Optical measurement and control of spin diffusion in n-doped GaAs quantum wells

Transient spin gratings are used to study spin diffusion in lightly n-doped GaAs quantum wells at low temperatures. The spin grating is shown to form in the excess electrons from doping, providing spin relaxation and transport properties of the carriers most relevant to spintronic applications. We demonstrate that spin diffusion of the these carriers is accelerated by increasing the density or energy of the optically excited carriers. These results can be used to better understand and even control spin transport in experiments that optically excite spin-polarized carriers.     

Resonant intrinsic spin hall effect in p-type GaAs quantum well structure

We study intrinsic spin Hall effect in p-type GaAs quantum well structure described by Luttinger Hamiltonian and a Rashba spin-orbit coupling arising from the structural inversion symmetry breaking. The Rashba term induces an energy level crossing in the lowest heavy hole subband, which gives rise to a resonant spin Hall conductance. The resonance may be used to identify the intrinsic spin Hall effect in experiments.     

Optical orientation and spin relaxation of resident electrons in n-doped InAs/GaAs self-assembled quantum dots

We report on optical orientation of electrons in n-doped InAs/GaAs quantum dots. Under non-resonant cw optical pumping, we measure a negative circular polarization of the luminescence of charged excitons (or trions) at low temperature (T=10 K). The dynamics of the recombination and of the circular polarization is studied by time-resolved spectroscopy. We discuss a simple theoretical model for the trion relaxation, that accounts for this remarkable polarization reversal. The interpretation relies on the bypass of Pauli blocking allowed by the anisotropic electron–hole exchange. Eventually, the spin relaxation time of doping electrons trapped in quantum dots is measured by a non-resonant pump–probe experiment.     

Optically driven spin memory in n-doped InAs-GaAs quantum dots

We show that the spin state of the resident electron in an n-doped self-assembled InAs-GaAs quantum dot can be written and read using nonresonant, circularly polarized optical pumping. A simple theoretical model is presented and accounts for the remarkable dynamics producing counterpolarized photoluminescence.     

Theory of the quantized hall conductivity plateaus of two-dimensional electron systems

The width of the plateaus of the quantized Hall conductivity of two- dimensional electron systems is expected to drop sharply as a function of μ_BH/kT from the zero point value of $\text{2mμ}{}_{\mathrm{B}}\text{H/π}\text{h}\text{?}\text{;}{}^2$, when it is expressed in terms of electron density.     

Optical spin hall effect

A remarkable analogy is established between the well-known spin Hall effect and the polarization dependence of Rayleigh scattering of light in microcavities. This dependence results from the strong spin effect in elastic scattering of exciton polaritons: if the initial polariton state has a zero spin and is characterized by some linear polarization, the scattered polaritons become strongly spin polarized. The polarization in the scattered state can be positive or negative dependent on the orientation of the linear polarization of the initial state and on the direction of scattering. Very surprisingly, spin polarizations of the polaritons scattered clockwise and anticlockwise have different signs. The optical spin Hall effect is possible due to strong longitudinal-transverse splitting and finite lifetime of exciton polaritons in microcavities.     

Optically enhanced hall mobility in GaAs

Photo-Hall effect measurements on n-type LPE GaAs: Si between 15 and 60 K indicate an enhancement of the Hall mobility in the ionized impurity scattering conduction regime. The mobility is enhanced by a factor of 2 upon illumination with band gap light of intensity 7 × 10¹⁴ photons/cm² sec. Quantitatively, this enhancement is partially accounted for in terms of a screening of impurities by photo-generated carriers. We discuss several other mechanisms which could also contribute to an enhancement, and conclude that an increase in carrier temperature and/or a decrease in the number of scattering centers through photo-neutralization are primarily responsible for the remainder of the enhancement.     

Mid-infrared heavily n-doped GaAs extraordinary transmission through subwavelength grooves

Transmission of an electromagnetic wave from a heavily doped n-type GaAs film is studied theoretically. The calculations are performed using the two-dimensional finite-different time-domain method. From the calculations, we find the extraordinary transmission of p-polarized waves through the film with subwavelength grooves on both surfaces at mid-infrared frequencies. By determining a set of groove parameters, we optimize the transmission to as high as 55.2%. We ascribe this extraordinary transmission to the coupling of the surface-plasmon polariton modes and waveguide modes. Such an enhanced transmission device can be useful for mid-infrared wave filters, emitters, and monitors.     

Electromagnetically induced transparency and slow light with n-doped GaAs

The suitability of bound exciton system in semiconductors is studied for use in nonlinear optical schemes based on EIT, such as “slow” or “stored” photons. We match the desired properties of such a system exhibiting EIT with the known physical realities of a semiconductor system, and suggest, in particular, two suitable schemes using donor impurities in GaAs. In addition to generic properties, we also focus on the influence of many neighboring levels and continuum levels, and on the effect of strong hole-mixing.     

Microwave-induced magnetic field state with zero conductivity in GaAs/AlAs Corbino disks and hall bars

The microwave photoconductivity of the 2D electron gas in GaAs/AlAs heterostructures has been investigated at a temperature of 4.2 K in magnetic fields up to 1.5 T. It has been found that the magnetic field state with zero conductivity appears in GaAs/AlAs Corbino disks irradiated by 130.70-GHz microwave radiation. This state was previously observed only in GaAs/AlGaAs Corbino disks with much higher electron mobility and lower density. It has been shown that the microwave photoconductivity measured in high magnetic fields on Corbino disks can significantly differ from the value calculated from the results of the measurements on Hall bars. This difference is explained by the fact that the conditions of the appearing magnetoplasmons that affect the magnitude and character of the microwave photoconductivity (photoresistance) in the Corbino disks are nonequivalent to those in the Hall bars.     

