Search for logarithmic factors near the two-dimensional percolation threshold

Monte Carlo data of Eschbach et al. indicate z = 0.06 ± 0.06 for a possible logarithmic factor in the correlation length ξ ∝ (|p ? p_cz.sfnc; log^zz.sfnc;p ? p_cz.sfnc;)^?v.     

The kinetics of relaxation of magnetization fluctuations near the phase transition from paramagnetic state to ferromagnetic state with domain structure

The equations of motion describing relaxation of magnetization fluctuations in the paramagnetic state for a uniaxial ferromagnet have been solved. In the Landau-Ginzburg functional the demagnetization energy has been taken into account. The finite dimensions of a sample have been considered. It has been shown, that a phase transition is generated by fluctuations containing information about the period of the domain structure at the phase transition point T_c¹. The obtained formulae describe T_c¹ and the period of the domain structure. The results can be used both for bulk materials and thin films.     

Critical scaling function for long-range interactions to order 1/n

A universal scaling function of the order-parameter correlation function for an n-vector system in d dimensions, with long-range interactions $\text{～1/z.sfnc}\text{r}\text{z.sfnc}{}^{\mathrm{d+\sigma }}$, is discussed on the basis of explicit zero-field calculations to order 1/n, for T?T_c and σ<d<2σ.     

Superconductivity in the exactly solvable model of pseudogap state: The absence of self-averaging

The features of the superconducting state are studied in the simple exactly solvable model of the pseudogap state induced by fluctuations of the short-range “dielectric” order in the model of the Fermi surface with “hot” spots. The analysis is carried out for arbitrary short-range correlation lengths ξ_corr. It is shown that the superconducting gap averaged over such fluctuations differs from zero in a wide temperature range above the temperature T _c of the uniform superconducting transition in the entire sample, which is a consequence of non-self-averaging of the superconducting order parameter over the random fluctuation field. In the temperature range T>T _c, superconductivity apparently exists in individual regions (drops). These effects become weaker with decreasing correlation length ξ_corr; in particular, the range of existence for drops becomes narrower and vanishes as ξ_corr → 0, but for finite values of ξ_corr, complete self-averaging does not take place.     

Dynamics of the random field Ising model

Dynamics of the kinetic Ising model in the presence of static random fields is investigated using a self-consistent method. It is shown that if the interface fluctuations of the low temperature phase are small the system at low temperatures stays in a state without long range order. For this state the spin correlation function 〈S_q(t)S_?q(O)> averaged over all configurations of random fields decays exponentially in time with a single wavevector dependent relaxation time which is finite at the transition temperature T₀ and remains very long below T₀. In the mean field approximation the correlation time at the magnetic Bragg peak and at T₀ scales with the magnitude of the random field as τ∞h^?z_h with z_h = 1 for d = 2 and $\text{z}{}_{\mathrm{h}}\text{=}\text{4}\text{3}$ for d = 3, respectively.     

Ultrasonic attenuation anomalies at the two transitions in antiferromagnetic FeGe2

The attenuation of longitudinal and shear sound waves is measured through the Néel temperature, T_N≈286 K, and the lower transition temperature, T_K≈265 K, of FeGe₂. Longitudinal sound with wavevector q along the [100] axis of this tetragonal antiferromagnetic metal shows an attenuation peak at T_N, which is reversibly suppressed by compressive uniaxial stress σ along [010]. The estimated pressure dependence of T_N is dT_N/dp=(?2.8±0.3) mK bar^?1. The peak at T_N shows thermal and stress hysteresis, which suggests that it is associated with domain wall motion and that this transition is first order.     

Spin fluctuations in the stacked-triangular antiferromagnet YMnO<Subscript>3</Subscript>

The spectrum of spin fluctuations in the stacked-triangular antiferromagnet YMnO₃ was studied above the Néel temperature using both unpolarized and polarized inelastic neutron scattering. We find an in-plane and an out-of-plane excitation. The in-plane mode has two components just above T _N : a resolution-limited central peak and a Debye-like contribution. The quasi-elastic fluctuations have a line width that increases with q as Dq ^z and the dynamical exponent z = 2.3. The out-of-plane fluctuations have a gap at the magnetic zone center and do not show any appreciable q dependence at small wave vectors.     

