Richard Deblock

The topology of WTe 2 , a transition-metal dichalcogenide with large spin-orbit interactions, is thought to combine type II Weyl semimetal and second-order topological insulator (SOTI) character. The SOTI character should endow WTe 2 multilayer crystals with topologically protected helical states at their hinges, and, indeed, one-dimensional states have been detected thanks to Josephson interferometry. However, the immunity to backscattering conferred to those states by their helical nature has so far not been tested. To probe the topological protection of WTe 2 edge states, we have fabricated superconducting quantum interference devices (SQUIDs) in which the supercurrent through a junction on the crystal edge interferes with the supercurrent through a junction in the bulk of the crystal. We find behaviors ranging from a symmetric SQUID to asymmetric SQUID patterns, including one in which the modulation by magnetic field reveals a sawtoothlike supercurrent versus phase relation for the edge junction, demonstrating that the supercurrent at the edge is carried by ballistic channels over 600 nm, a telltale sign of the SOTI character of multilayer WTe 2 .

We investigate quantum transport in micrometer-sized single crystals of Bi 4 Br 4 , a material predicted to be a second-order topological insulator. 1D topological states with long phase coherence times are revealed via the modulation of quantum interference with magnetic field and gate voltage. In particular, we demonstrate the existence of Aharonov-Bohm interference between 1D ballistic states several micrometers long, that we identify as phase-coherent hinge modes on neighboring step edges at the crystal surface. These Aharonov-Bohm oscillations are made possible by a disordered phase-coherent contact region, the existence of which is confirmed by scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy (STEM-EDX) of FIB lamellae. Their coherent nature modulates the transmission of the 1D edge states, leading to weak antilocalization and universal conductance fluctuations with surprisingly large characteristic fields and a strongly anisotropic behavior. These complementary experimental results provide a comprehensive, coherent description of quantum transport in Bi 4 Br 4 and establish the material as a second-order topological insulator with topologically protected 1D ballistic states.

In superconductor-normal-superconductor (SNS) junctions, supercurrent is mediated via Andreev bound states (ABSs) controlled by the phase difference between the two superconductors. Theory has long predicted significant noise of such supercurrent in equilibrium, due to thermal excitation between the ABSs. Via the fluctuation-dissipation theorem (FDT), this leads to a finite dissipative conductance that coexists with the supercurrent but is hidden in dc measurement. Here, we directly measure the supercurrent noise at radio frequency in a phase-biased SNS ring inductively coupled to a superconducting resonator. We also measure the admittance of the same system whose real part is the dissipative conductance, and quantitatively verify the FDT relation of the SNS ring. The dissipative conductance shows a 1/T temperature dependence, in contrast to the Drude conductance of an unproximitized metal. Using linear response theory, we attribute this behavior to the enhanced current correlations due to the electron-hole symmetry imposed by the proximity effect. Our results reveal the commonly overlooked role of thermal fluctuations in superconducting hybrid systems and other mesoscopic or quantum material systems with large orbital susceptibility. Published by the American Physical Society 2024

Irradiating normal-superconducting junctions with microwave photons produce spectacular effects, such as Shapiro steps and photoinduced modifications of the dc supercurrent. Moreover, microwave irradiation can also have other, hitherto unexplored consequences, such as a photoassisted dissipation which is phase dependent. Here we present a finite-frequency measurement of both the dissipation and the supercurrent of a phase-biased graphene-superconductor junction in response to microwave photons. We find that, while the supercurrent response is well described by existing theory, the dissipation exhibits unexpected effects which need new theoretical elucidation. Especially with high frequency photons, the dissipation is enhanced at phase zero, where it is minimum without irradiation. We attribute this enhancement to Andreev level transitions, made possible by microwave-induced nonequilibrium population of Andreev bound states. Our results demonstrate that dissipation is a more sensitive probe of microwave photons than is the supercurrent, and reveal the potential of measuring dissipation to improve superconducting photodetectors and investigate photoassisted physics in hybrid superconducting systems.

We probe the high frequency emission of a carbon nanotube based Josephson junction and compare it to its dc Josephson current. The ac emission is probed by coupling the carbon nanotube to an on-chip detector (a superconductor-insulator-superconductor junction), via a coplanar waveguide resonator. The measurement of the photoassisted current of the detector gives direct access to the signal emitted by the carbon nanotube. We focus on the gate regions that exhibit Kondo features in the normal state and demonstrate that when the dc supercurrent is enhanced by the Kondo effect, the ac Josephson effect is strongly reduced. This result is compared to numerical renormalization group theory and is attributed to a transition between the singlet ground state and the doublet excited state which is enabled only when the junction is driven out-of-equilibrium by a voltage bias.