Superconductor-normal Conductor Research Articles

High-Temperature Superconducting YBa2Cu3O7 – δ Josephson Junction Fabricated with a Focused Helium Ion Beam

As a newly developed method for fabricating Josephson junctions, a focused helium ion beam has the advantage of producing reliable and reproducible junctions. We fabricated Josephson junctions with a focused helium ion beam on our 50 nm YBa2Cu3O7 – δ (YBCO) thin films. We focused on the junction with irradiation doses ranging from 100 to 300 ions/nm and demonstrated that the junction barrier can be modulated by the ion dose and that within this dose range, the junctions behave like superconductor–normal conductor–superconductor junctions. The measurements of the I–V characteristics, Fraunhofer diffraction pattern, and Shapiro steps of the junctions clearly show AC and DC Josephson effects. Our findings demonstrate high reproducibility of junction fabrication using a focused helium ion beam and suggest that commercial devices based on this nanotechnology could operate at liquid nitrogen temperatures.

Properties of grooved Dayem bridge based YBa2Cu3O7−δ superconducting quantum interference devices and magnetometers

The transport properties of a YBa2Cu3O7−δ superconducting quantum interference device (SQUID) based on grooved Dayem bridge weak links are studied as a function of temperature: at high temperatures (60 K<T<Tc=89 K), the weak links show properties similar to superconductor–normal conductor–superconductor junctions, while at temperatures below 60 K, the weak links behave like short Dayem bridges. Using these devices, we have fabricated SQUID magnetometers with galvanically coupled in-plane pickup loops: at T = 77 K, magnetic field white noise levels as low as 63 fT/Hz have been achieved.

Commensurability and quantum interference magnetotransport oscillations in a two-dimensional electron gas sandwiched by superconductors

Magnetic-field dependence of the transport properties of ballistic two-dimensional electrons in a planar superconductor–normal-conductor–superconductor structure is numerically investigated. In the circumstance where the Andreev reflection from the normal-conductor–superconductor interfaces is almost perfect, two oscillatory behaviors occur for magnetic fields higher and lower than that for the coincidence of the cyclotron diameter with the separation between the superconductors. The oscillation period for the former and latter cases is proportional to the magnetic field and inverse of the magnetic field, respectively. The low-field oscillation originates from commensurability-driven guiding of Andreev-reflected trajectories along the interfaces between the normal conductor and the superconductors. If the Andreev reflection probability is considerably less than unity, the commensurability oscillation is suppressed in amplitude and is dwarfed by additionally emerged oscillations originating from the quantum interference between the Andreev- and normal-reflected components.

Andreev reflection in a proximity junction of graphene: Influence of a naturally formed pn junction

We theoretically study the electron transport in a proximity junction made by partially covering a graphene sheet with a bulk superconductor. Such a system serves as a normal conductor–superconductor (NS) junction and its electron transport properties are mainly governed by Andreev reflection (AR). In this NS junction, the charge neutrality point (CNP) of graphene must spatially vary as a result of the penetration of excess carriers induced from the superconductor. By taking this into account in the electron-doped case, we calculate the differential conductance GNS of the graphene NS junction. Owing to the variation of the CNP in the uncovered region, a smooth pn junction is naturally formed at the point where the Fermi energy crosses the CNP. The resulting pn junction causes unusual behavior of GNS. For example, a resonant peak structure appears in GNS reflecting the fact that quasi-bound states are created by the pn junction.

