According to the Bardeen, Cooper, Schrieffer theory of superconductivity a static electric field should not have any effect on the superconducting order parameter . Hence, it was a surprise when in a paper last year in Nature Nanotechnology , and subsequent publications [3, 4], Giazotto’s group presented a number of experimental works indicating that an electric field can significantly reduce the critical current in gated-superconducting Dayem bridges. According to the authors, the critical current of a Dayem bridge is suppressed on the application of a very strong electric field between the bridge and a capacitively-coupled gate electrode, separated by a gap of ∼ 100 nm. The effect is symmetrical with respect to the gate voltage, independent of the superconducting material used and is also observed in normal metal weak links under the influence of the proximity effect to the superconducting leads . The authors explain their experimental observations by the superconducting field effect (SFE), which is the major novel phenomena, and argue that the behaviour cannot be explained by current theory, and must imply an influence of the electric field on the superconducting order parameter. Since SFE has not been confirmed by other groups, we have made a complementary microwave experiments that shows similar behaviour to Giazotto’s observations, and can be straightforwardly explained by a simple overheating mechanism. This can also describe all the experimental results of Giazotto’s group [2, 4, 3, 5].  M. Tinkham, Introduction to Superconductivity: Second Edition., 2004.  Nature Nanotechnology 13, 802, (2018)  Nano Letters 18, 4195, (2018).  Phys. Rev. Applied 11, 024061 (2019).  ACS Nano 13, 7871 (2019).
I will give an overview of recent work on minimal models for quantum chaos in spatially extended many-body quantum systems, describing simple, solvable models. The detailed study of generic quantum systems -- ones without exact or approximate conservation laws -- goes back at least as far as investigations of highly-excited states in nuclei in the 1950s. It saw revivals in the 1980s with work on systems such as quantum billiards, that have only a few degrees of freedom, and also in the context of mesoscopic conductors. It is attracting renewed current interest with a focus on many-body systems that are spatially extended. The study of dynamics in spatially many-body systems introduces new questions that do not arise in finite systems such as quantum billiards, or in extended single- particle systems, such as mesoscopic conductors. These questions concern the dynamics of quantum information and the approach to equilibrium. I will discuss for minimal models both these new questions and established ways of characterising generic quantum systems via statistical properties of spectra. Based on: A. Chan, A. De Luca, and J. T. Chalker, Phys. Rev. Lett. 121, 060601 (2018) and Phys. Rev. X 8, 041019 (2018)
Optomagnonics studies the quantum-coherent coupling of light to collective magnetic excitations in solid state systems. The magnetic material hosting the magnetic excitations can be also used as an optical cavity if patterned appropriately. This not only enhances the magnon-photon coupling (making these systems promising for applications in quantum technologies) but also allows studying cavity-modified light-matter interaction in a novel platform. In my talk I will go over the basics of cavity optomagnonics and present results on recent theory developments in my group, including optomagnonics with magnetic textures, optical heralding of magnon Fock states, and antiferromagnetic cavity optomagnonics.
Spin liquids are collective phases of quantum matter that have eluded discovery in correlated magnetic materials for over half a century. In theory and experiment, there exist many beautiful models and plenty of promising candidate materials for their realization. However, one of the central challenges for the clear diagnosis of a quantum spin liquid (QSL) phase has been to understand the effect of disorder. From that perspective, this talk discusses three different roles of disorder: First, as a nuisance which obscures clear signatures of a QSL; Second, as a powerful new probe to uncover subtle QSL correlations; and Third, as a novel tuning parameter giving rise to qualitatively new disorder-QSL types. After a general introduction, the talk will focus on recent developments in Kitaev Spin Liquids.
Randomness leads to modification of the properties of low-dimensional spin-systems. Our study of time-reversal invariant quenched disorder on some frustrated spin-systems with non-collinear magnetic and spin-liquid ground states reveals how disorder acts to the detriment of the ordering, interferes with the topological properties of the ground state and causes the emergence of glassy behaviour. As examples, we have considered triangular lattice Heisenberg magnets and U(1) Dirac spin-liquids and found a pattern of disorder-induced destruction of the background order in two spatial dimensions. These results parallel the experimental efforts to uncover the low-temperature behaviour of various frustrated magnets.
Developing a theory of activated dynamics is one of the most challenging problems of disordered systems. Activated glassy dynamics is central in many different contexts both in physics and beyond, e.g. in computer science and biology. In this talk, after a general introduction, I will describe recent research works aimed at characterising the activated dynamics of mean-field glassy systems. In particular I will discuss numerical results on the random energy model and variants, and analytical results on the organization of barriers in the p-spin spherical model.