c-axis tunneling on YBCO films

We have measured I(V) characteristics of c-axis planar tunnel junctions on Y₁Ba₂Cu₃O _{7 - δ} films. Our results and their analysis provide experimental support for the importance of the two-dimensional character of the YBCO band structure, and a method to measure the ratio between the Fermi energy of YBCO and the barrier height. The analysis is based on the relation between the linear conductance background, related to the inelastic tunneling component, and the zero bias conductance, related to the elastic one. Received 24 September 2000 and Received in final form 15 November 2000     

Investigations on phase locking in a two-dimensional Josephson junction array within the Werthamer and the RCSJ model

The synchronization properties of a simple two-dimensional Josephson array consisting of two coupled SQUID cells are studied within the Werthamer as well as the RCSJ model. Special emphasis is placed on the role of inductances arranged perpendicular and parallel to the current bias direction for the phase locking behavior. The general behavior within the Werthamer model is found to be similar to that within the RCSJ model. However, there are quantitative differences, e.g. an enhanced phase shift between the voltage oscillations within one cell and a shift of the parameter range for the in-phase regime between different cells towards lower values of the McCumber parameter in the Werthamer model. Received: 23 March 1998 / Revised: 3 June 1998 / Accepted: 9 June 1998     

Electronic entanglement in the vicinity of a superconductor

A weakly biased normal-metal-superconductor junction is considered as a potential device injecting entangled pairs of quasi-particles into a normal-metal lead. The two-particle states arise from Cooper pairs decaying into the normal lead and are characterized by entangled spin- and orbital degrees of freedom. The separation of the entangled quasi-particles is achieved with a fork geometry and normal leads containing spin- or energy-selective filters. This solid state entangler is characterized by noise cross-correlations which are identical to the noise in one lead, a signature consistent with entanglement. A connection to Bell-type experiments is envisioned (cond-mat/0009193). Received 20 September 2001     

Evidence for pseudogap and phase-coherence gap separation by Andreev reflection experiments in Au/ La 2 - x Sr x CuO 4 point-contact junctions

We present new Au/La_2-xSr_xCuO₄ (LSCO) point-contact conductance measures as a function of voltage and temperature in samples with 0.08 ? x ? 0.2. Andreev reflection features disappear at about the bulk T _c , giving no evidence of gap for T > T _c . The fit of the normalized conductance at any T < T _c supports a (s + d)-wave symmetry of the gap, whose dominant low-T s component follows the T _c ( x ) curve in contrast with recent angle-resolved photoemission spectroscopy and quasiparticle tunneling data. These results prove the separation between pseudogap and phase-coherence superconducting gap in LSCO at x 0.2. Received 14 June 2001     

Temperature dependence of the YBa2Cu3O7 energy gap in differently oriented tunnel junctions

We have applied the break-junction technique to highly biepitaxial c-axis oriented YBa₂Cu₃O₇ thin films with T _C (ρ=0) = 91 K. Mechanically adjustable junctions with a good stability and tunneling current favored along the ab-planes have been realized. The conductance characteristics of these junctions show the presence of gap related maxima that move towards zero bias for increasing temperatures. Considering the misorientation angle α≈ 45 ^° ± 5 ^° of the junction, a maximum gap value at the Fermi level Δ ≈ 22 meV is inferred at T = 13 K. The temperature dependence of the gap related structures, shows a quasilinear behavior for T > 0.4 T _C similar to that observed in c-axis oriented, S-I-N type YBa₂Cu₃O₇ planar junctions. Received 20 July 2001     

Whole-wafer fabrication process for three-terminal double stacked tunnel junctions

A new fabrication process for three-terminal superconducting devices consisting of two Josephson junctions in a stacked configuration is reported. The process is based on the deposition of the whole Nb/Al_xO_y/Nb-Al/Al_xO_y/Nb multilayer on a Si crystalline wafer without any vacuum breaking. Lift-off techniques, anodization processes and a SiO film deposition have been adopted for patterning and insulating the two tunnel stacked junctions. Devices have been characterized in terms of current-voltage (I-V) curves and Josephson critical current vs. the externally applied magnetic field. They show high quality factors (V _m values up to 65 mV at 4.2 K), and good current uniformity. Received 5 June 2001     

Microscopic theory of non local pair correlations in metallic F/S/F trilayers

We consider a microscopic theory of F/S/F trilayers with metallic or insulating ferromagnets. The trilayer with metallic ferromagnets is controlled by the formation of non local pair correlations among the two ferromagnets which do not exist with insulating ferromagnets. The difference between the insulating and ferromagnetic models can be understood from lowest order diagrams. Metallic ferromagnets are controlled by non local pair correlations and the superconducting gap is larger if the ferromagnetic electrodes have a parallel spin orientation. Insulating ferromagnets are controlled by pair breaking and the superconducting gap is smaller if the ferromagnetic electrodes have a parallel spin orientation. The same behavior is found in the presence of disorder in the microscopic phase variables and also in the presence of a partial spin polarization of the ferromagnets. The different behaviors of the metallic and insulating trilayers may be probed in experiments. Received 4 July 2001 and Received in final form 8 November 2001     

