First-principles Many-body Calculations Research Articles

Revisiting the metal-to-metal transition in 2H-AgNiO2

The layered delafossite compound AgNiO2 with stacking symmetry undergoes a structural metal-to-metal transition at . It has been described in the past as a charge-ordering transition, where local S = 1 spins are formed on part of the Ni sites. By means of first-principles many-body calculations, we show that the transition is in fact a site-selective Mott transition on the Ni sublattice with only minor charge differentiation. Key to this finding is the uncovering of ligand-hole physics, rendering a Ni2+ instead of a formal Ni3+ oxidation state, in conjunction with strong local Coulomb repulsions.

The Structure of Terbium in the Ferromagnetic State.

Metals typically crystallize in highly symmetric structures due to their nondirectional and nonsaturated metallic bonds. Here, we report that terbium metal in its ferromagnetic state adopts an unusual low-symmetry orthorhombic structure with a Cmcm space group. A similar structure has been previously observed only in a few actinide metals with bonding 5f electrons at ambient pressure, such as uranium, neptunium, and plutonium, but with different nearest coordination numbers and bond-length variations. The Tb atom occupies the 4c site (0, ∼0.1661, 1/4), building up -[Tb-Tb]- layers stacking along the b-axis. Our first-principles many-body calculations of the crystal field splitting in the correlated Tb 4f-shell demonstrate that the Cmcm structure for ferromagnetic terbium is stabilized by magneto-elastic forces due to a secondary order of quadrupolar moments in the ferromagnetic state. These findings are significant for further understanding of the nature of terbium, including its electron structure, energy bands, phonons, and magnetism.

Polarization Resolved Optical Excitation of Charge-Transfer Excitons in PEN:PFP Cocrystalline Films: Limits of Nonperiodic Modeling.

Charge-transfer excitons (CTXs) at organic donor/acceptor interfaces are considered important intermediates for charge separation in photovoltaic devices. Crystalline model systems provide microscopic insights into the nature of such states as they enable microscopic structure-property investigations. Here, we use angular-resolved UV/vis absorption spectroscopy to characterize the CTXs of crystalline pentacene:perfluoro-pentacene (PEN:PFP) films allowing determination of the polarization of this state. This analysis is complemented by first-principles many-body calculations, performed on the three-dimensional PEN:PFP cocrystal, which confirm that the lowest-energy excitation is a CTX. Analogous simulations performed on bimolecular clusters are unable to reproduce this state. We ascribe this failure to the lack of long-range interactions and wave function periodicity in these cluster calculations, which appear to remain a valid tool for modeling properties of organic materials ruled by local intermolecular couplings.

Electrically controlled dielectric band gap engineering in a two-dimensional semiconductor

This work proposes and investigates a new physical concept of electrically controlled dielectric band gap engineering in a two-dimensional (2D) semiconductor. The authors show, using first-principles many-body calculations, that the band gap of a 2D semiconductor can be varied by several hundred meV's by varying the doping concentration in a supporting graphene sheet as illustrated in the accompanying image. This provides a method for continuous and dynamical $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0.333em}{0ex}}s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u$ tuning of the band gap. Importantly, the band gap adjustment is achieved via nonlocal Coulomb interactions and does not involve changes in the atomic structure, or even the shape or hybridization pattern, of the wave functions of the 2D semiconductor.

Interlayer Excitons with Large Optical Amplitudes in Layered van der Waals Materials.

Vertically stacked two-dimensional materials form an ideal platform for controlling and exploiting light-matter interactions at the nanoscale. As a unique feature, these materials host electronic excitations of both intra- and interlayer type with distinctly different properties. In this Letter, using first-principles many-body calculations, we provide a detailed picture of the most prominent excitons in bilayer MoS2, a prototypical van der Waals material. By applying an electric field perpendicular to the bilayer, we explore the evolution of the excitonic states as the band alignment is varied from perfect line-up to staggered (Type II) alignment. For moderate field strengths, the lowest exciton has intralayer character and is almost independent of the electric field. However, we find higher lying excitons that have interlayer character. They can be described as linear combinations of the intralayer B exciton and optically dark charge transfer excitons, and interestingly, these mixed interlayer excitons have strong optical amplitude and can be easily tuned by the electric field. The first-principles results can be accurately reproduced by a simple excitonic model Hamiltonian that can be straightforwardly generalized to more complex van der Waals materials.

