Supercurrent Branch Research Articles

Sensitive phonon detection using a single Cooper-pair transistor

We demonstrate that the supercurrent branch of a single Cooper-pair transistor (SCPT) can be used for sensitive phonon detection. We built a theoretical model of phonon detection based on quasiparticle poisoning and demonstrated its operation with an SCPT and a superconducting quantum interference device as a phonon source both composed of mesoscopic Al/AlO x /Al junctions. The sensitivity of the SCPT detector turned out to be approximately 103 times as high as that of the traditional superconducting-tunnel-junction detector.

Microwave irradiation on a-axis oriented Y123/Pr123 two-stacked Josephson junctions device

Two-stacked submicron Josephson junctions devices were fabricated in a-axis oriented YBaCu3O7 and PrBa2Cu3O7 (Y123/Pr123) multi layered thin films using focused ion beam milling technique. The transition temperature and critical current density (Jc) of the device are about 83 K and 5 × 105 A/cm2 at 20 K, respectively. The device was irradiated with external microwave of 10 GHz and studied at 20 K. The microwave induced voltage steps are observed in I–V characteristics. The supercurrent branch become resistive above a certain microwave power and also the Jc was suppressed as we increased the microwave power. Magnetic field modulation of critical current shows periodicity of about 2000 gauss correspond to the Josephson junctions in the stack.

Phase Diffusion in Graphene-Based Josephson Junctions

We report on graphene-based Josephson junctions with contacts made from lead. The high transition temperature of this superconductor allows us to observe the supercurrent branch at temperatures up to ∼2 K, at which point we can detect a small, but nonzero, resistance. We attribute this resistance to the phase diffusion mechanism, which has not been yet identified in graphene. By measuring the resistance as a function of temperature and gate voltage, we can further characterize the nature of the electromagnetic environment and dissipation in our samples.

Superconducting Junction of a Single-Crystalline Au Nanowire for an Ideal Josephson Device

We report on the fabrication and measurements of a superconducting junction of a single-crystalline Au nanowire, connected to Al electrodes. The current-voltage characteristic curve shows a clear supercurrent branch below the superconducting transition temperature of Al and quantized voltage plateaus on application of microwave radiation, as expected from Josephson relations. Highly transparent (0.95) contacts very close to an ideal limit of 1 are formed at the interface between the normal metal (Au) and the superconductor (Al). The very high transparency is ascribed to the single crystallinity of a Au nanowire and the formation of an oxide-free contact between Au and Al. The subgap structures of the differential conductance are well explained by coherent multiple Andreev reflections (MAR), the hallmark of mesoscopic Josephson junctions. These observations demonstrate that single crystalline Au nanowires can be employed to develop novel quantum devices utilizing coherent electrical transport.

Analysis of Possible Quantum Metastable States in Ballistic Graphene-Based Josephson Junctions

Graphene is a relatively new material (2004) made of atomic layers of carbon arranged in a honeycomb lattice. Josephson junction devices are made from graphene by depositing two parallel superconducting leads on a graphene flake. These devices have hysteretic current-voltage characteristics with a supercurrent branch and Shapiro steps appear when irradiated with microwaves. These properties motivate us to investigate the presence of quantum metastable states similar to those found in conventional current-biased Josephson junctions. We present work investigating the nature of these metastable states for ballistic graphene Josephson junctions. We model the effective Washboard potential for these devices and estimate parameters, such as energy level spacing and critical currents, to deduce the design needed to observe metastable states. We propose devices consisting of a parallel on-chip capacitor and suspended graphene. The capacitor is needed to lower the energy level spacing down to the experimentally accessible range of 1-20 GHz. The suspended graphene helps reduce the noise that may otherwise come from two-level states in the insulating oxide layer. Moreover, back-gate voltage control of its critical current introduces another knob for quantum control. We will also report on current experimental progress in the area of fabrication of this proposed device.

Incoherent microwave-induced resistive states of small Josephson junctions

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.

Asymmetric Noise Probed with a Josephson Junction

Fluctuations of the current through a tunnel junction are measured using a Josephson junction. The current noise adds to the bias current of the Josephson junction and affects its switching out of the supercurrent branch. The experiment is carried out in a regime where switching is determined by thermal activation. The variance of the noise results in an elevated effective temperature, whereas the third cumulant, related to its asymmetric character, leads to a difference in the switching rates observed for opposite signs of the current through the tunnel junction. Measurements are compared quantitatively with recent theoretical predictions.

