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We report Hall measurement of the normal state in K- and Co-doped BaFe$_2$As$_2$, as well NaFe$_{1-x}$Co$_x$As. We found that a power-law temperature dependence of Hall angle, cot$theta_{rm H}$$propto$ $T^beta$, prevails in normal state with temperature range well above the structural, spin-density-wave and superconducting transitions for the all samples with various doping levels. The power $beta$ is nearly 4 for the parent compounds and the heavily underdoped samples, while around 3 for the superconducting samples. The $beta$ suddenly changes from 4 to 3 at a doping level that is close to the emergence of superconductivity. It suggests that the $beta$ of $sim 3$ is clearly tied to the superconductivity. Our data suggest that, similar to cuprates, there exists a connection between the physics in the normal state and superconductivity of iron-pnictides. These findings shed light on the mechanism of high-temperature superconductivity.
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The normal-state Hall effect and magnetoresisitance (MR) have been measured in the quasi-2D heavy fermion superconductor CeCoIn_5. In the non-Fermi liquid region where the reistivity rho_xx exhibits an almost perfect T-linear dependence, the Hall angle varies as cot theta_H propto T^2 and the MR displays a strong violation of Kohlers rule. We demonstrate a novel relation between the MR and the Hall conductivity, Delta rho_xx/rho_xx propto (sigma_xy rho_xx)^2. These results bear a striking resemblance to the normal-state properties of high-T_c cuprates, indicating universal transport properties in the presence of quasi-2D antiferromagnetic fluctuations near a quantum critical point.
The recent discovery of superconductivity at moderately high temperature (26 K to 55 K) in doped iron-based pnictides (LnO_{1-x}F_xFeAs, where Ln = La, Ce, Sm, Pr, Nd, etc.), having layered-structure-like cuprates, has triggered renewed challenge towards understanding the pairing mechanism. After reviewing the current findings on these systems, a theoretical model of a combined mechanism is suggested in which the phonon-mediated and distortion-field-mediated pairing processes give the right order of superconducting critical temperature T_c. The distortion-field modes arise from Jahn-Teller or pseudo Jahn-Teller effects due to degenerate or near-degenerate iron 3d_{xz} and 3d_{yz} orbitals.
A growing list of experiments show orthorhombic electronic anisotropy in the iron pnictides, in some cases at temperatures well above the spin density wave transition. These experiments include neutron scattering, resistivity and magnetoresistance measurements, and a variety of spectroscopies. We explore the idea that these anisotropies stem from a common underlying cause: orbital order manifest in an unequal occupation of $d_{xz}$ and $d_{yz}$ orbitals, arising from the coupled spin-orbital degrees of freedom. We emphasize the distinction between the total orbital occupation (the integrated density of states), where the order parameter may be small, and the orbital polarization near the Fermi level which can be more pronounced. We also discuss light-polarization studies of angle-resolved photoemission, and demonstrate how x-ray absorption linear dichroism may be used as a method to detect an orbital order parameter.
The experimental consequences of different order parameters in iron-based superconductors are theoretically analyzed. We consider both nodeless and nodal order parameters, with an emphasis on the $cos(k_x)cdot cos(k_y)$ nodeless order parameter recently derived by two of us. We analyze the effect of this order parameter on the spectral function, density of states, tunneling differential conductance, penetration depth, and the NMR spin relaxation time. This extended s-wave symmetry has line-zeroes in between the electron and hole pockets, but they do not intersect the two Fermi surfaces for moderate doping, and the superconductor is fully gapped. However, this suggests several quantitative tests: the exponential decay of the penetration depth weakens and the density of states reveals a smaller gap upon electron or hole doping. Moreover, the $cos(k_x) cdot cos(k_y)$ superconducting gap is largest on the smallest (hole) Fermi surface. For the $1/T_1$ NMR spin relaxation rate, the inter-band contribution is consistent with the current experimental results, including a (non-universal) $T^{3}$ behavior and the absence of a coherence peak. However, the intra-band contribution is considerably larger than the inter-band contributions and still exhibits a small enhancement in the NMR spin relaxation rate right below $T_c$ in the clean limit.
We present the first infrared and optical study in the normal state of ab-plane oriented single crystals of the iron-oxypnictide superconductor LaFePO. We find that this material is a low carrier density metal with a moderate level of correlations and exhibits signatures of electron-boson coupling. The data is consistent with the presence of coherent quasiparticles in LaFePO.
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