149 research outputs found
The effect of Cr impurity to superconductivity in electron-doped BaFe2-xNixAs2
We use transport and magnetization measurements to study the effect of
Cr-doping to the phase diagram of the electron-doped superconducting
BaFe2-xNixAs2 iron pnictides. In principle, adding Cr to electron-doped
BaFe2-xNixAs2 should be equivalent to the effect of hole-doping. However, we
find that Cr doping suppresses superconductivity via impurity effect, while not
affecting the normal state resistivity above 100 K. We establish the phase
diagram of Cr-doped BaFe2-x-yNixCryAs2 iron pnictides, and demonstrate that
Cr-doping near optimal superconductivity restore the long-range
antiferromagnetic order suppressed by superconductivity.Comment: 10 pages, 5 figure
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Strain-Induced Spin-Nematic State and Nematic Susceptibility Arising from 2Γ2 Fe Clusters in KFe_{0.8}Ag_{1.2}Te_{2}.
Spin nematics break spin-rotational symmetry while maintaining time-reversal symmetry, analogous to liquid crystal nematics that break spatial rotational symmetry while maintaining translational symmetry. Although several candidate spin nematics have been proposed, the identification and characterization of such a state remain challenging because the spin-nematic order parameter does not couple directly to experimental probes. KFe_{0.8}Ag_{1.2}Te_{2} (K_{5}Fe_{4}Ag_{6}Te_{10}, KFAT) is a local-moment magnet consisting of well-separated 2Γ2 Fe clusters, and in its ground state the clusters order magnetically, breaking both spin-rotational and time-reversal symmetries. Using uniform magnetic susceptibility and neutron scattering measurements, we find a small strain induces sizable spin anisotropy in the paramagnetic state of KFAT, manifestly breaking spin-rotational symmetry while retaining time-reversal symmetry, resulting in a strain-induced spin-nematic state in which the 2Γ2 clusters act as the spin analog of molecules in a liquid crystal nematic. The strain-induced spin anisotropy in KFAT allows us to probe its nematic susceptibility, revealing a divergentlike increase upon cooling, indicating the ordered ground state is driven by a spin-orbital entangled nematic order parameter
Thermal properties of La2Zr2O7 double-layer thermal barrier coatings
La2Zr2O7 is a promising thermal barrier coating (TBC) material. In this work, La2Zr2O7 and 8YSZ-layered TBC systems were fabricated. Thermal properties such as thermal conductivity and coefficient of thermal expansion were investigated. Furnace heat treatment and jet engine thermal shock (JETS) tests were also conducted. The thermal conductivities of porous La2Zr2O7 single-layer coatings are 0.50β0.66β
Wβ
mβ1β
Β°Cβ1 at the temperature range from 100 to 900Β°C, which are 30β40% lower than the 8YSZ coatings. The coefficients of thermal expansion of La2Zr2O7 coatings are about 9β10βΓβ10β6β
Β°Cβ1 at the temperature range from 200 to 1200Β°C, which are close to those of 8YSZ at low temperature range and about 10% lower than 8YSZ at high temperature range. Double-layer porous 8YSZ plus La2Zr2O7 coatings show a better performance in thermal cycling experiments. It is likely because porous 8YSZ serves as a buffer layer to release stress
Doping evolution of antiferromagnetism and transport properties in the non-superconducting BaFe2-2xNixCrxAs2
We report elastic neutron scattering and transport measurements on the Ni and
Cr equivalently doped iron pnictide BaFeNiCrAs.
Compared with the electron-doped BaFeNiAs, the long-range
antiferromagnetic (AF) order in BaFeNiCrAs is
gradually suppressed with vanishing ordered moment and N\'{e}el temperature
near without the appearance of superconductivity. A detailed analysis
on the transport properties of BaFeNiAs and
BaFeNiCrAs suggests that the non-Fermi-liquid
behavior associated with the linear resistivity as a function of temperature
may not correspond to the disappearance of the static AF order. From the
temperature dependence of the resistivity in overdoped compounds without static
AF order, we find that the transport properties are actually affected by Cr
impurity scattering, which may induce a metal-to-insulator crossover in highly
doped BaFeNiCrAs.Comment: 10 pages, 12 figure
Two-dimensional Massless Dirac Fermions in Antiferromagnetic AFe2As2 (A = Ba, Sr)
We report infrared studies of AFeAs (A = Ba, Sr), two
representative parent compounds of iron-arsenide superconductors, at magnetic
fields (B) up to 17.5 T. Optical transitions between Landau levels (LLs) were
observed in the antiferromagnetic states of these two parent compounds. Our
observation of a dependence of the LL transition energies, the
zero-energy intercepts at B = 0 T under the linear extrapolations of the
transition energies and the energy ratio ( 2.4) between the observed LL
transitions, combined with the linear band dispersions in two-dimensional (2D)
momentum space obtained by theoretical calculations, demonstrates the existence
of massless Dirac fermions in antiferromagnetic BaFeAs. More
importantly, the observed dominance of the zeroth-LL-related absorption
features and the calculated bands with extremely weak dispersions along the
momentum direction indicate that massless Dirac fermions in
BaFeAs are 2D. Furthermore, we find that the total substitution of
the barium atoms in BaFeAs by strontium atoms not only maintains 2D
massless Dirac fermions in this system, but also enhances their Fermi velocity,
which supports that the Dirac points in iron-arsenide parent compounds are
topologically protected.Comment: Magneto-infrared study, Landau level spectroscopy, DFT+DMFT
calculation
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