Search Results (177 total)

We study the rectification properties of geometrically asymmetric metal-vacuum-metal junctions. In particular, we focus on systems in which the cathode metal supports a hemispherical protrusion. By using a transfer-matrix methodology to take account of three-dimensional aspects of the problem, we compute the forward and backward currents that flow in this device when it is subject to positive or negative external biases. These currents enable the calculation of the rectification properties of the device in the limit of quasistatic fields. We also determine the power this device could provide to an external load. We study in detail how these properties depend (i) on the magnitude of the bias established between the two metallic electrodes, (ii) on the separation between the two electrodes, (iii) on differences in the work function of the two metals, (iv) on differences in the temperature of the two metals, and (v) on the height of the protrusion. These calculations provide quantitative results for the use of these junctions as an energy converter and the efficiency with which the energy of incident radiation is being converted into a dc current and delivered to an external load.

The Paul trap simulator experiment is a compact laboratory Paul trap that simulates a long, thin charged-particle bunch coasting through a kilometers-long magnetic alternating-gradient (AG) transport system by putting the physicist in the beam’s frame of reference. The transverse dynamics of particles in both systems are described by similar equations, including all nonlinear space-charge effects. The time-dependent quadrupolar electric fields created by the confinement electrodes of a linear Paul trap correspond to the axially dependent magnetic fields applied in the AG system. Results are presented for experiments in which the lattice period and strength are changed over the course of the experiment to transversely compress a beam with an initial depressed tune of 0.9. Instantaneous and smooth changes are considered. Emphasis is placed on determining the conditions that minimize the emittance growth and the number of halo particles produced by the beam compression process. Both the results of particle-in-cell simulations performed with the warp code and envelope equation solutions agree well with the experimental data.

The transverse compression of a long charge bunch is investigated in the Paul trap simulator experiment (PTSX), which is a linear Paul trap that simulates the nonlinear transverse dynamics of an intense charged particle beam propagating through an equivalent kilometers-long magnetic alternating-gradient (AG) focusing system. Changing the voltage amplitude at fixed focusing frequency in the PTSX device corresponds to changing the field gradient of the quadrupole magnets with fixed axial periodicity in the AG transport system. In this work, we present experimental results on transverse compression of the charge bunch in which the amplitude of the applied oscillatory focusing voltage is changed instantaneously, and adiabatically. The experimental data are also compared with analytical estimates and 2D WARP particle-in-cell simulations.

The Paul Trap Simulator Experiment (PTSX) is a linear Paul trap whose purpose is to simulate the nonlinear transverse dynamics of intense charged particle beam propagation in periodic-focusing quadrupole magnetic transport systems. Externally created cesium ions are injected and trapped in the long central electrodes of the PTSX device. In order to have well-matched one-component plasma equilibria for various beam physics experiments, it is important to optimize the ion injection. From the experimental studies reported in this paper, it is found that the injection process can be optimized by minimizing the beam mismatch between the source and the focusing lattice, and by minimizing the number of particles present in the vicinity of the injection electrodes when the injection electrodes are switched from the fully oscillating voltage waveform to their static trapping voltage.

Oxygen vacancies in TbMnO₃ were created by monovalent Na⁺ doping. The effects of the interruption of the superexchange paths on the spin orderings were investigated. Chemical doping resulted in a much higher ordering temperature, which is probably associated with enhanced Tb-Tb coupling because of a reduction in Tb-Tb interatomic distances. The oxygen vacancies turned the modulated Mn spin structure into a simple commensurate one, presumably due to interruptions of the in-plane (Mn-O)–(O-Mn) superexchange paths, which reduced the significance of the next nearest-neighbor interactions of the Mn ions. The modulated spin structure of the Mn moments was found to reappear in systems with high Na doping, indicating that the anisotropic magnetic couplings had reemerged.

