Array Of Quantum Wells Research Articles

Топологические моды в неэрмитовых фононных кристаллах, индуцированных оптически в массиве квантовых ям

We study theoretically propagation of acoustic waves in an array of quantum wells, where a spatially modulated amplification and attenuation of sound is realized by optical excitation with a frequency close to the exciton resonance. The dispersion of sound waves near Bragg resonances corresponding to the modulation wave vector is calculated. The appearance of "imaginary" stop bands, where the imaginary parts of the Bloch mode frequencies are split, is described. The emergence of acoustic topological modes at the edge of the structure is demonstrated.

Collective Excitation and Quantum Depletion of a Bose—Einstein Condensate in a Periodic Array of Quantum Wells

With the help of a set of exact closed-form solutions to the stationary Gross—Pitaevskii (GP) equation, we calculate the collective excitation and quantum depletion of a weakly interacting Bose gas in the presence of a periodic array of quantum wells. The excitation spectrum (Bogoliubov spectrum) is obtained from the solution of the linearized time-dependent GP equation, which develops energy bands ħωj(p) periodic in quasi-momentum space. Moreover, we calculate the excitation strengths Zj (p) relative to the density operator and then the dynamic structure factor S(p, ω). Accordingly, the analytical expressions of quantum depletion of the system are obtained. We find that the quantum depletion is enhanced when the interatomic interactions become larger and the potential is sufficiently deep. The conditions for the possible experimental realization of our scenario are also proposed.

The thermoelectric power factor of a semiconductor superlattice with nanoparticle inclusions

We develop a phenomenological theory for cross-plane transport in a semiconductor superlattice (SL) doped with nanostructures to improve the thermoelectric properties. The SL consists of an array of quantum wells equally spaced along a spatial direction separated by narrow barriers, such that, in the presence of inter-well tunneling, a miniband energy structure is established. The semi-metallic ErAs nanoparticles are embedded inside the quantum wells in the process of growth of the SL, as reported in several recent experiments. Their effect on thermoelectric transport is considered through an additional contribution to the electron scattering rate, that is correlated with a process of resonant tunneling through the nanoparticle–semiconductor interface modeled as a Schottky barrier. In a semi-classical approximation of the miniband conduction regime, we calculate the electric conductivity, the thermopower and the power factor as a function of the barrier height and demonstrate the presence of a filtering effect, whereby the Seebeck coefficient is enhanced by the additional scattering.

Excitonic lattice control

Customizing the refractive index of wells and barriers in a periodic array of quantum wells yields a way to control the reflectivity and dispersion of an excitonic lattice. The result is a new method for slowing or modulating light.

Stability Diagrams of a Bose–Einstein Condensate in a Periodic Array of Quantum Wells

With the help of a set of exact closed-form solutions to the stationary Gross–Pitaevskii equation, we comprehensively investigate Landau and dynamical instabilities of a Bose–Einstein condensate in a periodic array of quantum wells. In the tight-binding limit, the analytical expressions for both Landau and dynamical instabilities are obtained in terms of the compressibility and effective mass of the BEC system. Then the stability phase diagrams are shown to be similar to the one in the case of the sinusoidal optical lattice.

Exact nonlinear Bloch-state solutions for Bose–Einstein condensates in a periodic array of quantum wells

A set of exact closed-form Bloch-state solutions to the stationary Gross–Pitaevskii equation are obtained for a Bose–Einstein condensate in a one-dimensional periodic array of quantum wells, i.e. a square-well periodic potential. We use these exact solutions to comprehensively study the Bloch band, the compressibility, effective mass and the speed of sound as functions of both the potential depth and interatomic interaction. According to our study, a periodic array of quantum wells is more analytically tractable than the sinusoidal potential and allows an easier experimental realization than the Krönig–Penney potential, therefore providing a useful theoretical model for understanding Bose–Einstein condensates in a periodic potential.

Switched wave packets: a route to nonperturbative quantum control.

The dynamic Stark effect due to a strong nonresonant but nonionizing laser field provides a route to quantum control via the creation of novel superposition states. We consider the creation of a field-free "switched" wave packet through adiabatic turn-on and sudden turn-off of a strong dynamic Stark interaction. There are two limiting cases for such wave packets. The first is a Raman-type coupling, illustrated by the creation of field-free molecular axis alignment. An experimental demonstration is given. The second case is that of dipole-type coupling, illustrated by the creation of charge localization in an array of quantum wells.

Approaches Toward Chemically Prepared Multiple Quantum Well Structures

AbstractAn enhanced thermoelectric figure of merit, ZT, has been predicted for Bi2Te3 in the form of 2-dimensional quantum wells. A new approach to making multiple quantum well (MQW) structures for thermoelectric applications utilizing a chemical intercalation technique is proposed and investigated. It is proposed that by starting from Li intercalated Bi2Te3 and Bi2Se3, the layers of these materials can be separated by chemical means. The layers of Bi2Te3 or Bi2 Se3 can then be restacked, by self-assembly, forming a non-periodic array of quantum wells. These chemically prepared MQWs are characterized by X-ray diffraction, SEM (scanning electron microscopy) and TEM (transmission electron microscopy) at various stages in the sample preparation to assess the degree to which the actual samples match the proposal. Experimental measurements of the Seebeck coefficient (S) and the electrical conductivity (σ) were performed over a range of temperatures for the initial bulk materials. It is found that some of the steps in the proposed fabrication have been achieved but still much improvement is needed before any practical thermoelectric 2D-system can be provided.

