Narrow Results

We carried out a systematic measurement and data analysis of low-temperature heat capacities of three BEDT-TTF-based superconductive compounds with $\kappa$-type structure, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene so as to compare the character of the quasi-particle excitations reflected in the electronic heat capacity. We used an original relaxation calorimetry cell with much reduced addenda heat capacity as compared with previous works. The three compounds, $\kappa$-(BEDT-TTF)₂Cu(NCS)₂, $\kappa$-(BEDT-TTF)₂Ag(CN)₂H₂O, $\kappa$-(BEDT-TTF)₄Hg${}_{2.89}$Br₈ show distinct quadratic temperature dependence in their electronic heat capacity obtained by subtracting normal states data obtained by applying magnetic fields from the 0 T data. The line-nodal gap due to $d$-wave pairing symmetry is suggested as common phenomena of the superconductivity of the $\kappa$-type compounds.

The electronic structures, lattice dynamics and thermodynamic properties of rare-earth intermetallic ScCd alloy are studied by the first-principles plane-wave pseudopotential method within the generalized gradient approximation in the framework of density functional pertubation theory. The band structure, density of states, phonon dispersion frequencies, vibrational free energy $F_{\mathrm{\text{vib}}}$, specific heat capacity $C_V$ and entropy are studied between 0 K and 1500 K. Finally, using the calculated phonon density of states, the thermodynamic properties are determined within the quasi-harmonic approximation and a value of 47.9 (J/mol⋅K) at 300 K for specific heat capacity of ScCd is predicted.

Structural, electronic, elastic and thermal properties of Al₄Si₂C₅ under constant pressure were investigated by using first-principles theory. The total volume of the cell decreased by almost 15.7% under 40 GPa which is smaller than that of Al₄SiC₄ (16%), while the linear compressibility along $a$- or $b$-axis direction showed better anti-deformation behavior than that of along $c$-axis direction. The peak heights of total density of state (TDOS) and partial density of state (PDOS) curves of Al₄Si₂C₅ are slightly lowered with forced high pressure. Meanwhile, the mechanical properties of Al₄Si₂C₅-like elastic constants and elastic moduli accelerate with the pressure increasing from 0 GPa to 40 GPa; the thermal expansion coefficient $\alpha$ increases rapidly at lower temperature and this tendency gradually approaches a linear increase when the temperature is above 1000 K. At particular temperature, $\alpha$ decreases continuously with the pressure accelerating. Heat capacity at constant volume ($C_V)$ with pressure was also evaluated, the results displayed that $C_V$ is sensitive with the temperature rather than the pressure. The elastic anisotropy and Debye temperature with pressure were successfully obtained and discussed.

Starting from the Holstein model, we have investigated the effects of Rashba spin–orbit coupling (RSOC) on density of states (DOS), electronic heat capacity (EHC) and magnetic susceptibility (MS) of boron nitride-doped (BN-doped) graphene beyond the Dirac cone approximation within the Green’s function approach. By using the self-consistent perturbation theory, retarded self-energy can be calculated. We have found that the band gap (the peak of EHC and MS) of the system increases (decreases) with RSOC and electron–phonon (e–ph) interaction. Also, the Schottky anomaly moves only to the higher temperatures for strong RSOCs and e–ph interactions. Also, our results show that the response of the system to an external magnetic field is scaled down in the presence of RSOC and e–ph interaction.

Barium chalcogenides are known for their high-technological importance and great scientific interest. Detailed studies of their elastic, mechanical, dynamical and thermodynamic properties were carried out using density functional theory and plane-wave pseudo potential method within the generalized gradient approximation. The optimized lattice constants were in good agreement when compared with experimental data. The independent elastic constants, calculated from a linear fit of the computed stress–strain function, were used to determine the Young’s modulus (E), bulk modulus (B), shear modulus (G), Poisson’s ratio ($\sigma$) and Zener’s anisotropy factor (A). Also, the Debye temperature and sound velocities for barium chalcogenides were estimated from the three independent elastic constants. The calculations of phonon dispersion showed that there are no negative frequencies throughout the Brillouin zone. Hence barium chalcogenides have dynamically stable NaCl-type crystal structure. Finally, their thermodynamic properties were calculated in the temperature range of 0–1000 K and their constant-volume specific heat capacities at room-temperature were reported.

The Stirling’s estimation to $ln$(N!) is typically introduced to students as a step in the derivation of the statistical expression for the heat capacity. However, naïve application of this estimation leads to wrong conclusions. In this paper, firstly, the heat capacity of some semiconductor compounds was calculated using exponential Boltzmann distribution and compared with experimental data. It has shown a disagreement between experimental results and those calculated. Secondly, by applying the more exact Stirling formula, an analytical formulation of Boltzmann statistics using Lambert W function is shown to be a very good model and proves an excellent agreement between calculated and experimental data for heat capacity over the entire temperature range.

