Pattern formation in Ferroelastic Transitions

We study pattern formation in ferroelastic materials using the Ginzburg–Landau approach. Since ferroelastic transitions are driven by strain, the nonlinear elastic free energy is expressed as an expansion in the appropriate (i.e., order parameter) strain variables. However, the displacement fields are the real independent variables, whereas the components of the strain tensor are related to each other through elastic compatibility relations. These constraints manifest as an anisotropic long-range interaction which drastically influences the underlying microstructure. The evolution of the microstructure is demonstrated for (i) a hexagonal-to-orthorhombic transition using a strain-based approach with explicit long-range interactions; and (ii) a cubic-to-tetragonal transition by solving the force-balance equations for the displacement fields.     

The ferroelastic transition of betaine borate

The ferroelastic transition of betaine borate at about 142 K and the nonlinear temperature dependence of the corresponding critical elastic modulus are related to the softening of optical phonons. The transition has to be considered a ferroelastic one of the optic type, i.e. pseudoproper one, therefore. It is shown by Raman spectroscopic results that the transition is induced by a strong bilinear coupling between a homogeneous strain and at least two optic lattice modes which are of the same symmetry as the strain. Optic and dielectric, investigations support the second-order character of and the classical behaviour at this transition, which have been reported in the literature. Differences and similarities to the ferroelastic transition of betaine fumarate are discussed.     

The ferroelastic transition of betaine borate

The ferroelastic transition of betaine borate at about 142 K and the nonlinear temperature dependence of the corresponding critical elastic modulus are related to the softening of optical phonons. The transition has to be considered a ferroelastic one of the optic type, i.e. pseudoproper one, therefore. It is shown by Raman spectroscopic results that the transition is induced by a strong bilinear coupling between a homogeneous strain and at least two optic lattice modes which are of the same symmetry as the strain. Optic and dielectric, investigations support the second-order character of and the classical behaviour at this transition, which have been reported in the literature. Differences and similarities to the ferroelastic transition of betaine fumarate are discussed.     

Vortex sublattice melting in a two-component superconductor

We consider the vortices in a superconductor with two individually conserved condensates in a finite magnetic field. The ground state is a lattice of cocentered vortices in both order parameters. We find two phase transitions: (i) a "vortex sublattice melting" transition where vortices in the field with lowest phase stiffness ("light vortices") lose cocentricity with the vortices with large phase stiffness ("heavy vortices"), entering a liquid state (the structure factor of the light vortices vanishes continuously; this transition is in the 3Dxy universality class); (ii) a first-order melting transition of the lattice of heavy vortices, in a liquid of light vortices.     

Features of sound propagation in the vicinity of “symmetry-dictated” isostructural phase transitions in ferroelastics

Using a proper ferroelastic phase transition of the tension-compression type as an example, it is shown that, if the order parameter characterizing a structural phase transition allows the existence of a third-power invariant in the Landau potential, then there must be “symmetry-dictated” isostructural phase transition lines in the vicinity of the line of that structural phase transition. These isostructural transitions may manifest themselves both directly and as supercritical anomalies in the behavior of elastic moduli and lattice parameters. These effects are discovered and investigated without invoking the perturbation theory in terms of which the second-order phase transitions are commonly described. A hypothesis is made on the basis of the results obtained that the sound velocity anomalies observed in orthoclase and sanidine crystals are due to the super-critical behavior of the lattice parameters in the vicinity of a symmetry-dictated isostructural phase transition in the prototype phase of these crystals.     

Collective mode in melting of a crystal with a face-centered lattice

Physics of the Solid State - The collective mechanism, which can lead to the loss of long-range order during melting of an ideal crystal with a face-centered lattice, has been studied using the...     

Soft optical phonon in ferroelastic BiVO4

We report the observation of an underdamped q～O soft optical phonon in the Raman spectra of the paraelastic and ferroelastic phases of BiVO₄. This mode has the same symmetry as the ferroelastic strain. The temperature dependence of the soft optical phonon energy indicates that the ferroelastic transition is continuous and that the order parameter has a Landau-type behavior over a wide temperature range.     

