142 research outputs found
Conduction spectroscopy of a proximity induced superconducting topological insulator
The combination of superconductivity and the helical spin-momentum locking at
the surface state of a topological insulator (TI) has been predicted to give
rise to p-wave superconductivity and Majorana bound states. The
superconductivity can be induced by the proximity effect of a an s-wave
superconductor (S) into the TI. To probe the superconducting correlations
inside the TI, dI/dV spectroscopy has been performed across such S-TI
interfaces. Both the alloyed BiSbTeSe and the
stoichiometric BiSbTeSe have been used as three dimensional TI. In the case
of BiSbTeSe, the presence of disorder induced
electron-electron interactions can give rise to an additional zero-bias
resistance peak. For the stoichiometric BiSbTeSe with less disorder, tunnel
barriers were employed in order to enhance the signal from the interface. The
general observations in the spectra of a large variety of samples are
conductance dips at the induced gap voltage, combined with an increased sub-gap
conductance, consistent with p-wave predictions. The induced gap voltage is
typically smaller than the gap of the Nb superconducting electrode, especially
in the presence of an intentional tunnel barrier. Additional uncovered
spectroscopic features are oscillations that are linearly spaced in energy, as
well as a possible second order parameter component.Comment: Semiconductor Science and Technology; Special Issue on Hybrid Quantum
Materials and Device
Josephson Effect and Charge Distribution in Thin Bi<sub>2</sub>Te<sub>3</sub> Topological Insulators
Thin layers of topological insulator materials are quasi-2D systems featuring a complex interplay between quantum confinement and topological band structure. To understand the role of the spatial distribution of carriers in electrical transport, the Josephson effect, magnetotransport, and weak anti-localization are studied in bottom-gated thin Bi2Te3 topological insulator films. The experimental carrier densities are compared to a model based on the solutions of the self-consistent Schrödinger–Poisson equations and they are in excellent agreement. The modeling allows for a quantitative interpretation of the weak antilocalization correction to the conduction and of the critical current of Josephson junctions with weak links made from such films without any ad hoc assumptions.</p
Josephson Effect and Charge Distribution in Thin BiTe Topological Insulators
Thin layers of topological insulator materials are quasi-two-dimensional
systems featuring a complex interplay between quantum confinement and
topological band structure. To understand the role of the spatial distribution
of carriers in electrical transport, we study the Josephson effect,
magnetotransport, and weak anti-localization in bottom-gated thin BiTe
topological insulator films.We compare the experimental carrier densities to a
model based on the solutions of the self-consistent Schr\"odinger-Poisson
equations and find excellent agreement. The modeling allows for a quantitative
interpretation of the weak antilocalization correction to the conduction and of
the critical current of Josephson junctions with weak links made from such
films without any ad hoc assumptions.Comment: 18 pages, 6 figure
Interface structure of SrTiO3-LaAlO3 at elevated temperatures studied in-situ by synchroton x-rays
The atomic interface structure between SrTiO3 and LaAlO3 was studied at elevated temperatures employing in situ surface x-ray diffraction. The results at 473 K indicate that the lattice distorts significantly in two ways. First, the interatomic distances between the cations across the interface become as large as 4.03(2) Å. Second, the TiO6 octahedra at the interface contract their principal axis along the surface normal considerably and the Ti displaces off center. These distortions can be ascribed to the charge inbalance introduced by the change in atomic species across the interface and to a Jahn-Teller effect. The latter distortion suggests the presence of extra electrons at the interface, which is important for understanding the electronic properties of this system
Hard superconducting gap and diffusion-induced superconductors in Ge-Si nanowires
We show a hard induced superconducting gap in a Ge-Si nanowire Josephson
transistor up to in-plane magnetic fields of mT, an important step
towards creating and detecting Majorana zero modes in this system. A hard
induced gap requires a highly homogeneous tunneling heterointerface between the
superconducting contacts and the semiconducting nanowire. This is realized by
annealing devices at C during which aluminium inter-diffuses and
replaces the germanium in a section of the nanowire. Next to Al, we find a
superconductor with lower critical temperature ( K) and a
higher critical field ( T). We can therefore selectively
switch either superconductor to the normal state by tuning the temperature and
the magnetic field and observe that the additional superconductor induces a
proximity supercurrent in the semiconducting part of the nanowire even when the
Al is in the normal state. In another device where the diffusion of Al rendered
the nanowire completely metallic, a superconductor with a much higher critical
temperature ( K) and critical field ( T) is
found. The small size of diffusion-induced superconductors inside nanowires may
be of special interest for applications requiring high magnetic fields in
arbitrary direction
An international, multicentre survey of β-lactam antibiotic therapeutic drug monitoring practice in intensive care units
