408 research outputs found
Unusual suppression of the superconducting energy gap and critical temperature in atomically thin NbSe2
It is well known that superconductivity in thin films is generally suppressed
with decreasing thickness. This suppression is normally governed by either
disorder-induced localization of Cooper pairs, weakening of Coulomb screening,
or generation and unbinding of vortex-antivortex pairs as described by the
Berezinskii-Kosterlitz-Thouless (BKT) theory. Defying general expectations,
few-layer NbSe2 - an archetypal example of ultrathin superconductors - has been
found to remain superconducting down to monolayer thickness. Here we report
measurements of both the superconducting energy gap and critical temperature in
high-quality monocrystals of few-layer NbSe2, using planar-junction tunneling
spectroscopy and lateral transport. We observe a fully developed gap that
rapidly reduces for devices with the number of layers N < 5, as does their
ctitical temperature. We show that the observed reduction cannot be explained
by disorder, and the BKT mechanism is also excluded by measuring its transition
temperature that for all N remains very close to Tc. We attribute the observed
behavior to changes in the electronic band structure predicted for mono- and
bi- layer NbSe2 combined with inevitable suppression of the Cooper pair density
at the superconductor-vacuum interface. Our experimental results for N > 2 are
in good agreement with the dependences of the gap and Tc expected in the latter
case while the effect of band-structure reconstruction is evidenced by a
stronger suppression of the gap and the disappearance of its anisotropy for N =
2. The spatial scale involved in the surface suppression of the density of
states is only a few angstroms but cannot be ignored for atomically thin
superconductors.Comment: 21 pages, including supporting informatio
Organisation and Governance of Urban Energy Systems:District Heating and Cooling in the UK
International audienc
High electron mobility, quantum Hall effect and anomalous optical response in atomically thin InSe
A decade of intense research on two-dimensional (2D) atomic crystals has revealed that their properties can differ greatly from those of the parent compound. These differences are governed by changes in the band structure due to quantum confinement and are most profound if the underlying lattice symmetry changes. Here we report a high-quality 2D electron gas in few-layer InSe encapsulated in hexagonal boron nitride under an inert atmosphere. Carrier mobilities are found to exceed 103cm2V-1s-1and 104cm2V-1s-1at room and liquid-helium temperatures, respectively, allowing the observation of the fully developed quantum Hall effect. The conduction electrons occupy a single 2D subband and have a small effective mass. Photoluminescence spectroscopy reveals that the bandgap increases by more than 0.5eV with decreasing the thickness from bulk to bilayer InSe. The band-edge optical response vanishes in monolayer InSe, which is attributed to the monolayer's mirror-plane symmetry. Encapsulated 2D InSe expands the family of graphene-like semiconductors and, in terms of quality, is competitive with atomically thin dichalcogenides and black phosphorus.EU, EPSRC. The Royal Societ
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Partial Oxidized Arsenene:Emerging Tunable Direct Bandgap Semiconductor
Arsenene, as a member of the Group V elemental two-dimensional materials appears on the horizon, has shown great prospects. However, its indirect bandgap limits the applications in optoelectronics. In this theoretical work, we reported that partial oxidation can tune the indirect bandgap of arsenene into the direct one. Attributed to the enthalpy decreasing linear to the oxygen ratio, partial oxidized arsenene can be controllably produced by the progressive oxidation under low temperature. Importantly, by increasing the oxygen content from 1O/18As to 18O/18As, the oxidation can narrow the direct bandgap of oxidized arsenene from 1.29 to 0.02 eV. The bandgap of partial oxidized arsenene is proportional to the oxygen content. Consequently, the partial oxidized arsenene with tunable direct bandgap has great potentials in the high efficient infra light emitter and photo-voltaic devices
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