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Papers by Ni Ni

Thermoelectric power of Ba(Fe 1−x Co x) 2 As 2 , 0 ≤ x ≤ 0.05, and Ba(Fe 1−x Rh x) 2 As 2 , (0 ≤ ... more Thermoelectric power of Ba(Fe 1−x Co x) 2 As 2 , 0 ≤ x ≤ 0.05, and Ba(Fe 1−x Rh x) 2 As 2 , (0 ≤ x ≤ 0.171

The parent compounds of iron-arsenide superconductors, AFe2As2 (A=Ca, Sr, Ba), undergo a tetragon... more The parent compounds of iron-arsenide superconductors, AFe2As2 (A=Ca, Sr, Ba), undergo a tetragonal to orthorhombic structural transition at a temperature TTO in the range 135 to 205 K depending on the alkaline earth element. Below TTO the free standing crystals split into equally populated structural domains, which mask intrinsic, in-plane, anisotropic properties of the materials. Here we demonstrate a way of mechanically detwinning CaFe2As2 and BaFe2As2. The detwinning is nearly complete, as demonstrated by polarized light imaging and synchrotron X-ray measurements, and reversible, with twin pattern restored after strain release. Electrical resistivity measurements in the twinned and detwinned states show that resistivity, ρ, decreases along the orthorhombic ao-axis but increases along the orthorhombic bo-axis in both compounds. Immediately below TTO the ratio ρ bo /ρao = 1.2 and 1.5 for Ca and Ba compounds, respectively. Contrary to CaFe2As2, BaFe2As2 reveals an anisotropy in the nominally tetragonal phase, suggesting that either fluctuations play a larger role above TTO in BaFe2As2 than in CaFe2As2, or that there is a higher temperature crossover or phase transition.

High-resolution x-ray diffraction measurements reveal an unusually strong response of the lattice... more High-resolution x-ray diffraction measurements reveal an unusually strong response of the lattice to superconductivity in Ba(Fe1−xCox)2As2. The orthorhombic distortion of the lattice is suppressed and, for Co-doping near x = 0.063, the orthorhombic structure evolves smoothly back to a tetragonal structure. We propose that the coupling between orthorhombicity and superconductivity is indirect and arises due to the magneto-elastic coupling, in the form of emergent nematic order, and the strong competition between magnetism and superconductivity.

High-resolution x-ray diffraction measurements reveal an unusually strong response of the lattice... more High-resolution x-ray diffraction measurements reveal an unusually strong response of the lattice to superconductivity in Ba(Fe1−xCox)2As2. The orthorhombic distortion of the lattice is suppressed and, for Co-doping near x = 0.063, the orthorhombic structure evolves smoothly back to a tetragonal structure. We propose that the coupling between orthorhombicity and superconductivity is indirect and arises due to the magneto-elastic coupling, in the form of emergent nematic order, and the strong competition between magnetism and superconductivity.

High-resolution x-ray diffraction measurements reveal an unusually strong response of the lattice... more High-resolution x-ray diffraction measurements reveal an unusually strong response of the lattice to superconductivity in Ba(Fe1−xCox)2As2. The orthorhombic distortion of the lattice is suppressed and, for Co-doping near x = 0.063, the orthorhombic structure evolves smoothly back to a tetragonal structure. We propose that the coupling between orthorhombicity and superconductivity is indirect and arises due to the magneto-elastic coupling, in the form of emergent nematic order, and the strong competition between magnetism and superconductivity.

Thermoelectric power of Ba(Fe 1−x Co x) 2 As 2 , 0 ≤ x ≤ 0.05, and Ba(Fe 1−x Rh x) 2 As 2 , (0 ≤ ... more Thermoelectric power of Ba(Fe 1−x Co x) 2 As 2 , 0 ≤ x ≤ 0.05, and Ba(Fe 1−x Rh x) 2 As 2 , (0 ≤ x ≤ 0.171

The parent compounds of iron-arsenide superconductors, AFe2As2 (A=Ca, Sr, Ba), undergo a tetragon... more The parent compounds of iron-arsenide superconductors, AFe2As2 (A=Ca, Sr, Ba), undergo a tetragonal to orthorhombic structural transition at a temperature TTO in the range 135 to 205 K depending on the alkaline earth element. Below TTO the free standing crystals split into equally populated structural domains, which mask intrinsic, in-plane, anisotropic properties of the materials. Here we demonstrate a way of mechanically detwinning CaFe2As2 and BaFe2As2. The detwinning is nearly complete, as demonstrated by polarized light imaging and synchrotron X-ray measurements, and reversible, with twin pattern restored after strain release. Electrical resistivity measurements in the twinned and detwinned states show that resistivity, ρ, decreases along the orthorhombic ao-axis but increases along the orthorhombic bo-axis in both compounds. Immediately below TTO the ratio ρ bo /ρao = 1.2 and 1.5 for Ca and Ba compounds, respectively. Contrary to CaFe2As2, BaFe2As2 reveals an anisotropy in the nominally tetragonal phase, suggesting that either fluctuations play a larger role above TTO in BaFe2As2 than in CaFe2As2, or that there is a higher temperature crossover or phase transition.

High-resolution x-ray diffraction measurements reveal an unusually strong response of the lattice... more High-resolution x-ray diffraction measurements reveal an unusually strong response of the lattice to superconductivity in Ba(Fe1−xCox)2As2. The orthorhombic distortion of the lattice is suppressed and, for Co-doping near x = 0.063, the orthorhombic structure evolves smoothly back to a tetragonal structure. We propose that the coupling between orthorhombicity and superconductivity is indirect and arises due to the magneto-elastic coupling, in the form of emergent nematic order, and the strong competition between magnetism and superconductivity.

High-resolution x-ray diffraction measurements reveal an unusually strong response of the lattice... more High-resolution x-ray diffraction measurements reveal an unusually strong response of the lattice to superconductivity in Ba(Fe1−xCox)2As2. The orthorhombic distortion of the lattice is suppressed and, for Co-doping near x = 0.063, the orthorhombic structure evolves smoothly back to a tetragonal structure. We propose that the coupling between orthorhombicity and superconductivity is indirect and arises due to the magneto-elastic coupling, in the form of emergent nematic order, and the strong competition between magnetism and superconductivity.

High-resolution x-ray diffraction measurements reveal an unusually strong response of the lattice... more High-resolution x-ray diffraction measurements reveal an unusually strong response of the lattice to superconductivity in Ba(Fe1−xCox)2As2. The orthorhombic distortion of the lattice is suppressed and, for Co-doping near x = 0.063, the orthorhombic structure evolves smoothly back to a tetragonal structure. We propose that the coupling between orthorhombicity and superconductivity is indirect and arises due to the magneto-elastic coupling, in the form of emergent nematic order, and the strong competition between magnetism and superconductivity.