arXiv subjects

G. Xu

Publications and source records attributed to G. Xu.

81 records · Page 5Linked to original sources

arXiv2004

Hypernuclear Spectroscopy using the (e,e'K+) Reaction

A pioneering experiment in Lambda hypernuclear spectroscopy, undertaken at the Thomas Jefferson National Accelerator Facility (Jlab), was recently reported. The experiment used the high- precision, continuous electron beam at Jlab, and a special arrangement of spectrometer magnets to measure the spectrum from {nat}C and 7Li targets using the (e,e' K+)reaction. The 12B hypernuclear spectrum was previously published. This experiment is now reported in more detail, with improved results for the 12B hypernuclear spectrum. In addition, unpublished results of the 7He hypernuclear spectrum are also shown. This later spectrum indicates the need for a more detailed few-body calculation of the hypernucleus and the reaction process. The success of this experiment demonstrates that the (e,e'K+) reaction can be effectively used as a high resolution tool to study hypernuclear spectra, ant its use should be vigorously pursued.

nucl-ex↗

arXiv2004

Quantum Magnetic Excitations from Stripes in Copper-Oxide Superconductors

In the copper-oxide parent compounds of the high-transition-temperature superconductors, the valence electrons are localized, one per copper site, due to strong intraatomic Coulomb repulsion. A symptom of the localization is antiferromagnetism, where the spins of localized electrons alternate between up and down. The superconductivity appears when mobile 'holes' are doped into this insulating state, and it coexists with antiferromagnetic fluctuations. In one approach to the coexistence, the holes are believed to self-organize into 'stripes' that alternate with antiferromagnetic (insulating) regions within copper-oxide planes. Such an unusual electronic state would necessitate an unconventional mechanism of superconductivity. There is an apparent problem with this picture, however: measurements of magnetic excitations in superconducting YBa(2)Cu(3)O(6+x) near optimum doping are incompatible with the naive expectations for a material with stripes. Here we report neutron scattering measurements on stripe-ordered La(1.875)Ba(0.125)CuO(4). We show that the measured excitations are, surprisingly, quite similar to those in YBa(2)Cu(3)O(6+x) (i.e., the predicted spectrum of magnetic excitations is wrong). We find instead that the observed spectrum can be understood within a stripe model by taking account of quantum excitations. Our results support the concept that stripe correlations are essential to high-transition-temperature superconductivity.

cond-mat.supr-con↗

arXiv2004

Accelerator Physics Issues in the Bepcii

The Beijing Electron-Positron Collider (BEPC) has been running for both high energy physics (HEP) and synchrotron radiation (SR) researches since 1989. Good performance of BEPC accomplishes a lot of achievements in the tau-charm region over the past decade, and in many fields of SR applications as well. As an upgrading scheme from BEPC, the BEPCII project was approved by the Chinese government in the early time of this year, with a micro-beta scheme and multi-bunch collision in a doublering machine installed in the current BEPC tunnel. The design work of the BEPCII is being carried on. This paper will describe the main progresses on the design of the BEPCII in the field of accelerator physics.

physics.acc-ph↗

arXiv2002

Structure of end states for a Haldane Spin Chain

Inelastic neutron scattering was used to probe edge states in a quantum spin liquid. The experiment was performed on finite length antiferromagnetic spin-1 chains in Y_2BaNi_{1-x}Mg_xO_5. At finite fields, there is a Zeeman resonance below the Haldane gap. The wave vector dependence of its intensity provides direct evidence for staggered magnetization at chain ends, which decays exponentially towards the bulk (xi = 8(1) at T=0.1K). Continuum contributions to the chain end spectrum indicate inter-chain-segment interactions. We also observe a finite size blue shift of the Haldane gap.

cond-mat.str-el↗

arXiv2002

A neutron scattering study of two-magnon states in the quantum magnet copper nitrate