Collective excitations in coupled quantum wells

The dielectric response of a modulated three-dimensional electron system composed of a periodic array of quantum wells with weak coupling and strong coupling are studied, and the dispersions of the collective excitations and the single particle excitations as functions of wave vectors are given. It is found that for the nearly isolated multiple-quantum-well case with several subbands occupation, there is a three-dimensional-like plasmon when q z =0 ( q z is the wave-vector component in the superlattice axis). There also exist inter-subband collective excitations in addition to one intra-subband mode when q z ≠0. The intra-subband mode has a linear dispersion relation with q | (the wave-vector component perpendicular to the superlattice axis) when q | is small. The inter-subband modes cover wider ranges in q | with increasing values of q z . The energies of inter-subband collective excitations are close by the corresponding inter-subband single-particle excitation spectra. The collective excitation dispersions show obvious anisotropy in the 2D quantum limit. The calculated results agree with the experiment. The coupling between quantum wells affects markedly both the collective excitations and the single particle excitations spectra. The system shows gradually a near-three-dimensional electron gas character with increasing coupling.

Acoustic wave amplification in quantum semiconductor structures

AbstractA microscopic theory of acoustic wave amplification is presented for an ideal, periodic array of quantum wells (superlattice) in the presence of an applied electric field. The effect of the external field is to produce a drift velocity of the charge carriers in the superlattice heterostructure such that when this drift velocity exceeds the acoustic wave velocity, the confined electrons can lose energy to the acoustic wave and an acoustic amplification rather than attenuation occurs. It is shown that for these quantum semiconductor structures provided they can be tailored, acoustic wave amplification can occur at much larger rate and at much lower electric fields as compared with the bulk structure.

Minibands in the continuum of multi-quantum-well superlattices.

We report the first observation of well defined minibands and minigaps between them, at energies above the barriers in multi-quantum-well structures. Using intersubband absorption spectroscopy, we have observed the optical transitions between confined electronic states of isolated quantum wells to minibands in the continuum of the periodic structure. By comparison with a theoretical model we spectroscopically identify all the observed transitions and explain their strength and spectral shape. In addition, we estimate the coherence length of the electronic wave function in these minibands comparing the absorption spectrum with that of a finite array of quantum wells.

Ultrasound attenuation in superlattices

The ultrasound attenuation coefficient is evaluated for an ideal, periodic array of quantum wells (superlattice) by taking matrix elements of the electron-phonon interaction between basis states that are plane-wave combinations of finite quantum-well wave functions. Contributions due to intrawell electron-phonon scattering as well as due to interwell transfer were found. In addition, a contribution due to the interference of intrawell and interwell electron-phonon scattering process is obtained. The total attenuation coefficient along the superlattice axis is found to be dependent upon the electron wave number (or the density of states) at the Fermi level.

Local plasmon modes of a semiconductor superlattice

The energy dispersion relation of the plasmon modes of an infinite periodic array of quantum wells where all the wells are doped with the same electron density except for one is calculated. In analogy to the local phonon modes of a vibrational lattice, we find that local plasmon modes exist. An explicit relation for the plasmon energy of the local mode is derived. Contrary to the surface plasmon mode of a semi-infinite superlattice predicted by Giuliani and Quinn [Phys. Rev. Lett. 51, 919 (1983)], these local modes exist at all values of the wavevector. A low-lying acoustic mode (ω≊qv) is found whose velocity is determined by the local density of the symmetry breaking well. Implications for obtaining amplification by drift instabilities in these structures are discussed.

Coupled interface-plasmon modes of a superlattice and doped overlayer.

We calculate the energy dispersion relation of the local interface-plasmon modes of a semi-infinite periodic array of quantum wells coupled to the surface-plasmon mode of a doped overlayer. For equal dielectric constants, an exact solution for the interface-plasmon modes is obtained. In the limit that the distance d separating the superlattice and overlayer is zero, one interface mode is found. This mode exists only for wave vectors larger than a critical value dependent upon the ratio of the density of the quantum well to the density of the overlayer. For arbitrary separation d, two local interface-plasmon eigenstates are found that correspond to the symmetric and antisymmetric combinations of the semi-infinite superlattice plasmon and the surface plasmon associated with the doped layer. These modes are shown to have a very intricate energy dispersion. They are distinctly different from the surface modes of Giuliani and Quinn [Phys. Rev. Lett. 51, 919 (1983)], which do not exist in this case of equal dielectric constants.

Effects of finite layer thickness on superlattice plasmons

In recent publications, the dispersion relation for the plasmon oscillations of a semiconductor superlattice, consisting of a periodic array of quantum wells, doped with electrons, was calculated in the limit that the well size approached zero. Here, we extend this theory to include wells of finite thickness. In most situations of physical interest, it is shown that the superlattice plasmon dispersion is modified by only a few percent because of the finite well size.