Site web ESPCI
Mardi 31 Mai 2011, 16h00
Amphi Boreau, Esc C, 2ème etage
Confinement-Induced Vortex Fusion and Giant Vortex States in Superconductors
Dimitri Roditchev
When put in rotation, macroscopic quantum condensates develop a very peculiar collective response : They split in a huge number of small quantum tornados - vortices - that organize in a lattice [1]. The vortex currents circulate owing to the gradient of the condensate wave function Ψ(r) that accumulates exactly 2π phase difference around each vortex core where Ψ(r) vanishes. This general quantum phenomenon was observed in superconductors [2], superfluids [3], Bose-Einstein condensates of ultra-cold atoms [4]. The confinement of quantum condensates to scales comparable to their characteristic coherence length should modify the vortex lattice, leading to novel vortex arrangements [5-9]. Moreover, new quantum objects - Giant Vortices, characterized by the phase accumulation L x 2π, L ≥ 2 (L being the orbital quantum number or winding factor) were predicted to exist owing to the spatial confinement of the superconducting condensate [8]. This prediction remained till now experimentally unconfirmed.
In my talk I will discuss the results of the recent direct Scanning Tunneling Spectroscopy (STS) experiment by our group in which we addressed the problem of the quantum vortex configurations in strongly confined superconductors. The strong confinement regime was achieved in in-situ grown ultra-thin single nanocrystals of Pb which lateral size was tuned to few coherence lengths. Upon an external magnetic field, the STS at 0.3K revealed novel ultra-dense arrangements of single Abrikosov vortices characterized by inter-vortex distance up to three times shorter than the bulk critical one. At yet stronger confinement we discovered the Giant Vortex phase ; the spatial evolution of the excitation tunneling spectra in the cores of these unusual quantum objects was explored. I will show that the magnetic energy is not relevant in the observed phenomena and thus, the observed Giant Vortex phase should be a common feature of all confined quantum condensates such as superfluids, Bose-Einstein condensates of cold atoms etc.
[1] A. A. Abrikosov, Soviet Physics JETP 5, 1174 (1957) ; Nobel Lecture (2003).
[2] U. Essmann, H. Trauble, Physics Letters 24A, 526 (1967).
[3] E. J. Yarmchuk, M. J. V. Gordon, R. E. Packard, Phys. Rev. Lett. 43, 214217 (1979).
[4] J. R. Abo-Shaeer, C. Raman, J. M. Vogels, W. Ketterle, 292, 476 (2001).
[5] D. Saint-James, Phys. Lett. 15, 1 13 (1965).
[6] H. J. Fink, A. G. Presson, Phys. Rev. 151, 219 (1966).
[7] P. S. Deo, V. A. Schweigert, F. M. Peeters, A. K. Geim, Phys. Rev. Lett. 79, 4653 (1997).
[8] V. A. Schweigert, F. M. Peeters, P. Singha Deo, Phys. Rev. Lett. 81, 2783 (1998).
[9] L. F. Chibotaru, A. Ceulemans, V. Bruyndoncx, V. V. Moshchalkov, Nature 408, 833 (2000).