Thermally enhanced optical nonlinearity in nematic liquid crystal close to phase transition temperature

This study investigates the beam profile and the liquid crystal (LC) arrangement affected by an optical field on LC thin films at a temperature close to nematic-isotropic phase transition temperature (T_NI). A combined microscopic and conoscopic technique was used in experiments as a convenient way to analyze the optical nonlinearity that is associated with the molecular configuration of nematic liquid crystal (NLC). An optical field combined with thermal enhancement enhances molecular reorientation and causes additional molecular excitation along the axis of propagation of the beam. The reorientational nonlinearity yields an undulating structure with multi-foci; the length between each pair of foci increases with time, as described.     

Changes of the Mössbauer effect caused by the excitation of the solitons in the organic molecular crystals at finite temperatures

The influence of the soliton on the properties of the Mössbauer effect arising from a gamma-active nucleus situated at a lattice site in the organic molecular crystal, which being in contact with a thermostat at T≠0, has been theoretically studied. An expression for the gamma-radiated Mössbauer transition probability as a function of temperature has been obtained. The results show that the transition probability is different from that in the case when the molecular crystal state is adequately described exclusively by thermal phonon modes. Finally, we give some properties of the kind of effect by numerical calculation in such a case which are helpful to facilitate further the experimental confirment of the soliton existence in the molecular crystals by using the method of Mössbauer effect.     

Structural transition of sheared-liquid metal in quenching state

In this Letter, the effects of shear rate on structural properties of liquid Al in quenching process were investigated via molecular dynamics (MD) simulations based on the EAM potential. Analyses in internal energy and pair correlation functions (PCF) reveal an increasing structural transition temperature as the shear rate is enhanced in the liquid. Results of pair analysis indicate that for liquid Al under normal condition, face center cubic (FCC) structure is clearly detected upon cooling; while in sheared liquid, structural transition from FCC to body center cubic (BCC) at temperature of 800 K is manifested, leading to the dominance of BCC structural order at low temperatures.     

Molecular field theory for nematic liquid crystal film with finite layers

The nematic liquid crystal film composed of n molecular layers is studied based upon a spatially anisotropic pair potential, which reproduces approximately the elastic free energy density. On condition that the system has perfect nematic order, as in the Lebwohl—Lasher model, the director in the film is isotropic. The effect of the temperature is investigated by means of molecular field theory. Some new results are obtained. Firstly, symmetry breaking takes place when taking account of the temperature, and the state with the director along the normal of the film has the lowest free energy. Secondly, the N—I phase transition temperature increases as an effect of finite sizes instead of decreasing as in the Lebwohl—Lasher model. Thirdly, the nematic order is induced in the layers near the surface in the isotropic phase.     

Molecular dynamics study of structure and glass forming ability of Zr<Subscript>70</Subscript>Pd<Subscript>30</Subscript> alloy

In this study, the temperature effects on the structural evolution of theZr₇₀Pd₃₀ binary alloy in the glassy and liquid states werestudied using the molecular dynamics simulations based on the many-body type tight-bindingpotential. We considered the following properties in detail: the temperature dependence ofthe volume, the partial and total pair distribution functions and the simulated glasstransition temperature. The effects of the cooling rates on the glass transitiontemperature were examined. The Wendt-Abraham parameter was calculated to determine theglass transition temperature of Zr₇₀Pd₃₀ glassy alloy. The pair analysis technique ofHoneycutt-Andersen was applied to define local atomic arrangements produced from moleculardynamics simulations. The results show that the icosahedral ordering in glassy state hasbeen composed during quenching period, and the simulated glass transition temperature andthe total pair distribution functions are in good agreement with the experimental data.     

Piezo-optical study of excitons in KI

The effects of a uniaxial stress along 〈001〉 or 〈110〉 direction on the excitonic structures of KI are studied between 5.5 and 7.5 eV at room temperature by a piezo-modulation technique. Stress dichroism leads to the conclusion that either the second or the third peak corresponds to a transition at the Γ point in the Brillouin zone forming a spin-orbit pair with the first exciton and the other to a transition at the L point. The fourth peak is ascribed to a transition at the L point with an overlaping one at the X point.     

The origin of anomalous temperature dependences of intensities of spin-forbidden d-d transitions in octahedral complexes

It is shown that anomalous temperature dependences of intensities of spin-forbidden d-d transitions, observed in octahedral magnetically-ordering complexes in a paramagnetic phase and in particular in hexagonal RbMnCl₃, investigated by the authors, can be accounted for by a pair-vibrational mechanism, which represents two simultaneous events: breaking of selection rules for spin during pair absorption by exchange coupled ions and breaking of those for parity during an ordinary electronic transition in one of the ions of the pair due to excitation of odd-parity vibrations.     

