Session E11: Electronic Structure and Magnetism in Fe-based Superconductors I

The role of Hund's coupling in the correlations and the nematicity of iron superconductors

Since their discovery in 2008 the strength and the nature of correlations in iron superconductors have been widely discussed [1]. Understanding the correlations is key to unveil the nature of the superconducting, nematic and magnetic instabilities which appear in the phase diagram. Due to their multi-orbital character, correlations in iron superconductors are strongly affected by Hund's coupling and these materials have been classified by some authors as Hund metals. For a long time there has been a strong controversy on the nature of correlations induced by Hund's coupling and its relation to Mott physics. While some authors describe Hund metals as strongly correlated systems which are not in proximity to a Mott insulating state, others, have described iron superconductors as doped Mott insulators. In the talk, after some introduction, I will first show our recent results which show that while the spin polarization of the atoms, promoted by Hund's coupling induces strong correlations, this does not necessary mean that the total charge is more localized [2]. On the contrary, in some cases~this polarization promotes itinerancy [2]. I will then present a generic framework to address the correlations in iron superconductors and discuss the role of Hund's coupling in the nematicity of iron superconductors, with special emphasis on FeSe. [1] Magnetic interactions in iron superconductors: a review, E. Bascones, B. Valenzuela and M.J. Calderon, (in press) arXiv:1503.04223 [2] Electronic correlations in Hund metals. L. Fanfarillo and E. Bascones, Phys. Rev. B 92, 075136 (2015)   

Itinerancy-Enhanced Quantum Fluctuation of Magnetic Moments in Iron-Based Superconductors

We investigate the influence of itinerant carriers on dynamics and fluctuation of local moments in Fe-based superconductors, via linear spin-wave analysis of a spin-fermion model containing both itinerant and local degrees of freedom.Surprisingly against the common lore, instead of enhancing the ($\pi$,0) order, itinerant carriers with well nested Fermi surfaces are found to induce a significant amount of $spatial$ and temporal quantum fluctuation that leads to the observed small ordered moment. Interestingly, the underlying mechanism is shown to be intra-pocket nesting-associated long-range coupling rather than the previously believed ferromagnetic double-exchange effect. This challenges the validity of ferromagnetically compensated first-neighbor coupling reported from short-range fitting to the experimental dispersion, which turns out to result instead from the ferro-orbital order that is also found instrumental in stabilizing the magnetic order. *Y.-T. Tam, D.-X. Yao and W. Ku, Phys. Rev. Lett. 115, 117001(2015)   

Competing magnetic fluctuations in iron pnictide superconductors: role of ferromagnetic spin correlations revealed by NMR

The role of magnetic fluctuations in iron pnictide superconductors has been extensively studied since their discovery. As the parent materials have antiferromagnetic (AFM) ground states, attention has been focused on stripe-type AFM fluctuations, which are widely believed to give rise to the Cooper pairing in the systems. On the other hand, according to density functional theory calculations, the static magnetic susceptibility is enhanced at not only the stripe-type AFM but also ferromagnetic (FM) wavevectors. Nevertheless, FM fluctuations have not been investigated microscopically. In this talk, based on $^{75}$As NMR data [1,2], we report clear evidence for the existence of strong FM correlations in the hole-doped (Ba$_{1-x}$K$_{x})$Fe$_{2}$As$_{2}$ and electron-doped Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$. We will discuss the role of FM spin correlations in the occurrence of superconductivity in these systems. [1] P. Wiecki, et al., Phys. Rev. B \textbf{91}, 220406 (2015). [2] P. Wiecki, et al., Phys. Rev. Lett. \textbf{115}, 137001 (2015).   

Spin-liquid polymorphism in an underdoped iron-chalcogenide superconductor

We report neutron scattering measurements which reveal spin-liquid polymorphism in an “11” iron chalcogenide superconductor. It occurs when a poorly metallic magnetic state of FeTe is driven toward superconductivity by substitution of a small amount of tellurium with isoelectronic sulfur. We observe a liquid-like magnetic response, which is described by the coexistence of two disordered magnetic phases with different local structures whose relative abundance depends on temperature. One is the ferromagnetic (FM) plaquette phase observed in undoped, nonsuperconducting FeTe, which preserves the C4 symmetry of the underlying square lattice and is favored at high temperatures, whereas the other is the antiferromagnetic plaquette phase with broken C4 symmetry, which emerges with doping and is predominant at low temperatures. These findings suggest the coexistence of and competition between two distinct liquid states, and a liquid–liquid phase transformation between these states, in the electronic spin system of FeTe$_{1-x}$(S,Se)$_x$. Our results shed light on many recent experimental data in unconventional superconductors. The phase with lower, C2 local symmetry, whose emergence precedes superconductivity, naturally accounts for a propensity to electronic nematic states.   

