Browsing by Subject "Electron"

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2D impurity scattering of electron in a magnetic field
(2020-01-30) Feng, Jingjing, 1990-; Niu, Qian
Magneto-transport of two-dimensional electrons is an interesting but yet complicated topic in condensed matter physics. While most existing theories in magneto-transport are based on phase space Berry curvature, the mechanism of impurity scattering is less mentioned, but actually plays an important role in magneto-transport. Our theory focuses on how single electron-impurity scattering is influenced by electromagnetic field. In order to address this problem, we consider two different regimes separately in transport: electron transport in weak magnetic field, electron transport in strong magnetic field. Classical electron transport in weak magnetic field: In order to address this difficulty, we introduce a new model recipe based on an abstraction of the actual impurity scattering process to redefine scattering parameters for the single elastic impurity scattering. It can introduce an appropriate set of scattering parameters in the presence of magnetic field to calculate the differential cross section. Specifically, the real scattering process can be abstracted into a sudden switch between the initial asymptotic and final asymptotic trajectory. Two effects emerges under this model: skew scattering and coordinate jump, which will eventually modify the Boltzmann equation. It results in anomalous Hall effect and Negative magnetoresistivity. Classical and Quantum electron transport in strong magnetic field: In strong magnetic field, the electron motion can be generally described by the migration of guiding center. During the electron-impurity scattering, the guiding centers suddenly shift its coordinate. Traditionally, the electric field is not addressed in the impurity scattering process, but only in the drifting process. Because here we consider the electric field effect, the cyclotron radius changes in each individual scattering and the cumulative scattering broadens the cyclotron radius to be multiple values. At the same time, the coordinate shift accumulatively contribute to a longitudinal current due to the symmetry breaking of the energy spectrum by the electric field. From quantum perspective, the broadened cyclotron radius is a classical manifestation of Landau Level broadening and shifting induced by electric field.
Geometrodynamics in crystals with space-time periodicity and deformation
(2021-08-12) Dong, Liang (Ph. D. in physics); Niu, Qian; Macdonald, Allan H; Sadun, Lorenzo A; Matzner, Richard A; Marder, Michael P
Wave-packet dynamics is a semiclassical theory which works in the classical limit and includes quantum corrections in the form of Berry phase effect. Its key result is the equations of motion describing a point paticle with internal multipole moments moving in phase space. It has been proved to be successful in describing the electronic response in a crystal to external fields, e.g., electro-magnetic field. In this thesis, we further develop the theory in three directions: (1). In addition to the electric current response given by the equations of motion, we derived a formula describing the response of arbitrary physical observables to electric field and temperature gradient. Within this framework, the Mott relation for general physical observables is proved; (2). we apply the original wave-packet theory to study the electronic property in a deformed crystal. Empowered by a novel lattice bundle picture where a deformed crystal is treated as a bundle of locally periodic lattices, we developed a geometrical formulation where the equations of motion in many ways resembles gravitational effect in the framework of general relativity. The energy-stress tensor and viscosity tensor of electrons are calculated; (3). we further developed the wave-packet theory such that space and time can be treated on equal footing. This is achieved by enabling the theory to introduce arbitrary parameter as ”time” to describe particle dynamics. The Lorentz invariant equations of motion of Dirac electron as a point particle is discussed with Berry curvature effect. The BMT equation of the Pauli-Lubanski vector is derived from Dirac field equation.
Laser wakefield and direct acceleration in the plasma bubble regime
(2017-08) Zhang, Xi, Ph. D.; Mahajan, Swadesh M.; Shvets, G.; Downer, Michael; Morrison, Philip; Milosavljevic, Milos
Laser wakefield acceleration (LWFA) and direct laser acceleration (DLA) are two different kinds of laser plasma electron acceleration mechanisms. LWFA relies on the laser-driven plasma wave to accelerate electrons. The interaction of ultra-short ultra-intensive laser pulses with underdense plasma leads the LWFA into a highly nonlinear regime (“plasma bubble regime”) that attracts particular interest nowadays. DLA accelerates electrons by laser electromagnetic wave in the ion channel or the plasma bubble through the Betatron resonance. This dissertation presents a hybrid laser plasma electron acceleration mechanism. We investigate its features through particle-in-cell (PIC) simulations and the single particle model. The hybrid laser plasma electron acceleration is the merging concept between the LWFA and the DLA, so called laser wakefield and direct acceleration (LWDA). The requirements of the initial conditions of the electron to undergo the LWDA are determined. The electron must have a large initial transverse energy. Two electron injection mechanisms that are suitable for the LWDA, density bump injection and ionization induced injection, are studied in detail. The features of electron beam phase space and electron dynamics are explored. Electron beam phase space appears several unique features such as spatially separated two groups, the correlation between the transverse energy and the relativistic factor and the double-peak spectrum. Electrons are synergistically accelerated by the wakefield as well as by the laser electromagnetic field in the laser-driven plasma bubble. LWDA are also investigated in the moderate power regime (10 TW) in regarding the effects of laser color and polarization. It is found that the frequency upshift laser pulse has better performance on avoiding time-jitter of electron energy spectra, electron final energy and electron charge yield. Some basic characters that related to the LWDA such as the effects of the subluminal laser wave, the effects of the longitudinal accelerating field, the electron beam emittance, the electron charge yield and potentially applications as radiation source are discussed.
Spectroscopic measurement of n[subscript e] and T[subscript e] profiles using atomic and kinetic models for Argon in the Texas Helimak
(2013-05) Dodd, Kenneth Carter; Gentle, Kenneth W.
Profiles for electron density and temperature were determined in a self-consistent way using line emission spectroscopy and collisional radiative models for neutral and singly ionized Argon (Ar I and Ar II) in the Texas Helimak. Neutral Argon density profiles were calculated using a kinetic gas model. Electron-impact excitation and Ionization rates were corrected to account for the electron velocity distribution deviating slightly from a true Maxwellian distribution due to inelastic electron-neutral collisions. Results show an electron temperature which roughly agrees with probe diagnostics. This method gives an electron density that is about twice as high, which may be possible from a power balance perspective.
Study of picosecond-scale electron dynamics in laser-produced plasmas with and without an external magnetic field
(2013-12) McCormick, Matthew Warren; Ditmire, Todd
The interaction of ultra-short laser pulses and cluster targets can be used to explore a number of interesting phenomena, ranging from nuclear fusion to astrophysical blast waves. In our experiments, we focused on exploring very fast plasma dynamics of a plasma created by ionizing clusters and monomer gas. By using a 115 fs laser pulse, we can even study sub-picosecond plasma dynamics. In addition, we also wanted to impose an external magnetic field on these plasmas to study how the plasma evolution would change. The results of this work produced two significant results. First, a new, extremely fast ionization mechanism, with velocities as high as 0.5 c, was discovered which allows for significant plasma expansion on a picosecond time-scale. Experimental studies measured the velocity of the ionization wave, while particle-in-cell simulations helped explain the source and longevity of the wave. It was also observed that this ionization wave was not affected by the external magnetic field. Second, the external field was shown to inhibit plasma expansion on a time-scale of tens of picoseconds, which seems to be one of the first demonstrations of magnetic confinement on such a fast time-scale. Simple 1D simulations tell us that the field appears to slow electron heat transport in the plasma as well as inhibiting collisional ionization of electrons expanding into the surrounding gas. The inhibition of plasma expansion by the field on this time-scale may provide some evidence that magnetic confinement of a fusion plasma created by exploding clusters could improve the fusion yield by slowing heat loss as well as possibly electrostatically confining the hot ions.