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Showing papers on "Landau theory published in 2021"


Journal ArticleDOI
TL;DR: In this article, the authors used a general continuum model and utilize renormalization group and mean field theory methods to identify mechanisms that can lead to the experimentally observed charge density wave ordering.
Abstract: The newly discovered family of kagome metals $A$V${}_{3}$Sb${}_{5}$ ($A$=K,Cs,Rb) has recently gained wide attention. Here, the authors study these materials using a general continuum model and utilize renormalization group and mean field theory methods to identify mechanisms that can lead to the experimentally observed charge density wave ordering. The authors find that the charge density wave is the primary density wave instability in certain cases but could also be induced by a leading orbital current or spin density wave order.

104 citations


Journal ArticleDOI
TL;DR: The universal framework of the Landau theory of phase transition is relevant for describing such ordering processes through the evolution of a symmetry-breaking order parameter η as discussed by the authors, and the coupling between these two types of instabilities is responsible for the emergence of various types of functions.
Abstract: Spin-transition materials, including the families of spin-crossover and charge-transfer systems, and more generally molecular-based materials exhibiting electronic and/or structural bistability, may undergo various types of phase transitions. The change of electronic state is stabilized by molecular reorganizations and both phenomena, which are usually non-symmetry breaking, can be described through the evolution of an order parameter q. Due to symmetry, q linearly couples to volume change. It is known that such elastic interactions are responsible for cooperative phenomena in non-symmetry-breaking spin-transitions. However, spin-transition materials may also exhibit symmetry-breaking phenomena related to various types of orders such as structural order as well as spin-state concentration waves. The universal framework of the Landau theory of phase transition is relevant for describing such ordering processes through the evolution of a symmetry-breaking order parameter η. The simultaneous or sequential occurrence of spin-transition and symmetry-breaking phenomena are reported for numerous spin-transition materials, and the coupling between these two types of instabilities is responsible for the emergence of various types of functions. In this work, we use the Landau approach to describe both symmetry-breaking phenomena and non-symmetry-breaking spin transition. We discuss how their coupling can generate sequences of phase transitions, from simple spin-crossover to spin-transition, continuous or discontinuous symmetry breaking, including ferroelasticity or stepwise spin transitions.

14 citations


Journal ArticleDOI
TL;DR: In this article, the authors use the Landau theory to study the elastic coupling between a spin-crossover instability and a discontinuous ferroelastic distortion and the different phase transition lines.
Abstract: Spin-transition materials can exhibit thermal hysteresis due to cooperative elastic interactions in between active molecular sites. It results from the coupling of the nonsymmetry-breaking high spin fraction order parameter to the lattice volume strain. However, a symmetry-breaking order parameter responsible for crystallographic phase transition, like a ferroelastic distortion, can also couple to the volume strain. Here, we use the Landau theory to study the elastic coupling between a spin-crossover instability and a discontinuous ferroelastic distortion and the different phase transition lines. Below the triple point, the first-order line involves simultaneous ferroelastic distortion and spin transition. Above, the purely first-order ferroelastic distortion and the spin crossover occur sequentially. Our model, related to the coupling and decoupling of the ferroelastic phase transition and spin crossover, explains exotic behaviors reported experimentally in the literature for diverse spin-crossover systems, plastic crystals, or Prussian blue analogues. The unsymmetric hysteresis loops and the stepwise evolution of the spin conversion or volume strain under pressure result from different sequences of phase transitions. The model shows that the ferroelastic phase transition is the driving force of the cooperative spin transition hysteresis in this case. © 2021 American Physical Society.