The new generalized Bose–Einstein condensation (GBEC) quantum-statistical theory starts from a noninteracting ternary boson-fermion (BF) gas of two-hole Cooper pairs (2hCPs) along with the usual two-electron Cooper pairs (2eCPs) plus unpaired electrons. Here we obtain the entropy and heat capacity and confirm once again that GBEC contains as a special case the Bardeen–Cooper–Schrieffer (BCS) theory. The energy gap is first calculated and compared with that of BCS theory for different values of a new dimensionless coupling parameter n/n${}_f$ where n is the total electron number density and n${}_f$ that of unpaired electrons at zero absolute temperature. Then, from the entropy, the heat capacity is calculated. Results compare well with elemental-superconductor data suggesting that 2hCPs are indispensable to describe superconductors (SCs).

In this paper, we used the first-principles method to investigate the structural, electronic, mechanical and thermodynamic parameters of the ternary $\beta$-Ti–15Nb–xSi alloys with $x=0.6,0.8,1,1.2,1.4,1.6$wt.%. We have carried out theoretical computations inside the density functional theory (DFT) utilizing the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) model. The random distribution of Nb atoms in the alloy was described by using both virtual crystal approximation (VCA), special quasirandom structure (SQS) and the coherent potential approximation (CPA) techniques, in combination with first-principles plane-wave pseudopotential (PW-PP) and exact muffin-tin orbital (EMTO) methods. We determined the elastic constants as well as the bulk, shear, Young’s modulus and Poisson’s ratio. Our structural results are in good agreement with the available experimental and theoretical results for the pure structure of the titanium. In addition, we have estimated the band structure and the density of state (DOS) for the electronic computations. Our findings demonstrate that all of the compounds are metallic, stable and meet the requirements for stability. Young’s modulus of Ti–15Nb–0.6Si and Ti–15Nb–1.6Si is 86.5GPa and 15.11GPa, respectively, which are similar to Young’s moduli of human bone (10–30GPa). All calculated parameters of the alloys decreased with increasing of Si concentration except for Poisson’s ratio, anisotropy and B/G ratio. Furthermore, all of the materials investigated showed ductile nature, and Young’s modulus values are needed for further applications. Excitations from the quasi-harmonic Debye approximation’s vibrational part were applied to the 0K free energy calculated via ab-initio calculations. The influence of temperatures up to 800 K on phase stability was investigated. These findings can be utilized to help designers create alternative low-modulus alloys for biomedical applications.

An analytical formulation that calculates the thermodynamic properties of zirconium carbide and zirconium nitride has been produced using the n-dimensional Debye function. The results of the heat capacity and entropy calculated here for ZrM (M=N and C) were compared with the literature data, and are found to be in agreement. This agreement shows that the used method would be useful to calculate the thermophysical properties (heat capacity, entropy, etc.) of materials such as ZrC and ZrN.

We present the concentration-temperature phase diagram, characteristic functions, thermal equation of state and heat capacity at constant volume for degenerate ideal gases of relativistic fermions and bosons. The nonrelativistic and ultrarelativistic limits of these laws are also discussed.

We analyze the heat capacity C_P for low and high-density amorphous ice below the transition temperature (T_C ≈ 140 K) using a power-law formula. The renormalized critical exponent α_R is extracted from the observed C_P data, which describes similar critical behavior for both low and high-density amorphous ice below T_C. Our analysis can also describe a glass transition in the low-density amorphous ice which is made from the high-density amorphous ice at 124 K, as observed experimentally.

This paper is devoted to the analysis of physical processes in composite matrix materials whose properties are greatly affected by the interphase interaction of the matrix and the modifier. Contribution of this interaction to thermodynamic and dielectric properties of such materials is investigated by the example of a model system which is a colloid solution of solid particles with charged surface in a polar liquid. Mechanisms underlying formation and stabilization of specific structures near the interphase boundaries of the examined system are discussed. Special attention is paid to the assessment of additional contribution to the internal energy and heat capacity related to the electric interaction of solid and liquid components. Results obtained within the proposed model show that for a certain concentration of liquid (about several percent) the interphase energy in a unit of volume magnificently increases to the values of about 10⁷–10⁸ J/m³ and therefore exceeds heat motion energy of polar molecules. Moreover it was revealed that the electrical part of heat capacity is comparable to self-capacity of the liquid matrix provided that the surface charge density of solid particles is high enough.

We report here a microscopic theory of the temperature dependence of the specific heat in heavy fermion (HF) systems. The standard model Hamiltonian consists of c–f spin exchange term and Heisenberg type inter-site spin–spin correlation in addition to the conduction electron and f-electron hopping terms. The Hamiltonian is treated in mean-field approximation (MFA) taking into account two mean-field parameters i.e. the Kondo singlet and f-electron correlation parameter . An attempt has been made in this present communication to calculate the temperature dependent of specific heat to study the peaks at Kondo and correlation temperatures. The evolution of peaks are studied by varying the model parameters and comparing its relation to the experimental data.

