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1.
This paper reports on a measurement of the heat capacity at constant pressure (C p ) in the temperature range 3–320 K and the sound velocity (v) at 77 K for the “light” heavy-fermion compound YbMgCu4. The present experimental data on C p and v of YbMgCu4, combined with our earlier phonon thermal conductivity data for YbMgCu4 in the range 5–300 K, have been used to calculate the phonon mean free path l in this compound. The temperature dependence of l obtained is found to be characteristic of classical amorphous materials.  相似文献   

2.
This paper reports on measurements of the thermal conductivity κ and the electrical conductivity σ of high-porosity (cellular pores) biocarbon precursors of white pine tree wood in the temperature range 5–300 K, which were prepared by pyrolysis of the wood at carbonization temperatures (T carb) of 1000 and 2400°C. The x-ray structural analysis has permitted the determination of the sizes of the nanocrystallites contained in the carbon framework of the biocarbon precursors. The sizes of the nanocrystallites revealed in the samples prepared at T carb = 1000 and 2400°C are within the ranges 12–35 and 25–70 Å, respectively. The dependences κ(T) and σ(T) are obtained for samples cut along the tree growth direction. As follows from σ(T) measurements, the biocarbon precursors studied are semiconducting. The values of κ and σ increase with increasing carbonization temperature of the samples. Thermal conductivity measurements have revealed that samples of both types exhibit a temperature dependence of the phonon thermal conductivity κph, which is not typical of amorphous (and amorphous to x-rays) materials. As the temperature increases, κph first varies proportional to T, to scale subsequently as ~T 1.7. The results obtained are analyzed.  相似文献   

3.
Samples of opal + NaCl nanocomposites with 100 and 80% filling of first-order opal voids by sodium chloride have been prepared. Their effective thermal conductivities, κeff, were measured in the temperature interval 5–300 K. The lattice thermal conductivity of NaCl loaded in the opal voids, κ ph op , was calculated from the measured κeff(T). The value of ph was found to be considerably smaller than the lattice thermal conductivity of bulk NaCl throughout the temperature interval studied. For T>20 K, this behavior of κ ph op (T) is accounted for by the presence of specific defects that form in NaCl loaded in opal voids. For T<20 K, κ ph op (T) is governed by boundary phonon scattering from bottlenecks in horn-shaped channels interconnecting the octahedral and tetrahedral first-order opal voids filled by sodium chloride. It was found that the value of κ ph op (T) in this temperature region depends substantially on the dimensions of the bottlenecks, whose thicknesses are related to the amount of the cristobalite forming in a near-surface layer of the amorphous SiO2 opal spheres in the course of preparation of the opal + NaCl nanocomposite.  相似文献   

4.
Heat capacity of the PbMg1/3Nb2/3O3 compound is measured using the methods of adiabatic and differential scanning calorimetry in the temperature range 80–750 K. Two blurred anomalies on the C p (T) dependence are observed in wide temperature intervals of 200–400 K and 500–700 K. The results of studies are discussed together with data on the structure and phonon spectrum in the framework of spherical random bond-random field model.  相似文献   

5.
We have studied the behavior of the thermal expansion coefficient α(T) (in a zero magnetic field and at H≈4 T), the heat capacity C(T), and the thermal conductivity κ(T) of magnesium boride (MgB2) in the vicinity of Tc and at lower temperatures. It was established that MgB2, like oxide-based high-temperature superconductors, exhibits a negative thermal expansion coefficient at low temperatures. The anomaly of α(T) in MgB2 is significantly affected by the magnetic field. It was established that, in addition to the well-known superconducting transition at Tc≈40 K, MgB2 exhibits an anomalous behavior of both heat capacity and thermal conductivity in the region of T≈10–12 K. The anomalies of C(T) and κ(T) take place in the same temperature interval where the thermal expansion coefficient of MgB2 becomes negative. The low-temperature anomalies are related to the presence of a second group of charge carriers in MgB2 and to an increase in the density of the Bose condensate corresponding to these carriers at Tc2≈10–12 K.  相似文献   

