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Tungsten diselenide (WSe2) and related transition metal dichalcogenides exhibit interesting optoelectronic properties owing to their peculiar band structures originating from the valley degree of freedom. Although the optical generation and detection of valley polarization has been demonstrated, it has been difficult to realize active valley-dependent functions suitable for device applications. We report an electrically switchable, circularly polarized light source based on the material's valley degree of freedom. Our WSe2-based ambipolar transistors emit circularly polarized electroluminescence from p-i-n junctions electrostatically formed in transistor channels. This phenomenon can be explained qualitatively by the electron-hole overlap controlled by the in-plane electric field. Our device demonstrates a route to exploit the valley degree of freedom and the possibility to develop a valley-optoelectronics technology.

A dome-shaped superconducting region appears in the phase diagrams of many unconventional superconductors. In doped band insulators, however, reaching optimal superconductivity by the fine-tuning of carriers has seldom been seen. We report the observation of a superconducting dome in the temperature—carrier density phase diagram of MoS₂, an archetypal band insulator. By quasi-continuous electrostatic carrier doping achieved through a combination of liquid and solid gating, we revealed a large enhancement in the transition temperature T c occurring at optimal doping in the chemically inaccessible low-carrier density regime. This observation indicates that the superconducting dome may anse even in doped band insulators.

In Cooper pairs--pairs of electrons responsible for the exotic properties of superconductors--the two electrons' spins typically point in opposite directions. A strong-enough external magnetic field will destroy superconductivity by making the spins point in the same direction. Lu et al. observed a two-dimensional superconducting state in the material MoS2 that was surprisingly immune to a magnetic field applied in the plane of the sample (see the Perspective by Suderow). The band structure of MoS2 and its spin-orbit coupling conspired to create an effective magnetic field that reinforced the electron pairing, with spins aligned perpendicular to the sample. Science, this issue p. 1353; see also p. 1316 The Zeeman effect, which is usually detrimental to superconductivity, can be strongly protective when an effective Zeeman field from intrinsic spin-orbit coupling locks the spins of Cooper pairs in a direction orthogonal to an external magnetic field. We performed magnetotransport experiments with ionic-gated molybdenum disulfide transistors, in which gating prepared individual superconducting states with different carrier dopings, and measured an in-plane critical field Bc2 far beyond the Pauli paramagnetic limit, consistent with Zeeman-protected superconductivity. The gating-enhanced Bc2 is more than an order of magnitude larger than it is in the bulk superconducting phases, where the effective Zeeman field is weakened by interlayer coupling. Our study provides experimental evidence of an Ising superconductor, in which spins of the pairing electrons are strongly pinned by an effective Zeeman field.

Using a liquid gate has allowed electrically induced superconductivity in a solid specimen by means of carrier accumulation on the surface. But this phenomenon was limited to materials that became superconductors at low carrier density. It is now shown that superconductivity can be induced in a much wider range of materials by using an ionic liquid. Liquid/solid interfaces are attracting growing interest not only for applications in catalytic activities and energy storage 1 , 2 , but also for their new electronic functions in electric double-layer transistors (EDLTs) exemplified by high-performance organic electronics 3 , 4 , 5 , 6 , 7 , field-induced electronic phase transitions 8 , 9 , 10 , 11 , as well as superconductivity in SrTiO 3 (ref.  12 ). Broadening EDLTs to induce superconductivity within other materials is highly demanded for enriching the materials science of superconductors. However, it is severely hampered by inadequate choice of materials and processing techniques 13 . Here we introduce an easy method using ionic liquids as gate dielectrics, mechanical micro-cleavage techniques for surface preparation, and report the observation of field-induced superconductivity showing a transition temperature T c =15.2 K on an atomically flat film of layered nitride compound, ZrNCl. The present result reveals that the EDLT is an extremely versatile tool to induce electronic phase transitions by electrostatic charge accumulation and provides new routes in the search for superconductors beyond those synthesized by traditional chemical methods.

Transition metal dichalcogenide monolayers can host Ising-type superconductivity and if two such layers are coupled, exotic superconducting phases may emerge. Here, the authors induce a coupled superconducting state with tuneable Ising protection by means of symmetric, double-side ionic liquid gating.

The interactions of gas-phase acetone, ethanol and benzene with smooth ice films and artificial snow have been studied. In one technique, the snow is packed into a cylindrical column and inserted into a low-pressure flow reactor coupled to a chemical-ionization mass spectrometer for gas-phase analysis. At 214 and 228K, it is found for acetone and ethanol that the adsorbed amounts per surface area match those for adsorption to thin films of ice formed by freezing liquid water, when the specific surface area of the snow (as determined from Kr adsorption at 77K) and the geometric surface area of the ice films are used. This indicates that freezing thin films of water leads to surfaces that are smooth at the molecular level. Experiments performed to test the effect of film growth on ethanol uptake indicate that uptake is independent of ice growth rate, up to 2.4µmmin−1. In addition, traditional Brunauer–Emmett–Teller (BET) experiments were performed with these gases on artificial snow from 238 to 266.5K. A transition from a BET type I isotherm indicative of monolayer formation to a BET type II isotherm indicative of multilayer uptake is observed for acetone at T≥263K and ethanol at T≥255K, arising from solution formation on the ice. When multilayer formation does not occur, as was the case for benzene at T≤263K and for acetone at T≤255K, the saturated surface coverage increased with increasing temperature, consistent with the quasi-liquid layer affecting adsorption prior to full dissolution/multilayer formation.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

We here report the result of in situ magnetization measurements of electrochemical cells at low temperatures. Upon applying voltages between the electrodes of the electrochemical cells, we observed shielding diamagnetic signals from several materials, indicating superconducting transitions. The superconducting states can be induced both electrochemically and electrostatically with appropriate combination of counter electrode materials and electrolytes. The present technique may become a powerful method for searching novel superconductors.

The Zeeman effect, which is usually detrimental to superconductivity, can be strongly protective when an effective Zeeman field from intrinsic spin-orbit coupling locks the spins of Cooper pairs in a direction orthogonal to an external magnetic field. We performed magnetotransport experiments with ionic-gated molybdenum disulfide transistors, in which gating prepared individual superconducting states with different carrier dopings, and measured an in-plane critical field Bc2 far beyond the Pauli paramagnetic limit, consistent with Zeeman-protected superconductivity. The gating-enhanced Bc2 is more than an order of magnitude larger than it is in the bulk superconducting phases, where the effective Zeeman field is weakened by interlayer coupling. Our study provides experimental evidence of an Ising superconductor, in which spins of the pairing electrons are strongly pinned by an effective Zeeman field.

High carrier density part of many materials could be accessed by a variation of the field effect transistor technique: electric double layer transistor. Carrier density regime of n∼1014 cm−2 can be easily accessed electrostatically realizing effective doping without chemical modification. In this study, we utilized micro-cleavage on a number of interesting layered materials. And realized high carrier density state and high performance transport on atomically flat surfaces.