arXiv subjects

Steven G. Louie

Publications and source records attributed to Steven G. Louie.

At least 145 records · Page 8Linked to original sources

arXiv2014

Satellite structures in the spectral functions of semiconductor quantum well two-dimensional electron gases: a GW plus cumulant study

We present theoretical calculations for the spectral functions and single-particle densities of states of the two-dimensional electron gas in semiconductor quantum wells at different electron densities using the GW+cumulant method. We compare our results to GW only calculations and find significant differences in the description of the satellites between the two theories: while GW theory predicts the existence of a plasmaron excitation, no such excitation is found in GW+cumulant theory. We compare our results to experimental tunneling spectra from semiconductor quantum wells and find good agreement for the satellite properties.

cond-mat.mes-hall↗

arXiv2014

Effect of spin fluctuations on quasiparticle excitations: first-principles theory and application to sodium and lithium

We present first-principles calculations for quasiparticle excitations in sodium and lithium including the effects of charge and spin fluctuations. We employ the Overhauser-Kukkonen form for the electron self energy arising from spin fluctuations and demonstrate that the coupling of electrons to spin fluctuations gives an important contribution to the quasiparticle lifetime, but does not significantly reduce the occupied bandwidth. Including correlation effects beyond the random-phase approximation in the screening from charge fluctuations yields good agreement with experiment.

cond-mat.mtrl-sci↗

arXiv2014

Ab initio quasiparticle bandstructure of ABA and ABC-stacked graphene trilayers

We obtain the quasiparticle band structure of ABA and ABC-stacked graphene trilayers through ab initio density functional theory (DFT) and many-body quasiparticle calculations within the GW approximation. To interpret our results, we fit the DFT and GW $π$ bands to a low energy tight-binding model, which is found to reproduce very well the observed features near the K point. The values of the extracted hopping parameters are reported and compared with available theoretical and experimental data. For both stackings, the self energy corrections lead to a renormalization of the Fermi velocity, an effect also observed in previous calculations on monolayer graphene. They also increase the separation between the higher energy bands, which is proportional to the nearest neighbor interlayer hopping parameter $γ_1$. Both features are brought to closer agreement with experiment through the self energy corrections. Finally, other effects, such as trigonal warping, electron-hole asymmetry and energy gaps are discussed in terms of the associated parameters.

cond-mat.mes-hall↗

arXiv2013

Tuning many-body interactions in graphene: The effects of doping on excitons and carrier lifetimes

The optical properties of graphene are strongly affected by electron-electron (e-e) and electron-hole (e-h) interactions. Here we tune these many-body interactions through varying the density of free charge carriers. Measurements from the infrared to the ultraviolet reveal significant changes in the optical conductivity of graphene for both electron and hole doping. The shift, broadening, and modification in shape of the saddle-point exciton resonance reflect strong screening of the many-body interactions by the carriers, as well as changes in quasi-particle lifetimes. Ab initio calculations by the GW Bethe-Salpeter equation (GW-BSE), which take into account modification of both the repulsive e-e and the attractive e-h interactions, provide excellent agreement with experiment. Understanding the optical properties and high-energy carrier dynamics of graphene over a wide range of doping is crucial for both fundamental graphene physics and for emerging applications of graphene in photonics.

cond-mat.mtrl-sci↗

arXiv2013

Systematic Determination of Absolute Absorption Cross-section of Individual Carbon Nanotubes

Determination of optical absorption cross-section is always among the central importance of understanding a material. However its realization on individual nanostructures, such as carbon nanotubes, is experimentally challenging due to the small extinction signal using conventional transmission measurements. Here we develop a technique based on polarization manipulation to enhance the sensitivity of single-nanotube absorption spectroscopy by two-orders of magnitude. We systematically determine absorption cross-section over broad spectral range at single-tube level for more than 50 chirality-defined single-walled nanotubes. Our data reveals chirality-dependent one-dimensional photo-physics through the behaviours of exciton oscillator strength and lifetime. We also establish an empirical formula to predict absorption spectrum of any nanotube, which provides the foundation to determine quantum efficiencies in important photoluminescence and photovoltaic processes.

cond-mat.mes-hall↗

arXiv2013

Optical Spectrum of MoS${}_2$: Many-body Effects and Diversity of Exciton States

We present first-principles calculations of the optical response of monolayer molybdenum disulfide employing the GW-Bethe Salpeter equation (GW-BSE) approach including self-energy, excitonic, and electron-phonon effects. We show that monolayer MoS2 possesses a large and diverse number of strongly bound excitonic states with novel k-space characteristics that were not previously seen experimentally or theoretically. The absorption spectrum is shown to be dominated by excitonic states with binding energy close to $1$~eV and by strong electron-phonon broadening in the visible to ultra-violet range. Our results explain recent experimental measurements and resolve inconsistencies between previous GW-BSE calculations.

