Phase Specific Heat Research Articles

Structure, charge ordering and physical properties ofLuFe2O4

Microstructural properties, phase transitions, and charge ordering of $\mathrm{Lu}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ have been extensively investigated by means of transmission electron microscopy (TEM) in a large temperature range from $20\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}550\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The experimental results demonstrate that the $\mathrm{Lu}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ crystal is commonly modulated by charge ordering (CO), which is often recognizable by superstructure reflections. The (001) twinning domains as a common defect often appear in the $\mathrm{Lu}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ crystals along the $c$-axis direction, with the crystals across each (001) boundary rotated by 180\ifmmode^\circ\else\textdegree\fi{} with respect to one another. The in situ cooling TEM observations from $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ down to $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ reveal remarkable alternations of the superstructures, suggesting a complex CO process in the present system. Careful analysis shows that the CO in the frustrated ground state is characterized by a modulation with a wave vector of ${\mathbf{q}}_{1}=(1∕3\phantom{\rule{0.3em}{0ex}}1∕3\phantom{\rule{0.3em}{0ex}}2)$. In situ heating TEM observations from $300\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}550\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ clearly reveal that the CO modulation in $\mathrm{Lu}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ becomes invisible above a critical temperature of about ${T}_{C}=530\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. These facts suggest that the CO should be the essential driving force for the structural transitions and ferroelectricity observed in this kind of layered material. Experimental measurements on the ferroelectricity show that the $\mathrm{Lu}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ material, in general, has a large dielectric constant of about 10 000 at room temperature. In order to understand the properties of low-temperature phase transitions, the magnetization and specific heat from $300\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ have been briefly discussed.

Phase diagram, magnetic properties and specific heat of Mn 1− xFe xPS 3

We have investigated the magnetism and the specific heat of transition metal phosphorous trisulfide Mn 1− x Fe x PS 3 and determined the magnetic phase diagram. A spin glass state appears in the range 0.5 ≤ x ≤ 0.6 . The remarkable feature of this spin glass phase is that the spin freezing is not suppressed up to a rather high magnetic field. Reentrant spin glass states appear in the ranges 0.3 ≤ x ≤ 0.45 and 0.6 < x ≤ 0.7; antiferrmomagnetic ordered states appear in the ranges 0.0 ≤ x ≤ 0.2 and 0.8 ≤ x ≤ 1.0 . The temperature dependence of the magnetization of the antiferromagnets except x = 0.0 and 1.0 does not approach zero at 0 K. The molar specific heat of Mn 1− x Fe x PS 3 below 4 K is larger than that of x = 0.0 and 1.0. These results strongly suggest that the spin fluctuation exists in Mn 1− x Fe x PS 3 even in the antiferromagnetic ordered state.

A Study on the Cooling Characteristics of TMA Clathrate Compound with Additives

This study aims to find out cooling characteristics of TMA 25wt%-water clathrate compound with ethanol such as supercooling, phase change temperature and specific heat. For this purpose, ethanol is added as per weight concentration and cooling experiment is performed at -6, -7 and -8, cooling heat source temperature, and it leads the following result. (1) Phase change temperature is decreased due to freezing point depression phenomenon. Especially, it is minimized as 5.1 and 5.0, 3.8 according to cooling source temperature in case that 0.5wt% of ethanol is added. (2) If 0.5wt% of ethanol is added, average supercooling degree is 0.9 and minimum supercooling is 0.8, 0.7 according to cooling heat source temperature. The restraint effect of supercooling is shown. (3) Specific heat shows tendency to decrease if ethanol is added. It is 3.013~3.048 kJ/kgK according to cooling heat source temperature if 0.5wt% of ethanol is added. Phase change temperature higher than that of water and inhibitory effect against supercooling can be confirmed through experimental study on cooling characteristics of TMA 25wt%-water clathrate compound by adding additive, ethanol.

