Author Correction: Superconductor to resistive state switching by multiple fluctuation events in NbTiN nanostrips.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Superconductor to resistive state switching by multiple fluctuation events in NbTiN nanostrips.

We report on measurements of the switching current distributions on two-dimensional superconducting NbTiN strips that are 5 nm thick and 80 nm wide. We observe that the width of the switching current distributions has a non-monotonous temperature dependence, where it is constant at the lowest temperatures up to about 1.5 K, after which it increases with temperature until 2.2 K. Above 2.5 K any increase in temperature decreases the distribution width which at 4.0 K is smaller than half the width observed at 0.3 K. By using a careful analysis of the higher order moments of the switching distribution, we show that this temperature dependence is caused by switching due to multiple fluctuations. We also find that the onset of switching by multiple events causes the current dependence of the switching rate to develop a characteristic deviation from a pure exponential increase, that becomes more pronounced at higher temperatures, due to the inclusion of higher order terms.

Tuning of Magnetic Activity in Spin-Filter Josephson Junctions Towards Spin-Triplet Transport.

The study of superconductor-ferromagnet interfaces has generated great interest in the last decades, leading to the observation of spin-aligned triplet supercurrents and 0-π transitions in Josephson junctions where two superconductors are separated by an itinerant ferromagnet. Recently, spin-filter Josephson junctions with ferromagnetic barriers have shown unique transport properties, when compared to standard metallic ferromagnetic junctions, due to the intrinsically nondissipative nature of the tunneling process. Here we present the first extensive characterization of spin polarized Josephson junctions down to 0.3 K, and the first evidence of an incomplete 0-π transition in highly spin polarized tunnel ferromagnetic junctions. Experimental data are consistent with a progressive enhancement of the magnetic activity with the increase of the barrier thickness, as neatly captured by the simplest theoretical approach including a nonuniform exchange field. For very long junctions, unconventional magnetic activity of the barrier points to the presence of spin-triplet correlations.

Geometrical vortex lattice pinning and melting in YBaCuO submicron bridges.

Since the discovery of high-temperature superconductors (HTSs), most efforts of researchers have been focused on the fabrication of superconducting devices capable of immobilizing vortices, hence of operating at enhanced temperatures and magnetic fields. Recent findings that geometric restrictions may induce self-arresting hypervortices recovering the dissipation-free state at high fields and temperatures made superconducting strips a mainstream of superconductivity studies. Here we report on the geometrical melting of the vortex lattice in a wide YBCO submicron bridge preceded by magnetoresistance (MR) oscillations fingerprinting the underlying regular vortex structure. Combined magnetoresistance measurements and numerical simulations unambiguously relate the resistance oscillations to the penetration of vortex rows with intermediate geometrical pinning and uncover the details of geometrical melting. Our findings offer a reliable and reproducible pathway for controlling vortices in geometrically restricted nanodevices and introduce a novel technique of geometrical spectroscopy, inferring detailed information of the structure of the vortex system through a combined use of MR curves and large-scale simulations.

Tunable spin polarization and superconductivity in engineered oxide interfaces.

Advances in growth technology of oxide materials allow single atomic layer control of heterostructures. In particular delta doping, a key materials' engineering tool in today's semiconductor technology, is now also available for oxides. Here we show that a fully electric-field-tunable spin-polarized and superconducting quasi-2D electron system (q2DES) can be artificially created by inserting a few unit cells of delta doping EuTiO3 at the interface between LaAlO3 and SrTiO3 oxides. Spin polarization emerges below the ferromagnetic transition temperature of the EuTiO3 layer (TFM = 6-8 K) and is due to the exchange interaction between the magnetic moments of Eu-4f and of Ti-3d electrons. Moreover, in a large region of the phase diagram, superconductivity sets in from a ferromagnetic normal state. The occurrence of magnetic interactions, superconductivity and spin-orbit coupling in the same q2DES makes the LaAlO3/EuTiO3/SrTiO3 system an intriguing platform for the emergence of novel quantum phases in low-dimensional materials.

Suspended InAs nanowire Josephson junctions assembled via dielectrophoresis.

We present a novel technique for the realization of suspended Josephson junctions based on InAs semiconductor nanowires. The devices are assembled using a technique of drop-casting guided by dielectrophoresis, which allows one to finely align the nanostructures on top of the electrodes. The proposed architecture removes the interaction between the nanowire and the substrate which is known to influence disorder and the orientation of the Rashba vector. The relevance of this approach in view of the implementation of hybrid Josephson junctions based on semiconducting nanowires coupled with high-temperature superconductors is discussed.

Macroscopic quantum tunnelling in spin filter ferromagnetic Josephson junctions.

The interfacial coupling of two materials with different ordered phases, such as a superconductor (S) and a ferromagnet (F), is driving new fundamental physics and innovative applications. For example, the creation of spin-filter Josephson junctions and the demonstration of triplet supercurrents have suggested the potential of a dissipationless version of spintronics based on unconventional superconductivity. Here we demonstrate evidence for active quantum applications of S-F-S junctions, through the observation of macroscopic quantum tunnelling in Josephson junctions with GdN ferromagnetic insulator barriers. We show a clear transition from thermal to quantum regime at a crossover temperature of about 100 mK at zero magnetic field in junctions, which present clear signatures of unconventional superconductivity. Following previous demonstration of passive S-F-S phase shifters in a phase qubit, our result paves the way to the active use of spin filter Josephson systems in quantum hybrid circuits.

