Microscopic theory of a strongly correlated two-dimensional electron gas

The rearrangement of the Fermi surface in a diluted two-dimensional electron gas beyond the topological quantum critical point has been examined within an approach based on the Landau theory of Fermi liquid and a nonperturbative functional method. The possibility of a transition of the first order in the coupling constant at zero temperature between the states with a three-sheet Fermi surface and a transition of the first order in temperature between these states at a fixed coupling constant has been shown. It has also been shown that a topological crossover, which is associated with the joining of two sheets of the Fermi surface and is characterized by the maxima of the density of states N(T) and ratio C(T)/T of the specific heat to the temperature, occurs at a very low temperature T _⋄ determined by the structure of a state with the three-sheet Fermi surface. A momentum region where the distribution n(p, T) depends slightly on the temperature, which is manifested in the maximum of the specific heat C(T) near T _*, appears through a crossover at temperatures T ∼ T _* > T _⋄. It has been shown that the flattening of the single-particle spectrum of the strongly correlated two-dimensional electron gas results in the crossover from the Fermi liquid behavior to a non-Fermi liquid one with the density of states N(T) ∝ T ^−α with the exponent α }~ 2/3.     

Investigation of thermal expansion,phase diagrams,and barocaloric effect in the (NH4)2WO2F4 and (NH4)2MoO2F4 oxyfluorides

The thermal expansion along the principal crystallographic axes of the (NH₄)₂WO₂F₄ and (NH₄)₂MoO₂F₄ oxyfluorides has been studied. The anomalous behavior of α _i (T) due to the phase transitions has been revealed at T ₁ = 271.4 K and T ₂ ≈ 180 K for the molybdate and at T ₁ = 201.5 K and T ₂ ≈ 161 K for the tungstate. The quantities dT/dp and dT/dσ _i , which characterize the dependence of the phase transition temperatures on the hydrostatic and uniaxial pressures, have been determined from analyzing the results of studies of the thermal expansion and heat capacity with the use of the Pippard relations. The p-T and α _i -T phase diagrams reflect different characters of the influence of the pressure on the stability of the initial and distorted phases of the oxyfluorides. The magnitudes of the extensive and intensive barocaloric effects determined in the vicinity of the structural phase transitions are as follows: ΔS _BCE varies from approximately −10 to −17 J/mol K and ΔT _AD ≈ 8−17 K for the molybdate and ΔS _BCE varies from approximately −10 to −17 J/mol K and ΔT _AD ≈ 8−13 K for the tungstate.     

Lattice thermal expansion of cesium plumbo chloride

The temperature variation of the lattice parameter of CsPbCl₃ in the cubic phase has been studied by x-ray method, from a determination of the precision lattice parameter at various temperatures, ranging from 50°C to 400°C. The coefficient of thermal expansion of CsPbCl₃ can be expressed by the quadratic equation,α _T = 21.6 × 10⁻⁶ + 2.44 × 10⁻⁹ T + 5.90 × 10⁻¹¹ T ².     

Quantum thermal discord in a two-spin-1/2 XXZ model

We study the quantum discord in a two-spin-1/2 XXZ model in thermal equilibrium at temperature T in the presence of an external magnetic field B. Special attention is paid to the dependence of quantum discord on the temperature T and magnetic field B. It is found that quantum thermal discord is more robust than concurrence against temperature, in the sense that quantum thermal discord does not vanish at finite temperatures, but concurrence vanishes completely at a critical temperature.     

