Specific heat of LiB3O5 crystals in the temperature interval 80–300 K

The specific heat of LiB₃O₅ crystals is measured by adiabatic calorimetry. The experimental data on the specific heat are used to calculate the change in the thermodynamic functions (the entropy, enthalpy, and Gibbs free energy) and the Debye temperature Θ_D(T) of crystalline LiB₃O₅. Fiz. Tverd. Tela (St. Petersburg) 39, 624–625 (April 1997)     

Temperature dependence of the electronic specific heat of superconductors with quantum defects

The electronic specific heat of metals with quantum defects in the superconducting state is examined. The role of the electron-polaron effect, as well as that of the level population factor of two-level states, is analyzed in an adiabatic approach. The cases of intermediate and strong coupling are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 413–418 (March 1998)     

Interaction of polarons in two and three dimensions

Two-dimensional and three-dimensional bipolarons of large radius, symmetrized with respect to the coordinate parts of the two-center wave functions, are investigated in the adiabatic approximation with allowance for dynamic interelectronic correlations. The adiabatic potential lines are plotted. It is shown that the quasimolecular configuration appearing in the Hartree-Fock approximation is unstable in both the two-dimensional and the three-dimensional cases. The ground state is a one-center configuration. Estimates are given for the binding energy and the heat of dissociation of a bipolaron. Fiz. Tverd. Tela (St. Petersburg) 39, 441–443 (March 1997)     

Specific heat of KTiOPO4 within the 80–300 K range

The specific heat of the KTiOPO₄ crystal has been measured with a vacuum adiabatic calorimeter within the 80–300 K range. A peak-shaped anomaly in the specific heat indicating a phase transition has been revealed in the C _p (T) curve at T≅279 K. Numerical integration of smoothened experimental C _p (T) curves yielded the thermodynamic functions of KTiOPO₄, namely, the entropy, enthalpy, and reduced Gibbs energy. The entropy and enthalpy of the observed transition have been determined. Fiz. Tverd. Tela (St. Petersburg) 41, 497–498 (March 1999)     

Specific heat and critical behavior of Tl0.3MoO3 near the Peierls phase transition

The specific heat of the quasi-one-dimensional charge-density-wave (CDW) compound Tl_0.3MoO₃ has been measured using an adiabatic continuous heating method from 100 to 220 K. A specific heat jump associated with the Peierls phase transition occurs at 172.3 K. A good scaling relation between the excess specific heat and the susceptibility is found between 140 K and 190 K. Further analysis indicates that the width of the critical region of Tl_0.3MoO₃ is about 10 K and the specific-heat critical behavior can be well described by the three-dimensional XY model.     

Optical potential discrete variable representation method in the adiabatic representation

The optical potential discrete variable representation method (OP-DVR) has been applied recently to calculate resonances in the framework of the diabatic representation [J. Chem. Phys. 101, 7580 (1994)]. This method is based on the conjoint use of the discrete variable representation (DVR) method and the properties of a complex absorbing potential (CAP). The OP-DVR method is the DVR version of the CAP stabilization method initially proposed by Jolicard and Austin [Chem. Phys. Lett. 121, 106 (1985)]. In the present study, we show that this efficient and accurate method can also be applied within the adiabatic representation since it allows one to overcome in a simple way, numerical difficulties associated with the first derivative operator which appears in the expression of non adiabatic couplings. Within the OP-DVR method, the choice of the representation (diabatic or adiabatic) is governed by physical arguments and by the fact that the potentials and the couplings are known in one or the other of these two representations. In the case where the potentials and the couplings are obtained in the adiabatic representation, we show in this paper that the transformation into the diabatic framework is not necessary. We demonstrate that the discrete variable representation can be a simple and an efficient way to deal with the adiabatic representation. Received: 30 April 1998 / Revised: 29 September 1998 /Accepted: 21 October 1998     

Effect of γ irradiation on the heat capacity of Rb2ZnBr4 in the vicinity of phase transitions

Heat capacity of Rb₂ZnBr₄ as a function of γ irradiation dose has been measured within the 85–300 K range by the adiabatic calorimeter technique. It is shown that, as the irradiation dose increases, the heat capacity peak in the vicinity of the incommensurate-commensurate first-order phase transition (PT) decreases, and the transition temperature T _c increases. The heat capacity peak in the region of the second-order PT at T ₃=112 K does not depend on γ irradiation, both in magnitude and in position, just as the heat capacity throughout the remainder of the temperature range studied. Fiz. Tverd. Tela (St. Petersburg) 40, 1106–1108 (June 1998)     

