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Register a new userWeak-link behavior of grain boundaries in superconducting Ba(Fe1-xCox)2As2 bicrystals
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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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An extensive calorimetric study of the normal- and superconducting-state properties of Ba(Fe1-xCox)2As2 is presented for 0 < x < 0.2. The normal-state Sommerfeld coefficient increases (decreases) with Co doping for x < 0.06 (x > 0.06), which illustrates the strong competition between magnetism and superconductivity to monopolize the Fermi surface in the underdoped region and the filling of the hole bands for overdoped Ba(Fe1-xCox)2As2. All superconducting samples exhibit a residual electronic density of states of unknown origin in the zero-temperature limit, which is minimal at optimal doping but increases to the normal-state value in the strongly under- and over-doped regions. The remaining specific heat in the superconducting state is well described using a two-band model with isotropic s-wave superconducting gaps.
Inelastic neutron scattering measurements have been performed on underdoped Ba(Fe1-xCox)2As2 (x = 4.7%) where superconductivity and long-range antiferromagnetic (AFM) order coexist. The broad magnetic spectrum found in the normal state develops into a magnetic resonance feature below TC that has appreciable dispersion along c-axis with a bandwidth of 3-4 meV. This is in contrast to the optimally doped x = 8.0% composition, with no long-range AFM order, where the resonance exhibits a much weaker dispersion [see Lumsden et al. Phys. Rev. Lett. 102, 107005 (2009)]. The results suggest that the resonance dispersion arises from interlayer spin correlations present in the AFM ordered state.
The angular-dependent critical current density, Jc(theta), and the upper critical field, Hc2(theta), of epitaxial Ba(Fe1-xCox)2As2 thin films have been investigated. No Jc(theta) peaks for H || c were observed regardless of temperatures and magnetic fields. In contrast, Jc(theta) showed a broad maximum at theta=90 degree, which arises from intrinsic pinning. All data except at theta=90 degree can be scaled by the Blatter plot. Hc2(theta) near Tc follows the anisotropic Ginzburg-Landau expression. The mass anisotropy increased from 1.5 to 2 with increasing temperature, which is an evidence for multi-band superconductivity.
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.
We investigate the nature of the SDW (Spin Density Wave) transition in the underdoped regime of an iron-based high Tc superconductor Ba(Fe1-xCox)2As2 by 75As NMR, with primary focus on a composition with x = 0.02 (T_SDW = 99 K).We demonstrate that critical slowing down toward the three dimensional SDW transition sets in at the tetragonal to orthorhombic structural phase transition, Ts = 105 K, suggesting strong interplay between structural distortion and spin correlations. In the critical regime between Ts and T_SDW, the dynamical structure factor of electron spins S(q,Wn) measured with the longitudinal NMR relaxation rate 1/T1 exhibits a divergent behavior obeying a power law, 1/T1~S(q, Wn)~(T/T_SDW-1)^a with the critical exponent a ~ 0.33.
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