Temperature dependence of the far infrared reflectivity spectra of a KD2PO4

Complete data on far infrared reflectivity spectra of a KD₂PO₄ single crystal in the temperature range 7–300 K, from the millimeter wave up to 400 cm^–1, for all crystallographic orientations of the crystal, in the ferro- and paraelectric phase, are given for the first time. The temperature dependence of the phonon spectra is similar to that we have observed and published for KH₂PO₄.     

Directional dependence of the phonon modes in biaxial sodium nitrite

The directional dependence of the phonon mode frequencies is calculated in the three principal planes of biaxial sodium nitrite at 7 and 300 K, using the lattice mode parameters determined from the simulation of the infrared reflectivity spectra polarised along the three cristallographic axes. Good agreement is obtained between the calculated and experimental infrared reflectivity spectra for different orientations of the wave vector in the a-c plane of sodium nitrite.     

Far infrared reflectivity spectra of a KH2PO4 single crystal

Conclusion The experimental results presented in this paper show that two heavily damped modes, due to proton tunneling, are active in the two phases of KDP: the well-known soft mode which attains a hard-core frequency at the transition temperature and a low frequency mode polarized in a direction perpendicular to the c axis. These modes are responsible for the temperature variation of the low frequency dielectric permittivity in the two crystallographic directions. Due to the size of the samples we do not obtain a single domain crystal, and it was impossible to distinguish the B₁ and B₂ modes. The 200–300 cm^–1 spectral range is very sensitive to the orientation of the incident wave, and certainly corresponds to the hydrogen bonds vibrations.     

Spectroscopic studies of optical materials in the far infrared: The case of crystal quartz

Spectroscopic studies using far infrared reflectivity and transmissivity measurements between 10 and 600 cm⁻¹, were performed on plane parallel quartz crystal plates, in the temperature range 10–300 K. The temperature dependence of the optical mode parameters is reporeted for both polarizations parallel and perpendicular to the c-axis. The extraordinary optical constants of quartz are determined at 10 and 300 K, from the A₂ transmissivity channeled spectrum, between 10 and 300 cm⁻¹.     

Temperature dependence of the reflectance spectrum of the single crystal of tetrathiafulvalene-chloranil

Near-infrared and visible reflectance spectrum was measured on the tetrathiafulvalene-chloranil (TTF-CA) crystal at temperatures in the range of 40–290 K. An abrupt change of reflectivity was found at about 80 K, and new dispersions associated with the intramolecular transitions of TTF⁺ and CA^? gradually grew up on lowering temperature below 80 K, indicating the increase of the ionized fraction.     

First-principles study of the high pressure phase transition and lattice dynamics of cerium

We present a first-principles study of the phase transition and lattice dynamics of Ce within the framework of the density functional theory using the GGA+U method. Our calculated results denote that under pressure the transition path is α-Ce (fcc)→α″-Ce (monoclinic, with two atoms per unit cell)→bct-Ce (body centered tetragonal), and the transition pressures are located at 5.36 and 14.37 GPa, respectively. The equation of state in a wide range of pressure is consistent with the experimental data. During the γ-α phase transition, the magnetic moment disappears gradually, which is mainly due to the strong interaction between the 4f and 5d electrons. By calculating the free energies from phonon dispersions including electronic contribution, the obtained γ-α transition temperature at zero pressure is 148 K. From the Blackman diagram of dimensionless elastic constant ratios, we can find that both γ- and α-Ce show negative Cauchy pressure—C₄₄>C₁₂.     

