We study the evolution of the band gap structure in few-layer MoTe2 crystals, by means of low-temperature microreflectance (MR) and temperature-dependent photoluminescence (PL) measurements. The analysis of the measurements indicate that in complete analogy with other semiconducting transition metal dichalchogenides (TMDs) the dominant PL emission peaks originate from direct transitions associated with recombination of excitons and trions. When we follow the evolution of the PL intensity as a function of layer thickness, however, we observe that MoTe2 behaves differently from other semiconducting TMDs investigated earlier. Specifically, the exciton PL yield (integrated PL intensity) is identical for mono and bilayer, decreases slightly for trilayer, and it is significantly lower in the tetralayer. The analysis of this behavior and of all our experimental observations is fully consistent with mono and bilayer MoTe2 being direct band gap semiconductors with tetralayer MoTe2 being an indirect gap semiconductor and with trilayers having nearly identical direct and indirect gaps. This conclusion is different from the one reached for other recently investigated semiconducting transition metal dichalcogenides for which monolayers are found to be direct band gap semiconductors, and thicker layers have indirect band gaps that are significantly smaller (by hundreds of meV) than the direct gap. We discuss the relevance of our findings for experiments of fundamental interest and possible future device applications.

Three-dimensional topological insulators are characterized by the presence of a bandgap in their bulk and gapless Dirac fermions at their surfaces. New physical phenomena originating from the presence of the Dirac fermions are predicted to occur, and to be experimentally accessible via transport measurements in suitably designed electronic devices. Here we study transport through superconducting junctions fabricated on thin Bi2Se3 single crystals, equipped with a gate electrode. In the presence of perpendicular magnetic field B, sweeping the gate voltage enables us to observe the filling of the Dirac fermion Landau levels, whose character evolves continuously from electron- to hole-like. When B=0, a supercurrent appears, whose magnitude can be gate tuned, and is minimum at the charge neutrality point determined from the Landau level filling. Our results demonstrate how gated nano-electronic devices give control over normal and superconducting transport of Dirac fermions at an individual surface of a three-dimensional topological insulators.

The electronic structure of Bi(2)Se(3) is studied by angle-resolved photoemission and density functional theory. We show that the instability of the surface electronic properties, observed even in ultrahigh-vacuum conditions, can be overcome via in situ potassium deposition. In addition to accurately setting the carrier concentration, new Rashba-like spin-polarized states are induced, with a tunable, reversible, and highly stable spin splitting. Ab initio slab calculations reveal that these Rashba states are derived from 5-quintuple-layer quantum-well states. While the K-induced potential gradient enhances the spin splitting, this may be present on pristine surfaces due to the symmetry breaking of the vacuum-solid interface.

Semiconducting transition metal dichalcogenides (TMDs) have emerged as materials that can be used to realize two-dimensional (2D) crystals possessing rather unique transport and optical properties. Most research has so far focused on sulfur and selenium compounds, while tellurium-based materials attracted little attention so far. As a first step in the investigation of Te-based semiconducting TMDs in this context, we have studied MoTe 2 crystals with thicknesses above 4 nm, focusing on surface transport and a quantitative determination of the gap structure. Using ionic-liquid gated transistors, we show that ambipolar transport at the surface of the material is reproducibly achieved, with hole and electron mobility values between 10 and 30 cm 2 /Vs at room temperature. The gap structure is determined through three different techniques: ionic-liquid gated transistors and scanning tunneling spectroscopy, that allow the measurement of the indirect gap (E ind ), and optical transmission spectroscopy on crystals of different thickness, that enables the determination of both the direct (E dir ) and the indirect gap. We find that at room temperature E ind = 0.88 eV and E dir = 1.02 eV. Our results suggest that thin MoTe 2 layers may exhibit a transition to a direct gap before mono-layer thickness. They should also drastically extend the range of direct gaps accessible in 2D semiconducting TMDs. Introduction

