Dynamic Conductance Research Articles

Ac transport through a resonant level between ferromagnetic electrodes

We report the investigation of the spin-valve effect through a resonant level between a ferromagnetic electrode in the presence of an external ac bias. We use the current conserving and gauge invariant theory developed by B\uttiker to calculate the dynamic conductance. Specifically, we have calculated the tunneling magnetoresistance (TMR) ratio as a function of various system parameters such as the angle between magnetization of the left and right leads, ac frequency, and the Fermi energy. We found that the TMR ratio can be modulated by ac frequency. At large frequency, the TMR ratio can be negative.

Room-temperature tunneling magnetoresistance in La0.7Sr0.3MnO3 step-edge junctions

La 0.7 Sr 0.3 MnO 3 tunneling magnetoresistance (TMR) junctions have been fabricated on step-edge (001) SrTiO3 substrates with a high step-edge angle. The step-edge junctions show nonvanishing TMR spikes in R(H) curves above room temperature (RT). The resistance, dynamic conductance, and TMR ratio are characterized to explore the possible conduction mechanism for the step-edge junctions. The temperature dependence of surface magnetization MS(T) deduced from the spin polarization P(T), following a (Tc−T)0.92±0.02 dependence, agrees with the theoretical models for MS(T). From these results, we conclude that tunneling is the dominating mechanism and that the charge carriers at the surface boundary govern the tunneling conductivity.

Deposition of nanostructured thin films using an inductively coupled plasma chemical vapor deposition technique

Nanostructured materials are the fundamental building block for the realization of nanotechnology. In this work, we propose a 13.56 MHz radio-frequency (RF) inductively coupled plasma chemical vapor deposition (ICP–CVD) system for the preparation of nanostructured materials. The material we deposited is tin(IV) oxide, which has found application as the gas sensing layer in the making of miniaturized chemical gas sensors. The deposited film was characterized for its structural properties using X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). For electrical properties, four-point probe and dynamic conductance temperature measurements were performed. In this paper, we describe on our novel system as a promising candidate in the synthesis of nanostructured materials, together with the preliminary results of tin(IV) oxide deposition.

Temperature and bias dependence of dynamic conductance—low resistive magnetic tunnel junctions

I–V curves of magnetic tunnel junctions were measured in a temperature range of 5–305 K. Effective barrier parameters were estimated by fitting the dynamical conductance G(V) with the Brinkman–Dynes–Rowell model and fitting the temperature dependence of zero-bias conductance G(T) with the Stratton model. A large discrepancy was discovered when comparing barrier parameters predicted by the two models. The inconsistency between the models can be explained by the presence of an inelastic, spin-independent conductance that is strongly dependent on both temperature and voltage as described by Glazman–Matveev theory of electron hopping. This additional hopping conductance helps explain the observed temperature dependence and bias dependence of magnetic tunnel junction conductance.

Nonlinear electrical transport through artificial grain-boundary junctions inLa0.7Ca0.3MnO3epitaxial thin films

We report investigation of non-linear electronic transport through artificial grain-boundary junctions made on epitaxial films of ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ on bicrystal ${\mathrm{SrTiO}}_{3}$ substrates. The experiments carried out over the temperature range 4.2 K--300 K in magnetic field up to 3 T allow us to identify some of the conduction mechanisms that may give rise to nonlinear transport in these grain boundary junctions. The nonlinear transport is associated with multistep inelastic processes in the grain-boundary region, which is moderately affected by the applied magnetic field. However the primary effect of the magnetic field is to enhance the zero-bias conductance $[{G}_{0}{=(dI/dV)}_{V=0}].$ The dominant voltage dependent contribution to the dynamic conductance $(G=dI/dV)$ comes from a term of the type ${V}^{4/3}$ at lower temperatures. Other voltage dependent contributions to G, which are of higher order in V, appear only for $T>~75\mathrm{K}.$ In addition we found a contribution to G arising from a ${V}^{0.5}$ term, which is likely to arise from the disordered region around the grain boundary (GB). The magnetoresistance in the GB depends on the bias used and it decreases at higher bias. The bias dependence is found to be reduced as temperature is increased. We discuss the physical origins of the various contributions to the nonlinear conduction.