Chaotic solitons in Sine-Gordon system

We extend the constant-variation method to the case of partial differential equations. Applying the method to periodically perturbed Sine-Gordon system, we find some novel solitons, which are embedded in a chaotic attractor and possess controllable velocity of motion. Taking periodically driven long Josephson junction as an example the corresponding chaotic region in parameter space and chaotic orbit are obtained analytically and numerically. Received 25 December 2000     

MO analysis of macroscopic supercurrent flow in MgB 2

We present results from an extended magneto-optical (MO) analysis of two samples cut from high-density pellets of MgB₂. The first sample was studied in order to show that no matter how large the sample is and despite the bulk granularity, the material enters into a critical state in a crystal-like fashion. The second sample was chosen for the quantitative analysis. A numerical approach based on an inverted 2D Biot-Savart model was used to calculate the current paths across the homogeneous polycrystalline bulk, as well as in the vicinity and across some morphological defects. Local current densities in the homogeneous part were estimated as a function of the applied magnetic field at different temperatures, in three regimes: below full penetration, at full penetration and above full penetration, respectively. A hypothesis of interpretation of the apparent absence of magnetic granularity inside the polycrystalline microstructure is presented. It is related to a critical state likely reached by a network of strongly coupled Josephson junctions. Received 31 May 2001 and Received in final form 5 December 2001     

Inelastic relaxation and noise temperature in S/N/S junctions

We studied electronic relaxation in long diffusive superconductor/normal metal/superconductor (S/N/S) junctions by means of current noise and transport measurements down to very low temperature (100mK). Samples with normal metal lengths of 4, 10 and 60μm have been investigated. In all samples the shot noise increases very rapidly with the voltage. This is interpreted in terms of enhanced heating of the electron gas confined between the two S/N interfaces. Experimental results are analyzed quantitatively taking into account electron-phonon interaction and heat transfer through the S/N interfaces. Transport measurements reveal that in all samples the two S/N interfaces are connected incoherently, as shown by the reentrance of the resistance at low temperature. The complementarity of noise and transport measurements allows us to show that the energy dependence of the reentrance at low voltage is essentially due to the increasing effective temperature of the quasiparticles in the normal metal. Received 5 February 2002 / Received in final form 6 September 2002 Published online 31 October 2002 RID="a" ID="a"e-mail: hoffmann@drfmc.ceng.cea.fr     

Coexistence of superconductivity and ferromagnetism in ferromagnet/superconductor proximity structures

The Nambu spinor Green's function approach is applied to calculating the density of states (DOS) and superconducting order parameter in normal-metal/insulator/ferromagnet/superconductor (NM/I/FM/SC) junctions. It is found that the s-wave superconductivity and ferromagnetism can coexist near the FM/SC interface, which is induced by proximity effect. On the SC side, the spin-dependent DOS appears both within and without the energy gap. On the FM side, the superconducting order parameter displays a damped oscillation and the DOS exhibits some superconducting behavior. The calculated result for the DOS in FM for “0 state” and “π state” can reproduce recent tunneling spectra in Al/Al₂O₃/PdNi/Nb tunnel junctions. Received 1st July 2002 Published online 19 November 2002     

Nanosize patterns as reference structures for macroscopic transport properties and vortex phases in YBCO films

This paper reports on the striking correlation between nanosize mosaic domain walls in YBCO films and 1D rows of parallel Josephson junctions, determining the J _c vs.B curves. From X-ray data analysis, it results that the average “hidden" domain wall, faceted at a nanometric scale, is almost mimicking the Josephson Junction (JJ) 1D array. The assumption that the JJs and the domain-wall arrays are coincident, enables to find out the particular scaling field, making the J _c vs.B curves independent of temperature. This scaling field can be interpreted in terms of the Josephson nature of the transport current across these particular patterns in the intermediate temperature range. By means of our model it is also possible to calculate two asymptotic behaviors of the pinning force as a function of field, for low and high fields, respectively. These behaviors are punctually repeated by the experimental results in the same asymptotic limit, so that two corresponding vortex regimes are clearly pointed out. All results can be interpreted by concluding that in the intermediate temperature range, the strong pinning observed in high quality YBCO films is due to the Josephson Junctions average patterns. These patterns are the counterpart related to the transport mechanisms of “hidden" structural nano-domains. Received 16 October 2000 and Received in final form 28 November 2000     

Quantum phase expectation values of a mesoscopic Josephson junction from quantum current measurements

A simple way to acquire information on the mean values of the phase operators sinϕ and cosϕ of an ultrasmall Josephson junction prepared in an arbitrary pure or not state is reported. Our proposal exploits the recently predicted occurrence of current spikes in the I-V characteristic of a mesojunction irradiated by a quantum single-mode low-intensity coherent electromagnetic field. A necessary condition for the validity of our treatment is presented and discussed. Received 11 April 2001     