We relate the reduced density matrices of quadratic fermionic and bosonic models to their Green’s function matrices in a unified way and calculate the scaling of the entanglement entropy of finite systems in an infinite universe exactly. For critical fermionic two-dimensional (2D) systems at T=0, two regimes of scaling are identified: generically, we find a logarithmic correction to the area law with a prefactor dependence on the chemical potential that confirms earlier predictions based on the Widom conjecture. If, however, the Fermi surface of the critical system is zero-dimensional, then we find an area law with a sublogarithmic correction. For a critical bosonic 2D array of coupled oscillators at T=0, our results show that the entanglement entropy follows the area law without corrections.

The superconducting parameters of zero-dimensional In nanoparticles were studied by measuring the magnetic susceptibility, electrical resistivity, and specific heat. Significant alternations of the superconducting parameters were found for particles with diameters smaller than 120 nm. The Anderson regime, where superconductivity was sharply suppressed by the discrete nature of the electronic levels, was seen for particle smaller than 7.5 nm. A regime prior to the Anderson one, in which superconductivity was found to vary in accordance with alternations of the structural parameters, due to the small size effect, was also observed and characterized. A nonignorable 5 % increase in T_C and a 400 times higher H_C were seen for the 39 nm particles. The appearance of this regime was closely related to the structural symmetry of the system.

We describe the transport characteristics and the observation of a type of magnetoresistive behavior in nonmagnetic nanocompacts, consisting of mixtures of Ag and PbO nanoparticles packed into compact units at various mass compositions and compacting densities. Semiconductorlike, intermediate, and metallic transports may all be revealed by tuning the compacting density. Crossovers from a positive magnetoresistance (MR) at low applied magnetic fields to a negative MR at high fields were observed in the loosely packed units. No evidence of MR saturation was seen up to an applied field of 9 T. Raising the temperature suppresses the MR responses. Surprisingly, no noticeable differences in the MR responses were found for samples with different Ag∕PbO compositions. The observed transport characteristics fit well to the tunneling transport, and the MR characteristics may be described as being the tunneling transport between Zeeman split discrete Fermi states of weakly linked nanoparticles.

We propose to use the rescaled range analysis to examine the records of rapidity-dependence of multiplicities in high-energy collision processes. We probe event by event the existence of global statistical dependence in the system of produced hadrons, and demonstrate the effectiveness of the above-mentioned statistical method by applying it to the cosmic-ray data of the JACEE collaboration, and by comparing the obtained results with other experimental results for similar reactions at accelerator and collider energies. We present experimental evidence for the validity of Hurst’s empirical law, and the evidence for the existence of global statistical dependence, fractal dimension, and scaling behavior in such systems of hadronic matter. None of these features is directly related to the basis of the conventional physical picture. Hence, it is not clear whether (and if yes, how and why) these striking empirical regularities can be understood in terms of the conventional theory.

Transverse momentum spectra and yields of hadrons are measured by the PHENIX collaboration in Au+Au collisions at sqrt[s_NN]=130  GeV at the Relativistic Heavy Ion Collider. The time-of-flight resolution allows identification of pions to transverse momenta of 2  GeV∕c and protons and antiprotons to 4  GeV∕c. The yield of pions rises approximately linearly with the number of nucleons participating in the collision, while the number of kaons, protons, and antiprotons increases more rapidly. The shape of the momentum distribution changes between peripheral and central collisions. Simultaneous analysis of all the p_T spectra indicates radial collective expansion, consistent with predictions of hydrodynamic models. Hydrodynamic analysis of the spectra shows that the expansion velocity increases with collision centrality and collision energy. This expansion boosts the particle momenta, causing the yield from soft processes to exceed that for hard to large transverse momentum, perhaps as large as 3  GeV∕c.

Results are presented on a measurement of the tt̅ pair production cross section in pp̅ collisions at sqrt[s]=1.8 TeV from nine independent decay channels. The data were collected by the DØ experiment during the 1992–1996 run of the Fermilab Tevatron Collider. A total of 80 candidate events is observed with an expected background of 38.8±3.3 events. For a top quark mass of 172.1 GeV/c², the measured cross section is 5.69±1.21(stat)±1.04(syst) pb.

