Ludger Wirtz

University of Luxembourg

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Alejandro Molina‐Sánchez

Instituto de Ciencia de Materiales de Sevilla

Joachim Burgdörfer

TU Wien

ÁR

Ángel Rubio

Max Planck Institute for the Structure and Dynamics of Matter

Henrique Miranda

UCLouvain

Andrea Marini

Institute of Structure of Matter

Claudio Attaccalite

Aix-Marseille Université

Christoph Stampfer

RWTH Aachen University

Sven Reichardt

University of Luxembourg

Francesco Mauri

Sapienza University of Rome

C. Lemell

TU Wien

Cooperative Institutions

Centre National de la Recherche Scientifique

University of Luxembourg

TU Wien

University of Cambridge

University of Vienna

RWTH Aachen University

Commissariat à l'Énergie Atomique et aux Énergies Alternatives

Institut d'électronique de microélectronique et de nanotechnologie

European Theoretical Spectroscopy Facility

National Institute for Materials Science

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Spatially Resolved Raman Spectroscopy of Single- and Few-Layer Graphene

Nano Letters (2007)

David Graf F. Molitor K. Ensslin Christoph Stampfer A. Jungen

We present Raman spectroscopy measurements on single- and few-layer graphene flakes. Using a scanning confocal approach we collect spectral data with spatial resolution, which allows us to directly compare Raman images with scanning force micrographs. Single-layer graphene can be distinguished from double- and few-layer by the width of the D' line: the single peak for single-layer graphene splits into different peaks for the double-layer. These findings are explained using the double-resonant Raman model based on ab-initio calculations of the electronic structure and of the phonon dispersion. We investigate the D line intensity and find no defects within the flake. A finite D line response originating from the edges can be attributed either to defects or to the breakdown of translational symmetry.

Line (geometry)

10.1021/nl061702a

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Citations (2,119)

Excitation-intensity dependence of shallow and deep-level photoluminescence transitions in semiconductors

Journal of Applied Physics (2019)

Conrad Spindler Thomas Galvani Ludger Wirtz Germain Rey Susanne Siebentritt

Photoluminescence characterization of semiconductors is a powerful tool for studying shallow and deep defects. Excitation-intensity-dependent measurements at low temperatures are typically analyzed to distinguish between exciton and defect related transitions. We have extended existing models based on rate equations to include the contribution of deep defects. Generally, it is observed that the photoluminescence intensity IPL follows a power law IPL∝ϕk with the excitation intensity ϕ. We show that the exponent k takes on values of multiples of 1/2. The values depend on the availability of additional recombination channels. Defect levels can saturate at high enough excitation intensities, leading to one or several crossover points from one power law behavior to another. Power law exponents different from n/2 can result from the transition region between two limiting cases of linear power laws. Model functions for the analytical description of these transitional excitation dependencies are derived and the analysis is applied to chalcopyrite thin films and to numerical data. The saturation effects of defects by excess carriers as well as the influence of deep recombination centers can be extracted with the help of the presented model, which extends existing theories.

Saturation (graph theory)

Intensity

Exponent

Rate equation

Limiting

10.1063/1.5095235

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Citations (57)

Gauge-invariant theory for semiclassical magnetotransport through ballistic microstructures

Physical review. B, Condensed matter (1999)

Ludger Wirtz Jian-Zhi Tang Joachim Burgdörfer

Within the semiclassical theory of magnetotransport through ballistic cavities, fluctuations in the transmission amplitude and in the conductance originate from the Aharonov-Bohm phase of directed areas. We formulate the semiclassical transmission amplitude in gauge-invariant form. The gauge invariant phases can be visualized in terms of areas enclosed by classical paths, which consist of the real path connecting the entrance point to the exit point and a virtual path leading back to the entrance point. We implement this method on different levels of a semiclassical description of magnetotransport with applications to magnetoconductance fluctuations and correlations. The validity of the semiclassical theories is analyzed.

Semiclassical physics

10.1103/physrevb.59.2956

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Citations (18)

Fast-atom diffraction at surfaces

Journal of Physics Conference Series (2009)

Florian Aigner N. S. Simonović B. Solleder Ludger Wirtz Joachim Burgdörfer

Fast helium atoms diffracted at alkali-halide surfaces under grazing angles of incidence exhibit intriguing diffraction patterns. The persistence of quantum coherence is remarkable, considering high surface temperatures and high (keV) kinetic energies of the incident atoms. Dissipative and decohering effects such as the momentum transfer between the incident helium atoms and the surface influence the diffraction patterns and control the width of the diffraction peaks, but they are weak enough to preserve the visibility of the diffration patterns. We perform an ab initio simulation of the quantum diffraction of fast helium beams at a LiF (100) surface in the (110) direction. Our results agree well with recent experimental diffraction data.