10 citations


Posted Content
TL;DR: In this paper, the authors assess experimentally and theoretically the character of the superfluid-supersolid quantum phase transition recently discovered in trapped dipolar quantum gases and find that one-row supersolids can have already two types of phase transitions, discontinuous and continuous, that are reminiscent of the first-and second-order transitions predicted in the thermodynamic limit in 2D and 1D, respectively.
Abstract: We assess experimentally and theoretically the character of the superfluid-supersolid quantum phase transition recently discovered in trapped dipolar quantum gases. We find that one-row supersolids can have already two types of phase transitions, discontinuous and continuous, that are reminiscent of the first- and second-order transitions predicted in the thermodynamic limit in 2D and 1D, respectively. The smooth crossover between the two regimes is peculiar to supersolids and can be controlled via the transverse confinement and the atom number. We justify our observations on the general ground of the Landau theory of phase transitions. The quasi-adiabatic crossing of a continuous phase transition opens new directions of investigation for supersolids.

10 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that a C-T-O-R quadruple point in a binary ferroelectric system can only exist in the form of a unique type of critical point at which two first-order transition lines and two second-order ones meet, and such critical quadruple points do not defy the thermodynamic phase rule.
Abstract: In experimentally measured temperature-composition ferroelectric phase diagrams of ${\mathrm{BaTiO}}_{3}$-based binary systems, a quadruple point where cubic (C), tetragonal (T), orthorhombic (O), and rhombohedral (R) phases converge has been frequently reported in previous work. More interestingly, the quadruple points are experimentally found to behave as a critical point with large enhancement in properties. However, it has remained a fundamental question as to whether a quadruple point in a binary ferroelectric system defies the thermodynamic phase rule and whether such a point necessarily goes critical. In this study, it is demonstrated by Landau theory that a C-T-O-R quadruple point in a binary ferroelectric system can only exist in the form of a unique type of critical point at which two first-order transition lines and two second-order ones meet, and such critical quadruple points do not defy the thermodynamic phase rule. It is further shown that at such a critical C-T-O-R quadruple point, the system exhibits infinitely large piezoelectric coefficients, which agrees with the high piezoelectricity observed at the C-T-O-R quadruple point in a number of ${\mathrm{BaTiO}}_{3}$-based binary ferroelectric systems and also helps to explain the large piezoelectricity obtained at the morphotropic phase boundaries of these quadruple point based systems.

8 citations


Journal ArticleDOI
TL;DR: In this article, the dependence of the phase diagram of a hypothetical isotope of helium with nuclear mass less than 4 atomic mass units was discussed, and it was shown that with decreasing nucleus mass, the temperature of the superfluid phase transition increases, while that of the liquid-gas critical point decreases.
Abstract: We discuss the dependence of the phase diagram of a hypothetical isotope of helium with nuclear mass less than 4 atomic mass units. We argue that with decreasing nucleus mass, the temperature of the superfluid phase transition (about 2.2 K in real He-4) increases, while that of the liquid-gas critical point (about 5.2 K in real He-4) decreases. We discuss various scenarios that may occur when the two temperatures approach each other and the order parameters of the superfluid and the liquid-gas phase transitions interact with each other. The simplest scenario, in which both order parameters become critical at particular values of the nuclear mass, temperature, and pressure, can be ruled out through on an analysis of the Landau theory. We argue that in the most likely scenario, as the nuclear mass decreases, first, a tricritical point appears on the line separating the superfluid and the normal fluid phase, then the critical point disappears under the first-order part of superfluid phase transition line, and in the end the tricritical point disappears. The last change in the phase diagram occurs when the two-body scattering length crosses zero, which corresponds to the nuclear mass of about 1.55 u. We develop a quantitative theory that allows one to determine the phase diagram in the vicinity of this point. Finally, we discuss several ways to physically realize such liquids.

