Session K12: 2D Materials (Metals, Superconductors, and Correlated Materials) -- TMDC

Probing and Tuning Interlayer Interactions to Control Electronic and Photonic Properties of 2D Heterostructures

Atomically thin, single crystalline 2D electronic materials have recently emerged, offering a remarkably wide range of building blocks of nanostructures, ranging from metals (e.g. graphene), large gap insulators (BN), to semiconductors (transition metal dichalcogenides and black phosphorous). One key advantage of these van der Waals materials lies in the flexibility of stacking different types of materials to form heterostructures, providing a design platform for achieving novel device functionality. In vdW hetero-bilayers, the interface encompasses the whole heterostructure and interlalyer interactions become the controlling parameter for the electronic structure.
In this talk I will first discuss directly probing the inter-layer interactions through the “lens” of moiré patterns using scanning tunneling microscopy and spectroscopy (STM/S). I will show that the interlayer coupling is strongly dependent on the inter-atomic alignment of the constituent layers. Moreover, as a consequence of moiré pattern formation, the energy band structure of the hetero-bilayer also shows lateral modulation, forming a 2D electronic superlattice. The moiré pattern “lens” also provides us with a means to measure the 2D strain tensor with high precision and high spatial resolution. In addition, the strain profile shows a direct correlation with the band gap modification.
As the periodic potential modulation also provides lateral confinement for excitons, an intriguing scenario occurs – the 2D lateral superlattices also form 2D exciton quantum dot arrays. Recent reports of ultra-sharp atomic like spectra provide a direct confirmation of such a scenario. Finally, I will add another control knob and show evidence for valley spin mediated interlayer couplings, and their effect on excitonic states of the hetero-bilayer.   

Insulating Bulk State in Monolayer WTe2

The layered semimetal WTe₂ displays a diverse set of properties in the two-dimensional limit. For example, using an electrostatic gate, one can control the carrier density and change the state of monolayer WTe₂ from a 2D topological insulator a 2D superconductor. We will discuss our recent work to better understand the properties of the insulating bulk state seen in monolayer WTe₂ near charge neutrality. Using graphene as a probe of the transverse penetrating electric field, we determine the chemical potential as a function of charge density, finding a thermodynamic gap of 45±5 meV at low temperatures. We relate the measurements to transport and angle-resolved photoemission measurements.   

Evidence of Metal-Insulator Transition in 2D Van der Waals layers of Copper Indium Selenide (CuIn7Se11)

Several recent reports have indicated the possibility of metal-insulator transition (MIT) in two-dimensional electron systems (2DES), which also includes atomically thick van der Waals layers of two-dimensional (2D) materials such as MoS₂, MoSe₂, ReS₂ etc. In general it appears that alteration of external control parameters such as pressure, charge carrier density etc. can give rise to such phenomenon in these materials. In this regard, this study will demonstrate the possibility of MIT in 2D van der Waals layers of Copper Indium Selenide (CuIn₇Se₁₁). A study of conductivity (σ) of 2D CuIn₇Se₁₁ FET devices (fabricated using flakes exfoliated from crystals grown using chemical vapor transport technique) as a function of temperature (T) and charge carrier density (n_2D) (modulated via electrostatic doping through SiO₂ gate) show strong features of MIT. The observed phenomenon will be discussed in light of existing theoretical models.   

Microfocus laser-ARPES on encapsulated mono-, bi- and few-layer 1T'-WTe2

The recent discoveries of a quantum spin Hall state and of ferroelectric polarization in exfoliated crystals of 1T'-WTe₂ demonstrate the potential of semimetallic van der Waals materials for the realization of novel phases. However, current understanding of these phases is largely based on theoretical simulations since little is known from experiment about the electronic structure of few-layer crystals. Here, we report direct electronic structure measurements of exfoliated mono- bi- and few-layer 1T'-WTe₂ by laser-based micro-focus angle resolved photoemission. This is achieved by encapsulating a flake of WTe₂ comprising regions of different thickness with monolayer graphene. Our data support the recent identification of a quantum spin Hall state in monolayer 1T'-WTe₂ and reveal strong signatures of the broken inversion symmetry in the bilayer. We finally discuss the sensitivity of encapsulated samples to contaminants following exposure to ambient atmosphere.   

