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1.
The temperature dependence of the specific (per atom) entropy and heat capacity of a nanocrystal is studied using a nanocrystal model in the form of a rectangular parallelepiped with variable surface shape. Accounting for the temperature dependence of the surface energy showed that the temperature dependence of the surface contribution to specific entropy is described by the same function that determines the temperature dependence of the isochoric heat capacity of a macrocrystal. Thus, at T → 0 K at T/Θ > 2 the surface contribution to the specific heat is zero. The maximal surface contribution to specific heat is reached at T/Θ = 0.2026 and is equal to c st/k B = 1.0115 (where k B is the Boltzmann constant, Θ is the characteristic temperature depending both on the size and the shape of the nanocrystal). The applicability of the Grüneisen rule for a nanocrystal both at low and high temperatures is studied. It has been found that a case when the surface contribution to specific heat would be negative c(N) < c(∞), i.e. c st(N) < 0 can occur for nanocrystals with a noncubic habitus.  相似文献   

2.
Measurements of the temperature dependence of the electrical resistance R(T) below the superconducting transition temperature have been performed at different values of the transport current in HTSC+CuO composites modeling a network of weak S-I-S Josephson junctions (S—superconductor, I—insulator). It has been shown experimentally that the temperature dependence R(T) at different values of the transport current is adequately described by means of the mechanism of thermally activated phase slippage developed by Ambegaokar and Halperin for tunnel structures. Within the framework of this model we have numerically calculated the temperature dependence of the critical current J c(T) as defined by various criteria. Qualitative agreement obtains between the measured and calculated temperature dependences J c(T). Fiz. Tverd. Tela (St. Petersburg) 41, 969–974 (June 1999)  相似文献   

3.
The behavior of the thermal conductivity k(T) of bulk faceted fullerite C60 crystals is investigated at temperatures T=8–220 K. The samples are prepared by the gas-transport method from pure C60, containing less than 0.01% impurities. It is found that as the temperature decreases, the thermal conductivity of the crystal increases, reaches a maximum at T=15–20 K, and drops by a factor of ∼2, proportional to the change in the specific heat, on cooling to 8 K. The effective phonon mean free path λ p, estimated from the thermal conductivity and known from the published values of the specific heat of fullerite, is comparable to the lattice constant of the crystal λ pd=1.4 nm at temperatures T>200 K and reaches values λp∼50d at T<15 K, i.e., the maximum phonon ranges are limited by scattering on defects in the volume of the sample in the simple cubic phase. In the range T=25−75 K the observed temperature dependence k(T) can be described by the expression k(T)∼exp(Θ/bT), characteristic for the behavior of the thermal conductivity of perfect nonconducting crystals at temperatures below the Debye temperature Θ (Θ=80 K in fullerite), where umklapp phonon-phonon scattering processes predominate in the volume of the sample. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 651–656 (25 April 1997)  相似文献   

4.
It is shown that the square-root van Hove singularity appearing in the density of states ν (E F )∼(E F −E 0)−1/2 as a result of extended saddle-point singularities in the electron spectrum of high-T c superconductors based on hole-type cuprate metal-oxide compounds gives a nonmonotonic dependence of the critical temperature T c on the position of the Fermi level E F relative to the bottom E 0 of the saddle. Because the divergence of ν(E F ) is canceled in the electron-electron interaction constant renormalized by strong-coupling effects, T c approaches zero as E F →E 0, in contrast to the weak-coupling approximation, where in this limit T c approaches a finite (close to maximum) value. The dependence obtained for T c as a function of the doped hole density in the strong-coupling approximation agrees qualitatively with the experimental data for overdoped cuprate metal oxides. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 7, 473–477 (10 April 1998)  相似文献   

5.
Results are presented of experimental studies of how the spontaneous polarization of β-BaB2O4 (BBO) varies with temperature in the range from 2.5 to 90 K. Values of the pyroelectric coefficient γ s are calculated, and effective values of the Debye temperature (Θ=112 K) and the Einstein temperature (ΘE=28 K) are estimated. It is shown that the dominant contribution to γ s comes from acoustic lattice modes. A mechanism is proposed to explain why the temperature dependence of the pyroelectric effect in linear pyroelectric materials is different from that in ferroelectrics. Fiz. Tverd. Tela (St. Petersburg) 39, 1631–1633 (September 1997)  相似文献   

