Low-lying excited states of quantum antiferromagnets on a triangular lattice

We study low-lying states of theXY and Heisenberg antiferromagnets on a triangular lattice to clarify whether spontaneous symmetry breaking occurs atT=0 in the thermodynamic limit. Approximate forms of low-lying states are proposed, in which degrees of freedom of the sublattice magnetization and of the chirality are separated. These approximate states have a long-range order and twofold structures. It is shown that low-lying states can be accurately described with the present approximation. It has been argued that low-lying states play an important role in symmetry breaking. With the help of this approximation, we discuss the contribution of low-lying states to symmetry breaking of two types, namely creation of the spontaneous sublattice magnetization and the spontaneous chirality. Furthermore, to show evidence for the occurrence of symmetry breaking, we numerically study the low-lying states of finite systems of theXY and Heisenberg antiferromagnets. It is found that the necessary conditions for the symmetry breaking to occur are satisfied in these models.     

Magnetization plateaus and sublattice ordering in easy-axis kagome lattice antiferromagnets

We study kagome lattice antiferromagnets where the effects of easy-axis single-ion anisotropy (D) dominates over the Heisenberg exchange J. For S> or =3/2, virtual quantum fluctuations help lift the extensive classical degeneracy. We demonstrate the presence of a one-third magnetization plateau for a broad range of magnetic fields J3/D2 < or = B < or = JS along the easy axis. The fully equilibrated system at low temperature on this plateau develops an unusual nematic order that breaks sublattice rotation symmetry but not translation symmetry; however, extremely slow dynamics associated with this ordering is expected to lead to glassy freezing of the system on intermediate time scales.     

Properties of phase transitions in easy-axis tetragonal antiferromagnets

Properties of phase transitions in a magnetic field H parallel to the easy axis have been investigated, and it has been shown, in particular, that the nature of the transition of the magnetic subsystem from the antiferromagnetic phase to the angular phase depends on the “sign” of the Dzyaloshinskii interaction. The conditions for orientation of the antiferromagnetism vector l in the basis plane in fields larger than the threshold field have been determined. It is shown that the transition from the angular phase to the state where the resulting magnetic moment m is parallel to the easy axis takes place in the field corresponding to a spin-flip transition. From an analysis of the configuration of the magnetic subsystem for arbitrary orientation of the external magnetic field, it follows that the values of the critical angle (ψ _cr ) for which a first-order phase transition takes place satisfy the condition in the case in which the anisotropy constant f in the basis plane is of the order of the first anisotropy constant b. Usually so that the tricritical point in the phase diagram H _y ,H _z satisfies the condition H _y ∼H _z . Fiz. Tverd. Tela (St. Petersburg) 41, 2044–2046 (November 1999)     

旋转方形散射体对三角晶格磁振子晶体带结构的优化

用改进的平面波展开法数值计算了正方形散射体三角排列的二维磁振子晶体当散射体旋转时的带结构. 结果显示, 同样的填充率下, 旋转正方柱散射体可以在新的频率范围内打开更多的带隙, 或者使低频带隙加宽. 说明旋转散射体可以有效地优化带隙.     

Detection of XY behavior in weakly anisotropic quantum antiferromagnets on the square lattice

We consider the Heisenberg antiferromagnet on the square lattice with S=1/2 and very weak easy-plane exchange anisotropy; by means of the quantum Monte Carlo method, based on the continuous-time loop algorithm, we find that the thermodynamics of the model is highly sensitive to the presence of tiny anisotropies and is characterized by a crossover between isotropic and planar behavior. We discuss the mechanism underlying the crossover phenomenon and show that it occurs at a temperature which is characteristic of the model. The expected Berezinskii-Kosterlitz-Thouless transition is observed below the crossover: a finite range of temperatures consequently opens for experimental detection of noncritical 2D XY behavior. Direct comparison is made with uniform susceptibility data relative to the S=1/2 layered antiferromagnet Sr2CuO2Cl2.     

Critical damping of the uniform mode in easy-axis antiferromagnets: MnF2

We report calculations of the critical damping of the uniform spin wave mode in MnF₂. Evaluation of the damping rate using empirical values for the parameters leads to results in reasonable agreement with experiment for 0.1 K < T_N ? T < 1K.     

The magnetic properties of three-dimensional spin-1 easy-axis single-ion anisotropic antiferromagnets

The ordered and disordered phases of spin-1 Heisenberg and Ising antiferromagnets with easy-axis single-ion anisotropy on a three-dimensional lattice are studied. By using of the double-time Green’s function method within the Tyablikov decoupling for the exchange anisotropy and Callen’s approximation for the single-ion anisotropy, the Néel temperature, magnetization and susceptibility are investigated. Their relations with the temperature and anisotropic parameter are analyzed over the entire range of temperature. It is found that our results agree well with spin wave theory results at low temperature, agree with the high temperature series results at high temperature, and compare reasonably well with the linked-cluster series approach and ratio method results at intermediate temperature.     

Yang-Lee zeros of triangular Ising antiferromagnets

In our previous research, by combining both the exact enumeration method (microcanonical transfer matrix) for a small system (L=9) with the Wang-Landau Monte Carlo algorithm for large systems (to L=30) we obtained the exact and approximate densities of states g(M,E), as a function of the magnetization M and exchange energy E, for a triangular-lattice Ising model. In this paper, based on the density of states g(M,E), the precise distribution of the Yang-Lee zeros of triangular-lattice Ising antiferromagnets is obtained in a uniform magnetic field as a function of temperature for a 9×9 lattice system. Also, the feasibility of the Yang-Lee zero approach combined with the Wang-Landau algorithm is demonstrated; as a result, we obtained the magnetic exponents for triangular Ising antiferromagnets at various temperatures.     

Resonating valence bond phase in the triangular lattice quantum dimer model

We study the quantum dimer model on the triangular lattice, which is expected to describe the singlet dynamics of frustrated Heisenberg models in phases where valence bond configurations dominate their physics. We find, in contrast to the square lattice, that there is a truly short ranged resonating valence bond phase with no gapless excitations and with deconfined, gapped, spinons for a finite range of parameters. We also establish the presence of crystalline dimer phases.     

Spin-wave interaction and renormalization of magnetic anisotropy in 2D antiferromagnets

The renormalization of the magnetic anisotropy is investigated in 2D easy-axis antiferromagnets using 1/S expansion. It was shown that in the spin-wave gap Delta appears as the 1/S2 infrared divergent correction which becomes large at T>Delta and demands analysis of the full 1/S expansion. Qualitative consequences of this gap renormalization are discussed in relation to the AFR and neutron scattering experiments.     

Supersolid hard-core bosons on the triangular lattice

We determine the phase diagram of hard-core bosons on a triangular lattice with nearest-neighbor repulsion, paying special attention to the stability of the supersolid phase. Similar to the same model on a square lattice we find that for densities rho<1/3 or rho>2/3 a supersolid phase is unstable and the transition between a commensurate solid and the superfluid is of first order. At intermediate fillings 1/3相似文献   

Three-dimensional folding of the triangular lattice

We study the folding of the regular triangular lattice in three-dimensional embedding space, a model for the crumpling of polymerised membranes. We consider a discrete model, where folds are either planar or form the angles of a regular octahedron. These “octahedral” folding rules correspond simply to a discretisation of the 3d embedding space as a Face Centred Cubic lattice. The model is shown to be equivalent to a 96-vertex model on the triangular lattice. The folding entropy per triangle ln q_3d is evaluated numerically to be q_3d = 1.43(1). Various exact bounds on q_3d are derived.