Specific heat and electrical resistivity of an icosahedral-structure Zr<Subscript>70</Subscript>Pd<Subscript>30</Subscript> alloy and of its amorphous and crystalline analogs

Binary icosahedral and crystalline phases of the Zr₇₀Pd₃₀ alloy were obtained in crystallization from the amorphous state during heat treatment. The specific heat and electrical resistivity of the icosahedral, amorphous, and crystalline phases were measured and compared. An increase in the electronic density of states on the Fermi surface, lattice softening, and an increase in the electron-phonon coupling constant were observed to occur with decreasing structural order. Despite the high valence electron density in the icosahedral phase, where the electronic densities of states are twice those in the crystal, the electrical resistivity of the icosahedral phase is ～50 times as high. Superconductivity was observed for the first time in the icosahedral phase of a binary system of transition metal atoms, Zr₇₀Pd₃₀.     

Photoluminescence and photoinduced magnetic phase transition in an organic radical TTTA crystal studied by two-photon absorption

Photoluminescence as well as photoinduced diamagnetic to paramagnetic phase transition in a strongly correlated electron system, an organic radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) crystal, was investigated under two-photon excitation with different photon densities. Below the threshold photon density to drive the photoinduced phase transition (PIPT), the diamagnetic phase shows a broad luminescence band with a large Stokes shift, whose intensity obeys almost second power law of the excitation photon density. Above the threshold photon density, on the other hand, the diamagnetic to paramagnetic phase transition effectively takes place with a large conversion yield and a steep response instead of an occurrence of the photoluminescence, indicating that the phase transition is optically induced by two-photon absorption. As far as we know, this is the first observation of the PIPT phenomena mediated by two-photon absorption.     

Density functional study of α–β phase transition of polyvinylidene difluoride

We perform density functional calculations to investigate structural and dynamical properties of crystalline polyvinylidene difluoride (PVDF) associated with the transition from α to β phase. We examine the change of the conformational energy and the corresponding structure of each phase depending on the lattice parameters of the orthorhombic crystalline structure. From this information, we construct the path that connects the point where the α phase is most stable to the point where the β phase is most stable, and identify the sub‐ region in the lattice parameter space where α and β phases have the same energy. In this sub‐region, we locate the point which gives the lowest conformation energy for both α and β phases, and examine the behaviour of the lowest energy profile and corresponding change of intermediate structures as the conformation of the PVDF chain transforms from α phase to β phase. Finally we perform ab‐initio molecular dynamics simulations and analyse the characteristic dynamics associated with transition from α to β phase. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Atomistic and lattice model of a grain boundary defaceting phase transition

Recent calculations have shown that grain boundary (GB) stress is too small to stabilize finite GB facets, suggesting that the existing theory of GB defaceting phase transitions is incomplete. We perform molecular dynamics calculations, which show a reversible phase transition at approximately 400 K with a concerted shuffle of two atoms at the facet junction as the elementary excitation. Based on this excitation we formulate an appropriate lattice model, perform Monte Carlo simulations, and establish an analytical relationship between the elementary excitation energy and the transition temperature.     

Optical properties in nonequilibrium phase transitions

An open question about the dynamical behavior of materials is how phase transition occurs in highly nonequilibrium systems. One important class of study is the excitation of a solid by an ultrafast, intense laser. The preferential heating of electrons by the laser field gives rise to initial states dominated by hot electrons in a cold lattice. Using a femtosecond laser pump-probe approach, we have followed the temporal evolution of the optical properties of such a system. The results show interesting correlation to nonthermal melting and lattice disordering processes. They also reveal a liquid-plasma transition when the lattice energy density reaches a critical value.     

Supersolid versus phase separation in atomic bose-fermi mixtures

We show that a two-dimensional atomic mixture of bosons and fermions cooled into their quantum degenerate states and subject to an optical lattice develops a supersolid phase characterized by the simultaneous presence of a nontrivial crystalline order and phase order. This transition is in competition with phase separation. We determine the phase diagram of the system and propose an experiment allowing for the observation of the supersolid phase.     

