共查询到20条相似文献,搜索用时 921 毫秒
1.
V. N. Likhachev T. Yu. Astakhova G. A. Vinogradov 《Russian Journal of Physical Chemistry B, Focus on Physics》2011,5(6):1023-1037
In this work, we propose a new approach to the computation of heat conductivity in nonlinear systems. The total heat conductivity
process is decomposed into two parts: one part is an equilibrium process at the same temperature T of either end of the lattice, which does not transfer energy and the other is a nonequilibrium process at temperature ΔT of one end and a zero temperature of the opposite end of the lattice. This approach makes it possible to somewhat reduce
the time of computation of heat conductivity at ΔT → 0. The threshold temperature T
thr is found to behave as T
thr ∼ N
−3, where N is the lattice length. The threshold temperature conventionally separates two mechanisms of heat conductivity: at T < T
thr, phonon heat conductivity is dominant; at T > T
thr, the contribution of soliton heat conductivity increases with increasing temperature. The problem of the computation of heat
conductivity in the limit ΔT → 0 reduces to the heat conductivity of a harmonic lattice with time-dependent bond rigidities determined by an equilibrium
process at temperature T. An exact expression for the temperature dependence of sound velocity in a lattice with a β-FPU potential at T < 10 is derived. A numerical experiment confirmed the existence of solitons and breathers that correspond to a modified Korteweg-de
Vries (KdV) equation. The problem of the quantitative contribution of solitons and breathers to heat conductivity requires
further study. 相似文献
2.
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 T ∼ T * > 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
3.
M. N. Magomedov 《Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques》2012,6(1):86-91
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
4.
V. P. D’yakonov V. I. Markovich V. L. Kovarskii A. V. Markovich M. Borowiec A. Jendrzejczak H. Szymczak 《Physics of the Solid State》1999,41(3):440-444
The investigation of the specific heat of a RbDy(WO4)2 single crystal at temperatures 0.2–2.5 K and in magnetic fields up to 2 T are reported. The temperature dependence of the
specific heat near T
N=0.818 K is compared with the predictions for different models. The 2D Ising model describes satisfactorily C(T) below T
N, while for T>T
N none of the theoretical models agree with the behavior of C(T) of RbDy(WO4)2. The H-T phase diagram for H∥c is complicated and possesses a triple point, where regions of existence of three magnetic phases converge. The magnetic ordering
is analyzed from the standpoint of the Jahn-Teller nature of the structural phase transitions occurring in RbDy(WO4)2 at higher temperatures. It is shown that the form of the phase diagram depends on the direction of the vector H, for the general case of an arbitrary direction of H, two phase transitions can occur with increasing field.
Fiz. Tverd. Tela (St. Petersburg) 41, 491–496 (March 1999) 相似文献
5.
We consider a Hamiltonian system made of weakly coupled anharmonic oscillators arranged on a three dimensional lattice , and subjected to stochastic forcing mimicking heat baths of temperatures T
1 and T
2 on the hyperplanes at 0 and N. We introduce a truncation of the Hopf equations describing the stationary state of the system which leads to a nonlinear
equation for the two-point stationary correlation functions. We prove that these equations have a unique solution which, for
N large, is approximately a local equilibrium state satisfying Fourier law that relates the heat current to a local temperature
gradient. The temperature exhibits a nonlinear profile.
Partially supported by the Academy of Finland. 相似文献
6.
M. N. Magomedov 《Technical Physics》2011,56(9):1277-1282
Expressions for the melting point (T
m
), freezing temperature (T
N
< T
m
), entropy change per atom (Δs), latent heat (Δh = T
m
Δs), and volume change (Δv) for the solid-liquid phase transition are derived from a model of a nanocrystal in the form of a parallelepiped with a variable
shape of the surface. These quantities are studied as a function of the number of atoms (N) and the shape of the nanoparticle. Calculations carried out for copper nanoparticles show good agreement with the results
of computational experiments. It is shown that functions Δs, Δh, and Δv vanish in a certain range of cluster dimension N
0 and a hysteresis between the melting point and freezing temperature disappears, T
N
(N
0) = T
m
(N
0). In such a cluster, the phases become physically identical. For nanocopper, this dimension falls into the range N
0 = 49–309 and grows when the shape of the nanoparticle deviates from the energetically most favorable one. 相似文献
7.
