| 1. |
- Cerbu, D., et al.
(author)
-
Vortex ratchet induced by controlled edge roughness
- 2013
-
In: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 15
-
Journal article (peer-reviewed)abstract
- We demonstrate theoretically and experimentally the generation of rectified mean vortex displacement resulting from a controlled difference between the surface barriers at the opposite borders of a superconducting strip. Our investigation focuses on Al superconducting strips where, in one of the two sample borders, a saw tooth-like array of micro-indentations has been imprinted. The origin of the vortex ratchet effect is based on the fact that (i) the onset of vortex motion is mainly governed by the entrance/nucleation of vortices and (ii) the current lines bunching produced by the indentations facilitates the entrance/nucleation of vortices. Only for one current direction the indentations are positioned at the side of vortex entry and the onset of the resistive regime is lowered compared to the opposite current direction. This investigation points to the relevance of ubiquitous border effects typically neglected when interpreting vortex ratchet measurements on samples with arrays of local asymmetric pinning sites.
|
|
| 2. |
- Moskalenko, S. A., et al.
(author)
-
Nambu-Goldstone modes of the two-dimensional Bose-Einstein condensed magnetoexcitons
- 2012
-
In: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 85:10, s. 359-
-
Journal article (peer-reviewed)abstract
- The collective elementary excitations of two-dimensional magnetoexcitons in a Bose-Einstein condensate (BEC) with wave vector κ = 0 were investigated in the framework of the Bogoliubov theory of quasiaverages. The Hamiltonian of the electrons and holes lying in the lowest Landau levels (LLLs) contains supplementary interactions due to virtual quantum transitions of the particles to the excited Landau levels (ELLs) and back. As a result, the interaction between the magnetoexcitons with κ = 0 does not vanish and their BEC becomes stable. The equations of motion for the exciton operators d(P) and d†(P) are interconnected with equations of motion for the density operators ρ(P) and D(P). Instead of a set of two equations of motion, as in the case of usual Bose gas, corresponding to normal and abnormal Green's functions, we have a set of four equations of motion. This means we have to deal simultaneously with four branches of the energy spectrum, the two supplementary branches being the optical plasmon branch represented by the operator ρ(P) and the acoustical plasmon branch represented by the operator D(P). The perturbation theory on the small parameter v 2(1 - v 2), where v 2 is the filling factor and (1 - v 2) is the phase space filling factor was developed. The energy spectrum contains only one gapless, true Nambu-Goldstone (NG) mode of the second kind with dependence ω(κ) ≈ κ 2 at small values κ describing the optical-plasmon-type oscillations. There are two exciton-type branches corresponding to normal and abnormal Green's functions. Both modes are gapped with roton-type segments at intermediary values of the wave vectors and can be named as quasi-NG modes. The fourth branch is the acoustical plasmontype mode with absolute instability in the region of small and intermediary values of the wave vectors. All branches have a saturation-type dependencies at great values of the wave vectors. The number and the kind of the true NG modes is in accordance with the number of the broken symmetry operators. The comparison of the results concerning two Bose-Einstein condensates namely of the coplanar magnetoexcitons and of the quantum Hall excitons in the bilayer electron systems reveals their similarity.
|
|
| 3. |
- Moskalenko, S. A., et al.
(author)
-
True, quasi and unstable Nambu-Goldstone modes of the two-dimensional Bose-Einstein condensed magnetoexcitons
- 2013
-
In: Solid State Communications. - : Elsevier BV. - 0038-1098 .- 1879-2766. ; 155, s. 57-61
-
Journal article (peer-reviewed)abstract
- The collective elementary excitations of two-dimensional magnetoexcitons in a Bose-Einstein condensate (BEC) with wave vector k→=0 were investigated in the framework of the Bogoliubov theory of quasi-averages. The Hamiltonian of the electrons and holes lying in the lowest Landau levels (LLLs) contains supplementary interactions due to virtual quantum transitions of the particles to the excited Landau levels (ELLs) and back. As a result, the interaction between the magnetoexcitons with k→=0 does not vanish and their BEC becomes stable. The energy spectrum contains only one gapless, true Nambu-Goldstone (NG) mode of the second kind with dependence ω(k)≈k2 at small values k describing the optical-plasmon-type oscillations. There are two exciton-type branches corresponding to normal and abnormal Green's functions. Both modes are gapped with roton-type segments at intermediary values of the wave vectors and can be named as quasi-NG modes. The fourth branch is the acoustical plasmon-type mode with absolute instability in the region of small and intermediary values of the wave vectors. All branches have a saturation-type dependencies at great values of the wave vectors. The number and the kind of the true NG modes are in accordance with the number of the broken symmetry operators.
|
|
