Theory of one-dimensional and quasi-one-dimensional heat conduction

It is shown that the heat conduction process in a one-dimensional flow of a fluid moving with a velocity V in a constant temperature field follows a law that is considerably more complicated than an “ordinary” exponential law. It is demonstrated that in the quasi-one-dimensional case the heat conduction process in an abstract space of dimension 1+ɛ, where ɛ varies from zero to unity, is described by a modified Fourier equation. Its solution for an infinite space is found. Zh. Tekh. Fiz. 67, 8–12 (July 1997)     

Specific heat of magnetic clusters with S=2 YBa2Cu3O7−δ

An analysis is made of published data from measurements of the temperature-and-field dependence of the specific heat in YBaCuO high-temperature superconductors at temperatures below 10 K. It is shown that the “excess” contribution to the specific heat can be explained by spin splittings in copper-oxygen clusters with S=2. The magnitudes of the necessary splittings agree with the available magnetic resonance data. Fiz. Tverd. Tela (St. Petersburg) 39, 1320–1322 (August 1997)     

Heat conductivity and the Lorentz number of the Sm1−x GdxS “black” phase

Measurement of the heat conductivity and electrical resistivity of two Sm_1−x Gd_xS compositions with x=0.1 and 0.14 is reported within the 80–300 K interval. An analysis of experimental data on the electronic component of heat conductivity permits a conclusion that the d subband of “heavy” carriers in the conduction band of these materials lies above the s “light”-carrier subband. Fiz. Tverd. Tela (St. Petersburg) 41, 26–29 (January 1999)     

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)     

Anisotropy of the upper critical field and specific heat in V3Si — a superconductor with cubic symmetry

The specific heat of a V₃Si single crystal (T _c=17 K, H _c2=20 T) in magnetic fields up to 8 T isinvestigated experimentally for three orientations of the field relative to the crystallographic directions — H∥〈001〉, H∥〈110〉, and H∥〈111〉. Both the upper critical magnetic field and the specific heat of the mixed state are observed to depend on the orientation of the magnetic field relative to the crystallographic directions (anisotropy): The critical field reaches its maximum value and the specific heat its minimum value in a field along the 〈001〉 direction. The anisotropy scale in both phenomena increases as the magnetic field and reaches 3% in a 6 T field. The interrelationship of the upper critical field anisotropy and the specific-heat anisotropy in type-II superconductors is studied. It is shown that the anisotropy of the specific heat in the mixed state in weak fields can serve as a criterion for nontrivial pairing. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 1, 26–29 (10 January 1999)     

Temperature dependence of the heat capacity and lattice constant of lanthanum and samarium hexaborides

The temperature dependence of isobaric heat capacity and [411] interplanar spacing in lanthanum and samarium hexaborides have been determined experimentally within the 5–300 K region. The variation of the lattice parameters and thermal expansion coefficients α(T) with temperature has been calculated. Fiz. Tverd. Tela (St. Petersburg) 40, 2051–2053 (November 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)     

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)     

Nonequilibrium phase transition in quenched samples of Fe0.1TiTe2

The influence of the heat treatment of Fe_0.1TiTe₂ samples on the appearance of thermally induced “levitation” is investigated. It is postulated that this phenomenon is most probably caused by undamped fluctuations of the electric charge on the surface of particles of the material. Fiz. Tverd. Tela (St. Petersburg) 41, 680–683 (April 1999)     

Specific heat of the elpasolite Pb2MgWO6

The specific heat of Pb₂MgWO₆ has been measured in the temperature interval 83–370 K. An anomaly in the specific heat associated with the phase transition at T ₀=312.8K has been discovered. The thermodynamic parameters of the structural phase transition Fm3m-Pmcn have been determined. Fiz. Tverd. Tela (St. Petersburg) 41, 1686–1688 (September 1999)     

Propagation of a heat pulse in a bounded conducting medium: Thermoelectric detection

The one-dimensional, non-steady thermal field on the surface of a bounded conducting sample arising from the absorption of a square heat pulse of arbitrary duration is calculated. It is shown that the spatial temperature distributions for long and short pulses are fundamentally different: After a short heat pulse is “switched off” regions of local heating arise; such regions do not occur in the case of a long pulse. The thermoelectric response is calculated and it is shown that data on the thermal conductivity and thermal diffusivity can be obtained in a single experiment. Fiz. Tverd. Tela (St. Petersburg) 41, 606–611 (April 1999)     

