53 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
Electrostatically confined monolayer graphene quantum dots with orbital and valley splittings
The electrostatic confinement of massless charge carriers is hampered by
Klein tunneling. Circumventing this problem in graphene mainly relies on
carving out nanostructures or applying electric displacement fields to open a
band gap in bilayer graphene. So far, these approaches suffer from edge
disorder or insufficiently controlled localization of electrons. Here we
realize an alternative strategy in monolayer graphene, by combining a
homogeneous magnetic field and electrostatic confinement. Using the tip of a
scanning tunneling microscope, we induce a confining potential in the Landau
gaps of bulk graphene without the need for physical edges. Gating the localized
states towards the Fermi energy leads to regular charging sequences with more
than 40 Coulomb peaks exhibiting typical addition energies of 7-20 meV. Orbital
splittings of 4-10 meV and a valley splitting of about 3 meV for the first
orbital state can be deduced. These experimental observations are
quantitatively reproduced by tight binding calculations, which include the
interactions of the graphene with the aligned hexagonal boron nitride
substrate. The demonstrated confinement approach appears suitable to create
quantum dots with well-defined wave function properties beyond the reach of
traditional techniques
Vertical Field Effect Transistor based on Graphene-WS2 Heterostructures for flexible and transparent electronics
The celebrated electronic properties of graphene have opened way for
materials just one-atom-thick to be used in the post-silicon electronic era. An
important milestone was the creation of heterostructures based on graphene and
other two-dimensional (2D) crystals, which can be assembled in 3D stacks with
atomic layer precision. These layered structures have already led to a range of
fascinating physical phenomena, and also have been used in demonstrating a
prototype field effect tunnelling transistor - a candidate for post-CMOS
technology. The range of possible materials which could be incorporated into
such stacks is very large. Indeed, there are many other materials where layers
are linked by weak van der Waals forces, which can be exfoliated and combined
together to create novel highly-tailored heterostructures. Here we describe a
new generation of field effect vertical tunnelling transistors where 2D
tungsten disulphide serves as an atomically thin barrier between two layers of
either mechanically exfoliated or CVD-grown graphene. Our devices have
unprecedented current modulation exceeding one million at room temperature and
can also operate on transparent and flexible substrates
In Situ TEM Imaging of Solution‐Phase Chemical Reactions Using 2D‐Heterostructure Mixing Cells
From Wiley via Jisc Publications RouterHistory: received 2021-01-26, rev-recd 2021-03-31, pub-electronic 2021-06-09Article version: VoRPublication status: PublishedFunder: Engineering and Physical Sciences Research Council (UK) EPSRC; Grant(s): EP/M010619/1, EP/S021531/1, EP/P009050/1Funder: European Commission H2020 ERC Starter grant EvoluTEM; Grant(s): 715502Funder: Henry Royce Institute for Advanced MaterialsFunder: EPSRC; Id: http://dx.doi.org/10.13039/501100000266; Grant(s): EP/R00661X/1, EP/S019367/1, EP/P025021/1, EP/P025498/1Funder: Royal Society FellowshipAbstract: Liquid‐phase transmission electron microscopy is used to study a wide range of chemical processes, where its unique combination of spatial and temporal resolution provides countless insights into nanoscale reaction dynamics. However, achieving sub‐nanometer resolution has proved difficult due to limitations in the current liquid cell designs. Here, a novel experimental platform for in situ mixing using a specially developed 2D heterostructure‐based liquid cell is presented. The technique facilitates in situ atomic resolution imaging and elemental analysis, with mixing achieved within the immediate viewing area via controllable nanofracture of an atomically thin separation membrane. This novel technique is used to investigate the time evolution of calcium carbonate synthesis, from the earliest stages of nanodroplet precursors to crystalline calcite in a single experiment. The observations provide the first direct visual confirmation of the recently developed liquid‐liquid phase separation theory, while the technological advancements open an avenue for many other studies of early stage solution‐phase reactions of great interest for both the exploration of fundamental science and developing applications
Thermopower in hBN/graphene/hBN superlattices
Thermoelectric effects are highly sensitive to the asymmetry in the density
of states around the Fermi energy and can be exploited as probes of the
electronic structure. We experimentally study thermopower in high-quality
monolayer graphene, within heterostructures consisting of complete hBN
encapsulation and 1D edge contacts, where the graphene and hBN lattices are
aligned. When graphene is aligned to one of the hBN layers, we demonstrate the
presence of additional sign reversals in the thermopower as a function of
carrier density, directly evidencing the presence of the moir\'e superlattice.
