Search Results (733 total)

The STAR Collaboration at the Relativistic Heavy Ion Collider presents measurements of J/ψ→e⁺e^- at midrapidity and high transverse momentum (p_T>5 GeV/c) in p+p and central Cu+Cu collisions at sqrt[s_NN]=200 GeV. The inclusive J/ψ production cross section for Cu+Cu collisions is found to be consistent at high p_T with the binary collision-scaled cross section for p+p collisions. At a confidence level of 97%, this is in contrast to a suppression of J/ψ production observed at lower p_T. Azimuthal correlations of J/ψ with charged hadrons in p+p collisions provide an estimate of the contribution of B-hadron decays to J/ψ production of 13%±5%.

The results of midrapidity (0<y<0.8) neutral pion spectra over an extended transverse momentum range (1<p_T<12 GeV/c) in sqrt[s_NN]=200 GeV Au+Au collisions, measured by the STAR experiment, are presented. The neutral pions are reconstructed from photons measured either by the STAR Barrel Electro-Magnetic Calorimeter or by the Time Projection Chamber via tracking of conversion electron-positron pairs. Our measurements are compared to previously published π^± and π⁰ results. The nuclear modification factors R_CP and R_AA of π⁰ are also presented as a function of p_T. In the most central Au+Au collisions, the binary collision scaled π⁰ yield at high p_T is suppressed by a factor of about 5 compared to the expectation from the yield of p+p collisions. Such a large suppression is in agreement with previous observations for light quark mesons and is consistent with the scenario that partons suffer considerable energy loss in the dense medium formed in central nucleus-nucleus collisions at the Relativistic Heavy Ion Collider.

We demonstrate the dramatic effect of film thickness on the ferroelectric phase transition temperature T_c in strained BaTiO₃ films grown on SrTiO₃ substrates. Using variable-temperature ultraviolet Raman spectroscopy enables measuring T_c in films as thin as 1.6 nm, and a film thickness variation from 1.6 to 10 nm leads to T_c tuning from 70 to about 925 K. Raman data are consistent with synchrotron x-ray scattering results, which indicate the presence of 180° domains below T_c, and thermodynamic phase-field model calculations of T_c as a function of thickness.

Forward-backward multiplicity correlation strengths have been measured with the STAR detector for Au+Au and p+p collisions at sqrt[s_NN]=200  GeV. Strong short- and long-range correlations (LRC) are seen in central Au+Au collisions. The magnitude of these correlations decrease with decreasing centrality until only short-range correlations are observed in peripheral Au+Au collisions. Both the dual parton model (DPM) and the color glass condensate (CGC) predict the existence of the long-range correlations. In the DPM, the fluctuation in the number of elementary (parton) inelastic collisions produces the LRC. In the CGC, longitudinal color flux tubes generate the LRC. The data are in qualitative agreement with the predictions of the DPM and indicate the presence of multiple parton interactions.

Theoretical analysis to the defect mediated discrete solitons in one- and two-dimensional periodical waveguide lattices is presented. The waveguide arrays with these functional defects are assumed to respond to the light field as an optically induced photorefraction and they are patterned by a holographic technique. It is found that the spatial energy distributions of the solitary waves can be controlled by the defects in the waveguide arrays, and this gives rise to an additional freedom to externally shaping the light field distribution to a special shape.

We present an analysis of the shear-banding dynamics in a bulk metallic glass (BMG), including the temperature rise in the band, the sliding speed of the band, and the time elapsed as well as the step size of the shear offset growth in a stop-and-go cycle. This model analysis quantitatively demonstrates that the major shear band can remain cold and slide in a stick-slip manner. We predict that the shear step (distance covered by a stop-and-go cycle) scales with the sample size and machine stiffness. We also illustrate the conditions when such serrated shear is unsustainable and a hot shear band directly develops in a runaway instability (catastrophe). These findings provide physical insight into the shear-instability processes and offer useful information for improving the plasticity of BMGs. The calculation results are used to explain several intriguing recent experimental observations, including the stick slip of the dominant shear-band and the sample-size effects on the plastic-flow behavior of BMGs.

