Valley density-wave and multiband superconductivity in Fe-pnictides

TL;DR

This paper explores the role of valley density-waves and multiband effects in Fe-pnictide superconductors, proposing a connection to a negative U Hubbard model and highlighting the importance of VDW fluctuations for high-Tc superconductivity.

Contribution

It introduces a theoretical framework linking valley density-waves to multiband superconductivity in Fe-pnictides, emphasizing the impact of doping and pressure on their electronic states.

Findings

Valley density-wave is the primary instability in Fe-based superconductors.

Doping and pressure induce a transition to FFLO and VDW states.

VDW fluctuations are crucial for understanding high-Tc superconductivity.

Abstract

The key feature of the Fe-based superconductors is their quasi 2D multiband Fermi surface. By relating the problem to a negative U Hubbard model and its superconducting ground state, we show that the defining instability of such a Fermi surface is the valley density-wave (VDW), a combined spin/charge density-wave at the wavevector connecting the electron and hole valleys. As the valley parameters change by doping or pressure, the fictitious superconductor experiences "Zeeman splitting", eventually going into a non-uniform "Fulde-Ferrell-Larkin-Ovchinikov" (FFLO) state, an itinerant and often incommensurate VDW of the real world, characterized by the metallic conductivity from the ungapped remnants of the Fermi surface. When "Zeeman splitting" exceeds the "Chandrasekhar-Clogston" limit, the "FFLO" state disappears, and the VDW is destabilized. Near this point, the VDW fluctuations and interband pair repulsion are essential ingredients of high-Tc superconductivity in Fe-pnictides.