Subgap spectroscopy of thermally excited quasiparticles in a Nb-contacted carbon nanotube quantum dot

Abstract

We present electronic transport measurements of a single wall carbon nanotube quantum dot coupled to Nb superconducting contacts. For temperatures comparable to the superconducting gap peculiar transport features are observed inside the Coulomb blockade and superconducting energy gap regions. The observed temperature dependence can be explained in terms of sequential tunneling processes involving thermally excited quasiparticles. In particular, these new channels give rise to two unusual conductance peaks at zero bias in the vicinity of the charge degeneracy point and allow to determine the degeneracy of the ground states involved in transport. The measurements are in good agreement with model calculations.

Nb/InAs nanowire proximity junctions from Josephson to quantum dot regimes

“…At zero magnetic field, peaks appear in conductance at resonant values of the bias voltage V , and these resonant values are modulated by changing the backgate voltage V g . The dependence of the resonances on V g strongly correlates with the Coulomb diamond structure, as previously seen in carbon nan- otube [20,21,22,23,3,24] and InAs nanowire devices [15,16,17,18,19]. Unlike previous studies, however, there appears to be little correlation between the shape of the resonances versus V g and the charge state of the quantum dot; i.e.…”

“…The junction phase dynamics are typically overdamped. In devices with more disorder in the potential landscape, the quantum dot (QD) regime is observed and reveals resonances associated with ABS up to temperatures ∼ 2 K. Unlike previously reported observations [15,16,17,18,19,20,21,22,23,3,24], the gate dependence of the ABS resonances does not appear to depend strongly on whether the QD is in a even-or odd-electron state. In order to explain this, we adapt an Anderson-type mode [25] originally developed for a phase-biased S-QD-S configuration to our experiment, which behaves as a current-biased S-N-QD-N-S configuration.…”

Thermally induced subgap features in the cotunneling spectroscopy of a carbon nanotube

“…Thermally assisted quasiparticle transport has up to now only been discussed in the context of sequential [17,18] and resonant [10,13] tunneling. Also responsible for the energy distribution of the fermionic quasiparticles, in addition to the BCS density of states (DOS), is the Fermi function.…”

“…The emergence of a zero-bias peak inside the Coulomb diamond by increasing the temperature [10,13] was explained in terms of resonant tunneling [11] of thermal quasiparticles. Recently the additional possibility of observing transport features due to sequential tunneling of thermally excited quasiparticles was theoretically proposed in [17] and experimentally confirmed in [18]. Such processes lead to thermal resonance lines within the Coulomb blockade region, parallel to the Coulomb diamond edges.…”

Transmission of a microwave cavity coupled to localized Shiba states

“…In the absence of photons, such an electronic system develops multiple Shiba states inside the superconducting gap [20]. They have been recently investigated by using subgap spectroscopy [37]. The competition between them drives the system through a quantum phase transition.…”