20,048 research outputs found
Optical conductivity of nodal metals
Fermi liquid theory is remarkably successful in describing the transport and
optical properties of metals; at frequencies higher than the scattering rate,
the optical conductivity adopts the well-known power law behavior
. We have observed an unusual non-Fermi
liquid response in the ground
states of several cuprate and iron-based materials which undergo electronic or
magnetic phase transitions resulting in dramatically reduced or nodal Fermi
surfaces. The identification of an inverse (or fractional) power-law behavior
in the residual optical conductivity now permits the removal of this
contribution, revealing the direct transitions across the gap and allowing the
nature of the electron-boson coupling to be probed. The non-Fermi liquid
behavior in these systems may be the result of a common Fermi surface topology
of Dirac cone-like features in the electronic dispersion.Comment: 8 pages including supplemental informatio
Phonon anomaly in BaFe2As2
The detailed optical properties of BaFe2As2 have been determined over a wide
frequency range above and below the structural and magnetic transition at T_N =
138 K. A prominent in-plane infrared-active mode is observed at 253 cm^{-1}
(31.4 meV) at 295 K. The frequency of this vibration shifts discontinuously at
T_N; for T < T_N the frequency of this mode displays almost no temperature
dependence, yet it nearly doubles in intensity. This anomalous behavior appears
to be a consequence of orbital ordering in the Fe-As layers.Comment: 4 pages, 3 figures and one table (minor revisions
Radiance and Doppler shift distributions across the network of the quiet Sun
The radiance and Doppler-shift distributions across the solar network provide
observational constraints of two-dimensional modeling of transition-region
emission and flows in coronal funnels. Two different methods, dispersion plots
and average-profile studies, were applied to investigate these distributions.
In the dispersion plots, we divided the entire scanned region into a bright and
a dark part according to an image of Fe xii; we plotted intensities and Doppler
shifts in each bin as determined according to a filtered intensity of Si ii. We
also studied the difference in height variations of the magnetic field as
extrapolated from the MDI magnetogram, in and outside network. For the
average-profile study, we selected 74 individual cases and derived the average
profiles of intensities and Doppler shifts across the network. The dispersion
plots reveal that the intensities of Si ii and C iv increase from network
boundary to network center in both parts. However, the intensity of Ne viii
shows different trends, namely increasing in the bright part and decreasing in
the dark part. In both parts, the Doppler shift of C iv increases steadily from
internetwork to network center. The average-profile study reveals that the
intensities of the three lines all decline from the network center to
internetwork region. The binned intensities of Si ii and Ne viii have a good
correlation. We also find that the large blue shift of Ne viii does not
coincide with large red shift of C iv. Our results suggest that the network
structure is still prominent at the layer where Ne viii is formed in the quiet
Sun, and that the magnetic structures expand more strongly in the dark part
than in the bright part of this quiet Sun region.Comment: 10 pages,9 figure
Observation of indirect ionization of W7+ in an electron-beam ion-trap plasma
In this work, visible and extreme ultraviolet spectra of W7+ are measured
using the high-temperature superconducting electron-beam ion trap (EBIT) at the
Shanghai EBIT Laboratory under extremely low-energy conditions (lower than the
nominal electron-beam energy of 130 eV). The relevant atomic structure is
calculated using the flexible atomic code package based on the relativistic
configuration interaction method. The GRASP2K code, in the framework of the
multiconfiguration Dirac-Hartree-Fock method, is employed as well for
calculating the wavelength of the M1 transition in the ground configuration of
W7+. A line from the W7+ ions is observed at a little higher electron-beam
energy than the ionization potential for W4+, making this line appear to be
from W5+. A hypothesis for the charge-state evolution of W7+ is proposed based
on our experimental and theoretical results; that is, the occurrence of W7+
ions results from indirect ionization caused by stepwise excitation between
some metastable states of lower-charge-state W ions, at the nominal
electron-beam energy of 59 eV
Upflows in the upper transition region of the quiet Sun
We investigate the physical meaning of the prominent blue shifts of Ne VIII,
which is observed to be associated with quiet-Sun network junctions (boundary
intersections), through data analyses combining force-free-field extrapolations
with EUV spectroscopic observations. For a middle-latitude region, we
reconstruct the magnetic funnel structure in a sub-region showing faint
emission in EIT-Fe 195. This funnel appears to consist of several smaller