10.1088/1742-6596/194/1/012057

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Citations (4)

Ab‐initio Calculations of Charge Exchange in Ion–surface Collisions: An Embedde–cluster Approach

Ludger Wirtz Michal Dallos Hans Lischka Joachim Burgdörfer

This chapter contains sections titled: Introduction Convergence of the Density of States as a Function of Cluster Size Going beyond Hartree–Fock Convergence of Potential Energy Curves as a Function of Cluster Size Conclusions References

Charge exchange

Charge density

10.1002/3527603425.ch10

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Citations (1)

Anisotropic excitonic effects in the energy loss function of hexagonal boron nitride

Physical Review B (2011)

S. Galambosi Ludger Wirtz J. A. Soininen J. Serrano Andrea Marini

We demonstrate that the valence energy-loss function of hexagonal boron nitride (hBN) displays a strong anisotropy in shape, excitation energy and dispersion for momentum transfer q parallel or perpendicular to the hBN layers. This is manifested by e.g. an energy shift of 0.7 eV that cannot be captured by single-particle approaches and is a demonstration of a strong anisotropy in the two-body electron-hole interaction. Furthermore, for in-plane directions of q we observe a splitting of the -plasmon in the M direction that is absent in the K direction and this can be traced back to band-structure effects.

Hexagonal boron nitride

10.1103/physrevb.83.081413

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Citations (37)

Nonadiabatic exciton-phonon coupling in Raman spectroscopy of layered materials

Science Advances (2020)

Sven Reichardt Ludger Wirtz

We develop a new computational approach to resonant Raman scattering to study nonadiabatic exciton-phonon scattering.

10.1126/sciadv.abb5915

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Citations (14)

Optical absorption and electron energy loss spectra of carbon and boron nitride nanotubes: a first-principles approach

Applied Physics A (2004)

A. G. Marinopoulos Ludger Wirtz Andrea Marini Valério Olevano Ángel Rubio

10.1007/s00339-003-2467-z

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Citations (115)

Liouville master equation for multielectron dynamics: Neutralization of highly charged ions near a LiF surface

Physical Review A (2003)

Ludger Wirtz C. O. Reinhold C. Lemell Joachim Burgdörfer

We present a simulation of the neutralization of highly charged ions in front of a lithium fluoride surface including the close-collision regime above the surface. The present approach employs a Monte Carlo solution of the Liouville master equation for the joint probability density of the ionic motion and the electronic population of the projectile and the target surface. It includes single as well as double particle-hole (de)excitation processes and incorporates electron correlation effects through the conditional dynamics of population strings. The input in terms of elementary one- and two-electron transfer rates is determined from classical trajectory Monte Carlo calculations as well as quantum-mechanical Auger calculations. For slow projectiles and normal incidence, the ionic motion depends sensitively on the interplay between image acceleration towards the surface and repulsion by an ensemble of positive hole charges in the surface (``trampoline effect''). For ${\mathrm{Ne}}^{10+}$ we find that image acceleration is dominant and no collective backscattering high above the surface takes place. For grazing incidence, our simulation delineates the pathways to complete neutralization. In accordance with recent experimental observations, most ions are reflected as neutral or even as singly charged negative particles, irrespective of the charge state of the incoming ions.

10.1103/physreva.67.012903

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Citations (31)

Electronic structure and radial breathing mode for carbon nanotubes with ultra‐high curvature

physica status solidi (b) (2010)

W. Plank R. Pfeiffer C. Scharman Ludger Wirtz Matteo Calvaresi

Abstract Ultra‐high curvature single‐walled carbon nanotubes (SWCNTs) with diameters down to 0.37 nm were prepared by transformation of FeCp 2 peapods to double‐walled CNTs (DWCNTs). Results from resonance Raman scattering and high resolution transmission electron microscopy (TEM) were compared to calculations on the molecular dynamical, many electron corrected extended tight binding, and density functional theory (DFT) level. The growth process was found to be catalytic from Fe 3 C particles inside the tubes with dimensions of a crystallographic unit cell. The electronic structure showed dramatic deviations from tight binding results. The family behavior leads to level crossing already for $E_{11} $ and $E_{22} $ transitions. Experimental results fitted well into a Kataura–Popov plot and allowed assignment for the observed Raman lines of the inner tubes. Experimental and calculated radial breathing mode (RBM) frequencies showed a systematic difference indicating a radial expansion of the smallest inner tubes and a radial compression for the larger tubes.

Tight binding

10.1002/pssb.201000357

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Citations (5)