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arXiv:cond-mat/0511061

Real-time formalism for studying the nonlinear response of "smart" materials to an electric field

James K. Freericks, V. Turkowski, and V. Zlatic

Published Nov 2, 2005• 1 citation
arXivHTMLPDF

Abstract

The nonlinear response of a material to a large electric field (steady or pulsed) often determines the ultimate performance of the material for electronics applications. The formalism for understanding nonlinear effects in conventional semiconductors is well understood. The formalism is less well developed for so-called ``smart'' materials that are tuned to lie close to the metal-insulator transition. Here we show that the nonlinear response of a strongly correlated electronic material can be calculated with a massively parallel algorithm by discretizing a continuous matrix operator on the Kadanoff-Baym contour in real time. We benchmark the technique by examining the solutions when the field vanishes and comparing the results to exact results from an equilibrium formalism. We briefly discuss the numerical issues associated with the case of a large electric field and present results that show how the Bloch oscillations become damped as the scattering due to electron correlations increases.

DOI
10.48550/arxiv.cond-mat/0511061

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The nonlinear response of a material to a large electric field (steady or pulsed) often determines the ultimate performance of the material for electronics applications. The formalism for understanding nonlinear effects in conventional semiconductors is well understood. The formalism is less well developed for so-called ``smart'' materials that are tuned to lie close to the metal-insulator transition. Here we show that the nonlinear response of a strongly correlated electronic material can be calculated with a massively parallel algorithm by discretizing a continuous matrix operator on the Kadanoff-Baym contour in real time. We benchmark the technique by examining the solutions when the field vanishes and comparing the results to exact results from an equilibrium formalism. We briefly discuss the numerical issues associated with the case of a large electric field and present results that show how the Bloch oscillations become damped as the scattering due to electron correlations increases.
The nonlinear response of a material to a large electric field (steady or pulsed) often determines the ultimate performance of the material for electronics applications. The formalism for understanding nonlinear effects in conventional semiconductors is well understood. The formalism is less well developed for so-called “smart” materials that are tuned to lie close to the metalinsulator transition. Here we show that the nonlinear response of a strongly correlated electronic material can be calculated with a massively parallel algorithm by discretizing a continuous matrix operator on the Kadanoff-Baym contour in real time. We benchmark the technique by examining the solutions when the field vanishes and comparing the results to exact results from an equilibrium formalism. We briefly discuss the numerical issues associated with the case of a large electric field and present results that show how the Bloch oscillations become damped as the scattering due to electron correlations increases.
Strongly correlated electron materials are materials where the electron-electron interaction is so strong it is of primary importance in determining how the materials behave. These materials may be tuned to pass through a metal-insulator transition as a function of chemical doping, pressure, or temperature. This tunability of their properties makes them good candidates for so-called "smart materials" that may change their properties to respond to the particular needs of a device. We employ a massively parallel algorithm to solve exactly for the response of these strongly correlated materials to the presence of a large electric field, including all nonlinear and non-equilibrium effects. The work was performed as a CAP Phase IIproject on the ERDC XT3 computer in the Winter of 2006. The code displayed good scalability up to approximately 3000 processors although most production runs employed about 1500 processors, because of machine load
The nonlinear response of noninteracting (Bloch) electrons is examined within a nonequilibrium formalism on the infinite-dimensional hypercubic lattice. We examine the effects of a spatially uniform, but time-varying electric field (ignoring magnetic-field effects). The electronic Green's functions, Wigner density of states, and time-varying current are all determined and analyzed. We study both constant and pulsed electric fields, focusing on the transient response region and on local properties. These noninteracting Green's functions are an important input into nonequilibrium dynamical mean field theory for the nonlinear response of strongly correlated electrons.
The electric-field response of a one-dimensional ring of interacting fermions, where the interactions are described by the extended Hubbard model, is investigated. By using an accurate real-time propagation scheme based on the Chebyshev expansion of the evolution operator, we uncover various nonlinear regimes for a range of interaction parameters that allows modeling of metallic and insulating (either charge density wave or spin density wave insulators) rings. The metallic regime appears at the phase boundary between the two insulating phases and provides the opportunity to describe either weakly or strongly correlated metals. We find that the fidelity susceptibility of the ground state as a function of magnetic flux piercing the ring provides a very good measure of the short-time response. Even completely different interacting regimes behave in a similar manner at short time scales as long as the ground-state fidelity susceptibility is the same. Depending on the strength of the electric field we find various types of responses: persistent currents in the insulating phase, a dissipative regime, or damped Bloch-like oscillations with varying frequencies or even irregular in nature. Furthermore, we also consider the dimerization of the ring and describe the response of a correlated band insulator. In this case the distribution of the energy levels is more clustered and the Bloch-like oscillations become even more irregular.
Dynamical mean-field theory is generalized to solve the nonequilibrium Keldysh boundary problem: a system is started in equilibrium at a temperature T=0.1T=0.1T=0.1, a uniform electric field is turned on at t=0t=0t=0, and the system is monitored as it approaches the steady state. The focus here is on the Bloch oscillations of the current and how they decay after their initial appearance near t=0t=0t=0. The system is evolved out to the longest time allowed by our computational resources---in most cases we are unable to reach the steady state. The strongly correlated material is modeled by the spinless Falicov-Kimball model at half-filling on a hypercubic lattice in $d=\ensuremath{\infty}$ dimensions, which has a Mott-like metal-insulator transition at U=2U=\sqrt{2}U=2​. The computational algorithm employed is highly efficient, parallelizes well, and scales to thousands of processors. For strong fields, we find beats develop with a period of $2\ensuremath{\pi}∕U$, while for strong interactions, the Bloch oscillations are sharply damped and become quite irregular in time.
Abstract Motivated by the recent developments in terahertz spectroscopy using pump-probe setups to study correlated electronic materials, we review the field theoretical formalism to compute finite frequency nonlinear electro-optical responses in centrosymmetric systems starting from basic time dependent perturbation theory. We express the nonlinear current kernel as a sum of several causal response functions. These causal functions cannot be evaluated using perturbative field theory methods, since they are not contour ordered. Consequently, we associate each response function with a corresponding imaginary time ordered current correlation function, since the latter can be factorized using Wick’s theorem. The mapping between the response functions and the correlation functions, suitably analytically continued to real frequencies, is proven exactly. We derive constraints satisfied by the nonlinear current kernel and we prove a generalized f-sum rule for the nonlinear conductivity, all of which are consequences of particle number conservation. The constraints guarantee that the nonlinear static responses are free from spurious divergences. We apply the theory to compute the gauge invariant nonlinear conductivity of a system of noninteracting electrons in the presence of weak disorder. As special cases of this generalized nonlinear response, we discuss its third harmonic and its instantaneous terahertz Kerr signals. The formalism can be used to compute the nonlinear conductivity in symmetry broken phases of electronic systems such as superconductors, density waves and nematic states.