External gates and transport in biased bilayer graphene

We formulate a theory of transport in graphene bilayers in the weak momentum scattering regime in such a way as to take into account contributions to the electrical conductivity to leading and next-to-leading order in the scattering potential. The response of bilayers to an electric field cannot be regarded as a sum of terms due to individual layers. Rather, interlayer tunneling and coherence between positive- and negative-energy states give the main contributions to the conductivity. At low energies, the dominant effect of scattering on transport comes from scattering within each energy band, yet a simple picture encapsulating the role of collisions in a set of scattering times is not applicable. Coherence between positive- and negative-energy states gives, as in monolayers, a term in the conductivity which depends on the order of limits. The application of an external gate, which introduces a gap between positive- and negative-energy states, does not affect transport. Nevertheless the solution to the kinetic equation in the presence of such a gate is very revealing for transport in both bilayers and monolayers.Comment: 6 pages, accepted for publication in Physical Review

Comment on "Froehlich Mass in GaAs-Based Structures"

The results of recent measurements of the cyclotron resonance (CR) spectra for a GaAs quantum well are interpreted in terms of the resonant magnetopolaron effect. Owing to this effect, the CR peaks split near the TO-phonon frequency and also change their positions with respect to those obtained without electron-phonon interaction. The theoretical peak positions of the CR spectra calculated within the many-polaron approach compare well with experimental data, as distinct from the CR energies calculated without electron-phonon interaction, which show no particular features in the region of the optical-phonon frequencies. We conclude that the Froehlich polaron concept is valid and even necessary to interpret the CR spectra of quantum wells.Comment: 1 page, 1 figure, E-mail addresses: [email protected], [email protected]

Relaxation of hole spins in quantum dots via two-phonon processes

We investigate theoretically spin relaxation in heavy hole quantum dots in low external magnetic fields. We demonstrate that two-phonon processes and spin-orbit interaction are experimentally relevant and provide an explanation for the recently observed saturation of the spin relaxation rate in heavy hole quantum dots with vanishing magnetic fields. We propose further experiments to identify the relevant spin relaxation mechanisms in low magnetic fields.Comment: 5 pages, 2 figure

Hole spin relaxation in semiconductor quantum dots

Hole spin relaxation time due to the hole-acoustic phonon scattering in GaAs quantum dots confined in quantum wells along (001) and (111) directions is studied after the exact diagonalization of Luttinger Hamiltonian. Different effects such as strain, magnetic field, quantum dot diameter, quantum well width and the temperature on the spin relaxation time are investigated thoroughly. Many features which are quite different from the electron spin relaxation in quantum dots and quantum wells are presented with the underlying physics elaborated.Comment: 10 pages, 10 figure

Mixing of two-electron spin states in a semiconductor quantum dot

We show that the low lying spin states of two electrons in a semiconductor quantum dot can be strongly mixed by electron-electron asymmetric exchange. This mixing is generated by the coupling of electron spin to its orbital motion and to the relative orbital motion of the two electrons. The asymmetric exchange can be as large as 50% of the isotropic exchange, even for cylindrical quantum dots. The resulting spin mixing contributes to understanding spin dynamics in quantum dots, including light polarization reversal

Atomistic theory of electronic and optical properties of InAs/InP self-assembled quantum dots on patterned substrates

We report on a atomistic theory of electronic structure and optical properties of a single InAs quantum dot grown on InP patterned substrate. The spatial positioning of individual dots using InP nano-templates results in a quantum dot embedded in InP pyramid. The strain distribution of a quantum dot in InP pyramid is calculated using the continuum elasticity theory. The electron and valence hole single-particle states are calculated using atomistic effective-bond-orbital model with second nearest-neighbor interactions, coupled to strain via Bir-Pikus Hamiltonian. The optical properties are determined by solving many-exciton Hamiltonian for interacting electron and hole complexes using the configuration-interaction method. The effect of positioning of quantum dots using nanotemplate on their optical spectra is determined by a comparison with dots on unpatterned substrates, and with experimental results. The possibility of tuning the quantum dot properties with varying the nano-template is explored.Comment: 9 pages, 12 figure

Strain distribution in quantum dot of arbitrary polyhedral shape: Analytical solution in closed form

An analytical expression of the strain distribution due to lattice mismatch is obtained in an infinite isotropic elastic medium (a matrix) with a three-dimensional polyhedron-shaped inclusion (a quantum dot). The expression was obtained utilizing the analogy between electrostatic and elastic theory problems. The main idea lies in similarity of behavior of point charge electric field and the strain field induced by point inclusion in the matrix. This opens a way to simplify the structure of the expression for the strain tensor. In the solution, the strain distribution consists of contributions related to faces and edges of the inclusion. A contribution of each face is proportional to the solid angle at which the face is seen from the point where the strain is calculated. A contribution of an edge is proportional to the electrostatic potential which would be induced by this edge if it is charged with a constant linear charge density. The solution is valid for the case of inclusion having the same elastic constants as the matrix. Our method can be applied also to the case of semi-infinite matrix with a free surface. Three particular cases of the general solution are considered--for inclusions of pyramidal, truncated pyramidal, and "hut-cluster" shape. In these cases considerable simplification was achieved in comparison with previously published solutions. A generalization of the obtained solution to the case of anisotropic media is discussed.Comment: revtex4, 12 pages, 6 figures; Ch. II rewritten, new Ch. V added, errors in Eq.(13) and Eq.(22) fixe

Effect of initial spin polarization on spin dephasing and electron g factor in a high-mobility two-dimensional electron system

We have investigated the spin dynamics of a high-mobility two-dimensional electron system (2DES) in a GaAs--Al$_{0.3}$Ga$_{0.7}$As single quantum well by time-resolved Faraday rotation (TRFR) in dependence on the initial degree of spin polarization, $P$, of the 2DES. From $P\sim 0$ to $P\sim 30$ %, we observe an increase of the spin dephasing time, $T_2^\ast$, by an order of magnitude, from about 20 ps to 200 ps, in good agreement with theoretical predictions by Weng and Wu [Phys. Rev. B {\bf 68}, 075312 (2003)]. Furthermore, by applying an external magnetic field in the Voigt configuration, also the electron $g$ factor is found to decrease for increasing $P$. Fully microscopic calculations, by numerically solving the kinetic spin Bloch equations considering the D'yakonov-Perel' and the Bir-Aronov-Pikus mechanisms, reproduce the most salient features of the experiments, {\em i.e}., a dramatic decrease of spin dephasing and a moderate decrease of the electron $g$ factor with increasing $P$. We show that both results are determined dominantly by the Hartree-Fock contribution of the Coulomb interaction.Comment: 4 pages, 4 figures, to be published in PR