We describe ²⁷Al NMR experiments on Ba₈Al_xGe_46−x type-I clathrates coupled with ab initio computational studies. For x=16, calculated spectra determined by the ab initio results gave good agreement with the measurements, with best-fitting configurations also corresponding to the computed lowest-energy atomic arrangements. Analysis of the NMR results showed that a distribution of Knight shifts dominates the central portion of the line. Computational results demonstrate that this stems from the large variation of carrier density on different sites. Al-deficient samples with x=12 and 13 exhibited a split central NMR peak, signaling two main local environments for Al ions, which we connected to the presence of vacancies. Modeling of the wide-line spectrum for x=12 indicates a configuration with more Al on the 24k site than for x=16. The results indicate the importance of nonbonding hybrids adjacent to the vacancies in the electronic structure near E_F. We also address the static distortions from Pm3̅ n symmetry in these structures.

We study the role of qubit dephasing in cooling a mechanical resonator by quantum back-action. With a superconducting flux qubit as a specific example, we show that ground-state cooling of a mechanical resonator can only be realized if the qubit dephasing rate is sufficiently low.

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.

We present a resonant inelastic scattering (RIXS) study using of CuB₂O₄, which contains a lattice of CuO₄ plaquettes electronically isolated by B⁺³ ions. The observed Cu K-edge spectra show a small number of well-separated features, and the simple electronic structure of CuB₂O₄ allows us to explore RIXS phenomenology. We find a low-energy feature that cannot be attributed to the same charge-transfer excitation discussed in other cuprates and is likely a d→d transition thought to be forbidden under common considerations of K-edge RIXS.

Background: Nucleon-nucleon (NN) bremsstrahlung processes NNγ (nnγ, npγ, and ppγ) have been extensively investigated. Neutrino-pair bremsstrahlung processes from nucleon-nucleon scattering NNνν̅ (nnνν̅ , npνν̅ , and ppνν̅ ) have recently attracted attention in studies of neutrino emission in neutron stars. The calculated NNνν̅ cross sections (or emissivities) are found to be sensitive to the two-nucleon dynamical model used in the calculations. Purpose and Method: A realistic one-boson-exchange (ROBE) model for NN interactions is used to construct the electroweak bremsstrahlung amplitudes using the well-known nucleon electromagnetic and weak interaction vertices. The constructed nnγ and nnνν̅ amplitudes are investigated by applying them to calculate nnγ and nnνν̅ cross sections, respectively. Results: (i) The 190-MeV ROBE nnγ cross sections agree well with those calculated using the TuTts amplitude, but they are in disagreement with those calculated using the Low amplitude. (ii) The calculated nnνν̅ cross sections using the ROBE amplitude at the neutrino-pair energy ω = 1 MeV are in quantitative agreement with those calculated by Timmermans [Phys. Rev. C 65, 064007 (2002)], who used the leading-order term of the soft neutrino-pair bremsstrahlung amplitude. Conclusions: The nnγ amplitude in the ROBE approach, which obeys the soft-photon theorem, has a predictive power similar to that of the TuTts amplitude. The nnνν̅ amplitude in the ROBE approach, which is consistent with the soft neutrino-pair bremsstrahlung theorem, has a predictive power similar to that of the soft neutrino-pair bremsstrahlung amplitude of Timmermans in the low neutrino-pair energy region.

An approach to microwave measurements is used in order to determine both, the in-plane and out-of-plane conductivity of the high-T_c superconductor HgBa₂CuO_4+δ near optimal doping. Unlike the ab-plane conductivity, the c-axis conductivity is highly sensitive to superconducting fluctuations. From a single c-axis data set, we can clearly discern the opening of the pseudogap at T^∗=185(15) K, the appearance of the superconducting fluctuations at a much lower temperature T^′=105(2) K, and the full transition to the superconducting state at the critical temperature T_c=94.3 K. Thus, with the present high sensitivity, we establish that the extent of the superconducting fluctuations is only about 10 K above T_c.

