876 research outputs found
Effect of Co doping and hydrostatic pressure on SrFe2As2
We report a pressure study on electron doped SrFeCoAs by
electrical-resistivity () and magnetic-susceptibility ()
experiments. Application of either external pressure or Co substitution rapidly
suppresses the spin-density wave ordering of the Fe moments and induces
superconductivity in SrFeAs. At the broad superconducting (SC)
dome in the phase diagram exhibits its maximum K at
a pressure of only GPa. In
SrFeCoAs no superconductivity is observed anymore up to 2.8
GPa. Upon increasing the Co concentration the maximum of the SC dome shifts
toward lower pressure accompanied by a decrease in the value of . Even though, superconductivity is induced by both tuning methods, Co
substitution leads to a much more robust SC state. Our study evidences that in
SrFeCoAs both, the effect of pressure and Co-substitution, have
to be considered in order to understand the SC phase-diagram and further
attests the close relationship of SrFeAs and its sister compound
BaFeAs.Comment: 6 pages, 6 figure
Enhancement of the upper critical field in codoped iron-arsenic high-temperature superconductors
We present the first study of codoped iron-arsenide superconductors of the
122 family (Sr/Ba)_(1-x)K_xFe_(2-y)Co_yAs_2 with the purpose to increase the
upper critical field H_c2 compared to single doped (Sr/Ba)Fe_2As_2 materials.
H_c2 was investigated by measuring the magnetoresistance in high pulsed
magnetic fields up to 64 T. We find, that H_c2 extrapolated to T = 0 is indeed
enhanced significantly to ~ 90 T for polycrystalline samples of
Ba_0.55K_0.45Fe_1.95Co_0.05As_2 compared to ~75 T for Ba_0.55K_0.45Fe_2As_2 and
BaFe_1.8Co_0.2As_2 single crystals. Codoping thus is a promising way for the
systematic optimization of iron-arsenic based superconductors for
magnetic-field and high-current applications.Comment: 7 pages, 5 figures, submitted to Journal of Applied Physic
Competition of local-moment ferromagnetism and superconductivity in Co-substituted EuFe2As2
In contrast to SrFe2As2, where only the iron possesses a magnetic moment, in
EuFe2As2 an additional large, local magnetic moment is carried by Eu2+. Like
SrFe2As2, EuFe2As2 exhibits a spin-density wave transition at high
temperatures, but in addition the magnetic moments of the Eu2+ order at around
20 K. The interplay of pressure-induced superconductivity and the Eu2+ order
leads to a behavior which is reminiscent of re-entrant superconductivity as it
was observed, for example, in the ternary Chevrel phases or in the rare-earth
nickel borocarbides. Here, we study the delicate interplay of the ordering of
the Eu2+ moments and superconductivity in EuFe1.9Co0.1As2, where application of
external pressure makes it possible to sensitively tune the ratio of the
magnetic (T_C) and the superconducting (T_{c,onset}) critical temperatures. We
find that superconductivity disappears once T_C > T_{c,onset}.Comment: 4 pages, 4 figures, submitted to the proceedings of SCES201
A Comparison of Multiple Selections Using Multiple Checkbox Selections and List Boxes
acceptedVersio
Magnetic impurity resonance states and symmetry of the superconducting order parameter in iron-based superconductors
We investigate the effect of magnetic impurities on the local quasiparticle
density of states (LDOS) in iron-based superconductors. Employing the
two-orbital model where 3 electron and hole conduction bands are hybridizing
with the localized -orbital of the impurity spin, we investigate how various
symmetries of the superconducting gap and its nodal structure influence the
quasiparticle excitations and impurity bound states. We show that the bound
states behave qualitatively different for each symmetry. Most importantly we
find that the impurity-induced bound states can be used to identify the nodal
structure of the extended s-wave symmetry () that is actively
discussed in ferropnictides.Comment: 7 pages, 5 figures, theory part is extended, figures are replace
Unconventional magnetism in multivalent charge-ordered YbPtGe probed by Pt- and Yb-NMR
