Realistic Superlattice Research Articles

Three-dimensional electron-hole superfluidity in a superlattice close to room temperature

Although there is strong theoretical and experimental evidence for electron-hole superfluidity in separated sheets of electrons and holes at low $T$, extending superfluidity to high $T$ is limited by strong 2D fluctuations and Kosterlitz-Thouless effects. We show this limitation can be overcome using a superlattice of alternating electron- and hole-doped semiconductor monolayers. The superfluid transition in a 3D superlattice is not topological, and for strong electron-hole pair coupling, the transition temperature $T_c$ can be at room temperature. As a quantitative illustration, we show $T_c$ can reach $270$ K for a superfluid in a realistic superlattice of transition metal dichalcogenide monolayers.

Towards Mott design by δ-doping of strongly correlated titanates

Doping the distorted-perovskite Mott insulators LaTiO3 and GdTiO3 with a single SrO layer along the [001] direction gives rise to a rich correlated electronic structure. A realistic superlattice study by means of the charge self-consistent combination of density functional theory with dynamical mean-field theory reveals layer- and temperature-dependent multi-orbital metal-insulator transitions. An orbital-selective metallic layer at the interface dissolves via an orbital-polarized doped-Mott state into an orbital-ordered insulating regime beyond the two conducting TiO2 layers. We find large differences in the scattering behavior within the latter. Breaking the spin symmetry in δ-doped GdTiO3 results in blocks of ferromagnetic itinerant and ferromagnetic Mott-insulating layers that are coupled antiferromagnetically.

Quantitative analysis of photon density of states for a realistic superlattice with omnidirectional light propagation

Omnidirectional light propagation in a realistic superlattice is investigated. This work complements two previous articles [Phys. Rev. E 59, 3624 (1999); 61, 5802 (2000)] that analyzed the cases of transverse electric (TE) and transverse magnetic (TM) polarization modes, respectively, of the dielectric superlattice modeled by means of Dirac delta functions. We present a quantitative analysis of the transmission functions, the band structures, the equifrequency surfaces, and the photon density of states (PDOS) for both TE and TM modes of the real superlattice without any approximations on the given dielectric function profiles. One of the advantages is that the Brewster effect can be manifested via our approach. In addition, the modes corresponding to TM evanescent waves that are absent from the Dirac comb model can be predicted. Finally, the exact PDOS of the realistic superlattice for the TE and TM modes can be obtained, respectively. These results are relevant to the spontaneous emission by an atom or to dipole radiation in one-dimensional periodic structures.

Spontaneous emission of Bloch oscillation radiation from a single energy band

A theory for the spontaneous emission of radiation for a Bloch electron traversing a single energy band under the influence of a constant external electric field is presented. The constant external electric field is described in the vector potential gauge. The quantum radiation field is described by the free space quantized electromagnetic field in the Coulomb gauge. The instantaneous eigenstates of the Bloch Hamiltonian are introduced as basis states to analyze the Bloch dynamics to all orders in the constant external electric field. The radiation field described by the quantized electromagnetic field provides the vacuum fluctuations that are responsible for the spontaneous emission of radiation of the single electron from the upper regions of the energy band. It is shown that the spontaneous emission occurs with frequencies equal to integral multiples of the Bloch frequency without any ad hoc assumptions made concerning the existence of Wannier-Stark ladder levels. An explicit expression for the spontaneous emission transition probability is derived to first order in the quantized radiation field; results show the explicit dependence upon the electron energy band structure, photon polarization, and the directionality of the radiation output. As an illustration, spontaneous emission probabilities are developed and illustrated for nearest-neighbor tight-binding and more realistic superlattice band structure models. For the GaAs-based superlattices, the power radiated into free space from spontaneous emission due to Bloch oscillations in the terahertz frequency range is estimated to be about one-tenth of a microwatt.

