Why Quantum Phase Transitions Are Interesting

This paper discusses why the usual notion that quantum phase transitions can be mapped onto classical phase transitions in a higher dimension, and that this makes the former uninteresting from a fundamental theoretical point of view, is in general misleading. It is shown that quantum phase transitions are often qualitatively different from their classical counterparts due to (1) long-ranged effective interactions that are induced by soft modes, and (2) in the presence of quenched disorder, an extreme anisotropy of space-time. These points are illustrated using various magnetic phase transitions as examples.     

Search for Electronic Phase Separation at Quantum Phase Transitions

Phase separation and extreme sensitivity to disorder and defects are key features of electronic order near quantum phase transitions. Neutron depolarization imaging and neutron Larmor diffraction are new experimental techniques capable of providing detailed real-space and reciprocal-space information, respectively, on the existence and nature of phase separations. Proof-of-principle depolarization imaging in Pd_1?x Ni _x , CePd_1?x Rh _x and NbFe₂ suggests distinct differences of the real-space distribution of ferromagnetic moments and Curie temperatures in materials at ferromagnetic quantum phase transitions. This compares with neutron Larmor diffraction which provides high-resolution reciprocal-space information of phase separation and the absence of quantum criticality in the itinerant helimagnet MnSi or the parasitic nature of small moment antiferromagnetism in URu₂Si₂.     

From Thermodynamically Driven Phase Transitions to Quantum Critical Phenomena

In this short review some basic concepts of phase transitions at non-zero and zero temperature are highlighted. For that the critical behavior of classical, thermodynamically driven magnetic phase transitions is exemplified for conventional universality classes as well as for systems reflecting chiral order. Finally the quantum critical phenomena for two magnetic-field-driven phase transitions are presented.     

Reentrant Orientational Phase Transitions and Critical Points at Quantum Orientational Melting

An effect is studied of the crystal field on a system of rotors under the conditions of quantum orientational melting. It is shown that even very small crystal fields can substantially change the behavior of the system: at a certain positive value of the crystal field, a critical point appears at the classical side of the phase separation line; another critical point appears at the quantum side with further increasing of the field; then these critical points degenerate into a multicritical point and then the phase separation line disappears. For negative crystal fields stable states of the system of rotors can exist with a negative order parameter which can be treated as the orientational analog of the easy-plane-type ordering in magnets.     

Quantum Phase Transitions and Finite Size Effects in Frustrated Two-Leg Spin Ladders

We studied the ground state properties of frustrated two-leg ladders in the framework of Heisenberg model. We considered the models of these ladders with all site spins s = 1/2 and the models with different values of spins on neighbor rungs. We found a quantum phase transition mediated by the value of the interactions between the neighbor rungs. For finite-size fragments, we also found a strong dependence of the ground state spin ordering on the size of these fragments. The region of model parameters which is characterized by such strong dependence was established numerically.

     

Quantum Griffiths Effects and Smeared Phase Transitions in Metals: Theory and Experiment

In this paper, we review theoretical and experimental research on rare region effects at quantum phase transitions in disordered itinerant electron systems. After summarizing a few basic concepts about phase transitions in the presence of quenched randomness, we introduce the idea of rare regions and discuss their importance. We then analyze in detail the different phenomena that can arise at magnetic quantum phase transitions in disordered metals, including quantum Griffiths singularities, smeared phase transitions, and cluster-glass formation. For each scenario, we discuss the resulting phase diagram and summarize the behavior of various observables. We then review several recent experiments that provide examples of these rare region phenomena. We conclude by discussing limitations of current approaches and open questions.     

Structural Phase Transitions I

Optical phase changes of visible radiation in transmission through evaporated gold and tellurium thin films were determined using the Rayleigh-Lowe refractometer. The variation of the phase change in transmission with thickness was utilized to determine the absorption coefficient, ku, for both films as well as the refractive index, nu.     

Phase Transitions in Four-Fermion Models

The most important advances in the physics of four-fermion theories are illustrated using the Nambu–Jona-Lasinio and Gross–Neveu models. Magnetic-field-catalyzed dynamical chiral symmetry breaking is considered, and the magnetic-field effect on the effective fermion mass is assessed. The models in question are used to interpret the recently discoveredPparity breaking in high-T _c superconductors. High-T _c materials are considered in which the superconducting transition can be interpreted within the Gross-Neveu model as due to the formation of Berezinskii–Kosterlitz–Thouless vortex–antivortex pairs. The generalized Gross–Neveu model with superconducting states is described. Examples are presented of nonperturbative kink–antikink solutions in the Gross–Neveu model, and their role in the restoration of spontaneously broken symmetry is discussed. A number of Gross–Neveu models on a sphere are considered with application to C₆₀ fullerenes and related matrices with T _c = 117 K. It is shown that the curvature effect restores chiral symmetry according to the critical phase dependence r _c(T _c).     

On Low-Temperature Structural Phase Transitions

We comment on zero- and low-temperature structural phase transitions, expecting that these comments might be relevant not only for this structural case. We first consider a textbook model whose classical version is the only model for which the Landau theory of phase transitions and the concept of “soft mode” introduced by Ginzburg are exact. Within this model, we reveal the effects of quantum fluctuations and thermal ones at low temperatures. To do so, the knowledge of the dynamics of the model is needed. However, as already was emphasized by Ginzburg et al. in eighties, a realistic theory for such a dynamics at high temperatures is lacking, what also seems to be the case in the low-temperature regime. Consequently, some theoretical conclusions turn out to be dependent on the assumptions on this dynamics. We illustrate this point with the low-temperature phase diagram, and discuss some unexpected shortcomings of the continuous medium approaches.     

