Search Results (38 total)

Target ionization and projectile charge changing were investigated for 20–500 keV/u C^q+, O^q++He (q=1–3) collisions. Double- to single-ionization ratios R₂₁ of helium associated with no projectile charge change (direct ionization), single-electron capture, and single-electron loss were measured. The cross-section ratio R₂₁ depends strongly on the collision velocity v, the projectile charge state q, and the outgoing reaction channel. Meanwhile, a model extended from our previous work [J. X. Shao, X. M. Chen, and B. W. Ding, Phys. Rev. A 75, 012701 (2007)] is presented to interpret the above-mentioned dependences. Good agreement is found between the model and the experimental data.

Inclusive electron-proton and electron-deuteron inelastic cross sections have been measured at Jefferson Lab (JLab) in the resonance region, at large Bjorken x, up to 0.92, and four-momentum transfer squared Q² up to 7.5 GeV² in the experiment E00-116. These measurements are used to extend to larger x and Q² precision, quantitative, studies of the phenomenon of quark-hadron duality. Our analysis confirms, both globally and locally, the apparent “violation” of quark-hadron duality previously observed at a Q² of 3.5 GeV² when resonance data are compared to structure function data created from CTEQ6M and MRST2004 parton distribution functions (PDFs). More importantly, our new data show that this discrepancy saturates by Q²~4 GeV², becoming Q² independent. This suggests only small violations of Q² evolution by contributions from the higher-twist terms in the resonance region that is confirmed by our comparisons to ALEKHIN and ALLM97. We conclude that the unconstrained strength of the CTEQ6M and MRST2004 PDFs at large x is the major source of the disagreement between data and these parametrizations in the kinematic regime we study and that, in view of quark-hadron duality, properly averaged resonance region data could be used in global quantum chromodynamics fits to reduce PDF uncertainties at large x.

The differential cross section for the process p(e,e^'p)η has been measured at Q²~5.7 and 7.0(GeV/c)² for center-of-mass energies from threshold to 1.8 GeV, encompassing the S₁₁(1535) resonance, which dominates the channel. This is the highest momentum-transfer measurement of this exclusive process to date. The helicity-conserving transition amplitude A_1/2, for the production of the S₁₁(1535) resonance, is extracted from the data. Within the limited Q² now measured, this quantity appears to begin scaling as Q^-3—a predicted, but not definitive, signal of the dominance of perturbative QCD at Q²~5 (GeV/c)².

We use spin-transfer-driven ferromagnetic resonance (ST-FMR) to measure the spin-transfer torque vector τ in MgO-based magnetic tunnel junctions as a function of the offset angle between the magnetic moments of the electrodes and as a function of bias, V. We explain the conflicting conclusions of two previous experiments by accounting for additional terms that contribute to the ST-FMR signal at large |V|. Including the additional terms gives us improved precision in the determination of τ(V), allowing us to distinguish among competing predictions. We determine that the in-plane component of dτ/dV has a weak but nonzero dependence on bias, varying by 30%–35% over the bias range where the measurements are accurate, and that the perpendicular component can be large enough to be technologically significant. We also make comparisons to other experimental techniques that have been used to try to measure τ(V).

Spin-polarized surface electronic states in Ni(111) have been examined using scanning tunneling microscopy (STM) and spectroscopy (STS) combined with high-resolution angle-resolved photoemission spectroscopy (HR-ARPES). Standing waves derived from the majority-spin Shockley surface state (SS) have been observed in the STM and dI/dV images. The fast Fourier transform (FFT)-dI/dV image at a different sample bias exhibited a circular contour in the reciprocal space. The radius of the FFT-dI/dV image was in agreement with that of the corresponding constant-energy contour given by the HR-ARPES. The majority-spin Shockley SS is partially occupied and disperses upward, crossing the Fermi level (E_F) at a wave number of k_F=0.081±0.005 Å⁻¹. The effective mass (m^∗) with respect to the free-electron mass (m_e) of the majority-spin Shockley SS was evaluated to be m^∗/m_e=0.19±0.03. The STS spectrum indicated a pair of the Shockley SS below and above E_F with an exchange splitting of ∼190 meV. By the line-shape analyses of the HR-ARPES spectrum, the lifetime broadening at the Γ̅ point was calculated to be 53.6 meV, which agrees well with the width (49 meV) of the steplike structure in the STS spectrum. The results from the STM/STS and HR-ARPES experiments were found to be mutually consistent.

