Josephson Tunnel Research Articles

Comparison of the superconducting transition properties of YBa2Cu3O7 nanobridges with and without a twin boundary

We have fabricated nanobridges from YBa2Cu3O7 films deposited on LaAlO3 substrates by using a focused ion beam technique and studied their superconducting transition properties with and without a twin boundary (TB) crossing the bridge. The nanobridge dimensions were 200–500 nm in width and length, and 100 nm in thickness. All the nanobridges studied with or without a TB had a superconducting transition temperature Tc of 90 K with a zero-resistance transition at 86.5 K and high critical current density of ∼1 × 107 A/cm2 at 77 K. However, the current-voltage curves for the nanobridges with a TB showed Josephson tunneling characteristics near the transition, which is in clear contrast to the flux-flow-type transition behavior of those without a TB. According to a crystallographic analysis with d-wave symmetry of YBCO, the small mismatch angle of the TB itself was not sufficient to cause the Josephson effect. Secondary effects, such as stress and strain, are believed to be the origin.

Ba0.6K0.4BiO3 single crystal as a multiple Josephson system: new coherent effect?

The existence of space inhomogeneous superconductor insulator state (SISIS) found out earlier in polycrystalline samples of high-TC system Ba0.6K0.4BiO3 (TC≍30 K) is confirmed on Ba0.6K0.4BiO3 single crystal. At T* (T*<TC, T*≍17 K) the transition from the homogeneous superconducting state into the SISIS occurs. SISIS is characterized by the appearance of two gapes on the Fermi surface: semi- and superconducting, that are modulated in space in antiphase, the electric transport between superconducting regions being carried out due to Josephson tunneling. Thus the whole sample becomes a multiple Josephson system. Nonlinear I-V curves, depended on temperature and magnetic field, that are typical to a Josephson system, are observed on Ba0.6K0.4BiO3 single crystal at temperatures below T*. Besides, a step like peculiarity at the values of voltage of the order of one and two superconducting gaps shows up. These peculiarities are suppressed by magnetic field much earlier then critical current. Perhaps the last phenomenon is the consequence of “coherent” state of several successive Josephson junctions, appeared in the exfoliation state.

The Josephson heat interferometer

The Josephson effect is perhaps the prototypical manifestation of macroscopic phase coherence, and forms the basis of a widely used electronic interferometer--the superconducting quantum interference device (SQUID). In 1965, Maki and Griffin predicted that the thermal current through a temperature-biased Josephson tunnel junction coupling two superconductors should be a stationary periodic function of the quantum phase difference between the superconductors: a temperature-biased SQUID should therefore allow heat currents to interfere, resulting in a thermal version of the electric Josephson interferometer. This phase-dependent mechanism of thermal transport has been the subject of much discussion but, surprisingly, has yet to be realized experimentally. Here we investigate heat exchange between two normal metal electrodes kept at different temperatures and tunnel-coupled to each other through a thermal 'modulator' (ref. 5) in the form of a direct-current SQUID. We find that heat transport in the system is phase dependent, in agreement with the original prediction. Our Josephson heat interferometer yields magnetic-flux-dependent temperature oscillations of up to 21 millikelvin in amplitude, and provides a flux-to-temperature transfer coefficient exceeding 60 millikelvin per flux quantum at 235 millikelvin. In addition to confirming the existence of a phase-dependent thermal current unique to Josephson junctions, our results point the way towards the phase-coherent manipulation of heat in solid-state nanocircuits.

Magnetic sensors based on long Josephson tunnel junctions

The properties of Josephson devices are strongly affected by geometrical effects. A loop-shaped superconducting electrode tightly couples a long Josephson tunnel junction with the surrounding electromagnetic field. Due to the fluxoid conservation, any change of the magnetic flux linked to the loop results in a variation of the shielding current circulating around the loop, which, in turn, affects the critical current of the Josephson junction. This method allows the realization of a novel family of robust superconducting devices (not based on the quantum interference) which can function as general-purpose magnetic sensors. The best performance is accomplished without compromising the noise performance by employing an in-line-type junction few times longer than its Josephson penetration length. The linear (rather than periodic) response to magnetic flux changes over a wide range is just one of its several advantages compared to the most sensitive magnetic detectors currently available, namely the superconducting quantum interference devices (SQUID). We will also comment on the drawbacks of the proposed system and speculate on its noise properties.

