41 research outputs found
Subharmonic gap structures and Josephson effect in MgB2/Nb micro-constrictions
Superconducting micro-constrictions between Nb tips and high quality
MgB pellets have been realized by means of a point-contact inset, driven
by a micrometric screw. Measurements of the current-voltage characteristics and
of the dynamical conductance versus bias have been performed in the temperature
range between 4.2 K and 500 K. Above the Nb critical temperature T,
the conductance of the MgB/normal-metal constrictions behaves as predicted
by the BTK model for low resistance contacts while high resistance junctions
show quasiparticle tunneling characteristics. Consistently, from the whole set
of data we infer the value meV for the
three-dimensional gap of MgB. Below T, low resistance contacts
show Josephson current and subharmonic gap structures (SGS), due to multiple
Andreev reflections. Simultaneous observations of both features, unambiguously
indicate coupling of the 3D band of MgB with the Nb superconducting order
parameter. We found that the temperature dependence of the Josephson critical
current follows the classical Ambegaokar-Baratoff behavior with a value
meV at low temperatures.Comment: 8 pages, 5 figures. Replaced with published versio
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
Dynamical I-V Characteristics of SNS Junctions
We have probed the switching dynamics of the Josephson critical current of a
superconducting weak link by measuring its voltage/current characteristics
while applying an ac current bias in the range 1-200 MHz. The weak link between
two Nb reservoirs is formed by an mesoscopic Al wire above its critical
temperature. We observe a dynamical phase transition as a function of the
frequency and amplitude of the ac current. While at low frequency the
transition driven by increasing the current bias is well described by the
standard Kramers theory, at high frequency the switching histograms become
hysteretic and much narrower than expected by thermal fluctuations. The
crossover frequency between the two regimes is set by the electron-phonon
interaction rate in the normal metal.Comment: 5 pages, 4 figure
Atomic scale shot-noise using broadband scanning tunnelling microscopy
We have developed a broadband scanning tunnelling microscope capable of
conventional, low frequency (<10 kHz), microscopy as well spectroscopy and
shot-noise detection at 1 MHz. After calibrating our AC circuit on a gold
surface, we illustrate our capability to detect shot-noise at the atomic scale
and at low currents (<1 nA) by simultaneously measuring the atomically resolved
differential conductance and shot-noise on the high temperature superconductor
BiSrCaCuO. We further show our direct sensitivity to
the temperature of the tunnelling electrons at low voltages. Our broadband
probe opens up the possibility to study charge and correlation effects at the
atomic scale in all materials accessible to STM
Spin-Orbit induced phase-shift in BiSe Josephson junctions
The transmission of Cooper pairs between two weakly coupled superconductors
produces a superfluid current and a phase difference; the celebrated Josephson
effect. Because of time-reversal and parity symmetries, there is no Josephson
current without a phase difference between two superconductors. Reciprocally,
when those two symmetries are broken, an anomalous supercurrent can exist in
the absence of phase bias or, equivalently, an anomalous phase shift
can exist in the absence of a superfluid current. We report on the
observation of an anomalous phase shift in hybrid Josephson
junctions fabricated with the topological insulator BiSe submitted to
an in-plane magnetic field. This anomalous phase shift is observed
directly through measurements of the current-phase relationship in a Josephson
interferometer. This result provides a direct measurement of the spin-orbit
coupling strength and open new possibilities for phase-controlled Josephson
devices made from materials with strong spin-orbit coupling
Observation of the Unconventional Photon Blockade in the Microwave Domain
We have observed the unconventional photon blockade effect for microwave
photons using two coupled superconducting resonators. As opposed to the
conventional blockade, only weakly nonlinear resonators are required. The
blockade is revealed through measurements of the second order correlation
function of the microwave field inside one of the two resonators.
The lowest measured value of is 0.4 for a resonator population of
approximately photons. The time evolution of exhibits an
oscillatory behavior, which is characteristic of the unconventional photon
blockade