6.
A new perovskite-like compound Er0.73Cu3V4O12 (space group Im \(\bar 3\), Z = 2, a = 7.266 Å) has been synthesized barothermally (P = 8.0 GPa, t = 1000°C). Its electrical and magnetic properties have been studied. It is found that the temperature dependence of the electrical conductivity (in the range 78–300 K) has of semiconductor type. The behavior of the impedance and admittance has been analyzed at 290 K and frequencies of 200 Hz to 200 kHz under atmospheric pressure and at high (15–42 GPa) pressures.  相似文献   

7.
The thermoelectric properties of n-Bi2 ? x Sb x Te3 ? y ? z Se y S z solid solutions are studied in the temperature range 300–550 K. It is shown that an increase in the parameter β determining the figure-of-merit Z of the material is observed in compositions with the optimally related effective mass of the density of states m/m 0, the carrier mobility μ0, and the lattice thermal conductivity κ L . Within the temperature range 300–350 K, the parameter β and the figure-of-merit Z are found to increase in solid solutions with substitutions in both bismuth telluride sublattices Bi → Sb and Te → Se, S (x = 0.16, y = z = 0.12) for optimum electron concentrations. An increase in the electron concentration and substitutions of atoms only in the tellurium sublattice bring about an increase in the β parameter and the value of Z at higher temperatures. Within the range 350–450 K, the parameters β and Z are observed to increase in a solid solution with a low content of substituted atoms in the tellurium sublattice Te → Se, S for y = z = 0.09 and, at higher temperatures up to 550 K, in compositions with tellurium substituted by selenium only, with increasing content of substituted atoms.  相似文献   

8.
Detailed measurements of the Seebeck coefficient S(T) in a broad range of temperatures (T = 2–300 K) have been performed for the first time for RB12 dodecaborides (R = Ho, Er, Tm, Lu) in paramagnetic (diamagnetic for LuB12) and antiferromagnetic states. At intermediate temperatures (10–300 K), the thermopower is determined by the interaction of carriers with phonon modes, which are related to the oscillations of rare-earth atoms in the framework of atomic clusters B12. A comparative analysis of the parameters determining photon drag the thermopower related to the phonon drag and the results of galvanomagnetic measurements shows evidence for a significant effect of spin fluctuations on the behavior of charge transport characteristics in RB12 compounds with strong electron correlations.  相似文献   

9.
The temperature and magnetic-field dependences of the heat capacity, thermal conductivity, thermopower, and electrical resistivity of the Sm0.55Sr0.45MnO3.02 ceramic material are studied in the temperature range 77–300 K and in magnetic fields up to 26 kOe. It is revealed that the quantities under investigation exhibit anomalous behavior due to a magnetic phase transition at the Curie temperature TC. An increase in the magnetic field strength H leads to an increase in the Curie temperature TC and a jump in the heat capacity ΔCp at TC. The temperature dependences of the measured quantities are characterized by hystereses that are considerably suppressed in a magnetic field of 26 kOe and depend neither on the thermocycling range nor on the rate of change in the temperature. The thermal conductivity K at temperatures above TC shows unusual behavior for crystalline solids (dK/dT>0) and, upon the transition to a ferromagnetic state, drastically increases as a result of a decrease in the phonon scattering by Jahn-Teller distortions. It is demonstrated that the hystereses of the studied properties of the Sm0.55Sr0.45MnO3.02 manganite are caused by a jumpwise change in the critical temperature due to variations in the lattice parameters upon the magnetic phase transition.  相似文献   

10.
Samples of various compositions were obtained in the homogeneity range of the Yb-In-Cu system (YbIn1?xCu4+x), from stoichiometric (YbInCu4) to YbIn0.905Cu4.095. Their lattice constant (at 300 K and in the range 20–100 K), total thermal conductivity, and electrical resistivity (from 4 to 300 K) were measured. All the compositions studied exhibited an isostructural phase transition at T v ?40–80 K driven by a change in the Yb ion valence state. It was shown that within the YbIn1?xCu4+x homogeneity range, the lattice thermal conductivity κph decreases with increasing x; at T>T v , κph grows with temperature and the Lorenz number (which enters the Wiedemann-Franz law for the electronic component of thermal conductivity) of the light heavy-fermion system, to which YbIn1?xCu4+x belongs for T<T v , behaves as it does in classical heavy-fermion systems. Thermal cycling performed through T v generates stresses in the YbIn1?xCu4+x lattice, which entails an increase in the electrical resistivity and a decrease in the thermal conductivity. “Soft anneal” (prolonged room-temperature aging of samples) makes the effect disappear. A conclusion is drawn as to the nature of the effects observed.  相似文献   

