Electron Heating Effect Research Articles

The resistive sensor: a device for high-power microwave pulsed measurements

In this paper we review a novel microwave power sensor, the operation of which is based on the electron-heating effect in semiconductors. The sensor has been specifically developed for the measurement of high-power microwave pulses. The sensor's principle of operation and its related circuitry are described. The influence of lattice heating on the sensor's performance is considered. The actual design and implementation of a waveguide-type sensor is presented, including details regarding its calibration. The application of the sensor for the measurement of microwave pulses at medium and high power levels is also presented.

Spin accumulation and Andreev reflection in a mesoscopic ferromagnetic wire

The electron transport though ferromagnetic metal-superconducting hybrid devices is considered in the non-equilibrium Green's function formalism in the quasiclassical approximation. Attention if focused on the limit in which the exchange splitting in the ferromagnet is much larger than the superconducting energy gap. Transport properties are then governed by an interplay between spin-accumulation close to the interface and Andreev reflection at the interface. We find that the resistance can either be enhanced or lowered in comparison to the normal case and can have a non-monotonic temperature and voltage dependence. In the non-linear voltage regime electron heating effects may govern the transport properties, leading to qualitative different behaviour than in the absence of heating effects. Recent experimental results on the effect of the superconductor on the conductance of the ferromagnet can be understood by our results for the energy-dependent interface resistance together with effects of spin- accumulation without invoking long range pairing correlations in the ferromagnet.

Multiple Andreev reflection and giant excess noise in diffusive superconductor/normal-metal/superconductor junctions

We have studied superconductor/normal-metal/superconductor $(\mathrm{SNS})$ junctions consisting of short Au or Cu wires between Nb or Al banks. The Nb based junctions display inherent electron heating effects induced by the high thermal resistance of the $\mathrm{NS}$ boundaries. The Al based junctions show in addition subharmonic gap structures in the differential conductance $dI/dV$ and a pronounced peak in the excess noise at very low voltages V. We suggest that the noise peak is caused by fluctuations of the supercurrent at the onset of Josephson coupling between the superconducting banks. At intermediate temperatures where the supercurrent is suppressed a noise contribution $\ensuremath{\propto}1/V$ remains, which suggests the presence of a long-range proximity effect in the noise.

Two-dimensional modeling of a transverse collisionless shock wave

Transverse collisionless shock waves in a plasma in which the initial β value is equal to zero for electrons and is small but nonzero for ions are studied in the two-dimensional approximation with allowance for anomalous resistivity. A hybrid model is applied such that the ions are treated in the kinetic approximation and the electrons are described in the hydrodynamic approximation. A collisionless shock wave is generated using a piston with a small two-dimensional perturbation. The ion distribution downstream of the shock front and the effect of electron and ion heating are analyzed. It is shown that, for Alfven-Mach numbers M A>2, ion heating is attributed primarily to the ions that have experienced a reflection from the shock front and whose velocities downstream of the front are very high. This conclusion agrees with the results of one-dimensional calculations. Solving the problem as formulated shows that two-dimensional effects are insignificant in the range of low Alfven-Mach numbers (M A≤5): the direction of the magnetic field is always close to its initial direction, the ions acquire low velocities along the magnetic field, and the quantitative parameters of the plasma downstream of the shock front are close to those obtained from the one-dimensional model. In the range of higher Alfven-Mach numbers, two-dimensional effects are more pronounced and the ion distribution function is less anisotropic.

Effect of electron heating and radiation pressure on tunneling across Schottky barrier due to giant near field of FIR laser radiation

The conductance of tunnel junctions formed by n-GaAs and a semitransparent metal electrode on its surface was shown to be changed by normally incident radiation with frequency below the plasma edge of n-GaAs. A spatial redistribution of the electrons due to the radiation pressure and the corresponding change in the shape of the self-consistent Schottky barrier are observed as a photoconductivity response. The comparison of the measured intensity dependence of the response to the theory has revealed an effective enhancement of the radiation field close to the n-GaAs/Me interface. The ratio K e of the effective radiation intensity to the incident plain-wave intensity has turned out to be as large as 10 5. The high local electric field results in strong heating of the degenerate electron gas in the Schottky-barrier region that is mostly apparent as a photo-e.m.f. An analysis of the intensity dependence of the photo-e.m.f. is carried out involving the modification of the Schottky barrier by the ponderomotive force, the electron gas heating in Γ- and L-valleys, as well as the heating of LO phonons. The results confirm the value of K e and allow to estimate the hot-electron temperature in Γ- and L-valleys.

