Subgap resonant quasiparticle transport in normal-superconductor quantum dot devices

J.  Gramich; A.  Baumgartner; C.  Schönenberger

doi:10.1063/1.4948352

ABSTRACT

We report thermally activated transport resonances for biases below the superconducting energy gap in a carbon nanotube

quantum dot

(QD) device with a superconducting Pb and a normal metal contact. These resonances are due to the superconductor's finite quasi-particle population at elevated temperatures and can only be observed when the QD life-time broadening is considerably smaller than the gap. This condition is fulfilled in our QD devices with optimized Pd/Pb/In multi-layer contacts, which result in reproducibly large and “clean” superconducting transport gaps with a strong conductance suppression for subgap biases. We show that these gaps close monotonically with increasing magnetic field and temperature. The accurate description of the subgap resonances by a simple resonant tunneling model illustrates the ideal characteristics of the reported Pb contacts and gives an alternative access to the tunnel coupling strengths in a QD.

This work was financially supported by the Swiss National Science Foundation (SNF), the Swiss Nanoscience Institute (SNI), the Swiss NCCR QSIT, the ERC Project QUEST, and the EU FP7 Project SE²ND.

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45.In the studied temperature range $k T \sim Δ \sim k T_{c}$ , the closing of the transport gap for $T ≲ T_{c}$ plays already a significant role.

Subgap resonant quasiparticle transport in normal-superconductor quantum dot devices

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