Doublon-Holon Pairing Phase in Photodoped Mott Insulators

Nonequilibrium systems induced by external fields have attracted attention as platforms that give rise to intriguing phenomena. When a Mott insulator is optically excited, doublons (doubly occupied sites) and holons (empty sites) are generated, and the excited state then settles into a quasi-steady photodoped state. The ladder structure is a fascinating system because the interchain spin-exchange interaction, which favors the formation of interchain spin singlets, can serve as a pairing glue for doped carriers. In this work, we investigate possible pairing states in photodoped ladder systems. Using the density matrix renormalization group method, we find a doublon-holon (DH) pairing phase, characterized by quasi-long-ranged correlations of doublon-holon pairs, between the spin singlet phase at zero doublon density and the CDW/η-pairing phase in heavily photodoped regions. This DH pairing phase is stabilized over a broad range where the interchain hopping is comparable to or exceeds the intrachain hopping. The DH pairing correlation exhibits a d-wave-like character with a staggered sign structure. This feature can be understood by connecting the system to a perturbatively obtained effective model. Our findings reveal that fluctuations among multiple degrees of freedom in photodoped systems can generate an exotic pairing state.