Frustration-induced valence bond crystal and its melting in Mo3Sb7

(121/123)Sb nuclear quadrupole resonance and muon spin relaxation experiments of Mo_3Sb_7 revealed symmetry breakdown to a nonmagnetic state below the transition recently found at T_S approximately 50 K. The transition is characterized by a distinct lattice dynamics suggested from narrowing of nuclear fields. We point out that the Mo sublattice is a unique three-dimensional frustrated lattice where nearest-neighbor and next-nearest-neighbor antiferromagnetic interactions compete, and propose that tetragonal distortion to release the frustration stabilizes long-range order of spin-singlet dimers, i.e., valence bond crystal, which is thermally excited to the dynamic state with cubic symmetry.     

磁通格子的有序-无序相变和反向熔化

考虑了无序钉扎、热涨落和磁通互作用, 用Monte Carlo分子动力学模拟方法研究二维磁通格子在无序钉扎强度和温度空间的相图, 以及由布拉格玻璃相到非晶磁通玻璃相和到磁通液体相的有序-无序相变.为了决定磁通格子的序，计算了静态结构因子和磁通格子位形的有限尺寸指数.计算结果表明，Bragg玻璃相在低温的无序磁通玻璃相和高温的磁通液体相之间 , 表现出磁通格子的反向熔化行为.分析后认为，这一反向熔化行为起因于磁通之间互作用的温度效应. 关键词： Ⅱ类超导体 磁通格子 相图 结构因子 反向熔化     

Two-dimensional melting far from equilibrium in a granular monolayer

We report an experimental investigation of the transition from a hexagonally ordered solid phase to a disordered liquid in a monolayer of vibrated spheres. The transition occurs as the intensity of the vibration amplitude is increased. Measurements of the density of dislocations and the positional and orientational correlation functions show evidence for a dislocation-mediated continuous transition from a solid phase with long-range order to a liquid with only short-range order. The results show a strong similarity to simulations of melting of hard disks in equilibrium, despite the fact that the granular monolayer is far from equilibrium due to the effects of interparticle dissipation and the vibrational forcing.     

Microscale deformations in a two-dimensional lattice: Structural transitions and bifurcations at critical shear

A two-dimensional model of a crystal lattice with an essentially nonlinear interaction between atoms for arbitrary mutual displacements under shear conditions is considered. The energy of deformation contains a periodic term and gradient terms. The equilibrium equation in the sine-Helmholtz form (with two characteristic coherence lengths) is solved exactly. It is shown that a large uniform shear deformation is unstable and is accompanied by periodic modulations emerging in the plane of the layer and leading to nonuniform extensions and compressions of atomic chains, i.e., to a change in the long-range order. The chain slip by a lattice half-period or a larger value changes the long-range order as well. The obtained relations connecting the unit cell size and the amplitude of the displacement field determine the conditions for the existence of a modulated structure. A bifurcation transition from a purely elastic deformation of the lattice to its elastoplastic deformation, as well as a transition to its disordered (amorphous-type) state, is observed.     

Disordered ground state and magnetic field-induced long-range order in an S=3/2 antiferromagnetic honeycomb lattice compound Bi3Mn4O12(NO3)

Bi3Mn4O12(NO3), in which the Mn4+ ions carry S=3/2, is the first honeycomb lattice system that shows no long-range magnetic order. Using neutron scattering, we have determined that short-range antiferromagnetic correlations develop at low temperatures. Applied magnetic fields induce a magnetic transition, in which the short-range order abruptly expands into a long-range order.     

Mott Transition and Superconductivity in Quantum Spin Liquid Candidate NaYbSe_2

The Mott transition is one of the fundamental issues in condensed matter physics,especially in the system with antiferromagnetic long-range order.However,such a transition is rare in quantum spin liquid(QSL) systems without long-range order.Here we report the experimental pressure-induced insulator to metal transition followed by the emergence of superconductivity in the QSL candidate NaYbSe₂ with a triangular lattice of 4 f Yb³⁺ ions.Detail analysis of transport properties...     

