Excitonic density waves, biexcitons, and orbital-selective pairing in two-orbital correlated chains

Abstract

We present a comprehensive study of a one-dimensional two-orbital model at and below quarterfilling that realizes a number of unconventional phases. In particular, we find an excitonic density wave in which excitons quasi-condense with finite center of mass momentum and an order parameter that changes phase with wave-vector Q. In this phase, excitons behave as hard-core bosons without charge order. In addition, excitons can pair to form bi-excitons in a state that is close to a charge density-wave instability. When pairing dominates over the inter-orbital repulsion, we encounter a regime in which one orbital is metallic, while the other forms a spin gapped superconductor, a genuine orbital selective paired state. All these results are supported by both, analytical and numerical calculations. By assuming a quasi-classical approximation, we solve the three-body holeelectron-spinon problem and show that excitons are held together by forming a bound state with spinons. In order to preserve the antiferromagnetic background, excitons acquire a dispersion that has a minimum away from k = 0. The full characterization of the different phases is obtained by means of extensive density matrix renormalization group calculations.

Excitonic wave-packet evolution in a two-orbital Hubbard model chain: A real-time real-space study

“…We believe this is because increasing J H /U reduces the interorbital interaction U , which results in a less-tightly bound electron-hole pair and, thus, the exciton becomes less heavier and can move at a faster rate |v τ |. On the other hand, at small J H /U the inter-orbital interaction U increases and results in heavier excitons, which naturally are more localized [7,29]. The larger value of orbiton velocity was observed in RIXS experiments because of the large Hund's coupling in Ca 2 CuO 3 [22].…”

“…2(a) and (c)]. The reason is that the inter-orbital interaction U = U −2J H acts as an effective attraction between the hole in orbital b and electron in orbital a [29]. Intuitively, when the hole of orbital b and the electron in orbital a are in the same site, the strong inter-orbital repulsion energy U is not active (as compared to the case where two electrons are on the same site).…”

Biexciton Condensation in Electron-Hole-Doped Hubbard Bilayers: A Sign-Problem-Free Quantum Monte Carlo Study

“…While exciton condensation and charge-2e SC have been scrutinized both theoretically and experimentally, Bi-EC and charge-4e SC remain relatively under-explored, mainly due to the four-particle nature of their order parameters, and are energetically less favorable than their two-particle counterparts in most cases. Recent developments include identification of Bi-EC in a two-orbital t − J chain [51] and exploration of charge-4e SC using Majorana quantum Monte Carlo [52].…”