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Journal of Physics Condensed Matter

Exact ground and excited states of an antiferromagnetic quantum spin model

Indrani Bose

Published Nov 20, 1989• 13 citations
Publisher

Abstract

A quasi-one-dimensional spin model that consists of a chain of octahedra of spins has been suggested, for which in a certain parameter regime of the Hamiltonian the ground state can be written down exactly. The ground state is highly degenerate and can be other than a singlet. Also, several excited states can be constructed exactly. The ground state is a local resonating valence bond state for which resonance is confined to rings of spins. Some exact numerical results for an octahedron of spins have also been reported.

DOI
10.1088/0953-8984/1/46/018

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A quasi-one-dimensional spin model that consists of a chain of octahedra of spins has been suggested, for which in a certain parameter regime of the Hamiltonian the ground state can be written down exactly. The ground state is highly degenerate and can be other than a singlet. Also, several excited states can be constructed exactly. The ground state is a local resonating valence bond state for which resonance is confined to rings of spins. Some exact numerical results for an octahedron of spins have also been reported.
An antiferromagnetic spin model (S=1/2) consisting of a chain of octahedra is considered. In a certain parameter regime of the Hamiltonian the exact ground state is a local resonating-valence-bond state. We start with the t-J Hamiltonian for this model describing two holes in the same octahedron, one in the base and the other at one of the vertex sites of the octahedron. The t-J Hamiltonian consists of three parts: Ht1{\mathit{H}}_{\mathit{t}1}Ht1​, describing the motion of holes along the chain, Ht2{\mathit{H}}_{\mathit{t}2}Ht2​, describing the motion of holes in the bases of the octahedra, and HJ{\mathit{H}}_{\mathit{J}}HJ​, describing the antiferromagnetic spin-spin interaction. Some exact eigenstates of Ht2{\mathit{H}}_{\mathit{t}2}Ht2​ and HJ{\mathit{H}}_{\mathit{J}}HJ​ are constructed. Successive applications of Ht1{\mathit{H}}_{\mathit{t}1}Ht1​ on some of these states show two types of localization for one of the holes. In one type of localization one of the holes is localized at its original location while the other hole moves. In the other type of localization, a hole, located in the base of an octahedron, cannot move away from the other hole sitting at a vertex site of the same octahedron. The localization in both cases is produced by the topology of the lattice on which the t-J model is defined.
Two spin models in two dimensions are constructed for which the exact ground states can be written down in a certain parameter regime of the Hamiltonian. The ground state of the first model is a local resonating valence bond (RVB) state in which not all the spins participate in resonance. The ground state is highly degenerate and there is a gap in the excitation spectrum. The second model is shown to have a ground state which is a quantum spin liquid with all the spins taking part in resonance. The ground state obtained is non-degenerate and there is no gap in the excitation spectrum.
An antiferromagnetic spin model (S=1/2) consisting of a chain of octahedra is considered. In a certain parameter regime of the Hamiltonian the exact ground state is a local resonating-valence-bond (RVB) state. We start with the t-J Hamiltonian for this model describing a single hole in the system and derive exact ground and excited states of the Hamiltonian when the hole is static. The local spin distortion around a hole is termed a spin polaron. The dynamics of the hole is studied through construction of exact eigenstates describing spin polarons in extended states. Coherent states for all momentum wave vectors in the chain direction exist for certain spin polarons. In the Nagaoka limit, the RVB polaronic states have energy greater than that of the ferromagnetic state. For exchange interaction J\ensuremath{\ne}0, the RVB states have lower energy. Other exact states obtained include states in which the hole motion is localized. Structural similarity of the present model to the arrangement of Cu spins in La2{\mathrm{La}}_{2}La2​CuO4{\mathrm{CuO}}_{4}CuO4​ is also highlighted.
We outline the recent results on the ground state for a class of one- and two-dimensional frustrated quantum spin models with competing ferro(F)- and antiferromagnetic (AF) interactions. Frustrated spin systems are known to have many interesting properties due to large quantum fluctuations. As a result of these fluctuations usual mean-field approach gives quite crude (if not false) description of these systems. Therefore, exactly solvable models are very instructive at investigations of such systems. The exact ground state wave function of proposed models has a structure of the valence-bond state (VBS) type. One of the 1D model describes the transition line between the F and AF phase. The exact singlet ground state on this line has a double-spiral ordering. Using different approximation methods we study the magnetization curve in the AF phase. The second considered set of the 1D and 2D models has an exact non-degenerate ground state with exponentially decaying spin correlations. We also proposed the 1D and 2D electronic models with exact ground state represented in terms of singlet bond functions which are the generalization of the RVB functions including ionic states.
The experimental observation of a spin gap in the antiferromagnetic spin-12\frac{1}{2}21​ compound CaV4{}_{4}4​O9{}_{9}9​ has been attributed to the formation of a plaquette-resonating-valence-bond (PRVB) state in the underlying 1/5-depleted square lattice. We construct a spin-12\frac{1}{2}21​ model on the 1/5-depleted lattice for which the PRVB state can be shown to be the exact ground state in a particular parameter regime. In a subspace of this parameter regime, the first excited state can be calculated exactly. In a different parameter regime, the dimer state is shown to be the exact ground state.
We present exact ground states in spin models with orbital generacy in one and higher dimensions. A method to obtain the exact ground states of the models when the Hamiltonians are composed of the products of two commutable operators is proposed. For the case of the spin-1/2 model with two-fold degeneracy some exact ground states are given, such as the Valence-Bond (VB), the magnetically ordered, and the orbitally ordered states under particular parameter regimes. We also find the models with the higher spin and degeneracy which have the new types of VB ground states in the spin and the orbital sectors.