Interlayer Transfer Research Articles

Green's-function study of the electron tunneling in a double-barrier heterostructure.

The quasi-one-dimensional aspects of the electron tunneling through AlAs/GaAs/AlAs (001) double-barrier structures have been investigated using the tunneling formalism based on the nonequilibrium Green's function developed by Keldysh. In describing the electronic structure of a double-barrier heterostructure, Green's-function formulation of scattering theory incorporated with the multiorbital tight-binding model is used, so as to take into account the effects of indirect band gap, band nonparabolicity, and multiple orbitals in the aperiodic layered structure. The effect of high external bias on the junction is solved exactly within this model. The electron-tunneling situation and the effect of the X-valley originated barrier confined states on the electron tunneling are analyzed with the spectral local density of states of the double-barrier structure in the [001] direction under the external bias. The calculated current density J(${\mathbf{k}}_{\mathrm{\ensuremath{\parallel}}}$=0) versus voltage curve exhibits the structures for the interlayer intervalley electron transfer, which are manifested as the electron propagation through \ensuremath{\Gamma}-valley originated states and through X-valley barrier confined states. The smaller current peak-to-valley ratios are found in the double-barrier structures with thicker barriers, which is consistent with the experimental results. The enhancement of \ensuremath{\Gamma}-X interlayer intervalley electron transfer and reduction of the \ensuremath{\Gamma}-\ensuremath{\Gamma} resonant electron interlayer transfer is found with the increase of the phenomenological imaginary potential term.

Bound states of two holes in the double-layer two-dimensional two-band model

The bound states of p holes are studied within the double-layer two-dimensional two-band model with the antiferroomagnetic exchange interaction between d and p spins, J. The binding energies of p holes are computed as a function of the interlayer transfer, t 1, and one of J within the mean-field theory with periodic boundary conditions. It is negative for 0 ≤ t 1 ≤1 eV and has a minimum at t 1=0.1 eV. The amplitude of spin density at each Cu site is small, as a result, p holes pair in a singlet state.

Determination of the parallel and perpendicular intermolecular tunnelling rates in two Langmuir-Blodgett quantum well systems

The first experimental determination of the anisotropy of the electron transfer rate in two Langmuir-Blodgett insulator multilayer systems is presented. A key feature of these two multilayer systems is the presence of the planar conjugated phthalocyanine ring. The first system is composed of tetra-tertiary-butyl phthalocyanine monosulphonic acid. In it the ring planes are approximately normal to the layer plane. The second system is composed of the two-ring phthalocyanine HOSiPcOSiPcOSI( nC 6H 13) 3. In it the rings lie in the layer plane. The anisotropy of the systems is determined from a model of the recombination rate, at high density, of photoelectrons and photoholes on the same layer. The ratio of the intralayer to interlayer transfer rates between adjacent rings is found to be 1500 and 570 respectively. Coupled with a previous determination of the interlayer transfer time of 1.6 and 0.30 ns, the intralayer transfer times are then determined as 1.07 and 0.56 ps respectively. The in-plane mobilities can then be determined as 0.34 and 1.2 cm 2 s −1 V −1 respectively. The results justify regarding these systems as organic multiple-quantum wells.

Plasmons in an Ultrathin Film of Weakly-Coupled Layered Materials

Electronic collective modes of an ultrathin film of transition metal dicalcogenides are investigated based on models in which the interlayer transfer integrals are neglected. First we study the two-dimensional plasmons in the film consisting of the different materials from layer to layer. Then, taking into account the intralayer polarization effect in the c direction, we show that the bulk-plasmon-like modes can appear in the quasi-two-dimensional systems. Analytical expression of the dispersion relation is given in the long wave limit while numerical calculations are performed for the whole range of wavelength in the random phase approximation.

Determination of anisotropic electron transport properties of two Langmuir-Blodgett organic multiple quantum wells

The first experimental determination of the anisotropy of the electron transfer rate in two- Langmuir-Blodgett insulator multilayer systems is presented. A key feature of these two multilayer systems is the presence of the planar-conjugated phthalocyanine ring. The first system is composed of tetra-tertiary-butyl phthalocyanine monosulphonic acid. In it the ring planes are approximately normal to the layer plane. The second system is composed of the two-ring phthalocyanine, HOSiPcOSiPcOSi(n-C 6H 13) 3. In it the rings lie in the layer plane. The anisotropy of the systems is determined from a model of the recombination rate, at high density, of photoelectrons and photoholes on the same layer. The ratio of the intralayer to interlayer transfer rate between adjacent rings is found to be 1500 and 570 respectively. Coupled with a previous determination of the interlayer transfer time of 1.6 and 0.15 ns, the intralayer transfer times are then determined as 1.07 and 0.28 ps respectively. The in-plane mobilities can then be determined as 0.34 and 2.4 cm 2 s -1 V -1 respectively. The results justify regarding these systems as organic multiple quantum wells.

Core-electron spectra and electronic structure of CeNbS3.

