Lower critical field and magnetization of strong-coupling,type II superconductors in the dirty limit

Usadel's dirty-limit version of the Gorkov-Eilenberger theory including strong-coupling effects is used to calculate the lower critical field of superconducting alloys. The results of the (rigorous) numerical study, which is valid for all temperatures and Ginzburg-Landau parameters , are compared with experiments and with the existing weak-coupling theory. One achieves agreement with the bulk of the experimental results only if is properly adjusted. In addition, it is shown that a circular-cell approximation for the vortex state yields highly accurate magnetization curves, which are influenced in a characteristic way by strong electron-phonon coupling.     

Crossover from dirty to clean superconducting limit in dc magnetron-sputtered thin Nb films

High-quality Nb (110) thin films with residual resistance ratios up to 60 and critical temperatures T_c ≈ 9.27 K have been prepared by conventional dc-magnetron sputtering on α-Al₂O₃ by careful selection of the sputtering conditions. This allowed for a systematic study of the influence of the growth rate on the structural quality and the superconducting properties of the films. The optimized growth conditions were revealed at the substrate temperature T_s = 850 °C, Ar pressure P_s = 0.4 Pa, and the growth rate g ? 0.5 nm/s. The results of the films' structural characterization by X-ray diffraction, reflection high-energy electron diffraction, and atomic force microscopy are presented. In terms of the electron mean free path l and the superconducting coherence length ξ, deduced from the magneto-resistivity data, the clean superconducting limit (l > ξ) is realized in the high-purity films. For comparison, in impure Nb films sputtered at room temperature while keeping the rest of the sputtering parameters unvaried, the opposite dirty limit (ξ ? l) ensues. The merits of these findings are discussed in the context of the demands of present-day fluxonics devices regarding the normal-state and flux-flow properties of superconducting films they are made of.     

Geometrical resonance effects in HTSC break junctions

A periodic structure has been observed in the tunneling characteristics of HTSC break junctions. This structure is explained in terms of the electron-hole interference effect in the surface layer, which causes the formation of bound states in the normal regions N of the SNINS-type junctions. Good qualitative agreement has been found between experimental data and the predictions of the Arnolds proximity model. Several parameters of the SNINS-type structures have been derived from studies of Fiske resonances in HTSC Josephson break junctions in weak magnetic fields.     

Hot-electron effects in InAs nanowire Josephson junctions

The controlled tailoring of the energy distribution in an electron system opens the way to interesting new physics and device concepts, as demonstrated by research on metallic nanodevices during recent years. Here we investigate how Josephson coupling in a superconductor-InAs nanowire junction can be tuned by means of hot-electron injection and we show that a complete suppression of superconductive effects can be achieved using a power as low as 100 pW. Nanowires offer a novel design freedom as they allow axial and radial heterostructures to be defined as well as control over doping profiles, which can be crucial in the development of devices—such as nanorefrigerators—where precisely controlled and predictable energy barriers are mandatory. Our work provides estimates for unknown key thermal and electrical parameters, such as the electron-phonon coupling, in our InAs nanostructures.      

AC electric field-induced alignment and long-range assembly of multi-wall carbon nanotubes inside aqueous media

The present work examines the behavior of multiwall carbon nanotubes (MWCNT) inside AC electric fields created by three-dimensional electrodes. The response of carbon nanotubes stably suspended in water with the aid of a nonionic surfactant is monitored by combining microscopic observations with on-line measurements of the suspension resistivity. It is found that polarization effects induced by the externally applied AC electric field on MWCNTs can cause their unidirectional orientation and end-to-end contact that result in formations of spatially distributed, long-range, three-dimensional and electrically conducting structures that span the entire gap between the electrodes. The length of the formed structures, which in the present case was approximately 30 times larger than that of an individual carbon nanotube, can be controlled by adjusting the spacing between the electrodes. The influence of main experimental parameters, namely, MWCNT concentration, applied voltage, AC field frequency, and electrode surface topography on the suspension behavior is experimentally examined. Results are demonstrated for applied voltage values, AC field frequencies, and carbon nanotube concentrations in the range 4-40 Vptp, 10 Hz-5 MHz, and 0.001-2.0 wt%, respectively. While higher electric field strengths accelerate the formation of aligned structures, higher frequency values were found to result in suspensions that exhibit smaller electrical resistivity. Carbon nanotube dispersions exposed to an AC electric field exhibit a 100-fold or more decrease in their electrical resistivity, even when carbon nanotube concentrations as low as 0.005 wt% are used.     