We report measurements of the two-magnon states in a dimerized antiferromagnetic chain material, copper nitrate (Cu(NO3)2*2.5D2O). Using inelastic neutron scattering we have measured the one and two magnon excitation spectra in a large single crystal. The data are in excellent agreement with a perturbative expansion of the alternating Heisenberg Hamiltonian from the strongly dimerized limit. The expansion predicts a two-magnon bound state for q ~ (2n+1)pi*d which is consistent with the neutron scattering data.

cond-mat.str-el↗

arXiv2000

Two-magnon states in Cu(NO3)2*2.5D2O using inelastic neutron scattering

We report measurements of the two-magnon states in a dimerized antiferromagnetic chain material, copper nitrate (\QTR{group}{Cu(NO$_{3}$)$_{2}\cdot 2.5$D$_{2}$O}). Using inelastic neutron scattering, we have studied the one- and two-magnon excitation spectra in a large single crystal of this material. We compare this new data with perturbative expansions of the alternating Heisenberg chain and find good agreement with these calculations. The data may also show evidence for the recently proposed S=1 two-magnon bound state (Phys. Rev. B54, R9624 (1996)).

cond-mat.str-el↗

arXiv2000

Photoluminescence Study of Carbon Nanotubes

ultiwalled carbon nanotubes, prepared by both electric arc discharge and chemical vapor deposition methods, show a strong visible light emission in photoluminescence experiments. All the samples employed in the experiments exhibit nearly same super-linear intensity dependence of the emission bands on the excitation intensity, and negligible temperature dependence of the central position and the line shapes of the emission bands. Based upon theoretical analysis of the electronic band structures and optical transition, it is suggested that besides the electronic transitions across the fundamental gap, the transitions between pi and sigma conduction bands are the major source of the light emissions. A two-step transition mechanism is proposed.

cond-mat.mtrl-sci↗

arXiv1999

The Santa Barbara cluster comparison project: a comparison of cosmological hydrodynamics solutions

We have simulated the formation of an X-ray cluster in a cold dark matter universe using 12 different codes. The codes span the range of numerical techniques and implementations currently in use, including SPH and grid methods with fixed, deformable or multilevel meshes. The goal of this comparison is to assess the reliability of cosmological gas dynamical simulations of clusters in the simplest astrophysically relevant case, that in which the gas is assumed to be non-radiative. We compare images of the cluster at different epochs, global properties such as mass, temperature and X-ray luminosity, and radial profiles of various dynamical and thermodynamical quantities. On the whole, the agreement among the various simulations is gratifying although a number of discrepancies exist. Agreement is best for properties of the dark matter and worst for the total X-ray luminosity. Even in this case, simulations that adequately resolve the core radius of the gas distribution predict total X-ray luminosities that agree to within a factor of two. Other quantities are reproduced to much higher accuracy. For example, the temperature and gas mass fraction within the virial radius agree to about 10%, and the ratio of specific kinetic to thermal energies of the gas agree to about 5%. Various factors contribute to the spread in calculated cluster properties, including differences in the internal timing of the simulations. Based on the overall consistency of results, we discuss a number of general properties of the cluster we have modelled.

astro-ph↗

arXiv1997

A Linear Quantum Dynamic Theory for Coherent Output of Bose-Einstain Condensation

A model for the coherent output coupler of the Bose-Einstein condensed atoms from a trap in the recent MIT experiment (Phys. Rev. Lett., 78 (1997) 582) is established with a simple many-boson system of two states with linear coupling. Its exact solution for the many-body problem shows a factorization of dynamical evolution process, i.e., the wave function initially prepared in a direct product of a vacuum state and a coherent state remains in a direct product of two coherent states at any instance in the evolution of the total system. This conclusion always holds even for a system with a finite average particle number in the initial state. Its thermodynamical limit can be directly dealt with in the Bogoliubov approximation and manifests that an ideal condensate in the trap will remain in a coherent state after the r.f. interaction while the output-coupler pulse of atoms is also in a coherent state, which means a coherent output of atomic beam to form a macroscopic quantum state in a propagating mode.

quant-ph↗