arXiv subjects

E. Pehlke

Publications and source records attributed to E. Pehlke.

13 recordsLinked to original sources

arXiv2013

Comment on "Discussion on `Novel attractive force between ions in quantum plasmas -- failure of simulations based on a density functional approach"

In a recent article [P.K. Shukla, B. Eliasson and M. Akbari-Moghanjoughi, Physica Scripta {\bf 87}, 018202 (2013)] the authors criticized our analysis of the screened proton potential in dense hydrogen that was based on {\em ab initio} density functional theory (DFT) simulations [M. Bonitz, E. Pehlke, and T. Schoof, Phys. Rev. E {\bf 87}, 037102 (2013)]. In particular, they attributed the absence of the Shukla-Eliasson attractive force between protons in the DFT simulations to a failure of DFT. Here we discuss in detail their arguments and show that their conclusions are incorrect.

physics.plasm-ph↗

arXiv2013

On "Novel attractive forces" between ions in quantum plasmas -- failure of linearized quantum hydrodynamics

In a recent letter [P.K. Shukla and B. Eliasson, Phys. Rev. Lett. 108, 165007 (2012)] the discovery of a new attractive force between protons in a hydrogen plasma was reported that would be responsible for the formation of molecules and of a proton lattice. Here we show, based on ab initio density functional theory calculations, that these predictions are wrong and caused by using linearized quantum hydrodynamics beyond the limits of its applicability.

physics.plasm-ph↗

arXiv2013

Reply to comment of Shukla et al. on 'On "Novel attractive forces" between ions in quantum plasmas - failure of linearized quantum hydrodynamics'

In an earlier paper we demonstrated that the "novel attractive force" between protons in dense hydrogen reported by Shukla and Eliasson based on linearized quantum hydrodynamics (LQHD) is wrong. As benchmark results we used state of the art density functional theory (DFT) simulations. In their response to that paper Shukla et al. (arXiv:1112.5556) claimed that the disagreement is due to a failure of DFT. Here we show - by comparing the properties and limitations of DFT and QHD - that their statements have no scientific basis and that DFT is a suitable benchmark for more approximate approaches such as QHD and LQHD.

physics.plasm-ph↗

arXiv2007

Unoccupied states of individual silver clusters and chains on Ag(111)

Size-selected silver clusters on Ag(111) were fabricated with the tip of a scanning tunneling microscope. Unoccupied electron resonances give rise to image contrast and spectral features which shift toward the Fermi level with increasing cluster size. Linear assemblies exhibit higher resonance energies than equally sized compact assemblies. Density functional theory calculations reproduce the observed energies and enable an assignment of the resonances to hybridized atomic 5s and 5p orbitals with silver substrate states.

cond-mat.mtrl-sci↗

arXiv2004

Surface-State Localization at Adatoms

Low-temperature scanning tunneling spectroscopy of magnetic and non-magnetic metal atoms on Ag(111) and on Cu(111) surfaces reveals the existence of a common electronic resonance at an energy below the binding energies of the surface states. Using an extended Newns-Anderson model, we assign this resonance to an adsorbate-induced bound state, split off from the bottom of the surface-state band, and broadened by the interaction with bulk states. A lineshape analysis of the bound state indicates that native adatoms decrease the surface-state lifetime, while a cobalt adatom causes no significant change.

cond-mat.str-el↗

arXiv2002

Quantum Monte Carlo calculations of H$_2$ dissociation on Si(001)

We present quantum Monte Carlo calculations for various reaction pathways of H$_2$ with Si(001), using large model clusters of the surface. We obtain reaction energies and energy barriers noticeably higher than those from approximate exchange-correlation functionals. In improvement over previous studies, our adsorption barriers closely agree with experimental data. For desorption, the calculations give barriers for conventional pathways in excess of the presently accepted experimental value, and pinpoint the role of coverage effects and desorption from steps.

cond-mat.mtrl-sci↗

arXiv1999

Equilibrium shapes and energies of coherent strained InP islands

The equilibrium shapes and energies of coherent strained InP islands grown on GaP have been investigated with a hybrid approach that has been previously applied to InAs islands on GaAs. This combines calculations of the surface energies by density functional theory and the bulk deformation energies by continuum elasticity theory. The calculated equilibrium shapes for different chemical environments exhibit the {101}, {111}, {\=1\=1\=1} facets and a (001) top surface. They compare quite well with recent atomic-force microscopy data. Thus in the InP/GaInP-system a considerable equilibration of the individual islands with respect to their shapes can be achieved. We discuss the implications of our results for the Ostwald ripening of the coherent InP islands. In addition we compare strain fields in uncapped and capped islands.

