# Workshops

## Novel Phenomena in Multi-Condensate Superconductors, Superfluids, and Ultracold Gases

### Organisers

- Milorad Milosevic
*(Universiteit Antwerpen, Belgium)* - François Peeters
*(University of Antwerp, Belgium)* - Andrea Perali
*(University of Camerino, Italy)* - Kazushige Machida
*(Okayama University, Japan)*

### Supports

CECAM

Flemish Science Foundation

### Description

**Multiband superconductivity** arises when the gap amplitudes on different sheets of the Fermi surface are radically disparate, e.g. due to different dimensionality of the bands for the usual phonon- mediated pairing, as is the case in MgB_{2} [1,2], or due to the repulsive pairing interaction, as it appears to be the case in recently discovered iron-pnictides [3]. The other examples of multi-gap materials include OsB_{2}, iron silicides such as Lu_{2}Fe_{3}Si_{5}, and chalcogenides (NbSe_{2}). In all these materials it is of abiding interest to understand how multiple electronic condensates affect the macroscopic properties of the samples, but also in which cases and for which phenomena the condensates work cooperatively and in which cases destructively, and this is one of the key objectives of our Workshop. We will first discuss the effect of the multiband character of a material on its critical properties – i.e. critical field, current and temperature depending on the microscopic parameters of the material and the introduced nanoengineered pinning, in analogy with what is known for single-band superconductors. However, the field of vortex matter is probably the best polygon for testing the interplay of multiple, coupled condensates in multiband materials, and it will also be represented in the Workshop. Namely, in multiband mesoscopic superconductors, one has a tantalizing possibility of stabilization of fractional-flux vortices [4]. The basis for this phenomenon comes from different length scales of different condensates in the sample [5], and therefore their different interaction with the imposed confinement. Even in bulk, vortex matter may hold surprises, in cases when different condensates have very different magnetic properties, which can lead to vortex behavior and complicated vortex patterns - unobtainable otherwise [6]. Finally, due to all above properties of penetrating flux quanta different from conventional (single-band) understanding, one expects special dynamic properties of bulk and nanopatterned materials, and those will also be addressed during the Workshop.

Such interplay of multiple electronic condensates in a superconductor is a very rich topic, but it is also very timely to study, since the latest discovered high-T_{c} superconductors, iron-pnictides often exhibit signatures of multi-band superconductivity [7], as do the nanoscale-confined conventional superconductors (such as few monolayers thick Pb [8-10]) and confined superfluids. The latter is induced via the quantization of the electron motion perpendicular to the nanoscale confinement. Quantum confinement of the perpendicular motion of electrons splits the conduction band of the sample into a series of subbands. This leads naturally to quantum-confinement-engineered multiband superconductivity or superfluidity! In the case of strongly confined atomic Fermi gases the single-particle levels can be widely and very precisely manipulated by laser light or magnetic fields through, e.g., changing the spatial dimensions of a trap, and the physical picture (see Refs. [11-13]) turns out to be similar to the case of high-quality superconducting nanowires and nanofilms. In this Workshop, we will discuss how those single-electron subbands can be manipulated in energy depending on the characteristic dimensions, fabrication conditions, etc., and how such manipulation can lead to resonances [13], or a crossover from the Bardeen-Cooper-Schrieffer (BCS) regime to the Bose-Einstein condensation (BEC) induced by quantum confinement.

When mentioning BEC, it is known that recent experiments with ultracold trapped atoms offer a systematic probe of the effects of fermionic and bosonic strong many-body correlations. The knowledge of the Hamiltonian of the atomic systems and its tunability in terms of parameters of the interactions or kinematic properties make it possible to compare theoretical predictions and experimental findings with high level of accuracy, and investigate novel quantum phases of matter which cannot be realized in solid state systems [15]. In the last years research efforts have been shifting to **multicomponent **ultracold atomic systems where several quantum particles with different statistics or with different properties (mass, density, magnetic moment, dipolarity, interaction with other particles, etc.) participate in the occurrence of novel and interesting quantum collective phenomena. In this Workshop, we are going to consider: (i) fermions with spin population imbalance [16]; (ii) fermi-fermi and fermi-bose mixtures with equal or unequal masses and equal or unequal spin populations [17,18]; (iii) dipolar molecules in multiple pancake geometries [19]; etc.

All these issues are of fundamental importance not only for the field of ultracold gases and superfluids but also for theorists and experimentalists in the field of the multiband superconductivity, which gives an enclosed form to this Workshop. Note also that this makes our Workshop very interdisciplinary, linking different scientific communities, with typically weak interaction but actually closely bound by scientific interests. The main goal of this workshop is to bring together key experimentalists and theorists working or about to work on above subjects. Although mentioned systems are closely related to each other, their scientific communities act very separately. In this Workshop, we aim to change the latter, and cross-fertilize each other fields by reaching agreement on fundamental issues, by discussing recent achievements and possibilities in experiments and computational models, and by defining emerging topics of common interest for the years to come.

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