Effects of low particle fuelling and electron heating on the internal transport barrier in ICRF heated reversed magnetic shear plasmas of JT-60U

Plasmas with an internal transport barrier (ITB) were heated with a second-harmonic ion cyclotron range of frequency (ICRF) minority heating during the reversed magnetic shear (RS) experiments in JT-60U. In order to control the central particle fuelling, the input power of the neutral beam injection (NBI) heating during ICRF heating was scanned shot-by-shot. The electron temperature gradient around the ITB increased with the total input power. Steeper gradients were obtained with additional ICRF heating by avoiding collapses induced by the steep pressure gradient. The difference of the time derivative at the ICRF injection timing indicated a fast ion loss of about 40% during the RS operation. The density gradient was increased with the central fuelling by the NBI heating and the shoulder position of the slope at the ITB moved outward with the particle fuelling. The particle confinement time at the ITB increased with the central particle fuelling while the particle confinement time of the low-fuelling discharges were similar to that of the normal shear L-mode discharge.

Non-linear effects in hopping conduction of single-crystal La2CuO4+δ

The unusual non-linear effects in hopping conduction of single-crystal La2CuO4+δ with excess oxygen has been observed. The resistance is measured as a function of the applied voltage U (voltage controlled regime) in the temperature range 5 K⩽T⩽300 K and voltage range 10−3−25 V. At relatively high voltage (approximately at U>0.1 V) the conduction of sample investigated corresponds well to variable-range hopping (VRH). That is, in the range 0.1 V<U⩽1 V the conductivity does not depend on U (Ohmic behavior) and the temperature dependence of resistance R(T) follows closely Mott’s law of VRH [R∝exp(T0/T)1/4]. In the range of highest applied voltage the conduction has been non-Ohmic: the resistance decreases with increasing U. This non-linear effect is quite expected in the frame of VRH mechanism, since the applied electric field increases the hopping probability. A completely different and unusual conduction behavior is found, however, in the low voltage range (approximately below 0.1 V), where the influence of electric field and (or) electron heating effect on VRH ought to be neglected. Here we have observed strong increase in resistance at increasing U at T⩽20 K, whereas at T>20 K the resistance decreases with increasing U. The magnetoresistance of the sample below 20 K has been positive at low voltage and negative at high voltage. The observed unusual non-Ohmic behavior at low voltage range is attributable to inhomogeneity of the sample, namely, to the enrichment of sample surface with oxygen during the course of the heat treatment of the sample in helium and air atmosphere before measurements. At low enough temperature (below ≈20 K) the surface layer with increased oxygen concentration is presumed to consist of disconnected superconducting regions in a poorly conducting (dielectric) matrix. This allows us to explain the observed unusual non-linear effects in the conduction of sample studied. The results obtained demonstrate that in some cases the measured transport properties of cuprate oxides cannot be attributed to the intrinsic bulk properties.

Picosecond response of a superconducting hot-electron NbN photodetector

Abstract The ps optical response of ultrathin NbN photodetectors has been studied by electro-optic sampling. The detectors were fabricated by patterning ultrathin (3.5 nm thick) NbN films deposited on sapphire by reactive magnetron sputtering into either a 5×10 μm2 microbridge or 25 1 μm wide, 5 μm long strips connected in parallel. Both structures were placed at the center of a 4 mm long coplanar waveguide covered with Ti/Au. The photoresponse was studied at temperatures ranging from 2.15 K to 10 K, with the samples biased in the resistive (switched) state and illuminated with 100 fs wide laser pulses at 395 nm wavelength. At T=2.15 K, we obtained an approximately 100 ps wide transient, which corresponds to a NbN detector response time of 45 ps. The photoresponse can be attributed to the nonequilibrium electron heating effect, where the incident radiation increases the temperature of the electron subsystem, while the phonons act as the heat sink. The high-speed response of NbN devices makes them an excellent choice for an optoelectronic interface for superconducting digital circuits, as well as mixers for the terahertz regime. The multiple-strip detector showed a linear dependence on input optical power and a responsivity R =3.9 V/W.

