ИСТИНА

Abstract When put in rotation, macroscopic quantum condensates develop a very peculiar collective response: Instead of rotating as a whole, they split in a huge number of small quantum tornados – vortices [1]. The vortex currents circulate owing to the gradient of the condensate wave function that accumulates exactly 2π phase difference around each vortex core - the eye of the cyclone - where the wave function vanishes. This general quantum phenomenon was observed in superconductors, superfluids, Bose-Einstein condensates of ultra-cold atoms [2]. Vortices may be seen as individual molecules: they repel each other and form a regular triangular lattice in bulk superconductors [1]. The confinement of quantum condensates to the scales comparable to their characteristic coherence length should modify the vortex lattice, leading to novel configurations. Despite of 45 years of theoretical efforts, till recently there have not been relevant experiments on this topic. Importantly, since the pioneering work by Hess et al. [3] the Scanning Tunneling Spectroscopy (STS) at low temperatures is widely used to visualize the vortices in superconductors and to study their cores. Till now however, the STS was mainly performed on atomically flat (very smooth) surfaces, the STS on high-relief samples being an important experimental challenge. Owing our home-made UHV STM/STS working down to 0.3K in magnetic fields up to 10T we succeeded to visualize the vortex phases strongly confined in individual superconducting nanocrystals of Pb deposited in-situ onto Si(111)-7x7 [4], or even pinned at individual atomic steps of superconducting single atomic layers of Pb on Si(111) [5]. Starting from the simplest case of a single vortex confined in a superconducting box [6], in our talk we will show how the confinement influences the vortex lattice leading to novel ultra-dense vortex configurations, impossible in bulk superconductors, such as vortex lines, molecules and clusters. At even higher confinement the Giant Vortices - quantum tornados characterized by a multiple phase accumulation L x 2π, L = 2; 3; 4 - are experimentally revealed by STS; their unusual cores will be discussed. References [1] Abrikosov, A. A. Sov. Physics JETP 5, 1174 (1957). [2] Essmann, U. & Trauble, H. Physics Letters 24A, 526 (1967); Yarmchuk, E. J., Gordon, M. J. V. & Packard, R. E. Phys. Rev. Lett. 43, 214217 (1979); Abo-Shaeer, J. R., Raman, C., Vogels, J. M. & Ketterle, Science 292, 476 (2001). [3] H. F. Hess et al., Phys. Rev. Lett. 62, 214 (1989). [4] Cren, T., et al., Phys. Rev. Lett. 107, 097202 (2011). [5] Brun, Ch., Nature Phys.10, 444–450 (2014) [6] Cren, T., et al., Phys. Rev. Lett. 102, 207002 (2009).