Through the quasi-harmonic Debye model, the pressure and temperature dependences of linear expansion coefficient, bulk modulus, Debye temperature and heat capacity have been investigated. The calculated thermodynamic properties were compared with experimental data and satisfactory agreement is reached.

We report here a microscopic theory of the temperature dependence of specific heat in high-T_c cuprate superconductors. The system is described by a model Hamiltonian consisting the antiferromagnetic spin fluctuations due to the impurity f-electrons as well as the conduction electrons, besides the superconducting interaction due to the itinerant electrons. The Hamiltonian is treated within a mean-field approach. The transverse spin fluctuation parameters and the superconducting gap are calculated by Zubarev's Green's function technique and solved self-consistently. The temperature dependent specific heat is calculated from the free energy in order to study the anomalies appearing at the spin fluctuation and superconducting transition temperatures. The evolutions of these anomalies are studied by varying the model parameters of the systems and results are discussed in reference to experimental observations.

A brief overview of changes in the superconducting transition temperature under pressure and evolution of specific heat capacity jump at T_c for two related families of iron-based superconductors, Ba_1-xK_xFe₂As₂ (0.2 ≤ x ≤ 1.0) and Ba_1-xNa_xFe₂As₂ (0.2 ≤ x ≤ 0.9) will be given. For Ba_1-xK_xFe₂As₂ the specific heat capacity jump at T_c measured over the whole extent of the superconducting dome shows clear deviation from the empirical, , scaling (known as the BNC scaling) for x > 0.7. At the same concentrations range apparent equivalence of effects of pressure and K-substitution on T_c fails. These observations suggests a significant change of the superconducting state for x > 0.7. In contrast, the data for the large portion of Ba_1-xNa_xFe₂As₂ (0.2 ≤ x ≤ 0.9) series follow the BNC scaling. However, the pressure dependence of T_c (measured up to ~12 kbar) have clear nonlinearities for Na concentration in 0.2–0.25 region, that may be consistent with T_c crossing the phase boundaries of the emergent, narrow, antiferromagnetic/tetragonal phase. Results will be discussed in context other studies of these two related families of iron-based superconductors.

A theoretical study is conducted by first-principles theory to study the structural, electronic, elastic and thermal properties of Al₄SiC₄, Al₄C₃ and 4H-SiC phases under pressure. The calculated results indicated that the volumetric shrinkage of Al₄SiC₄ declines to 16% compared with 4H-SiC for 12% and its length of lattice parameter along c-axis decreases faster than that of along other axes in cell structures. The mechanical properties of Al₄SiC₄ like elastic constants and elastic moduli increase continuously under pressure. The thermal expansion coefficient of three compounds under pressure are studied first. When temperature is lower than 500 K, the coefficient increases rapidly first then gradually tends to a linear accession at higher temperature and the propensity of increment becomes moderate. The $C_V$ data decreases slightly with pressure but increases dramatically with temperature for all compounds.

In this work, differential scanning calorimetric (DSC) analyses were performed in an Ar inert medium in the temperature range of 100–750 K for the Y₂O₃ compound with the purity rate of 99.99%, the density in powder form is 0.069 g/cm³, the specific surface area is 100–150 m²/g, the particle size is in the range of 8–10 nm irradiated with fast neutrons with different intensities ($E<1$ MeV). Using mathematical approximation methods, the equations of dependence of the heat flux function on temperature and heat capacity after irradiation at different intensities for the Y₂O₃ compound over a wide temperature range were determined. It was found that in the temperature range of $150\le T\le 750$ K, the value of the heat flux increases by 16.6%, up to 86 mW for the case of maximum radiation. At all radiation intensities, anomalies recorded with very small changes were observed in the spectra of the heat flux function related to the internal structural transitions.

We address the problem of identifying the critical temperature in a crossover from the Bose–Einstein condensed (BEC) phase to the normal phase. For this purpose we study the temperature dependence of magnetization of spin-gapped quantum magnets described by BEC of triplons. We have calculated the heat capacity $C_H$ at constant field and fluctuations in magnetization in a spin-gapped quantum magnet using the Hartree–Fock–Bogouliubov approximation and found optimized parameters of the Hamiltonian of triplon gas. In the region of phase transition, the heat capacity $C_H$ is smeared out due to the Dzyaloshinsky–Moriya (DM) interaction. The sharp maximum of the fluctuations in the magnetization is identified as the critical temperature of the crossover.

One general consequence of the Nernst theorem is derived, i.e. the various heat capacities of a thermodynamic system under different constraints approach zero as the temperature approaches absolute zero. The temperature dependence of the heat capacity of any thermodynamic system at ultra-low temperatures is revealed through this consequence. Moreover, the general form and the simplest expression of the heat capacities of thermodynamic systems at ultra-low temperatures are deduced. Some discussion and results are given. One new research method is provided by using this consequence. Finally, the equivalence between the Nernst theorem and its consequence is proved, so that this consequence may be referred to another description of the third law of thermodynamics.