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Applied Physics Letters

Spatial modulation of critical current density in niobium based Josephson junctions induced by selective heating

C. Granata, Lucia Petti, Massimo Rippa, S. Rombetto, B. Ruggiero, M. Russo, Roberto Russo, Antonio Vettoliere

Published Jun 3, 2013• 6 citations
Publisher

Abstract

Results about induced spatial modulation of critical current density in niobium based Josephson devices by a selective radiation heating are presented. The localized heating inside a single circuit element with a good spatial resolution (less than 1 μm) is made possible by depositing a carbon film on the desired region exploiting the larger absorbance coefficient of carbon (close to 1) with respect to niobium (about 0.2). Measurements of critical current as a function of the external magnetic field on high quality Josephson junctions at T = 4.2 K evidenced that the critical current density was changed only in the absorber covered region, leaving unchanged the critical current density outside that region and ensuring the capability to locally modify the Josephson critical current density. The experimental data have been compared with theoretical predictions obtained by using a suitable junction barrier model based on step-like barrier shape. Such results are very interesting in view of applications in quantum computing, Majorana fermions detection, and superconducting magnetic sensors.

DOI
10.1063/1.4809737

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Results about induced spatial modulation of critical current density in niobium based Josephson devices by a selective radiation heating are presented. The localized heating inside a single circuit element with a good spatial resolution (less than 1 μm) is made possible by depositing a carbon film on the desired region exploiting the larger absorbance coefficient of carbon (close to 1) with respect to niobium (about 0.2). Measurements of critical current as a function of the external magnetic field on high quality Josephson junctions at T = 4.2 K evidenced that the critical current density was changed only in the absorber covered region, leaving unchanged the critical current density outside that region and ensuring the capability to locally modify the Josephson critical current density. The experimental data have been compared with theoretical predictions obtained by using a suitable junction barrier model based on step-like barrier shape. Such results are very interesting in view of applications in quantum computing, Majorana fermions detection, and superconducting magnetic sensors.
A thermal annealing technique for Nb∕Al–AlOx∕Nb Josephson devices based on laser heating is presented. This technique allows “locally” modifying the Josephson critical current density, which is not possible using standard procedures based on oven or hot plate heating. In fact, the heating of a single circuit element with a good spatial resolution is possible. At room temperature, the selected junction is exposed to a focused Ar+ laser beam aligned by an optical system. A thermographic imaging allows controlling the temperature distribution on the whole chip. Experimental results on high quality Josephson junction measured in liquid helium have shown a continuous reduction of the critical current density up to about 40%. Neighboring junctions have not exhibited any measurable change ensuring the capability to locally modify the Josephson critical current density. As first application, the present technique has been employed to recover noisy dc superconducting quantum interference device magnetometers with nonoptimal critical current values obtaining a reduction of the spectrum density of magnetic field noise from about 30to2.5fT∕Hz1∕2.
The effect of a focused laser beam on the critical junction current of a one-dimensional Josephson tunnel junction is analyzed for both short and long junctions. The laser heating locally alters the critical current density and London penetration depth, giving rise to a change in the critical current which is proportional to the local current density of the irradiated region. In addition, the relative pair phase is modified in a nonlocal manner which gives rise to an additional contribution to the change in the critical current of the junction. For short junctions, this nonlocal phase modification contributes two parts to the junction critical current. One has the same spatial dependence as that of the current distribution itself, while the other is independent of the position of the laser beam. This latter contribution depends upon the applied magnetic field and is a signature of the nonlocal effect. For a long junction, the nonlocal phase contribution to the change in the junction critical current can give rise to dramatic effects when the applied magnetic field is less than a critical value Hc{H}_{c}Hc​=\ensuremath{\Elzxh}c/(2ed${\ensuremath{\lambda}}_{J}$). In this case the spatial dependence of the laser-induced change in the critical current can be qualitatively different from the unperturbed current distribution.
Stacked Josephson junction arrays located at the rf current antinodes of a superconducting microstripline resonator were investigated. The circuits were designed to obtain highly accurate voltages at cm-waveband microwave driving and low rf-power level. Nb-Al/AlO/sub x/-Nb Josephson junctions with tunneling area of 60 /spl mu/m/spl times/300 /spl mu/m and critical current density up to 7 A/cm/sup 2/ were used. We compared the average voltages of the rf induced maximal current steps in arrays with up to three junctions per stack. Quantized current steps up to 300 mV were observed under microwave irradiation at frequency of 19.20 GHz in circuits with 384 three-junction stacks. The dynamics of stacked tunnel junctions in the presence of rf-currents will be discussed with respect to application as voltage standards.
We present electrical characterization data of sputtered Nb/a-Si/Nb Josephson junctions (JJs) for high-speed and high-density superconducting circuits. Junctions were studied with critical current densities (Jc) ranging from 0.01 to 3 mA/μm2 at 4 K. For junctions deposited at room temperature and processed to a maximum temperature of 150 °C, the dependence of Jc on barrier thickness d is exponential, Jc∝exp (−d/d0), with d0 constant over the entire range of Jc values studied. Junctions were annealed at temperatures up to 300 °C to study changes in their electrical properties and possible compatibility with high temperature fabrication processes. Current–voltage characteristics, critical current uniformity, critical current modulation with in-plane magnetic field, and sub-gap resistance behavior of these junctions were measured at 4 K and demonstrate that the junction properties do not degrade with annealing. These data indicate that Nb/a-Si/Nb JJs are a potential candidate for higher speed and higher density superconducting circuits.
Josephson junctions enable dissipation-less electrical current through metals and insulators below a critical current. Despite being central to quantum technology based on superconducting quantum bits and fundamental research into self-conjugate quasiparticles, the spatial distribution of super current flow at the junction and its predicted evolution with current bias and external magnetic field remain experimentally elusive. Revealing the hidden current flow, featureless in electrical resistance, helps understanding unconventional phenomena such as the nonreciprocal critical current, i.e., Josephson diode effect. Here we introduce a platform to visualize super current flow at the nanoscale. Utilizing a scanning magnetometer based on nitrogen vacancy centers in diamond, we uncover competing ground states electrically switchable within the zero-resistance regime. The competition results from the superconducting phase re-configuration induced by the Josephson current and kinetic inductance of thin-film superconductors. We further identify a new mechanism for the Josephson diode effect involving the Josephson current-induced phase. The nanoscale super current flow emerges as a new experimental observable for elucidating unconventional superconductivity, and optimizing quantum computation and energy-efficient devices. Diract imaging of supercurrent flow at a Josephson junction has been inaccessible in experiment. Here, using nanoscale magnetometry, the authors find large kinetic inductance of thin film superconductors can lead to competing Josephson vortex states hidden below the critical current, and also provide a new route towards the Josephson diode effect.
Experimental measurements on critical current noise in underdamped niobium based Josephson devices by a technique based on the switching current measurements is reported. By sweeping the junction with a current ramp we measure the critical current switching as a function of the time using the standard time of flight technique. In such a way it is possible to obtain the critical current fluctuations ΔIc=Ic(t)-<Ic(t)> and the relative standard deviations which corresponds to the root square of the current fluctuation power. Pointing at the white noise fluctuations (above few Hz) and taking into account the physical frequency of the device, it is possible to evaluate the power spectral density of the critical current. The analysis has involved high quality underdamped Josephson junctions having an area ranging from (4x4) μm2 to (40x40) μm2 in the temperature range from 4.2 K to few tenth of mK. These measurement provide very useful information about the intrinsic noise of Josephson devices involving SQUIDs and qubits.