On the frequency dependence of the transition temperature in spin glasses

For the first time, the frequency dependence of T_f (temperature of the maximum of the a.c. susceptibility of spin-glasses) is shown to obey a Fulcher law $\text{τ = τ}{}_{\mathrm{o}}\text{exp}\text{[}\text{E}{}_{\mathrm{a}}\text{k(}\text{T}{}_{\mathrm{f}}\text{?}\text{T}{}_{\mathrm{f}}\text{)}\text{]}$. This is observed as well in the case of dilute alloys (or R.K.K.Y. spin-glasses : $\text{Cu}$Mn, $\text{Au}$Fe, …) as for frustrated systems (Eu_1?xGd_xS, Eu_xSr_1?xS …). For R.K.K.Y. spin-glasses, only in the case of a very small amplitude, V_o of the R.K.K.Y. interaction, this time dependence approaches an Arrhenius law. In the case of “frustrated” spin-glasses the concentration is the main parameter to determine the kind of frequency dependence of T_f. These properties are evidence for a glass-like phase transition in spin-glasses. The scaling of the frequency dependence of T_f with V_o is justified for R.K.K.Y. spin-glasses from present data.     

Chaotic temperature dependence in a model of spin glasses

We address the problem of chaotic temperature dependence in disordered glassy systems at equilibrium by following states of a random-energy random-entropy model in temperature; of particular interest are the crossings of the free-energies of these states. We find that this model exhibits strong, weak or no temperature chaos depending on the value of an exponent. This allows us to write a general criterion for temperature chaos in disordered systems, predicting the presence of temperature chaos in the Sherrington-Kirkpatrick and Edwards-Anderson spin glass models, albeit when the number of spins is large enough. The absence of chaos for smaller systems may justify why it is difficult to observe chaos with current simulations. We also illustrate our findings by studying temperature chaos in the naıve mean field equations for the Edwards-Anderson spin glass. Received 27 March 2002 Published online 19 July 2002     

The temperature dependence of the magnetic susceptibility of spin glasses

The temperature dependence of the magnetic susceptibility of spin glasses is calculated exactly on the basis of a magnetic cluster model, following Néel. The susceptibility of a single cluster below its blocking temperature is taken to be negligibly small compared to its value above. A distribution of blocking temperatures may be obtained from observations on spin glasses. Conditions for the sharpness of the susceptibility peak are discussed and the low-temperature variation of the susceptibility is also obtained.     

Correction to the concentration dependence of the very low temperature specific heat of RKKY spin glasses

We show that there are important corrections to the “concentration independence” of the very low temperature specific heat of RKKY spin-glasses. It is argued that this correction, if confirmed experimentally, gives important information on the mechanism causing the excess low temperature specific heat.     

Influence of the spectral distribution of relaxation times in spin glasses on the frequency dependence of the freezing temperature

Neutron scattering measurements on spin glasses show evidence for a wide spectrum of relaxation times including the dominant influence of the Korringa relaxation mechanism in some systems. It is argued that the magnitude of the latter relative to relaxation processes arising through solute-solute interactions plays a major role in the observed weak or strong frequency dependence of the temperature of the a.c. susceptibility maximum.     

Generalized Bose-Einstein phase transition in large-m component spin glasses

It is proposed to understand finite dimensional spin glasses using a 1/m expansion, where m is the number of spin components. It is shown that this approach predicts a replica symmetric state in finite dimensions. The point about which the expansion is made, the infinite-m limit, has been studied in the mean-field limit in detail and has a very unusual phase transition, rather similar to a Bose-Einstein phase transition but with N(2/5) macroscopically occupied low-lying states.     

Quantum phase transition in spin glasses with multi-spin interactions

We examine the phase diagram of the p-interaction spin glass model in a transverse field. We consider a spherical version of the model and compare with results obtained in the Ising case. The analysis of the spherical model, with and without quantization, reveals a phase diagram very similar to that obtained in the Ising case. In particular, using the static approximation, reentrance is observed at low temperatures in both the quantum spherical and Ising models. This is an artifact of the approximation and disappears when the imaginary time dependence of the order parameter is taken into account. The resulting phase diagram is checked by accurate numerical investigation of the phase boundaries.     

Single spin and chiral glass transition in vector spin glasses in three dimensions

Results of Monte Carlo simulations of XY and Heisenberg spin glass models in three dimensions are presented. A finite-size scaling analysis of the correlation length of the spins and chiralities of both models shows that there is a single, finite-temperature transition at which both spins and chiralities order.     

Magnetic field dependence of the spin—Peierls transition

We consider the effect of a magnetic field on the spin—Peierls transition within mean-field theory. Using a mathematical analogy to the regular Peierls transition and existing results, we derive formulas for the magnetic field dependence of the transition temperature and of the distortion periodicity.     

