Pb Junctions Research Articles

Temperature dependence of differential conductance in Co-based Heusler alloy Co2TiSn and superconductor Pb junctions

We investigated temperature dependence of differential conductance G(V) in planar junctions consisting of Co-based Heusler alloy Co2TiSn and superconductor Pb. Ferromagnetic Co2TiSn was predicted to be half-metal by first-principles band calculations. The spin polarization P of Co2TiSn was deduced to be 60.0% at 1.4K by the Andreev reflection spectroscopy. The G(V) spectral shape was smeared gradually with increasing temperature and its structure was disappeared above the superconducting transition temperature Tc. Theoretical model analysis revealed that the superconducting energy gap Δ was 1.06meV at 1.4K and the Tc was 6.8K, indicating that both values were suppressed from bulk values. However, the temperature dependent Δ(T) behavior was in good agreement with that of the Bardeen-Cooper-Schrieffer (BCS) theory. The experimental results exhibit that the superconductivity of Pb attached to half-metallic Co2TiSn was kept the conventional BCS mechanism characterized strong-coupling superconductors while its superconductivity was slightly suppressed by the superconducting proximity effect at the Co2TiSn/Pb interface.

Andreev bound states in superconductor/ferromagnet point contact Andreev reflection spectra

As charge carriers traverse a single superconductor ferromagnet interface they experience an additional spin-dependent phase angle which results in spin mixing and the formation of a bound state called the Andreev Bound State. This state is an essential component in the generation of long range spin triplet proximity induced superconductivity and yet the factors controlling the degree of spin mixing and the formation of the bound state remain elusive. Here we demonstrate that point contact Andreev reflection can be used to detect the bound state and extract the resulting spin mixing angle. By examining spectra taken from La1.15Sr1.85Mn2O7 single crystal - Pb junctions, together with a compilation of literature data on highly spin polarised systems, we show that the existence of the Andreev Bound State both resolves a number of long standing controversies in the Andreev literature as well as defining a route to quantify the strength of spin mixing at superconductor-ferromagnet interfaces. Intriguingly we find that for these high transparency junctions, the spin mixing angle appears to take a relatively narrow range of values across all the samples studied. The ferromagnets we have chosen to study share a common property in terms of their spin arrangement, and our observations may point to the importance of this property in determining the spin mixing angle under these circumstances.

Plasticity of single-atom Pb junctions

A low-temperature scanning tunneling microscope was used to fabricate atomic contacts on Pb(111). Conductance characteristics of the junctions were simultaneously recorded with forming and subsequent breaking of the contacts. A pronounced hysteresis effect in conductance traces was observed from junctions comprising the clean Pb(111) surface. The hysteretic behavior was less profound in contacts to single Pb atoms adsorbed to Pb(111). Density-functional calculations reproduced the experimental results by performing a full ab initio modeling of plastic junction deformations. A comprehensive description of the experimental findings was achieved by considering different atomic tip apex geometries.

Origin of the smaller conductances of Rh, Pb, and Co atomic junctions in hydrogen environment

We study theoretically the structural and electronic origins of the smaller conductances (one conductance quantum, G0, and smaller) of Rh, Pb, and Co metal atomic junctions (MAJs) in a hydrogen environment, as were measured in recent experiments. For the Rh MAJs, the 1G0 conductance is attributed to a stable contact bridged by a single hydrogen molecule whose antibonding state provides a single transport channel. For the Pb and Co MAJs the 1G0 conductance is, however, ascribed to a linear atomic chain adsorbing two dissociated H atoms, which largely reduces the density of states at the Fermi energy with respect to the pure ones. On the other hand, the small conductances of 0.3G0 (Rh) and 0.2G0 (Co) are due to H-decorated atomic chains connected to electrodes by a H atom.

Spin polarized conductance in ferromagnet / insulator / conventional superconductor junctions

A theory of differential conductance in ferromagnet / insulator / conventional superconductor (F/I/CS) junctions is presented and an experimental conductance is analyzed by the theory. With regard to line shapes of calculated differential conductance, as the magnitude of the exchange interaction in ferromagnets is increased, the differential conductance for the half-metallic state below the energy gap is always reduced in contrast to that for the normal metallic state. This is due to the fact that the retro-reflectivity of the Andreev reflection in the case of CS's is broken by the influence from the exchange interaction. The experimental conductance of ferromagnetic Ru2-xFexCrSi Heusler alloy / superconducting Pb junctions was in good agreement with calculational results of the theory for the F/I/CS junctions.