6 citations


Posted Content
TL;DR: In this paper, the authors employ a quantum theory of dissipative phase transitions to capture the genuine non-equilibrium phase transition of the Scully-Lamb laser model (SLLM), and show that the phase transition corresponds to the emergence of a dynamical multistability even without SSB.
Abstract: Phase transitions of thermal systems and the laser threshold were first connected more than forty years ago. Despite the non-equilibrium nature of the laser, the Landau theory of thermal phase transitions, applied directly to the Scully-Lamb laser model (SLLM), indicated that the laser threshold is a second-order phase transition, associated with a $U(1)$ spontaneous symmetry breaking (SSB). To capture the genuine non-equilibrium phase transition of the SLLM (i.e., a single-mode laser without saturable absorber), here we employ a quantum theory of dissipative phase transitions. Our results confirm that the $U(1)$ SSB can occur at the lasing threshold but, in contrast to the Landau theory and semiclassical approximation, they signal that the SLLM "fundamental" transition is a different phenomenon, which we call Liouvillian spectral collapse; that is, the emergence of diabolic points of infinite degeneracy. By considering a generalized SLLM with additional dephasing, we witness a second-order phase transition, with a Liouvillian spectral collapse, but in the absence of symmetry breaking. Most surprisingly, the phase transition corresponds to the emergence of a dynamical multistability even without SSB. Normally, bistability is suppressed by quantum fluctuations, while in this case the very presence of quantum fluctuations enable bistability. This rather anomalous bistability, characterizing the truly dissipative and quantum origin of lasing, can be an experimental signature of our predictions.

6 citations


Journal ArticleDOI
TL;DR: In this paper, the magnetocaloric effect in nanosystems based on exchange-coupled ferromagnets with different Curie temperatures is calculated within the mean-field theory.
Abstract: The magnetocaloric effect in nanosystems based on exchange-coupled ferromagnets with different Curie temperatures is calculated within the mean-field theory. Good agreement between the results of the mean-field theory and the Landau theory, valid near the critical phase transition temperature, is demonstrated for a flat-layered Fe/Gd/Fe structure. We show that a high magnetic cooling efficiency in this system is attainable in principle and prove the validity of the Maxwell relation, enabling an experimental verification of the predictions made. The theory developed for flat-layered structures is generalized to a granular medium.

6 citations


Journal ArticleDOI
TL;DR: In this paper, a phase transition from the high-temperature ferromagnetic (FM) to the lowtemperature antiferromagnetic phase is considered and the dependence of the phase transition field on the temperature of solid has been computed; quantitative agreement between the theoretical results and experimental data obtained in the recent experiments with Fe49Rh51 alloy has been achieved.

5 citations


Posted Content
TL;DR: In this paper, a Landau-Ginzburg theory of one-form global symmetries, called mean string field theory, has been proposed, which can be used to describe the creation, annihilation, and condensation of effective strings.
Abstract: By analogy with the Landau-Ginzburg theory of ordinary zero-form symmetries, we introduce and develop a Landau-Ginzburg theory of one-form global symmetries, which we call mean string field theory. The basic dynamical variable is a string field -- defined on the space of closed loops -- that can be used to describe the creation, annihilation, and condensation of effective strings. Like its zero-form cousin, the mean string field theory provides a useful picture of the phase diagram of broken and unbroken phases. We provide a transparent derivation of the area law for charged line operators in the unbroken phase and describe the dynamics of gapless Goldstone modes in the broken phase. The framework also provides a theory of topological defects of the broken phase and a description of the phase transition that should be valid above an upper critical dimension, which we discuss. We also discuss general consequences of emergent one-form symmetries at zero and finite temperature.

5 citations


Journal ArticleDOI
TL;DR: The Landau theory of phase transitions with single-order parameter (second rank alignment tensor) has been applied to study the formation of modulated nematic phases in a melt of the symmetric V-shores as mentioned in this paper.
Abstract: The Landau theory of phase transitions with single-order parameter (second rank alignment tensor) has been applied to study the formation of modulated nematic phases in a melt of the symmetric V-sh...

