Mixed spin- and spin- Ising models with random nearest-neighbour interactions

Using the effective field theory with correlations, we study mixed spin?3/2 and spin?1/2 Ising models with random bonds and crystal-field interactions on the honeycomb lattice. The nearest-neighbour couplings J_ij are taken as random variables with distribution P(J_ij) = pδ(J_ij ? J)+(1 ? p)δ(J_ij ? αJ), where J > 0 and |α| ≤ 1. In a certain range of negative values of α, the phase diagrams exhibit re-entrant behaviour. In detail, we investigate separately two kinds of disorder: Bond dilution (α = 0) and random ±J interactions (α = ?1). In both cases, the influence of the an-isotropy on the phase diagrams shows some new outstanding features.     

Quantum Monte Carlo Investigation of the magnetic properties of weakly interacting antiferromagnetic chains with an alternating exchange interaction with spin S = 1/2

An approximation dependence of the spontaneous magnetic moment at a site, σ/σ(0) ? 1 = and the antiferromagnet-singlet state phase boundary, J ₂/J ₁ = 0.52(3)δ, are determined by the quantum Monte Carlo method in the self-consistent sublattice molecular field approximation for weakly inter-acting (J ₂) antiferromagnetic chains with spin S = 1/2 and alternating exchange interaction (J ₁ ± δ). The Néél temperature and a number of critical temperatures which could be related with the filling energy of two singlets (ΔS ^z = 0) and one triplet (ΔS ^z = 1) spin bands, each of which is split by the sublattice field (h ^{x, y} ≠ h ^z into two subbands, are determined on the basis of the computed correlation radii of the two-and four-spin correlation function, the squared total spin 〈 (S ^z)²〉 with respect to the longitudinal components, the dimerization parameter, and the correlation functions between the nearest neighbors with respect to longitudinal and transverse spin components. On the basis of the Monte Carlo calculations, the critical temperatures and possible energy gaps at the band center are determined for the antiferromagnets CuWO₄ and Bi₂CuO₄ and for the singlet compounds (VO)₂P₂O₇ and CuGeO₃, agreeing satisfactorily with existing results, and new effects are also predicted.     

非晶态Fe87-xSixB13合金的磁化强度、电阻与温度的关系

本文研究了非晶态Fe_87-xSi_xB₁₃(x=0,9.6,14.5)合金的饱和磁化强度、电阻率与温度的关系。得到样品的居里温度T_C和晶化温度T_cr随Si含量的增加而明显提高。低温下的热磁关系符合布洛赫的T^3/2定律,计算出自旋波劲度系数D从x=0时的62meV·A²增加到x=14.5时的111meV·A²。从D值和Handrich理 关键词：     

块体非晶合金的黏度与玻璃形成能力的关系

基于经典结晶理论讨论了非晶合金的晶化动力学因素和晶化热力学因素对玻璃形成能力（GFA）的影响.分析表明,合金的等温转变（TTT）曲线“鼻尖”温度T_n对应的黏度与晶化阻力因子成正比;重新加热时晶化开始温度T_x对应的黏度与晶化驱动力因子成反比.由此得到了新的GFA参数ω₀=（T_g-T₀）/（T_x-T₀）-（T_g-T₀）/（T_n-T₀）,其中T_g为玻璃转变温度,T₀为理想玻璃转变温度.统计结果显示,ω₀与临界冷却速率具有较高的相关性,R²高达09626.进一步分析表明：新提出的ω₀参数可以合理地解释过冷熔体的黏度、脆性、液相稳定性、热稳定性以及T_rg、ΔT_x、γ、γ_m、ΔT_rg、α、β、δ和φ等参数与GFA的关系. 关键词： 块体非晶合金 黏度 脆性 玻璃形成能力     

A simulation of the mixed spin 3–spin 3/2 ferrimagnetic Ising model

The mixed spin 3–spin 3/2 ferrimagnetic Ising model was simulated using cooling algorithm on cellular automaton (CA). The simulations were carried out in the intervals ?4 ≤ D_A/J ≤ 8 and ?4 ≤ D_B/J ≤ 8 for the square lattices with periodic boundary conditions. The ground-state phase diagram of the model has different types of ferrimagnetic phases. Although only the antiferromagnetic nearest-neighbor interaction was contained in the Hamiltonian, the compensation points emerged through D_A/J = 2 at kT/J = 0. The values of the critical exponents (ν, α , β and γ) were estimated within the framework of the finite-size scaling theory and power-law relations for the selected D_A/J values (?2, 0, 1, 2, and 4). The estimated critical exponent values were in good agreement with the universal values of the two-dimensional Ising model (ν = 1, α = α′ = 0, β = 0.125, β′ = 0.875 and γ = γ′ = 1.75).     

