Recent Publications

Chen, M. ; Baer, R. ; Neuhauser, D. ; Rabani, E. Overlapped Embedded Fragment Stochastic Density Functional Theory for Covalently Bonded Materials. Journal of Chemical Physics In Press. Publisher's VersionAbstract

The stochastic density functional theory (DFT) [Phys. Rev. Lett. 111, 106402 (2013)] is a valuable linear scaling approach to Kohn-Sham DFT that does not rely on the sparsity of the density matrix. Linear (and often sub-linear) scaling is achieved by introducing a controlled statistical error in the density, energy and forces. The statistical error (noise) is proportional to the inverse square root of the number of stochastic orbitals and thus decreases slowly, however, by dividing the system to fragments that are embedded stochastically, the statistical error can be reduced significantly. This has been shown to provide remarkable results for non-covalently bonded systems, however, the application to covalently bonded systems had limited success, particularly for delocalized electrons. Here, we show that the statistical error in the density correlates with both the density and the density matrix of the system and propose a new fragmentation scheme that elegantly interpolates between overlapped fragments. We assess the performance of the approach for bulk silicon of varying supercell sizes (up to \$N\_\e\=16384\$ electrons) and show that overlapped fragments reduce significantly the statistical noise even for systems with a delocalized density matrix.

Baer, R. ; Kronik, L. Time-dependent generalized Kohn–Sham theory. The European Physical Journal B 2018, 91, 170. Publisher's VersionAbstract

Generalized Kohn–Sham (GKS) theory extends the realm of density functional theory (DFT) by providing a rigorous basis for non-multiplicative potentials, the use of which is outside original Kohn–Sham theory. GKS theory is of increasing importance as it underlies commonly used approximations, notably (conventional or range-separated) hybrid functionals and meta-generalized-gradient-approximation (meta-GGA) functionals. While this approach is often extended in practice to time-dependent DFT (TDDFT), the theoretical foundation for this extension has been lacking, because the Runge–Gross theorem and the van Leeuwen theorem that serve as the basis of TDDFT have not been generalized to non-multiplicative potentials. Here, we provide the necessary generalization. Specifically, we show that with one simple but non-trivial additional caveat – upholding the continuity equation in the GKS electron gas – the Runge–Gross and van Leeuwen theorems apply to time-dependent GKS theory. We also discuss how this is manifested in common GKS-based approximations.

Ghosh, T. ; Fabian, M. D. ; Dehnel, J. ; Lifshitz, E. ; Baer, R. ; Ruhman, S. Spin blockades to relaxation of hot multi-excitons in nanocrystals. arXiv:1809.08581 [cond-mat] 2018. Publisher's VersionAbstract

The rates of elementary photophysical processes in nanocrystals, such as carrier cooling, multiexciton generation, Auger recombination etc., are determined by monitoring the transient occupation of the lowest exciton band. The underlying assumption is that hot carriers relax rapidly to their lowest quantum level. Using femtosecond transient absorption spectroscopy in CdSe/CdS nanodots we challenge this assumption. Results show, that in nanodots containing a preexisting cold exciton "spectator", \textbackslashemph\only half of the photoexcited electrons\relax directly to the band-edge and the complementary half is blocked in an excited state level due to Pauli exclusion. Full relaxation occurs only after \textbackslashtextasciitilde 15 ps, as the blocked electrons flip spin. This novel spin-blockade effect may offer the key for the long-sought-for bottleneck mechanism for multiexciton energy dissipation.

Hernandez, S. ; Xia, Y. ; Vlček, V. ; Boutelle, R. ; Baer, R. ; Rabani, E. ; Neuhauser, D. First principles absorption spectra of Au nanoparticles: from quantum to classical. Molecular Physics 2018. Publisher's VersionAbstract

Absorption cross-section spectra for gold nanoparticles were calculated using fully quantum Stochastic Density Functional Theory and a classical Finite-Difference Time Domain Maxwell solver. Spectral shifts were monitored as a function of size (1.3–3.1nm) and shape (octahedron, cubeoctahedron and truncated cube). Even though the classical approach is forced to fit the quantum time-dependent density functional theory at 3.1nm, at smaller sizes there is a significant deviation as the classical theory is unable to account for peak splitting and spectral blue shift seven after quantum spectral corrections. We attribute the failure of classical methods at predicting these features to quantum effects and low density of states in small nanoparticles. Classically, plasmon resonances are modelled as collective conduction electron excitations, but at small nanoparticle size these excitations transition to few or even individual conductive electron excitations, as indicated by our results.