The transmission of 18-keV O⁻ ions through Al₂O₃ nanocapillaries with 50 nm in diameter and 12 μm in length is studied in this work. By measuring angular distribution of transmitted particles when capillaries were tilted with respect to incident ion beam, two peaks were observed. It is distinguished that one of them is composed by direct transmitted ions and another one is composed by scattered ions. A phenomenon referred to as guiding effect, as found for highly charged ions and low-energy electrons, was observed. When negative ions (18-keV O⁻) are transmitted through nanocapillaries, most of them were ionized to neutral atoms and even positive ions. The intensity of transmitted particles (O⁻, O⁰, and O⁺) decreased when the tilt angle increased. In transmitted particles, the fraction of O⁻ declined but that of O⁰ and O⁺ ions grew when the tilt angle grew. Both elastic collision and electrostatic scattering were found in scattered ions.

Cosmic strings can be created in the early universe during symmetry-breaking phase transitions, such as might arise if the gauge structure of the standard model is extended by additional U(1) factors at high energies. Cosmic strings presented in the early universe form a network of long horizon-length segments, as well as a population of closed string loops. The closed loops are unstable against decay, and can be a source of nonthermal particle production. In this work we compute the density of weakly-interacting massive particle dark matter formed by the decay of gauge theory cosmic string loops derived from a network of long strings in the scaling regime or under the influence of frictional forces. We find that for symmetry-breaking scales larger than 10¹⁰  GeV, this mechanism has the potential to account for the observed relic density of dark matter. For symmetry-breaking scales lower than this, the density of dark matter created by loop decays from a scaling string network lies below the observed value. In particular, the cosmic strings originating from a U(1) gauge symmetry broken near the electroweak scale, that could lead to a massive Z^′ gauge boson observable at the LHC, produces a negligibly small dark matter relic density by this mechanism.

Valence-band electronic structure of Co₂MnSi has been revealed by photoelectron spectroscopy utilizing hard x-ray synchrotron radiation. We have obtained a good correspondence between experimental valence-band spectra and theoretical density of states calculated with generalized gradient aprroximation. However, no distinct temperature dependence has been observed for the experimental valence band, which requires reexamination of the explanation based on the dynamical mean-field theory for the rapid decrease in tunneling magnetoresistance ratio with increasing temperature.

Core-level photoemission spectra of Fe_3−xV_xSi alloys with inequivalent Fe_I and Fe_II sites are investigated via hard x-ray photoemission spectroscopy over the entire doping range x=0–1. All the measured 1s core-level peaks are found to shift to higher binding energy with increasing V concentration. First-principles, all electron charge- and spin-self-consistent electronic structure computations within the framework of the local-spin-density approximation are used to interpret the experimental results. The measured size of energy shift in going from x=0 to 1 is consistent with the corresponding theoretical value for the Fe_II and Si 1s core levels, whereas for the Fe_I and V core levels the computed shifts are generally larger than the experimental values. We ascribe these discrepancies to the effects of the core-hole screening in the final state which are not accounted for in the computations. In a rigid-band model the chemical potential and the core-level binding energies are expected to decrease with V doping as electrons are depleted from the Fermi energy. The observed increase in the binding energy of core levels thus supports a picture of the electronic structure where V doping induces a “pseudogap” or a region of reduced density of states in the vicinity of the Fermi energy.

