Environmental Control of Charge Density Wave Order in Monolayer 2H-TaS₂

Abstract

For quasi-freestanding 2H-TaS 2 in monolayer thickness grown by in situ molecular beam epitaxy on graphene on Ir(111), we find unambiguous evidence for a charge density wave close to a 3 × 3 periodicity. Using scanning tunneling spectroscopy, we determine the magnitude of the partial charge density wave gap. Angle-resolved photoemission spectroscopy, complemented by scanning tunneling spectroscopy for the unoccupied states, makes a tight-binding fit for the band structure of the TaS 2 monolayer possible. As hybridization with substrate bands is absent, the fit yields a precise value for the doping of the TaS 2 layer. Additional Li doping shifts the charge density wave to a 2 × 2 periodicity. Unexpectedly, the bilayer of TaS 2 also displays a disordered 2 × 2 charge density wave. Calculations of the phonon dispersions based on a combination of density-functional theory, density-functional perturbation theory, and many-body perturbation theory enable us to provide phase diagrams for the TaS 2 charge density wave as functions of doping, hybridization and interlayer potentials, and offer insight into how they affect lattice dynamics and stability. Our theoretical considerations are consistent with the experimental work presented and shed light on previous experimental and theoretical investigations of related systems.

Ab initio phonon self-energies and fluctuation diagnostics of phonon anomalies: Lattice instabilities from Dirac pseudospin physics in transition metal dichalcogenides

“…4. In line with former theoretical results [66] and experiments [62] we find that electron doping pushes the wave vector of the leading lattice instability towards the M point. Hole doping of µ = −119 meV on the other hand shifts the instabilities further away from M and also lets additional "fragile" instabilities between Γ and K emerge, which depend very sensitively on the thermal broadening k B T of the electronic Fermi distribution function.…”

“…Charge doping is known to affect CDW instabilities by shifting the ordering wave vectors and suppressing or supporting CDW order in many materials from high-T c superconductors [4] to TMDCs [44,[81][82][83][84] and 1H-TaS 2 in particular [62,66,85,86]. We studied the dependence of the LA phonon mode on charge doping in the phonon self-energy formalism [Eqs.…”

Patterns and driving forces of dimensionality-dependent charge density waves in 2H-type transition metal dichalcogenides

“…For monolayer TaSe 2 , due to its extended charge distribution, charge transfer tends to accumulate between TaSe 2 and graphene, so that the intralayer covalency is reduced; CDW is therefore enhanced. Interestingly, CDWs were found to be absent in epitaxial monolayer TaS 2 on Au(111) substrate 37 , but persist in MBEgrown monolayer TaS 2 on graphene/Ir(111) 38 as well as in the exfoliated monolayer studied here. These findings provide further evidence that the CDW formation in such atomically thin TMDs is, in general, highly susceptible to the surrounding environment, which could be a neighboring layer or even a substrate.…”

“…Meanwhile, the trigonal symmetry of Au(111) substrate enforces the original symmetry of pristine monolayer TaS 2 , thereby avoiding any lattice deformation. Of course, hybridization between monolayer TaS 2 and the Au(111) substrate could significantly affect its electronic band structure [38][39][40] , and correspondingly, its intrinsic properties. Alternatively, when the substrate is made of chemically inactive orbitals (such as p π states in graphene), the CDW is still inevitable.…”

Intercalated phases of transition metal dichalcogenides

“…FFT, fast Fourier transform; TEM, transmission electron microscope concentration ( Figure 5F). 100 Calculations showed that charge doping from Li atoms disturbed phonon dispersions, which changed CDW structures ( Figure 5G).…”