of stripemore » conducting phase.« less

  2. Short-range Ising spin glasses: the metastate interpretation of replica symmetry breaking.

    PubMed

    Read, N

    2014-09-01

    Parisi's formal replica-symmetry-breaking (RSB) scheme for mean-field spin glasses has long been interpreted in terms of many pure states organized ultrametrically. However, the early version of this interpretation, as applied to the short-range Edwards-Anderson model, runs into problems because as shown by Newman and Stein (NS) it does not allow for chaotic size dependence, and predicts non-self-averaging that cannot occur. NS proposed the concept of the metastate (a probability distribution over infinite-size Gibbs states in a given sample that captures the effects of chaotic size dependence) and a nonstandard interpretation of the RSB results in which the metastate is nontrivial and is responsible for what was called non-self-averaging. In this picture, each state drawn from the metastate has the ultrametric properties of the old theory, but when the state is averaged using the metastate, the resulting mixed state has little structure. This picture was constructed so as to agree both with the earlier RSB results and with rigorous results. Here we use the effective field theory of RSB, in conjunction with the rigorous definitions of pure states and the metastate in infinite-size systems, to show that the nonstandard picture follows directly from the RSB mean-field theory. In addition, the metastate-averaged state possesses power-law correlations throughout the low-temperature phase; the corresponding exponent ζ takes the value 4 according to the field theory in high dimensions d, and describes the effective fractal dimension of clusters of spins. Further, the logarithm of the number of pure states in the decomposition of the metastate-averaged state that can be distinguished if only correlations in a window of size W can be observed is of order W(d-ζ). These results extend the nonstandard picture quantitatively; we show that arguments against this scenario are inconclusive. More generally, in terms of Parisi's function q(x), if q(0)≠∫(0)(1)dxq(x), then the

  3. Spin-glass phase transition and behavior of nonlinear susceptibility in the Sherrington-Kirkpatrick model with random fields

    NASA Astrophysics Data System (ADS)

    Morais, C. V.; Zimmer, F. M.; Lazo, M. J.; Magalhães, S. G.; Nobre, F. D.

    2016-06-01

    The behavior of the nonlinear susceptibility χ3 and its relation to the spin-glass transition temperature Tf in the presence of random fields are investigated. To accomplish this task, the Sherrington-Kirkpatrick model is studied through the replica formalism, within a one-step replica-symmetry-breaking procedure. In addition, the dependence of the Almeida-Thouless eigenvalue λAT (replicon) on the random fields is analyzed. Particularly, in the absence of random fields, the temperature Tf can be traced by a divergence in the spin-glass susceptibility χSG, which presents a term inversely proportional to the replicon λAT. As a result of a relation between χSG and χ3, the latter also presents a divergence at Tf, which comes as a direct consequence of λAT=0 at Tf. However, our results show that, in the presence of random fields, χ3 presents a rounded maximum at a temperature T* which does not coincide with the spin-glass transition temperature Tf (i.e., T*>Tf for a given applied random field). Thus, the maximum value of χ3 at T* reflects the effects of the random fields in the paramagnetic phase instead of the nontrivial ergodicity breaking associated with the spin-glass phase transition. It is also shown that χ3 still maintains a dependence on the replicon λAT, although in a more complicated way as compared with the case without random fields. These results are discussed in view of recent observations in the LiHoxY1 -xF4 compound.

  4. Doping of ZnO nanowires using phosphorus diffusion from a spin-on doped glass source

    SciTech Connect

    Bocheux, A.; Robin, I. C.; Bonaimé, J.; Hyot, B.; Feuillet, G.; Kolobov, A. V.; Fons, P.; Mitrofanov, K. V.; Tominaga, J.; Tamenori, Y.

    2014-05-21

    In this article, we report on ZnO nanowires that were phosphorus doped using a spin on dopant glass deposition and diffusion method. Photoluminescence measurements suggest that this process yields p-doped ZnO. The spatial location of P atoms was studied using x-ray near-edge absorption structure spectroscopy and it is concluded that the doping is amphoteric with P atoms located on both Zn and O sites.

  5. Doping of ZnO nanowires using phosphorus diffusion from a spin-on doped glass source

    NASA Astrophysics Data System (ADS)

    Bocheux, A.; Robin, I. C.; Bonaimé, J.; Hyot, B.; Kolobov, A. V.; Mitrofanov, K. V.; Fons, P.; Tominaga, J.; Tamenori, Y.; Feuillet, G.

    2014-05-01

    In this article, we report on ZnO nanowires that were phosphorus doped using a spin on dopant glass deposition and diffusion method. Photoluminescence measurements suggest that this process yields p-doped ZnO. The spatial location of P atoms was studied using x-ray near-edge absorption structure spectroscopy and it is concluded that the doping is amphoteric with P atoms located on both Zn and O sites.

  6. PREFACE: ELC International Meeting on Inference, Computation, and Spin Glasses (ICSG2013)

    NASA Astrophysics Data System (ADS)

    Kabashima, Yoshiyuki; Hukushima, Koji; Inoue, Jun-ichi; Tanaka, Toshiyuki; Watanabe, Osamu

    2013-12-01

    The close relationship between probability-based inference and statistical mechanics of disordered systems has been noted for some time. This relationship has provided researchers with a theoretical foundation in various fields of information processing for analytical performance evaluation and construction of efficient algorithms based on message-passing or Monte Carlo sampling schemes. The ELC International Meeting on 'Inference, Computation, and Spin Glasses (ICSG2013)', was held in Sapporo 28-30 July 2013. The meeting was organized as a satellite meeting of STATPHYS25 in order to offer a forum where concerned researchers can assemble and exchange information on the latest results and newly established methodologies, and discuss future directions of the interdisciplinary studies between statistical mechanics and information sciences. Financial support from Grant-in-Aid for Scientific Research on Innovative Areas, MEXT, Japan 'Exploring the Limits of Computation (ELC)' is gratefully acknowledged. We are pleased to publish 23 papers contributed by invited speakers of ICSG2013 in this volume of Journal of Physics: Conference Series. We hope that this volume will promote further development of this highly vigorous interdisciplinary field between statistical mechanics and information/computer science. Editors and ICSG2013 Organizing Committee: Koji Hukushima Jun-ichi Inoue (Local Chair of ICSG2013) Yoshiyuki Kabashima (Editor-in-Chief) Toshiyuki Tanaka Osamu Watanabe (General Chair of ICSG2013)

  7. Overlap and activity glass transitions in plaquette spin models with hierarchical dynamics

    NASA Astrophysics Data System (ADS)

    Turner, Robert M.; Jack, Robert L.; Garrahan, Juan P.

    2015-08-01

    We consider thermodynamic and dynamic phase transitions in plaquette spin models of glasses. The thermodynamic transitions involve coupled (annealed) replicas of the model. We map these coupled-replica systems to a single replica in a magnetic field, which allows us to analyze the resulting phase transitions in detail. For the triangular plaquette model (TPM), we find for the coupled-replica system a phase transition between high- and low-overlap phases, occurring at a coupling ɛ*(T ) , which vanishes in the low-temperature limit. Using computational path sampling techniques, we show that a single TPM also displays "space-time" transitions between active and inactive dynamical phases. These first-order dynamical transitions occur at a critical counting field sc(T ) ≳0 that appears to vanish at zero temperature in a manner reminiscent of the thermodynamic overlap transition. In order to extend the ideas to three dimensions, we introduce the square pyramid model, which also displays both overlap and activity transitions. We discuss a possible common origin of these various phase transitions, based on long-lived (metastable) glassy states.

