16,210 research outputs found
Superconductivity-induced Phonon Renormalization on NaFeCoAs
We report a study of the lattice dynamics in superconducting NaFeAs (Tc = 8
K) and doped NaFe0.97Co0.03As (Tc = 20 K) using Raman light scattering. Five of
the six phonon modes expected from group theory are observed. In contrast with
results obtained on iso-structural and iso-electronic LiFeAs, anomalous
broadening of Eg(As) and A1g(Na) modes upon cooling is observed in both
samples. In addition, in the Co-doped sample, a superconductivity-induced
renormalization of the frequency and linewidth of the B1g(Fe) vibration is
observed. This renormalization can not be understood within a single band and
simple multi-band approaches. A theoretical model that includes the effects of
SDW correlations along with sign-changing s-wave pairing state and interband
scattering has been developed to explain the observed behavior of the B1g(Fe)
mode.Comment: 10 pages; 6 figure
Raman Scattering Study of the Lattice Dynamics of Superconducting LiFeAs
We report an investigation of the lattice dynamical properties of LiFeAs
using inelastic light scattering. Five out of the six expected phonon modes are
observed. The temperature evolution of their frequencies and linewidths is in
good agreement with an anharmonic-decay model. We find no evidence for
substantial electron-phonon coupling, and no superconductivity-induced phonon
anomalies.Comment: 5 pages, 3 figures, 1 tabl
Electron-boson spectral density of LiFeAs obtained from optical data
We analyze existing optical data in the superconducting state of LiFeAs at 4 K, to recover its electron-boson spectral density. A maximum entropy
technique is employed to extract the spectral density from
the optical scattering rate. Care is taken to properly account for elastic
impurity scattering which can importantly affect the optics in an -wave
superconductor, but does not eliminate the boson structure. We find a robust
peak in centered about 8.0 meV or 5.3 (with 17.6 K). Its position in energy agrees well with a similar
structure seen in scanning tunneling spectroscopy (STS). There is also a peak
in the inelastic neutron scattering (INS) data at this same energy. This peak
is found to persist in the normal state at 23 K. There is evidence that
the superconducting gap is anisotropic as was also found in low temperature
angular resolved photoemission (ARPES) data.Comment: 17 pages, 6 figure
The Gluon Spin in the Chiral Bag Model
We study the gluon polarization contribution at the quark model
renormalization scale to the proton spin, , in the chiral bag model. It
is evaluated by taking the expectation value of the forward matrix element of a
local gluon operator in the axial gauge . It is shown that the confining
boundary condition for the color electric field plays an important role. When a
solution satisfying the boundary condition for the color electric field, which
is not the conventionally used but which we favor, is used, the has a
positive value for {\it all} bag radii and its magnitude is comparable to the
quark spin polarization. This results in a significant reduction in the
relative fraction of the proton spin carried by the quark spin, which is
consistent with the small flavor singlet axial current measured in the EMC
experiments.Comment: Corrections to figure
Electronic structure of YbB: Is it a Topological Insulator or not?
To resolve the controversial issue of the topological nature of the
electronic structure of YbB, we have made a combined study using density
functional theory (DFT) and angle resolved photoemission spectroscopy (ARPES).
Accurate determination of the low energy band topology in DFT requires the use
of modified Becke-Johnson exchange potential incorporating the spin-orbit
coupling and the on-site Coulomb interaction of Yb electrons as large
as 7 eV. We have double-checked the DFT result with the more precise GW band
calculation. ARPES is done with the non-polar (110) surface termination to
avoid band bending and quantum well confinement that have confused ARPES
spectra taken on the polar (001) surface termination. Thereby we show
definitively that YbB has a topologically trivial B 2-Yb 5
semiconductor band gap, and hence is a non-Kondo non-topological insulator
(TI). In agreement with theory, ARPES shows pure divalency for Yb and a -
band gap of 0.3 eV, which clearly rules out both of the previous scenarios of
- band inversion Kondo TI and - band inversion non-Kondo TI. We
have also examined the pressure-dependent electronic structure of YbB,
and found that the high pressure phase is not a Kondo TI but a
\emph{p}-\emph{d} overlap semimetal.Comment: The main text is 6 pages with 4 figures, and the supplementary
information contains 6 figures. 11 pages, 10 figures in total To be appeared
in Phys. Rev. Lett. (Online publication is around March 16 if no delays.
Density functional calculations of the electronic structure and magnetic properties of the hydrocarbon K3picene superconductor near the metal-insulator transition
We have investigated the electronic structures and magnetic properties of of
K3picene, which is a first hydrocarbon superconductor with high transition
temperature T_c=18K. We have shown that the metal-insulator transition (MIT) is
driven in K3picene by 5% volume enhancement with a formation of local magnetic
moment. Active bands for superconductivity near the Fermi level E_F are found
to have hybridized character of LUMO and LUMO+1 picene molecular orbitals.
Fermi surfaces of K3picene manifest neither prominent nesting feature nor
marked two-dimensional behavior. By estimating the ratio of the Coulomb
interaction U and the band width W of the active bands near E_F, U/W, we have
demonstrated that K3picene is located in the vicinity of the Mott transition.Comment: 5 pages, 5 figure
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