Spin-triplet p-wave pairing in a three-orbital model for iron pnictide superconductors (original) (raw)
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Orbital coupling and superconductivity in the iron pnictides
Physical Review B, 2009
We demonstrate that strong inter-orbital interaction is very efficient to achieve superconductivity due to magnetic fluctuations in the iron pnictides. Fermi surface states that are coupled by the antiferromagnetic wave vector are often of different orbital nature, causing pair-hopping interactions between distinct Fe-3d orbitals to become important. Performing a self-consistent FLEX calculation below TcT_{c}Tc we determine the superconducting order parameter as function of intra- and inter-orbital couplings. We find an spms^{\pm}spm-pairing state with Tcsimeq80mathrmKT_{c}\simeq80\mathrm{K}Tcsimeq80mathrmK for realistic parameters.
Unconventional pairing originating from disconnected Fermi surfaces in the iron-based superconductor
New Journal of Physics, 2009
For the iron-based high T c superconductor LaFeAsO 1−x F x , we construct a minimal model, where all of the five Fe d bands turn out to be involved. We then investigate the origin of superconductivity with a five-band random-phase approximation by solving the Eliashberg equation. We conclude that the spin fluctuation modes arising from the nesting between the disconnected Fermi pockets realise, basically, an extended s-wave pairing, where the gap changes sign across the nesting vector.
Tetrahedral and Orbital Pairing: A Fully Gapped Pairing Scenario for the Iron-Based Superconductors
Physical Review Letters, 2013
Motivated by the fully gapped superconductivity in iron-based superconductors with uncompensated electron pockets, we propose a spin singlet, but orbital triplet analogue of the superfluid phase of 3 He-B. We show that orbital triplets with a nominal d-wave symmetry at the iron sites can transform as s-wave pairs under rotations about the selenium sites. Linear combinations of such dxy and d x 2 −y 2 triplets form a fully gapped, topological superconductor. Raman-active excitations are predicted to develop below the superconducting transition temperature.
Superconductivity in multi-orbital t-J 1 -J 2 model and its implications for iron pnictides
EPL (Europhysics Letters), 2010
PACS 74.20.-z-Theories and models of superconducting state PACS 74.20.Mn-Nonconventional mechanisms PACS 74.70.Xa-Pnictides and chalcogenides Abstract.-Motivated by the bad metal behavior of the iron pnictides, we study a multi-orbital t − J1 − J2 model and investigate possible singlet superconducting pairings. Magnetic frustration by itself leads to a large degeneracy in the pairing states. The kinetic energy breaks this into a quasi-degeneracy among a reduced set of pairing states. For small electron and hole Fermi pockets, an A1g state dominates over the phase diagram but a B1g state has close-by energy. In addition to the nodeless A1g s x 2 y 2 channel, the nodal A1g s x 2 +y 2 and B1g d x 2 −y 2 channels are also competitive in the magnetically frustrated J1 ∼ J2 parameter regime. An A1g + iB1g state, which breaks time-reversal symmetry, occurs at low temperatures in part of the phase diagram. Implications for the experiments in the iron pnictides are discussed.
Journal of the Physical Society of Japan, 2009
We study whether Fermi-surface (FS) nesting can give rise to high-temperature superconductivity in iron pnictides. Starting with ab initio construction of an effective four-orbital model, we employ the fluctuation-exchange approximation to show that FS does not necessarily favor the stripe antiferromagnetic order observed in experiments, especially for realistic electronic correlations. If superconductivity in iron pnictides is magnetically mediated and has fully-gapped sign-reversing s-wave symmetry, our results suggest that the pairing interaction does not arise only from FS nesting and exchange interactions between local moments in the Fe 3d orbitals may play a crucial role.
