# Unexpected Enhancement of Three-Dimensional Low-Energy Spin Correlations in Quasi-Two-Dimensional Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ System at High Temperature

## TL;DR

This study reveals an unexpected increase in three-dimensional low-energy spin correlations in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ at high temperatures, challenging existing models based solely on spin degrees of freedom.

## Contribution

It demonstrates a temperature-induced transition from 2D to 3D spin correlations linked to orbital effects, providing new insights into the magnetic behavior of the system.

## Key findings

- Spin correlations are 2D at low temperatures in superconducting samples.
- Spin correlations become more 3D at temperatures above $T_c$, with increased correlation length perpendicular to Fe planes.
- The transition from 2D to 3D correlations cannot be explained by spin degrees alone, indicating orbital contributions.

## Abstract

We report inelastic neutron scattering measurements of low energy ($\hbar \omega < 10$ meV) magnetic excitations in the "11" system Fe$_{1+y}$Te$_{1-x}$Se$_{x}$. The spin correlations are two-dimensional (2D) in the superconducting samples at low temperature, but appear much more three-dimensional when the temperature rises well above $T_c \sim 15$ K, with a clear increase of the (dynamic) spin correlation length perpendicular to the Fe planes. The spontaneous change of dynamic spin correlations from 2D to 3D on warming is unexpected and cannot be naturally explained when only the spin degree of freedom is considered. Our results suggest that the low temperature physics in the "11" system, in particular the evolution of low energy spin excitations towards %better satisfying the nesting condition for mediating superconducting pairing, is driven by changes in orbital correlations.