Studying the morphology of the magnetic C$_{\mathrm{4}}$ phase in the 122 superconductors.

The iron based superconductors continue to prove an exciting system for the study of superconductivity: the recent discovery of a reentrant tetragonal phase with SDW magnetic ordering has opened new avenues to study the competition between microscopically coexistent superconductivity and magnetism. This intriguing new phase is not only an exceedingly rare example of a magnetic structure with two ordering vectors, and consequently a confirmation of itinerate magnetism, but has also allowed for the determination of spin fluctuations as the driving mechanism behind the phase evolution in these materials. Evidence has been mounting of the universality of C$_{\mathrm{4}}$ in the hole doped iron pnictides providing a useful playground for the comparison of how this phase behaves as it is stabilized out of different parent compounds and through different dopant atoms. Here all members of the hole doped family which show the C$_{\mathrm{4}}$ phase will be compared and the parameters which appear to tune the phase's extent in temperature and phase space will be discussed.   

Itinerant Double-Q Spin-Density Wave in Iron Arsenide Superconductors

The recent observation of a tetragonal magnetic ($C_4$) phase in hole-doped iron arsenide superconductors has provided evidence of a magnetic origin for the electronic nematicity in the $C_2$ phase of these compounds. Now, M\"ossbauer data shows that the new phase also establishes the itinerant character of the antiferromagnetism of these materials and the primary role played by magnetic over orbital degrees of freedom. Neutron diffraction had shown that the magnetic order in the $C_4$ phase was compatible with a double-Q structure arising from a collinear spin-density wave along both the X and Y directions simultaneously. The coherent superposition of the two modulations produces a non-uniform magnetic structure, in which the spin amplitudes vanish on half of the sites and double on the others, a uniquely itinerant effect that is incompatible with local moment magnetism. M\"ossbauer spectra in the $C_4$ phase confirm this double-Q structure, with 50\% of the spectral weight in a zero-moment peak and 50\% with double the magnetic splitting seen in the $C_2$ phase.   

Molecular stripping in the $NF\kappa B/I\kappa B/DNA$ genetic regulatory network

Genetic switches based on the $NF\kappa B/I\kappa B/DNA$ system are master regulators of an array of cellular responses. Recent kinetic experiments have shown that $I \kappa B$ can actively remove NF$\kappa$B bound to its genetic sites via a process called "molecular stripping". This allows the $NF\kappa B/I\kappa B/DNA$ switch to function under kinetic control rather than the thermodynamic control contemplated in the traditional models of gene switches. Using molecular dynamics simulations of coarse grained predictive energy landscape models for the constituent proteins by themselves and interacting with the DNA we explore the functional motions of the transcription factor $NF\kappa B$ and its various binary and ternary complexes with DNA and the inhibitor I$\kappa$B. These studies show that the function of the $NF\kappa B/I\kappa B/DNA$ genetic switch is realized via an allosteric mechanism. Molecular stripping occurs through the activation of a domain twist mode by the binding of $I\kappa B$ which occurs through conformational selection. Free energy calculations for DNA binding show that the binding of $I\kappa B$ not only results in a significant decrease of the affinity of the transcription factor for the DNA but also kinetically speeds DNA release. Projections of the   

Emergent Ising degrees of freedom in the J$_1$-J$_2$-J$_3$ model for the iron tellurides

The iron-telluride family of superconductors form a double-stripe $[\mathbf{Q}=(\pi/2, \pi/2)]$ magnetic order, which can be captured within a $J_1-J_2-J_3$ Heisenberg model in the regime $J_3 \gg J_2 \gg J_1$. Intriguingly, besides breaking spin-rotational symmetry, the ground state manifold has three additional Ising degrees of freedom. Via their coupling to the lattice, they give rise to a monoclinic distortion and to two non-uniform lattice distortions with wave-vector $(\pi, \pi)$. Because the ground state is four-fold degenerate (mod rotations in spin space), only two of these Ising order parameters are independent. Here we introduce an effective field theory to treat all Ising order parameters, as well as magnetic order. All three transitions (corresponding to the condensations of two Ising and one magnetic order parameter) are simultaneous and first order in three dimensions, but lower dimensionality (or equivalently weaker interlayer coupling) and weaker magnetoelastic coupling can split the three transitions, and in some cases allows for two separate Ising phase transitions.   

