277 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
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
Charge-Doping driven Evolution of Magnetism and non-Fermi-Liquid Behavior in the Filled Skutterudite CePt4Ge12-xSbx
The filled-skutterudite compound CePt4Ge12 is situated close to the border
between intermediate-valence of Ce and heavy-fermion behavior. Substitution of
Ge by Sb drives the system into a strongly correlated and ultimately upon
further increasing the Sb concentration into an antiferromagnetically ordered
state. Our experiments evidence a delicate interplay of emerging Kondo physics
and the formation of a local 4f moment. An extended non-Fermi-liquid region,
which can be understood in the framework of a Kondo-disorder model, is
observed. Band-structure calculations support the conclusion that the physical
properties are governed by the interplay of electron supply via Sb substitution
and the concomitant volume effects.Comment: 5 pages, 3 Figur
Evidence for time-reversal symmetry breaking in superconducting PrPt4Ge12
Zero and longitudinal field muon spin rotation (muSR) experiments were
performed on the superconductors PrPt4Ge12 and LaPt4Ge12. In PrPt4Ge12 below Tc
a spontaneous magnetization with a temperature variation resembling that of the
superfluid density appears. This observation implies time-reversal symmetry
(TRS) breaking in PrPt4Ge12 below Tc = 7.9 K. This remarkably high Tc for an
anomalous superconductor and the weak and gradual change of Tc and of the
related specific heat anomaly upon La substitution in La_(1-x)Pr_xPt_4Ge_(12)
suggests that the TRS breaking is due to orbital degrees of freedom of the
Cooper pairs.Comment: To appear in Phys. Rev. B. 5 pages, 3 figure
Field dependence of the Eu2+ spin relaxation in EuFe(2-x)CoxAs2
The layered compound EuFe2As2 is an interesting model system to investigate
the effects of well defined local Eu2+ 4f states on the itinerant electronic
and magnetic properties of the FeAs layers. To address this subject, we
investigated the series EuFe2-xCoxAs2 (0.1 <= x <=0.75) by electron spin
resonance (ESR) of Eu2+ to probe the spin dynamics of the itinerant subsystem.
We relate the results to dc-susceptibility measurements and band structure
calculations. As a consequence of the weak coupling between the local and
itinerant subsystems we found that the spin relaxation is well understood in
terms of the exchange coupling among the local Eu2+ spins. A pronounced field
dependence of the Eu2+ spin relaxation demonstrates the direct influence of
magnetic fluctuations at the Fe2-xCoxAs2 layers.Comment: 13 pages, 5 figure
Hard X-ray spectroscopy of the itinerant magnets FeSb (Na, K, Ca, Sr, Ba)
Ordered states in itinerant magnets may be related to magnetic moments
displaying some weak local moment characteristics, as in intermetallic
compounds hosting transition metal coordination complexes. In this paper, we
report on the Fe -edge X-ray absorption spectroscopy (XAS) of the itinerant
magnets FeSb ( Na, K, Ca, Sr, Ba), aiming at exploring the
electronic and structural properties of the octahedral building block formed by
Fe and the Sb ligands. We find evidence for strong hybridization between the Fe
and Sb states at the Fermi level, giving experimental support to
previous electronic structure calculations of the FeSb
skutterudites. The electronic states derived from Fe 3 Sb mixing are
shown to be either more occupied and/or less localized in the cases of the
magnetically ordered systems, for which Na or K, connecting the local Fe
electronic structure to the itinerant magnetic properties. Moreover, the
analysis of the extended region of the XAS spectra (EXAFS) suggests that bond
disorder may be a more relevant parameter to explain the suppression of the
ferromagnetic ordered state in CaFeSb than the decrease of the
density of states.Comment: 7 pages, 5 figures, submitte
On Electron Transport in ZrB12, ZrB2 and MgB2
We report on measurements of the temperature dependence of resistivity,
, for single crystal samples of ZrB, ZrB and
polycrystalline samples of MgB. It is shown that cluster compound
ZrB behaves like a simple metal in the normal state, with a typical
Bloch -- Gr\"uneisen dependence. However, the resistive Debye
temperature, , is three times smaller than obtained from
specific heat data. We observe the term in of these borides,
which could be interpreted as an indication of strong electron-electron
interaction. Although the dependence of ZrB reveals a sharp
superconductive transition at , no superconductivity was observed
for single crystal samples of ZrB down to .Comment: 5 pages, 4 figure
Superconductivity in SrFe2As2 with Pt Doping
We have synthesized polycrystalline samples of Pt-substituted SrFe2As2 and
measured the temperature dependence of magnetization and electrical
resistivity. We have observed the superconducting transition at Tc = 17 K with
the maximum shielding volume fraction at x = 0.125 in Sr(Fe1-xPtx)2As2. It is
found that the maximum Tc depends on the substituted element, so it is
important to substitute various elements to explore new iron-based
superconductors with higher Tc.Comment: 2 pages, 1 figure, to appear in J. Phys. Soc. Jpn. Vol. 79 No. 9
(2010
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