Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session K24: ManyBody Perturbation Theory for Electronic Excitations: Theoretical SpectroscopyFocus

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Sponsoring Units: DMP Chair: Claudia Draxl, Humboldt University Room: 323 
Wednesday, March 16, 2016 8:00AM  8:12AM 
K24.00001: DFT$+U(\omega)$: Frequencydependent Hubbard $U$ correction David D. O'Regan, Nicola Marzari In contemporary firstprinciples atomistic simulation based on DFT, the augmentation of approximate exchangecorrelation functionals with spatially or energetically localized corrections, such as DFT$+U$, is a successful approach for improving its applicability to strongly interacting systems. Electronic screening is a dynamical process, and since the Hubbard $U$ parameter, in particular, is a measure of the screened Coulomb interaction, its frequencydependent generalisation for the dynamical regime is possible. We introduce a conceptually pragmatic and computationally straightforward method, named DFT$+U(\omega)$, for calculating and incorporating strong dynamical screening effects in spectroscopic calculations based on KohnSham DFT. Our method is designed to be a minimal dynamical extension of DFT$+U$, one in which computing approximate dynamical Hubbard $U$ functions only requires functionality that is widely available. We demonstrate our effective plasmon fitting and selfenergy approximation scheme for DFT$+U(\omega)$, which enables the resulting lowenergy dynamical model to be solved at the $G_0 W_0$ level, and beyond, efficiently and effectively. [Preview Abstract] 
Wednesday, March 16, 2016 8:12AM  8:24AM 
K24.00002: RIXS of Ammonium Nitrate using OCEAN John Vinson, Terrence Jach, Matthias Mueller, Rainer Unterumsberger, Burkhard Beckhoff The {\sc ocean} code allows for calculations of nearedge xray spectroscopies using a {\it GW}/BetheSalpeter equation (BSE) approach. Here we present an extension of the code for calculating resonant inelastic xray scattering (RIXS). Recent work has shown that peakspecific broadening of nitrogen K$\alpha$ emission in nitrates is due to a valenceband lifetime that is an order of magnitude shorter than that of the nitrogen 1s hole, an inversion of the usual assumption that valence holes have longer lifetimes than corelevel holes. Using the BSE, including {\it GW} corrections to the DFT energies, as implemented in {\sc ocean} we are able to compare calculations of RIXS with measured spectra of the same. By utilizing an approach free from fitting parameters we are able to identify the origins of various broadening effects observed in experiment. [Preview Abstract] 
Wednesday, March 16, 2016 8:24AM  8:36AM 
K24.00003: Signatures of correlation in transition metal oxides Matteo Gatti, Matteo Guzzo, Lucia Reining Photoemission satellites are a genuine fingerprint of electronic correlation that cannot be interpreted within the quasiparticle bandstructure picture. Here we show that they can be understood in terms of the coupling between different elementary excitations, as in the case of plasmon sidebands. Using examples from different correlated materials, we discuss how this coupling can be explained by advanced calculations based on firstprinciples manybody perturbation theory that combine GWlike approximations for the selfenergy with the cumulant expansion of the Green's function [13]. This approach is not limited to lowenergy satellites, but allows for a consistent explanation of signatures of correlation over a wide range of binding energies. [1] M. Guzzo {\it et al.}, Phys. Rev. Lett. {\bf 107}, 166401 (2011). [2] M. Gatti and M. Guzzo, Phys Rev B {\bf 87}, 155147 (2013). [3] M. Gatti {\it et al,}, Phys. Rev. Lett. {\bf 114}, 116402 (2015). [Preview Abstract] 
Wednesday, March 16, 2016 8:36AM  8:48AM 
K24.00004: Highresolution Valence and Core Excitation Spectra via FirstPrinciples Calculations and Experiment Eric Shirley, F. Fossard, K. Gilmore, G. Hug, J.J. Kas, J.J. Rehr, F. Vila We calculate the optical and C Kedge near edge spectra of crystalline and molecular C$_{\mathrm{60}}$ measured with highresolution electron energyloss spectroscopy. The calculations are carried out using at least three different methods: BetheSalpeter calculations using the NIST BetheSalpeter Equation solver (NBSE) in the valence and OCEAN (Obtaining Core Excitation with Ab initio methods and NBSE) suite [Gilmore et al., Comp. Phys. Comm., (2015)]; excitedcorehole calculations using XCH [D. Prendergast and G. Galli, Phys. Rev. Lett. 96, 215502 (2006)]; and constrained occupancy using StoBe ( StockholmBerlin coreexcitation code) [StoBedeMon version 3.0, K. Hermann et al. (2009)]. They include selfenergy effects, lifetimedamping, and DebyeWaller effects. A comparison of spectral features to those observed illustrates the sensitivity of certain features to computation details (e.g., selfenergy corrections and corehole screening). This may point to limitations of various approximations, e.g. in conventional BSE paradigm and/or the incomplete treatment of vibrational effects. [Preview Abstract] 