Non-Fermi liquid behavior in quantum critical iron-pnictide metal Ba(Fe,Ni,Co)$_2$As$_2$

The breakdown of Landau's Fermi liquid theory has been believed to be induced by quantum fluctuations in the vicinity of a quantum critical point (QCP), occasionally accompanied by exotic superconductivity in the strongly correlated electron systems, such as cuprate and iron pnictide superconductors [1]. However, the superconducting dome of such materials with high Tc precludes us from investigating the interplay between quantum fluctuations and the exotic superconductivity. We report non-Fermi liquid behavior associated with quantum fluctuations in the transport and thermodynamic properties of the non-superconducting iron pnictide Ba(Fe,Co,Ni)$_2$As$_2$, which allows us to elucidate the behavior on cooling down to near absolute zero without distractions from the superconductivity. We will discuss the evolution of non-Fermi liquid behavior with magnetic field, highlighting the presence of field tuned QCP. [1] T. Shibauchi et al., Annu. Rev. Condens. Matter Phys. 5, 113 (2014).   

Spin-fluctuation induced non-Fermi-liquid behaviour with suppressed superconductivity in LiFe1-xCoxAs

We study a series of LiFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$As compounds with different Co concentrations by transport, optical spectroscopy, angle-resolved photoemission spectroscopy, and nuclear magnetic resonance. We observe a Fermi-liquid to non-Fermi-liquid to Fermi-liquid (FL-NFL-FL) crossover alongside a monotonic suppression of the superconductivity with increasing Co content. In parallel to the FL-NFL-FL crossover, we find that both the low-energy spin fluctuations and Fermi surface nesting are enhanced and then diminished, strongly suggesting that the NFL behaviour in LiFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$As is induced by low-energy spin fluctuations that are very likely tuned by Fermi surface nesting. Our study reveals a unique phase diagram of LiFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$As where the region of NFL is moved to the boundary of the superconducting phase, implying that they are probably governed by different mechanisms.   

Study of non-Fermi Liquid behavior from partial nesting in multi-orbital superconductors.

Partial nesting between two connected or disconnected regions of the Fermi surface leads to fractional powers of the Coulomb scattering lifetime as a function of temperature and frequency. This result is first demonstrated for a toy band structure where partial nesting occurs within a single band and between different regions of the Brillouin zone. A comparison is then made to a multiband scenario by studying the scattering rate of an effective two orbital model that was proposed in the context of multi-orbital superconductors. In the process, various model independent factors affecting the temperature exponent, n, are identified. The logarithmically divergent contributions of the lowest order vertex correction to the multi-orbital susceptibility, and the role played by nesting in suppressing these divergences is analyzed. The relevance of these results is discussed keeping the recently observed anomalous resistivity in the Co doped Iron superconductor LiFeAs as a backdrop.   

Sensitivity of quantum critical pairing to Fermi surface topology: a Quantum Monte Carlo study

In many iron-based and copper-based materials, unconventional superconductivity appears in close proximity to an antiferromagnetic instability. This fact has motivated intense theoretical investigations of the impact of magnetic fluctuations, particularly those associated with the putative quantum critical point (QCP), on the formation of the Cooper pairs. Although significant advance has been achieved using analytical methods to solve the so-called spin-fermion model, in which low-energy electronic states couple to quantum critical bosonic fluctuations, there remain significant challenges in establishing a perturbative scheme that accounts for both non-Fermi liquid behavior and superconductivity near the QCP. Here we present a sign-problem-free Quantum Monte Carlo (QMC) study of the spin-fermion model for a generic two-band Hamiltonian. We show that properties of the Fermi surface topology beyond the existence of hot spots play a fundamental role in determining the superconducting properties. In particular, we find that proximity to perfect nesting strongly suppresses the enhancement of the pairing susceptibility promoted by the QCP. We also compare our QMC results with an Eliashberg analysis of the quantum critical problem.   

Density Functional Theory insights into the mechanism of noncollinear incommensurate spin density waves in Iron Arsenide.

Iron arsenide intersects interesting physics between novel superconductors and other helical magnetic ordering in Pnma metal arsenide materials. Recent diffraction data has found a more complex ordering than a simple helical incommensurate spin density wave. Instead iron arsenide exhibits a definite chirality to the helimagnetism, an ellipticity in the spiral not aligned with the crystal axis, and resonant diffraction peaks forbidden by the Pnma symmetry. From non-magnetic and collinear density functional theory calculations we present insight into the mechanisms for the formation of this helimagnetic state. We find that ferromagnetic superexchange is a likely mechanism for the spin ordering and that the noncollinear ordering under this regime is caused by the spins on neighbouring irons arranging to minimise direct exchange between iron atoms, and also minimize disruption of the ferromagnetic superexchange between priveleged iron-arsenic pairs. To explain the forbidden peaks in the diffraction we have performed second-order spin-orbit perturbation calculations on the nonmagnetic calculation, which finds that the orbital ordering on the iron atoms coupled to the helimagnetism will lead to the otherwise symmetry-forbidden peaks.