On the limiting velocity and forced motion of ferromagnetic domain walls in an external field perpendicular to the easy-magnetization axis

A theory is constructed for the dynamics and braking of domain walls in ferromagnets when a magnetic field is applied perpendicular to the axis of easy magnetization (i.e., a transverse field H _⊥). The theory is valid for velocities v up to the limiting domain wall velocity v c. The Landau-Lifshitz equations in the dissipationless approximation are used to investigate the motion of domain walls and the change in the character of the wall motion as its velocity v approaches v _c. The force acting on a domain wall due to viscous friction is calculated within the framework of generalized relaxation theory, and the dependence of the domain wall velocity v on the forcing field H _z is investigated. Calculations of the braking force show that the contributions of various dissipation mechanisms to the friction force have different dependences on the domain wall velocity, which affects the form of the function v=v(H _z). The shapes of the curves v(H _z) differ very markedly from one another for different values of the field H _⊥. The theory developed here can be used to describe the experimental results, in particular the almost linear behavior of v=v(H _z) for small H _⊥ and its strongly nonlinear behavior when H _⊥∼H _a, whereas these data cannot be reconciled within the standard theory based on relaxation terms of Hilbert type. Zh. éksp. Teor. Fiz. 112, 953–974 (September 1997)     

Charge density wave transition and nonlinear conductivity in NbS3

We report dc conductivity (σ) measurements on the linear chain compound NbS₃. The temperature dependence of σ indicates a phase trànsition at T_MI = 155 K with strong one-dimensional fluctuations above T_MI. Below T_MI the conductivity is strongly increasing with increasing electric field above a threshold field E_T, and is also strongly frequency dependent. We argue that the nonlinear conductivity is due to sliding charge density waves.     

Specific features of the ferroelastic phase transition in CsLiS1−z CrzO4 solid solutions

Single crystals of the CsLiS_1?z Cr_zO₄ solid solutions (z = 0, 0.1, 0.4, 0.5, 0.9, 1.0) are grown, optical polarization studies are performed, and the birefringence and the angle of rotation of the optical indicatrix φ(T) are measured. A (z?T) phase diagram is constructed in the temperature range from 77 to ～550 K. It is established that substitution of chromium for sulfur has practically no effect on the shape of the optical indicatrix in the initial phase with Pmcn symmetry but produces strong distortions in the monoclinic phase with P112₁/n symmetry. The unusual shape of the curve φ(T) for compositions with z ≈ 1 is accounted for by the interaction of the polarization-constant tensor components a _ij. No radical differences in the patterns of the ferroelastic phase transition between CsLiSO₄ and CsLiCrO₄ are revealed (the critical exponent for the transition parameter is β = 0.33 ± 0.01). These crystals differ only in the magnitudes of the anomalies, which is associated with the large difference between the ionic radii of sulfur and chromium.     

Scattering of acoustic phonons by rare earth substitutional atoms in yttrium aluminum garnets

The diffusive maxima of phonon signals, and in particular their arrival timest _m are examined for a number of solid solutions of rare earth atoms in yttrium aluminum garnets. The phonon pulses are generated by metallic films of the characteristic lengthl _h heated by current pulses to the temperatureT _h slightly higher than the ambient temperatureT. The injected phonons travel in wafers of the thicknessL _z. They are scattered by substitutional atoms of rare earth occupying the yttrium dodecahedral sites, rare earth and yttrium atoms occupying the aluminum octahedral sites and by another lattice imperfections generated in the process of sample growing. The qualitative analysis based on our exact formula for the diffusion coefficientD allows us to extract the contribution of rare earth atoms substituting the Y atoms toD. Considering the dependence oft _m/L _z ² on the temperature and the ratiol _h/L_z we conclude thatD～T _h ^?4 and that the energy of phonons forming the diffusive maxima ranges from 3.2k _BT_H to 4.2k _BT_H, which is in reasonable agreement with the existing estimates.     

The exact solution of an elimination problem in kinetic theory: I. The one-dimensional hard sphere gas

A class of initial value problems for a one-dimensional hard sphere gas is considered where a specified particle has a given distribution f⁽¹⁾(z₁; 0) and the rest are in equilibrium at t=0. An exact expansion is obtained for a certain n-particle reduced distribution function f⁽ⁿ⁾(z₁;…;z_n; t) in terms of the 1-particle reduced distribution function f⁽¹⁾(z₁; t) for the specified particle by starting with separate expressions for these functions in terms of f⁽¹⁾(z₁; 0). Expansions for the corresponding cluster functions are first obtained and then graph theoretic methods applied to obtain a solution.     

