SFB 767 - TP C03 "Time-dependent transport in correlated in electron nanostructures"
The goal of the project was to develop new theoretical methods for time-dependent transport through nanoscale junctions. We have first shown that the measurement of the quantum electric noise of the current is an intrinsic quantum mechanical measurement problem, which requires to take into account the measurement apparatus and implies that an ideal detector has to add a certain amount of noise to avoid the problem of backaction. Next we have investigated the effect of a time-dependent bias voltage on the statistics of the charge transfer and were able to show that in general at finite temperature it is impossible to separate the statistics into only single- and two-particle events. Finally we have considered the correction to the current noise of a molecular contact due to electron-phonon interaction. We have predicted that the noise for a junction consisting of a deuterium molecule between metallic contacts increases significantly.
- FB Physik
- SFB 767 Kontrollierte Nanosysteme
- AG Belzig (Theoretische Physik mit SP Quantentransport)
|(2018): Non-equilibrium charge and spin transport in superconducting–ferromagnetic–superconducting point contacts Philosophical Transactions of the Royal Society of London, Series A : Mathematical, Physical and Engineering Sciences. 2018, 376(2125), 20150229. ISSN 1364-503X. eISSN 1471-2962. Available under: doi: 10.1098/rsta.2015.0229||
Non-equilibrium charge and spin transport in superconducting–ferromagnetic–superconducting point contacts
The conventional Josephson effect may be modified by introducing spin-active scattering in the interface-layer of the junction. Here, we discuss a Josephson junction consisting of two s-wave superconducting leads coupled over a classical spin that precesses with the Larmor frequency due to an external magnetic field. This magnetically active interface results in a time-dependent boundary condition with different tunnelling amplitudes for spin-up and -down quasiparticles and where the precession produces spin-flip scattering processes. As a result, the Andreev states develop sidebands and a non-equilibrium population that depend on the details of the spin precession. The Andreev states carry a steady-state Josephson charge current and a time-dependent spin current, whose current-phase relations could be used for characterising the precessing spin. The spin current is supported by spin-triplet correlations induced by the spin precession and creates a feed-back effect on the classical spin in the form of a torque that shifts the precession frequency. By applying a bias voltage, the Josephson frequency adds another complexity to the situation and may create resonances together with the Larmor frequency. These Shapiro resonances are manifested as torques and are, under suitable conditions, able to reverse the direction of the classical spin in sub-nanosecond time. Another characteristic feature is the subharmonic gap structure in the dc charge current displaying an even-odd effect that is attributable to precession-assisted multiple Andreev reflections.
|(2016): Ground-State Cooling of a Mechanical Oscillator by Interference in Andreev Reflection Physical Review Letters. 2016, 117, 197202. ISSN 0031-9007. eISSN 1079-7114. Available under: doi: 10.1103/PhysRevLett.117.197202||
We study the ground-state cooling of a mechanical oscillator linearly coupled to the charge of a quantum dot inserted between a normal metal and a superconducting contact. Such a system can be realized, e.g., by a suspended carbon nanotube quantum dot with a capacitive coupling to a gate contact. Focusing on the subgap transport regime, we analyze the inelastic Andreev reflections which drive the resonator to a nonequilibrium state. For small coupling, we obtain that vibration-assisted reflections can occur through two distinct interference paths. The interference determines the ratio between the rates of absorption and emission of vibrational energy quanta. We show that ground-state cooling of the mechanical oscillator can be achieved for many of the oscillator's modes simultaneously or for single modes selectively, depending on the experimentally tunable coupling to the superconductor.
|Laufzeit:||01.01.2008 – 31.12.2019|