A new mechanism of surface polariton formation at a ferroelastic phase transition

Conditions are determined at which the combined effect of the lattice and the spatial dispersion gives rise to the formation of a surface polariton with the participation of a soft mode in the vicinity of a dipole ferroelastic phase transition.     

Supersolidity for hard-core-bosons coupled to optical phonons

The coexistence of diagonal long-range order (DLRO) and off-diagonal long-range order (ODLRO), manifested in some systems, is still a theoretical enigma. Here, we present a novel microscopic mechanism for supersolidity or the homogeneous coexistence of charge-density-wave state (an example of DLRO) and superfluidity/superconductivity (a realization of ODLRO). We derive an effective d-dimensional Hamiltonian for a system of hard-core-bosons coupled to optical phonons in a lattice. At non-half-fillings, a superfluid/superconductor to a supersolid transition occurs at intermediate boson–phonon couplings, while at strong-couplings the system phase separates. We demonstrate explicitly that the presence of next-nearest-neighbor hopping and nearest-neighbor repulsion leads to supersolidity.     

Studying the ferroelastic phase transition in Sb5O7I crystals

The specific heat of a ferroelastic Sb₅O₇I crystal in the vicinity of the ferroelastic phase transition is studied. It is shown that the phase transition in such a crystal is of the first order.     

3-d Model for strain ordering in steel: I Static effects

We present a spring-defect model in 3-dimensions to describe the connection between elastic distortion and interstitial carbon ordering associated with phase transition from a body centred cubic (BCC) to body centered tetragonal (BCT) structure in BCC metals such as α-iron. The presence or the absence of the carbon is modelled in terms of a pseudo spinŝ=+1or -l.An Ising interaction between carbon atoms is recovered after eliminating the lattice degrees of freedom, which is longranged. The coupling between the spin and lattice degrees of freedom allows for a systematic study of ferroelasticity and the variation of the lattice parameter with carbon concentration. The mean field results for the paraelastic to ferroelastic transition, lattice parameter and static compliance are presented. The significant feature of this calculation is not only a derivation of the defect-defect interaction, but also an explicit calculation of the strain dipole tensor associated with each defect, from a microscopic model.     

Dynamical phase transition of a driven vortex lattice with disordered pinning

We have numerically solved the overdamped equation of vortex motion in a two-dimensional driven vortex lattice with disordered pinning, in which the driving Lorentz force, the pinning force due to point defects, the intervortex interacting force, and the thermal fluctuation force are taken into account. It is found that the vortex density and pinning strength are two important factors of affecting the melting transition of a vortex lattice. At low magnetic fields, there exist hysteresis loops of the average vortex velocity and the average pinning force vs. the driving force, from which the feature of a first-order melting transition of the vortex motion can be clearly seen. As the magnetic field is increased beyond a critical value, the hysteresis loops disappear and the melting transition is replaced by a second-order glass transition. We have also studied the influence of intervortex interactions on the vortex melting transition by comparing several forms of repulsive forces between the vortices.     

Phase transition in the three-state Potts antiferromagnet on the diced lattice

We prove that the 3-state Potts antiferromagnet on the diced lattice (dual of the kagome lattice) has entropically driven long-range order at low temperatures (including zero). We then present Monte Carlo simulations, using a cluster algorithm, of the 3-state and 4-state models. The 3-state model has a phase transition to the high-temperature disordered phase at v=e;{J}-1=-0.860 599+/-0.000 004 that appears to be in the universality class of the 3-state Potts ferromagnet. The 4-state model is disordered throughout the physical region, including at zero temperature.     

Effective spin model for the spin-liquid phase of the Hubbard model on the triangular lattice

We show that the spin-liquid phase of the half-filled Hubbard model on the triangular lattice can be described by a pure spin model. This is based on a high-order strong coupling expansion (up to order 12) using perturbative continuous unitary transformations. The resulting spin model is consistent with a transition from three-sublattice long-range magnetic order to an insulating spin-liquid phase, and with a jump of the double occupancy at the transition. Exact diagonalizations of both models show that the effective spin model is quantitatively accurate well into the spin-liquid phase, and a comparison with the Gutzwiller projected Fermi sea suggests a gapless spectrum and a spinon Fermi surface.