Showing papers by "M. Knupfer published in 2013"

Anisotropic Eliashberg function and electron-phonon coupling in doped graphene

Abstract: We investigate, with high-resolution angle-resolved photoemission spectroscopy, the spectral function of potassium-doped quasi-free-standing graphene on Au. Angle-dependent x-ray photoemission and density functional theory calculations demonstrate that potassium intercalates into the graphene/Au interface, leading to an upshift of the K-derived electronic band above the Fermi level. This empty band is what makes this system perfectly suited to disentangle the contributions to electron-phonon coupling coming from the pi band and K-derived bands. From a self-energy analysis we find an anisotropic electron-phonon coupling strength lambda of 0.1 (0.2) for the K Gamma (K M) high-symmetry directions in momentum space, respectively. Interestingly, the high-energy part of the Eliashberg function which relates to graphene's optical phonons is equal in both directions but only in K M does an additional low-energy part appear. (Less)

Influence of film thickness and air exposure on the transport gap of manganese phthalocyanine

Abstract: The interface formation between manganese phthalocyanine (MnPc) and cobalt was investigated combining ultraviolet photoelectron spectroscopy and inverse photoelectron spectroscopy. The transport band gap of the MnPc increases with the film thickness up to a value of (1.2 ± 0.3) eV while the optical band gap as determined from spectroscopic ellipsometry amounts to 0.5 eV. The gap values are smaller compared to other phthalocyanines due to metallic Mn 3d states close to the Fermi level. The transport band gap was found to open upon air exposure as a result of the disappearance of the occupied 3d electronic states.

Loss spectroscopy of molecular solids: combining experiment and theory

Abstract: The nature of the lowest-energy electronic excitations in prototyp- ical molecular solids is studied here in detail by combining electron energy loss spectroscopy (EELS) experiments and state-of-the-art many-body calcula- tions based on the Bethe-Salpeter equation. From a detailed comparison of the spectra in picene, coronene and tetracene we generally find a good agreement 8 Authors to whom any correspondence should be addressed. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Doping dependence of the plasmon dispersion in 2 H -TaSe 2

Abstract: Inelastic electron scattering is applied to investigate the impact of potassium intercalation on the charge-carrier plasmon energy and dispersion in the charge- density wave (CDW) bearing compound 2H-TaSe2. We observe an unususal doping dependence of the plasmon dispersion, which even changes sign on alkali addition. In contrast to the continous energy shift of the plasmon position on doping at lowest momentum transfer, its dispersion changes in a rather discontinuous manner. We argue that the observed dynamics can be explained only in a picture, where complex phenomena are taken into account, including the suppression of the CDW on doping as well as the interplay of the CDW and the plasma resonance.

Doping Dependence of the Plasmon Dispersion in 2H-TaSe2

Abstract: Inelastic electron scattering is applied to investigate the impact of potassium intercalation on the charge-carrier plasmon energy and dispersion in the charge-density wave (CDW) bearing compound 2H-TaSe2. We observe an unususal doping dependence of the plasmon dispersion, which even changes sign on alkali addition. In contrast to the continous energy shift of the plasmon position on doping at lowest momentum transfer, its dispersion changes in a rather discontinuous manner. We argue that the observed dynamics can be explained only in a picture, where complex phenomena are taken into account including the suppression of the CDW on doping as well as the interplay of the CDW and the plasma resonance.

Organic [6,6]-phenyl-C61-butyric-acid-methyl-ester field effect transistors: Analysis of the contact properties by combined photoemission spectroscopy and electrical measurements

Abstract: Carrier injection barriers determined by photoemission spectroscopy for organic/metal interfaces are widely accepted to determine the performance of organic field-effect transistors (OFET), which strongly depends on this interface at the source/drain contacts. This assumption is checked here in detail, and a more sophisticated connection is presented. According to the preparation process described in our recently published article [S. Scheinert, J. Appl. Phys. 111, 064502 (2012)], we prepared PCBM/Au and PCBM/Al samples to characterize the interface by photoemission and electrical measurements of PCBM based OFETs with bottom and top (TOC) contacts, respectively. The larger drain currents for TOC OFETs indicate the presence of Schottky contacts at source/drain for both metals. The hole injection barrier as determined by photoemission is 1.8 eV for both Al and Au. Therefore, the electron injection barriers are also the same. In contrast, the drain currents are orders of magnitude larger for the transistors ...

π and π + σ plasmon localization in single-walled carbon nanotube meta-materials.

Abstract: We have investigated the plasmon excitations and interband transitions in nematic nanotube meta-materials with different nanotube mean diameters. While the characteristic optical low-energy transitions give testimony to the differing mean diameters, the full momentum resolved π as well as π + σ plasmon response of the meta-materials is found to be entirely invariant with the nanotube diameters 1 nm < d < 2 nm. We hence demonstrate that plasmons on single-walled carbon nanotubes are fully channeled to be one dimensional in the typical range of nanotube diameters. The internal morphology of the meta-material offers a knob to uniformly tune the plasmon resonances.

Band dependent emergence of heavy quasiparticles in CeCoIn5

Abstract: We investigate the low temperature (T $<$ 2 K) electronic structure of the heavy fermion superconductor CeCoIn5 (T$_c$ = 2.3 K) by angle-resolved photoemission spectroscopy (ARPES). The hybridization between conduction electrons and f-electrons, which ultimately leads to the emergence of heavy quasiparticles responsible for the various unusual properties of such materials, is directly monitored and shown to be strongly band dependent. In particular the most two-dimensional band is found to be the least hybridized one. A simplified multiband version of the Periodic Anderson Model (PAM) is used to describe the data, resulting in semi-quantitative agreement with previous bulk sensitive results from de-Haas-van-Alphen measurements.