13 research outputs found
Pressure Effect on the superconducting properties of LaO_{1-x}F_{x}FeAs(x=0.11) superconductor
Diamagnetic susceptibility measurements under high hydrostatic pressure (up
to 1.03 GPa) were carried out on the newly discovered Fe-based superconductor
LaO_{1-x}F_{x}FeAs(x=0.11). The transition temperature T_c, defined as the
point at the maximum slope of superconducting transition, was enhanced almost
linearly by hydrostatic pressure, yielding a dT_c/dP of about 1.2 K/GPa.
Differential diamagnetic susceptibility curves indicate that the underlying
superconducting state is complicated. It is suggested that pressure plays an
important role on pushing low T_c superconducting phase toward the main
(optimal) superconducting phase.Comment: 7 pages, 4 figure
Angular dependence of resistivity in the superconducting state of NdFeAsOF single crystals
We report the results of angle dependent resistivity of
NdFeAsOF single crystals in the superconducting state. By
doing the scaling of resistivity within the frame of the anisotropic
Ginzburg-Landau theory, it is found that the angle dependent resistivity
measured under different magnetic fields at a certain temperature can be
collapsed onto one curve. As a scaling parameter, the anisotropy can
be determined for different temperatures. It is found that
increases slowly with decreasing temperature, varying from 5.48
at T=50 K to 6.24 at T=44 K. This temperature dependence can be
understood within the picture of multi-band superconductivity.Comment: 7 pages, 4 figure
Nernst effect of the new iron-based superconductor LaOFFeAs
We report the first Nernst effect measurement on the new iron-based
superconductor LaOFFeAs . In the normal state, the
Nernst signal is negative and very small. Below a large positive peak
caused by vortex motion is observed. The flux flowing regime is quite large
compared to conventional type-II superconductors. However, a clear deviation of
the Nernst signal from normal state background and an anomalous depression of
off-diagonal thermoelectric current in the normal state between and 50
K are observed. We propose that this anomaly in the normal state Nernst effect
could correlate with the SDW fluctuations.Comment: 8 pages, 4 figures; Latex file changed, references adde
Point-Contact Spectroscopy of Iron-Based Layered Superconductor LaOFFeAs
We present point-contact spectroscopy data for junctions between a normal
metal and the newly discovered F-doped superconductor
LaOFFeAs (F-LaOFeAs). A zero-bias conductance peak was
observed and its shape and magnitude suggests the presence of Andreev bound
states at the surface of F-LaOFeAs, which provides a possible evidence of an
unconventional pairing symmetry with a nodal gap function. The maximum gap
value meV was determined from the measured spectra,
in good agreement with the recent experiments on specific heat and lower
critical field.Comment: 5 pages, 4 figure
A Generic Two-band Model for Unconventional Superconductivity and Spin-Density-Wave Order in Electron and Hole Doped Iron-Based Superconductors
Based on experimental data on the newly synthesized iron-based
superconductors and the relevant band structure calculations, we propose a
minimal two-band BCS-type Hamiltonian with the interband Hubbard interaction
included. We illustrate that this two-band model is able to capture the
essential features of unconventional superconductivity and spin density wave
(SDW) ordering in this family of materials. It is found that bound
electron-hole pairs can be condensed to reveal the SDW ordering for zero and
very small doping, while the superconducting ordering emerges at small finite
doping, whose pairing symmetry is qualitatively analyzed to be of nodal d-wave.
The derived analytical formulas not only give out a nearly symmetric phase
diagram for electron and hole doping, but also is likely able to account for
existing main experimental results. Moreover, we also derive two important
relations for a general two-band model and elaborate how to apply them to
determine the band width ratio and the effective interband coupling strength
from experimental data.Comment: 6 pages, 4 figures, refs. added, typos correcte
Nuclear magnetic relaxation and superfluid density in Fe-pnictide superconductors: An anisotropic \pm s-wave scenario
We discuss the nuclear magnetic relaxation rate and the superfluid density
with the use of the effective five-band model by Kuroki et al. [Phys. Rev.
