Electron Heating Effect Research Articles

Finite shot noise and electron heating at quantized conductance in high-mobility quantum point contacts

We report a precise experimental study on the shot noise of a quantum point contact (QPC) fabricated in a GaAs/AlGaAs based high-mobility two-dimensional electron gas (2DEG). The combination of unprecedented cleanliness and very high measurement accuracy has enabled us to discuss the Fano factor to characterize the shot noise with a precision of 1 %. We observed that the shot noise at zero magnetic field exhibits a slight enhancement exceeding the single particle theoretical prediction, and that it gradually decreases as a perpendicular magnetic field is applied. We also confirmed that this additional noise completely vanishes in the quantum Hall regime. These phenomena can be explained by the electron heating effect near the QPC, which is suppressed with increasing magnetic field.

Multi-scale modelling for HEDP experiments on Orion

The Orion laser at AWE couples high energy long-pulse lasers with high intensity short-pulses, allowing material to be compressed beyond solid density and heated isochorically. This experimental capability has been demonstrated as a platform for conducting High Energy Density Physics material properties experiments. A clear understanding of the physics in experiments at this scale, combined with a robust, flexible and predictive modelling capability, is an important step towards more complex experimental platforms and ICF schemes which rely on high power lasers to achieve ignition. These experiments present a significant modelling challenge, the system is characterised by hydrodynamic effects over nanoseconds, driven by long-pulse lasers or the pre-pulse of the petawatt beams, and fast electron generation, transport, and heating effects over picoseconds, driven by short-pulse high intensity lasers. We describe the approach taken at AWE; to integrate a number of codes which capture the detailed physics for each spatial and temporal scale. Simulations of the heating of buried aluminium microdot targets are discussed and we consider the role such tools can play in understanding the impact of changes to the laser parameters, such as frequency and pre-pulse, as well as understanding effects which are difficult to observe experimentally.

Modification of the lower ionospheric conductivity by thunderstorm electrostatic fields

AbstractThis paper reports a modeling study of the modifications of the nighttime lower ionospheric conductivity by electrostatic fields produced by underlying thunderstorms. The model used combines Ohm's law with a simplified lower ionospheric ion chemistry model to self‐consistently calculate the steady state nighttime conductivity above a thunderstorm. The results indicate that although the electron density is generally increased, the lower ionospheric conductivity can be reduced by up to 1–2 orders of magnitude because electron mobility is significantly reduced due to the electron heating effect. For a typical ionospheric density profile, the resulting changes in the reflection heights of extremely low frequency and very low frequency waves are 5 and 2 km, respectively.

Equations of state, energy transport and two-temperature hydrodynamic simulations for femtosecond laser irradiated copper and gold

In this work, the model of thermodynamic and transport properties of copper and gold at electron-ion non-equilibrium state is presented. Accepted here ranges of electron temperature and pressure are enough to describe the experimentally achievable states. The changes in electron spectra due to electron heating and compression or expansion are taken into account using two-parabolic model. In our previous works, thermal conductivity and electron-ion coupling were considered as dependencies on electron and ion temperatures. Now the dependence on density for these coefficients is taken into account. To include exchange-correlation effects on electron-electron collisions we have found out how this effect can be included in electron screening. In addition, we have renewed our approach for heat conductivity calculation to include thermoelectric phenomena, which are significant at high electron temperatures. The effect of electron heating on sound velocities in aforementioned metals is investigated. The two-temperature hydrodynamics simulation of film expansion was provided with the use of the model presented here.

Triple-probe Diagnostic Measurements in Plasma of GLAST Spherical Tokamak

Triple-probe has been developed and operated successfully to characterize ECRH-assisted argon as well as hydrogen microwave plasmas in GLAST Spherical Tokamak. This technique enables to determine transient plasma parameters such as floating potential, electron temperature and electron number density in rapidly time-varying plasmas. An effective electron heating mechanism is applied to produce microwave plasma by injecting radiofrequency (RF) radiation at a frequency of 2.45 GHz in the presence of resonant toroidal magnetic field. Plasma parameters and corresponding fluctuations are measured as a function of time in different gas fill pressures for various applied magnetic fields. The results demonstrate the dependence of plasma parameters such as Vf, Te, ne and their fluctuations on gas fill pressure during the pre-ionization phase of the GLAST operation. Plasma behavior is observed to be closely depending on the coupling of RF power during microwave discharge. Additionally, the hydrogen plasma shows pronounced fluctuations in comparison with argon plasma with some decrease in electron temperature and densities.

