Nuclear magnetic resonance studies of magnetic fluctuations and nematic order in iron-based superconductors

This thesis illustrates the use of the nuclear magnetic resonance (NMR) technique as a local probe for the study of static and dynamic magnetism in the iron-based superconductors. First, a Korringa ratio analysis of 59Co and 75As NMR data reveals the existence of ferromagnetic (FM) spin fluctuations in SrCo2As2 and the hole- and electron-doped BaFe2As2 families of iron-pnictide superconductors. The analysis further shows that the FM fluctuations compete with AFM fluctuations to suppress superconductivity in these materials. The FM fluctuations are thus a crucial ingredient to understanding the variability of the superconducting transition temperature (Tc) and the shape of the superconducting dome in these and other iron-pnictide families. Secondly, a study of KFe2As2 under pressures up to 2.1 GPa reveals a crossover between a high-temperature incoherent, local-moment behavior and a low-temperature coherent behavior at a crossover temperature, T*. The T* is found to increase monotonically with pressure, consistent with increasing hybridization between localized 3d orbital-derived bands with the itinerant electron bands. No anomaly in T* is seen at the critical pressure where a change of slope of Tc(p) has been observed. In the superconducting state, two-component nuclear spin-lattice relaxation is observed at low temperatures, suggesting the existence of two distinct local electronic environments. Finally, 77Se-NMR studies of FeSe subjected to external pressure and sulfur doping are presented. In pure FeSe under pressure, the NMR spectra reveal the existence of a short-range, local nematic ordered state above the bulk nematic ordering temperature. Furthermore, this local nematic order does not compete with low-energy AFM spin fluctuations. In sulfur-doped FeSe(1−x)Sx, the observed behavior of the magnetic fluctuations parallels the Tc, providing strong evidence for the primary importance of magnetic fluctuations for superconductivity, despite the presence of nematic quantum criticality in this system

Competing Magnetic Fluctuations in Iron Pnictide Superconductors: Role of Ferromagnetic Spin Correlations Revealed by NMR

In the iron pnictide superconductors, theoretical calculations have consistently shown enhancements of the static magnetic susceptibility at both the stripe-type antiferromagnetic (AFM) and in-plane ferromagnetic (FM) wavevectors. However, the possible existence of FM fluctuations has not yet been examined from a microscopic point of view. Here, using $^{75}$As NMR data, we provide clear evidence for the existence of FM spin correlations in both the hole- and electron-doped BaFe$_2$As$_2$ families of iron-pnictide superconductors. These FM fluctuations appear to compete with superconductivity and are thus a crucial ingredient to understanding the variability of $T_{\rm c}$ and the shape of the superconducting dome in these and other iron-pnictide families.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev. Let

Pressure dependence of coherence-incoherence crossover behavior in KFe2As2 observed by resistivity and 75As-NMR/NQR

We present the results of 75 As nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), and resistivity measurements in KFe 2 As 2 under pressure ( p ). The temperature dependence of the NMR shift, nuclear spin-lattice relaxation time ( T 1 ), and resistivity show a crossover between a high-temperature incoherent, local-moment behavior and a low-temperature coherent behavior at a crossover temperature ( T ∗ ). T ∗ is found to increase monotonically with pressure, consistent with increasing hybridization between localized 3 d orbital-derived bands with the itinerant electron bands. No anomaly in T ∗ is seen at the critical pressure p c = 1.8 GPa where a change of slope of the superconducting (SC) transition temperature T c ( p ) has been observed. In contrast, T c ( p ) seems to correlate with antiferromagnetic spin fluctuations in the normal state as measured by the NQR 1 / T 1 data, although such a correlation cannot be seen in the replacement effects of A in the A Fe 2 As 2 ( A = K , Rb, Cs) family. In the superconducting state, two T 1 components are observed at low temperatures, suggesting the existence of two distinct local electronic environments. The temperature dependence of the short T 1 s indicates a nearly gapless state below T c . On the other hand, the temperature dependence of the long component 1 / T 1 L implies a large reduction in the density of states at the Fermi level due to the SC gap formation. These results suggest a real-space modulation of the local SC gap structure in KFe 2 As 2 under pressure

Coexistence of antiferromagnetic and ferromagnetic spin correlations in Ca(Fe1−xCox)2As2 revealed by 75As nuclear magnetic resonance

Recent nuclear magnetic resonance (NMR) measurements revealed the coexistence of stripe-type antiferromagnetic (AFM) and ferromagnetic (FM) spin correlations in both the hole- and electron-doped BaFe2As2 families of iron-pnictide superconductors by a Korringa ratio analysis. Motivated by the NMR work, we investigate the possible existence of FM fluctuations in another iron-pnictide superconducting family, Ca(Fe1−xCox)2As2. We reanalyzed our previously reported data in terms of the Korringa ratio and found clear evidence for the coexistence of stripe-type AFM and FM spin correlations in the electron-doped CaFe2As2 system. These NMR data indicate that FM fluctuations exist in general in iron-pnictide superconducting families and thus must be included to capture the phenomenology of the iron pnictides

Persistent correlation between superconductivity and antiferromagnetic fluctuations near a nematic quantum critical point in FeSe1−xSx

We present 77Se-NMR measurements on FeSe1−xSx samples with sulfur content x=0%, 9%, 15%, and 29%. Twinned nematic domains are observed in the NMR spectrum for all samples except x=29%. The NMR spin-lattice relaxation rate shows that antiferromagnetic (AFM) fluctuations are initially enhanced between x=0% and x=9%, but are strongly suppressed for higher x values. The observed behavior of the AFM fluctuations parallels the superconducting transition temperature Tc in these materials, providing strong evidence for the primary importance of AFM fluctuations for superconductivity, despite the presence of nematic quantum criticality in the FeSe1−xSx system

Impact of Nematicity on the Relationship between Antiferromagnetic Fluctuations and Superconductivity in FeSe0.91S0.09 Under Pressure

The sulfur substituted FeSe system, FeSe$_{1-x}$S$_{x}$, provides a versatile platform for studying the relationship between nematicity, antiferromagnetism, and superconductivity. Here, by nuclear magnetic resonance (NMR) and resistivity measurements up to 4.73 GPa on FeSe$_{0.91}$S$_{0.09}$, we established the pressure($p$)-temperature($T$) phase diagram in which the nematic state is suppressed with pressure showing a nematic quantum phase transition (QPT) around $p$ = 0.5 GPa, two SC regions, separated by the QPT, appear and antiferromagnetic (AFM) phase emerges above $\sim$3.3 GPa. From the NMR results up to 2.1 GPa, AFM fluctuations are revealed to be characterized by the stripe-type wavevector which remains the same for the two SC regions. Furthermore, the electronic state is found to change in character from non-Fermi liquid to Fermi liquid around the nematic QPT and persists up to $\sim$ 2.1 GPa. In addition, although the AFM fluctuations correlate with $T_{\rm c}$ in both SC states, demonstrating the importance of the AFM fluctuations for the appearance of SC in the system, we found that, when nematic order is absent, $T_{\rm c}$ is strongly correlated with the AFM fluctuations, whereas $T_{\rm c}$ weakly depends on the AFM fluctuations when nematic order is present. Our findings on FeSe$_{0.91}$S$_{0.09}$ were shown to be applied to the whole FeSe$_{1-x}$S$_{x}$ system and also provide a new insight into the relationship between AFM fluctuations and SC in Fe-based superconductors.Comment: 10 pages, 10 figures including 4 pages and 6 figures of Supplementary Material, accepted for publication in Phys. Rev. B rapid communicatio