Lithium halide monolayer sheets: First-principles many-body calculations

The geometric, electronic and optical properties of two-dimensional lithium halide (LiF, LiCl, LiBr) monolayers are systematically explored by using ab initio density functional theory, the HSE06 functional, the GWA method and Bethe–Salpeter equation calculations. The stabilities of these structures have been further evaluated by cohesive energy, phonon spectra, mechanical stability analysis and ab initio molecular dynamics. The structural results exhibit that the buckled configurations of these monolayer systems are stable; therefore, it is possible to synthesize these structures in experiments. The electronic properties were analyzed by HSE06 functional and quasi-particle many-body perturbation theory (MBPT) via GW approach. It is found that all these nanostructures are indirect band gap insulators. The HSE06 and G0W0 gave remarkably larger band gaps than the PBE functional. The optical properties and excitonic effects of these materials are investigated in independent-particle, independent-quasiparticle and including excitonic effects (BSE). The formation of first exciton peaks at 8.48, 7.7, and 6.92 eV with large binding energy of 2.92, 2.13 and 1.70 eV was observed for LiF, LiCl and LiBr monolayers, respectively. The strong Coulomb interaction between electrons and holes in these compounds leads to the large binding energy and small exciton Bohr radius. The interesting stability, electronic and optical properties of these monolayers are potentially useful for future electronic and optoelectronic device design such as VUV transmitter and X-ray monochromator plates.

Quasi-particle energies and optical excitations of novel porous graphene phases from first-principles many-body calculations

Using ab-initio density functional theory, the HSE06 functional, the quasi-particle G0W0 method and Bethe–Salpeter equation calculations, we systematically explore the electronic and optical properties of two recently proposed systems, namely hydrogenated porous graphene (HPG) and biphenyl carbon (BPC). Our results indicate that both structures have a direct band gap at the K-point. The HSE06 and G0W0 gave remarkably larger band gaps than the PBE functional for BPC and HPG, while they cannot open the band gap in graphene. The optical properties and excitonic effects of these materials are investigated in independent-quasiparticle (G0W0+RPA) and including excitonic effects (G0W0+BSE). The formation of first exciton peaks at 1.14 and 3.65 with binding energy of and 0.06 and 1.35eV was observed for BPC and HPG nanostructures, respectively. It is found that the optical conductivity for the BPC system in the 570–750nm range is larger than the graphene one. The enhanced optical conductivity can be useful in designing photonic and optoelectronic devices such as solar cells and light-emitters. The first absorption peak of HPG is located in UV-A range which can be used in the UV-absorbing devices.

Importance of Excitonic Effect in Charge Separation at Quantum-Dot/Organic in-Terface: First-Principles Many-Body Calculations

Charge separation of excitons in materials is one of the most important physical processes to utilize the solar energy in diverse devices including solar cells and photo-catalysts. Heterogeneous interfaces with the so-called type-II character are often employed to infer the interfacial charge transfer in this context. As a simple criterion for designing such an interface, the energy alignment of the quasi-particle states together with the exciton binding energy of electron-donating materials is often discussed in the literature. However, an accurate description of the effect of exciton binding at the interface has not been investigated extensively. Although density functional theory (DFT) is a powerful method to investigate various elec-tronic properties of materials, incomplete description of many-body interactions often leads to an incorrect interpretation of the energy level alignment. While Many-Body Perturbation Theory and Quantum Monte Carlo (QMC) are promising in this context, much more work is necessary to assess how well these methods perform in practice. In this talk, I will discuss how significant the excitonic effect can affect charge separation process at heterogeneous interfaces between PbS quantum dot and thiophene oligomers by employing QMC calculations. Our study shows that charge transfer excitons can act as a trap states for electron-hole recombination instead of facilitating charge separation. This work has been published in Nano Letters.

Strong second harmonic generation in SiC, ZnO, GaN two-dimensional hexagonal crystals from first-principles many-body calculations.