The process ep→epπ⁰ has been measured at Q²=6.4 and 7.7 (GeV/c²)² in Jefferson Lab's Hall C. Unpolarized differential cross sections are reported in the virtual photon-proton center-of-mass frame considering the process γ^*p→pπ⁰. Various details relating to the background subtractions, radiative corrections, and systematic errors are discussed. The usefulness of the data with regard to the measurement of the electromagnetic properties of the well-known Δ(1232) resonance is covered in detail. Specifically considered are the electromagnetic and scalar-magnetic ratios R_EM and R_SM along with the magnetic transition form factor G_M^*. It is found that the rapid falloff of the Δ(1232) contribution continues into this region of momentum transfer and that other resonances may be making important contributions in this region.

This paper explores the physics of the electromagnetic diffraction radiation of a subwavelength holes array excited by a set of evanescent waves generated by a line charge of electron beam moving parallel to the array. Activated by a uniformly moving line charge, numerous physical phenomena occur such as the diffraction radiation on both sides of the array as well as the electromagnetic penetration or transmission below or above the cut-off through the holes. As a result the subwavelength holes array becomes a radiation array. Making use of the integral equation with relevant Green’s functions, an analytical theory for such a radiation system is built up. The results of the numerical calculations based on the theory agree well with that obtained by the computer simulation. The relation among the effective surface plasmon wave, the electromagnetic penetration or transmission of the holes and the diffraction radiation is revealed. The energy dependence of and the influence of the hole thickness on the diffraction radiation and the electromagnetic penetration or transmission are investigated in detail. Therefore, a distinct diffraction radiation phenomenon is discovered.

Precision saturation spectroscopy of the ⁸⁸Sr ¹S₀-³P₁ is performed in a vapor cell filled with various rare gases including He, Ne, Ar, and Xe. By continuously calibrating the absolute frequency of the probe laser, buffer-gas-induced collision shifts of ∼kHz are detected with gas pressure of 1–20 mTorr. Helium gave the largest fractional shift of 1.6×10⁻⁹ Torr⁻¹. Comparing with a simple impact calculation and a Doppler-limited experiment of Holtgrave and Wolf [J. C. Holtgrave and P. J. Wolf, Phys. Rev. A 72, 012711 (2005)], our results show larger broadening and smaller shifting coefficient, indicating effective atomic loss due to velocity-changing collisions. The applicability of the result to the ¹S₀-³P₀ optical lattice clock transition is also discussed.

Charge transfer processes due to collisions of ground-state C³⁺(1s²2s ²S) ions with He atoms are studied using the quantum-mechanical molecular-orbital close-coupling method for energies in the range 10⁻⁴–2×10³ eV/u. The ab initio adiabatic potentials and radial couplings utilized in the calculations are obtained from the multireference single- and double-excitation configuration interaction approach. Total and state-selective single-electron capture cross sections and rate coefficients are calculated and compared with the available experimental and theoretical data. A good agreement is found between the measured cross sections and the present calculations. However, the previous calculations of total rate coefficients using the Landau-Zener model are one to two orders of magnitude smaller than the present results.

We report K/π fluctuations from Au+Au collisions at sqrt[s_NN]=19.6, 62.4, 130, and 200 GeV using the STAR detector at the Relativistic Heavy Ion Collider. K/π fluctuations in central collisions show little dependence on incident energy and are on the same order as those from NA49 at the Super Proton Synchrotron in central Pb+Pb collisions at sqrt[s_NN]=12.3 and 17.3 GeV. We report results for the collision centrality dependence of K/π fluctuations and results for charge-separated fluctuations. We observe that the K/π fluctuations scale with the charged particle multiplicity density.

We present a systematic analysis of two-pion interferometry in Au+Au collisions at sqrt[s_NN]=62.4 GeV and Cu+Cu collisions at sqrt[s_NN]=62.4 and 200 GeV using the STAR detector at the Relativistic Heavy Ion Collider (RHIC). The multiplicity and transverse momentum dependences of the extracted correlation lengths (radii) are studied. The scaling with charged particle multiplicity of the apparent system volume at final interaction is studied for the RHIC energy domain. The multiplicity scaling of the measured correlation radii is found to be independent of colliding system and collision energy.