cond-mat.mtrl-sci↗

arXiv2013

Atomically perfect torn graphene edges and their reversible reconstruction

The atomic structure of graphene edges is critical in determining the electrical, magnetic, and chemical properties of truncated graphene structures, notably nanoribbons. Unfortunately, graphene edges are typically far from ideal and suffer from atomic-scale defects, structural distortion, and unintended chemical functionalization, leading to unpredictable properties. Here we report that graphene edges fabricated by electron-beam-initiated mechanical rupture or tearing in high vacuum are clean and largely atomically perfect, oriented in either the armchair or zigzag direction. Via aberration-corrected transmission electron microscopy, we demonstrate reversible and extended pentagon-heptagon (5-7) reconstruction at zigzag edges, and explore experimentally and theoretically the dynamics of the transitions between configuration states. Good theoretical-experimental agreement is found for the flipping rates between 5-7 and 6-6 zigzag edge states. Our study demonstrates that simple ripping is remarkably effective in producing atomically clean, ideal terminations thus providing a valuable tool for realizing atomically-tailored graphene and facilitating meaningful experimental study.

cond-mat.mtrl-sci↗

arXiv2013

Theory of the Raman spectrum of rotated double-layer graphene

We study theoretically the Raman spectrum of the rotated double-layer graphene, consisting of two graphene layers rotated with respect to each other by an arbitrary angle θ. We find a relatively simple dependence of the Raman G peak intensity on the angle θ. On the other hand, the Raman 2D peak position, intensity, and width show a much more complicated dependence on the angle θ. We account for all of these effects, including dependence on the incoming photon energy, in good agreement with the experimental data. We find that it is sufficient to include the interaction between the graphene layers on the electronic degrees of freedom (resulting in the occurrence of van Hove singularities in the density of states). We assume that the phonon degrees of freedom are unaffected by the interaction between the layers. Furthermore, we decompose the Raman 2D peak into two components having very different linewidths; these widths are almost independent of the angle θ. The change in the intensity and the peak position of one of these two components gives insight into the dependence of the overall Raman 2D peak features as a function of the angle θ. Additionally, we show regions in the phonon spectrum giving rise to the Raman 2D peak signal. This work provides an insight into the interplay between the mechanical degree of freedom (angle θ) and the electronic degrees of freedom (singularities in the density of states) in rotated double-layer graphene. Additionally, this work provides a way to establish experimentally the value of the rotation angle θusing Raman spectroscopy measurement. This procedure becomes even more robust if one repeats the Raman spectroscopy measurement with a different incoming photon energy.

cond-mat.mes-hall↗

arXiv2013

Intermolecular interactions and substrate effects for an adamantane monolayer on the Au(111) surface

We study theoretically and experimentally the infrared (IR) spectrum of an adamantane monolayer on a Au(111) surface. Using a new STM-based IR spectroscopy technique (IRSTM) we are able to measure both the nanoscale structure of an adamantane monolayer on Au(111) as well as its infrared spectrum, while DFT-based ab initio calculations allow us to interpret the microscopic vibrational dynamics revealed by our measurements. We find that the IR spectrum of an adamantane monolayer on Au(111) is substantially modified with respect to the gas-phase IR spectrum. The first modification is caused by the adamantane--adamantane interaction due to monolayer packing and it reduces the IR intensity of the 2912 cm$^{-1}$ peak (gas phase) by a factor of 3.5. The second modification originates from the adamantane--gold interaction and it increases the IR intensity of the 2938 cm$^{-1}$ peak (gas phase) by a factor of 2.6, and reduces its frequency by 276 cm$^{-1}$. We expect that the techniques described here can be used for an independent estimate of substrate effects and intermolecular interactions in other diamondoid molecules, and for other metallic substrates.

cond-mat.mes-hall↗

arXiv2013

Imprint of transition metal d-orbitals on graphene Dirac cone

We investigate the influence of SiO2, Au, Ag, Cu, and Pt substrates on the Raman spectrum of graphene. Experiments reveal particularly strong modifications to the intensity, position, width, and shape of the Raman signal of graphene on platinum, compared to that of suspended graphene. The modifications strongly depend on the relative orientation of the graphene and platinum lattices. These observations are theoretically investigated and shown to originate from hybridization of electronic states in graphene and d-orbitals in platinum. It is expected that, quite generally, hybridization between graphene and any material with d-orbitals near the Fermi level will result in an imprint on the graphene Dirac cone which depends sensitively on the relative orientation of the respective lattices.

cond-mat.mtrl-sci↗

arXiv2013

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.

