Recent Publications

Ruan, Z. ; Baer, R. Unraveling open-system quantum dynamics of non-interacting Fermions. Submitted 2018.Abstract

The Lindblad equation is commonly used for studying quantum dynamics in open systems that cannot be completely isolated from an environment, relevant to a broad variety of research fields, such as atomic physics, materials science, quantum biology and quantum information and computing. For electrons in condensed matter systems, the Lindblad dynamics is intractable even if their mutual Coulomb repulsion could somehow be switched off. This is because they would still be able to affect each other by interacting with the bath. Here, we develop an approximate approach, based on the Hubbard-Stratonovich transformation, which allows to evolve non-interacting Fermions in open quantum systems. We discuss several applications for systems of trapped 1D Fermions showing promising results.

Cytter, Y. ; Rabani, E. ; Neuhauser, D. ; Baer, R. Stochastic Density Functional Theory at Finite Temperatures. arXiv:1801.02163 [cond-mat.mtrl-sci] 2018.Abstract

Simulations in the warm dense matter regime using finite temperature Kohn-Sham density functional theory (FT-KS-DFT), while frequently used, are extremely expensive computationally due to the partial occupation of a very large number of high-energy KS eigenstates which are obtained from subspace diagonalization. We have developed a stochastic method for applying FT-KS-DFT, that overcomes the bottleneck of calculating the occupied KS orbitals by directly obtaining the density from KS Hamiltonian. The proposed algorithm, scales as $O\left(NT^{-1}\right)$ and is compared with the high-temperature limit scaling $O\left(N^{3}T^{3}\right)$ of the deterministic approach, where $N$ is the system size (number of electrons, volume etc.) and $T$ is the temperature. The method has been implemented in a plane-waves code within the local density approximation (LDA); we demonstrate its efficiency, statistical errors and bias in the estimation of the free energy per electron for a diamond structure silicon. The bias is small compared to the fluctuations, and is independent of system size. In addition to calculating the free energy itself, one can also use the method to calculate its derivatives and obtain the equations of state.

Luzon, I. ; Jagtap, K. ; Livshits, E. ; Lioubashevski, O. ; Baer, R. ; Strasser, D. Single-photon Coulomb explosion of methanol using broad bandwidth ultrafast EUV pulses. Phys. Chem. Chem. Phys. 2017, 19, 13488–13495.Abstract

Single-photon Coulomb explosion of methanol is instigated using the broad bandwidth pulse achieved through high-order harmonics generation. Using 3D coincidence fragment imaging of one molecule at a time, the kinetic energy release (KER) and angular distributions of the products are measured in different Coulomb explosion (CE) channels. Two-body CE channels breaking either the C–O or the C–H bonds are described as well as a proton migration channel forming H2O+, which is shown to exhibit higher KER. The results are compared to intense-field Coulomb explosion measurements in the literature. The interpretation of broad bandwidth single-photon CE data is discussed and supported by ab initio calculations of the predominant C–O bond breaking CE channel. We discuss the importance of these findings for achieving time resolved imaging of ultrafast dynamics.

Takeshita, T. Y. ; de Jong, W. A. ; Neuhauser, D. ; Baer, R. ; Rabani, E. Stochastic Formulation of the Resolution of Identity: Application to Second Order Møller–Plesset Perturbation Theory. J. Chem. Theory Comput. 2017, 13, 4605. Publisher's VersionAbstract

A stochastic orbital approach to the resolution of identity (RI) approximation for 4 index electron repulsion integrals (ERIs) is presented. The stochastic RI-ERIs are then applied to second order Møller-Plesset perturbation theory (MP2) utilizing a multiple stochastic orbital approach. The introduction of multiple stochastic orbitals results in an O(N_AO^3 ) scaling for both the stochastic RI-ERIs and stochastic RI-MP2, NAO being the number of basis functions. For a range of water clusters we demonstrate that this method exhibits a small prefactor and observed scalings of O(Ne^2.4) for total energies and O(Ne^3.1) for forces (Ne being the number of correlated electrons), outperforming MP2 for clusters with as few as 21 water molecules.

Vlček, V. ; Baer, R. ; Rabani, E. ; Neuhauser, D. Self-consistent band-gap renormalization GW. arXiv preprint arXiv:1701.02023 2017.Abstract

Partially-self-consistent gap-renormalization GW (grGW) is introduced to calculate quasiparticle (QP) energies within the many-body perturbation theory of Hedin. Self-consistency of the Green's function is obtained by renormalization of the band gap, removing the most significant approximation of the single-shot G0W0 approach. The formalism is performed as a post-processing step and thus, can be implemented within any GW algorithm which calculates the full frequency-dependent self-energies. Here, the grGW approach is combined with the stochastic GW (sGW) formalism developed in [Phys. Rev. Lett. 113, 076402 (2014)]. The computational cost and scaling are similar to that of the single-shot sGW. We illustrate the approach for two confined semiconducting systems with open boundary conditions: Silicon nanocrystal (NC) quantum dots (QDs) and 2D planar and bent phosphorene nanoribbons (PNRs).