Effect of island length on the Coulomb modulation in single-electron transistors

We have studied single-electron transistors with island lengths of 2, 10, 20, 30, and 40 \ensuremath{\mu}m and with high-resistance tunnel junctions to minimize the effects of cotunneling and electron self-heating. With longer islands, there is a marked reduction of the gate modulation of the Coulomb blockade width. According to orthodox theory, the width of the Coulomb blockade at $T=0$ is equal to ${e/C}_{\ensuremath{\Sigma}},$ and it falls approximately exponentially with ${k}_{B}{T/E}_{c},$ where ${E}_{c}{=e}^{2}{/2C}_{\ensuremath{\Sigma}},$ and ${C}_{\ensuremath{\Sigma}}$ is the total capacitance. Based on numerical calculations and analytic estimates, we conclude that the modulation reduction is mainly due to the large increase in stray capacitance between the island and the leads as the island length is increased, and not to some more subtle nonequilibrium effect. When the increased capacitance is combined with the effect of electron heating, the Coulomb modulation is rapidly reduced. This work also demonstrates the need to take stray capacitance into account in addition to intrinsic junction capacitance even in structures that are only a few \ensuremath{\mu}m long.

Particle simulation and electron heating effects in plasmas produced by laser pulse

The processes of resonant absorption, vacuum heating, and anomalous skin effect in plasmas produced by a laser pulse are studied using the two-dimensional multi-time-scale, fully electromagnetic relativistic particle simulation code with mobile ions. The mechanisms of electron heating are expounded and compared.

Electron heating effects in diffusive metal wires

We have investigated the electron heating in metallic diffusive wires of varying length at liquid-helium temperature by measuring the electric noise. The local increase of the electron temperature can be essential already for small currents and is well described by a heat-diffusion equation for the electrons. Depending on the electron thermal conductance and the electron–phonon coupling in the wire, different length regimes are identified. The quantitative knowledge of the electron temperature is important for analysis of nonequilibrium effects involving current heating in mesoscopic wires.

Light scattering by electrons in the excitonic absorption region of GaAs

A study has been made of the effect of the additional generation of photoexcited electrons on the excitonic absorption and luminescence spectra of ultrapure GaAs samples at T=2 K. The observed increase in the absorption coefficient for the ground (n=1) excitonic state is shown to originate from the polariton character of the energy spectrum of this state and to be due to an increase of polariton damping. The increased damping observed under electron generation is caused by polariton scattering from hot electrons as the latter undergo thermalization. As a result, the polaritons are heated. The changes observed in the luminescence spectra are produced by the reverse effect of electron heating and polariton cooling.

High-power microwave pulse measurement using resistive sensors

Resistive sensors based on the electron heating effect in semiconductors are used for microwave pulse-power measurement in the waveguide. The substitution procedure of replacing a microwave electric field by a dc electric field is proposed for the calibration of the resistive sensor in the medium-power range. The difference of readings between the calibrated resistive sensors and the former Soviet Union pulse-power standard was found to be less than 4%. Two calibrated sensors have been compared with the standard since 1982. A deviation in the range of /spl plusmn/4% from the standard power meter was found during this long-term experiment. A diaphragm-type resistive sensor has also been developed for the microwave pulse measurement at high power.

Measurement of reduced shot noise in a quantum point contact

We present a comprehensive experimental study of mesoscopic noise in a GaAs quantum point contact. In agreement with existing theory, we find that the measured noise power is strongly reduced from the full shot noise 2 eI ( I is the time-averaged current) expected for completely uncorrelated electron transport. Study of the temperature dependence of the noise clearly demonstrates the transition from thermal noise at low bias voltage to shot noise at high bias. For nearly total shot noise suppression, such as on a conductance plateau, we find it necessary to include the effect of electron heating to understand the current dependence of the noise.

Semiclassical theory of shot noise in mesoscopic conductors

A semiclassical theory is developed for time-dependent current fluctuations in mesoscopic conductors. The theory is based on the Boltzmann-Langevin equation for a degenerate electron gas. The low-frequency shot-noise power is related to classical transmission probabilities at the Fermi level. For a disordered conductor with impurity scattering, it is shown how the shot noise crosses over from zero in the ballistic regime to one-third of the Poisson noise in the diffusive regime. In a conductor consisting of n tunnel barriers in series, the shot noise approaches one-third of the Poisson noise as n goes to infinity, independent of the transparency of the barriers. The analysis confirms that phase coherence is not required for the occurrence of the one-third suppression of the shot noise. The effects of electron heating and inelastic scattering are calculated, by inserting charge-conserving electron reservoirs between segments of the conductor.

