Valence bands spin splitting in GaAs/AlGaAs quantum wells

A quantitative comparison between different model calculations of valence band states in GaAs/AlGaAs heterostructures is presented. We demonstrate that a 14-band k.p Hamiltonian using a completely new parameterization based on fits of the tight-binding band structure leads to energy dispersion relations in excellent agreement with experiment, whereas previous parameterizations result in significant deviations. The relevance of the present results to the calculation of spin-related phenomena is discussed.     

Integrable structure of melting crystal model with two -parameters

This paper explores integrable structures of a generalized melting crystal model that has two q-parameters q₁,q₂. This model, like the ordinary one with a single q-parameter, is formulated as a model of random plane partitions (or, equivalently, random 3D Young diagrams). The Boltzmann weight contains an infinite number of external potentials that depend on the shape of the diagonal slice of plane partitions. The partition function is thereby a function of an infinite number of coupling constants t₁,t₂,… and an extra one Q. There is a compact expression of this partition function in the language of a 2D complex free fermion system, from which one can see the presence of a quantum torus algebra behind this model. The partition function turns out to be a tau function (times a simple factor) of two integrable structures simultaneously. The first integrable structure is the bigraded Toda hierarchy, which determines the dependence on t₁,t₂,…. This integrable structure emerges when the q-parameters q₁,q₂ take special values. The second integrable structure is a q-difference analogue of the 1D Toda equation. The partition function satisfies thisq-difference equation with respect to Q. Unlike the bigraded Toda hierarchy, this integrable structure exists for any values of q₁,q₂.     

Fabrication of polyaniline/ heterojunction structure using plasma enhanced polymerization technique

The work reports on the fabrication of a p–n heterojunction structure comprised of polyaniline (PANI) and TiO₂ nanoparticles. PANI was deposited by plasma enhanced polymerization on TiO₂ thin film substrates. The structural and the crystalline properties demonstrated the coherence and the substantive interaction of the plasma polymerized PANI molecules with the TiO₂ nanoparticle thin film. The UV–Vis studies of PANI/TiO₂ thin film supported the internalization of PANI with TiO₂ nanoparticles due to π–π^* transition of the phenyl rings with the lone pair electrons () of the nitrogen atom present in the PANI molecules. The I–V characteristics of the PANI/TiO₂ heterojunction structure were obtained in the forward and the reverse biased at applied voltage ranging from −1 V to +1 V with a scan rate of 2 mV/s. The proficient current in the PANI/TiO₂ heterojunction structure was attributed to the well penetration of PANI molecules into the pores of the TiO₂ nanoparticle thin film. The I–V characteristics ensured an efficient charge movement at the junction of PANI/TiO₂ interface and thus, behaved as a typical ohmic system.     

Cold nuclear matter effects on production: Intrinsic and extrinsic transverse momentum effects

Cold nuclear matter effects on J/ψ production in proton–nucleus and nucleus–nucleus collisions are evaluated taking into account the specific J/ψ-production kinematics at the partonic level, the shadowing of the initial parton distributions and the absorption in the nuclear matter. We consider two different parton processes for the -pair production: one with collinear gluons and a recoiling gluon in the final state and the other with initial gluons carrying intrinsic transverse momentum. Our results are compared to RHIC observables. The smaller values of the nuclear modification factor R_AA in the forward rapidity region (with respect to the mid rapidity region) are partially explained, therefore potentially reducing the need for recombination effects.     

On the calculation of group characters

It is known that characters of irreducible representations of finite Lie algebras can be obtained using the Weyl character formula including Weyl group summations which make actual calculations almost impossible except for a few Lie algebras of lower rank. By starting from the Weyl character formula, we show that these characters can be re-expressed without referring to Weyl group summations. Some useful technical points are given in detail for the instructive example of G₂ Lie algebra.     