On the order-disorder transition of chemisorbed H on Ni (111)

The order-disorder transition of chemisorbed H on Ni (111) has been analyzed by Kittler and Bennemann in terms of the lattice gas model. The main purpose of this paper is to refine the above calculation. The expression with regard to adlayer domains is given for the half-order LEED spot intensity, from which the dependence of the transition temperature and of the LEED spot intensity on H-coverages is calculated. In particular, the H-coverage dependence of the transition-temperature T_c and the LEED intensity I for θ? 0.25 is restudied. We present results obtained by using only pair interactions between the H-atoms and no 3H-interactions. It is shown that the observed asymmetric phase diagram with respect to H-coverages can result from the two kinds of pair interactions between chemisorbed H atoms.     

Ising models with two- and three-spin interactions: Mean field equation of state

The Ising model with pair and triplet interactions on the triangular lattice is solved in the mean-field approximation. With a sufficiently strong triplet interaction two first-order transitions take place at low temperature, and at intermediate temperatures one transition, terminating in a critical point. For J₂ > 0.75J₃ only the latter transition remains.     

Free energy of mixing of an Fe–Co liquid alloy taking into account nondiagonal <Emphasis Type="Italic">d−d</Emphasis>-electron coupling in the framework of the Willis−Harrison model

Within the framework of the Willis?Harrison model, the effect of taking into account d?d-electron couplings nondiagonal in the magnetic quantum number between the neighboring atoms in a transition metal on the partial pair potentials and the free energy of mixing of an Fe–Co liquid alloy near the melting temperature is investigated. It is found that an increase in the fraction of nondiagonal couplings results in a decrease in the depth of the first minimum of the partial pair potentials and in the displacement of its position towards larger r. It is shown that taking this factor into account considerably improves the agreement with experimental data of the concentration dependence of the free energy of mixing of the system under consideration.     

Thermodynamic properties of noble metal clusters: molecular dynamics simulation

The thermodynamics properties of noble metal clusters Au_N, Ag_N, Cu_N, and Pt_N (N = 80, 106, 140, 180, 216, 256, 312, 360, 408, 500, 628, 736, and 864) are simulated by micro-canonical molecular dynamics simulation technique. The potential energy and heat capacities change with temperature are obtained. The results reveal that the phase transition temperature of big noble metal clusters (N ? 312 for Au, 180 for Ag and Cu, and 360 for Pt) increases linearly with the atom number slowly and approaches gently to bulk crystals. This phenomenon indicates that clusters are intermediate between single atoms and molecules and bulk crystals. But for the small noble clusters, the phase transition temperature changes irregularly with the atom number due to surface effect. All noble metal clusters have negative heat capacity around the solid-liquid phase transition temperature, and hysteresis in the melting/freezing circle is derived in noble metal clusters.     

Mechanical, vibrational, and dynamical properties of amorphous systems near jamming

Amorphous systems undergo the jamming transition when the density increases, temperature drops, or external shear stress decreases, as described by the jamming phase diagram which was proposed to unify different processes such as the glass transition, random close packing, and yielding under shear stress. At zero temperature and shear stress, the jamming transition occurs at a critical density at Point J. In this paper, we review recent studies of the material properties of marginally jammed solids and the glassy dynamics in the vicinity of Point J. As the only singular point in the jamming phase diagram, Point J exhibits special criticality in both mechanical and vibrational quantities. Dynamics approaching the glass transition in the vicinity of Point J show critical scalings, suggesting that the molecular glass transition and the colloidal glass transition are equivalent in the hard sphere limit. All these studies shed light on the long-standing puzzles of the glass transition and unusual properties of amorphous solids.     

Mössbauer spectroscopy in molecular magnetism

The aim of this paper is to highlight some selected research activities on molecular magnetic materials using Mössbauer spectroscopy as a technique carried out in our laboratory in recent years. The first part of the present article is devoted to the studies of the various magnetic interactions, metal-to-metal electron-transfer phenomenon, glass transition occurring in molecular magnetic materials, whereas the second part deals with the iron(II) high spin (S = 2)–low spin (S = 0) transition phenomenon occurring in some isoxazole ligand based iron(II) compounds as examples with unusually complicated spin transition behaviour. Also, an example of a dinuclear a spin crossover compound of iron(II) is described, where Mössbauer spectroscopy has most convincingly unraveled the mechanism of the spin transition process. Finally, an example from our most recent studies of spin crossover materials exhibiting both thermal spin crossover and liquid crystalline properties in the same temperature interval near room temperature will be presented.     

