Anisotropic random networks of semiflexible polymers

Motivated by the organization of cross-linked cytoskeletal biopolymers, we present a semimicroscopic replica field theory for the formation of anisotropic random networks of semiflexible polymers. The networks are formed by introducing random permanent cross-links which fix the orientations of the corresponding polymer segments to align with one another. Upon increasing the cross-link density, we obtain a continuous gelation transition from a fluid phase to a gel where a finite fraction of the system gets localized at random positions. For sufficiently stiff polymers, this positional localization is accompanied by a continuous isotropic-to-nematic (IN) transition occurring at the same cross-link density. As the polymer stiffness decreases, the IN transition becomes first order, shifts to a higher cross-link density, and is preceded by an amorphous solid where the average polymer orientations freeze in random directions.     

Induced moment antiferromagnet in an external magnetic field—II: Green's function theory

A two level two sublattice antiferromagnetic induced moment system is examined in the Green's function theory in the random phase approximation. Attentions are drawn to the effects of an externally applied magnetic field. It is shown that a “soft mode” behavior occurs at the transition field similar to that observed at the transition temperature in the absence of a field. The transition becomes first order in the RPA. The sublattice magnetizations and the transition field are significantly lowered compared to the molecular field theory results.     

Photoinduced phase transitions in a Peierls system

A theory is constructed that explains photoinduced phase transitions in a Peierls system being irradiated by light with a finite width of the optical spectrum and a central frequency close to the upper van Hove singularity of the first kind in the combined density of electron states. The electron spectrum and the matrix elements of the dipole-moment operator are calculated by Bogolyubov’s method of canonical transformations. The interaction with the light is described by the Liouville equation for the density matrix of the electron subsystem in the dipole approximation. The light field is considered a quasimonochromatic time-independent random process with a Lorentzian spectrum. The derived equations are analyzed for two limits: (1) when the width of the optical spectrum tends to zero (a monochromatic light field), and (2) when the width of the optical spectrum is close to the upper limit (a bifurcation point) at which a photoinduced phase transition can still be observed. An existence criterion for such a transition is obtained, and the main parameters of the transition (the critical points and the size of the hysteresis loop) are calculated. The broadening of the optical spectrum of the incident light is shown to narrow the range of values of the central frequency of the light field and to reduce the size of the hysteresis loop. Finally, near the phase transition point, cavityless optical bistability sets in in the system, with light absorption increasing in the process. Zh. éksp. Teor. Fiz. 114, 1407–1420 (October 1998)     

Condensate density and superfluid mass density of a dilute Bose-Einstein condensate near the condensation transition

We derive via diagrammatic perturbation theory the scaling behavior of the condensate and superfluid mass density of a dilute Bose gas just below the condensation temperature, T(c). Sufficiently below T(c) particle excitations are described by mean field (Bogoliubov). Near T(c), however, mean field fails, and the system undergoes a second order phase transition, rather than first order as predicted by Bogoliubov theory. Both condensation and superfluidity occur at the same T(c), and have similar scaling functions below T(c), but different finite size scaling at T(c) to leading order in the system size. A self-consistent two-loop calculation yields the condensate fraction critical exponent, 2beta approximately 0.66.     

The Electroweak Transition at Hot and Density Condition

The effective potential of the Weinbefg-Salam (WS) model is calculated at finite temperatureand in the chemical potential of top quarks according to the grand canonical ensembletemperature field theory. The electroweak transition is studied at hot and density conditions.It is indicated that the symmetry restoring transition can take place either by increasing thetemperature or chemical potential of top quarks, but at high temperature and in the lowchemical potential the transition is of the first order, and it becomes the second order at acold density condition. The phase diagrams are showed.     

Low-energy band structure very sensitive to the interlayer distance in Bernal-stacked tetralayer graphene

We have investigated Bernal-stacked tetralayer graphene as a function of interlayer distance and perpendicular electric field by using density functional theory calculations. The low-energy band structure was found to be very sensitive to the interlayer distance, undergoing a metal-insulator transition. It can be attributed to the nearest-layer coupling that is more sensitive to the interlayer distance than are the next-nearest-layer couplings. Under a perpendicular electric field above a critical field, six electric-field-induced Dirac cones with mass gaps predicted in tight-binding models were confirmed, however, our density functional theory calculations demonstrate a phase transition to a quantum valley Hall insulator, contrasting to the tight-binding model prediction of an ordinary insulator.     

The spin density waves in chromium detected by hyperfine field measurements

The magnetic hyperfine field of tantalum nuclei in a high purity chromium matrix has been measured using the Time Differential Perturbed Angular Correlation technique. The spectra show that the hyperfine field is proportional to the amplitude of the spin density wave of chromium and that the tantalum probe nuclei do not clamp the phase of the spin density wave. The incommensurate antiferromagnetic first order phase transition as well as the spin flip transition have been observed. The temperature dependence of the hyperfine field is shown to deviate from the temperature dependence of the maximum magnetization of the spin density wave.     