Wednesday, March 16, 2016 8:48AM  9:00AM 
K24.00005: Obtaining Xray absorption nearedge structure for transition metal oxides via the BetheSalpeter equation YUFENG LIANG, John Vinson, Sri Pemmaraju, Eric Shirley, David Prendergast Transition metal oxides are an important class of materials featured with strongly correlated effects. Most interesting and yet tobeunveiled physics is associated with the metal 3d orbitals, which can be probed by Xray absorption nearedge spectroscopy. A thorough interpretation of the xray spectroscopy is often accompanied with firstprinciples simulations of structures, electronic properties and the corresponding xray spectra. However, the simulation for TMOs is particularly challenging with the localized 3d orbitals. Most previous studies relied on the groundstate calculations without the corehole as a compromise. Other treated the excited atom as a charged impurity but the calculated spectra turn out to be even more deviated from experiments [1]. Here, we present the first study for the O Kedge for several typical TMOs via solving the BetheSalpeter equation (BSE). We have found that electroncorehole interactions can alter the absorption spectra significantly. Our study helps to disentangle corehole effects from the intrinsic electron correlations and hence facilitates the development of more advanced manyelectron theories. [1] Isao Tanaka, Teruyasu Mizoguchi, and Tomoyuki Yamamoto J. Am. Ceram. Soc., 88 [8] 2013–2029 (2005) [Preview Abstract] 
Wednesday, March 16, 2016 9:00AM  9:12AM 
K24.00006: FirstPrinciples Study of FrequencyDependent Resonant Raman Scattering Yannick Gillet, Stefan Kontur, Matteo Giantomassi, Claudia Draxl, Xavier Gonze A resonance phenomenon appears in the Raman response when the exciting light has frequency close to electronic transitions. Unlike for molecules and for graphene, the theoretical prediction of such frequencydependent Raman response of crystalline systems has remained a challenge. Indeed, many Raman intensity firstprinciple calculations are nowadays done at vanishing light frequency, using static DensityFunctional Perturbation Theory, thus neglecting the frequency dependence and excitonic effects. Recently, we proposed a finitedifference method for the computation of the firstorder frequencydependent Raman intensity [1], with excitonic effects described by the BetheSalpeter equation. We found these to be crucial for the accurate description of the experimental enhancement for laser photon energies around the gap. In this work, we generalize this approach to the more complex secondorder Raman intensity, with phonon losses coming from the entire Brillouin zone. Interestingly, even without excitonic effects, one is able to capture the main relative changes in the frequencydependent Raman spectrum at fixed laser frequencies. The excitonic effects are discussed. [1] Y. Gillet, M. Giantomassi, X. Gonze, Phys. Rev. B 88, 094305 (2013). [Preview Abstract] 
Wednesday, March 16, 2016 9:12AM  9:48AM 
K24.00007: Cumulant approach for electronic excitations in xray and electron spectra Invited Speaker: J J Rehr A quantitative treatment of electronic excitations and other manybody effects in xray and electron spectra has long been challenging. Physically, electronic correlations and atomic vibrations lead to inelastic losses and damping effects that are ignored in ground state methods or approximations such as TDDFT. Quasiparticle (QP) approaches such as the GW approximation yield significant improvements, as demonstrated in realspace Green’s function [1] and GW/BetheSalpeter equation [2] calculations, but still ignore multielectron excitations. Recently such excitations have been treated with considerable success using cumulant expansion techniques and the quasiboson approximation [3,4]. In this beyond QP approach, excitations such as plasmons and electronhole excitations appear