5,329 research outputs found
Computer Program for the Calculation of Multicomponent Convective Diffusion Deposition Rates from Chemically Frozen Boundary Layer Theory
The computer program based on multicomponent chemically frozen boundary layer (CFBL) theory for calculating vapor and/or small particle deposition rates is documented. A specific application to perimter-averaged Na2SO4 deposition rate calculations on a cylindrical collector is demonstrated. The manual includes a typical program input and output for users
Theoretical search for superconductivity in Sc3XB perovskites and weak ferromagnetism in Sc3X (X = Tl, In, Ga, Al)
A possibility for a new family of intermetallic perovskite superconductors
Sc3XB, with X = Tl, In, Ga and Al, is presented as a result of KKR electronic
structure and pseudopotential phonon calculations. The large values of computed
McMillan--Hopfield parameters on scandium suggest appearance of
superconductivity in Sc3XB compounds. On the other hand, the possibility of
weak itinerant ferromagnetic behavior of Sc3X systems is indicated by the small
magnetic moment on Sc atoms in two cases of X =~ l and In. Also the electronic
structure and resulting superconducting parameters for more realistic case of
boron--deficient systems Sc3XB_x are computed using KKR--CPA method, by
replacing boron atom with a vacancy. The comparison of the calculated
McMillan--Hopfield parameters of the Sc3XB series with corresponding values in
MgCNi3 and YRh3B superconductors is given, finding the favorable trends for
superconductivity.Comment: 13 pages, 13 figures. v3 - revise
Specific heat of CaNaFeAs single crystals: unconventional s multi-band superconductivity with intermediate repulsive interband coupling and sizable attractive intraband couplings
We report a low-temperature specific heat study of high-quality single
crystals of the heavily hole doped superconductor
CaNaFeAs. This compound exhibits bulk
superconductivity with a transition temperature \,K, which is
evident from the magnetization, transport, and specific heat measurements. The
zero field data manifests a significant electronic specific heat in the normal
state with a Sommerfeld coefficient mJ/mol K. Using a
multi-band Eliashberg analysis, we demonstrate that the dependence of the zero
field specific heat in the superconducting state is well described by a
three-band model with an unconventional s pairing symmetry and gap
magnitudes of approximately 2.35, 7.48, and -7.50 meV. Our analysis
indicates a non-negligible attractive intraband coupling,which contributes
significantly to the relatively high value of . The Fermi surface averaged
repulsive and attractive coupling strengths are of comparable size and outside
the strong coupling limit frequently adopted for describing high- iron
pnictide superconductors. We further infer a total mass renormalization of the
order of five, including the effects of correlations and electron-boson
interactions.Comment: 8 Figures, Submitted to PR
Atomic Parity Violation and Precision Electroweak Physics - An Updated Analysis
A new analysis of parity violation in atomic cesium has led to the improved
value of the weak charge, . The implications
of this result for constraining the Peskin-Takeuchi parameters S and T and for
guiding searches for new Z bosons are discussed.Comment: 8 pages, LaTeX, 3 figures, Submitted to Physical Review D. Updated
experimental inputs and references; clarification of notatio
Tight-binding parameters and exchange integrals of Ba_2Cu_3O_4Cl_2
Band structure calculations for Ba_2Cu_3O_4Cl_2 within the local density
approximation (LDA) are presented. The investigated compound is similar to the
antiferromagnetic parent compounds of cuprate superconductors but contains
additional Cu_B atoms in the planes. Within the LDA, metallic behavior is found
with two bands crossing the Fermi surface (FS). These bands are built mainly
from Cu 3d_{x^2-y^2} and O 2p_{x,y} orbitals, and a corresponding tight-binding
(TB) model has been parameterized. All orbitals can be subdivided in two sets
corresponding to the A- and B-subsystems, respectively, the coupling between
which is found to be small. To describe the experimentally observed
antiferromagnetic insulating state, we propose an extended Hubbard model with
the derived TB parameters and local correlation terms characteristic for
cuprates. Using the derived parameter set we calculate the exchange integrals
for the Cu_3O_4 plane. The results are in quite reasonable agreement with the
experimental values for the isostructural compound Sr_2Cu_3O_4Cl_2.Comment: 5 pages (2 tables included), 4 ps-figure
Quantum-Chromodynamic Potential Model for Light-Heavy Quarkonia and the Heavy Quark Effective Theory
We have investigated the spectra of light-heavy quarkonia with the use of a
quantum-chromodynamic potential model which is similar to that used earlier for
the heavy quarkonia. An essential feature of our treatment is the inclusion of
the one-loop radiative corrections to the quark-antiquark potential, which
contribute significantly to the spin-splittings among the quarkonium energy
levels. Unlike and , the potential for a light-heavy
system has a complicated dependence on the light and heavy quark masses and
, and it contains a spin-orbit mixing term. We have obtained excellent
results for the observed energy levels of , , , and , and
we are able to provide predicted results for many unobserved energy levels. Our
potential parameters for different quarkonia satisfy the constraints of quantum
chromodynamics.