单壁碳纳米管晶格振动模及红外吸收光谱的研究

碳纳米管晶格动力学的研究可以提供碳纳米管结构和螺旋性等方面的相关信息。在室温下测量了单壁碳纳米管的红外吸收光谱,首次在透射法模式下观察到了其晶格振动模的红外吸收峰,通过与晶格动力学理论计算值对照,做出了振动模的初步归属。讨论了单壁碳纳米管高频振动模与管径的关系,并与高取向热解石墨振动模进行了对比。结果表明,红外吸收光谱包含有碳纳米管的许多信息,是研究其晶格动力学的有效方法。     

The effect of molecular dynamics on basic parameters of cw-epr spectrum of pyridinium tetrafluoroborate single crystal

This paper presents an investigation of the phase transitions and molecular dynamics of radicals in pyridinium tetrafluoroborate single crystals by CW-EPR combined with the Computer Resolution Enhancement Method. We present the advantages following from the computer method for enhancement of resolution by the example of applying it in the analysis of complex CW-EPR spectra of organic radicals. The method enabled determination of basic parameters such as spectroscopic splitting factor, hyperfine coupling constants and widths of individual spectral lines in CW-EPR spectra of pyridinium tetrafluoroborate recorded in a wide range of temperatures. Phase transitions and changes in molecular dynamics of radicals may imply changes in their electronic structure, which may affect the above-mentioned parameters of CW-EPR spectra.     

Temperature dependence of the reflectance spectrum of tetrathiafulvalenium iodide (TTF)I0.71 single crystal

The reflectance spectrum of (TTF)I_0.71 exhibits a Drude-like plasma edge near 10 × 10³ cm^-1, and the reflectivity below this region increases as the temperature is lowered from room temperature to 210 K. However, the near-infrared reflectivity decreases with lowering temperature in the semiconductive phase below 210 K, and an additional peak attributable to the charge transfer between (TTF)⁺ ions grows up as the temperature is lowered. Discussion is given on these behaviors of (TTF)I_0.71.     

Density–driven superfluid transition of the constrained bosons on a lattice

We formulate the Landau theory and study its consequences for lattice bosons, which are stabilized by a three-body on-site constraint. This is relevant for experiments with cold atoms in optical lattices, where the presence of three-body dissipative loss processes renders the occupation number n for bosons on short timescales effectively constrained to $n\le 2$. We elaborate on the Landau free energy expansion resulting from the Hamiltonian of bosons in the restricted Hilbert space, and present the phase diagram of the constrained Bose lattice gas.     

Effects of the bond dilution on the phase diagrams of a spin-1 transverse Ising model with crystal field interaction on a honeycomb lattice

A bond diluted spin-1 Ising model with crystal and transverse field interactions is examined for honeycomb lattice by introducing an effective field approximation that takes into account the correlations between different spins that emerge when expanding the identities. The effects of the crystal field as well as the transverse field and dilution on the phase diagrams and order parameter m_z are discussed in detail. A number of interesting and unusual phenomena such as reentrant phenomena and three successive phase transitions originating from the crystal field as well as the transverse field and bond dilution have been found.     

Phase transition in a lattice gas of hard spheres with second-neighbor exclusions on the simple cubic lattice

Using reflection positivity and the Peierls argument, we prove the existence of an ordered phase at sufficiently high activity for a lattice gas of hard spheres on the simple cubic lattice with first- and second-neighbor exclusions.     

Ionic diffusion on a lattice: Effects of the order-disorder transition on the dynamics of non-equilibrium systems

We examine the behaviour of the concentration profiles of particles with repulsive interactions diffusing on a host lattice. At low temperature, the diffusion process is strongly influenced by the presence of ordered domains. We use mean field equations and Monte-Carlo simulations to describe the various effects which influence the kinetic behaviour. An effective diffusion coefficient is determined analytically and is compared with the simulations. Finite gradient effects on the ordered domains and on the diffusion are discussed. The kinetics studied is relevant for superionic conductors, for intercalation and also for the diffusion of particles adsorbed on a substrate. Received: 26 June 1997 / Revised: 18 September 1997 / Accepted: 10 November 1997     

The temperature-dependent phase diagrams of the spin-3/2 Ising model on a FM/AFM two-layer lattice with a crystal field