Ultracold atoms in a disordered crystal of light: towards a bose glass

We use a bichromatic optical lattice to experimentally realize a disordered system of ultracold strongly interacting 87Rb bosons. In the absence of disorder, the atoms are pinned by repulsive interactions in the sites of an ideal optical crystal, forming one-dimensional Mott-insulator states. We measure the excitation spectrum of the system as a function of disorder strength and characterize its phase-coherence properties with a time-of-flight technique. Increasing disorder, we observe a broadening of the Mott-insulator resonances and the transition to a state with vanishing long-range phase coherence and a flat density of excitations, which suggest the formation of a Bose-glass phase.     

Nuclear spin ferromagnetic phase transition in an interacting two dimensional electron gas

Electrons in a two-dimensional semiconducting heterostructure interact with nuclear spins via the hyperfine interaction. Using a a Kondo lattice formulation of the electron-nuclear-spin interaction, we show that the nuclear-spin system within an interacting two-dimensional electron gas undergoes a ferromagnetic phase transition at finite temperatures. We find that electron-electron interactions and non-Fermi liquid behavior substantially enhance the nuclear-spin Curie temperature into the mK range with decreasing electron density.     

Role of lateral interaction in the homoepitaxy of GaAs on the (001)-β(2 × 4) surface

The homoepitaxy of GaAs on the (001)-β(2 × 4) surface during molecular beam epitaxy is considered as a twodimensional first-order phase transition from the lattice gas of adsorbed growth components to the two-dimensional crystalline phase. In the context of the mean field theory of phase transitions, the parameters of lateral interaction between filled cells of the lattice gas are determined. The causes for the completion of the growth of a particular monolayer (self-ordering) before the beginning of growth of a new monolayer are clarified. The oscillations observed in the reflection high-energy electron diffraction experiments support the conclusions of the suggested theory of the phase transition for homoepitaxy.     

Structural instabilities of a two-dimensional Wigner crystal in a lateral superlattice

We consider the instability of a two-dimensional Wigner crystal in a short-period lateral superlattice. To find instabilities, we calculate the phonon spectrum of the electron lattice deformed by a periodic potential. We show that one of the transverse modes of the deformed electron crystal becomes soft when the electron density exceeds a critical value. This can result in a phase transition with the formation of a charge-density wave.     

Squeezing superfluid from a stone: coupling superfluidity and elasticity in a supersolid

Starting from the assumption that the normal solid to supersolid (NS-SS) phase transition is continuous, we develop a phenomenological Landau theory of the transition in which superfluidity is coupled to the elasticity of the crystalline lattice. We find that the elasticity does not affect the universal properties of the superfluid transition, so that in an unstressed crystal the well-known anomaly in the heat capacity of the superfluid transition should also appear at the NS-SS transition. We also find that the onset of supersolidity leads to anomalies in the elastic moduli and thermal expansion coefficients near the transition and, conversely, that inhomogeneous lattice strains can induce local variations of the superfluid transition temperature, leading to a broadened transition.     

Absence of an abrupt phase change from polycrystalline to amorphous in silicon with deposition temperature.

Using fluctuation electron microscopy, we have observed an increase in the mesoscopic spatial fluctuations in the diffracted intensity from vapor-deposited silicon thin films as a function of substrate temperature from the amorphous to polycrystalline regimes. We interpret this increase as an increase in paracrystalline medium-range order in the sample. A paracrystal consists of topologically crystalline grains in a disordered matrix; in this model the increase in ordering is caused by an increase in the grain size or density. Our observations are counter to the previous belief that the amorphous to polycrystalline transition is a discontinuous disorder-order phase transition.     

