Influence of the diffusion current on the hysteretic behavior in the system of coupled Josephson junctions

The detailed investigation of the phase dynamics and the I–V curves in the system of coupled Josephson junctions have been carried out. The superconducting, quasiparticle, diffusion, and displacement currents have been calculated as functions of the total current through the system. The role of the diffusion current in the formation of the I–V curves has been studied and the influence of this quantity on the I–V curve branching and the magnitude of the return current has been revealed. The calculation results agree qualitatively with the experimental data.     

First-principles calculation on the conductance of ruthenium-quasi cumulene-ruthenium molecular junctions

The conductance of a family of ruthenium-quasi cumulene-ruthenium molecular junctions including different numbers of carbon atoms, both in even numbers and odd numbers, are investigated using a fully self-consistent ab initio approach which combines the non-equilibrium Green’s function formalism with density functional theory. Our calculations demonstrate that although the overall transport properties of the Ru-quasi cumulene-Ru junctions with an even number of carbon atoms are different from those of the junctions with an odd number of carbon atoms, the difference between the corresponding current-voltage (I–V) characteristics of these molecular junctions declines to lesser than 16% when the voltage goes up. In each group, the molecular junctions give a large transmission around the Fermi level since the Ru-C π bonds can extend the π conjugation of the carbon chains into the Ru electrodes, and their I–V characteristics are almost linear and independent of the chain length, illustrating potential applications as conducting molecular wires in future molecular electronic devices and circuits.      

Nb/BiSCCO point conctact junctions

Summary Preparation procedure and sample characterization of 112 BiSCCO pellets exhibiting high-T _c superconductivity are discussed. TypicalI–V and dV/dI curves of Nb/BiSCCO point conctact junctions at various temperatures are presented.     

Pulse measurement of the I–V characteristics in Bi-2212 intrinsic junctions

Current-voltage (I–V) characteristics are studied in the intrinsic Josephson junctions of Bi₂Sr₂CaCu₂O_y single crystals. In order to examine the influence of self-heating, a current pulse (∼0.2 μsec) is applied to the mesas of 40 μmϕx0.15 μm patterned on the crystal. As a consequence, in contrast to small characteristic voltageV _c in the continuous-current measurement, theV _c data is found comparable to the BCS value. Moreover, theI–V curve is nearly ohmic forl>l _c , implying that the nonlinearity under the continuous current is due to heating. The quasiparticle resistance forT<T _c is also presented by an estimate from the characteristic voltage.     

Electron resonances in VLEED from Cu(111)

The sensitivity of the VLEED I–V curves to the shape and the position of the barrier is shown for the image- type surface barriers. For demonstration and comparison with experimental data the intensities of specularly reflected electron beams from Cu(111) are computed by dynamical theory of electron diffraction. Image plane position changes and modifications of the saturation shape of the surface barrier induce pronounced changes in the I–V curves calculated with the image-type barrier. Presented at the X-th Symposium on Suface Physics, Prague, Czech Republic, July 11–15, 2005.     

Zero-field current-voltage characteristics in high-Tc superconductors

Zero-field current-voltage (I–V) characteristics of various high-temperature superconductor samples are analyzed in the context of the current-temperature (I−T) phase diagram. After establishing the validity and relevance of the phase diagram to these materials, the anisotropy factor is extracted from the slope ofI _c ¹ (T) (the current defined by the onset of resistance). It is concluded that studying theI−V characteristics of amples in the context of theI−T phase diagram is a simple, useful tool for comparing samples. Work supported by the Office of Naval Research.     

On a current mechanism in Ta2O5 thin films

Electrical conduction in the temperature range of 120–370 K has been studied in sandwiched structures of Al/Ta₂O₅/Si. The tantalum oxide films were prepared by evaporation of tantalum on a p-Si crystal substrate, followed by oxidation at a temperature of 600°C. The temperature-dependent current-voltage (I–V) characteristics are explained on the basis of a phonon-assisted tunnelling model. The same explanation is given for I–V data measured on Ta₂O₅ films by other investigators. From the comparison of experimental data with theory the density of states in the interface layer is derived and the electron-phonon interaction constant is assessed.      

