168 research outputs found
Anisotropy in the electrical resistivity and susceptibility of superconducting BaFeAs single crystals
Sizable single crystals of have been grown with self-flux
method. The crystals are plate-like with c-axis perpendicular to the plane. The
size can be as large as 3 x 5 x 0.2 . The resistivity anisotropy
() is as large as about 150, and independent of temperature.
The transport in ab plane and along c-axis direction shares the same scattering
mechanism. In contrast to the magnetic behavior of polycrystalline samples, no
Curie-Weiss behavior are observed, a temperature linear dependent
susceptibility occurs above spin-density-wave (SDW) transition. The
susceptibility behavior is very similar to that of antiferromagnetic SDW
chromium. Magnetic behavior of single crystal definitely gives evidence for
existence of local moment except for the contribution to susceptibility from
itinerant electrons. A resistivity minimum strongly dependent on magnetic field
is observed. A log(1/T) divergency, similar to that of the underdoped cuprates,
happens at low temperature. Here we first present intrinsic transport and
magnetic properties, and their anisotropy from high quality single crystal.Comment: 12 pages, 4 figures, Physical Review Letters (2009) in pres
Evidence for Local Moment by Electron Spin Resonance Study on Polycrystalline LaFeAsOF (x=0 and 0.13)
The temperature dependence of electron spin resonance (ESR) was studied in
the oxypnictide superconductors LaFeAsOF (x = 0.0 and 0.13). In the
samples, the ESR signal indicates that the g factor and peak-to-peak linewidth
strongly depend on temperature, especially at low temperatures. It indicates a
strong coupling picture with existence of local moment. The dependence
mentioned above gradually attenuates, and tends to saturation around
room-temperature. This behavior could be ascribed to "bottleneck" effect due to
coupling of local moment and itinerant electron. In addition, a Curie-Weiss
like behavior is also observed in temperature dependent integral intensity for
the two samples. Our results strongly support the existence of local moments in
these materials while its origin is still unclear. The results also indicate
strong magnetic frustration in this system, and magnetic fluctuation mechanism
for superconductivity is suggested.Comment: 5 pages, 5 figure
Superconductivity induced by oxygen deficiency in Sr-doped LaOFeAs
We synthesized Sr-doped sample with single phase,
and systematically studied the effect of oxygen deficiency in the Sr-doped
LaOFeAs system. It is found that substitution of Sr for La indeed induces the
hole carrier evidenced by positive thermoelectric power (TEP), but no bulk
superconductivity is observed. The superconductivity can be realized by
annealing the as-grown sample in vacuum to produce the oxygen deficiency. With
increasing the oxygen deficiency, the superconducting transition temperature
() increases and maximum reaches about 26 K the same as that in
La(O,F)FeAs. TEP dramatically changes from positive to negative in the
nonsuperconducting as-grown sample to the superconducting samples with oxygen
deficiency. While is always negative for all samples (even for Sr-doped
as grown sample). It suggests that the is
still electron-type superconductor.Comment: 4 pages, 4 figure
Transport properties and superconductivity in (M=La and K) with double FeAs layers
We synthesized the samples (M=La and K) with
-type structure. These samples were systematically characterized by
resistivity, thermoelectic power (TEP) and Hall coefficient ().
shows an anomaly in resistivity at about 140 K. Substitution of La
for Ba leads to a shift of the anomaly to low temperature, but no
superconducting transition is observed. Potassium doping leads to suppression
of the anomaly in resistivity and induces superconductivity at 38 K as reported
by Rotter et al.\cite{rotter}. The Hall coefficient and TEP measurements
indicate that the TEP is negative for and La-doped ,
indicating n-type carrier; while potassium doping leads to change of the sign
in and TEP. It definitely indicates p-type carrier in superconducting
with double FeAs layers, being in contrast to the case of
with single FeAs layer. A similar superconductivity is also
observed in the sample with nominal composition .Comment: 4 pages, 4 figure
Thermodynamic properties of Ba1-xMxFe2As2 (M = La and K)
The specific heat of BaFeAs single crystal, electron-doped
BaLaFeAs and hole-doped BaKFeAs
polycrystals were measured. For undoped BaFeAs single crystal, a very
sharp specific heat peak was observed at 136 K. This is attributed to the
structural and antiferromagnetic transitions occurring at the same temperature.
of the electron-doped non-superconducting
BaLaFeAs also shows a small peak at 120 K, indicating a
similar but weaker structural/antiferromagnetic transition. For the hole-doped
superconducting BaKFeAs, a clear peak of was
observed at = 36 K, which is the highest peak seen at superconducting
transition for iron-based high- superconductors so far. The electronic
specific heat coefficient and Debye temperature of these
compounds were obtained from the low temperature data
Different resistivity response to spin density wave and superconductivity at 20 K in
We report that intrinsic transport and magnetic properties, and their
anisotropy from high quality single crystal . The resistivity
anisotropy () is , and less than 150 of
, which arises from the strong coupling along c-axis due to an
apparent contraction of about 0.13 nm compared to . Temperature
independent anisotropy indicates that the transport in ab plane and along
c-axis direction shares the same scattering mechanism. In sharp contrast to the
case of parent compounds (R=rare earth) and (M=Ba and Sr),
spin-density-wave (SDW) ordering (or structural transition) leads to a steep
increase of resistivity in . Such different resistivity response to
SDW ordering is helpful to understand the role played by SDW ordering in
Fe-based high- superconductors. The susceptibility behavior is very
similar to that observed in single crystal . A linear temperature
dependent susceptibility occurs above SDW transition of about 165 K. Partial
substitution of Na for Ca suppresses the SWD ordering (anomaly in resistivity)
and induces occurrence of superconductivity at K.Comment: 5 pages, 6 figure
- β¦