Point contact spectroscopy studies of the iron based superconductors
Point contact spectroscopy (PCS) is used to probe both the normal and superconducting phases of the iron based superconductors. It is shown that apart from probing superconductors by Andreev reflection, PCS is also a useful technique to study exotic electron matter in correlated materials. The point contact junctions are made by two separate techniques: needle-anvil PCS and soft PCS. $dI/dV$ measured in the superconducting phase is sensitive to the magnitude and symmetry of the superconducting order parameter. Andreev reflection spectra for the 122 and 111 families of the iron based superconductors is presented. The 122 crystals probed include electron doped $\rm{Ba(Fe_{1-x}Co_x)_2As_2}$ ($\rm{x = 0.05, 0.055, 0.07, 0.08}$), hole doped $\rm{Ba_{0.8}K_{0.2}Fe_2As_2}$, and isoelectronic doped $\rm{BaFe_2(As_{1-x}P_{x})_2}$ ($\rm{x = 0.24, 0.43}$). The 111 crystals studied are electron doped $\rm{NaFe_{1-x}Co_xAs}$ ($\rm{x = 0.02, 0.06}$). The Andreev spectra show clear features corresponding to multiple, nodeless superconducting gaps. The $dI/dV$ curves are fit to the independent two band BTK model assuming isotropic s-wave order parameters. The normal state spectra of certain iron based superconductors shows a conductance enhancement around zero bias above the structural transition temperature, $T_S$. Theoretical analysis showing that this enhancement is likely a consequence of orbital fluctuations is discussed. The nonsuperconducting compounds probed are $\rm{AEFe_2As_2}$ ($\rm{AE=Ca,Sr,Ba}$) and $\rm{Fe_{1+y}Te}$. For $\rm{AE=Sr,Ba}$ orbital fluctuations are detected above $T_S$ while for $\rm{AE=Ca}$ these fluctuations start below $T_S$. Co doping preserves the orbital fluctuations while K doping suppresses it. The fluctuations are only seen at those dopings and temperatures where for detwinned crystals, an in-plane resistive anisotropy is known to exist. The normal state spectra thus provides evidence that PCS is sensitive to orbital fluctuations in the iron based superconductors. A thorough analysis of $\rm{Ba(Fe_{1-x}Co_x)_2As_2}$ is performed and a new region on the phase diagram is defined where PCS picks up orbital fluctuations. Diagnostics performed to ensure the quality of the PCS junctions are discussed in detail. Preliminary data for $dI/dV$ under applied magnetic field and compressive stress is also presented.
Superconductivity, Point Contact Spectroscopy, Iron Based Superconductors, Andreev Reflection, Orbital Fluctuations, Nematicity, Correlated Electron Matter
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign