Competition between superconductivity and magnetic/nematic order as a source of anisotropic superconducting gap in underdoped Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$

Abstract

The in-plane London penetration depth, $\Delta \lambda (T)$, was measured using a tunnel diode resonator technique in single crystals of Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ with doping levels $x$ ranging from heavily underdoped, $x$=0.16 ($T_c$=7~K) to nearly optimally doped, $x$= 0.34 ($T_c=$39 K). Exponential saturation of $\Delta \lambda (T)$ in the $T\to 0$ limit is found in optimally doped samples, with the superfluid density ${\rho }_s(T)\equiv (\lambda (0)/\lambda (T){)}^2$ quantitatively described by a self-consistent $\gamma$-model with two nodeless isotropic superconducting gaps. As the doping level is decreased towards the extreme end of the superconducting dome at $x$=0.16, the low-temperature behavior of $\Delta \lambda (T)$ becomes non-exponential and best described by the power-law $\Delta \lambda (T)\propto T^2$, characteristic of strongly anisotropic gaps. The change between the two regimes happens within the range of coexisting magnetic/nematic order and superconductivity, $x<0.25$, and is accompanied by a rapid rise in the absolute value of $\Delta \lambda (T)$ with underdoping. This effect, characteristic of the competition between superconductivity and other ordered states, is very similar to but of significantly smaller magnitude than what is observed in the electron-doped Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ compounds. Our study suggests that the competition between superconductivity and magnetic/nematic order in hole-doped compounds is weaker than in electron-doped compounds, and that the anisotropy of the superconducting state in the underdoped iron pnictides is a consequence of the anisotropic changes in the pairing interaction and in the gap function promoted by both magnetic and nematic long-range order.

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• Nodeless multiband superconductivity in stoichiometric single crystalline CaKFe${}_4$As${}_4$
• Nematic superconducting state in iron pnictide superconductors
• Effects of electron irradiation on resistivity and London penetration depth of Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ ($x\le$ 0.34) iron - pnictide superconductor
• Using electron irradiation to probe iron - based superconductors
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• Magnetic penetration depth in disordered iron-based superconductors
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• Robust s+/- pairing in CaK[Fe(1-x)Ni(x)]4As4$ (x = 0 and 0.05) from the response to electron irradiation
• Muon spin rotation and infrared spectroscopy study of $B{a}_{1-x}N{a}_{x}F{e}_{2}A{s}_2$
• Effect of proton irradiation on the normal state low-energy excitations of Ba(Fe${}_{1-x}$Rh${}_x$)${}_2$As${}_2$ superconductors
• Infrared probe of the gap evolution across the phase diagram of Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$
• Dependence of the absolute value of the penetration depth in $(\mathbf{B}{\mathbf{a}}_{1-\mathbf{x}}{\mathbf{K}}_{\mathbf{x}})\mathbf{F}{\mathbf{e}}_2\mathbf{A}{\mathbf{s}}_2$ on doping
• Trion formation and unconventional superconductivity in a three-dimensional model with short-range attraction
• Interplay between nematic fluctuation and superconductivity in the two-orbital Hubbard model: A quantum Monte Carlo study
• On the existence of nematic-superconducting states in the Ginzburg-Landau regime
• Quasiclassical theory of $C_4$-symmetric magnetic order in disordered multiband metals