Recent Publications

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.

Vlček, V. ; Li, W. ; Baer, R. ; Rabani, E. ; Neuhauser, D. Swift $GW$ beyond $10,000 $ electrons using fractured stochastic orbitals. arXiv preprint arXiv:1805.10554 2018.Abstract

We introduce the concept of fractured stochastic orbitals (FSOs), short vectors that sample a small number of space points and enable an efficient stochastic sampling of any general function. As a first demonstration, FSOs are applied in conjunction with simple direct-projection to accelerate our recent stochastic GW technique; the new developments enable accurate prediction of G 0 W 0 quasiparticle energies and gaps for systems with up to Ne > 10,000 electrons, with small statistical errors of ±0.05eV and using less than 2000 core CPU hours. Overall, stochastic GW scales linearly (and often sub-linearly) with N.

Ruan, Z. ; Baer, R. Unravelling open-system quantum dynamics of non-interacting Fermions. Mol. Phys. 2018, 1–7. Publisher's VersionAbstract

ABSTRACTThe 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 HubbardStratonovich 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. Phys. Rev. B 2018, 97, 115207. Publisher's VersionAbstract

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.

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