Phase-controlled superconducting heat-flux quantum modulator

We theoretically put forward the concept of a phase-controlled superconducting heat-flux quantum modulator. Its operation relies on phase-dependent heat current predicted to occur in temperature-biased Josephson tunnel junctions. The device behavior is investigated as a function of temperature bias across the junctions, bath temperature, and junctions asymmetry as well. In a realistic Al-based setup the structure could provide temperature modulation amplitudes up to ∼50 mK with flux-to-temperature transfer coefficients exceeding ∼125 mK/Φ0 below 1 K, and temperature modulation frequency of the order of a few MHz. The proposed structure appears as a promising building-block for the implementation of caloritronic devices operating at cryogenic temperatures.

Sliding phase in randomly stacked 2d superfluids/superconductors

Using large-scale quantum Monte Carlo simulations of lattice bosonic models, we precisely investigate the effect of weak Josephson tunneling between 2D superfluid or superconducting layers. In the clean case, the Kosterlitz-Thouless transition immediately turns into 3DXY, with phase coherence and superflow in all spatial directions, and a strong enhancement of the critical temperature. However, when disorder is present, rare regions fluctuations can lead to an intermediate finite-temperature phase —the so-called sliding regime— where only 2D superflow occurs within the layers without any transverse superfluid coherence, while a true 3D Bose-Einstein condensate exists. Critical properties of such an unconventional regime are carefully investigated.

Finite-temperature phase-locking transition in three-dimensional arrays of Josephson coupled Bose–Einstein condensates

We consider theoretically a phase-locking transition in Bose–Einstein condensate in an optical lattice in the regime where system can realized as a three-dimensional Josephson junction array. The coherence between adjacent Bose condensates (trapped in the valleys of the periodic potential) caused by the Josephson tunneling can lead to a phase transition with a global phase coherence at certain critical temperature. Using a model Hamiltonian of Josephson weakly coupled Bose condensates we calculate the critical temperature for the three-dimensional system placed in a simple cubic lattice and discuss the result in the context of system parameters and possible experiments.

Defect Formation in Superconducting Rings: External Fields and Finite-Size Effects

Consistent with the predictions of Kibble and Zurek, scaling behaviour has been seen in the production of fluxoids during temperature quenches of superconducting rings. However, deviations from the canonical behaviour arise because of finite-size effects and stray external fields. Technical developments, including laser heating and the use of long Josephson tunnel junctions, have improved the quality of data that can be obtained. With new experiments in mind we perform large-scale 3D simulations of quenches of small, thin rings of various geometries with fully dynamical electromagnetic fields, at nonzero externally applied magnetic flux. We find that the outcomes are, in practice, indistinguishable from those of much simpler Gaussian analytical approximations in which the rings are treated as one-dimensional systems and the magnetic field fluctuation-free.

Interlayer phase coherence and Josephson effects in bilayer quantum Hall systems

When an electron is confined within the lowest Landau level, its position is described solely by the guiding center, whose X and Y coordinates do not commute with one another. The equations of motion do not follow from the kinetic Hamiltonian but from the noncommutative property of the space. Based on this microscopic theory, we analyze the bilayer QH system at the filling factor ν = 1, and show that there develops an interlayer phase coherence. It is interpreted that the phase coherence occurs due to the Bose-Einstein condensation of composite bosons, which are single electrons bound to magnetic flux quanta. The phase coherence can induce the Josephson inplane current as well as the Josephson tunneling current, which are dissipationless as in superconductor. We demonstrate that the Josephson inplane current provokes anomalous behaviors in the Hall resistance in counterflow and drag experiments. Furthermore, we investigate the condition on the input current for the tunneling current to be coherent and dissipationless. We predict also how the condition changes when the sample is tilted in the magnetic field.

