We report a systematic study of thickness-dependent superconductivity and carrier transport properties in exfoliated layered 2H-NbS2. Hall-effect measurements reveal 2H-NbS2 is a p-type metal with hole mobility of 1–3 cm2 V−1 s−1. The superconducting transition temperature is found to decrease with thickness. However, we find that superconductivity is suppressed due to disorder resulting from the incorporation of atmospheric oxygen. Cross-section transmission electron microscope imaging reveals a chemical change of NbS2 in the ambient, resulting in the formation of amorphous oxide layers sandwiching crystalline layered NbS2. Though few-nm-thick 2H-NbS2 completely converts to amorphous oxide in the ambient, PMMA encapsulation prevents further chemical change and preserves superconductivity in thicker samples.
The design and performance of a fast-scanning, low- and variable-temperature, scanning tunneling microscope (STM) incorporated in an ultrahigh vacuum system is described. The sample temperature can be varied from 25 to 350 K by cooling the sample using a continuous flow He cryostat and counter heating by a W filament. The sample temperature can be changed tens of degrees on a time scale of minutes, and scanning is possible within minutes after a temperature change. By means of a software implemented active drift compensation the drift rate can be as low as 1 nm/day. The STM is rigid, very compact, and of low weight, and is attached firmly to the sample holder using a bayonet-type socket. Atomic resolution on clean metal surfaces can be achieved in the entire temperature range. The performance of the instrument is further demonstrated by images of adsorbed hexa-tert-butyl-decacyclene molecules on Cu(110), by STM movies, i.e., sequential STM images with a time resolution down to 1 s/image (100×100 Å2 with 256×256 pixels), of the mobility of these molecules, and finally by constant current images of standing waves in the electronic local density of states on Cu(110).
Solid-state magnetic field sensors are important for applications in commercial electronics and fundamental materials research. Most magnetic field sensors function in a limited range of temperature and magnetic field, but Hall sensors in principle operate over a broad range of these conditions. Here, we evaluate ultraclean graphene as a material platform for highperformance Hall sensors. We fabricate micrometer-scale devices from graphene encapsulated with hexagonal boron nitride and few-layer graphite. We optimize the magnetic field detection limit under different conditions. At 1 kHz for a 1 μm device, we estimate a detection limit of 700 nT Hz −1/2 at room temperature, 80 nT Hz −1/2 at 4.2 K, and 3 μT Hz −1/2 in 3 T background field at 4.2 K. Our devices perform similarly to the best Hall sensors reported in the literature at room temperature, outperform other Hall sensors at 4.2 K, and demonstrate high performance in a few-Tesla magnetic field at which the sensors exhibit the quantum Hall effect.
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