Local effects across core junctions in sandwich panels

Sandwich beams and panels with symmetric faces and cores of varying stiffness are investigated. The paper presents a theoretical and experimental study of the local effects that occur in the vicinity of intersections between cores of different stiffness in such sandwich panels. These local effects manifest themselves by a significant rise of the bending stresses in the faces in the vicinity of the core junctions. Closed-form estimates of the stress/strain fields induced by local effects are presented for sandwich beams and panels loaded in cylindrical bending. The accuracy of the derived closed-form estimates is verified experimentally for the case of a sandwich beam in three-point bending.     

Static Josephson effects in circularly symmetric annular junctions

Static Josephson effects in circularly symmetric annular junctions are discussed. When the annular width W of a junction is smaller than _J, analytic solutions can be obtained. For annular width W of a junction larger than _J, the self-field effects must be considered; the solutions can be obtained numerically. For these two cases, called "small junction" and "large junction," respectively, the magnetic field dependence of the critical current I _m of a circularly symmetric annular junction is obtained. Further, for a width W larger than _J, the current density distribution is also given for several magnetic fields. In addition, for a given external magnetic field, when the feeding current I is smaller than the critical current I _m of the circular symmetric annular junction, there are multisolutions under such physical conditions. Finally, the stability of these solutions is also discussed.     

Damage effects in as-prepared recoil-implanted shallow p-n junctions

This work investigated the damage effects in shallow Si p⁺-n junctions formed by recoil implantation. By implanting through a thin aluminium film, it was possible to achieve very shallow p-n junctions (<100 nm). Using different implant dosaqes, both the junction depths as well as the damage densities were varied.I-V curves were used to study the electrical characteristics after low temperature heat-treatment at 450 °C. It was observed that the devices changed from Schottky diodes to shallow p-n junctions as the implant dosages increased. At the intermediate dosage, recombination current was found to be important. There were evidence that the densities of the recombination centers could be related to those of the displaced Si atoms. At the very high implant dosages, diffusion current dominated.     

Iron saturation effects in PM AC motors

Iron saturation causes the lumped circuit parameters to vary with the point of operation. Experimental evidence shows that the field excitation voltage and the direct axis reactance are not affected significantly by the armature reaction. This can be attributed to the presence in the direct axis of the magnets whose thickness represents a large effective air gap. In the quadrature axis, instead, the contribution of the air gap to the reluctance is small. Therefore, the quadrature axis reactance depends strongly on both the quadrature and direct axis components of armature magnetomotive force. This suggests a simple, albeit approximate, approach for predicting the parameter variation with operating point. As a result, both the design and analysis of synchronous motors excited by permanent magnets (PM's) are facilitated.     

Influence of long-range and strong correlation effects on the electron-phonon coupling in high-T c oxides

Calculations of the Eliashberg functionα ²F and the corresponding transport functionα _tr ² F for high-T _c oxides are presented using a screened ionic model (rigidly shifted ionic potentials screened by charge carriers in the CuO₂ planes within the RPA) for the electron-phonon coupling. It is shown that this model yields a large difference between the transport and the superconducting electron-phonon interaction due to imperfect screening and contributions beyond nearest-neighbor interactions. Using these results, the electron and the lattice heat conductivities are calculated both in the normal and the superconducting state and compared with experiment. Finally, effects due to a strong on-site electron-electron repulsion are included in leading order in an 1/N expansion, whereN is the number of spin degrees of freedom. In particular, it is shown for the infiniteU, one-band Hubbard model that correlations tend to suppressα _tr ² F strongly andα ² F somewhat.     

