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arXiv:1607.07520 · Physical review. B./Physical review. B

Analytic expression for the entanglement entropy of a two-dimensional topological superconductor

Jan Borchmann, T. Pereg-Barnea

Published Feb 27, 2017• 4 citations
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Abstract

In this work we study a two-dimensional topological superconductor on a square lattice. It contains hopping, spin-orbit coupling and a time reversal symmetry breaking Zeeman term. This realizes, in combination with a singlet pairing term, an effective triplet pairing that leads to a topological superconductor. As previously found numerically, the transitions are seen in the entanglement entropy as cusps as a function of model parameters. We complement this through studying the entanglement entropy analytically by keeping only its most important components. Our study is based on the intuition that the number of Fermi surfaces in the system controls the topological invariant. With our approximate expression for the entanglement entropy we are able to extract the divergent entanglement entropy derivative close to the phase transition.

DOI
10.1103/physrevb.95.075152

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In this work we study a two-dimensional topological superconductor on a square lattice. It contains hopping, spin-orbit coupling and a time reversal symmetry breaking Zeeman term. This realizes, in combination with a singlet pairing term, an effective triplet pairing that leads to a topological superconductor. As previously found numerically, the transitions are seen in the entanglement entropy as cusps as a function of model parameters. We complement this through studying the entanglement entropy analytically by keeping only its most important components. Our study is based on the intuition that the number of Fermi surfaces in the system controls the topological invariant. With our approximate expression for the entanglement entropy we are able to extract the divergent entanglement entropy derivative close to the phase transition.
We analyze the entanglement entropy properties of a 2D p-wave superconductor with Rashba spin-orbit coupling, which displays a rich phase-space that supports non-trivial topological phases, as the chemical potential and the Zeeman term are varied. We show that the entanglement entropy and its derivatives clearly signal the topological transitions and we find numerical evidence that for this model the derivative with respect to the magnetization provides a sensible signature of each topological phase. Following the area law for the entanglement entropy, we systematically analyze the contributions that are proportional to or independent of the perimeter of the system, as a function of the Hamiltonian coupling constants and the geometry of the finite subsystem. For this model, we show that even though the topological entanglement entropy vanishes, it signals the topological phase transitions in a finite system. We also observe a relationship between a topological contribution to the entanglement entropy in a half-cylinder geometry and the number of edge states, and that the entanglement spectrum has robust modes associated with each edge state, as in other topological systems.
We analyze the entanglement entropy properties of a 2D p-wave superconductor with Rashba spin-orbit coupling, which displays a rich phase-space that supports non-trivial topological phases, as the chemical potential and the Zeeman term are varied. We show that the entanglement entropy and its derivatives clearly signal the topological transitions and we find numerical evidence that for this model the derivative with respect to the magnetization provides a sensible signature of each topological phase. Following the area law for the entanglement entropy, we systematically analyze the contributions that are proportional to or independent of the perimeter of the system, as a function of the Hamiltonian coupling constants and the geometry of the finite subsystem. For this model, we show that even though the topological entanglement entropy vanishes, it signals the topological phase transitions in a finite system. We also observe a relationship between a topological contribution to the entanglement entropy in a half-cylinder geometry and the number of edge states, and that the entanglement spectrum has robust modes associated with each edge state, as in other topological systems.
Topological insulators and topological superconductors display various topological phases that are characterized by different Chern numbers or by gapless edge states. In this work we show that various quantum information methods such as the von Neumann entropy, entanglement spectrum, fidelity, and fidelity spectrum may be used to detect and distinguish topological phases and their transitions. As an example we consider a two-dimensional ppp-wave superconductor, with Rashba spin-orbit coupling and a Zeeman term. The nature of the phases and their changes are clarified by the eigenvectors of the kkk-space reduced density matrix. We show that in the topologically nontrivial phases the highest weight eigenvector is fully aligned with the triplet pairing state. A signature of the various phase transitions between two points on the parameter space is encoded in the kkk-space fidelity operator.
In this paper we study the phase diagram of a disordered, spin-orbit coupled superconductor with sss-wave or d+idd+idd+id-wave pairing symmetry in symmetry class DDD. We analyze the topological phase transitions by applying three different methods which include a disorder averaged entanglement entropy, a disorder averaged real-space Chern number, and an evaluation of the momentum space Chern number in a disorder averaged effective model. We find evidence for a disorder-induced topological state. While in the clean limit there is a single phase transition from a trivial phase with a Chern number C=4C=4C=4 to a topological phase with C=1C=1C=1, in the disordered system there is an intermediate phase with C=3C=3C=3. The phase transition from the trivial C=4C=4C=4 phase into the intermediate phase with C=3C=3C=3 is seen in the real-space calculation of the Chern number. In spite of this, this phase transition is not detectable in the entanglement entropy. A second phase transition from the disorder induced C=3C=3C=3 into the C=1C=1C=1 phase is seen in all three quantities.
We consider a model proposed before for a time-reversal-invariant topological superconductor which contains a hopping term ttt, a chemical potential $\ensuremath{\mu}$, an extended sss-wave pairing $\mathrm{\ensuremath{\Delta}}$, and spin-orbit coupling $\ensuremath{\lambda}$. We show that for $|\mathrm{\ensuremath{\Delta}}|=|\ensuremath{\lambda}|$, $\ensuremath{\mu}=t=0$, the model has an exact analytical solution defining new fermion operators involving nearest-neighbor sites. The many-body ground state is fourfold degenerate due to the existence of two zero-energy modes localized exactly at the first and last sites of the chain. These four states show entanglement in the sense that creating or annihilating a zero-energy mode at the first site is proportional to a similar operation at the last site. By continuity, this property should persist for general parameters. Using these results, we discuss some statements related to the so-called time-reversal anomaly. The addition of a small hopping term ttt for a chain with an even number of sites breaks the degeneracy, and the ground state becomes unique with an even number of particles. We also consider a small magnetic field BBB applied to one end of the chain. We compare the many-body excitation energies and spin projection along the spin-orbit direction for both ends of the chains obtained treating ttt and BBB as small perturbations with numerical results in a short chain, obtaining good agreement.
We theoretically investigate a tight-binding model of fermions hopping on the square-octagon lattice which consists of a square lattice with plaquette corners themselves decorated by squares. Upon the inclusion of second-neighbor spin-orbit coupling or non-Abelian gauge fields, time-reversal symmetric topological Z2{Z}_{2}Z2​ band insulators are realized. Additional insulating and gapless phases are also realized via the non-Abelian gauge fields. Some of the phase transitions involve topological changes to the Fermi surface. The stability of the topological phases to various symmetry-breaking terms is investigated via the entanglement spectrum. Our results enlarge the number of known exactly solvable models of Z2{Z}_{2}Z2​ band insulators and are potentially relevant to the realization and identification of topological phases in both the solid-state and cold atomic gases.