The spin-peierls critical temperature in an XY quasi-one-dimensional system with nearest and next-nearest neighbour interactions

The mean field critical temperature for the spin-Peierls phase transition in the XY antiferromagnetic chain is obtained for nearest and next nearest neighbour exchange interaction. An increase in T_c is predicted for next nearest neighbour antiferromagnetic exchange and a decrease is obtained for ferromagnetic exchange. This model is applied to the alkali-TCNQ salts, which are treated in the framework of the highly correlated Hubbard model.     

用Monte-Carlo方法计算L12结构的有序度

本文应用Monte-Carlo统计物理计算方法,计算了L1₂结构有序度随温度的变化。计算中考虑了最近邻有序相互作用J₁和次近邻有序相互作用J₂=0,-0.3J₁,0.3J₁三种情况。计算结果显示存在有序无序一级相变,临界温度T_c与准化学方法等理论结果相符,同时发现次近邻有序相互作用J₂敏感地影响到有序无序转变温度T_c,为间隙原子对L1₂结构金属间化合物塑性的作用提供了一种可能的解释。 关键词：     

Absolute coverage and isostemc heat of adsorption of deuterium on Pt(111) studied by nuclear microanalysis

The absolute coverage (θ) of deuterium adsorbed on Pt(111) in the ranges 180< T<440 K and 5 × 10^?6 < P < 5 × 10^?2 Pa D₂ has been determined by nuclear microanalysis using the D(³He, p)⁴He reaction. From these data, the isosteric heat of adsorption (E_a) has been determined to be 67 ± 7 kJ mol^?1 at θ ? 0.3. This heat of adsorption yields values of the pre-exponential for desorption (10^?5 to 10^?2 cm² atom^?1 s^?1) that lie much closer to the normal range for a second order process than those determined from previous isosteric heat measurements. The E_a versus θ relationship indicates that the adsorbed D atoms are mobile and that there is a repulsive interaction of 6–8 kJ mol^?1 at nearest neighbour distances. At 300 K the coverage decreases to ? 0.05 monolayer (? 8 × 10¹³ D atoms cm^?2) as P→ 0, apparently invalidating a recent model of site exchange in the adsorbed layer.     

Re-entrant superconductivity in a magnetically ordered superconductor: La1-xGdxRu2

We have measured the critical temperature (T_c) and the upper critical magnetic field (H_c2) of La_1-xGdxRu₂. At low concentrations of the magnetic impurity (Gd), the suppression of T_c follows the expected Abrikosov-Gorkov (A-G) pair breaking curve. However, for larger concentrations, strong deviations below A-G are observed. Samples in this region (4. ? × ? 5. at. %) exhibit two T_c's. La_1-xGd_xRu₂ is known to order magnetically, probably as a spin glass, and the magnetic ordering temperature (T_M) has been measured in the normal state. This T_M curve intersects the T_c curve in the concentration range where the T_c curve is re-entrant and we therefore attribute the re-entrant T_c behavior to the magnetic ordering of the Gd³⁺ ions.     

Phase transition in a linear chain of classical spins with a logarithmic nearest neighbour pair potential

We present the complete calculation of the partition function and correlation functions of a linear array of classical spins coupled by a nearest neighbour logarithmic pair potential. In the case of a ferromagnetic coupling there occurs a phase transition at T_c > 0. The critical exponents of the specific heat C and the magnetic susceptibility χ (in the absence of an external field) are shown to have the non-classical value α = 2 and classical value γ = 1 respectively. The underlying mathematical mechanism of the phase transition is the complete degeneracy of all the eigenvalues of the corresponding integral equation (Kac's mechanism). Below T_c the partition function becomes complex. For antiferromagnetic coupling the free energy is analytic in the whole temperature range and so no phase transition occurs in this case.     