Analytical model of iron losses in power transformers

The authors have improved a circuit model of transformers by defining a conductance whose losses will be close to the iron losses under any circumstance. Connecting a constant conductance G/sub cs/ in parallel with the main magnetizing inductance is very popular in the literature; however, it is acceptable only if the flux amplitude and frequency are reasonably constant during the time of operation, and if the value can be properly adjusted. Here, they replace G/sub cs/ with a nonlinear dynamic conductance G/sub cd/, whose losses are a function of flux amplitude. This conductance is based on the concept of dynamical hysteresis. Its definition is such that it is determined by the same test results used for G/sub cs/. Knowing the values of losses given by the manufacturer of the sheets, and the measured values of iron losses of a transformer, one can then analytically express the building factor as a function of the maximum flux density. The authors have applied the method to a three-leg 10-kVA transformer. It allows one to identify the cases when iron losses may be ignored, when they may be represented by a constant conductance, and when they must be represented by a nonlinear conductance.

Tunnel Junctions Using Different High- T c Superconductors

We present the results on the measurements of the ramp-edge type hetero-junctions with different high-T c superconductor electrodes, YBa2Cu3O7−δ and Bi2Sr2CaCu2O8+δ. The dynamic conductance dI/dV in junctions showed a two-gap-like structure, which can be explained in terms of the sum and difference of two different superconducting gaps. The BTK model fitting yielded a good agreement with the experimental results. Subharmonic gap structure was also observed in several junctions with lower tunnel resistance.

Reverse-bias safe operation area of large area MCT and IGBT

A comprehensive investigation of the reverse-bias safe operation area (RBSOA) of large area MOS controlled thyristor (MCT) and insulated gate bipolar transistor (IGBT) was performed and results are reported in this paper. Multi-cell devices turn-off failure due to non-uniform gate delay was first investigated. Fundamental device characteristic difference between MCT and IGBT was discussed. It is found that, under isothermal and homogeneity condition, the RBSOAs of both devices are determined by the sustain-mode dynamic avalanche limitation and the maximum controllable current density limitation. If device inhomogeneity exists, the turn-off failure will occur at power densities that are much lower than the RBSOA decided by these two limitations. A new parameter, called dynamic avalanche conductance ( g dynamic), was defined to describe the characteristic of dynamic avalanche of the two devices. Finally, the RBSOAs of large area MCT and IGBT are summarized and compared.

Currents conversion in the submicron CDW-N-CDW structures

Using selective area irradiation by electrons with high energies the submicron CDW-N -CDW heterostructures were obtained on NbSe, single crystals. In sliding CDW regime an excess voltage appears on this narrow normal region that depends neither on a sample, nor on the length of the irradiated area l and represents threshold phase slip (PS) voltage Vps, induced on CW-N boundaries. The obtained Vps, data for lower CDW strongly differ (5-6 times higher) from the known results for NbSe, obtained in the conventional configuration. This suggests that CDW-N boundaries created by irradiation represent the "ideal" face contacts introducing small perturbations. At low temperature (T<7 K) and at l<0.5 μm we observe narrow (-0.5 mV width) dynamic conductance peak at zero bias voltage. The peak amplitude grows with decreasing temperature and length of the irradiated area.

Transport in magnetically doped magnetic tunnel junctions

We report striking transport behavior of magnetic tunnel junctions which are engineered to incorporate localized magnetic impurities inside the tunnel barrier. Such junctions exhibited zero-bias anomalies (giant conductance dip or equivalently, giant resistance peak) in dynamic conductance curves, and striking bias and temperature dependencies of tunneling magnetoresistance (TMR), i.e., suppression of TMR at zero-bias and the decrease of TMR with decreasing temperature. Logarithmic dependencies of the conductance on bias and temperature agree well with Applebaum's theory describing Kondo-type spin flip scattering between tunneling electrons and impurity spins. The systematic variation of the transport anomalies with oxidation time confirmed that an over-oxidation of the tunnel barrier leads to diffusion of Co (Fe) ions from the bottom electrode into the barrier, which is consistent with the Cabrera-Mott oxidation model.

Temperature dependence of magnetoresistance and nonlinear conductance of the bicrystal grain boundary in epitaxial La0.67Ba0.33MnO3 thin films

We investigated conduction through an artificial grain-boundary junction made in La0.67Ba0.33MnO3 thin films, deposited on a 36.7° SrTiO3 bicrystal substrate using a laser ablation technique. The grain boundary exhibits substantial magnetoresistance at low temperatures and also shows nonlinear I–V characteristics. Analysis of temperature dependence of the dynamic conductance allows us to identify three carrier transport mechanisms across the grain boundary. These mechanisms exist in parallel, and at a given temperature one mechanism may dominate. Particularly, at higher temperatures (T>175 K) the transport across the grain boundary involves spin–flip scattering, which we establish leads to decrease of the bicrystal grain-boundary contribution in magnetoresistance. At lower temperature (4.2–45 K), tunneling through a disordered oxide at the grain boundary dominates, whereas in the temperature range from 100 to 175 K, carrier transport is dominated by inelastic tunneling via pairs of manganese atoms.