Detailed Pt- and Yb nuclear magnetic resonance (NMR) studies
on the heterogeneous mixed valence system YbPtGe are reported. The
temperature dependence of the Pt-NMR shift indicates the
opening of an unusual magnetic gap below 200\,K. was analyzed by a
thermal activation model which yields an isotropic gap \,K. In contrast, the spin-lattice relaxation rate () does
not provide evidence for the gap. Therefore, an intermediate-valence picture is
proposed while a Kondo-insulator scenario can be excluded. Moreover,
() follows a simple metallic behavior, similar to the reference
compound YPtGe. A well resolved NMR line with small shift is assigned to
divalent Yb. This finding supports the proposed model with two sub-sets
of Yb species (di- and trivalent) located on the Yb2 and Yb1 site of the
YbPtGe lattice.Comment: Submitted in Physical Review B (Rapid Communication
Superconductivity in the New Platinum Germanides MPt4Ge12 (M = Rare-earth and Alkaline-earth Metals) with Filled Skutterudite Structure
New germanium-platinum compounds with the filled-skutterudite crystal
structure were synthesized. The structure and composition were investigated by
X-ray diffraction and microprobe analysis. Magnetic susceptibility, specific
heat, and electrical resistivity measurements evidence superconductivity in
LaPt4Ge12 and PrPt4Ge12 below 8.3K. The parameters of the normal and
superconducting states were established. Strong coupling and a crystal electric
field singlet groundstate is found for the Pr compound. Electronic structure
calculations show a large density of states at the Fermi level. Similar
behavior with lower T_c was observed for SrPt4Ge12 and BaPt4Ge12.Comment: RevTeX, 4 figures, submitted to Physical Review Letters July 12, 200
Superconducting gap structure of the skutterudite LaPt4Ge12 probed by specific heat and thermal transport
We investigated the superconducting order parameter of the filled
skutterudite LaPt4Ge12, with a transition temperature of Tc = 8.3 K. To this
end, we performed temperature and magnetic-field dependent specific-heat and
thermal-conductivity measurements. All data are compatible with a single
superconducting s-wave gap. However, a multiband scenario cannot be ruled out.
The results are discussed in the context of previous studies on the
substitution series Pr1-xLaxPt4Ge12. They suggest compatible order parameters
for the two end compounds LaPt4Ge12 and PrPt4Ge12. This is not consistent with
a single s-wave gap in LaPt4Ge12 considering previous reports of unconventional
and/or multiband superconductivity in PrPt4Ge12.Comment: 8 pages, 4 figure
Superconductivity in SrFe_(2-x)Co_xAs_2: Internal Doping of the Iron Arsenide Layers
In the electron doped compounds SrFe_(2-x)Co_xAs_2 superconductivity with T_c
up to 20 K is observed for 0.2 < x < 0.4. Results of structure determination,
magnetic susceptibility, electrical resistivity, and specific heat are
reported. The observation of bulk superconductivity in all thermodynamic
properties -- despite strong disorder in the Fe-As layer -- favors an itinerant
picture in contrast to the cuprates and renders a p- or d-wave scenario
unlikely. DFT calculations find that the substitution of Fe by Co (x > 0.3)
leads to the suppression of the magnetic ordering present in SrFe_2As_2 due to
a rigid down-shift of the Fe-3d_(x^2-y^2) related band edge in the density of
states.Comment: 5 pages, 3 figure
Annealing effects on superconductivity in SrFe2-xNixAs2
Superconductivity has been explored in single crystals of the Ni-doped
FeAs-compound SrFe2-xNixAs2 grown by self-flux solution method. The
antiferromagnetic order associated with the magnetostructural transition of the
parent compound SrFe2As2 is gradually suppressed with increasing Ni
concentration x and bulk-phase superconductivity with full diamagnetic
screening is induced near the optimal doping of x = 0.15 with a maximum
transition temperature Tc ~9.8 K. An investigation of high-temperature
annealing on as-grown samples indicate that the heat treatment can enhance Tc
as much as ~50 %
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