Interacting one dimensional Ising spin chains as a model of the layered antiferroelectric squaric acid

Starting from the structure of the layered antiferroelectric squaric acid and assuming an ice rule type interaction mechanism of the protons it is shown that aboveT _c the molecular layers may be considered as a system of parallel and orthogonal nearly independent linear Ising spin chains with strong intrachain and comparatively weak interchain interactions. Whereas the former are treated exactly as nearest neighbour interactions the interchain interactions are taken into account either by the molecular field approximation or, in computer simulations, by nearest neighbour interactions. The parameters of the model are related to the unusually small transition entropy, to the high and nearly temperature independent dielectric constant aboveT _c ,and to neutron scattering data.     

Iron above the curie temperature

A quantitative discussion is given of recent paramagnetic diffuse neutron scattering data on bcc Fe above T_c. It is found that although the spatial spin correlation function has a weak tail of fairly long range the correlation between nearest neighbour spin directions is very small at 1.24T_c. It is concluded that the data are inconsistent with the giant short-range order hypothesis of Prange and Korenman, Capellmann and Sokoloff and another interpretation is given.     

Low energy electron diffraction beam profiles and phase transition at Si(111)-7 X 7 surface

Angular profiles of low energy electron diffraction (LEED) beams from Si(111)-7 × 7 are measured for various crystal temperatures T near the phase transition with apparent critical temperature T_c ≈ 1140 K. From analyses of the profiles it is concluded that (1) long range superstructure order persists for T up to at least 50 K above T_c and (2) with increasing T the correlation length characterizing the short-range order peaks for T ≈ T_c ? 100 K and decreases rapidly for T >T_c. Conclusion (1) is discussed with reference to a dislocation network model of Si(111)-7 × 7 reconstruction.     

Correlations in inhomogeneous Ising models

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.     

On the possible mechanism of structural transition in cuprates

A possible mechanism of tetragonal to orthorhombic transition in high-T_c cuprates based on the removal of orbital degeneracy of p states in the CuO₂ cell by electron lattice interaction is proposed. Spontaneous distortion creates a finite energy gap or a pseudogap in the density of states depending on the relative strength of the next-near and nearest neighbour hopping strengths. The gap is a function of electron density and vanishes beyond the structural transition temperature. The growth of the gap leads to a metal semiconductor transition as temperature decreases with attendant stripe and orbital ordering. The phase diagram for the distorted phase is examined in detail in the parameter space.     

X-ray and neutron investigations of the phase instabilities in K2SnCl6

From X-ray and neutron diffraction work with single crystals and powders we found two phase transitions at T_c1 = 260.5 ± 1 K and at T_c2 = 255 ± 1 K. The transition at T_c1 is characterised by the occurence of superstructure reflections breaking the f.c.c. translational symmetry, whereas the second one at T_c2 is indicated by a monoclinic lattice distortion without any further loss of translational symmetry. Both transitions can be explained by the softening of optical branches at the X- and the Γ-point of the Brillouin zone, corresponding to librations of the SnCl₆-octahedra. Evidence for this low energy phonon branch is offered from the observation of diffuse streaks with X-rats and from their inelastic nature as revealed by the neutron triple axes technique.     

Surface phases of Se deposited on Ni(001)

Reflection electron diffraction and Auger electron spectroscopy have been used to study the temperature versus coverage (T versus θ) phase diagram for Se submonolayers on Ni(001). Disordered, p(2×2) and c(2×2) phases are observed for θ < 0.5. (2×1) and c(4×2) phases were nerver observed. High Se exposures (θ > 0.5) lead to bulk compound formation for substrate temperatures T>350 K with the only distinguishable diffraction pattern being c(4×4). The p(2×2)-disorder and c(2×2)-disorder transitions appear to be continuous (second-order) over the range observed. The character of the p(2×2)−c(2×2) transition may also be continuous but this cannot be unambiguously determined from the data. Symmetry arguments predict the Se/Ni(001) system to be a realization of the Ashkin-Teller model and thus make specific predictions which need to be confirmed in future studies. Good agreement with the phase diagram is obtained using a transfer matrix scaling calculation with up to fourth-nearest-neighbor pairwise interactions.     

