arXiv subjects

Johannes Lischner

Publications and source records attributed to Johannes Lischner.

At least 91 records · Page 5Linked to original sources

arXiv2018

Core electron binding energies of adsorbates on Cu(111) from first-principles calculations

Core-level X-ray Photoelectron Spectroscopy (XPS) is often used to study the surfaces of heterogeneous copper-based catalysts, but the interpretation of measured spectra, in particular the assignment of peaks to adsorbed species, can be extremely challenging. In this study we demonstrate that first principles calculations using the delta Self Consistent Field (delta-SCF) method can be used to guide the analysis of experimental core-level spectra of complex surfaces relevant to heterogeneous catalysis. Specifically, we calculate core-level binding energy shifts for a series of adsorbates on Cu(111) and show that the resulting C1s and O1s binding energy shifts for adsorbed CO, CO2, C2H4, HCOO, CH3O, H2O, OH and a surface oxide on Cu(111) are in good overall agreement with the experimental literature. In the few cases where the agreement is less good, the theoretical results may indicate the need to re-examine experimental peak assignments.

cond-mat.mtrl-sci↗

arXiv2018

Strong Correlations and d+id Superconductivity in Twisted Bilayer Graphene

We compute the phase diagram of twisted bilayer graphene near the magic angle where the occurrence of flat bands enhances the effects of electron-electron interactions and thus unleashes strongly-correlated phenomena. Most importantly, we find a crossover between d+id superconductivity and Mott insulating behavior near half-filling of the lowest electron band when the temperature is increased. This is consistent with recent experiments. Our results are obtained using unbiased many-body renormalization group techniques combined with a mean-field analysis of the effective couplings.

cond-mat.str-el↗

arXiv2018

Bound States of Charged Adatoms on MoS2: Screening and Multivalley Effects

Adsorbate engineering is a promising route for controlling the electronic properties of monolayer transition-metal dichalcogenide materials. Here, we study shallow bound states induced by charged adatoms on MoS$_2$ using large-scale tight-binding simulations with screened adatom potentials obtained from ab initio calculations. The interplay of unconventional screening in two-dimensional systems and multivalley effects in the transition-metal dichalcogenide (TMDC) band structure results in a rich diversity of bound impurity states. We present results for impurity state wavefunctions and energies, as well as for the local density of states in the vicinity of the adatom which can be measured using scanning tunnelling spectroscopy. We find that the presence of several distinct valleys in the MoS$_2$ band structure gives rise to crossovers of impurity states at critical charge strengths, altering the orbital character of the most strongly bound state. We compare our results to simpler methods, such as the 2D hydrogen atom and effective mass theory, and we discuss limitations of these approaches.

cond-mat.mes-hall↗

arXiv2018

Material, size and environment dependence of plasmon-induced hot carriers in metallic nanoparticles

Harnessing hot electrons and holes resulting from the decay of localized surface plasmons in nanomaterials has recently led to new devices for photovoltaics, photocatalysis and optoelectronics. Properties of hot carriers are highly tunable and in this work we investigate their dependence on the material, size and environment of spherical metallic nanoparticles. In particular, we carry out theoretical calculations of hot carrier generation rates and energy distributions for six different plasmonic materials (Na, K, Al, Cu, Ag and Au). The plasmon decay into hot electron-hole pairs is described via Fermi's Golden Rule using the quasistatic approximation for optical properties and a spherical well potential for the electronic structure. We present results for nanoparticles with diameters up to 40 nm, which are embedded in different dielectric media. We find that small nanoparticles with diameters of 16 nm or less in media with large dielectric constants produce most hot carriers. Among the different materials, Na, K and Au generate most hot carriers. We also investigate hot-carrier induced water splitting and find that simple-metal nanoparticles are useful for initiating the hydrogen evolution reaction, while transition-metal nanoparticles produce dominantly holes for the oxygen evolution reaction.