Phase transitions in a driven lattice gas in two planes

We report on a Monte Carlo study of ordering in a nonequilibrium system. The system is a lattice gas that comprises two equal, parallel square lattices with stochastic particle-conserving irreversible dynamics. The particles are driven along a principal direction under the competition of the heat bath and a large, constant external electric field. There is attraction only between particles on nearest-neighbor sites within the same lattice. Particles may jump from one plane to the other; therefore, density fluctuations have an extra mechanism to decay and build up. It helps to obtain the steady-state accurately. Spatial correlations decay with distance according to a power law at high enough temperature, as for the ordinary two-dimensional case. We find two kinds of nonequilibrium phase transitions. The first one has a critical point for half occupation of the lattice, and seems to be related to the anisotropic phase transition reported before for the plane. This transition becomes discontinuous for low enough density. The difference of density between the planes changes discontinuously for any density at a lower temperature. This seems to correspond to a phase transition that does not have a counterpart in equilibrium nor in the two-dimensional nonequilibrium case.     

CDW patterns of two-dimensional electrons in strong magnetic fields

The phase diagram of a two-dimensional electron system in a strong magnetic field is studied in the Hartree-Fock approximation. Special attention is paid to the consequences of the electron-hole symmetry in the system. When the ground Landau level is half-filled, the high-temperature gaseous phase undergoes a second-order transition to a square CDW with a period dependent on temperature. This transition preserves the electron-hole symmetry in the sense that the square CDW is self-dual in contrast to a triangular CDW where the dual CDW has a honeycomb pattern. If the density is slightly less than the half-filled case, the gaseous phase first undergoes a first-order transition to a triangular CDW, followed by another first-order transition to a square CDW. We discuss the role of quantum effects which are responsible for this unusual phase diagram.     

The field induced magnetization reorientation in weak ferromagnets—III: The case of non-equivalent single ion anisotropy

The phase transitions in a single-ion anisotropy type weak ferromagnet at 0°K are studied using a two sublattice model and molecular field theory. In an external field applied along the antiferromagnetic axis, there exist three stable phases, denoted as weak ferromagnetic (WF), spin-flop (SF), and metamagnetic (MM). The character of the WF to SF transition changes from first to second order as the angle between a sublattice uniaxial anisotropy axis and the antiferromagnetic axis is increased. The tricritical field for this phase transition is proportional to the one-half power of the uniaxial anisotropy field, when the latter is much smaller than the exchange field. The transitions to and from the MM phase are always of first order. The former can result from either a WF or a SF phase instability threshold being reached. The latter always results in a transition to the WF phase. All three phases can, under specified conditions, coexist for a range of applied field values. In this case, an instability of the WF phase always results in a first order transition to the SF phase.     

Thermodynamic signature of a two-dimensional metal-insulator transition

We present a study of the compressibility kappa of a two-dimensional hole system which exhibits a metal-insulator phase transition at zero magnetic field. It has been observed that dkappa/dp changes sign at the critical density for the metal-insulator transition. Measurements also indicate that the insulating phase is incompressible for all values of B. Finally, we show how the phase transition evolves as the magnetic field is varied and construct a phase diagram in the density-magnetic field plane for this system.     

Magnetic properties of charged spin-1 Bose gases with ferromagnetic coupling

The magnetic properties of a charged spin-1 Bose gas with ferromagnetic interactions are investigated within mean-field theory. It is shown that a competition between paramagnetism, diamagnetism and ferromagnetism exists in this system. It is shown that diamagnetism, being concerned with spontaneous magnetization, cannot exceed ferromagnetism in a very weak magnetic field. The critical value of reduced ferromagnetic coupling of the paramagnetic phase to ferromagnetic phase transition [Formula: see text] increases with increasing temperature. The Landé-factor g is introduced to describe the strength of the paramagnetic effect which comes from the spin degree of freedom. The magnetization density [Formula: see text] increases monotonically with g for fixed reduced ferromagnetic coupling [Formula: see text] as [Formula: see text]. In a weak magnetic field, ferromagnetism makes an immense contribution to the magnetization density. On the other hand, at a high magnetic field, the diamagnetism tends to saturate. Evidence for condensation can be seen in the magnetization density at a weak magnetic field.     

外壳层最近邻交换相互作用对Blume-Capel模型相变行为的影响

利用有效场理论研究了纳米管上双模随机晶场中混合自旋Blume-Capel模型的相变行为。结果表明,系统相变行为与取值概率、晶场强度比值、晶场参数、温度以及外壳层最近邻交换相互作用密切相关。取值概率、晶场强度比值、晶场参数和外壳层最近邻交换相互作用等诸多因素相互竞争,影响系统的一级和二级相变以及临界点。     