On an adiabatic invariant in the thermodynamics of solids

A thermodynamic invariant in the form of the ratio of a vibrational frequency in an anharmonic solid to the temperature in adiabatic processes is derived. The adiabatic invariance established is used to derive in a simple manner an expression for the temperature change due to elastic adiabatic loading of solids (Kelvin's equation). Fiz. Tverd. Tela (St. Petersburg) 41, 134–136 (January 1999)     

Thermomagnetic properties of very heavy fermions suitable for adiabatic demagnetisation refrigeration at low temperature

ABSTRACT

With the aim to improve the performance of classical paramagnetic salts for adiabatic refrigeration processes at the sub-Kelvin range, relevant thermodynamic parameters of some new Yb-based intermetallic compounds are analysed and compared. Two main applications are considered: (i) those requiring temperature fixed-points to be reached applying magnetic fields of moderate intensity for satellite applications, and (ii) those which can be used for controlled thermal drifts in laboratory applications. Different thermomagnetic trajectories of the entropy are identified depending on respective specific heat behaviours and the constraints imposed by the Nernst postulate at the sub-Kelvin range. Some simple relationships are proposed to compare the diverse magnetocaloric characteristics of different systems. This procedure allows to include the classical Cerium-Magnesium-Nitride (CMN) salt in that comparison.     

Exponential decrease of thermodynamic functions of photon gas in cubic and spherical cavities with low adiabatic invariants

The Planck radiation spectrum of ideal cubic and spherical cavities with low adiabatic invariants, γ = TV ^1/3, is discrete and strongly dependent on the cavity geometry and temperature. This behavior is a consequence of the random distribution of the state weights in the cubic cavity and of the random overlapping of successive multiplet components, in the case of a spherical cavity. The total energy density of cavities with low adiabatic invariant, γ (obtained by summing up the exact contributions of the eigenvalues and their weights) no longer obeys the Stefan-Boltzmann law. The new law includes a corrective factor depending on γ, which imposes an exponential decrease of the total energy density to zero, when γ → 0. A similar behavior is demonstrated for specific heat and for all other thermodynamic functions of photon gas in cubic and spherical cavities with low adiabatic invariants. This special quantum regime, defined by the limits of principal quantum numbers or by adiabatic invariants, is shown to be similar for cubic and spherical cavities.     

Magnetocaloric properties of LaFe13−x−yCoxSiy and commercial grade Gd

The magnetocaloric properties of three samples of LaFe_13−x−yCo_xSi_y have been measured and compared to measurements of commercial grade Gd. The samples have (x=0.86, y=1.08), (x=0.94, y=1.01) and (x=0.97, y=1.07) yielding Curie temperatures in the range 276-288 K. The magnetization, specific heat capacity and adiabatic temperature change have been measured over a broad temperature interval. Importantly, all measurements were corrected for demagnetization, allowing the data to be directly compared. In an internal field of 1 T the maximum specific entropy changes were 6.2, 5.1 and 5.0 J/kg K, the specific heat capacities were 910, 840 and 835 J/kg K and the adiabatic temperature changes were 2.3, 2.1 and 2.1 K for the three LaFeCoSi samples respectively. For Gd in an internal field of 1 T the maximum specific entropy change was 3.1 J/kg K, the specific heat capacity was 340 J/kg K and the adiabatic temperature change was 3.3 K. The adiabatic temperature change was also calculated from the measured values of the specific heat capacity and specific magnetization and compared to the directly measured values. In general an excellent agreement was seen.     

Investigation of the specific heat of bismuth tellurite in the temperature range 100–370 K

The temperature dependence of the specific heat C _p of bismuth tellurite is measured. The experimental data are compared with the results of a calculation that takes account of the Debye and Einstein mechanisms of the specific heat. Fiz. Tverd. Tela (St. Petersburg) 41, 629–631 (April 1999)     

Energetics of the thermoelastic effect in solids

The dependence of the temperature on the external adiabatic deformation is determined for a one-dimensional model of a solid — chains of atoms with an anharmonic interaction. The resulting dependences of the average kinetic and potential components of the internal energy on this deformation are compared with a model of adiabatic loading of a single oscillator. Fiz. Tverd. Tela (St. Petersburg) 40, 1548–1551 (August 1998)     

Experimental investigation of the behavior of the specific heat in finite systems in the vicinity of the critical mixing point