Possibility of utilizing the D-3He fuel cycle with 3He production in a spherical tokamak reactor

Possible operating regimes of a spherical tokamak reactor based on the D-³He fuel cycle with ³He production are considered. The parameters of the plasma and magnetic system are calculated for several versions corresponding to the high power efficiency (with a power gain factor in plasma of Q = 20) in a reactor with an aspect ratio of A = 1.5. According to calculations, for an axial magnetic field in vacuum of B ₀ = 2 T, a plasma radius of a = 3 m, an average 〈β〉 value of 0.53, and a plasma temperature of 〈T〉 = 48 keV, the reactor power can reach P _fus = 500 MW. In order to achieve a power of P _fus = 1500 MW in a reactor with a = 2 m, 〈β〉 = 0.36, and 〈T〉 = 40 keV, the magnetic field should be increased to B ₀ = 5 T.     

Discontinuities in disordered systems

The entropyS _T (j) of a two-dimensional Ising spin glass with an independent distribution of the random couplingp(J)=x·δ(J+1)+(1-x)δ(J-j) is discontinuous for temperatureT=0 and rationalj>0 and continuous elsewhere. The integrated density of frequenciesk _M (ω ²) of an one-dimensional chain of coupled oscillators with an independent distribution of the random massesp(m)=x·δ(m-1)+(1-x)δ(m-M) has the same behaviour, whereω ² corresponds toj andM to 1/T. The discontinuity points for infiniteM are, for sufficiently large but finiteM, special, frequencies, wherek _M (ω ²) has a Lifshitz singularity.     

Effects of impurity on the entanglement of the three-qubit Heisenberg XXX spin chain

We investigate the entanglement of the three-qubit Heisenberg XXX chain in the presence of impurity and obtain the analytical expressions of the concurrence C. It is found that for impurity entanglement, C appears only when J ₁ > J for J > 0, and J ₁ > 0 for J < 0, and in these two regions C increases with the increase of J ₁, so is the critical temperature T _c. When J ₁ ≫ | J |, C reaches its maximum value 0.5 and T _c reaches the asymptotic value T _c = 3.41448J ₁. For entanglement between the normal lattices, C appears only when J > 0 and −2J < J ₁ < J, and initially increases with the increase of J ₁ and arrives at the maximum value C _max = (e^4J/T −3)/(e^4J/T + 3) before it decays to zero gradually, so is the critical temperature T _c with, however, the maximum value T _cmax = 4J/In3. Supported by the Natural Science Research Project of Shaanxi Province (Grant No. 2004A15)     

Metal-insulator transition in perovskite oxides: A low-temperature perspective

A review is presented of recent experimental results of low temperature studies of composition driven metal-insulator transition in perovskite oxides of the ABO₃ class. The evolution of physical properties like conductivity, tunnelling, density of states and magnetoconductivity has been studied at low temperatures (T < 10 K) because composition is varied so that the sample goes from the metallic state to the critical region through a weakly localized region. The results show an interesting interplay of disorder and correlation effects. Special attention has been paid to the critical region which is marked by very low conductivity and dσ/dT>0. In this region the following important observations emerge. (1) It is possible to have a metallic state [σ(T = 0) = σ ₀ ≠ 0] with σ ₀/σ _Mott ? 1 and dσ/dT > 0. (2) At T < 2 K the conductivity follows a power law σ ～T^ν , where the exponent can be related to the finite frequency response of a zero temperature phase transition. (3) The Coulomb interaction plays a major role and evidence from tunnelling experiments suggests that a gap in the density of states at the Fermi level opens up continuously as the critical region is approached from the metallic side. (4) The magnetoconductivity is relatively smaller in the metallic and the weakly localized region (except the hole-doped LaMnO₃ and related systems) but becomes very large at the critical region.     

