arXiv subjects

Herbert Pfnür

Publications and source records attributed to Herbert Pfnür.

5 recordsLinked to original sources

arXiv2025

Ultrafast thermal boundary conductance under large temperature discontinuities of ultrathin epitaxial Pb films on Si(111)

Heat transfer is a critical aspect of modern electronics, and a deeper understanding of the underlying physics is essential for building faster, smaller, and more powerful devices with an improved performance and efficiency. In such nanoscale structures, the heat transfer between two materials is limited by the finite thermal boundary conductance across their interface. Using ultrafast electron diffraction under grazing incidence we investigated the heat transfer from ultrathin epitaxial Pb films to an Si(111) substrate under strong non-equilibrium conditions. Applying an intense femtosecond laser pulse, the 5-7 ML thin Pb film experiences a strong heat up by 10-120 K while the Si substrate remains cold at $\approx$ 10 K. At such large temperature discontinuities we observe a significantly faster cooling for stronger excited Pb films. The decrease of the corresponding cooling time constant is explained through the thermal boundary conductance in the framework of the diffuse mismatch model. The thermal boundary conductance is reduced by more than a factor of three in comparison with Pb films grown on H-terminated substrates, pointing out the importance of the morphology of substrate, heterofilm and their interface.

cond-mat.mes-hall↗

arXiv2016

Two-dimensional crossover and strong coupling of plasmon excitations in arrays of one-dimensional atomic wires

The collective electronic excitations of arrays of Au chains on regularly stepped Si(553) and Si(775) surfaces were studied using electron loss spectroscopy with simultaneous high energy and momentum resolution (ELS-LEED) in combination with low energy electron diffraction (SPA-LEED) and tunneling microscopy. Both surfaces contain a double chain of gold atoms per terrace. Although one-dimensional metallicity and plasmon dispersion is observed only along the wires, two-dimensional effects are important, since plasmon dispersion explicitly depends both on the structural motif of the wires and the terrace width. The electron density on each terrace turns out to be modulated, as seen by tunneling spectroscopy (STS). The effective wire width of 7.5\,Å for Si(553)-Au -- 10.2\,Å for Si(775)-Au -- , determined from plasmon dispersion is in good agreement with STS data. Clear evidence for coupling between wires is seen beyond nearest neighbor coupling.

cond-mat.mes-hall↗

arXiv2011

Organic Molecules on Wide-Gap Insulators: Electronic Excitations

The electronic excitation of a conjugated molecule-insulator interface, as exemplified by the adsorption of benzoic acid and its phenolic derivative on NaCl(001) surface, is addressed by many-body Green's function methods. By solving the two-particle Bethe-Salpeter equation on top of the $GW$ quasiparticle energies, it turns out that instead of the intramolecular $π-π^\ast$ transition of the adsorbate, the lowest singlet excited state of the adsorbate system, being a charge transfer excitonic effect, is essentially assigned to the transition from the surface valence band maximum to the $π-π^\ast$ state of the molecule. An accurate description of this lowest electronic excitations confined at the interface requires the knowledge of a full excitonic Hamiltonian due to the sizable electron-hole exchange interaction.

cond-mat.mes-hall↗

arXiv2010

Color centers in NaCl by hybrid functionals

We present in this work the electronic structure and transition energies (both thermodynamic and optical) of Cl vacancies in NaCl by hybrid density functionals. The underestimated transition energies by the semi-local functional inherited from the band gap problem are recovered by the PBE0 hybrid functional through the non-local exact exchange, whose amount is adjusted to reproduce the experimental band gap. The hybrid functional also gives a better account of the lattice relaxation for the defect systems arising from the reduced self-interaction. On the other hand, the quantitative agreement with experimental vertical transition energy cannot be achieved with hybrid functionals due to the inaccurate descriptions of the ionization energies of the localized defect and the positions of the band edges.

cond-mat.mtrl-sci↗

arXiv2010

Anomalous molecular orbital variation upon adsorption on wide band gap insulator

It is commonly believed that organic molecules are physisorbed on the ideal non-polar surfaces of wide band gap insulators with limited variation of the electronic properties of the adsorbate molecule. On the basis of first principles calculations within density functional theory (DFT) and $GW$ approximation, we show that this is not generally true. We find that the molecular frontier orbitals undergo significant changes when a hydroxy acid (here we chose gluconic acid) is adsorbed on MgSO$_4$$\cdot$H$_2$O(100) surface due to the complex interaction between the molecule and the insulating surface. The predicted trend of the adsorption effect on the energy gap obtained by DFT is reversed when the surface polarization effect is taken into account via the many-body corrections.

cond-mat.mtrl-sci↗