Mekata M. Kikuchi H. Watanabe I. Nagamine K. Itoh S. Mamiya H. Kojima K. M. 《Hyperfine Interactions》2001,136(3-8):263-268
The magnetic ordering process of Ising spins on diluted square lattice was studied by muon spin relaxation using model compounds
Rb2Co
c
Mg1−c
F4. Muon relaxation shows an anomaly at a remarkably higher temperature T
N
μSR than the transition temperature determined by neutron Bragg scattering T
N
ND near the percolation threshold for square lattice (c
p=0.593). The difference between the two temperatures amounts to 50% of T
N
ND just above c
p. The field cooling effect of DC magnetic susceptibility is appreciable below T
N
ND while the temperature of the anomaly in AC susceptibility approaches to T
N
μSR as the frequency is increased. It was concluded that there is a crossover from two-dimensional ordering at T
N
μSR to three-dimensional ordering at T
N
ND but the two-dimensional order between T
N
μSR and T
N
ND has slow fluctuations due to the fractal structure with a plenty of weak links.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
8.
The heavy-fermion metal CePd1−x
Rh
x
can be tuned from ferromagnetism at x = 0 to the nonmagnetic state at some critical concentration x
c
. The non-Fermi liquid behavior (NFL) at x ≃ x
c
is recognized by the power-law dependence of the specific heat C(T) given by the electronic contribution susceptibility X(T) and volume expansion coefficient α(T) at low temperatures: C/T ∝ X(T) ∝ α(T)/T∝ 1/ √T. We also demonstrate that the behavior of the normalized effective mass M
N
*
observed in CePd1−x
Rh
x
at x ≃ 0.8 agrees with that of M
N
*
observed in paramagnetic CeRu2Si2 and conclude that these alloys exhibit the universal NFL thermodynamic behavior at their quantum critical points. We show
that the NFL behavior of CePd1−x
Rh
x
can be accounted for within the frameworks of the quasiparticle picture and fermion condensation quantum phase transition,
while this alloy exhibits a universal thermodynamic NFL behavior that is independent of the characteristic features of the
given alloy such as its lattice structure, magnetic ground state, dimension, etc.
The text was submitted by the authors in English. 相似文献
9.
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
10.
The incorporation of deep levels in high purity vapour phase epitaxy (VPE) GaAs is studied as a function of the crystal growth
conditions. Two deep levels, at 0.4 eV and 0.75 eV above the valence band, are investigated using photocapacitance. It is
shown that their concentrations are always equal and vary together as a function of the AsCl3 mole fractionX. Two regimes are observed, respectively, characterized by different variations of the total deep level concentrationN
T:N
TαX for lowX, andN
TαX−2 for highX. In this last range,N
T andN
D are found to vary similarly.
This work has been supported by the DRME. 相似文献
11.
本文采用第一性原理框架下的赝势平面波方法结合振动类德拜模型研究了α,β和γ-Si3N4在高温下的点阵常数,弹性常数和弹性模量.研究发现三种同质异相体的体模量都很高.β-Si3N4在低温下表现出脆性,在高温下则表现出延展性.γ-Si3N4在低温和高温下都是脆性的共价化合物.β → γ 相变的相界斜率为正值,说明在较高温度时合成γ-Si3N4所需的压强也较高.α → γ 相变的相界可以表示成 P=16.29- 1.835-10-2 T+9.33945-10-5T2-2.16759-10-7T3+2.91795-10-10T4.本文还分析了Si3N4同质异相体在高压下的态密度和能带.在α-Si3N4中主要是Si-s, p和N-s,p的轨道杂化对晶体的稳定性起作用.α和β-Si3N4都具有ΓV-ΓC类型的间接带隙(分别是4.9~eV和4.4~eV)而γ-Si3N4具有直接带隙(3.9~eV). 研究还发现α-Si3N4和β-Si3N4的价带顶分别沿着Γ-M和Γ-A方向.本文的计算结果和已有的实验数据是一致的. 相似文献
12.