Photoproduction of beauty quarks in events with two jets and an electron associated with one of the jets has been studied with the ZEUS detector at HERA using an integrated luminosity of 120  pb^-1. The fractions of events containing b quarks, and also of events containing c quarks, were extracted from a likelihood fit using variables sensitive to electron identification as well as to semileptonic decays. Total and differential cross sections for beauty and charm production were measured and compared with next-to-leading-order QCD calculations and Monte Carlo models.

Inclusive K_S⁰K_S⁰ production in ep collisions at the DESY ep collider HERA was studied with the ZEUS detector using an integrated luminosity of 0.5  fb^-1. Enhancements in the mass spectrum were observed and are attributed to the production of f₂(1270)/a₂⁰(1320), f₂^′(1525) and f₀(1710). Masses and widths were obtained using a fit which takes into account theoretical predictions based on SU(3) symmetry arguments, and are consistent with the Particle Data Group values. The f₀(1710) state, which has a mass consistent with a glueball candidate, was observed with a statistical significance of 5 standard deviations. However, if this state is the same as that seen in γγ→K_S⁰K_S⁰, it is unlikely to be a pure glueball state.

Jet cross sections were measured in charged-current deep inelastic e^±p scattering at high boson virtualities Q² with the ZEUS detector at HERA II using an integrated luminosity of 0.36  fb^-1. Differential cross sections are presented for inclusive-jet production as functions of Q², Bjorken x and the jet transverse energy and pseudorapidity. The dijet invariant mass cross section is also presented. Observation of three- and four-jet events in charged-current e^±p processes is reported for the first time. The predictions of next-to-leading-order (NLO) QCD calculations are compared to the measurements. The measured inclusive-jet cross sections are well described in shape and normalization by the NLO predictions. The data have the potential to constrain the u and d valence-quark distributions in the proton if included as input to global fits.

We measure the microwave signals produced by spin-torque-driven magnetization dynamics excited by direct currents in patterned magnetic multilayer devices at room temperature as a function of the angle of a magnetic field applied in the sample plane. We find strong variations in the frequency linewidth of the signals, with a decrease by more than a factor of 20 as the field is rotated from the magnetic easy axis to the in-plane hard axis. Based on micromagnetic simulations, we identify these variations as due to a transition from spatially incoherent to coherent precession.

The torque generated by the transfer of spin angular momentum from a spin-polarized current to a nanoscale ferromagnet can switch the orientation of the nanomagnet much more efficiently than a current-generated magnetic field and is therefore in development for use in next-generation magnetic random access memory (MRAM). Up to now, experiments have focused on spin-torque switching driven by simple square-wave current pulses. Here we present measurements showing that spin transfer from a microwave-frequency current pulse can produce a resonant excitation of a nanomagnet and improved switching characteristics in combination with a square current pulse. With the assistance of a microwave-frequency pulse, the switching time is reduced and achieves a narrower distribution than when driven by a square current pulse alone, and this can permit significant reductions in the integrated power required for switching. Resonantly excited switching may also enable alternative, more compact MRAM circuit architectures.

A symmetrical E-shaped metamaterial is investigated in this paper. Numerical simulations disclose the two electromagnetic resonances and the superwide negative-refractive band of this structure. The distributions of the induced current in the E-shaped copper wires show that these two electromagnetic resonances originate from the current flowing in the different C-shaped rings. The retrieved negative-refractive band is superwide, and the negative refraction with high transmission occurs in the different bands. The metamaterial with two or even more electromagnetic resonances offers an opportunity for the realization of wide-band negative refraction.

