本文证明了量子Heisenberg反铁磁体的二聚体和格构价键键簇基态的精确表示可以通过通常的耦合团簇方法(CCM)从独立自旋产物(如Néel)模型态得到.我们表明,我们能够提供良好的结果为二聚体和格片价键相内的CCM的基态能量和亚晶格磁化。作为第一个例子,我们调查的自旋半$J_1$ - $J_2$模型的线性链,我们表明,我们能够准确地再现二聚基态(KET)在$J_2/J_1=0.5$。二聚相在$J_2/J_1$约0.5的值范围内是稳定的。我们提出的对称性破缺的证据,考虑作为$J_2/J_1$的函数的ket和bra状态的相关系数。然后,我们考虑Shastry-Sutherland模型,并证明CCM可以跨越正确的基态在Néel和二聚化阶段。最后,我们考虑了一个自旋半系统与最近邻键的基础晶格对应的磁性材料CaV $_4$ O $_9$(CAVO)。我们表明,我们能够提供良好的结果,基态能量在每一个的格序,Néel有序,和二聚制度的这个模型。精确的元胞体和二聚体基态由CCM ket态在其相应的限制下再现。
英文摘要:
In this article, we prove that exact representations of dimer and plaquette valence-bond ket ground states for quantum Heisenberg antiferromagnets may be formed via the usual coupled cluster method (CCM) from independent-spin product (e.g. Néel) model states. We show that we are able to provide good results for both the ground-state energy and the sublattice magnetization for dimer and plaquette valence-bond phases within the CCM. As a first example, we investigate the spin-half $J_1$--$J_2$ model for the linear chain, and we show that we are able to reproduce exactly the dimerized ground (ket) state at $J_2/J_1=0.5$. The dimerized phase is stable over a range of values for $J_2/J_1$ around 0.5. We present evidence of symmetry breaking by considering the ket- and bra-state correlation coefficients as a function of $J_2/J_1$. We then consider the Shastry-Sutherland model and demonstrate that the CCM can span the correct ground states in both the Néel and the dimerized phases. Finally, we consider a spin-half system with nearest-neighbor bonds for an underlying lattice corresponding to the magnetic material CaV$_4$O$_9$ (CAVO). We show that we are able to provide excellent results for the ground-state energy in each of the plaquette-ordered, Néel-ordered, and dimerized regimes of this model. The exact plaquette and dimer ground states are reproduced by the CCM ket state in their relevant limits.
High-Order Coupled Cluster Method Calculations using Three-Dimensional Model States: An Illustration for the Triangular-Lattice Antiferromagnet in an External Field
Coplanar model states for applications of the coupled cluster method (CCM) to problems in quantum magnetism are those in which all spins lie in a plane, whereas three-dimensional (3D) model states are, by contrast, non-coplanar ones in which all the spins do not lie in any single plane. Here we extend the CCM to non-coplanar / 3D model states and we present results for three cases: (a) the spin-half one-dimensional Ising ferromagnet in an applied transverse magnetic field (as an exactly solvable test model to use as a yardstick for the viability and accuracy of our new methodology); (b) the sp…
Coupled cluster treatment of one quasi-one-dimensional coupled spin triangles
利用耦合集团方法(CCM)和数值精确对角化(ED)方法,研究了准一维耦合自旋三角形的性质。ED结果表明,当a<ac 1时,体系处于严格的四聚体-二聚体(TD)态。我们证明了在该参数范围内利用CCM可以形成精确的TD态。更有趣的是,我们发现通过CCM也可以得到TD态的精确自旋能隙。在临界点ac 1处,发生一级跃迁,系统的基态从TD态演化到铁磁长程序态。在a> ac 1的参数范围内,CCM对基态能、磁化强度和反铁磁能隙等物理量的计算结果与ED的结果吻合得很好,因此CCM可以用来精确地分析整个参数范围内准一维耦合自旋三角形的性质.我们相信它也可以可靠地应用于研究其他准单自旋系统的性质。
Transverse Magnetic Susceptibility of a Frustrated Spin-$\frac{1}{2}$ $J_{1}$--$J_{2}$--$J_{1}^{\perp}$ Heisenberg Antiferromagnet on a Bilayer Honeycomb Lattice
A molecular Mott insulator $κ$-(ET)$_2$B(CN)$_4$ [ET = bis(ethylenedithio)tetrathiafulvalene] with a distorted triangular lattice exhibits a quantum disordered state with gapped spin excitation in the ground state. $^{13}$C nuclear magnetic resonance, magnetization, and magnetic torque measurements reveal that magnetic field suppresses valence bond order and induces long-range magnetic order above a critical field $\sim 8$ T. The nuclear spin-lattice relaxation rate $1/T_1$ shows persistent evolution of antiferromagnetic correlation above the transition temperature, highlighting a quantum spin…
Mixed valence on a pyrochlore lattice - LiV2O4 as a geometrically frustrated magnet
在40 K以上,重费米子尖晶石LiV_2O_4的磁化率与几何阻挫磁绝缘体的磁化率具有许多共同特征,而其室温电阻率舒适地超过Mott-Regel极限。这表明,当地的磁矩,以及烧绿石晶格的基本几何形状,在确定其磁性能发挥了重要作用。我们最近推出的四面体平均场论绝缘烧绿石反铁磁体的情况下,其中个别四面体包含不同长度的自旋,并以此为出发点,讨论三种不同的情况下,在LiV 2 O 4的磁性和电子的转变。
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We present results of magnetic susceptibility and thermal expansion measurements performed on high-quality single crystals of EuPd$_2$(Si$_{1-x}$Ge$_x$)$_2$ for 0 $\leq$ x $\leq$ 0.2 and temperatures 2 K $\leq T \leq$ 300 K. Data were taken at ambient pressure and finite He-gas pressure $p$ $\leq$ 0.5 GPa. For x = 0 and ambient pressure we observe a pronounced valence-change crossover centred around $T'_V$ $\approx$ 160 K with a non-magnetic ground state. This valence-change crossover is characterized by an extraordinarily strong pressure dependence of d$T'_V$ /d$p$ = (80 $\pm 10)$ K/GPa. We o…
The phase diagram of the filled skutterudite CeOs$_4$Sb$_{12}$ has been mapped in fields $H$ of up to 60 T and temperatures $T$ down to 0.5 K using resistivity, magnetostriction, and MHz conductivity. The valence transition separating the semimetallic, low-$H$, low-$T$, $\cal{L}$ phase from the metallic high-$H$, high-$T$ $\cal{H}$ phase exhibits a very unusual, wedge-shaped phase boundary, with a non-monotonic gradient alternating between positive and negative. This is quite different from the text-book "elliptical" phase boundary usually followed by valence transitions. Analysis of Shubnikov…
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