Reconcile the Bulk Metallic and Surface Insulating state in 1T-TaSe$_2$

TL;DR

This study uses scanning tunneling microscopy and spectroscopy to investigate the surface electronic states of bulk 1T-TaSe$_2$, revealing that stacking order influences the coexistence of metallic and insulating states, and reconciling previous conflicting observations.

Contribution

It demonstrates that different stacking orders in 1T-TaSe$_2$ lead to either metallic or insulating surface states, clarifying the origin of these states and unifying bulk and surface electronic behaviors.

Findings

Insulating and metallic states coexist in different areas of the same sample.

Stacking order determines whether the surface state is metallic or insulating.

The insulating state is likely a single-layer property modulated by stacking.

Abstract

The transition metal dichalcogenides 1T-TaS$_2$ and 1T-TaSe$_2$ have been extensively studied for the complicated correlated electronic properties. The origin of different surface electronic states remains controversial. We apply scanning tunneling microscopy and spectroscopy to restudy the surface electronic state of bulk 1T-TaSe$_2$. Both insulating and metallic states are identified in different areas of the same sample. The insulating state is similar to that in 1T-TaS$_2$, concerning both the dI/dV spectrum and the orbital texture. With further investigations in single-step areas, the discrepancy of electronic states is found to be associated with different stacking orders. The insulating state is most possibly a single-layer property, modulated to a metallic state in some particular stacking orders. Both the metallic and large-gap insulating spectra, together with their corresponding stacking orders, are dominant in 1T-TaSe$_2$. The connected metallic areas lead to the metallic transport behavior. We then reconcile the bulk metallic and surface insulating state in 1T-TaSe$_2$. The rich phenomena in 1T-TaSe$_2$ deepen our understanding of the correlated electronic state in bulk 1T-TaSe$_2$ and 1T-TaS$_2$.