Optical properties and g factors of GaAs quantum rods

The Hamiltonian of the zinc-blende quantum rods in the framework of eight-band effective-mass approximation in the presence of external homogeneous magnetic field is given. The electronic structure, optical properties and electron g factors of GaAs quantum rods are investigated. We found that the electron g factors are very sensitively dependent on the dimensions of the quantum rods. As some of the three dimensions increase, the electron g factors decrease. The more the dimensions increase, the more the electron g factors decrease. The dimensions perpendicular to the direction of the magnetic field affect the electron g factors more than the other dimension.     

Phase transitions in the quantum Hall liquid in a quantum wire

Two scenarios for the collapse of the ν=1 quantum Hall liquid (QHL) state, with the effective quantum wire (QW) width defined by the Fermi vector k_F, are studied. Here, ν for the QW is defined as the filling factor of Landau levels (LL) at the center of the QW. In the first one there is no electron redistribution at critical magnetic field , where the Fermi energy, E_F, coincides with the bottom of the empty upper spin-split LL. For the ν=1 state is unstable due to exchange-correlation effects and lateral confinement. In the second scenario, a transition to the ν=2 state occurs, with much smaller width, at . The latter scenario is analyzed in the Hartree–Fock approximation (HFA). Here the Hartree contribution to the total energy affects drastically due to strong electron redistribution in the QW. In both scenarios, the exchange-enhanced g-factor is suppressed at B_cr. The critical fields, activation energy, and optical g-factor obtained in the first scenario are very close to the measured ones.     

Second-order eikonal corrections for

The first-order eikonal approximation is frequently adopted in interpreting the results of A(e,e^′p) measurements. Glauber calculations, for example, typically adopt the first-order eikonal approximation. We present an extension of the relativistic eikonal approach to A(e,e^′p) which accounts for second-order eikonal corrections. The numerical calculations are performed within the relativistic optical model eikonal approximation. The nuclear transparency results indicate that the effect of the second-order eikonal corrections is rather modest, even at Q²≈0.2 (GeV/c)². The same applies to polarization observables, left–right asymmetries, and differential cross sections at low missing momenta. At high missing momenta, however, the second-order eikonal corrections are significant and bring the calculations in closer agreement with the data and/or the exact results from models adopting partial-wave expansions.     

Nuclear isospin mixing and elastic parity-violating electron scattering

The influence of nuclear isospin mixing on parity-violating elastic electron scattering is studied for the even–even, N=Z nuclei ¹²C, ²⁴Mg, ²⁸Si, and ³²S. Their ground-state wave functions have been obtained using a self-consistent axially-symmetric mean-field approximation with density-dependent effective two-body Skyrme interactions. Some differences from previous shell-model calculations appear for the isovector Coulomb form factors which play a role in determining the parity-violating asymmetry. To gain an understanding of how these differences arise, the results have been expanded in a spherical harmonic oscillator basis. Results are obtained not only within the plane-wave Born approximation, but also using the distorted-wave Born approximation for comparison with potential future experimental studies of parity-violating electron scattering. To this end, for each nucleus the focus is placed on kinematic ranges where the signal (isospin-mixing effects on the parity-violating asymmetry) and the experimental figure-of-merit are maximized. Strangeness contributions to the asymmetry are also briefly discussed, since they and the isospin mixing contributions may play comparable roles for the nuclei being studied at the low momentum transfers of interest in the present work.     

Structural and optical properties of thermally evaporated thin films

Chalcogenide glass Se₅₅Ge₃₀As₁₅ have amorphous structure in both as-deposited and annealed conditions. The optical properties of the as-deposited and annealed films were studied using spectrophotometric measurements of transmittance, T(λ), and reflectance, R(λ), at normal incidence of light in the wavelength range 200–2500 nm. Neither annealing temperature nor film thickness can influence spectral response on refractive index and absorption index of films. The type of electronic transition responsible for optical properties is indirectly allowed transition with energy gap of 1.94 eV and phonon energy of 40 meV. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple–Didomenico (WD) model. The width of band tails of localized states into the gap (ΔE), the single oscillator energy (E_o), the dispersion energy (E_d), the optical dielectric constant (ε_∞), the lattice dielectric constant (ε_L), the plasma frequency (ω_p) and the free charge carrier concentration (N) were estimated.     