Concentration dependence of impurity absorption in chalcogenide glasses of the Ge-Sb-S system

We have investigated the IR spectra of chalcogenide glasses of the triple Ge-Sb-S system in Sb₂S₃-GeS₂ and Sb₂S₃-Ge₂S₃ sections. We have analyzed the specific features of the appearance of intense vibrational bands of absorption of various structural fragments depending on the chemical composition of glasses. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 4, pp. 499–502, July–August, 2000.     

The magnetic phase transition in amorphous ferromagnets and in spin glasses

The magnetic phase transition in materials with exchange disorder (amorphous ferromagnets, spin glasses) is discussed. In the critical temperature range the behavior of amorphous ferromagnetic transition metal-metalloid glasses is found to be similar to the one derived for a three-dimensional homogeneous Heisenberg ferromagnet. The most prominent difference between disordered and homogeneous materials is manifested in a large temperature range of deviations from the mean field behavior beyond the critical region, as observed experimentally for the temperature dependence of the linear susceptibility of amorphous ferromagnets and of the nonlinear susceptibility of spin glasses. A molecular field theory with correlations in space and time is developed, which relates the deviations from the mean field behavior to the interplay between the temperature dependent thermal correlations in the spin system and the spatial fluctuations of the material. Application to dynamical processes (kinetic critical slowing down) is discussed.     

Connection between the low temperature anomaly in glasses and the glass transition temperature

The low temperature specific heat anomaly in water-doped K-Ca nitrate glass has been found to scale with the reciprocal glass transition temperature. Similarly, the anomaly in Na₂O-containing and in neutron-irradiated silica scales with the reciprocal fictive temperature. These observations lead to the suggestion that the frozen-in disorder is connected with the low temperature anomaly characteristic of the glassy state.     

Low temperature linear dynamical susceptibility of dipolar spin glasses

The dissipative part of the linear magnetic dynamic susceptibility of dipolar spin glasses is considered. Due to the transition of the system (at enough high concentration of the magnetic dipoles) from a paramagnetic phase to a magnetic dipolar one, an anomalous temperature dependence of the dissipative part of the magnetic susceptibility is found. Some considerations related to the experimental results for LiHo_xY _1?xF₄ are made.     

Why temperature chaos in spin glasses is hard to observe

The overlap length of a three-dimensional Ising spin glass on a cubic lattice with Gaussian interactions has been estimated numerically by transfer matrix methods and within a Migdal-Kadanoff renormalization group scheme. We find that the overlap length is large, explaining why it has been difficult to observe spin glass chaos in numerical simulations and experiment.     

Sudden transition from finite temperature spin environments

We investigate the phenomenon of sudden transition from finite temperature critical environments in the study of quantum correlations of a two-qubit system coupled to independent thermal Ising baths. The influence of the temperature and external field of bath on the critical time of sudden transition is also explored. It is found that the phenomenon of sudden transition can be used to detect the critical points of thermal spin environments. How to protect quantum correlations of the system is also examined by applying a series of π-phase pulses.     

Average coordination-number dependence of glass-transition temperature in Ge-In-Se chalcogenide glasses

Twenty-six glassy compositions, belonging to the Ge_xIn₆Se_94-x and Ge_xIn₁₂Se_88-x families of the Ge-In-Se system, were prepared from high purity constiment elements. Differential Scanning Calorimetry (DSC) measurements were carried out on these glasses. The DSC thermograms were used to determine the glass transition temperatures (T _g) for the compositions examined. TheT _g-composition dependence of the Ge_xIn₆Se_94-x family shows a minimum inT _g at a value of the average coordination number, r, equal to 2.4. For both families of the glasses examined, theT _g-r dependence displays maxima at r=2.75 and r=2.83 for Ge_xIn₆Se_94-x and Ge_xIn₁₂Se_88-x families, respectively. These results are interpreted by using the Phillips model of rigidity percolation and Chemically Ordered Covalent Network (COCN) model.     

Universal temperature dependence of the magnetization of gapped spin chains

A Haldane chain under applied field is analyzed numerically, and a clear minimum of magnetization is observed as a function of temperature. We elucidate its origin using the effective theory near the critical field and propose a simple method to estimate the gap from the magnetization at finite temperatures. We also demonstrate that there exists a relation between the temperature dependence of the magnetization and the field dependence of the spin-wave velocity. Our arguments are universal for general axially symmetric one-dimensional spin systems.     

An explanation of the anomalous temperature dependence of optical linewidths in glasses

Recently photochemical holeburning experiments have been carried out in organic glasses down to 0.4 K. Here we give a theoretical explanation of theT ^1.3-dependence of the holewidth. It is based on the strain field coupling between impurity molecules and two-level tunneling states leading to a kind of spectral diffusion of the optical excitation energy.