Vertical hall effect in GaAs quantum wells

Applying orthogonal in-plane electric and magnetic fields in a 2D system leads to the development of a Hall voltage across the width of the quantum well when the cyclotron orbit is greater than the well width. Tang and Butcher [1] have calculated the developed Hall voltage for a parabolic quantum well where they find that the Hall voltage is dependent on the frequency associated with the harmonic potential in the well. The limitation of this model is that it does not enable one to determine the well width dependence of the Hall Voltage, nor is it a particularly good model for a quantum well. It is also difficult to compare their model with the bulk result which would apply at large well widths. In this work we present a model calculation which considers a square quantum well and hence is able to predict the well width dependence of the Hall Voltage and compare the large well width case to the bulk result. An electro-optic probing method previously used to measure bulk Hall voltages [2] is shown to be capable of measuring the Hall 'voltage across a quantum well, and therefore can be used to confirm the prediction of the model presented here.     

High frequency conductivity in the quantum hall regime

We have measured the complex conductivity sigma(xx) of a two-dimensional electron system in the quantum Hall regime up to frequencies of 6 GHz at electron temperatures below 100 mK. Using both its imaginary and real part we show that sigma(xx) can be scaled to a single function for different frequencies and several transitions between plateaus in the quantum Hall effect. Additionally, the conductivity in the variable-range hopping regime is used for a direct evaluation of the localization length xi. Even for large filling factor distances deltanu from the critical point we find xi approximately equals deltanu(-gamma) with a scaling exponent gamma = 2.3.     

Phonon conductivity of GaAs

A modified Holland's and three-phonon processes having different temperature dependences in the varius temperature ranges have been used to analyse the lattice thermal conductivity of GaAs. A very good agreement with the experimental results has been obtained     

On the theory of hall conductivity in disordered systems

A general formula for the Hall conductivity, which is exact in the one-particle approximation, has been obtained. Making use of this formula, the Hall coefficient for small polarons has been calculated.     

Spin hall effect in the presence of spin diffusion

Hirsch [Phys. Rev. Lett. 83, 1834 (1999)] recently proposed a spin Hall effect based on the anomalous scattering mechanism in the absence of spin-flip scattering. Since the anomalous scattering causes both anomalous currents and a finite spin-diffusion length, we derive the spin Hall effect in the presence of spin diffusion from a semiclassical Boltzmann equation. When the formulation is applied to certain metals and semiconductors, the magnitude of the spin Hall voltage due to the spin accumulation is found to be much larger than that of magnetic multilayers. An experiment is proposed to measure this spin Hall effect.     

Role of electron spin in integer quantum hall photoluminescence

We investigate numerically the photoluminescence (PL) spectrum in the integer quantum Hall regime and find that the electron spins play important roles. The spectra for the left circularly polarized light show peak splittings when the Fermi levels lies in the excited Landau level, which is caused by the inter Landau level scattering between electrons with anti-parallel spins. At around ν_e∼1 the PL energy is strongly affected by the interplay between the screening and multiple spin flipping (skyrmion) effects.     

The hall conductivity of a two-dimensional confined system

The Hall conductivity of a two-dimensional system of free, independent electrons is calculated using linear response theory. Emphasis is on the influence of the edge states introduced by a confining potential. It is shown that within the Kubo formalism the quantized values of the Hall conductivity may be described by bulk states as well as by edge states only. The influence of a random potential is discussed.     

Time-evolution of low-temperature photoconductivity and hall mobility in semi-insulating GaAs

The time-evolution of photo-Hall mobility, μ_H, during low-temperature illumination with 1.55 eV photons is presented as determined experimentally. At first μ_H decreases gradually during illumination, dropping almost to zero, and then increases back to high values, reaching an equilibrium value. The same behavior of μ_H is predicted from calculations of time-evolutions of free electrons and holes, derived from the time-evolution of independently measured thermoelectric effect signal and photoconductivity. Results are compatible with the model in which the dynamics of trap filling and changes of relative occupancies of traps with different sign are responsible for transient phenomena observed in photoconductivity, Hall mobility and thermoelectric signal during low-temperature, low-intensity illumination.     

Monte Carlo simulation of the hall effect in degenerate GaAs

This paper describes a simulation of the Hall effect in GaAs using a single particle Monte Carlo method. The Pauli exclusion principle and Fermi-Dirac statistics are included in the simulation so that the calculations are extended to the case of transport in degenerate materials with electron concentrations up to 10¹⁹ cm⁻³. Hall mobility and drift mobility for electrons in the low-field ohmic transport regime are calculated from the Monte Carlo results and compared with available typical experimental data. These results demonstrate the importance of including electron-electron interactions in Monte Carlo simulations for GaAs with carrier concentrations above 10¹⁷ cm⁻³. They also suggest the need for an evaluation of current models for ionized impurity scattering applied to degenerate materials and an examination of the role of energy band structure details in Monte Carlos simulations in degenerate materials.