11.
Tm x Cu3V4O12, a perovskite-like oxide (space group, Im-3; Z = 2; a = 7.279–7.293 Å) containing vacancies in its cationic sublattice, was obtained barothermally (P = 7.0–9.0 GPa, t = 1000–1100°C) for the first time. The temperature dependences on the electrical resistivity (10–300 K) and the magnetic susceptibility (0–300 K) were investigated. It was shown that the oxide Tm x Cu3V4O12 is characterized by metal-type conductivity and paramagnetic properties.  相似文献   

12.
The galvanomagnetic and magnetic properties of EuB6 single crystal have been measured over wide temperature (1.8–300 K) and magnetic-field (up to 70 kOe) ranges, and the parameters of charge carriers and the characteristics of the magnetic subsystem are estimated in the paramagnetic and ferromagnetic (T < T C ≈ 13.9 K) phases of this compound with strong electron correlations. In the temperature range T < T* ≈ 80 K, a magnetoresistance hysteresis Δρ(H)/ρ(0) is detected; it reaches a maximum amplitude of about 5% at T ≈ 12 K. The anomalies of charge transport observed in the temperature range T C < T < T* are shown to be related to the magnetic scattering of charge carriers (m eff = (15–30)m 0, where m 0 is the free-electron mass) that results from a short-range magnetic order appearing upon the formation of ferromagnetic nanoregions (ferrons).  相似文献   

13.
Samples of the opal + HgSe nanocomposite with 100% filling of the first-order opal pores by mercury selenide were prepared. The effective thermal conductivity κeff and electrical resistivity ρeff were measured in the temperature range T=5–200 K, and the thermopower coefficient α was measured in the interval 80–300 K. The coefficient α of HgSe in opal was shown to remain the same as that in bulk mercury selenide samples with similar carrier concentrations. The mechanism of carrier scattering in the HgSe loaded in opal also did not change. The total thermal conductivity κ tot 0 and electrical resistivity ρ0 were isolated from κeff and ρeff, and the electronic (κ e 0 ) and lattice (κ ph 0 ) components of thermal conductivity of HgSe in opal were determined. The magnitude of κ ph 0 was found to be considerably smaller than κph of bulk HgSe with the same carrier concentration throughout the temperature interval studied (5–200 K). For T>20 K, this behavior of κ ph 0 (T) is accounted for by the presence of specific impurities and defects forming in HgSe, and for T<20 K, by the onset of boundary scattering of phonons in the bottlenecks of the horn-shaped channels connecting first-order octahedral and tetrahedral opal pores loaded by mercury selenide.  相似文献   

14.
Temperature dependences of specific heat Cp(T) and coefficient of thermal expansion ;(T) for Na0.95Li0.05NbO3 sodium-lithium niobate ceramic samples are investigated in the temperature range of 100–800 K. The Cp(T) and α(T) anomalies at T3 = 310 ± 3 K, T2 = 630 ± 8 K, and T1 = 710 ± 10 K are observed, which correspond to the sequence of phase transitions N ? Q ? S(R) ? T2(S). The effect of heat treatment of the samples on the sequence of structural distortions was established. It is demonstrated that annealing of the samples at 603 K leads to splitting of the anomaly corresponding to the phase transition QR/S in two anomalies. After sample heating to 800 K, the only anomaly is observed in both the Cp(T) and ;(T) dependence. Possible mechanisms of the observed phenomena are discussed.  相似文献   

15.
This paper reports on the results of measurements of the internal friction Q?1 and the shear modulus G of Li2B4O7 single crystals along the crystallographic directions [100] and [001] in the temperature range 300–550 K for strain amplitudes of (2–10)×10?5 at infralow frequencies. The anomalies observed in Q?1 and G in the temperature range 390–410 K are due to thermal activation of the mobility of lithium cations and their migration from one energetically equivalent position to another. A jump in the internal friction background is revealed in the vicinity of the Q?1 and G anomalies for the Li2B4O7 crystal. The magnitude of this jump depends on the crystallographic direction.  相似文献   

