Charge Berezinskii-Kosterlitz-Thouless transition in superconducting NbTiN films

A half-century after the discovery of the superconductor-insulator transition (SIT), one of the fundamental predictions of the theory, the charge Berezinskii-Kosterlitz-Thouless (BKT) transition that is expected to occur at the insulating side of the SIT, has remained unobserved. The charge BKT transition is a phenomenon dual to the vortex BKT transition, which is at the heart of the very existence of two-dimensional superconductivity as a zero-resistance state appearing at finite temperatures. The dual picture points to the possibility of 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, identified by the critical behavior of the resistance. We find that the critical temperature of the charge BKT transition depends on the magnetic field exhibiting first the fast growth and then passing through the maximum at fields much less than the upper critical field. 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.Comment: 9 pages, 6 figure

Superconductivity in a disordered metal with Coulomb interactions

We study the electronic densities of states (DOS) of strongly disordered superconducting thin films of TiN. We find, using Scanning Tunneling Microscopy (STM) that the DOS decreases towards the Fermi level in the normal phase obtained by applying magnetic fields. The DOS shows spatial fluctuations whose length scale is related to the energy dependent DOS and is similar in normal and superconducting phases. This suggests that Coulomb interactions lead to a spatially varying DOS in the normal phase of a disordered superconductor

Multiple crossing points and possible quantum criticality in the magnetoresistance of thin TiN films

We have measured R(T,B) of a TiN thin film very close to the disorder-driven superconductor-insulator transition but still superconducting at zero field and low temperatures. In a magnetic field we find that three distinct crossing points of the magnetoresistance isotherms occur at magnetic fields BcX in three different temperature regions. Each crossing point in R(T,B) corresponds to a plateau in R(T,BcX). We systematically study the evolution of these crossing points near the disorder-induced superconductor/insulator transition, identify the most promising candidate for a quantum phase transition, and provide estimates for the two critical exponents z and ν

Gate-tunable electron interaction in high-κ dielectric films

The two-dimensional (2D) logarithmic character of Coulomb interaction between charges and the resulting logarithmic confinement is a remarkable inherent property of high dielectric constant (high-kappa) thin films with far reaching implications. Most and foremost, this is the charge Berezinskii-Kosterlitz-Thouless transition with the notable manifestation, low-temperature superinsulating topological phase. Here we show that the range of the confinement can be tuned by the external gate electrode and unravel a variety of electrostatic interactions in high-k films. We find that by reducing the distance from the gate to the film, we decrease the spatial range of the 2D long-range logarithmic interaction, changing it to predominantly dipolar or even to exponential one at lateral distances exceeding the dimension of the film-gate separation. Our findings offer a unique laboratory for the in-depth study of topological phase transitions and related phenomena that range from criticality of quantum metal- and superconductor-insulator transitions to the effects of charge-trapping and Coulomb scalability in memory nanodevices

Content of polyunsaturated fatty acids essential for fish nutrition in zoobenthos species

Content of eicosapentaenoic acid (EPA, 20 ∶ 5n-3) and docosahexaenoic acid (DHA, 22 ∶ 6n-3) and the n-3/n-6 ratio are important indicators of nutritive value of aquatic invertebrates as food for fish. We studied fatty acid (FA) content and composition of 68 zoobenthic species. Benthic invertebrates differed significantly in their contents of EPA and DHA and n-3/n-6 ratios. The most valuable food for fish were Insecta, especially Ephemeroptera and Trichoptera, Gammaridae, and Turbellaria (Dendrocaelopsis sp.). The invertebrates of low food quality for fish were Hirudinea, Mollusca, Oligochaeta, Bryozoa (Plumatella emarginata), and Coleoptera (Dytiscus lapponicus). Our data suggest that the recent and ongoing global replacement of native species, mainly Insecta, by invaders, mostly represented by Mollusca, Crustacea, Polychaeta, Oligochaeta, Bryozoa, and Hirudinea, probably will reduce the nutritive value of food sources for benthivorous fish. In addition, some of the taxa studied have peculiar FAs, which may be useful as their markers in trophic webs. Thus, 20 ∶ 1n-13, 20 ∶ 2n-6, 22 ∶ 5n-3, and 22 ∶ 6n-3 are likely to be considered markers of Mollusca, Hirudinea, Turbellaria, and Gammaridae, respectively, for tracing food webs in freshwater ecosystem