Physical Review B, 2014
We study the orbital-dependent superconducting pairing in a five-orbital t-J1-J2 model for iron pnictides. Depending on the orbital selectivity of electron correlations and the orbital characters along the Fermi surface, the superconducting gap in an A1g pairing state may exhibit anisotropy. This anisotropy varies with the degree of J1-J2 magnetic frustration. We have also calculated the dynamical spin susceptibility in the superconducting state. The frequency dependence of the susceptibility at the antiferromagnetic wavevector (π, 0) shows a resonance, whose width is enhanced by the orbital dependence of the superconducting gap; when the latter is sufficiently strong, the resonance peak may be split into two. We discuss the implications of our results on the recent angle-resolved photoemission and neutron-scattering measurements in several superconducting iron pnictides.
Unconventional superconductivity in iron pnictides: Magnon mediated pairing
Physica C: Superconductivity and its Applications
We study the phenomenon of unconventional superconductivity in iron pnictides on the basis of localized-itinerant model. In this proposed model, superconductivity arises from the itinerant part of electrons, whereas antiferromagnetism arises from the localized part. The itinerant electrons move over the sea of localized electrons in antiferromagnetic alignment and interact with them resulting in excitation of magnons. We find that triplet pairing of itinerant electrons via magnons is possible in checkerboard antiferromagnetic spin configuration of the substances CaFe 2 As 2 and BaFe 2 As 2 in pure form for umklapp scattering with scattering wave vector Q = (1 , 1) , in the unit of π / a where a being one orthorhombic crystal parameter, which is the nesting vector between two Fermi surfaces. The interaction potential figured out in this way, increases with the decrease in nearest neighbour (NN) exchange couplings. Under ambient pressure, with stripe antiferromagnetic spin configuration, a very small value of coupling constant is obtained which does not give rise to superconductivity. The critical temperature of superconductivity of the substances CaFe 2 As 2 and BaFe 2 As 2 in higher pressure checkerboard antiferromagnetic spin configuration are found to be 12.12 K and 29.95 K respectively which are in agreement with the experimental results.
Journal of the Physical Society of Japan, 2008
The iron-based LaFeAsO 1Àx F x recently discovered by Hosono's group is a fresh theoretical challenge as a new class of high-temperature superconductors. Here we describe the electronic structure of the material and the mechanism of superconductivity. We start with constructing a tight-binding model in terms of the maximally localised Wannier orbitals from a first-principles electronic structure calculation, which has turned out to involve all the five Fe 3d bands. This is used to calculate the spin and charge susceptibilities with the random phase approximation. The spin susceptibility has peaks around k ¼ ðp; 0Þ; ð0; pÞ arising from a nesting across disconnected Fermi surface pockets. We have then plugged the susceptibilities into the linearised Eliashberg equation. For the doping concentration x ¼ 0:1, we obtain an unconventional s-wave pairing, which is roughly an extended s in that the gap changes sign between the Fermi pockets, but the gap function is actually a 5 Â 5 matrix. Its experimental implications are also discussed.
The Role of Orbital Nesting in the Superconductivity of Iron-Based Superconductors
Condensed Matter
We analyze the magnetic excitations and the spin-mediated superconductivity in iron-based superconductors within a low energy model that operates in the band basis, but fully incorporates the orbital character of the spin excitations. We show how the orbital selectivity, encoded in our low energy description, simplifies substantially the analysis and allows for analytical treatments, while retaining all the main features of both spin excitations and gap functions computed using multiorbital models. Importantly, our analysis unveils the orbital matching between the hole and electron pockets as the key parameter to determine the momentum dependence and the hierarchy of the superconducting gaps, instead of the Fermi surface matching, as in the common nesting scenario.
Direct observation of spin-orbit coupling in iron-based superconductor
In iron-based superconductors its role has so far been considered insignificant with the models based on spin-or orbital fluctuations pairing being the most advanced in the field. Using angle-resolved photoemission spectroscopy we directly observe a sizeable spin-orbit splitting in a paradigm material LiFeAs (T c~1 8K) and demonstrate its decisive impact on the low-energy electronic structure and details of the Fermi surface topology. Intriguingly, the largest pairing gap is supported exactly by SOC-induced three-dimensional Fermi surface.