as satellites in the spectral function. The method naturally accounts for multiplesatellites and can be extended to include extrinsic losses and interference effects. Extensions for effects of vibrations and strong correlations including chargetransfer satellites may also be possible [5]. These advances are illustrated with a number of applications. \\[4pt] [1] John J. Rehr et al., Comptes Rendus Physique \textbf{10}, 548 (2009). \\[0pt] [2] K. Gilmore et al., Comput. Phys. Comm. \textbf{197}, 109 (2015). \\[0pt] [3] L. Hedin, J. Phys.: Condens. Matter \textbf{11}, R489 (1999). \\[0pt] [4] Jianqiang Sky Zhou et al., J. Chem. Phys. \textbf{143}, 194109 (2015). \\[0pt] [5] J. J. Kas, et al., Phys. Rev. B \textbf{91}, 121112(R) (2015). [Preview Abstract] 
Wednesday, March 16, 2016 9:48AM  10:00AM 
K24.00008: Realtime cumulant approach for inelastic losses in xray spectra J. J. Kas, J. J. Rehr, J. B. Curtis Intrinsic inelastic losses in core level xray absorption (XAS), emission (XES), and xray photoemission spectra (XPS), arise from excitations of the system due to the sudden creation or annihilation of a deep core hole. Additional extrinsic losses arise during the propagation of the photoelectron, and interference processes are also important. These excitations are reflected in the satellite peaks observed in XPS. Formally the distribution of these excitations are described in terms of the corehole spectral function, which can be calculated in terms of the corehole Green's function represented in exponential form. Here we discuss an approach for calculating the exponent, or cumulant in terms of local density fluctuations via realspace, realtime timedependent density functional theory.\footnote{J. J. Kas, F. D. Vila, J. J. Rehr, and S. A. Chambers, Phys. Rev. B {\bf 91}, 121112 (2015).} The role of extrinsic and interference terms is also discussed. Our method is illustrated in calculations of XAS and XPS for number of systems, including weakly correlated as well as $d$ and $f$electron materials. [Preview Abstract] 
Wednesday, March 16, 2016 10:00AM  10:12AM 
K24.00009: Approaching the quantum limit for plasmonics: linear atomic chains Emily Townsend, Garnett Bryant Linear atomic chains, such as atom chains on surfaces, linear arrays of dopants in semiconductors, or linear molecules, provide ideal testbeds for studying quantum plasmonics in nanosystems. We study the manybody excitations of finite (1025) linear atomic chains. We use both timedependent density functional theory (TDDFT) and exact diagonalization to analyze the excitations. TDDFT reveals optically driven excitations that can be singleparticlelike, plasmonlike or mixed states. Such states can have very different dependencies on the electronelectron interaction strength, which can be used to help identify the states. TDDFT can identify plasmonic resonances, but it does not reveal how to quantize them. Exact diagonalization is used to get the full quantum description. However, exact diagonalization results can be very different from TDDFT results. Highly correlated, multiexcitonic states, also strongly dependent on the electronelectron interaction strength, appear in the exact response but not in TDDFT excitation spectra. These excitonic manybody states make it hard to identify plasmonic excitations. Exact results are also strongly dependent on the strength of the exchange interaction. We present these results to show how quantum plasmons appear in linear atomic chains. [Preview Abstract] 
Wednesday, March 16, 2016 10:12AM  10:24AM 
K24.00010: On the Role of Fe$_{\mathrm{2}}$O$_{\mathrm{3\thinspace }}$Surface States for Water Splitting Maytal Caspary Toroker Understanding the chemical nature and role of electrode surface states is crucial for improved electrochemical cell operation. For iron (III) oxide ($\alpha $Fe$_{\mathrm{2}}$O$_{\mathrm{3}})$, which is one of the most widely studied anode electrodes used for water splitting, surface states were related to the appearance of a dominant absorption peak during water splitting. The chemical origin of this signature is still unclear and this open question has provoked tremendous debate. In order to pin down the origin and role of surface states, we perform first principle calculations with density functional theory $+$U on several possible adsorbates at the $\alpha $Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$(0001) surface. We show that the origin of the surface absorption peak could be a FeO\textbullet type bond that functions as an essential intermediate of water oxidation [Preview Abstract] 
