Quasiparticle excitations in the hole-doped Hubbard model with orthogonal-dimer structure

We study the hole-doping effects on the half-filled Hubbard ladder model with the frustrated orthogonal-dimer structure. By using the density matrix renormalization group method, we discuss how strong geometrical frustration affects the nature of spin excitations upon hole doping. We find that for large U with strong frustration, a spin-gap metallic phase appears, for which excitations can be described by two independent quasiparticles