We have also used our investigation to test the accuracy of the heavy quark
effective theory. We find that the heavy quark expansion yields generally good
results for the and energy levels provided that and
corrections are taken into account in the quark-antiquark
interactions. It does not, however, provide equally good results for the energy
levels of and , which indicates that the effective theory can be
applied more accurately to the quark than the quark.Comment: 17 pages of LaTeX. To appear in Physical Review D. Complete
PostScript file is available via WWW at
http://gluon.physics.wayne.edu/wsuhep/jim/heavy.p
Electronic structure and magnetic properties of the spin-1/2 Heisenberg system CuSe2O5
A microscopic magnetic model for the spin-1/2 Heisenberg chain compound
CuSe2O5 is developed based on the results of a joint experimental and
theoretical study. Magnetic susceptibility and specific heat data give evidence
for quasi-1D magnetism with leading antiferromagnetic (AFM) couplings and an
AFM ordering temperature of 17 K. For microscopic insight, full-potential DFT
calculations within the local density approximation (LDA) were performed. Using
the resulting band structure, a consistent set of transfer integrals for an
effective one-band tight-binding model was obtained. Electronic correlations
were treated on a mean-field level starting from LDA (LSDA+U method) and on a
model level (Hubbard model). In excellent agreement of experiment and theory,
we find that only two couplings in CuSe2O5 are relevant: the nearest-neighbour
intra-chain interaction of 165 K and a non-frustrated inter-chain coupling of
20 K. From a comparison with structurally related systems (Sr2Cu(PO4)2,
Bi2CuO4), general implications for a magnetic ordering in presence of
inter-chain frustration are made.Comment: 20 pages, 8 figures, 3 table
Structure and magnetism of Cr2BP3O12: Towards the quantum-classical crossover in a spin-3/2 alternating chain
Magnetic properties of the spin-3/2 Heisenberg system Cr2BP3O12 are
investigated by magnetic susceptibility chi(T) measurements, electron spin
resonance, neutron diffraction, and density functional theory (DFT)
calculations, as well as classical and quantum Monte Carlo (MC) simulations.
The broad maximum of chi(T) at 85K and the antiferromagnetic Weiss temperature
of 139 K indicate low-dimensional magnetic behavior. Below TN = 28 K, Cr2BP3O12
is antiferromagnetically ordered with the k = 0 propagation vector and an
ordered moment of 2.5 muB/Cr. DFT calculations, including DFT+U and hybrid
functionals, yield a microscopic model of spin chains with alternating
nearest-neighbor couplings J1 and J1' . The chains are coupled by two
inequivalent interchain exchanges of similar strength (~1-2 K), but different
sign (antiferromagnetic and ferromagnetic). The resulting spin lattice is
quasi-one-dimensional and not frustrated. Quantum MC simulations show excellent
agreement with the experimental data for the parameters J1 ~= 50 K and J1'/J1
~= 0.5. Therefore, Cr2BP3O12 is close to the gapless critical point (J1'/J1 =
0.41) of the spin-3/2 bond-alternating Heisenberg chain. The applicability
limits of the classical approximation are addressed by quantum and classical MC
simulations. Implications for a wide range of low-dimensional S = 3/2 materials
are discussed.Comment: Published version: 13 pages, 7 figures, 5 tables + Supplementary
informatio
Consequences of critical interchain couplings and anisotropy on a Haldane chain
Effects of interchain couplings and anisotropy on a Haldane chain have been
investigated by single crystal inelastic neutron scattering and density
functional theory (DFT) calculations on the model compound SrNiVO.
Significant effects on low energy excitation spectra are found where the
Haldane gap (; where is the intrachain exchange
interaction) is replaced by three energy minima at different antiferromagnetic
zone centers due to the complex interchain couplings. Further, the triplet
states are split into two branches by single-ion anisotropy. Quantitative
information on the intrachain and interchain interactions as well as on the
single-ion anisotropy are obtained from the analyses of the neutron scattering
spectra by the random phase approximation (RPA) method. The presence of
multiple competing interchain interactions is found from the analysis of the
experimental spectra and is also confirmed by the DFT calculations. The
interchain interactions are two orders of magnitude weaker than the
nearest-neighbour intrachain interaction = 8.7~meV. The DFT calculations
reveal that the dominant intrachain nearest-neighbor interaction occurs via
nontrivial extended superexchange pathways Ni--O--V--O--Ni involving the empty
orbital of V ions. The present single crystal study also allows us to
correctly position SrNiVO in the theoretical - phase
diagram [T. Sakai and M. Takahashi, Phys. Rev. B 42, 4537 (1990)] showing where
it lies within the spin-liquid phase.Comment: 12 pages, 12 figures, 3 tables PRB (accepted). in Phys. Rev. B (2015
Antiferromagnetic fluctuations in the normal state of LiFeAs
We present a detailed study of 75As NMR Knight shift and spin-lattice
relaxation rate in the normal state of stoichiometric polycrystalline LiFeAs.
Our analysis of the Korringa relation suggests that LiFeAs exhibits strong
antiferromagnetic fluctuations, if transferred hyperfine coupling is a dominant
interaction between 75As nuclei and Fe electronic spins, whereas for an on-site
hyperfine coupling scenario, these are weaker, but still present to account for
our experimental observations. Density-functional calculations of electric
field gradient correctly reproduce the experimental values for both 75As and
7Li sites.Comment: 5 pages, 3 figures, thoroughly revised version with refined
experimental data, accepted for publication as a Rapid Communication in
Physical Review B
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