Applying BCS–BEC crossover theory to high-temperature superconductors and ultracold atomic Fermi gases (Review Article)

This review is written at the time of the twentieth anniversary of the discovery of high-temperature superconductors, which nearly coincides with the important discovery of the superfluid phases of ultracold trapped fermionic atoms. We show how these two subjects have much in common. Both have been addressed from the perspective of the BCS—Bose–Einstein condensation (BEC) crossover scenario, which is designed to treat short coherence length superfluids with transition temperatures which are “high” with respect to the Fermi energy. A generalized mean field treatment of BCS–BEC crossover at general temperatures T, based on the BCS–Leggett ground state, has met with remarkable success in the fermionic atomic systems. Here we summarize this success in the context of four different cold atom experiments, all of which provide indications, direct or indirect, for the existence of a pseudogap. This scenario also provides a physical picture of the pseudogap phase in the underdoped cuprates which is a central focus of high Tc research. We summarize successful applications of BCS–BEC crossover to key experiments in high Tc systems, including the phase diagram, specific heat, and vortex core STM data, along with the Nernst effect, and exciting recent data on the superfluid density in very underdoped samples.

Bose–Einstein Condensation of Magnons in TlCuCl3: Phase Diagram and Specific Heat from a Self-consistent Hartree–Fock Calculation with a Realistic Dispersion Relation

We extend the self-consistent Hartree–Fock–Popov calculations by Nikuni et al. [Phys. Rev. Lett. 84 (2000) 5868] concerning the Bose–Eistein condensation of magnons in TlCuCl 3 to include a realistic dispersion of the excitations. The result for the critical field as a function of temperature behaves as H c ( T )- H c (0)∼ T 3/2 below 2 K but deviates from this simple power-law at higher temperature and is in very good agreement with the experimental results. The specific heat is computed as a function of temperature for different values of the magnetic field. It shows a λ-like shape at the transition and is in good qualitative agreement with the results of Oosawa et al. [Phys. Rev. B 63 (2001) 134416].

Effects of the applied transverse field on biaxial spin system

Magnetic properties of a biaxial spin-1 system in an applied transverse field are investigated by using the effective-field theory with correlations on the basis of the differential technique. We have studied the effects of the transverse field on the phase diagrams, internal energy, and specific heat on the honeycomb lattice.

Effects of Biaxial Crystal-Field on Magnetic System**The project supported by Science and Technology Key Foundation of the Ministry of Education of China under Grant No. 99026

The spin-1 Ising model with biaxial crystal-field on the honeycomb lattice is investigated by using the effective-field theory with self-spin correlations and the differential operator technique. The effects of biaxial crystal-field described by two uniaxial anisotropy parameters Dx and Dy on the phase transition and specific heat and susceptibility are studied numerically. Some interesting results are observed in the system such as tricritical behavior depending sensitively on the strength of the biaxial crystal-field.

Thermodynamic properties of thin films of superfluid3He−A

The pairing correlations in superfluid He-3 are strongly modified by quasiparticle scattering off a surface or an interface. We present theoretical results and predictions for the order parameter, the quasiparticle excitation spectrum and the free energy for thin films of superfluid He-3. Both specular and diffuse scattering by a substrate are considered, while the free surface is assumed to be a perfectly reflecting specular boundary. The results are based on self-consistent calculations of the order parameter and quasiparticle excitation spectrum at zero pressure. We obtain new results for the phase diagram, free energy, entropy and specific heat of thin films of superfluid He-3.

Effects of biaxial crystal-field on spin-3/2 honeycomb lattice

The method of the effective-field theory with self-spin correlations and the differential operator technique is used to study the magnetic properties of the spin-3/2 Ising model with biaxial crystal-field on the honeycomb lattice. The effects of a biaxial crystal-field described by two uniaxial anisotropy parameters D x and D y on the phase transition and specific heat and susceptibility are studied analytically and numerically. Some characteristic phenomena are found in the system such as tricritical behavior depending sensitively on the strength of the biaxial crystal-field.

