Motivated by recent experiments in Ba1−xKxFe2As2 [A. E. Böhmer et al, to be published], we analyze the type of spin-density wave (SDW) order in doped iron-pnictides and the discontinuities of the superconducting transition temperature Tc in the coexistence phase with SDW magnetism. By tracking the magnetic transition line TN (x) towards optimal doping within an itinerant fermionic model, we find a sequence of transitions from the stripe-orthorhombic (C2) SDW order to the tetragonal (C4) order and then back to the C2 order. We argue that the superconducting Tc has two discontinuities -it jumps to a smaller value upon entering the coexistence region with the C4 magnetic phase, and then jumps to a larger value inside the SDW state when it crosses the boundary between the C4 and C2 SDW orders. The full agreement with the experimental phase diagram provides a strong indication that the itinerant approach is adequate to describe the physics of weakly/moderately doped iron-pnictides.Introduction. One of the key features of the ironbased superconductors is the proximity or coexistence of superconductivity with a spin-density wave (SDW) magnetic order [1]. The stripe-type magnetic order (spins aligned ferromagnetically in one direction and antiferromagnetically in the other) has been observed experimentally in numerous undoped and weakly doped materials below T N ∼ 150K [2,6]. Such an order breaks the O(3) spin-rotational symmetry and also breaks the tetragonal (C 4 ) lattice rotational symmetry down to orthorhombic (C 2 ). Theoretically, the stripe order has been found in both itinerant [3][4][5][7][8][9] and localized spin [10][11][12] approaches to Fe-pnictides. In the itinerant scenario, stripe order originates from the interaction between fermions near hole and electron pockets, which are separated by Q 1 = (0, π) and Q 2 = (π, 0) in the Fe-only Brillouin zone. Fluctuations of the stripe order above the magnetic ordering temperature T N were further argued [4,13] to split the O(3) and C 4 transitions and give rise to the observed nematic-type order at T N < T < T nem in which the C 4 lattice rotational symmetry is broken down to C 2 but the O(3) spin-rotational symmetry remains unbroken.

where J is the electrical current operator corresponding to Eq. (1), obtained through replacing c αk → c αk+eA and taking J = ∂H/∂A. To lowest order in λ or in 1/N ,. Within the memory matrix method, the dynamics is projected onto a set of "slow operators" that are nearly conserved by the Hamiltonian. We briefly review the method in Appendix A 1. In our model, the operators n αk = c † αk c αk are nearly-conserved in the limit of either small λ or large N [58]. The optical conductivity can then be cast in the form:where χ Jx,αk ≡´β 0 dτ J x (τ )n αk (0) and χ αk,βk ≡

In several unconventional superconductors, the highest superconducting transition temperature Tc is found in a region of the phase diagram where the antiferromagnetic transition temperature extrapolates to zero, signaling a putative quantum critical point. The elucidation of the interplay between these two phenomena -high-Tc superconductivity and magnetic quantum criticality -remains an important piece of the complex puzzle of unconventional superconductivity. In this paper, we combine sign-problem-free Quantum Monte Carlo simulations and field-theoretical analytical calculations to unveil the microscopic mechanism responsible for the superconducting instability of a general low-energy model, called spin-fermion model. In this approach, low-energy electronic states interact with each other via the exchange of quantum critical magnetic fluctuations. We find that even in the regime of moderately strong interactions, both the superconducting transition temperature and the pairing susceptibility are governed not by the properties of the entire Fermi surface, but instead by the properties of small portions of the Fermi surface called hot spots. Moreover, Tc increases with increasing interaction strength, until it starts to saturate at the crossover from hot-spots dominated to Fermi-surface dominated pairing. Our work provides not only invaluable insights into the system parameters that most strongly affect Tc, but also important benchmarks to assess the origin of superconductivity in both microscopic models and actual materials.

Most iron-based superconductors undergo a transition to a magnetically ordered state characterized by staggered stripes of parallel spins. With ordering vectors (π, 0) or (0, π), this magnetic state breaks the high-temperature tetragonal symmetry of the system, which is manifested by a splitting of the lattice Bragg peaks. Remarkably, recent experiments in hole-doped iron arsenides reported an ordered state that displays magnetic Bragg peaks at (π, 0) and (0, π) but remains tetragonal. Despite being inconsistent with a magnetic stripe configuration, this unusual magnetic phase can be described in terms of a double-Q magnetic structure consisting of an equal-weight superposition of the ordering vectors (π, 0) and (0, π). Here we show that a non-collinear double-Q magnetic configuration, dubbed orthomagnetic, arises naturally within an itinerant three-band microscopic model for the iron pnictides. In particular, we find that strong deviations from perfect nesting and residual interactions between the electron pockets favor the orthomagnetic over the stripe magnetic state. Using an effective low-energy model, we also calculate the spin-wave spectrum of the orthomagnetic state. In contrast to the stripe state, there are three Goldstone modes, manifested in all diagonal and one off-diagonal component of the spin-spin correlation function. The total magnetic structure factor displays two anisotropic gapless spin-wave branches emerging from both (π, 0) and (0, π) momenta, in contrast to the case of domains of stripe order, where only one gapless spin-wave branch emerges from each momentum. We propose that these unique features of the orthomagnetic state can be used to unambiguously distinguish it from the stripe state via neutron scattering experiments, and discuss the implications of its existence to the nature of the magnetism of the iron arsenides.

We investigate the fate of the orthorhombic stripe-type magnetic state [ordering vectors (π,0)/(0,π )], observed in most iron-pnictide superconductors, in the presence of localized magnetic moments that tend to form a Néel state [ordering vector (π,π)]. We show that before long-range Néel order sets in, the coupling between the conduction electrons and the fluctuations of the local moments favors an unusual magnetic state consisting of a coherent superposition of the (π,0) and (0,π ) orders that preserves tetragonal symmetry. The magnetization of this state is nonuniform and induces a simultaneous checkerboard charge order. We discuss signatures of this magnetic configuration on the electronic spectrum and its impact on the superconducting state, showing that its phase space for coexistence with the s +− state is smaller than the stripe-type state. Our results shed light on recent experimental observations on Ba(Fe 1−x Mn x ) 2 As 2 compounds, where the Néel-type local Mn moments interact with the Fe conduction electrons.