Journal ArticleDOI
TL;DR: In this paper, a sensitive second harmonic generation (SHG) microscope was used to detect long-range Neel antiferromagnetic (AFM) order and Neel vector switching down to the monolayer in MnPSe$_3$.
Abstract: The family of monolayer two-dimensional (2D) materials hosts a wide range of interesting phenomena, including superconductivity, charge density waves, topological states and ferromagnetism, but direct evidence for antiferromagnetism in the monolayer has been lacking. Nevertheless, antiferromagnets have attracted enormous interest recently in spintronics due to the absence of stray fields and their terahertz resonant frequency. Despite the great advantages of antiferromagnetic spintronics, controlling and detecting Neel vectors have been limited in bulk materials. In this work, we developed a sensitive second harmonic generation (SHG) microscope and detected long-range Neel antiferromagnetic (AFM) order and Neel vector switching down to the monolayer in MnPSe$_3$. Temperature-dependent SHG measurement in repetitive thermal cooling surprisingly collapses into two curves, which correspond to the switching of an Ising type Neel vector reversed by the time-reversal operation, instead of a six-state clock ground state expected from the threefold rotation symmetry in the structure. We imaged the spatial distribution of the Neel vectors across samples and rotated them by an arbitrary angle irrespective of the lattice in the sample plane by applying strain. By studying both a Landau theory and a microscopic model that couples strain to nearest-neighbor exchange, we conclude that the phase transition of the XY model in the presence of strain falls into the Ising universality class instead of the XY one, which could explain the extreme strain tunability. Finally, we found that the 180° AFM domain walls are highly mobile down to the monolayer after thermal cycles, paving the way for future control of the antiferromagnetic domains by strain or external fields on demand for ultra-compact 2D AFM terahertz spintronics.

Journal ArticleDOI
TL;DR: In this paper, the authors show simulation results of first-order phase transitions between high and low-density liquid (HDL and LDL) in confined water in both positive and negative pressures.
Abstract: The stories behind supercooled bulk and confined water can be different. Bulk water has a metastable liquid-liquid phase transition at deeply supercooled conditions, but the existence of such a phenomenon in confined water is in question. Herein we show simulation results of first-order phase transitions between high- and low-density liquid (HDL and LDL) in confined water in both positive and negative pressures. A mid-density state between these two local states appears, which lets the transition show the hysteresis loop with transiently stable intermediate states. On the basis of Landau theory that we have adapted for mixing of moieties with high- and low-density states, we explain the phase transitions with the order parameter-dependent free energy change which is governed by second- to higher-order interactions among those moieties.

Journal ArticleDOI
TL;DR: In this paper, the authors have preliminarily realized the Landau continuous phase transition theory in black hole thermodynamics by introducing the general construction of Landau free energy of the van der Waals system and charged AdS black hole system.
Abstract: By introducing the general construction of Landau free energy of the van der Waals system and charged AdS black hole system, we have preliminarily realized the Landau continuous phase transition theory in black hole thermodynamics. The results show that the Landau free energy constructed in present paper can directly reflect the physical process of black hole phase transition. Specifically, the splitting of the global minimum of the Landau free energy corresponds to the second-order phase transition of the black hole, and the transformation of the global minimum reflects the first-order phase transition of the black hole.

Journal ArticleDOI
17 Jun 2021
TL;DR: In this article, a comprehensive study of structure, magnetic phase transition, magnetocaloric effect and thermomechanical properties for MnCoGe1−x6 is reported, and it is shown that the temperature dependence of the magnetization curves overlaps completely, indicating that there is no thermal hysteresis.
Abstract: MnCoGe-based compounds have been increasingly studied due to their possible large magnetocaloric effect correlated to the magnetostructural coupling. In this research, a comprehensive study of structure, magnetic phase transition, magnetocaloric effect and thermomechanical properties for MnCoGe1−xSix is reported. Room temperature X-ray diffraction indicates that the MnCoGe1−xSix (x = 0, 0.05, 0.1, 0.15 and 0.2) alloys have a major phase consisting of an orthorhombic TiNiSi-type structure with increasing lattice parameter b and decreasing others (a and c) with increasing Si concentration. Along with M-T and DSC measurements, it is indicated that the Tc value increased with higher Si concentration and decreased for structural transition temperature Tstr. The temperature dependence of the magnetization curves overlaps completely, indicating that there is no thermal hysteresis, and it is shown that the transition is the second-order type. It is also shown that the decreased magnetization on the replacement of Si for Ge decreases the value of −ΔSM from −ΔSM~8.36 J kg−1 K−1 at x = 0 to −ΔSM~5.49 J kg−1 K−1 at x = 0.2 with 5 T applied field. The performed Landau theory has confirmed the second-order transition in this study, which is consistent with the Banerjee criterion. The magnetic measurement and thermomechanical properties revealed the structural transition that takes place with Si substitution of Ge.

