Premonitory effects near critical transition temperature in ordering systems – a Monte Carlo simulation

Premonitory effects manifest themselves in an ordering transition of the first kind (order) in the form of anomalously high short-range order (SRO) intensity at temperatures marginally above T _c, the critical transition temperature. This intensity located at the superlattice positions of the long-range ordered (LRO) phase is often attributed to the formation of ‘heterophase fluctuations’ resembling clusters of the LRO phase. Monte Carlo simulations in a hypothetical system showing FCC-to-L1₂ ordering transition have been carried out here to shed some light on this phenomenon and to look into the atomic configurations that make up these fluctuations.     

Computational investigations of inhomogeneous elastic effects near phase transitions

Abstract

We present different approaches to computer simulations of the order-disorder phase transition in thiocyanate crystals. In these systems the order parameter is coupled to the strain by coupling of the type ?²ε. This coupling plays the dominant role in the behavior of the system below T _c. Molecular Dynamics and Monte Carlo techniques give the proper temperature dependence of the order parameter and the behavior of the fluctuations similar to that observed experimentally. The ab initio calculations allow us to render properly the structural properties of crystals like KSCN and additionally can become the basis for more realistic phenomenological models.     

Electric-field influence on the neutron diffuse scattering near the ferroelectric transition of Sr0.61Ba0.39Nb2O6

ABSTRACT

Uniaxial relaxor ferroelectric Sr_0.61Ba_0.39Nb₂O₆ single crystal has been investigated in the vicinity of its phase transition using neutron scattering and dielectric spectroscopy. A global-type thermal hysteresis is evidenced by both techniques in the ferroelectric phase and up to about 15 K above T_c. In addition, a part of the transverse neutron diffuse scattering in the 001 Brillouin zone, presumably related to static nanodomain structure, can be suppressed by prior poling the crystal in electric field of 3 kV/cm. The remaining part of the transverse neutron diffuse scattering and the real part of permittivity show a similar temperature dependence. The temperature position of the maximal scattering intensity T_max depends significantly on the scattering wave vector. T_max shifts monotonically to higher temperature with the increasing wave vector in all investigated cooling and heating regimes. It is concluded that the critical fluctuations have space correlations which depend on frequency and wave vector.     

Optical study of the antiferromagnetic-paramagnetic phase transition in chromium oxide Cr2O3

Abstract

Optical effects of the first and second order with respect to the order parameter (1 is antiferromagnetic vector) have been studied in Cr₂O₃ around its phase transition at T_N = 306 K from the antiferromagnetic to paramagnetic state. The magnetic linear birefringence is characterized by a rather large magnitude Δn _sp ? 10^?3 and by a large contribution of fluctuations of the order parameter to the birefringence. The study of the nonreciprocal optical rotation induced by an electric field has shown that the phase transition has a well defined first-order character. This result is also supported by the observation of a nonlinear (quadratic in the electric field) nonreciprocal rotation in a narrow temperature region ΔT = 0.15 K around T_N . The temperature variation of the order parameter l(T) below T_N is well described by a power low l where τ^β = (T_N - T)/T_N and β = 0.355. We also observed a very reproducible effect of the rotation of the optical indicatrix in opposite directions for two types of antiferromagnetic domains. The possible explanation of this effect could be related to the so-called gyrotropic birefringence, an effect related to k_il_i terms in the dielectric permeability.     

The vortex-like excitations detected by Nernst signals in high-Tc superconductors

The nature of the pseudogap state and its relation to the d-wave superconductivity in high-T _c superconductors is still an open issue. The vortex-like excitations detected by the Nernst effect measurements exist in a certain temperature range above superconducting transition temperature T _c, which strongly support that the pseudogap phase is characterized by finite pairing amplitude with strong phase fluctuations and imply that the phase transition at T _c is driven by the loss of long-range phase coherence. We first briefly introduce the electronic phase diagram and pseudogap state of high-T _c superconductors, and then review the results of Nernst effect for different high-T _c superconductors. Related theoretical models are also discussed.     