cond-mat.mtrl-sci↗

arXiv2017

Universal Scaling in Intrinsic Resistivity of Two-Dimensional Metal Borophene

Two-dimensional boron sheets (borophenes) have been successfully synthesized in experiments and are expected to exhibit intriguing transport properties such as the emergence of superconductivity and Dirac Fermions. However, quantitative understanding of intrinsic electrical transport of borophene has not been achieved. Here, we report a comprehensive first-principles study on electron-phonon driven intrinsic electrical resistivity (\r{ho}) of emerging borophene structures. We find that the resistivity is highly dependent on the atomic structures and electron density of borophene. Low-temperature resistivity of borophene \r{ho} exhibits a universal scaling behavior, which increases rapidly with temperature T (\r{ho}~T^4), while \r{ho} increases linearly for a large temperature window T > 100 K. It is observed that this universal behavior of intrinsic resistivity is well described by Bloch-Grünesisen model. Different from graphene and conventional three-dimensional metals, the intrinsic resistivity of borophenes can be easily tuned by adjusting carrier densities while the Bloch-Grünesisen temperature is nearly fixed at ~100 K. Our work suggests monolayer boron can serve as an intriguing platform for realizing high-tunable two-dimensional electronic devices.

physics.comp-ph↗

arXiv2017

Spatially resolving density-dependent screening around a single charged atom in graphene

Electrons in two-dimensional graphene sheets behave as interacting chiral Dirac fermions and have unique screening properties due to their symmetry and reduced dimensionality. By using a combination of scanning tunneling spectroscopy (STM/STS) measurements and theoretical modeling we have characterized how graphene's massless charge carriers screen individual charged calcium atoms. A back-gated graphene device configuration has allowed us to directly visualize how the screening length for this system can be tuned with carrier density. Our results provide insight into electron-impurity and electron-electron interactions in a relativistic setting with important consequences for other graphene-based electronic devices.

cond-mat.mes-hall↗

arXiv2017

First-principles multiscale modelling of charged adsorbates on doped graphene

Adsorbed atoms and molecules play an important role in controlling and tuning the functional properties of two-dimensional (2D) materials. Understanding and predicting this process from theory is challenging because of the need to capture the complex interplay between the local chemistry and the long-range screening response. To address this problem, we present a first-principles multiscale approach that combines linear-scaling density-functional theory, continuum screening theory and large-scale tight-binding simulations into a seamless parameter-free theory of adsorbates on 2D materials. We apply this method to investigate the electronic structure of doped graphene with a single calcium (Ca) adatom and find that the Ca atom acts as a Coulomb impurity which modifies the graphene local density of states (LDOS) within a distance of several nanometres in its vicinity. We also observe an important doping dependence of the graphene LDOS near the Ca atom, which gives insights into electronic screening in graphene. Our multiscale framework opens up the possibility of investigating complex mesoscale adsorbate configurations on 2D materials relevant to real devices.

cond-mat.mes-hall↗

arXiv2016

Dispersion and lineshape of plasmon satellites in one, two and three dimensions

Using state-of-the-art many-body Green's function calculations based on the "GW plus cumulant" approach, we analyze the properties of plasmon satellites in the electron spectral function resulting from electron-plasmon interactions in one-, two- and three-dimensional systems. Specifically, we show how their dispersion relation, lineshape and linewidth are related to the properties of the constituent electrons and plasmons. To gain insight into the many-body processes giving rise to the formation of plasmon satellites, we connect the "GW plus cumulant" approach to a many-body wavefunction picture of electron-plasmon interactions and introduce the coupling-strength weighted electron-plasmon joint-density states as a powerful concept for understanding plasmon satellites.

cond-mat.mes-hall↗

arXiv2016

Accurate determination of the valence band edge in hard x-ray photoemission spectra using GW theory

We introduce a new method for determining accurate values of the valence-band maximum in x-ray photoemission spectra. Specifically, we align the sharpest peak in the valence-band region of the experimental spectrum with the corresponding feature of a theoretical valence-band density of states curve from ab initio GW theory calculations. This method is particularly useful for soft and hard x-ray photoemission studies of materials with a mixture of valence-band characters, where strong matrix element effects can render standard methods for extracting the valence-band maximum unreliable. We apply our method to hydrogen-terminated boron-doped diamond, which is a promising substrate material for novel solar cell devices. By carrying out photoemission experiments with variable light polarizations, we verify the accuracy of our analysis and the general validity of the method.

cond-mat.mtrl-sci↗

arXiv2015

Spectral Functions of the Uniform Electron Gas via Coupled-Cluster Theory and Comparison to the $GW$ and Related Approximations