Internal mode dynamics in driven nonlinear Klein-Gordon systems

We study analytically and numerically the action of a constant force on the propagation of kinks in the φ⁴ and sine-Gordon systems, with and without dissipation. We specifically investigate the relation of the external force with the oscillations of the kink width due to excitation of its internal mode or quasimode. We demonstrate that both dc force and dissipation, either jointly or separately, damp the oscillations of the kink width. We further prove that, in contrast to earlier predictions, those oscillations can only arise if we use a distorted kink as initial condition for the evolution. Finally, we show that for the φ⁴ system the oscillations of the kink width come from the excitation of its internal mode, whereas in the sG equation they originate in the excitation of the lowest radiational modes and an internal mode induced by the discreteness of the numerical simulations. Received 6 June 2000     

Large grain boundary area superconductors

For many applications of polycrystalline high-T _c superconductors the small critical currents of the grain boundaries pose a severe problem. To solve this problem, we derive novel designs for the microstructure of coated conductors. Received 25 February 2002 Published online 6 June 2002     

Phase oscillations in superfluid 3 He - B weak links

Oscillations in quantum phase about a mean value of π, observed across micropores connecting two ³ He - B baths, are explained in a Ginzburg-Landau phenomenology. The dynamics arises from the Josephson phase relation,the interbath continuity equation, and helium boundary conditions. The pores are shown to act as Josephson tunnel junctions, and the dynamic variables are the inter bath phase difference and fractional difference in superfluid density at micropores. The system maps onto a non-rigid, momentum-shortened pendulum, with inverted-orientation oscillations about a vertical tilt angle φ = π, and other modes are predicted. Received 19 March 2001     

Positive and negative Hanbury-Brown and Twiss correlations in normal metal-superconducting devices

In the light of the recent analogs of the Hanbury-Brown and Twiss experiments [1] in mesoscopic beam splitters, negative current noise correlations are recalled to be the consequence of an exclusion principle. Here, positive (bosonic) correlations are shown to exist in a fermionic system, composed of a superconductor connected to two normal reservoirs. In the Andreev regime, the correlations can either be positive or negative depending on the reflection coefficient of the beam splitter. For biases beyond the gap, the transmission of quasiparticles favors fermionic correlations. The presence of disorder enhances positive noise correlations. Potential experimental applications are discussed. Received 1 June 1999     

Semiclassical theory of integrable and rough Andreev billiards

We study the effect on the density of states in mesoscopic ballistic billiards to which a superconducting lead is attached. The expression for the density of states is derived in the semiclassical S-matrix formalism shedding light onto the origin of the differences between the semiclassical theory and the corresponding result derived from random matrix models. Applications to a square billiard geometry and billiards with boundary roughness are discussed. The saturation of the quasiparticle excitation spectrum is related to the classical dynamics of the billiard. The influence of weak magnetic fields on the proximity effect in rough Andreev billiards is discussed and an analytical formula is derived. The semiclassical theory provides an interpretation for the suppression of the proximity effect in the presence of magnetic fields as a coherence effect of time reversed trajectories. It is shown to be in good agreement with quantum mechanical calculations. Received 21 August 1999 and Received in final form 21 March 2001     

Josephson effect through a ferromagnetic layer

We report transport measurements on Superconductor/Ferromagnet/Superconductor (S/F/S) junctions: Nb/Al/Gd/Al/Nb where gadolinium (Gd) is a weakly polarized ferromagnet. A sizeable critical current I _c is observed in the I(V) characteristics. This current can be modulated by a weak magnetic field, as expected for a Josephson current. With these experiments, we establish that superconducting coherent transport survives across a small ferromagnetic layer. The penetration depth of Cooper pairs in Gd has been measured. An extensive study of the Josephson critical current in temperature for different thicknesses of magnetic compounds is presented. A comparison of transport measurements with S/N/S junction is given through measurements made on Nb/Al/Y/Al/Nb, where yttrium (Y) is used as non magnetic rare earth metal. Received 20 September 2000 and Received in final form 22 February 2001     

0-π oscillations in nanostructured Nb/Fe/Nb Josephson junctions

The physics of the π phase shift in ferromagnetic Josephson junctions may enable a range of applications for spin-electronic devices and quantum computing. We investigate transitions from “0” to “π” states in Nb/Fe/Nb Josephson junctions by varying the Fe barrier thickness from 0.5 nm to 5.5 nm. From magnetic measurements we estimate for Fe a magnetic dead layer of about 1.1 nm. By fitting the characteristic voltage oscillations with existing theoretical models we extrapolate an exchange energy of 256 meV, a Fermi velocity of 1.98 ×10⁵ m/s and an electron mean free path of 6.2 nm, in agreement with other reported values. From the temperature dependence of the I_CR_N product we show that its decay rate exhibits a nonmonotonic oscillatory behavior with the Fe barrier thickness.