On the second-order phase transition initiated by a strong electron-phonon interaction

A model of a crystal with a strong electron-phonon interaction that initiates a second-order phase transition has been considered. The purpose of the study is to determine the temperature dependence of the thermodynamic potential of the symmetric phase in the temperature range in the immediate vicinity of the transition temperature T _C for this model. The problem has been solved using the quantum Matsubara Green’s function approach, which takes into account the influence of both thermal and quantum fluctuations. It has been demonstrated that fluctuation coherent deformations of the crystal lattice with the same symmetry as in the ordered phase appear to be energetically favorable at T > T _C due to the interaction with the electronic subsystem. The results obtained have made it possible to construct the model of the second-order phase transition near the Curie point T _C.     

Muon spin relaxation measurements in kagomé lattice system SrCr8Ga4O19

Dynamical spin fluctuations in SrCr_8–xGa_4+xO₁₉ a frustrated spin system on a kagomé lattice, is examined by the longitudinal field muon spin relaxation technique. This system shows a spin-glass (SG)-like cusp in the susceptibility atT _g=3.5(2) K. The slowing down of Cr spin fluctuations is found to occur over a very wide temperature rangeT _g<T<30T _g. AsT/T _g 0 these fluctuations remain without static polarization (order parameter). Such strong fluctuations belowT _g have not been observed before in a conventional SG system.     

Fermi-liquid instability at magnetic–nonmagnetic quantum phase transitions

In metals with strong electronic correlations such as heavy-fermion systems or itinerant-electron magnets it is possible to change from a magnetically ordered to a nonmagnetic groundstate by variation of an external parameter such as composition or pressure. In principle a transition between these groundstates can occur at zero temperature. In case of a continuous transition quantum fluctuations take the role of thermal fluctuations in finite-temperature transitions. The abundance of low-lying magnetic excitations leads in the vicinity of the quantum critical point to unusual behavior of thermodynamic and transport properties at low temperatures T not envisioned by the classical Fermi-liquid behavior that is observed even in strongly correlated electron systems away from the quantum phase transition. We discuss in detail a few examples of this ‘non-Fermi-liquid behavior', viz., CeCu_6−xAu_x, Ce_1−xLa_xRu₂Si₂, Ce₇Ni₃, CeCu₂Si₂ and CeCu₂Ge₂, CePd₂Si₂, and UCu_1−xPd_x. In CeCu_6−xAu_x the very unusual low-T behavior of the linear specific-heat coefficient C/T−ln(T/T₀) and of the resistivity ΔρT can be attributed to quasi-two-dimensional fluctuations as determined from inelastic neutron scattering. The systems CeCu₂Ge₂ and CePd₂Si₂ are particuarly interesting since here the magnetic order which is suppressed under hydrostatic pressure gives way to superconductivity, suggesting that spin fluctuations mediate the formation of Cooper pairs at least in the latter system.     

Behavior of sound velocities in the compounds La1−x SrxMnO3 near magnetic and structural phase transitions

The temperature dependences of the transverse V _t and longitudinal V _l sound velocities in single crystals of the perovskites La_1−x Sr_xMnO₃ (x=0.1, 0.175, 0.2, 0.25) in the temperature interval T=70–350 K are investigated by the resonance method. Anomalies — small minima and kinks in the temperature dependences V _l (T) and V _t (T) — are observed at the Curie points. A strong jumplike increase (by up to 30%) in both the longitudinal and transverse sound velocities, which attests to sizable hardening of the acoustic branches of the phonon spectrum, is observed near the temperatures of the structural transitions between the rhombohedral and orthorhombic phases (for x=0.175, 0.2, 0.25) and at the phase transition to the polaron-ordered state (for x=0.1). Pis’ma Zh. éksp. Teor. Fiz. 68, No. 2, 141–146 (25 July 1998)     

Second order elastic anomalies in barium titanate from cubic to tetragonal phase transition

The anomalies of the second order elastic constants have been derived for barium titanate for the phase transition from cubic to tetragonal. The equilibrium values of the components of the order parameter and the strain variables have been obtained from the stability conditions. The fluctuations in the order parameter have been derived from the Landau-Khalatnikov equations. Expression for the shift in the zero point energy in the tetragonal phase is obtained and is shown to be proportional to (T −T _c)². The anomalies for all the second order elastic constants have been derived and relations among them reported. It is shown that the second order elastic anomalies suffer a discontinuity at the transition temperature.     

