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Two terminal devices have traditionally provided band-structure based high frequency operation. Third terminal control often involves hybrid design approaches. The presence of diluted magnetic semiconductor layers in device fabrication should permit the magnetic field to function as a pseudothird terminal. This is discussed for single barrier, double barrier and superlattice structures, where control is demonstrated. The limits of high frequency operation are discussed in general terms with application to barrier devices and superlattices containing DMS layers.

The Casimir force between inhomogeneous slabs that exhibit a band-like structure is calculated. The slabs are made of basic unit cells each made of two layers of different materials. As the number of unit cells increases the Casimir force between the slabs changes, since the reflectivity develops a band-like structure characterized by frequency regions of high reflectivity. This is also evident in the difference of the local density of states between free and boundary distorted vacuum, that becomes maximum at frequencies corresponding to the band gaps. The calculations are restricted to vacuum modes with wave vectors perpendicular to the slabs.

Superlattices consisting of ferromagnetic SrRuO₃ and superconducting YBa₂Cu₃O₇ [YBCO] layers of a thickness between 5 nm and 60 nm have been analyzed with respect to their magnetic [T_mag] as well superconducting [T_C] ordering temperature. The reduction of both is compared with previous results obtained in YBCO/La_2/3Ca_1/3MnO₃ superlattices. Whereas the reduction of T_C is comparable for both ferromagnetic materials the suppression of ferromagnetism is more pronounced in the case of the SrRuO₃-based superlattices. This result is discussed in terms of a competition of the order parameters giving rise to ferromagnetism and superconductivity and the role of interface roughness.

Metal-insulator transitions (MITs) in doped uncompensated systems are investigated in the Mott–Hubbard and Anderson impurity models by considering the intercarrier correlation and screening effect of carriers in the same hydrogenic impurity center, the formation of the superlattices with different coordination numbers (z=6, 8 and 12) and by studying the effect of randomness in impurity distribution. We have obtained simple and quite general criteria for the Mott and Anderson transitions and used these criteria to describe quantitatively the correlation and disorder-induced MITs in doped semiconductors and high-T_c cuprates. We examine the validity of the obtained criteria for the Mott and Anderson MITs in these doped systems. It is found that the newly derived criteria for the Mott MIT are well satisfied in doped semiconductors, but they cannot be used to describe the observed MITs in the hole-doped high-T_c cuprates, whereas the newly derived criteria for the Anderson MIT are applicable equally to describe the MITs observed both in doped semiconductors (at weak and intermediate disorders) and in doped cuprates (at intermediate and strong disorders). The new criteria for the Anderson MIT are extended to the polaronic systems in p-type cuprates. Our results are in quantitative agreement with the existing well-established experimental data and shed more light on the different types of MITs that occur in doped uncompensated semiconductors and cuprates.

The electronic properties of ZnS/CdS (001) superlattices (SLs) are investigated using the sp³s* tight-binding method versus substrate composition and valence-band offset (VBO). The results show that the electron is always confined within the CdS slabs; and also show a more striking feature due to the prominent localization of the top hole state near the interface region independently from the substrate composition (strain state). Theoretically, we have varied the VBO and inspected its effect on the interface state, and found that this latter persists to exist for VBO values within the interval [-1.4 eV, 0.6 eV]. This interface state is a manifestation of strong band mixing effects induced by the biaxial strain, and is expected to enhance the radiative efficiency of the SLs. The modelling of some available PL data, obtained for thin CdS quantum well embedded inside ZnS, has shown that the samples are of type-I with high energy bandgaps lying within the blue to ultraviolet spectral region.

