285 research outputs found
Probing the pairing symmetry in the over-doped Fe-based superconductor Ba_0.35Rb_0.65Fe_2As_2 as a function of hydrostatic pressure
We report muon spin rotation experiments on the magnetic penetration depth
lambda and the temperature dependence of lambda^{-2} in the over-doped Fe-based
high-temperature superconductor (Fe-HTS) Ba_{1-x}Rb_ xFe_2As_2 (x = 0.65)
studied at ambient and under hydrostatic pressures up to p = 2.3 GPa. We find
that in this system lambda^{-2}(T) is best described by d-wave scenario. This
is in contrast to the case of the optimally doped x = 0.35 system which is
known to be a nodeless s^{+-}-wave superconductor. This suggests that the
doping induces the change of the pairing symmetry from s^{+-} to d-wave in
Ba_{1-x}Rb_{x}Fe_{2}As_{2}. In addition, we find that the d-wave order
parameter is robust against pressure, suggesting that d is the common and
dominant pairing symmetry in over-doped Ba_{1-x}Rb_{x}Fe_{2}As_{2}. Application
of pressure of p = 2.3 GPa causes a decrease of lambda(0) by less than 5 %,
while at optimal doping x = 0.35 a significant decrease of lambda(0) was
reported. The superconducting transition temperature T_c as well as the gap to
T_c ratio 2Delta/k_BT_c show only a modest decrease with pressure. By combining
the present data with those previously obtained for optimally doped system x =
0.35 and for the end member x = 1 we conclude that the SC gap symmetry as well
as the pressure effects on the SC quantities strongly depend on the Rb doping
level. These results are discussed in the light of the putative Lifshitz
transition, i.e., a disappearance of the electron pockets in the Fermi surface
of Ba_{1-x}Rb_{x}Fe_{2}As_{2} upon hole doping.Comment: Accepted for publication in Physical Review
Muon Spin Rotation Study of the Intercalated Graphite Superconductor CaC6 at Low Temperatures
Muon spin rotation (μSR) experiments were performed on the intercalated graphite CaC6 in the normal and superconducting state down to 20mK. In addition, AC magnetization measurements were carried out resulting in an anisotropic upper critical field , from which the coherence lengths ξ ab (0)=36.3(1.5)nm and ξ c (0)=4.3(7)nm were estimated. The anisotropy parameter increases monotonically with decreasing temperature. Asingle isotropic s-wave description of superconductivity cannot account for this behavior. From magnetic field dependent μSR experiments, the absolute value of the in-plane magnetic penetration depth λ ab (0)=78(3)nm was determined. The temperature dependence of the superfluid density ρ s (T) is slightly better described by a two-gap than a single-gap mode
Field Dependent Superfluid Density in the Optimally Doped SmFeAsO_(1-x)F_y Superconductor
The magnetic field dependence of the in-plane magnetic penetration depth for
optimally doped SmFeAsO_(1-x)F_y was investigated by combining torque
magnetometry, SQUID magnetometry, and muon-spin rotation. The results obtained
from these techniques show all a pronounced decrease of the superfluid density
as the field is increased up to 1.4 T. This behavior is analysed within a
two-band model with self-consistently derived coupled gaps, where the
superfluid density related to the larger gap is field independent and the
superfluid density related to the smaller gap is strongly suppressed with
increasing field.Comment: 7 pages, 5 figure
Common effect of chemical and external pressures on the magnetic properties of RECoPO (RE = La, Pr)
We report a detailed investigation of RECoPO (RE = La, Pr) and LaCoAsO
materials performed by means of muon spin spectroscopy. Zero-field measurements
show that the electrons localized on the Pr ions do not play any role in
the static magnetic properties of the compounds. Magnetism at the local level
is indeed fully dominated by the weakly-itinerant ferromagnetism from the Co
sublattice only. The increase of the chemical pressure triggered by the
different ionic radii of La and Pr, on the other hand, plays a
crucial role in enhancing the value of the magnetic critical temperature and
can be mimicked by the application of external hydrostatic pressure up to 24
kbar. A sharp discontinuity in the local magnetic field at the muon site in
LaCoPO at around 5 kbar suggests a sizeable modification in the band structure
of the material upon increasing pressure. This scenario is qualitatively
supported by \emph{ab-initio} density-functional theory calculations.Comment: 13 pages, 10 figure
Oxygen-isotope effect on the in-plane penetration depth in cuprate superconductors
Muon-spin rotation (muSR) studies of the oxygen isotope (^{16}O/^{18}O)
effect (OIE) on the in-plane magnetic field penetration depth lambda_{ab} in
cuprate high-temperature superconductors (HTS) are presented. First, the doping
dependence of the OIE on the transition temperature T_c in various HTS is
briefly discussed. It is observed that different cuprate families show a
similar doping dependence of the OIE on T_c. Then, bulk muSR, low-energy muSR,
and magnetization studies of the total and site-selective OIE on lambda_{ab}
are described in some detail. A substantial OIE on lambda_{ab} was observed in
various cuprate families at all doping levels, suggesting that cuprate HTS are
non-adiabatic superconductors. The experiments clearly demonstrate that the
total OIE on T_c and lambda_{ab} arise from the oxygen sites within the
superconducting CuO_2 planes, demonstrating that the phonon modes involving the
movement of planar oxygen are dominantly coupled to the supercarriers. Finally,
it is shown that the OIE on T_c and lambda_{ab} exhibit a relation that appears
to be generic for different families of cuprate HTS. The observation of these
unusual isotope effects implies that lattice effects play an essential role in
cuprate HTS and have to be considered in any realistic model of
high-temperature superconductivity.Comment: 22 pages, 12 figures. To be published in a special issue of J. Phys.
Cond. Ma
Low-temperature magnetic fluctuations in the Kondo insulator SmB6
We present the results of a systematic investigation of the magnetic
properties of the three-dimensional Kondo topological insulator SmB6 using
magnetization and muon-spin relaxation/rotation (muSR) measurements. The muSR
measurements exhibit magnetic field fluctuations in SmB6 below 15 K due to
electronic moments present in the system. However, no evidence for magnetic
ordering is found down to 19 mK. The observed magnetism in SmB6 is homogeneous
in nature throughout the full volume of the sample. Bulk magnetization
measurements on the same sample show consistent behavior. The agreement between
muSR, magnetization, and NMR results strongly indicate the appearance of
intrinsic bulk magnetic in-gap states associated with fluctuating magnetic
fields in SmB6 at low temperature.Comment: 5 pages, 5 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
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