12 research outputs found
Dissociation of a Hubbard--Holstein bipolaron driven away from equilibrium by a constant electric field
Using a variational numerical method we compute the time-evolution of the
Holstein-Hubbard bipolaron from its ground state when at t=0 the constant
electric field is switched on. The system is evolved taking into account full
quantum effects until it reaches a quasi-stationary state. In the zero-field
limit the current shows Bloch oscillations characteristic for the adiabatic
regime where the electric field causes the bipolaron to evolve along the
quasiparticle band. Bipolaron remains bound and the net current remains zero in
this regime. At larger electric fields the system enters the dissipative regime
with a finite steady-state current. Concomitantly, the bipolaron dissociates
into two separate polarons. By examining different parameter regimes we show
that the appearance of a finite steady-state current is inevitably followed by
the dissociation of the bipolaron.Comment: 9 pages, 7 figure
Optical manipulation of bipolarons in a system with nonlinear electron-phonon coupling
We investigate full quantum mechanical evolution of two electrons nonlinearly
coupled to quantum phonons and simulate the dynamical response of the system
subject to a short spatially uniform optical pulse that couples to
dipole-active vibrational modes. Nonlinear electron-phonon coupling can either
soften or stiffen the phonon frequency in the presence of electron density. In
the former case, an external optical pulse tuned just below the phonon
frequency generates attraction between electrons and leads to a long-lived
bound state even after the optical pulse is switched off. It originates from a
dynamical modification of the self-trapping potential that induces a metastable
state. By increasing the pulse frequency, the attractive electron-electron
interaction changes to repulsive. Two sequential optical pulses with different
frequencies can switch between attractive and repulsive interaction.
Pulse-induced binding or repulsion of electrons is shown to be efficient also
for weakly dispersive optical phonons and in the presence of weak Coulomb
repulsion.Comment: 10 page
Adiabatic Preparation of a Correlated Symmetry‐Broken Initial State with the Generalized Kadanoff–Baym Ansatz
A fast time propagation method for nonequilibrium Green's functions based on the generalized Kadanoff--Baym Ansatz (GKBA) is applied to a lattice system with a symmetry-broken equilibrium phase, namely an excitonic insulator. The adiabatic preparation of a correlated symmetry-broken initial state from a Hartree--Fock wave function within GKBA is assessed by comparing with a solution of the imaginary-time Dyson equation. We find that it is possible to reach a symmetry-broken correlated initial state with nonzero excitonic order parameter by the adiabatic switching procedure. We discuss under which circumstances this is possible in practice within reasonably short switching times
Ultrafast Doublon Dynamics in Photoexcited 1T-TaS2
Strongly correlated materials exhibit intriguing properties caused by intertwined microscopic interactions that are hard to disentangle in equilibrium. Employing nonequilibrium time-resolved photoemission spectroscopy on the quasi-two-dimensional transition-metal dichalcogenide 1T-TaS2, we identify a spectroscopic signature of doubly occupied sites (doublons) that reflects fundamental Mott physics. Doublon-hole recombination is estimated to occur on timescales of electronic hopping ℏ/J≈14 fs. Despite strong electron-phonon coupling, the dynamics can be explained by purely electronic effects captured by the single-band Hubbard model under the assumption of weak hole doping, in agreement with our static sample characterization. This sensitive interplay of static doping and vicinity to the metal-insulator transition suggests a way to modify doublon relaxation on the few-femtosecond timescale
Snapshots of the retarded interaction of charge carriers with ultrafast fluctuations in cuprates
One of the pivotal questions in the physics of high-temperature superconductors is whether the low-energy dynamics of the charge carriers is mediated by bosons with a characteristic timescale. This issue has remained elusive as electronic correlations are expected to greatly accelerate the electron-boson scattering processes, confining them to the very femtosecond timescale that is hard to access even with state-of-the-art ultrafast techniques. Here we simultaneously push the time resolution and frequency range of transient reflectivity measurements up to an unprecedented level, enabling us to directly observe the ∼16 fs build-up of the effective electron-boson interaction in hole-doped copper oxides. This extremely fast timescale is in agreement with numerical calculations based on the t-J model and the repulsive Hubbard model, in which the relaxation of the photo-excited charges is achieved via inelastic scattering with short-range antiferromagnetic excitations