16.
New data on the specific heat, thermal expansion, and magnetization of the CaMn7O12 phase require a revision of the current concepts of the sequence of phase transitions in this compound. It is found that a spin-glass phase transition occurs in CaMn7O12 at T M = 49 K, whereas the transition at T S = 89 K exhibits the features of a first-order phase transition and thereby is apparently of structural origin. In the range T M < T< T S , the CaMn7O12 compound exhibits negative thermal expansion, which is also indicative of structural changes.  相似文献   

17.
The anisotropy in the superconducting properties of single-crystal Nd1.85Ce0.15CuO4 was studied from measurements of the heat capacity within the temperature interval 2–40 K in zero magnetic field and in a magnetic field of 8 T. We report on the first observation of heat capacity jumps occurring at the superconducting transition for various magnetic field orientations with respect to the crystallographic axes and on a strong anisotropy of the magnetic contribution to heat capacity in magnetic fields oriented in the a-b plane and perpendicular to it. These measurements yielded the anisotropy in the electronic heat capacity coefficient γn(H) and in the superconducting transition temperature Tc(H). The angular dependence of the Sommerfeld coefficient γn in the a-b plane observed in a magnetic field of 8 T exhibits four-lobe symmetry and zero gap direction of the order parameter. A comparison of the results obtained on the Nd1.85Ce0.15CuO4 single crystal with the data available for La1.85Sr0.15CuO4 permits one to conclude that the mechanisms of superconductivity in the electron-and hole-doped superconductors are similar.  相似文献   

18.
The transmission spectra of HoFe3(BO3) multiferroic single crystals are studied by optical Fourier-transform spectroscopy at temperatures of 1.7–423 K in polarized light in the spectral range 500–10 000 cm–1 with a resolution up to 0.1 cm–1. A new first-order structural phase transition close to the second-order transition is recorded at Tc = 360 K by the appearance of a new phonon mode at 976 cm–1. The reasons for considerable differences in Tc for different samples of holmium ferroborate are discussed. By temperature variations in the spectra of the f–f transitions in the Ho3+ ion, we studied two magnetic phase transitions, namely, magnetic ordering into an easy-plane structure as a second-order phase transition at TN = 39 K and spin reorientation from the ab plane to the c axis as a first-order phase transition at TSR = 4.7 ± 0.2 K. It is shown that erbium impurity in a concentration of 1 at % decreases the spin-reorientation transition temperature to TSR = 4.0 K.  相似文献   

19.
The unit cell parameters a, b, and c of [N(CH3)4]2ZnCl4 have been measured by x-ray diffraction in the temperature range 80–293 K. Temperature dependences of the thermal expansion coefficients αa, αb, and αc along the principal crystallographic axes and of the unit cell thermal expansion coefficient αV were determined. It is shown that the a=f(T), b=f(T), and c=f(T) curves exhibit anomalies in the form of jumps at phase transition temperatures T1=161 K and T2=181 K and that the phase transition occurring at T3=276 K manifests itself in the a=f(T) and b=f(T) curves as a break. A slight anisotropy in the coefficient of thermal expansion of the crystal was revealed. The phase transitions occurring at T1=161 K and T2=181 K in [N(CH3)4]2ZnCl4 were established to be first-order.  相似文献   

20.
The temperature dependences of the specific heat C(T) and thermal conductivity K(T) of MgB2 were measured at low temperatures and in the neighborhood of T c . In addition to the well-known superconducting transition at T c ≈40 K, this compound was found to exhibit anomalous behavior of both the specific heat and thermal conductivity at lower temperatures, T≈10–12 K. Note that the anomalous behavior of C(T) and K(T) is observed in the same temperature region where MgB2 was found to undergo negative thermal expansion. All the observed low-temperature anomalies are assigned to the existence in MgB2 of a second group of carriers and its transition to the superconducting state at Tc2≈10?12 K.  相似文献   

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