Near room-temperature ferromagnetism from double-exchange in the van der Waals material CrGeTe$_3$: evidence from optical conductivity under pressure

TL;DR

This study combines experimental and theoretical methods to reveal that double-exchange interactions drive near room-temperature ferromagnetism in pressurized CrGeTe$_3$, with evidence from optical conductivity and electronic structure analysis.

Contribution

It provides the first comprehensive evidence linking double-exchange mechanisms to high-temperature ferromagnetism in a van der Waals material under pressure.

Findings

Optical spectra show insulator-metal transition with pressure.

Strong orbital correlations develop in high-pressure ferromagnetic phase.

Charge-transfer gap persists, contradicting previous collapse hypothesis.

Abstract

The unexpected discovery of intrinsic ferromagnetism in layered van der Waals materials has sparked interest in both their fundamental properties and their potential for novel applications. Recent studies suggest near room-temperature ferromagnetism in the pressurized van der Waals crystal CrGeTe$_3$. We perform a comprehensive experimental and theoretical investigation of magnetism and electronic correlations in CrGeTe$_3$, combining broad-frequency reflectivity measurements with density functional theory and dynamical mean-field theory calculations. Our experimental optical conductivity spectra trace the signatures of developing ferromagnetic order and of the insulator-to-metal transition (IMT) as a function of temperature and hydrostatic pressure. With increasing pressure, we observe the emergence of a mid-infrared feature in the optical conductivity, indicating the development of strong orbital-selective correlations in the high-pressure ferromagnetic phase. We find a distinct relationship between the plasma frequency and Curie temperature of CrGeTe$_3$, which strongly suggests that a double-exchange mechanism is responsible for the observed near room-temperature ferromagnetism. Our results clearly demonstrate the existence of a charge-transfer gap in the metallic phase, ruling out its previously conjectured collapse under pressure.