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1.
The tetragonal compound UNi2Si2 exhibits in zero magnetic field three different antiferromagnetic phases belowT N =124 K. They are formed by ferromagnetic basal planes, which are antiferromagnetically coupled along thec-axis with the propagation vectorq=(0, 0, q z ). Two additional order-order magnetic phase transitions are observed below T N , namely atT 1=108 K and T 2=40 K in zero magnetic field. All three phases exhibit strong uniaxial anisotropy confining the U moments to a direction parallel to the c-axis. UNi2Si2 single crystals were studied in detail by measuring bulk thermodynamic properties, such as thermal expansion, resistivity, susceptibility, and specific heat. A microscopic study using neutron diffraction was performed in magnetic fields up to 14.5 T parallel to the c-axis, and a complex magnetic phase diagram has been determined. Here, we present the analysis of specific-heat data measured in magnetic fields up to 14 T compared with the results of the neutron-diffraction study and with other thermodynamic properties of UNi2Si2.  相似文献   

2.
The results of the three-dimensional liquid droplet model are compared with experiment and other phenomenological theories. The homogeneity assumption of the scaling laws holds both above and belowT c , and various series expansions can be derived. But there is a nonanalyticity near the critical isotherm aboveT c for fixed “field” μ?μ(p c ,T); and liquid and gas are not symmetric about the critical isochore aboveT c . Both results contradict the usual scaling assumptions and experiment. The equation of state is fixed if the density on the coexistence curve and the critical pressure are known. Therefore we can derive various relations between critical quantities. They are compared with experiment and the corresponding relations in the Vicentini-Missoni ansatz, the parametric representation, and the generalized Landau ansatz. The disagreement ranges from about 20% to one order of magnitude.  相似文献   

3.
Using general methods developed in a previous treatment we study correlations in inhomogeneous Ising models on a square lattice. The nearest neighbour couplings can vary both in strength and sign such that the coupling distribution is translationally invariant in diagonal direction. We calculate correlations parallel to the layering in the diagonally layered model with periodv=2, the so-called “general square lattice” model (GS). If the model has a finite critical temperature,T c>0, we have a spontaneous magnetization belowT c vanishing atT c with the Ising exponent β=1/8. AtT c correlations decay algebraically with critical exponnet η=1/4 and exponentially forT>T c. In the frustrated case we have oscillatory behaviour superposed on the exponential decay where the wavevector of the oscillations changes at some “disorder temperature”T D(>T c) from commensurate to temperature-dependent in commensurate periods. If the critical temperature vanishes,T c=0 we always have exponential decay at finite temperatures, while atT=T c=0 we encounter either long-range order or algebraic decay with critical index η=1/2, i.e.T=0 is thus a critical point.  相似文献   

4.
In this paper, we examine the interacting dark energy model in f(T) cosmology. We assume dark energy as a perfect fluid and choose a specific cosmologically viable form f(T) = ????T. We show that there is one attractor solution to the dynamical equation of f(T) Friedmann equations. Further we investigate the stability in phase space for a general f(T) model with two interacting fluids. By studying the local stability near the critical points, we show that the critical points lie on the sheet u* = (c ? 1)v* in the phase space, spanned by coordinates (u, v, ??, T). From this critical sheet, we conclude that the coupling between the dark energy and matter c ?? (?2, 0).  相似文献   

5.
Scattering of x-rays at grazing angles is studied for semi-infinite systems which exhibit a second order phase transition. We calculate the scaling functions for the diffuse scattering intensity aboveT c and for the Bragg-intensity belowT c, using -expansion techniques. Corrections to the leading critical surface behaviour and the size of the asymptotic scaling region are discussed. In addition, we obtain the crossover function for the excess susceptibility s.This work was supported by Bundesministerium für Forschung und Technologie  相似文献   

6.
Prompted by a recent article of Chakravarty, we reexamine theO(N) vector model with twisted boundary conditions ind dimensions in the various frameworks of the =d–2 expansion, the =4–d expansion, and the large-N expansion. These continuum models describe the physics below the critical temperatureT c and nearT c of a latticeO(N) spin model. We determine the effect of the twisting on finite-size scaling functions, for various geometries.On leave from G. Nadjakov Institute of Solid State Physics, 1784 Sofia, Bulgaria.  相似文献   

7.
In order to elucidate the anisotropic pressure effect on superconductivity in an iron-based superconductor, magnetization measurements have been performed in Ba(Fe0.92Co0.08)2As2 single crystals under uniaxial pressures applied along the c-axis. Gigantic Tc suppression, dTc/dP//c = −15 K/GPa, was observed when the anisotropic deformation with the a-expansion and c-compression was induced by the c-pressure, which should be compared with dTc/dP < +1 K/GPa in the isotropic pressure case. This suggests that the a-axis (c-axis) compression has a positive (negative) contribution to Tc.  相似文献   

8.
We propose an approximate set of kinetic equations to describe the time evolution of spin correlation functions below Tc in a system with long-range interactions. In the ordered region, these equations lead to weakly damped spin waves, in exact agreement with the previous work of Vaks, Lakkin and Pikin. When Tc is approached, spin waves persist for q ? κ_, where κ_is the inverse correlation length but, for q ? κ_, these kinetic equations continuously transform into the non-Markoffian equation previously derived by one of the authors and De Leener above Fc. Moreover, dynamical scaling is microscopically justified in our model. Beyond the assumption of a large number of interacting neighbours, we also need the phenomenological hypothesis that, close to Tc, the equilibrium properties appearing in our kinetic equations can be described by their correct (nonclassical) critical indices.  相似文献   

9.
Thermal fluctuations of the order parameter in the superconducting state are investigated, particularly near the transition temperature, using the time dependent Ginzburg-Landau theory. These fluctuations give rise to a contribution of the dynamical conductivity, which strongly increases as the temperature is raised towardsT c , in contrast to the temperature dependence of the electromagnetic response due to the static order parameter. At the transition temperature this contribution joins continuously to the extra response (conductivity, susceptibility) which has been calculated and observed in the temperature region aboveT c , where it represents the onset of superconductivity in the normal state. Particularly the dynamical conductivity due to fluctuations is calculated and discussed for bulk material, thin films and thin wires belowT c . The temperature and frequency dependence should be observable in microwave experiments.  相似文献   

10.
With the help of the two-fluid model developed by Götze and Michel for phonons it is shown for a simple model Hamiltonian that in the low temperature phase the optical soft mode becomes isothermal, the heat diffusion mode is dominant near the transition temperatureT c and the quasiparticle interaction is of great importance in determining the thermodynamic quantities nearT c. Green function techniques are applied to describe the two-fluid model functions in a microscopic way. The simplest approximations are discussed for the model equations describing nonequilibrium phenomena of the soft optical phonon mode in the low temperature phase. The quasiparticle interaction operator can be related to the interaction operator between quasiparticles and the condensed mode. This relation enables one to understand the behaviour of the thermodynamic quantities near the transition temperature on a microscopic way. The first order displacive phase transition is also discussed.  相似文献   

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