Charge Berezinskii-Kosterlitz-Thouless transition in superconducting NbTiN films.

Three decades after the prediction of charge-vortex duality in the critical vicinity of the two-dimensional superconductor-insulator transition (SIT), one of the fundamental implications of this duality-the charge Berezinskii-Kosterlitz-Thouless (BKT) transition that should occur on the insulating side of the SIT-has remained unobserved. The dual picture of the process points to the existence of a superinsulating state endowed with zero conductance at finite temperature. Here, we report the observation of the charge BKT transition on the insulating side of the SIT in 10 nm thick NbTiN films, identified by the BKT critical behavior of the temperature and magnetic field dependent resistance, and map out the magnetic-field dependence of the critical temperature of the charge BKT transition. Finally, we ascertain the effects of the finite electrostatic screening length and its divergence at the magnetic field-tuned approach to the superconductor-insulator transition.

A modular reactor design for in situ synchrotron x-ray investigation of atomic layer deposition processes.

Synchrotron characterization techniques provide some of the most powerful tools for the study of film structure and chemistry. The brilliance and tunability of the Advanced Photon Source allow access to scattering and spectroscopic techniques unavailable with in-house laboratory setups and provide the opportunity to probe various atomic layer deposition (ALD) processes in situ starting at the very first deposition cycle. Here, we present the design and implementation of a portable ALD instrument which possesses a modular reactor scheme that enables simple experimental switchover between various beamlines and characterization techniques. As first examples, we present in situ results for (1) X-ray surface scattering and reflectivity measurements of epitaxial ZnO ALD on sapphire, (2) grazing-incidence small angle scattering of MnO nucleation on silicon, and (3) grazing-incidence X-ray absorption spectroscopy of nucleation-regime Er2O3 ALD on amorphous ALD alumina and single crystalline sapphire.

Point contact tunneling spectroscopy apparatus for large scale mapping of surface superconducting properties.

We describe the design and testing of a point contact tunneling spectroscopy device that can measure material surface superconducting properties (i.e., the superconducting gap Δ and the critical temperature T(C)) and density of states over large surface areas with size up to mm(2). The tip lateral (X,Y) motion, mounted on a (X,Y,Z) piezo-stage, was calibrated on a patterned substrate consisting of Nb lines sputtered on a gold film using both normal (Al) and superconducting (PbSn) tips at 1.5 K. The tip vertical (Z) motion control enables some adjustment of the tip-sample junction resistance that can be measured over 7 orders of magnitudes from a quasi-ohmic regime (few hundred Ω) to the tunnel regime (from tens of kΩ up to few GΩ). The low noise electronic and LabVIEW program interface are also presented. The point contact regime and the large-scale motion capabilities are of particular interest for mapping and testing the superconducting properties of macroscopic scale superconductor-based devices.

Self-assembled tunable networks of sticky colloidal particles.

Surfaces decorated with dense arrays of microscopic fibres exhibit unique materials properties, including superhydrophobicity and low friction. Nature relies on 'hairy' surfaces to protect blood capillaries from wear and infection (endothelial glycocalyx). Here we report on the discovery of self-assembled tunable networks of microscopic polymer fibres ranging from wavy colloidal 'fur' to highly interconnected networks. The networks emerge via dynamic self-assembly in an alternating electric field from a non-aqueous suspension of 'sticky' polymeric colloidal particles with a controlled degree of polymerization. The resulting architectures are tuned by the frequency and amplitude of the electric field and surface properties of the particles. We demonstrate, using atomic layer deposition, that the networks can serve as a template for a transparent conductor. These self-assembled tunable materials are promising candidates for large surface area electrodes in batteries and organic photovoltaic cells, as well as for microfluidic sensors and filters.