Elevated Electron Temperatures Research Articles

Quantum-circuit refrigerator

Quantum technology promises revolutionizing applications in information processing, communications, sensing and modelling. However, efficient on-demand cooling of the functional quantum degrees of freedom remains challenging in many solid-state implementations, such as superconducting circuits. Here we demonstrate direct cooling of a superconducting resonator mode using voltage-controllable electron tunnelling in a nanoscale refrigerator. This result is revealed by a decreased electron temperature at a resonator-coupled probe resistor, even for an elevated electron temperature at the refrigerator. Our conclusions are verified by control experiments and by a good quantitative agreement between theory and experimental observations at various operation voltages and bath temperatures. In the future, we aim to remove spurious dissipation introduced by our refrigerator and to decrease the operational temperature. Such an ideal quantum-circuit refrigerator has potential applications in the initialization of quantum electric devices. In the superconducting quantum computer, for example, fast and accurate reset of the quantum memory is needed.

Modeling the electron temperature distribution in F2 region of high-latitude ionosphere for winter solstice conditions

Using the three-dimensional model of the high-latitude ionosphere in Euler variables, which takes into account the mismatch between geographic and geomagnetic poles, we study the behavior of the electron temperature Te in the F2 region as a function of universal time. We present results of the numerical modeling of spatial-temporal distribution of electron temperature in the F2 region for winter solstice, minimum solar activity, and moderate geomagnetic activity. The electron temperature distribution in the F2 region of the high-latitude ionosphere in winter is shown to be characterized by a Te increase in dawn and dusk sectors. Further, the mismatch between the poles leads to regular longitudinal features in Te distribution during Earth’s daily rotation. Thus, at 05 UT, when the Eastern Hemisphere is illuminated, the elevated Te zone is formed only in the dawn sector, and at 17 UT, when the Western Hemisphere is illuminated such zones are observed in both the sectors. We discuss reasons for the formation of the regions with elevated electron temperature depending on the universal time. Results of numerical experiments are compared with similar results obtained with other models.

Моделирование распределения температуры электронов в области F2 высокоширотной ионосферы для условий зимнего солнцестояния

Using the three-dimensional model of the high-latitude ionosphere in Euler variables, which takes into account the mismatch between geographical and geomagnetic poles, we study the behavior of the electron temperature Te in the F2 region as a function of universal time. We present results of the numerical modeling of spatial-temporal distribution of electron temperature in the F2 region for winter solstice, minimum solar activity, and moderate geomagnetic activity. The electron temperature distribution in the F2 region of the high-latitude ionosphere in winter is shown to be characterized by a Te increase in dawn and dusk sectors. Further, the mismatch between the poles leads to regular longitudinal features in Te distribution during Earth’s daily rotation. Thus, at 05 UT, when the Eastern Hemisphere is illuminated, the elevated Te zone is formed only in the dawn sector, and at 17 UT, when the Western Hemisphere is illuminated, such zones are observed in both the sectors. We discuss reasons for the formation of the regions with elevated electron temperature depending on the universal time. The results of numerical experiments are compared with similar results obtained with other models.

Hot-Electron Dynamics and Thermalization in Small Metallic Nanoparticles

The important role played by hot electrons in photocatalysis and light harvesting has attracted great interest in their dynamics and mechanisms of generation. Here, we theoretically study the temporal evolution of optically excited conduction electrons in small plasmon-supporting gold and silver nanoparticles. We describe the electron dynamics through a master equation incorporating transition rates for optical excitations and electron–electron collisions that are calculated using the screened interaction within an independent-electron picture. Upon optical excitation of the particle by a light pulse, a nonthermal electron distribution is produced, which takes 10s fs to thermalize at an elevated electron temperature due to electron–electron collisions and eventually relaxes back to ambient temperature via coupling to phonons and thermal diffusion. Phonons and diffusion are introduced through a phenomenological inelastic attenuation rate. We find the temporal evolution of the electron energy distribution t...

