Dynamic Conductance Research Articles

DYNAMIC PROPERTIES OF SEQUENTIAL TUNNELING THROUGH DOUBLE-BARRIER SYSTEMS

In this paper, we present an analytical stochastical approach to the dynamic properties of sequential tunneling through double-barrier systems. The effect of charge accumulation is included in the investigation of dynamic conductance and noise current power density at finite frequency. Albeit in the sequential tunneling limit the quantum phase coherence of electron waves is destroyed by inelastic scattering while traversing the junction, the occupation of resonance states by fermionic particles sojourning in the well along with the charge accumulation within and around the structure still give rise to strong correlation among tunneling events. This correlation determines the characteristic frequency of the system and leads to significant suppression of shot noise. The low frequency noise current power density compares agreeably with experimental measurements.

Dynamic conductance and the scattering matrix of small conductors.

The current response to oscillating electric or magnetic fields acting on the carriers in the probes of a multichannel, mutlilead conductor is investigated. For a noninteracting system we find a frequency-dependent admittance matrix which is expressed in terms of scattering matrices. A self-consistent potential method is used to include Coulomb interactions. The low-frequency departure of the admittance away from the dc conductance is discussed in terms of phase-delay times and RC times.

Potential of NdCeCuO thin films for electronic applications

Films of NdCeCuO with high crystalline quality (channeling yield of 6%) were deposited on the low-loss material LaAlO/sub 3/, making it possible to measure the microwave surface losses at 18.29 GHz. An encouraging value of 1 m Omega was found at 4.2 K. The T/sub c/ is still lower than on SrTiO/sub 3/, but this is attributed to the fact that the deposition parameters have not yet been explored as systematically as for SrTiO/sub 3/. The deposition on artificial SrTiO/sub 3/ bicrystals yields grain boundaries with nonlinear I-V characteristics. The dynamic conductance looks similar to that observed on tunnel junctions of other high-T/sub c/ compounds, suggesting that quasiparticle tunneling is taking place for high-angle grain boundaries.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Fabrication and tunneling characteristics of low-leakage All-YBa/sub 2/Cu/sub 3/O/sub 7-x/ edge junctions

All-YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) superconductor-insulator superconductor tunnel edge junctions have been fabricated. Y/sub 2/O/sub 3/ is used as the barrier material. YBCO and Y/sub 2/O/sub 3/ are deposited by electron-beam evaporation under the same deposition conditions. Special attention is paid to creating a smooth base-electrode edge by ion-mill etching. The junction width is reduced to submicron size in order to avoid defects in the junction region. The YBCO superconductive gap structure observed in dynamic conductance, measurements is very pronounced. Quasi-particle tunneling is observed with very low leakage in the subgap region. No zero-bias anomalies are present. The edge junction fabrication and the junction tunneling characteristics are discussed. >

Pauli-limited critical field of Ba/sub 1-x/K/sub x/BiO/sub 3/ determined from quasiparticle tunneling

A planar tunnel diode with Ba/sub 1-x/K/sub x/BiO/sub 3/ (BKBO) and Ag electrodes were fabricated. The dynamic conductance of the junction was similar to that found with conventional BCS superconductors. Using weak coupling theory, the undepaired transition temperature, spin-orbit scattering rate, and diffusion constant of BKBO were determined. The Pauli-limited critical field was calculated and compared to the measured resistive transitions of the BKBO film in magnetic fields. The Pauli-limited field is observed to occur generally at the field where the normal-state resistance is restored.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Superconducting and structural properties of homogeneous BiSrCaCuO (2212) single crystals prepared by solution growth

Abstract The modified method for growing of single crystal BSCCO (2212) from solution-melt in KCl was developed. The dependency of crystal properties on growth conditions was studied. The crystals have platelet-like form with thickness of about 200 μm and lateral sizes up to 3 mm. Chemical composition corresponds to formula Bi2.4Sr1.5Ca0.9 Cu2.2O8+δ. X-ray diffraction showed the block structure of the crystals with block sizes 100–200 μm and its misorientation 1°–2°. Crystals have lattice parameters a = b = 5.39 A , c = 30.731 A . The Tc and ΔTc measured by d.c. resistive and a.c. susceptibility techniques were Tc = 80 ± 3 K and ΔTc (10%–90%) = 2–6 K. The measuring of energy gap parameter by break-junction tunneling technique results in value 2Δ(0) k B T c ≅ 7 . The dynamic conductance of tunneling junction showed the additional structures due to peaks in the BSCCO phonon density of states. The analysis of the results allows us to conclude that the presence of atmospheric oxygen during crystal lattice forming has more effect than post-annealing in oxygen of as-grown crystals.