Magneto-elastic phase transition in a linear chain within the double and super-exchange model

In this work a magneto-elastic phase transition in a linear chain was obtained due the interplay between magnetism and lattice distortion in a double and super-exchange model. We consider a linear chain consisting of classical localized spins interacting with itinerant electrons. Due to the double exchange interaction, localized spins align ferromagnetically. This ferromagnetic tendency is expected to be frustrated by the anti-ferromagnetic super-exchange interaction between neighbor localized spins. Additionally, the lattice parameter is allowed to have small changes, which contributes harmonically to the energy of the system. The phase diagram is obtained as a function of the electron density and the super-exchange interaction using a Monte Carlo minimization. At low super-exchange interaction energy phase transition between electron-full ferromagnetic distorted and electron-empty anti-ferromagnetic undistorted phases occurs. In this case all electrons and lattice distortions were found within the ferromagnetic domain. For high super-exchange interaction energy, phase transition between two site distorted periodic arrangement of independent magnetic polarons ordered anti-ferromagnetically and the electron-empty anti-ferromagnetic undistorted phase was found. For this high interaction energy, Wigner crystallization, lattice distortion and charge distribution inside two-site polarons were obtained.     

Excitation energy as a basic variable to control nuclear disassembly

Thermodynamical features of Xe system is investigated as functions of temperature and freeze-out density in the frame of lattice gas model. The calculation shows different temperature dependence of physical observables at different freeze-out density. In this case, the critical temperature when the phase transition takes place depends on the freeze-out density. However, a unique critical excitation energy %and the same excitation reveals regardless of freeze-out density when the excitation energy is used as a variable instead of temperature. Moreover, the different behavior of other physical observables with temperature due to different ρ_f vanishes when excitation energy replaces temperature. It indicates that the excitation energy can be seen as a more basic quantity to control nuclear disassembly. Received: 25 November 1998 /Revised version: 20 January 1999     

Plasma annealing state of semiconductors; plasma condensation to a superconductivity- like state at 1000 K?

The high reflectivity, fluid “plasma annealing” phase of semiconductors, particularly Si, subjected to short, intense pulses of laser or electron or ion beam irradiation is known to exhibit a combination of properties for which no adequate explanation has previously been given. These include an almost crystalline Raman spectrum, a lattice temperature that is an extremely non-linear function of absorbed energy density, an optical band gap with no detectable free carrier absorption, a flat absorption spectrum above the gap. We propose a bose condensation of the carriers excited by the irradiation into a state having properties similar to those of a superconductor to explain these anomalies.     

Electric quadrupole interactions and the γ–α phase transition in Ce: the role of conduction electrons

We present a theoretical model of the “isostructural" - phase transition in Ce which is based on quadrupolar interactions due to coupled charge density fluctuations of 4f electrons and of conduction electrons. The latter are treated in tight-binding approximation. The - transition is described as an orientational ordering of quadrupolar electronic densities in a structure. The quadrupolar order of the conduction electron densities is complementary to the quadrupolar order of 4f electron densities. The inclusion of conduction electrons leads to an increase of the lattice contraction at the - transition in comparison to the sole effect of 4f electrons. We calculate the Bragg scattering law and suggest synchrotron radiation experiments in order to check the structure. Received 21 September 1999 and Received in final form 2 May 2000     

Coupling-matrix approach to the Chern number calculation in disordered systems

The Chern number is often used to distinguish different topological phases of matter in two-dimensional electron systems. A fast and efficient coupling-matrix method is designed to calculate the Chern number in finite crystalline and disordered systems. To show its effectiveness, we apply the approach to the Haldane model and the lattice Hofstadter model, and obtain the correct quantized Chern numbers. The disorder-induced topological phase transition is well reproduced, when the disorder strength is increased beyond the critical value. We expect the method to be widely applicable to the study of topological quantum numbers.     

Changes of the electron dynamics in hydrogen inductively coupled plasma

Changes of the electron dynamics in hydrogen （H2） radio-frequency （RF） inductively coupled plasmas are investigated using a hairpin probe and an intensified charged coupled device （ICCD）. The electron density, plasma emission intensity, and input current （voltage） are measured during the E to H mode transitions at different pressures. It is found that the electron density, plasma emission intensity, and input current jump up discontinuously, and the input voltage jumps down at the E to H mode transition points. And the threshold power of the E to H mode transition decreases with the increase of the pressure. Moreover, space and phase resolved optical emission spectroscopic measurements reveal that, in the E mode, the RF dynamics is characterized by one dominant excitation per RF cycle, while in the H mode, there are two excitation maxima within one cycle.     

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.