6.
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)  相似文献   

7.
The spin-wave resonance method is used to investigate the temperature dependence of the exchange interaction constant A in iron garnet films. The experimental results are compared with theoretical results based on the theory of second-order phase transitions. It is shown that the temperature dependence A(T) for films of pure yttrium iron garnet is different from A(T) for films exhibiting a point of compensation of the magnetic moment. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 528–532 (10 April 1996)  相似文献   

8.
The effective Debye temperatures Θeff determined for solids by different physical methods have been analyzed and compared. Attention has been focused on the original parameter of the Debye theory of heat capacity, i.e., the translational calorimetric Debye temperature Θ c t (0), and the X-ray Debye temperature Θ x in the framework of the Debye-Waller theory for the C60 fullerite. It has been established that the true Debye law T 3 is satisfied for the C60 fullerite over a very narrow range of temperatures: 0.4 K ≤ T ≤ 1.8 K. For this reason, the experimental Debye temperatures Θ c t (0) obtained for the C60 fullerite by different authors in the range T > 4.2 K are characterized by a large scatter (by a factor of ∼5). It has been revealed that the value Θ c t (0) = 77.12 K calculated in this paper with the use of the six-term Betts formula from the harmonic elastic constants $ \tilde C_{ijkl} $ \tilde C_{ijkl} of the C60 single crystal in the limit T = 0 K is closest to the true Debye temperature. It has been demonstrated using the method of equivalent moments that the real spectral frequency distribution of translational lattice vibrations g(ω) for the C60 fullerite deviates from a parabolic distribution. The effective Debye temperatures Θeff involved in applied problems of thermodynamics of crystals and elastic scattering of different radiations from lattice vibrations have been determined. The quantitative measure of anharmonicity of translational and librational lattice vibrations of the C60 fullerite has been determined. This has made it possible to empirically evaluate the lattice thermal conductivity κ of the C60 fullerite at T ≈ 300 K: κ(300) = 0.80 W (m/K), which is in good agreement with the experimental thermal conductivity κexp = 0.78 W (m/K) at T ≈ 250 K.  相似文献   

9.
The rearrangement of the Fermi surface in a diluted two-dimensional electron gas beyond the topological quantum critical point has been examined within an approach based on the Landau theory of Fermi liquid and a nonperturbative functional method. The possibility of a transition of the first order in the coupling constant at zero temperature between the states with a three-sheet Fermi surface and a transition of the first order in temperature between these states at a fixed coupling constant has been shown. It has also been shown that a topological crossover, which is associated with the joining of two sheets of the Fermi surface and is characterized by the maxima of the density of states N(T) and ratio C(T)/T of the specific heat to the temperature, occurs at a very low temperature T determined by the structure of a state with the three-sheet Fermi surface. A momentum region where the distribution n(p, T) depends slightly on the temperature, which is manifested in the maximum of the specific heat C(T) near T *, appears through a crossover at temperatures TT * > T . It has been shown that the flattening of the single-particle spectrum of the strongly correlated two-dimensional electron gas results in the crossover from the Fermi liquid behavior to a non-Fermi liquid one with the density of states N(T) ∝ T −α with the exponent α }~ 2/3.  相似文献   

10.
We make a new proposal to describe the very low temperature susceptibility of the doped Haldane gap compound Y2BaNi1-xZnxO5. We propose a new mean field model relevant for this compound. The ground state of this mean field model is unconventional because antiferromagnetism coexists with random dimers. We present new susceptibility experiments at very low temperature. We obtain a Curie-Weiss susceptibility χ( T ) ∼ C /(Θ + T ) as expected for antiferromagnetic correlations but we do not obtain a direct signature of antiferromagnetic long range order. We explain how to obtain the “impurity” susceptibility ( T ) by subtracting the Haldane gap contribution to the total susceptibility. In the temperature range [1 K, 300 K] the experimental data are well fitted by T ( T ) = C imp 1 + T imp / T . In the temperature range [100 mK, 1 K] the experimental data are well fitted by T ( T ) = A ln( T / T c ), where T c increases with x. This fit suggests the existence of a finite Néel temperature which is however too small to be probed directly in our experiments. We also obtain a maximum in the temperature dependence of the ac-susceptibility ( T ) which suggests the existence of antiferromagnetic correlations at very low temperature. Received 17 July 2001  相似文献   

11.
The effect of an electric field on the differential thermopower α(E) of a one-dimensional superlattice is investigated in the semiclassical approximation. A nonmonotonic temperature dependence of α(0) is established for a degenerate electron gas. It is shown that, in principle, an electric field can be used to control the thermoelectric properties of superlattices. Fiz. Tverd. Tela (St. Petersburg) 41, 1314–1316 (July 1999)  相似文献   

