145 research outputs found
Superconductivity at 25 K in hole doped
By partially substituting the tri-valence element La with di-valence element
Sr in , we introduced holes into the system. For the first time, we
successfully synthesized the hole doped new superconductors
. The maximum superconducting transition temperature at
about 25 K was observed at a doping level of x = 0.13. It is evidenced by Hall
effect measurements that the conduction in this type of material is dominated
by hole-like charge carriers, rather than electron-like ones. Together with the
data of the electron doped system , a generic phase diagram
is depicted and is revealed to be similar to that of the cuprate
superconductors.Comment: 5 pages, 5 figure
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
Enhanced Orbital Degeneracy in Momentum Space for LaOFeAs
The Fermi surfaces (FS) of LaOFeAs (in =0 plane) consist of two
hole-type circles around point, which do not touch each other, and two
electron-type co-centered ellipses around M point, which are degenerate along
the M-X line. By first-principles calculations, here we show that additional
degeneracy exists for the two electron-type FS, and the crucial role of
F-doping and pressure is to enhance this orbital degeneracy. It is suggested
that the inter-orbital fluctuation is the key point to understand the
unconventional superconductivity in these materials.Comment: 4 pages, 5 figure
Epitaxial growth and anisotropy of La(O,F)FeAs thin films deposited by Pulsed Laser Deposition
LaFeAsO1-xFx thin films were deposited successfully on (001)-oriented LaAlO3
and MgO substrates from stoichiometric LaFeAsO1-xFx polycrystalline targets
with fluorine concentrations up to x = 0.25 by PLD. Room temperature deposition
and post annealing of the films yield nearly phase pure films with a pronounced
c-axis texture and a strong biaxial in-plane orientation. Transport
measurements show metallic resistance and onset of superconductivity at 11 K.
Hc2(T) was determined by resistive measurements and yield Hc2 values of 3 T at
3.6 K for B||c and 6 T at 6.4 K for B||ab.Comment: 11 pages, 5 figure
Structural properties, defects and structural phase transition in the ROFeM (R=La, Nd; M=As, P) materials
The structural properties of the ROFeM (R=La, Nd; M=As, P) materials have
been analyzed by means of electron diffraction, high-resolution
transmission-electron microscopy (TEM) and in-situ cooling TEM observations.
The experimental results demonstrate that the layered ROFeM crystals often
contain a variety of structural defects, such as stacking faults and
small-angle boundaries. The in-situ TEM investigations reveal that, in
association with the remarkable spin-density-wave (SDW) instability near 150 K,
complex structural transitions can be clearly observed in both crystal symmetry
and local microstructure features.Comment: 17 pages, 6 figure
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
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
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
Crystal growth and superconductivity of FeSe_x
Single crystals FeSe_x have been grown in evacuated sealed quartz tube using
a NaCl/KCl flux. The products include two crystal structures of tetragon and
hexagon. The electronic transport and magnetic properties measurements of
FeSe_x single crystal exhibits a superconducting transition at about 10K.Comment: 9 pages, 4 Figure
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