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Low Temperature Physics

Incoherent microwave-induced resistive states of small Josephson junctions

Yuri Koval, M. V. Fistul, Alexey Vladimirovich Ustinov

Published Oct 1, 2010• 2 citations
Publisher

Abstract

We report an experimental and theoretical study of low-voltage resistive states observed in small tunnel Josephson junctions subject to microwave radiation. The observed features result from Shapiro steps in the current–voltage characteristics and appear when both thermal fluctuations and high frequency dissipation are strong. Without microwave radiation, Josephson junctions have a phase diffusion supercurrent branch characterized by a finite small resistance and hysteretic switching to a higher voltage range under these conditions. When microwave radiation is applied, three different types of resistive states are observed in the current-voltage characteristics. First, a phase diffusion branch steadily evolves and its maximum voltage Vm increases with the microwave power. Another interesting observed feature is a zero-crossing resistive state characterized by a negative resistance. Finally, we find that the low-voltage resistive state can split into numerous hysteretic fine branches resembling incoherent Shapiro-like steps. The appearance of a particular resistive state depends on the interrelations among the Josephson energy EJ, the energy kBT of thermal fluctuations, and the microwave frequency ω. A theoretical analysis based on incoherent multi-photon absorption by a junction biased in the Josephson phase diffusion regime is in good agreement with the experimental observations.

DOI
10.1063/1.3518082

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We report an experimental and theoretical study of low-voltage resistive states observed in small tunnel Josephson junctions subject to microwave radiation. The observed features result from Shapiro steps in the current–voltage characteristics and appear when both thermal fluctuations and high frequency dissipation are strong. Without microwave radiation, Josephson junctions have a phase diffusion supercurrent branch characterized by a finite small resistance and hysteretic switching to a higher voltage range under these conditions. When microwave radiation is applied, three different types of resistive states are observed in the current-voltage characteristics. First, a phase diffusion branch steadily evolves and its maximum voltage Vm increases with the microwave power. Another interesting observed feature is a zero-crossing resistive state characterized by a negative resistance. Finally, we find that the low-voltage resistive state can split into numerous hysteretic fine branches resembling incoherent Shapiro-like steps. The appearance of a particular resistive state depends on the interrelations among the Josephson energy EJ, the energy kBT of thermal fluctuations, and the microwave frequency ω. A theoretical analysis based on incoherent multi-photon absorption by a junction biased in the Josephson phase diffusion regime is in good agreement with the experimental observations.
We report an experimental and theoretical study of the phase diffusion in small Josephson junctions under microwave irradiation. A peculiar enhancement of the phase diffusion by microwaves is observed. The enhancement manifests itself by a pronounced current peak in the current-voltage characteristics. The voltage position V(top) of the peak increases with the power P of microwave radiation as V(top) proportional to sqrt[P], while its current amplitude weakly decreases with P. As the microwave frequency increases, the peak feature evolves into Shapiro steps with a finite slope. Our theoretical analysis, taking into account the enhancement of incoherent superconducting current by multiphoton absorption, is in good agreement with experimental data.
Experimental I-V curves of microwave-driven Josephson tunnel junctions with resistive shunts are reported. The results are in very good agreement with numerical calculations using the resistively shunted junction model. In the low-frequency regime there are three distinct regions in the I-V curves and two different types of Shapiro steps. It is demonstrated that the overall shape of the I-V curves can be explained by using an adiabatic interpretation of the junction response. The two different types of Shapiro steps are related to a compensation effect between dc and rf bias currents.
We investigate the noise-affected I-V curves of small-area Josephson junctions through experiment, simulation, and theory. In particular, we consider I-V curves in which two different states of finite voltage coexist at the same dc bias: a high-voltage state that corresponds to the usual quasiparticle branch and a low-voltage state that is characterized by thermally activated phase diffusion. The observed hysteresis between the phase-diffusion and quasiparticle branches cannot be explained within the context of the simple resistively and capacitively shunted junction (RCSJ) model but is explained by extended models in which the damping increases with frequency. Frequency-dependent damping is shown to produce a qualitative change in the attractors of the noise-free system which allows the two voltage states to be simultaneously stable. This picture is confirmed by Monte Carlo simulations which accurately reproduce the experimental I-V curves of two different samples over a wide range of temperatures. In addition we develop analytic expressions for three key parameters of the I-V curve of junctions displaying hysteresis between the phase-diffusion and quasiparticle branches: the initial slope of the phase-diffusion branch, the bias level at which the junction switches from the phase-diffusion branch to the quasiparticle branch, and the bias level at which it returns to the phase-diffusion branch.
We theoretically demonstrate that in a nonhysteretic Josephson junction (JJ) with a $2\ensuremath{\pi}$ periodic supercurrent in the presence of radio-frequency (RF) radiation one or several low-lying Shapiro steps could be absent in the voltage-current (VI)(VI)(VI) characteristic. The effect exists at low RF power and at frequencies fff approximately ten times smaller than ${f}_{0}={I}_{c}{R}_{N}/{\mathrm{\ensuremath{\Phi}}}_{0}$ (where Ic{I}_{c}Ic​ is the critical current, RN{R}_{N}RN​ is the normal-state resistance of the Josephson junction, and ${\mathrm{\ensuremath{\Phi}}}_{0}$ is the magnetic flux quantum). The origin of the effect is related to the large differential resistance Rdiff=dV/dI{R}_{\mathrm{diff}}=dV/dIRdiff​=dV/dI near $I\ensuremath{\simeq}{I}_{c}$, which is significantly greater than the value predicted by the resistively capacitance shunted junction (RCSJ) model in the overdamped limit. Our theoretical results are supported by experiments on Nb/NbSi/Nb Josephson junctions with nonhysteretic VIVIVI curves, where we observe the absence of one or more low-lying Shapiro steps, depending on the frequency and power of the RF radiation.
We consider the dynamics of a quantum phase-slip junction (QPSJ), a dual Josephson junction, connected to a microwave source with frequency ${\ensuremath{\omega}}_{\mathrm{mw}}$. With respect to an ordinary Josephson junction, a QPSJ can sustain dual Shapiro steps, consisting of well-defined current plateaus at multiple integers of $e{\ensuremath{\omega}}_{\mathrm{mw}}/\ensuremath{\pi}$ in the current-voltage $(I\text{\ensuremath{-}}V)$ characteristic. The experimental observation of these plateaus has been elusive up to now. We argue that thermal as well as quantum fluctuations can smear the $I\text{\ensuremath{-}}V$ characteristic considerably. In order to understand these effects, we study a current-biased QPSJ under microwave irradiation and connected to an inductive and resistive environment. We find that the effect of the fluctuations is governed by the resistance of the environment and by the ratio of the phase-slip energy and the inductive energy. Our results are of interest for experiments aiming at the observation of dual Shapiro steps in QPSJ devices for the definition of a new quantum current standard.