  8. Overlap and activity glass transitions in plaquette spin models with hierarchical dynamics.

    PubMed

    Turner, Robert M; Jack, Robert L; Garrahan, Juan P

    2015-08-01

    We consider thermodynamic and dynamic phase transitions in plaquette spin models of glasses. The thermodynamic transitions involve coupled (annealed) replicas of the model. We map these coupled-replica systems to a single replica in a magnetic field, which allows us to analyze the resulting phase transitions in detail. For the triangular plaquette model (TPM), we find for the coupled-replica system a phase transition between high- and low-overlap phases, occurring at a coupling ɛ*(T), which vanishes in the low-temperature limit. Using computational path sampling techniques, we show that a single TPM also displays "space-time" transitions between active and inactive dynamical phases. These first-order dynamical transitions occur at a critical counting field sc(T)≳0 that appears to vanish at zero temperature in a manner reminiscent of the thermodynamic overlap transition. In order to extend the ideas to three dimensions, we introduce the square pyramid model, which also displays both overlap and activity transitions. We discuss a possible common origin of these various phase transitions, based on long-lived (metastable) glassy states. PMID:26382352

  9. Exact Solution of the Gauge Symmetric p-Spin Glass Model on a Complete Graph

    NASA Astrophysics Data System (ADS)

    Korada, Satish Babu; Macris, Nicolas

    2009-07-01

    We consider a gauge symmetric version of the p-spin glass model on a complete graph. The gauge symmetry guarantees the absence of replica symmetry breaking and allows to fully use the interpolation scheme of Guerra (Fields Inst. Commun. 30:161, 2001) to rigorously compute the free energy. In the case of pairwise interactions ( p=2), where we have a gauge symmetric version of the Sherrington-Kirkpatrick model, we get the free energy and magnetization for all values of external parameters. Our analysis also works for even p≥4 except in a range of parameters surrounding the phase transition line, and for odd p≥3 in a more restricted region. We also obtain concentration estimates for the magnetization and overlap parameter that play a crucial role in the proofs for odd p and justify the absence of replica symmetry breaking. Our initial motivation for considering this model came from problems related to communication over a noisy channel, and is briefly explained.

  10. On one-step replica symmetry breaking in the Edwards–Anderson spin glass model

    NASA Astrophysics Data System (ADS)

    Del Ferraro, Gino; Wang, Chuang; Zhou, Hai-Jun; Aurell, Erik

    2016-07-01

    We consider a one-step replica symmetry breaking description of the Edwards–Anderson spin glass model in 2D. The ingredients of this description are a Kikuchi approximation to the free energy and a second-level statistical model built on the extremal points of the Kikuchi approximation, which are also fixed points of a generalized belief propagation (GBP) scheme. We show that a generalized free energy can be constructed where these extremal points are exponentially weighted by their Kikuchi free energy and a Parisi parameter y, and that the Kikuchi approximation of this generalized free energy leads to second-level, one-step replica symmetry breaking (1RSB), GBP equations. We then proceed analogously to the Bethe approximation case for tree-like graphs, where it has been shown that 1RSB belief propagation equations admit a survey propagation solution. We discuss when and how the one-step-replica symmetry breaking GBP equations that we obtain also allow a simpler class of solutions which can be interpreted as a class of generalized survey propagation equations for the single instance graph case.

  11. Spin glass behavior in the weberite related structure Dy3-xYxTaO7

    NASA Astrophysics Data System (ADS)

    Gomez Garcia, Jose; Tavizon, Gustavo; Duran, Alejandro; Escudero, Roberto

    2013-03-01

    Crystalline structures with tetrahedral arrangement of magnetic cations are susceptible to present non-collinear magnetism. Dy3TaO7 with weberite-type crystal structure has this arrangement and could display non-conventional magnetic coupling. Previous magnetic studies on Dy3TaO7 have characterized this as an antiferromagnetic system with TN of about 3 K. In this work magnetic properties of polycrystalline samples of the Dy3-xYxTaO7, with weberite structure are presented. X-Ray diffraction of our samples are single phase in all range of compositions. Magnetic properties measured from 2 - 300 K shown a typical Curie-Weiss behavior with the Dy3+ effective magnetic moment about 10.35 μB. The compositions x =0.66, 0.33, and 0.0 display a maximum in the susceptibility vs. temperature at 2.3, 2.7, and 3 K respectively. This behavior has been previously assigned to an antiferromagnetic transition; however our AC magnetic measurements as a function of frequency indicate a spin glass behavior. Since magnetic cations have tetrahedral arrangement for x =0, a magnetic frustrated state is anticipated for this composition.

  12. Cooling field and temperature dependent exchange bias in spin glass/ferromagnet bilayers

    PubMed Central

    Rui, W. B.; Hu, Y.; Du, A.; You, B.; Xiao, M. W.; Zhang, W.; Zhou, S. M.; Du, J.

    2015-01-01

    We report on the experimental and theoretical studies of cooling field (HFC) and temperature (T) dependent exchange bias (EB) in FexAu1 − x/Fe19Ni81 spin glass (SG)/ferromagnet (FM) bilayers. When x varies from 8% to 14% in the FexAu1 − x SG alloys, with increasing T, a sign-changeable exchange bias field (HE) together with a unimodal distribution of coercivity (HC) are observed. Significantly, increasing in the magnitude of HFC reduces (increases) the value of HE in the negative (positive) region, resulting in the entire HE ∼ T curve to move leftwards and upwards. In the meanwhile, HFC variation has weak effects on HC. By Monte Carlo simulation using a SG/FM vector model, we are able to reproduce such HE dependences on T and HFC for the SG/FM system. Thus this work reveals that the SG/FM bilayer system containing intimately coupled interface, instead of a single SG layer, is responsible for the novel EB properties. PMID:26348277

  13. Properties of the multicritical point of ±J Ising spin glasses on the square lattice

    NASA Astrophysics Data System (ADS)

    Lessa, Jean C.; de Queiroz, S. L. A.

    2006-10-01

    We use numerical transfer-matrix methods to investigate properties of the multicritical point of binary Ising spin glasses on a square lattice, whose location we assume to be given exactly by a conjecture advanced by Nishimori and Nemoto. We calculate the two largest Lyapunov exponents, as well as linear and nonlinear zero-field uniform susceptibilities, on strip of widths 4⩽L⩽16 sites, from which we estimate the conformal anomaly c , the decay-of-correlations exponent η , and the linear and nonlinear susceptibility exponents γ/ν and γnl/ν , with the help of finite-size scaling and conformal invariance concepts. Our results are c=0.46(1) ; 0.187≲η≲0.196 ; γ/ν=1.797(5) ; γnl/ν=5.59(2) . A direct evaluation of correlation functions on the strip geometry, and of the statistics of the zeroth moment of the associated probability distribution, gives η=0.194(1) , consistent with the calculation via Lyapunov exponents. Overall, these values tend to be inconsistent with the universality class of percolation, though by small amounts. The scaling relation γnl/ν=2γ/ν+d (with space dimensionality d=2 ) is obeyed to rather good accuracy, thus showing no evidence of multiscaling behavior of the susceptibilities.

  14. Implosion of indirectly driven reentrant cone shell target

    SciTech Connect

    R.B. Stephens; S.P. Hatchett; R.E. Turner; K.A. Tanaka; R. Kodama

    2003-10-31

    In an x-ray driven reentrant cone fast ignition target the x-ray spectrum contains a high energy component that casuses preheating of the reentrant cone and mixing of the gold into the collapsing shell. Direct laser drive might avoid this problem.

