Ital Dx Research Articles

Microscopic Structure ofDXCenters in Cd0.8Zn0.2Te:Cl

Photoexcitation of chlorine {ital DX} centers induces a transition of the Cl atoms to the shallow-donor state and persistent photoconductivity at low temperature in Cd{sub 0.8 }Zn{sub 0.2}Te:Cl. The relaxation of the substitutional Cl atoms to the {ital DX} state at 140K is coincident with a decrease of the positron line-shape parameter and an increase of annihilation with high-momentum core electrons. The results indicate positron trapping and annihilation at {ital DX} centers and at chlorine {ital A} centers. The data support the bond breaking model of the {ital DX} centers and the outward relaxation of the Cl and Cd(Zn) atoms along the [111] direction. The thermal barrier for the shallow-deep transition was found to be 0.44eV. {copyright} {ital 1997} {ital The American Physical Society}

Pressure Induced Deep Gap State of Oxygen in GaN

O and Si donors in GaN are studied by Raman spectroscopy under hydrostatic pressure p. The ground state of O is found to transfer from a shallow level to a deep gap state at p{gt}20 GPa reminiscent of {ital DX} centers in GaAs. Transferred to Al{sub x}Ga {sub 1-x}N we predict that O induces a deep gap state for x{gt}0.40. In GaN:Si no such state is induced up to the highest pressure obtained (p=25 GPa ) equivalent to x=0.56 in Al{sub x}Ga {sub 1-x}N and possibly higher. We attribute this distinction to the lattice sites of the dopants. O substituting for N is found to be the origin of high free electron concentration in bulk GaN crystals. {copyright} {ital 1997} {ital The American Physical Society}

Thermal spike effects in low-energy single-ion impacts on graphite.

Monte Carlo simulations have been used to obtain three-dimensional distributions of cascade defects and energy deposition due to single-ion impacts in graphite. This energy deposition profile serves as the starting point for the formation and evolution of the thermal spike. In this case the minimum deposited energy density per target atom for the spike formation is the order of the atomic binding energy. An effective spike created by a single-ion impact in the near-surface region is introduced to account for the bump formation on the highly oriented pyrolytic graphite surface. A linear relationship between the bump volume and the effective spike energy is obtained based on the thermal spike model, which agrees well with the scanning tunnel microscope results. It is suggested that the one-dimensional energy deposition rate {ital dE}/{ital dx} is not suitable to describe spike effects. On the other hand, a three-dimensional parameter corresponding to the nature of the spike formation should be used. {copyright} {ital 1996 The American Physical Society.}

Electron Paramagnetic Resonance Study of Se-Doped AlSb: Evidence for Negative-U of the DX Center

A metastable electron-paramagnetic-resonance (EPR) signal is observed in Se-doped AlSb. When cooled in the dark, the sample is nonparamagnetic. After illumination with white light from a tungsten source a strong, persistent EPR signal with an isotropic {ital g} factor of 1.949 is observed. The metastability of the signal is consistent with earlier work done on {ital DX}-like centers. The absence of any EPR when cooled in the dark is direct evidence for the negative-{ital U} model. The EPR arises from the effective-mass state of the defect, which is not filled at thermal equilibrium. A detailed analysis of the line shape reveals that the linewidth is determined by hyperfine interactions. The extent of the wave function is found to be comparable to the predicted value of the effective-mass state.

Electron-paramagnetic-resonance study of Se-doped AlSb: Evidence for negative U of the DX center.

A metastable electron-paramagnetic-resonance (EPR) signal is observed in Se-doped AlSb. When cooled in the dark, the sample is nonparamagnetic. After illumination with white light from a tungsten source a strong, persistent EPR signal with an isotropic {ital g} factor of 1.949 is observed. The metastability of the signal is consistent with earlier work done on {ital DX}-like centers. The absence of any EPR when cooled in the dark is direct evidence for the negative-{ital U} model. The EPR arises from the effective-mass state of the defect, which is not filled at thermal equilibrium. A detailed analysis of the line shape reveals that the linewidth is determined by hyperfine interactions. The extent of the wave function is found to be comparable to the predicted value of the effective-mass state.

Dispersion relation and the dieletric tensor for magnetized plasmas with inhomogeneous magnetic field.

We investigate the dispersion relation for a magnetized plasma with weak magnetic field gradients perpendicular to the ambient magnetic field. An explicit expression for the effective dielectric tensor is derived, incorporating the relevant contributions due to the inhomogeneity, which include corrections to all orders in the small parameter [epsilon], where [epsilon]=[(1/[ital B][sub 0])([ital dB][sub 0]/[ital dx])]. It is shown that this effective dielectric tensor satisfies the required symmetry conditions and is the tensor which should be utilized in the dispersion relation, in order to describe correctly wave-particle interactions in media with inhomogeneous magnetic field. The case of high frequency oscillations propagating perpendicularly to the magnetic field in a Maxwellian plasma is considered as an example and the effect of inhomogeneities in the magnetic field upon the absorption coefficient and the optical depth of ordinary mode waves is discussed. A region of negative absorption coefficient is predicted near the electron cyclotron frequency for sufficiently high inhomogeneity. Moreoever, it is shown that significant differences may exist between the absorption coefficient evaluated with the present formulation and results from other approaches found in the literature which do not exhibit correct symmetry properties.