Low-leakage superconductive tunneling in all-YBa2Cu3O7−x edge junctions

All-YBa2Cu3O7−x (YBCO) superconductor-insulator-superconductor (SIS) edge-type tunnel junctions have been fabricated. This leads to the observation of superconductive tunneling with very low small-bias conductance for the first time in fabricated all-YBCO junctions. The presence of a superconductive gap of YBCO with a complete suppression of the density of states at the Fermi surface is evident from dynamic conductance measurements. The approach to all-YBCO SIS edge junctions is discussed.

Quasi-particle and Josephson tunnelling characteristics of the single-grain boundary in BaPb1-xBixO3 bicrystals

Bulk bicrystals of the oxide superconductor BaPb1-xBixO3, with parameter x=0.27 and critical temperature 10.9 K, were grown by crystallization in a PbO-Bi2O3-BaO melt. Current-voltage (I-V) characteristics of the single-grain boundary, constituting a plane between superconducting blocks, were measured. The I-V characteristics had a shape typical of the Josephson junction with a critical current density of lower than 0.1 A cm2 at 4.2 K. The temperature dependences of the critical current density of the single-grain boundary were measured. Clear peaks on the dynamic conductance against bias voltage dependences of the bicrystals were observed and interpreted as energy-gap peculiarities. The temperature dependence of the energy gap of BaPb1-xBixO3 and the ratio 2 Delta (0)/kTc=3.6 were determined.

Inelastic electron tunneling spectrometer for complete calibrated measurements of any two or four terminal junctions

The conventional tunneling spectrometer gives the uncalibrated second derivative d2V/dI2 versus applied voltage (V) of the junction current-voltage curve (I-V). However, the calibrated second derivative d2I/dV2 is more useful for accurate comparison with theory and can be applied to negative resistance devices. We report here a single electronic instrument for calibrated measurements of all relevant tunneling junction parameters, i.e., dynamic conductance (G), dynamic capacitance (C), current-voltage curve, and its first and second calibrated derivatives. Moreover, it can measure the derivative of the dynamic capacitance (dC/dV) versus applied potential, which is useful for various types of semiconductor devices. This design is versatile enough to find many laboratory applications where current-voltage curves are of interest. The circuit, based on a simple design, is accurate to 1% and allows spectral acquisition in about 15 min.

Sulfur dioxide-induced bronchoconstriction in the isolated perfused and ventilated guinea-pig lung.

SO2 exposure (50-500 ppm) of isolated, perfused and ventilated guinea pig lungs, via the air passages, caused a concentration-related reduction in dynamic compliance and conductance. No changes in pulmonary perfusion flow was noted at any SO2 concentration. Formed sulfite was detected in lung lavage fluid as well as in the perfusate. Pretreatment of the lungs with a low concentration of SO2 (10 ppm) for 30 min protected against bronchoconstriction by a high concentration of SO2 (250 ppm). A similar protective effect was noted by pretreatment with sodium sulfite (3 mM) in the lung perfusate.

Microshort-to-tunneling transition in Au-YBa2Cu3O7- delta single-crystal point contacts.

We report on the evolution of dynamic conductance curves [(dI/dV)-V] as a function of junction resistance in $Au-YBa_2Cu_3O_{7-\delta}$ single-crystal point-contact junctions. We have demonstrated that it is possible to evolve continuously from a microshort (N-S) to a tunnel junction (N-I-S) by adjusting the contact pressure in the point-contact junction. At the lowest junction resistances, we see a peak in dI/dV at zero bias, which we explain as due to Andreev reflection. As the junction resistance is gradually increased, the zero-bias peak diminishes and gaplike features appear in the (dI/dV)-V curves. At even higher resistance, the zero-bias peak vanishes, all features get smeared out, and a linear dI/dV variation with the bias voltage is seen. Based on the similarity of (dI/dV)-V curves obtained in high-resistance $Au-YBa_2Cu_3O_{7-\delta}$ junctions with those obtained in a nonsuperconducting perovskite oxide metal ($Au-Na_xWO_3$ junctions, x$\simeq$0.45), we suggest that the linear (dI/dV)-V curves arise due to inelastic scattering of electrons at a ‘‘metallic’’ layer on the surface of the superconductor.

