394 research outputs found
Superconductivity emerging from an electronic phase separation in the charge ordered phase of RbFeAs
As, Rb and Rb nuclear quadrupole resonance (NQR) and
Rb nuclear magnetic resonance (NMR) measurements in RbFeAs
iron-based superconductor are presented. We observe a marked broadening of
As NQR spectrum below K which is associated with the
onset of a charge order in the FeAs planes. Below we observe a power-law
decrease in As nuclear spin-lattice relaxation rate down to K. Below that temperature the nuclei start to probe different dynamics
owing to the different local electronic configurations induced by the charge
order. A fraction of the nuclei probes spin dynamics associated with electrons
approaching a localization while another fraction probes activated dynamics
possibly associated with a pseudogap. These different trends are discussed in
the light of an orbital selective behaviour expected for the electronic
correlations.Comment: 5 pages, 3 figures and 4 pages of supplemental materia
Nernst effect in single crystals of the pnictide superconductor CaFe1.92Co0.08As2 and parent compound CaFe2As2
We report a combined study of the Nernst coefficient (v), Hall effect and
thermoelectric power of CaFe2As2 and CaFe1.92Co0.08As2 single crystals. The
absolute value of v in both samples is enhanced, probably due to ambipolar flow
of electron- and hole-like quasiparticles. The onset of spin-density-wave order
in CaFe2As2 causes further rapid rise of the Nernst coefficient. On the
contrary, in the CaFe1.92Co0.08As2 crystal we do not see any feature of v(T),
which could be clearly associated with SDW fluctuations. In this Co-doped
sample there is also no noticeable increase in of v in the vicinity of
superconducting transition, despite the expectation of such due to vortex
movement.Comment: 6 pages, 3 figures; (v2) slight changes and clarifications; to appear
in Phys. Rev.
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
Hydrostatic pressure effects on the static magnetism in Eu(FeCo)As
The effects of hydrostatic pressure on the static magnetism in
Eu(FeCo)As are investigated by complementary
electrical resistivity, ac magnetic susceptibility and single-crystal neutron
diffraction measurements. A specific pressure-temperature phase diagram of
Eu(FeCo)As is established. The structural phase
transition, as well as the spin-density-wave order of Fe sublattice, is
suppressed gradually with increasing pressure and disappears completely above
2.0 GPa. In contrast, the magnetic order of Eu sublattice persists over the
whole investigated pressure range up to 14 GPa, yet displaying a non-monotonic
variation with pressure. With the increase of the hydrostatic pressure, the
magnetic state of Eu evolves from the canted antiferromagnetic structure in the
ground state, via a pure ferromagnetic structure under the intermediate
pressure, finally to a possible "novel" antiferromagnetic structure under the
high pressure. The strong ferromagnetism of Eu coexists with the
pressure-induced superconductivity around 2 GPa. The change of the magnetic
state of Eu in Eu(FeCo)As upon the application
of hydrostatic pressure probably arises from the modification of the indirect
Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the Eu moments
tuned by external pressure.Comment: 9 pages, 6 figure
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
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