Journal ArticleDOI
TL;DR: This work combines double photoexcitation with an X-ray free-electron laser (XFEL) probe to control and detect the lifetime and magnitude of the intermediate vibrational state near the critical point of the SDW in chromium.
Abstract: The spin-phonon interaction in spin density wave (SDW) systems often determines the free energy landscape that drives the evolution of the system. When a passing energy flux, such as photoexcitation, drives a crystalline system far from equilibrium, the resulting lattice displacement generates transient vibrational states. Manipulating intermediate vibrational states in the vicinity of the critical point, where the SDW order parameter changes dramatically, would then allow dynamical control over functional properties. Here we combine double photoexcitation with an X-ray free-electron laser (XFEL) probe to control and detect the lifetime and magnitude of the intermediate vibrational state near the critical point of the SDW in chromium. We apply Landau theory to identify the mechanism of control as a repeated partial quench and sub picosecond recovery of the SDW. Our results showcase the capabilities to influence and monitor quantum states by combining multiple optical photoexcitations with an XFEL probe. They open new avenues for manipulating and researching the behaviour of photoexcited states in charge and spin order systems near the critical point. Precise control of vibrational states coupled to electronic degrees of freedom could enable control over charge or magnetic order in a material. Here, the authors use a double-pulse photoexcitation combined with an X-ray probe to control vibrational states near the critical point of spin density wave in Cr films.

Journal ArticleDOI
TL;DR: In this article, the structural, magnetic, and magnetoclaoric properties of nanocrystalline iron-rich intermetallic Pr2Fe14B, prepared using ball milling, were studied using Landau theory around Curie temperature.
Abstract: We report on the structural, magnetic, and magnetoclaoric properties of nanocrystalline iron-rich intermetallic Pr2Fe14B, prepared using ball milling The annealed sample is single-phase with the tetragonal structure [P42/mnm space group (136)] The magnetic and magnetocaloric properties were studied using Landau theory around Curie temperature Reasonable agreement was obtained between the magnetic entropy (− ∆SM) determined using Landau theory and that extracted using classical Maxwell relation Pr2Fe14B system depicts an interesting value of maximum magnetic entropy change of 36 J/kg K at 1 T and a relative cooling power about 36 J/kg Universal curves based on re-scale entropy change curves as well as Landau theory reveal the first-order magnetic transition of Pr2Fe14B intermetallic

Proceedings ArticleDOI
16 May 2021
TL;DR: In this article, a phenomenological framework that can explain the most pertinent features of ferroelectricity and phase transitions with doping and composition remains conspicuously absent, despite much insight generated from ab initio and DFT calculations.
Abstract: Causes for the emergence of ferroelectricity and field-induced phase transitions in doped HfO 2 , Hf 1-x Zr x O 2 , and ZrO 2 are a rich topic of scientific debate for these important technological materials. A phenomenological framework that can explain the most pertinent features of ferroelectricity and phase transitions with doping and composition remains conspicuously absent, despite much insight generated from ab initio and DFT calculations. From the Landau theory of phase transitions, it is shown that electronic and bonding contributions from dopants can directly impact the formation of the ferroelectric phase. The size of the dopant ions and the appearance of field-induced phase transitions are found to be inextricably linked.

