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Register a new userPoint contact spectroscopy in the superconducting and normal state of $mathrm{NaFe_{1-textit{x}}Co_textit{x}As}$
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We use point contact spectroscopy to probe the superconducting and normal state properties of the iron-based superconductor $rm{NaFe_{1-textit{x}}Co_{textit{x}}As}$ with $rm{textit{x} = 0, 0.02, 0.06}$. Andreev spectra corresponding to multiple superconducting gaps are detected in the superconducting phase. For $rm{textit{x} = 0.02}$, a broad conductance enhancement around zero bias voltage is detected in both the normal and the superconducting phase. Such a feature is not present in the $rm{textit{x} = 0.06}$ samples. We suspect that this enhancement is caused by orbital fluctuations, as previously detected in underdoped $rm{Ba(Fe_{1-textit{x}}Co_textit{x})_2As_2}$ (Phys. Rev. B 85, 214515 (2012)). Occasionally, the superconducting phase shows a distinct asymmetric conductance feature instead of Andreev reflection. We discuss the possible origins of this feature. NaFeAs (the parent compound) grown by two different techniques is probed. Melt-grown NaFeAs shows a normal state conductance enhancement. On the other hand, at low temperatures, flux-grown NaFeAs shows a sharp dip in the conductance at zero bias voltage. The compounds are very reactive in air and the different spectra are likely a reflection of their different oxidation and purity levels.
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We report comprehensive temperature and doping-dependences of the Raman scattering spectra for $mathrm{BaFe_{2}}(mathrm{As}_{1-x}mathrm{P}_{x}mathrm{)_{2}}$ ($x =$ 0, 0.07, 0.24, 0.32, and 0.38), focusing on the nematic fluctuation and the superconducting responses. With increasing $x$, the bare nematic transition temperature estimated from the Raman spectra reaches $T =$ 0 K at the optimal doping, which indicates a quantum critical point (QCP) at this composition. In the superconducting compositions, in addition to the pair breaking peaks observed in the $A_{mathrm{1g}}$ and $B_{mathrm{1g}}$ spectra, another strong $B_{mathrm{1g}}$ peak appears below the superconducting transition temperature which is ascribed to the nematic resonance peak. The observation of this peak indicates significant nematic correlations in the superconducting state near the QCP in this compound.
Point contact spectroscopy reveals a gap-like feature above the magnetic and structural transition temperatures for underdoped $Ba(Fe_{1-x}Co_x)_2As_2$, $SrFe_2As_2$ and $Fe_{1+y}Te$. The conductance spectrum starts showing an enhancement at temperatures as high as 177 K for $BaFe_2As_2$ ($T_N$ $sim$ 132 K) and 250 K for $SrFe_2As_2$ ($T_N$ $sim$ 192 K). Possible origins for this enhancement are discussed in light of recent experimental claims of nematicity in these materials. We construct a modified phase diagram for Co-doped Ba122 showing a gap-like feature existing above $T_N$ and $T_S$ for the underdoped regime.
The normal state and superconducting properties are investigated in the phase diagram of K_xSr_{1-x}Fe_2As_2 for 0<x<1. The ground state upper critical field, H_{c2}(0), is extrapolated from magnetic field dependent resistivity measurements. H_{c2}(0) scales with the critical temperature, T_c, of the superconducting transition. In the normal state the Seebeck coefficient is shown to experience a dramatic change near a critical substitution of x=0.3. This is associated with the formation of a spin density wave state above the superconducting transition temperature. The results provide strong evidence for the reconstruction of the Fermi surface with the onset of magnetic order.
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