High-pressure infrared spectroscopy: tuning of the low-energy excitations in correlated electron systems

TL;DR

This paper introduces a new high-pressure infrared spectroscopy method that extends measurement capabilities into the far-infrared, enabling detailed study of pressure-induced phase transitions in correlated electron systems, exemplified by 1T-TaS2.

Contribution

The authors developed a novel optical pressure cell with a large diamond window, allowing accurate far-infrared measurements up to 26 kbar, advancing the study of electronic-structural changes under pressure.

Findings

Pressure stabilizes the semi-metallic phase of 1T-TaS2 above 14 kbar.

Low-energy optical conductivity remains temperature independent below 300 K.

Insulating phase emergence at intermediate pressures shows as reflectivity drops and phonon peaks.

Abstract

We have extended the range of the high-pressure optical spectroscopy to the far-infrared region keeping the accuracy of ambient-pressure experiments. The newly-developed method offers a powerful tool for the study of pressure-induced phase transitions and electronic-structural changes in correlated electron systems. The novel-type optical pressure cell, equipped with large free-aperture diamond window, allows the measurement of optical reflectivity down to $\omega\approx20-30$ cm$^{-1}$ for hydrostatic pressures up to $p\approx26$ kbar. The efficiency of the technique is demonstrated by the investigation of the 2-dimensional charge-density-wave 1$T$-TaS$_2$ whose electronic structure shows high sensitivity to external pressure. The room-temperature semi-metallic phase of 1$T$-TaS$_2$ is effectively extended by application of pressure and stabilized as the ground state above $p=14$ kbar. The corresponding fully incoherent low-energy optical conductivity is almost temperature independent below T=300 K. For intermediate pressures, the onset of the low-temperature insulating phase is reflected by the sudden drop of the reflectivity and by the emergence of sharp phonon resonances.