Transient thermal and nonthermal electron and phonon relaxation after short-pulsed laser heating of metals

Several dynamic thermal and nonthermal scattering processes affect ultrafast heat transfer in metals after short-pulsed laser heating. Even with decades of measurements of electron-phonon relaxation, the role of thermal vs. nonthermal electron and phonon scattering on overall electron energy transfer to the phonons remains unclear. In this work, we derive an analytical expression for the electron-phonon coupling factor in a metal that includes contributions from equilibrium and nonequilibrium distributions of electrons. While the contribution from the nonthermal electrons to electron-phonon coupling is non-negligible, the increase in the electron relaxation rates with increasing laser fluence measured by thermoreflectance techniques cannot be accounted for by only considering electron-phonon relaxations. We conclude that electron-electron scattering along with electron-phonon scattering have to be considered simultaneously to correctly predict the transient nature of electron relaxation during and after short-pulsed heating of metals at elevated electron temperatures. Furthermore, for high electron temperature perturbations achieved at high absorbed laser fluences, we show good agreement between our model, which accounts for d-band excitations, and previous experimental data. Our model can be extended to other free electron metals with the knowledge of the density of states of electrons in the metals and considering electronic excitations from non-Fermi surface states.

Determining energy relaxation length scales in two-dimensional electron gases

We present measurements of the energy relaxation length scale ℓ in two-dimensional electron gases (2DEGs). A temperature gradient is established in the 2DEG by means of a heating current, and then the elevated electron temperature Te is estimated by measuring the resultant thermovoltage signal across a pair of deferentially biased bar-gates. We adapt a model by Rojek and König [Phys. Rev. B 90, 115403 (2014)] to analyse the thermovoltage signal and as a result extract ℓ, Te, and the power-law exponent αi for inelastic scattering events in the 2DEG. We show that in high-mobility 2DEGs, ℓ can attain macroscopic values of several hundred microns, but decreases rapidly as the carrier density n is decreased. Our work demonstrates a versatile low-temperature thermometry scheme, and the results provide important insights into heat transport mechanisms in low-dimensional systems and nanostructures. These insights will be vital for practical design considerations of future nanoelectronic circuits.

Energy flows in graphene: hot carrier dynamics and cooling

Long lifetimes of hot carriers can lead to qualitatively new types of responses in materials. The magnitude and time scales for these responses reflect the mechanisms governing energy flows. We examine the microscopics of two processes which are key for energy transport, focusing on the unusual behavior arising due to graphene's unique combination of material properties. One is hot carrier generation in its photoexcitation dynamics, where hot carriers multiply through an Auger type carrier–carrier scattering cascade. The hot-carrier generation manifests itself through elevated electronic temperatures which can be accessed in a variety of ways, in particular optical conductivity measurements. Another process of high interest is electron-lattice cooling. We survey different cooling pathways and discuss the cooling bottleneck arising for the momentum-conserving electron–phonon scattering pathway. We show how this bottleneck can be relieved by higher-order collisions—supercollisions—and examine the variety of supercollision processes that can occur in graphene.

Hot-carrier photocurrent effects at graphene–metal interfaces

Photoexcitation of graphene leads to an interesting sequence of phenomena, some of which can be exploited in optoelectronic devices based on graphene. In particular, the efficient and ultrafast generation of an electron distribution with an elevated electron temperature and the concomitant generation of a photo-thermoelectric voltage at symmetry-breaking interfaces is of interest for photosensing and light harvesting. Here, we experimentally study the generated photocurrent at the graphene–metal interface, focusing on the time-resolved photocurrent, the effects of photon energy, Fermi energy and light polarization. We show that a single framework based on photo-thermoelectric photocurrent generation explains all experimental results.

