729 research outputs found
Spectroscopy of bulk and few-layer superconducting NbSe with van der Waals tunnel junctions
Tunnel junctions, a well-established platform for high-resolution
spectroscopy of superconductors, require defect-free insulating barriers with
clean engagement to metals on both sides. Extending the range of materials
accessible to tunnel junction fabrication, beyond the limited selection which
allows high-quality oxide formation, requires the development of alternative
fabrication techniques. Here we show that van-der-Waals (vdW) tunnel barriers,
fabricated by stacking layered semiconductors on top of the transition metal
dichalcogenide (TMD) superconductor NbSe, sustain a stable, low noise
tunneling current, and exhibit strong suppression of sub-gap tunneling. We
utilize the technique to measure the spectra of bulk (20 nm) and ultrathin (3-
and 4-layer) devices at 70 mK. The spectra exhibit two distinct energy gaps,
the larger of which decreases monotonously with thickness and , in
agreement with BCS theory. The spectra are analyzed using a two-band model
modified to account for depairing. We show that in the bulk, the smaller gap
exhibits strong depairing in an in-plane magnetic field, consistent with a high
Fermi velocity. In the few-layer devices, depairing of the large gap is
negligible, consistent with out-of-plane spin-locking due to Ising spin-orbit
coupling. Our results demonstrate the utility of vdW tunnel junctions in
mapping the intricate spectral evolution of TMD superconductors over a range of
magnetic fields.Comment: This submission contains the first part of arxiv:1703.07677 with the
addition of spectra taken on this devices. The second part of 1703.07677 will
be published separatel
Enhancing the excitation gap of a quantum-dot-based Kitaev chain
Connecting double quantum dots via a semiconductor-superconductor hybrid
segment offers a platform for creating a two-site Kitaev chain that hosts a
pair of "poor man's Majoranas" at a finely tuned sweet spot. However, the
effective couplings, which are mediated by Andreev bound states in the hybrid,
are generally weak in the tunneling regime. As a consequence, the excitation
gap is limited in size, presenting a formidable challenge for using this
platform to demonstrate non-Abelian statistics of Majoranas and realizing
error-resilient topological quantum computing. In this work, we systematically
study the effects of increasing the coupling between the dot and the hybrid
segment. In particular, the proximity effect transforms the dot orbitals into
Yu-Shiba-Rusinov states, forming a new spinless fermion basis for a Kitaev
chain, and we derive a theory for their effective coupling. As the coupling
strength between the dots and the hybrid segment increases, we find a
significant enhancement of the excitation gap and reduced sensitivity to local
perturbations. Although the hybridization of the Majorana wave function with
the central Andreev bound states increases strongly with increasing coupling,
the overlap of Majorana modes on the outer dots remains small, which is a
prerequisite for potential qubit experiments. We discuss how the
strong-coupling regime shows in experimentally accessible quantities, such as
the local and non-local conductance, and provide a protocol for tuning a
double-dot system into a sweet spot with a large excitation gap.Comment: 12 pages, 9 figure
The key project managers’ competences for different types of projects
This paper describes a quantitative research approach for identifying
key project managers’ competences for different types of projects. By
identifying the perceived most valuable project manager competences, as
having the most potential for increased contribution to project management
(PM) performance, practitioners and organizations can select their priorities
when developing their PM practices. The 46 competences (technical,
behavioural and contextual) provided by IPMA (International Project
Management Association) were surveyed through an online questionnaire.
Three dimensions to distinguish project types were used: application area,
innovation and complexity. Completed questionnaires were received from 96
project managers from Portugal. The results showed that 13 key competences
(20%) were common to the majority of the projects. Most of these are
behavioural competences, such as: ethics, reliability, engagement, openness,
and leadership. It was also observed a clear correlation between technical
competences and project complexity
Tribological behavior of shape-specific microplate-enriched synovial fluids on a linear two-axis tribometer
Nano- and micro-particles are being increasingly used to tune interfacial frictional properties in diverse applications, from friction modifiers in industrial lubrication to enhanced biological fluids in human osteoarthritic joints. Here, we assessed the tribological properties of a simulated synovial fluid enriched with non-spherical, poly lactic-co-glycolic acid (PLGA) microparticles (μPL) that have been previously demonstrated for the pharmacological management of osteoarthritis (OA). Three different μPL configurations were fabricated presenting a 20 μm 20 μm square base and a thickness of 5 μm (thin, 5H μPL), 10 μm (10H μPL), and 20 μm (cubical, 20H μPL). After extensive morphological and physicochemical characterizations, the apparent Young’s modulus of the μPL was quantified under compressive loading returning an average value of 6 kPa, independently of the particle morphology. Then, using a linear two-axis tribometer, the static (μs) and dynamic (μd) friction coefficients of the μPL-enriched simulated synovial fluid were determined in terms of particle configuration and concentration, varying from 0 (fluid only) to 6105 μPL/mL. The particle morphology had a modest influence on friction, possibly because the μPL were fully squeezed between two mating surfaces by a 5.8 N normal load realizing boundary-like lubrication conditions. Differently, friction was observed to depend on the dimensionless parameter , defined as the ratio between the total volume of the μPL enriching the simulated synovial fluid and the volume of the fluid itself. Both coefficients of friction were documented to grow with reaching a plateau of μs 0.4 and μd 0.15, already at 210-3. Future investigations will have to systematically analyze the effect of sliding velocity, normal load, and rigidity of the mating surfaces to elucidate in full the tribological behavior of μPL in the context of osteoarthritis
Crossed Andreev reflection and elastic co-tunneling in a three-site Kitaev chain nanowire device
The formation of a topological superconducting phase in a quantum-dot-based
Kitaev chain requires nearest neighbor crossed Andreev reflection and elastic
co-tunneling. Here we report on a hybrid InSb nanowire in a three-site Kitaev
chain geometry - the smallest system with well-defined bulk and edge - where
two superconductor-semiconductor hybrids separate three quantum dots. We
demonstrate pairwise crossed Andreev reflection and elastic co-tunneling
between both pairs of neighboring dots and show sequential tunneling processes
involving all three quantum dots. These results are the next step towards the
realization of topological superconductivity in long Kitaev chain devices with
many coupled quantum dots
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