Lett. 101, 087004 (2008)] in Fe-based superconductors. We show that a
fully-gapped anisotropic \pm s-wave superconductivity consistently explains
experimental observations. In our phenomenological model, the gaps are assumed
to be anisotropic on the electron-like \beta Fermi surfaces around the M point,
where the maximum of the anisotropic gap is about four times larger than the
minimum.Comment: 10 pages, 8 figures; Submitted versio
Near-degeneracy of several pairing channels in multiorbital models for the Fe-pnictides
Weak-coupling approaches to the pairing problem in the iron pnictide
superconductors have predicted a wide variety of superconducting ground states.
We argue here that this is due both to the inadequacy of certain approximations
to the effective low-energy band structure, and to the natural near-degeneracy
of different pairing channels in superconductors with many distinct Fermi
surface sheets. In particular, we review attempts to construct two-orbital
effective band models, the argument for their fundamental inconsistency with
the symmetry of these materials, and the comparison of the dynamical
susceptibilities in two- and five-orbital models. We then present results for
the magnetic properties, pairing interactions, and pairing instabilities within
a five-orbital Random Phase Approximation model. We discuss the robustness of
these results for different dopings, interaction strengths, and variations in
band structure. Within the parameter space explored, an anisotropic,
sign-changing s-wave state and a d_x2-y2 state are nearly degenerate, due to
the near nesting of Fermi surface sheets.Comment: 17 pages, 23 figure
Point contact Andreev reflection spectroscopy of NdFeAsO_0.85
The newly discovered oxypnictide family of superconductors show very high
critical temperatures of up to 55K. Whilst there is growing evidence that
suggests a nodal order parameter, point contact Andreev reflection spectroscopy
can provide crucial information such as the gap value and possibly the number
of energy gaps involved. For the oxygen deficient NdFeAsO0.85 with a Tc of
45.5K, we show that there is clearly a gap value at 4.2K that is of the order
of 7meV, consistent with previous studies on oxypnictides with lower Tc.
Additionally, taking the spectra as a function of gold tip contact pressure
reveals important changes in the spectra which may be indicative of more
complex physics underlying this structure.Comment: 11 pages, 3 figures. New references included, extra discussion. This
version is accepted in Superconductor Science and Technolog
Spectroscopic scanning tunneling microscopy insights into Fe-based superconductors
In the first three years since the discovery of Fe-based high Tc
superconductors, scanning tunneling microscopy (STM) and spectroscopy have shed
light on three important questions. First, STM has demonstrated the complexity
of the pairing symmetry in Fe-based materials. Phase-sensitive quasiparticle
interference (QPI) imaging and low temperature spectroscopy have shown that the
pairing order parameter varies from nodal to nodeless s\pm within a single
family, FeTe1-xSex. Second, STM has imaged C4 -> C2 symmetry breaking in the
electronic states of both parent and superconducting materials. As a local
probe, STM is in a strong position to understand the interactions between these
broken symmetry states and superconductivity. Finally, STM has been used to
image the vortex state, giving insights into the technical problem of vortex
pinning, and the fundamental problem of the competing states introduced when
superconductivity is locally quenched by a magnetic field. Here we give a
pedagogical introduction to STM and QPI imaging, discuss the specific
challenges associated with extracting bulk properties from the study of
surfaces, and report on progress made in understanding Fe-based superconductors
using STM techniques.Comment: 36 pages, 23 figures, 229 reference
The sequence and de novo assembly of the giant panda genome
Using next-generation sequencing technology alone, we have successfully generated and assembled a draft sequence of the giant panda genome. The assembled contigs (2.25 gigabases (Gb)) cover approximately 94% of the whole genome, and the remaining gaps (0.05 Gb) seem to contain carnivore-specific repeats and tandem repeats. Comparisons with the dog and human showed that the panda genome has a lower divergence rate. The assessment of panda genes potentially underlying some of its unique traits indicated that its bamboo diet might be more dependent on its gut microbiome than its own genetic composition. We also identified more than 2.7 million heterozygous single nucleotide polymorphisms in the diploid genome. Our data and analyses provide a foundation for promoting mammalian genetic research, and demonstrate the feasibility for using next-generation sequencing technologies for accurate, cost-effective and rapid de novo assembly of large eukaryotic genomes. © 2010 Macmillan Publishers Limited. All rights reserved.link_to_OA_fulltex