Degrading effects of the lower atmosphere on long-range airborne synthetic aperture radar imaging

The imaging performance of airborne synthetic aperture radar (SAR) systems has advanced to the point that the effects of clear-air refractive index perturbations cannot be ignored. Operating at long ranges, and low grazing angles, in particular, require propagation geometries through regions of the lower atmosphere that may cause noticeable and, sometimes, severe degradation of the images. The range of image anomalies that can be attributed to the atmospheric boundary layer (ABL) is illustrated, the pertinent characteristics of the ABL is discussed, a first-order SAR imaging model that incorporates the refractive index perturbations associated with the ABL is developed and the magnitude of the image anomalies resulting from measured refractive index perturbations is estimated. The model predictions correlate well with the observed image anomalies and measured properties of the ABL. On the basis of theory and measurements, it is expected that the degrading effect of clear-air atmospheric refractive index perturbations is much more common than previously thought and may be a limiting factor for long-range SAR imaging performance.     

Electronic transport in crystalline magnetotunnel junctions: effects of structural disorder

We have modeled the electrical properties of an Fe–MgO–Fe magnetotunnel junction (MTJ), using a combination of density functional theory and non-equilibrium Green’s functions. We have performed calculations for two series of random perturbations to the atomic coordinates perpendicular to the Fe/MgO interface, and report on the effects of such random variations of the structure on the calculated conductance. We find that the variations lead to significant differences in conductance.     

Phenomenological treatment of exchange effects in solid3He in the high-temperature limit

Using an expansion in1/T andH ² for the free energy, we phenomenologically obtain a good fit to the existing high-temperature thermodynamic data for bcc ³ He at both high and low magnetic field. In addition, we show that our coefficients can be obtained by forming a synthesis of the independent theories resulting from (1) next-near neighbor exchange and (2) spin—phonon coupling.Alfred P. Sloan Fellow.     

Hall effect in the vortex state of dirty type-II superconductors

The off-diagonal component of the conductivity tensor of dirty type-II superconductors is calculated in the magnetic field nearH _c2 . Contributions come from both the fluctuation and static parts. The former is the Ginzburg-Landau (GL) term, whose origin is the motion of Cooper pairs, and is obtained to the order ofT/ _F as the result of the energy dependence of the density of states near the Fermi surface. The latter is obtained to the order of _c . By use of the diagonal component of the conductivity tensor given by Thompson, Takayama, and Ebisawa, the derivative of the Hall angle with respect toH is calculated; it agrees qualitatively with the observed behavior.A part of Ph.D. thesis submitted to University of Tokyo, 1971.Work supported by the Research Institute for Fundamental Physics, Kyoto University.     

On some effects in a system of two interacting Josephson junctions

We consider two long, weakly interacting Josephson junctions in an external magnetic field. A uniformly distributed direct current applied to the system excites the movement of a vortex chain within each junction. The weak interaction between the vortex chains results in monochromatical radiation from the system and some peculiarities in theI–V curve.     

Ultrasonic attenuation in the mixed state of a dirty superconductor

Transport coefficients in the mixed state have simple expressions when the response is local. For dirty materials the ultrasonic attenuation is given by the local expression over the whole mixed-state regime because of the Coulomb interaction between the electrons, and responses of the order parameter and magnetic field to the sound wave do not contribute to the attenuation. Computations based on the theory are compared with experiment.On leave of absence from Research Institute of Electrical Communication, Tohoku University, Sendai, Japan.     

