Superconductivity in low dimensional systems

Resume : The electron liquid at the LaAlO3/SrTiO3 interface is a model system for the study of superconductivity as it provides a two-dimensional superconductor whose properties can be tuned with an electrical gate field. We developed planar tunnel junctions to study the superconductivity spectroscopically. Our tunnel junctions give access to two important physical parameters: the size of the superconducting gap and the electron-phonon spectral function. The gap increases with decreasing charge density in the entire phase diagram and persists up to temperatures far above Tc in the underdoped regime. These results are analogous to the pseudogap behavior of the high-transition-temperature copper oxide superconductors. The electron-phonon spectral function is independent of the carrier density and the likelihood of the conventional electron-phonon coupling mechanism for superconducting pairing will be discussed.

Resume : The role of the local charge carrier concentration profiles is paramount for the occurrence of superconductivity in low-dimensional systems. Atomic layer-by-layer oxide MBE technique together with state-of-the-art characterization methods have disclosed new ways for the synthesis of novel superconducting epitaxial heterostructures. In this contribution, we present the exciting findings of the study of Sr delta-doped La2CuO4. Delta-doping, which represents a novel approach in the field of layered complex oxides, here relies on the substitution of a full La-O atomic layer by a Sr-O layer. Artificial superlattices having different spacing between two subsequent Sr-O layers were grown on LaSrAlO4 substrates; by appropriately tuning the superlattice structure, high-Tc superconductivity up to about 40 K was obtained. In order to rationalize this effect, detailed studies were performed with a spherical aberration-corrected STEM, which allows imaging and spectroscopic analyses (EDX as well as EELS) of each atomic column. The results indicate that high-Tc superconductivity, confined in few unit cells across the delta-doped atomic layer, can be attributed to holes enrichment on both sides of the interface, while Sr cations undergo an asymmetric spatial redistribution from the nominal Sr-O plane, confined within ~3 unit cells in the growth direction. These findings, suggesting a rather complex mechanism of charge rearrangement, are discussed.

Resume : The conducting interface between the two band insulators LaAlO3 and SrTiO3 has drawn a large share of attention, as it presents a variety of exciting electronic properties that are tunable by an electric field [1]. At low temperatures, magnetotransport analysis has revealed a strong Rashba spin-orbit interaction originating from the breaking of inversion symmetry [2] and, in field effect devices, the ground state has been tuned from an insulating to a superconducting state. In this presentation I will discuss field effect experiments to probe the superconducting state and map the phase diagram of the system and its link with doped bulk SrTiO3. I will also present recent experiments on nanostructures that reveal a remarkable tuning of the electronic properties and allow weak localization and weak anti-localization as a function of doping and temperature to be followed [3]. Finally, I will discuss an approach that should allow superconducting coupling between different gases to be studied [4]. [1] A. D. Caviglia et al., Nature 456, 624 (2008). [2] A. D. Caviglia et al., Phys. Rev. Lett. 104, 126803 (2010); A. Fête et al., Phys. Rev. B 86, 201105 (2012). [3] D. Stornaiuolo et al. Appl. Phys. Lett 101, 222601 (2012). [4] D. Li et al. Appl. Phys Lett. Mat. 2, 012102 (2014).

Resume : Surfaces of three-dimensional (3D) topological insulators have emerged as one of the most remarkable states of condensed quantum matter. These surfaces host robust spin-protected helical Dirac particles that emerge from topological order and can support unusual electronic phases when electron correlations are at play. One particularly exciting phase is topological superconductivity which, if found, could lead to a paradigm change in quantum information. Here we report on high-temperature surface superconductivity of hole carriers we discovered in a 3D topological material Sb2Te3 by fine-tuning the crystal growth protocol. This new superconducting state has low bulk carrier density, huge carrier mobility, and a sharp diamagnetic (Meissner) onset near 55 K, at which also the local superconducting gap is observed by STM. The unconventional nature of this state is witnessed by the persistence of quantum oscillations inside the superconducting state and two-dimensional (2D) quantum beats arising from strong spin-orbit coupling. This unusual superconductivity resides in surface Dirac puddles, with global coherence mediated by interpuddle diffusion of quasiparticles. Our findings suggest that by tuning the material's parameters (e.g., the quasiparticle mean free path and the system's Fermi velocity) of this mesoscopic structure even higher temperature surface superconductivity may be achieved. (This work was supported in part by the NSF DMR-1122594 and DOD-W911NF-13-1-0159).