- AG Belzig (Theoretische Physik mit SP Quantentransport)
|(2024): Interplay of Andreev Reflection and Coulomb Blockade in Hybrid Superconducting Single-Electron Transistors Physical Review Letters. Americal Physical Society (APS). 2024(132), 057001. ISSN 0031-9007. eISSN 1079-7114. Available under: doi: 10.1103/PhysRevLett.132.057001
Interplay of Andreev Reflection and Coulomb Blockade in Hybrid Superconducting Single-Electron Transistors
We study the interplay between Coulomb blockade and superconductivity in a tunable superconductor–superconductor–normal-metal single-electron transistor. The device is realized by connecting the superconducting island via an oxide barrier to the normal-metal lead and with a break junction to the superconducting lead. The latter enables Cooper pair transport and (multiple) Andreev reflection. We show that these processes are relevant also far above the superconducting gap and that signatures of Coulomb blockade may reoccur at high bias while they are absent for small bias in the strong-coupling regime. Our experimental findings agree with simulations using a rate equation approach in combination with the full counting statistics of multiple Andreev reflection.
|(2020): Electron cooling by phonons in superconducting proximity structures Physical Review B. American Physical Society. 2020, 102(21), 214514. ISSN 2469-9950. eISSN 2469-9969. Available under: doi: 10.1103/PhysRevB.102.214514
We investigate the electron-phonon cooling power in disordered electronic systems with a special focus on mesoscopic superconducting proximity structures. Employing the quasiclassical Keldysh Green's function method, we obtain a general expression for the cooling power perturbative in the electron-phonon coupling, but valid for arbitrary electronic systems out of equilibrium. We apply our theory to several disordered electronic systems valid for an arbitrary relation between the thermal phonon wavelength and the electronic mean free path due to impurity scattering. Besides recovering the known results for bulk normal metals and BCS superconductors, we consider two experimentally relevant geometries of superconductor-normal metal proximity contacts. Both structures feature a significantly suppressed cooling power at low temperatures related to the existence of a minigap in the quasiparticle spectrum. This improved isolation low cooling feature in combination with the high tunability makes such structures highly promising candidates for quantum calorimetry.
|(2019): Optimized proximity thermometer for ultrasensitive detection Physical Review Applied. American Physical Society (APS). 2019, 13(5), 054001. eISSN 2331-7019. Available under: doi: 10.1103/PhysRevApplied.13.054001
We present a set of experiments to optimize the performance of a noninvasive thermometer based on proximity superconductivity. Current through a standard tunnel junction between an aluminum superconductor and a copper electrode is controlled by the strength of the proximity induced to this normal metal, which in turn is determined by the position of a direct superconducting contact from the tunnel junction. Several devices with different distances are tested. We develop a theoretical model based on Usadel equations and dynamic Coulomb blockade that reproduces the measured results and yields a tool to calibrate the thermometer and to optimize it further in future experiments. We also propose an analytic formula that reproduces the experimental data for a wide range of temperatures.
|(2019): Local density of states in clean two-dimensional superconductor–normal metal–superconductor heterostructures Physical Review Research. 2019, 1(3), 033031. eISSN 2643-1564. Available under: doi: 10.1103/PhysRevResearch.1.033031
Local density of states in clean two-dimensional superconductor–normal metal–superconductor heterostructures
Motivated by recent advances in the fabrication of Josephson junctions in which the weak link is made of a low-dimensional nonsuperconducting material, we present here a systematic theoretical study of the local density of states (LDOS) in a clean two-dimensional normal metal (N) coupled to two s-wave superconductors (S). To be precise, we employ the quasiclassical theory of superconductivity in the clean limit, based on Eilenberger's equations, to investigate the phase-dependent LDOS as a function of factors such as the length or the width of the junction, a finite reflectivity, and a weak magnetic field. We show how the spectrum of Andreev bound states that appear inside the gap shape the phase-dependent LDOS in short and long junctions. We discuss the circumstances when a gap appears in the LDOS and when the continuum displays a significant phase dependence. The presence of a magnetic flux leads to a complex interference behavior, which is also reflected in the supercurrent-phase relation. Our results agree qualitatively with recent experiments on graphene SNS junctions. Finally, we show how the LDOS is connected to the supercurrent that can flow in these superconducting heterostructures and present an analytical relation between these two basic quantities.
|01.01.2018 – 31.12.2021