Configurational electronic states in layered transition metal dichalcogenides

Abstract

Mesoscopic irregularly ordered and even amorphous self-assembled electronic structures were recently reported in two-dimensional metallic dichalcogenides (TMDs), created and manipulated with short light pulses or by charge injection. Apart from promising new all-electronic memory devices, such states are of great fundamental importance, since such aperiodic states cannot be described in terms of conventional charge-density-wave (CDW) physics. In this paper, we address the problem of metastable mesoscopic configurational charge ordering in TMDs with a sparsely filled charged lattice gas model in which electrons are subject only to screened Coulomb repulsion. The model correctly predicts commensurate CDW states corresponding to different TMDs at magic filling fractions = / / / / / f 1 3, 1 4, 1 9, 1 13, 1 16.mDoping away from f m results either in multiple neardegenerate configurational states, or an amorphous state at the correct density observed by scanning tunnelling microscopy. Quantum fluctuations between degenerate states predict a quantum charge liquid at low temperatures, revealing a new generalized viewpoint on both regular, irregular and amorphous charge ordering in transition metal dichalcogenides.

A time-domain phase diagram of metastable states in a charge ordered quantum material

“…Monte-Carlo simulations using this model give a theoretical phase diagram (Figure 3c) that is consistent with the experimentally observed C (1/13 lling) and H states at ~ 4 % nominal doping 24,25 , observed at the experimental photodoping of 0.09 photons/unit cell. Remarkably, they also predict the A state towards 1/11 lling (at nominal doping, observed at a threshold of ~0.3 photons/unit cell) 18,24 . Simulations also predict the existence of a uniform ordered state with 1/12 lling and close to 1/12 domain states around it, but these states are not observed experimentally.…”

“…To obtain insight into the origin of different phases we compare the observed experimental phases on the STM timescale with the equilibrium con gurational states obtained from theoretical treatment. The model considers the ordering of electrons subject to screened Coulomb interaction on a triangular atomic lattice, and was previously successfully applied to describe both irregular domain patterns 24,25 and hyperuniform polaron orders 18,24 in the H and A states, respectively. It's predictions can be compared with the experiment by assuming a correspondence between the photoexcited carrier density (which is proportional to incident photon density) and electron lling.…”

“…It's predictions can be compared with the experiment by assuming a correspondence between the photoexcited carrier density (which is proportional to incident photon density) and electron lling. The model de nes the lling of the system as the number of electrons at the Fermi level divided by the number of atoms 24 . In 1T-TaS 2 with one electron per Ta atom, a reconstruction of the CCDW state gaps 12 out of 13 electrons, resulting in 1/13 lling by the remaining electron 38,39 .…”

“…Doping is de ned as a change of the lling with respect to 1/13. Experimentally, the lling is obtained by counting the number of polarons per unit area in an STM image (1 polaron equals 1 electron) 18,24 .…”

“…Its' phase diagram includes different structural polytypes (1T-TaS 2 , 2H-TaS 2 etc) 17 , different con gurational charge-ordered states 13,[18][19][20][21] , superconductivity 22 and a quantum spin liquid candidate state 23 . The 1T polytype is metallic above K. In the range K it displays an incommensurate (IC) charge density wave (CDW), which undergoes a transition to a nearly-commensurate (NC) phase below K. Below K, the material becomes insulating and fully commensurate (C), discussed either in terms of a CCDW, a polaronic Wigner crystal [24][25][26] or a Mott state 22 . Upon heating, the material goes through a triclinic domain state in the range K, whereupon it reverts to the NC state.…”