Search Results (41 total)

The magneto-photoluminescence of type-II (Zn,Mn)Te quantum dots is presented. As a result of the type-II band alignment, Aharonov-Bohm (AB) oscillations in the photoluminescence intensity are evident. In addition, an interesting interplay between the AB effect and the spin polarization in these diluted magnetic semiconductor quantum dots is observed. The intensity of the AB oscillations increases with both magnetic field and the degree of optical polarization, indicating that the suppression of spin fluctuations improves the coherence of the system.

This study elucidates the theory of phonon-induced decoherence of a double dot charge qubit that is embedded inside a suspended semiconductor slab. The influences of the lattice temperature, width of the slab, and positions of the dots on the decoherence are analyzed. Numerical results indicate that the decoherence in the slab system is weaker than that in a bulk environment. In particular, the decoherence is markedly suppressed by the inhibition of the electron-phonon coupling. Such a system with low decoherence may be useful for manipulating the qubits.

Orientation of single and two coupled polar molecules irradiated by a single laser pulse under a conical-well model is investigated theoretically. The orientation of a single hindered rotor shows a periodic behavior. In particular, the amplitude of the oscillation is sensitive to the degree of alternation of the laser field. Crossover from field-free to hindered rotation is observed by varying the hindering angle for different heights of conical wells. For a small hindering potential and angle, time-averaged orientation differs greatly from that for an infinite one. The orientation at a large hindering angle shows irregularlike behavior under strong dipole-dipole interaction. Entanglement induced by the dipole-dipole interaction is also calculated for the coupled-rotor system, in which the time-averaged entropy increases monotonically as the hindering angle is increased. The competition between the confinement effect and dipole interaction is found to dominate the behavior of the coupled-rotor system.

We have theoretically studied the optical absorption coefficient of quantum wires in electric and strain fields with intermixing interfaces. The potential profiles are governed by the intermixing heterojunctions, internal strain due to the lattice mismatch, the external electric field and stress. The second Fick’s law describes the wires’ intermixing heterojunctions due to the alloy interdiffusion. We adopt the Green’s function method to solve the Poisson equation for the displacement to determine the internal strain due to the lattice mismatch. The single-band Schrödinger equation in the effective mass approximation is used to describe the conduction subband structures while the four-band Kohn-Luttinger Hamiltonian is used to describe the valence subband structures. In solving the Schrödinger equation and the Kohn-Luttinger Hamiltonian, we expand the wave functions for the electrons and holes by using linear combinations of the two-dimensional harmonic oscillator wave functions to yield two matrix equations. These matrix equations are numerically solved for their eigen-energies and their corresponding eigenfunctions. We investigate some physical properties of the unstrained quantum wire (GaAs∕AlAs) and the strained wire (CdSe∕ZnSe) as examples. When the interdiffusion becomes stronger, on one hand, the internal strain is relaxed leading to the band gap shrinkage (if the initial strain ε₀<0) and, on the other hand, the potential profiles are deformed and the effective band gap becomes wider. Variations of the transition energies with external stress show the anticrossing effect and variations of the transition energies with external electric field show the field emission effect. The hole effective masses can be enhanced or become electronlike by applying stress to the wire. The oscillator strengths of the dipole-allowed intersubband transitions for the y and z polarizations are calculated to infer possible optical transitions. The optical absorption coefficients together with the joint density of states are calculated.

We describe the status of our effort to realize a first neutrino factory and the progress made in understanding the problems associated with the collection and cooling of muons towards that end. We summarize the physics that can be done with neutrino factories as well as with intense cold beams of muons. The physics potential of muon colliders is reviewed, both as Higgs factories and compact high-energy lepton colliders. The status and time scale of our research and development effort is reviewed as well as the latest designs in cooling channels including the promise of ring coolers in achieving longitudinal and transverse cooling simultaneously. We detail the efforts being made to mount an international cooling experiment to demonstrate the ionization cooling of muons.

