- Este evento ha pasado.
diciembre 4, 2018 @ 2:30 pm - 4:00 pm
Fecha y hora: Martes 4 de diciembre, 14:30
Lugar: Seminario 3ª planta
Ponente: Adrian E. Feiguin, Northeastern University, Boston, USA
Título:The spectral function of Mott-insulating Hubbard ladders: From fractional excitations to coherent quasi-particles
Abstract:When electrons are confined in one spatial dimension, they may loose their identity as individual particles, and “split” 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 half-filled Hubbard 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). The high-resolution spectrum displays features of spin-charge separation and a scattering continuum of excitations with coherent bands of bound states “leaking” from it. As the inter-leg hopping is increased, the continuum in the bonding channel moves to higher energies and spinon and holon branches merge into a single coherent quasi-particle band. Simultaneously, the spectrum undergoes a crossover from a regime with two minima at incommensurate values of momentum (a Mott insulator), to one with a single minimum at k_x=pi (a band insulator). We identify the presence of a continuum of scattering states consisting of a triplon and a polaron. The sharp edge of the continuum in the anti-bonding sector indicates the formation of bound states with higher spin $S=3/2$, ferromagnetic “spin bags” in which the hole can move freely. We analyze the processes leading to quasiparticle formation by studying the time evolution of charge and spin degrees of freedom in real space after the hole is created. At short times, incoherent holons and spinons are emitted but after a characteristic time, charge and spin form polarons that propagate coherently. In addition, we infer the full spectrum of the 2D Hubbard model in the thermodynamic limit by coupling ladders in the perpendicular direction using cluster perturbation theory. A remarkable advantage of this approach is that unlike small clusters, the one-dimensional systems are already “infinite”. Excellent agreement with quantum Monte Carlo and other cluster methods is obtained.