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Physical review. B, Condensed matter

Nonlocal response of Josephson tunnel junctions to a focused laser beam

Jhy-Jiun Chang, C. H. Ho, Douglas J. Scalapino

Published May 1, 1985• 17 citations
Publisher

Abstract

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 ${H}_{c}$=\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.

DOI
10.1103/physrevb.31.5826

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Journal of Applied Physics•May 1, 1982•3 citations
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\ensuremathλJ{\ensuremath{\lambda}}_{J}\ensuremathλJ​). In this case the spatial dependence of the laser-induced change in the critical current can be qualitatively different from the unperturbed current distribution.
A perturbation method is used to calculate the change of the total zero-voltage current ΔIc(y0) flowing through a one-dimensional Josephson tunnel junction due to a focused laser beam irradiated at y0. Nonlocal response of the phase difference function φ as well as the local response of the amplitude J to the laser beam are included. In the junction configuration we considered, it is found that the nonlocal effect can, depending upon the external magnetic field, make the spatial dependences of ΔIc(y0) and the current distribution different.
Perturbative calculations of the voltage response of constant-current biased Josephson tunnel junctions to a focused laser beam have been performed. The dependence of the voltage modification as a function of the laser beam position was calculated. The results were found to be qualitatively different for junctions biased on the first zero-field current step but with either vortex-mode or cavity-mode excitations. Therefore, the recently developed scanning focused laser beam technique operated in this mode can be a simple and effective method for probing the nature of the dynamic Josephson tunneling state.
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.
Low βc tunnel junctions have current-voltage characteristics which deviate from the intrinsic quasiparticle characteristic because of the ac Josephson effect. This effect has previously been analyzed for the special case of a pointlike junction in zero applied field. We demonstrate that deviations still occur when the critical current is suppressed by an applied field to a small fraction of its zero field value. The effects are significant for junctions whose length l is comparable to the Josephson penetration length λJ. These are often the junctions of practical interest. We present a simple model to explain the physical origin of these deviations, and also present detailed computer simulations using up to nine segments to model the distributed junction. For l = λJ, and an applied magnetic field which corresponds to the first minimum of the junction threshold characteristic, we find time-averaged voltages which deviate from the quasiparticle characteristic by up to 5%. Their magnitude is proportional to (l/λJ)2 and inversely proportional to the square of the applied field. This dependence can be used experimentally to confirm the observation of the effect.
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.
The microwave response of the dc quasiparticle tunneling current in Josephson tunnel junctions, where the Josephson current is suppressed by an external magnetic field, has been studied quantitatively in order to clarify its characteristics as a probe for the measurement of the junction capacitance. Extensive experiments for both small and long junctions are carried out for distinguishing between microwave behaviors of lumped and distributed constant junctions. It is shown that the observed voltage dependence of the dc quasiparticle tunneling current modified by an applied rf field is in good agreement with a theoretical result which takes into account the influence of the microwave circuit connected to the junction. The comparison between theory and experiment gives the magnitude of the internal rf field in the junction. Together with the applied rf field, this internal rf field leads to the junction rf impedance which is dominated by the junction capacitance in our experimental condition. In the case of lumped junctions, this experimental rf impedance is in reasonable agreement with the theoretical one with the junction capacitance estimated from the Fiske step of the distributed junction fabricated on the same substrate; the obtained ratio of the experimental impedance to the theoretical one is approximately 0.6–1.7. In the case of distributed junctions, however, experimental values of their characteristic impedances are approximately 0.2–0.3 of theoretical values calculated by assuming the one-dimensional junction model and taking account of the standing-wave effect in the junction.