Spin state and exchange in the quasi-one-dimensional antiferromagnet KFeS2

We report the optical spectra and single crystal magnetic susceptibility of the one-dimensional antiferromagnet KFeS₂. Measurements have been carried out to ascertain the spin state of Fe³⁺ and the nature of the magnetic interactions in this compound. The optical spectra and magnetic susceptibility could be consistently interpreted using aS=1/2 spin ground state for the Fe³⁺ ion. The features in the optical spectra have been assigned to transitions within thed-electron manifold of the Fe³⁺ ion, and analysed in the strong field limit of the ligand field theory. The high temperature isotropic magnetic susceptibility is typical of a low-dimensional system and exhibits a broad maximum at ∼565K. The susceptibility shows a well defined transition to a three dimensionally ordered antiferromagnetic state atT _N=250 K. The intra and interchain exchange constants,J andJ′, have been evaluated from the experimental susceptibilities using the relationship between these quantities, andχ _max,T _max, andT _N for a spin 1/2 one-dimensional chain. The values areJ=−440.71 K, andJ′=53.94 K. Using these values ofJ andJ′, the susceptibility of a spin 1/2 Heisenberg chain was calculated. A non-interacting spin wave model was used belowT _N. The susceptibility in the paramagnetic region was calculated from the theoretical curves for an infiniteS=1/2 chain. The calculated susceptibility compares well with the experimental data of KFeS₂. Further support for a one-dimensional spin 1/2 model comes from the fact that the calculated perpendicular susceptibility at 0K (2.75×10⁻⁴ emu/mol) evaluated considering the zero point reduction in magnetization from spin wave theory is close to the projected value (2.7×10⁻⁴ emu/mol) obtained from the experimental data.     

Flow equations for the spin-boson problem

Using continuous unitary transformations recently introduced by Wegner [1], we obtain flow equations for the parameters of the spin-boson Hamiltonian. Interactions not contained in the original Hamiltonian are generated by this unitary transformation. Within an approximation that neglects additional interactions quadratic in the bath operators, we can close the flow equations. Applying this formalism to the case of Ohmic dissipation at zero temperature, we calculate the renormalized tunneling frequency. We find a transition from an untrapped to trapped state at the critical coupling constant α _c =1. We also obtain the static susceptibility via the equilibrium spin correlation function. Our results are both consistent with results known from the Kondo problem and those obtained from mode-coupling theories. Using this formalism at finite temperature, we find a transition from coherent to incoherent tunneling atT ₂ ^* ≈2T ₁ ^* , whereT ₁ ^* is the crossover temperature of the dynamics known from the NIBA.     

One-norm geometric quantum discord and critical point estimation in the XY spin chain

In contrast with entanglement and quantum discord (QD), we investigate the thermal quantum correlation in terms of Schatten one-norm geometric quantum discord (GQD) in the XY spin chain, and analyze their capabilities in detecting the critical point of quantum phase transition. We show that the one-norm GQD can reveal more properties about quantum correlation between two spins, especially for the long-range quantum correlation at finite temperature. Under the influences of site distance, anisotropy and temperature, one-norm GQD and its first derivative make it possible to detect the critical point efficiently for a general XY spin chain.     

Quantum correlations in a long range interaction spin chain

We propose a new type of long range interaction spin chain. The quantum correlations such as quantum discord, entanglement, and structure factor are investigated in the thermal state with considering them both in zero temperature and finite temperature. Based on our results, we compare the differences and show the relations between the three types of quantum correlations in this long range interaction model.     

Anomalous light-induced drift of linear molecules

The sudden approximation in energy is used to derive analytic formulas that describe the anomalous light-induced drift (LID) of linear molecules absorbing radiation in the rovibrational transition nJ _i −mJ _f (n and m are the ground and excited vibrational states, and J _α is the rotational quantum number in the vibrational state α=m, n). It is shown that for all linear molecules with moderate values B≲1 cm⁻¹ of the rotational constant, anomalous LID can always by observed under the proper experimental conditions; temperature T, rotational quantum number J _i , and type of transition (P or R). The parameter γ=B[J _i (J _i +1)−J _f (J _f +1)] ν _n /2k _BT (ν _m −ν _n ) is used to derive a condition for observing anomalous LID: γ∼1 (k _B is the Boltzmann constant and ν _α is the transport rate of collisions of molecules in the vibrational state α and buffer particles at moderate molecular velocities , where is the most probable velocity of the buffer particles). For ν _m >ν _n anomalous LID can be observed only in P-transitions, while for ν _m <ν _n it can be observed only in R-transitions. It is shown that anomalous LID is possible for all ratios β=M _b /M of the masses of the buffer particles (M _b ) and of the resonant particles (M) and any absorption-line broadening (Doppler or homogeneous). The optimum conditions for observing anomalous LID are realized when the absorption line is Doppler-broadened in an atmosphere of medium-weight (β∼1) and heavy (β≫1) buffer particles. In this case, anomalous LID can be observed in the same transition within a broad temperature interval ΔT∼T. If the buffer particles are light (β≪1) or if the broadening of the absorption line is homogeneous, anomalous LID in the same transition can be observed only within a narrow temperature range ΔT≪T. Zh. éksp. Teor. Fiz. 115, 1664–1679 (May 1999)     