The electron thermal structure in the dayside Martian ionosphere implied by the MGS radio occultation data

AbstractWe propose a revised Chapman model for the ionosphere of Mars by allowing for vertical variation of electron temperature. An approximate energy balance between solar EUV heating and CO2 collisional cooling is applied in the dayside Martian ionosphere, analogous to the method recently proposed by Withers et al. (2014). The essence of the model is to separate the contributions of the neutral and electron thermal structures to the apparent width of the main ionospheric layer. Application of the model to the electron density profiles from the Mars Global Surveyor (MGS) radio occultation measurements reveals a clear trend of elevated electron temperature with increasing solar zenith angle (SZA). It also reveals that the characteristic length scale for the change of electron temperature with altitude decreases with increasing SZA. These observations may imply enhanced topside heat influx near the terminator, presumably an outcome of the solar wind interactions with the Martian upper atmosphere. Our analysis also reveals a tentative asymmetry in electron temperature between the northern and southern hemispheres, consistent with the scenario of elevated electron temperature within minimagnetospheres.

Electron–lattice coupling after high-energy deposition in aluminum

This paper presents an analysis of the parameters of highly-excited electron subsystem of aluminum, appearing e.g. after swift heavy ion impact or laser pulse irradiation. For elevated electron temperatures, the electron heat capacity and the screening parameter are evaluated. The electron–phonon approximation of electron–lattice coupling is compared with its precise formulation based on the dynamic structure factor (DSF) formalism. The DSF formalism takes into account collective response of a lattice to excitation including all possible limit cases of this response. In particular, it automatically provides realization of electron–phonon coupling as the low-temperature limit, while switching to the plasma-limit for high electron temperatures. Aluminum is chosen as a good model system for illustration of the presented methodology.

Ultrafast high-power microwave window breakdown: Nonlinear and postpulse effects

The time- and space-dependent optical emissions of nanosecond high-power microwave discharges near a dielectric-air interface have been observed by nanosecond-response four-framing intensified-charged-coupled device cameras. The experimental observations indicate that plasma developed more intensely at the dielectric-air interface than at the free-space region with a higher electric-field amplitude. A thin layer of intense light emission above the dielectric was observed after the microwave pulse. The mechanisms of the breakdown phenomena are analyzed by a three-dimensional electromagnetic-field modeling and a two-dimensional electromagnetic particle-in-cell simulation, revealing the formation of a space-charge microwave sheath near the dielectric surface, accelerated by the normal components of the microwave field, significantly enhancing the local-field amplitude and hence ionization near the dielectric surface. The nonlinear positive feedback of ionization, higher electron mobility, and ultraviolet-driven photoemission due to the elevated electron temperature are crucial for achieving the ultrafast discharge. Following the high-power microwave pulse, the sheath sustains a glow discharge until the sheath collapses.

Nonequilibrium phonon effects in midinfrared quantum cascade lasers

We investigate the effects of nonequilibrium phonon dynamics on the operation of a GaAs-based midinfrared quantum cascade laser over a range of temperatures (77–300 K) via a coupled ensemble Monte Carlo simulation of electron and optical-phonon systems. Nonequilibrium phonon effects are shown to be important below 200 K. At low temperatures, nonequilibrium phonons enhance injection selectivity and efficiency by drastically increasing the rate of interstage electron scattering from the lowest injector state to the next-stage upper lasing level via optical-phonon absorption. As a result, the current density and modal gain at a given field are higher and the threshold current density lower and considerably closer to experiment than results obtained with thermal phonons. By amplifying phonon absorption, nonequilibrium phonons also hinder electron energy relaxation and lead to elevated electronic temperatures.

TWISTED MAGNETIC FLUX TUBES IN THE SOLAR WIND

Magnetic flux tubes in the solar wind can be twisted as they are transported from the solar surface, where the tubes are twisted owing to photospheric motions. It is suggested that the twisted magnetic tubes can be detected as the variation of total (thermal+magnetic) pressure during their passage through observing satellite. We show that the total pressure of several observed twisted tubes resembles the theoretically expected profile. The twist of isolated magnetic tube may explain the observed abrupt changes of magnetic field direction at tube walls. We have also found some evidence that the flux tube walls can be associated with local heating of the plasma and elevated proton and electron temperatures. For the tubes aligned with the Parker spiral, the twist angle can be estimated from the change of magnetic field direction. Stability analysis of twisted tubes shows that the critical twist angle of the tube with a homogeneous twist is 70$^0$, but the angle can further decrease owing to the motion of the tube with regards to the solar wind stream. The tubes with a stronger twist are unstable to the kink instability, therefore they probably can not reach 1 AU.