cond-mat.mtrl-sci↗

arXiv1998

Highly site-specific H2 adsorption on vicinal Si(001) surfaces

Experimental and theoretical results for the dissociative adsorption of H_2 on vicinal Si(001) surfaces are presented. Using optical second-harmonic generation, sticking probabilities at the step sites are found to exceed those on the terraces by up to six orders of magnitude. Density functional theory calculations indicate the presence of direct adsorption pathways for monohydride formation but with a dramatically lowered barrier for step adsorption due to an efficient rehybridization of dangling orbitals.

cond-mat↗

arXiv1998

Density-functional study of hydrogen chemisorption on vicinal Si(001) surfaces

Relaxed atomic geometries and chemisorption energies have been calculated for the dissociative adsorption of molecular hydrogen on vicinal Si(001) surfaces. We employ density-functional theory, together with a pseudopotential for Si, and apply the generalized gradient approximation by Perdew and Wang to the exchange-correlation functional. We find the double-atomic-height rebonded D_B step, which is known to be stable on the clean surface, to remain stable on partially hydrogen-covered surfaces. The H atoms preferentially bind to the Si atoms at the rebonded step edge, with a chemisorption energy difference with respect to the terrace sites of >sim 0.1 eV. A surface with rebonded single atomic height S_A and S_B steps gives very similar results. The interaction between H-Si-Si-H mono-hydride units is shown to be unimportant for the calculation of the step-edge hydrogen-occupation. Our results confirm the interpretation and results of the recent H_2 adsorption experiments on vicinal Si surfaces by Raschke and Hoefer described in the preceding paper.

cond-mat↗

arXiv1998

The influence of surface stress on the equilibrium shape of strained quantum dots

The equilibrium shapes of InAs quantum dots (i.e., dislocation-free, strained islands with sizes >= 10,000 atoms) grown on a GaAs (001) substrate are studied using a hybrid approach which combines density functional theory (DFT) calculations of microscopic parameters, surface energies, and surface stresses with elasticity theory for the long-range strain fields and strain relaxations. In particular we report DFT calculations of the surface stresses and analyze the influence of the strain on the surface energies of the various facets of the quantum dot. The surface stresses have been neglected in previous studies. Furthermore, the influence of edge energies on the island shapes is briefly discussed. From the knowledge of the equilibrium shape of these islands, we address the question whether experimentally observed quantum dots correspond to thermal equilibrium structures or if they are a result of the growth kinetics.

cond-mat.mtrl-sci↗

arXiv1996

The Equilibrium Shape of Quantum Dots

The formation of dislocation-free three-dimensional islands during the heteroepitaxial growth of lattice-mismatched materials has been observed experimentally for several material systems. The equilibrium shape of the islands is governed by the competition between the surface energy and the elastic relaxation energy of the islands as compared to the uniform strained film. As an exemplification we consider the experimentally intensively investigated growth of InAs quantum dots on a GaAs(001) substrate, deriving the equilibrium shape as a function of island volume. For this purpose InAs surface energies have been calculated within density-functional theory, and a continuum approach has been applied to compute the elastic relaxation energies.

cond-mat.mtrl-sci↗

arXiv1996

The GaAs Equilibrium Crystal Shape from First-Principles

Surface energies for different GaAs surface orientations have been calculated as a function of the chemical potential. We use an energy density formalism within the first-principles pseudopotential density-functional approach. The equilibrium crystal shape (ECS) has been derived from the surface energies for the (110), (100), (111), and (-1-1-1) orientations. Under As-rich conditions all four considered surface orientations exist in thermodynamic equilibrium, in agreement with experimental observations. Moreover, our calculations allow us to decide on previous contradictory theoretical values for the surface energies of the (111) and (-1-1-1) facets.

mtrl-th↗

arXiv1994

Theory of adsorption and desorption of H_2/Si(001)

While the small sticking coefficient for molecular hydrogen on the Si(001) surface apparently requires a large energy barrier of adsorption, no such barrier is observed in desorption experiments. We have calculated the potential-energy surface of an H_2 molecule in front of a Si(001) surface. If we relax the Si substrate, we find an optimum desorption path with a low (\lesssim 0.3 eV) adsorption energy barrier. While molecules impinging on the surface will mostly be reflected at the larger barrier of some frozen-substrate, molecules adsorbed on the surface can desorb along the low-barrier path.

mtrl-th↗