Journal ArticleDOI
TL;DR: In this article, the properties of polaron-polaritons are investigated in terms of their energy and the interactions between them, and they can be measured in a non-demolition way via the light transmission/reflection spectrum of the system.
Abstract: Two-dimensional semiconductors inside optical microcavities have emerged as a versatile platform to explore new hybrid light-matter quantum states. The strong light-matter coupling leads to the formation of exciton-polaritons, which in turn interact with the surrounding electron gas to form quasiparticles called polaron-polaritons. Here, we develop a general microscopic framework to calculate the properties of these quasiparticles such as their energy and the interactions between them. From this, we give microscopic expressions for the parameters entering a Landau theory for the polaron-polaritons, which offers a simple yet powerful way to describe such interacting light-matter many-body systems. As an example of the application of our framework, we then use the ladder approximation to explore the properties of the polaron-polaritons. We furthermore show that they can be measured in a non-demolition way via the light transmission/reflection spectrum of the system. Finally, we demonstrate that the Landau effective interaction mediated by electron-hole excitation is attractive leading to red shifts of the polaron-polaritons. Our work provides a systematic framework to study exciton-polaritons in electronically doped two-dimensional materials such as novel van der Waals heterostructures.

Journal ArticleDOI
TL;DR: In this paper, a differential model is proposed to describe the hysteresis in magnetic shape memory alloys (MSMAs) caused by magnetic field induced martensite reorientation based on Landau theory of phase transitions.
Abstract: Magnetic shape memory alloys (MSMAs) are materials with strong nonlinearity, which will show hysteresis when it works. In the current paper, a differential model is proposed to describe the hysteresis in MSMAs caused by magnetic field induced martensite reorientation based on Landau theory of phase transitions. First, the three-dimensional model is simplified into a one-dimensional case, and the hysteresis in MSMAs is described by three martensite variant orientations. Then, a traditional Landau free energy is introduced, and the traditional Landau model is obtained by using the Euler–Lagrange equation. However, it is found that the prediction accuracy of the model is not ideal compared with the experimental data. Thus, an improved model is proposed, and the traditional Landau free energy is replaced by a function that can be determined by the easily measured physical quantities in the experimental curve. Numerical experiments show that the prediction effect of the improved model is much better than the traditional Landau model, and the stress dependence is also demonstrated. Moreover, the proposed improved model has the advantages for dynamic analysis and control with its differential form. Therefore, the frequency dependence of the improved model is demonstrated and a feedback linearization control methodology is proposed to make the system develops in the desired trajectory.

Journal ArticleDOI
TL;DR: In this article, a phenomenological constitutive model is constructed involving phase transformation in one-dimensional SMA, taking into account the tension-compression asymmetry, an asymmetric potential energy function is constructed, according to the three-dimensional deformation of the crystalline lattice.

Journal ArticleDOI
TL;DR: In this paper, a detailed analysis of the atomic structural parameters across the transition in combination with ab initio studies allow to pinpoint a microscopic mechanism driven by a rearrangement of point defects initially present in the structure.

Journal ArticleDOI
TL;DR: In this paper, the authors studied the elastic behavior of single crystals of the Ni-Fe-Ga(Co) ferromagnetic shape memory alloy in a broad temperature range using tensile stress-strain measurements and ultrasonic characterization.

Journal ArticleDOI
TL;DR: In this article, it was shown that interacting electrons on triangular lattice structures arrange themselves in various competing magnetic and possible topological orders, leading to paradoxical electron localization as temperature increases.
Abstract: Interacting electrons on triangular lattice structures arrange themselves in various competing magnetic and possible topological orders, leading to paradoxical electron localization as temperature increases.