Flat directions are a generic feature of the scalar potential in supersymmetric gauge field theories. They can arise, for example, from D-terms associated with an extra Abelian gauge symmetry. Even when supersymmetry is broken softly, there often remain directions in the scalar field space along which the potential is almost flat. Upon breaking a gauge symmetry along one of these almost-flat directions, cosmic strings may form. Relative to the standard cosmic string picture based on the Abelian Higgs model, these flat-direction cosmic strings have the extreme type-I properties of a thin gauge core surrounded by a much wider scalar field profile. We perform a comprehensive study of the microscopic, macroscopic, and observational characteristics of this class of strings. We find many differences from the standard string scenario, including stable higher winding-mode strings, the dynamical formation of higher mode strings from lower ones, and a resultant multitension scaling string network in the early universe. These strings are only moderately constrained by current observations, and their gravitational wave signatures may be detectable at future gravity wave detectors. Furthermore, there is the interesting but speculative prospect that the decays of cosmic string loops in the early universe could be a source of ultrahigh-energy cosmic rays or nonthermal dark matter. We also compare the observational signatures of flat-direction cosmic strings with those of ordinary cosmic strings as well as (p,q) cosmic strings motivated by superstring theory.

The cross section for high-E_T dijet production in photoproduction has been measured with the ZEUS detector at HERA using an integrated luminosity of 81.8  pb^-1. The events were required to have a virtuality of the incoming photon, Q², of less than 1  GeV² and a photon-proton center-of-mass energy in the range 142<W_γp<293  GeV. Events were selected if at least two jets satisfied the transverse-energy requirements of E_T^jet1>20  GeV and E_T^jet2>15  GeV and pseudorapidity (with respect to the proton beam direction) requirements of -1<η^jet1,2<3, with at least one of the jets satisfying -1<η^jet<2.5. The measurements show sensitivity to the parton distributions in the photon and proton and to effects beyond next-to-leading order in QCD. Hence these data can be used to constrain further the parton densities in the proton and photon.

The current-voltage (I-V) characteristics of various MgB₂ films have been studied at different magnetic fields parallel to the c axis. At fields μ₀H between 0 and 5  T, vortex liquid-glass transitions were found in the I-V isotherms. Consistently, the I-V curves measured at different temperatures show a scaling behavior in the framework of quasi-two-dimension (quasi-2D) vortex-glass theory. However, at μ₀H≥5  T, a finite dissipation was observed down to the lowest temperature here, T=1.7  K, and the I-V isotherms did not scale in terms of any known scaling law, of any dimensionality. We suggest that this may be caused by a mixture of σ band vortices and π band quasiparticles. Interestingly, the I-V curves at zero magnetic field can still be scaled according to the quasi-2D vortex-glass formalism, indicating an equivalent effect of self-field due to persistent current and applied magnetic field.

The collisions of the isocharged sequence ions of q=6 (C⁶⁺, N⁶⁺, O⁶⁺, F⁶⁺, Ne⁶⁺, Ar⁶⁺, and Ca⁶⁺), q=7 (F⁷⁺, Ne⁷⁺, S⁷⁺, Ar⁷⁺, and Ca⁷⁺), q=8 (F⁸⁺, Ne⁸⁺, Ar⁸⁺, and Ca⁸⁺), q=9 (F⁹⁺, Ne⁹⁺, Si⁹⁺, S⁹⁺, Ar⁹⁺, and Ca⁹⁺) and q=11 (Si¹¹⁺, Ar¹¹⁺, and Ca¹¹⁺) with helium at the same velocities were investigated. The cross-section ratios of the double-electron transfer (DET) to the single-electron capture (SEC) σ^DET∕σ^SEC and the true double-electron capture (TDC) to the double-electron transfer σ^TDC∕σ^DET were measured. It shows that for different ions in an isocharged sequence, the experimental cross-section ratio σ^DET∕σ^SEC varies by a factor of 3. The results confirm that the projectile core is another dominant factor besides the charge state and the collision velocity in slow (0.35−0.49v₀; v₀ denotes the Bohr velocity) highly charged ions (HCIs) with helium collisions. The experimental cross-section ratio σ^DET∕σ^SEC is compared with the extended classical over-barrier model (ECBM) [A. Bárány , Nucl. Instrum. Methods Phys. Res. B 9, 397 (1985)], the molecular Coulombic barrier model (MCBM) [A. Niehaus, J. Phys. B 19, 2925 (1986)], and the semiempirical scaling laws (SSL) [N. Selberg , Phys. Rev. A 54, 4127 (1996)]. It also shows that the projectile core properties affect the initial capture probabilities as well as the subsequent relaxation of the projectiles. The experimental cross-section ratio σ^TDC∕σ^DET for those lower isocharged sequences is dramatically affected by the projectile core structure, while for those sufficiently highly isocharged sequences, the autoionization always dominates, hence the cross-section ratio σ^TDC∕σ^DET is always small.

