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Journal of Contemporary Physics (Armenian Academy of Sciences)

Simple Extension of Kitaev Chain for $${{\mathcal{Z}}_{3}}$$ Parafermions

Tigran Hakobyan, Raffi Varosyan, G. Harutunyan

Published Mar 1, 2024• 0 citations
Publisher

Abstract

A simple quantum chain with a topological phase is constructed. It is formed by parafermions (quasiparticles obeying the fractional statistics) with angular parameter $$\theta = {{2\pi } \mathord{\left/ {\vphantom {{2\pi } 3}} \right. \kern-0em} 3}$$ . The model is derived from the one-dimensional spin model with topological order protected by the $${{\mathcal{Z}}_{3}} \times {{\mathcal{Z}}_{3}}~$$ symmetry (generalized cluster model). Parafermionic zero modes arising at the chain boundaries, which remain stable because of the vacuum gap and topological order, are constructed and studied.

DOI
10.1134/s1068337224700075

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Simple Extension of Kitaev Chain for Z3{{\mathcal{Z}}_{3}}Z3​ Parafermions
A simple quantum chain with a topological phase is constructed. It is formed by parafermions (quasiparticles obeying the fractional statistics) with angular parameter θ=2π/2π33\theta = {{2\pi } \mathord{\left/ {\vphantom {{2\pi } 3}} \right. \kern-0em} 3}θ=2π/2π33 . The model is derived from the one-dimensional spin model with topological order protected by the Z3×Z3 {{\mathcal{Z}}_{3}} \times {{\mathcal{Z}}_{3}}~Z3​×Z3​  symmetry (generalized cluster model). Parafermionic zero modes arising at the chain boundaries, which remain stable because of the vacuum gap and topological order, are constructed and studied.
Parafermions are exotic quasiparticles with non-Abelian fractional statistics that can be realized and stabilized in one-dimensional models that are generalizations of the Kitaev ppp-wave wire. We study the simplest generalization, i.e., the Z3{Z}_{3}Z3​ parafermionic chain. Using a Jordan-Wigner transform we focus on the equivalent three-state chiral clock model, and study its rich phase diagram using the density matrix renormalization group technique. We perform our analyses using quantum entanglement diagnostics which allow us to determine phase boundaries, and the nature of the phase transitions. In particular, we study the transition between the topological and trivial phases, as well as to an intervening incommensurate phase which appears in a wide region of the phase diagram. The phase diagram is predicted to contain a Lifshitz type transition which we confirm using entanglement measures. We also attempt to locate and characterize a putative tricritical point in the phase diagram where the three above mentioned phases meet at a single point.
Parafermion modes are non-Abelian anyons which were introduced as ZN{\mathbb{Z}}_{N}ZN​ generalizations of Z2{\mathbb{Z}}_{2}Z2​ Majorana states. In particular, Z3{\mathbb{Z}}_{3}Z3​ parafermions can be used to produce Fibonacci anyons, laying a path towards universal topological quantum computation. Due to their fractional nature, much of the theoretical work on Z3{\mathbb{Z}}_{3}Z3​ parafermions has relied on bosonization methods or parafermionic quasiparticles. In this paper, we introduce a representation of Z3{\mathbb{Z}}_{3}Z3​ parafermions in terms of purely fermionic models. We establish the equivalency of a family of lattice fermionic models written in the basis of the $t\ensuremath{-}J$ model with a Kitaev-like chain supporting free Z3{\mathbb{Z}}_{3}Z3​ parafermionic modes at its ends. By using density matrix renormalization group calculations, we are able to characterize the topological phase transition and study the effect of local operators (doping and magnetic fields) on the spatial localization of the parafermionic modes and their stability. Moreover, we discuss the necessary ingredients towards realizing Z3{\mathbb{Z}}_{3}Z3​ parafermions in strongly interacting electronic systems.
Parafermion zero-modes, which are present in several one-dimensional topological phases, generalize widely studied Majorana fermions and present a promising route for topological quantum computation. In this paper, analytical and numerical methods are used to find the regimes in which exact parafermion zero-modes exist in a lattice model which is an extension of the Kitaev chain. The authors find that the stability conditions for the zero-modes in parafermion chains differs markedly from that of the Majorana case.
Topological orders and associated topological protected excitations satisfying Abelian or non-Abelian statistics have been widely explored in various platforms. In recent years, the Majorana zero modes with these features have been realized in experiments based on semiconductor and superconducting hybrid structures, opening an exciting avenue for realization of fault-tolerant topological quantum computations. The Z3\mathbb{Z}_3Z3​ parafermions are regarded as the most natural generation of the Majorana fermions for realizing of more intriguing topological orders. In this work, we investigate the topological phase and emergent fermions in an extended parafermion chain using exact numerical methods. This model exhibits rich variety of phases, including threefold degenerate topological ferromagnetic phase, trivial paramagnetic phase, XX phase, dimer phase, chiral phase and commensurate phase. We present a smooth connection between this model and the extended spin-12{1\over 2}21​ XX model, by which we can generalize their measurements, such as the order parameters and long-range spin-spin correlations {\it etc}, to fully characterize the properties of these phases and map out their corresponding phase diagram. Strikingly, we find that all the phase boundaries finally merge to a single supercritical point, determined by the extended three-state Potts model. This approach opens a wide range of intriguing applications in investigating the phases and emergent phenomena in other Zk\mathbb{Z}_kZk​ parafermion models, and may be also illuminating for their experimental realizations.
In the Fock representation, we propose a framework to construct the generalized matrix product states (MPS) for topological phases with Zp{\mathbb{Z}}_{p}Zp​ parafermions. Unlike the Z2{\mathbb{Z}}_{2}Z2​ Majorana fermions, the Zp{\mathbb{Z}}_{p}Zp​ parafermions form intrinsically interacting systems. Here we explicitly construct two topologically distinct classes of irreducible Z3{\mathbb{Z}}_{3}Z3​ parafermionic MPS wave functions, characterized by one or two parafermionic zero modes at each end of an open chain. Their corresponding parent Hamiltonians are found as the fixed point models of the single Z3{\mathbb{Z}}_{3}Z3​ parafermion chain and two-coupled parafermion chains with ${\mathbb{Z}}_{3}\ifmmode\times\else\texttimes\fi{}{\mathbb{Z}}_{3}$ symmetry. Our results thus pave the road to investigate all possible topological phases with Zp{\mathbb{Z}}_{p}Zp​ parafermions within the matrix product representation in one dimension.
Parafermions are the simplest generalizations of Majorana fermions that realize topological order. We propose a less restrictive notion of topological order in one-dimensional open chains, which generalizes the seminal work by Fendley [J. Stat. Mech. (2012) P11020]. The first essential property is that the ground states are mutually indistinguishable by local, symmetric probes, and the second is a generalized notion of zero edge modes which cyclically permute the ground states. These two properties are shown to be topologically robust, and applicable to a wider family of topologically ordered Hamiltonians than has been previously considered. As an application of these edge modes, we formulate a notion of twisted boundary conditions on a closed chain, which guarantees that the closed-chain ground state is topological, i.e., it originates from the topological manifold of the open chain. Finally, we generalize these ideas to describe symmetry-breaking phases with a parafermionic order parameter. These exotic phases are condensates of parafermion multiplets, which generalize Cooper pairing in superconductors. The stability of these condensates is investigated on both open and closed chains.