Nonlinear dynamics of terahertz‐driven quantum‐dot miniband superlattices

We have theoretically studied terahertz-driven nonlinear dynamics of a quantum-dot miniband superlattice with a realistic treatment of the scatterings by impurity, acoustic phonon, and polar optic phonons. The electron motion that incorporates the influence of the self-consistent field within the miniband superlattice produces a cooperative nonlinear oscillatory mode, which can lead to complicated chaotic dynamics with the driving amplitude, driving frequency, and the relaxation frequency of the external circuit as the controlling parameters. The two-dimensional driving amplitude–frequency phase diagrams are calculated, which distinguishes the chaotic region from the periodic region. The present result provides a useful guidance of controlling chaos in the realistic superlattice device.

Rabi Oscillations in Realistic Superlattice with Finite Bloch Bands

We investigate the dynamical processestaking place in nanodevices by high-frequency dc-ac fields. We found thatRabi oscillations between minibands are clearly identified undertheoretical resonant conditions derived by an ideal two-band superlatticemodel, the resonant conditions have broadened, and the amount ofbroadening is about four times of the Rabi oscillation frequency. We alsowant to elucidate the role of different mechanisms that could lead to lossof quantum coherence. Our results show how the dephasing effects ofdisorder of interface roughness and doping fluctuation that after someperiods destroy coherent oscillations, such as Rabi oscillations, can bereduced dramatically if we apply a bias static electric field to thesuperlattice system. The doping fluctuation dephasing effect is muchstronger than that of interface roughness in the coherent process ofrealistic superlattices.

Effect of superlattice structure on the thermoelectric figure of merit.

The electrical conductivity, the thermoelectric power, and the electrical contribution to the thermal conductivity of superlattices have been studied within the envelope-function approach for the electrons. The effects of tunneling through the barriers due to finite potential offsets and of the thermal currents through the barrier layers are shown to be essential to describe properly the thermoelectric figure of merit of realistic superlattices. The figure of merit is calculated as a function of superlattice period, potential barrier offset, and barrier width. It is found that the figure of merit has a maximum as a function of superlattice period and that its value there can be somewhat larger than that of the corresponding bulk. For larger periods the figure of merit is generally less than that of the correponding bulk.

Simulation of electron transport in a (GaAs)12/(AlAs)12 superlattice

A Monte Carlo simulation of electron transport in a GaAs/AlAs superlattice is presented. The simulation is based on realistic superlattice band structure which is calculated by a method which matches complex empirical pseudopotential bulk band structure at the heterointerfaces. The structure considered here is a (GaAs)12/(AlAs)12 superlattice in which significant Gamma -X mixing is observed. The pseudopotential band structure method has been used to obtain the wavevector locations and effective masses of the six lowest sets of conduction band valleys. These valleys were then used as the basis of ensemble Monte Carlo simulations of electron transport and relaxation in the superlattice under the action of phonon scattering. The drift velocity of electrons in the superlattice as a function of electric field applied parallel to the layers is predicted using the simulation and compared with bulk GaAs. The drift velocity exhibits a much broader and lower maximum than bulk GaAs and the negative differential mobility at higher fields is also much weaker. Simulations have also been carried out to study the thermalization of electrons which are initially in excited states of the system. The results show that the rates at which the initial valley empties and the ground valley becomes occupied are strongly dependent on the initial valley.

Superlattice k

A quantitative comparison is presented of two realistic superlattice k\ensuremath{\cdot}p electronic structure calculations. The first is an analytic approach based on an extended bulk Kane model; the second is an extended-basis treatment, developed by McGill and collaborators, based on bulk pseudopotential calculations. Both approaches are applied to HgTe/CdTe superlattices. Energies, wave functions, effective masses, and oscillator strengths are found to agree within 10%. The limited-basis approach based on the Kane model is seen to be adequate for superlattices whose bulk constituents have direct gaps in the conduction- and valence-band regions near the superlattice band gap.