Session P24: Many-Body Perturbation Theory for Electronic Excitations: Electronic Structure

Electronic structure from relativistic quasiparticle self-consistent $GW$ calculations

Most theoretical studies of topological insulators (TIs) are based on tight-binding descriptions and density functional theory (DFT). But recently, many-body calculations within the $GW$ approximation attract much attention in the study of these materials. We present an implementation of the quasiparticle self-consistent (QS) $GW$ method where the spin-orbit coupling (SOC) is fully taken into account in each iteration rather than added a posteriori. Within the all-electron FLAPW formalism, we show DFT, one-shot $GW$, and QS$GW$ calculations for several, well-known TIs. We present a comparison of the calculations to photoemission spectroscopy and show that the $GW$ corrected bands agree much better with experiment. For example, we show that Bi$_2$Se$_3$ [1,2] is a direct gap semiconductor, in contrast to what was believed for many years by interpreting experimental results on the basis of DFT and that small strains in Bi can lead to a semimetal-to-semiconductor or trivial-to-topological transitions [3]. Quasiparticle calculations for low-dimensional systems are still very demanding. In order to study the topological surface states with an approach based on $GW$, we use Wannier functions to construct a Hamiltonian that reproduces the many-body band structure of the bulk, and that is used to construct a slab Hamiltonian. With this approach, we discuss the effect of quasiparticle corrections on the surface states of TIs and on the interaction between bulk and surface states. [1] I. Aguilera \textit{et al.}, PRB {\bf 88}, 045206 (2013), \textit{ibid}., PRB {\bf 88}, 165136 (2013). [2] M. Michiardi \textit{et al.}, PRB \textbf{90}, 075105 (2014). [3] I.\ Aguilera \textit{et al.}, PRB \textbf{91}, 125129 (2015).   

Quasiparticle electronic structure of Bi$_2$Se$_3$ via the sc-COHSEX+GW approach

We present ab initio calculations of the quasiparticle electronic band structure of three-dimensional topological insulator material Bi$_2$Se$_3$ using the full spinor GW approach. The mean-field is initially computed at the DFT level in the local density approximation (LDA) using fully-relativistic pseudopotentials. We then improve the mean-field electronic structure by solving Dyson's equation in the static COHSEX approximation, self-consistently updating the eigenvalues, eigenvectors, and dielectric screening. After a few iterations, we then perform a GW calculation to determine the quasiparticle energies. We compare our calculated results to experimental values of the band gaps and effective masses.   

Ubiquitous electron-plasmon coupling in doped semiconductors

The interplay between electrons and bosonic excitations [as, e.g., phonons, collective charge-density fluctuations (plasmons), and magnons] is pervasive in matter and underlies an extremely broad spectrum of physical phenomena, as, for instance, current dissipation, superconductivity, hot-carrier thermalisation, and band structure replicas [1]. At variance with phonons, however, questions pertaining the strength of electron-plasmon coupling in solids are still awaiting further investigations. We developed and implemented a first-principles theory of electron-plasmon coupling based on many-body perturbation theory. Our first-principles calculations reveal that electron-plasmon coupling alters ubiquitously the dynamical and optical properties of semiconductors at high doping concentrations. This behaviour stems from the emergence of low-energy extrinsic plasmons which may couple electronic states in the vicinity of the Fermi energy. [1] F. Caruso, H. Lambert, and F. Giustino, Phys. Rev. Lett. {\bf 114}, 146404 (2015).   

Quasiparticle excitations of adsorbates on doped graphene

Adsorbed atoms and molecules can modify the electronic structure of graphene, but in turn it is also possible to control the properties of adsorbates via the graphene substrate. In my talk, I will discuss the electronic structure of F4-TCNQ molecules on doped graphene and present a first-principles based theory of quasiparticle excitations that captures the interplay of doping-dependent image charge interactions between substrate and adsorbate and electron-electron interaction effects on the molecule. The resulting doping-dependent quasiparticle energies will be compared to experimental scanning tunnelling spectra. Finally, I will also discuss the effects of charged adsorbates on the electronic structure of doped graphene.   