12.
A study of the thermal expansion coefficient (TEC) of the ScxTi1−x Fe2 itinerant magnets has been made within the 5–1200 K range at the transition from the TiFe2 antiferromagnet (T N=270 K) to the ScFe2 ferromagnet (T C=540 K). A negative TEC magnetic contribution α m(T) has been found, which is associated with the formation of spin-fluctuation-induced local magnetic moments in both the magnetically ordered and the paramagnetic state. The specific features in the α m(T) dependence are shown to be due to the shape of the density-of-states function near the Fermi level. Fiz. Tverd. Tela (St. Petersburg) 41, 2174–2178 (December 1999)  相似文献   

13.
The effect of a bias field E on the value and temperature dependence of permittivity ɛ of ceramic samples of solid solutions 0.6NaNbO3-0.4 NaTaO3 (I), and 0.97[0.9NaNbO3-0.1Na0.5Bi0.5TiO3]-0.03LiNbO3 (II) possessing an anomalously large temperature hysteresis of ɛ(T) dependence is studied. The character of the dependences of both the height and the temperature of the ɛ(T) maximum on the field show that composition (I) has antiferroelectric properties, while composition (II) has ferroelectric properties. It is found that up to 8 kV/cm, the strength of the bias field has actually no effect on the width of the ɛ(T) temperature hysteresis of the compositions under study.  相似文献   

14.
The magnetic moment M, the magnetic susceptibility χ, and the thermal conductivity of chalcopyrite CuFeS2, which is a zero-gap semiconductor with antiferromagnetic ordering, have been measured in the temperature range 10–310 K. It has been revealed that the quantities χ(T) and M(T) increase anomalously strongly at temperatures below ∼100 K. The temperature dependence M(T) is affected by the magnetic prehistory of the sample. An analysis has demonstrated that the magnetic anomalies are associated with the presence of a system of noninteracting magnetic clusters in the CuFeS2 sample under investigation. The formation of the clusters is most likely caused by the disturbance of the ordered arrangement of Fe and Cu atoms in the metal sublattice of the chalcopyrite, which is also responsible for the phase inhomogeneity of the crystal lattice. The inhomogeneity brings about strong phonon scattering, and, as a result, the temperature dependence of the thermal conductivity coefficient exhibits a behavior characteristic of partially disordered crystals.  相似文献   

15.
A theoretical framework for treating the effects of magnetic fieldH on the pairing theory of superconductivity is considered, where the field is taken in an arbitrary direction with respect to crystal axes. This is applicable to closed, as well as open normal state Fermi surface (FS), including simple layered metals. The orbital effects of the magnetic field are treated semiclassically while retaining the full anisotropic paramagnetic contribution. Explicit calculations are presented in the limits |H| → |H c2(T)|,T ∼ 0 andTT c(|H|), |H| ∼ 0. Effects of weak nonmagnetic impurity scattering, without vertex corrections, have also been taken into account in a phenomenological way. The final results for the case of open FS and layered materials are found to differ considerably from those of the closed FS. For example, an important parameter,h(T=0)=|Hc2(0)|/[-Tδ|H c2 TT]T{s0} for the case of a FS open ink z-direction with thek z-bandwidth, 4t 3, very small compared to the Fermi energy,E F, is close to 0.5906, compared to 0.7273 for the closed FS, in the clean limit. Analytical results are given for the magnetic field dependence ofT c and the temperature dependence of H c2 for a model of layered superconductors with widely open FS. For a set of band structure parameters for YBa2Cu3O7 used elsewhere, we find reasonable values for the upper critical fieldH c2(0), the slope (dH c2/dT)T c0, anisotropic coherence lengths ζi(T=0),i=x, y, z, and (dT c/d|H|)|H| → 0.  相似文献   

16.
The temperature dependences of the order parameter η(T) for sodium nitrite NaNO2 embedded in porous glasses with average pore diameters of 320 and 20 nm, as well as in artificial opals, have been investigated. It has been demonstrated that the dependence η(T) for sodium nitrite in the porous glass almost coincides with that for the bulk material, whereas this dependence for NaNO2 in opals differs substantially from that observed in the bulk material and from those previously determined for sodium nitrite in porous glasses with average pore diameters of 3 and 7 nm. It has been revealed that the dependence of the order parameter for sodium nitrite in opals exhibits a temperature hysteresis (approximately equal to 8 K). The temperature dependence η(T) has been described using a simple model, which takes into account the nanopore diameter distribution existing in artificial opals.  相似文献   

