Josephson Junction Devices Research Articles

A Survey of Recent Patents on THz Radiation Sources

There is expanding interest in developing THz spectrum instruments for biochemical agent spectroscopy, environmental monitoring and medical imaging. This paper presents a brief review of recently published patents that target innovations for making improvements to THz radiation sources, which is one of the essential modules for building sensing and imaging systems. This analysis will summarize the novel physical aspects and/or innovative design methodology associated with each patent concept. The approaches presented include photonic crystal, ultrafast photoconductive switch, slow-wave structure, photo-mixing in piezoelectric material, stimulated Raman effect, multiquantum well multiplier, and Josephson Junction device.

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.

Transport driven by biharmonic forces: Impact of correlated thermal noise

We study an inertial brownian particle moving in a symmetric periodic substrate, driven by a zero-mean biharmonic force and correlated thermal noise. The brownian motion is described in terms of a generalized Langevin equation with an exponentially correlated gaussian noise term, obeying the fluctuation-dissipation theorem. We analyze impact of nonzero correlation time of thermal noise on transport properties of the brownian particle. We identify regimes where the increase of the correlation time intensifies long-time transport of the brownian particle. The opposite effect is also found: longer correlation time reduces the stationary velocity of the particle. The correlation time induced multiple current reversal is detected. We reveal that thermal noise of nonzero correlation time can radically enhance long-time velocity of the brownian particle in regimes where in the white noise limit the velocity is extremely small. All transport properties can be tested in the setup consisting of a resistively and capacitively shunted Josephson junction device.

Experimental Studies to Realize Josephson Junctions and Qubits in Cuprate and Fe-based Superconductors

Cuprate superconductors are very promising in terms of Josephson junction device because of the large energy scale of high-T c superconductivity. In particular, fabrication of qubits attracts lots of attention because of its primary importance for future computer technology. We will present some of our recent activities pointing to this purpose. (1) We succeeded in fabricating Josephson junction of cuprate without making any extra oxide barrier layer, rather putting Fe islands on the small strip of cuprate superconductors. This is very promising, since the fabrication of good Josephson junction was unsuccessful so far. Together with the introduction of the data, we discuss the possible mechanism of the weak-link fabrication in this structure. (2) We investigated the switching events in the I-V characteristics of the intrinsic Josephson junctions of Bi-cuprate superconductor, where macroscopic quantum tunneling (MQT) observation is well established recently. In addition to confirm the MQT for the first switching at 1 K in the multiple-branched current-voltage characteristics, we found that the temperature independence of the switching distribution for the second switching up to higher temperatures (10 K) is not due to the trivial Joule heating. We discuss the mechanism of the phenomena, including the possibility of MQT. (3) New Fe-based superconductors are also promising in terms of the application of superconductivity, since the anisotropy looks rather weak, in contrast to cuprates. We will introduce our trial to fabricate epitaxial thin films as the initial step to fabricate Josephson junction of this material.

Modeling and Characterization of Bloch Oscillating Junction Transistors

We have investigated quantum dynamics in small Josephson junction devices where Cooper pair and quasiparticle tunneling can be combined in an intricate way to provide amplification of current. These devices, called Bloch oscillating junction transistors (BOT), employ quasiparticle tunneling to trigger the transistor from a Bloch oscillating state in the coherent limit to a Coulomb blockaded state. We describe the theory of BOTs in terms of the phase fluctuation theory and the energy band structure, which arises from the delocalization of the phase variable. Experimentally measured device characteristics are compared with theory and extensive simulations. The current gain of our devices obtained in a stable (non-hysteretic) operating mode amounts to as high as 30. Measurements on input impedance, noise temperature, and power gain show that the BOT operates best as an amplifier in the mid-range impedances, extending from 100 kΩ−−10 MΩ.

New ideas for long Josephson junctions realistic devices simulation

In the paper, we consider the problem of numerical simulation of long Josephson Junction (JJ) devices. Our interest to the problem is motivated by need of erective parameters calculation of Flux Flow Oscillator (FFO). Practical FFOs have complicated structures and traditional model in form of simple perturbed sine-Gordon equation is inadequate because it doesn't capture important physical details. In this work, we evaluate variable coeficient 1D sine-Gordon equation and boundary conditions for this 1D equation starting from full 2D model of junction and 3D magnetic field problem for bias and control line currents. Then this 1D problem is solved numerically using implicit time stepping numerical procedure. As result program FFOSIM was developed. It is a general tool that allow calculations of critical current, I-V curves and other values. We present results of calculations of junctions inside microstrips where boundary conditions were evaluated using 3D magnetic field calculation program 3DMLSI.

Current statistics of correlated charge tunnelling through an impurity in a 1D wire

We review recent advances in the full counting statistics of a generic transport model of a quantum Brownian particle. The model applies to charge transfer through an impurity embedded in a Luttinger liquid environment, to a coherent one-channel conductor in a resistive environment, and to a resistively shunted Josephson junction device. We derive analytic expressions for the cumulant generating function (CGF) at two particular values of the Luttinger parameter which are related by self-duality. We determine the self-duality relations between the CGFs and between the cumulants.

