arXiv subjects

Kurt Hingerl

Publications and source records attributed to Kurt Hingerl.

12 recordsLinked to original sources

arXiv2024

Corrections of Electron-Phonon coupling for Second-Order Structural Phase Transitions

Structural phase transitions are accompanied by a movement of one nucleus (or a few) in the crystallographic unit cell. If the nucleus movement is continuous, a second order phase transition without latent heat results, whereas an abrupt nucleus displacement indicates a first order phase transition with accompanying latent heat. In this paper an Hamiltonian including electron-phonon coupling (EPC) as proposed by Kristoffel and Konsel\cite{Kristoffel1973} is taken. Contrary to their treatment, both the kinetic energy of the nucleus and its position are treated. The interaction of the many electron system with the single nucleus is taken into account by the Born-Oppenheimer approximation and perturbative expressions for the free energies are derived. The nuclei corrections due to the entangled electrons are found to be minor, but highlight the importance of the symmetry breaking at low temperature. Furthermore the free energy for a canonical ensemble is computed, whereas Kristoffel and Konsel used a grand canonical ensemble, which allows to derive more stringent bounds on the free energy. For the zero-order nucleus correction the shift of the phase transition temperature by evaluating the free energy is deduced.

cond-mat.stat-mech↗

arXiv2020

Bulk dipole contribution to second harmonic generation in diamond lattices

It is generally argued that material classes with inversion symmetry do not produce bulk dipole related second harmonic generation (SHG). So, SHG is then either ascribed to surface effects or bulk related electric quadrupole or magnetic dipole effects. Using symmetry and \emph{ab-initio} potentials we show analytically that due to the fact of the decaying harmonic electric field certain diamond crystal orientations, as e.g. Si(111), produce a bulk dipole SHG response. For fcc and bcc lattices with a single atom basis, i.e. for the most important metals, however, SHG can purely arise due to the disturbance induced by the surface. Finally we propose an experiment, exploiting the different dispersion for the fundamental as well as frequency doubled radiation to determine this effect.

physics.optics↗

arXiv2020

Analysis of carbon content in direct-write plasmonic Au structures by nanomechanical scanning absorption microscopy

The determination of the chemical content is crucial for the quality control in high-precision device fabrication and advanced process development. For reliable chemical composition characterization, certain interaction volume of the target material is necessary for conventional techniques such as energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS). This remains however a challenge for nanostructures. We hereby propose an alternative technique for measuring chemical composition of nanostructures with limited volume. By measuring the differences in the optical absorption of the nanostructure due to the differences in the chemical composition with the resonance frequency detuning of a nanomechanical resonator as well as the assistance of the analytical optical modelling, we demonstrate the possibility of characterizing the carbon content in the direct-write focused electron beam induced deposition (FEBID) gold nanostructures.

physics.optics↗

arXiv2015

Quasi-particle band structures and optical properties of the "magic sequence" SiGe superstructure

The quasi-particle band structure and dielectric function for the so-called magic sequence SiGe$_2$Si$_2$Ge$_2$SiGe$_{12}$ (or $α_{12}$) structure [PRL 108, 027401 (2012)] are calculated by many-body perturbation theory (MBPT) within an ab-initio framework. On top of density functional calculations within the local density approximation (LDA) leading to a fundamental band gap of 0.23 eV, we have computed the quasi-particle band structure within the G$_0$W$_0$ approach opening the gap to 0.61 eV. Moreover, we have calculated the optical properties by solving the Bethe-Salpeter equation (BSE) for the electron-hole two-particle correlation function. When comparing the imaginary part of the dielectric function obtained at various levels of approximation, \emph{i.e.} the independent particle approximation (or random phase approximation) based on (i) the LDA or (ii) GW band structures, and (iii) the BSE including local field effects and electron-hole correlations, we observe that the important first transition is better explained with taking into account excitonic effects. Moreover, the onset transition originating from the direct transition of the magic sequence structure is also investigated.

cond-mat.mtrl-sci↗

arXiv2014

Strain Enhanced Superconductivity in Li-Doped Graphene

We present a new way to enhance the electron-phonon coupling constant and the critical superconducting temperature of graphene, significantly beyond all reported values. Using density functional theory, we explore the application effects of the tensile biaxial strain on the lithium intercalated graphene. Both effects together, the presence of adatom and the strain, trigger enhancement of critical temperature, up to 300\%, compared to non-strained lithium intercalated graphene.

cond-mat.supr-con↗

arXiv2014

Bond Model and Group Theory of Second Harmonic Generation in GaAs(001)

A comprehensive analysis of second harmonic generation (SHG) in a diatomic zincblende crystal based on the Simplified Bond Hyperpolarizability Model (SBHM), Group Theory (GT) is presented. The third rank tensor between the two approaches are compared and applied to reproduce rotational anisotropy (RA) SHG experimental result.It is well known that such a crystal is noncentrosymmetric, therefore the second harmonic generation source is dominated by bulk dipoles with a SHG polarization $P_{BD}=χ_{ijk}^{(2)}E(ω)E(ω)$. We show that the SBHM previously applied to mono crystal system such as Silicon can also be applied to an atomic cell containing two different atoms e.g. GaAs by introducing an effective hyperpolarizability $α_{2GaAs}=\left(α_{2Ga}-α_{2As}\right)$ . Comparison with GT yields the same third rank tensor for a $T_d$ point group corresponding to bulk tetrahedral structure. Interestingly, SBHM gives good agreement with RASHG experiment of GaAs using only one single fitting parameter when Fresnel analysis is also incorporated in the model.

