# Bound states of fractionalized excitations in a modulated Kitaev spin liquid

## TL;DR

This paper investigates how microscopic interactions in a Kitaev honeycomb model with lattice distortion can lead to bound states of fractional excitations, resulting in sharp features in the magnetic response.

## Contribution

It demonstrates the formation of bound states between flux pairs and Majorana fermions in a modulated Kitaev spin liquid, revealing new sharp modes in the magnetic spectrum.

## Key findings

- Bound states form in Majorana band gaps due to lattice modulation.
- Sharp magnetic response modes encode the symmetry of bound states.
- Isolated fluxes can host zero-energy Majorana bound states.

## Abstract

Fractionalization is a hallmark of spin-liquid behavior; it typically leads to response functions consisting of continua instead of sharp modes. However, microscopic processes can enable the formation of short-distance bound states of fractionalized excitations, despite asymptotic deconfinement. Here we study such bound-state formation for the $Z_2$ spin liquid realized in Kitaev's honeycomb compass model, supplemented by a kekule distortion of the lattice. Bound states between flux pairs and Majorana fermions form in the Majorana band gaps. We calculate the dynamic spin susceptibility and show that bound states lead to sharp modes in the magnetic response of the spin liquid, with the momentum dependence of the corresponding spectral weight encoding internal symmetry of the bound state. As a byproduct, we also show that isolated fluxes may produce Majorana bound states at exactly zero energy. Generalizations and implications of the results are discussed.