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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Cited by in corpus (6)

• 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
• Nodeless superconductivity in the presence of spin-density wave in pnictide superconductors: The case of BaFe${}_{2-x}$Ni${}_x$As${}_2$
• 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${}_2$
• On the existence of nematic-superconducting states in the Ginzburg-Landau regime