  15. Spin glass behavior in the Dy3-xYxTaO7 (0≤x≤1) system

    NASA Astrophysics Data System (ADS)

    Gomez-Garcia, J. Francisco; Escudero, Roberto; Tavizon, Gustavo

    2014-09-01

    Several x-compositions of the polycrystalline Dy3-xYxTaO7 system, crystallizing in the weberite-type structure, were synthesized and structurally characterized using Rietveld refinements based on X-ray diffraction data. In previous magnetic characterization of Dy3TaO7 (x=0), with the same crystal structure, an antiferromagnetic transition at T=2.3 K has been assigned to this compound. On the basis of DC and AC magnetic susceptibilities analyses, we show in this work that all compounds in the range of 0≤x≤1.0 exhibit a spin glass behavior. The nature of the spin glass behavior in Dy3-xYxTaO7, can be attributed to the highly frustrated antiferromagnetic interaction of the Dy3+ sublattice and to the Dy3+-Dy3+ distorted tetrahedra array in the weberite-type structure of this system. By fitting AC susceptibility data, using dynamical scaling theory equations, we conclude that a cluster spin glass is present in Dy3-xYxTaO7 in the low temperature range. Depending on the x-composition, Tg~2.2-3.2 K. In the range 15-300 K the system obeys a Curie-Weiss magnetic behavior.

  16. Slow dynamics in the geometrically frustrated magnet ZnFe2O4: Universal features of aging phenomena in spin glasses

    NASA Astrophysics Data System (ADS)

    Mamiya, H.; Tsujii, N.; Terada, N.; Nimori, S.; Kitazawa, H.; Hoshikawa, A.; Ishigaki, T.

    2014-07-01

    To clarify the universal features of spin glasses, we carefully studied slow dynamics in a geometrically frustrated magnet ZnFe2O4 with slight disorders, regarded as an "unconventional" Heisenberg spin glass, using time-resolved neutron diffractometry and magnetometry. The results indicate that "aging" can be attributed not to growth of the short-range order detected by a diffuse scattering but to aging of a hidden aperiodic correlation, as expected from theories for spin glasses. Concerning aging, peculiar behavior was found; the decay of thermoremanent magnetization is extremely accelerated if the sample is heated/cooled briefly midway through the isothermal slow relaxation. Conversely, magnetization surprisingly increases despite the absence of a magnetic field when the temperature returns after the brief heating/cooling. The behavior can be explained as a destabilization of the aged spin configuration due to the thermal perturbations and subsequent spontaneous restoration of the original spin configuration after the destabilization. Whereas such destabilization and restoration do not occur during freezing into numerous metastable states in a fixed energy landscape, these are possible in an energy landscape with a temperature-sensitive funnel-like structure. These features, consistent with the ghost domain scenario of the droplet picture, are the same as for conventional Heisenberg spin glasses such as dilute magnetic alloys and dilute magnetic semiconductors. In other words, they are universal features in Heisenberg spin glasses including unconventional ones.

  17. Reentrant Phase Coherence in Superconducting Nanowire Composites.

    PubMed

    Ansermet, Diane; Petrović, Alexander P; He, Shikun; Chernyshov, Dmitri; Hoesch, Moritz; Salloum, Diala; Gougeon, Patrick; Potel, Michel; Boeri, Lilia; Andersen, Ole Krogh; Panagopoulos, Christos

    2016-01-26

    The short coherence lengths characteristic of low-dimensional superconductors are associated with usefully high critical fields or temperatures. Unfortunately, such materials are often sensitive to disorder and suffer from phase fluctuations in the superconducting order parameter which diverge with temperature T, magnetic field H, or current I. We propose an approach to overcome synthesis and fluctuation problems: building superconductors from inhomogeneous composites of nanofilaments. Macroscopic crystals of quasi-one-dimensional Na2-δMo6Se6 featuring Na vacancy disorder (δ ≈ 0.2) are shown to behave as percolative networks of superconducting nanowires. Long-range order is established via transverse coupling between individual one-dimensional filaments, yet phase coherence remains unstable to fluctuations and localization in the zero (T,H,I) limit. However, a region of reentrant phase coherence develops upon raising (T,H,I). We attribute this phenomenon to an enhancement of the transverse coupling due to electron delocalization. Our observations of reentrant phase coherence coincide with a peak in the Josephson energy EJ at nonzero (T,H,I), which we estimate using a simple analytical model for a disordered anisotropic superconductor. Na2-δMo6Se6 is therefore a blueprint for a future generation of nanofilamentary superconductors with inbuilt resilience to phase fluctuations at elevated (T,H,I). PMID:26727335

  18. Asymmetric electroresistance of cluster glass state in manganites

    SciTech Connect

    Lourembam, James; Lin, Weinan; Ding, Junfeng; Bera, Ashok; Wu, Tom

    2014-03-31

    We report the electrostatic modulation of transport in strained Pr{sub 0.65}(Ca{sub 0.75}Sr{sub 0.25}){sub 0.35}MnO{sub 3} thin films grown on SrTiO{sub 3} by gating with ionic liquid in electric double layer transistors (EDLT). In such manganite films with strong phase separation, a cluster glass magnetic state emerges at low temperatures with a spin freezing temperature of about 99 K, which is accompanied by the reentrant insulating state with high resistance below 30 K. In the EDLT, we observe bipolar and asymmetric modulation of the channel resistance, as well as an enhanced electroresistance up to 200% at positive gate bias. Our results provide insights on the carrier-density-dependent correlated electron physics of cluster glass systems.

  19. Nature of the spin-glass phase in models with long-range interactions

    NASA Astrophysics Data System (ADS)

    Wittmann, Matthew C.

    Despite decades of effort, our understanding of low-temperature phase of spin glass models with short-range interactions remains incomplete. Replica symmetry breaking (RSB) theory, based on the solution of the Sherrington-Kirkpatrick mean-field model, predicts many pure states; meanwhile, competing theories of short-range systems, such as the droplet picture, predict a single pair of pure states related by time-reversal symmetry, analogously to the ferromagnet. Since RSB certainly holds for the mean-field (infinite-range) model, it is interesting to study short-range models in high dimensions to observe whether RSB also holds here; however, computer simulations of short-range models in high dimensions are difficult because the number of spins to equilibrate grows so rapidly with the linear size of the system. A relatively recent idea which has been fruitful is to instead study one-dimensional models with long-range (power-law) interactions, which are argued to have the same critical behavior as corresponding short-range models in high dimensions, but for which simulations for a range of sizes (crucial for finite-size scaling analysis) are feasible. For these one-dimensional long-range (1DLR) models, we fill in a previously unexplored region of parameter space where the interactions become sufficiently long-range that they must be rescaled with the system size to maintain the thermodynamic limit. We find strong evidence that detailed behavior of the 1DLR models everywhere in this "nonextensive regime" is identical to that of the Sherrington-Kirkpatrick model, lending support to a recent conjecture. In an attempt to distinguish the RSB and droplet pictures, we study recently-proposed observables based on the statistics of individual disorder samples, rather than simply averaging over the disorder as is most frequently done in previous studies. We compare Monte Carlo results for 1DLR models which are proxies for short-range models in 3, 4, and 10 dimensions with

  20. Molecular Dynamics in a Liquid Crystal with Reentrant Mesophases

    NASA Astrophysics Data System (ADS)

    Sebastião, P. J.; Ribeiro, A. C.; Nguyen, H. T.; Noack, F.