The anisotropic 3D Ising model

An expression for the free energy of an (001) oriented domain wall of the anisotropic 3D Ising model is derived. The order--disorder transition takes place when the domain wall free energy vanishes. In the anisotropic limit, where two of the three exchange energies (e.g. J_x and J_y ) are small compared to the third exchange energy (J_z ), the following asymptotically exact equation for the critical temperature is derived, sinh(2J_z /k _B T _c)sinh(2(J_x ?+?J_y )/k _B T _c))?=?1. This expression is in perfect agreement with a mathematically rigorous result (k _B T _c/J_z ?=?2[ln(J_z /(J_x ?+?J_y ))?ln(ln(J_z /(J_x ?+?J_y ))?+?O(1)]^?1) derived earlier by Weng, Griffiths and Fisher (Phys. Rev. 162, 475 (1967)) using an approach that relies on a refinement of the Peierls argument. The constant that was left undetermined in the Weng et al. result is estimated to vary from ～0.84 at ((H_x ?+?H_y )/H_z )?=?10^?2 to ～0.76 at ((H_x ?+?H_y )/H_z )?=?10^?20.     

Repulsive and attractive critical Casimir forces

When confining vacuum fluctuations between two identical walls, the Casimir force manifests itself as a mutual attraction of the walls. When confining concentration fluctuations of a binary liquid mixture, an analogous force should exist near the critical temperature T_C; it is called the critical Casimir force. Here we show experimentally that this purely entropic force can be either attractive or repulsive, depending on the boundary conditions for the fluctuations. For symmetrical boundary conditions an attractive force is found while asymmetrical ones lead to a repulsive force. This is observed directly by confining the fluctuations in a thin wetting film. Depending on the boundary conditions either a thinning or a thickening of the film is observed when T→T_C.     

Domain wall theory of elementary excitations in anisotropic antiferromagneticS=1 chains

We present a theoretical investigation of elementary excitations in anisotropic antiferromagneticS=1 chains using the concept of domain walls in string (hidden) order. Domain walls are classified by the internal spin projectionS _dw ^z . We calculate energies and string correlation functions of low lying excited states of the domain wall type in the Haldane phase and compare the results to those of numerical computations. The boundaries of the Haldane phase are determined from the instability of these excitations with respect to the ground state. The interaction between two domain walls is found to be proportional to the productS _dw ^z , S _dw ^z ₂, it is effectively repulsive for equal spin projections.     

Phase diagram of YBa2Cu3O7−y at T<T c according to transverse nuclear relaxation data for Cu(2)

Two peaks are observed at T=35 and 47 K in the transverse relaxation rate for Cu(2) nuclei in YBa₂Cu₃O_7?y . A comparison of the relaxation rates for isotopes ⁶³Cu(2) and ⁶⁵Cu(2) at T=47 K indicates the magnetic nature of relaxation. The enhancement of local magnetic field fluctuations perpendicular to CuO₂ planes at T=47 K is associated with critical fluctuations of orbital currents. The peak at T=35 K is attributed to the emergence of an inhomogeneous superconducting phase. The obtained experimental results and the available data from the literature made it possible to propose a qualitatively new phase diagram of the superconducting state.     

Magneto-optical properties of the iron garnet Tb3Fe5O12 near the magnetic compensation temperature

The magneto-optical susceptibility and magnetic hysteresis loops of the Faraday effect, which accompany the technical magnetization of the iron garnet Tb₃Fe₅O₁₂, have been investigated experimentally in the temperature region near the magnetic compensation point of this ferrimagnet T _c = 249 K. It has been found that, during the technical magnetization as the temperature approaches the magnetic compensation point T _c , the velocity of domain walls increases, whereas the magneto-optical susceptibility has local maxima to the right and to the left from T _c . Mentioned features of magneto-optical properties of the iron garnet Tb₃Fe₅O₁₂ are associated with the resonance natural frequency of the domain wall oscillations in the crystal with the frequency of the alternating magnetic field. It has been shown that the used theoretical model of the magnetic resonance of domain walls makes it possible to consistently describe (at the qualitative level) the revealed regularities of the variation of the Faraday effect in the iron garnet Tb₃Fe₅O₁₂ near T _c .     

Pairing-isopairing competition in odd-odd nuclei

We discuss the systematics of 0⁺, v = 0, T = |T_Z| + 1 levels in odd-odd nuclei and conclude about the ground state isospin and seniority of these nuclei. For N = Z nuclei starting from the $\text{1f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ shell v_g.s = 0 |T_g.s = |T_z + 1. Otherwise v_g.s = 2, T_g.s = |T_z|.     

New results in the study of Brillouin scattering in thiourea

Below T₂ = 202 K, in the incommensurate phase, a Debye relaxation appears for c₃₃; it can be attributed to a linear coupling between an acoustical mode and a phason. The relaxation time is τ = τ₀/(T₀?T) with τ₀ = 6.2 x 10^?12 sec deg and T₀ = 200.9 K. The same phenomenon appears more weakly for c₁₁. The c₆₆ elastic constant has a double discontinuity around T₁ = 169 K; this shows that in the vicinity of T₁ there are two transitions, separated by a temperature interval of 3°.