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Register a new user2D superconductivity and vortex dynamics in 1T-MoS2
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Two-dimensional (2D) superconductivity is a fascinating phenomenon packed with rich physics and wide technological application. The vortices and their dynamics arising from classical and quantum fluctuations give rise to Berezinskii-Kosterlitz-Thouless (BKT) transition and 2D Bose metallic phase both of which are of fundamental interest. In 2D, observation of superconductivity and the associated phenomena are sensitive to material disorders. Highly crystalline and inherently 2D van der Waals (vW) systems with carrier concentration and conductivity approaching metallic regime have been a potential platform. The metallic 1T phase of MoS2, a widely explored vW material system controllably, engineered from the semiconducting 2H phase, is a tangible choice. Here, we report the observation of 2D superconductivity accompanied by BKT transition and Bose metallic state in a few-layer 1T-MoS2. Structural characterization shows excellent crystallinity over extended lateral dimension. The electrical characterization confirms the metallic nature down to 4 K and a transition to a superconducting state below 1.2 K with a Tc ~ 920 mK. The 2D nature of the superconducting state is confirmed from the magneto-transport anisotropy against field orientations and the presence of BKT transition. In addition, our sample showcases a manifold increase in the parallel upper-critical-field above the Pauli limit. The inherent two-dimensionality and possibility of scalably engineering semiconducting, metallic and superconducting phases makes MoS2 a potential candidate for hosting monolithic all-two-dimensional hybrid quantum devices.
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In view of their immensely intriguing properties, two dimensional materials are being intensely researched in search of novel phenomena and diverse application interests, however, studies on the realization of nanocomposites in the application-worthy thin-film platform are rare. Here we have grown MoS2-hBN composite thin films on different substrates by the pulsed laser deposition technique and made comparative studies with the pristine MoS2 and hBN films. The Raman, XPS and HRTEM confirm the concomitant presence of both the 1T (conducting) and 2H (semiconducting) polymorphs of MoS2 in the composite film. Interestingly, a peculiar reentrant semiconductor-metal-insulator transition is seen in the composite film which is absent in the MoS2 film, and it correlates well with the signatures of phonon softening seen in temperature-dependent Raman spectroscopy. Furthermore, electrostatic force microscopy reveals the presence of three distinct regions (metallic, semiconducting, and insulating) in the composite film with differing contact potentials and enhanced propensity for charge transfer with respect to pristine MoS2. A triboelectric nanogenerator device containing biphasic composite film as an electron acceptor exhibits more than twofold (sixfold) enhancement in peak-to-peak output voltage as compared to the pristine MoS2 (hBN) film. These observations bring out the potential of nanocomposite thin films for unfolding emergent phenomena and technological applications.
The quantum-spin-Hall (QSH) phase of 2D topological insulators has attracted increased attention since the onset of 2D materials research. While large bulk gaps with vanishing edge gaps in atomically thin layers have been reported, verifications of the QSH phase by resistance measurements are comparatively few. This is partly due to the poor uniformity of the bulk gap induced by the substrate over a large sample area and/or defects induced by oxidation. Here, we report the observation of the QSH phase at room-temperature in the 1T-phase of few-layer MoS2 patterned onto the 2H semiconducting phase using low-power and short-time laser beam irradiation. Two different resistance measurements reveal hallmark transport conductance values, ~e2/2h and e2/4h, as predicted by the theory. Magnetic-field dependence, scanning tunneling spectra, and calculations support the emergence of the room-temperature QSH phase. Although further experimental verification is still desirable, our results provide feasible application to room-temperature topological devices.
We report a new strategy for fabricating 2D/2D low-resistance ohmic contacts for a variety of transition metal dichalcogenides (TMDs) using van der Waals assembly of substitutionally doped TMDs as drain/source contacts and TMDs with no intentional doping as channel materials. We demonstrate that few-layer WSe2 field-effect transistors (FETs) with 2D/2D contacts exhibit low contact resistances of ~ 0.3 k ohm.um, high on/off ratios up to > 109, and high drive currents exceeding 320 uA um-1. These favorable characteristics are combined with a two-terminal field-effect hole mobility ~ 2x102 cm2 V-1 s-1 at room temperature, which increases to >2x103 cm2 V-1 s-1 at cryogenic temperatures. We observe a similar performance also in MoS2 and MoSe2 FETs with 2D/2D drain and source contacts. The 2D/2D low-resistance ohmic contacts presented here represent a new device paradigm that overcomes a significant bottleneck in the performance of TMDs and a wide variety of other 2D materials as the channel materials in post-silicon electronics.
Within a relativistic quantum formalism we examine the role of second-order corrections caused by the application of magnetic fields in two-dimensional topological and Chern insulators. This allows to reach analytical expressions for the change of the Berry curvature, orbital magnetic moment, density of states and energy determining their canonical grand potential and transport properties. The present corrections, which become relevant at relatively low fields due to the small gap characterizing these systems, unveil a zero-field diamagnetic susceptibility which can be tuned by the external magnetic field.
We report on spin-vortex pair dynamics measured at temperatures low enough to suppress stochastic core motion, thereby uncovering the highly non-linear intrinsic dynamics of the system. Our analysis shows that the decoupling of the two vortex cores is resonant and can be enhanced by dynamic chaos. We detail the regions of the relevant parameter space, in which the various mechanisms of the resonant core-core dynamics are activated. We show that the presence of chaos can reduce the thermally-induced spread in the switching time by up to two orders of magnitude.
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