A study of Peierls instabilities for a two-dimensional t-t' model

In this paper we study Peierls instabilities for a half-filled two-dimensional tight-binding model with nearest-neighbour hopping t and next nearest-neighbour hopping t' at zero and finite temperatures. Two dimerization patterns corresponding to the same phonon vector (π,π) are considered to be realizations of Peierls states. The effect of imperfect nesting introduced by t' on the Peierls instability, the properties of the dimerized ground state, as well as the competition between two dimerized states for each t' and temperature T, are investigated. It is found: (i). The Peierls instability will be frustrated by t' for each of the dimerized states. The Peierls transition itself, as well as its suppression by t', may be of second- or first-order. (ii). When the two dimerized states are considered jointly, one of them will dominate the other depending on parameters t' and T. Two successive Peierls transitions, that is, the system passing from the uniform state to one dimerized state and then to the other may take place with decrease of temperature. Implications of our results to real materials are discussed. Received 31 July 2001     

Abrupt increase in thermal diffusion belowT c in YBa2Cu3O7−δ

We report the variation of the thermal diffusivity in the high temperature superconductor YBa₂Cu₃O_7−δ below the transition temperature measured using photo-acoustic technique α is found to increase sharply belowT _c. Qualitative explanations are given.     

Order-disorder phase transition in a cliquey social network

We investigate the network model of community by Watts, Dodds and Newman (D.J. Watts et al., Science 296, 1302 (2002)) as a hierarchy of groups, each of 5 individuals. A homophily parameter α controls the probability proportional to exp (-αx) of selection of neighbours against distance x. The network nodes are endowed with spin-like variables s_i = ± 1, with Ising interaction J > 0. The Glauber dynamics is used to investigate the order-disorder transition. The transition temperature T_c is close to 3.8 for α < 0.0 and it falls down to zero above this value. The result provides a mathematical illustration of the social ability to a collective action via weak ties, as discussed by Granovetter in 1973.     

Weak exchange interactions in the heteronuclear crystal CuPr2(CCl3COO)8·6H2O

The new heteronuclear crystal CuPr₂(CCl₃COO)₈·6H₂O, constructed of chains containing copper and praseodymium atoms, has been synthesized and investigated by EPR at 9.3 GHz at temperatures ranging from room temperature down to 10 K. At temperatures T∼300–130 K, EPR spectra are observed which are characteristic of isolated polyhedra of copper ions with g _z=2.330±0.005, g _x,y =2.053±0.005, A _z=139×10⁻⁴ cm⁻¹, and A _x,y <26×10⁻⁴ cm⁻¹. At temperatures T<130 K a complex spectrum is observed, associated with the appearance of weak exchange interactions between the copper ions in the chain (J _Cu-CuΣS _i·S _i+1), comparable in magnitude with the hyperfine interactions J _Cu-Cu=0.015 cm⁻¹ at T=10 K. The magnitude of the exchange interaction decreases smoothly as the temperature is raised. It is conjectured that orbitals of the praseodymium ions participate in the process of indirect exchange between the copper ions. Fiz. Tverd. Tela (St. Petersburg) 41, 2154–2157 (December 1999)     

Effect of the “negative chemical” pressure on the temperatures of phase transitions in the TlInS<Subscript>2</Subscript> layered crystal

The temperature dependences of the permittivity of TlInS_{2(1 −x)}Se_2x solid solutions have been experimentally investigated in the temperature range including the points of structural phase transitions in the solid solutions. It has been established that the isovalent substitution of selenium for sulfur in the anion sublattice of the TlInS_{2(1 − x)}Se_2x solid solutions shifts the phase transition temperatures T _i and T _c toward the low-temperature range with a simultaneous decrease in the temperature interval of the existence of the incommensurate phase. The T-x phase diagram is constructed for the solid solutions under study and the coordinates are determined for a critical point (of the Lifshitz type) at which the lines T _i (x) and T _c (x) converge in the phase diagram. The pattern of the T-x phase diagram for the TlInS_{2(1 − x)}Se_2x solid solutions has been analyzed within the phenomenological model of a virtual crystal.     