We show that the temperature dependence of the thermopower enables the
assessment of the role of built-in strain variation and van Hove singularities
and hints at the presence of Umklapp electron-electron scattering processes. As
the thermopower peaks around the neutrality point, this allows to probe the
energy spectrum degeneracy. Further, when graphene is double-aligned with the
top and bottom hBN crystals, the thermopower exhibits features evidencing
multiple cloned Dirac points caused by the differential super-moir\'e lattice.
For both cases we evaluate how well the thermopower agrees with Mott's
equation. Finally, we show the same superlattice device can exhibit a
temperature-driven thermopower reversal from positive to negative and vice
versa, by controlling the carrier density. The study of thermopower provides an
alternative approach to study the electronic structure of 2D superlattices,
whilst offering opportunities to engineer the thermoelectric response on these
heterostructures.Comment: 9 pages, 3 figure
Atomically thin boron nitride: a tunnelling barrier for graphene devices
We investigate the electronic properties of heterostructures based on
ultrathin hexagonal boron nitride (h-BN) crystalline layers sandwiched between
two layers of graphene as well as other conducting materials (graphite, gold).
The tunnel conductance depends exponentially on the number of h-BN atomic
layers, down to a monolayer thickness. Exponential behaviour of I-V
characteristics for graphene/BN/graphene and graphite/BN/graphite devices is
determined mainly by the changes in the density of states with bias voltage in
the electrodes. Conductive atomic force microscopy scans across h-BN terraces
of different thickness reveal a high level of uniformity in the tunnel current.
Our results demonstrate that atomically thin h-BN acts as a defect-free
dielectric with a high breakdown field; it offers great potential for
applications in tunnel devices and in field-effect transistors with a high
carrier density in the conducting channel.Comment: 7 pages, 5 figure
Tuning the pseudospin polarization of graphene by a pseudo-magnetic field
One of the intriguing characteristics of honeycomb lattices is the appearance
of a pseudo-magnetic field as a result of mechanical deformation. In the case
of graphene, the Landau quantization resulting from this pseudo-magnetic field
has been measured using scanning tunneling microscopy. Here we show that a
signature of the pseudo-magnetic field is a local sublattice symmetry breaking
observable as a redistribution of the local density of states. This can be
interpreted as a polarization of graphene's pseudospin due to a strain induced
pseudo-magnetic field, in analogy to the alignment of a real spin in a magnetic
field. We reveal this sublattice symmetry breaking by tunably straining
graphene using the tip of a scanning tunneling microscope. The tip locally
lifts the graphene membrane from a SiO support, as visible by an increased
slope of the curves. The amount of lifting is consistent with molecular
dynamics calculations, which reveal a deformed graphene area under the tip in
the shape of a Gaussian. The pseudo-magnetic field induced by the deformation
becomes visible as a sublattice symmetry breaking which scales with the lifting
height of the strained deformation and therefore with the pseudo-magnetic field
strength. Its magnitude is quantitatively reproduced by analytic and
tight-binding models, revealing fields of 1000 T. These results might be the
starting point for an effective THz valley filter, as a basic element of
valleytronics.Comment: Revised manuscript: streamlined the abstract and introduction, added
methods to supplement, Nano Letters, 201
Niobium diselenide superconducting photodetectors
We report the photoresponse of niobium diselenide (NbSe), a transition
metal dichalcogenide (TMD) which exhibits superconducting properties down to a
single layer. Devices are built by using micro-mechanically cleaved 2 to 10
layers and tested under current bias using nano-optical mapping in the 350mK-5K
range, where they are found to be superconducting. The superconducting state
can be broken by absorption of light, resulting in a voltage signal when the
devices are current biased. The response found to be energy dependent making
the devices useful for applications requiring energy resolution, such as
bolometry, spectroscopy and infrared imaging.Comment: 6 pages, 6 figure
Non-minimally Coupled Cosmological Models with the Higgs-like Potentials and Negative Cosmological Constant
We study dynamics of non-minimally coupled scalar field cosmological models
with Higgs-like potentials and a negative cosmological constant. In these
models the inflationary stage of the Universe evolution changes into a
quasi-cyclic stage of the Universe evolution with oscillation behaviour of the
Hubble parameter from positive to negative values. Depending on the initial
conditions the Hubble parameter can perform either one or several cycles before
to become negative forever.Comment: 22 pages, 6 figures, v4:Section 2 expanded, references added,
accepted for publication in Class. Quant. Gra
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