Friday 31 July 2015- Jim Valles – Summary talk

Jim starts his talk by proposing a general scheme to classify the several topics covered by the workshop:

1-LAO/STO interfaces, status and advances

2- Pursuing related issues in superconductivity

3- SIT Physics in Thin films

4- Disorder and the SIT

5- Novel experimental probes

6- Interfaces matter

1- LAO/STO interfaces, status and advances. Jim remarks his personal surprise in discovering the beautiful world of interfaces, where the beauty actually come out from plenty of different features at play: SC, magnetism, spin-orbit (SO) scattering, gate tunability. The superconductivity is not so conventional, as shown by experiments of tunneling (Mannhart), by the Hc anisotropy (Gariglio) and the critical behavior (Bergeal). In the last case the beauty is that one can scale several samples because of the tunability in Vg, not only one or two, and this makes definitively a big advantage with respect to other system. Magnetism: it can be enhanced with a layer of ETO put in between (Stornaiuolo), giving rise to an anomalous Hall effect. One can also use Nb doped STO to boost the superconductivity (Hwang). How can we use this tunability? One proposal came out of an intrinsic spin-Hall effect induced by a modulated Rashba coupling (Seibold), motivated by a general phase-separation model (Grilli) with a possible QCP.

2- Pursuing related issues in superconductivity. Jim reviews then some other fundamental issues of superconductivity, triggered or not by the LAO/STO physics. One is the effect of strong SO coupling on superconductivity (Michaeli). The mechanisms leading to a SC dome: the one shown for granular AL (Pracht) resembles something we have seen, is it the same as in other systems or not?  Can superconductivity be triggered by a Van Hove singularity in Nanotubes (Barbara)? Can we go towards artificially-generated “molecular” superconductors (Ilani)? Superconductivity can even occurs at the nanoscale (Bose): even rare-grain effects (Mason) can be relevant to the Tc .

3- SIT Physics in thin films. Here Jim shows something exciting from himself! One can pattern Bi films with holes. According to the modulation of the underlying substrate one obtains different R(T) (direct SIT or percolation). Jim then compares flat films with hilly ones. In the latter ones he sees quantum oscillations with 2e periodicity even on the insulating side of the transition, while these are not seen in the flat films. Conclusion: if one wants a Bose insulator one needs inhomogeneity. The persistence of pairing on the I side of the SIT has been seen often during the workshop. For example, in InOx samples with very different Tc’s loose the magnetoresistance peak at the same field (Shahar), real signatures of bosons surviving across the SIT. The same occurs in decorated graphene (Bouchiat), even if here strictly speaking there is a metal in between (and plateaus..).

4- Disorder and the SIT. The route is difficult (as shown by  an explicative slide by Svortsov on Monday!). Short-scale effects can affect strongly the non-universality. So while in Leridon’s talk we have seen that the fluctuations seem to suggest OD physics (due to grains?), different disorder realizations can lead to inhomogeneous currents (Castellani), they can affect the STM DOS (Misha), and they can explain the failure of the Mattis-Bardeen picture of the conductivity near the SIT (Armitage). Here again Jim goes back to his own work in NHC films and addresses a different type of disorder, i.e. the flux disorder. How does it affect the quantum critical transport at the SIT? Jim shown that this can be realized by fabricating hole arrays with varying geometrical order. The spreading in the hole size induces magnetic-field oscillations or flux disorder (it is not ‘exactly’ one flux for hole, up to 10-20 %). Such a flux disorder increases with the magnetic field B. This implies that one can identify plateaus in the resistance at several B, like if one had multiple B-induced SIT. More remarkable, one finds out that the critical resistance as a function of flux disorder increases up to the ‘universal’ Rc, so the expectation of an universal Rc is not realized.  

5- Novel experimental probes. Here we had several nice examples: scanning critical current microscopy on a nanowire (Driessen), non-Gaussian noise in NbN (Ghosh), light-induced superconductivity in cuprates (Wanzheng Hu), a novel four-probe technique for measuring resistance in ultrathin FeSe films (Jia).

6- Interfaces matter. Jim concludes with a very nice comment: “interfaces are our friends”. They can help: one can design a Josephson junction to explore the properties of a topological superconductor (Brinkman), or one has to use a STO substrate to enhance the (tunneling) Tc of FeSe (Wang).

Overall, Jim talk showed that a common language is possible for scientists working apparently on different materials, with different experimental probes, and speaking different theoretical ‘slangs’.  And people like to discuss when there are the right conditions to do that. With Jim’s talk we close this exciting workshop and we agree that we should definitively plan to have something similar in few more years.

Blogged by Lara Benfatto