Deb Moumita, Ghosh Asim Kumar
Department of Physics, Jadavpur University, 188 Raja Subodh Chandra Mallik Road, Kolkata 700032, India.
J Phys Condens Matter. 2020 Jun 12;32(36). doi: 10.1088/1361-648X/ab85f7.
A number of topological nodes including Dirac, quadratic and three-band touching points as well as a pair of degenerate Dirac line nodes are found to emerge in the triplet plaquette excitations of the frustrated spin-1/2-antiferromagnetic Heisenberg honeycomb model when the ground state of the system lies in a spin-disordered plaquette-valence-bond-solid phase. A six-spin plaquette operator theory of this honeycomb model has been developed for this purpose by using the eigenstates of an isolated Heisenberg hexagonal plaquette. Spin-1/2 operators are thus expressed in the Fock space spanned by the plaquette operators those are obtained in terms of exact analytic form of eigenstates for a single frustrated Heisenberg hexagon. Ultimately, an effective interacting boson model of this system is obtained on the basis of low energy singlets and triplets plaquette operators by employing a mean-field approximation. The values of ground state energy and spin gap of this system have been estimated and the validity of this formalism has been tested upon comparison with the known results. Emergence of topological point and line nodes on the basis of spin-disordered ground state noted in this investigation is very rare on any frustrated system as well as the presence of triplet flat band. Evolution of those topological nodes is studied throughout the full frustrated regime. Finally, emergence of topological phases has been reported upon adding a time-reversal-symmetry breaking term to the Hamiltonian. Coexistence of spin gap with either topological nodes or phases turns this honeycomb model an interesting one.
当系统基态处于自旋无序的面元价键固体相时，人们发现，在受挫的自旋1/2反铁磁海森堡蜂窝模型的三重态面元激发中，会出现包括狄拉克点、二次型和三能带接触点在内的多个拓扑节点，以及一对简并狄拉克线节点。为此，通过使用孤立海森堡六边形的本征态，开发了该蜂窝模型的六自旋面元算符理论。自旋1/2算符因此在由面元算符所张成的福克空间中表示出来，这些面元算符是根据单个受挫海森堡六边形本征态的精确解析形式得到的。最终，通过采用平均场近似，在低能单重态和面元三重态算符的基础上，得到了该系统的有效相互作用玻色子模型。估计了该系统的基态能量和自旋能隙的值，并通过与已知结果比较检验了该形式体系的有效性。在本研究中指出的基于自旋无序基态的拓扑点和线节点的出现，在任何受挫系统中都非常罕见，同样罕见的还有三重态平带的存在。在整个完全受挫区域研究了这些拓扑节点的演化。最后，报道了在哈密顿量中加入时间反演对称性破缺项后拓扑相的出现。自旋能隙与拓扑节点或相的共存，使得这个蜂窝模型变得很有趣。