Phase diffusion in single-walled carbon nanotube Josephson transistors

We investigate electronic transport in Josephson junctions formed by individual single-walled carbon nanotubes coupled to superconducting electrodes. We observe enhanced zero-bias conductance (up to 10e 2/h) and pronounced sub-harmonic gap structures in differential conductance, which arise from the multiple Andreev reflections at superconductor/nanotube interfaces. The voltage-current characteristics of these junctions display abrupt switching from the supercurrent branch to the resistive branch, with a gate-tunable switching current ranging from 65 pA to 2.5 nA. The finite resistance observed on the supercurrent branch and the magnitude of the switching current are in good agreement with the classical phase diffusion model for resistively and capacitively shunted junctions.

Gate-Controlled Superconductivity in a Diffusive Multiwalled Carbon Nanotube

We have investigated electrical transport in a diffusive multiwalled carbon nanotube contacted using superconducting leads made of an Al/Ti sandwich structure. We find proximity-induced superconductivity with measured critical currents up to I(cm)=1.3 nA, tunable by the gate voltage down to 10 pA. The supercurrent branch displays a finite zero bias resistance which varies as R(0) proportional to I(cm){-alpha} with alpha=0.74. Using IV characteristics of junctions with phase diffusion, a good agreement is obtained with the Josephson coupling energy in the long, diffusive junction model of A. D. Zaikin and G. F. Zharkov [Sov. J. Low Temp. Phys. 7, 184 (1981)].

Time-domain measurements of quasiparticle tunneling rates in a single-Cooper-pair transistor

We have measured, in real time, individual quasiparticle tunneling in a single Cooper pair transistor, using rf reflectometry on the supercurrent branch. We have extracted the even-to-odd and odd-to-even transition rates directly by analyzing the distributions of dwell times in the even and odd states of the transistor. We discuss both the measurement and analysis techniques and report on the temperature and gate dependence of the quasiparticle tunneling rates.

Resonant Readout of a Persistent Current Qubit

We have implemented a resonant circuit that uses a SQUID as a flux-sensitive Josephson inductor for qubit readout. In contrast to the conventional switching current measurement that generates undesired quasi-particles when the SQUID switches to the voltage state, our approach keeps the readout SQUID biased along the supercurrent branch during the measurement. By incorporating the SQUID inductor in a high-Q resonant circuit, we can distinguish the two flux states of a niobium persistent-current (PC) qubit by observing a shift in the resonant frequency of both the magnitude and the phase spectra. The readout circuit was also characterized in the nonlinear regime to investigate its potential use as a nonlinear amplifier.

Collective Josephson vortex dynamics in long Josephson junction stacks

We investigate the collective phase dynamics in conventional long Josephson junction (LJJ) stacks and in layered superconductors, exhibiting intrinsic LJJ behaviors. Using a theoretical model which accounts for both the magnetic induction effect and the breakdown of local charge neutrality (i.e., charging effect), we show that the collective motion of Josephson vortices, including the dispersion of Josephson plasma mode and the Swihart-type velocity, in an intrinsic LJJ stack such as Bi 2Sr 2CaCu 2O 8+ y (BSCCO) is significantly modified from those in a conventional LJJ stack. In BSCCO, the strength of the charging effect α is small (i.e., α∼0.1–0.4), but it leads to notable changes in collective phase dynamics, including changes to the stability condition. Also, we show that splitting of the supercurrent branch in the resistive state is due to collective motion of Josephson vortices. The width of spread of these sub-branches in the linear current–voltage regime depends on α, suggesting another way to measure the charging effect in BSCCO.

Dynamics of microwave-induced fluxons in HgI 2-intercalated Bi 2Sr 2CaCu 2O 8+ δ Josephson stacks

The microwave response of intrinsic Josephson junctions (IJJs) in mesa structures formed on HgI 2-intercalated Bi 2Sr 2CaCu 2O 8+ δ single crystals was studied in a range of microwave frequencies above the junction plasma frequency. Under 73–76 GHz microwave irradiation, the supercurrent branch becomes resistive above a certain onset microwave power. At a low current bias, the current–voltage characteristics show linear behavior, while at a high current bias, the resistive branch splits into multiple sub-branches. The voltage spacing between neighboring sub-branches increases with the microwave power and the total number of sub-branches is almost identical to the number of IJJs in the mesa. All the experimental results suggest that each sub-branch represents a specific mode of collective motion of Josephson vortices generated by the microwave irradiation in a stack of IJJs.

