Transverse Voltage Research Articles

Level anticrossing effect on electron properties of coupled quantum wells under an in-plane magnetic field

The influence of an in-plane magnetic field on the energy spectrum and zero-temperature equilibrium properties of tunnel-coupled double and triple quantum wells is studied. Both the appearance of the gap due to anticrossing of two energy branches and the peculiarities of the third-order crossing point (for symmetric triple quantum well case) are discussed. As results, magnetization of two-dimensional electrons in double and triple quantum wells is modified essentially if the Fermi level is localized near such peculiarities. Another effect under consideration is the interlevel charge redistribution between quantum wells and the transverse voltage induced by the in-plane magnetic field. Self-consistent numerical calculations for double and triple quantum wells, which take into account the modifications of energy spectra under gate voltage, are presented.

Resonant terahertz excitation of tunnel-coupled quantum wells: Nonlinear absorption and photovoltage

The redistribution of nonequilibrium electrons between tunnel-coupled double quantum wells under terahertz pump is described by using a self-consistent two-level basis with ``rigid'' orbitals. The photoinduced component of the self-consistent potential, that changes both energy spectra (i.e., transition energies and oscillator strengths) and efficiency of tunnel relaxation, causes such redistribution. The intersubband photoexcitation and relaxation of electrons due to quasielastic scattering are described by the system of balance equations for electron concentrations over the tunnel-coupled pair of levels. In order to calculate the electron population numbers we use self-consistent wave functions, which take into account the transverse potential obtained from Poisson equation. Such equations describe also the transverse voltage induced by the redistribution of electrons between the quantum wells under pumping (rectification effect) and nonlinear modifications of intersubband absorption peak. Parameters of nonequilibrium electron redistribution significantly depend on the intersubband transitions energies, broadening and intensity of pumping (because of resonant excitation). Calculating them, we have found the photoinduced level splitting and considered the transition from the linear regime excitation to saturation one. We have a bistable solution in the transition region. Also, we present results for the nonlinear absorption and for the photoinduced voltage. Comparisons to experimental data are presented as well.

Guiding of vortices in YBa 2Cu 3O 7− δ single crystals with unidirected twins

Guided motion (GM) of vortices in vortex liquid phase of YBa 2Cu 3O 7− δ (YBaCuO) single crystal with unidirected twins is studied by measuring transverse voltages in geometry where the current is flowing in the a– b plane and the external magnetic field H is oriented along the c-axis. Field inversion was used for separation of the even component of the measured transverse and longitudinal voltages in order to select new even contribution to magnetoresistivity caused by GM of vortices. The results are discussed in terms of the phenomenological approach to the anisotropic pinning caused by unidirected twins.

Experimental evidence for vortex lines in the vortex-liquid phase ofYBa2Cu3O6.95in a geometry of tilted vortices

Using a transport method with a geometry of tilted magnetic field, we have studied the direction of the electric field generated by the motion of vortices. This experiment gives evidence of uncut vortex lines in slightly overdoped untwinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.95}$ above the melting first order transition, showing that the first order transition does not correspond to a decoupling of the vortex lines. The gradual loss of the line correlation appears close to ${T}_{c2}$ on a width which is typically that of the critical fluctuations. Those results are similar to what we have found previously in a heavily twinned sample and lead to the conclusion that the twin planes are not necessary to induce uncut vortex lines in the liquid phase, even in a thick sample. Moreover, we observe a nonzero transverse voltage below the critical current, which is not explained unless referring to surface nondissipative currents.

Transverse magnetovoltage in epitaxial La 0.67Ca 0.33MnO 3 thin films

We present detailed Hall effect, planar Hall effect and resistance measurements of an epitaxial La 0.67Ca 0.33MnO 3 thin film. In the ferromagnetic regime the transverse voltage shows a similar dependence on the external magnetic field when it is applied perpendicular to (Hall effect) and in the plane of the film (planar Hall effect). By this observation it is established that the anomalous Hall effect is not the main origin of the transverse magnetovoltage. It is the anisotropic magnetoresistance (AMR), which we observe in both the longitudinal and the planar Hall configuration that causes the transverse magnetovoltage.

Determination of deep levels' parameters in epi-GaAs by a transient acoustoelectric technique

A new acoustoelectric method is developed in order to study solid state layered structures. The method is based on the transient transverse acoustoelectric voltage (TAV) measurements. The decay of the TAV signal is read when a surface acoustic wave (SAW) producing the signal is switched off. The shape of the transient voltage and its spectral dependence on the wavelength of a light irradiation are studied. A theoretical model is presented to explain the transient TAV data. The results demonstrate that the transient acoustoelectric technique is an effective means of characterizing trapping centres in the bulk and at surfaces or interfaces of epitaxial semiconductor structures.

