Spin transport in the Néel and collinear antiferromagnetic phase of the two dimensional spatial and spin anisotropic Heisenberg model on a square lattice

We analyze and compare the effect of spatial and spin anisotropy on spin conductivity in a two dimensional S = 1/2 Heisenberg quantum magnet on a square lattice. We explore the model in both the Néel antiferromagnetic (AF) phase and the collinear antiferromagnetic (CAF) phase. We find that in contrast to the effects of spin anisotropy in the Heisenberg model, spatial anisotropy in the AF phase does not suppress the zero temperature regular part of the spin conductivity in the zero frequency limit–rather it enhances it. In the CAF phase (within the non-interacting approximation), the zero frequency spin conductivity has a finite value, which is suppressed as the spatial anisotropy parameter is increased. Furthermore, the CAF phase displays a spike in the spin conductivity not seen in the AF phase. We also explore the finite temperature effects on the Drude weight in the AF phase (within the collisionless approximation). We find that enhancing spatial anisotropy increases the Drude weight value and increasing spin anisotropy decreases the Drude weight value. Based on these studies, we conclude that antiferromagnets with spatial anisotropy are better spin conductors than those with spin anisotropy at both zero and finite temperatures.     

Spin supersolid in an anisotropic spin-one Heisenberg chain

We consider an S=1 Heisenberg chain with strong exchange (Delta=J(z)/J(perpendicular)) and single-ion uniaxial anisotropy (D) in a magnetic field (B) along the symmetry axis. The low-energy spectrum is described by an effective S=1/2 XXZ model that acts on two different low-energy sectors for a finite range of fields. The vacuum of each sector exhibits Ising-like antiferromagnetic ordering coexisting with the finite spin stiffness obtained from the exact solution of the XXZ model. In this way, we demonstrate the existence of a spin supersolid phase. We also compute the full Delta-B quantum phase diagram using a quantum Monte Carlo method.     

Magnetoelectric memory effect in the Y-type hexaferrite BaSrZnMgFe_(12)O_(22)

We report on the magnetic and magnetoelectric properties of the Y-type hexaferrite BaSrZnMgFe_(12)O_(22),which undergoes transitions from a collinear ferrimagnetic phase to a proper screw phase at 310 K and to a longitudinal conical phase at 45 K.Magnetic and electric measurements revealed that the magnetic structure with spiral spin order can be modified by applying a magnetic field,resulting in magnetically controllable electric polarization.It was observed that BaSrZnMgFe_(12)O_(22)exhibits an anomalous magnetoelectric memory effect:the ferroelectric state can be partially recovered from the paraelectric phase with collinear spin structure by reducing magnetic field at 20 K.We ascribe this memory effect to the pinning of multiferroic domain walls,where spin chirality and structure are preserved even in the nonpolar collinear spin state.     

Spin liquid phases for spin-1 systems on the triangular lattice

Motivated by recent experiments on material Ba3NiSb2O9, we propose two novel spin liquid phases (A and B) for spin-1 systems on a triangular lattice. At the mean field level, both spin liquid phases have gapless fermionic spinon excitations with quadratic band touching; thus, in both phases the spin susceptibility and γ=C(v)/T saturate to a constant at zero temperature, which are consistent with the experimental results on Ba3NiSb2O9. On the lattice scale, these spin liquid phases have Sp(4)~SO(5) gauge fluctuation, while in the long wavelength limit this Sp(4) gauge symmetry is broken down to U(1)×Z(2) in the type A spin liquid phase, and broken down to Z(4) in the type B phase. We also demonstrate that the A phase is the parent state of the ferroquadrupole state, nematic state, and the noncollinear spin density wave state.     

Low-temperature broken-symmetry phases of spiral antiferromagnets

We study Heisenberg antiferromagnets with nearest- (J1) and third- (J3) neighbor exchange on the square lattice. In the limit of spin S-->infinity, there is a zero temperature (T) Lifshitz point at J(3)=1/4J(1), with long-range spiral spin order at T=0 for J3>1/4J(1). We present classical Monte Carlo simulations and a theory for T>0 crossovers near the Lifshitz point: spin rotation symmetry is restored at any T>0, but there is a broken lattice reflection symmetry for 0< or =T相似文献   

Nematic and chiral order for planar spins on a triangular lattice

We propose a variant of the antiferromagnetic XY model which includes a biquadratic (J2) as well as the quadratic (J1) interaction on the triangular lattice. The phase diagram for large J2/J1 exhibits a phase with coexisting quasi-long-range nematic, and long-ranged vector spin chirality orders in the absence of magnetic order, which qualifies our model as the first instance of a classical spin model that exhibits a vector chiral spin liquid phase. The interplay of nematic and spin chirality orders is discussed. A variety of critical properties are derived by means of Monte Carlo simulation.     

Persistent spin currents without spin-orbit coupling in a spin-1/2 ring

We show that a spatially dependent magnetic field can induce a persistent spin current in a spin-1/2 Heisenberg antiferromagnetic ring, proportional to the solid angle subtended by the magnetic field on a unit sphere. The magnitude of the spin current is determined by the ratio of longitudinal and transverse exchange interactions J(parallel)/J(perpendicular) and by the magnetic field magnitude.     