Critical disorder effects in Josephson-coupled quasi-one-dimensional superconductors

Effects of non-magnetic randomness on the critical temperature T _c and diamagnetism are studied in a class of quasi-one dimensional superconductors. The energy of Josephson-coupling between wires is considered to be random, which is typical for dirty organic superconductors. We show that this randomness destroys phase coherence between the wires and T _c vanishes discontinuously when the randomness reaches a critical value. The parallel and transverse components of the penetration depth are found to diverge at different critical temperatures T _c ⁽¹⁾ and T _c , which correspond to pair-breaking and phase-coherence breaking. The interplay between disorder and quantum phase fluctuations results in quantum critical behavior at T = 0, manifesting itself as a superconducting-normal metal phase transition of first-order at a critical disorder strength.     

Quenchedn-vectorp-spin model

A disorderedn-vector model withp spin interactions previously introduced is studied for the quenched case by means of the replica method and a generalized Parisi theory. We present formal solutions for generaln andp and then study the casep . The high-temperature solution is stable at all temperatures and there is only one phase transition at a temperatureT _g. Only longitudinal lowtemperature solutions are possible. There is one spin-glass solution, and it is stable for allT _g. The phase transition atT _g is of first order and displays a jump discontinuity in the order parametersq _j ^(L) andd. The spin-glass free energy is temperature dependent forn > 1 while it is constant whenn = 1.     

Coupling of orientational and translational modes in solid C60 and C70

The orientational phase transitions in solid C₆₀ and C₇₀ are accompanied by quite different anomalies in the crystalline strains. In solid C₆₀ the phase transition Fm3m→Pa3 is primarily an orientational effect (antiferro-rotational), which is driven by the condensation of orientational modes belonging to X₅ ⁺ irreducible representation (irreps) of Fm3m. These modes are the primary order parameters (oops) and their number is equal to the number of irreps of T_2g and T_1g symmetry within the manifolds under consideration. Taking into account irreps up to the manifold 1=12, we have studied the rotation-rotation-translation (RRT) coupling between the oops and the lattice displacements. We have investigated the resulting lattice contraction and the change of the elastic constant c₁₁ at the phase transition. In solid C₇₀ (fcc-phase) we investigate the bilinear coupling of orientational fluctuations of T_2g symmetry to transverse acoustic lattice displacements. This coupling is the driving mechanism for the ferroelastic phase transition Fm3m → R3m. Finally we investigate the transition from the rhombohedral phase to a low temperature monoclinic phase. This transition in antiferro-rotational.     

Effect of high electric fields on the nematic to isotropic transition in a material exhibiting large negative dielectric anisotropy

We report the experimental high electric field phase diagram of a nematic liquid crystal which exhibits a large negative dielectric anisotropy. We measure simultaneously the birefringence (Δn) and the dielectric constant (epsilon_⊥) at various applied fields as functions of the local temperature of an aligned sample. We also measure the higher harmonics of the electrical response of the medium. The following experimental results are noted: (i) enhancement of orientational order parameter S in the nematic phase due to both the Kerr effect and quenching of director fluctuations; (ii) enhancement in the paranematic to nematic transition temperature (T_PN) with field; (iii) divergence of the order parameter susceptibility beyond the tricritical point as measured by third harmonic electrical signal; (iv) a small second harmonic electrical signal which also diverges near T_PN, indicating the presence of polarised domains. Our measurements show that ΔT_PN(= T_PN(E)-T_NI(0)) varies linearly with |E| whereas the Landau de Gennes theory predicts a dependence on E². It is argued that the quenching of director fluctuations by the field makes the dominant contribution to all the observations, including the thermodynamics of the transition.     

