Session L1: Design and Control of the Superconducting Order Parameter in Low Dimensions

Gate tunability and collapse of superconductivity in hybrid tin-graphene Josephson junction arrays

The accessible and surface-exposed 2D electron gas offered by graphene provides indeed an ideal platform on which to tune, via application of an electrostatic gate, the coupling between adsorbates deposited on its surface. We have experimentally studied the case of graphene transistors which channel is decorated with an array of superconducting tin nanoparticles. They induce via percolation of proximity effect a global 2D superconducting state which critical temperature Tc can be tuned by gate voltage. When the Graphene show strong disorder, it is possible to tune via the applied gate voltage the system towards an insulating state, demonstrating the possibility to trigger a superconducting to insulator transition [1], which features ressembles those found in granular superconductors. In this work, graphene monolayers are surface-conjugated to regular arrays of superconducting disk-shaped metal islands, whose inter-island distances were patterned to be in the quasi-ballistic limit of the underlying 2D electron gas. Arrays can be made on a large range of geometry and density, up to the highly diluted limit with less than 5{\%} surface coverage and few micrometers in between islands. In the lower temperature limit (\textless 100 mK) , despite of the long distance (2 microns) in between islands, a supercurrent was observed among the whole graphene sheet. Interestingly, the superconducting state vanishes exponentially in gate voltage and rests in a metallic state [2], caused by quantum fluctuations of phase is found for diluted and regular arrays. This peculiar behaviour provides evidence for recently developed theory, and may provide a hint to the understanding of long-standing issue of ``zero-temperature'' bosonic metallic state. [1] A. Allain, et al. Nat. Mat.,\textbf{11}, 590, (2012). [2] Z. Han et al., Nat. Phys. \textbf{10}, 380 (2014).   

The Emergence of Superconductivity in Inhomogeneous, Mesoscopic Systems

Although low-dimensional, inhomogeneous superconductors have been intensely studied, the nature of the onset of superconductivity in these systems is still largely unknown. In this talk we present transport measurements on mesoscopic disks of granular, inhomogeneous Nb, where we determine the superconducting transition temperature as a function of disk diameter. We observe an unexpected suppression of superconductivity at micron diameters, length scales that are considerably longer than the coherence length of Nb. This suppression does not appear in large-scale films, and cannot be explained by single-grain small-size effects. By considering the diameter-dependence of the transition, as well as observations of strong fluctuations in the transition temperature as disk diameters decrease, we are able to explain this long length scale dependence by an extremal-grain model, where superconducting order first appears in unusually large grains and, due to proximity coupling, spreads to other grains. The extremal-grain onset of superconductivity has not previously been observed experimentally, and explains how superconductivity can emerge in granular or inhomogeneous superconductors.   

Cooper Pair Insulators

One of the recent advances in the field of the Superconductor to Insulator Transition (SIT) has been the discovery and characterization of the Cooper Pair Insulator phase. This bosonic insulator, which consists of localized Cooper pairs, exhibits activated transport and a giant magneto-resistance peak. These features differ markedly from the weakly localized transport that emerges as pairs break at a ``fermionic'' SIT. I will describe how our experiments on films nano-patterned with a nearly triangular array of holes have enabled us to 1) distinguish bosonic insulators from fermionic insulators, 2) show that Cooper pairs, rather than quasi-particles dominate the transport in the Cooper Pair insulator phase, 3) demonstrate that very weak, sub nano-meter thickness inhomogeneities control whether a bosonic or fermionic insulator forms at an SIT and 4) reveal that Cooper pairs disintegrate rather than becoming more tightly bound deep in the localized phase. We have also developed a method, using a magnetic field, to tune flux disorder reversibly in these films. I will present our latest results on the influence of magnetic flux disorder and random gauge fields on phenomena near bosonic SITs.   

Designing Quantum Matter with Superconducting Nanowires

Superconducting nanowires are an experimental realization of a model quantum system that features collective degrees of freedom and exhibits a host of non-equilibrium and non-local phenomena. The nature of the quantum states in nanowires is particularly sensitive to size and shape quantization, coupling with the environment and proximity effects. I will demonstrate how we can utilize these features to tailor the quantum states in nanowires in desirable ways. Specifically for this purpose, we have developed a unique nanoprinting method for fabrication of ultranarrow nanowires with unprecedented control over their physical texture and their transport properties. I will show how short nanowires exhibit a tunable vortex-in-a-box blockade phenomenon, and how tunable interfaces with graphene and topological insulators lead to unusual properties. Finally, I will discuss the bigger picture for how the texture of the superconducting wavefunction can be precisely controlled by the size, shape, magnetic field and tunable interfaces with materials that exhibit unconventional order, spin texture or topological properties.   

Proximity and Anti-proximity effects in nanowires

Near a superconductor/normal-metal interface, the leakage of Cooper pairs extends superconducting behavior into the metal. The spatial range of this proximity effect in a normal metal can be as long as 1 \textmu m. However, when a ferromagnet is placed in contact with a superconductor, the Cooper pairs from the superconductor are not expected to survive beyond at most a few nanometers into the ferromagnet. Surprisingly we find when a cobalt nanowire as long as 600 nm is sandwiched between superconducting electrodes, it attains zero resistance at low temperature. For even longer wires, the transition to incomplete superconductivity via this (long range) proximity effect is foreshadowed by a large resistance peak (1). On the other hand when Zn nanowires of 40 nm diameter are contacted by superconducting electrodes, their superconductivity is unexpectedly suppressed (2). 1. Wang et al., PRL 102, 247003(2009); Nature Phys. 6, 389 ( 2010) 2. Tian et al., PRL 95, 076802 (2005); PRB 88, 064511 (2013)