Single Josephson Junction Research Articles

A new definition of the critical current in low T c superconductors based on the Ambegaokar-Halperin model

We report on a new way to determine I c for low temperature superconductors. We have analyzed a series of I–V curves taken at 4.2 K in magnetic fields from 0 to 10 Tesla. Our results on multifilamentary NbTi wire in an Al matrix are used to illustrate this technique. The I–V curves are analyzed using a model based on the Ambegaokar-Halperin equation for a single Josephson junction. The critical current defined in this fashion is larger than that defined from the traditional E-field criterion.

Transverse thermomagnetic effects in the mixed state of Josephson-coupled superconducting systems

Transverse thermomagnetic effects (Ettingshausen and Nernst effects) are discussed for a variety of phenomenological models of Josephson-coupled superconducting systems. Analytic expressions for a possible thermal force acting on Josephson vortices in a single Josephson junction are derived. The temperature dependence of the associated transport line entropy is illustrated. The thermal force appropriate to vertices in multilayer and granular structures is mentioned.

Noise in thin-film YBa2Cu3O7 step-edge junction RF SQUIDS

We describe the noise performance of thin-film YBa2Cu3O7 RF SQUIDS fabricated on MgO substrates with single step-edge Josephson junctions. A substantial range of noise levels has been observed for devices having similar electrical parameters (inductance, critical current, RF frequency). The best devices have white-noise levels below 10-4 phi 0 Hz-1/2 when operated at a frequency of 170-200 MHz and have 1/f noise corners of a few hertz. Measurements of the VRF versus LRF characteristics of one SQUID, indicate that intrinsic device noise dominates the measured total noise of these devices. The white noise of all devices exceeds the predictions of the theory of Kurkijarvi at al. by a large factor, apparently because of non-ideal behaviour of step-edge Josephson junctions.

Current-voltage characteristics of a Bi 2Sr 2Ca 2Cu 3O 8+δ ceramic superconductor

A special contactless technique in combination with the four-probe method allowed current-voltage characteristics in a very wide range of electric fields E∼10 -13÷10 -1 V/cm to be measured on a ring-shaped specimen of the Bi 2Sr 2Ca 2Cu 3O 8+δ ceramics at T=77 K and different magnetic fields. Experimental data are characterized with an S-like behavior that corresponds qualitatively to the characteristic of a single Josephson junction in the presence of thermal fluctuations. At low magnetic fields, a scaling behavior of magnetic field dependences of critical currents determined by different voltage levels has been revealed which fails on exceeding an intragrain lower critical field.

Single electron tunnelling oscillations in a current-biased Josephson junction

Searching for single electron tunnelling (SET) oscillations has been done in single ultrasmall Josephson junctions of area S≈0.01 μm 2 at low temperatures, T≈50 mK. The current-biased regime of the junctions was achieved by insertion of high-Ohmic metallic resistors with R≈500 kΩ into dc leads and additional tunnel junctions of the same area into the voltage leads. Existence of the SET oscillations was proved by observation of the peaks in the dV/dI curves under irradiation of the junctions by an external RF signal when the critical current was suppressed by a magnetic field. The dc current position of the peaks was proportional to the frequency of the RF signal, following a relation I=± ef in correspondence to predictions of the Orthodox theory. Without magnetic field, clear peaks occurred at I=±2 ef, following the Bloch relation.

Dynamical properties of an undedamped single-plaquette Josephson-junction array

We have studied, both analytically and numerically, the dynamical properties of a single-plaquette square array consisting of 2×2 superconducting islands coupled by resistively and capacitively shunted Josephson junctions (RCSJ). In zero magnetic field the dynamics of the system becomes identical to that of a single Josephson junction. But in an external magnetic field it shows a transition to chaos even in a dc bias current only.

Spatially resolved studies of the microwave properties of superconducting devices

Low temperature scanning electron microscopy (LTSEM) offers the possibility for spatially resolved investigations of microwave properties of single Josephson junctions, superconducting striplines, resonators, and complex cryoelectric circuits during their operation. The spatial resolution of about 1 μm is at least two orders of magnitude better than the typical wavelength (≳100 μm in the elements) in the frequency range of interest (50–500 GHz). We demonstrate that LTSEM is a well suited tool for the experimental investigation of superconducting microwave devices and cryoelectronic circuits.

Crises and transient chaos in a driven Josephson junction

Nonlinear dynamics of a single Josephson junction under radio-frequency excitation is analyzed by a model which takes into account interference effects and by means of numerical simulations. The study emphasizes the existence of a crisis, which is followed by chaotic transients over a portion of its state space. The mean lifetime is found to scale as |ω-ω c| −δ, where ω is the frequency of the external excitation, ω c is its value at the crisis, and δ is the critical exponent.

