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Register a new userJosephson effect between electron-doped and hole-doped iron pnictide single crystals
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We have observed the Josephson effect in junctions formed between single crystals of SrFe1.74Co0.26As2 and Ba0.23K0.77Fe2As2. I-V curves showed resistively-shunted junction characteristics, and the ac Josephson effect was observed under microwave irradiation. By applying an in-plane magnetic field, the critical current is completely modulated and shows a relatively symmetric diffraction pattern, consistent with the intermediate junction limit. The observation of the Josephson effect in the p-n bicrystal structure not only has significant implications for designing phase-sensitive junctions to probe the pairing symmetry of iron pnictide superconductors, but also represents an important step in developing all iron pnictide devices for applications.
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The electrodynamic properties of Ba(Fe$_{0.92}$Co$_{0.08})_2$As$_{2}$ and Ba(Fe$_{0.95}$Ni$_{0.05})_As$_{2}$ single crystals have been investigated by reflectivity measurements in a wide frequency range. In the metallic state, the optical conductivity consists of a broad incoherent background and a narrow Drude-like component which determines the transport properties; only the latter contribution strongly depends on the composition and temperature. This subsystem reveals a $T^2$ behavior in the dc resistivity and scattering rate disclosing a hidden Fermi-liquid behavior in the 122 iron-pnictide family. An extended Drude analysis yields the frequency dependence of the effective mass (with $m^*/m_bapprox 5$ in the static limit) and scattering rate that does not disclose a simple power law. The spectral weight shifts to lower energies upon cooling; a significant fraction is not recovered within the infrared range of frequencies.
We present a polarization resolved study of the low energy band structure in the optimally doped iron pnictide superconductor Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ (T$_c$=37K) using angle resolved photoemission spectroscopy. Polarization-contrasted measurements are used to identify and trace all three low energy hole-like bands predicted by local density approximation (LDA) calculations. The photoemitted electrons reveal an inconsistency with LDA-predicted symmetries along the $Gamma$-X high symmetry momentum axis, due to unexpectedly strong rotational anisotropy in electron kinetics. We evaluate many-body effects such as Mott-Hubbard interactions that are likely to underlie the anomaly, and discuss how the observed deviations from LDA band structure affect the energetics of iron pnictide Cooper pairing in the hole doped regime.
We report on the nonequilibrium quasiparticle dynamics in BaFe$_2$As$_2$ on both the hole doped (Ba$_{1-x}$K$_x$Fe$_2$As$_2$) and electron doped (BaFe$_{2-y}$Co$_y$As$_2$) sides of the phase diagram using ultrafast pump-probe spectroscopy. Below $T_c$, measurements conducted at low photoinjected quasiparticle densities in the optimally and overdoped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ samples reveal two distinct relaxation processes: a fast component whose decay rate increases linearly with excitation density and a slow component with an excitation density independent decay rate. We argue that these two processes reflect the recombination of quasiparticles in the two hole bands through intraband and interband processes. We also find that the thermal recombination rate of quasiparticles increases quadratically with temperature in these samples. The temperature and excitation density dependence of the decays indicates fully gapped hole bands and nodal or very anisotropic electron bands. At higher excitation densities and lower hole dopings, the dependence of the dynamics on quasiparticle density disappears as the data are more readily understood in terms of a model which accounts for the quasiequilibrium temperature attained by the sample. In the BaFe$_{2-y}$Co$_y$As$_2$ samples, dependence of the recombination rate on quasiparticle density at low dopings (i.e., $y=0.12$) is suppressed upon submergence of the inner hole band and quasiparticle relaxation occurs in a slow, density independent manner.
We constructed an effective tight-binding model with five Cr $3d$ orbitals for LaOCrAs according to first-principles calculations. Basing on this model, we investigated possible superconductivity induced by correlations in doped LaOCrAs using the functional renormalization group (FRG). We find that there are two domes of superconductivity in electron-doped LaOCrAs. With increasing electron doping, the ground state of the system evolves from G-type antiferromagnetism in the parent compound to an incipient $s_pm$-wave superconducting phase dominated by electron bands derived from the $d_{3z^2-r^2}$ orbital as the filling is above $4.2$ electrons per site on the $d$-orbitals of Cr. The gap function has strong octet anisotropy on the Fermi pocket around the zone center and diminishes on the other pockets. In electron over-doped LaOCrAs, the system develops $d_{x^2-y^2}$-wave superconducting phase and the active band derives from the $d_{xy}$ orbital. Inbetween the two superconducting domes, a time-reversal symmetry breaking $s+id$ SC phase is likely to occur. We also find $s_pm$-wave superconducting phase in the hole-doped case.
Low temperature specific heat (SH) was measured on the FeAs-based superconducting single crystals Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ and high pressure synthesized polycrystalline samples SmFeAsO$_{0.9}$F$_{0.1}$. It is found that the sharp SH anomaly $Delta C/T|_{T_c}$ in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ reaches an unexpected high value of 98 mJ/mol K$^2$, about one order of magnitude larger than that of SmFeAsO$_{0.9}$F$_{0.1}$ ($6sim8$ mJ/mol K$^2$) samples, suggesting very high normal state quasiparticle density of states in FeAs-122 than in FeAs-1111. Furthermore, we found that the electronic SH coefficient $gamma_e(T)$ of Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ is weakly temperature dependent and increases almost linearly with the magnetic field in low temperature region, which may indicate that the hole-doped FeAs-122 system contains a dominant component with a full superconducting gap, although we cannot rule out the possibility of a small component with anisotropic or nodal gap. A detailed analysis reveals that the $gamma_e(T)$ of Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ cannot be fitted with a single gap of s-wave symmetry probably due to the multigap effect. These results indicate clear difference between the properties of the superconducting state of the holed-doped Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ and the F-doped LnFeAsO (Ln = rare earth elements) systems, which we believe is originated from the complex Fermi surface structures in different systems.
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