23 research outputs found
Universal lineshape of the Kondo zero-bias anomaly in a quantum dot
Encouraged by the recent real-time renormalization group results we carried
out a detailed analysis of the nonequilibrium Kondo conductance observed in an
InAs nanowire-based quantum dot and found them to be in excellent agreement. We
show that in a wide range of bias the Kondo conductance zero-bias anomaly is
scaled by the Kondo temperature to a universal lineshape predicted by the
numerical study. The lineshape can be approximated by a phenomenological
expression of a single argument . The knowledge of
an analytical expression for the lineshape provides an alternative way for
estimation of the Kondo temperature in a real experiment, with no need for time
consuming temperature dependence measurements of the linear conductance.Comment: 5 pages, 3 figure
Wide-band current preamplifier for conductance measurements with large input capacitance
A wide-band current preamplifier based on a composite operational amplifier
is proposed. It has been shown that the bandwidth of the preamplifier can be
significantly increased by enhancing the effective open-loop gain of the
composite preamplifier. The described preamplifier with current gain 10 V/A
showed the bandwidth of about 100 kHz with 1 nF input shunt capacitance. The
current noise of the amplifier was measured to be about 46 fA/
at 1 kHz, close to the design noise minimum. The voltage noise was found to be
about 2.9 nV/ at 1 kHz, which is in a good agreement with the
value expected for the operational amplifier used in the input stage. By
analysing the total noise produced by the preamplifier we found the optimal
frequency range suitable for the fast lock-in measurements to be from 1 kHz to
2 kHz. To get the same signal-to-noise ratio, the reported preamplifier
requires roughly 10% of the integration time used in measurements made with a
conventional preamplifier.Comment: 5 pages, 4 figure
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
Spin-1/2 Kondo effect in an InAs nanowire quantum dot: the Unitary limit, conductance scaling and Zeeman splitting
We report on a comprehensive study of spin-1/2 Kondo effect in a
strongly-coupled quantum dot realized in a high-quality InAs nanowire. The
nanowire quantum dot is relatively symmetrically coupled to its two leads, so
the Kondo effect reaches the Unitary limit. The measured Kondo conductance
demonstrates scaling with temperature, Zeeman magnetic field, and
out-of-equilibrium bias. The suppression of the Kondo conductance with magnetic
field is much stronger than would be expected based on a g-factor extracted
from Zeeman splitting of the Kondo peak. This may be related to strong
spin-orbit coupling in InAs.Comment: 12 pages, 7 figures. Revised version accepted for publicatio
Nanoscale mapping and spectroscopy of non-radiative hyperbolic modes in hexagonal boron nitride nanostructures
The inherent crystal anisotropy of hexagonal boron nitride (hBN) sustains
naturally hyperbolic phonon polaritons, i.e. polaritons that can propagate with
very large wavevectors within the material volume, thereby enabling optical
confinement to exceedingly small dimensions. Indeed, previous research has
shown that nanometer-scale truncated nanocone hBN cavities, with deep
subwavelength dimensions, support three-dimensionally confined optical modes in
the mid-infrared. Due to optical selection rules, only a few of many such modes
predicted theoretically have been observed experimentally via far-field
reflection and scattering-type scanning near-field optical microscopy. The
Photothermal induced resonance (PTIR) technique probes optical and vibrational
resonances overcoming weak far-field emission by leveraging an atomic force
microscope (AFM) probe to transduce local sample expansion due to light
absorption. Here we show that PTIR enables the direct observation of previously
unobserved, dark hyperbolic modes of hBN nanostructures. Leveraging these
optical modes could yield a new degree of control over the electromagnetic
near-field concentration, polarization and angular momentum in nanophotonic
applications.Comment: 14 pages with references, 4 figure
Organisation and Governance of Urban Energy Systems:District Heating and Cooling in the UK
International audienc
Gate-defined quantum confinement in InSe-based van der Waals heterostructures
Indium selenide, a post-transition metal chalcogenide, is a novel two-dimensional (2D) semiconductor with interesting electronic properties. Its tunable band gap and high electron mobility have already attracted considerable research interest. Here we demonstrate strong quantum confinement and manipulation of single electrons in devices made from few-layer crystals of InSe using electrostatic gating. We report on gate-controlled quantum dots in the Coulomb blockade regime as well as one-dimensional quantization in point contacts, revealing multiple plateaus. The work represents an important milestone in the development of quality devices based on 2D materials and makes InSe a prime candidate for relevant electronic and optoelectronic applications
Influence of Metal Deposition on Exciton–Surface Plasmon Polariton Coupling in GaAs/AlAs/GaAs Core–Shell Nanowires Studied with Time-Resolved Cathodoluminescence
The
coupling of excitons to surface plasmon polaritons (SPPs) in
Au- and Al-coated GaAs/AlAs/GaAs core–shell nanowires, possessing
diameters of ∼100 nm, was probed using time-resolved cathodoluminescence
(CL). Excitons were generated in the metal coated nanowires by injecting
a pulsed high-energy electron beam through the thin metal films. The
Purcell enhancement factor (<i>F</i><sub>P</sub>) was obtained
by direct measurement of changes in the temperature-dependent radiative
lifetime caused by the nanowire exciton-SPP coupling and compared
with a model that takes into account the dependence of <i>F</i><sub>P</sub> on the distance from the metal film and the thickness
of the film covering the GaAs nanowires