Objectives Emerging evidence supports the use of therapeutic drug monitoring (TDM) of β-lactams for intensive care unit (ICU) patients to optimize drug exposure, although limited detail is available on how sites run this service in practice. This multicentre survey study was performed to describe the various approaches used for β-lactam TDM in ICUs. Methods A questionnaire survey was developed to describe various aspects relating to the conduct of β-lactam TDM in an ICU setting. Data sought included: β-lactams chosen for TDM, inclusion criteria for selecting patients, blood sampling strategy, analytical methods, pharmacokinetic (PK)/pharmacodynamic (PD) targets and dose adjustment strategies. Results Nine ICUs were included in this survey. Respondents were either ICU or infectious disease physicians, pharmacists or clinical pharmacologists. Piperacillin (co-formulated with tazobactam) and meropenem (100% of units surveyed) were the β-lactams most commonly subject to TDM, followed by ceftazidime (78%), ceftriaxone (43%) and cefazolin (43%). Different chromatographic and microbiological methods were used for assay of β-lactam concentrations in blood and other biological fluids (e.g. CSF). There was significant variation in the PK/PD targets (100% fT>MIC up to 100% fT>4×MIC) and dose adjustment strategies used by each of the sites. Conclusions Large variations were found in the type of β-lactams tested, the patients selected for TDM and drug assay methods. Significant variation observed in the PK/PD targets and dose adjustment strategies used supports the need for further studies that robustly define PK/PD targets for ICU patients to ensure a greater consistency of practice for dose adjustment strategies for optimizing β-lactam dosing with TD
The XMM-SSC survey of hard-spectrum XMM-Newton sources 1: optically bright sources
We present optical and X-ray data for a sample of serendipitous XMM-Newton
sources that are selected to have 0.5-2 keV vs 2-4.5 keV X-ray hardness ratios
which are harder than the X-ray background. The sources have 2-4.5 keV X-ray
flux >= 10^-14 cgs, and in this paper we examine a subsample of 42 optically
bright (r < 21) sources; this subsample is 100 per cent spectroscopically
identified. All but one of the optical counterparts are extragalactic, and we
argue that the single exception, a Galactic M star, is probably a coincidental
association. The X-ray spectra are consistent with heavily absorbed power laws
(21.8 < log NH < 23.4), and all of them appear to be absorbed AGN. The majority
of the sources show only narrow emission lines in their optical spectra,
implying that they are type-2 AGN. Only a small fraction of the sources (7/42)
show broad optical emission lines, and all of these have NH < 10^23 cm^-2. This
implies that ratios of X-ray absorption to optical/UV extinction equivalent to
> 100 times the Galactic gas-to-dust ratio are rare in AGN absorbers (at most a
few percent of the population), and may be restricted to broad absorption-line
QSOs. Seven objects appear to have an additional soft X-ray component in
addition to the heavily absorbed power law. We consider the implications of our
results in the light of the AGN unified scheme. We find that the soft
components in narrow-line objects are consistent with the unified scheme
provided that > 4 per cent of broad-line AGN have ionised absorbers that
attenuate their soft X-ray flux by >50 per cent. In at least one of the X-ray
absorbed, broad-line AGN in our sample the X-ray spectrum requires an ionised
absorber, consistent with this picture.Comment: accepted for publication in MNRA
Stencil lithography of superconducting contacts on MBE-grown topological insulator thin films
Topological insulator (Bi0.06Sb0.94)2Te3 thin films grown by molecular beam epitaxy have been capped in-situ with a 2 nm Al film to conserve the pristine topological surface states. Subsequently, a shadow mask - structured by means of focus ion beam - was in-situ placed underneath the sample to deposit a thick layer of Al on well-defined microscopically small areas. The 2 nm thin Al layer fully oxidizes after exposure to air and in this way protects the TI surface from degradation. The thick Al layer remains metallic underneath a 3–4 nm thick native oxide layer and therefore serves as (super-) conducting contacts. Superconductor-Topological Insulator-Superconductor junctions with lateral dimensions in the nm range have then been fabricated via an alternative stencil lithography technique. Despite the in-situ deposition, transport measurements and transmission electron microscope analysis indicate a low transparency, due to an intermixed region at the interface between topological insulator thin film and metallic Al
Spectral and transport properties of doped Mott-Hubbard systems with incommensurate magnetic order
We present spectral and optical properties of the Hubbard model on a
two-dimensional square lattice using a generalization of dynamical mean-field
theory to magnetic states in finite dimension. The self-energy includes the
effect of spin fluctuations and screening of the Coulomb interaction due to
particle-particle scattering. At half-filling the quasiparticles reduce the
width of the Mott-Hubbard `gap' and have dispersions and spectral weights that
agree remarkably well with quantum Monte Carlo and exact diagonalization
calculations. Away from half-filling we consider incommensurate magnetic order
with a varying local spin direction, and derive the photoemission and optical
spectra. The incommensurate magnetic order leads to a pseudogap which opens at
the Fermi energy and coexists with a large Mott-Hubbard gap. The quasiparticle
states survive in the doped systems, but their dispersion is modified with the
doping and a rigid band picture does not apply. Spectral weight in the optical
conductivity is transferred to lower energies and the Drude weight increases
linearly with increasing doping. We show that incommensurate magnetic order
leads also to mid-gap states in the optical spectra and to decreased scattering
rates in the transport processes, in qualitative agreement with the
experimental observations in doped systems. The gradual disappearence of the
spiral magnetic order and the vanishing pseudogap with increasing temperature
is found to be responsible for the linear resistivity. We discuss the possible
reasons why these results may only partially explain the features observed in
the optical spectra of high temperature superconductors.Comment: 22 pages, 18 figure
- …