Investigation of nanoSQUID designs for practical applications

In the recent past, considerable effort was spent on the development of superconducting quantum interference device (SQUID) designs for magnetic detection in the micro- and nano-scale. Where these novel nanoSQUIDs were mostly of a simple format, real applications require more elaborate designs including auxiliary components such as coils and transformers. Therefore, we have developed SQUID designs based on a Nb/HfTi/Nb thin-film technology which offers both, nano-patterning and waferscale manufacturing of complex design structures. Employing e-beam lithography and chemical–mechanical polishing process steps, the area of the superconductor–normal conductor–superconductor Josephson junctions of these nanoSQUIDs has been varied between 150 nm × 150 nm and 200 nm × 200 nm, whereas the thickness of the barrier is about 20 nm. In order to enable real practical applications, nanoSQUID designs with a number of implemented auxiliary components and design features, such as gradiometric feedback loops, gradiometric transformers, and rf filters, have been carefully investigated. This paper combines a summary of recent achievements with a presentation of detailed measurements of the device performance in magnetic fields of up to a few tens of millitesla. This investigation is intended to pave the way for future practical complex nanoSQUID tools.

HfTi-nanoSQUID gradiometers with high linearity

We have developed a family of HfTi nanoSQUID gradiometers for different applications. These Nb-based nanoSQUIDs contain overdamped superconductor–normal conductor–superconductor (SNS) Josephson junctions with HfTi as a normal conducting barrier. The lateral dimensions of the junctions are about 200 nm × 200 nm, and the barrier thickness is nominally 30 nm. In order to enhance their practical use, the nanoSQUIDs are implemented with gradiometric SQUID and feedback loops, gradiometric transformers, and rf filters. The devices can be operated in an excitation field of up to a few mT with very low levels of nonlinearity. Due to the small loop size and the resulting low loop inductance, a white noise level down to 110 nΦ0/√Hz was achieved. The 1/f noise with a typical corner frequency below 1 kHz is dominated by critical current fluctuations. It can be reduced by applying bias reversal. A noise level of 600 nΦ0/√Hz was achieved at 1 Hz in a two-stage flux locked loop with bias reversal.

A superconducting 180° hybrid ring coupler for circuit quantum electrodynamics

Superconducting circuit quantum electrodynamics experiments with propagating microwaves require devices acting as beam splitters. Using niobium thin films on silicon and sapphire substrates, we fabricated superconducting 180° microstrip hybrid ring couplers, acting as beam splitters with center frequencies of about 6 GHz. For the magnitude of the coupling and isolation, we find −3.5±0.5 dB and at least −15 dB, respectively, in a bandwidth of 2 GHz. We also investigate the effect of reflections at the superconductor-normal conductor contact by means of low temperature laser scanning microscopy. Our measurements show that our hybrid rings are well suited for on-chip applications in circuit quantum electrodynamics experiments.

Contact superconductivity in In–PbTe junctions

We report on electron transport studies on superconductor/semiconductor hybrid structures of indium and n-type PbTe quantum wells embedded between Pb0.92Eu0.08Te barriers. In/PbTe contacts form by spontaneous alloying, which occurs already at room temperature. The alloyed phase penetrates deeply into the semiconductor and forms metallic contacts even in the presence of depletion layers at the semiconductor’s surface. Although the detailed structure of this phase is unknown, we observe that it exhibits a superconducting transition at temperatures below 7 K. This causes such substantial reduction in the contact resistances that they even become comparable to those predicted for ideal superconductor-normal conductor contacts. Our findings suggest that the new superconducting phase is result of Josephson coupling arising between tiny superconducting precipitates randomly distributed in the interface region. In analogy to the granular superconducting systems, the coupling would lead to the occurrence of the global superconductivity only when the Coulomb charging energies of the precipitates are sufficiently small. This condition is exceptionally well fulfilled in PbTe because of a huge static dielectric constant of this material, ε>1000.

Stacked SNS Josephson junction of all boron doped diamond

It is important for a new superconducting device application to demonstrate Josephson effect by homoepitaxial all diamond junction. Using only CVD diamond, we have fabricated stacked SNS (superconductor–normal conductor superconductor) Josephson junction where normal state diamond film is sandwiched by superconducting diamond films. The current–voltage (I–V) characteristics of the junction showed clear Josephson properties at 2K.