Interplay of electron heating and saturable absorption in ultrafast extreme ultraviolet transmission of condensed matter

High intensity pulses obtained by modern extreme ultraviolet (EUV) and x-ray photon sources allows the observation of peculiar phenomena in condensed matter. Experiments performed at the Fermi@Elettra FEL-1 free-electron-laser source at 23.7, 33.5, and 37.5 eV on Al thin films, for an intermediate-fluence range up to about $20\phantom{\rule{0.28em}{0ex}}\mathrm{J}/{\mathrm{cm}}^{2}$, show evidence for a nonmonotonic EUV transmission trend. A decreasing transmission up to about $5--10\phantom{\rule{0.28em}{0ex}}\mathrm{J}/{\mathrm{cm}}^{2}$ is followed by an increase at higher fluence, associated with saturable absorption effects. The present findings are interpreted within a simplified three-channel model, showing that an account of the interplay between ultrafast electron heating and saturation effects is required to explain the observed transmission trend.

Sawtooth control in JET with ITER relevant low field side resonance ion cyclotron resonance heating and ITER-like wall

New experiments at JET with the ITER-like wall show for the first time that ITER-relevant low field side resonance first harmonic ion cyclotron resonance heating (ICRH) can be used to control sawteeth that have been initially lengthened by fast particles. In contrast to previous (Graves et al 2012 Nat. Commun. 3 624) high field side resonance sawtooth control experiments undertaken at JET, it is found that the sawteeth of L-mode plasmas can be controlled with less accurate alignment between the resonance layer and the sawtooth inversion radius. This advantage, as well as the discovery that sawteeth can be shortened with various antenna phasings, including dipole, indicates that ICRH is a particularly effective and versatile tool that can be used in future fusion machines for controlling sawteeth. Without sawtooth control, neoclassical tearing modes (NTMs) and locked modes were triggered at very low normalised beta. High power H-mode experiments show the extent to which ICRH can be tuned to control sawteeth and NTMs while simultaneously providing effective electron heating with improved flushing of high Z core impurities. Dedicated ICRH simulations using SELFO, SCENIC and EVE, including wide drift orbit effects, explain why sawtooth control is effective with various antenna phasings and show that the sawtooth control mechanism cannot be explained by enhancement of the magnetic shear. Hybrid kinetic-magnetohydrodynamic stability calculations using MISHKA and HAGIS unravel the optimal sawtooth control regimes in these ITER relevant plasma conditions.

Investigation of structural disorder using electron temperature in VHF-PECVD on hydrogenated amorphous silicon films for thin film solar cell applications.

Electrode distances and gas flow ratios are important parameters for fabricating intrinsic (i-type) layers of hydrogenated amorphous silicon (a-Si:H) films using a very high frequency plasma-enhanced chemical-vapor deposition (VHF-PECVD) system. In this work, we investigated the relationship between the electrode distances and gas flow ratios on the properties of i-type a-Si:H films. The electrical, chemical and structural properties are improved with decreasing electrode distances (20-40 mm) at a hydrogen ratio [R (H2/SiH4) = 4], due to the low electron temperature and heating effect. A low electron temperature generates silane-related-reactive species (SiH3) and decreases structural disorder resulting in high quality i-type a-Si:H films. The electrical, chemical and structural properties of the a-Si:H films are confirmed using Al coplanar electrodes, FTIR, Raman spectroscopy, and spectroscopy ellipsometry (SE). When a solar cell is fabricated using the a-Si:H film, J(sc) of 13.2-14.8 mA/cm2, photoconductivity of 1.5 x 10(-5)-8.6 x 10(-6) S/cm, Si--H2 content of 0-1.24 at.%, and hydrogen content of about 10 at.% are obtained. These results together with a model of the plasma chemistry indicate that H atoms and SiH3 radicals play an important role in the deposition process.

Ion cyclotron resonance frequency heating in JET during initial operations with the ITER-like wall

In 2011/12, JET started operation with its new ITER-Like Wall (ILW) made of a tungsten (W) divertor and a beryllium (Be) main chamber wall. The impact of the new wall materials on the JET Ion Cyclotron Resonance Frequency (ICRF) operation is assessed and some important properties of JET plasmas heated with ICRF are highlighted. A ∼ 20% reduction of the antenna coupling resistance is observed with the ILW as compared with the JET carbon (JET-C) wall. Heat-fluxes on the protecting limiters close the antennas, quantified using Infra-Red thermography (maximum 4.5 MW/m2 in current drive phasing), are within the wall power load handling capabilities. A simple RF sheath rectification model using the antenna near-fields calculated with the TOPICA code can reproduce the heat-flux pattern around the antennas. ICRF heating results in larger tungsten and nickel (Ni) contents in the plasma and in a larger core radiation when compared to Neutral Beam Injection (NBI) heating. The location of the tungsten ICRF specific source could not be identified but some experimental observations indicate that main-chamber W components could be an important impurity source: for example, the divertor W influx deduced from spectroscopy is comparable when using RF or NBI at same power and comparable divertor conditions, and Be evaporation in the main chamber results in a strong reduction of the impurity level. In L-mode plasmas, the ICRF specific high-Z impurity content decreased when operating at higher plasma density and when increasing the hydrogen concentration from 5% to 15%. Despite the higher plasma bulk radiation, ICRF exhibited overall good plasma heating performance; the power is typically deposited at the plasma centre while the radiation is mainly from the outer part of the plasma bulk. Application of ICRF heating in H-mode plasmas has started, and the beneficial effect of ICRF central electron heating to prevent W accumulation in the plasma core has been observed.