    1995-11-01

    It is well known that liquid crystalline compounds with a cyano terminal group can present peculiar polymorphisms in particular different types of smectic A mesophases and a reentrant behaviour for both nematic and smectic A mesophases. In this work we study by proton NMR relaxation the influence of these features on the molecular dynamics of the compound 4-cyanobenzoate-4'-octylbenzoyloxyphenyl (DB8CN Sym) in its nematic (N), partial bilayer smectic A (SAd), reentrant nematic (Nre) and reentrant smectic A (SA1) mesophases. Standard and fast field-cycling techniques were used for our spin-lattice relaxation's study over a broad frequency range of 6 decades (200 Hz up to 300 MHz). It was found that the molecular dynamics in the nematic mesophases is rather different from the molecular dynamics in the smectic A mesophases. However, the reentrant aspect present in both nematic and smectic A states is not associated to a major difference on the molecular dynamics of the nematic and reentrant nematic or smectic and reentrant smectic A mesophases. Order director fluctuations and rotations/reorientations are the most important relaxation mechanisms in the nematic mesophases in the lower and higher frequency limits, respectively, while self-diffusion has a very small contribution to the overall relaxation. As for the smectic A mesophases, self-diffusion and rotations/reorientations are the predominant relaxation mechanisms for frequencies above 20 kHz. The collective motions, which for these mesophases have to be associated with layer undulations with the frequency law T_1sim ν, are only important to the spin-lattice relaxation on the low part of the frequency spectrum (ν<10 kHz). The inclusion in the relaxation study of a contribution from the cross-relaxation between protons and nitrogen nuclei improves the quality of the 1/T_1 data fits in both kinds of mesophases. The combined study of the molecular dynamics in the N, SAd, Nre and SA1 mesophases of DB8CN Sym reveals

  1. Spin-glass model predicts metastable brain states that diminish in anesthesia

    PubMed Central

    Hudetz, Anthony G.; Humphries, Colin J.; Binder, Jeffrey R.

    2014-01-01

    Patterns of resting state connectivity change dynamically and may represent modes of cognitive information processing. The diversity of connectivity patterns (global brain states) reflects the information capacity of the brain and determines the state of consciousness. In this work, computer simulation was used to explore the repertoire of global brain states as a function of cortical activation level. We implemented a modified spin glass model to describe UP/DOWN state transitions of neuronal populations at a mesoscopic scale based on resting state BOLD fMRI data. Resting state fMRI was recorded in 20 participants and mapped to 10,000 cortical regions (sites) defined on a group-aligned cortical surface map. Each site represented the population activity of a ~20 mm2 area of the cortex. Cross-correlation matrices of the mapped BOLD time courses of the set of sites were calculated and averaged across subjects. In the model, each cortical site was allowed to interact with the 16 other sites that had the highest pair-wise correlation values. All sites stochastically transitioned between UP and DOWN states under the net influence of their 16 pairs. The probability of local state transitions was controlled by a single parameter T corresponding to the level of global cortical activation. To estimate the number of distinct global states, first we ran 10,000 simulations at T = 0. Simulations were started from random configurations that converged to one of several distinct patterns. Using hierarchical clustering, at 99% similarity, close to 300 distinct states were found. At intermediate T, metastable state configurations were formed suggesting critical behavior with a sharp increase in the number of metastable states at an optimal T. Both reduced activation (anesthesia, sleep) and increased activation (hyper-activation) moved the system away from equilibrium, presumably incompatible with conscious mentation. During equilibrium, the diversity of large-scale brain states was

  2. Response to a field of the D = 3 Ising spin glass with Janus and JanusII dedicated computers

    NASA Astrophysics Data System (ADS)

    Seoane, Beatriz; Janus Collaboration Collaboration

    Using the Janus dedicated computer, and its new generation JanusII, we study the linear response to a field of the Edwards-Anderson model for times that cover twelve orders of magnitude. The fluctuation-dissipation relations are investigated for several values of tw. We observe that the violations of the fluctuation-dissipation theorem can be directly related to the P (q) measured in equilibrium at finite sizes, although a simple statics-dynamics dictionary L <--> ξ (tw) is not enough to account for the behavior at large times. We show that the equivalence can be easily restored by taking into account the growth of ξ (t +tw) . Interestingly, experimental measurements of the spin glass correlation length rely precisely on the response of a spin glass to a field, although a direct relation between the measured object and the real ξ has never been established. In this work, we mimic the experimental protocol with Janus data, which lets us relate the experimental ξ with the length extracted from the spatial correlation function. These results allow us for the first time to make a quantitative comparison between experiments and simulations, finding a surprising good agreement with measurements in superspin glasses. This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 654971, the ERC grant CRIPHERASY (no. 247328) and from the MINECO(Spain) (No. FIS2012-35719-C02).

  3. Spin-glass behaviors in carrier polarity controlled Fe{sub 3−x}Ti{sub x}O{sub 4} semiconductor thin films

    SciTech Connect

    Yamahara, H.; Seki, M.; Adachi, M.; Takahashi, M.; Nasu, H.; Tabata, H.; Horiba, K.; Kumigashira, H.

    2015-08-14

    Carrier-type control of spin-glass (cluster spin-glass) is studied in order to engineer basic magnetic semiconductor elements using the memory functions of spin-glass. A key of carrier-polarity control in magnetite is the valence engineering between Fe(II) and Fe(III) that is achieved by Ti(IV) substitution. Single phases of (001)-oriented Fe{sub 3−x}Ti{sub x}O{sub 4} thin films have been obtained on spinel MgAl{sub 2}O{sub 4} substrates by pulsed laser deposition. Thermoelectric power measurements reveal that Ti-rich films (x = 0.8) show p-type conduction, while Ti-poor films (x = 0.6–0.75) show n-type conduction. The systematic Fe(III) reduction to Fe(II) followed by Ti(IV) substitution in the octahedral sublattice is confirmed by the X-ray absorption spectra. All of the Fe{sub 3−x}Ti{sub x}O{sub 4} films (x = 0.6–0.8) exhibit ferrimagnetism above room temperature. Next, the spin-glass behaviors of Ti-rich Fe{sub 2.2}Ti{sub 0.8}O{sub 4} film are studied, since this magnetically diluted system is expected to exhibit the spin-glass behaviors. The DC magnetization and AC susceptibility measurements for the Ti-rich Fe{sub 2.2}Ti{sub 0.8}O{sub 4} film reveal the presence of the spin glass phase. Thermal- and magnetic-field-history memory effects are observed and are attributed to the long time-decay nature of remanent magnetization. The detailed analysis of the time-dependent thermoremanent magnetization reveals the presence of the cluster spin glass state.