Periodic Superconducting Microstrip Line With Nonlinear Kinetic Inductance

This paper investigates linear and nonlinear microwave propagation in a periodic superconductor-normal conductor microstrip transmission line (S-N muTL). We derive closed-form equations for complex propagation constant, input impedance and complex resonant frequencies for linear infinite and finite periodic S-N muTL. Jumps on the curves of attenuation constants, input impedances and eigenfrequencies occur exactly at the Bragg frequencies. We demonstrate that the finite S-N muTL shows the same behavior as infinite periodic SmuTL, when there is enough number of unit cells. Furthermore, nonlinear microwave signal propagation analyses are carried out based on the self-consistent method, when the kinetic inductance of the superconducting segments is in the nonlinear regime.

Induced Voltage Measurements on a YBa2Cu3O7−δ Superconducting Thin Film Due to a Pair of Rotating Magnets

In this study, the induced voltages due to the rotation of a pair of magnets on an YBCO superconducting thin film sample (SS) with and without bias current were measured. It was found that the induced root mean square voltage (Vrms) was a constant (V0) at temperatures higher than the critical temperature (Tc) of the SS, as expected from Faraday’s law. At a temperature in the superconducting transition region, the induced Vrms is a sensitive function of both the motion of the magnet and the bias current applied to the SS. These results can be understood by considering the superposition of the two kinds of induced voltages. One is induced according to Faraday’s law, and the other one is induced by the vortex movements inside the SS which is caused by the bias current. At temperatures below the transition region, the induced Vrms had a value equal to V0 and remained unchanged as the temperature further decreased. An explanation based on the distribution of the magnetic flux inside the SS was given, and it was concluded that the superconductor-normal conductor loop acted like a normal-normal conductor loop to the moving magnetic fields in this low temperature region.

Interface dependence of the Josephson-current fluctuations in short mesoscopic superconductor/normal-conductor/superconductor junctions

We discuss the dependence of the Josephson current correlations in mesoscopic superconductor/normal-conductor/superconductor (SNS) devices on the transparency of the superconductor/normal-conductor (SN) interfaces. Focusing on short junctions we apply the supersymmetry method to construct an effective field theory for mesoscopic SNS devices which is evaluated in the limit of highly and weakly transparent interfaces. We show that the two-point Josephson-current correlator differs by an universal factor 2 in these two cases.

Resonant Reflections in a Particle Conducting Through a Normal-Conductor–Superconductor Interface in Magnetic Fields

A quantum resonance in the conduction through the normal-conductor–superconductor junction is investigated in the presence of a magnetic field using a numerical transfer matrix method. It is shown that dip-pairs in the conductance curve occur at a certain geometry of the system owing to the resonant reflection caused by the interference of the reflected electron and hole waves under a finite magnetic field. By examining a local density of quasiparticles, we found that the feature of the resonant state giving the dip-pair in the conductance (in a magnetic field) is very different from that in a finite bias voltage.

Conductance Fluctuations in a Superconductor-Dirty Normal Conductor-Superconductor Junction

We obtain an analytical expression for the conductance fluctuation in a superconductor-dirty normal conductor-superconductor (SNS) junction using the Green's function method. It is shown that the magnitude of the fluctuation is enhanced by a factor of ( L φ / L x ) (4- d )/2 compared with the universal value of the order of \(e^{2}/\hbar\) ( L φ : the phase coherence length, L x : the separation between two superconductors, d : the effective dimension of the normal region). It is also shown that the energy correlation range is of the order of the inverse of the phase relaxation time, in contrast to ordinary cases where the energy correlation range is of the order of the Thouless energy. We calculate the phase coherence length L φ as a function of the sample size and find that L φ becomes longer with the reduction of the sample size. This indicates that the conductance fluctuation in an SNS junction can be strongly enhanced beyond the universal value.