Lifetimes of the first 4⁺ and 6⁺ states in ¹⁰⁴Mo and ¹⁰⁶Mo have been measured using the recoil distance method following spontaneous fission of ²⁵²Cf. The experiment was performed at the 88-inch cyclotron of the Lawrence Berkeley National Laboratory employing the Gammasphere array and the New Yale Plunger Device. Reduced transition probabilities in the ground state band of ¹⁰⁴Mo are compared with predictions of the critical point symmetry X(5) for phase transitional nuclei between rotational and vibrational shape. While known level energies of ¹⁰⁴Mo are in good agreement with the X(5) predictions, the analysis of the measured B(E2) values favors a rotor interpretation.

In this paper states in ¹⁰²Pd populated in the ε/β⁺ decay of ¹⁰²Ag, produced in the ⁸⁹Y(¹⁶O,3n) reaction, were studied with high efficiency Ge detectors. The level scheme of ¹⁰²Pd is in good overall agreement with the predictions of the E(5) critical point symmetry, with the exception of the 0₂⁺ state, which may be of intruder character.

We investigate the achievable degree of agreement between spectroscopic predictions of the geometric collective model (GCM) and the interacting boson model-1 (IBM-1). First, the GCM potential-energy surfaces are made locally equivalent with the IBM-1 semiclassical potentials close to the minimum. The GCM kinetic term is then optimized to reproduce some of the IBM-1 energy ratios. Differences between the spectra, as predicted by the two models, are discussed for the transitional region from the prolate axially symmetric rotor to the spherical vibrator.

A simple compact correlation of the energies of the lowest 0⁺ excitation with the yrast energy ratio R_4/2(g.s.)≡E(4₁⁺)/E(2₁⁺) is found to characterize all collective nuclei, regardless of structure. This correlation can be reproduced by interacting bosom approximation calculations. Collective signature observables for the states based on 0₂⁺ excitations indicate a different structure than for the ground state configuration but systematic inconsistencies in their global behavior and discrepancies with calculations prevent a definite interpretation.

Lifetimes in the neutron-rich nuclei ¹⁰⁴Mo, ¹¹⁰Ru, and ¹⁴⁴Ba were measured using the differential recoil distance method. The experiment was performed with a ²⁵²Cf fission source inside the New Yale Plunger Device. γ rays were detected by the SPEctrometer for Experiments with Doppler shifts at Yale (SPEEDY) while fission fragments with the appropriate kinematics were detected by an array of photocells.

A systematic study of predictions of the interacting boson approximation (IBA)-2 model leads to the identification of two parameter invariants such that calculations with the same values of the invariants yield results that are identical in certain cases and show close similarities under a wider set of conditions. The invariants validate a much-used form of the IBA-2 Hamiltonian and provide a systematic method to simplify IBA-2 calculations by choosing a truncated Hamiltonian with the same invariant values as more general Hamiltonians.

Molecular beam epitaxy and optical investigation of Zn_1-xMn_xSe epilayers over a large composition range (x=0–0.78), grown on GaAs (001) substrates, are reported. The far-infrared (FIR) reflectance and Raman scattering were performed to characterize the film quality and study their optical and electrical properties. FIR and Raman data provide experimental evidence on the intermediate-mode phonon behavior for Zn_1-xMn_xSe with x up to 0.78. Theoretical modeling fits of FIR spectra lead to the determination of optical parameters such as mode frequency, strength, damping constant, and electrical properties of dielectric constant, carrier concentration, mobility, conductivity, and effect mass, and their dependence on the Mn composition. Values of force constants of F_MnSe=17.1×10⁴ dyn cm¹ and F_ZnSe=14.7×10⁴ dyn cm^-1, and the high-frequency limit dielectric constant of MnSe, ε_∞=5.4, are obtained.

