Tatyana I. Baturina

Disorder-induced inhomogeneities of the superconducting state close to the superconductor-insulator transition.

Scanning tunneling spectroscopy at very low temperatures on homogeneously disordered superconducting titanium nitride thin films reveals strong spatial inhomogeneities of the superconducting gap Delta in the density of states. Upon increasing disorder, we observe suppression of the superconducting critical temperature Tc towards zero, enhancement of spatial fluctuations in Delta, and growth of the Delta/Tc ratio. These findings suggest that local superconductivity survives across the disorder-driven superconductor-insulator transition.

Superinsulator and quantum synchronization.

Synchronized oscillators are ubiquitous in nature, and synchronization plays a key part in various classical and quantum phenomena. Several experiments have shown that in thin superconducting films, disorder enforces the droplet-like electronic texture—superconducting islands immersed into a normal matrix—and that tuning disorder drives the system from superconducting to insulating behaviour. In the vicinity of the transition, a distinct state forms: a Cooper-pair insulator, with thermally activated conductivity. It results from synchronization of the phase of the superconducting order parameter at the islands across the whole system. Here we show that at a certain finite temperature, a Cooper-pair insulator undergoes a transition to a superinsulating state with infinite resistance. We present experimental evidence of this transition in titanium nitride films and show that the superinsulating state is dual to the superconducting state: it is destroyed by a sufficiently strong critical magnetic field, and breaks down at some critical voltage that is analogous to the critical current in superconductors.

Pseudogap in a thin film of a conventional superconductor

A superconducting state is characterized by the gap in the electronic density of states, which vanishes at the superconducting transition temperature T(c). It was discovered that in high-temperature superconductors, a noticeable depression in the density of states, the pseudogap, still remains even at temperatures above T(c). Here, we show that a pseudogap exists in a conventional superconductor, ultrathin titanium nitride films, over a wide range of temperatures above T(c). Our study reveals that this pseudogap state is induced by superconducting fluctuations and favoured by two-dimensionality and by the proximity to the transition to the insulating state. A general character of the observed phenomenon provides a powerful tool to discriminate between fluctuations as the origin of the pseudogap state and other contributions in the layered high-temperature superconductor compounds.

Collective Cooper-pair transport in the insulating state of Josephson-junction arrays.

We investigate collective Cooper-pair transport of one- and two-dimensional Josephson-junction arrays. We derive an analytical expression for the current-voltage characteristic revealing thermally activated conductivity at small voltages and threshold voltage depinning. The activation energy and the related depinning voltage represent a dynamic Coulomb barrier for collective charge transfer over the whole system and scale with the system size. We show that both quantities are nonmonotonic functions of the magnetic field. We propose that formation of the dynamic Coulomb barrier and its size scaling are consequences of the mutual Josephson phase synchronization across the system. We apply the results for interpretation of experimental data in disordered films near the superconductor-insulator transition.

Superconducting phase transitions in ultrathin TiN films

Building on the complete account of quantum contributions to conductivity, we demonstrate that the resistance of thin superconducting films exhibits a non-monotonic temperature behaviour due to the competition between weak localization, electron-electron interaction, and superconducting fluctuations. We show that superconducting fluctuations give rise to an appreciable decrease in the resistance even at temperatures well exceeding the superconducting transition temperature, Tc, with this decrease being dominated by the Maki-Thompson process. The transition to a global phase-coherent superconducting state occurs via the Berezinskii-Kosterlitz-Thouless (BKT) transition, which we observe both by power-law behaviour in current-voltage characteristics and by flux flow transport in the magnetic field. The ratio TBKT/Tc follows the universal relation.

Superinsulator-Superconductor Duality in Two Dimensions

Abstract For nearly a half century the dominant orthodoxy has been that the only effect of the Cooper pairing is the state with zero resistivity at finite temperatures, superconductivity. In this work we demonstrate that by the symmetry of the Heisenberg uncertainty principle relating the amplitude and phase of the superconducting order parameter, Cooper pairing can generate the dual state with zero conductivity in the finite temperature range, superinsulation. We show that this duality realizes in the planar Josephson junction arrays (JJA) via the duality between the Berezinskii–Kosterlitz–Thouless (BKT) transition in the vortex–antivortex plasma, resulting in phase-coherent superconductivity below the transition temperature, and the charge-BKT transition occurring in the insulating state of JJA and marking formation of the low-temperature charge-BKT state, superinsulation. We find that in disordered superconducting films that are on the brink of superconductor–insulator transition the Coulomb forces between the charges acquire two-dimensional character, i.e. the corresponding interaction energy depends logarithmically upon charge separation, bringing the same vortex-charge-BKT transition duality, and realization of superinsulation in disordered films as the low-temperature charge-BKT state. Finally, we discuss possible applications and utilizations of superconductivity–superinsulation duality.

Direct observation of the superconducting gap in thin film of titanium nitride using terahertz spectroscopy

We report on the charge carrier dynamics of superconducting titanium nitride (TiN) in the frequency range 90–510 GHz (3–17 cm−1). The experiments were performed on an 18-nm thick TiN film with a critical temperature of Tc=3.4 K. Measurements were carried out from room temperature down to 2 K, and in magnetic fields up to B=7 T. We extract the real and imaginary parts of the complex conductivity σ as a function of frequency and temperature, directly providing the superconducting energy gap 2Δ. Further analysis yields the superconducting London penetration depth λL. The findings as well as the normal-state properties strongly suggest conventional BCS superconductivity, underlined by the ratio 2Δ(0)/kBTc=3.44. Detailed analysis of the charge carrier dynamics of the silicon substrate is also discussed.

Nanopattern-stimulated superconductor-insulator transition in thin TiN films

We present the results of the comparative study of transport properties of continuous and nanoperforated TiN films, enabling us to separate the disorder and the geometry effects. Nanopatterning transforms a thin TiN film into an array of superconducting weak links and eo ipso stimulates the disorder– and magnetic-field–driven superconductor-to-insulator transitions, shifting both transitions to a lower degree of microscopic disorder. We observe magnetoresistance oscillations reflecting collective phase-frustration behaviour of the multiconnected superconducting weak link network in a wide range of temperatures. We find that nanopatterning enhances the role of the two-dimensional Coulomb interaction and changes the characteristic energies of the film on length scales significantly larger than the mean free path or the superconducting coherence length.

Two-dimensional array of diffusive SNS junctions with high-transparent interfaces

We report a comparative study of the properties of two-dimensional arrays and single superconducting-film--normal-wire--superconducting-film (SNS) junctions. Superconducting and normal metal parts are made from the same material (superconducting polycrystalline PtSi film), ensuring that the NS interfaces of our SNS junctions are highly transparent. We have found the two-dimensional arrays to reveal some unusual features: (i) the significant narrowing of the zero bias anomaly in comparison with single SNS junctions, (ii) the strengthening of subharmonic energy gap structure, with up to

Out-of-equilibrium heating of electron liquid: fermionic and bosonic temperatures.

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