Journal ArticleDOI
TL;DR: In this paper, the polycrystalline manganite Pr0.55CaxSr0.45-xMnO3 (x = 0.20) has been prepared by the solid state reaction technique to investigate its crystal structure, magnetic, and magnetocaloric properties.
Abstract: The polycrystalline manganite Pr0.55CaxSr0.45-xMnO3 (x = 0.00, 0.05, 0.1, and 0.2) has been prepared by the solid state reaction technique to investigate its crystal structure, magnetic, and magnetocaloric properties. Critical behaviour around PM-FM phase transition has also been analyzed through various methods including modified Arrott plots (MAP), and critical isotherm analysis of Pr0.55CaxSr0.45-xMnO3 (x = 0.00, 0.05, 0.1, and 0.2). XRD analysis reveals that all the samples are found to be crystallized in the orthorhombic system with Pnma space group and lattice parameters a, b and c as well as the cell volume are found to decrease with increasing Ca (x) content. Microstructure is observed with the field emission scanning electron microscopy (FESEM) photograph and elemental compositions are determined by energy dispersive X-ray diffractometer (EDX). Temperature and field dependent magnetization measurement disclose that all the samples undergo second-order FM to PM phase transition but the Curie temperature (TC) value decreases from 290 K to 245 K with the increase in Ca (x) content from 0.00 to 0.20. The magnetocaloric effect (MCE) in terms of maximum entropy change, (-ΔSm)max and relative cooling power (RCP) was calculated from isothermal magnetization measurements around TC, using Maxwell’s thermodynamic relations. Both (−ΔSm)max and RCP increases with increasing Ca content suggests the suitability of this compound as a potential solid state refrigerent. Contribution of itinerant electron in the entropy change is found from modulating MCE with Landau theory of phase transition.

Journal ArticleDOI
TL;DR: In this paper, the effect of carbon nanotubes (CNTs) in ferroelectric liquid crystal (FLC) was studied using a combination of Flory-Huggins theory and Landau theory.
Abstract: Effect of carbon nanotubes (CNTs) in ferroelectric liquid crystal (FLC) is studied using a combination of Flory-Huggins theory and Landau theory. An appreciable change in polarisation, tilt angle a...

Journal ArticleDOI
TL;DR: In this article, a path-integral approach based on the faithful Schwinger fermion representation is proposed to determine the phase boundary of spin and boson models, which can be used to study the universality of the Rabi and Dicke models.
Abstract: The Dicke model and the Rabi model can undergo phase transitions from the normal phase to the superradiant phase at the same boundary, which can be accurately determined using some approximated approaches. The underlying mechanism for this coincidence is still unclear, and the universality class of these two models is elusive. Here we prove this phase transition exactly using the path-integral approach based on the faithful Schwinger fermion representation, and give a unified phase boundary condition for these models. We demonstrate that at the phase boundary, the fluctuation of the bosonic field is vanished, thus, it can be treated as a classical field, based on which a much simplified method to determine the phase boundary is developed. This explains why the approximated theories by treating the operators as classical variables can yield the exact boundary. We use this method to study several similar spin and boson models, showing its much wider applicability than the previously used approaches. Our results demonstrate that these phase transitions belong to the same universality by the classical Landau theory of phase transition from a more general way, which can be confirmed using the platforms in the recent experiments.


Journal ArticleDOI
TL;DR: In this paper, the authors have preliminarily realized the Landau continuous phase transition theory in black hole thermodynamics by introducing the general construction of Landau free energy of the van der Waals system and charged AdS black hole system.
Abstract: By introducing the general construction of Landau free energy of the van der Waals system and charged AdS black hole system, we have preliminarily realized the Landau continuous phase transition theory in black hole thermodynamics. The results show that the Landau free energy constructed in present paper can directly reflect the physical process of black hole phase transition. Specifically, the splitting of the global minimum of the Landau free energy corresponds to the second-order phase transition of the black hole, and the transformation of the global minimum reflects the first-order phase transition of the black hole.

Journal ArticleDOI
TL;DR: In this paper, a bulk system of ferroelectric bilayer with an antiferroelectric interface coupling between two similar materials which have second-order phase transitions is considered, and the second order phase transition is considered.
Abstract: We consider a bulk system of ferroelectric bilayer with an antiferroelectric interface coupling between two similar materials which have second-order phase transitions. Landau theory and Landau–Kha...