51 research outputs found
Phase-dependent microwave response of a graphene Josephson junction
Gate-tunable Josephson junctions embedded in a microwave environment provide a promising platform to in-situ engineer and optimize novel superconducting quantum circuits. The key quantity for the circuit design is the phase-dependent complex admittance of the junction, which can be probed by sensing an rf SQUID with a tank circuit. Here, we investigate a graphene-based Josephson junction as a prototype gate-tunable element enclosed in a SQUID loop that is inductively coupled to a superconducting resonator operating at 3 GHz. With a concise circuit model that describes the dispersive and dissipative response of the coupled system, we extract the phase-dependent junction admittance corrected for self-screening of the SQUID loop. We decompose the admittance into the current-phase relation and the phase-dependent loss and as these quantities are dictated by the spectrum and population dynamics of the supercurrent-carrying Andreev bound states, we gain insight to the underlying microscopic transport mechanisms in the junction. We theoretically reproduce the experimental results by considering a short, diffusive junction model that takes into account the interaction between the Andreev spectrum and the electromagnetic environment, from which we deduce a lifetime of ~17 ps for non-equilibrium populations
Phase-dependent microwave response of a graphene Josephson junction
Gate-tunable Josephson junctions embedded in a microwave environment provide a promising platform to in situ engineer and optimize novel superconducting quantum circuits. The key quantity for the circuit design is the phase-dependent complex admittance of the junction, which can be probed by sensing a radio frequency SQUID with a tank circuit. Here, we investigate a graphene-based Josephson junction as a prototype gate-tunable element enclosed in a SQUID loop that is inductively coupled to a superconducting resonator operating at 3 GHz. With a concise circuit model that describes the dispersive and dissipative response of the coupled system, we extract the phase-dependent junction admittance corrected for self-screening of the SQUID loop. We decompose the admittance into the current-phase relation and the phase-dependent loss, and as these quantities are dictated by the spectrum and population dynamics of the supercurrent-carrying Andreev bound states, we gain insight to the underlying microscopic transport mechanisms in the junction. We theoretically reproduce the experimental results by considering a short, diffusive junction model that takes into account the interaction between the Andreev spectrum and the electromagnetic environment, from which we estimate lifetimes on the order of ∼10 ps for nonequilibrium populations
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
Trajectories for the Wave Function of the Universe from a Simple Detector Model
Inspired by Mott's (1929) analysis of particle tracks in a cloud chamber, we
consider a simple model for quantum cosmology which includes, in the total
Hamiltonian, model detectors registering whether or not the system, at any
stage in its entire history, passes through a series of regions in
configuration space. We thus derive a variety of well-defined formulas for the
probabilities for trajectories associated with the solutions to the
Wheeler-DeWitt equation. The probability distribution is peaked about classical
trajectories in configuration space. The ``measured'' wave functions still
satisfy the Wheeler-DeWitt equation, except for small corrections due to the
disturbance of the measuring device. With modified boundary conditions, the
measurement amplitudes essentially agree with an earlier result of Hartle
derived on rather different grounds. In the special case where the system is a
collection of harmonic oscillators, the interpretation of the results is aided
by the introduction of ``timeless'' coherent states -- eigenstates of the
Hamiltonian which are concentrated about entire classical trajectories.Comment: 37 pages, plain Tex. Second draft. Substantial revision
Global versus task-specific postoperative feedback in surgical procedure learning
Background: Task-specific checklists and global rating scales are both recommended assessment tools to provide constructive feedback on surgical performance. This study evaluated the most effective feedback tool by comparing the effects of the Observational Clinical Human Reliability Analysis (OCHRA) and the Objective Structured Assessment of Technical Skills (OSATS) on surgical performance in relation to the visual-spatial ability of the learners. Methods: In a randomized controlled trial, medical students were allocated to either the OCHRA (n = 25) or OSATS (n = 25) feedback group. Visual-spatial ability was measured by a Mental Rotation Test. Participants performed an open inguinal hernia repair procedure on a simulation model twice. Feedback was provided after the first procedure. Improvement in performance was evaluated blindly using a global rating sca
A NOTE ON AN EXAMPLE BY VAN MILL
Improving on an earlier example by J. van Mill, we prove that there exists a zero-dimensional compact space of countable #-weight and uncountable character which is homogeneous under MA+CH, but not under CH. 1
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