Néel temperature of quasi-low-dimensional Heisenberg antiferromagnets

The Néel temperature T(N) of quasi-one- and quasi-two-dimensional antiferromagnetic Heisenberg models on a cubic lattice is calculated by Monte Carlo simulations as a function of interchain (interlayer) to intrachain (intralayer) coupling J(')/J down to J(')/J approximately = 10(-3). We find that T(N) obeys a modified random-phase approximationlike relation for small J(')/J with an effective universal renormalized coordination number, independent of the size of the spin. Empirical formulas describing T(N) for a wide range of J(') and useful for the analysis of experimental measurements are presented.     

Spontaneous plaquette dimerization in the J1-J2 heisenberg model

We investigate the nonmagnetic phase of the spin-half frustrated Heisenberg antiferromagnet on the square lattice using exact diagonalization (up to 36 sites) and quantum Monte Carlo techniques (up to 144 sites). The spin gap and the susceptibilities for the most important crystal symmetry breaking operators are computed. A genuine and somehow unexpected "plaquette resonating valence bond," with spontaneously broken translation symmetry and no broken rotation symmetry, comes out from our numerical simulations as the most plausible ground state for J(2)/J(1) approximately 0.5.     

Short-range magnetic order in a spin density wave in Cr-based multilayers

A mechanism of the formation of the short antiferromagnetic order with a spin density wave (SDW) in the vicinity of the interfaces in the Fe/Cr type multilayers is proposed. The main reason behind the emergence of magnetic ordering with SDWs is the redistribution of charge (and, hence, spin) density in the vicinity of Fe/Cr interfaces, which leads to the paramagnetic phase instability at a temperature considerably higher than the Néel temperature in chromium. The Ginzburg-Landau expansion for the free energy of the system is used for determining the inhomogeneous collinear structures of CDWs and for constructing the phase diagram (the dependence of the transition temperature on the thickness of the antiferromagnetic interlayer). The obtained results are used for discussing the experimental data on neutron scattering and tunnel microscopy.     

Ordering in spatially anisotropic triangular antiferromagnets

We investigate the phase diagram of the anisotropic spin-1/2 triangular lattice antiferromagnet, with interchain diagonal exchange J' much weaker than the intrachain exchange J. We find that fluctuations lead to a competition between (commensurate) collinear antiferromagnetic and (zigzag) dimer orders. Both states differ in symmetry from the spiral order known to occur for larger J', and are therefore separated by quantum phase transitions from it. The zero-field collinear antiferromagnet is succeeded in a magnetic field by magnetically ordered spin-density-wave and cone phases, before reaching the fully polarized state. Implications for the anisotropic triangular magnet Cs2CuCl4 are discussed.     

Competition between helimagnetism and commensurate quantum spin correlations in LiCu2O2

Neutron diffraction and bulk measurements are used to determine the nature of the low-temperature ordered state in LiCu2O2, a S=1/2 spin-chain compound with competing interactions. The spin structure is found to be helimagnetic, with a propagation vector (0.5,zeta,0), zeta=0.174. The nearest-neighbor exchange constant and frustration ratio are estimated to be J(1)=5.8 meV and J(2)/J(1)=0.29, respectively. For idealized quantum spin chains, these parameter values would signify a gapped spin-liquid ground state with commensurate spin correlations. The observed temperature dependence of the magnetic propagation vector in LiCu2O2 is attributed to a competition between incommensurate helimagnetism in the classical spin model and commensurability in the quantum case. It is also proposed that long-range ordering in LiCu2O2 is facilitated by intrinsic nonstoichiometry.     

Incommensurate phases in ferromagnetic spin-chains with weak antiferromagnetic interchain interaction

We study planar ferromagnetic spin-chain systems with weak antiferromagnetic inter-chain interaction and dipole-dipole interaction. The ground state depends sensitively on the relative strengths of antiferromagnetic exchange and dipole energies κ = J′a ² c/(g _L μ _B )². For increasing values of κ, the ground state changes from a ferromagnetic via a collinear antiferromagnetic and an incommensurate phase to a 120° structure for very large antiferromagnetic energy. Investigation of the magnetic phase diagram of the collinear phase, as realized in CsNiF₃, shows that the structure of the spin order depends sensitivly on the direction of the magnetic field in the hexagonal plane. For certain angular domains of the field incommensurate phases appear which are seperated by commensurate phases. When rotating the field, the wave vector characterizing the structure changes continously in the incommensurate phase, whereas in the commensurate phase the wave vector is locked to a fixed value describing a two-sublattice structure. This is a result of the competition between the exchange and the dipole-dipole interaction.     

