9,593 research outputs found
Temperature dependence and resonance effects in Raman scattering of phonons in NdFeAsOF single crystals
We report plane-polarized Raman scattering spectra of iron oxypnictide
superconductor NdFeAsOF single crystals with varying fluorine
content. The spectra exhibit sharp and symmetrical phonon lines with a weak
dependence on fluorine doping . The temperature dependence does not show any
phonon anomaly at the superconducting transition. The Fe related phonon
intensity shows a strong resonant enhancement below 2 eV. We associate the
resonant enhancement to the presence of an interband transition around 2 eV
observed in optical conductivity. Our results point to a rather weak coupling
between Raman-active phonons and electronic excitations in iron oxypnictides
superconductors.Comment: 4 pages, 3 figures, to appear in Phys. Rev.
Measurement of a Sign-Changing Two-Gap Superconducting Phase in Electron-Doped Ba(Fe_{1-x}Co_x)_2As_2 Single Crystals using Scanning Tunneling Spectroscopy
Scanning tunneling spectroscopic studies of (x =
0.06, 0.12) single crystals reveal direct evidence for predominantly two-gap
superconductivity. These gaps decrease with increasing temperature and vanish
above the superconducting transition . The two-gap nature and the slightly
doping- and energy-dependent quasiparticle scattering interferences near the
wave-vectors and are consistent with
sign-changing -wave superconductivity. The excess zero-bias conductance and
the large gap-to- ratios suggest dominant unitary impurity scattering.Comment: 4 pages, 4 figures. Paper accepted for publication in Physical Review
Letters. Contact author: Nai-Chang Yeh ([email protected]
Anomalous Meissner effect in pnictide superconductors
The Meissner effect has been studied in Ba(Fe0.926Co0.074)2As2 and
Ba0.6K0.4Fe2As2 single crystals and compared to well known, type-II
superconductors LuNi2B2C and V3Si. Whereas flux penetration is mostly
determined by the bulk pinning (and, perhaps, surface barrier) resulting in a
large negative magnetization, the flux expulsion upon cooling in a magnetic
field is very small, which could also be due to pinning and/or surface barrier
effects. However, in stark contrast with the expected behavior, the amount of
the expelled flux increases almost linearly with the applied magnetic field, at
least up to our maximum field of 5.5 T, which far exceeds the upper limit for
the surface barrier. One interpretation of the observed behavior is that there
is a field-driven suppression of magnetic pair-breaking
Noise bridges dynamical correlation and topology in coupled oscillator networks
We study the relationship between dynamical properties and interaction
patterns in complex oscillator networks in the presence of noise. A striking
finding is that noise leads to a general, one-to-one correspondence between the
dynamical correlation and the connections among oscillators for a variety of
node dynamics and network structures. The universal finding enables an accurate
prediction of the full network topology based solely on measuring the dynamical
correlation. The power of the method for network inference is demonstrated by
the high success rate in identifying links for distinct dynamics on both model
and real-life networks. The method can have potential applications in various
fields due to its generality, high accuracy and efficiency.Comment: 2 figures, 2 tables. Accepted by Physical Review Letter
Collective Excitations, Nambu-Goldstone Modes and Instability of Inhomogeneous Polariton Condensates
We study non-equilibrium microcavity-polariton condensates (MPCs) in a
harmonic potential trap theoretically. We calculate and analyze the steady
state, collective-excitation modes and instability of MPCs. Within excitation
modes, there exist Nambu-Goldstone modes that can reveal the pattern of the
spontaneous symmetry breaking of MPCs. Bifurcation of the stable and unstable
modes is identified in terms of the pumping power and spot size. The unstable
mechanism associated with the inward supercurrent flow is characterized by the
existence of a supersonic region within the condensate.Comment: 16 pages, 3 figure
Hall effect and magnetoresistance in single crystals of NdFeAsOF
Hall effect and magnetoresistance have been measured on single crystals of
with x = 0 ( = 0 K) and x = 0.18 ( = 50 K). For the undoped samples, strong Hall effect and magnetoresistance with
strong temperature dependence were found below about 150 K. The
magnetoresistance was found to be as large as 30% at 15 K at a magnetic field
of 9 T. From the transport data we found that the transition near 155 K was
accomplished in two steps: first one occurs at 155 K which may be associated
with the structural transition, the second one takes place at about 140 K which
may correspond to the spin-density wave like transition. In the superconducting
sample with = 50 K, it is found that the Hall coefficient also reveals
a strong temperature dependence with a negative sign. But the magnetoresistance
becomes very weak and does not satisfy the Kohler's scaling law. These
dilemmatic results (strong Hall effect and very weak magnetoresistance) prevent
to understand the normal state electric conduction by a simple multi-band model
by taking account the electron and hole pockets. Detailed analysis further
indicates that the strong temperature dependence of cannot be easily
understood with the simple multi-band model either. A picture concerning a
suppression to the density of states at the Fermi energy in lowering
temperature is more reasonable. A comparison between the Hall coefficient of
the undoped sample and the superconducting sample suggests that the doping may
remove the nesting condition for the formation of the SDW order, since both
samples have very similar temperature dependence above 175 K.Comment: 8 pages, 9 figure
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