Interplay between static and dynamic properties of semifluxons in YBa2Cu3O(7-delta) 0-pi Josephson junctions.

We have investigated the static and dynamic properties of long YBa2Cu3O(7-delta) 0-pi Josephson junctions and compared them with those of conventional 0 junctions. Scanning SQUID microscope imaging has revealed the presence of a semifluxon at the phase discontinuity point in 0-pi Josephson junctions. Zero field steps have been detected in the current-voltage characteristics of all junctions. Comparison with simulation allows us to attribute these steps to fluxons traveling in the junction for conventional 0 junctions and to fluxon-semifluxon interactions in the case of 0-pi Josephson junctions.

Evidence for a minigap in YBCO grain boundary Josephson junctions.

Self-assembled YBaCuO diffusive grain boundary submicron Josephson junctions offer a realization of a special regime of the proximity effect, where normal state coherence prevails on the superconducting coherence in the barrier region. Resistance oscillations from the current-voltage characteristic encode mesoscopic information on the junction and more specifically on the minigap induced in the barrier. Their persistence at large voltages is evidence of the long lifetime of the antinodal (high energy) quasiparticles.

Direct measurement of sheet resistance Rsquare in cuprate systems: evidence of a fermionic scenario in a metal-insulator transition.

The metal-insulator transition (MIT) has been studied in Ba(0.9)Nd(0.1)CuO(2+x)/CaCuO2 ultrathin cuprate structures. Such structures allow for the direct measurement of the 2D sheet resistance R( square), eliminating ambiguity in the definition of the effective thickness of the conducting layer in high temperature superconductors. The MIT occurs at room temperature for experimental values of R(square) close to the 25.8 kOmega universal quantum resistance. All data confirm the assumption that each CaCuO2 layer forms a 2D superconducting sheet within the superconducting block, which can be described as weak-coupled equivalent sheets in parallel.

Macroscopic quantum tunneling in d-wave YBa2Cu3O7-delta Josephson junctions.

The escape rate from the zero voltage state in a superconducting Josephson junction (JJ) is determined by the temperature, but it saturates at low temperature due to macroscopic quantum tunneling (MQT). Complications due to d-wave symmetry in a high temperature superconductor, like low energy quasiparticles and an unconventional current-phase relation, may influence the escape rate. We report, for the first time to our knowledge, the observation of MQT in a YBa(2)Cu(3)O(7-delta) grain boundary biepitaxial JJ. This proves that dissipation can be significantly reduced by a proper junction configuration, which is of significance for quantum coherence.

Magnetic imaging of pearl vortices in artificially layered (Ba0.9Nd0.1CuO2+x)m/(CaCuO2)n systems.

We have used scanning SQUID magnetometry to image vortices in ultrathin (Ba0.9Nd0.1CuO2+x)(m)/(CaCuO2)(n) high temperature superconductor samples, with as few as three superconducting CuO2 planes. The Pearl lengths (Lambda=2lambda(2)(L)/d, lambda(L) the London penetration depth, d the superconducting film thickness) in these samples, as determined by fits to the vortex images, agree with those by local susceptibility measurements, and can be as long as 1 mm. The in-plane penetration depths lambda(ab) inferred from the Pearl lengths are longer than many bulk cuprates with comparable critical temperatures. We speculate on the causes of the long penetration depths, and on the possibility of exploiting the unique properties of these superconductors for basic experiments.

Thermally activated spontaneous fluxoid formation in superconducting thin film rings.

We have observed spontaneous fluxoid generation in thin film rings of the amorphous superconductor Mo3Si, cooled through the normal-superconducting transition, as a function of quench rate and externally applied magnetic field, using a variable sample temperature scanning SQUID microscope. Our results can be explained using a model of freeze-out of thermally activated fluxoids, mediated by the transport of bulk vortices across the ring walls. This mechanism is complementary to a mechanism proposed by Kibble and Zurek, which only relies on causality to produce a freeze-out of order parameter fluctuations.

Intrinsic d-wave effects in YBa(2)Cu(3)O(7-delta) grain boundary Josephson junctions.

We have measured the angular dependence of the Josephson critical current density (J(C)) in c-axis tilt biepitaxial grain boundary YBa(2)Cu(3)O(7-delta) junctions. We observe for the first time intrinsic d-wave pairing symmetry effects manifested as an oscillatory dependence of J(C) on angle. This intrinsic effect is evident even though spontaneous currents, possibly induced by faceting or barrier impurities, are observed in the grain boundaries.

Flux flow of Abrikosov-Josephson vortices along grain boundaries in high-temperature superconductors.

Low-angle grain boundaries (GBs) in superconductors exhibit intermediate Abrikosov vortices with Josephson cores, whose length l along GB is smaller than the London penetration depth, but larger than the coherence length. We found an exact solution for a periodic vortex structure moving along GBs in a magnetic field H and calculated the flux flow resistivity R(F)(H), and the nonlinear voltage-current characteristics. The predicted R(F)(H) dependence describes well our experimental data on 7 unirradiated and irradiated YBa(2)Cu(3)O(7) bicrystals, from which the core size l(T), and the intrinsic depairing density J(b)(T) on nanoscales of a few GB dislocations were measured for the first time. The observed J(b)(T) = J(b0)(1-T/T(c))(2) indicates a significant order parameter suppression on GB.