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The spectral function of Mott-insulating Hubbard ladders: From fractional excitations to coherent quasi-particles
Feiguin, Adrian - Northeastern University
Presentation on Thursday, Feb. 21, 2019, 7 a.m.
Location: MIT Room 26-214
When electrons are confined in one spatial dimension, they may loose their identity as individual particles, and ``break'' into separate excitations carrying spin (a spinon), and charge (a holon) and, as a consequence, Landau's quasi-particle picture breaks down. Whether spin-charge separation survives in dimensions greater than d=1 has been a topic of great debate, particularly in the context of high-temperature superconductivity. A peculiar case that can shed some light on this problem consists of coupling two chains into a ladder geometry. When the Mott insulator is doped, it is expected that polaronic quasiparticles (a bound state of a spinon and a holon) will form, restoring Landau's paradigm. We study the spectral function of two-leg Mott insulating Hubbard ladders using the time-dependent density matrix renormalization group method (tDMRG), that allows us to resolve fine details with unprecedented resolution. The spectrum displays features of both spin-charge separation and coherent bound states. As the inter-leg hopping is increased, spinon and holon branches merge into a single coherent quasi-particle band and the spectrum undergoes a crossover from a regime with two incommensurate minima (a Mott insulator), to one with a single minimum (a band insulator). Interestingly, while the bonding sector of the spectrum realizes quasi-particles, the anti-bonding one displays a broad scattering continuum. We identify the processes leading to quasiparticle formation by studying the time evolution of charge and spin degrees of freedom in real space after a hole is created. At short times, incoherent holons and spinons are emitted but charge and spin quickly form polarons that propagate coherently.