Quantum charge density fluctuations and the phase transition in Ce

We have calculated the quantum quadrupolar interaction due to charge density fluctuations of localized 4f-electrons in Ce by taking into account the angular dependence, the degeneracy of the localized 4f -orbitals and the spin-orbit coupling. The calculated crystal field of 4 f electronic states is in good agreement with neutron diffraction measurements. We show that orientational ordering of quantum quadrupoles drives a phase transition at K which we assign with the transformation. In the phase the centers of mass of the Ce atoms still form a face centered cubic lattice. The theory accounts for the first order character of the transition and for the cubic lattice contraction which accompanies the transition. The transition temperature increases linearly with pressure. Our approach does not involve Kondo spin fluctuations as the significant process for the phase transition. Received 19 October 1998     

Phase transition and critical properties of spin-orbital interacting systems

Phase transition and critical properties of Ising-like spin-orbital interacting systems in 2-dimensional triangular lattice are investigated. We first show that the ground state of the system is a composite spin-orbital ferro-ordered phase. Though Landau effective field theory predicts the second-order phase transition of the composite spin-orbital order, however, the critical exponents obtained by the renormalization group approach demonstrate that the spin-orbital order-disorder transition is far from the second-order, rather, it is more close to the first-order. The unusual critical behavior near the transition point is attributed to the fractionalization of the composite order parameter.     

Bose glass and superfluid phase transitions of exciton-polaritons in GaN microcavities

Microcavity exciton-polaritons within GaN-based structures are the object of the present work. The impact of the structural imperfections on the properties of the two-dimensional polariton gas is investigated through the calculation of its phase diagram. We demonstrate that the presence of disorder first induces a quasi-phase transition of the polariton system towards a Bose-glass phase before it becomes superfluid as its density increases. Calculations of the density of states as well as the condensate wavefunction and the related spectrum of elementary excitations in the framework of the Gross-Pitaevskii theory provide further insight into the properties of exciton-polaritons in GaN-based microcavities.     

Critical and tricritical behavior of a selectively diluted triangular Ising antiferromagnet in a field

We study a geometrically frustrated triangular Ising antiferromagnet in an external magnetic field which is selectively diluted with nonmagnetic impurities employing an effective-field theory with correlations and Monte Carlo simulations. We focus on the frustration-relieving effects of such a selective dilution on the phase diagram and find that it can lead to rather intricate phase diagrams in the dilution-field parameters space. In particular, in a highly (weakly) diluted system the frustration is greatly (little) relieved and such a system is found to display only the second(first)-order phase transitions at any field. On the other hand, for a wide interval of intermediate dilution values the transition remains second-order at low fields but it changes to first-order at higher fields and the system displays a tricritical behavior. The existence of the first-order transition in the region of intermediate dilution and high fields is verified by Monte Carlo simulations.     

Notes on Black Hole Phase Transitions

In these notes we present a summary of existing ideas about phase transitions of black hole spacetimes in semiclassical gravity and offer some thoughts on three possible scenarios or mechanisms by which these transitions could take place. We begin with a review of the thermodynamics of a black hole system and emphasize that the phase transition is driven by the large entropy of the black hole horizon. Our first theme is illustrated by a quantum atomic black hole system, generalizing to finite-temperature a model originally offered by Bekenstein. In this equilibrium atomic model, the black hole phase transition is realized as the abrupt excitation of a high energy state, suggesting analogies with the study of two-level atoms. Our second theme argues that the black hole system shares similarities with the defect-mediated Kosterlitz–Thouless transition in condensed matter. These similarities suggest that the black hole phase transition may be more fully understood by focusing upon the dynamics of black holes and white holes, the spacetime analogy of vortex and antivortex topological defects. Finally, we compare the black hole phase transition to another transition driven by an (exponentially) increasing density of states, the Hagedorn transition first found in hadron physics in the context of dual models or the old string theory. In modern string theory the Hagedorn transition is linked by the Maldacena conjecture to the Hawking–Page black hole phase transition in Anti-de Sitter (AdS) space, as observed by Witten. Thus, the dynamics of the Hagedorn transition may yield insight into the dynamics of the black hole phase transition. We argue that characteristics of the Hagedorn transition are already contained within the dynamics of classical string systems. Our third theme points to carrying out a full nonperturbative and nonequilibrium analysis of the large N behavior of classical SU(N) gauge theories to understand its Hagadorn transition. By invoking the Maldacena conjecture we can then gain valuable insight into black hole phase transitions in AdS space.     

双模随机交错晶场对spin-1和spin-1/2混合自旋纳米管中Blume-Capel模型磁化强度的影响

利用有效场理论研究了纳米管上双模随机交错晶场中混合自旋Blume-Capel模型格点的平均磁化强度,得到了系统格点的平均磁化强度与双模随机晶场的取值概率、外磁场、晶场参数和晶场强度比值的关系.结果表明:取值概率、外磁场、交换相互作用、晶场强度比值和晶场强度等诸多因素相互竞争,使系统表现出比恒定晶场作用的Blume-Capel模型更为丰富的磁化现象;双模随机交错晶场会抑制系统的平均磁化强度,使其基态饱和值小于5/6;外磁场导致系统的二级相变消失;一定条件下系统发生一级相变;系统的平均磁化强度呈现部分缺失和负值现象.