Electron heating and current scaling in a two-dimensional electron system in close proximity to nanoscale scatterers

We have performed magneto-transport measurements on a two-dimensional electron system (2DES) which is in close proximity to nanoscaled scatterers. Weak localization, as evidenced by negative magnetoresistance, is observed. In this work, we use the extracted phase coherence rate as a thermometer to measure the electron’s effective temperature T e in our 2DES when a high driving current I flows through the device. We find that T e ∝ I ~0.52, consistent with 1/τ ep ~ T 2 in two dimensions, where 1/τ ep is the electron-phonon scattering rate. However, the phase coherence rate 1/τ φ ~ T, with a very small offset, is consistent with zero-temperature dephasing. Most importantly, our experimental results are in agreement with the fact that at low temperatures, the dominant phase-breaking mechanism is electron-electron scattering, not electron-phonon scattering. Therefore we are able to investigate both electron-electron scattering and electron-phonon scattering which are, in most cases, difficult to study independently in the linear region. Our data show that the electron heating effect is a very powerful tool for studying semiconductor devices.

Effect of Internal Antenna Coil Power on the Plasma Parameters in 13.56 MHz/60 MHz Dual-Frequency Sputtering

The plasma property of a hybrid ICP/sputtering discharge driven by 13.56 MHz/60 MHz power sources was investigated by Langmuir probe measurement. For the pure sputtering discharge, the low electron density and ion flux, the rise of floating potential and plasma potential with increasing power, as well as the bi-Maxwellian distribution of electron energy distributions (EEDFs) were obtained. The assistance of ICP discharge led to the effective increases of electron density and ion flux, the suppression of rise of floating potential and plasma potential, as well as the change of EEDFs from bi-Maxwellian distribution into Maxwellian distribution. The increase of electron density and ion flux, and the EEDFs evolution were related to the effective electron heating by the induced electric field.

Wide band waveguide sensor for microwave pulse measurement

A wide band resistive sensor, the performance of which is based on electron heating effect in semiconductors, has been implemented in a double ridge waveguide WRD250 in a frequency range of 2.6–7.8 GHz. The dependences of the output signal on microwave pulse power and frequency response of the sensor were measured. The measured results of frequency response have been compared with the calculated ones obtained using a finite-difference time-domain method, and a reasonable agreement between them was found.

Wide band waveguide sensor for microwave pulse measurement

A wide band resistive sensor, the performance of which is based on electron heating effect in semiconductors, has been implemented in a double ridge waveguide WRD250 in a frequency range of 2.6–7.8 GHz. The dependences of the output signal on microwave pulse power and frequency response of the sensor were measured. The measured results of frequency response have been compared with the calculated ones obtained using a finite-difference time-domain method, and a reasonable agreement between them was found.

Effect of electron heating on self-induced transparency in relativistic-intensity laser-plasma interactions

The effective increase of the critical density associated with the interaction of relativistically intense laser pulses with overcritical plasmas, known as self-induced transparency, is revisited for the case of circular polarization. A comparison of particle-in-cell simulations to the predictions of a relativistic cold-fluid model for the transparency threshold demonstrates that kinetic effects, such as electron heating, can lead to a substantial increase of the effective critical density compared to cold-fluid theory. These results are interpreted by a study of separatrices in the single-electron phase space corresponding to dynamics in the stationary fields predicted by the cold-fluid model. It is shown that perturbations due to electron heating exceeding a certain finite threshold can force electrons to escape into the vacuum, leading to laser pulse propagation. The modification of the transparency threshold is linked to the temporal pulse profile, through its effect on electron heating.