Precision Measurement of the Proton Spin Structure Functiong1p

We have measured the ratio [ital g][sup [ital p]][sub 1]/[ital F][sup [ital p]][sub 1] over the range 0.029[lt][ital x][lt]0.8 and 1.3[lt][ital Q][sup 2][lt]10 (GeV/[ital c])[sup 2] using deep-inelastic scattering of polarized electrons from polarized ammonia. An evaluation of the integral [integral][ital g][sup [ital p]][sub 1]([ital x],[ital Q][sup 2])[ital dx] at fixed [ital Q][sup 2]=3 (GeV/[ital c])[sup 2] yields 0.127[plus minus]0.004(stat)[plus minus]0.010(syst), in agreement with previous experiments, but well below the Ellis-Jaffe sum rule prediction of 0.160[plus minus]0.006. In the quark-parton model, this implies [Delta][ital q]=0.27[plus minus]0.10.

Nuclear dependence of neutral-D-meson production by 800 GeV/c protons.

The nuclear dependence for 800 GeV/[ital c] proton production of neutron [ital D] mesons has been measured near [ital x][sub [ital F]]=0 in Experiment 789 at Fermilab. [ital D] mesons from beryllium and gold targets were detected with a pair spectrometer and a silicon vertex detector via their decay [ital D][r arrow][ital K][pi]. No nuclear dependence is found, with a measured [alpha]=1.02[plus minus]0.03[plus minus]0.02. The measured differential cross section, [ital d][sigma]/[ital dx][sub [ital F]], for neutral-[ital D]-meson production at [l angle][ital x][sub [ital F]][r angle]=0.031 is 58[plus minus]3[plus minus]7 [mu]b/nucleon. The integrated cross section obtained by extrapolation of the measured cross section to all [ital x][sub [ital F]] is 17.7[plus minus]0.9[plus minus]3.4 [mu]b/nucleon and is consistent with previous measurements.

Psi production in p-barN and pi -N interactions at 125 GeV/c and a determination of the gluon structure functions of the p-bar and the pi -

We have measured the cross section for production of [psi] and [psi][prime] in [ital [bar p]] and [pi][sup [minus]] interactions with Be, Cu, and W targets in experiment E537 at Fermilab. The measurements were performed at 125 GeV/[ital c] using a forward dimuon spectrometer in a closed geometry configuration. The gluon structure functions of the [ital [bar p]] and [pi][sup [minus]] have been extracted from the measured [ital d][sigma]/[ital dx][sub [ital F]] spectra of the produced [psi]'s. From the [ital [bar p]]W data we obtain, for [ital [bar p]], [ital xG]([ital x])=(2.15[plus minus]0.7)[1[minus][ital x]][sup (6.83[plus minus]0.5)] [1+(5.85[plus minus]0.95)[ital x]]. In the [pi][sup [minus]] case, we obtain, from the W and the Be data separately, [ital xG]([ital x])=(1.49[plus minus]0.03)[1[minus][ital x]][sup (1.98[plus minus]0.06)] (for [pi][sup [minus]]W), [ital xG]([ital x])=(1.10[plus minus]0.10)[1[minus][ital x]][sup (1.20[plus minus]0.20)] (for [pi][sup [minus]]Be).

What can we learn from the recent measurements of the spin-dependent structure function g1(x)?

In this paper, we argue that the effects of the Drell-Hearn-Gerasimov (DHG) sum rule should be included to extract the sum rule of polarized neutrons from the recent Spin Muon Collaboration measurement of the spin-dependent structure function [ital g][sub 1]([ital x]) of the deuteron, since the measurement was obtained at fairly low [ital Q][sup 2]. The value [Gamma][equivalent to][integral][sub 0][sup 1][ital g][sub 1]([ital x])[ital dx] could be modified significantly at [ital Q][sup 2] around 2--4 GeV[sup 2] by the high twist effects connected with the DHG sum rule. More accurate data for the spin-dependent structure function in the resonance region are needed to obtain reliable information on [Gamma] for the polarized protons and neutrons.

Limit on the d-bar/u-bar asymmetry of the nucleon sea from Drell-Yan production.

