# Semimetal-to-semiconductor transition and charge-density-wave melting in $1T$-TiSe$_{2-x}$S$_x$ single crystals

## TL;DR

This study investigates how sulfur substitution in $1T$-TiSe$_2$ affects its electronic structure and phase transitions, revealing a semimetal-to-semiconductor transition and the melting of charge density waves using combined experimental and theoretical methods.

## Contribution

It provides new insights into the relationship between band structure evolution and charge density wave stability in $1T$-TiSe$_{2-x}$S$_x$ through combined ARPES, STM, and DFT analysis.

## Key findings

- CDW state remains stable up to $x=0.34$ S substitution.
- Continuous decrease in electron-hole band overlap with S doping.
- Semimetal-to-semiconductor transition predicted at $x_c=0.9$.

## Abstract

The transition metal dichalcogenide $1T$-TiSe$_2$ is a quasi-two-dimensional layered material with a phase transition towards a commensurate charge density wave (CDW) at a critical temperature T$_{c}\approx 200$K. The relationship between the origin of the CDW instability and the semimetallic or semiconducting character of the normal state, i.e., with the non-reconstructed Fermi surface topology, remains elusive. By combining angle-resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations, we investigate $1T$-TiSe$_{2-x}$S$_x$ single crystals. Using STM, we first show that the long-range phase coherent CDW state is stable against S substitutions with concentrations at least up to $x=0.34$. The ARPES measurements then reveal a slow but continuous decrease of the overlap between the electron and hole ($e$-$h$) bands of the semimetallic normal-state well reproduced by DFT and related to slight reductions of both the CDW order parameter and $T_c$. Our DFT calculations further predict a semimetal-to-semiconductor transition of the normal state at a higher critical S concentration of $x_c$=0.9 $\pm$0.1, that coincides with a melted CDW state in TiSeS as measured with STM. Finally, we rationalize the $x$-dependence of the $e$-$h$ band overlap in terms of isovalent substitution-induced competing chemical pressure and charge localization effects. Our study highlights the key role of the $e$-$h$ band overlap for the CDW instability.