Nonlinear magnetotransport phenomena in high-mobility two-dimensional electrons in InGaAs/InP and GaAs/AlGaAs

This paper reports on the observation and analysis of magnetotransport phenomena in the nonlinear differential resistance ${r}_{xx}=d{V}_{xx}/dI$ of high-mobility In${}_{x}$Ga${}_{1\ensuremath{-}x}$As/InP and GaAs/Al${}_{x}$Ga${}_{1\ensuremath{-}x}$As Hall bar samples driven by direct current ${I}_{\mathrm{dc}}$. Specifically, it is observed that Shubnikov--de Haas (SdH) oscillations at large filling factors invert their phase at sufficiently large values of ${I}_{\mathrm{dc}}$. This phase inversion is explained as being due to an electron heating effect. In the quantum Hall effect regime the ${r}_{xx}$ oscillations transform into diamond-shaped patterns with different slopes corresponding to odd and even filling factors. The diamond-shaped features at odd filling factors can be used as a probe to determine spin energy gaps. A zero current anomaly (ZCA) which manifests itself as a narrow dip in the ${r}_{xx}({I}_{\mathrm{dc}})$ characteristics at zero current is also observed. The ZCA effect strongly depends upon temperature, vanishing above 1 K while the transport diamonds persist to higher temperatures. The transport diamonds and ZCA are fully reproduced in a higher mobility GaAs/AlGaAs Hall bar structure confirming that these phenomena reflect intrinsic properties of two-dimensional systems.

Effect of Electron Beam Irradiation and Heating on the Structural Stability of Sulphide-Filled Carbon Nanotubes

In recent years there has been an increased interest in characterisation methods capable of evaluating the response of nanostructured materials such as quantum dots and nanotubes to externally applied stimuli. However, due to issues concerning sample homogeneity and non-uniform orientation, the study of collective responses can lead to ambiguous results. Therefore, it is crucial to perform a complementary evaluation at the single nanostructure level which requires the use of sophisticated analytical instrumentation. In addition to high spatial resolution, the equipment employed must permit sample manipulation and real-time imaging of the in situ stimulus response. Transmission electron microscopes (TEM) observe all these requirements. Furthermore, novel developments in sample holder design and manufacturing have opened a window to an unprecedented range of new in situ experiments. As a result, varied reports are increasingly populating the in situ TEM literature examining such different matters as temperature-related structural phase changes of nanoparticles or dopant diffusion in nanoscaled batteries.

Shot noise suppression in InGaAs/InGaAsP quantum channels

We have measured the shot noise in a quantum point contact (QPC) fabricated by using InGaAs/InGaAsP heterostructure, whose conductance can be electrically tuned by the gate voltages. The reduced shot noise is observed when the QPC conductance equals to N(2e2/h) (N = 4, 5, and 6), which is the direct experimental evidence of the coherent quantized channel formation in the QPC. The deviation of the observed Fano factor from the theory is explained by the electron heating effect generated at the QPC.

Polarisation insensitive in-situ resistive sensor for high-power microwave pulse measurement in Te 11 mode circular waveguide

A resistive sensor (RS) based on the electron heating effect in semiconductors is considered for high-power microwave pulse measurements in TE 11 (H 11 ) mode circular waveguide. A plate-shaped semiconductor sample with contacts on its top and bottom is used as the sensing element (SE). A three-dimensional finite-difference time-domain method was applied for the calculation of the electric field strength in the SE. The optimal dimensions and specific resistance of the SE have been found, providing a flat frequency response for the RS when the SE position in the waveguide coincides with a polarisation plane of the wave. A polarisation-insensitive layout of the RS consisting of two SEs placed at 90° from each other has been proposed. It is shown that the sum of their signals (within °7°) is independent of the position of the polarisation plane in the waveguide.

Interaction of semiconductor sample with TE<sub>10</sub> mode in double ridged waveguide

An interaction of a semiconductor sample inserted in the centre of a double ridge waveguide between its metal ridges with TE10 mode was investigated. A three-dimensional finite-difference time-domain method was applied for the calculation of the electromagnetic field components in the waveguide section with a semiconductor sample. The average electric field strength in the sample and the reflection coefficient were determined. This sample is considered a prototype of the sensing element (SE) of a resistive sensor (RS) the performance of which is based on electron heating effect in the semiconductor. The optimal dimensions and specific resistance of the SE have been found, providing frequency response as flat as possible for the RS in the WRD250 waveguide covering the frequency range of 2.60–7.80 GHz.

Interaction of semiconductor sample with TE<sub>10</sub> mode in double ridged waveguide

An interaction of a semiconductor sample inserted in the centre of a double ridge waveguide between its metal ridges with TE10 mode was investigated. A three-dimensional finite-difference time-domain method was applied for the calculation of the electromagnetic field components in the waveguide section with a semiconductor sample. The average electric field strength in the sample and the reflection coefficient were determined. This sample is considered a prototype of the sensing element (SE) of a resistive sensor (RS) the performance of which is based on electron heating effect in the semiconductor. The optimal dimensions and specific resistance of the SE have been found, providing frequency response as flat as possible for the RS in the WRD250 waveguide covering the frequency range of 2.60–7.80 GHz.