cond-mat.mtrl-sci↗

arXiv2014

Simplified Bond Hyperpolarizability Model of Second Harmonic Generation, Group Theory and Neumann's Principle

We discuss the susceptibility third-rank tensor for second harmonic and sumfrequency generation, associated with low index surfaces of silicon (Si(001), Si(011) and Si(111)) from two different approaches: the Simplified Bond-Hyperpolarizablility Model(SBHM) and Group Theory (GT). We show that SBHM agrees very well with the experimental results for simple surfaces because the definitions of the bond vectors implicitly include the geometry of the crystal and therefore the symmetry group. However, for more complex surfaces it is shown that one can derive from GT the SBHM tensor, if Kleinman symmetry is allowed.

cond-mat.mtrl-sci↗

arXiv2013

Simplified bond-hyperpolarizability model of second-harmonic-generation in Si(111): theory and experiment

Second-harmonic-generation (SHG) in centrosymmetric material such as Si(111) is usually understood either from the phenomenology theory or more recently using the Simplified Bond-Hyperpolarizability Model (SBHM) [G. D. Powell, J. F. Wang, and D. E. Aspnes, Phys. Rev. B 65, 205320/1 (2002)]. Although SBHM is derived from a classical point of view, it has the advantage over the former that it gives-especially for lower symmetry systems- a clear physical picture and a more efficient explanation of how nonlinearity is generated. In this paper we provide a step-by-step description of the SBHM in Si(111) for the linear and second harmonic case. We present a SHG experiment of Si(111) and show how it can be modelled by summing up the contribution of the fields produced by the anharmonic motion along the bonds.

cond-mat.mtrl-sci↗

arXiv2010

GHZ and W Entanglement Witnesses for the Non-interacting Fermi gas

The existence and nature of tripartite entanglement of a noninteracting Fermi gas (NIFG) is investigated. Three new classes of parameterized entanglement witnesses (EWs) are introduced with the aim of detecting genuine tripartite entanglement in the three-body reduced density matrix and discriminating between the presence of the two types of genuine tripartite entanglement, W\B and GHZ\W. By choosing appropriate EW operators, the problem of finding GHZ and W EWs is reduced to linear programming. Specifically, we devise new W EWs based on a spin-chain model with periodic boundary conditions, and we construct a class of parametrized GHZ EWs by linearly combining projection operators corresponding to all the different state-vector types arising for a three-fermion system. A third class of EWs is provided by a GHZ stabilizer operator capable of distinguishing W\B from GHZ\B entanglement, which is not possible with $W$ EWs. Implementing these classes of EWs, it is found that all states containing genuine tripartite entanglement are of W type, and hence states containing GHZ\W genuine tripartite entanglement do not arise. Some genuine tripartite entangled states that have a positive partial transpose (PPT) with respect to some bipartition are detected. Finally, it is demonstrated that a NIFG does not exhibit "pure" W\B genuine tripartite entanglement: three-party entanglement without any separable or biseparable admixture does not occur.

quant-ph↗

arXiv2009

Coordinate transformation based design of confined metamaterial structures

The coordinate transformation method is applied to bounded domains to design metamaterial devices for steering spatially confined electromagnetic fields. Both waveguide and free-space beam applications are considered as these are analogous within the present approach. In particular, we describe devices that bend the propagation direction and squeeze confined electromagnetic fields. Two approaches in non-magnetic realization of these structures are examined. The first is based on using a reduced set of material parameters, and the second on finding non-magnetic transformation media. It is shown that transverse-magnetic fields can be bent or squeezed to an arbitrary extent and without reflection using only dielectric structures.

physics.optics↗

arXiv2006

All-angle left-handed negative refraction in Kagome and honeycomb lattice photonic crystals

Possibilities of all-angle left-handed negative refraction in 2D honeycomb and Kagome lattices made of dielectric rods in air are discussed for the refractive indices 3.1 and 3.6. In contrast to triangular lattice photonic crystals made of rods in air, both the honeycomb and Kagome lattices show all-angle left-handed negative refraction in the case of the TM2 band for low normalized frequencies. Certain advantages of the honeycomb and Kagome structures over the triangular lattice are emphasized. This specially concerns the honeycomb lattice with its circle-like equifrequency contours where the effective indices are close to -1 for a wide range of incident angles and frequencies.

cond-mat.mtrl-sci↗

arXiv2006

Refraction and rightness in photonic crystals

We present a study on relation between the refraction and rightness effects in photonic crystals applied on a 2D square lattice photonic crystal. The plane wave (the band and equifrequency contour analyses) and FDTD calculations for both TM and TE modes revealed all possible refraction and rightness cases in photonic crystal structures in the first three bands. In particular, we show for the first time, a possibility of the left-handed positive refraction. This means that left-handedness does not necessarily imply negative refraction in photonic crystals.

cond-mat.mtrl-sci↗