No abstract

van der Waals two-dimensional (2D) semiconductors have emerged as a class of materials with promising device characteristics owing to the intrinsic band gap. For realistic applications, the ideal is to modify the band gap in a controlled manner by a mechanism that can be generally applied to this class of materials. Here, we report the observation of a universally tunable band gap in the family of bulk 2H transition metal dichalcogenides (TMDs) by in situ surface doping of Rb atoms. A series of angle-resolved photoemission spectra unexceptionally shows that the band gap of TMDs at the zone corners is modulated in the range of 0.8–2.0 eV, which covers a wide spectral range from visible to near-infrared, with a tendency from indirect to direct band gap. A key clue to understanding the mechanism of this band-gap engineering is provided by the spectroscopic signature of symmetry breaking and resultant spin-splitting, which can be explained by the formation of 2D electric dipole layers within the surface bilayer of TMDs. Our results establish the surface Stark effect as a universal mechanism of band-gap engineering on the basis of the strong 2D nature of van der Waals semiconductors.

We investigate background charge fluctuation in a GaAs quantum dot device by measuring 1/f noise in the single-electron tunneling current through the dot. The current noise is understood as fluctuations of the confinement potential and tunneling barriers. The estimated potential fluctuation increases almost linearly with temperature, which is consistent with a simple model of the 1/f noise. We find that the fluctuation increases very slightly when electrons are injected into excited states of the quantum dot.Comment: to be published in Appl. Phys. Let

The layered van der Waals antiferromagnet MnBi2Te4 has been predicted to combine the band ordering of archetypical topological insulators like Bi2Te3 with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topological states. We have systematically investigated the surface electronic structure of MnBi2Te4(0001) single crystals by use of spin-and angle-resolved photoelectron spectroscopy (ARPES) experiments. In line with theoretical predictions, the results reveal a surface state in the bulk band gap and they provide evidence for the influence of exchange interaction and spin-orbit coupling on the surface electronic structure.The hallmark of a topological insulator is a single spinpolarized Dirac cone at the surface which is protected by time reversal-symmetry and originates from a band inversion in the bulk [1,2]. Notably, breaking time-reversal symmetry by magnetic order does not necessarily destroy the non-trivial topology but instead may drive the system into another topological phase. One example is the quantum anomalous Hall (QAH) state that has been observed in magnetically doped topological insulators [3]. The QAH state, in turn, may form the basis for yet more exotic electronic states, such as axion insulators [4,5] and chiral Majorana fermions [6]. Another example is the antiferromagnetic topological insulator state which is protected by a combination of time-reversal and lattice translational symmetries [7].Magnetic order in a topological insulator has mainly been achieved by doping with 3d impurities [3,8], which however inevitably gives rise to increased disorder. By contrast, the layered van der Waals material MnBi 2 Te 4 [9, 10] has recently been proposed to realize an intrinsic magnetic topological insulator [11][12][13][14], i.e. a compound that features magnetic order and a topologically non-trivial bulk band structure at the arXiv:1903.11826v2 [cond-mat.str-el]

Two-dimensional (2D) crystals have emerged as a class of materials with tunable carrier density. Carrier doping to 2D semiconductors can be used to modulate many-body interactions and to explore novel composite particles. The Holstein polaron is a small composite particle of an electron that carries a cloud of self-induced lattice deformation (or phonons), which has been proposed to play a key role in high-temperature superconductivity and carrier mobility in devices. Here we report the discovery of Holstein polarons in a surface-doped layered semiconductor, MoS, in which a puzzling 2D superconducting dome with the critical temperature of 12 K was found recently. Using a high-resolution band mapping of charge carriers, we found strong band renormalizations collectively identified as a hitherto unobserved spectral function of Holstein polarons. The short-range nature of electron-phonon (e-ph) coupling in MoS can be explained by its valley degeneracy, which enables strong intervalley coupling mediated by acoustic phonons. The coupling strength is found to increase gradually along the superconducting dome up to the intermediate regime, which suggests a bipolaronic pairing in the 2D superconductivity.