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TT: Fachverband Tiefe Temperaturen

TT 16: 2D Materials I: Electronic Structure (joint session O/TT)

TT 16.6: Talk

Monday, March 18, 2024, 16:15–16:30, MA 005

Formation of complex CDW patterns in single-layer TiSe2 — •Wen Wan, Paul Dreher, Maria Gastiasoro, Fernando de Juan, and Miguel Ugeda — Donostia International Physics Center, San Sebastián, Spain

While the CDW in TiSe2 emerges naturally with a commensurate 2x2x2 symmetry below ~200 K, superconductivity only develops induced by pressure, doping, and intercalation. Interestingly, these external stimuli triggering superconductivity also induce a CDW phase transition from a commensurate (CCDW) to an incommensurate state (ICCDW). Therefore, it has been speculated that superconductivity in TiSe2 is triggered by the CCDW to ICCDW transition in an unprecedented, exotic mutual interplay.

In this work, we explore the CDW state of TiSe2 in the single-layer limit by means of low-temperature STM/STS measurements. nm-scale STM imaging reveals the presence of a CDW with a local 2x2 atomic periodicity as in bulk TiSe2 which, in contrast, is markedly inhomogeneous in space. On a larger scale, we find that such inhomogeneity is due to incommensuration of the CDW, which leads to the formation of complex 4-fold 2D matrix of CDW domains. Our high-resolution large-scale STM images allow us to spatially map the full complex CDW order parameter (amplitude and phase) with sub-nm precision. This analysis reveals that the CDW develops π-phase shifts at the domain walls. Lastly, while the matrix of CDW domains is unaffected by point defects in the Se layer, artificially introduced defects in the Ti plane pin the domain walls.

Keywords: CDW; TMD; STM