Spectral flow invariants and twisted cyclic theory for the Haar state on

In [A.L. Carey, J. Phillips, A. Rennie, Twisted cyclic theory and an index theory for the gauge invariant KMS state on Cuntz algebras. arXiv:0801.4605], we presented a K-theoretic approach to finding invariants of algebras with no non-trivial traces. This paper presents a new example that is more typical of the generic situation. This is the case of an algebra that admits only non-faithful traces, namely SU_q(2) and also KMS states. Our main results are index theorems (which calculate spectral flow), one using ordinary cyclic cohomology and the other using twisted cyclic cohomology, where the twisting comes from the generator of the modular group of the Haar state. In contrast to the Cuntz algebras studied in [A.L. Carey, J. Phillips, A. Rennie, Twisted cyclic theory and an index theory for the gauge invariant KMS state on Cuntz algebras. arXiv:0801.4605], the computations are considerably more complex and interesting, because there are non-trivial ‘eta’ contributions to this index.     

Ambiversion of

An analysis including most recent Belle data on X(3872) is performed, using coupled channel Flatté formula. A third sheet pole close to but below D⁰D^*0 threshold is found, besides the bound state/virtual state pole discussed in previous literature. The co-existence of two poles near the D⁰D^*0 threshold indicates that the X(3872) may be of ordinary 2³P₁ state origin, distorted by strong coupled channel effects. The latter manifests itself as a molecular bound state (or a virtual state).     

Magnetic octupole order in : A polarized neutron diffraction study

Recently, in phase IV of Ce_xLa_1-xB₆, weak but distinct superlattice reflections from the order parameter of phase IV have been detected by our unpolarized neutron scattering experiment [K. Kuwahara, K. Iwasa, M. Kohgi, N. Aso, M. Sera, F. Iga, J. Phys. Soc. Japan 76 (2007) 093702]. The scattering vector dependence of the intensity of superlattice reflections is quite unusual; the intensity is stronger for high scattering vectors. This result strongly indicates that the order parameter of phase IV is the magnetic octupole. However, the possibility that the observed superlattice reflections are due to lattice distortions could not be completely ruled out only on the basis of the unpolarized neutron scattering experiment. To confirm that the superlattice reflections are magnetic, therefore, we have performed a single crystal polarized neutron diffraction experiment on Ce_0.7La_0.3B₆. The obtained result has clearly shown that the time reversal symmetry is broken by the order parameter of phase IV. This is further evidence for the magnetic octupole order in Ce_xLa_1-xB₆.     

Non-linear Shot Noise: Lévy, Noah, & Joseph

We introduce and study a generic non-linear Shot Noise system-model. Shots of random magnitudes arrive to the system stochastically, following an arbitrary time-homogeneous Poisson point process. After ‘hitting’ the system, the magnitude of an arriving shot decays to zero. The decay is governed by an arbitrary differential-equation dynamics. Shots are independent, and their overall effect on the system is additive: the system's noise level at time t equals the sum of the magnitudes, at time t, of all the shots arriving to the system prior to time t.The resulting Shot Noise is: (i) a Lévy process when the decay-dynamics are degenerate; (ii) a Lévy-driven Ornstein–Uhlenbeck process when the decay-dynamics are linear; and, (iii) a stationary non-Markov process when the decay-dynamics are non-linear.The resulting Shot Noise admits an underlying Lévy structure—which we explicitly compute, and can yield both the Noah effect and the Joseph effect. Closed-form analytic formulae for various statistics are derived, including: the log-Laplace transform and cumulants of the stationary noise level; the process’ auto-covariance function; and, the process’ range-of-dependence.     

