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arXiv:1905.11090

Exact Solution for an Interacting Spin One-Half Chain at the Symmetric Point

Gianluca Francica, Paola Gentile, Mario Cuoco

Published May 27, 2019• 0 citations
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Abstract

The p-wave superconductive wire with Hubbard interaction $U$ is solved exactly at the symmetry point $\Delta=t$ and chemical potential $\mu=0$ in an open boundary condition. By applying two Jordan-Wigner transformations and individuating non-local constant of motions such interacting model is mapped onto a noninteracting fermion model which can be diagonalized exactly. In the lowest energy subspace the mapping is onto a Kitaev chain. The system enters in a topologically trivial phase due to a second order quantum phase transition at $U=\pm 2t$. The features of the quantum phases are characterized with the help of the exact solution.

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The p-wave superconductive wire with Hubbard interaction UUU is solved exactly at the symmetry point Δ=t\Delta=tΔ=t and chemical potential μ=0\mu=0μ=0 in an open boundary condition. By applying two Jordan-Wigner transformations and individuating non-local constant of motions such interacting model is mapped onto a noninteracting fermion model which can be diagonalized exactly. In the lowest energy subspace the mapping is onto a Kitaev chain. The system enters in a topologically trivial phase due to a second order quantum phase transition at U=±2tU=\pm 2tU=±2t. The features of the quantum phases are characterized with the help of the exact solution.
The Kitaev chain model with a nearest neighbor interaction U is solved exactly at the symmetry point Δ=t and chemical potential μ=0 in an open boundary condition. By applying two Jordan-Wigner transformations and a spin rotation, such a symmetric interacting model is mapped onto a noninteracting fermion model, which can be diagonalized exactly. The solutions include a topologically nontrivial phase at |U|<t and a topologically trivial phase at |U|>t. The two phases are related by dualities. Quantum phase transitions in the model are studied with the help of the exact solution.
A superconducting wire described by a ppp-wave pairing and a Kitaev Hamiltonian exhibits Majorana fermions at its edges and is topologically protected by symmetry. We consider two Kitaev wires (chains) coupled by a Coulomb-type interaction and study the complete phase diagram using analytical and numerical techniques. A topological superconducting phase with four Majorana fermions occurs until moderate interactions between chains. For large interactions, both repulsive and attractive, by analogy with the Hubbard model, we identify Mott phases with Ising-type magnetic order. For repulsive interactions, the Ising antiferromagnetic order favors the occurrence of orbital currents spontaneously breaking time-reversal symmetry. By strongly varying the chemical potentials of the two chains, quantum phase transitions towards fully polarized (empty or full) fermionic chains occur. In the Kitaev model, the quantum critical point separating the topological superconducting phase and the polarized phase belongs to the universality class of the critical Ising model in two dimensions. When increasing the Coulomb interaction between chains, then we identify an additional phase corresponding to two critical Ising theories (or two chains of Majorana fermions). We confirm the existence of such a phase from exact mappings and from the concept of bipartite fluctuations. We show the existence of negative logarithmic corrections in the bipartite fluctuations, as a reminiscence of the quantum critical point in the Kitaev model. Other entanglement probes such as bipartite entropy and entanglement spectrum are also used to characterize the phase diagram. The limit of large interactions can be reached in an equivalent setup of ultracold atoms and Josephson junctions.
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A Spin 1/21/21/2 Fermions Chain with XYXYXY Interaction
We consider a chain of spinful fermions with nearest neighbor hopping in the presence of a XYXYXY antiferromagnetic interaction. The XYXYXY term is mapped onto a Kitaev chain at half-filling such that displays a bosonic zero mode topologically protected and long-range order. As the strength of the hopping amplitude is changed, the system undergoes a quantum phase transition from the topological non-trivial to the trivial phase. We apply the finite-size scaling method to determinate the phase diagram of the model.
It was recently shown that an interacting Kitaev topological superconductor model is exactly solvable based on two-step Jordan-Wigner transformations together with one spin rotation. We generalize this model by including the dimerization, which is shown also to be exactly solvable. We analytically determine the topological phase diagram containing seven distinct phases. It is argued that the system is topological when a fermionic many-body Majorana zero-energy edge state emerges. It is intriguing that there are two tetracritical points, at each of which four distinct phases touch.
Applying the Jordan-Wigner transformation to spin-1/2 operators on special quasi-one dimensional paths, we show that the two-dimensional Kitaev model can be exactly mapped to a free Majorana fermion model without any redundant degrees of freedom. Via duality transformation, it can be further shown that the quantum phase transitions of the Kitaev model are described by non-local topological order parameters, which become Landau type local order parameters in the dual space. A closed relationship between conventional and topological quantum phase transitions is revealed, and the validity of conventional Landau phase transition theory with spontaneous symmetry breaking and local order parameters is also extended.