Enhanced Superconductivity in Nanosized Tips of Scanning Tunnelling Microscope

Nanosized Pb junctions exhibit superconducting correlations at magnetic fields more than an order of magnitude higher than the zero-temperature critical field of the bulk Pb. The strongly enhanced critical field is a spectacular demonstration of nanosize effect where the Meissner screening currents become ineffective for junction’s tip smaller than the London penetration depth (32 nm for Pb). From the enhanced critical field we characterize the geometry of a particular junction using the variable radius pair breaking theoretical model of Suderow et al.

Peculiarity of non-equilibrium superconductivity in half-metallic ferromagnet/insulator/superconductor Co2CrAl/I/Pb heterostructures

The peculiarities of electron transport trough the tunnel half-metallic ferromagnet(Co2CrAl)/insulator/superconductor(Pb) (HMF/I/S) junctions have been investigated. It was experimentally shown that the effective life-time of the spin-polarized quasi-particles ensemble in superconductor before their recombination into singlet cooper pairs is much longer than that for non-polarized ones. It was also found that the tunnel injection of spin-polarized current from HMF into S in HMF/I/S junction in its superconductive state the effect of “giant blocking” of the tunnel current is appeared. In this case at bias voltage, Vt, close to zero a very significant increase in differential resistance of the HMF/I/S junction in comparison with that of the N/I/S one is observed (where N is normal metal). Additionally, it was also established that at Vt→0 the ratio of differential resistance in superconductive state to that of in the normal state for the Co2CrAl/I/Pb junction is much bigger than corresponding ratio for N/I/S junctions obtained experimentally as well as in the frame of Bardeen, Cooper and Schrieffer theory.

MoRe-BASED AND NbN-BASED TUNNEL JUNCTIONS AND THEIR CHARACTERISTICS

Perspective for qubits Josephson Mo - Re alloy-oxide– Pb , Mo - Re alloy-normal metal–oxide– MoRe alloy, and NbN -normal metal–oxide– Pb junctions have been fabricated and investigated.

Niobium nitride Josephson junctions: Experiment and computer simulations

Highly asymmetric Josephson NbN–NbN*–Nb oxide–Pb junctions have been fabricated. A prominent feature, known as a ‘knee’, has been observed in the energy-gap region of quasiparticle current–voltage curves. It is discussed and explained within a conventional scattering approach to the phase-coherent charge transport in a quasiballistic double-barrier S1–I1–N–I2–S2 heterostructure with an extremely great difference between the two barrier transparencies.

Realization of a Good Contact Between Au and<tex>$rm Bi_2rm Sr_2rm CaCu_2rm O_8+rm y$</tex>for HTS Ramp-Edge Junctions

The fabrication process of high-T/sub C/ ramp-edge type junctions with a Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+y/ (Bi-2212) base electrode and a conventional superconducting counter electrode (Nb, Pb or Pb-In alloy) was investigated. An Au film was deposited as a layer between Bi-2212 and Nb (or Pb, Pb-In). Most of thin films were prepared by a pulsed laser deposition (PLD) method. By optimizing the annealing condition for the junction fabrication process, the higher T/sub C/'s (T/sub C//spl sim/80 K) of Bi-2212 thin films were stably available after all processes. Resistivity of Au thin film could be reduced to about 1 /spl mu//spl Omega/cm at 4.2 K by heating the sample at 300-400/spl deg/C during the in-situ Au deposition. There were difficulties to realize Bi-2212/Au/Nb (Pb or Pb-In) junctions. However, in case of Bi-2212/Au junctions, the junction interface was much improved by an interlayer process. Good homogeneous conductivity found to be kept in the fabricated Bi-2212/Au junctions, which was reproducible.

YBaCuO/PrBaCuO/YBaCuO trilayer junctions with the polished YBaCuO base electrodes

Superconducting wiring layer is indispensable to actualize digital circuits constructed with trilayer junctions. However, it is complicated to fabricate the junctions with the superconducting wiring layer. Thus, we have formed Josephson junctions on mechanical polished surface of YBaCuO films for introducing the simple device structure, such as a Pb junction, to the YBaCuO junction. The surface of YBaCuO film sputtered on vicinal SrTiO 3(0 0 1) substrate was polished by 12 nm depth. Peak–valley roughness of the surface decreased from 8 to 2 nm. After that YBaCuO film with 2 nm thickness for re-growth, PrBaCuO film with 13 nm thickness for barrier layer, and YBaCuO film for counter electrodes were consecutively deposited on the polished surface of the YBaCuO base electrodes. The junctions with the size of 10 μm × 10 μm showed resistively shunted junction like I– V curves with no excess current. The spreads of I c were 12% for 16 junctions on a chip. Applying mechanical polishing may improve the uniformity of the barrier thickness.