Interpretation of effects at the static fatigue limit of soda-lime-silicate glass

Crack growth in soda-lime-silicate glass near the static fatigue limit is rationalized by a fracture mechanics model of the crack tip, in which a stressed layer is built up on the crack surface as a consequence of ion exchange at the crack tip. This model extends the one presented earlier by Bunker and Michalske. Ion exchange, between hydronium (H₃O⁺) ions in the solution and sodium (Na⁺) ions in the glass, gives rise to compressive stresses at the tips of cracks in soda-lime-silicate glasses. These compressive stresses are responsible for (1) the occurrence of a fatigue limit in glass, (2) for the fact that crack tips remain sharp at the fatigue limit even though the walls of the crack are corroded by the basic solutions that form as a consequence of ion exchange, (3) for the crack tip bifurcation often observed when cracks are held at the fatigue limit for a while and then restarted at higher loads, and (4) for the fact that a delay time to restart the crack is often observed after the crack is held under load at the static fatigue limit. Most of the predictions are in quantitative agreement with experimental observations on crack growth and crack tip structure for soda-lime-silicate glass. The prediction of the time required to restart the crack is, however, only qualitatively correct, as experimental data report a sharp peak centered at the fatigue limit in the plot of restart time versus hold stress intensity factor, whereas the model gives a broad maximum on such a plot. Clearly, further development of the model will be needed for a better representation of the experimental data.     

Modelling the effects of surface finish on fatigue limit in austenitic stainless steels

Existing short fatigue crack models have been reviewed to determine the most suitable fatigue model to analyse the effect of the surface finish on the fatigue limit of Type 304 austenitic stainless steels. A mechanistic model firstly proposed by Navarro and Rios (N‐R model) was selected as the most suitable generic model, because the model can include the effects of surface finishing parameters such as surface roughness and residual stress depth profile on the fatigue limit. The N‐R model has been implemented for fatigue specimens with various surface finishing conditions, and the effect of the surface finish on the fatigue limit was simulated. The material/surface properties required for the implementation were fully characterized by experiments. The applicability of the model to this study was also discussed. It is concluded that a development of the model would be required for proper prediction of the surface effects on fatigue in austenitic stainless steels.     

Image force effects on trapezoidal barrier parameters in metal-insulator-metal tunnel junctions

A one-dimensional trapezoidal barrier potential U₀(x) with and without the image potential U_i(x) was used to model three tunnel junctions: Al-Al₂O₃-Au (-Ag, -Cu) and calculate the junction I-V curves. As the effective trapezoidal barrier parameters are determined by fitting the calculated I-V curves to the experimental ones, it is found that if the I-V curves are calculated based on the barrier potential U₀(x)+U_i(x), the parameters of the actual barrier U₀(x)−U_i(x) are obtained. Thus, the parameters of the actual trapezoidal barrier U₀(x)+U_i(x) can be obtained by extrapolating. The results show that the area of the actual U₀(x)+U_i(x) is smaller than that of the U₀(x) and for the Au-, Ag- and Cu-junction the area reduction is − 6.9, −5.4 and −4.2%, respectively. The variation of the barrier area reduction with the electrodes can be ascribed to the diffusion and oxidation reaction in the region of the junction interfaces, which results in smeared and indistinct interfaces and an increase in the actual distance between the metal mirrors of the two electrodes.     

Peculiarities of the long-range effects in GaAs-Based transistor structures upon combined irradiation with ions of various masses

Anomalous distinctions in the depth profiles of charge carrier density in the active regions of GaAs-based structures were observed for samples irradiated from the rear side (through substrate) with molecular hydrogen and argon ions either separately or in a certain combined sequence. The maximum effect of ion irradiation was observed in the structures characterized by increased density of defects in the buffer layer. The experimental results are explained by reconstruction of the impurity-defect complexes at the layer interfaces under the action of elastic waves arising both as a result of relaxation of the atomic displacement peaks in the ion stopping zone and due to the reverse piezoelectric effect.