Josephson effect between a two-band superconductor withs++ors±pairing symmetry and a conventionals-wave superconductor

In this work, we investigate the Josephson effect between a two-band superconductor either with the $s++$ (two energy gaps have the same sign and are fully gapped) pairing symmetry or $s\pm$ (two energy gaps have $\pi$ phase difference and are fully gapped) pairing symmetry and a conventional s-wave superconductors. The ground state, critical current, plasma modes, flux flow dynamics, and response to external ac electric field, possible soliton solutions are investigated. For junctions with the charge neutrality breaking, we find a new plasma mode for junctions, which gives rise to new resonance peaks in the Josephson flux flow region. Because of the frustrated interaction in junctions with $s\pm$ pairing symmetry, time-reversal-symmetry (TRS) can be broken if the frustration is optimized. In the TRS broken (TRSB) state, there is a non-trivial phase difference between the two Josephson tunnelling channels, which results in a non-trivial interference. Furthermore, we find a novel massless plasma mode at the TRSB transition for junctions with the charge neutrality breaking. In the TRSB state, a spontaneous magnetic flux appears where there is a spatial inhomogeneity in the Josephson coupling, thus provides a possible smoking-gun evidence for the underlying pairing symmetry.

Sub-fT/Hz1/2 resolution and field-stable SQUID magnetometer based on low parasitic capacitance sub-micrometer cross-type Josephson tunnel junctions

We review the development of low parasitic capacitance sub-micrometer cross-type Josephson tunnel junctions for their use in highly sensitive and field-stable SQUID magnetometers.The potential of such junctions is shown on I–V characteristics as well as on Fraunhofer diffraction patterns. The evaluation of Fiske steps lead to a specific junction capacitance of about 62fF/μm2 for a critical current density of about 1.7kA/cm2.The avoidance of any idle-region – the undesired overlap between superconducting electrodes around the junction – due to a self-aligned junction definition process lead to highly sensitive SQUIDs; multiloop SQUID magnetometers exhibiting exceptionally low magnetic field noise levels as low as 0.3fT/Hz1/2, as well as large usable voltage swings of more than 150μVpp. Furthermore, junction dimensions in the sub-micrometer range allowed for very high tolerable background fields during cool-down of up to 6.5mT. In operation mode, the SQUID magnetometers recovered completely from magnetization pulses of up to 76mT.With respect to their easy and reliable usage as well as their high sensitivity, the presented SQUID sensors are adequate for many applications, like in geophysics, biomagnetism or low-field magnetic resonance imaging.

Enhanced Josephson tunneling between high-temperature superconductors through a normal pseudogap underdoped cuprate with a finite-energy cooperon

The Josephson coupling between optimally cuprate superconductors separated by a spacer with a finite-energy cooperon excitation, which contributes to the Josephson coupling strength, is examined. For an underdoped-cuprate barrier in its normal state, the YRZ model gives a good description of the temperature-dependent enhanced Josephson coupling. A detailed examination of the origin of the enhancement shows a significant contribution from the cooperon excitation which is comparable to that from nodal quasiparticles.

Josephson tunneling in bilayer quantum Hall system

A Bose–Einstein condensation is formed by composite bosons in the quantum Hall state. A composite boson carries the fundamental charge (−e). We investigate Josephson tunneling of such charges in the bilayer quantum Hall system at the total filling ν=1. We show the existence of the critical current for the tunneling current to be coherent and dissipationless. Our results explain recent experiments due to [L. Tiemann, Y. Yoon, W. Dietsche, K. von Klitzing, W. Wegscheider, Phys. Rev. B 80 (2009) 165120] and due to [Y. Yoon, L. Tiemann, S. Schmult, W. Dietsche, K. von Klitzing, Phys. Rev. Lett. 104 (2010) 116802]. We predict also how the critical current changes as the sample is tilted in the magnetic field.

Probing interfacial pair breaking in tunnel junctions based on the first and the second harmonics of the Josephson current

It will be shown that a pronounced interfacial pair breaking can be identified in Josephson tunnel junctions provided the first j_{1c} and the second j_{2c} harmonics of the supercurrent, as well as the depairing current in the bulk j_{dp}, are known. Namely, within the Ginzburg-Landau theory a strong interfacial pair breaking results in the relation j_{2c}j_{dp}>>j_{1c}^2, while in standard junctions, with negligibly small pair breaking, the relation of opposite character takes place.