We study Bi2Se3 by polarization-dependent angle-resolved photoemission spectroscopy (ARPES) and density-functional theory slab calculations. We find that the surface state Dirac fermions are characterized by a layer-dependent entangled spin-orbital texture, which becomes apparent through quantum interference effects. This explains the discrepancy between the spin polarization from spin-resovled ARPES -ranging from 20 to 85% -and the 100% value assumed in phenomenological models. It also suggests a way to probe the intrinsic spin texture of topological insulators, and to continuously manipulate the spin polarization of photoelectrons and photocurrents all the way from 0 to ±100% by an appropriate choice of photon energy, linear polarization, and angle of incidence.PACS numbers: 71.10.Pm, 73.20.At, 73.22.Gk Topological insulators (TIs) define a new state of matter in which strong spin-orbit interaction (SOI) leads to the emergence of a metallic topological surface state (TSS) formed by spin-nondegenerate Dirac fermions [1][2][3][4][5][6]. To capture the physics of TIs, a spin-momentum locking with 100% spin polarization is usually assumed for the TSS in time-reversal invariant models [3][4][5]. The successful realization of topological insulating behavior in quantum wells [7,8] and crystalline materials such as Bi 2 Se 3 [9-11] brings us closer to the practical implementation of theoretical concepts built upon novel topological properties. However, the large discrepancy in the degree of TSS spin polarization determined for Bi 2 Se 3 by spin-resolved ARPES (angle-resolved photoemission spectroscopy) -ranging from 20 to 85% [12-16] -challenges the hypothesis of a 100% spin polarization for real TIs. First principle density-functional theory (DFT) also indicates that the TSS spin polarization in members of the Bi 2 X 3 material family (X=Se, Te) can be substantially reduced from 100% [17,18]. Based on general symmetry arguments, it was shown that the spin polarization direction of photoelectrons in spin-resolved ARPES can be very different from that of the TSS wavefunction [19]. However, the role played by the intrinsic properties of the TSS wavefunction in defining the highest spin polarization that could be achieved, for instance in d.c. and photoinduced electrical currents, has remained elusive.We report here that the TSS many-layer-deep extension into the material's bulk -in concert with strong SOI -gives rise to a layer-dependent, entangled spin-orbital texture of the Dirac fermions in Bi 2 Se 3 . A remarkable consequence, specifically exploited in this study, is that one can gain exquisite sensitivity to the internal structure of the TSS wavefunction, Ψ TSS , via quantum interference effects in ARPES. The spin-orbital texture is captured directly in the linear-polarization dependence of the ARPES intensity maps in momentum space, and can be fully resolved with the aid of ab-initio DFT slabcalculations. This has also major consequences in the interpretation of spin-resolved ARPES results, explicitly solving ...

The transition-metal dichalcogenide 1T-TiSe2 is a quasi-two-dimensional layered material with a charge density wave (CDW) transition temperature of T(CDW) ≈ 200 K. Self-doping effects for crystals grown at different temperatures introduce structural defects, modify the temperature-dependent resistivity, and strongly perturbate the CDW phase. Here, we study the structural and doping nature of such native defects combining scanning tunneling microscopy or spectroscopy and ab initio calculations. The dominant native single atom dopants we identify in our single crystals are intercalated Ti atoms, Se vacancies, and Se substitutions by residual iodine and oxygen.

BiTeI is a giant Rashba spin splitting system, in which a noncentrosymmetric topological phase has recently been suggested to appear under high pressure. We investigated the optical properties of this compound, reflectivity and transmission, under pressures up to 15 GPa. The gap feature in the optical conductivity vanishes above p ∼ 9 GPa and does not reappear up to at least 15 GPa. The plasma edge, associated with intrinsically doped charge carriers, is smeared out through a phase transition at 9 GPa. Using high-pressure Raman spectroscopy, we follow the vibrational modes of BiTeI, providing additional clear evidence that the transition at 9 GPa involves a change of crystal structure. This change of crystal structure possibly inhibits the high-pressure topological phase from occurring. DOI: 10.1103/PhysRevLett.112.047402 PACS numbers: 78.20.hb, 62.50.-p, 78.30.Am, 78.40.Fy Interest in the noncentrosymmetric semiconductor BiTeI surged when it was found that this compound hosts the largest known Rashba spin splitting in bulk form [1][2][3]. While this material is structurally related to the recently discovered bismuth chalcogenide topological insulators [4,5], it is an insulator of the common variety at ambient pressure. Recent first-principles band structure calculations suggested that BiTeI undergoes a transition to the topological insulating phase under pressure [6], through which BiTeI would become the first example of noncentrosymmetric topological insulator. Moreover, such a bandstructure topology change realizes a remarkable example of topological phase transition. While several examples of topological phase transitions occurring upon varying chemical composition have been reported in the literature [7][8][9], the pressure-induced transition in BiTeI would present the advantage of being controllable and reversible.Optical conductivity is well suited to probe the band structure of BiTeI under pressure. In this Letter, we determine the high-pressure optical properties by measuring transmission and reflectivity of BiTeI up to 15 GPa. We follow the optical gap under pressure and find that it decreases monotonically until 9 GPa. At this pressure the plasma edge associated with the doped carriers is strongly broadened due to a sudden increase of σ 1 ðωÞ at the plasma frequency. Above this pressure the gap feature in the optical conductivity has disappeared, and it does not reappear to the highest pressure reached. The high-pressure phase appears to be metallic. Using Raman spectroscopy, we observe a sudden change in the number and frequency of the vibrational modes at 9 GPa, which shows that a structural transition occurs at this pressure.Single crystals of BiTeI were grown by the floating zone method, starting from the stoichiometric ratio of metallic bismuth, tellurium and bismuth iodide. The unit cell of BiTeI is composed of triple layers, Te-Bi-I, stacked along the polar c-axis [1]. The triple layers are bound by a weak van der Waals interaction. The structure is described by the noncentrosymmetric space...

No abstract