23 research outputs found
Multi-band superconductivity in LaFeAsO_{0.9}F_{0.1} single crystals probed by high-field vortex torque magnetometry
To probe manifestations of multiband superconductivity in oxypnictides, we
measured the angular dependence of the magnetic torque in the
mixed state of LaOFFeAs single crystals as a function of
temperature and magnetic fields up to 18 T. The paramagnetic
contribution of the Fe ions is properly treated in order to extract the
effective mass anisotropy parameter from
. We show that depends strongly on both and ,
reaching a maximum value of 10 followed by a decrease towards values
close to 1 as is lowered. The observed field dependencies of the London
penetration depth and suggest the onset of suppression
of a superconducing gap at .Comment: 7 pages, 7 figure
Single crystal of superconducting SmFeAsO1-xFy grown at high pressure
Single crystals of SmFeAsO1-xFy of a size up to 120 micrometers have been
grown from NaCl/KCl flux at a pressure of 30 kbar and temperature of 1350-1450
C using the cubic anvil high-pressure technique. The superconducting transition
temperature of the obtained single crystals varies between 45 and 53 K.Obtained
crystals are characterized by a full diamagnetic response in low magnetic
fields and by a high critical current density in high magnetic fields.
Structural refinement has been performed on single crystal. Differential
thermal analysis investigations at 1 bar Ar pressure show decomposition of
SmFeAsO1-xFy at 1302 C.Comment: 12 pages, 3 tables, 6 figure
Quasiparticle relaxation dynamics in spin-density-wave and superconducting SmFeAsO_{1-x}F_{x} single crystals
We investigate the quasiparticle relaxation and low-energy electronic
structure in undoped SmFeAsO and near-optimally doped SmFeAsO_{0.8}F_{0.2}
single crystals - exhibiting spin-density wave (SDW) ordering and
superconductivity respectively - using pump-probe femtosecond spectroscopy. In
the undoped single crystals a single relaxation process is observed, showing a
remarkable critical slowing down of the QP relaxation dynamics at the SDW
transition temperature T_{SDW}\simeq125{K}. In the superconducting (SC)
crystals multiple relaxation processes are present, with distinct SC state
quasiparticle recombination dynamics exhibiting a BCS-like T-dependent
superconducting gap, and a pseudogap (PG)-like feature with an onset above 180K
indicating the existence of a pseudogap of magnitude
2\Delta_{\mathrm{PG}}\simeq120 meV above T_{\mathrm{c}}. From the pump-photon
energy dependence we conclude that the SC state and PG relaxation channels are
independent, implying the presence of two separate electronic subsystems. We
discuss the data in terms of spatial inhomogeneity and multi-band scenarios,
finding that the latter is more consistent with the present data.Comment: Replaced by the correct versio
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
Superconductivity at 23 K and Low Anisotropy in Rb-Substituted BaFe_2As_2 Single Crystals
Single crystals of Ba_{1-x}Rb_{x}Fe_2As_2 with x=0.05-0.1 have been grown
from Sn flux and are bulk superconductors with T_c up to 23 K. The crystal
structure was determined by X-ray diffraction analysis, and Sn is found to be
incorporated for 9% Ba, shifted by 1.1 Angstroem away from the Ba site towards
the (Fe_2As_2)-layers. The upper critical field deduced from resistance
measurements is anisotropic with slopes of 7.1(3) T/K (H || ab-plane) and
4.2(2) T/K (H || c-axis), sufficiently far below T_c. The extracted upper
critical field anisotropy of 3 close to T_c, is in good agreement with the
estimate from magnetic torque measurements. This indicates that the electronic
properties in the doped BaFe_2As_2 compound are significantly more isotropic
than those in the LnFeAsO family. The in-plane critical current density at 5 K
exceeds 10^6 A/cm^2, making Ba_{1-x}Rb_xFe_2As_2 a promising candidate for
technical applications.Comment: accepted for publication in Phys. Rev. B (9 pages, 11 figures
Th substituted SmFeAsO: structural details and superconductivity with Tc above 50 K
Superconducting poly- and single-crystalline samples of Sm1-xThxFeAsO with
partial substitution of Sm3+ by Th4+ were synthesized and grown under high
pressure and their structural, magnetic and transport properties are studied.
The superconducting Tc reaches values higher than 50 K. Bulk superconducting
samples (x = 0.08, 0.15, 0.3) do not show any signs of a phase transition from
tetragonal to orthorhombic crystal structure at low temperatures. With Th
substitution the unit cell parameters a and c shrink and the fractional atomic
coordinate of the As site zAs remains almost unchanged, while that of Sm/Th
zSm/Th increases. Upon warming from 5 K to 295 K the expansion of the FeAs
layer thickness is dominant, while the changes in the other structural building
blocks are smaller by a factor of 1/5, and they compensate each other, since
the As-Sm/Th distance appears to contract by about the same amount as the
O-Sm/Th expands. The poly- and single-crystalline samples are characterized by
a full diamagnetic response in low magnetic field, by a high intergrain
critical-current density for polycrystalline samples, and by a critical current
density of the order of 8 x 105 A/cm2 for single crystals at 2 K in fields up
to 7 T. The magnetic penetration depth anisotropy {\gamma}{\lambda} increases
with decreasing temperature, a similar behavior to that of SmFeAsO1-xFy single
crystals. The upper critical field estimated from resistance measurements is
anisotropic with slopes of 5.4 T/K (H//ab plane) and 2.7 T/K (H//c axis), at
temperatures sufficiently far below Tc. The upper critical field anisotropy
{\gamma}H is in the range of 2, consistent with the tendency of a decreasing
{\gamma}H with decreasing temperature, already reported for SmFeAsO1-xFy single
crystals.Comment: 30 pages, 2 tables, 15 figure
{Rearrangement of the antiferromagnetic ordering at high magnetic fields in SmFeAsO and SmFeAsOF single crystals
The low-temperature antiferromagnetic state of the Sm-ions in both
nonsuperconducting SmFeAsO and superconducting SmFeAsOF single
crystals was studied by magnetic torque, magnetization, and magnetoresistance
measurements in magnetic fields up to 60~T and temperatures down to 0.6~K. We
uncover in both compounds a distinct rearrangement of the antiferromagnetically
ordered Sm-moments near ~T. This is seen in both, static and pulsed
magnetic fields, as a sharp change in the sign of the magnetic torque, which is
sensitive to the magnetic anisotropy and hence to the magnetic moment in the
-plane, ({\it i.e.} the FeAs-layers), and as a jump in the magnetization
for magnetic fields perpendicular to the conducting planes. This rearrangement
of magnetic ordering in ~T is essentially temperature independent and
points towards a canted or a partially polarized magnetic state in high
magnetic fields. However, the observed value for the saturation moment above
this rearrangement, suggests that the complete suppression of the
antiferromagnetism related to the Sm-moments would require fields in excess of
60~T. Such a large field value is particularly remarkable when compared to the
relatively small N\'{e}el temperature ~K, suggesting very
anisotropic magnetic exchange couplings. At the transition, magnetoresistivity
measurements show a crossover from positive to negative field-dependence,
indicating that the charge carriers in the FeAs planes are sensitive to the
magnetic configuration of the rare-earth elements. This is indicates a finite
magnetic/electronic coupling between the SmO and the FeAs layers which are
likely to mediate the exchange interactions leading to the long range
antiferromagnetic order of the Sm ions.Comment: 10 pages, 7 figures, accepted in Phys. Rev.