Electron heating effect on transport properties in [formula omitted] modulation doped structures grown by gas source molecular beam epitaxy

We have grown Si SiGe modulation doped structures by gas source molecular beam epitaxy. Magnetotransport measurements were carried out on Hall bar structures at 0.4 K. Pronounced Shubnikov-de Haas oscillations were observed in the longitudinal magnetoresistance, indicative of a high quality two-dimensional electron gas in the Si channel. Electron mobilities up to 95 000 cm 2/V · s with sheet densities of about 8 × 10 11 cm −2 were obtained at low temperature. The oscillations in the magnetoresistance showed obvious damping as the electric field applied to the Hall bar was increased, indicating an effect of electron heating. Electron temperatures were derived from the oscillation ampitude damping and examined as a function of energy loss rate, which can be determined from the input power per electron in the steady state.

Raman studies of plasmon modes in a drifting two-dimensional electron gas

We present the first Raman-scattering studies of the behavior of the intrasubband plasmon mode of a two-dimensional electron gas which is undergoing lateral drift in an applied electric field. The data clearly show the expected Doppler shifts of the modes traveling up- and downstream, together with the expected dependence on the wave vector, but at higher drift velocities, the expected linear shift is distorted because of electron heating effects.

Random telegraph signals in deep submicron n-MOSFET's

Random telegraph signals (RTS) in the drain current of deep-submicron n-MOSFET's are investigated at low and high lateral electric fields. RTS are explained both by number and mobility fluctuations due to single electron trapping in the gate oxide. The role of the type of the trap (acceptor or donor), the distance of the trap from the Si-SiO/sub 2/ interface, the channel electron concentration (which is set by the gate bias) and the electron mobility (which is affected by the drain voltage) is demonstrated. The effect of capture and emission on average electron mobility is demonstrated for the first time. A simple theoretical model explains the observed effect of electron heating on electron capture. The mean capture time depends on the local velocity and the nonequilibrium temperature of channel electrons near the trap. The difference between the forward and reverse modes (source and drain exchanged) provides an estimate of the effective trap location along the channel.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Equatorial heating and hemispheric decoupling effects on inner magnetospheric core plasma evolution

We have extended our previous semikinetic study of early stage plasmasphere refilling with perpendicular ion heating (Lin et al., 1992) by removing the restriction that the northern and southern boundaries are identical and incorporating a generalized transport description for the electrons (e.g., Brown et al., 1991). This allows investigation of the effects of electron heating and a more realistic calculation of electric fields produced by ion and electron temperature anisotropies. The combination of perpendicular ion heating and parallel electron heating leads to an equatorial electrostatic potential peak, which tends to shield and decouple ion flows in the northern and southern hemispheres. Unequal ionospheric upflows in the northern and southern hemispheres lead to the development of distinctly asymmetric densities and other bulk parameters. At t= 5 hour after the initiation of refilling with different source densities (Nnorth = 100 cm−3, Nsouth = 50 cm−3), the maximum potential drops of the northern and southern hemispheres are 0.6 and 1.3 V, respectively. At this time the minimum ion densities are 11 and 7 cm−3 for the northern and southern hemispheres. DE 1 observations of asymmetric density profiles by Olsen (1992) may be consistent with these predictions. Termination of particle heating causes the reduction of equatorial potential and allows interhemispheric coupling. When the inflows from the ionospheres are reduced (as may occur after sunset), decreases in plasma density near the ionospheric regions are observed while the heated trapped ion population at the equator persists.

Edge channels and the quantum-Hall-effect breakdown

Quantum Hall devices have been investigated in the regime of the breakdown of dissipationless current flow by means of a photoresistance imaging technique. It is possible to distinguish three stages in development of the breakdown: (i) the initial rise of the longitudinal resistance due to a change with the Hall electric field of the percolation threshold leading to electron backscattering between the edges; (ii) at higher bias currents a strong response from the edges is observed in two-dimensional images, in agreement with the edge-state model; (iii) on increasing the bias current further, electron heating effects are seen to prevail.