Spin Susceptibility, Phase Diagram, and Quantum Criticality in the Eelctron-Doped High T_c Superconductor Ba(Fe_1-xCo_x)₂As₂

Abstract

We report a systematic investigation of Ba(Fe 1Àx Co x ) 2 As 2 based on transport and 75 As NMR measurements, and establish the electronic phase diagram. We demonstrate that doping progressively suppresses the uniform spin susceptibility and low frequency spin fluctuations. The optimum superconducting phase emerges at x c ' 0:08 when the tendency toward spin ordering completely diminishes. Our findings point toward the presence of a quantum critical point near x c between the SDW (spin density wave) and superconducting phases.KEYWORDS: iron pnictide superconductor, high temperature superconductivity, NMR, quantum criticality DOI: 10.1143/JPSJ.78.013711The recent discovery of iron-pnictide high T c superconductors 1-6) poses a new intellectual challenge in condensed matter physics. Although the superconducting mechanism remains unknown, it has become apparent that ironpnictides share remarkable similarities with high T c cuprates: the undoped parent phases LaFeAsO and BaFe 2 As 2 have FeAs square-lattice sheets, and itinerant electrons in these layers undergo antiferromagnetic long range order (AFLRO) below modest temperatures, T SDW $ 140 K; [7][8][9] electron or hole doping suppresses the AFLRO and induces superconductvity. There exist clear dissimilarities, too. For example, all five 3d orbitals of Fe atoms contribute to the multiple Fermi surfaces, hence to superconductivity, in ironpnictides. 10,11) In contrast, only the Cu 3d x 2 Ày 2 orbital plays a role in curates. Furthermore, substitution of Co atoms into Fe sites of the parent phases results in electron doping and induces superconductivity [12][13][14] without creating localized moments, 15) while Zn 2þ ions doped into Cu 2þ sites induce localized moments and destroy superconductivity in cuprates. Sorting out these similarities and dissimilarities may lead us to an understanding the mechanism of high T c superconductivity in iron-pnictides as well as in cuprates.In this letter, we utilize transport and NMR techniques to probe the evolution of the electronic properties of Ba(Fe 1Àx -Co x ) 2 As 2 single crystals from the undoped SDW phase (x ¼ 0 with T SDW ¼ 135 K), underdoped SDW phase (x ¼ 0:02 with T SDW ¼ 100 K, and x ¼ 0:04 with T SDW ¼ 66 K), optimally doped superconducting phase (x ¼ 0:08 with T c ¼ 22 K), to the slightly overdoped regime (x ¼ 0:105 with T c ¼ 15 K). We establish the electronic phase diagram which has a quantum critical point near the optimum concentration x c $ 0:08, and explore the possible relation between paramagnetic spin fluctuations and the mechanism of superconductivity. From NMR Knight shift and spin-lattice relaxation rate measurements, we show that electron doping progressively suppresses the uniform spin susceptibility spin and low frequency spin fluctuations. The optimally doped superconducting phase emerges at x c $ 0:08 as soon as doped electrons completely suppress the tendency toward AFLRO.We grew single crystals of Ba(Fe 1Àx Co x ) 2 As 2 from FeAs flux, 13) and determined the actual Co concentration x by energ...

Antiferromagnetic order and spin dynamics in iron-based superconductors

“…In addition to determining the electronic phase diagram of Ba(Fe 1−x Co x ) 2 As 2 as shown in Fig. 35(a), the 1/T 1 T obtained by NMR measurements is also related to the wave vector integral of the low-energy spin dynamic susceptibility χ ′′ (Q, f ) via 1/T 1 T = A/(T − θ) ∼ Q |A(Q)| 2 χ ′′ (Q, f )/f , where θ is the Curie-Weiss temperature (the temperature at which a plot of the reciprocal molar magnetic susceptibility against the absolute temperature T intersects the T -axis), f is the NMR frequency, a is the lattice constant, |A(Q)| 2 = |A cos(Q x a/2) cos(Q y a/2)| 2 is the form factor of transferred hyperfine coupling at the 75 As sites, and the wave vector summation of Q is taken over the entire first Bril-louin zone (Ning et al, 2009). By measuring the Codoping dependence of 1/T 1 T , one can fit the data with Curie-Weiss law and obtain the electron doping dependence of θ.…”

Suppression of spin density wave by isoelectronic substitution in

“…This is not typically observed in doped 1111-type materials; however, a similar change in resistivity behavior at this transition upon alloying has been observed upon substitution of Co and Cr in BaFe 2−x Co x As 2 . [32,33,34] The upturn in resistivity is present in materials with x as high as 0.67 ( Figure 3b, inset), indicating a phase transition persists to quite high Ru concentrations. Figure 4 shows the results of thermal transport measurements on PrFe 1−x Ru x AsO.…”

Effect of inelastic scattering on the nuclear magnetic relaxation rate 1/T1Tin iron-based superconductors

“…28 In the Fe based superconductors, many experimental results of 1/T 1 have been published. [29][30][31][32][33][34][35] It was found that the coherence peak in 1/T 1 is absent in many compounds, like electron-doped Ba(Fe 1−x Co x ) 2 As 2 and hole-doped Ba 1−x K x Fe 2 As 2 .…”