Weak link behavior of single grain boundary Josephson junction in BaPb1−xBixO3 bicrystals

The single grain boundaries in bulk bicrystals of the oxide superconductor BaPb 1−x Bi x O 3 show properties of Josephson junctions. Josephson supercurrent and the system of self-excited current resonances were found in I–V characteristic, measured critical current oscillating in applied magnetic field. The temperature dependence of the critical current exhibit anomalous non monotonous behavior at low temperature and a power-law behavior with the critical exponent close to 3 2 near critical temperature. Auger microscopy studies of the grain boundary were carried out to investigate a structure of the junction. Experimental findings are discussed from the point of view of the tunneling barrier with a semiconducting properties in the BaPb 1−x Bi x O 3 bicrystals.

Experimental evidence for the autonomous Bloch oscillations in single Josephson junctions

The charging effects in single ultrasmall Josephson junctions with area S approximately=0.01 mu m/sup 2/ have been studied at low temperatures, T approximately=50 mK. The junctions were isolated from the electromagnetic environment by high-ohmic metallic resistors with R approximately=500 k Omega inserted into the current and voltage leads. For improvement of the isolation by a factor of two, additional tunnel junctions of the same area were inserted into the current leads. Existence of Bloch oscillations was demonstrated by observation of the self-selective videoresponse of the junction under irradiation by a small amplitude-modulated RF signal. The amplitude of the signal was 2-3 orders smaller than in previous experiments. The amplitude of the response had a linear dependence on the power of the signal, and the DC current position of its peak was proportional to the frequency of the signal with the relation I=+or-2 ef. All these observations can be explained as an interaction of the small external signal with the narrowband Bloch oscillations generated by the junction. >

Large RF-controlled voltage steps in DC SQUIDs applicable to voltage standards and sources

The current height of RF-induced voltage steps in nonhysteretic single Josephson junctions is usually a small fraction of the critical current. It is demonstrated that when a resistively shunted DC superconducting quantum interference device (SQUID) is subjected to an RF magnetic flux, a voltage step with a large current height is obtained. In contrast to weak selection rules for single Josephson junctions, strong selection of the step harmonic number occurs in a shunted DC SQUID for appropriate choice of RF amplitude and DC flux offset, with odd harmonic numbers selected by half-integer flux quantum offsets and even harmonic numbers by integer flux quantum offsets. This pronounced effect has immediate implications for voltage standards and fast, high-precision digital-to-analog converters.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Vortex dynamics in one-dimensional parallel arrays of underdamped Josephson junctions

The dynamics of discrete-one-dimensional parallel arrays of Josephson junctions has been studied experimentally and numerically. The energy barriers for vortex motion from cell to cell have been calculated as a function of the discreteness parameter, the ratio of the Josephson inductance to the self-inductance of the cell. The experimental data show resonances in the current-voltage characteristic, analogous to Fiske steps in a single long Josephson junction. These steps provide a novel experimental method for measuring the self-inductance of 1D parallel arrays.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Fraunhofer oscillations in a multilayer system with Josephson coupling of layers.

The equations for gauge-invariant phases are obtained for superconductors with Josephson coupling of the layers. Using these equations, we find the condition under which Fraunhofer oscillatory dependence of the critical current on parallel magnetic field H can be observed in such compounds: The width of the sample along the ab plane in the direction perpendicular to H should be smaller than the Josephson length ${\ensuremath{\lambda}}_{\mathit{J}}$=${\ensuremath{\gamma}}_{\mathit{s}}$, where \ensuremath{\gamma} is the penetration depth anisotropy ratio and s is the interlayer spacing. We show that in the presence of weak disorder such oscillations are more pronounced in a multilayer system than in a single Josephson junction with the same level of disorder. We present the expression for the c-axis critical density that is inherent to the intrinsic interlayer Josephson effect.

Experimental evidence for the Coulomb blockade of Cooper pair tunneling and Bloch oscillations in single Josephson junctions

We report on measurements of the current-voltage characteristics of ultrasmall Josephson junctions. The junctions were made of either Al or Pb alloy, and the leads connecting the junctions to the outside world were high resistance thin film microstrips fabricated on the chip very close to the junction. The high frequency impedance of these leads was sufficiently large to enable the observation in a single Josephson junction of the Coulomb blockade of Cooper pair tunneling, as well as evidence for the time correlation of Cooper pair tunneling (Bloch Oscillations).

Coulomb blockade and incoherent tunneling of Cooper pairs in ultrasmall junctions affected by strong quantum fluctuations.

We have fabricated and investigated submicron-size (0.01 and 0.04 \ensuremath{\mu}${\mathrm{m}}^{2}$) Pb-alloy single Josephson junctions attached to high-resistance thin-film NiCr or Cr leads which provide the necessary conditions for a strong charging effect. When the Josephson coupling energy is less than the charging energy, we find a novel shape of the current-voltage curves, with a Coulomb gap at small voltages, and a current peak at larger voltages. A theory based on the concept of incoherent Cooper-pair tunneling has explained the data well.