Temperature and junction-type dependency of Andreev reflection in MgB 2

We studied the voltage and temperature dependency of the dynamic conductance of normal metal–MgB 2 junctions obtained either with the point-contact technique (with Au and Pt tips) or by making Ag-paint spots on the surface of MgB 2 samples. The fit of the conductance curves with the generalized BTK model gives evidence of pure s-wave gap symmetry. The temperature dependency of the gap, measured in Ag-paint junctions (dirty limit), follows the standard BCS curve with 2 Δ/ k B T c=3.3. In out-of-plane, high-pressure point-contacts we obtained almost ideal Andreev reflection characteristics showing a single small s-wave gap Δ=2.6±0.2 meV (clean limit).

Charge and low-frequency response of normal-superconducting heterostructures

Charge distribution is a basic aspect of electrical transport. In this work we investigate the self-consistent charge response of normal-superconducting heterostructures. Of interest is the variation of the charge density due to voltage changes at contacts and due to changes in the electrostatic potential. We present response functions in terms of functional derivatives of the scattering matrix. We use these results to find the dynamic conductance matrix to lowest order in frequency. We illustrate similarities and differences between normal systems and heterostructures for specific examples such as a ballistic wire and a quantum point contact.

Evidence of spin-polarized tunneling in phase-separated manganites La1/3Sr2/3−xBaxMnO3 (0⩽x⩽0.67)

Polycrystalline La1/3(Sr,Ba)2/3MnO3 (0⩽x⩽0.67) bulk samples were fabricated by solid-state reaction. Structural analysis by Rietveld refinement shows the phase separation while x exceeds 0.1. This phase separation leads to dramatic changes in transport properties for the samples. For the sample La1/3Ba2/3MnO3, the current–voltage and dynamic conductance characteristics demonstrate a spin-polarized tunneling behavior below ∼150 K. The spin-polarized tunneling induces a large magnetoresistance under low magnetic field.

Pseudogap features of intrinsic tunneling in Bi2212 single crystals

The c-axis intrinsic tunneling properties of Bi2212 and HgBr 2–Bi2212 single crystals have been measured in the temperature range 4.2–250 K. The 7–15-unit-cell-high mesa structures on the surfaces of these single crystals were investigated. Clear superconductor–insulator–superconductor like tunneling curves were observed for current applied in the c-axis direction. The dynamic conductance shows both sharp peaks at the superconducting energy gap (SG), followed by dips and wide maxima at larger voltages. The maxima together with depressed conductance at zero voltage persist in the whole temperature range, illustrating the presence of the pseudogap in the quasiparticle excitation spectra, while the SG peaks decrease in voltage and lose their distinctiveness at T→ T c. The intercalation of Bi2212 with HgBr 2 molecules results in a drastic increase of the c-axis lattice constant and resistivity ρ c . Despite this, the overall shape of the ρ c ( T)-dependence and the characteristic temperature T * at which an upturn in the ρ c ( T)-dependence sets in upon cooling do not change. This implies that T * is not affected by coupling between the CuO 2 bilayers.

Surface state trapping models for SnO2-based microhotplate sensors

Due to their small size, SnO2-based microhotplate gas sensors can be used to develop a portable, sensitive, and low-cost gas monitoring system to detect, for example, leakage of hazardous gases. These devices, because of their low thermal mass, allow rapid temperature changes of the sensing material as a mode of sensor operation. To gain insight into the conductance response of microhotplate sensors, the basic physical and chemical processes involved in the sensing operation have been modeled. In this paper, intrinsic and extrinsic surface state trapping models are presented to describe the dynamic conductance responses of microhotplate gas sensors to argon and to air, respectively. These models relate the change in the conducting electron density to the change of the intrinsic/extrinsic surface state density based on potential barrier theory. Model parameters are estimated from one set of experiments, and then the models are used to predict output signals in a different set of experiments. Excellent agreement is achieved between the predicted and measured responses. The models can predict the fast temperature programmed sensing responses of microhotplate sensors on a time scale ranging from seconds to milliseconds. One interesting aspect of this modeling is that it correctly predicts that a transient conductivity response will occur when the temperature is cycled even if only argon is present. This paper also shows evidence for the effect of surface states on the conductance response of tin oxide films to these rapid temperature changes.

