Heating in mesa structures

Analytic and computational methods are used to consider the e®ects of heating on IV mea- surements on commonly used mesa structures fabricated from single crystals of highly anisotropic superconductors. We address the time dependence of the temperature rise as well as its ultimate value and discuss the relative advantages for measurements of employing small mesa-size, short pulses and ns-to-¹s measurements

Quasiparticle tunnelling and field-dependent critical current in 2212-BSCCO

Intrinsic c-axis tunnelling in the superconducting state has been measured in zero and finite fields in small mesa structures fabricated on the surface of 2212-BSCCO single crystals. The temperature dependence of the zero-field critical current and quasi-particle conductance is related to microscopic d-wave models in the presence of impurity scattering. The strong field dependence of the c-axis critical current provides information on the correlation of flux pancakes across adjacent superconducting bi-layers. An instability in the IV characteristics is observed below 20K, which accounts for the apparent drop in critical current at low temperatures previously reported

Intrinsic c-axis transport in 2212-BSCCO

We describe two experimental approaches to circumvent the problem of self-heating in IV measurements on small mesa samples of 2212-BSCCO. Simultaneous dc and temperature measurements have been performed, allowing corrections for heating to be made. Short pulse measurements have also been made, where the IV characteristics and the mesa temperature can be measured on a $\mu$s time-scale enabling intrinsic IV characteristics to be derived, even in the presence of appreciable self-heating. Self-heating leads to an appreciable depression of the apparent energy gap and also accounts, in major part, for the s-shaped characteristics often reported at high currents. By correcting for the temperature rise, we derive the intrinsic temperature dependence of the tunnelling characteristics for crystals with a range of doping. Results are compared with recent theoretical models for c-axis transport in d-wave superconductors

Interlayer tunnelling in Bi2Sr2CaCu2O8+d single crystals

We present measurements of the intrinsic quasi-particle conductivity along the c-axis of 2212-BSCCO single-crystal mesa structures in the superconducting and normal states. Direct measurement of the mesa temperature enables corrections to be made for self-heating and permits the acquisition of reliable I-V characteristics over a wide range of temperatures and voltages. Unlike a conventional superconductor, there is no evidence for any change in the quasiparticle conductivity at Tc, consistent with precursor pairing of electrons in the normal state. At low temperatures the initial low-voltage linear conductivity exhibits a T2 dependence, approaching a limiting value at zero temperature

System for fast time-resolved measurements of c-axis quasiparticle conductivity in intrinsic Josephson junctions of 2212-BSCCO

A wide-band cryogenic ampli¯er measurement system for time-resolved 4-point VI-characteristic measurements on Bi2Sr2CaCu2O8+± mesa structures is described. We present measurements which demonstrate the importance of self-heating on » 50 ns time scales. Such heating is likely to have been very signi¯cant in many previously published measurements, where the reported nonlinear VI characteristics have been used to derive superconducting energy gaps

Superconducting NbN nanowires and coherent quantum phase-slips in dc transport

A quantum current standard dual to the Josephson voltage standard has been trumpeted as a primary application for the phenomenon of coherent quantum phase-slips (CQPS) in superconducting nanowires, but it requires a dc transport geometry, distinct from the ring geometry in which CQPS have been probed by microwave spectroscopy measurements. We present measurements on NbN nanowires with width 15~nm and including integrated thin-film resistors to provide a high-impedance environment. Analysis shows that the nanowire behaviour is in line with expectations. The results demonstrate that the process is promising both for future dc transport CQPS measurements and for the fabrication of other circuits in which low-width superconducting nanowires are required

Materials for superconducting nanowires for quantum phase-slip devices

Quantum phase-slip processes in superconducting nanowires of suitably small cross-section have been proposed as the basis for a new current standard, based on physics dual to that for the Josephson voltage standard. The practical realisation of such devices presents several challenges. We consider the requirements which need to be met in constructing a nanowire quantum-phase-slip device and in particular the need to maximise R, the normalstate resistance of a length of nanowire equal to the superconducting coherence length. Titanium and niobium–silicon are promising materials for the nanowires

Controllable Tunneling of Single Flux Quanta Mediated by Quantum Phase Slip in Disordered Superconducting Loops

Quantum phase slip (QPS) is the exact dual to the well-known Josephson effect. Although there are numerous proposals for applications of QPS devices, experimental work to develop these remains in the relatively early stages. Significant barriers to exploiting QPS nanowires for useful technologies still exist, such as establishing robust nanowire-fabrication methods that allow coupling to low-loss circuits, and demonstrating control over the QPS process with an experimenter-controlled external bias. Here we report experiments that show that both of these barriers have been overcome. We present measurements at 300 mK of Nb N coplanar-waveguide (CPW) resonators embedded with nanowires fabricated using a neon focused ion beam. The internal quality factor exceeds 2 × 10 4 —significantly higher than previously reported in comparable experiments. The resonator frequency tunes periodically with an applied magnetic field, revealing tunneling of the order parameter that always occurs at half-integer values of the applied flux. In contrast to previous studies of single QPS, the order-parameter tunneling is shown to be adiabatic, demonstrating improved control over energy dissipation in nanowire QPS circuits. Our results highlight a promising pathway towards realizing low-loss nanowire-based QPS devices

Embedding NbN Nanowires Into Quantum Circuits With a Neon Focused Ion Beam

Parasitic two-level systems are generally present in superconducting circuits as a result of conventional fabrication and processing, and these lead to noise and loss of coherence in quantum systems. We examine the use of a Ne focused ion beam for producing nanowires integrated into superconducting circuits with a minimized density of parasitic two-level systems. We report measurements of nanowires produced by Ne focused-ion-beam milling in NbN resonators. The resonator losses increase after the nanowire is fabricated, with the Q factor decreasing by ~30% to ~10 5, which is a factor 10-100 higher than in the most closely comparable circuits previously measured. This indicates that the Ne focused ion beam is a promising route for creating superconducting-nanowire-based devices with low levels of decoherence

Superconductivity of ultra-fine tungsten nanowires grown by focused-ion-beam direct-writing

The electrical properties of lateral ultra-fine tungsten nanowires, which were grown by focused-ion-beam-induced deposition with 1 pA ion-beam current, were investigated. Temperature-dependent electrical measurements show that the wires are conducting and have a superconducting transition with a transition temperature (T-c) about 5.1 K. Resistance vs. temperature measurements reveal that, with decreasing cross-sectional area, the wires display an increasingly broad superconducting transition. A residual resistive tail extending down to the low-temperature region is found only for the thinnest tungsten nanowire, which is 10 nm thick and 19 nm wide. The logarithm of the residual resistance of this wire appears as two linear sections as a function of temperature, one within 300 mK below T-c and the other extending down to the lowest measuring temperature of 4.26 K. Such features have previously been identified with phase slip processes. Our results are suggestive that the focused-ion-beam technique might be a potential approach to fabricate ultra-thin and ultra-narrow nanowires for the study of superconducting suppression in nanoscale materials and for maskless superconducting device fabrication. (C) 2011 Elsevier B.V. All rights reserved