Effect of disorder on the transport properties of the high-T c superconductor Nd2− x CexCuO4+δ

The temperature dependences ρ_ab(T) of Nd_2?x Ce_xCuO_4+δ single crystals with 0≤x≤0.20 are studied and analyzed on the basis of the concepts in the theory of disordered 2D systems. The results are compared with the data obtained for other copper-oxide HTSC. It is found that a transition to the superconducting state in the optimal doping region 0.14≤x≤0.18 occurs only in crystals with a fairly small degree of disorder (k _Fl≥2, where l is the mean free path). This transition is compatible with the weak 2D-localization mode as long as the localization radius is longer than the characteristic size of a Cooper pair. The superconducting transition temperature in the optimal doping region increases monotonically with the parameter k _Fl characterizing the degree of disorder in the crystal. The degradation of superconducting properties upon a further increase in the doping level (x>0.18) is apparently associated with a transition from 2D to 3D conductivity in the single crystal.     

Pressure dependence of the Kondo resistance anomaly and the pair breaking effect in La-Ce alloys

The pressure dependence of the parr breaking effect and of the resistance anomaly was measured in LaCe alloys. The results indicate that the maximum in the pressure dependent pair breaking effect is due to a monotonic shift of the Kondo temperatureT _k with pressure from valuesT _k > ?T _c0 toT _k ?T _c0, whereT _c0 is the superconducting transition temperature of pure lanthanum.     

On the bethe approximation for the random bond Ising model

A random bond Ising model is considered in terms of the pair approximation, which is equivalent to the Bethe approximation, of the cluster variation method. On taking the configurational average over the random distribution of bonds ±J, we take into account the nearest neighbor correlations between effective fields and bonds. We investigate their effects to the phase transition temperature from the paramagnetic phase to the ferro- (or antiferro-) magnetic phase and to the spin glass phase for the Ising model on the square lattice. It turns out that the correlation effects act favorably to the spin glass phase and bend upward the line of transition temperature from the paramagnetic phase to the spin glass phase as the concentration being apart from 0.5. In the appendix, we derive the expression of free energy in the weak interaction limit.     

Superconducting plate in a transverse magnetic field: New state

A model is proposed for describing Cooper pairs near the transition (in temperature and magnetic field) point when their spacing is larger than their size. The essence of the model is as follows: the Ginzburg-Landau functional is written in operator form in terms of field operators of the Bose type so that the average value of the density operator gives the concentration of Cooper pairs, and the same Ginzburg-Landau expression is obtained for the Bose condensate. The model is applied to a superconducting plate with a thickness smaller than the size of a pair in a transverse magnetic field near its upper critical value H _c2. A new state is discovered that is energetically more advantageous in a certain interval in the vicinity of the transition point as compared to the Abrikosov vortex state. The wavefunction of the system in this state is of the type of the Laughlin function used in the fractional quantum Hall effect (naturally, as applied to Cooper pairs as Bose particles in our case) and corresponds to a homogeneous incompressible fluid. The energy of this state is proportional to the first power of quantity (1 ? H/H _c2) in contrast to the energy of the vortex state containing the square of this quantity. The interval of the existence of the new state is the larger, the dirtier the sample.     

Temperature dependent energy bands and the metal-nonmetal transition in NiS

The LCMTO (linear combination of muffin-tin orbitals) technique has been used to calculate the first temperature dependent energy bands for a transtion metal compound. The particular compound studied was hexagonal NiS which undergoes a first-order metal-to-nonmetal transition as the temperature is lowered below 264K. We find the bands to agree well with XPS data and that the Sp bands overlap the bottom of the d bands as predicted by White and Mott. This p-d overlap, which reduces the correlation energy, is found to increase with temperature as a result of lattice vibrations whereas the lattice distortion accompanying the transition has very little effect on the energy bands. This suggests that the Mott-Hubbard transition in NiS is driven by the Debye-Waller modification of the potential and not the lattice distortion as suggested by White and Mott.     

Infinite range Ising spin glass model with a transverse field

The infinite range Ising spin glass model with a transverse field is studied by the pair approximation extended to include quantum effects. A mean field equation is obtained and the transition temperature T_c is shown to increase linearly with the transverse field Γ when Γ is small.     

Influence of Quantal and Statistical Fluctuations on Phase Transitions in Finite Nuclei

Investigations on thermal evolution of pairing-phase transition and shape-phase transition in light nuclei are made as a function of pair gap, deformation, temperature and angular momentum using a finite temperature statistical approach with main emphasis to fluctuations. The occurrence of a peak structure in the specific heat predicted as signals of the pairing-phase and shape-phase transitions are reviewed and it is found that they are not actually true phase
transitions and it is only an artifact of the mean field models. Since quantal number and spin fluctuations and statistical fluctuations in pair gap, deformation degrees of freedom and energy when incorporated, it wash out the pairing-phase transition and smooth out the shape-phase transition. Phase transitions due to collapse of pair gap and deformation is discussed and a clear picture of pairing-phase transition in light nuclei is presented in which pairing transition is reconciled.