Analyses of intrinsic magnetoelectric properties in spin-valve-type tunnel junctions with high magnetoresistance and low resistance

A series of experimental data was obtained systematically for a spin-valve-type tunnel junction of ${\mathrm{Ta}\mathrm{}(5\mathrm{}\mathrm{n}\mathrm{m})/\mathrm{N}\mathrm{i}}_{79}{\mathrm{Fe}}_{21}$ (3 nm)/Cu ${(20\mathrm{}\mathrm{n}\mathrm{m})/\mathrm{N}\mathrm{i}}_{79}{\mathrm{Fe}}_{21}$ ${(3\mathrm{}\mathrm{n}\mathrm{m})/\mathrm{I}\mathrm{r}}_{22}{\mathrm{Mn}}_{78}{\mathrm{}(10\mathrm{}\mathrm{n}\mathrm{m})/\mathrm{C}\mathrm{o}}_{75}{\mathrm{Fe}}_{25}$ (4 nm)/Al ${(0.8\mathrm{}\mathrm{n}\mathrm{m})\ensuremath{-}\mathrm{o}\mathrm{x}\mathrm{i}\mathrm{d}\mathrm{e}/\mathrm{C}\mathrm{o}}_{75}{\mathrm{Fe}}_{25}$ ${(4\mathrm{}\mathrm{n}\mathrm{m})/\mathrm{N}\mathrm{i}}_{79}{\mathrm{Fe}}_{21}$ (20 nm)/Ta (5 nm). Analyses of (i) temperature dependence of tunnel magnetoresistance (TMR) ratio and resistance from 4.2 K to room temperature, (ii) applied dc bias-voltage dependence of TMR ratio and resistance at 6.0 K and room temperature, and (iii) tunnel current I and dynamic conductance $(dI/dV)$ as functions of dc bias voltage at 6.0 K were carried out. High-TMR ratio of 64.7% at 4.2 K and 44.2% at room temperature were observed for this junction after annealing at 300 \ifmmode^\circ\else\textdegree\fi{}C for an hour. An anisotropic wavelength cutoff energy of spin-wave spectrum in magnetic tunnel junctions, which is essential for self-consistent calculations, was suggested based on a series of inelastic electron tunnel spectra obtained. The main intrinsic magnetoelectric properties in such spin-valve-type tunnel junction with high magnetoresistance and low resistance can be evaluated based on the magnon-assisted inelastic excitation model and theory.

Dynamic conductance of mesoscopic waveguides

We report a theoretical investigation of dynamic conductance G(ω), for general ac frequency ω, of two-dimensional mesoscopic waveguides whose transport is characterized by antiresonances. We calculate G(ω) by numerically evaluating nonequilibrium Green’s functions. By tuning the ac frequency we observe photon-assisted resonant transport as well as a gradual smearing out of the antiresonances. The antiresonance causes the dynamic response to vary between capacitive-like behavior to that of the inductive-like behavior.

An axial monotron with rippled wall resonator

Summary form only given. Consisting of an electron beam that traverses a standing-wave cavity resonator, the monotron ranks as the simplest microwave tube. The effect of the beam on the cavity fields can be described in terms of the functional dependence of the dynamic beam conductance upon the transit time of the election through the resonator Negative values of the conductance represent work done by the beam on the fields so that the beam acts as a source, that can be used for self-excitation or regenerative amplification of electromagnetic oscillations in the cavity. Besides its simplicity and compactness, recent studies have indicated that the monotron operating in an azimuthally symmetric TM mode can reach 20.0 percent conversion efficiency at weakly relativistic beam energies, unlike past predictions of a maximal theoretical efficiency of 14.5 percent. Devoted to exploring novel concepts of monotron resonant structures, the present work proposes a sinusoidally rippled wall cavity which distinguishes itself by providing efficient coupling to a coaxial output circuit. The synthesis of the cavity relics on the properties of periodic guiding structures from which we formulate the dispersion relation for waveguides with sinusoidally rippled wall. On the basis of 2.5-D particle-in-cell (PIC) simulation, we discuss the operation of the corrugated-cavity monotron thus envisaged to operate at 10.5 GHz with coaxial loading. Then using a centered, solid beam at the injection parameters of 50A current and 34keV input energy, the simulation gives the average power output of 260kW, which amounts to 15.3 percent overall efficiency. The conclusions of the paper are presented in the last part, which reviews the monotron design by highlighting its attributes.

Emittance fluctuations in a mesoscopic diffusive conductor

We report a first principles analysis of low frequency dynamic conductance fluctuations for disordered two-dimensional mesoscopic conductors. In the transport regime where dc conductance shows the familiar universal conductance fluctuations, we discovered that the low frequency emittance also fluctuates with an amplitude that is independent of the impurity scattering strength, showing a degree of generic behavior. When the impurity density is increased such that the dynamic response of the conductor changes from inductivelike to capacitivelike, the emittance distribution is found to cross over from Gaussian-like to non-Gaussian-like; the latter is qualitatively consistent with random matrix theory.