Optical intersubband transitions in double Si -doped GaAs under an applied magnetic field

For different applied magnetic fields, the intersubband transitions of double Si δ-doped GaAs structures is theoretically investigated for a uniform donor distribution. The electronic structure has been calculated by solving the Schrödinger and Poisson equations self-consistently. It is found that the intersubband optical absorption and mobility are sensitive to the applied magnetic field: for all allowed intersubband transitions the intersubband absorption spectra show blueshifts. The results open the possibility to design devices for use as optical filters controlled by an applied magnetic field, depending on the δ-doped structure. It is hoped that these results will provide important improvement in device applications, for a suitable choice of magnetic field.     

-Factor tuning of 2D electrons in double-gated Si/SiGe quantum wells

We report on the design and first experiments of Si/SiGe heterostructures that allow gate-operated shifting of a 2D electron gas between two channels with different Landé g-factors. This allows gate-operated moving of electrons in and out of resonance in an electron spin resonance (ESR) experiment, which can act as a building block of a proposed solid-state quantum computer. We use MBE-grown modulation-doped quantum-wells (QWs) on SiGe pseudosubstrates with up to 30% Ge and low-temperature electron mobilities up to . A double QW structure with two different Ge contents separated by a thin barrier was optimized for this purpose with self-consistent simulations. The band structure simulations show that by applying gate voltages one can completely shift the wave function from one well to the other. First experiments on pure Si channels show the working of the gate setup. Both carrier density and mobility can be increased by using the back gate which corresponds to shifting the wave function in the channel.     

Effect of the -function potential on the electron transport in a magnetic nanostructure

In this paper, the spin-dependent electron transport is studied in detail in a magnetic nanostructure with a δ-function potential. It is shown that the large spin-polarization can be achieved in such a device, and the degree of the spin-polarization strongly depends on the height of the δ-function potential. It is also shown that the conductance-polarization apparently has the bigger oscillatory magnitudes with the height of δ-function potential increasing. These interesting features will be more helpful for developing new types of devices.     

First-principles study of fundamental properties and electronic structure of alloys

We have performed first-principles calculations using full-potential augmented-plane-wave method to investigate the fundamental properties of the Cd_1–xZn_xTe alloys. The composition dependence of the lattice constant and the bulk modulus have been estimated from total energy calculations. By means of the analytical fitting the band structures in the vicinity of the Brillouin center a complete set of effective electron- and hole-masses have also been derived. In order to further understand the effects of the chemical bonding on the above macroscopic properties we then studied the relaxation behaviors and the changes of the electronic states upon alloying for x=0.25 system. The results presented here yield a general understanding of the fundamental properties for the Cd_1–xZn_xTe crystals studies.     

Interrelation between electronic structure and interatomic distances for compounds

The heat capacity was studied for LaMn₂Si₂, La_0.75Y_0.25Mn₂Si₂, La_0.7Y_0.3Mn₂Si₂, YMn₂Si₂ and LaFe₂Si₂ isostructural intermetallic compounds in the temperature range 1.8–360 K. The electronic, magnetic and lattice contributions to the heat capacity of the compounds were determined and analyzed. The interrelation was found between values of the electronic contribution to the heat capacity (density of states at the Fermi level) and crystal lattice parameters of R(Mn,Fe,Ni)₂Si₂ compounds. The electronic contribution and the density of states at Fermi level increase with increasing lattice parameters of the compounds. The change of interlayer Mn–Mn exchange interactions with change of Y concentration in La_1-xY_xMn₂Si₂ compounds is not accompanied by considerable changes in the electronic contribution to the heat capacity and density of states at the Fermi level. The performed analysis of the magnetic contribution shows that no essential differences exist between the behavior of the heat capacity of the compounds with d_Mn–Mnd_c and with d_Mn–Mn<d_c upon various types of the magnetic phase transitions.