Pressure-induced phase transformation in In2Bi

We have made measurements of the pressure dependence of the superconducting transition temperature, T_c, for In₂Bi and related alloys. For In₂Bi- phase alloys, a large discontinuity in T_c is seen at 15–20 kbar, which we associate with a phase transformation first seen by Bridgman [1]. Our measurements suggest that this transformation is produced by the decomposition of In₂Bi into In₅Bi₃ and an In-rich phase. In the low pressure phase, T_c shows a minimum at 9–15 kbar whereas it depends linearly on pressure in the high pressure phase with ?T_c/?P equal to -4.9 × 10^-5 K bar^-1.     

Critical temperature for a system with Heisenberg-like nearest neighbour and Ising-like next nearest neighbour interactions

A ferromagnetic system with $\text{S =}\text{1}\text{2}$, where the interactions between nearest neighbours are isotropic (having Heisenberg character) and those between the next nearest neighbours anisotropic (Ising-like), is investigated. Using the effective hamiltonian approach we find the change in the critical temperature due to the inclusion of next nearest neighbour interactions of different character. The change is greatest in two dimensions where the critical temperature is shifted up from its zero value for a system with the Ising-like interactions switched off. We also calculate the T_c for a system with both nearest and next nearest neighbours interactions of the Ising-type. The results for the two models are compared.     

NMR studies of hydrogen diffusion in hydrogen uranyl phosphate tetrahydrate (HUP)

¹H NMR spin-lattice relaxation times, T₁ (Zeeman) and T_1ϱ (rotating frame) and spin-spin relaxation times, T₂, and ³¹P NMR solid-echoes are reported for phase I and II of hydrogen uranyl phosphate tetrahydrate (HUP) at temperatures in the range 200–323 K. The spectral density functions extracted from the measured relaxation times for phases I and II are consistent with a 2D diffusion mechanism for hydrogen motion. ³¹P second moments determined from the solid-echoes show that all the hydrogens diffuse rapidly in phase I, and that the hydrogen-bond site nearest to the phosphate oxygen is not occupied in phase II. The mechanism for diffusion in phase II is discussed.     

Pyroelectric properties of a ferroelectric single crystal [NH2(CH3)2]3Sb2Cl9 (DMACA)

The pyroelectric properties of DMACA single crystals have been measured in the range 135–293 K, revealing the existence of ferroelectric second order phase transition at T_c = 243 K. The saturation value of spontaneous polarization P_s along a-axis amounts to 6.8 × 10⁻³ Cm⁻² at about 203 K. Critical exponent β = 0.5 has been found in the region 0.5–10 K away from T_c.     

Phase diagram of oxygen on Ni(100)

We have studied the coverage-temperature phase diagram of chemisorbed oxygen on Ni(100) with LEED and AES. We find that the oxygen p(2 × 2) structure undergoes a nearly reversible order- disorder phase transition for coverages between 0.15 and 0.30 monolayer (T_c ≈ 560 K for θ = 0.25). For these coverages, it is also found that the chemisorbed oxygen begins dissolving into the bulk nickel as the overlayer disorders. A reversible order-disorder transition is not seen for the oxygen c(2 × 2) structure, but there is evidence that bulk dissolution here also occurs coincident with the overlayer disordering.     

Absence of re-entrant phase transition of the antiferromagnetic Ising model on the simple cubic lattice: Monte Carlo study of the hard-sphere lattice gas

We perform Monte Carlo simulations of the hard-sphere lattice gas on the simple cubic lattice with nearest neighbour exclusion. The critical activity is estimated, z_c = 1.0588 ± 0.0003. Using a relation between the hard-sphere lattice gas and the antiferromagnetic Ising model in an external magnetic field, we conclude that there is no re-entrant phase transition of the latter on the simple cubic lattice.     

The onset of deconfinement inSU(2) lattice gauge theory

InSU(2) lattice gauge theory, we study deviations from ideal gas behaviour near the deconfinement point. On lattices of sizeN _σ ³ ×4,N _σ=8, 12, 18 and 26, we calculate the quantityΔ≡(ε?3P)/T ⁴. It increases sharply just aboveT _c , peaks atT/T _c =1.15 ±0.05 and then drops quickly. This form of behaviour is shown to be the consequence of a second order phase transition. Dynamically it could arise because just aboveT _c , the low momentum states of the system are remnant massive modes rather than deconfined massless gluons.