The oxidation state at tunnel junction interfaces

The oxidation state at the interfaces of Nb/Al–AlO x /Pb junctions is discussed. Conductance–voltage curves below and above the superconducting temperature suggest tunneling conduction, while X-ray photoelectron spectroscopy shows the existence of a thin AlO x layer at the Nb/Al interface. We demonstrate that at the usual 10 −7 Torr range of base pressures in the sputtering chamber, this is due to the time elapsed between the deposition of the Nb and Al bottom layers, in both Nb/Al–AlO x /Pb tunnel junctions and Nb/Al bilayers. We also give some direct evidence of the oxidation of the top Pb electrode on the Nb electrode surface. Such oxidation probably occurs at the pinholes of the intermediate Al–AlO x layer of the tunnel junctions, as a consequence of the oxidation state at the Nb/Al interface, which helps to avoid barrier shorts and enhances the quality of the tunnel barrier. We therefore suggest that there is oxide formation in other places besides where there is an actual oxide deposited. This is relevant for the performance of magnetic tunnel junctions since in most tunneling studies it is assumed that once the oxide is deposited, that is the only place where there is an oxide. This is also a very general statement applicable to thin film systems.

Theory of the d.c. Josephson Effect ins-wave/p-wave/s-wave Superconductor Junction

To clarify the origin of anomalous behaviors in Pb/Sr 2 RuO 4 /Pb junctions in terms of the pairing symmetry, a theory of the d.c. Josephson current in s -wave superconductor/ p -wave superconductor/ s -wave superconductor junctions is developed. Calculated results on the temperature dependence of the critical Josephson current show nonmonotonic behavior when the thickness of the p -wave superconductor is comparable to the coherence length. The consistency between the present results with recent experimental measurements support the possibility of a unitary p -wave pairing state in Sr 2 RuO 4 .

Measurement of Surface Energy States in Polyimide Langmuir-Blodgett Tunneling Barrier

We fabricated Au/polyimide (PI)/Pb junctions with the number of deposited PI Langmuir-Blodgett (LB) layers of 19–23 as tunneling barrier, and measured the current–voltage (I–V) and dV/dI–V characteristics of these junctions. Then, we estimated the distribution of the density of surface energy states in PI LB film. It was suggested that electron accepting states exist at the interface. Integration of the density of the surface energy states in PI LB film gave a value higher than 2×1024 m-3. This result supports the conclusion of our previous study, which was obtained from surface potential measurement.

Evidence for a Josephson tunnel current in all in situ c-YBa2Cu3O7/Pb junctions

All in situ ${\mathrm{YBu}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$/Pb and ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$/Ag/Pb planar tunnel junctions have been successfully made on c-axis-oriented ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ thin films grown by a coevaporation technique, which show very-low-leakage I-V characteristics. The junctions are hysteretic and display a Josephson current ${\mathrm{I}}_{\mathrm{c}}$ below 7.2 K (lead ${\mathrm{T}}_{\mathrm{c}}$). The only modification introduced by Ag is a substantial lowering of the normal-state resistance ${\mathrm{R}}_{\mathrm{n}}$, but the ${\mathrm{I}}_{\mathrm{c}}$${\mathrm{R}}_{\mathrm{n}}$ product remains roughly constant. The magnetic-field modulation of ${\mathrm{I}}_{\mathrm{c}}$ shows that the current distribution is rather homogeneous over the whole junction area (0.5 ${\mathrm{mm}}^{2}$), and follows the expected behavior for a short junction on a thin film (thickness ${\ensuremath{\lambda}}_{\mathrm{ab}}$). The Josephson coupling between a conventional superconductor and the high-${\mathrm{T}}_{\mathrm{c}}$ compound ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ along its c axis indicates that an s component is present in the superconducting order parameter of this material.

Conductance regimes in superconducting junctions of atomic size.

A scanning tunneling microscope has been used to study the different conductance regimes of superconducting Pb junctions of atomic size. Measurement of the apparent tunneling barrier and imaging of probed areas before and after the spectroscopic measurement give information on the surface condition and the geometry of the junction. The evolution of the differential conductance curves is followed from the tunneling regime to contact as the junction is varied in a continuous and controlled manner. The oberved evolution of the conductance follows quantitatively Octavio-Tinkham-Blander-Klapwijk theory with the addition of a lifetime broadening parameter, which accounts for the observed smearing of the superconducting gap features.