Thermal Conductivity during Phase Transitions

Thermal conductivity is a very basic property that determines how fast a material conducts heat, which plays an important and sometimes a dominant role in many fields. However, because materials with phase transitions have been widely used recently, understanding and measuring temperature‐dependent thermal conductivity during phase transitions are important and sometimes even questionable. Here, the thermal transport equation is corrected by including heat absorption due to phase transitions to reveal how a phase transition affects the measured thermal conductivity. In addition to the enhanced heat capacity that is well known, it is found that thermal diffusivity can be abnormally lowered from the true value, which is also dependent on the speed of phase transitions. The extraction of the true thermal conductivity requires removing the contributions from both altered heat capacity and thermal diffusivity during phase transitions, which is well demonstrated in four selected kinds of phase transition materials (Cu₂Se, Cu₂S, Ag₂S, and Ag₂Se) in experiment. This study also explains the lowered abnormal thermal diffusivity during phase transitions in other materials and thus provides a novel strategy to engineer thermal conductivity for various applications.     

Quantum Phase Transitions and Superconductivity in Single- and Two-Layer Cuprates in the Multiband Theory of Hubbard Fermions

We consider the doping dependence of the normal and superconducting properties of La₂Sr_2?x CuO₂ and YBa₂Cu₃O₇ in the low energy effective model based on the ab initio LDA+GTB calculations. With doping we have found a concentration region with electronic instability of the uniform state. We have shown that two quantum phase transitions (QPT) of the Lifshitz type correspond well to the experimental phase diagram. For superconducting state we have considered both magnetic and phonon mechanisms of pairing.     

Phonons and Phase Transitions in Finite Nuclei

The nature and evolution of collectivity and coherence in nuclei is one of the most fundamental issues in nuclear structure and its evolution with N and Z. Despite many experiments, the nature of nuclear vibrational modes in deformed nuclei and the nature of nuclear phase/shape transitions are not at all understood. We discuss new experiments on phonon and multi-phonon states in the rare earth nuclei and on new evidence for phase coexistence in Sm that relates to the possible existence of phase transitional behavior in finite nuclei.     

Magnetic Phase Transitions in bcc 3He

We study magnetic phase transitions in the bcc solid ³He by means of Monte Carlo simulations. We employ a classical spin model on the bcc lattice with multiple ring-exchange interactions. In the present study, we take into account up to 4-spin ring exchanges. In order to clarify the character of phase transitions we examine energy histograms generated by Monte Carlo simulations. It is shown that the phase transition between the cnaf and the paramagnetic phases is discontinuous at low magnetic fields while it is continuous at high magnetic fields. This result agrees qualitatively with experimental observations.     

Phase Transitions of Water in Zeolite Pores

Technical Physics Letters - Phase transition temperatures of water are experimentally determined for the case in which ice melts in the pores of zeolite (natural mineral) with a pore size of...     

Magnetic Phase Transitions in Cobalt Chromite Nanoparticles

Cobalt chromite (CoCr₂O₄), an insulating normal spinel compound, is a potential multiferroic material. We report that pure, nanosize CoCr₂O₄ particles are synthesized through a conventional coprecipiation technique by controlling the pH of the precipitation. Both single-crystal and polycrystalline samples develop long-range ferrimagnetic order below the Curie temperature, T _c (97 K), and a sharp phase transition at T _s??31 K, attributed to the onset of long-range spiral magnetic order. However, we observed a transition from paramagnetic to superparamagnetic phase at T _c. Further lowering the temperature below T _c (97 K), the superparamagnetic phase transforms to ferrimagnetic phase at blocking temperature, T _b, which is found to be between 50 and 60 K. This intermediate superparamagnetic phase in between paramagnetic and long-range ferrimagnetic phases is attributed to a nanosize effect.     

Dynamical Phase Slipping in Superconducting Nanowires

In the present work a study of resist state dynamics of superconducting nanowires on the basis of simulation of the one-dimensional time-dependent Ginzburg-Landau equation is carried out. The parameter u characterizing the purity of a superconducting substance was introduced in this equation. It was found out that an area of the current density, in which a dynamical phase slip process is observed, exists at u > 1. This process depends on the length of the nanowires and is not defined by thermodynamic fluctuations of conductivity and order parameter in contrast to thermal activated phase slips and quantum phase slip mechanisms. Also their arising does not depend on heterogeneity of nanowire substances.     

Phase Formation and Phase Transitions in Nonstoichiometric Sodium Bismuth Titanate Ceramics

We have studied the phase formation, microstructure, and dielectric and ferroelectric properties of (Na_0.5–xBi_0.5)TiO₃ and (Na_0.5Bi_{0.5 + x})TiO₃ nonstoichiometric ceramics with Na/Bi < 1 and x = 0–0.1. The grain size of the ceramics has been shown to decrease with increasing x. The temperature dependences of dielectric permittivity for the samples studied have anomalies near ~400 K and peaks at ~600 K, corresponding to ferroelectric phase transitions. The phase transitions near 400 K demonstrate relaxor behavior, indicative of the presence of polar regions in a nonpolar matrix, as supported by laser second harmonic generation measurements. In addition, the (Na_0.5Bi_{0.5 + x})TiO₃ samples with x > 0.05 have anomalies near 900 K, confirming the presence of Bi₄Ti₃O₁₂ as an impurity phase, which is accompanied by an increase in the spontaneous polarization of these samples.