We have applied photoluminescence (PL) microscopy with scanning laser excitation wavelength for imaging and characterizing individual, millimeter-long, single-walled carbon nanotubes (SWNTs) grown by chemical-vapor deposition on structured Si/SiO₂ substrates. Trenches etched into the substrates allowed a direct comparison of the PL properties of air-suspended (across the trenches) and on-SiO₂ segments of the same semiconducting nanotubes. For the on-SiO₂ segments, we found an ∼10–20-fold decrease in PL intensity and redshifts of the emission and excitation transitions by 7–27 meV and 5–24 meV, respectively, compared to air-suspended regions of the same SWNTs. Furthermore, PL imaging was applied to SWNTs fractured by dragging an atomic force microscope tip across on-SiO₂ segments. Strong, localized changes in the emission properties were observed. These included the appearance of PL blinking at the fracture site and evidence for residual axial and to a lesser extent torsional strain extending tens of microns away from the fracture site. We also discuss how PL measurements of torsional strain can be used to determine the handedness of a luminescent nanotube.

The Advanced Photon Source (APS) injector complex includes an option for rf photocathode (PC) gun beam injection into the 450-MeV S-band linac. At the 150-MeV point, a four-dipole chicane was used to compress the micropulse bunch length from a few ps to sub-0.5 ps (FWHM). Noticeable enhancements of the optical transition radiation (OTR) signal sampled after the APS chicane were then observed as has been reported in the Linac Coherent Light Source (LCLS) injector commissioning. A far-infrared (FIR) coherent transition radiation detector and interferometer were used to monitor the bunch compression process and correlate the appearance of localized spikes of OTR signal (5 to 10 times brighter than adjacent areas) within the beam-image footprint. We have performed spectral-dependency measurements at 375 MeV with a series of bandpass filters centered in 50-nm increments from 400 to 700 nm and with an imaging spectrometer and observed a broadband enhancement in these spikes. Mitigation concepts of the observed coherent OTR, which exhibits an intensity enhancement in the red part of the visible spectrum as compared to incoherent OTR, are described.

We demonstrate experimentally the existence of two stability regimes of Ag nanoislands grown on a Si(111)-(4×1)-In surface: a conventional regime at low temperature where only one island shape is stable, and an unconventional regime at room temperature (RT) where isotropic compact islands coexist with anisotropic elongated ones. First-principles calculations show the unusual bistability at RT arises from the fact that the Ag nanoislands are under anisotropic stress, supporting a recent theoretical prediction by Zandvliet and van Gastel [Phys. Rev. Lett. 99, 136103 (2007)].

Ferroelectric 180° domain walls are well-known to be predominantly Ising-like. Using density functional theory, and molecular dynamics simulations, the 180° domain walls in prototypical ferroelectrics lead titanate (PbTiO₃) and lithium niobate (LiNbO₃) are shown to have mixed character; while predominantly Ising-like, they also manifest some Bloch- and Néel-like character. Phase-field calculations show that such mixed wall character can be dramatically enhanced in nanoscale thin film heterostructures such as BaTiO₃/SrTiO₃, where the internal wall structure can form polarization vortices. Such mixed character walls can be expected to exhibit dynamical wall properties distinct from pure Ising walls.

We investigate the effects of transition metals (TM) on the electronic doping and scattering in graphene using molecular-beam epitaxy combined with in situ transport measurements. The room-temperature deposition of TM onto graphene produces clusters that dope n type for all TM investigated (Ti, Fe, and Pt). We also find that the scattering by TM clusters exhibits different behavior compared to 1/r Coulomb scattering. At high coverage, Pt films are able to produce doping that is either n type or weakly p type, which provides experimental evidence for a strong interfacial dipole favoring n-type doping as predicted theoretically.