Atomic scale investigations of the gate controlled tunneling effect in graphyne nanoribbon

Configuration and transport properties of zigzag graphyne nanoribbon (n = 2) are investigated by means of the first-principles calculations and non-equilibrium Green's function in this work. We demonstrated the controllability of the graphyne's conductivity by gate bias, and the tunneling behavior induced by gate and drain voltages was investigated systemically. The characteristics of Id-Vd, Id-Vg, as well as the evolutions of current with electron temperature elevation were explored. The device exhibits a tunneling ratio around 103, and the state art of tunneling operations of the tunneling field effect transistor in this split-new material was achieved.

Theoretical analysis of Poynting flux and polarization for ELF‐VLF electromagnetic waves in the Earth's magnetosphere

We discuss properties of extremely low‐frequency (ELF) and very low‐frequency (VLF) electromagnetic waves in the Earth's magnetosphere. General expressions for wave magnetic field polarization and for the angle θP between the direction of the Poynting flux vector and the ambient magnetic field are derived for low‐amplitude (linear) waves taking into account first‐order finite gyroradius effects. The wave magnetic field is always in a plane perpendicular to the direction of the wave propagation, and the polarization depends on wave frequency (or wavelength), dispersion, and plasma parameters. In a warm plasma, the Poynting flux is not aligned with the group velocity and generally deviates from the wave propagation direction, except for parallel propagation. Numerical estimates for θP and wave polarization are made for plasmaspheric hiss (at L=2, 4, and 6) for typical plasma parameters and for the case of elevated electron temperature (5 keV). The plasmaspheric hiss wave magnetic field is right‐hand circularly polarized, except for highly oblique hiss waves which are elliptically polarized. We note a possible transition to the magnetosonic wave regime at short wavelengths. The maximum Poynting flux angle is ∼20°. Estimates for daytime outer zone chorus (L=6) show two regions of oblique Poynting flux corresponding to a low‐frequency band (ω<ωce/2) and to a high‐frequency band (ωce/2<ω<ωce) where ω and ωce are the wave frequency and the electron cyclotron frequency, respectively. The wave magnetic field polarization varies from circular to elliptical at shorter wavelengths. In a hot plasma, chorus is elliptically polarized. The maximum θP in the low‐frequency band is ∼20°. θP can reach ∼60° in the high‐frequency band. Our results are consistent with the Poynting flux statistics of Polar measurements.

Elevated electron temperatures around twin sporadic E layers at low latitude: Observations and the case for a plausible link to currents parallel to the geomagnetic field

We present data from nighttime sounding rocket flights in the low latitude E region. The payloads carried a sweeping Langmuir probe, a plasma impedance probe, and electric field probes. A detailed examination of the plasma density, temperature, and electric field measurements show two strong sporadic E (Es) layers with very high electron temperatures (∼1000 K) on each side of the upper layer. The lower layer was consistent with the presence of a strong zonal neutral wind shear. The upper layer was strongly influenced by the presence of a strongly negative vertical electric field, with zonal winds and their shears also contributing. A strong downward motion of the plasma from the combined action of the downward electric field and negative zonal wind advected the upper layer far below the region of maximum growth. We have attributed the more puzzling high electron temperatures to frictional heating from parallel currents and shown that the F region nighttime dynamo could easily generate the necessary parallel current densities (1 μA m−2) near the electron density troughs. The electron temperature was also elevated in the Es layers themselves, implying parallel current densities of the order of 15 μA m−2 around the Es peaks. Those parallel currents were attributed to strong Hall current divergences driven by the zonal electric field around the Es peaks.