We describe a search for evidence of minimal supergravity (MSUGRA) in 92.7 pb^-1 of data collected with the DØ detector at the Fermilab Tevatron pp̅ collider at sqrt[s]=1.8 TeV. Events with a single electron, four or more jets, and large missing transverse energy were used in this search. The major backgrounds are from W+jets, misidentified multijet, tt̅ , and WW production. We observe no excess above the expected number of background events in our data. A new limit in terms of MSUGRA model parameters is obtained.

Two-particle azimuthal correlation functions are presented for charged hadrons produced in Au+Au collisions at the Relativistic Heavy Ion Collider (sqrt[s_NN]=130  GeV). The measurements permit determination of elliptic flow without event-by-event estimation of the reaction plane. The extracted elliptic flow values (v₂) show significant sensitivity to both the collision centrality and the transverse momenta of emitted hadrons, suggesting rapid thermalization and relatively strong velocity fields. When scaled by the eccentricity of the collision zone ε, the scaled elliptic flow shows little or no dependence on centrality for charged hadrons with relatively low p_T. A breakdown of this ε scaling is observed for charged hadrons with p_T >1.0  GeV/c.

We present results of a search for R-parity-violating decay of the neutralino χ ˜₁⁰, taken as the lightest supersymmetric particle, to a muon and two jets. The decay proceeds through a lepton-number violating coupling λ_2jk^′ (j=1,2; k=1,2,3), with R-parity conservation in all other production and decay processes. In the absence of candidate events from 77.5±3.9   pb^-1 of data collected by the D0 experiment at the Fermilab Tevatron in pp̅ collisions at sqrt[s]=1.8  TeV, and with an expected background of 0.18±0.03±0.02 events, we set limits on squark and gluino masses within the framework of the minimal low-energy supergravity-supersymmetry model.

Based on 85 pb^-1 data of pp̅ collisions at sqrt[s]=1.8 TeV collected using the DØ detector at Fermilab during the 1994–1995 run of the Tevatron, we present a direct measurement of the total decay width of the W boson Γ_W. The width is determined from the transverse mass spectrum in the W→e+ν_e decay channel and found to be Γ_W=2.23_-0.14^+0.15(stat)±0.10(syst) GeV, consistent with the expectation from the standard model.

We present results on the measurement of Λ and Λ̅ production in Au+Au collisions at sqrt[s_NN]=130  GeV with the PHENIX detector at the Relativistic Heavy Ion Collider. The transverse momentum spectra were measured for minimum bias and for the 5% most central events. The Λ̅ /Λ ratios are constant as a function of p_T and the number of participants. The measured net Λ density is significantly larger than predicted by models based on hadronic strings (e.g., HIJING) but in approximate agreement with models which include the gluon-junction mechanism.

Data from Au+   Au interactions at sqrt[s_NN]=130   GeV, obtained with the PHENIX detector at the Relativistic Heavy-Ion Collider, are used to investigate local net charge fluctuations among particles produced near midrapidity. According to recent suggestions, such fluctuations may carry information from the quark-gluon plasma. This analysis shows that the fluctuations are dominated by a stochastic distribution of particles, but are also sensitive to other effects, like global charge conservation and resonance decays.

Distributions of event-by-event fluctuations of the mean transverse momentum and mean transverse energy near mid-rapidity have been measured in Au+Au collisions at sqrt[s_NN]=130 GeV at the Relativistic Heavy-Ion Collider. By comparing the distributions to what is expected for statistically independent particle emission, the magnitude of nonstatistical fluctuations in mean transverse momentum is determined to be consistent with zero. Also, no significant nonrandom fluctuations in mean transverse energy are observed. By constructing a fluctuation model with two event classes that preserve the mean and variance of the semi-inclusive p_T or e_T spectra, we exclude a region of fluctuations in sqrt[s_NN]=130 GeV Au+Au collisions.

