4,900 research outputs found
The S=1/2 chain in a staggered field: High-energy bound-spinon state and the effects of a discrete lattice
We report an experimental and theoretical study of the antiferromagnetic
S=1/2 chain subject to uniform and staggered fields. Using inelastic neutron
scattering, we observe a novel bound-spinon state at high energies in the
linear chain compound CuCl2 * 2((CD3)2SO). The excitation is explained with a
mean-field theory of interacting S=1/2 fermions and arises from the opening of
a gap at the Fermi surface due to confining spinon interactions. The mean-field
model also describes the wave-vector dependence of the bound-spinon states,
particularly in regions where effects of the discrete lattice are important. We
calculate the dynamic structure factor using exact diagonalization of finite
length chains, obtaining excellent agreement with the experiments.Comment: 16 pages, 7 figures, accepted by Phys. Rev.
Phase diagram and spin Hamiltonian of weakly-coupled anisotropic S=1/2 chains in CuCl2*2((CD3)2SO)
Field-dependent specific heat and neutron scattering measurements were used
to explore the antiferromagnetic S=1/2 chain compound CuCl2 * 2((CD3)2SO). At
zero field the system acquires magnetic long-range order below TN=0.93K with an
ordered moment of 0.44muB. An external field along the b-axis strengthens the
zero-field magnetic order, while fields along the a- and c-axes lead to a
collapse of the exchange stabilized order at mu0 Hc=6T and mu0 Hc=3.5T,
respectively (for T=0.65K) and the formation of an energy gap in the excitation
spectrum. We relate the field-induced gap to the presence of a staggered
g-tensor and Dzyaloshinskii-Moriya interactions, which lead to effective
staggered fields for magnetic fields applied along the a- and c-axes.
Competition between anisotropy, inter-chain interactions and staggered fields
leads to a succession of three phases as a function of field applied along the
c-axis. For fields greater than mu0 Hc, we find a magnetic structure that
reflects the symmetry of the staggered fields. The critical exponent, beta, of
the temperature driven phase transitions are indistinguishable from those of
the three-dimensional Heisenberg magnet, while measurements for transitions
driven by quantum fluctuations produce larger values of beta.Comment: revtex 12 pages, 11 figure
The contribution of ultracompact dark matter minihalos to the isotropic radio background
The ultracompact minihalos could be formed during the earlier epoch of the
universe. The dark matter annihilation within them is very strong due to the
steep density profile, . The high energy electrons and
positrons from the dark matter annihilation can inverse Compton scatter (ICS)
with the background photons, such as CMB photons, to acquire higher energy. On
the other hand, the synchrotron radiation can also be produced when they meet
the magnetic field. In this paper, we study the signals from the UCMHs due to
the dark matter annihilation for the radio, X-ray and -ray band. We
found that for the radio emission the UCMHs can provide one kind of source for
the radio excess observed by ARCADE 2.
But the X-ray signals due to the ICS effect or the -ray signals
mainly due to the prompt emission from dark matter would exceed the present
observations, such as Fermi, COMPTEL and CHANDRA. We found that the strongest
limits on the fraction of UCMHs come from the X-ray observations and the
constraints from the radio data are the weakest.Comment: 6 pages, 8 figures, Comments Welcome! Some Refs. are added, some
presentation have been corrected. The conclusions remain unchanged. One
important reference has been corrected. Some presentations are changed and
added according to the referee's comments. Accepted for publication in PR
Raman imaging and electronic properties of graphene
Graphite is a well-studied material with known electronic and optical
properties. Graphene, on the other hand, which is just one layer of carbon
atoms arranged in a hexagonal lattice, has been studied theoretically for quite
some time but has only recently become accessible for experiments. Here we
demonstrate how single- and multi-layer graphene can be unambiguously
identified using Raman scattering. Furthermore, we use a scanning Raman set-up
to image few-layer graphene flakes of various heights. In transport experiments
we measure weak localization and conductance fluctuations in a graphene flake
of about 7 monolayer thickness. We obtain a phase-coherence length of about 2
m at a temperature of 2 K. Furthermore we investigate the conductivity
through single-layer graphene flakes and the tuning of electron and hole
densities via a back gate
Neutron scattering from a coordination polymer quantum paramagnet
Inelastic neutron scattering measurements are reported for a powder sample of
the spin-1/2 quantum paramagnet . Magnetic neutron
scattering is identified above an energy gap of 1.9 meV. Analysis of the sharp
spectral maximum at the onset indicates that the material is magnetically
quasi-one-dimensional. Consideration of the wave vector dependence of the
scattering and polymeric structure further identifies the material as a
two-legged spin-1/2 ladder. Detailed comparison of the data to various models
of magnetism in this material based on the single mode approximation and the
continuous unitary transformation are presented. The latter theory provides an
excellent account of the data with leg exchange meV and
rung exchange meV.Comment: 10 pages, 11 figures, 1 tabl
G213.00.6, a true supernova remnant or just an HII region?
G213.00.6 is a faint extended source situated in the anti-center region of
the Galactic plane. It has been classified as a shell-type supernova remnant
(SNR) based on its shell-like morphology, steep radio continuum spectrum, and
high ratio of [S II]/H. With new optical emission line data of
H, [S II], and [N II] recently observed by the Large Sky Area
Multi-Object Fiber Spectroscopic Telescope, the ratios of [S II]/H and
[N II]/H are re-assessed. The lower values than those previously
reported put G213.00.6 around the borderline of SNR-HII region
classification. We decompose the steep-spectrum synchrotron and the
flat-spectrum thermal free-free emission in the area of G213.00.6 with
multi-frequency radio continuum data. G213.00.6 is found to show a flat
spectrum, in conflict with the properties of a shell-type SNR. Such a result is
further confirmed by TT-plots made between the 863-MHz, 1.4-GHz, and 4.8-GHz
data. Combining the evidence extracted in both optical and radio continuum, we
argue that G213.00.6 is possibly not an SNR, but an HII region instead. The
pertaining to the H filaments places G213.00.6
approximately 1.9 kpc away in the Perseus Arm.Comment: 9 pages, 4 figures, accepted for publication in MNRA
On the Nature of Part Time Radio Pulsars
The recent discovery of rotating radio transients and the quasi-periodicity
of pulsar activity in the radio pulsar PSR B193124 has challenged the
conventional theory of radio pulsar emission. Here we suggest that these
phenomena could be due to the interaction between the neutron star
magnetosphere and the surrounding debris disk. The pattern of pulsar emission
depends on whether the disk can penetrate the light cylinder and efficiently
quench the processes of particle production and acceleration inside the
magnetospheric gap. A precessing disk may naturally account for the
switch-on/off behavior in PSR B193124.Comment: 9 pages, accepted to ApJ
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