Magnetically driven suppression of nematic order in an iron-based superconductor.

A theory of superconductivity in the iron-based materials requires an understanding of the phase diagram of the normal state. In these compounds, superconductivity emerges when stripe spin density wave (SDW) order is suppressed by doping, pressure or atomic disorder. This magnetic order is often pre-empted by nematic order, whose origin is yet to be resolved. One scenario is that nematic order is driven by orbital ordering of the iron 3d electrons that triggers stripe SDW order. Another is that magnetic interactions produce a spin-nematic phase, which then induces orbital order. Here we report the observation by neutron powder diffraction of an additional fourfold-symmetric phase in Ba1-xNaxFe2As2 close to the suppression of SDW order, which is consistent with the predictions of magnetically driven models of nematic order.

Enhancement of the London penetration depth in pnictides at the onset of spin-density-wave order under superconducting dome.

Recent measurements of the doping dependence of the London penetration depth λ(x) at low T in clean samples of isovalent BaFe2(As(1-x)P(x))2 at T≪T(c) [Hashimoto et al., Science 336, 1554 (2012)] revealed a peak in λ(x) near optimal doping x=0.3. The observation of the peak at T≪T(c), points to the existence of a quantum critical point beneath the superconducting dome. We associate such a quantum critical point with the onset of a spin-density-wave order and show that the renormalization of λ(x) by critical magnetic fluctuations gives rise to the observed feature. We argue that the case of pnictides is conceptually different from a one-component Galilean invariant Fermi liquid, for which correlation effects do not cause the renormalization of the London penetration depth at T=0.

Interpocket pairing and gap symmetry in Fe-based superconductors with only electron pockets.

We analyze the pairing symmetry in Fe-based superconductors AFe2Se2 (A=K, Rb, Cs) which contain only electron pockets. We argue that the pairing condensate in such systems contains not only intrapocket component but also interpocket component, made of fermions belonging to different electron pockets. We analyze the interplay between intrapocket and interpocket pairing, depending on the ellipticity of electron pockets and the strength of their hybridization. We show that with increasing hybridization, the system undergoes a transition from a d-wave state to an s+- state, in which the gap changes sign between hybridized pockets. This s+- state has the full gap and at the same time supports spin resonance, in agreement with the data. Near the boundary between d and s+- states, we found a long-sought s+id state which breaks time-reversal symmetry.

Evolution of the superconducting state of Fe-based compounds with doping.

We introduce an effective low-energy pairing model for Fe-based superconductors with s- and d-wave interaction components and a small number of input parameters and use it to study the doping evolution of the symmetry and the structure of the superconducting gap. We argue that the model describes the entire variety of pairing states found so far in the Fe-based superconductors and allows one to understand the mechanism of the attraction in s(±) and d(x(2)-y(2)) channels, the competition between s- and d-wave solutions, and the origin of superconductivity in heavily doped systems, when only electron or only hole pockets are present.

Effect of Fermi surface nesting on resonant spin excitations in Ba(1-x)K(x)Fe2As2.

We report inelastic neutron scattering measurements of the resonant spin excitations in Ba(1-x)K(x)Fe(2)As(2) over a broad range of electron band filling. The fall in the superconducting transition temperature with hole doping coincides with the magnetic excitations splitting into two incommensurate peaks because of the growing mismatch in the hole and electron Fermi surface volumes, as confirmed by a tight-binding model with s(±)-symmetry pairing. The reduction in Fermi surface nesting is accompanied by a collapse of the resonance binding energy and its spectral weight, caused by the weakening of electron-electron correlations.

Nonanalytic spin susceptibility of a Fermi liquid: the case of Fe-based pnictides.

We propose an explanation of the peculiar linear temperature dependence of the uniform spin susceptibility chi(T) in ferropnictides. We argue that the linear in T term appears to be due to the nonanalytic temperature dependence of chi(T) in a two-dimensional Fermi liquid. We show that the prefactor of the T term is expressed via the square of the spin-density-wave (SDW) amplitude connecting nested hole and electron pockets. Because of an incipient SDW instability, this amplitude is large, which, along with a small value of the Fermi energy, makes the T dependence of chi(T) strong. We demonstrate that this mechanism is in quantitative agreement with the experiment.

Spin resonance in three-dimensional superconductors: the case of CeCoIn5.

The recent observation of resonance spin excitation at (1/2, 1/2, 1/2) in the superconducting state of CeCoIn5 [C. Stock, Phys. Rev. Lett. 100, 087001 (2008)10.1103/Phys. Rev. Lett.100.087001] was interpreted as evidence for d{x{2}-y{2}} gap symmetry, by analogy with cuprates. This is true if the resonance is a spin exciton. We argue that such a description is undermined by the three dimensionality of CeCoIn5. We show that in 3D systems the excitonic resonance only emerges at strong coupling, and is weak. We argue in favor of the alternative, magnon scenario, which does not require a d{x{2}-y{2}} gap.

Phenomenological theory of the underdoped phase of a high-Tc superconductor.

We model the Fermi surface of the cuprates by one-dimensional nested parts near (0, pi) and (pi, 0) and unnested parts near the zone diagonals. Fermions in the nested regions form 1D spin liquids and develop spectral gaps below some approximately T*, but superconducting order is prevented by 1D phase fluctuations. We show that the Josephson coupling between these order parameters locks their relative phase at pi at the crossover scale T**

Effect of fermi surface curvature on low-energy properties of fermions with singular interactions.

We discuss the effect of Fermi surface curvature on long-distance or time asymptotic behaviors of two-dimensional fermions interacting via a gapless mode described by an effective gauge-field-like propagator. By comparing the predictions based on the idea of multidimensional bosonization with those of the strong-coupling Eliashberg approach, we demonstrate that an agreement between the two requires a further extension of the former technique.

Novel neutron resonance mode in dx2-y2-wave superconductors.

We show that a new resonant magnetic excitation at incommensurate momenta, observed recently by inelastic neutron scattering experiments on YBa2Cu3O6.85 and YBa2Cu3O6.6, is a spin exciton. Its location in the Brillouin zone and its frequency are determined by the momentum dependence of the particle-hole continuum. We identify several features that distinguish this novel mode from the previous resonance mode observed near Q=(pi,pi).

Neutron resonance in the cuprates and its effect on fermionic excitations.

We argue that the exciton scenario for the magnetic resonance in the cuprate superconductors yields a small spectral weight of the resonance, in agreement with experiment. We show that the small weight is related to its concentration in a small region of momentum and energy. Despite this, we find that a large fermionic self-energy can indeed be generated by a resonance with such properties, i.e., the scattering from the resonance substantially affects the electronic properties of the cuprates below T(c).