A remarkable set of correlations of yrast and excited quasiband energies in collective nonrotor and rotor nuclei is discussed. These correlations are essentially independent of the nature of the states (e.g., yrast, quasi-γ-vibrational, superdeformed), of mass region, and of the type of nucleus (even-even, odd-A, or odd-odd), and indicate a universal behavior which apparently describes all collective low energy structures. The results also show that, for most cases in odd-A deformed nuclei, the band energies do not follow the rotational formula. The nature of spherical-deformed phase/shape transitions is also discussed and related to the seemingly (but not actually) different behavior of the neighboring even-even core nuclei. Finally, theoretical treatments that reproduce these correlations in even-even and odd-A nuclei are discussed, and new calculations with the geometric collective model are presented.

The status of the research on muon colliders is discussed and plans are outlined for future theoretical and experimental studies. Besides work on the parameters of a 3–4 and 0.5 TeV center-of-mass (COM) energy collider, many studies are now concentrating on a machine near 0.1 TeV (COM) that could be a factory for the s-channel production of Higgs particles. We discuss the research on the various components in such muon colliders, starting from the proton accelerator needed to generate pions from a heavy-Z target and proceeding through the phase rotation and decay (π→μν_μ) channel, muon cooling, acceleration, storage in a collider ring, and the collider detector. We also present theoretical and experimental R&D plans for the next several years that should lead to a better understanding of the design and feasibility issues for all of the components. This report is an update of the progress on the research and development since the feasibility study of muon colliders presented at the Snowmass '96 Workshop [R. B. Palmer, A. Sessler, and A. Tollestrup, Proceedings of the 1996 DPF/DPB Summer Study on High-Energy Physics (Stanford Linear Accelerator Center, Menlo Park, CA, 1997)].

The lattice vibration of ZnSe_1-xTe_x epilayers grown using molecular-beam epitaxy was investigated with Raman as well as Fourier transform infrared reflectance spectroscopy. The dependence of longitudinal-optical (LO) and transverse-optical (TO) phonon frequency on the Te concentration was found to follow previous theoretical predictions. However, additional vibration modes were observed at the energy between that of the LO and TO phonons. The microscopic force constants F_ZnTe and F_ZnSe were evaluated to be 6.46×10⁶ amu×(cm^-1)² and 2.91×10⁶ amu×(cm^-1)², respectively. In addition, the Raman spectra were recorded at high pressure up to 20 GPa. The pressure at which the semiconductor to metal transition occurred is characterized by the disappearance of the LO phonon and found to decrease with the Te concentration. Current results imply the decreasing crystal stability with the Te concentration.

The evolution of nuclear structure along the Xe and Ba isotopes with N<82 is studied in comparison with that in the Pt-Os isotopes and with interacting boson approximation calculations. As is well known, the Pt isotopes from ^188-196Pt exhibit a stable structure very close to O(6), the lighter Pt isotopes and the Os nuclei evolve towards a rotor, and the heaviest Pt nuclei, ^198,200Pt, show a tendency towards vibrational character. In contrast, we show that the Ba nuclei are best described by a trajectory from U(5) towards SU(3) that passes through an intermediate structure that resembles O(6) for N∼72–76. The Xe appears to evolve from U(5) to O(6)-like.

It was recently demonstrated that the empirical linear correlation of E(4₁⁺) with E(2₁⁺) is reproduced by the interacting boson model 1 (IBA-1) if a constant value for the coefficient κ of the Q⋅Q term is used. This constancy motivates the present investigation of whether it is possible to find sets of IBA-1 parameters, with fixed κ, that reproduce the data for a more general set of observables and a wide range of nuclei. A consistent procedure, based on contour plots of key observables, was used to extract ε and χ values for 145 nuclei spanning the Z = 50–82 shell, which give reasonable overall agreement with the data with smoothly varying parameters. The constant κ constraint sometimes leads to parameter values different than normal: finite ε values (∼ 0.1 MeV) even for rotational nuclei and large χ values for transitional and vibrational nuclei that even increase towards the vibrational region. Finally, a systematic correlation of ε values with E(2₁⁺) was found.