Thermodynamic Property Fields of an Adsorbate−Adsorbent System

We have developed the complete thermodynamic property fields for a single-component adsorbent + adsorbate system. These equations enable us to compute the actual specific heat capacity, the partial enthalpy, and the entropy, which are essential for the analyses of single-component adsorption processes. Conventionally, the specific heat capacity of the adsorbate is assumed to correspond to its liquid phase specific heat capacity and more recently to that of its gas phase. We have shown that the actual specific heat capacity of the adsorbate could differ significantly from what has been conventionally assumed. A simple but improved expression for the adsorbate specific heat capacity is also proposed.

Low-temperature electronic properties ofSr2RuO4.I. Microscopic model and normal-state properties

Starting from the quasi one-dimensional kinetic energy of the d_{yz} and d_{zx} bands we derive a bosonized description of the correlated electron system in Sr_2RuO_4. At intermediate coupling the magnetic correlations have a quasi one-dimensional component along the diagonals of the basal plane of the tetragonal unit cell that accounts for the observed neutron scattering results. Together with two-dimensional correlations the model consistently accounts for the normal phase specific heat, cyclotron mass enhancement, static susceptibility, and Wilson ratio and implies an anomalous high temperature resistivity.

Phase diagrams and tricritical behavior in a spin- [formula omitted] Ising model with transverse crystal field

Within the framework of the effective-field theory with correlations, the ferromagnetism of spin- 3 2 Ising model with a transverse crystal field on honeycomb and square and simple cubic lattices is studied. Tricritical phenomena have been observed for the square and simple cubic lattices, but there is no tricritical point for the honeycomb lattice. We have investigated the effect of the transverse crystal field on the phase diagrams, magnetization, quadrupolar moments, internal energy, and specific heat of the system on the square lattice in detail.

Network simulation method for solving phase-change heat transfer problems with variable thermal properties

The transient heat conduction equation in a finite slab undergoing phase change (two-phase problem of melting and solidification), with isothermal, adiabatic or convective boundary conduction is studied by the network simulation method; solid phase conductivity and specific heat are assumed to be dependent on temperature. Ablation, as a particular case, is also analysed. A network model is established for a cell and boundary conditions are added to complete the whole network model. No restrictions exist, as to the kinds of linear and non-linear boundary conditions, Stefan number values or the initial conditions (when hypotheses concern of the Stefan problem, numerical and exact solutions are compared for a large interval of Stefan numbers; simulation values show good agreement). Movement of the solid–liquid boundary and thermal fields are determined in all cases.

Exact results of a generalized Wu model with two- and four-spin interactions.

The mixed spin-1/2 and spin-S(B) Ising model with two- and four-spin interactions on the honeycomb lattice is studied by the use of a generalized star-triangle transformation. The exact results for the phase diagrams, magnetization, correlation functions, internal energy, specific heat, and quadrupolar susceptibility are obtained and discussed.

Transverse Ising Model with a Crystal Field for the Spin-2

The properties of the spin-2 transverse Ising model with a crystal field are studied by using the effective-field theory with correlations. A new method is developed to investigate the model with spin >1. The phase diagrams, magnetization curves, internal energy and specific heat are investigated numerically for the honeycomb lattice.

A model for freeze concentration and glass transition of carbohydrate solutions as determined by differential scanning calorimetry

Differential scanning calorimetry (DSC) is a common method for the determination of certain thermophysical properties of matter, such as phase transition heat, specific heat, or phase and glass transition temperatures. In particular, the freeze concentration and glass transition of carbohydrate solutions have been extensively studied by DSC. In order to verify the reliability of differential scanning calorimetry in the characterisation of such processes, the correct assignment of the features of the DSC curves to real physical effects is essential. Based on the results of one-dimensional numerical calculations of the material redistribution upon cooling, a new approach is proposed for the interpretation of the double-step feature appearing in the DSC curves. Because of the strong dependence of the molecular diffusitivies on temperature and composition, below a certain temperature the homogeneous distribution of the more and more concentrated solute cannot be established in the unfrozen volume upon fast cooling, and a thin, high-concentration (maximally freeze-concentrated) boundary layer develops at the ice surfaces. This boundary layer acts as a diffusion barrier for the molecules and inhibits water freezing upon cooling and delays ice melting upon heating. The model is illustrated by experimental data obtained by DSC.