The <Emphasis Type="Bold">±</Emphasis><Emphasis Type="Bold">J</Emphasis> spin glass in Migdal-Kadanoff approximation

We study the low-temperature phase of the three-dimensional ± J Ising spin glass in Migdal-Kadanoff approximation. At zero temperature, T = 0, the properties of the spin glass result from the ground-state degeneracy and can be elucidated using scaling arguments based on entropy. The approach to the asymptotic scaling regime is very slow, and the correct exponents are only visible beyond system sizes around 64. At T > 0, a crossover from the zero-temperature behaviour to the behaviour expected from the droplet picture occurs at length scales proportional to T ^-2/ds where d_s is the fractal dimension of a domain wall. Canonical droplet behaviour is not visible at any temperature for systems whose linear dimension is smaller than 16 lattice spacings, because the data are either affected by the zero-temperature behaviour or the critical point behaviour. Received 18 February 2001     

Effective-field theory of spin glasses and the coherent-anomaly method. I

A new cluster-effective-field theory of spin glasses is formulated. Basic formulas for the spin-glass transition point and the spin-glass susceptibility in the high-temperature phase are obtained. The present theory combined with the coherent-anomaly method is shown to be useful to estimate the true critical point and the nonclassical critical exponent of a spin-glass transition. Concerning the two-dimensional ±J model, we have _s =5.2(1) forT _SG=0, which agrees well with the data by some other authors. As for the threedimensional±J model, the present tentative analysis givesT _SG=1.2(1)(J/k _B) and _s =4(1), but more extensive calculations are needed.     

Limits on the impact parameters occuring in specific K−p interactions at 8.25 GeV/c

We have determined a lower limit of the impact parameter for the reactions associated with the channels K^?p → K^?pπ⁺π^?K^?p → λπ⁺π^?π⁰. The limit was found to be highest for the diffractive parts of the first channel (～ 0.52 fm) and smallest for the forward Λ in the second channel (～0.21 fm).We have also examined the elements of the inverse correlation matrix for the transfer: the off-diagonal elements are small (consistent with zero) for n = 4?7 body products. The log 〈Q_i·Q_j〉 of the correlation matrix as a function of $\text{Гi?jГ}$ falls on a straight line and the eigenvalue λ₁ of the transverse momentum transfer eigenfunction was found to increase slightly from ～0.6 for 4-body to ～0.7 for 7-body products. The 〈cosφ_ij〉 of the Q_i, Q_j shows similar behaviour to the 〈Q_i·Q_j〉     

Avoided critical behavior in a uniformly frustrated system

We study the effects of weak long-ranged antiferromagnetic interactions of strength Q on a spin model with predominant short-ranged ferromagnetic interactions. In three dimensions, this model exhibits an avoided critical point in the sense that the critical temperature T_c(Q = 0) is strictly greater than lim_Q→0T_c(Q). The behavior of this system at temperatures less than T_c(Q = 0) is controlled by the proximity to the avoided critical point. We also quantize the model in a novel way to study the interplay between charge-density wave and superconducting order.     

Study of the three-dimensional Ising model on film geometry with the cluster Monte Carlo method

Topological properties of clusters are used to extract critical parameters. This method is tested for the bulk properties ofd=2 percolation and thed=2, 3 Ising model. For the latter we obtain an accurate value of the critical temperatureJ/k _B T _c=0.221617(18). In the case of thed=3 Ising model with film geometry the critical value of the surface coupling at the special transitions is determined as J_1c/J=1.5004(20) together with the critical exponents ₁ ^m =0.237(5) and=0.461(15).     

A comparison of πππ, Kππ, πKKKK system produced in the reactions meson +p→3 mesons + p

A comparison is made of the low-mass three-meson systems (πππ), (Kππ), $\text{(π}\text{K}\text{K}\text{)}$ and $\text{(}\text{K}\text{K}\text{K}\text{)}$ diffractively produced in the reaction meson + proton → three mesons + proton. Several striking similarities and a few important differences are observed: (i) the reactions are consistent with the assumption that the three mesons decay entirely into a 0^? meson and a 0⁺, 1^? or 2⁺ resonance; (ii) the three-meson mass spectra have a peak ≈ 250 MeV above the effective threshold M_eff of the dominant decay mode and then fall off approximately as (mass)^?3;(iii) the average spin 〈J〉 = 0.55 + 1.1 Q_eff, where Q_eff = M - M_eff; (iv) the average orbital angular momentum 〈l〉 increases according to 〈l〉 = 0.75 Q_eff; (v) the three-meson states are produced dominantly in unnatural spin-parity states and no evidence for their being resonant is found; (vi) the only natural spin-parity states found are the well-established 2⁺ resonances A₂ and K¹ (1420); they have similar properties to the non-resonant unnatural parity states except for a dip at t = 0 in the dσ/dt distributions; (vii) both the unnatural and natural spin-parity states are produced mostly by an exchange of natural parity; (viii) there is evidence for two types of production mechanism with different polarization properties, one approximately conserving helicity in the t-channel and the other in the s-channel.     

