Nonequilibrium GW+EDMFT : Antiscreening and Inverted Populations from Nonlocal Correlations

Abstract

We study the dynamics of screening in photo-doped Mott insulators with long-ranged interactions using a nonequilibrium implementation of the GW plus extended dynamical mean field theory (GW +EDMFT) formalism. Our study demonstrates that the complex interplay of the injected carriers with bosonic degrees of freedom (charge fluctuations) can result in long-lived transient states with properties that are distinctly different from those of thermal equilibrium states. Systems with strong nonlocal interactions are found to exhibit a self-sustained population inversion of the doublons and holes. This population inversion leads to low-energy antiscreening which can be detected in time-resolved electron-energy loss spectra. PACS numbers: 71.10.Fd,72.10.Di,05.70.Ln The development of time-resolved spectroscopic techniques provided important insights into the properties of complex materials [1][2][3][4][5], where charge, spin, orbital and lattice degrees of freedom are intertwined. A particularly exciting prospect is the nonequilibrium manipulation of material properties on electronic time scales, and the exploration of transient states that cannot be realized under equilibrium conditions. Prominent examples of this development are the laserinduced switching to a hidden state [6] in 1T-TaS 2 , and an apparent increase of the superconducting T c in phonon-driven cuprates and fulleride superconductors [7, 8].Essential for the understanding of such experiments and phenomena is the ability to simulate relevant model systems using techniques that capture correlation effects in highly nonthermal states. Of particular importance is a proper description of the time-dependent screening processes, which determine the interaction parameters in such model Hamiltonians. The photo-induced change of screening was considered, e.g., as the cause of the collapse of the band gap in VO 2 [9], or for an enhancement of excitonic order in Ta 2 NiSe 5 [10]. Moreover, screening originates from charge fluctuations, which, similar to other bosonic modes like phonons [11][12][13][14] or spin fluctuations [15,16], profoundly affect the relaxation pathway of the electronic distribution. As we will show in this paper, the fermionic dynamics and the bosonic screening modes are strongly coupled, so that their mutual interplay can lead to long-lived transient states which are entirely different from those characterizing equilibrium phases. These non-thermal states, with partially inverted populations, thus provide an intriguing pathway to novel light-induced properties.A promising formalism to address these questions in strongly correlated solids is the combination of the GW method and extended dynamical mean field theory (GW +EDMFT) [17,18]. Hedin's GW method [19,20] is a weak coupling approach in which the self-energy is approximated by the product of the Green's function G and the screened interaction W . It captures nonlocal physics resulting from charge fluctuations, like screening, plasmonic collective modes and charge density waves. It however f...

Photoenhanced excitonic correlations in a Mott insulator with nonlocal interactions

“…These excitonic features can originate either from the non-local interactions [16][17][18] or the modification of the spin background [10]. Methods for infinite lattices based on single-site dynamical mean field theory (DMFT) [19], such as extended DMFT [20][21][22][23] or the combination of GW and extended DMFT [24][25][26], can capture the dynamical screening of the local Coulomb interaction resulting from nonlocal interactions, but they cannot describe exciton formation. Here, we combine the two approaches by implementing a cluster extension of nonequilibrium DMFT [8,27], with local and nonlocal interactions on the cluster.…”

Quantum Boltzmann equation for strongly correlated systems: Comparison to dynamical mean field theory

“…In particular, doped Mott insulators in equilibrium display bad metallic behavior above some rather low coherence temperature, with a scattering length of the order of the lattice spacing, 5 and also photodoped Mott insulators do not quickly relax to a Fermi liquid. 6,7 Furthermore, in correlated systems, the electronic spectrum itself can depend strongly on the nonequilibrium distribution, which may result in a photoinduced renormalization [8][9][10] or filling of the Mott gap, similarly to what happens by increasing temperature. 11 For these reasons, the dynamics of correlated systems is often studied using formally exact but computation-ally expensive non-equilibrium Green's function (NEGF) techniques.…”

Influence of Fock exchange in combined many-body perturbation and dynamical mean field theory

“…The denominator in the susceptibility [Eq. (16)] imposes that the charge-ordering transition should occur for negative values of v q , since the polarization q is always negative (for the parameters studied here). For the square lattice, this implies V c > U 4 , which is the classical energy estimate [Eq.…”

“…[15] (see also the appendix of Ref. [16]), the GW +EDMFT method is formally obtained by constructing an energy functional of G and W , the Almbladh [17] functional , and by approximating as a sum of two terms, one containing all local diagrams (corresponding to EDMFT) and the other containing the simplest nonlocal correction (corresponding to the GW approximation [18]). This functional construction rules out double counting of local terms in the self-energy and polarization [15,19].…”