17.
Data on the thermal conductivity ϰ ph of the cluster lattice of synthetic opals are analyzed. All opals are divided into two groups according to the temperature dependence of their ϰ ph. These are opals-1, whose thermal conductivity behaves like ϰ ph(T) of quasi-crystalline materials, and opals-2, with a ϰ ph(T) dependence typical of classical amorphous solids. Possible reasons for this difference are considered. An attempt is made to explain the complex temperature hysteresis in ϰ ph(T) observed earlier in opals-2. Fiz. Tverd. Tela (St. Petersburg) 39, 392–398 (February 1997)  相似文献   

18.
The dependences of the magnetization M and the magnetic susceptibility χ = ∂M/∂H of pure gadolinium (the concentration of foreign impurities is lower than 0.1 wt %) on the temperature T and the magnetic field H have been measured using a Quantum Design MPMS-5XL SQUID magnetometer. In this material, inhomogeneities of the atomic structure should not lead to a nonuniform distribution of the magnetic characteristics (including the Curie temperature T C) over the volume of the sample. The obtained dependences of M and χ have been used to investigate the possibility of suppressing magnetic inhomogeneities of other types by magnetic fields with a strength of up to 50 kOe. It has been assumed that these inhomogeneities are suppressed when the specific relationship, namely, the 21/3 rule is fulfilled. The rule relates the portions of the dependence χ(T, H) which at the temperature T = T C and at the maximum in the curve χ(T) (T = T m ) depend on H in accordance with the H 2/3 law. It has been shown that the portions separated from the experimental curves χ(T C, H) and χ(T m , H) obey the 21/3 rule.  相似文献   

19.
The magnetoresistance Δρ/ρ of single-crystal samples of praseodymium and neodymium hexaborides (PrB6 and NdB6) has been measured at temperatures ranging from 2 to 20 K in a magnetic field of up to 80 kOe. The results obtained have revealed a crossover of the regime from a small negative magnetoresistance in the paramagnetic state to a large positive magnetoresistive effect in magnetically ordered phases of the PrB6 and NdB6 compounds. An analysis of the dependences Δρ(H)/ρ has made it possible to separate three contributions to the magnetoresistance for the compounds under investigation. In addition to the main negative contribution, which is quadratic in the magnetic field (−Δρ/ρ ∝ H 2), a linear positive contribution (Δρ/ρ ∝ H) and a nonlinear ferromagnetic contribution have been found. Upon transition to a magnetically ordered state, the linear positive component in the magnetoresistance of the PrB6 and NdB6 compounds becomes dominant, whereas the quadratic contribution to the negative magnetoresistance is completely suppressed in the commensurate magnetic phase of these compounds. The presence of several components in the magnetoresistance has been explained by assuming that, in the antiferromagnetic phases of PrB6 and NdB6, ferromagnetic nanoregions (ferrons) are formed in the 5d band in the vicinity of the rareearth ions. The origin of the quadratic contribution to the negative magnetoresistance is interpreted in terms of the Yosida model, which takes into account scattering of conduction electrons by localized magnetic moments of rare-earth ions. Within the approach used, the local magnetic susceptibility χloc has been estimated. It has been demonstrated that, in the temperature range T N < T < 20 K, the behavior of the local magnetic susceptibility χloc for the compounds under investigation can be described with good accuracy by the Curie-Weiss dependence χloc ∝ (T − Θ p )−1.  相似文献   

20.
Calculations of the complex elastic moduli Ĉ(T) in dilute Cr alloys are compared to measurements of the velocity and damping of sound near T N and at high temperatures T>T N (T N — Néel temperature). The thermodynamic calculation is based on the covalent bond model of 3d ions in a state with different numbers n of covalent electrons. The parameters A ij (n) of indirect exchange between the ions of the i and j sublattices are expressed in terms of the covalent bond energy Γ ij (n) . The stability of the charge and spin density waves (CDWs and SDWs) is found by a variational method and is determined by the dispersion of Γ ij (n) and by the Coulomb parameters U n. For a small structural vector Q the phase diagram contains a superantiferromagnetic phase (SAFM) at temperatures T N<T≲2T N. The peak of the defect |ΔE(T)| of the modulus and of the sound damping Δh(T N) near the first-order SDW-SAFM transition is determined by the structure of the transitional domain. Measurements of the anomalous growth of E(T) at temperatures T>T N make it possible to determine the magnetostriction constants λ(T) of Ce alloys in the SAFM phase on the basis of the SAFM theory. Fiz. Tverd. Tela (St. Petersburg) 41, 1467–1472 (August 1999)  相似文献   

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