  4. Nicholas Metropolis Award for Outstanding Doctoral Thesis Work in Computational Physics Lecture: The Janus computer, a new window into spin-glass physics

    NASA Astrophysics Data System (ADS)

    Yllanes, David

    2013-03-01

    Spin glasses are a longstanding model for the sluggish dynamics that appears at the glass transition. They enjoy a privileged status in this context, as they provide the simplest model system both for theoretical and experimental studies of glassy dynamics. However, in spite of forty years of intensive investigation, spin glasses still pose a formidable challenge to theoretical, computational and experimental physics. The main difficulty lies in their incredibly slow dynamics. A recent breakthrough has been made possible by our custom-built computer, Janus, designed and built in a collaboration formed by five universities in Spain and Italy. By employing a purpose-driven architecture, capable of fully exploiting the parallelization possibilities intrinsic to these simulations, Janus outperforms conventional computers by several orders of magnitude. After a brief introduction to spin glasses, the talk will focus on the new physics unearthed by Janus. In particular, we recall our numerical study of the nonequilibrium dynamics of the Edwards-Anderson Ising Spin Glass, for a time that spans eleven orders of magnitude, thus approaching the experimentally relevant scale (i.e. seconds). We have also studied the equilibrium properties of the spin-glass phase, with an emphasis on the quantitative matching between non-equilibrium and equilibrium correlation functions, through a time-length dictionary. Last but not least, we have clarified the existence of a glass transition in the presence of a magnetic field for a finite-range spin glass (the so-called de Almeida-Thouless line). We will finally mention some of the currently ongoing work of the collaboration, such as the characterization of the non-equilibrium dynamics in a magnetic field and the existence of a statics-dynamics dictionary in these conditions.

  5. Spin-glass-like freezing of inner and outer surface layers in hollow γ-Fe2O3 nanoparticles

    SciTech Connect

    Khurshid, Hafsa; Lampen-Kelley, Paula; Iglesias, Òscar; Alonso, Javier; Phan, Manh-Huong; Sun, Cheng-Jun; Saboungi, Marie-Louise; Srikanth, Hariharan

    2015-10-27

    Disorder among surface spins largely dominates the magnetic response of ultrafine magnetic particle systems. In this work, we examine time-dependent magnetization in high-quality, monodisperse hollow maghemite nanoparticles (NPs) with a 14.8±0.5 nm outer diameter and enhanced surface-to-volume ratio. The nanoparticle ensemble exhibits spin-glass-like signatures in dc magnetic aging and memory protocols and ac magnetic susceptibility. The dynamics of the system slow near 50 K, and becomes frozen on experimental time scales below 20 K. Remanence curves indicate the development of magnetic irreversibility concurrent with the freezing of the spin dynamics. A strong exchange-bias effect and its training behavior point to highly frustrated surface spins that rearrange much more slowly than interior spins with bulk coordination. Monte Carlo simulations of a hollow particle reproducing the experimental morphology corroborate strongly disordered surface layers with complex energy landscapes that underlie both glass-like dynamics and magnetic irreversibility. Calculated hysteresis loops reveal that magnetic behavior is not identical at the inner and outer surfaces, with spins at the outer surface layer of the 15 nm hollow particles exhibiting a higher degree of frustration. Lastly, our combined experimental and simulated results shed light on the origin of spin-glass-like phenomena and the important role played by the surface spins in magnetic hollow nanostructures.

  6. Spin-glass-like freezing of inner and outer surface layers in hollow γ-Fe2O3 nanoparticles

    SciTech Connect

    Khurshid, Hafsa; Lampen-Kelley, Paula; Iglesias, Òscar; Alonso, Javier; Phan, Manh-Huong; Sun, Cheng-Jun; Saboungi, Marie-Louise; Srikanth, Hariharan

    2015-10-27

    Disorder among surface spins largely dominates the magnetic response of ultrafine magnetic particle systems. In this work, we examine time-dependent magnetization in high-quality, monodisperse hollow maghemite nanoparticles (NPs) with a 14.8±0.5 nm outer diameter and enhanced surface-to-volume ratio. The nanoparticle ensemble exhibits spin-glass-like signatures in dc magnetic aging and memory protocols and ac magnetic susceptibility. The dynamics of the system slow near 50 K, and becomes frozen on experimental time scales below 20 K. Remanence curves indicate the development of magnetic irreversibility concurrent with the freezing of the spin dynamics. A strong exchange-bias effect and its training behavior point to highly frustrated surface spins that rearrange much more slowly than interior spins with bulk coordination. Monte Carlo simulations of a hollow particle reproducing the experimental morphology corroborate strongly disordered surface layers with complex energy landscapes that underlie both glass-like dynamics and magnetic irreversibility. Calculated hysteresis loops reveal that magnetic behavior is not identical at the inner and outer surfaces, with spins at the outer surface layer of the 15 nm hollow particles exhibiting a higher degree of frustration. Our combined experimental and simulated results shed light on the origin of spin-glass-like phenomena and the important role played by the surface spins in magnetic hollow nanostructures.

  7. Laser-induced radial birefringence and spin-to-orbital optical angular momentum conversion in silver-doped glasses

    SciTech Connect

    Amjad, Jafar Mostafavi; Khalesifard, Hamid Reza; Slussarenko, Sergei; Karimi, Ebrahim; Santamato, Enrico; Marrucci, Lorenzo

    2011-07-04

    Samples of Ag{sup +}/Na{sup +} ion-exchanged glass that have been subject to intense laser irradiation may develop novel optical properties, as a consequence of the formation of patterns of silver nanoparticles and other structures. Here, we report the observation of a laser-induced permanent transverse birefringence, with the optical axis forming a radial pattern, as revealed by the spin-to-orbital angular momentum conversion occurring in a probe light beam. The birefringence pattern can be modeled well as resulting from thermally-induced stresses arising in the silver-doped glass during laser exposure, although the actual mechanism leading to the permanent anisotropy is probably more complex.

  8. Spin-Glass Properties of (terbium, Yttrium)silver, (gadolinium, Yttrium)silver and (terbium, Yttrium)zinc

    NASA Astrophysics Data System (ADS)

    Said, Moh'd. Rida

    1990-01-01

    Standard magnetization measurements on Tb _{0.15}Y_{0.85 }Zn, Tb_{x} Y_{1-x}Ag, and Gd_{x}Y _{1-x}Ag for x <= 0.5 show that these compounds exhibit spin-glass behavior, as evidenced by the irreversibility of the magnetization (M) versus temperature (T) curves and by the thermoremanence (TRM) induced by field cooling. In the case of Tb_{x}Y _{1-x}Ag, spin -glass ordering coexists with the long-range antiferromagnetic order seen by neutron diffraction, which presumably also exists in Gd_{x}Y _{1-x}Ag. In the case of Tb_{0.15}Y _{0.85}Zn, our magnetization measurements show no spontaneous moment and thus the absence of the ferromagnetic order that exists in undiluted TbZn. Detailed measurements of the TRM versus T for different fields applied during cooling (H_{cool}), produce isotherms of TRM versus H_{cool}, which are found to scale to a universal curve for each sample. Resistivity measurements (rho) as a function of temperature were performed on Tb _{x}Y_ {1-x}Ag and Gd_ {x}Y_{1-x}Ag, from which we found that the residual resistivity at T(<spin-glass anisotropy in Tb _{0.3}Y_ {0.7}Ag, Tb_{0.15 }Y_{0.85}Ag, Tb_{0.15}Y_ {0.85}Zn, and Gd_{0.3 }Y_{0.7}Ag by the rotational magnetization-vector measurements technique. When a thin disk sample was field cooled and rotated in various fixed fields (vec H) directed in the plane of the sample, we found that the anisotropy field (vec H_{K}) rotated rigidly and reversibly with the sample if the magnitude of vec H was below a certain critical value. But if H was larger than this critical value, which decreased with increasing temperature, vec H_{K} turned with the sample up to some angle relative to vec H where it remained fixed as the sample continued to rotate. Thus, the direction of vec H _{K} changed relative to the sample in a frictional manner. The magnitude of vec H_{K} was determined from the results of a

  9. A Note on the Guerra and Talagrand Theorems for Mean Field Spin Glasses: The Simple Case of Spherical Models

    NASA Astrophysics Data System (ADS)

    Franz, Silvio; Tria, Francesca

    2006-01-01

    The aim of this paper is to discuss the main ideas of the Talagrand proof of the Parisi Ansatz for the free-energy of Mean Field Spin Glasses with a physicist's approach. We consider the case of the spherical p-spin model, which has the following advantages: (1) the Parisi Ansatz takes the simple "one step replica symmetry breaking form," (2) the replica free-energy as a function of the order parameters is simple enough to allow for numerical maximization with arbitrary precision. We present the essential ideas of the proof, we stress its connections with the theory of effective potentials for glassy systems, and we reduce the technically more difficult part of the Talagrand's analysis to an explicit evaluation of the solution of a variational problem.

