Origin of the abnormal electrical transport behavior in Cu-intercalated 1 T − TaS 2

Layered transition-metal dichalcogenide (TMD) materials host a variety of quantum states and exhibit fascinating physical phenomena, such as charge density waves (CDWs) and a metal-insulator transition, which have attracted considerable attention in condensed matter physics. $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ is an intriguing TMD and has multiple CDW phases. In particular, the commensurate (C) CDW phase, an insulating ground state, has attracted much attention. Herein, we systematically investigate and verify the effect of electron injection through intercalation of Cu atoms in single crystals of the TMD material $1T\text{\ensuremath{-}}{\mathrm{Cu}}_{x}{\mathrm{TaS}}_{2}$ ($0\ensuremath{\le}x\ensuremath{\le}0.05$) by combining electrical transport measurements, scanning tunneling microscopy and spectroscopy measurements, and first-principles calculations. Our experiment results reveal that the charge transfer from Cu atoms to adjacent ${\mathrm{TaS}}_{2}$ layers, which suppresses the CCDW-insulating state, destroys the long-range CDW order, changes the dominant carrier (from $p$ type to $n$ type), and renders $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ metallic. The itinerant electrons of the intercalated Cu atoms, which induce the metallic state, primarily act as electron donors to raise the Fermi level. Our intercalation results support a scenario in which the doping electron primarily fills an existing band gap, consistent with a band-insulator picture. In addition, the stacking order is important for obtaining an accurate electronic structure, namely, the density of states. Our work provides opportunities for enriching the electronic phase diagram and exploring exotic physics in $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ and other TMD materials.