Spatial orientation of molecules by a heat flux

An effect of spatial orientation of molecules by a heat flux is predicted. Under typical experimental conditions the magnitude of the constant electric field arising due to static polarization of a gas of oriented molecules can reach values ∼10⁻⁴ V/cm. Zh. Tekh. Fiz. 67, 139–141 (September 1997)     

Low-temperature specific heat of crystalline and amorphous Eu2(MoO4)3

The specific heat C _total of crystalline and amorphous Eu₂(MoO₄)₃ is measured in the temperature interval 4.5–30 K. The amorphous state is obtained by applying pressure ∼7 GPa at room temperature. It is found that the specific heat of the crystal at T⩽7.5 K is described by a cubic function of temperature, while the specific heat of the amorphous sample has a strongly non-Debye character in the entire experimental temperature interval. The curve of C _total for amorphous europium molybdate is analyzed in a model of soft atomic potentials, and it is shown that it agrees well with universal low-temperature anomalies of the specific heat of classical glasses obtained by quenching from the liquid. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 8, 623–627 (25 October 1998)     

Solid deuterium source of ultracold neutrons based on a pulsed spallation source

A new type of source of ultracold neutrons (UCNs) is proposed. The source operates on the basis of a pulsed spallation source. Solid deuterium makes it possible to obtain UCN density 10⁴ neutrons/cm³ as a result of high gain at low temperatures and the possibility of withstanding high pulsed heat loads as a result of the high specific heat of solid deuterium. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 12, 765–770 (25 December 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)     

Unusual behavior of thermal conductivity of a crystalline-NaCl-opal nanocomposite

An opal-based nanocomposite has been prepared with NaCl incorporated in its pores. The nanocomposite was produced by impregnating the opal with a NaCl solution at room temperature. Thermal conductivity of the nanocomposite has been measured in the temperature range 4.2–300 K. The effective heat conductivity of the nanocomposite was found to be equal to that of pure opal. The observed phenomenon can be explained by assuming that NaCl resides in opal pores in the form of noncontacting needles, thus precluding heat propagation through it. Fiz. Tverd. Tela (St. Petersburg) 40, 379–380 (February 1998)     

Specific heat and thermal conductivity of superionic conductors in the superionic phase

The temperature dependences of the specific heat and the thermal conductivity of crystalline superionic conductors LnF₃ (Ln = La, Ce, Pr), Li₂B₄O₇ and α-LiIO₃ in the superionic phase have been investigated experimentally. The specific heat C _p and the thermal conductivity K are observed to increase monotonically over a wide range of temperatures above the Debye temperature Θ_D. This increase is attributed to the relaxational interaction of high-frequency phonons with two-level systems. Fiz. Tverd. Tela (St. Petersburg) 39, 1548–1553 (September 1997)     

Reply to the Comment on¶Low temperature specific heat of blue bronze K0.30MoO3

Eur. Phys. J. B 24, 315 (2001) In our Reply to the Comment [#!1!#] we refute the “straightforward” interpretation of the excess low-temperature specific heat, C_p, contribution we have measured in our study of CDW systems K_0.3MoO₃ and (TaSe₄)₂I [#!2!#] as originating solely from normal phonon modes. The specific sensitivity of the bump in C _p / T ³ at low temperatures to the impurity content is consistent with the increased value of the phason pinning gap while the dispersion of normal phonons remains unaffected. We ascribe at least this part of the anomaly to the phason contribution. As stated in reference [3] that the phonon density of states extracted from neutron scattering measurements is the least reliable in this energy range (<0.5 meV), we conclude that C_p measurements are more accurate for detecting the phason contribution. Received 17 April 2002 Published online 19 July 2002     

Anomalies of the low-temperature specific heat of the cuprates La2CuO4 and La2−x MxCuO4 (M = Sr, Ba)

The low-temperature specific heat of the cuprates La₂CuO₄, La_2−x M_xCuO₄ (M = Sr, Ba) in the temperature interval 2–45 K has been investigated by the technique of pulsed differential calorimetry. It is found that the coefficient of the residual linear term in the specific heat remains constant in the entire experimental temperature interval. It is shown that La atoms play a special role in the formation of the anomaly, associated with the specific nature of the interaction of these atoms with their environment, in the acoustic region of the phonon spectrum of these objects near 6 meV. Fiz. Tverd. Tela (St. Petersburg) 39, 1000–1004 (June 1997)     

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)