Experimental evidence for a d-wave pairing state in YBa2Cu3O7-y from a study of YBa2Cu3O7-y/insulator/Pb Josephson tunnel junctions.

Detailed measurements of Josephson tunneling between ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{y}}$ (YBCO) and Pb using YBCO/[MgO (or native barrier)]/Pb junctions with YBCO films either c-axis or a-axis oriented are presented. In the case of the c-axis-oriented films, the dependence of the Josephson critical current on the applied magnetic field exhibits diffraction patterns close to that characteristic of a d-wave superconductor. The temperature dependence of the Josephson critical current also shows an anomalous behavior. Incidentally, the diffraction pattern exhibits less-sensitive behavior to magnetic fields at low temperatures. These results suggest the possibility of d-wave symmetry for a YBCO superconductor.

Electron tunneling spectroscopy of single-crystal Bi2Sr2CaCu2O8.

The T-dependent conductance G(V,T) of evaporated Pb junctions on cleaved ab planes of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ (2:2:1:2) crystals is presented. Strong superconducting ${\mathit{G}}_{\mathit{s}}$(V,T) structures imply a large gap, approximately ${\mathrm{\ensuremath{\Delta}}}_{0}$(0)=26 meV, or 2${\mathrm{\ensuremath{\Delta}}}_{0}$(0)/${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$=6.7. An analysis of G(V,T) near ${\mathit{T}}_{\mathit{c}}$, assuming a lifetime-broadened BCS ${\mathit{N}}_{\mathit{s}}$(E), reveals a broadening \ensuremath{\Gamma}(T) such that \ensuremath{\Delta}(${\mathit{T}}_{\mathit{c}}$)\ensuremath{\approxeq}\ensuremath{\Gamma}(${\mathit{T}}_{\mathit{c}}$)\ensuremath{\approxeq}2${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$. The fitted \ensuremath{\Delta}(${\mathit{T}}_{\mathit{c}}$), unlike typical BCS values, is \ensuremath{\approxeq}0.8\ensuremath{\Delta}(0). $G(V,T>T sub c )--- has a minimum at V=0, suggestive of known c-direction localization in 2:2:1:2 compounds. G(V,T) also reveals the expected fluctuation superconductivity above ${\mathit{T}}_{\mathit{c}}$. 4.2-K scanning-tunneling-microscope (STM) spectra are mentioned in comparison. A larger gap, about 40 meV, and dip at 80 meV, found in the STM G(V) are tentatively attributed to ${\mathrm{CuO}}_{2}$ planes.

Origin of the linear tunneling conductance background.

Tunnel junctions often have conductances that are linear in voltage over wide voltage ranges. Although this linear conductance background has received much attention recently in connection with the high-${\mathit{T}}_{\mathit{c}}$ superconductors, it is found in many systems. Previous work has shown that the discontinuity in slope of the linear conductance background at zero bias is thermally smeared by \ensuremath{\sim}5kT, consistent with inelastic tunneling from a broad, flat continuum of states. In this paper we concentrate on a particular system, junctions with ${\mathrm{Cr}}_{2}$${\mathrm{O}}_{3}$ in the barrier region, in which the size of the effect can be adjusted by changing the barrier composition. Careful analysis of the Pb strong-coupling phonon structure in Al-${\mathrm{Cr}}_{2}$${\mathrm{O}}_{3}$-Pb junctions is consistent with the inelastic-tunneling hypothesis, and inconsistent with a voltage-dependent matrix-element explanation for this effect. Suppression of the Al gap signature, as well as Zeeman splitting of the conductance background near zero bias, indicates that strong spin interactions occur in the barrier region in these junctions. We present Monte Carlo simulations of multiple-scattering strong inelastic tunneling to explain the very wide voltage range over which the linear background occurs. We argue that the same basic mechanism is probably responsible for the linear background in junctions involving high-${\mathit{T}}_{\mathit{c}}$ junctions as for the junctions studied in detail here.

Magnetic field dependence of the critical currents in high Tc superconductors

Abstract It is shown, within a simple model calculation, that the critical current density of an S / N / S junction in a high paralel magnetic field decays exponentially. The comparison of the theoretical results with experimental data for the Pb / Cd / Pb junction and YBCO films is given.