525 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
Reversible Engineering of Topological Insulator Surface State Conductivity through Optical Excitation
Despite the broadband response, limited optical absorption at a particular
wavelength hinders the development of optoelectronics based on Dirac fermions.
Heterostructures of graphene and various semiconductors have been explored for
this purpose, while non-ideal interfaces often limit the performance. The
topological insulator is a natural hybrid system, with the surface states
hosting high-mobility Dirac fermions and the small-bandgap semiconducting bulk
state strongly absorbing light. In this work, we show a large photocurrent
response from a field effect transistor device based on intrinsic topological
insulator Sn-Bi1.1Sb0.9Te2S. The photocurrent response is non-volatile and
sensitively depends on the initial Fermi energy of the surface state, and it
can be erased by controlling the gate voltage. Our observations can be
explained with a remote photo-doping mechanism, in which the light excites the
defects in the bulk and frees the localized carriers to the surface state. This
photodoping modulates the surface state conductivity without compromising the
mobility, and it also significantly modify the quantum Hall effect of the
surface state. Our work thus illustrates a route to reversibly manipulate the
surface states through optical excitation, shedding light into utilizing
topological surface states for quantum optoelectronics
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
Role of Aminoalcoholphosphotransferases 1 and 2 in Phospholipid Homeostasis in Arabidopsis
Aminoalcoholphosphotransferase (AAPT) catalyzes the synthesis of phosphatidylcholine (PC) and phosphotidylethanolamine (PE), which are the most prevalent membrane phospholipids in all eukaryotic cells. Here, we show that suppression of AAPTs results in extensive membrane phospholipid remodeling in Arabidopsis thaliana. Double knockout (KO) mutants that are hemizygous for either aapt1 or aapt2 display impaired pollen and seed development, leading to embryotic lethality of the double KO plants, whereas aapt1 or aapt2 single KO plants show no overt phenotypic alterations. The growth rate and seed yield of AAPT RNA interference (RNAi) plants are greatly reduced. Lipid profiling shows decreased total galactolipid and phospholipid content in aapt1-containing mutants, including aapt1, aapt1/aapt1 aapt2/AAPT2, aapt1/AAPT1 aapt2/aapt2, and AAPT RNAi plants. The level of PC in leaves was unchanged, whereas that of PE was reduced in all AAPT-deficient plants, except aapt2 KO. However, the acyl species of PC was altered, with increased levels of C34 species and decreased C36 species. Conversely, the levels of PE and phosphatidylinositol were decreased in C34 species. In seeds, all AAPT-deficient plants, including aapt2 KO, displayed a decrease in PE. The data show that AAPT1 and AAPT2 are essential to plant vegetative growth and reproduction and have overlapping functions but that AAPT1 contributes more than AAPT2 to PC production in vegetative tissues. The opposite changes in molecular species between PC and PE and unchanged PC level indicate the existence of additional pathways that maintain homeostatic levels of PC, which are crucial for the survival and proper development of plants
Role of Aminoalcoholphosphotransferases 1 and 2 in Phospholipid Homeostasis in Arabidopsis
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