Emergence of Superconductivity in Doped Multiorbital Hubbard Chains

TL;DR

This paper introduces a variational valence bond state for one-dimensional two-orbital Hubbard models, explaining the emergence of superconductivity upon doping through a resonating valence bond mechanism.

Contribution

It proposes a novel variational state, Orbital Resonant Valence Bond, that captures pairing and superconductivity in doped multiorbital Hubbard chains.

Findings

Doped systems exhibit mobile singlet pairs leading to superconductivity.

The variational state accurately describes the undoped and doped phases.

Material conditions for spin-singlet and spin-triplet pairing are identified.

Abstract

We introduce a variational state for one-dimensional two-orbital Hubbard models that intuitively explains the recent computational discovery of pairing in these systems when hole doped. Our Ansatz is an optimized linear superposition of Affleck-Kennedy-Lieb-Tasaki valence bond states, rendering the combination a valence bond liquid dubbed Orbital Resonant Valence Bond. We show that the undoped (one electron/orbital) quantum state of two sites coupled into a global spin singlet is exactly written employing only spin-1/2 singlets linking orbitals at nearest-neighbor sites. Generalizing to longer chains defines our variational state visualized geometrically expressing our chain as a two-leg ladder, with one orbital per leg. As in Anderson's resonating valence-bond state, our undoped variational state contains preformed singlet pairs that via doping become mobile leading to superconductivity. Doped real materials with one-dimensional substructures, two near-degenerate orbitals, and intermediate Hubbard U/W strengths -- W the carrier's bandwidth -- could realize spin-singlet pairing if on-site anisotropies are small. If these anisotropies are robust, spin-triplet pairing emerges.