Microscopic Model Calculations Research Articles

Controllable kinetic magnetoelectric effect in two-dimensional electron gases with both Rashba and Dresselhaus spin-orbit couplings

A microscopic model calculation of the electric-field-induced nonequilibrium spin accumulation due to kinetic magnetoelectric effect in two-dimensional electron gases with both Rashba and Dresselhaus spin-orbit couplings is presented. The results show that by modulating the relative strength of the two different spin-orbit couplings, both the magnitude and the polarization direction of the nonequilibrium spin accumulation can be effectively controlled.

Excitation function of elliptic flow in Au + Au collisions and the nuclear matter equation of state

We present measurements of the excitation function of elliptic flow at midrapidity in Au+Au collisions at beam energies from 0.09 to 1.49 GeV per nucleon. For the integral flow, we discuss the interplay between collective expansion and spectator shadowing for three centrality classes. A complete excitation function of transverse momentum dependence of elliptic flow is presented for the first time in this energy range, revealing a rapid change with incident energy below 0.4 AGeV, followed by an almost perfect scaling at the higher energies. The equation of state of compressed nuclear matter is addressed through comparisons to microscopic transport model calculations.

Giant resonances — experiments at highest resolution, wavelets and scales

Recent high-resolution experiments using electron and proton scattering and charge-exchange reactions covering a wide mass range of nuclei establish the fine structure of giant resonances as a global phenomenon. Experimental evidence includes a systematic study of the isoscalar giant quadrupole resonance (ISGQR) in 58Ni, 89Y, 90Zr, 120Sn and 208Pb, the first observation of fine structure of the Gamow–Teller resonance in a heavy nucleus, 90Nb, as well as of the isovector giant dipole (IVGDR) and magnetic quadrupole (M2) modes. Novel methods based on wavelet transforms are introduced for the extraction of scales characterizing the fine structure and evaluating their relative importance. Comparison of those scales to predictions from microscopic model calculations including 2p–2h degrees of freedom indicate surface vibration coupling as the underlying mechanism for their appearance. A generic pattern is also found for the stochastic coupling to the background of the more complex states.

Theoretical study of phosphorousδ-doped silicon for quantum computing

We present microscopic model calculations of phosphorous $\ensuremath{\delta}$-doped silicon. Using the planar Wannier orbitals [Y. C. Chang and G. Li, Comput. Phys. Commun. 95, 158 (1996)] obtained based on the pseudopotential method, we calculate the electronic structures of a large slab (1000 ML) self-consistently (for doping electrons), taking into account both the long-range Coulomb potential in the direction perpendicular to the doping plane and the short-range interaction due to the $1∕4$ monolayer (ML) phosphor $\ensuremath{\delta}$ doping. Our results show that the Fermi level after doping is about $100\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ below silicon conduction band minimum (CBM). We also find that the short-range interaction due to P dopants only has a minor influence on the Fermi level.

Correlation effects on resonant tunneling in one-dimensional quantum wires

We study resonant tunneling in a Luttinger liquid with a double barrier enclosing a dot region. Within a microscopic model calculation the conductance G as a function of temperature T is determined over several decades. We identify parameter regimes in which the peak value G_p(T) shows distinctive power-law behavior. For intermediate dot parameters G_p behaves in a non-universal way.

Fine structure in the energy region of the isoscalar giant quadrupole resonance: characteristic scales from a wavelet analysis.

Fine structure in the energy region of the isoscalar giant quadrupole resonance in nuclei is observed in high-resolution proton scattering experiments at iThemba LABS over a wide mass range. A novel method based on wavelet transforms is introduced for the extraction of scales characterizing the fine structure. A comparison with microscopic model calculations including two-particle two-hole (2p2h) degrees of freedom identifies the coupling to surface vibrations as the main source of the observed scales. A generic pattern is also found for the stochastic coupling to the background of the more complex states.

How to extract theΛNscattering length from production reactions

A dispersion integral is derived that allows one to relate directly (spin-dependent) $\ensuremath{\Lambda}N$ invariant mass spectra, measured in a large-momentum transfer reaction such as $pp\ensuremath{\rightarrow}{K}^{+}p\ensuremath{\Lambda}$ or $\ensuremath{\gamma}d\ensuremath{\rightarrow}{K}^{+}n\ensuremath{\Lambda}$, to the scattering length for elastic $\ensuremath{\Lambda}N$ scattering. The involved systematic uncertainties are estimated and could be $0.3\phantom{\rule{0.3em}{0ex}}\text{fm}$ or even smaller. This estimate is confirmed by comparing results of the proposed formalism with those of microscopic model calculations. We also show, for the specific reaction $pp\ensuremath{\rightarrow}{K}^{+}\ensuremath{\Lambda}p$, how polarization observables can be used to separate the two spin states of the $\ensuremath{\Lambda}N$ system.

