965 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
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
Microscopic magnetic modeling for the =1/2 alternating chain compounds NaCuSbO and NaCuTeO
The spin-1/2 alternating Heisenberg chain system NaCuSbO features
two relevant exchange couplings: within the structural CuO
dimers and between the dimers. Motivated by the controversially
discussed nature of , we perform extensive density-functional-theory
(DFT) calculations, including DFT+ and hybrid functionals. Fits to the
experimental magnetic susceptibility using high-temperature series expansions
and quantum Monte Carlo simulations yield the optimal parameters =
217 K and = 174 K with the alternation ratio 1.25. For the closely related system
NaCuTeO, DFT yields substantially enhanced , but weaker
. The comparative analysis renders the buckling of the chains as the
key parameter altering the magnetic coupling regime. Numerical simulation of
the dispersion relations of the alternating chain model clarify why both
antiferromagnetic and ferrromagnetic can reproduce the experimental
magnetic susceptibility data.Comment: published version: 11 pages, 8 figures, 5 tables + Supplemental
materia
Crystal water induced switching of magnetically active orbitals in CuCl2
The dehydration of CuCl2*2(H2O) to CuCl2 leads to a dramatic change in
magnetic behavior and ground state. Combining density functional electronic
structure and model calculations with thermodynamical measurements we reveal
the microscopic origin of this unexpected incident -- a crystal water driven
switching of the magnetically active orbitals. This switching results in a
fundamental change of the coupling regime from a three-dimensional
antiferromagnet to a quasi one-dimensional behavior. CuCl2 can be well
described as a frustrated J1-J2 Heisenberg chain with ferromagnetic exchange J1
and J2/J1 ~ -1.5 for which a helical ground state is predicted.Comment: 6 pages, 5 figures, 1 table (PRB, accepted
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
Scaling Study and Thermodynamic Properties of the cubic Helimagnet FeGe
The critical behavior of the cubic helimagnet FeGe was obtained from
isothermal magnetization data in very close vicinity of the ordering
temperature. A thorough and consistent scaling analysis of these data revealed
the critical exponents , , and . The
anomaly in the specific heat associated with the magnetic ordering can be well
described by the critical exponent . The values of these
exponents corroborate that the magnetic phase transition in FeGe belongs to the
isotropic 3D-Heisenberg universality class. The specific heat data are well
described by ab initio phonon calculations and confirm the localized character
of the magnetic moments.Comment: 10 pages, 8 figure
Interpreting Safety Outcomes: Waymo's Performance Evaluation in the Context of a Broader Determination of Safety Readiness
This paper frames recent publications from Waymo within the broader context
of the safety readiness determination for an Automated Driving System (ADS).
Starting from a brief overview of safety performance outcomes reported by Waymo
(i.e., contact events experienced during fully autonomous operations), this
paper highlights the need for a diversified approach to safety determination
that complements the analysis of observed safety outcomes with other estimation
techniques. Our discussion highlights: the presentation of a "credibility
paradox" within the comparison between ADS crash data and human-derived
baselines; the recognition of continuous confidence growth through in-use
monitoring; and the need to supplement any aggregate statistical analysis with
appropriate event-level reasoning
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