Anomalous transport in biased ac-driven Josephson junctions: Negative conductances

We investigate classical anomalous electrical transport in a driven, resistively, and capacitively shunted Josephson junction device. Intriguing transport phenomena are identified in chaotic regimes when the junction is subjected to both a time-periodic (ac) and a constant biasing (dc) current. The dependence of the voltage across the junction on the dc exhibits a rich diversity of anomalous transport characteristics. In particular, depending on the chosen parameter regime, we can identify the so termed absolute negative conductance around zero dc bias, the occurrence of negative differential conductance, and, after crossing a zero conductance, the emergence of a negative nonlinear conductance in the nonequilibrium response regime remote from zero dc bias.

Critical current fluctuation in a microwave-driven Josephson junction

Josephson junction devices are good candidates for quantum computation. A large energysplitting was observed in the spectroscopy of a superconducting Josephson junction. Thepresence of the critical current fluctuation near the energy splitting indicated couplingbetween the junction and a two-level system. Furthermore, we find that this fluctuation ismicrowave dependent. It only appears at certain microwave frequency. This relationsuggested that the decoherence of qubits is influenced by the necessary computingoperations.

Absolute Negative Mobility Induced by Thermal Equilibrium Fluctuations

A novel transport phenomenon is identified that is induced by inertial Brownian particles which move in simple one-dimensional, symmetric periodic potentials under the influence of both a time periodic and a constant, biasing driving force. Within tailored parameter regimes, thermal equilibrium fluctuations induce the phenomenon of absolute negative mobility (ANM), which means that the particle noisily moves backwards against a small constant bias. When no thermal fluctuations act, the transport vanishes identically in these tailored regimes. ANM can also occur in the absence of fluctuations on grounds which are rooted solely in the complex, inertial deterministic dynamics. The experimental verification of this new transport scheme is elucidated for the archetype symmetric physical system: a convenient setup consisting of a resistively and capacitively shunted Josephson junction device.

STM and transport measurements of highly charged ion modified materials

Careful measurements of highly charged ions (HCIs) colliding with gases and surfaces have provided glimpses of intense electronic interactions, but a comprehensive model for the interaction mechanisms, time scales, and resultant nano-features that bridges materials systems is yet to be realized. At the National Institute of Standards and Technology (NIST) electron beam ion trap (EBIT) facility, new apparatus is now connected to the HCI beamline to allow preparation of clean, atomically flat surfaces of single crystals, e.g. gold, tungsten and silicon, and deposition and patterning of thin films, e.g. high resistivity oxides, ferromagnetic metals, normal metals and superconductors. Experiments reported here focus on the electronic and morphological structure of HCI induced nano-features. Current activities are focused on using in situ scanning tunneling microscope (STM) on Au(111) and (separately) ex situ transport measurements to study electronic properties within HCI modified magnetic multilayer systems. Specifically, we are fabricating magnetic multilayers similar to magnetic tunnel junctions (MTJs) (important in advanced magnetic field sensors and superconducting Josephson junction devices) and using HCIs to adjust critical electronic properties. The electrical response of the tunnel junction to HCIs provides a novel approach to performing HCI-induced nanostructure ensemble measurements.

Short-time decoherence of a quantum Hadamard gate with Josephson charge qubits

We investigate the short-time decoherence of a quantum Hadamard gate with the Josephson-junction device (JJD). It is shown that the maximum norm of the deviation operator has the same expression as in the previous literature. We demonstrate that the elementary gate operation can be completed under the low decoherence criterion no matter what the initial state is. Thus, the JJD can be a good candidate of quantum computating devices.

Interlayer Transport in Bilayer Quantum Hall Systems

Bilayer quantum Hall systems have a broken symmetry ground state at a filling factor which can be viewed either as an excitonic superfluid or as a pseudospin ferromagnet. We present a theory of interlayer transport in quantum Hall bilayers that highlights remarkable similarities and critical differences between transport in Josephson junction and ferromagnetic metal spin-transfer devices. Our theory is able to explain the size of the large but finite low-bias interlayer conductance and the voltage width of this collective transport anomaly.

Six-Layer Process for the Fabrication of<tex>$rm Nb/Al-rm AlO_x/rm Nb$</tex>Josephson Junction Devices

A six-layer process is described for the fabrication of Josephson junction devices and Superconducting QUantum Interference Devices (SQUIDs) on silicon wafers up to 150 mm in diameter. The Nb/Al-AlO/sub x//Nb trilayers are sputter-deposited in a load-locked vacuum chamber with base pressure in the low 10/sup -7/ Pa (10/sup -9/ Torr) range and a very low leak up rate of 5/spl times/10/sup -6/ Pa/min (3.5/spl times/10/sup -5/ mTorr/min). The barrier process is optimized for a critical current density of 100 A/cm/sup 2/, and all resistors are fabricated from palladium with a sheet resistivity of 1 /spl Omega//sq. A charge dissipative dielectric layer is used in order to reduce the probability of damage caused by electrostatic charge build-up and discharge. The trilayer films are wet etched, while the subsequent metal and dielectric films are patterned using fluorine-based reactive ion etch, Ar ion milling, or lift-off techniques. This process is currently being used to fabricate dc SQUIDs for various instrumentation applications, integrated SQUID magnetometers and planar gradiometers, and single tunnel junction detectors.

