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Register a new userLow-energy interband transitions in the infrared response of Ba(Fe1-xCox)2As2
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We studied the doping and temperature (T) dependence of the infrared (IR) response of Ba(Fe1-xCox)2As2 single crystals. We show that a weak band around 1000 cm-1, that was previously interpreted in terms of interaction of the charge carriers with magnetic excitations or of a pseudogap, is rather related to low-energy interband transitions. Specifically, we show that this band exhibits a similar doping and T-dependence as the hole pockets seen by angle resolved photoemission spectroscopy (ARPES). Notably, we find that it vanishes as a function of doping near the critical point where superconductivity is suppressed in the overdoped regime. Our IR data thus provide bulk specific information (complementary to the surface sensitive ARPES) for a Lifshitz transition. Our IR data also reveal a second low-energy band around 2300 cm-1 which further emphasizes the necessity to consider the multiband nature of these iron arsenides in the analysis of the optical response.
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We report the temperature dependence of the resistivity and thermoelectric power under hydrostatic pressure of the itinerant antiferromagnet BaFe2As2 and the electron-doped superconductor Ba(Fe0.9Co0.1)2As2. We observe a hole-like contribution to the thermopower below the structural-magnetic transition in the parent compound that is suppressed in magnitude and temperature with pressure. Pressure increases the contribution of electrons to transport in both the doped and undoped compound. In the 10% Co-doped sample, we used a two-band model for thermopower to estimate the carrier concentrations and determine the effect of pressure on the band structure.
We report muon spin rotation ($mu$SR) measurements of single crystal Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$. From measurements of the magnetic field penetration depth $lambda$ we find that for optimally- and over-doped samples, $1/lambda(Tto 0)^2$ varies monotonically with the superconducting transition temperature T$_{rm C}$. Within the superconducting state we observe a positive shift in the muon precession signal, likely indicating that the applied field induces an internal magnetic field. The size of the induced field decreases with increasing doping but is present for all Co concentrations studied.
The magnetic excitations in the paramagnetic-tetragonal phase of underdoped Ba(Fe0.953Co0.047)2As2, as measured by inelastic neutron scattering, can be well described by a phenomenological model with purely diffusive spin dynamics. At low energies, the spectrum around the magnetic ordering vector Q_AFM consists of a single peak with elliptical shape in momentum space. At high energies, this inelastic peak is split into two peaks across the direction perpendicular to Q_AFM. We use our fittings to argue that such a splitting is not due to incommensurability or propagating spin-wave excitations, but is rather a consequence of the anisotropies in the Landau damping and in the magnetic correlation length, both of which are allowed by the tetragonal symmetry of the system. We also measure the magnetic spectrum deep inside the magnetically-ordered phase, and find that it is remarkably similar to the spectrum of the paramagnetic phase, revealing the strongly overdamped character of the magnetic excitations.
75As NMR and susceptiblity were measured in a Ba(Fe1-xCox)2As2 single crystal for x=6% for various field H values and orientations. The sharpness of the superconducting and magnetic transitions demonstrates a homogeneity of the Co doping x better than +-0.25%. On the nanometer scale, the paramagnetic part of the NMR spectra is found very anisotropic and very narrow for H//ab which allows to rule out the interpretation of Ref.[6] in terms of strong Co induced electronic inhomogeneities. We propose that a distribution of hyperfine couplings and chemical shifts due to the Co effect on its nearest As explains the observed linewidths and relaxations. All these measurements show that Co substitution induces a very homogeneous electronic doping in BaFe2As2, from nano to micrometer lengthscales, on the contrary to the K doping.
We show that despite the low anisotropy, strong vortex pinning and high irreversibility field Hirr close to the upper critical field Hc2 of Ba(Fe1-xCox)2As2, the critical current density Jgb across [001] tilt grain boundaries (GBs) of thin film Ba(Fe1-xCox)2As2 bicrystals is strongly depressed, similar to high-Tc cuprates. Our results suggest that weak-linked GBs are characteristic of both cuprates and pnictides because of competing orders, low carrier density, and unconventional pairing symmetry.
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