Scaling of the physical properties in Ba(Fe,Ni) ₂As ₂ single crystals: Evidence for quantum fluctuations

We report on local magnetization, tunnel diode oscillators, and specific-heat measurements in a series of Ba(Ni _xFe _1-x) ₂As ₂ single crystals (0.26≤x≤0.74). We show that the London penetration depth λ(T)=λ(0) +Δλ(T) scales as λ(0) 1/Tc0.85±0.2, Δλ(T) T2.3 ^±0.3 (for T<T _c/3), and Δλ/T2 1/Tc2.8±0.3 in both underdoped and overdoped samples. Moreover, the slope of the upper critical field [Hc2′=-(dH _c2dT)|T→T _c] decreases with T _c in overdoped samples but increases with decreasing T _c in underdoped samples. The remarkable variation of λ(0) with T _c and the nonexponential temperature dependence of Δλ clearly indicates that pair-breaking effects are important in this system. We show that the observed scalings strongly suggest that those pair-breaking effects could be associated with quantum fluctuations near three-dimensional superconducting critical points.