30,093 research outputs found
Optical study of phase transitions in single-crystalline RuP
RuP single crystals of MnP-type orthorhombic structure were synthesized by
the Sn flux method. Temperature-dependent x-ray diffraction measurements reveal
that the compound experiences two structural phase transitions, which are
further confirmed by enormous anomalies shown in temperature-dependent
resistivity and magnetic susceptibility. Particularly, the resistivity drops
monotonically upon temperature cooling below the second transition, indicating
that the material shows metallic behavior, in sharp contrast with the
insulating ground state of polycrystalline samples. Optical conductivity
measurements were also performed in order to unravel the mechanism of these two
transitions. The measurement revealed a sudden reconstruction of band structure
over a broad energy scale and a significant removal of conducting carriers
below the first phase transition, while a charge-density-wave-like energy gap
opens below the second phase transition.Comment: 5 pages, 6 figure
Non-magnetic B-site Impurities Induce Ferromagnetic Tendencies in CE Manganites
Using a two-orbital model and Monte Carlo simulations, we investigate the
effect of nonmagnetic B-site substitution on half-doped CE-type manganites. The
lattice defects induced by this substitution destabilize the CE phase, which
transforms into (1) the ferromagnetic (FM) metallic competing state, or (2) a
regime with short-range FM clusters, or (3) a spin-glass state, depending on
couplings and on the valence of the B-site substitution. While a C-type
antiferromagnetic state is usually associated with an average
charge density less than 0.5, the nonmagnetic B-site substitution that lowers
the charge density is still found to enhance the FM tendency in our
simulations. The present calculations are in qualitative agreement with
experiments and provide a rationalization for the complex role of nonmagnetic
B-site substitution in modulating the phase transitions in manganites.Comment: 8 pages, 5 figure
Optical spectroscopy study of the collapsed tetragonal phase of CaFe(AsP) single crystals
We present an optical spectroscopy study on P-doped CaFeAs which
experiences a structural phase transition from tetragonal to collapsed
tetragonal (cT) phase near 75 K. The measurement reveals a sudden reduction of
low frequency spectral weight and emergence of a new feature near 3200 \cm (0.4
eV) in optical conductivity across the transition, indicating an abrupt
reconstruction of band structure. The appearance of new feature is related to
the interband transition arising from the sinking of hole bands near
point below Fermi level in the cT phase, as expected from the density function
theory calculations in combination with the dynamical mean field theory.
However, the reduction of Drude spectral weight is at variance with those
calculations. The measurement also indicates an absence of the abnormal
spectral weight transfer at high energy (near 0.5-0.7 eV) in the cT phase,
suggesting a suppression of electron correlation effect.Comment: 6 pages, 4 figure
2-D DOA Estimation for L-Shaped Array With Array Aperture and Snapshots Extension Techniques
A two-dimensional (2-D) direction of arrival estimation method for L-shaped array with automatic pairing is proposed. It exploits the conjugate symmetry property of the array manifold matrix to increase the effective array aperture and the number of virtual snapshots simultaneously, and then applies the principle of MUSIC to construct an angle cost function and transforms the conventional 2-D search into 1-D via a Rayleigh quotient, which can greatly reduce the computation complexity. Finally, the azimuth and elevation angles are estimated without pair matching. Simulation results show that the proposed method has a better performance and can resolve more sources than some existing computationally efficient methods
ε-Strong Simulation of Fractional Brownian Motion and Related Stochastic Differential Equations
Consider a fractional Brownian motion (fBM) BH={BH(t):t∈[0,1]} with Hurst index H∈(0,1). We construct a probability space supporting both BH and a fully simulatable process B⌢Hε such that
supt∈[0,1]∣∣∣BH(t)−B⌢H∈(t)∣∣∣≤ε
with probability one for any user-specified error bound ɛ>0. When H>1/2, we further enhance our error guarantee to the α-Hölder norm for any α∈(1/2,H). This enables us to extend our algorithm to the simulation of fBM-driven stochastic differential equations Y={Y(t):t∈[0,1]}. Under mild regularity conditions on the drift and diffusion coefficients of Y, we construct a probability space supporting both Y and a fully simulatable process Y⌢ε such that
supt∈[0,1]∣∣Y(t)−Y⌢ε(t)∣∣≤ε
with probability one. Our algorithms enjoy the tolerance-enforcement feature, under which the error bounds can be updated sequentially in an efficient way. Thus, the algorithms can be readily combined with other advanced simulation techniques to estimate the expectations of functionals of fBMs efficiently
Heavy and Light Quarks with Lattice Chiral Fermions
The feasibility of using lattice chiral fermions which are free of
errors for both the heavy and light quarks is examined. The fact that the
effective quark propagators in these fermions have the same form as that in the
continuum with the quark mass being only an additive parameter to a chirally
symmetric antihermitian Dirac operator is highlighted. This implies that there
is no distinction between the heavy and light quarks and no mass dependent
tuning of the action or operators as long as the discretization error is negligible. Using the overlap fermion, we find that the
(and ) errors in the dispersion relations of the pseudoscalar and
vector mesons and the renormalization of the axial-vector current and scalar
density are small. This suggests that the applicable range of may be
extended to with only 5% error, which is a factor of
larger than that of the improved Wilson action. We show that the generalized
Gell-Mann-Oakes-Renner relation with unequal masses can be utilized to
determine the finite errors in the renormalization of the matrix elements
for the heavy-light decay constants and semileptonic decay constants of the B/D
meson.Comment: final version to appear in Int. Jou. Mod. Phys.
Effects of losses in the hybrid atom-light interferometer
Enhanced Raman scattering can be obtained by injecting a seeded light field
which is correlated with the initially prepared collective atomic excitation.
This Raman amplification process can be used to realize atom-light hybrid
interferometer. We numerically calculate the phase sensitivities and the
signal-to-noise ratios of this interferometer with the method of homodyne
detection and intensity detection, and give their differences between this two
methods. In the presence of loss of light field and atomic decoherence the
measure precision will be reduced which can be explained by the break of the
intermode decorrelation conditions of output modesComment: 9 pages, 7 figure
Quantum criticality and nodal superconductivity in the FeAs-based superconductor KFe2As2
The in-plane resistivity and thermal conductivity of
FeAs-based superconductor KFeAs single crystal were measured down to 50
mK. We observe non-Fermi-liquid behavior at =
5 T, and the development of a Fermi liquid state with when
further increasing field. This suggests a field-induced quantum critical point,
occurring at the superconducting upper critical field . In zero field
there is a large residual linear term , and the field dependence of
mimics that in d-wave cuprate superconductors. This indicates that
the superconducting gaps in KFeAs have nodes, likely d-wave symmetry.
Such a nodal superconductivity is attributed to the antiferromagnetic spin
fluctuations near the quantum critical point.Comment: 4 pages, 4 figures - replaces arXiv:0909.485
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