Many body theory of photoemission in the quadratic response formalism

S. Di Sabatino

Carl-Olof Almbladh, Claudio Verdozzi

Pina Romaniello

Ayan Pal, Erik G. C. P. van Loon, F. Aryasetiawan

Nonequilibrium quantum physics greatly simplifies in the case of time-periodic Hamiltonians, since Floquet theory provides an analog to Bloch's theorem in the time domain. Still, the formal properties of Floquet many-body theory remain underexplored. Here, we study what happens to the uniform electron gas when a space-time periodic potential of arbitrary strength is applied, which leads to band splitting. The case of a potential traveling at constant speed is special, since the problem can be made time independent by transforming to a comoving frame. We perform linear-response theory, in the sense that we apply an additional perturbatively small but nonperiodic probing field, for which we illustrate the analogy of Fermi's golden rule and the photoemission spectrum of a many-electron system. As in the equilibrium case, the latter is related to the spectral function which is positive definite. We also analyze the parameter dependence of the controllable photoemission spectra by virtue of Floquet-Bloch engineering.

Gabor J. Kalman, J. M. Rommel

While the theory of linear response functions in many body theory has developed into a substantial lore over the past decades, rather little is known about the properties and behavior of higher order (quadratic, etc.) response functions of many body systems. In this poster we first review essential elements of quadratic response theory such as the quadratic fluctuation dissipation theorems and sum rules for the quadratic density response function. Next we show how the correlational local field G(k,ω) in the linear response is fundamentally linked to the quadratic response function which opens a way to a non-perturbational analysis of the former. We proceed to present the results of the explicit calculation of the quadratic density response of 2D and 3D electron gas. Both, real and imaginary part of the latter reveal an extremely rich behavior. Of particular interest is the comparison between the two- and the three-dimensional case.

Denis J. Evans

We show that the formal results of many-body, isothermal response theory can be generated from thermodynamic extremum principles. It is shown that this method yields the correct time-dependent response to a perturbing external field.