  10. On the exchange bias effect in NiO nanoparticles with a core(antiferromagnetic)/shell (spin glass) morphology

    NASA Astrophysics Data System (ADS)

    Rinaldi-Montes, N.; Gorria, P.; Martínez-Blanco, D.; Fuertes, A. B.; Fernández Barquín, L.; Rodríguez Fernández, J.; de Pedro, I.; Fdez-Gubieda, M. L.; Alonso, J.; Olivi, L.; Aquilanti, G.; Puente-Orench, I.; Blanco, J. A.

    2015-11-01

    The unexpected appearance of magnetic hysteresis and exchange bias effects in nominally antiferromagnetic NiO nanoparticles is usually explained in terms of a core/shell morphology, where a spin glass-like shell is exchange coupled to an antiferromagnetic core. However, recent studies have challenged the validity of this assumption for small enough NiO nanoparticles. In this work we present proof of the core/shell model for NiO nanoparticles with sizes below 10 nm by combining neutron powder diffraction and magnetic measurements. In addition, we have verified that the exchange bias effect persists even when the particle size is reduced down to 4 nm.

  11. Ferromagnetic superexchange interaction and spin glass behaviour in Sm{sub 2}CoMnO{sub 6} nanoparticles

    SciTech Connect

    Maiti, R. P. Mitra, M. K.; Chakravorty, Dipankar

    2014-04-24

    Sm{sub 2}CoMnO{sub 6} samples with particle sizes ∼70 nm were synthesized by a sol-gel technique. Field cooled magnetization study exhibited a ferromagnetic transition with a Curie temperature at ∼139K. This was attributed to ferromagnetic Co{sup 2+}−O−Mn{sup 4+} superexchange interaction. The results of magnetic relaxation measurements could be fitted to a stretched exponential function indicating the presence of both ferromagnetic and spin glass components in the system.

  12. Sample-to-sample fluctuations of the overlap distributions in the three-dimensional Edwards-Anderson spin glass

    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.

  13. Classification, Electrophysiological Features and Therapy of Atrioventricular Nodal Reentrant Tachycardia

    PubMed Central

    Josephson, Mark E

    2016-01-01

    Atrioventricular nodal reentrant tachycardia (AVNRT) should be classified as typical or atypical. The term ‘fast-slow AVNRT’ is rather misleading. Retrograde atrial activation during tachycardia should not be relied upon as a diagnostic criterion. Both typical and atypical atrioventricular nodal reentrant tachycardia are compatible with varying retrograde atrial activation patterns. Attempts at establishing the presence of a ‘lower common pathway’ are probably of no practical significance. When the diagnosis of AVNRT is established, ablation should be only directed towards the anatomic position of the slow pathway. If right septal attempts are unsuccessful, the left septal side should be tried. Ablation targeting earliest atrial activation sites during typical atrioventricular nodal reentrant tachycardia or the fast pathway in general for any kind of typical or atypical atrioventricular nodal reentrant tachycardia, are not justified. In this review we discuss current concepts about the tachycardia circuit, electrophysiologic diagnosis, and ablation of this arrhythmia.

  14. Cluster-spin-glass behavior in layered LiNi0.4Mn0.4Co0.2O2

    NASA Astrophysics Data System (ADS)

    Du, Fei; Bie, Xiaofei; Chen, Yan; Wei, Yingjin; Liu, Lina; Wang, Chunzhong; Zou, Guangtian; Chen, Gang

    2009-09-01

    Layered LiNi0.4Mn0.4Co0.2O2 has been synthesized by citrate precursor method. Its magnetic properties are investigated by dc magnetization. The high-temperature susceptibility curve follows the Curie-Weiss law with Curie and Weiss constant 1.435(2) emu K/mol Oe and -112(4) K, respectively, larger than those values reported in previous researches, which possibly results from the difference in the synthesis process and sintered temperature. Our dc susceptibility differs from that of the homogeneous spin glass in that below Tirr field cooled (FC) curve continues to rise, while the FC curve is almost flat for homogeneous spin glass. Together with the de Almeida-Thouless line analysis, cluster spin glass is suggested to be the ground state of LiNi0.4Mn0.4Co0.2O2. Frustration parameter |θ|/Tf in this system is estimated to be about four, lower than the value that frustration effect is strong enough to give rise to spin glass state. This fact indicates that the cluster-spin-glass results from the short-range structure disorder rather than the geometrical frustration.

  15. Spin glass freezing and superconductivity in YBa2(Cu(1-x)Fe(x))3O7 alloys

    NASA Technical Reports Server (NTRS)

    Mirebeau, I.; Hennion, M.; Dianoux, J.; Caignaert, V.; Phillips, T. E.; Moorjani, K.

    1991-01-01

    The dynamics were studied of the iron spins in superconducting YBa2(Cu(0.94)Fe(0.06))3O7 by neutron time of flight measurements. Two samples were studied with slightly different characteristics, as shown by resistivity and neutron diffraction measurements. The same dynamical anomalies are observed by neutrons in both samples. Differences appear qualitative but not quantitative. In the whole temperature range, the q-dependence of the magnetic intensity mainly reflects the magnetic form factor of iron which shows that the iron spins are almost uncorrelated. The elastic and quasielastic intensities strongly vary with temperature. A spin glass like freezing is revealed at low temperature by a sharp decrease of the quasielastic intensity, an increase of the 'elastic' or resolution limited intensity and a minimum in the quasielastic width. The freezing temperature (T sub f - 18 K) corresponds to that already determined by a magnetic splitting in Mossbauer experiments. Above T sub f, the relaxation of the iron spins in the paramagnetic state is modified by the occurrence of superconductivity. An increase was observed of the quasielastic intensity and of the quasielastic width at the superconducting transition.

  16. Contrasting energy scales of reentrant integer quantum Hall states

    NASA Astrophysics Data System (ADS)

    Deng, Nianpei; Watson, J. D.; Rokhinson, L. P.; Manfra, M. J.; Csáthy, G. A.

    2012-11-01

    We report drastically different onset temperatures of the reentrant integer quantum Hall states in the second and third Landau level. This finding is in quantitative disagreement with the Hartree-Fock theory of the bubble phases which is thought to describe these reentrant states. Our results indicate that the number of electrons per bubble in either the second or the third Landau level is likely different than predicted.