The specific heat of a 2,6-lutidine-water mixture is measured in the vicinity of the lower critical mixing point in the bulk and in porous media. The results of the measurements are interpreted by finite-size scaling. In particular, a universal function of the ratio t/t* [t is the dimensionless deviation of the temperature from the specific-heat maximum T _m (L), and t* is the characteristic dimensionless temperature, which depends on the pore size] describing the behavior of the specific heat in the vicinity of the critical point in porous media with different pore sizes is obtained. The results obtained are consistent with the predictions of finite-size scaling and with the data from a numerical calculation of the specific heat of the finite three-dimensional Ising model. Zh. éksp. Teor. Fiz. 113, 1071–1080 (March 1998)     

Chaleur specifique du mononitrure de thorium de 7 a 300K

The specific heat of ThN has been measured between 7 and 300K by an adiabatic method with continuous heating, and has been compared to that of UN. The magnetic specific heat of UN between 50 and 300K is certainly lower than that of a U⁴⁺ ions gas submitted to an octahedral crystal field of about 500 cm⁻¹.     

Specific heat below 1k some examples in magnetism

The measurement of specific heat below 1 K by means of a quasi-adiabatic calorimeter, employing adiabatic demagnetization for cooling, is described. The lattice, hyperfine and electronic contributions in the temperature domain 0.05 < T < 4 K are discussed briefly as an introduction to the analysis and exploitation of the magnetic contribution to the specific heat in magnetic insulators. The comparison with simple magnetic models is illustrated by the analysis of the specific heat data of the NdMO₃ perovskites, with M = nonmagnetic atom. The action of a magnetic field, albeit an internal field, is discussed using NdMO₃, M = Fe, Cr and Ni perovskite as examples. Finally, two molecular magnetic materials are discussed; Gd₂(ox)[Cu(pba)]₃[Cu(H₂O)₅]· 20H₂O as a candidate for ferromagnetic order in a rare earth transition metal complex, and Gd(hfac)₃NITR, with the nitronyl nitroxide magnetic radical interacting with the rare earth.     

Dependence of the specific heat of a La1.85Sr0.15CuO4 superconducting single crystal on the magnetic field direction in the a-b plane

The low-temperature specific heat of a La_1.85Sr_0.15CuO₄ superconducting single crystal was investigated in magnetic fields up to 8 T and with four orientations — in the a-b plane (along the (100) and (110) directions) and at angles of 45° and 90° with respect to the a-b plane (along the (103) and (001) directions). Anisotropy was observed in the field dependence of the specific heat in the a-b plane. The specific heat was found to be minimum with the field oriented in the direction of the a axis and maximum with the field oriented in a direction making an angle of 45° with the a axis. This can be explained by the anisotropy of the energy gap, whose minimum lies along the (110) direction. For all orientations of the magnetic field the specific heat of the mixed state at low temperatures is a nonlinear function of the magnetic field strength. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 683–687 (10 December 1997)     

Moment-volume instabilities in alloys with compositions around the C14 laves phase

We measured the thermal expansion and the specific heat of Ti_xFe_100-x alloys with x = 30.5, 32.5 and 35, all with hexagonal C14 laves phase structure (MgZn₂) like TiFe₂, and determine the temperature dependence of the magnetic contributions to the thermal expansion and the specific heat c_mag. For fixed composition and c mag ( T ) show the same type of behavior, demonstrating that both anomalies are of the same microscopic nature. They originate from moment-volume fluctuations (antiferromagnetic Invar-effect) as a comparison with total energy calculations as a function of atomic volume and moment for TiFe₂ reveals. Received: 26 January 1998 / Accepted: 17 April 1998     

Anomalous behavior of the specific heat and upper critical magnetic field in the superconducting single crystal La1.85Sr0.15CuO4

The specific heat and resistive upper critical magnetic field of the single crystal La_1.85Sr_0.15CuO₄ are investigated in the temperature range 2–50 K in magnetic fields up to 8 T for two directions of the magnetic field, parallel and normal to the ab crystalline plane. For both orientations a nonlinear (close to square root) magnetic field dependence of the mixed-state specific heat and a positive curvature of the temperature dependence of the upper critical magnetic field are observed. Neither of these anomalies is described by standard theories of superconductivity. Within the framework of the thermodynamic relations it is shown that in a type-II superconductor a relationship exists between the temperature dependence of the critical magnetic field and the field dependence of the specific heat. The anomalies observed in these phenomena are interrelated. Zh. éksp. Teor. Fiz. 112, 1386–1395 (October 1997)