Suchergebnisse

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

6 pag., 4 figs. ; The understanding of the dynamics of nonequilibrium cooling and heating processes at the nanoscale is still an open problem. These processes can follow surprising relaxation paths due to, e.g., memory effects, which significantly alter the expected equilibration routes. The Kovacs effect can take place when a thermalization process is suddenly interrupted by a change of the bath temperature, leading to a nonmonotonic evolution of the energy of the system. Here, we demonstrate that the Kovacs effect can be observed in the thermalization of the center of mass motion of a levitated nanoparticle. The temperature is controlled during the experiment through an external source of white Gaussian noise that mimics an effective thermal bath at a temperature that can be changed faster than any relaxation time of the system. We describe our experiments in terms of the dynamics of a Brownian particle in a harmonic trap without any fitting parameter, suggesting that the Kovacs effect can appear in a large variety of systems. ; This research has been supported by European Union's Horizon 2020 Research and Innovation Programme under Grant No. 863132 (iQLev) and by the Swiss National Science Foundation through Grant No. 200021L-169319. A. L. acknowledges financial support by the FEDER/Ministerio de Ciencia, Innovación y Universidades—Agencia Estatal de Investigación, under Grants No. MTM2017-84446-C2-2-R and No. PID2020–116567GB-C22. L. L. B. acknowledges financial support by the FEDER/Ministerio de Ciencia, Innovación y Universidades—Agencia Estatal de Investigación, under Grants No. MTM2017-84446-C2-2-R and No. PID2020-112796RB-C22, the Madrid Government (Comunidad de Madrid-Spain) under the Multiannual Agreement with UC3M in the line of Excellence of University Professors (EPUC3M23), and in the context of the V PRICIT (Regional Programme of Research and Technological Innovation). R. A. R. acknowledges financial support from FEDER/Junta de Andalucía-Consejería de Economía y Conocimiento/Projects No. C-FQM-410- UGR18 and No. P18-FR-3583