High-frequency modulation of current in tunnel-coupled quantum wells by transverse radiation in the terahertz range

The modulation of a longitudinal current in double quantum wells by a transverse voltage in the terahertz frequency range is calculated. A quantum kinetic equation which allows for the contributions of a pair of asymmetric tunnel-coupled levels to the resonant response is solved (the difference in effective mass or the asymmetric scattering in the left-and right-hand quantum wells is also taken into account). These structural features lead to conversion of the modulating voltage into a high-frequency longitudinal current. The calculated longitudinal-transverse nonlinear susceptibility determines the conditions needed for creating an efficient field-effect transistor circuit that would use pumping in the terahertz range instead of a transverse control voltage.

Observation of an odd longitudinal magnetoresistance in YBa 2Cu 3O 7− δ single crystals with unidirected twins

Unidirected twin planes are the cause of anisotropic pinning of vortices in YBa 2Cu 3O 7− δ (below YBaCuO) single crystals. In contrast to the case of only isotropic pinning, two new effects may appear due to the pinning anisotropy: guiding of vortices along twin planes and anisotropy of the Hall resistivity. The former leads to the appearance of the even (with respect to the reversal of a magnetic field direction) transverse resistivity and influences the Hall angle, while the last causes new additional odd contribution to the longitudinal resistivity. The paper presents magnetoresistivity measurements in the vortex liquid phase of YBaCuO single crystals with different angles between unidirected twins and current direction in the magnetic fields up to 12 T. Field inversion was used for separation of the even and odd components of the measured transverse and longitudinal voltages in order to select new contributions to the resistivity in the case of anisotropic pinning. The results are consistent with the model of anisotropic pinning where the anisotropy of the Hall drag coefficient is caused by twins.

Composite fermion signature in the acoustoelectric effect in 2DES and 2DHS

The integer and fractional quantum Hall effects have been investigated using the acoustoelectric effect in both two-dimensional hole and electron systems fabricated by MBE on GaAs. Measurements have been made of the longitudinal and transverse acoustoelectric voltages as a function of magnetic field up to 16 T at a temperature of 340 mK. We report behaviour close to ν= 1 2 in the dispersion and acoustoelectric effect that cannot be explained on the basis of quasi-DC measurements. Finally, we find that a tensor relationship exists in the acoustoelectric effect similar to that reported recently in thermoelectric measurements.

An experimental investigation of the off-diagonal Seebeck effect on textured YBCO high-Tc superconductor

The off-diagonal Seebeck effect was investigated on a melt-textured YBa 2 Cu 3 O 7− δ (YBCO) high- T c superconductor. It was found that the transverse voltage V x was proportional to the longitudinal temperature difference Δ T z , measured directly with a pair of differential thermocouples, for textured samples with their c -axes tilted with respect to the surface’s normal. This supported the idea that the off-diagonal thermoelectric effect accounts for the anomalously high laser-induced transverse voltage on the oriented YBCO superconducting thin films. The variation of the V x against the sample’s thickness d , at a given Δ T z , deviated from the inversely proportional relationship when the sample was too thin. The deviation was discussed qualitatively in terms of deteriorated surface layers. The data of V x ( d ) was fitted to obtain a reasonably reliable ∣ S c − S ab ∣ value of 12 μ V K −1 .

Anisotropic pinning and the mixed-state galvanothermomagnetic properties of superconductors—a phenomenological approach

In the presence of isotropic and anisotropic pinning the vortex dynamics is discussed in terms of phenomenologically introduced, nonlinear viscosities. The formulas for linear galvanothermomagnetic effects are derived and analyzed under the condition at which the transport current or temperature gradient is directed at arbitrary angle with respect to the unidirected twins, which cause the anisotropic pinning. It is shown that two new effects which appear due to the anisotropic pinning, namely (with respect to the reversal of the magnetic field direction) even transverse and odd longitudinal voltage, have a distinct origin. The first is due to the guided vortex motion, while the second appears only when anisotropic (in contrast with isotropic) pinning changes the Hall drag coefficient. We also show that the last effect might be masked in the experiment by a large, odd contribution, which has the same angular dependence and which appears due to the Ettingshausen effect. In order to clarify the problem of influence of the twins on the Hall drag coefficient, we discuss the possibility of separating these two contributions in the experiment.

Characteristics of a submicrosecond transverse discharge in mixtures of helium with N2 and CO molecules and xenon atoms

An investigation is made of the electrical and optical characteristics of a moderate-pressure transverse discharge in typical active media of infrared CO lasers. The discharge was ignited in a system of unprofiled “ grid-plane” electrodes with automatic ultraviolet preionization by a corona discharge and had a 2×3 cm aperture. The pulse repetition frequency was 1–10 Hz, the current pulse length was ≤0.5 μs, and the pressure of the working mixtures was 10–100 kPa. Studies were made of the transverse discharge current voltage pulses, and also the spectral and temporal characteristics of the plasma radiation in N2:CO, He:N2:CO, and He: N2:CO:Xe mixtures in the ultraviolet and visible ranges.

