Structure and control of charge density waves in two-dimensional 1T-TaS ₂

Abstract

The layered transition metal dichalcogenides host a rich collection of charge density wave phases in which both the conduction electrons and the atomic structure display translational symmetry breaking. Manipulating these complex states by purely electronic methods has been a long-sought scientific and technological goal. Here, we show how this can be achieved in 1T-TaS 2 in the 2D limit. We first demonstrate that the intrinsic properties of atomically thin flakes are preserved by encapsulation with hexagonal boron nitride in inert atmosphere. We use this facile assembly method together with transmission electron microscopy and transport measurements to probe the nature of the 2D state and show that its conductance is dominated by discommensurations. The discommensuration structure can be precisely tuned in few-layer samples by an in-plane electric current, allowing continuous electrical control over the discommensuration-melting transition in 2D.two-dimensional materials | strongly correlated systems | charge density waves L ayered 1T-TaS 2 exhibits a number of unique structural and electronic phases. At low temperature and ambient pressure, the ground state is a commensurate (C) charge density wave (CDW). On heating, it undergoes a sequence of first-order phase transitions to a nearly commensurate (NC) CDW at 225 K, to an incommensurate (IC) CDW at 355 K, and finally to a metallic phase at 545 K. Each transition involves both conduction electron and lattice degrees of freedom-large changes in electronic transport properties occur, concomitant with structural changes to the crystal. By either chemical doping or applying high pressures, it is possible to suppress the CDWs and induce superconductivity (1-3). For device applications, it is desirable to control these phases by electrical means, but this capability is difficult to achieve in bulk crystals due to the high conduction electron density. Recent efforts to produce thin samples by mechanical exfoliation provide a new avenue for manipulating the CDWs in 1T-TaS 2 (4-8). These studies have demonstrated the suppression of CDW phase transitions using polar electrolytes, as well as resistive switching between the different phases. As the material approaches the 2D limit, however, significant changes have been observed in the transport properties (4,5,8). However, the microscopic nature of the 2D state remains unclear. In this work, we use transmission electron microscopy (TEM) together with transport measurements to develop a systematic understanding of the CDW phases and phase transitions in ultrathin 1T-TaS 2 . We find that charge ordering disappears in flakes with few atomic layers due to surface oxidation. When samples are instead environmentally protected, the CDWs persist and their transitions can be carefully tuned by electric currents.Both the atomic and CDW structure of 1T-TaS 2 can be visualized in reciprocal space by TEM electron diffraction (9, 10). In Fig. 1A, we show diffraction images taken from a bulk-like, 50-nm-thick crystal at low and room tem...

Ab initio tight-binding Hamiltonian for transition metal dichalcogenides

“…The off-diagonal hopping matrix elements between the same type of atoms (M-M and X-X) can be classified into two categories depending on the symmetry of the i and j orbitals under M 2 : for (i,j )=(3,5), (6,8), (9,11), the symmetry is (+), giving (5) while for (i,j )=(1,2), (3,4), (4,5), (6,7), (7,8), (9,10), (10,11), the symmetry is (−), giving…”

“…while for the pairs (i,j )=(4,1), (3,2), (5,2), (9,6), (11,6), (10,7), (9,8), (11,8), the symmetry is (−), giving…”

“…In contrast to graphene, layered transition metal dichalcogenides (TMDCs) 2-5 with chemical formula MX 2 (representative examples being M=Mo,W and X=S,Se), are semiconductors with direct or indirect band-gaps that depend on the number of layers and are in the range of visible light (1-2 eV). These materials also display a whole spectrum of phenomena such as superconductivity 6 , magnetism 7 , charge density waves (in TaS 2 ) observed by experiment 8,9 and topological insulator phases predicted by theory 10,11 . For monolayer TMDCs, the broken inversion symmetry enables optical control of valley degrees of freedom 12 and the strong spin-orbit coupling leads to valley-spin coupling 13 .…”

Manipulating charge density waves in1T−TaS2by charge-carrier doping: A first-principles investigation

“…Yu et al reported a gate-controlled Li ion intercalation can suppress CCDW and introduce superconducivity [21]. Tsen et al [22], Hollander et al [23], Yoshida et al [24], and Mihailovic et al [25,26] reported the in-plane electric field induced transition from CCDW state to NCCDW or some metastable hidden state. Cho et al applied perpendicular electric pulse on 1T -TaS 2 and found positive electric pulse can introduce IC network to suppress Mott state at low temperature [27].…”

Dimensionality-driven orthorhombic MoTe2 at room temperature

“…1(b) shows an optical image of a representative device with the MoTe2 flake outlined by a dashed line. In order to avoid the effects of surface oxidation [26][27][28], MoTe2 was exfoliated onto a polymer stamp within a nitrogen-filled glovebox, transferred onto gold electrodes, and covered with hexagonal boron nitride (hBN) before being moved out to the ambient environment. Since hBN conforms to the features of the underlying surface, an atomic force microscope was used to determine the thickness of the buried MoTe2.…”