Estimation of Kauzmann temperature and isenthalpic temperature of metallic glasses

We propose expressions for the estimation of the isenthalpic temperature T ₀ (T ₀ = αT _m , α is a semi-empirical parameter and 0 ⩽ α < 1, T _m is the solidus temperature) and the Kauzmann temperature T _k (T _k = T _m exp(α−1)) for glass forming alloys. It is found that T _k estimated by T _k = T _m exp(α−1) is in agreement with that directly calculated from the heat capacity data, indicating that T _k = T _m exp(α − 1) can be used to estimate T _k of glass forming alloys. T ₀ estimated by T ₀ = αT _m , on the other hand, widely deviates from that of directly calculated from the heat capacity data. This suggests that the enthalpy difference of the under-cooled liquid and the crystal might be a nonlinear function of the temperature below T _k . Moreover, the Gibbs free energy difference ΔG is not sensitive to the deviation of α.     

Universal behavior of CePd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Rh<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> ferromagnet at the quantum critical point

The heavy-fermion metal CePd_1−x Rh _x can be tuned from ferromagnetism at x = 0 to the nonmagnetic state at some critical concentration x _c . The non-Fermi liquid behavior (NFL) at x ≃ x _c is recognized by the power-law dependence of the specific heat C(T) given by the electronic contribution susceptibility X(T) and volume expansion coefficient α(T) at low temperatures: C/T ∝ X(T) ∝ α(T)/T∝ 1/ √T. We also demonstrate that the behavior of the normalized effective mass M _N ^* observed in CePd_1−x Rh _x at x ≃ 0.8 agrees with that of M _N ^* observed in paramagnetic CeRu₂Si₂ and conclude that these alloys exhibit the universal NFL thermodynamic behavior at their quantum critical points. We show that the NFL behavior of CePd_1−x Rh _x can be accounted for within the frameworks of the quasiparticle picture and fermion condensation quantum phase transition, while this alloy exhibits a universal thermodynamic NFL behavior that is independent of the characteristic features of the given alloy such as its lattice structure, magnetic ground state, dimension, etc. The text was submitted by the authors in English.     

Magnetism,exchange and crystal field parameters in the orbitally unquenched Ising antiferromagnet FePS3

FePS₃ is a layered antiferromagnet (T _N=123 K) with a marked Ising anisotropy in magnetic properties. The anisotropy arises from the combined effect of the trigonal distortion from octahedral symmetry and spin-orbit coupling on the orbitally degenerate⁵ T _2g ground state of the Fe²⁺ ion. The anisotropic paramagnetic susceptibilities are interpreted in terms of the zero field Hamiltonian, ℋ=Σ_i [δ(L _iz ² −2)+|λ|L _i .S _i ]−Σ _ij J _ij S _i .S _j . The crystal field trigonal distortion parameter Δ, the spin-orbit coupling λ and the isotropic Heisenberg exchange,J _ij, were evaluated from an analysis of the high temperature paramagnetic susceptibility data using the Correlated Effective Field (CEF) theory for many-body magnetism developed by Lines. Good agreement with experiment were obtained for Δ/k=215.5 K; λ/k=166.5 K;J _nn k=27.7 K; andJ _nnn k=−2.3 K. Using these values of the crystal field and exchange parameters the CEF predicts aT _N=122 K for FePS₃, which is remarkably close to the observed value of theT _N. The accuracy of the CEF approximation was also ascertained by comparing the calculated susceptibilities in the CEF with the experimental susceptibility for the isotropic Heisenberg layered antiferromagnet MnPS₃, for which the high temperature series expansion susceptibility is available.     

The μSR study of the relaxation of Ho and Er magnetic moments in high-Tc 1-2-3 compounds

High temperature superconductors HoBa₂Cu₃O_7−δ (T _c ≅93 K), Ho_0.5Y_0.5Ba₂Cu₃O_7−δ (T _c ≅93 K) and ErBa₂Cu₃O_7−δ (T _c ≅95 K) were investigated by the zero-field μSR-technique. The muon spin depolarisation rate connected with the fluctuation frequency of rare-earth ion magnetic moments was measured. It was found that the samples with holmium show a fast increase of the muon spin depolarisation rate at temperatures below 20 K, while in ErBa₂Cu₃O_7−δ the depolarisation rate remains low in the whole temperature region studied (4.2 K-270 K). The sharp difference between the behaviours of the muon spin depolarisation rate may be explained by the difference between the ground state of Ho³⁺ and Er³⁺ ions in the crystalline field of the lattice.