Effect of surface on the conductance characteristics of Au-Bi2Sr2CaCu2O8+δ (single crystal) point contact junctions

We report the effect of surface treatments on the dynamic conductance curves (G=dI/dV-V) of Au-Bi2Sr2CaCu2O8+δ (single crystal) point contact junctions of variable junction conductances (100 mS≳G≳100 μS). We find that if the crystal surface is cleaved freshly just prior to making contacts, all irreproducible sharp multiple features often observed in tunneling data of Bi(2212) oxide superconductors disappear. If the cleaved crystal surfaces are left under ambient conditions for a few days and the tunneling experiments are repeated, these multiple features reappear. We also find that if the current in the junction is made to pass predominantly through the bulk (and not along the surface), gap features are sharper. The observed conductance curves are fitted to a modified model [G. E. Blonder et al., Phys. Rev. B 25, 4515 (1982)] and estimated gap values are Δ≂28 to 30 meV corresponding to the ratio 2Δ/kBTc ≂ 7.5 with lifetime broadening Γ/Δ≂0.2. We conclude that the sharp multiple features observed in Bi(2212) tunneling curves has no intrinsic origin in the bulk and they arise from the surface only.

Magnetic field tuned energy of a single two-level system in a meso- scopic metal.

We study the effect of an external magnetic field H on an individual tunneling system by measuring the time-dependent bistable conductance of a submicron Bi sample near 1 K. The amplitude of the dynamic conductance fluctuations is a random function of H. The transition rates are significantly modified by the field, from which we infer an H-dependent energy splitting. This splitting is symmetric near H=0, but exhibits nonmonotonic features at higher fields, possibly arising from the coupling of local electron-density fluctuations to the defect.

A comparison of barrier type tunnel junction and point contact tunnel junction formed on the same highT c material

By making a combination of both point contact and barrier type tunnel junctions on a single sample of the high $T_c$ superconductor BSCCO (2212) single crystal, we have shown that as the tunneling tip is slowly retracted from the surface a point contact junction gradually evolves from a N-S short to a high resistance tunnel junction. The scaled dynamic conductance (dl/dV) of this point contact tunnel junction becomes almost identical to that of a conventional barrier type tunnel junction and both show a linear dl/dV — V curve. The observation implies that at high resistance a point contact junction behaves in the same way as a barrier type tunnel junction. We suggested that the almost linear tunneling conductance obtained in both the cases most likely arises due to an intrinsic characteristic of the surface of the crystal comprising of a mosaic of superconducting regions of the order of a few nanometers. We also conclude that the barrierless (N-S) point contact obtained by piercing the surface oxide layer of the crystal shows Andreev reflection which we suggest as the origin of the zero bias anomaly often observed in point contact junctions.

YBa2Cu3O7−δ-Ag-Al/Al2O3/Pb tunnel junctions based on the superconducting proximity effect

YBa2Cu3O7−δ-Ag-Al/Al2O3/Pb superconducting tunnel junctions with high subgap resistance were fabricated using the proximity effect induced superconductivity in the Ag-Al layer by the YBa2Cu3O7−δ film. At low temperature an energy gap of 9–10 meV is found to be induced in the aluminum layer. Above 20 K the induced gap disappears mainly due to the decreased normal metal coherence length. At higher voltages (V≤120 meV) the dynamic conductance dI/dV∝V, characteristic for the normal tunneling density of states of the oxide superconductor.

Quantum susceptance and its effects on the high-frequency response of superconducting tunnel junctions.