Kinetic Studies of Bulk Ge22Se78−xBix (x=0, 4 and 8) Semiconducting Glasses

Abstract Results of phase transformations, enthalpy released and specific heat of Ge22Se78–xBix(x=0, 4 and 8) chalcogenide glasses, using differential scanning calorimetry (DSC), under non-isothermal condition have been reported and discussed. The glass transition temperature, T g, is found to increase with an average coordination number and heating rates. Following Gibbs—Dimarzio equation, the calculated values of T g (i.e. 462.7, 469.7 and 484.4 K) and the experimental values (i.e. 463.1, 467.3 and 484.5 K) increase with Bi concentration. Both values of T g, at a heating rate of 5 K min–1, are found to be in good agreement. The glass transition activation energy increases i.e. 102±2, 109±3 and 115±8 kJ mol–1 with Bi concentration. The demand for thermal stability has been ensured through the temperature difference T c–T g and the enthalpy released during the crystallization process. Below T g, specific heat has been observed to be temperature independent but highly compositional dependent. The growth k...

Characterization ofRCo2B2C(R=rare earth,Y):Phase stability, nonsuperconductivity, and breakdown of the de Gennes scaling

We investigated the phase stability, crystal structure, magnetism, specific heat, and resistivity of $R{\mathrm{Co}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ $(R=\mathrm{La},$ Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Y, Er). $R{\mathrm{Co}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ crystallizes in the ${\mathrm{LuNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$-type structure and the evolution of their unit-cell metric properties with the R size is governed by the lanthanide contraction: a faithful replica, though with a shrinking unit cell, of the behavior observed in the isomorphous $R{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ series. In spite of such structural similarity, their magnetic and superconducting features are distinctly different. For all the studied $R{\mathrm{Co}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ and down to 30 mK, no superconductivity was detected though their Sommerfeld constants $\ensuremath{\gamma}$ are comparable to those of the superconducting $R{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}\mathrm{C}.$ ${T}_{N}$ of all $({\mathrm{PrCo}}_{2}{\mathrm{B}}_{2}\mathrm{C},$ due to hybridization, is an exception) are systematically lower than their Ni-based isomorphs and, furthermore, do not obey the de Gennes scaling. Such a breakdown, confirmed also in $({\mathrm{Dy}}_{1\ensuremath{-}x}{\mathrm{Gd}}_{x}){\mathrm{Co}}_{2}{\mathrm{B}}_{2}\mathrm{C},$ is attributed to crystal field effects on the R ions (Co ions are nonmagnetic). Below ${T}_{N},$ a zero-field order-to-order phase transition was observed in $R=\mathrm{Nd},$ Sm, Tb, Dy, Ho, Er but not in the spherical $R=\mathrm{Gd};$ emphasizing the role of the magnetic anisotropy. Finally, the character and dimensionality of the low-temperature spin waves are discussed.

Phase Diagrams and Tricritical Behavior of Spin-2 Ising Model with a Transverse Crystal Field

Within the framework of the effective-field theory with correlations, the ferromagnetic spin-2 Ising model with a transverse crystal field on honeycomb, square and simple cubic lattices is studied. We have investigated the effect of the transverse crystal field on the phase diagrams, magnetization, quadrupolar moments, internal energy and specific heat of the system. The tricritical phenomena have been observed in our system.

Specific Heat Jump at T c of High T c Superconductors: Effect of Van Hove Singularity

In the BCS framework, exact expressions for the ratio between the jump in the specific heat at Tc and the normal phase specific heat are derived within the Van Hove singularity scenario. Analytical results are obtained for an isotropic s-wave and anisotropic d-wave pairing symmetries. Graphical solutions of the ratio as functions of ωD/Tc and EF/Tc, where ωD is the cutoff energy and EF is the Fermi energy, show significant deviations from the BCS value of 1.43.