|October 20||Niravkumar D. Patel (UT)||Magnetic and pairing tendencies in quasi 1D multi-orbital models|
Niravkumar D. Patel (UT)
Magnetic and pairing tendencies in quasi 1D multi-orbital models
The recent discovery of superconductivity under high pressure in the two-leg ladder compound BaFe2S3  opens a broad avenue of research because it represents the first report of pairing tendencies in a quasi-one-dimensional iron-based high-critical-temperature superconductor. Similarly, as in the case of the cuprates, ladders and chains can be far more accurately studied using many-body techniques and model Hamiltonians than their layered counterparts, particularly if several orbitals are active. As a first step, we derive a two-orbital Hubbard model from first principles that describes individual ladders of BaFe2S3 . The model is then studied with the density matrix renormalization group technique . Three main results are found: (i) at half-filling, ferromagnetic order emerges as the dominant magnetic pattern along the rungs of the ladder, and antiferromagnetic order along the legs, in excellent agreement with neutron experiments; (ii) with hole doping, pairs form in the intermediate/strong coupling regime, as found by studying the binding energy of two holes doped on the half-filled system; (iii) projector analysis of the ground state show that same orbital rung and diagonal pairs are most probable. In addition, recently pairing tendencies were also found by our group in a 1D chain of two-orbitals (that can be mapped into a single-orbital two-leg ladder). The analysis of pair-pair correlations show that pairs are formed involving inter-orbital singlets on neighboring sites and different orbitals . In addition, pairing tendencies can be enhanced by increasing the Hund coupling and also by adding an inter-orbital nearest-neighbor antiferromagnetic Heisenberg coupling. These results clearly suggest that magnetic fluctuations are crucial to superconductivity in the iron-based ladder superconductors. These exciting new results pave the way for our understanding of pairing in the iron family of high-Tc materials.