  17. Pressure variation of reentrant transition temperature in liquid crystals.

    PubMed

    Srivastava, A; Sa, D; Singh, S

    2007-02-01

    High pressure experimental studies show that in certain mesogenic materials, the nematic-smectic A (N-Sm A) transition temperature T(AN) exhibits nonlinear pressure dependence. As a consequence, the material shows reentrant phenomena that is a phase sequence nematic -- smectic A -- reentrant nematic appears. The characteristic features of this phenomenon have been addressed here within the framework of Landau-de-Gennes theory, where the coupling between nematic and smectic A order parameters (gamma, lambda(eff)) plays an important role. The cubic coupling gamma is chosen to be negative in order to form Sm A phase whereas the biquadratic coupling lambda(eff) is made large and positive to obtain reentrant behaviour. In the present work, we incorporate the pressure dependence in the theory through gamma and lambda(eff) which justifies the experimental pressure dependence in the reentrant transition temperature [Formula: see text]. The pressure dependence of gamma and lambda(eff) are employed in the calculation of excess specific heat capacity near the reentrant transition. The computed heat capacity shows strong pressure dependence near the reentrant transition which can be confirmed from high pressure measurement. PMID:17342375

  18. Replica analysis of the generalized p-spin interaction glass model.

    SciTech Connect

    Schelkacheva, T. I.; Chtchelkatchev, N. M.

    2011-10-13

    We investigate the stability of replica symmetry breaking solutions in generalized p-spin models. It is shown that the kind of the transition to the one-step replica symmetry breaking state depends not only on the presence or absence of the reflection symmetry of the generalized 'spin'-operators {cflx U} but on the number of interacting operators and their individual characteristics.

  19. Critical temperature oscillations and reentrant superconductivity due to the FFLO like state in F/S/F trilayers

    NASA Astrophysics Data System (ADS)

    Kehrle, J.; Zdravkov, V. I.; Obermeier, G.; Garcia-Garcia, J.; Ullrich, A.; Müller, C.; Morari, R.; Sidorenko, A. S.; Horn, S.; Tagirov, L. R.; Tidecks, R.

    2012-01-01

    Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers, in which the establishing of a Fulde-Ferrell Larkin-Ovchinnikov (FFLO) like state leads to interference effects of the superconducting pairing wave function, form the core of the superconducting spin valve. The realization of strong critical temperature oscillations in such trilayers, as a function of the ferromagnetic layer thicknesses or, even more efficient, reentrant superconductivity, are the key condition to obtain a large spin valve effect, i.e. a large shift in the critical temperature. Both phenomena have been realized experimentally in the Cu 41 Ni 59 /Nb/Cu 41 Ni 59 trilayers investigated in the present work.

  20. Interface free-energy exponent in the one-dimensional Ising spin glass with long-range interactions in both the droplet and broken replica symmetry regions.

    PubMed

    Aspelmeier, T; Wang, Wenlong; Moore, M A; Katzgraber, Helmut G

    2016-08-01

    The one-dimensional Ising spin-glass model with power-law long-range interactions is a useful proxy model for studying spin glasses in higher space dimensions and for finding the dimension at which the spin-glass state changes from having broken replica symmetry to that of droplet behavior. To this end we have calculated the exponent that describes the difference in free energy between periodic and antiperiodic boundary conditions. Numerical work is done to support some of the assumptions made in the calculations and to determine the behavior of the interface free-energy exponent of the power law of the interactions. Our numerical results for the interface free-energy exponent are badly affected by finite-size problems. PMID:27627255

  1. Interface free-energy exponent in the one-dimensional Ising spin glass with long-range interactions in both the droplet and broken replica symmetry regions

    NASA Astrophysics Data System (ADS)

    Aspelmeier, T.; Wang, Wenlong; Moore, M. A.; Katzgraber, Helmut G.

    2016-08-01

    The one-dimensional Ising spin-glass model with power-law long-range interactions is a useful proxy model for studying spin glasses in higher space dimensions and for finding the dimension at which the spin-glass state changes from having broken replica symmetry to that of droplet behavior. To this end we have calculated the exponent that describes the difference in free energy between periodic and antiperiodic boundary conditions. Numerical work is done to support some of the assumptions made in the calculations and to determine the behavior of the interface free-energy exponent of the power law of the interactions. Our numerical results for the interface free-energy exponent are badly affected by finite-size problems.

  2. Spin-glass behavior and anomalous magnetoresistance in ferromagnetic Ge{sub 1-x}Fe{sub x}Te epilayer

    SciTech Connect

    Liu, Jindong; Cheng, Xiaomin Tong, Fei; Miao, Xiangshui

    2014-07-28

    We report that the Ge{sub 1-x}Fe{sub x}Te thin film exhibits spin-glass behavior when the Fe concentration increases to 0.08. A large bifurcation between the zero-field cooling and field cooling temperature-dependent magnetization was observed. The hysteresis loops after zero-field cooling and field cooling show an exchange bias effect. A time-dependent thermoremanent magnetization follows power-law decay, which confirms the existence of spin glass. The anomalous magnetotranport properties present a further evidence for spin-glass behavior and give a freezing temperature T{sub g} ∼ 5 K in the Ge{sub 0.92}Fe{sub 0.08}Te thin film.

  3. Spin glass and semiconducting behavior in one-dimensional BaFe2-dSe3 (d~2) crystals

    SciTech Connect

    Saparov, Bayrammurad I; Calder, Stuart A; Sipos, Balazs; Cao, Huibo; Chi, Songxue; Singh, David J; Christianson, Andrew D; Lumsden, Mark D; Sefat, A. S.

    2011-01-01

    We investigate the physical properties and electronic structure of BaFe{sub 1.79(2)}Se{sub 3} crystals, which were grown out of tellurium flux. The crystal structure of the compound, an iron-deficient derivative of the ThCr{sub 2}Si{sub 2}-type, is built upon edge-shared FeSe{sub 4} tetrahedra fused into double chains. The semiconducting BaFe{sub 1.79(2)}Se{sub 3} ({rho}{sub 295K} = 0.18 {Omega} {center_dot} cm and E{sub g} = 0.30 eV) does not order magnetically; however, there is evidence for short-range magnetic correlations of spin glass type (T{sub f} {approx} 50 K) in magnetization, heat capacity, and neutron diffraction results. A one-third substitution of selenium with sulfur leads to a slightly higher electrical conductivity ({rho}{sub 295K } = 0.11 {Omega} {center_dot} cm and E{sub g} = 0.22 eV) and a lower spin glass freezing temperature (T{sub f} {approx} 15 K), corroborating with higher electrical conductivity reported for BaFe{sub 2}S{sub 3}. According to the electronic structure calculations, BaFe{sub 2}Se{sub 3} can be considered as a one-dimensional ladder structure with a weak interchain coupling.

  4. (Tb_1-xY_x)Ni_2Ge_2: From Ising Antiferromagnet to Ising Spin Glass.

    NASA Astrophysics Data System (ADS)

    Canfield, P. C.; Wiener, T.; Bud'Ko, S. L.

    2000-03-01

    Recent measurements [1,2] on single crystals of RNi_2Ge2 compounds have revealed complex, anisotropies and H-T phase diagrams. In addition recent work on (Gd_1-xEu_x)Ni_2Ge2 has demonstrated our ability to change the band filling so as to tune the ordering wave-vector of the magnetically ordered state [3]. In this talk we will present the results of measurements on another pseudo- ternary series: (Tb_1-xY_x)Ni_2Ge_2. The Tb local moments are aligned along the crystallographic c-axis for all concentrations of Y and as Y replaces Tb the two magnetic transition temperatures decrease rapidly. For x > 0.6 the low temperature ground state of the magnetic sublattice is that of an Ising spin glass. T -x phase diagrams and detailed characterization of the spin glass state will be presented. [1]S.L.Bud'ko et al. JMMM 205, 53 (1999). [2]Z.Islam et al. PRB 58, 8522 (1998). [3]Z.Islam et al. PRL 83, 2817 (1999). Ames Laboratory is operated for the US Department of Energy by Iowa State University under Contract No. W-7405-Eng-82. This work was supported by the Director for Energy Research, Office of Basic Energy Sciences.