Synthesis and characterization of transition metal chalcogenide superlattice compounds

The transition metal dichalcogenides MX₂ have recently attracted significant attention for their novel electronic properties in monolayer form. These include optical control of valley polarization, the valley Hall effect, and novel forms of superconductivity. While bulk crystals of MX₂ materials have a quasi-two-dimensional character reflected in their electronic structure, the coupling between the layers dilutes the many exotic effects appearing in monolayer systems. Here, we report the synthesis of bulk single crystal superlattice materials that physically separate the MX₂ layers in the 2H-MX₂ structure resulting in enhancements of the 2D nature of their electronic properties observable in their transport behavior.   

Transport study of quasi-one dimensional transition metal trichalcogenides down to single-chain limit

With the successful isolation of few-to-single layer of graphene and transition metal dichalcogenides (TMDs), two-dimensional van der Waals (vdW) materials have been a central focus in condensed matter physics and materials science.
In addition to 2D vdWs, a different class of 1D vdW materials, transition metal trichalcogenides (TMTs) such as NbSe₃, is emerging with recent advances in isolation and structural characterization of their few-to-single chain encapsulated within carbon and boron nitride nanotubes. The single chain limit of TMTs may exhibit novel physical phenomena, including unusual ground state and collective-mode electronic transport, for instance sliding charge density wave and superconductivity, as found in their bulk counterparts. However, the experimental investigation of these potential properties has been hindered by the challenge of nanodevice fabricating on such small samples. Here, we propose a new approach, correlating the structure-property of the few-to-single chain vdW materials by tranmission electron microscopy and device fabrication on the same individual chain-tube sample on a thin SiN membrane. Our work will open up new opportunities for study and application of 1D TMTs based electronics.   

Pump-probe spectroscopy of ultrathin 1T'-MoTe2

Type II Weyl semimetal candidate MoTe₂ in bulk form undergoes a transition at ~250 K from the monoclinic phase (1T’-MoTe₂) to the inversion-symmetry breaking, orthorhombic phase (T_d-MoTe2). Previous transport and Raman measurements on thin MoTe₂ flakes have shown a dimensionally driven transition to the orthorhombic phase at room temperature for flakes less than ~12 nm. Using femtosecond broadband pump-probe spectroscopy, we impulsively excited MoTe₂ flakes of varying thickness that are protected from oxidation. The temporal evolution of the vibrational wave packet causes spectral modulations of electronic transitions. A probe pulse following the pump reveals any pump-induced time dependence of these signals, including oscillations of the initially excited coherent nuclear motion. Using some novel data analysis tools, and after background subtraction, we were able to follow characteristic phonon modes of each phase resolved in time and frequency domains.   

Local Probe Studies of the Orbital Texture of a Single-Layer Mott Insulator

Mott insulators are insulating phases induced by strong electron correlation that can lead to exotic states such as high-temperature superconductivity and quantum spin liquids. Recent advances in van der Waals material synthesis enable the exploration of novel Mott systems in the two-dimensional limit. Here we present the characterization of the single-layer Mott insulator 1T-TaSe₂ via energy-, spatial-, and momentum-resolved measurement of its electronic structure by scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy. Our study establishes single-layer 1T-TaSe₂ as a novel Mott insulator, thus providing an ideal experimental platform for investigating strong correlation physics in two-dimensional materials.   

Electronic structure of 3d transition-matal dichalcogenide thin films grown by molecular-beam epitaxy