We present an analysis of 800-GeV proton-induced Drell-Yan production data from isoscalar targets {sup 2}H and C, and from W, which has a large neutron excess. The ratio of cross sections per nucleon, {ital R}{minus}{sigma}{sub W}/{sigma}{sub IS}, is sensitive to the difference between the {ital {bar d}}({ital x}) and {ital {bar u}}({ital x}) structure functions of the proton. We find that {ital R} is close to unity in the range 0.04{le}{ital x}{le}0.27, allowing upper limits to be set on the {ital {bar d}}-{ital {bar u}} asymmetry. Additionally, the shape of the differential cross section {ital m}{sup 3} {ital d}{sup 2}{sigma}/{ital dx}{sub {ital F}} {ital dm} for {sup 2}H at {ital x}{sub {ital F}}{approx}0 shows no evidence of an asymmetric sea in the proton. We examine the implications of these data for various models of the violation of the Gottfried sum rule in deep-inelastic lepton scattering.

Is the qq-bar sea in the nucleon isospin symmetric?

The New Muon Collaboration (NMC) has recently performed a detailed analysis of the difference between the proton and neutron structure functions. They found that the Gottfried sum {integral}{ital dx}({ital F}{sub 2}{sup {ital p}}({ital x}){minus}{ital F}{sub 2}{sup {ital n}}({ital x}))/{ital x} is significantly below the simple quark-parton model prediction of 1/3, which indicates a sizable isospin asymmetry of the {ital q{bar q}} sea in the nucleon. We have carried out a theoretical analysis of the Gottfried sum on the basis of a chiral-quark-soliton model, and obtained a satisfactory agreement with the NMC measurement.

Quantum holonomy and link invariants.

It is shown that, in a non-Abelian quantum field theory without an anomaly and broken symmetry, the set of all matrix-valued quantum holonomies $\ensuremath{\Psi}[\ensuremath{\gamma}]\ensuremath{\equiv}〈P\mathrm{exp}(i\ensuremath{\oint}\ensuremath{\int}{\ensuremath{\gamma}}^{}\mathrm{Adx})〉$ for closed contours $\ensuremath{\gamma}$ form a commutative semigroup, whereas $〈P\mathrm{exp}(i\ensuremath{\int}{\ensuremath{\alpha}}^{}\mathrm{Adx})〉=0$ for every open path $\ensuremath{\alpha}$. The eigenvalues $\ensuremath{\Phi}[\ensuremath{\gamma}]$ of $\ensuremath{\Psi}[\ensuremath{\gamma}]$ are classified according to the irreducible representations of the gauge group. In an irreducible representation $\ensuremath{\rho}$, $\mathrm{Tr}(\ensuremath{\Psi}[\ensuremath{\gamma}])=\ensuremath{\Phi}[\ensuremath{\gamma}]\mathrm{Tr}({1}_{\ensuremath{\rho}})$ is a Wilson loop. This equation solves a puzzle in the relation between link invariants and Wilson loops in the Chern-Simons theory in three dimensions when the gauge group is $\mathrm{SU}(N|N)$, and provides useful insight in understanding nonperturbative quantum chromodynamics as a string theory.

Substitutionality of Te- and Sn-related DX centers in AlxGa1-xAs.

The lattice locations of Te and Sn atoms forming {ital DX} centers in Al{sub {ital x}}Ga{sub 1{minus}{ital x}}As were determined by particle-induced x-ray emission and ion-beam-channeling methods. The Te atoms were found to be in the As substitutional sites while the Sn atoms were in the Ga(Al) sites. No off-center displacement of Te and Sn larger than 0.14 A from the substitutional sites was observed in either system.

Magnetic studies of persistent photoconductivity in n-AlxGa1-xAs.

Low-temperature magnetic susceptibility measurements were made during illumination of {ital n}-Al{sub 2}Ga{sub 1{minus}{ital x}}As with {ital DX} centers to investigate the microscopic origin of persistent photoconductivity. Experiments using a dc SQUID magneto-optic microsusceptometer revealed the change of electronic susceptibility as a function of illumination time and temperature. The results indicate that the {ital DX} center is a paramagnetic donor with one unpaired electron.

Lattice relaxation of DX-like donors in ZnxCd1-xTe.

The configuration-coordinate diagram was constructed for deep relaxed In impurities in Zn{sub {ital x}}Cd{sub 1{minus}{ital x}}Te. On the basis of the Hall effect, deep-level transient spectroscopy, photoionization, and photoluminescence measurements it was shown that In donors in Zn{sub {ital x}}Cd{sub 1{minus}{ital x}}Te possess analogous properties to the so-called {ital DX} center in Al{sub {ital x}}Ga{sub 1{minus}{ital x}}As. The absence of paramagnetic resonance from In donors suggests that In atoms form a mixture of In{sup +} and In{sup 3+} ions, in equal numbers. A persistent photo-EPR signal from mobile photoelectrons was observed after illumination at {ital g}=1.316 and could be thermally annealed. A strong broad, donor-acceptor-pair luminescence was found, with periodically spaced phonon lines at high excitation powers.