Voltage dependence of the pattern and frequency of discrete Ca2+ release events after brief repriming in frog skeletal muscle.

Applying a brief repolarizing pre-pulse to a depolarized frog skeletal muscle fiber restores a small fraction of the transverse tubule membrane voltage sensors from the inactivated state. During a subsequent depolarizing test pulse we detected brief, highly localized elevations of myoplasmic Ca2+ concentration (Ca2+ "sparks") initiated by restored voltage sensors in individual triads at all test pulse voltages. The latency histogram of these events gives the gating pattern of the sarcoplasmic reticulum (SR) calcium release channels controlled by the restored voltage sensors. Both event frequency and clustering of events near the start of the test pulse increase with test pulse depolarization. The macroscopic SR calcium release waveform, obtained from the spark latency histogram and the estimated open time of the channel or channels underlying a spark, exhibits an early peak and rapid marked decline during large depolarizations. For smaller depolarizations, the release waveform exhibits a smaller peak and a slower decline. However, the mean use time and mean amplitude of the individual sparks are quite similar at all test depolarizations and at all times during a given depolarization, indicating that the channel open times and conductances underlying sparks are essentially independent of voltage. Thus, the voltage dependence of SR Ca2+ release is due to changes in the frequency and pattern of occurrence of individual, voltage-independent, discrete release events.

Current-induced vortex dynamics in Josephson-junction arrays: Imaging experiments and model simulations

We study the dynamics of current-biased Josephson-junction arrays with a magnetic penetration depth ${\ensuremath{\lambda}}_{\ensuremath{\perp}}$ smaller than the lattice spacing. We compare the dynamics imaged by low-temperature scanning electron microscopy to the vortex dynamics obtained from model calculations based on the resistively shunted junction model, in combination with Maxwell's equations. We find three bias current regions with fundamentally different array dynamics. The first region is the subcritical region, i.e., below the array critical current ${I}_{c}.$ The second, for currents $I$ above ${I}_{c},$ is a ``vortex region,'' in which the response is determined by the vortex degrees of freedom. In this region, the dynamics is characterized by spatial domains where vortices and antivortices move across the array in opposite directions in adjacent rows and by transverse voltage fluctuations. In the third, for still higher currents, the dynamics is dominated by coherent-phase motion, and the current-voltage characteristics are linear.

Activation energies for depinning of vortices from intrinsic and extrinsic pinning centers in high Tc superconductors

The anisotropy of the vortex motion has been studied on textured samples of Bi based superconductors. The Hall (transverse) voltage which is even function of the magnetic field has been discussed in terms of the guided motion of vortices due to the intrinsic pinning thanks to the layered structure of material under study. An increase of the temperature suppresses the anisotropy of pinning and the even transverse voltage disappears. The proposed thermal activation mechanism of vortex motion has been used for study of the depinning of vortices from intrinsic pinning centres connected with the presence of CuO2 planes and from the other pinning centers present in the sample.

Incoherent mesoscopic phenomena in semiconductor structure of macroscopic size

In Si : B MOS-structures ( N B = 1.2 × 10 18 cm −3), the formation of quasi-2D channel of hopping conductivity occurs in the region, where Fermi energy crosses the impurity band, shifted by gate voltage V g . Rising gate voltage above zero ( V g > 0, the case of depletion) results in reproducible fluctuations of transverse voltage V y between Hall probes, at temperatures 4.2–30 K and magnetic fields B up to 1 T, in spite of the macroscopic size of the sample (150 x 50 μm 2) and the potential probe ledges (15 x 5 μm 2).

Influence of disorder on luminescence from pseudorandomized strained Si1−xGex/Si superlattices

Carrier localization in a disordered potential was studied in type-I strained Si1−xGex/Si superlattices (SLs) where layer widths have been artificially pseudorandomized. The photoluminescence (PL) intensity under a transverse bias voltage varies monotonically and exhibits a folded increase as the bias is increased as opposed to an ordered SL where only steady drop-off of PL intensity, symmetric with respect to bias polarity, was observed with increasing field strength. The results indicate that electron and hole wave functions are located spatially apart at zero bias due to disorder-induced localization. We point out that such a wave-function displacement is even pronounced for type-II SL potentials in the presence of monolayer thickness fluctuation.