First Principles Charge Transfer Excitations in Curved Aromatic Materials

Understanding excitation properties and charge transport phenomena of curved $\pi $-conjugated materials is critical for a rational utilization of buckybowls as electrically active materials in solid-state devices. In this respect, the class of materials based on the smallest bowl-shaped fullerene fragment, corannulene, C$_{20}$H$_{10}$, offers a unique possibility for building up scaffolds with a tunable spectrum of structural and electronic properties.[1] Here, GW-BSE based approaches are applied to investigation and prediction of charge transfer excitations of C$_{20}$H$_{10}$ materials systems at functional interfaces, [1-3] with a special emphasis on design aspects of materials relevant in the experimental domain. Theoretical predictions together with experimental findings illustrate the possibility of integrating corannulene electronic functions in molecular devices. [1] L. Zoppi, L. Martin-Samos, K. K. Baldridge, \textit{Acc. Chem. Res}., 47, 3310--3320 (2014) [2] L. Zoppi, L. Martin-Samos, K. K. Baldridge\textit{, J. Am. Chem. Soc.} 133, 14002-14009 (2011) [3] L. Zoppi, L. Martin Samos, K. K. Baldridge, \textit{Phys. Chem. Chem. Phys.} 17, 6114-6121 (2015)   

Electronic and optical excitations in building blocks of the metal organic framework MOF-5

Metal organic frameworks (MOFs) are a relatively new class of materials which are made of metal-oxide clusters linked by organic bridging ligands. In recent years, MOFs have received considerable attention due to their widely tunable structural, chemical and physical properties. We investigate one of the well characterized MOFs, MOF-5, whose framework consists of tetrahedral [Zn$_4$0]$^{6+}$ units linked by rigid arylcarboxylate ligands. We use many-body perturbation (GW$+$BSE) and time-dependent DFT methods in real space to examine the electronic and optical excitations in the building blocks of MOF-5, such as Zn$_4$O(COOH)$_6$, basic zinc acetate [Zn$_4$O(CH$_3$COO)$_6$], and tetranuclear zinc benzoate [Zn$_4$O(C$_6$H$_5$COO)$_6$]. The calculated spectra are compared with available experimental measurements and existing calculations to shed light on the controversy regarding the nature (metal-ligand versus ligand-ligand) of low-energy electronic and optical excitations in MOF-5.   

Energy level alignment at hybridized organic-metal interfaces from a GW projection approach

Energy level alignments at organic-metal interfaces are of profound importance in numerous (opto)electronic applications. Standard density functional theory (DFT) calculations generally give incorrect energy level alignments and missing long-range polarization effects. Previous efforts to address this problem using the many-electron GW method have focused on physisorbed systems where hybridization effects are insignificant. Here, we use state-of-the-art GW methods to predict the level alignment at the amine-Au interface, where molecular levels do hybridize with metallic states. This non-trivial hybridization implies that DFT result is a poor approximation to the quasiparticle states. However, we find that the self-energy operator is approximately diagonal in the molecular basis, allowing us to use a projection approach to predict the level alignments. Our results indicate that the metallic substrate reduces the HOMO-LUMO gap by 3.5~4.0 eV, depending on the molecular coverage/presence of Au adatoms. Our GW results are further compared with those of a simple image charge model that describes the level alignment in physisorbed systems.   

Nonequilibrium transport in the Anderson-Holstein model with interfacial screening

Image charge effects in nanoscale junctions with strong electron-phonon coupling open the way to unexplored physical scenarios. Here we present a comprehensive study of the transport properties of the Anderson-Holstein model in the presence of dot-lead repulsion. We propose an accurate many-body approach to deal with the simultaneous occurrence of the Franck-Condon blockade and the screening-induced enhancement of the polaron mobility. Remarkably, we find that a novel mechanism of negative differential conductance origins from the competition between the charge blocking due to the electron-phonon interaction and the charge deblocking due to the image charges. An experimental setup to observe this phenomenon is discussed. References [1] E. Perfetto, G. Stefanucci and M. Cini, Phys. Rev. B 85, 165437 (2012). [2] E. Perfetto and G. Stefanucci, Phys. Rev. B 88, 245437 (2013). [3] E. Perfetto and G. Stefanucci, Journal of Computational Electronics 14, 352 (2015).   

Many-body effects and ultraviolet renormalization in three-dimensional Dirac materials

We develop a theory for electron-electron interaction-induced many-body effects in three dimensional (3D) Weyl or Dirac semimetals, including interaction corrections to the polarizability, electron self-energy, and vertex function, up to second order in the effective fine structure constant of the Dirac material. These results are used to derive the higher-order ultraviolet renormalization of the Fermi velocity, effective coupling, and quasiparticle residue, revealing that the corrections to the renormalization group (RG) flows of both the velocity and coupling counteract the leading-order tendencies of velocity enhancement and coupling suppression at low energies. This in turn leads to the emergence of a critical coupling above which the interaction strength grows with decreasing energy scale. In addition, we identify a range of coupling strengths below the critical point in which the Fermi velocity varies non-monotonically as the low-energy, non-interacting fixed point is approached. Furthermore, we find that while the higher-order correction to the flow of the coupling is generally small compared to the leading order, the corresponding correction to the velocity flow carries an additional factor of the Dirac cone flavor number relative to the leading-order result.   

