Ionic liquid gated field-effect transistors (FETs) based on semiconducting transition metal dichalcogenides (TMDs) are used to study a rich variety of extremely interesting physical phenomena, but important aspects of how charge carriers are accumulated in these systems are not understood. We address these issues by means of a systematic experimental study of transport in monolayer MoSe2 and WSe2 as a function of magnetic field and gate voltage, exploring accumulated densities of carriers ranging from approximately 10 14 cm -2 holes in the valence band to 4·10 14 cm -2 electrons in the conduction band. We identify the conditions when the chemical potential enters different valleys in the monolayer band structure (the K and Q valley in the conduction band and the two spin-split K-valleys in the valence band) and find that an independent electron picture describes the occupation of states well. Unexpectedly, however, the experiments show very large changes in the device capacitance when multiple valleys are occupied that are not at all compatible with the commonly expected quantum capacitance contribution of these systems, Q = 2 / . A theoretical analysis of all terms responsible for the total capacitance shows that -under general conditions-a term is present besides the usual quantum capacitance, which becomes important for very small distances between the capacitor plates. This term -that we call cross quantum capacitance-originates from screening of the electric field generated by charges on one plate from charges sitting on the other plate. The effect is negligible in normal capacitors, but large in ionic liquid FETs, because of the atomic proximity between the ions in the gate and the accumulated charges on the TMD, and it accounts for all our experimental observations. Our findings therefore reveal an important contribution to the capacitance of physical systems that had been virtually entirely neglected until now.
The ability to gate-induce superconductivity by electrostatic charge accumulation is a recent breakthrough in physics and nanoelectronics. With the exception of LaAlO/SrTiO interfaces, experiments on gate-induced superconductors have been largely confined to resistance measurements, which provide very limited information about the superconducting state. Here, we explore gate-induced superconductivity in MoS by performing tunnelling spectroscopy to determine the energy-dependent density of states (DOS) for different levels of electron density n. In the superconducting state, the DOS is strongly suppressed at energy smaller than the gap Δ, which is maximum (Δ ~2 meV) for n of ~1 × 10 cm and decreases monotonously for larger n. A perpendicular magnetic field B generates states at E < Δ that fill the gap, but a 20% DOS suppression of superconducting origin unexpectedly persists much above the transport critical field. Conversely, an in-plane field up to 10 T leaves the DOS entirely unchanged. Our measurements exclude that the superconducting state in MoS is fully gapped and reveal the presence of a DOS that vanishes linearly with energy, the explanation of which requires going beyond a conventional, purely phonon-driven Bardeen-Cooper-Schrieffer mechanism.
We have produced ultrathin lead (Pb) nanowires in the 6 nm pores of SBA-15 mesoporous silica substrates by chemical vapor deposition. The nanowires form regular and dense arrays. We demonstrate that bulk Pb (a type-I superconductor below Tc = 7.2 K with a critical field of 800 Oe) can be tailored by nanostructuring to become a type-II superconductor with an upper critical field (Hc2) exceeding 15 T and signs of Cooper pairing 3-4 K above the bulk Tc. The material undergoes a crossover from a one-dimensional fluctuating superconducting state at high temperatures to three-dimensional long-range-ordered superconductivity in the low-temperature regime. We show with our data in an impressive way that superconductivity in elemental metals can be greatly enhanced by nanostructuring.
We present electrical and thermal specific heat measurements that show superconductivity in double-wall carbon nanotube (DWCNT) bundles. Clear evidence, comprising a resistance drop as a function of temperature, magnetoresistance and differential resistance signature of the supercurrent, suggest an intrinsic superconducting transition below 6.8 K for one particular sample. Additional electrical data not only confirm the existence of superconductivity, but also indicate the Tc distribution that can arise from the diversity in the diameter and chirality of the DWCNTs. A broad superconducting anomaly is observed in the specific heat of a bulk DWCNT sample, which yields a Tc distribution that correlates well with the range of the distribution obtained from the electrical data. As quasi one dimensionality of the DWCNTs dictates the existence of electronic density of state peaks, confirmation of superconductivity in this material system opens the exciting possibility of tuning the Tc through the application of a gate voltage.
We have fabricated extraordinary magnetoresistance (EMR) device, comprising a monolayer graphene with an embedded metallic disk, that exhibits large room temperature magnetoresistance (MR) enhancement of up to 55,000% at 9 T. Finite element simulations yield predictions in excellent agreement with the experiment and show possibility for even better performance. Simplicity, ease of implementation and high sensitivity of this device imply great potential for practical applications.
We present a case of pneumatosis intestinalis (PI) of the colon in the setting of inflammatory bowel disease that was treated with medical management rather than emergent surgery. While the reflex response to extraluminal air in the abdomen is abdominal exploration, consideration of the clinical context in which PI is discovered and an understanding of a complete differential diagnosis of the sources of PI is critical to avoiding unnecessary surgery.
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