We use, for the first time, ab initio coupled-cluster theory to compute the spectral function of the uniform electron gas at a Wigner-Seitz radius of $r_\mathrm{s}=4$. The coupled-cluster approximations we employ go significantly beyond the diagrammatic content of state-of-the-art $GW$ theory. We compare our calculations extensively to $GW$ and $GW$-plus-cumulant theory, illustrating the strengths and weaknesses of these methods in capturing the quasiparticle and satellite features of the electron gas. Our accurate calculations further allow us to address the long-standing debate over the occupied bandwidth of metallic sodium. Our findings indicate that the future application of coupled-cluster theory to condensed phase material spectra is highly promising.

cond-mat.mtrl-sci↗

arXiv2015

Satellite Band Structure in Silicon Caused by Electron-Plasmon Coupling

We report the first angle-resolved photoemission measurement of the wave-vector dependent plasmon satellite structure of a three-dimensional solid, crystalline silicon. In sharp contrast to nanomaterials, which typically exhibit strongly wave-vector dependent, low-energy plasmons, the large plasmon energy of silicon facilitates the search for a plasmaron state consisting of resonantly bound holes and plasmons and its distinction from a weakly interacting plasmon-hole pair. Employing a first-principles theory, which is based on a cumulant expansion of the one-electron Green's function and contains significant electron correlation effects, we obtain good agreement with the measured photoemission spectrum for the wave-vector dependent dispersion of the satellite feature, but without observing the existence of plasmarons in the calculations.

cond-mat.mtrl-sci↗

arXiv2015

First-principles theory of electron-spin fluctuation coupling and superconducting instabilities in iron selenide

We present first-principles calculations of the coupling of quasiparticles to spin fluctuations in iron selenide and discuss which types of superconducting instabilities this coupling gives rise to. We find that strong antiferromagnetic stripe-phase spin fluctuations lead to large coupling constants for superconducting gaps with $s_\pm$-symmetry, 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 lead to a value of the coupling constant for an $s_\pm$-symmetric gap which can produce a superconducting transition temperature consistent with experimental measurements.

cond-mat.mtrl-sci↗

arXiv2014

Effects of self-consistency and plasmon-pole models on GW calculations for closed-shell molecules

We present theoretical calculations of quasiparticle energies in closed-shell molecules using the GW method. We compare three different approaches: a full-frequency $G_0W_0$ (FF-$G_0W_0$) method with density functional theory (DFT-PBE) used as a starting mean field; a full-frequency $GW_0$ (FF-$GW_0$) method where the interacting Green's function is approximated by replacing the DFT energies with self-consistent quasiparticle energies or Hartree-Fock energies; and a $G_0W_0$ method with a Hybertsen-Louie generalized plasmon-pole model (HL GPP-$G_0W_0$). While the latter two methods lead to good agreement with experimental ionization potentials and electron affinities for methane, ozone, and beryllium oxide molecules, FF-$G_0W_0$ results can differ by more than one electron volt from experiment. We trace this failure of the FF-$G_0W_0$ method to the occurrence of incorrect self-energy poles describing shake-up processes in the vicinity of the quasiparticle energies.

cond-mat.mtrl-sci↗

arXiv2014

Numerical integration for ab initio many-electron self energy calculations within the GW approximation

We present a numerical integration scheme for evaluating the convolution of a Green's function with a screened Coulomb potential on the real axis in the GW approximation of the self energy. Our scheme takes the zero broadening limit in Green's function first, replaces the numerator of the integrand with a piecewise polynomial approximation, and performs principal value integration on subintervals analytically. We give the error bound of our numerical integration scheme and show by numerical examples that it is more reliable and accurate than the standard quadrature rules such as the composite trapezoidal rule. We also discuss the benefit of using different self energy expressions to perform the numerical convolution at different frequencies.

physics.comp-ph↗

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↗

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↗

arXiv2012

First-principles calculations of the electronic structure of open-shell condensed matter systems

We develop a Green's function approach to quasiparticle excitations of open-shell systems within the GW approximation. It is shown that accurate calculations of the characteristic multiplet structure require a precise knowledge of the self energy and, in particular, its poles. We achieve this by constructing the self energy from appropriately chosen mean-field theories on a fine frequency grid. We apply our method to a two-site Hubbard model, several molecules and the negatively charged nitrogen-vacancy defect in diamond, and obtain good agreement with experiment and other high-level theories.

cond-mat.mtrl-sci↗