T. I. Baturina

Localized superconductivity in the quantum-critical region of the disorder-driven superconductor-insulator transition in TiN thin films.

We investigate low-temperature transport properties of thin TiN superconducting films in the vicinity of the disorder-driven superconductor-insulator transition. In a zero magnetic field, we find an extremely sharp separation between superconducting and insulating phases, evidencing a direct superconductor-insulator transition without an intermediate metallic phase. At moderate temperatures, in the insulating films we reveal thermally activated conductivity with the magnetic field-dependent activation energy. At very low temperatures, we observe a zero-conductivity state, which is destroyed at some depinning threshold voltage V{T}. These findings indicate the formation of a distinct collective state of the localized Cooper pairs in the critical region at both sides of the transition.

Magnetic field-induced dissipation-free state in superconducting nanostructures

A superconductor in a magnetic field acquires a finite electrical resistance caused by vortex motion. A quest to immobilize vortices and recover zero resistance at high fields made intense studies of vortex pinning one of the mainstreams of superconducting research. Yet, the decades of efforts resulted in a realization that even promising nanostructures, utilizing vortex matching, cannot withstand high vortex density at large magnetic fields. Here, we report a giant reentrance of vortex pinning induced by increasing magnetic field in a W-based nanowire and a TiN-perforated film densely populated with vortices. We find an extended range of zero resistance with vortex motion arrested by self-induced collective traps. The latter emerge due to order parameter suppression by vortices confined in narrow constrictions by surface superconductivity. Our findings show that geometric restrictions can radically change magnetic properties of superconductors and reverse detrimental effects of magnetic field.

Quantum metallicity on the high-field side of the superconductor-insulator transition.

We investigate ultrathin superconducting TiN films, which are very close to the localization threshold. Perpendicular magnetic field drives the films from the superconducting to an insulating state, with very high resistance. Further increase of the magnetic field leads to an exponential decay of the resistance towards a finite value. In the limit of low temperatures, the saturation value can be very accurately extrapolated to the universal quantum resistance h/e2. Our analysis suggests that at high magnetic fields a new ground state, distinct from the normal metallic state occurring above the superconducting transition temperature, is formed. A comparison with other studies on different materials indicates that the quantum metallic phase following the magnetic-field-induced insulating phase is a generic property of systems close to the disorder-driven superconductor-insulator transition.

Superconductivity on the localization threshold and magnetic-field-tuned superconductor-insulator transition in TiN films

Temperature-and magnetic-field-dependent measurements of the resistance of ultrathin superconducting TiN films are presented. The analysis of the temperature dependence of the zero-field resistance indicates an underlying insulating behavior, when the contribution of Aslamazov-Larkin fluctuations is taken into account. This demonstrates the possibility of the coexistence of the superconducting and insulating phases and of a direct transition from the one to the other. The scaling behavior of magnetic field data is in accordance with a superconductor-insulator transition (SIT) driven by quantum phase fluctuations in two-dimensional superconductor. The temperature dependence of the isomagnetic resistance data on the high-field side of the SIT has been analyzed, and the presence of an insulating phase is confirmed. A transition from the insulating to a metallic phase is found at high magnetic fields, where the zero-temperature asymptotic value of the resistance is equal to h/e2.

Stimulation of the Fluctuation Superconductivity by PT Symmetry

We discuss fluctuations near the second-order phase transition where the free energy has an additional non-Hermitian term. The spectrum of the fluctuations changes when the odd-parity potential amplitude exceeds the critical value corresponding to the PT-symmetry breakdown in the topological structure of the Hilbert space of the effective non-Hermitian Hamiltonian. We calculate the fluctuation contribution to the differential resistance of a superconducting weak link and find the manifestation of the PT-symmetry breaking in its temperature evolution. We successfully validate our theory by carrying out measurements of far from equilibrium transport in mesoscale-patterned superconducting wires.

Hyperactivated resistance in TiN films on the insulating side of the disorder-driven superconductor-insulator transition

We investigate the insulating phase that forms in a titanium nitride film in a close vicinity of the disorder-driven superconductor-insulator transition. In zero magnetic field the temperature dependence of the resistance reveals a sequence of distinct regimes upon decreasing temperature crossing over from logarithmic to activated behavior with the variable-range hopping squeezing in between. In perpendicular magnetic fields below 2 T, the thermally activated regime retains at intermediate temperatures, whereas at ultralow temperatures, the resistance increases faster than that of the thermally activated type. This indicates a change of the mechanism of the conductivity. We find that at higher magnetic fields the thermally activated behavior disappears and the magnetoresistive isotherms saturate towards the value close to quantum resistance h/e2.

Josephson junction arrays on the basis of superconducting PtSi films

Abstract We present the results of low-temperature transport measurements on Josephson junction arrays fabricated on the basis of superconducting polycrystalline PtSi films of thickness 6 nm . To fabricate a two-dimensional array of superconductor—normal-metal–superconductor Josephson weak links, we patterned a square lattice of holes with a period of 600 nm by means of electron lithography and subsequent plasma etching. A periodic variation of the resistance of these arrays with a period corresponding to the magnetic flux quantum per unit cell, including a secondary minimum at the half-quantum points, has been observed.

Anomalous behavior near Tc and synchronization of Andreev reflection in two-dimensional arrays of SNS junctions

The temperature dependences of resistance and the current-voltage characteristics of two-dimensional arrays of superconductor-normal metal-superconductor (SNS) junctions have been measured at low temperatures. It has been found that, in two-dimensional arrays of SNS junctions the following occur: (i) a change in the energy spectrum within an interval of the order of the Thouless energy is observed even when the thermal spread far exceeds the Thouless energy for a single SNS junction; (ii) the manifestation of the subharmonic gap structure with high harmonic numbers is possible even when the energy relaxation length is smaller than that required for the realization of a multiple Andreev reflection in a single SNS junction. These results point to the synchronization of a great number of SNS junctions. A possible mechanism that may be responsible for the features observed in the behavior of two-dimensional arrays of SNS junctions is discussed.

Non-linear conduction in the critical region of the superconductor-insulator transition in TiN thin films

We investigate experimentally electric transport at both the superconducting and insulating sides of the superconductor to insulator transition in TiN thin films. Relatively deeply on the superconducting side we find sharp transitions of R(T) to a zero-resistance state. Near the transition temperature the current-voltage I(V)-characteristics develops power-law type of non-linearity even for low currents. Close to the transition the superconducting state is destroyed by small magnetic fields and develops into an insulator with a thermally activated conductance. At low temperatures and low bias voltage we observe I ∝ Vα dependences in the insulating state with an exponent α rapidly growing with decreasing temperature. This behavior is strikingly similar to the V(I)s observed in the superconducting state, if I and V are interchanged.

Nonequilibrium transport near the superconducting transition in TiN films

The linear and nonlinear conductions of titanium nitride films with the thickness d ≤ 10 nm have been studied experimentally in the region of superconducting transitions. It has been shown that the inclusion of all quantum contributions to the conductivity at temperatures above the critical temperature of the superconducting transition Tc makes it possible to completely describe the temperature dependence of the conductivity measured in the linear regime, and the nonlinear behavior of the current-voltage characteristics is in complete agreement with the classical model of the heating of an electron gas in metals. The electron-phonon coupling constant has been determined. The analysis of the linear and nonlinear conductivities at temperatures below Tc shows that the transition to a superconducting state occurs through the Berezinskii-Kosterlitz-Thouless mechanism.