Observation of intra- and inter-band transitions in the transient optical response of graphene

The transient optical conductivity of freely suspended graphene was examined by femtosecond time-resolved spectroscopy using pump excitation at 400 nm and probe radiation at 800 nm. The optical conductivity (or, equivalently, absorption) changes abruptly upon excitation and subsequently relaxes to its initial value on the time scale of 1 ps. The form of the induced change in the optical conductivity varies strongly with excitation conditions, exhibiting a crossover from enhanced to decreased optical conductivity with increasing pump fluence. We describe the graphene response in terms of transient heating of the electrons, with the characteristic relaxation time of the transient conductivity reflecting the cooling of the electron system and the strongly coupled optical phonons through emission of lower energy phonons. The change in the optical conductivity is attributed to a combination of induced absorption from intra-band transitions of the photo-generated carriers and bleaching of the inter-band transitions by Pauli blocking. The former effect, which corresponds to the high-frequency wing of the Drude response, dominates at low pump fluence. In this regime of a limited rise in the electron temperature, an increase in the optical conductivity is observed. At high pump fluence, elevated electron temperatures are achieved. The decrease in the inter-band bleaching then dominates the transient response, the intra-band contribution being overwhelmed despite an increase in the Drude scattering rate with temperature. The temporal evolution of the optical conductivity in all the regimes can be described within a model including the intra- and inter-band contributions with a time-varying electronic temperature. An increased Drude scattering rate is inferred for high electron temperature and mechanisms for this enhancement are considered. The calculated scattering rate for interactions of the carriers with zone-center and zone-edge optical phonons agrees well with the rates obtained from experiment.

Photoconductivity of biased graphene

Graphene is a promising candidate for optoelectronic applications such as photodetectors, terahertz imagers, and plasmonic devices. The origin of photoresponse in graphene junctions has been studied extensively and is attributed to either thermoelectric or photovoltaic effects. In addition, hot carrier transport and carrier multiplication are thought to play an important role. Here we report the intrinsic photoresponse in biased but otherwise homogeneous graphene. In this classic photoconductivity experiment, the thermoelectric effects are insignificant. Instead, the photovoltaic and a photo-induced bolometric effect dominate the photoresponse due to hot photocarrier generation and subsequent lattice heating through electron-phonon cooling channels respectively. The measured photocurrent displays polarity reversal as it alternates between these two mechanisms in a backgate voltage sweep. Our analysis yields elevated electron and phonon temperatures, with the former an order higher than the latter, confirming that hot electrons drive the photovoltaic response of homogeneous graphene near the Dirac point.

Simulated Raman scattering instability of laser beam in a plasma channel

A high power laser beam propagating through collisional plasma creates a low density channel through which it propagates self-guided. This equilibrium, however, is unstable to stimulated Raman back scattering above a threshold power. The self focusing, leads to enhancement of wave intensity, elevation of electron temperature and reduction of local electron density, leading to diminished attenuation rate. Stimulated Raman scattering instability is treated for laser beam propagating through collisional plasma with thermal conduction in a self-focused filament. Thermal conduction could play a dominant role in determining the energy dissipation of electrons. Inside a filament, the laser undergoes stimulated Raman backscattering. Since the temperature inside a filament is higher and density lower than those outside, the collisional damping rates of the decay waves are lowered and hence the threshold power for B-SRS is reduced. It remains an important process in laser produced plasma. It is also a limiting process in a plasma- loaded free electron laser.

Electron temperature in the cusp as measured with the SCIFER‐2 sounding rocket

It is expected that energy deposited by soft auroral electron precipitation in the ionosphere should result in heating of ionospheric electrons in that location, and this heating is an important step in the ion outflow process. We present coordinated observations from the SCIFER‐2 sounding rocket in the cusp region overflying optical observing sites in Svalbard. The rocket payload included a sensor which is designed to measure the temperature of thermal electrons. We show that elevated electron temperatures measured in situ are correlated with electron precipitation as inferred from auroral emissions during the 60–120 s preceding the passage of the rocket. This integrated “cooking time” is an important factor in determining the origin and resulting flux of outflowing ions.