Fermi gas with attractive potential and spinS=32in a one-dimensional trap: Response functions for superfluidity and FFLO signatures

In the context of a gas of ultracold atoms with effective spin $S=3/2$ confined to an elongated trap, we study the one-dimensional Fermi gas interacting via an attractive $\ensuremath{\delta}$-function potential within the grand-canonical ensemble. The particles can be either unbound or clustered in bound states of two, three, and four fermions. The rich $\ensuremath{\mu}$ versus $H$ ground-state phase diagram ($\ensuremath{\mu}$ is the chemical potential and $H$ the external magnetic field) consists of the four basic states and the various possible mixed phases in which some these states coexist. Extending the analysis of K. Yang [Phys. Rev. B 63, 140511(R) (2001)] for $S=1/2$, we study the correlation functions of the generalized Cooper clusters of bound states of two, three, and four particles using conformal field theory and the exact Bethe Ansatz solution. The correlation functions consist of a power law with distance times a sinusoidal term oscillating with distance. In an array of tubes with weak Josephson tunneling, the type of superfluid order is determined by these correlation functions. The wavelength of the oscillations is related to the periodicity of a generalized Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state for higher spin particles. All the relevant states are analyzed for $S=3/2$.

Josephson Tunneling Current in Spin Polaron Theory

Using an effective Hamiltonian derived from spin polaron theory, the Josephson tunneling current between two high temperature superconductors is calculated. The method makes use of the Matsubara Green’s function and a canonical transformation analogous to the Schrieffer–Wolff transformation. The Josephson tunneling current is then obtained in terms of the energy gap function and the tunneling matrix, and it was found to have some similarities with the BCS result.

Dc Josephson current for d-wave superconductors with charge density waves

dc Josephson tunnel current Ic between a dx2−y2-wave superconductor and charge density waves (CDWs)—e.g. a high-Tc oxide and a conventional isotropic superconductor—was considered theoretically. Directionality of tunneling was taken into account. It was found that the dependence of Ic on the angle γ between the direction of the superconducting lobe and the plane normal to the junction is significantly altered by CDWs. For certain doping levels it may be nonmonotonic, as can be readily determined experimentally. For comparison, presented here are the corresponding results obtained for the tunnel junction between CDWs in an s-superconductor and a conventional isotropic superconductor.

Long Josephson tunnel junctions with doubly connected electrodes

In order to mimic the phase changes in the primordial Big Bang, several "cosmological" solid-state experiments have been conceived, during the last decade, to investigate the spontaneous symmetry breaking in superconductors and superfluids cooled through their transition temperature. In one of such experiments the number of magnetic flux quanta spontaneously trapped in a superconducting loop was measured by means of a long Josephson tunnel junction built on top of the loop itself. We have analyzed this system and found a number of interesting features not occurring in the conventional case with simply connected electrodes. In particular, the fluxoid quantization results in a frustration of the Josephson phase, which, in turn, reduces the junction critical current. Further, the possible stable states of the system are obtained by a self-consistent application of the principle of minimum energy.

Highly sensitive miniature SQUID magnetometer fabricated with cross-type Josephson tunnel junctions

We report on a new method suitable for the fabrication of highly sensitive SQUID magnetometers exhibiting very small effective areas. In contrast to commonly used approaches, which mostly make use of constrictions in a superconducting material to form the Josephson junctions, we have been able to fabricate such sensors with SIS cross-type Josephson tunnel junctions in an Nb/AlOx/Nb trilayer process. The small junction size and vanishing idle region of our cross-type junctions and a therefore very low capacitance lead to a high sensitivity of the resulting SQUIDs. First results on fabricated miniature SQUID magnetometer within this technology are shown. We found, that even with SQUID loop dimensions of about 10×10μm2 very low flux noise levels could be achieved, which clearly demonstrate the high potential of our approach. Furthermore we estimate the required SQUID parameter which may allow the detection of single electron spin-flips with such devices in the future.

Bi-SQUIDs with submicron cross-type Josephson tunnel junctions

We present and discuss results for the experimental evaluation of bi-SQUIDs with shunted submicron cross-type Josephson tunnel junctions characterized by low capacitance. Such a technology, meant for SQUID sensors, was developed to enable unshielded operation of the sensors and to avoid unwanted flux trapping. The obtained voltage response swing is as high as 60 µV. The observed differences between the measured and theoretically predicted voltage responses are analyzed and the possible ways to approach a triangular voltage response shape are discussed. An improved design of such bi-SQUIDs with gradiometric topology is suggested to compensate for the influence of the sizable parasitic three-junction loop, which is the main drawback of the implemented bi-SQUID topology.