Puddle formation and persistent gaps across the non-mean-field breakdown of superconductivity in overdoped (Pb,Bi)2Sr2CuO6+δ.

The cuprate high-temperature superconductors exhibit many unexplained electronic phases, but the superconductivity at high doping is often believed to be governed by conventional mean-field Bardeen-Cooper-Schrieffer theory1. However, it was shown that the superfluid density vanishes when the transition temperature goes to zero2,3, in contradiction to expectations from Bardeen-Cooper-Schrieffer theory. Our scanning tunnelling spectroscopy measurements in the overdoped regime of the (Pb,Bi)2Sr2CuO6+δ high-temperature superconductor show that this is due to the emergence of nanoscale superconducting puddles in a metallic matrix4,5. Our measurements further reveal that this puddling is driven by gap filling instead of gap closing. The important implication is that it is not a diminishing pairing interaction that causes the breakdown of superconductivity. Unexpectedly, the measured gap-to-filling correlation also reveals that pair breaking by disorder does not play a dominant role and that the mechanism of superconductivity in overdoped cuprate superconductors is qualitatively different from conventional mean-field theory.

4π-periodic Andreev bound states in a Dirac semimetal.

Although signatures of superconductivity in Dirac semimetals have been reported, for instance by applying pressure or using point contacts, our understanding of the topological aspects of Dirac semimetal superconductivity is still developing. Here, we utilize nanoscale phase-sensitive junction technology to induce superconductivity in the Dirac semimetal Bi1-xSbx. Our radiofrequency irradiation experiments then reveal a significant contribution of 4π-periodic Andreev bound states to the supercurrent in Nb-Bi0.97Sb0.03-Nb Josephson junctions. The conditions for a substantial 4π contribution to the supercurrent are favourable because of the Dirac cone's very broad transmission resonances and a measurement frequency faster than the quasiparticle poisoning rate. In addition, we show that a magnetic field applied in the plane of the junction allows tuning of the Josephson junctions from 0 to π regimes. Our results open the technologically appealing avenue of employing the topological bulk properties of Dirac semimetals for topological superconductivity research and topological quantum computer development.

Electrochemical Scanning Tunneling Microscopy Study of Bismuth Chalcogenide Single Crystals.

We use electrochemical scanning tunneling microscopy (EC-STM) to image single-crystal surfaces of the layered bismuth chalcogenide Sn0.01Bi1.99Te2Se in situ under electrochemical control for the first time. The Bi chalcogenides are of interest for their thermoelectric properties and as model topological insulators (TIs). We show that oxidative dissolution takes place via the progressive nucleation of pits in the initially smooth surface terraces rather than at their edges. Nanometer-resolution EC-STM images show that the pit depth is generally equal to the thickness of a complete chalcogenide quintuple layer. The preferential redeposition of dissolved components at step and defect edges on application of a more negative potential after oxidation is observed. Our work demonstrates the ability to control and characterize the surface morphology of single-crystal TIs in an electrochemical environment.

Electrochemical Modification and Characterization of Topological Insulator Single Crystals.

We compare electrochemically modified or thiol-functionalized single-crystal samples of the topological insulator (TI) Bi2Te0.9Se2.1 to freshly cleaved/air-exposed control samples and use X-ray photoelectron spectroscopy (XPS) to investigate the extent of any surface oxidation. XPS spectra for a TI sample maintained at an appropriate potential for 2 h demonstrate the feasibility of protecting the TI surface from oxidation while working in an electrochemical environment. Deliberate electrochemical oxidation, in contrast, generates prominent Bi, Te, and Se peaks associated with oxidation. However, this change is reversible, as further XPS spectra following electrochemical reduction are similar to those measured for an in situ cleaved sample. XPS also shows that adsorption of pentanedithiol (PDT) protects the TI surface from oxidation. Cyclic voltammetry shows that PDT adsorption suppresses electrochemical oxidation and reduction, while electrochemical impedance spectroscopy shows that it increases the charge transfer resistance significantly. Our work demonstrates the ability to control and characterize the surface chemistry of single-crystal TIs in an electrochemical environment for the first time.

Existence, character, and origin of surface-related bands in the high temperature iron pnictide superconductor BaFe(2-x)Co(x)As2.

Low energy electron diffraction (LEED) experiments, LEED simulations, and finite slab density functional calculations are combined to study the cleavage surface of Co doped BaFe(2-x)Co(x)As2 (x = 0.1,0.17). We demonstrate that the energy dependence of the LEED data can only be understood from a terminating 1/2 Ba layer accompanied by distortions of the underlying As-Fe2-As block. As a result, surface-related Fe 3d states are present in the electronic structure, which we identify in angle resolved photoemission spectroscopy (ARPES) experiments. The close proximity of the surface-related states to the bulk bands inevitably leads to broadening of the ARPES signals, which excludes the use of the BaFe(2-x)Co(x)As2 system for accurate determination of self-energies using ARPES.