We have measured the W boson mass using the DØ detector and a data sample of 82 pb^-1 from the Fermilab Tevatron collider. This measurement uses W→eν decays, where the electron is close to a boundary of a central electromagnetic calorimeter module. Such “edge” electrons have not been used in any previous DØ analysis, and represent a 14% increase in the W boson sample size. For these electrons, new response and resolution parameters are determined, and revised backgrounds and underlying event energy flow measurements are made. When the current measurement is combined with previous DØ W boson mass measurements, we obtain M_W=80.483±0.084 GeV. The 8% improvement from the previous DØ measurement is primarily due to the improved determination of the response parameters for non-edge electrons using the sample of Z bosons with non-edge and edge electrons.

Identified π^+/-, K^+/-, p, and p̅ transverse momentum spectra at midrapidity in sqrt[s_NN]  =  130  GeV  Au+Au collisions were measured by the PHENIX experiment at RHIC as a function of collision centrality. Average transverse momenta increase with the number of participating nucleons in a similar way for all particle species. Within errors, all midrapidity particle yields per participant are found to be increasing with the number of participating nucleons. There is an indication that K^+/-, p, and p̅ yields per participant increase faster than the π^+/- yields. In central collisions at high transverse momenta (p_T≳2 GeV/c), p̅ and p yields are comparable to the π^+/- yields.

Transverse momentum spectra of electrons from Au+Au collisions at sqrt[s_NN]  =  130 GeV have been measured at midrapidity by the PHENIX experiment at the Relativistic Heavy Ion Collider. The spectra show an excess above the background from photon conversions and light hadron decays. The electron signal is consistent with that expected from semileptonic decays of charm. The yield of the electron signal dN_e/dy for p_T>0.8 GeV/c is 0.025±0.004(stat)±0.010(syst) in central collisions, and the corresponding charm cross section is 380±60(stat)±200(syst)  μb per binary nucleon-nucleon collision.

Two-pion correlations in sqrt[s_NN]  =  130 GeV Au+Au collisions at RHIC have been measured over a broad range of pair transverse momentum k_T by the PHENIX experiment at RHIC. The k_T dependent transverse radii are similar to results from heavy-ion collisions at sqrt[s_NN]  =  4.1, 4.9, and 17.3 GeV, whereas the longitudinal radius increases monotonically with beam energy. The ratio of the outwards to sidewards transverse radii (R_out/R_side) is consistent with unity and independent of k_T.

We present the results of a search for leptoquark (LQ) pairs in (85.2±3.7)  pb^-1 of pp̅ collider data collected by the D0 experiment at the Fermilab Tevatron. We observe no evidence for leptoquark production and set a limit on σ(pp̅ →LQLQ̅ →νν+jets) as a function of the mass of the leptoquark (m_LQ). Assuming the decay LQ→νq, we exclude scalar leptoquarks for m_LQ<98 GeV/c², and vector leptoquarks for m_LQ<200 GeV/c² and coupling which produces the minimum cross section, at a 95% confidence level.

We have performed a search for scalar top quark (stop) pair production in the inclusive electron-muon-missing transverse energy final state, using a sample of pp̅ events corresponding to 108.3 pb^-1 of data collected with the D0 detector at Fermilab. The search is done in the framework of the minimal supersymmetric standard model assuming that the sneutrino is the lightest supersymmetric particle. For the dominant decays of the lightest stop, t̃→bχ̃₁⁺ and t̃→bℓν̃, no evidence for signal is found. We derive cross-section limits as a function of stop ( t̃ ), chargino ( χ̃₁⁺), and sneutrino ( ν̃) masses.

We present a search for charged Higgs bosons in decays of pair-produced top quarks in pp̅ collisions at sqrt[s]  =  1.8 TeV recorded by the D0 detector at the Fermilab Tevatron collider. With no evidence for signal, we exclude most regions of the ( M_H^±,tanβ) parameter space where the decay t→ H⁺b has a branching fraction >0.36 and B(H^±→τν_τ) is large.