Philippe A. Jacquet 《Journal of statistical physics》2009,134(4):709-748
We introduce a model for charge and heat transport based on the Landauer-Büttiker scattering approach. The system consists
of a chain of N quantum dots, each of them being coupled to a particle reservoir. Additionally, the left and right ends of the chain are
coupled to two particle reservoirs. All these reservoirs are independent and can be described by any of the standard physical
distributions: Maxwell-Boltzmann, Fermi-Dirac and Bose-Einstein. In the linear response regime, and under some assumptions,
we first describe the general transport properties of the system. Then we impose the self-consistency condition, i.e. we fix
the boundary values (T
L,μ
L) and (T
R,μ
R), and adjust the parameters (T
i
,μ
i
), for i=1,…,N, so that the net average electric and heat currents into all the intermediate reservoirs vanish. This condition leads to
expressions for the temperature and chemical potential profiles along the system, which turn out to be independent of the
distribution describing the reservoirs. We also determine the average electric and heat currents flowing through the system
and present some numerical results, using random matrix theory, showing that these currents are typically governed by Ohm
and Fourier laws. 相似文献
13.
D. P. Kozlenko N. T. Dang Z. Jirák S. E. Kichanov E. V. Lukin B. N. Savenko L. S. Dubrovinsky C. Lathe C. Martin 《The European Physical Journal B - Condensed Matter and Complex Systems》2010,77(3):407-411
The crystal and magnetic structures of Pr0.15Sr0.85MnO3
manganite have been studied by means of powder X-ray and neutron diffraction
in the temperature range 10–400 K at high external pressures up to 55 and 4 GPa, respectively. A structural phase transition
from cubic to tetragonal
phase upon compression was observed, with large positive pressure
coefficient of transition temperature dT
ct
/dP = 28(2) K/GPa. The C-type
antiferromagnetic (AFM) ground state is formed below T
N
≈ 260 K
at ambient pressure. While at ambient pressure the structural and magnetic
transition temperatures are close, T
ct
~ T
N
, upon compression
they become decoupled with T
N
≪ T
ct
due to much weaker
T
N
pressure dependence with coefficient dT
N
/dP = 3.8(1) K/GPa. 相似文献
14.
E. I. Golovenchits V. A. Sanin A. V. Babinskii 《Journal of Experimental and Theoretical Physics》1997,85(1):156-162
The magnetic and dielectric properties of EuMn2O5 and BiMn2O5 crystals are investigated over a wide range of temperatures 4.2–250 K, including the range T≫T
N
≃40 K. Significant departures from the Curie-Weiss law are observed in both crystals for the magnetic susceptibility in the
paramagnetic range; they are attributed to the presence of correlated domains of magnetic order over a wide range of temperatures.
Anomalies in the dielectric properties of the crystals are observed in the same temperature range T>T
N
and, as in the case T<T
N
, are correlated with the magnetic properties.
Zh. éksp. Teor. Fiz. 112, 284–295 (July 1997) 相似文献
15.
E. Veuhoff H. Bruch K. -H. Bachem P. Balk 《Applied Physics A: Materials Science & Processing》1980,23(1):37-40
Electron Hall mobilities were measured on a series of intentionally compensated vapor phase epitaxy (VPE) GaAs layers. Using
Sn and Zn as dopants, compensation ratiosK=(ND+NA)/(ND-NA) as high as 50 were obtained. Already for samples with the lowestK values the 300 K mobilities are higher than the 77 K values. In the range 20<T<100 [K] the data may be represented by μ∼T
α with α increasing from 0.6 to 1.1 with compensation. The experimental μ values are smaller than those predicted from current
models in all cases. It appears that scattering at ionized impurities is the dominant process also at temperatures well above
77 K, and that this scattering process is quantitatively underestimated in current models. 相似文献
16.
N. V. Dalakova B. I. Belevtsev A. S. Panfilov E. Yu. Belyaev 《Bulletin of the Russian Academy of Sciences: Physics》2011,75(5):692-694
The temperature dependence of the resistance of a La2CuO4 + δ (δ ≈ 0.05) single crystal with the Neel temperature T
N ≈ 205 K was investigated in order to establish the correlation between the transport and magnetic properties of the crystal.
