Ising domain wall networks from intertwined charge density waves in single-layer TiSe2

TL;DR

This study reveals a complex near-commensurate charge density wave state in monolayer TiSe2, characterized by an intricate network of Ising-type domain walls and emergent nematicity, advancing understanding of CDW phase transitions.

Contribution

It provides the first imaging and detailed analysis of a near-commensurate CDW state in TiSe2, showing intertwined real modulations and nematicity, with a Ginzburg-Landau model explaining the observed phenomena.

Findings

Observation of a near-commensurate CDW with domain wall networks

Identification of Ising-type sign-changing domain walls

Detection of emergent nematic modulation

Abstract

When the period of an incommensurate charge density wave (ICDW) approaches an integer multiple of a lattice vector, the energy gain obtained from locking the period to the lattice can lead to a fascinating transition into a commensurate state. This transition actually occurs through an intermediate near-commensurate (NC) phase, with locally commensurate regions separated by an ordered array of phase slips of a complex CDW order parameter. TiSe2 is a paradigmatic CDW system where incommensuration is believed to be induced by carrier doping, yet its putative NC state has never been imaged or its nature established. Here we report the observation of a striking NC state in ultraclean, slightly doped monolayers of TiSe2, displaying an intricate network of coherent, unidirectional CDW domain walls over hundreds of nanometers. Detailed analysis reveals these are not phase slips of a complex CDW, but rather sign-changing Ising-type domain walls of two coupled real CDWs of previously known symmetry, consistent with the period doubling nature of the parent commensurate state. In addition, we observe an unexpected nematic modulation at the original lattice Bragg peaks which couples to the CDW order parameters. A Ginzburg-Landau analysis naturally explains the couplings and relative modulations of all order parameters, unveiling TiSe2 as a rare example of an NC-CDW of two intertwined real modulations and emergent nematicity.