The electronic band structures of (ZnSe)_m(CdSe)_n(001) superlattices (SLs) versus slab thicknesses (m, n) monolayers, bi-axial strain (substrate), and valence-band offsets (VBO) are investigated. The calculations are based on the sp³s* tight-binding models with inclusion of spin–orbit interactions. The results show that the electron is always localized within the CdSe slabs, whereas the behavior of the hole is dependent on the interface specific effects and specifically on the VBO, which is controlled mainly by the bi-axial strain (substrate composition). For instance: (i) for VBO < -0.1 eV, the hole is localized within ZnSe slabs and the SL is of type-II; (ii) for -0.1 eV < VBO < +0.1 eV, the hole is localized at the interface; and (iii) for VBO > 0.1 eV, the hole is confined within the CdSe layers and the SL is of type-I. The comparison of our theoretical results with the photoluminescence data of single and multiple quantum wells yields valuable information about the structural and optical qualities of the experimental samples.

The characteristics of the localized Wannier exciton in defect layer (GaAs) embedded between two semi-infinite superlattices (GaAs/Al_xGa_1-xAs) are investigated theoretically using a variational approach. It can be clearly seen the exciton changes in character between three- and quasi-two-dimensional states from the variation of exciton binding energy, in-plane radius, and probability in the superlattices (SLs) growth direction. We find that the extensions of exciton in directions both parallel and perpendicular to the interface of SLs almost approach their minimums as the exciton binding energy reaches peak value at a certain defect width. Our results show that the binding energy of the ground exciton state is sensitive to Al concentration x in Al_xGa_1-xAs and thicknesses of the constituent layers. The comparison between excitonic behavior in structural defect SLs and single quantum well is made.

We present the modification of optical transition probability in the superlattices (SLs) due to the presence of structural defect and the variation of layer thickness. The results show that optical transition probability between two bound states is mainly determined by the thicknesses of layer in defect region and its nearest constituent layers for a symmetric structure. When the well layer thickness varies, the variation of transition probabilities between two bound states depend on the parity of the index of minigap n, while those from the bound states to the delocalized scattering is irrelevant to n. It is believed that applying appropriate structure and layer thickness may enhance the optical transition probability in designing optical device.

We investigate on the infrared (IR) optoelectronic properties in short-period InAs/GaSb type-II superlattices (SLs) by a modified eight-band $K\cdot P$ model. The electronic mini-band structures for such SLs are evaluated by the modified eight-band $K\cdot P$ model, incorporating the microscopic interface effect. We find that with varying the values around 20/25 Å for the InAs/GaSb layer widths, the tunable mid-IR bandgaps can be achieved effectively. The SL bandgap from 275 to 346 meV can be achieved by decreasing the InAs layer thickness from 23 to 17 Å at a fixed GaSb layer thickness of 24 Å, or from 254 to 313 meV by increasing the GaSb layer thickness from 18 to 27 Å at a fixed InAs layer thickness of 21 Å. Correspondingly the optical absorptions in such systems can be tuned evidently. Our theoretical results are in good agreement with experimental data over a series of SL samples. This study confirms further that short-period InAs/GaSb type-II SLs are of great importance for IR applications.

Current perpendicular-to-plane (CPP) magnetoresistance (MR) of La_0.7Ca_0.3MnO₃/LaNiO₃ superlattices sandwiched between two YBa₂Cu₃O₇ thin film electrodes is reported. The CPP-MR in the temperature window of 20 K to 80 K is larger by a factor of 7~15 compared to current-in-plane (CIP) MR. Both CPP and CIP-magnetoresistance decrease with temperature below ~ 20 K. The MR at T<40 K is also strongly hysteretic and does not saturate even at 4 Tesla. Measurements of saturation moment and modeling of the perpendicular-to-plane resistance suggest disorder at the ferromagnetic non-magnetic layer interfaces which dominate the MR. The relative orientation of magnetization in the ferromagnetic layers seems to play only a subservient role in the creation of magnetoresistance.

We theoretically investigate the influence of external magnetic fields on current oscillations in weakly coupled GaAs/Al_xGa_1-xAs superlattices when the system is driven by a pure DC voltage. The formation of unstable electric-field domains under the action of a small magnetic field indicates the current oscillation occurs in superlattices. Increasing magnetic fields, the stationary electric-field domain is created and current oscillations disappear. Thus, the system state is transferred from dynamic to stationary by the control of magnetic fields. When the system is subject to an AC voltage in the presence of small magnetic fields, a variety of nonlinear behaviors are obtained due to the coupling between the internal oscillations and external signals.