We have made the first direct measurement of the quantum susceptance that arises from the nondissipative part of quasiparticle tunneling in a superconductor-insulator-superconductor tunnel junction. The junction is coupled to an antenna and a superconducting microstrip stub to form a resonator; the resonant frequency is determined from the response of the junction to broadband radiation from a Fourier-transform spectrometer. A 19% shift of the resonant frequency, from 73 to 87 GHz, is observed, which arises from the change of the quantum susceptance of the junction with dc bias voltage. This shift is in excellent agreement with calculations based on the Werthamer-Tucker theory, which includes the quantum susceptance. We also demonstrate that it is essential to include the quantum susceptance in our theoretical computation to explain the photon-assisted-tunneling steps, which have negative dynamic conductance. Such steps are observed when the junction is pumped at slightly below the resonant frequency of the capacitor and the stub. The quantum susceptance should exist in all tunnel devices whose nonlinear I-V characteristics are due to elastic tunneling.

Scanning tunneling microscopy and tunneling spectroscopy of the titania(001) surface

The (001) surface of a highly doped ({approximately} 10{sup 19} cm{sup {minus}3}) n-type TiO{sub 2} crystal, following polishing, wet etching, and cleaning, was imaged in air with a Pt-Ir tip with a scanning tunneling microscope (STM). Good images of the surface could be obtained with the sample held at negative voltage (positive tip bias), with the high-resolution image showing only Ti atoms. Tunneling spectroscopy, involving the recording of i-V curves and dynamic conductance (di/dV)-V curves with the tip held over a given spot on the TiO{sub 2} surface, was also carried out. The resulting curves show electron tunneling into the conduction band and from the valence band, as well as a prominent peak identified as a surface state about 0.3 eV below the conduction band edge.

Leakage current in GaInAs/InP photodiodes grown by OMVPE

Reverse-bias dark current and zero-bias dynamic conductance are studied for large area p-i-n photodiodes of Ga 0.47In 0.53As grown on InP substrates by organometallic vapor phase epitaxy (OMVPE). The leakage in GaInAs homojunction mesa diodes is dominated by generation current near the intersection of the mesa surface with the GaInAs depletion region. The dynamic conductance at zero volts depends on both the length and orientation of the perimeter for these diodes. Double heterostructure InP/GaInAs/InP photodiodes with the p-n junction in the InP demonstrate lower zero-bias conductance which depends on diode area. The temperature dependence of leakage current in a double heterostructure p-i-n diode on a p-substrate is approximately fit by generation current in the GaInAs as well but with a much larger effective generation lifetime. The lowest leakage current measured is J = 3.6 × 10 -7 A/ cm 2 at - 10 V for this structure. Responsivities of these diodes at 1300 nm are 0.9 A/W for reverse biases 6 V. Using InP regrowth, 600 μm diodes with 300 K dynamic conductance < 7 × 10 -7 Ω -1 and responsivity 0.95 A/W at 0 V are fabricated.

Evidence for spin fluctuations in vanadium from a tunneling study of Fermi-liquid effects.

The Fermi-liquid parameter ${G}_{0}$ is found to be close to zero in vanadium, providing new experimental evidence for the importance of spin fluctuations in this material. Al/${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$/V and V/${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$/Fe tunnel junctions were used to observe the renormalization caused by Fermi-liquid interactions of the spin splitting of the vanadium's superconducting density of states in a magnetic field. This was done by fitting the theory of Rainer to the measured dynamic tunneling conductance. Excellent agreement was found with this theory, and the results were consistent with critical field measurements made on the same vanadium films. The spin-orbit scattering rate ${b}_{\mathrm{so}}$ was determined to be 0.07\ifmmode\pm\else\textpm\fi{}0.03.

Tunneling study of Fermi-liquid effects in amorphous gallium.

The Fermi-liquid parameter ${G}_{0}$ has been determined in amorphous gallium. Al/${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$/a-Ga tunnel junctions were used to observe the renormalization caused by Fermi-liquid interactions of the spin splitting of the a-Ga superconducting density of states in a magnetic field. This was done by fitting the dynamic tunneling conductance to the theory of Rainer. Excellent agreement was found with this theory, and the results were consistent with critical-field measurements made on the same films. A large renormalization was observed (${G}_{0}$=0.81\ifmmode\pm\else\textpm\fi{}0.14), as expected for this strongly coupled material. Correlation effects were also seen. The spin-orbit scattering rate ${b}_{\mathrm{SO}}$ was found to be 0.18\ifmmode\pm\else\textpm\fi{}0.03.