Investigation of transverse Peltier effect on top-seeded melt textureYBa2Cu3O7−δ

The transverse Peltier effect is investigated on the top-seeded melt texture superconductor YBa2Cu3O7−δ (YBCO). By restricting the heat absorbing or evolving on one of the sample’s surfaces, the Peltier heat flow is converted into a temperature difference for measurement. The temperature difference is found proportional to the current applied, which is in accordance with the prediction of transverse Peltier effect. Based on a simplified model, the difference of the Seebeck coefficients between the ab plane and the c axis, |Sab−Sc|, is about 35 μV/K. It is in good agreement with that of large single crystal [I. Terasaki, Y. Sato, S. Tajima, S. Miyamoto, and S. Tanaka, Physica C 235-240, 1413 (1994)]. The transverse Peltier effect is verified. This supports the idea that the off-diagonal thermoelectric effect is responsible for the anomalously high laser-induced transient transverse voltage on the oriented YBCO superconducting thin films.

The acousto-electric effect in the 2-D hole system using SAW

Experiments have been made using surface acoustic waves (SAW) propagating on two GaAs wafers on which a 2-dimensional hole system (2DHS) had been fabricated by MBE. On a GaAs (100) surface we used SAW at a frequency of 120 MHz. Amplitude modulation and a lock-in amplifier were used to measure the longitudinal and transverse acousto-electric voltages from a Hall-bar structure. Measurements have been made in a magnetic field up to 5 T and at temperatures between 4.2 and 2 K. A large longitudinal voltage was found which increased linearly with SAW power and also increased with applied magnetic field. We have also used a GaAs (311) wafer and SAW at a frequency of 113 MHz. Using pulse modulation, measurements have been made of the SAW attenuation and dispersion as a function of magnetic field up to 13 T and at temperatures below 100 mK. Square wave amplitude modulation and a lock-in amplifier were also used to measure the longitudinal and transverse acousto-electric voltages from a Hall-bar structure. The acousto-electric signals are compared with theoretical predictions using measured values of the longitudinal and transverse resistivity.

Kinetic isoforms of intramembrane charge in intact amphibian striated muscle.

The effects of the ryanodine receptor (RyR) antagonists ryanodine and daunorubicin on the kinetic and steady-state properties of intramembrane charge were investigated in intact voltage-clamped frog skeletal muscle fibers under conditions that minimized time-dependent ionic currents. A hypothesis that RyR gating is allosterically coupled to configurational changes in dihydropyridine receptors (DHPRs) would predict that such interactions are reciprocal and that RyR modification should influence intramembrane charge. Both agents indeed modified the time course of charging transients at 100-200-microM concentrations. They independently abolished the delayed charging phases shown by q gamma currents, even in fibers held at fully polarized, -90-mV holding potentials; such waveforms are especially prominent in extracellular solutions containing gluconate. Charge movements consistently became exponential decays to stable baselines in the absence of intervening inward or other time-dependent currents. The steady-state charge transfers nevertheless remained equal through the ON and the OFF parts of test voltage steps. The charge-voltage function, Q(VT), shifted by approximately +10 mV, particularly through those test potentials at which delayed q gamma currents normally took place but retained steepness factors (k approximately 8.0 to 10.6 mV) that indicated persistent, steeply voltage-dependent q gamma contributions. Furthermore, both RyR antagonists preserved the total charge, and its variation with holding potential, Qmax (VH), which also retained similarly high voltage sensitivities (k approximately 7.0 to 9.0 mV). RyR antagonists also preserved the separate identities of q gamma and q beta species, whether defined by their steady-state voltage dependence or inactivation or pharmacological properties. Thus, tetracaine (2 mM) reduced the available steady-state charge movement and gave shallow Q(VT) (k approximately 14 to 16 mV) and Qmax (VH) (k approximately 14 to 17 mV) curves characteristic of q beta charge. These features persisted with exposure to test agent. Finally, q gamma charge movements showed steep voltage dependences with both activation (k approximately 4.0 to 6.5 mV) and inactivation characteristics (k approximately 4.3 to 6.6 mV) distinct from those shown by the remaining q beta charge, whether isolated through differential tetracaine sensitivities, or the full approximation of charge-voltage data to the sum of two Boltzmann distributions. RyR modification thus specifically alters q gamma kinetics while preserving the separate identities of steady-state q beta and q gamma charge. These findings permit a mechanism by which transverse tubular voltage provides the primary driving force for configurational changes in DHPRs, which might produce q gamma charge movement. However, they attribute its kinetic complexities to the reciprocal allosteric coupling by which DHPR voltage sensors and RyR-Ca2+ release channels might interact even though these receptors reside in electrically distinct membranes. RyR modification then would still permit tubular voltage change to drive net q gamma charge transfer but would transform its complex waveforms into simple exponential decays.