Extremity of the disordered phase in the Ising model on the Bethe lattice

We prove that the disordered Gibbs distribution in the ferromagnetic Ising model on the Bethe lattice is extreme forTT _c ^SG , whereT _c ^SG is the critical temperature of the spin glass model on the Bethe lattice, and it is not extreme forT _c ^SG .     

Spin glass ordering of Zn0.75Co0.25Fe0.5Cr1.5O4 cubic spinel

The linear and nonlinear low field AC susceptibilities of Zn_0.75Co_0.25Fe_0.5Cr_1.5O₄ show peaks due to non-critical contributions, which mask the peak due to spin glass ordering. They extend into the region of temperatures in which Mössbauer spectra do not show any magnetic component. When a DC field of 200 Oe suppresses the non-critical contributions, peak due to spin glass ordering is clearly visible. The spin glass ordering is thus shown to be a thermodynamic transition. The critical exponent is found to fall within the range found using other spin glasses. Mössbauer spectra in zero fields provide T_SG, which agrees with the peak temperature of AC susceptibilities in the absence of non-critical contributions. 〈S_Z〉 determined using Mössbauer spectra does not show any anomaly. In the presence of a field of 5 T, the spectra show SG ordering at 4.2 K, which converts into ferrimagnetic ordering at higher temperatures.     

Effects of L-spin longitudinal quadrupolar relaxation in heteronuclear recoupling and S-spin magic-angle spinning NMR

In experiments on S–L heteronuclear spin systems with evolution of the S-spin magnetization under the influence of a quadrupolar nucleus (L-spin), effects of longitudinal quadrupolar (T_1Q) relaxation of the L-spin coherence on the sub-millisecond time scale have been documented and explored, and methods for minimizing their effect have been demonstrated. The longitudinal relaxation results in heteronuclear dephasing even in the reference signal S₀ of S{L} REDOR, REAPDOR, RIDER, or SPIDER experiments, due to T_1Q-relaxation of the transiently generated S_yL_z coherence, reducing or even eliminating the observable dephasing ΔS. Pulse sequences for measuring an improved reference signal S₀₀ with minimal heteronuclear recoupling but the same number of pulses as for S₀ and S have been demonstrated. From the observed intensity ΔS₀ = S₀₀ − S₀ and the SPIDER signal ΔS/S₀, T_1Q can be estimated. Accelerated decays analogous to the dipolar S₀ curves will occur in T₂ measurements for J-coupled S–L spin pairs. Even in the absence of recoupling pulses, fast T_1Q relaxation of the unobserved nucleus shortens the transverse relaxation time T_2S,MAS of the observed nucleus, in particular at low spinning frequencies, due to unavoidable heteronuclear dipolar evolution during a rotation period. The observed spinning-frequency dependence of T_2S,MAS matches the theoretical prediction and may be used to estimate T_1Q. The effects are demonstrated on several ¹³C{¹⁴N} spin systems, including an arginine derivative, the natural N-acetylated polysaccharide chitin, and a model peptide, (POG)₁₀.     

A cluster Monte Carlo algorithm for 2-dimensional spin glasses

A new Monte Carlo algorithm for 2-dimensional spin glasses is presented. The use of clusters makes possible global updates and leads to a gain in speed of several orders of magnitude. As an example, we study the 2-dimensional ±J Edwards-Anderson model. The new algorithm allows us to equilibrate systems of size 100² down to temperature T = 0.1. Our main result is that the correlation length diverges as an exponential ( ξ∼e ^2βJ) and not as a power law as T↦T _c = 0. Received 10 January 2001 and Received in final form 29 May 2001