A large data set of charged-pion (π^±) electroproduction from both hydrogen and deuterium targets has been obtained spanning the low-energy residual-mass region. These data conclusively show the onset of the quark-hadron duality phenomenon, as predicted for high-energy hadron electroproduction. We construct several ratios from these data to exhibit the relation of this phenomenon to the high-energy factorization ansatz of electron-quark scattering and subsequent quark→pion production mechanisms.

Matter fields in the minimal supersymmetric standard model are chiral supermultiplets in fundamental (or singlet) representations of the standard model gauge group. In this paper we introduce chiral superfields in the adjoint representation of SU(3)_C and study the effective field theory and phenomenology of them. These states are well motivated by intersecting D-brane models in which additional massless adjoint chiral supermultiplets appear generically in the low-energy spectrum. Although it has been pointed out that the existence of these additional fields may make it difficult to obtain asymptotic freedom, we demonstrate that this consideration does not rule out the existence of adjoints. The QCD gauge coupling can be perturbative up to a sufficiently high scale, and therefore a perturbative description for a D-brane model is valid. The full supersymmetric and soft SUSY breaking Lagrangians and the resulting renormalization group equations are given. Phenomenological aspects of the adjoint matter are also studied, including the decay and production processes. The similarity in gauge interaction between the adjoint fermion and gluino facilitates our study on these aspects. It is found that these adjoint multiplets can give detectable signals at colliders and satisfy the constraints from cosmology.

Studies of the dynamics of longitudinal space-charge waves in space-charge dominated beams propagating through a transport channel with a long solenoid are performed at the University of Maryland. In this paper, we report some experimental results on the energy modulations converted from density modulations. By changing the working conditions of the electron gun, pure initial density modulations are generated. Energy perturbation waveforms are measured with a high-resolution energy analyzer. The experimental results are compared with both the linear theory and the simulation results. Good agreements are achieved for the relationship between the energy and current perturbation strengths.

Using the color-magnetic interaction Hamiltonian with SU(3) flavor symmetry breaking, we perform a schematic study of the masses of the J^P=1⁺ tetraquarks in the antidecuplet representation. After diagonalizing the mass matrix, we find the uds̅ s̅ tetraquark could lie as low as about 1350 MeV. It decays into K⁺K⁰π⁰, K⁺K⁺π^-, K⁰K⁰π⁺ via P-wave. The dual suppression from the not-so-big three-body phase space and P-wave decay barrier may render this exotic state rather narrow. Future experimental exclusion of this state will cast doubt on the validity of applying the simple color-magnetic Hamiltonian to the multiquark system.

The low-frequency (10²–10⁵ Hz) dielectric properties and dc resistivity of oxygen-doped La₂CuO_4+y ceramic samples were investigated as a function of temperature (23–300 K). The as-prepared sample featuring phase separation shows very high dielectric constant ε^′>10⁵ above 150 K, which was found to originate from the hopping motion of localized holes. Below 150 K, the relaxation process due to the condensation of the holes can be observed in the dielectric spectra. When the sample was annealed at different temperatures in reduced atmosphere to tune the oxygen content, the phase separation disappears accompanied by the absence of the relaxation in the annealed sample. Our results give strong support to the phase separation model and evidence the inhomogeneous distribution of holes in the oxygen-doped La₂CuO_4+y ceramic samples investigated.