Triangular antiferromagnets with a layered structure in a uniform field

We study the magnetic states and phase transitions in layered triangular antiferromagnets and show that in compounds of the VBr₂ (or VCl₂) type the quantum effects alter the structure of the ground state and initiate a series of transitions as the magnetic field strength is increased. We establish that planar structures with different spin configurations are realized when the magnetic field strength is far from the saturation value, while a nonplanar structure of the umbrella type is realized in fields close to the saturation value. Finally, we build the phase diagram of the ground state and indicate a finite range of field strengths where a collinear phase is possible, too. Zh. éksp. Teor. Fiz. 111, 627–643 (February 1997)     

二维反铁磁Heisenberg模型(s=1/2)的低能性质研究

利用一种有效的Monte Carlo簇团迭代方法进行了关于二维反铁磁Heisenberg量子模型的数值研究。比较手征微扰理论的有关解析结果,在有限温度和有限体积下精确地确定了该系统的重要低能参数:基态能量密度e_o=-0.6693(1)J/a²,参差磁化强度M_s=0.3076(4)/a²,自旋波速度?c=1.68(1)Ja和自旋块度(spin stiffness)ρ_s=0.185(5)J。计算结果与 关键词：     

Correlation-length-exponent relation for the two-dimensional random ising model

We consider the two-dimensional (2D) random Ising model on a diagonal strip of the square lattice, where the bonds take two values, J1>J2, with equal probability. Using an iterative method, based on a successive application of the star-triangle transformation, we have determined at the bulk critical temperature the correlation length along the strip xi(L) for different widths of the strip L相似文献   

Phase transitions in quantum spin systems with isotropic and nonisotropic interactions

We prove the existence of spontaneous magnetization at sufficiently low temperature, and hence of a phase transition, in a variety of quantum spin systems in three or more dimensions. The isotropic spin 1/2x-y model and the Heisenberg antiferromagnet with spin 1, 3/2,...and with nearest neighbor interactions on a simple cubic lattice are included.Research supported by U.S. National Science Foundation under grants GP-40768X (F.J.D.), MCS 75-21684 (E.H.L.), and GP-39048 (B.S.).Alfred Sloan Fellow.     

Zero Temperature Phases of the Frustrated <Emphasis Type="Italic">J</Emphasis><Subscript>1</Subscript>–<Emphasis Type="Italic">J</Emphasis><Subscript>2</Subscript> Antiferromagnetic Spin-1/2 Heisenberg Model on a Simple Cubic Lattice

At zero temperature magnetic phases of the quantum spin-1/2 Heisenberg antiferromagnet on a simple cubic lattice with competing first and second neighbor exchanges (J ₁ and J ₂) is investigated using the non-linear spin wave theory. We find existence of two phases: a two sublattice Néel phase for small J ₂ (AF), and a collinear antiferromagnetic phase at large J ₂ (CAF). We obtain the sublattice magnetizations and ground state energies for the two phases and find that there exists a first order phase transition from the AF-phase to the CAF-phase at the critical transition point, p _c =0.56 or J ₂/J ₁=0.28. We also show that the quartic 1/S corrections due spin-wave interactions enhance the sublattice magnetization in both the phases which causes the intermediate paramagnetic phase predicted from linear spin wave theory to disappear.     

Li2VO(Si,Ge)O4, a prototype of a two-dimensional frustrated quantum heisenberg antiferromagnet

NMR and magnetization measurements in Li2VOSiO4 and Li2VOGeO4 are reported. The analysis of the susceptibility shows that both compounds are two-dimensional S = 1/2 Heisenberg antiferromagnets on a square lattice with a sizable frustration induced by the competition between the superexchange couplings J1 along the sides of the square and J2 along the diagonal. Li2VOSiO4 undergoes a low-temperature phase transition to a collinear order, as theoretically predicted for J2/J1>0.5. Just above the magnetic transition the degeneracy between the two collinear ground states is lifted by the onset of a structural distortion.     

Nonequilibrium Phase Transition in Spin-S Ising Ferromagnet Driven by Propagating and Standing Magnetic Field Wave

The dynamical response of spin-S(S=1, 3/2, 2, 3) Ising ferromagnet to the plane propagating wave, standing magnetic field wave and uniformly oscillating field with constant frequency are studied separately in two dimensions by extensive Monte Carlo simulation. Depending upon the strength of the magnetic field and the value of the spin state of the Ising spin lattice two different dynamical phases are observed. For a fixed value of S and the amplitude of the propagating magnetic field wave the system undergoes a dynamical phase transition from propagating phase to pinned phase as the temperature of the system is cooled down. Similarly in case with standing magnetic wave the system undergoes dynamical phase transition from high temperature phase where spins oscillate coherently in alternate bands of half wavelength of the standing magnetic wave to the low temperature pinned or spin frozen phase. For a fixed value of the amplitude of magnetic field oscillation the transition temperature is observed to decrease to a limiting value as the value of spin S is increased. The time averaged magnetisation over a full cycle of the magnetic field oscillation plays the role of the dynamic order parameter. A comprehensive phase boundary is drawn in the plane of magnetic field amplitude and dynamic transition temperature. It is found that the phase boundary shrinks inwards for high value of spin state S.Also in the low temperature(and high field) region the phase boundaries are closely spaced.