We propose qubits based on shallow donor electron spins in germanium. Spin-orbit interaction for donor spins in germanium is in many orders of magnitude stronger than in silicon. In a uniform bulk material it leads to very short spin lifetimes. Howev
er the lifetime increases dramatically when the donor is placed into a quasi-2D phononic crystal and the energy of the Zeeman splitting is tuned to lie within a phonon bandgap. In this situation single phonon processes are suppressed by energy conservation. The remaining two-phonon decay channel is very slow. The Zeeman splitting within the gap can be fine tuned to induce a strong, long-range coupling between the spins of remote donors via exchange by virtual phonons. This, in turn, opens a very efficient way to manipulate the quits. We explore various geometries of phononic crystals in order to maximize the coherent qubit-qubit coupling while keeping the decay rate minimal. We find that phononic crystals with unit cell sizes of 100-150 nm are viable candidates for quantum computing applications and suggest several spin-resonance experiments to verify our theoretical predictions.
Mechanism of transient population inversion in graphene with multi-splitted (interdigitated) top-gate and grounded back gate is suggested and examined for the mid-infrared (mid-IR) spectral region. Efficient stimulated emission after fast lateral spr
eading of carriers due to drift-diffusion processes is found for the case of a slow electron-hole recombination in the passive region. We show that with the large gate-to-graphene distance the drift process always precedes the diffusion process, due to the ineffective screening of the inplane electric field by the gates. Conditions for lasing with a gain above 100 cm$^{-1}$ are found for cases of single- and multi-layer graphene placed in the waveguide formed by the top and back gates. Both the waveguide losses and temperature effects are analyzed.
The electron-hole symmetry in the structure graphene - insulating substrate -semiconductor gate is violated due to an asymmetrical drop of potential in the semiconductor gate under positive or negative biases. The gate voltage dependencies of concent
ration and conductivity are calculated for the case of SiO_2 substrate placed over low- (moderate-) doped p-Si. Similar dependencies of the optical conductivity are analyzed for the case of high-kappa substrates (AlN, Al_2O_3, HfO_2, and ZrO_2). The comparison of our results with experimental data shows a good agreement for both cases.
The tunneling current between independently contacted graphene sheets separated by boron nitride insulator is calculated. Both dissipative tunneling transitions, with momentum transfer due to disorder scattering, and non-dissipative regime of tunneli
ng, which appears due to intersection of electron and hole branches of energy spectrum, are described. Dependencies of tunneling current on concentrations in top and bottom graphene layers, which are governed by the voltages applied through independent contacts and gates, are considered for the back- and double-gated structures. The current-voltage characteristics of the back-gated structure are in agreement with the recent experiment [Science 335, 947 (2012)]. For the double-gated structures, the resonant dissipative tunneling causes a ten times enhancement of response which is important for transistor applications.
The absorption of heavily doped graphene in the terahertz (THz) and mid-infrared (MIR) spectral regions is considered taking into account both the elastic scattering due to finite-range disorder and the variations of concentration due to long-range d
isorder. Interplay between intra- and interband transitions is analyzed for the high-frequency regime of response, near the Pauli blocking threshold. The gate voltage and temperature dependencies of the absorption efficiency are calculated. It is demonstrated that for typical parameters, the smearing of the interband absorption edge is determined by a unscreened part of long-range disorder while the intraband absorption is determined by finite-range scattering. The latter yields the spectral dependencies which deviate from those following from the Drude formula. The obtained dependencies are in good agreement with recent experimental results. The comparison of the results of our calculations with the experimental data provides a possibility to extract the disorder characteristics.
The diffusion of electron-hole pairs, which are excited in an intrinsic graphene by the ultrashort focused laser pulse in mid-IR or visible spectral region, is described for the cases of peak-like or spread over the passive region distributions of ca
rriers. The spatio-temporal transient optical response on a high-frequency probe beam appears to be strongly dependent on the regime of diffusion and can be used for verification of the elasic relaxation mechanism. Sign flip of the differential transmission coefficient takes place due to interplay of the carrier-induced contribution and weak dynamic conductivity of undoped graphene.
Low-energy continuous states of electron in heterosrtucture with periodically placed quantum-dot sheets are studied theoretically. The Greens function of electron is governed by the Dyson equation with the self-energy function which is determined the
boundary conditions at quantum-dot sheets with weak damping in low-energy region. The parameters of superlattice formed by quantum-dot sheets are determined using of the short-range model of quantum dot. The density of states and spectral dependencies of the anisotropic absorption coefficient under mid-IR transitions from doped quantum dots into miniband states of superlattice strongly depend on dot concentration and on period of sheets. These dependencies can be used for characterization of the multi-layer structure and they determine parameters of different optoelectronic devices exploiting vertical transport of carriers through quantum-dot sheets.
Low-energy electronic states in heterosrtuctures formed by ultranarrow layer (single or several monolayers thickness) are studied theoretically. The host material is described within the effective mass approximation and effect of ultranarrow layers i
s taken into account within the framework of the transfer matrix approach. Using the current conservation requirement and the inversion symmetry of ultranarrow layer, the transfer matrix is written through two phenomenological parameters. The binding energy of localized state, the reflection (transmission) coefficient for the single ultranarrow layer case, and the energy spectrum of superlattice are determined by these parameters. Spectral dependency of absorption in superlattice due to photoexcitation of electrons from localized states into minibands is strongly dependent on the ultranarrow layers characteristics. Such a dependency can be used for verification of the transfer matrix parameters.