The temperature-dependent phase diagrams of the spin-3/2 Ising model on a two-layer Bethe lattice with ferromagnetic (FM)/antiferromagnetic (AFM) intra-layer and either FM or AFM type inter-layer interactions are investigated under a constant magnetic field (H) and in the presence of a crystal field (D) by using exact recursion equations in a pairwise approach for coordination numbers q=3,4 and 6, in detail. In the light of the ground-state (GS) phase diagrams, the temperature-dependent phase diagrams of the model are obtained by studying the thermal variations of the order parameters, response functions and free energy. Then, they are illustrated on the (kT/J₁,J₃/J₁) and (kT/J₁,J₂/J₁) planes for the given system parameters. It is observed that the system exhibits first- and second-order phase transitions for all q values, and hence, in some cases, tricritical points. The existence of critical-end points and that of isolated points are also observed. The re-entrant behavior owes its presence to the two Néel temperatures, T_N, that are present for all q.     

Effects of oxygen deficiency on the acoustic anomalies and phase transition behaviors of barium titanate single crystals

The acoustic behaviors of oxygen-reduced barium titanate (BaTiO_3-δ) single crystals with δ∼0.04 were investigated as a function of temperature by using Brillouin spectroscopy. The longitudinal acoustic mode of the moderately-reduced BaTiO₃ (BTO) showed two pronounced anomalies at approximately 112 °C and −11 °C, which correspond to the cubic-tetragonal and tetragonal-orthorhombic phase transition temperature, respectively. These temperatures were lower by more than 10 °C compared to those of the pure BaTiO₃ suggesting that the disorder introduced by oxygen vacancies lowers the phase transition temperatures. The paraelectric phase of the reduced BaTiO₃ were characterized by substantial softening of the longitudinal acoustic mode and the growth of central peaks centered at zero frequency. These anomalies were observed in a certain temperature range above the Curie temperature, indicating that pretransitional precursor polar clusters exist in the cubic phase and that their dynamics are responsible for the acoustic anomalies caused by electrostrictive coupling between the strain and the polarization. The relaxation time of the precursor polar clusters derived from the central peak exhibited a critical slowing-down behavior showing that their dynamics becomes more sluggish as temperature approaches the Curie point.     

Ground state phase diagram of a spin-2 antiferromagnet on the square lattice

We use the vertex state model approach to construct optimum ground states for a large class of quantum spin-2 antiferromagnets on the square lattice. Optimum ground states are exact ground states of the model which minimize all local interaction operators. The ground state contains two continuous parameters and exhibits a second order phase transition from a disordered phase with exponentially decaying correlation functions to a Néel ordered phase. The behaviour is very similar to that of the corresponding ground state of a quantum spin-3/2 model on the hexagonal lattice, which has been investigated in an earlier paper. Received 8 April 1999     

Effect of gamma rays irradiation on ferroelectric phase transition and domain defect interaction in lead meta niobate single crystal

The objective of our study is to identify the role of gamma rays to control the existing limit of ferroelectric nature of lead meta niobate (PbNb₂O₆) single crystals in terms of domain-defect interaction. The critical value of density of oxygen vacancy sites and average distance between two successive oxygen vacancies, which are responsible for ferroelectric nature in lead niobate single crystal have been calculated. The disappearance of Ferro electricity, which is associated with domain-defect interaction as well as an absence of obeying the Curie-Weiss law have been discussed in gamma rays irradiated lead meta niobate single crystal.     

Kinetics of a rough crystal face: Temperature dependence of the growth rate

We present measurements of the linear growth rate of the (110)-face of naphthalene growing from toluene solution at various equilibrium temperatures. Above a critical supersaturation the growth rate depends linear on the supersaturation and the (110)-face becomes round, which both indicates kinetic roughening. Within a temperature interval of 50° C the growth rate changes by nearly two orders of magnitude. The slope of the linear part of the growth curve can be described by an activated process, the activation energy turns out to be of the order of the evaporation enthalpy of naphthalene.