The scattering properties of particles in a one-dimensional Fibonacci sequence based potential have been analyzed by means of the Transfer Matrix Method. The electronic band gaps are examined comparatively with those obtained using the corresponding periodic potentials. The reflection coefficient shows self-similar properties for the Fibonacci superlattices. Moreover, by using the generalized Bragg's condition, the band gaps positions are derived from the golden mean involved in the design of the superlattice structure.

Bi₂Te₃ has attracted attention due to its potential applications in the microfabrication of integrated thermoelectric devices. It is also interesting to study the metallization process of this compound. Metallic nanostructures were deposited by means of an electron gun evaporator in ultra high vacuum (UHV) conditions (10^-8 Pa) on the freshly cleaved 0001 surface of the crystal Bi₂Te₃. Measurements were conducted using the commercially available Omicron UHV scanning tunneling microscope (STM). Scanning tunneling spectroscopy (STS) measurements were performed using current imaging tunneling spectroscopy (CITS), and subsequent calculation of the dI/dV maps. Metallic characteristics were observed on nickel islands since early stages of the growth. CITS and dI/dV maps showed distinct contrast between the substrate and metallic islands. Similar contrast was not observed in the case of titanium, most probably due to an intercalation process. Occurring of such a process was confirmed by the appearance of the superlattice structure.

We report the study of the effect of miniband width on polarization properties of the hot electron photoluminescence (HPL) in SLs. It is demonstrated that the energy and magnetic field dependencies of the HPL polarization are very sensitive to the electron miniband widths. We show that high magnetic field applied perpendicular to the SL growth direction (Voigt geometry) induces transition (magnetic field breakdown) between electron minibands. The vertical transport of spin polarized and momentum aligned hot electrons is studied.

Recently, self-assembly of colloidal semiconductor nanocrystals (NCs) have attracted a great interest due to their flexible synthesis with tunable band gaps and shape-dependent optical and electronic properties. In particular, nanorods (NRs) superlattice is receiving considerable attention. Typically, the NRs superlattice is prepared by guiding the process of self-assembly through external forces. In this article, recent development of self-assembly approaches at work in fabricating NRs superlattices was reviewed. Despite those effective self-assembly techniques through external controls to obtain NCs assemblies during deposition were widespread used. But these techniques are time consuming, and cannot get rid of the organic capping insulated molecules surrounding the NCs. So there is still a challenge to guarantee the electron/hole dissociation as well as the charge transport of NCs. Here, thermal annealing method that applies selectivity even in the presence of organic molecules will be adopted to obtain colloidal NRs superlattices, and the self-assembly mechanism of NRs were briefly addressed.

The authors first establish a quantum microscopic scattering matrix model in multidimensional wave-vector space, which relates the phase space density of each superlattice cell with that of the neighbouring cells. Then, in the limit of a large number of cells, a SHE (Spherical Harmonics Expansion)-type model of diffusion equations for the particle number density in the position-energy space is obtained. The crucial features of diffusion constants on retaining the memory of the quantum scattering characteristics of the superlattice elementary cell (like e.g. transmission resonances) are shown in order. Two examples are treated with the analytically computation of the diffusion constants.

Two terminal devices have traditionally provided band-structure based high frequency operation. Third terminal control often involves hybrid design approaches. The presence of diluted magnetic semiconductor layers in device fabrication should permit the magnetic field to function as a pseudo-third terminal. This is discussed for single barrier, double barrier and superlattice structures, where control is demonstrated. The limits of high frequency operation are discussed in general terms with application to barrier devices and superlattices containing DMS layers.

We study multiple quasi-phase interactions in periodically poled grating configurations which are designed as one-dimensional multilayered media consisting of layers with different χ⁽²⁾ susceptibilities and refractive indexes. We analyze the spectral properties of photon pairs produced by means of pulsed parametric down-conversion in a modified periodically poled structure that additionally involve dispersive segments. The method of superlattices in multilayered media is used for designing of a phase-reversed configuration that simultaneously phase-match two interactions.