News on strangeness at ultrarelativistic energies—review of microscopic models

We review recent developments in the field of microscopic transport model calculations for ultrarelativistic heavy ion collisions. In particular, we focus on the strangeness production, for example, the ϕ-meson and its role as a messenger of the early phase of the system evolution. Moreover, we discuss the important effects of the (soft) field properties on the multiparticle system. We outline some current problems of the models as well as possible solutions to them.

Nature of low-energy dipole strength in nuclei: the case of a resonance at particle threshold in 208Pb.

A high-resolution (gamma,gamma') study of the electric dipole response in 208Pb at the S-DALINAC reveals a resonance structure centered around the neutron emission threshold. Microscopic quasiparticle phonon model calculations in realistic model spaces including the coupling to complex configurations are able to describe the data in great detail. The resonance is shown to result from surface density oscillations of the neutron skin relative to an approximately isospin-saturated core. It also forms an integral part of a toroidal E1 mode representing an example of vortex collective motion in nuclei.

Coulomb Correlations and Biexciton Signatures in Coherent Excitation Spectroscopy of Semiconductor Quantum Wells

The influence of coupling, correlation, and disorder on exciton and biexciton signatures is investigated using coherent excitation spectroscopy. Different biexciton-induced transitions of a single biexciton state are spectrally resolved. One of the transitions is found to be particularly sensitive to disorder. Mixed biexciton resonances, resulting from attractively interacting heavy and light hole exciton states, are identified. The observations are analyzed on the basis of microscopic model calculations.

Bridging the gap between micromagnetism and ab initio calculations: basic hysteretic properties of CoPt-type magnets

We demonstrate how first-principles, microstructural and microscopic model calculations may be combined to obtain material-specific input for micromagnetic calculations. We elucidate the generic links between microstructural features and coercivity of polytwinned CoPt-type magnets. It is shown with the help of ab initio calculations that domain walls in these alloys are strongly attracted to antiphase boundaries, and, using microscopic simulations, that at twin boundaries they are split in magnetostatically coupled segments. We predict a peculiar mechanism of magnetization reversal via DW splitting in ‘partial walls’. General features of magnetization reversal are explained and estimates of coercivity given for the strong and weak pinning cases.

Direct evidence for an orbital magnetic quadrupole twist mode in nuclei

The reactions Ni-58(e, e') and M-58(p, p) have been studied at kinematics favorable for the excitation of J(pi) = 2(-) states by isovector spin-flip transitions with DeltaL = 1. There are states at an excitation energy E-X approximate to 10 MeV which are strongly excited in electron scattering but not in proton scattering, suggesting a predominantly orbital character. This is taken as direct evidence for the so-called twist mode in nuclei in which different layers of nuclear fluid in the upper and lower hemisphere counterrotate against each other. Microscopic quasiparticle-phonon model calculations which predict sizable orbital M2 strength at this excitation energy yield indeed a current flow pattern of the strongest transitions consistent with a twist-like motion. (C) 2002 Published by Elsevier Science B.V.

Comment on “Origin and consequences of the compensation (Meyer-Neldel) law”

In Yelon, Movaghar, and Branz [Phys. Rev. B 46, 12 244 (1992); see also Phys. Rev. Lett. 65, 618 (1990); Appl. Phys. Lett. 71, 3549 (1997)] the authors have made an attempt to give a microscopic explanation for a well-known empirical ``compensation rule'' exhibited experimentally by a number of activated processes. In their effort to relate some of the important general features of these processes to first-principles quantum-mechanical calculations of the elemental multiphonon transition rates, the authors have made some unfortunate errors that invalidate their quantitative, experimentally verifiable conclusions. Also, the authors seem to ignore some already existing, important work (related to microscopic models of activated transport coefficients) that has a direct bearing on the subject. These errors and omissions are pointed out with reference to microscopic model calculations.

Spectral signatures of χ^(5) processes in four-wave mixing of homogeneously broadened excitons

The influence of fifth-order coherences on the spectrally resolved four-wave mixing response of predominantly homogeneously broadened quasi-two-dimensional excitons is studied. Fifth-order signatures are discussed as a function of spectral position and excitation polarization. An exciton–biexciton beating for positive delay times is the dominant effect, which is pronounced at the exciton–biexciton transition for collinearly polarized excitation and at the exciton transition for cross-linearly polarized excitation. For negative delay times and collinearly polarized excitation a pronounced exciton–biexciton beating at the exciton resonance is observed that is vanishing for long negative delays owing to the faster dephasing in the two-exciton continuum compared with the bound biexciton state. These results are in qualitative agreement with microscopic model calculations that include the coherent dynamics of one- and two-exciton resonances up to the fifth order in the optical field.