  5. Reentrant stability of BEC standing wave patterns

    SciTech Connect

    Kalas, Ryan M; Solenov, Dmitry; Timmermans, Eddy M

    2009-01-01

    We describe standing wave patterns induced by an attractive finite-ranged external potential in a large Bose-Einstein Condensate (BEC). As the potential depth increases, the time independent Gross-Pitaevskii equation develops pairs of solutions that have nodes in their wavefunction. We elucidate the nature of these states and study their dynamical stability. Although we study the problem in a two-dimensional BEC subject to a cylindrically symmetric square-well potential of a radius that is comparable to the coherence length of the BEC, our analysis reveals general trends, valid in two and three dimensions, independent of the symmetry of the localized potential well, and suggestive of the behavior in general, short- and large-range potentials. One set of nodal BEC wavefunctions resembles the single particle n node bound state wavefunction of the potential well, the other wavefunctions resemble the n - 1 node bound-state wavefunction with a kink state pinned by the potential. The second state, though corresponding to the lower free energy value of the pair of n node BEC states, is always unstable, whereas the first can be dynamically stable in intervals of the potential well depth, implying that the standing wave BEC can evolve from a dynamically unstable to stable, and back to unstable status as the potential well is adiabatically deepened, a phenomenon that we refer to as 'reentrant dynamical stability'.

  6. Capillary surface discontinuities above reentrant corners

    NASA Technical Reports Server (NTRS)

    Korevaar, H. J.

    1982-01-01

    A particular configuration of a vertical capillary tube for which S is the equilibrium interface between two fluids in the presence of a downward pointing gravitational field was investigated. S is the graph a function u whose domain is the (horizontal) cross section gamma of the tube. The mean curvature of S is proportional to its height above a fixed reference plane and lambda is a prescribed constant and may be taken between zero and pi/2. Domains gamma for which us is a bounded function but does not extend continuously to d gamma are sought. Simple domains are found and the behavior of u in those domains is studied. An important comparison principle that has been used in the literature to derive many of the results in capillarity is reviewed. It allows one to deduce the approximate shape of a capillary surface by constructing comparison surfaces with mean curvature and contact angle close to those of the (unknown) solution surface. In the context of nonparametric problems the comparison principle leads to height estimates above and below for the function u. An example from the literature where these height estimates have been used successfully is described. The promised domains for which the bounded u does not extend continuously to the boundary are constructed. The point on the boundary at which u has a jump discontinuity will be the vertext of a re-entrant corner having any interior angle theta pi. Using the comparison principle the behavior of u near this point is studied.

  7. Magnetic Field Reentrant Superconductivity in Aluminum Nanowires

    NASA Astrophysics Data System (ADS)

    Bretz-Sullivan, Terence; Goldman, Allen

    Reentrance to the superconducting state through the application of a magnetic field to quasi-one dimensional superconductors driven resistive by current, is counter to the expected properties of superconductors. It was not until recently that a microscopic mechanism explaining the phenomenon was proposed in which superconductivity and phase slip driven dissipation coexist in a non-equilibrium state. Here we present additional results of magnetic field induced reentrance into the superconducting state in quasi-one-dimensional aluminum nanowires with an in-plane magnetic field both transverse to, and along the wire axis. The reentrant behavior is seen in the magnetic field dependence of the I-V characteristic and resistance vs. temperature, and in the wire's magnetoresistance at 450mK. This work was supported by DOE Basic Energy Sciences Grant DE-FG02-02ER46004. Samples were fabricated at the Minnesota Nanofabrication Center. Parts of this work were carried out in the University of Minnesota Characterization Facility, a member of the Materials Research Facilities Network (www.mrfn.org) funded via the NSF MRSEC program.

  8. Light scattering by a reentrant fractal surface.

    PubMed

    Mendoza-Suárez, A; Méndez, E R

    1997-05-20

    Recently, rigorous numerical techniques for treating light scattering problems with one-dimensional rough surfaces have been developed. In their usual formulation, these techniques are based on the solution of two coupled integral equations and are applicable only to surfaces whose profiles can be described by single-valued functions of a coordinate in the mean plane of the surface. In this paper we extend the applicability of the integral equation method to surfaces with multivalued profiles. A procedure for finding a parametric description of a given profile is described, and the scattering equations are established within the framework of this formalism. We then present some results of light scattering from a sequence of one-dimensional flat surfaces with defects in the form of triadic Koch curves. Beyond a certain order of the prefractal, the scattering patterns become stationary (within the numerical accuracy of the method). It can then be argued that the results obtained correspond to a surface with a fractal structure. These constitute, to our knowledge, the first rigorous calculations of light scattering from a reentrant fractal surface. PMID:18253371

  9. Strong magnetic frustration and anti-site disorder causing spin-glass behavior in honeycomb Li2RhO3

    NASA Astrophysics Data System (ADS)

    Katukuri, Vamshi M.; Nishimoto, Satoshi; Rousochatzakis, Ioannis; Stoll, Hermann; van den Brink, Jeroen; Hozoi, Liviu

    2015-10-01

    With large spin-orbit coupling, the electron configuration in d-metal oxides is prone to highly anisotropic exchange interactions and exotic magnetic properties. In 5d5 iridates, given the existing variety of crystal structures, the magnetic anisotropy can be tuned from antisymmetric to symmetric Kitaev-type, with interaction strengths that outsize the isotropic terms. By many-body electronic-structure calculations we here address the nature of the magnetic exchange and the intriguing spin-glass behavior of Li2RhO3, a 4d5 honeycomb oxide. For pristine crystals without Rh-Li site inversion, we predict a dimerized ground state as in the isostructural 5d5 iridate Li2IrO3, with triplet spin dimers effectively placed on a frustrated triangular lattice. With Rh-Li anti-site disorder, we explain the observed spin-glass phase as a superposition of different, nearly degenerate symmetry-broken configurations.

  10. Identical band gaps in structurally re-entrant honeycombs.

    PubMed

    Zhu, Zhu-Wei; Deng, Zi-Chen

    2016-08-01

    Structurally re-entrant honeycomb is a sort of artificial lattice material, characterized by star-like unit cells with re-entrant topology, as well as a high connectivity that the number of folded sheets jointing at each vertex is at least six. In-plane elastic wave propagation in this highly connected honeycomb is investigated through the application of the finite element method in conjunction with the Bloch's theorem. Attention is devoted to exploring the band characteristics of two lattice configurations with different star-like unit cells, defined as structurally square re-entrant honeycomb (SSRH) and structurally hexagonal re-entrant honeycomb (SHRH), respectively. Identical band gaps involving their locations and widths, interestingly, are present in the two considered configurations, attributed to the resonance of the sketch folded sheets, the basic component elements for SSRH and SHRH. In addition, the concept of heuristic models is implemented to elucidate the underlying physics of the identical gaps. The phenomenon of the identical bandgaps is not only beneficial for people to further explore the band characteristics of lattice materials, but also provides the structurally re-entrant honeycombs as potential host structures for the design of lattice-based metamaterials of interest for elastic wave control. PMID:27586722