There has been increasing interest in atomically-thin transition-metal dichalcogenides (TMDs) hosting intriguing properties absent in their bulk form. For example, in 4d and 5d transition-metal TMDs such as MoS₂, WSe₂, and NbSe₂, valley degree of freedom plays an important role on novel two-dimensional properties. 3d TMDs, on the other hand, are expected to exhibit more varieties of phenomena involving excitonic physics, charge density wave, and magnetism, due to stronger electron-electron, electron-lattice and exchange interactions. Recently, emergent ferromagnetism in monolayer VSe₂ was reported, although the situation is still controversial. To unambiguously characterize physical properties of atomically thin TMDs and further understand the origin of the emergent two-dimensional phenomena, the direct observation of electronic structures is crucial. We have fabricated atomically thin films of 3d TMDs by molecular-beam epitaxy with our growth recipe [1] and clarified electronic structures by angle-resolved photoemission spectroscopy (ARPES). In this presentation, we will discuss the physical properties appearing in two-dimensional 3d TMDs by comparison with the ARPES results and band calculations. [1] M. Nakano, et al., Nano Lett. 17, 5595 (2017).   

Scanning tunnelling microscopy of ultrathin 1T-TaS2

1T-TaS₂ is a two-dimensional transition metal dichalcogenide that shows multiple charge density wave (CDW) transitions. Scanning tunnelling microscopy (STM) has been used to extensively study these transitions in bulk 1T-TaS₂. We utilize a device geometry that allows for in-plane transport measurements of ultrathin 1T-TaS₂ samples (less that 30 nm thick) prepared via mechanical exfoliation in an inert environment, in conjunction with imaging the CDW at the atomic scale. We also investigate the effects of current-driven transitions on the CDW.   

Exciton Condensation in Electron-hole Doped Hubbard Bilayers -- A Sign-problem-free Quantum Monte Carlo Study

A long-sought state of matter, exciton condensation, recently has been realized experimentally in several systems. We develop a sign-problem-free Determinant Quantum Monte Carlo (DQMC) algorithm for bilayer Hubbard model with electron-hole doping, which is an ideal platform to study exciton condensation due to the suppression of electron-hole recombination. In square lattice systems, we demonstrate a tendency for exciton condensation at momentum (π, π), and this excitonic order appears to compete with a charge ordered state in the parameter regime we study. Exciton condensation on hexagonal lattices, presumably more pertinent to systems used in previous experimental realizations, has also been investigated and the results will be shown in comparison with square lattice results.   

A monolayer transition metal dichalcogenide as a topological excitonic insulator

Monolayer transition metal dichalcogenides in the T’ phase might realize the quantum spin Hall effect at room temperature, since they have a large bulk hybridization gap between the inverted valence bands that provides topological order with robustness. Here we demonstrate that T’-MoS₂ is unstable against the spontaneous generation of excitons by using first-principles many-body perturbation theory, as the computed exciton binding energy is larger than the quasiparticle gap. We predict that the true ground state of T’-MoS2 is a novel correlated insulator in which both excitonic and topological orders coexist by reducing the spatial point group symmetry, whereas typically interactions tend to disrupt the topological order. A self-consistent calculation provides us with clear-cut signatures of this excitonic topological insulator, such as an enhanced bulk quasiparticle gap (and hence increased topological robustness), spontaneous inversion symmetry breaking, spin-splitting of quasiparticle bands. The phase diagram, in the space whose axes are temperature and strain, includes a second---topologically trivial---excitonic phase that spontaneously breaks mirror symmetry while changing discontinuously the quasiparticle gap, which surprisingly never closes.   

Suppression of Exciton Condensation in Copper-Doped TiSe2 Measured with M-EELS

Excitons were predicted to form a Bose condensate, but for many years there was no direct experimental verification of this condensation in a solid. Using momentum-resolved electron energy-loss spectroscopy (M-EELS), we demonstrated the presence of an exciton condensate in 1T-TiSe₂ by observing a soft electronic collective mode at the charge density wave (CDW) transition temperature, T_C = 190 K. As TiSe₂ is doped with copper to form Cu_xTiSe₂, the CDW transition temperature is suppressed and a superconducting dome emerges around x = 0.04. Using M-EELS, we found that the electronic collective mode softens partially near T_C for very low dopings (x < 0.004), but does not soften above x ~ 0.01. Our results indicate that the exciton condensate is rapidly destroyed in Cu_xTiSe₂ by screening from the Cu atoms and that a conventional Peierls CDW persists above x ~ 0.01. We conclude that the excitonic state is not directly related to the superconductivity in Cu_xTiSe₂.