cond-mat.mes-hall↗

arXiv2013

Theoretical study of solid iron nanocrystal movement inside a carbon nanotube

We use a first-principles based kinetic Monte Carlo simulation to study the movement of a solid iron nanocrystal inside a carbon nanotube driven by the electrical current. The origin of the iron nanocrystal movement is the electromigration force. Even though the iron nanocrystal appears to be moving as a whole, we find that the core atoms of the nanocrystal is completely stationary, and only the surface atoms are moving. Movement in the contact region with the carbon nanotube is driven by electromigration forces, and the movement on the remaining surfaces is driven by diffusion. Results of our calculations also provide a simple model which can predict the center of mass speed of the iron nanocrystal over a wide range of parameters. We find both qualitative and quantitative agreement of the iron nanocrystal center of mass speed with experimental data.

cond-mat.mes-hall↗

arXiv2013

Photoelectron spin-flipping and texture manipulation in a topological insulator

Recently discovered materials called three-dimensional topological insulators constitute examples of symmetry protected topological states in the absence of applied magnetic fields and cryogenic temperatures. A hallmark characteristic of these non-magnetic bulk insulators is the protected metallic electronic states confined to the material's surfaces. Electrons in these surface states are spin polarized with their spins governed by their direction of travel (linear momentum), resulting in a helical spin texture in momentum space. Spin- and angle-resolved photoemission spectroscopy (spin-ARPES) has been the only tool capable of directly observing this central feature with simultaneous energy, momentum, and spin sensitivity. By using an innovative photoelectron spectrometer with a high-flux laser-based light source, we discovered another surprising property of these surface electrons which behave like Dirac fermions. We found that the spin polarization of the resulting photoelectrons can be fully manipulated in all three dimensions through selection of the light polarization. These surprising effects are due to the spin-dependent interaction of the helical Dirac fermions with light, which originates from the strong spin-orbit coupling in the material. Our results illustrate unusual scenarios in which the spin polarization of photoelectrons is completely different from the spin state of electrons in the originating initial states. The results also provide the basis for a novel source of highly spin-polarized electrons with tunable polarization in three dimensions.

cond-mat.mtrl-sci↗

arXiv2013

Physical origin of satellite in photoemission of doped graphene: An ab-initio GW plus cumulant study

We calculate the photoemission spectra of suspended and epitaxial doped graphene using an ab initio cumulant expansion of the Green's function based on the GW self-energy. Our results are compared to experiment and to standard GW calculations. For doped graphene on a silicon carbide substrate, we find, in contrast to earlier calculations, that the spectral function from GW only does not reproduce experimental satellite properties. However, ab initio GW plus cumulant theory combined with an accurate description of the substrate screening results in good agreement with experiment, but gives no plasmaron (i.e., no extra well-defined excitation satisfying Dyson's equation).

cond-mat.mes-hall↗

arXiv2013

Experimentally Engineering the Edge Termination of Graphene Nanoribbons

The edges of graphene nanoribbons (GNRs) have attracted much interest due to their potentially strong influence on GNR electronic and magnetic properties. Here we report the ability to engineer the microscopic edge termination of high quality GNRs via hydrogen plasma etching. Using a combination of high-resolution scanning tunneling microscopy and first-principles calculations, we have determined the exact atomic structure of plasma-etched GNR edges and established the chemical nature of terminating functional groups for zigzag, armchair and chiral edge orientations. We find that the edges of hydrogen-plasma-etched GNRs are generally flat, free of structural reconstructions and are terminated by hydrogen atoms with no rehybridization of the outermost carbon edge atoms. Both zigzag and chiral edges show the presence of edge states.

cond-mat.mes-hall↗

arXiv2012

Spin Polarization of Photoelectrons from 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. 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; a rich set of predicted phenomena have recently been confirmed by spin- and angle-resolved photoemission experiments.

cond-mat.mes-hall↗

arXiv2012

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 excited-state properties that is based on the 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 ground-state properties, including PARATEC, PARSEC, Quantum ESPRESSO, OCTOPUS and SIESTA. 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 10,000's of CPUs and can be used to study systems containing up to 100's of atoms.

cond-mat.mtrl-sci↗

arXiv2012

Coulomb-hole summations and energies for GW calculations with limited number of empty orbitals: a modified static remainder approach

Ab initio GW calculations are a standard method for computing the spectroscopic properties of many materials. The most computationally expensive part in conventional implementations of the method is the generation and summation over the large number of empty orbitals required to converge the electron self energy. We propose a scheme to reduce the summation over empty states by the use of a modified static-remainder approximation, which is simple to implement and yields accurate self energies for both bulk and molecular systems requiring a small fraction of the typical number of empty orbitals.

cond-mat.mtrl-sci↗