Suchergebnisse

We investigate the occurrence of Majorana modes in semiconductor quantum wires in close proximity with a superconductor and when both Rashba interaction and magnetic field are present. We consider long, but finite, multiband wires (namely, planar wires with dimensions L xL y). Our results demonstrate that interband mixing coming from the Rashba spin-orbit term hybridizes Majorana pairs originating from different transverse modes while simultaneously closing the effective gap. Consequently, multiple Majorana modes do not coexist in general. On the contrary, Majorana physics is robust provided that only one single transverse mode contributes to a Majorana pair. Finally, we analyze the robustness of Majorana physics with respect to magnetic orbital effects. © 2012 American Physical Society. ; This work was supported by Grants No. FIS2008-00781, No. FIS2009-08744, No. FIS2011-23526, and No. CSD2007- 00042 (CPAN) of the Spanish Government. ; Peer Reviewed

[EN] Zero-energy Andreev levels in hybrid semiconductor-superconductor nanowires mimic all expected Majorana phenomenology, including 2 e∕ h conductance quantisation, even where band topology predicts trivial phases. This surprising fact has been used to challenge the interpretation of various transport experiments in terms of Majorana zero modes. Here we show that the Andreev versus Majorana controversy is clarified when framed in the language of non-Hermitian topology, the natural description for quantum systems open to the environment. This change of paradigm allows one to understand topological transitions and the emergence of zero modes in more general systems than can be described by band topology. This is achieved by studying exceptional point bifurcations in the complex spectrum of the system's non-Hermitian Hamiltonian. Within this broader topological classification, Majoranas from both conventional band topology and a large subset of Andreev levels at zero energy are in fact topologically equivalent, which explains why they cannot be distinguished. ; Research supported by the Spanish Ministry of Science, Innovation and Universities through Grants PGC2018-097018-B-I00, FIS2015-65706-P, FIS2015-64654-P, FIS2016-80434-P (AEI/FEDER, EU), the FPI programme BES-2016-078122, the Ramón y Cajal programme Grants RYC-2011-09345, RYC-2013-14645, the María de Maeztu Programme for Units of Excellence in R&D (MDM-2014-0377), and the European Union's Horizon 2020 research and innovation programme under the FETOPEN Grant Agreement No. 828948. We also acknowledge support from CSIC Research Platform on Quantum Technologies PTI-001.

Self-sustained current oscillations in weakly coupled superlattices are studied by means of a self-consistent microscopic model of sequential tunneling, naturally including boundary conditions. Well-to-well hopping and recycling of charge monopole domain walls produce current spikes - high-frequency modulation - superimposed on the oscillation. For highly doped injecting contacts, the self-oscillations are due to the dynamics of monopoles. As the contact doping decreases, a lower-frequency oscillatory mode due to recycling and motion of charge dipoles is predicted. For low contact doping, this mode dominates and monopole oscillations disappear. At intermediate doping, both oscillation modes coexist as stable solutions and hysteresis between them is possible. ©1999 The American Physical Society. ; We thank Rosa Lo´pez and Professor Holger Grahn for helpful discussions. This work has been supported by the DGES ~Spain!, Grant Nos. PB97-0088 and PB96-0875, by the European Union TMR Contract Nos. ERB FMBX-CT97- 0157 and FMRX-CT98-0180, and by the Community of Madrid, Project No. 07N/0026/1998. ; Peer Reviewed