The bicritical phase diagram of two-dimensional antiferromagnets with and without random fields

Neutron scattering measurements have been made of the phase diagrams of the nearly two-demensional antiferromagnets Rb₂MnF₄ and Rb₂Mn_0.7Mg_0.3F₄ in a magnetic field applied along thec-axis. In Rb₂MnF₄ there is at low temperatures a spin-flop phase at fields above 5.5 T which has long range order. The observation of true long range order rather than the algebraic decay of the order characteristic of the two-dimensional XY model is presumably due to subtle anisotropy effects in the plane as well as weak three-dimensional coupling. The phase boundaries of the uniaxial and transverse phases are shown to be consistent with renormalization group predictions for two-dimensional systems. The two lines become exponentially close to each other at low temperatures. The weak three-dimensional coupling moves the bicritical point fromT=0 to a non-zero temperature. The situation is more complex in Rb₂Mn_0.7Mg_0.3F₄ because of Ising random field effects. At low fields we observe typical random field metastable behavior with a sharp metastability boundary and a gange of length scales which are time independent below that boundary. At higher fields there are substantial uniaxial fluctuations. The transverse phase boundary and the metastability line appear to intercept atT=0 showing that the random field fluctuations do have a large effect on the phase diagram. The theory of the phase diagrams has been extended to include the random field fluctuations and good agreement is obtained with the observed transverse phase boundary. Unfortunately, there is as yet no theory of the metastable uniaxial phase with which to compare our results.     

Effects of electron localization in V2 − y O3

The effect of nonstoichiometry on the metal-insulator phase transition in V₂O₃ is studied. It is established that an increase in the vanadium deficiency in V2 ? yO₃ brings about a shift in the phase transition temperature toward lower temperatures and an increase in the width of the temperature hysteresis loop of the electrical conductivity. As the vanadium deficiency increases to a level corresponding to the composition ～V_1.974O₃, the phase transition completely disappears and the sample remains metallic down to T = 1.6 K. The magnetoresistance is measured for samples of this composition in longitudinal and transverse magnetic fields at T = 4.2 K.     

Dynamics of cooperative Jahn-TellerT-systems

We consider a cubic crystal with a triply degenerate electronicT-state in each unit cell which is slightly split by an additional trigonal crystal field. A linear Jahn-Teller interaction ofT-e nature is assumed, involving predominantly acoustic phonons and the macroscopic strain. Below a critical temperatureT _c a tetragonal distortion is established. The phase transition is accompanied by a strong temperature dependence of the elastic constants. The latter are calculated under various thermodynamic conditions. Both static and dynamic approaches are discussed and related to each other.Supported by a grant from the Deutsche Forschungs-gemeinschaft     

Magnetostriction of the spin-Peierls compound CuGeO3

The longitudinal and transverse magnetostriction λ of the spin-Peierls compound CuGeO₃ in the b-c plane is measured in magnetic fields up to 20 T both above and below the transition temperature T _sp=14.3K. It is found that for a given crystallographic direction the value of magnetostriction is weakly dependent on the magnetic field direction. In the uniform U phase at T⩾T _sp, λ is negative and approximately equal in the b and c directions, while in the dimerized D phase at T<T _sp, λ is positive and λ _b >λ _c . At low temperatures, λ increases sharply at the magnetic-field-induced transition from the dimerized to the magnetic M phase. The experimental data allow estimation of the stress derivatives of the antiferromagnetic intrachain exchange interaction parameter and of the stress dependence of the critical field of the D-Mphase transition. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 156–159 (10 August 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Thermal physical properties of the La0.825Sr0.175MnO3 single crystals

The heat capacity (C _P), the thermal diffusion (η), the thermal conductivity (κ), and the electrical resistance of the La_0.825Sr_0.175MnO₃ single crystal have been measured in the temperature range 80–350 K in magnetic fields to 40 kOe. Dependences C _P(T), κ(T), and η(T) have anomalies near T _C, which are suppressed in magnetic field. The minima in dependences κ(T) and η(T) near T _C are explained by the phonon scattering on fluctuations of the magnetic order parameter. Dependences κ(T) and η(T) have anomalies near T _S = 200 K related to the structural transition from the rhombohedral (R) to the orthorhombic (O*) phase.

     

X-ray study of microcrystalline Hg<Subscript>2</Subscript><Emphasis Type="Italic">Hal</Emphasis><Subscript>2</Subscript> ferroelastics

The rhombic splitting of basal plane reflections and the thermal behavior of fundamental and diffuse reflections from X-points of the Brillouin zone characterizing the behavior of an order parameter and its fluctuations, respectively, were studied in polycrystalline Hg₂Cl₂ and Hg₂Br₂ samples. In the case of polycrystalline samples, a strong broadening of phase transition effects was observed, due to damaged surface layers and elastic and plastic stress fields inducing order parameter fluctuations over a wide temperature range.