(Author Not Attending)

K24.00011: Many body calculations of the optoelectronic properties of $h$AlN: from 3D to 2D Deniz Kecik, Cihan Bacaksiz, Engin Durgun, Tugrul Senger Outstanding electronic and optical properties of graphene, $h$BN, MoS$_{2}$ etc. motivate the further discovery of novel 2D materials such as AlN, a IIIV compound, with remarkable features for potential optoelectronic applications, due to its wide indirect band gap. The layer and strain dependent optoelectronic properties of the recently synthesized monolayer hexagonal AlN ($h$AlN) were investigated using density functional and many body perturbation theories, where RPA and BSE were employed on top of the QP$G_{0}W_{0}$ method. The optical spectra of 14 layered $h$AlN revealed prominent absorption beyond the visible light regime; absorbance within the UV range increasing with the number of layers. In addition, the applied tensile strain ($17\%$) was observed to gradually redshift the absorption spectra. While the many body corrections induced significant blueshift to the optical spectra, evidence of bound excitons were also found for the layered structures. Hence, the optoelectronic properties of layered $h$AlN can be tuned by modifying their structure and applying strain, moreover are greatly altered when electronhole interactions are considered. [Preview Abstract] 
Wednesday, March 16, 2016 10:36AM  10:48AM 
K24.00012: Putting DFT to the Test: A FirstPrinciples Study of Electronic, Magnetic, and Optical Properties of Co$_{\mathrm{3}}$O$_{\mathrm{4}}$ Vijay Singh, Monica Kosa, Koushik Majhi, Dan Thomas Major Firstprinciples density functional theory (DFT) and a manybody Green's function method have been employed to elucidate the electronic, magnetic, and photonic properties of a spinel compound, Co$_{\mathrm{3}}$O$_{\mathrm{4}}$. Co$_{\mathrm{3}}$O$_{\mathrm{4}}$ is believed to be a strongly correlated material, where the onsite Coulomb interaction ($U)$ on Co d orbitals is presumably important, although this view has recently been contested. The suggested optical band gap for this material ranges from 0.8 to 2.0 eV, depending on the type of experiments and theoretical treatment. Thus, the correlated nature of the Co d orbitals in Co$_{\mathrm{3}}$O$_{\mathrm{4}}$ and the extent of the band gap are still under debate, raising questions regarding the ability of DFT to correctly treat the electronic structure in this material. To resolve the above controversies, we have employed a range of theoretical methods, including pure DFT, DFT$+$U, and a rangeseparated exchangecorrelation functional (HSE06) as well as manybody Green's function theory (i.e., the GW method). We compare the electronic structure and band gap of Co$_{\mathrm{3}}$O$_{\mathrm{4}}$ with available photoemission spectroscopy and optical band gap data and confirm a direct band gap of ca. 0.8 eV. Furthermore, we have also studied the optical properties of Co$_{\mathrm{3}}$O$_{\mathrm{4}}$ by calculating the imaginary part of the dielectric function (Im($\varepsilon ))$, facilitating direct comparison with the measured optical absorption spectra. [Preview Abstract] 
Wednesday, March 16, 2016 10:48AM  11:00AM 
K24.00013: Objective performance of the GW approximation and the BetheSalpeter Equation for molecules Fabien Bruneval, Samia M. Hamed, Tonatiuh RangelGordillo, Jeffrey B. Neaton We have evaluated the quality of the quasiparticle energies obtained within the $GW$ approximation and of the optical excitations with the solution of the BetheSalpeter equation (BSE) for molecules. The calculations have been performed with a recently developed code based on Gaussian [1,2] that allowed us to use the exact same techniques as the one employed in traditional quantum chemistry. We demonstrate [3] the extreme sensitivity of the $GW$ and BSE results upon the KohnSham starting point. Most of the starting point dependence in BSE is to be ascribed to the underlying $GW$ band structure. We highlight the problem of the triplet excitations that are equally underestimated in timedependent densityfunctional theory and in BSE. [1] F. Bruneval, J. Chem. Phys. \textbf{136}, 194107 (2012). [2] F. Bruneval and M.A.L. Marques, J. Chem. Theory Comput. \textbf{9}, 324 (2013). [3] F. Bruneval, S. M. Hamed and J. B. Neaton, J. Chem. Phys. \textbf{142}, 244101 (2015). [Preview Abstract] 
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