Theoretical investigation of a protected quantum bit in a small Josephson junction array with tetrahedral symmetry

We consider a Josephson junction device which has a symmetry of a tetrahedron; it can be visualized as a tetrahedron that contains two Josephson junctions at each edge. We find the conditions for which the ground state of the system is degenerate or almost degenerate; in this case, the low-energy degrees of freedom can be mapped on a quantum spin $1∕2$. We evaluate the effect of the physical perturbations and imperfections on the level splitting in this system and find that they are small for most perturbations. We argue that this system can provide a possible physical implementation of a protected quantum bit with a built-in error correction. We propose a way of manipulating the effective quantum spin by means of electrical potentials and an experimental scheme to read the information that it contains.

Quantum holonomies with Josephson-junction devices

We examined properties of a Josephson-junction system composed of two coupled Cooper-pair boxes (charge qubits) as a candidate for observation of quantum holonomies. We construct a universal set of transformations in a two-fold degenerate ground state, and discuss the effects of noise in the system.

Effects of Quantum and Thermal Fluctuations on the Stability of Mutual Phase-Locking in Long Josephson Junction Stacks

We report the effects of quantum and thermal fluctuations on the stability of phase-locking in one-dimensional long Josephson junction (LJJ) devices, involving high temperature superconductors. Accounting for both the magnetic induction effect and the charging effect, we determine the zero temperature (T = 0) stability phase diagram using renormalization group (RG) analysis. This phase diagram shows that the in-phase mode is stable but some out-of-phase modes are unstable against quantum fluctuations. At finite T, all stable phase-locked modes (at T = 0) are unstable, but its stability is maintained within the decoherence length which decreases inversely with T.

Bloch oscillating transistor—a new mesoscopic amplifier

Bloch oscillating transistor (BOT) is a novel, three-terminal Josephson junction device. Its operating principle utilizes the fact that Zener tunneling up to a higher band will lead to a blockade of coherent Cooper-pair tunneling, Bloch oscillation, in a suitably biased Josephson junction. The Bloch oscillation is resumed only after the junction has relaxed to the lowest band by quasiparticle tunneling. In this paper we present a simple model for the operation of the BOT and calculate its gain in terms of the interband transition rates.

Growth and characterisation of NdBa 2Cu 3O 7− δ/NdBa 2TaO 6/NdBa 2Cu 3O 7− δ multilayer films on (1 0 0) SrTiO 3

Epitaxial multilayer thin films of NdBa 2Cu 3O 7− δ /NdBa 2TaO 6/NdBa 2Cu 3O 7− δ (NBCO/NBTO/NBCO) have been in situ grown on (1 0 0) SrTiO 3 substrate by pulsed laser deposition with the aim of investigating the suitability of using NBTO as an interlayer for Josephson junction devices. NBTO, a dielectric ceramic material, has a cubic perovskite structure (A 2BB ′O 6) and is reported to have good chemical compatibility with 123 Superconductors. The structural characterisation of the films was carried out by θ–2 θ and four axis X-ray diffraction. The θ–2 θ X-ray scans of the multilayer films showed that both NBCO and NBTO films were (0 0 l) oriented and the φ-scan X-ray diffraction patterns revealed that the films were having excellent in-plane orientation. The temperature-resistance measurements of the top NBCO film gave a T c(0) of 91.5 K with a transition width of 0.5 K. Our results indicate that NBTO is a potential dielectric interlayer material for its use in NBCO superconductor multilayer structures for electronic device applications.

Formation processes of Bi-2212 films prepared on Ag( [formula omitted]) substrate by an atomization technique

We have studied in detail the growth of Bi-2212 ribbon-like thin film by melting Bi-2212 powders dispersed on flat, sputter-deposited Ag(0 0 1) films with order of tens of nm root-mean-square surface roughness. The formation processes of Bi-2212 ribbon-like thin films were studied by in situ high-temperature microscope observations. Because the powders melt incongruently, the liquid phase disperses with residual solid phases on the substrate. The residual solid phases act as the barrier for the melting phase to diffuse. Nearly monophasic Bi-2212 with excellent c-axis orientation in these thin films, proved by X-ray diffraction results, is the result of shortened diffusion length of the liquid phase. These techniques can be used especially to synthesize high quality Bi-based superconducting thin films for intrinsic Josephson junction devices.