Conductance anomalies of small tunnel junctions inside the Coulomb gap

A small-capacitance normal tunnel deviates significantly from equilibrium because each tunneling event turns the junction voltage almost upside-down. If such a sudden perturbation occurs locally, Fermi liquid theory guarantees that infinitely many electron-hole pairs should be created near the Fermi surface. It is predicted that such an infrared-divergent shake-up combined with the electromagnetic environment leads to subgap conductance anomalies for two categories of junctions. For symmetric junctions whose electrodes have the same electronic properties, a nonvanishing subgap conductance is shown to be inevitable even if the environmental impedance is infinite. This effect smoothes the current-voltage (I–V) characteristic and shifts the Coulomb offset extrapolated back from the high-voltage part of theI–V curve. For asymmetric junctions, whose electrodes have different electronic affinities, tunneling conductance is enhanced in one direction and suppressed in the other; that is, the junctions exhibit a diode effect. In particular, when the tunneling resistance is much smaller than the resistance quantum and the current flows in the favorable direction, a strong tendency towards establishing phase coherence is shown to emerge, as in Josephson junctions, resulting in infinite differential conductance at zero bias voltage.     

YBCO Josephson junctions based on artificially generated biepitaxial grain boundary

Summary In this paper we report the in-plane fabrication and the characterization of artificially engineered biepitaxial grain boundaries (GB) obtained by partly interposing a MgO seed layer between a bare or even-buffered substrate and a CeO₂ thin layer. The main peculiarity of this technique can be summarized by the capability to locate and engineer a single 45°-tilted GB at any stage of the device preparation. The junctions, realized by patterning the grain boundary occurring in the overhanging YBCO film, show Josephson current modulation in a large temperature range with anI _c R _n value of about 200 μV at 4.2K, theR _n value being constant over the whole superconducting region. Under microwave irradiation, theI–V characteristics display several Shapiro steps while, according to the Resistively Shunted Junction (RSJ) behaviour, the step heights have the typical current biased junction dependence. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

A variable electron beam and its irradiation effect on optical and electrical properties of CdS thin films

A low energy electron accelerator has been constructed and tested. The electron beam can operate in low energy mode (100 eV to 10 keV) having a beam diameter of 8–10 mm. Thin films of CdS having thickness of 100 nm deposited on ITO-coated glass substrate by thermal evaporation method have been irradiated by electron beam in the above instrument. The I–V characteristic is found to be nonlinear before electron irradiation and linear after electron irradiation. The TEP measurement confirms the n-type nature of the material. The TEP and I–V measurements also confirm the modification of ITO/CdS interface with electron irradiation.      

High-T c Josephson junctions for electronic applications

Summary The current status of the electronic applications of high-T _c Josephson junctions is briefly reviewed. Recent results obtained by the authors on devices employing step-edge junctions are reported. In particular the design of a microwave oscillator based on a parallel array of junctions is discussed and preliminary experimental results are presented. Paper presented at the ?VII Congresso SATT? Torino, 4–7 October 1994.     

On the existence of multistable resistive states at current injection into a high-temperature superconductor

The stability of resistive states in Bi2223 superconductor without a stabilizing matrix and with decreasing temperature dependence of the resistive-transition index of its I–V characteristic is studied in a steady-state approximation. Analysis is carried out for a superconductor placed in a constant magnetic field when it is slightly cooled by a coolant with a varying working temperature. It is shown that additional stable resistive states may arise in the case of high electric fields and overheats. These (multistable) states are characterized by stable steps in the electric field intensity and temperature, which do not transfer the supercon-ductor into the normal state. The multistable resistive states may be related to the nontrivial variation of the superconductor’s differential resistance. In this case, a decrease in the critical current density of the super-conductor and its related decrease in the resistive-transition index of the I–V characteristic with increasing temperature play the key role in the formation of these states. Their presence should be taken into consideration both in analyzing the stable conditions of high-temperature superconductors and in describing processes observed in superconducting magnets when they pass into the normal state.     

Capacitance spectroscopic study of the Si/HF-electrolyte interface

Thep-Si/HF-electrolyte interface was characterized by capacitance–voltage (C–V) and current–voltage (I–V) studies. At low frequency, the measured capacitance exhibits two maxima: one in the weak accumulation regime (around 0.8 V [SCE]) and the other in the strong accumulation regime (around 2.6 V [SCE]), both of which disappear at high frequency. The disappearance of the two capacitance maxima is attributed to the slow response of interface traps to high frequencies. The flat-band potential, V_FB, is found to be frequency dependent. The surface state densities corresponding to the two capacitance maxima are estimated to be 3.2×10¹¹ cm^-2 and 2.4×10¹¹ cm^-2, respectively. The in situ I–V characteristics distinguish pore formation, transition and electropolishing regions. Porous Si synthesized at 50 mA cm^-2 gives a broad photoluminescence peak around 2.04 eV at 300 K. Received: 4 September 2000 / Accepted: 9 February 2001 / Published online: 26 April 2001     