The R(T) dependence near T
N reveals a kink related to the enhancement of sample’s conductivity upon the transition from the antiferromagnetic to paramagnetic
state. With an increase in temperature far above T
N, the transition from the dielectric (dR/dT < 0) to metal (dR/dT > 0) occurs. The observed behavior of resistance is attributed to delocalization of carriers above T
N. 相似文献
17.
A. Schenck P. Birrer Z. Fisk F. N. Gygax B. Hitti E. Lippelt H. R. Ott M. Weber 《Hyperfine Interactions》1991,64(1-4):511-516
The 1 K phase transition in UCu5, which shows up in specific heat data within the antiferromagnetic state belowT
N
=16 K, was investigated by zero field μSR. The data reveal a pronounced increase in the internal field spread at theμ
+ sites below 1.2 K while the average fields seem to be unaffected. These results are compatible with the onset of a charge
density wave state below 1.2 K. 相似文献
18.
Bulk superconducting samples of type Tl0.5Pb0.5Sr1.6Ba0.4CaCu2−x
Ru
x
O7−δ, (Tl, Pb)/Sr-1212, with 0.0 ≤ x ≤ 0.525 were prepared by the conventional one-step solid-state reaction technique. The prepared samples were investigated
using X-ray powder diffraction, electrical resistivity and electron paramagnetic resonance (EPR) measurements. Enhancement
of the phase formation, superconducting transition temperature T
c and hole carriers concentration P was observed up to x = 0.075. For x > 0.075, a reverse trend was observed. EPR spectra were measured at different temperatures (120–290 K) for all prepared samples.
The number of spins N participating in the resonance and the paramagnetic susceptibility χ were calculated as a function of both Ru-content and
temperature. N and χ increased as the Ru-content increased. A linear relationship between logN and 1/T was established, from which the activation energy E
a was calculated as a function of the Ru-content. The temperature dependence of χ was fitted according to Curie–Weiss type
of magnetic behavior. Curie constant C, Curie temperature θ, the effective magnetic moment μ and the electronic specific heat γ were estimated as a function of the Ru-content. 相似文献
19.
B. I. Belevtsev V. B. Krasovitsky D. G. Naugle K. D. D. Rathnayaka Glenn Agnolet I. Felner 《Bulletin of the Russian Academy of Sciences: Physics》2010,74(8):1049-1052
Magnetic and superconducting properties of polycrystalline samples of RuSr2Gd1.5Ce0.5Cu2O(10 − δ), asprepared (by solid-state reaction) and annealed in pure oxygen at different pressure are presented. Specific heat and
magnetization were investigated in the temperature range 1.8–300 K with a magnetic field up to 8 T. Specific heat, C (T), shows a jump at the superconducting transition (with onset at T ≈ 37.5 K) and a Schottky-type anomaly below 20 K. It is found that curves C(T) taken for different values of magnetic field have the same crossing point (at T
* ≈ 2.7 K) for all samples studied. At the same time, C(H) curves taken for different temperatures have a crossing point at a characteristic field H
* ≈ 3.7 T. These effects are manifestations of the crossing-point phenomenon, which is supposed to be inherent for strongly
correlated electron systems. 相似文献
20.
Schnell B Meyer H Fond C Wittmer JP Baschnagel J 《The European physical journal. E, Soft matter》2011,34(9):97
By means of molecular-dynamics simulation we study a flexible and a semiflexible bead-spring model for a polymer melt on cooling
through the glass transition. Results for the glass transition temperature T
g and for the elastic properties of the glassy state are presented. We find that T
g increases with chain length N and is for all N larger for the semiflexible model. The N dependence of T
g is compared to experimental results from the literature. Furthermore, we characterize the polymer glass below T
g via its elastic properties, i.e., via the Lamé coefficients λ and μ. The Lamé coefficients are determined from the fluctuation formalism which allows to split λ and μ into affine (Born term) and nonaffine (fluctuation term) contributions. We find that the fluctuation term represents a substantial
correction to the Born term. Since the Born terms for λ and μ are identical, the fluctuation terms are responsible for the different temperature dependence of the Lamé coefficients. While
λ decreases linearly on approaching T
g from below, the shear modulus μ displays a much stronger decrease near T
g. From the present simulation data it is not possible to decide whether μ takes a finite value at T
g, as would be expected from mode-coupling theory, or vanishes continuously, as suggested by recent work from replica theory. 相似文献