Electron-electron interactions in Dirac and Weyl semimetals: collective modes and stability of the ground state

Three-dimensional Dirac and Weyl semimetals host linearly dispersive low-energy electronic bands in the bulk, and exotic Fermi-Arc states at the surface. Following theoretical proposals of candidate material classes [1,2], experimental observation of anomalous transport [3] and Fermi-Arc surface states [4] have recently been reported. Using time-dependent Hartree-Fock and renormalization group methods, we study collective mode dispersion and the influence of electron-electron interactions on the stability of the ground state. This work was supported by the DOE Division of Materials Sciences and Engineering under grant DE-FG02-ER45118. [1] Z. Wang et al., Phys. Rev. B. 88, 125427 (2013) [2] S.-M. Huang et al., Nat. Comm. 6, 7373 (2015) [3] T. Liang et al., Nat. Mater. 14, 280 (2014) [4] S.-Y. Xu et al., Science 347, 294 (2015)   

First-principles DFT+GW study of oxygen doped CdTe

The role of oxygen doping in CdTe is addressed by first-principles calculations. Formation energies, charge transition levels and quasiparticle defect states are calculated within the DFT+GW formalism. The formation of a new defect is identified, the $(\text{O}_{\text{Te}}-\text{Te}_\text{Cd})$ complex. This complex is energetically favored over both isovalent (O$_{\text{Te}}$) and interstitial oxygen (O$_{\text{i}}$). We find that incorporation of oxygen passivates the harmful deep energy levels derived from Te antisites, suggesting an improvement in the efficiency of CdTe based solar cells. Our calculations indicate that both (O$_{\text{Te}}$) and (O$_{\text{i}}$) have low formation energies. Moreover, $(\text{O}_{\text{Cd}})$ is only stable in the neutral charge state and undergoes a Jahn-Teller distortion. The (V$_{\text{Cd}}-$O$_{\text{Te}}$) complex is found to be a shallow acceptor with a high formation energy. We also report an oxygen-related interstitial defect, which plays a key role in the diffusion mechanism of oxygen in CdTe.   

Quasi-particle band structure of potassium-doped few-layer black phosphorus with GW approximation

We calculate the quasi-particle band structure of pristine and potassium-doped black phosphorus (BP) by using the GW approximation. We obtain band gaps of pristine bulk and few-layer BP and compare them with the result of the density functional calculations and experimental measurements. For potassium-doped cases, we calculate the electronic band structure of potassium-doped few-layer BPs with various doping densities. We obtain the critical doping density for the band-gap closing, and the energy-band dispersions when the band gap is inverted. We discuss Dirac semimetal properties of doped few-layer BPs obtained by the GW approximation. This work was supported by NRF of Korea (Grant No. 2011-0018306) and KISTI supercomputing center (Project No. KSC-2015-C3-039).   

Quasiparticle and optical band gaps of Sr$_{n+1}$Ti$_{n}$O$_{3n+1}$ from \textit{ab-initio} many-body perturbation theory

The Ruddlesden Popper homologous series Sr$_{n+1}$Ti$_{n}$O$_{3n+1}$ provides a unique opportunity to study the effect of dimensionality and confinement on the band gap and absorption spectrum of the complex oxide SrTiO$_3$. In this work, we use many-body perturbation theory within the \textit{GW} approximation and the Bethe-Salpeter equation (BSE) approach to study the electronic and optical properties of Sr$_{n+1}$Ti$_{n}$O$_{3n+1}$. We find that our \textit{GW}/BSE direct and indirect band gaps are in excellent agreement with measured direct and indirect optical gaps. We discuss technical aspects of the calculations such as convergence and starting-point dependence, and compare to higher levels of theory. In addition, we find a relatively large exciton binding energy of 500 meV for Sr$_2$TiO$_4$ ($n=1$). We explore the role of structural distortions and epitaxial strain in the properties of the localized exciton. Our work suggests that layered structures can provide a viable route for the design of complex oxide materials with desirable optoelectronic properties. This work is supported by DOE.