Paper published in PRB: Electronic and structural reconstructions of the polar (111) SrTiO 3 surface X. Torrelles, G. Cantele, G. M. De Luca, R. Di Capua, J. Drnec, R. Felici, D. Ninno, G. Herranz, and M. Salluzzo Phys. Rev. B 99, 205421 – Published 16 May 2019


Polar surfaces are known to be unstable due to the divergence of the surface electrostatic energy. Here we report on the experimental determination, by grazing incidence x-ray diffraction, of the surface structure of polar Ti-terminated (111) SrTiO3 single crystals. We find that the polar instability of the 1×1 surface is solved by a pure electronic reconstruction mechanism, which induces out-of-plane ionic displacements typical of the polar response of SrTiO3 layers to an electron confining potential. On the other hand, the surface instability can be also eliminated by a structural reconstruction driven by a change in the surface stoichiometry, which induces a variety of 3×3 (111) SrTiO3 surfaces consisting in an incomplete Ti (surface)–O2 (subsurface) layer covering the 1×1 Ti-terminated (111) SrTiO3 truncated crystal. In both cases, the TiO6octahedra are characterized by trigonal distortions affecting the structural and the electronic symmetry of several unit cells from the surface. These findings show that the stabilization of the polar (111) SrTiO3 surface can lead to the formation of quasi-two-dimensional electron systems characterized by radically different ground states which depend on the surface reconstructions.

Paper published in Nature Materials: Gap suppression at a Lifshitz transition in a multi-condensate superconductor

In a superconductor, electrons form Cooper pairs that merge to build a quantum collective state or condensate, adopting the same phase as if all the particles could be described as a unique wave with the same energy. Put in other words, all particles become identical from a physical point of view, and the whole group starts behaving as though it were a single particle. The idea of a large number of quantum particles forming a collective state was developed by Bose and Einstein in the 1920’s for the case of diluted atomic gases and the concept has been extended since then to other systems, including superconductors and superfluids.

Contrary to an ordinary metal, where a vanishingly small energy is enough to excite an electron and change its energy, a nonzero energy is required to break the electron pairs in a superconductor condensate. This is precisely why a superconductor has a zero electric resistance, since any energy below a critical value cannot break the electron pair, so that electrons cannot be scattered out of the collective quantum state. The minimum energy to break a Cooper pair is called the gap. In the simplest case, a single electronic band contributes to form the Cooper pair, and the gap is defined univocally. However, when Cooper pairs can form from different electron bands, a multi-condensate superconductor (also called multi-gapped) may emerge.

Several materials may exhibit multi-condensate behavior, including cuprates, pnictides or 2D- superconductors. A particular case is the 2D-electon gas (2DEG) at the interface between SrTiO3 (STO) and LaAlO3 (LAO). Figure 1 shows a schematic depiction of the different electronic states and bands associated with this 2DEG for two different crystallographic orientations (G. Herranz et al., Nature Communications 2015). Because of the multiple-band structure, the 2DEG at LAO/STO is a candidate for a multi-condensate state.

Figure 5

Figure 1. From  Herranz et al., Nature Comms 2015

So far, it has been predicted theoretically that the superconductivity of LAO/STO may involve two condensates and that a repulsive coupling exists between the two condensates. These predictions have awaited so far an experimental confirmation.

This is precisely the fundamental novelty of our work, namely, the first experimental demonstration of multi-condensate superconductivity in a SrTiO3-based system. We stress that SrTiO3 is a unique superconductor, as it the most dilute known superconductor, with the onset of superconductivity at carrier densities orders of magnitude lower than any other superconductor. Unsurprisingly, the origin of superconductivity in this material is still nowadays a matter of intense debate that started 50 years ago.

Our work provides novel and fundamental pieces of information relevant to this longstanding debate. A key aspect of our study is the observation, for the first time, of a transition between a regime of single-condensate superconductivity and a regime of two-condensate superconductivity. The experiments were done using resonant microwave transport characterization and were carried out in the Laboratoire de Physique et d’Etude des Matériaux, ESPCI Paris (Dr. Nicolas Bergeal), enabling continuous and reversible transitions between the two condensate regimes via electrostatic gating, which enables to control electrostatically the orbital occupation of the electronic states depicted in Figure 1. Our results can be interpreted consistently with unconventional s+-wave pairing, where the interaction between the two condensates is repulsive. This result is of broad interest for the field of superconductivity since this exotic state is the subject of considerable attention in other superconductor families including iron-pnictides and chalcogenides.



Xavi Domingo: Project “Neuromorphic Photonic Devices using Photoconductive Quantum Wells

Xavi will work on designing photoconductive wells to generate output electric signals mimicking the spiking activity of biological neurons.



Òscar Díaz: Project “Neuromorphic Photonic Devices using Photoconductive Quantum Wells

Òscar will work on the simulation of photoconductive responses of nonbiological synapses based on photoconductive quantum wells



Guillem Müller: Project “Topological Photonic Metasurfaces


Guillem will work on the design of topological photonic metasurfaces, with chiral and helical edge propagation modes

Paper published in Nature Physics about “Giant topological Hall Effect”

Our group has collaborated in a study published in Nature Physics about a “Giant topological Hall Effect”

The study, led by the group of Manuel Bibes, from Unité Mixte de Physique CNRS/Thales (France), published in Nature Physics on October 15, 2018, describes the discovery of a giant topological Hall effect arising from the topology of spatial distribution of spin textures.