The low energy octupole states in ¹³⁴Ba were examined using the inelastic scattering of 22 MeV protons. Strong fragmentation of the low energy octupole strength, which was observed in ^196,198Pt and was identified as a signature of the O(6) dynamical symmetry limit of the interacting boson approximation (IBA), does not occur in ¹³⁴Ba, which has been proposed to have O(6) structure. The model calculations demonstrate that there is, in fact, little difference in the structure of the low-lying positive parity levels between certain nuclei in a U(5)→SU(3) (vibrator→rotor) transition and an O(6) nucleus (γ-soft nucleus), but that the octupole fragmentation has an enhanced sensitivity to the d-boson composition of the wave functions and thereby reveals differences in structure that are not easily discernible with other observables.

Highly oriented Cd_1-xMn_xS thin films with a wurtzite structure were grown by the radio-frequency sputtering technique. The grain size of the Cd_1-xMn_xS thin films was found to decrease with an increase in the Mn concentration x. Moreover, the lattice softening effect due to the quantum size effect, the band-bowing phenomena accounted for by the band-gap correction arising from the chemical disorder, and the exchange interaction between the band carrier and the d electron of the magnetic Mn ions have been investigated. © 1996 The American Physical Society.

It is shown that the simple correlation of E(2₁⁺) and E(4₁⁺) energies can be a sensitive indicator of underlying shell structure and, in particular, that recent suggestions of radically different structure near the neutron drip line may be most easily signaled by this observable.

The negative parity state of ^172-180Os are studied in the framework of the interacting boson approximation model. The negative parity states are formed by including one f boson or allowing one boson to break into two fermions which are placed in orbits with opposite parity. The calculated level energies and B(E2)’s are in reasonable agreements with experiments. From the comparison of intrinsic structure it suggests that the relative energy of the quasiparticles may be the most important factor in the calculations.

We describe a general method of calculating exciton energies in shallow quantum wells. This technique is applicable to both single- and multiple-quantum-well systems, and is valid for both type-I and type-II systems, even under circumstances where the valence- and/or conduction-band offsets may be small compared to the exciton binding energy. Quantitative predictions of excitonic energies and relative intensities are made and compared with experimental data on a number of different shallow-well diluted magnetic semiconductor quantum well and superlattice systems. Based on these comparisons, it is shown that the model indeed provides a detailed description and analysis of the type-I–type-II transition and the behavior of excitons in a spin superlattice. In addition, the model also predicts and describes certain additional phenomena, such as metastable ‘‘above-gap’’ excitons which should exist in certain type-II systems. Also, reentrant type-I–type-II–type-I transitions are predicted under some circumstances.

A method for extracting effective p-n monopole matrix elements from experimental data is developed and applied to nuclei just below ²⁰⁸Pb. Eight effective monopole matrix elements are extracted from a fit to 38 experimental data with an overall rms deviation of 116 keV. The resulting matrix elements are compared with those from semirealistic effective interactions.

A comprehensive analysis is made of the experimental information on Gamow-Teller beta decay for the light (A≤18) nuclei. Experimental data on half-lives, Q values, and branching ratios are tabulated. Experimental logft values and beta-decay matrix elements are deduced from these data. The one-body-transition densities necessary to predict the beta-decay matrix elements are then calculated using a recently constructed shell-model Hamiltonian operating in the first four major shells. Using these parameters, effective Gamow-Teller operators are deduced for the 0p shell from a least-squares fit to 16 experimental matrix elements. The effective operators are used to calculate 83 decay matrix elements. Some specific decays are discussed.

Level lifetimes were measured for the first time for the neutron-rich isotope ⁷⁸Ge by the fast electronic scintillation timing technique and B(E2) values were extracted. The energies of the low-lying states and the B(E2) values between the 0⁺ and 2⁺ states disclose a rapidly changing structure near N=40, followed by a stabilization, with ^74-78Ge best understood as asymmetric rotors.