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Register a new userUltrafast observation of critical nematic fluctuations and giant magnetoelastic coupling in iron pnictides
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Many of the iron pnictides have strongly anisotropic normal-state characteristics, important for the exotic magnetic and superconducting behavior these materials exhibit. Yet, the origin of the observed anisotropy is unclear. Electronically driven nematicity has been suggested, but distinguishing this as an independent degree of freedom from magnetic and structural orders is difficult, as these couple together to break the same tetragonal symmetry. Here we use time-resolved polarimetry to reveal critical nematic fluctuations in unstrained Ba(Fe_(1-x)Co_x)_2As_2. The femtosecond anisotropic response, which arises from the two-fold in-plane anisotropy of the complex refractive index, displays a characteristic two-step recovery absent in the isotropic response. The fast recovery appears only in the magnetically ordered state, whereas the slow one persists in the paramagnetic phase with a critical divergence approaching the structural transition temperature. The dynamics also reveal a gigantic magnetoelastic coupling that far exceeds electron-spin and electron-phonon couplings, opposite to conventional magnetic metals.
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By applying density functional theory, we find strong evidence for an itinerant nature of magnetism in two families of iron pnictides. Furthermore, by employing dynamical mean field theory with continuous time quantum Monte Carlo as an impurity solver, we observe that the antiferromagnetic metal with small magnetic moment naturally arises out of coupling between unfrustrated and frustrated bands. Our results point to a possible scenario for magnetism in iron pnictides where magnetism originates from a strong instability at the momentum vector ($pi$, $pi$, $pi$) while it is reduced by quantum fluctuations due to the coupling between weakly and strongly frustrated bands.
Quantum criticality in iron pnictides involves both the nematic and antiferromagnetic degrees of freedom, but the relationship between the two types of fluctuations has yet to be clarified. Here we study this problem in the presence of a small external uniaxial potential, which breaks the $C_4$-symmetry in the B$_{1g}$ sector. We establish an identity that connects the spin excitation anisotropy, which is the difference of the dynamical spin susceptibilities at $vec{Q}_1=left(pi,0right)$ and $vec{Q}_2=left(0,piright)$, with the dynamical magnetic susceptibility and static nematic susceptibility. Using this identity, we introduce a scaling procedure to determine the dynamical nematic susceptibility in the quantum critical regime, and illustrate the procedure for the case of the optimally Ni-doped BaFe$_2$As$_2$[Y. Song textit{et al.}, Phys. Rev. B 92, 180504 (2015)]. The implications of our results for the overall physics of the iron-based superconductors are discussed.
Raman scattering experiments on LaFeAsO with splitted antiferromagnetic (T_AFM = 140 K) and tetragonal-orthorhombic (T_S = 155 K) transitions show a quasi-elastic peak (QEP) in B2g symmetry (2 Fe tetragonal cell) that fades away below ~T_AFM and is ascribed to electronic nematic fluctuations. A scaling of the reported shear modulus with the T-dependence of the QEP height rather than the QEP area indicates that magnetic degrees of freedom drive the structural transition. The large separation between T_S and T_AFM in LaFeAsO compared with their coincidence in BaFe2As2 manifests itself in slower dynamics of nematic fluctuations in the former.
While in strongly correlated materials one often focuses on local electronic correlations, the influence of non-local exchange and correlation effects beyond band-theory can be pertinent in systems with more extended orbitals. Thus in many compounds an adequate theoretical description requires the joint treatment of local and non-local self-energies. Here, I will argue that this is the case for the iron pnictide and chalcogenide superconductors. As an approach to tackle their electronic structure, I will detail the implementation of the recently proposed scheme that combines the quasi-particle self-consistent GW approach with dynamical mean-field theory: QSGW+DMFT. I will showcase the possibilities of QSGW+DMFT with an application on BaFe2As2. Further, I will discuss the empirical finding that in pnictides dynamical and non-local correlation effects separate within the quasi-particle band-width.
There are two prerequisites for understanding high-temperature (high-T$_c$) superconductivity: identifying the pairing interaction and a correct description of the normal state from which superconductivity emerges. The nature of the normal state of iron-pnictide superconductors, and the role played by correlations arising from partially screened interactions, are still under debate. Here we show that the normal state of carefully annealed electron-doped BaFe$_{2-x}$Co$_{x}$As$_2$ at low temperatures has all the hallmark properties of a local Fermi liquid, with a more incoherent state emerging at elevated temperatures, an identification made possible using bulk-sensitive optical spectroscopy with high frequency and temperature resolution. The frequency dependent scattering rate extracted from the optical conductivity deviates from the expected scaling $M_{2}(omega,T)propto(hbaromega)^{2}+(ppi k_{B}T)^{2}$ with $papprox$ 1.47 rather than $p$ = 2, indicative of the presence of residual elastic resonant scattering. Excellent agreement between the experimental results and theoretical modeling allows us to extract the characteristic Fermi liquid scale $T_{0}approx$ 1700 K. Our results show that the electron-doped iron-pnictides should be regarded as weakly correlated Fermi liquids with a weak mass enhancement resulting from residual electron-electron scattering from thermally excited quasi-particles.
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