66 research outputs found
Microscopic Study of the Superconducting State of the Iron Pnictide RbFe_2As_2
A study of the temperature and field dependence of the penetration depth
\lambda of the superconductor RbFe_2As_2 (T_c=2.52 K) was carried out by means
of muon-spin rotation measurements. In addition to the zero temperature value
of the penetration depth \lambda(0)=267(5) nm, a determination of the upper
critical field B_c2(0)=2.6(2) T was obtained. The temperature dependence of the
superconducting carrier concentration is discussed within the framework of a
multi-gap scenario. Compared to the other "122" systems which exhibit much
higher Fermi level, a strong reduction of the large gap BCS ratio 2\Delta/k_B
T_c is observed. This is interpreted as a consequence of the absence of
interband processes. Indications of possible pair-breaking effect are also
discussed.Comment: 5 pages, 4 figure
Pressure cycle of superconducting Cs0.8Fe2Se2: a transport study
We report measurements of the temperature and pressure dependence of the
electrical resistivity of single crystalline iron-based chalcogenide
Cs0.8Fe2Se2. In this material superconductivity Tc~30K develops from a normal
state with extremely large resistivity. At ambient pressure a large "hump" in
the resistivity is observed around 200K. Under pressure, the resistivity
decreases by two orders of magnitude, concomitant with a sudden Tc suppression
around p~8GPa. Even at 9GPa a metallic resistivity state is not recovered, and
the {\rho}(T) "hump" is still detected. A comparison of the data measured upon
increasing and decreasing the external pressure leads us to suggest that
superconductivity is not related to this hump
Muon-spin rotation measurements of the magnetic penetration depth in the Fe-based superconductor Ba_(1-x)Rb_(x)Fe2As2
Measurements of the magnetic penetration depth (lambda) in the Fe-based
superconductor Ba_(1-x)Rb_xFe_2As_2 (x=0.3, 0.35, 0.4) were carried out using
the muon-spin rotation muSR technique. The temperature dependence of lambda is
well described by a two-gap s+s-wave scenario with a small gap Delta_1 = 1 - 3
mev and a large gap Delta_2 = 7 - 9 mev. By combining the present data with
those obtained for RbFe_2As_2 a decrease of the BCS ratio 2Delta_2/(k_B)(T_c)
with increasing Rb content x is observed. On the other hand, the BCS ratio
2Delta_1/(k_B)(T_c) is almost independent of x. In addition, the contribution
of Delta_1 to the superfluid density is found to increase with x. These results
are discussed in the light of the suppression of interband processes upon hole
doping.Comment: 7 pages, 6 figure
Tuning of competing magnetic and superconducting phase volumes in LaFeAsO$_0.945F_0.055 by hydrostatic pressure
The interplay between magnetism and superconductivity in LaFeAsO_0.945F_0.055
was studied as a function of hydrostatic pressure up to p~2.4GPa by means of
muon-spin rotation (\muSR) and magnetization measurements. The application of
pressure leads to a substantial decrease of the magnetic ordering temperature
T_N and a reduction of the magnetic phase volume and, at the same time, to a
strong increase of the superconducting transition temperature T_c and the
diamagnetic susceptibility. From the volume sensitive \muSR measurements it can
be concluded that the superconducting and the magnetic areas which coexist in
the same sample are inclined towards spatial separation and compete for phase
volume as a function of pressure.Comment: 4 pages, 4 figure
Superconductivity and magnetism in RbxFe2-ySe2: Impact of thermal treatment on mesoscopic phase separation
An extended study of the superconducting and normal-state properties of
various as-grown and post-annealed RbxFe2-ySe2 single crystals is presented.
Magnetization experiments evidence that annealing of RbxFe2-ySe2 at 413 K, well
below the onset of phase separation Tp=489 K, neither changes the magnetic nor
the superconducting properties of the crystals. In addition, annealing at 563
K, well above Tp, suppresses the superconducting transition temperature Tc and
leads to an increase of the antiferromagnetic susceptibility accompanied by the
creation of ferromagnetic impurity phases, which are developing with annealing
time. However, annealing at T=488K=Tp increases Tc up to 33.3 K, sharpens the
superconducting transition, increases the lower critical field, and strengthens
the screening efficiency of the applied magnetic field. Resistivity
measurements of the as-grown and optimally annealed samples reveal an increase
of the upper critical field along both crystallographic directions as well as
its anisotropy. Muon spin rotation and scanning transmission electron
microscopy experiments suggest the coexistence of two phases below Tp: a
magnetic majority phase of Rb2Fe4Se5 and a non-magnetic minority phase of
Rb0.5Fe2Se2. Both microscopic techniques indicate that annealing the specimens
just at Tp does not affect the volume fraction of the two phases, although the
magnetic field distribution in the samples changes substantially. This suggests
that the microstructure of the sample, caused by mesoscopic phase separation,
is modified by annealing just at Tp, leading to an improvement of the
superconducting properties of RbxFe2-ySe2 and an enhancement of Tc.Comment: 13 pages, 12 figure
Extended Magnetic Dome Induced by Low Pressures in Superconducting FeSeS
We report muon spin rotation (SR) and magnetization measurements under
pressure on FeSeS with x
.Above GPa we find microscopic coexistence of
superconductivity with an extended dome of long range magnetic order that spans
a pressure range between previously reported separated magnetic phases. The
magnetism initially competes on an atomic scale with the coexisting
superconductivity leading to a local maximum and minimum of the superconducting
. The maximum of corresponds to the onset of
magnetism while the minimum coincides with the pressure of strongest
competition. A shift of the maximum of for a series of single
crystals with x up to 0.14 roughly extrapolates to a putative magnetic and
superconducting state at ambient pressure for x .Comment: 10 pages, 6 figures, including supplemental materia
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