Simulated performance of SIS and SIN junctions as heterodyne receivers in waveguide and open antenna mixer mounts

Tucker's quantum theory of mixing (in the 3-port approximation) is employed to calculate the gain over a wide range of frequencies of model mixers employing SIS and SIN junctions with both real and idealI–V characteristics. A comparison is made between the performance of junctions in waveguide and open antenna mounts. It is concluded that ideal junctions give gain 1.5 to 2 times higher than real ones, SIS junctions have gain approximately three times greater than otherwise similar SIN junctions, and that junction areas need to be typically three times smaller in open antenna structures to provide comparable gain to those in waveguide mounts.     

Current-voltage characteristics of porous-silicon structures

Summary I–V DC characteristics have been measured on metal/ porous-silicon structures. In particular, the measurements on metal/ free-standing porous-silicon film/metal devices confirmed the result, already obtained, that the metal/poroussilicon interface plays a crucial role in the transport of any device. Four-contacts measurements on free-standing layers showed that the current linearly depends on the voltage and that the conduction process is thermally activated, the activtion energy depending on the porous silicon film production parameters. Finally, annealing experiments performed in order to improve the conduction of rectifiyng contacts, are described. Paper presented at the III INSEL (Incontro Nazionale sul Silicio Emettitore di Luce), Torino, 12–13 October 1995     

Precise estimates of modulational reconstruction of the response function and its derivatives

Formulas are obtained which estimate the errors arising in the modulational reconstruction of the response function and its first and second derivatives in the hysteresis-free case. The algorithm used can yield formulas for estimating the errors in the presence of hysteresis. The results presented can be used to recover the magnetization and the I–V characteristics of high-temperature superconductors from the amplitude spectrum of the harmonics of the response signal obtained by subjecting the sample to modulated magnetic field and current. Zh. Tekh. Fiz. 67, 124–127 (August 1997)     

Emission properties of a silicon-polymer-vacuum planar structure

Self-sustained electron emission from a silicon-polymer planar structure is studied experimentally. It is found that emission can be initiated by a short-term action (e.g., a vacuum microdischarge). The I–V characteristics and time dependence of the emission current are recorded. It is shown that the value and stability of the emission current depend on the thickness of the polymer film. A model explaining the origin of and conditions for self-sustained electron emission is proposed. It is based on today’s concepts of electron emission from insulating layers (Malter effect) with regard to the structure and properties of polymer films.     

Electrophysical characteristics of nonlinear polymer-silicon composite resistors

The results of analysis of the current-voltage (I–V) characteristic and resistivity of composite resistors (varistors) based on nonpolar propylene and single-crystalline silicon (Si) are considered. The effect of the concentration of the composite components on the IV characteristic and resistivity is detected.     

Electronic properties investigation of silicon supersaturated with tellurium

Hall measurements as a function of temperature and current–voltage (I–V) measurements have been used to study the electronic properties of silicon supersaturated with tellurium (Te). The samples were fabricated by ion implantation, followed by thermal annealing at different temperatures in N₂ and by rapid quenching. The experiment results show that the effects on the resistivity and on the Hall effect of samples implanted with high doses of Te are almost insensitive to temperature. This is not consistent with those of low dose Te implanted samples. The dark I–V characteristics and white light I–V characteristics of the junction between the high dose Te implanted layer and the substrate were also investigated. The pn junctions of samples with non equilibrium dopant concentrations exhibit decreased rectification, but increased photon to electron conversion efficiency.     

Understanding of a-Si:H(p)/c-Si(n) heterojunction solar cell through analysis of cells with point-contacted p/n junction

We fabricated point-contacted a-Si:H(p)/c-Si(n) heterojunction solar cells using patterned SiO₂ and investigated their electrical properties using the light current–voltage (I–V) curve and Suns-V_oc measurements. The light I–V curves showed bias-dependent changes according to the applied voltage in the point-contacted cells, especially in the samples with a long distance between the point-contacted junctions. The Suns-V_oc measurements showed that the bias-dependence of the light I–V curves did not originate from the recombination in the SiO₂/Si or a-Si:H(p)/c-Si(n) interface, but from the series resistances. It is possible to explain the bias-dependent light I–V curve in terms of the conductivity of a-Si:H(p) and difference in the electrical contact properties between a-Si:H(p), ZnO and c-Si(n). These results mean that the electrical properties of the a-Si:H(p) layer and the contact properties with this layer are also critical to obtain a high J_sc and fill factor in n-type based Si heterojunction solar cells.