Harmonic generation by current modulation of the microwave transmission through granular high- Tc superconducting films

We report on microwave transmission through thin superconducting films of Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O in the presence of a low-frequency bias current I 0 = IDC + IAC cos 2 τft in the plane of the film. We found that the alternating current can modulate the microwave transmission with the generation of many even harmonics (2 f, 4 f, 6 f…) for I AC larger or equal to the critical current, I c, and for samples which contain a significant amount of weak links. Additional odd harmonics are observed upon applying a DC current I DC. The presence of weak links was determined from the variation of the I– V curves upon microwave irradiation. The harmonic generation is attributed to the nonlinear response of the microwave complex resistivity of the Josephson medium to the bias current. A model based on a single Josephson junction can account qualitatively for the experimental data.

Design and simulation of a fast Josephson junction on-chip gated clock for frequency and time analysis

As the sophistication and speed of digital communication systems increase, there is a corresponding demand for more sophisticated and faster measurement instruments. One such instrument on the market is the HP 5371A frequency and time interval analyzer (FTIA). This instrument is analogous to a conventional oscilloscope. Whereas the oscilloscope measures waveform amplitudes as a function of time, the FTIA measures phase, frequency, or timing events as function of time. These applications are useful in such areas as spread-spectrum radar, chirp filter designs, disk-head evaluation, and timing jitter analysis. The on-chip clock designed for this application uses a single Josephson junction as the clock and a resonator circuit to fix the frequency. A zero-crossing detector is used to start and stop the clock. A SFQ counter is used to count the pulses generated by the clock and a reset circuit is used to reset the clock. Extensive simulations and modeling have been done based on measured values obtained from an Nb-Al/sub 2/O/sub 3/-Al-Nb process.

Quantized voltage plateaus in Josephson-junction arrays: A numerical study.

We study numerically the quantized voltage plateaus in an N\ifmmode\times\else\texttimes\fi{}N array of resistively shunted Josephson junctions subjected to a combined dc and ac applied current ${\mathit{I}}_{\mathrm{dc}}$+${\mathit{I}}_{\mathrm{ac}}$sin(2\ensuremath{\pi}\ensuremath{\nu}t), and a transverse magnetic field equal to p/q==f flux quanta per plaquette (p and q relatively prime integers). With periodic transverse boundary conditions, we find plateaus at all voltages satisfying 〈V〉=nNh\ensuremath{\nu}/(2eq), where n is an integer, and the angular brackets 〈...〉 denote a time average. With free transverse boundary conditions, additional steps at 〈V〉=Nh\ensuremath{\nu}/(4eq) sometimes appear. For f=1/5 and 2/5, we study the motion of the vortex lattice on the steps. At both fields, on every step, the lattice moves an integer number of array lattice constants per cycle of the ac field. For both zero and finite applied transverse magnetic field, the width of the steps varies sinusoidally with ${\mathit{I}}_{\mathrm{ac}}$, in a manner reminiscent of that seen in single Josephson junctions. At a given field and current, the steps ``melt'' at a temperature no higher than the transition temperature of the underlying array at the same field and zero current. On the steps, the time-dependent voltage across the array has strong harmonics at multiples of the fundamental frequency. Off the steps, the power spectrum of the voltage has an apparently broad band with possible subharmonic structure.

A simple and robust niobium Josephson junction integrated circuit process

A simple and robust process for fabricating low-T/sub c/ Josephson junction integrated circuits has been developed. The process is designed around the Nb/Al/sub 2/O/sub 3/-Al/Nb trilayer, and utilized nine masking steps to form two separate levels of trilayer Josephson junctions, as well as resistors, capacitors, and transmission lines. Materials used for interlayer dielectrics and passivation layers are silicon dioxide and silicon nitride formed by plasma-enhanced chemical vapor deposition (PECVD). The PECVD equipment yields a high deposition rate at moderate substrate temperatures. No degradation of the junction characteristics due to these depositions is seen. The measured loss tangent of this dielectric at 10 GHz using a parallel plate technique is 4.44*10/sup -4/. The dielectric constant of this material is 5.1 in the range of 50 to 400 GHz, measured using an on-chip resonator capacitively coupled to a single shunted Josephson junction. The physical quality of the oxide has been investigated using a variety of tests and has proven to be excellent. A variety of simple circuits using this process technology have been fabricated and tested. More complex circuits are currently under development.

Quantum resistive ground state of a current biased Josephson junction of small capacitance

A single current biased Josephson junction of very small capacitance is described within the Ginzburg-Landau (GL) approach including the variation of the modulus of the order parameter, to account for the state of charge of the junction. Although no conventional dissipation is included, a resistive Josephson conduction (i.e. for I < I c) is found due to quantum fluctuations.