May 2017: Since the discovery of the quantum Hall effects, topological insulators and superconductors have attracted considerable interest in the field of condensed-matter physics. Such topological states of matter can be generated through intrinsic properties of materials; this is the case of topological insulators, which arise due to the large intrinsic spin-orbit coupling of specific materials. Another intriguing route towards topological phases consists in subjecting materials to time-periodic modulations, a strategy referred to as Floquet engineering. This was proposed as a powerful tool to induce topological effects in graphene, through well-designed irradiation. Recently, this method was demonstrated in artificial materials, such as 2D ultracold gases and photonic crystals.
However, optimizing the loading of particles into a given (target) Floquet band (i.e. a band associated with the effective band structure of a driven system) still constitutes a crucial issue, both for real and artificial systems. Indeed, changing the topological nature of Floquet Bloch bands from trivial to non-trivial, by progressively launching the time-modulation, is necessarily accompanied with gap-closing processes: this has important consequences for the loading of particles into a target Floquet band with non-trivial topology, and hence, on the subsequent transport measurements.
In a recent paper, published in the journal 2D Materials, ICFO researcher Alexandre Dauphin, ICREA Prof. Maciej Lewenstein at ICFO, in collaboration with researchers Duc Thanh Tran and Prof. Nathan Goldman from Université Libre de Bruxelles, have analysed how loading protocols can be optimized in view of probing the topology of 2D Floquet bands. In addition, they have investigated how realistic loading sequences can influence transport measurements. In particular, they have highlighted how distinct observables can react very differently to undesired effects, such as those induced by micro-motion due to the drive and inter-band effects associated with the loading sequence.
Most importantly, their study demonstrates how observing the center-of-mass displacement of a 2D atomic cloud (artificial graphene) can offer a reliable method to detect the topology of Floquet bands, after realistic loading sequences, in contrast to current measurements, which are shown to be highly irregular due to the aforementioned detrimental effects associated with the drive.
A. Dauphin, D.-T. Tran, M. Lewenstein, N. Goldman, “Loading ultracold gases in topological Floquet bands: the fate of current and center-of-mass responses”, 2D Mater. 4, 024010 (2017).
See also the web page of the ICFO group