Subgap conductance structures of a normal-conductor–superconductor junction induced by a magnetic impurity

The transport properties in a normal-conductor–superconductor (NS) junction are investigated when a magnetic impurity is present in the normal region. As the spin of quasiparticles is reversed when they are Andreev-reflected from the NS interface, spin-flip scattering takes place regardless the spin alignment of an incident quasiparticle with respect to the magnetic impurity. The magnetic coupling between the quasiparticle and the impurity gives rise to subgap conductance peaks. The resonance state responsible for the subgap structures is found to be a hybrid of singlet and triplet states.

Quantum interference effects on a particle conduction through a normal-conductor–superconductor interface

We investigate a quantum resonance in a conductance of a normal-conductor–superconductor junction under a finite bias voltage. Resonant transmissions for an electron-like and hole-like quasiparticles change to resonant reflections by a structure introduced. The resonances for the quasiparticles are examined through a local density of the particles.

Critical currents and voltages in weakly nonlinear inhomogeneous media

A random mixture of two components is considered. It is assumed that both these components have current-voltage characteristics which contain weak nonlinear terms of a power-law type. General results for the effective nonlinear susceptibility as well as for critical current and voltage, defined as the crossovers from linear to nonlinear behaviour are obtained, both above and below the percolation threshold. They agree with the results obtained previously for some less general composites. New results for the mixture of `nonlinear insulator'+`linear metal' are found. All these results are valid in the low-field limit. For larger fields it is shown that the exponent x describing the scaling of critical current as a function of conductance obeys the relation: x(d-1)/t for a random metal-insulator composite and x1-/q for a superconductor-normal conductor composite (d is dimensionality, is the percolation correlation length exponent and t and q are conductivity critical exponents for metal-insulator and superconductor-normal conductor percolation, respectively).

An origin of 1/ f fluctuations at a nonequilibrium phase transition

We present the results of experimental study of an intensive source of wide-band 1/ f noise whose generation appears in a system of two interacting nonequilibrium phase transitions. Such a process has been realized in the region of superposition of a superconductor–normal conductor phase transition on a liquid–vapor crisis transition in a boiling cooler. A mathematical model describing phase transitions in a dynamical system with two interacting order parameters, converting “white” noise into two stationary stochastic processes with 1/ f and 1/ f 2 spectra has been suggested. The properties of model oscillations with 1/ f spectrum are qualitatively adequate to those observed in experiment. The characteristic difference of model oscillations with a 1/ f 2 spectrum from classic random walks has also been noted.

Quantum transmission in narrow ballistic wires containing a superconductor island

We investigate the transmission properties in quantum wires with a superconducting island placed in the center of the channel. The conductance quantization is destroyed, unless the Fermi energies in the quantum wire and the superconductor are identical as a consequence of the single-particle reflection from the normal-conductor–superconductor interface. The conductance exhibits fluctuations due to the quantum interference effect in the island. The fluctuation amplitude is found to depend strongly on the pair potential amplitude in the superconductor.

Ballistic transport of electrons and composite fermions in narrow cross junctions

We describe the magnetotransport properties in cross junctions of narrow wires in the ballistic regime. The resistance is determined solely by the scattering from the geometry. It is unconventionally nonlocal and can be negative. These properties arise from the four-terminal nature of the resistance measurements. We consider the ballistic transmission under two circumstances: around zero magnetic field and in the vicinity of the half-filling of the lowest Landau level, where an electron and a composite fermion are the relevant particles, respectively. The electron trajectories are made random and chaotic by smooth corners in cross junctions. The magnetic field dependence of the negative bend resistance reflects the nature of the trajectories in the rounded junctions. For the composite fermion, nonuniform effective magnetic fields additionally modify the ballistic trajectories. We also investigate quantum ballistic transmission of quasiparticle excitations in the normalconductor-superconductor (NS) junctions. In hybrid NS systems, an electron (a hole) is reflected as a hole (an electron) from the NS interface. This Andreev reflection leads to a rich variety of behaviors in the composite structures.