Core-electron-spectroscopy studies have been made for CeNbS 3 , which is one of the misfit-layer compounds. The Ce 3d and 4d x-ray photoemission (XPS) spectra are quite similar to those in CeSe. Ce exists mainly as a trivalent ion and the 4f electrons hybridize with other conduction and valence electrons. The S 2p XPS spectrum exhibits only a broad peak which consists of plural spin-orbit-split structures at different chemical environments. We find some evidence which indicates interlayer interaction and electron transfer from the Ce 5d band to the Nb d z 2 band. The Nb n 2,3 inner-shell-electron energy-loss spectrum exhibits a small peak just above the threshold, indicating that the d z 2 band is still incompletely filled after charge transfer

Optical conductivity in the superconducting state of layered cuprate oxides

The optical conductivity for very low frequencies is investigated in the superconducting state of YBa 2Cu 3O 7 using a layered structure model that some pairing interaction generates superconductivity in the CuO 2-plane layers and the interlayer transfer interaction induces weak superconductivity in the CuO-chain layers. The optical conductivity has contributions from the plane and chain layers which show a striking difference in the temperature dependence below T c . The optical conductivity of the plane layers has a peak just below T c , which is similar to the Hebel-Slichter peak in the nuclear relaxarion rate, and decrease rapidly below the temperature of the peak, while the optical conductivity of the chain layers monotonically increases with desreasing temperature. The temperature dependence of the optical conductivity is compared with that of the nuclear relaxation rate.

Knight shift and nuclear relaxation rate in layered cuprate oxides

We calculate the spin-Knight shifts and the nuclear relaxation rates of the Cu and O ions in the superconducting state of YBa 2Cu 3O 7, using a layer model in which some pairing interaction generates superconductivity in the CuO 2-plane layers and the interlayer transfer interaction induces weak superconductivity in other layers. If the experimental values of the spin-Knight shift and the nuclear relaxation rate are normalized at T c , these values for the Cu and O ions in each layer are on universal curves in all temperature range of the superconducting state. The normalized spin-Knight shifts and relaxation rates of the Cu and O ions in the plane layer decrease much faster than those in the chain layer in the superconducting state. The present theoretical result well explains the temperature dependence of the normalized spin-Knight shift and nuclear relaxation rate.

Effect of magnetic impurities on the BCS and Josephson interlayer pair transfer in high-T C superconductors

Our previous investigation of BCS and Josephson tunneling as interlayer transfer mechanisms is used to study the effect of magnetic impurities between the layers. With proper choice of parameters the Josephson mechanism can fit the strong decrease of Tc observed by Winzer in the Tl compound. We give arguments why the BCS coupling is relevant for the Y compound but not for the more distant planes in the Tl and Bi compounds.

Transport properties of organic conductor (BEDT-TTF) 2KHg(SCN) 4: II. Shubnikov-de Haas oscillations and spin-splitting effect

The magnetoresistance, the Shubnikov-de Haas (SdH) effect and their orientation dependence of an organic conductor (BEDT-TTF) 2KHg(SCN) 4 have been measured at temperatures down to 0.5 K and under magnetic fields up to 13 T. The SdH frequency F=670T, the cyclotron mass m c =1.4 m 0 and the Dingle temperature T D =1.6 K are obtained when the fields are applied perpendicular to the conducting a-c plane. The dependence of F on the angle θ between the field and the b ∗-axis (⊥ a-c plane), which deviates slightly from cosθ, indicates the presence of the weakly warped cylindrical Fermi surface with an interlayer transfer integral estimated to be about 1 meV. Below 1 K and above 10 T, the spin-splitting effect is clearly observed and the direct analysis of the splitting oscillations results in the series of the g-factor; g=1.4 n±0.6 ( n is an odd number). The rigorous g-factor of 2.0 ( n=1, + sign), being consistent with ESR measurements, is obtained.

Birth-death models of epitaxy: I. Diffraction oscillations from low index surfaces

Simple mathematical modes are presented that exhibit features observed in reflection high-energy electron diffraction (RHEED) studies of crystal growth by molecular beam epitaxy. These models are based on a birth-death analysis of growth on low-index surfaces. These are mean field models which after an initial transient give RHEED intensity oscillations with varying degrees of damping. For moderate values of interlayer transfer the oscillations are nearly sinusoidal with maxima that do not necessarily correspond to the deposition of integral numbers of layers. As the transfer is increased, the oscillations become more cusp-like, as would be expected for perfect layer-by-layer growth. The results agree qualitatively with the intensity oscillations that are observed on low-index surfaces.

Phenomenological studies on molten core-concrete interactions

Recent assessments of risk due to severe accidents in light water reactors indicate that molten core-concrete interactions (MCCI) may dominate containment loads and performance, as well as the release of non-volatile fission product aerosols to the containment building. This program has investigated several important aspects of MCCIs in an effort to support the USNRC integral melt-concrete programs as well as the CORCON and VANESA computer code development and verification program. Among these are interlayer heat and mass transfer and liquid-liquid boiling processes. The issues of interlayer heat and mass transfer address phenomena that occur primarily interior to the molten core debris itself. Models have been developed to predict the onset of entrainment between liquid layers, the rate of entrainment, and the rate of heat transfer, both with and without entrainment. Application to the reactor case indicates that entrainment and mixing may or may not occur, depending upon the prevailing thermal hydraulic conditions in the melt. The issues of liquid-liquid film boiling address phenomena that occur primarily at the melt-coolant interface. Results from melt-water film boiling studies have suggested that the coolant heat flux increases above the flat plate limit with non-condensable gas injection from below. Steam explosions occur frequently; the frequency and magnitude vary with gas injection rate.