Coherent mode splitting of microwave-induced fluxons inHgI2−intercalatedBi2Sr2CaCu2O8+δsingle crystals

The microwave response of small stacks of intrinsic Josephson junctions in ${\mathrm{HgI}}_{2}\ensuremath{-}\mathrm{intercalated}$ ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ single crystals was studied. When irradiated with a microwave of frequencies higher than the junction plasma frequency, the supercurrent branch becomes resistive and splits into multiple subbranches. The number of subbranches turns out to be identical to the number of intrinsic Josephson junctions in the mesa within the experimental error. The behavior of the subbranches fits well the predicted Josephson fluxon dynamics in a strongly coupled stack of junctions.

The Josephson Effect in Nanoscale Tunnel Junctions

In nanoscale Josephson junctions, the Josephson coupling energy is usually comparable with the charging energy of the junction and with the typical energy of thermal fluctuations. Under these circumstances, phase fluctuations imposed by the electromagnetic environment of the junction crucially affect the junction electrical behavior. In particular, they determine the maximum “supercurrent” the junction can sustain. We discuss this quantity in the case where the junction is not resistively shunted, so that the I V characteristics of the junction remains hysteretic. For a simple, yet realistic, unshunted junction model, we obtain detailed predictions of the shape of the supercurrent branch of the I V characteristic. Finally, we present experimental results supporting the theoretical analysis and which demonstrate that the supercurrent in an unshunted nanoscale Josephson junction can indeed be of the order of its critical current.

Interband Transitions and Band Gap Measurements in Bloch Transistors

We have measured the switching current from the supercurrent branch of Bloch (superconducting single electron) transistors. Switching events occurring from the ground and first excited bands can be distinguished, allowing the band index of the system to be determined, thus giving the first direct evidence of the predicted excited bands. Both thermally assisted and microwave induced transitions have been observed. The latter provide a direct measurement of the band gap, which agrees well with theoretical predictions using independent estimates of the capacitance.

Effect of temperature on the resonant macroscopic quantum tunneling in small Josephson junctions

The study of the quantum behaviour of Josephson junctions is interesting not only by itself, but also for the technological importance in electronics employing superconducting weak links. Recently great attention has been devoted to the effects of Resonant Macroscopic Quantum Tunneling (RMQT) between levels the same energy in neighbouring wells of the potential shape describing the junction. This can produce voltage spikes along the supercurrent branch of the I-V curves and peaks in the current switching distributions. Schmidt, Clealand, and Clarke have presented a detailed analysis of the resonant tunnel dynamics for the T=0 case. The present paper is based on a different theoretical approach, first proposed by Larkin and Ovchinnikov, which allows one to include explicitly the temperature. We study the phenomenon in its time dependence and this allows us to consider the lifetime of the voltage spikes as well as the effect of the RMQT on the quantum escape rate out of the V=0 state.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A mechanism of zero-temperature dissipation in ultra-small Josephson junctions

Effect of electromagnetic environment on the phase dynamics of small Josephson junctions is studied in connection with the dissipation in supercurrent branch observed by Iansiti et al. It is found that the dissipation is determined by the tunnel resistance of quasi-particles excited above the superconducting gap by the high-frequency quantum fluctuations in electromagnetic fields. This offers a possible explanation of the temperature-independent dissipation observed experimentally.

Observation of parity-induced suppression of Josephson tunneling in the superconducting single electron transistor.

We have measured the supercurrent branch of a superconducting single electron transistor as a function of gate charge, temperature, and magnetic field. At low temperature and magnetic field, the switching current goes from a minimum to a maximum when the gate charge is varied from 0 to e, as expected for an island in the ground state with an even electron number. When the odd electron number ground state becomes populated by an increase of temperature or field, the Josephson tunneling is strongly suppressed, in agreement with theoretical predictions.

Resonant tunneling in small current-biased Josephson junctions.

We investigate the theory of resonant tunneling of the phase difference \ensuremath{\varphi} in current-biased, small-capacitance Josephson junctions. Tunneling of \ensuremath{\varphi} occurs between states localized in adjacent wells of the tilted cosine (``washboard'') potential when the states are nearly degenerate, and leads to unique junction behavior. Most notable are the presence of voltage spikes along the supercurrent branch of the current-voltage characteristic, the reduction of the bias current at which the junction switches to the free-running state to a value well below the thermodynamic value, and peaks in the distribution of rates at which this switching occurs as a function of bias current. For a range of junction parameters, we estimate the magnitude and width of the first (lowest bias current) voltage spike and the rate of switching to the free-running state in the zero temperature, low damping limit. Experimentally, the most readily observable signature of resonant tunneling should be the characteristic peaks in the switching distribution.