# Hierarchy of correlations for the Ising model in the Majorana representation

## TL;DR

This paper investigates the quantum Ising model using a hierarchy of correlations in the Majorana representation, revealing how thermal fluctuations and magnons influence phase transitions and critical behavior.

## Contribution

It introduces a correlation hierarchy approach in the Majorana representation that captures both mean field results and the effects of correlations on quantum phase transitions.

## Key findings

- Thermal fluctuations destroy order in 1D at B=0.
- Correlations significantly alter the critical behavior near quantum critical points.
- The method connects with Dyson's equation and self-energy formalism.

## Abstract

We study the quantum Ising model in D dimensions with the equation of motion technique, in combination with the Majorana representation for spins. The decoupling scheme used for the Green's functions is based on the hierarchy of correlations in position space. To lowest order this method reproduces the well known mean field phase diagram and critical exponents. When correlations between spins are included, we show how the appearance of thermal fluctuations and magnons strongly affect the physical properties. We demonstrate that in 1D and for B=0, thermal fluctuations completely destroy the ordered phase, and that near the quantum critical point, the quantum model displays different behavior than its classical counterpart. We discuss the connection with the Dyson's equation formalism and the explicit form of the self-energies.