funnels that originate from network lanes, expand with height and finally merge
into a single wide open-field region. However, the large blue shifts of Ne VIII
are generally not associated with open fields, but seem to be associated with
the legs of closed magnetic loops. Moreover, in most cases significant upflows
are found in both of the funnel-shaped loop legs. These quasi-steady upflows
are regarded as signatures of mass supply to the coronal loops rather than the
solar wind. Our observational result also reveals that in many cases the
upflows in the upper transition region (TR) and the downflows in the middle TR
are not fully cospatial. Based on these new observational results, we suggest
different TR structures in coronal holes and in the quiet Sun.Comment: 4 pages, 4 figures, will appear in the Proceedings of the Solar wind
12 conferenc
Molecular Motor of Double-Walled Carbon Nanotube Driven by Temperature Variation
An elegant formula for coordinates of carbon atoms in a unit cell of a
single-walled nanotube (SWNT) is presented and a new molecular motor of
double-walled carbon nanotube whose inner tube is a long (8,4) SWNT and outer
tube a short (14,8) SWNT is constructed. The interaction between inner an outer
tubes is analytically derived by summing the Lennard-Jones potentials between
atoms in inner and outer tubes. It is proved that the molecular motor in a
thermal bath exhibits a directional motion with the temperature variation of
the bath.Comment: 9 pages, 4 figures, revtex
Multispace and Multilevel BDDC
BDDC method is the most advanced method from the Balancing family of
iterative substructuring methods for the solution of large systems of linear
algebraic equations arising from discretization of elliptic boundary value
problems. In the case of many substructures, solving the coarse problem exactly
becomes a bottleneck. Since the coarse problem in BDDC has the same structure
as the original problem, it is straightforward to apply the BDDC method
recursively to solve the coarse problem only approximately. In this paper, we
formulate a new family of abstract Multispace BDDC methods and give condition
number bounds from the abstract additive Schwarz preconditioning theory. The
Multilevel BDDC is then treated as a special case of the Multispace BDDC and
abstract multilevel condition number bounds are given. The abstract bounds
yield polylogarithmic condition number bounds for an arbitrary fixed number of
levels and scalar elliptic problems discretized by finite elements in two and
three spatial dimensions. Numerical experiments confirm the theory.Comment: 26 pages, 3 figures, 2 tables, 20 references. Formal changes onl
Observation of an in-plane magnetic-field-driven phase transition in a quantum Hall system with SU(4) symmetry
In condensed matter physics, the study of electronic states with SU(N)
symmetry has attracted considerable and growing attention in recent years, as
systems with such a symmetry can often have a spontaneous symmetry-breaking
effect giving rise to a novel ground state. For example, pseudospin quantum
Hall ferromagnet of broken SU(2) symmetry has been realized by bringing two
Landau levels close to degeneracy in a bilayer quantum Hall system. In the past
several years, the exploration of collective states in other multi-component
quantum Hall systems has emerged. Here we show the conventional pseudospin
quantum Hall ferromagnetic states with broken SU(2) symmetry collapsed rapidly
into an unexpected state with broken SU(4) symmetry, by in-plane magnetic field
in a two-subband GaAs/AlGaAs two-dimensional electron system at filling factor
around . Within a narrow tilting range angle of 0.5 degrees, the
activation energy increases as much as 12 K. While the origin of this puzzling
observation remains to be exploited, we discuss the possibility of a
long-sought pairing state of electrons with a four-fold degeneracy.Comment: 13 pages, 4 figure
Specific heats of dilute neon inside long single-walled carbon nanotube and related problems
An elegant formula for coordinates of carbon atoms in a unit cell of a
single-walled nanotube (SWNT) is presented and the potential of neon (Ne)
inside an infinitely long SWNT is analytically derived out under the condition
of the Lennard-Jones potential between Ne and carbon atoms.
Specific heats of dilute Ne inside long (20, 20) SWNT are calculated at
different temperatures. It is found that Ne exhibits 3-dimensional (3D) gas
behavior at high temperature but behaves as 2D gas at low temperature.
Especially, at ultra low temperature, Ne inside (20, 20) nanotubes behaves as
lattice gas. A coarse method to determine the characteristic temperature
for low density gas in a potential is put forward. If
, we just need to use the classical statistical
mechanics without solving the Shr\"{o}dinger equation to consider the thermal
behavior of gas in the potential. But if , we
must solve the Shr\"{o}dinger equation. For Ne in (20,20) nanotube, we obtain
K.Comment: 14 pages, 7 figure
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