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Register a new userFirst-principles-based $pm s$-wave modelling for iron-based superconductors:Studies for specific heat and nuclear magnetic relaxation rate
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In order to consistently explain controversial experimental results on superconducting states observed by different probes in typical iron-based superconductors, we construct a realistic multi-band $pm s$-wave pairing model by combining the quasiclassical formalism with the first-principles calculation. The model successfully resolves the controversies in contrast to the fact that simplified models such as two-band $pm s$-wave one fail to do. A key in the model is the existence of relatively small gaps which leads to material-dependent peculiarities.
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Nuclear magnetic relaxation rate 1/T_1 in iron-pnictide superconductors is calculated using the gap function obtained in a microscopic calculation. Based on the obtained results, we discuss the issues such as the rapid decrease of 1/T_1 just below the transition temperature and the difference between nodeless and nodal s-wave gap functions. We also investigate the effect of Coulomb interaction on 1/T_1 in the random phase approximation and show its importance in interpreting the experimental results.
We discuss the nuclear magnetic relaxation rate and the superfluid density with the use of the effective five-band model by Kuroki et al. [Phys. Rev. Lett. 101, 087004 (2008)] in Fe-based superconductors. We show that a fully-gapped anisotropic pm s-wave superconductivity consistently explains experimental observations. In our phenomenological model, the gaps are assumed to be anisotropic on the electron-like beta Fermi surfaces around the M point, where the maximum of the anisotropic gap is about four times larger than the minimum.
We study hydrogen doping effects in an iron-based superconductor LaFeAsO_(1-y) by using the first-principles calculation and explore the reason why the superconducting transition temperature is remarkably enhanced by the hydrogen doping. The present calculations reveal that a hydrogen cation stably locating close to an iron atom attracts a negatively-charged FeAs layer and results in structural distortion favorable for further high temperature transition. In fact, the lattice constant a averaged over the employed supercell shrinks and then the averaged As-Fe-As angle approaches 109.74 degrees with increasing the hydrogen doping amount. Moreover, the calculations clarify electron doping effects of the solute hydrogen and resultant Fermi-level shift. These insights are useful for design of high transition-temperature iron-based superconductors.
The strong power law behavior of the specific heat jump $Delta C$ vs. $T_c$ ($Delta C/T_c sim T_c ^{alpha}, alphaapprox 2$), first observed by Budko, Ni, and Canfield (BNC)[1], has been confirmed with several families of the Fe-based superconducting compounds with a series of doping. We show here that this anomalous non-BCS behavior is an intrinsic property of the multiband superconducting state paired by a dominant interband interaction ($V_{inter} > V_{intra}$) reflecting the relation $frac{Delta_h}{Delta_e} sim sqrt{frac{N_e}{N_h}}$ near $T_c$, as in the $pm$S-wave pairing state. Then this $Delta C$ vs. $T_c$ relation can continuously change from the perfect BNC scaling to a considerable deviation at lower $T_c$ region with a moderate variation of the impurity scattering rate.
We report specific heat capacity measurements on a LiFeAs single crystal at temperatures down to 400 mK and magnetic fields up to 9 Tesla. A small specific heat jump at Tc and finite residual density of states at T=0 K in the superconducting (SC) state indicate that there are strong unitary scatterers that lead to states within the SC gap. A sub-linear magnetic field dependence of the Sommerfeld coefficient gamma(H) at T=0 K is equally well fitted by both a nodal d-wave gap as well as a sign changing multiband pm s-wave gap. When impurity effects are taken into account, however, the linear temperature dependence of the electronic specific heat C_{el}/T at low temperatures argues against a nodal d-wave superconducting gap. We conclude that the SC state of LiFeAs is most compatible with the multiband pm s-wave SC state with the gap values Delta_{small}=0.46 Delta_{large}.
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