# Quantum Ising model in a period-2 modulated transverse field

## TL;DR

This paper analyzes a quantum Ising chain with a period-2 modulated transverse field, deriving exact energies, energy gaps, and correlation lengths, providing insights into its quantum phase transitions and critical behavior.

## Contribution

It introduces a method to exactly compute ground-state energies, energy gaps, and correlation lengths for a period-2 modulated quantum Ising model.

## Key findings

- Derived closed-form expressions for energy gaps between parity subspaces.
- Analyzed the energy gap behavior across different regimes of the applied field.
- Proposed a correlation length expression consistent with known critical behaviors.

## Abstract

We study a finite spin-$\frac{1}{2}$ Ising chain with a spatially alternating transverse field of period 2. By means of a Jordan-Wigner transformation for even and odd sites, we are able to map it into a one-dimensional model of free fermions. We determine the ground-state energies in the positive- and negative-parity subspaces (subspaces with an even or odd total number of down spins, respectively) and compare them in order to establish the ground-state energy for the entire Hamiltonian. We derive closed-form expressions for this energy gap between the different parity subspaces and analyze its behavior and dependence on the system size in the various regimes of the applied field. Finally, we suggest an expression for the correlation length of such a model that is consistent with the various values found in the literature for its behavior in the vicinity of critical points.