The second harmonic generation (SHG) intensity spectrum of SiC, ZnO, GaN two-dimensional hexagonal crystals is calculated by using a real-time first-principles approach based on Green's function theory [Attaccalite et al., Phys. Rev. B: Condens. Matter Mater. Phys. 2013 88, 235113]. This approach allows one to go beyond the independent particle description used in standard first-principles nonlinear optics calculations by including quasiparticle corrections (by means of the GW approximation), crystal local field effects and excitonic effects. Our results show that the SHG spectra obtained using the latter approach differ significantly from their independent particle counterparts. In particular they show strong excitonic resonances at which the SHG intensity is about two times stronger than within the independent particle approximation. All the systems studied (whose stabilities have been predicted theoretically) are transparent and at the same time exhibit a remarkable SHG intensity in the range of frequencies at which Ti:sapphire and Nd:YAG lasers operate; thus they can be of interest for nanoscale nonlinear frequency conversion devices. Specifically the SHG intensity at 800 nm (1.55 eV) ranges from about 40-80 pm V(-1) in ZnO and GaN to 0.6 nm V(-1) in SiC. The latter value in particular is 1 order of magnitude larger than values in standard nonlinear crystals.

Importance of excitonic effect in charge separation at quantum-dot/organic interface: first-principles many-body calculations.

The staggered alignment of quasiparticle energy levels is widely regarded to be the key criterion necessary for electron-hole charge separation to occur at heterogeneous material interfaces. However, staggered energy levels at nanoscale interfaces, such as those between organic molecules and inorganic quantum dots, do not necessarily imply charge separation across the interface because the excitonic effect is often significant. Using quantum Monte Carlo calculations, we perform a detailed study of the role of the excitonic effects on charge separation across a representative set of interfaces between organic molecules and quantum dots. We find that the exciton binding energy of charge transfer excitons is significantly larger than would be estimated from a simple Coulombic analysis and, at these nanoscale interfaces, can be as significant as that of Frenkel excitons. This implies that charge transfer excitons can act as trap states and facilitate electron-hole recombination instead of charge separation. We conclude that in general, for nanoscale interfaces, high-fidelity quantum many-body calculations are essential for an accurate evaluation of the detailed energetic balance between localized and delocalized excitons and, thus, are crucial for the predictive treatment of interfacial charge separation processes.

How dielectric screening in two-dimensional crystals affects the convergence of excited-state calculations: Monolayer MoS2

We present first-principles many-body calculations of the dielectric constant, quasiparticle band structure, and optical absorption spectrum of monolayer MoS$_2$ using a supercell approach. As the separation between the periodically repeated layers is increased, the dielectric function of the layer develops a strong $q$ dependence around $q=0$. This implies that denser $k$-point grids are required to converge the band gap and exciton binding energies when large supercells are used. In the limit of infinite layer separation, here obtained using a truncated Coulomb interaction, a $45\times45$ $k$-point grid is needed to converge the G$_0$W$_0$ band gap and exciton energy to within 0.1 eV. We provide an extensive comparison with previous studies and explain agreement and variations in the results. It is demonstrated that too coarse $k$-point sampling and the interactions between the repeated layers have opposite effects on the band gap and exciton energy, leading to a fortuitous error cancellation in the previously published results.

Ultrafast Electron Dynamics in Metals

During the last decade, significant progress has been achieved in the rapidly growing field of the dynamics of hot carriers in metals. Here we present an overview of the recent achievements in the theoretical understanding of electron dynamics in metals, and focus on the theoretical description of the inelastic lifetime of excited hot electrons. We outline theoretical formulations of the hot-electron lifetime that originates in the inelastic scattering of the excited quasiparticle with occupied states below the Fermi level of the solid. First-principles many-body calculations are reviewed. Related work and future directions are also addressed.

Hole Dynamics in Noble Metals

Hole dynamics in noble metals (Cu and Au) is investigated by means of first-principles many-body calculations. While holes in a free-electron gas are known to live shorter than electrons with the same excitation energy, our results indicate that d holes in noble metals exhibit longer inelastic lifetimes than excited sp electrons, in agreement with experiment. The density of states available for d-hole decay is larger than that for the decay of excited electrons; however, the small overlap between d and sp states below the Fermi level increases the d-hole lifetime. The impact of d-hole dynamics on electron-hole correlation effects is also addressed.