Research

Our group is engaged in research in the fields of theoretical condensed matter physics and materials science. Our research covers a broad range of materials systems, from bulk materials (metals, semiconductors, and insulators) to those of finite size, such as organic and inorganic nanostructures, to complex multifunctional oxides; phenomena of interest include optical properties, superconductivity, conductance of nanostructures at finite bias, pressure and temperature effects, and dynamics. Particular emphasis is placed on the study of the role of many-particle effects in determining experimentally observed properties.

Our primary goal is to understand and predict materials properties at the most fundamental level using atomistic first principles (or “ab initio”) quantum-mechanical calculations. A variety of different computational approaches are used that require only the atomic number and positions as input. These first principles methods have, in the past, resulted in excellent quantitative agreement with experiment and have predicted with good accuracy materials properties that were later verified experimentally.

We present below some highlights on recent research projects. For more details, please see our list of publications.

Probing excitonic dark states in single-layer tungsten disulphide

In this work by applying GW-BSE theory, in combination with experimental two-photon excitation spectroscopy, we study a series of excitonic dark states in single-layer WS2. We prove that the excitons are of Wannier type, meaning that each exciton wavefunction extends over multiple unit cells, but with extraordinarily large binding energy (~0.7 electronvolts). These strongly bound exciton states are observed to be stable even at room temperature. We theoretically reveal an exciton series that deviates substantially from hydrogen models, with a novel energy dependence on the orbital angular momentum.

Optical properties of molybdenum dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are emerging as a new platform for exploring 2D semiconductor physics. Reduced screening in two dimensions results in markedly enhanced electron–electron interactions, which have been predicted to generate giant bandgap renormalization and excitonic effects. We present first-principles calculations of the optical response of different geometries of molybdenum dichalcogenides, employing the GW-Bethe-Salpeter equation (GW-BSE) approach including self-energy, excitonic, and electron-phonon effects. The resulting optical properties are shown to be dominated by excitonic states with very large binding energy. The optical properties are also strongly related to structures and substrates.

First-principles calculations of spin fluctuations

We develop the first-principles method to calculation spin fluctuations in real materials by employing the Overhauser-Kukkonen form for the electron self-energy arising from spin fluctuations. We demonstrate that in sodium and lithium, the coupling of electrons to spin fluctuations gives an important contribution to the quasiparticle lifetime but does not significantly reduce the occupied bandwidth. In iron selenide, we find that strong antiferromagnetic stripe-phase spin fluctuations lead to large coupling constants for superconducting, but these coupling constants are significantly reduced by other spin fluctuations with small wave vectors. An accurate description of this competition and an inclusion of band-structure and Stoner parameter renormalization effects can produce a superconducting transition temperature consistent with experimental measurements.

Photoelectron spin flipping and spin transport in topological insulators

We show that the degree of spin polarization of photoelectrons from the surface states of topological insulators is 100% if fully polarized light is used as in typical photoemission measurements, and, hence, can be significantly higher than that of the initial state, i.e. the surface states, ~50% according to our first-principles calculations. Further, the spin orientation of these photoelectrons in general can also be very different from that of the initial surface state and is controlled by the photon polarization. We also studied the quasiparticle effects in topological insulators.

Ab Initio Study of Hot Carriers

Hot carrier thermalization is a major source of efficiency loss in solar cells. We develop and apply an ab initio approach based on density functional theory and many-body perturbation theory to investigate hot carriers in semiconductors. Our calculations include electron-electron and electron-phonon interactions, and require no experimental input other than the structure of the material. We demonstrate that a hot carrier distribution characteristic of Si under solar illumination thermalizes within 350 fs, in excellent agreement with pump-probe experiments. Our work sheds light on the subpicosecond time scale after sunlight absorption in Si, and constitutes a first step towards ab initio quantification of hot carrier dynamics in materials.

Structures and electronic transport properties of polycrystalline graphene

We develop a theory of charge carrier transmission through grain boundaries composed of a periodic array of dislocations in graphene based on the momentum conservation principle. Depending on the grain-boundary structure we find two distinct transport behaviours—either high transparency, or perfect reflection of charge carriers over remarkably large energy ranges. Our study sheds light on the transport properties of large-area graphene samples. Furthermore, purposeful engineering of periodic grain boundaries with tunable transport gaps would allow for controlling charge currents without the need to introduce bulk bandgaps in otherwise semimetallic graphene. The proposed approach can be regarded as a means towards building practical graphene electronics.

Recent developments in GW-BSE methodologies and their applications

We show some of our recent developments in GW-BES methods encoded in our BerkeleyGW package. We have applied those new techniques to different systems that were difficult to dealt with before, and get consistent results with experiments.

BerkeleyGW

BerkeleyGW: A Massively Parallel Computer Package for the Calculation of the Quasiparticle and Optical Properties of Materials and Nanostructures

BerkeleyGW is a massively parallel computational package for electron excitedstate properties that is based on many-body perturbation theory employing the ab initio GW and GW plus Bethe-Salpeter equation methodology. It can be used in conjunction with many density-functional theory codes for groundstate properties, including PARATEC, PARSEC, Quantum ESPRESSO, SIESTA, and Octopus. The package can be used to compute the electronic and optical properties of a wide variety of material systems from bulk semiconductors and metals to nanostructured materials and molecules. The package scales to 10000s of CPUs and can be used to study systems containing up to 100s of atoms. The program is freely available at www.berkeleygw.org.

Surface Atom Motion to Move Iron Nanocrystals through Constrictions in Carbon Nanotubes under the Action of an Electric Current

Under the application of electrical currents, metal nanocrystals inside carbon nanotubes can be bodily transported. We examine experimentally and theoretically how an iron nanocrystal can pass through a constriction in the carbon nanotube with a smaller cross-sectional area than the nanocrystal itself. Remarkably, through in situ transmission electron imaging and diffraction, we find that, while passing through a constriction, the nanocrystal remains largely solid and crystalline and the carbon nanotube is unaffected. We account for this behavior by a pattern of iron atom motion and rearrangement on the surface of the nanocrystal. The nanocrystal motion can be described with a model whose parameters are nearly independent of the nanocrystal length, area, temperature, and electromigration force magnitude. We predict that metal nanocrystals can move through complex geometries and constrictions, with implications for both nanomechanics and tunable synthesis of metal nanoparticles.

Electron-phonon coupling: first-principles method and applications

Electron-phonon coupling is very important in many physics phenomena. Here we show some of our work about electron-phonon coupling effects in superconductivity and band gap renormalization in different materials. We have also developed the package EPW (Electron–Phonon coupling using Wannier functions), which is a program for calculating the electron–phonon coupling in periodic systems using density-functional perturbation theory and maximally localized Wannier functions. EPW can calculate electron–phonon interaction self-energies, electron–phonon spectral functions, and total as well as mode-resolved electron–phonon coupling strengths.