The influence of the matrix element on angle-resolved photoemission spectra of insulating cuprates

Microscopic model calculations of the matrix element of a dipole moment are carried out in terms of the cluster model. This matrix element determines the transition probability of electron photoemission from a one-electron orbital with symmetry γμ to a free state. The effect of the matrix element on the angular and polarization dependences in the angle-resolved photoemission spectra of insulating cuprates, such as Sr2CuO2Cl2 and Ca2CuO2Cl2, is analyzed under the assumption of a well-isolated ground state of the two-hole CuO45− cluster, namely, the Zhang-Rice singlet. The angular k dependence of the matrix element gives rise to effects, such as the residual Fermi surface, which are typical of metallic systems. An analysis of the experimental data reveals the presence of another electronic state (with different symmetry) in the vicinity of the Zhang-Rice singlet.

Lattice model calculations for d x2− y2-wave Josephson junctions

The Josephson current–phase relations in d-wave/d-wave superconductor junctions in the tunnel limit are studied using the two-dimensional t- J model. The order parameters are determined in a self-consistent way within the Gutzwiller approximation and the Bogoliubov–de Gennes theory. We find that the anomalous current–phase relation in ( 1 1 0 ) junctions obtained within a quasiclassical theory is qualitatively reproduced with microscopic model calculations. Our method can be applied to grain boundary junctions in which the atomic scale roughness should be taken into account.

Measurement of neutron total cross sections up to 560 MeV

We have completed a new set of total cross section measurements of 31 elements and isotopes spanning the periodic table from A=1 to 238. We employed the same technique as in Finley et al. [Phys. Rev. C 47, 237 (1993)] with refinements intended to allow measurements on separated isotopes and improved systematic error control. The goal of the new measurement was 1% statistical accuracy in 1% energy bins with systematic errors less than 1%. This was achieved for all but the thinnest samples. Stringent checks of systematic errors in this measurement resulted in a reassignment of systematic uncertainties to the neutron total cross sections reported in Finley et al. Microscopic optical model calculations were carried out to interpret the results of the experiment. Two specific types of optical models were employed. The Jeukenne-Lejeune-Mahaux model was used in the range of 5--160 MeV, and a model based on the empirical effective interaction of Kelly was used from 135 to 650 MeV. These models are shown to be useful for predicting both neutron total cross sections and proton reaction cross sections. They are particularly important for light nuclei, for which standard global phenomenological parametrizations of the optical potential are insufficiently accurate.

Threshold contribution to the Gerasimov–Drell–Hearn sum rule in 3He

Measurements of the cross section, vector and tensor analyzing powers, and gamma-ray polarization in the radiative capture reactions D(p,γ)3He and p(d,γ)3He at cm. energies in the range 0–53 keV allow the determination of the reduced matrix elements (RMEs) relevant for these transitions. The RMEs provide values of the doublet and quartet Ml cross sections, as well as the asymptotic D-to-S state ratio. These quantities are in good agreement with theoretical predictions. The RMEs can also be used to compute the value of the integral which determines the Gerasimov-Drell-Hearn sum rule for 3He in the energy region of our experiment. The results are compared with the predictions of an ab initio microscopic three-body model calculation. The theoretical predictions for the value of this integral based on a ”nucleons-only” assumption are an order-of-magnitude smaller than experiment. The discrepancy is reduced to about a factor of two when meson-exchange currents are taken into account.

Quantum protectorates in the cuprate superconductors

Following the identification of the pairing state, the major challenge in understanding the cuprate superconductors has been determining the evolution with doping and temperature of their anomalous normal state behavior. Key to this understanding is the experimentally determined magnetic phase diagram for the cuprates, which provides information on the protected magnetic properties of the normal state, generic behavior that is reliably the same one system to the next, regardless of details. I discuss the constraints this places on candidate quantum protectorates, and the status of microscopic model calculations for a protectorate consistent with these constraints, the nearly antiferromagnetic Fermi liquid.

Structure and evolution of electric dipole strength in 204,206,208Pb below the neutron emission threshold

The electric dipole strength distributions below threshold in stable even-mass Pb isotopes have been extracted from photon scattering experiments at the superconducting Darmstadt electron linear accelerator S-DALINAC. Between 4 and 6.5 MeV excitation energy a resonance-like clustering of strength is observed. A strong evolution of a fragmentation of the dipole strength with opening the neutron shell is found. The fine structure of the strength is compared to microscopic quasiparticle-phonon model calculations that quantitatively reproduce the data quite well and explain the fragmentation of the dipole strength. Models suggesting the oscillation of excess neutrons with respect to an N≃ Z core as a possible origin of low-lying E1 strength are critically examined.