arXiv subjects

Cornelius Krellner

Publications and source records attributed to Cornelius Krellner.

76 records · Page 5Linked to original sources

arXiv2010

Interplay between Co-3d and Ce-4f magnetism in CeCoAsO

We have investigated the ground state properties of polycrystalline CeCoAsO by means of magnetization, specific heat and solid state NMR. Susceptibility and specific-heat measurements suggest a ferromagnetic order at about, $T_\mathrm{C}$=75 K. No further transitions are found down to 2 K. At 6.5 K a complex Schottky type of anomaly shows up in the specific heat results. The interplay between Ce-4f and Co-3d magnetism being responsible for that anomaly is discussed. Furthermore $^{75}$As NMR investigations have been performed to probe the magnetism on a microscopic scale. As-NMR spectra are analysed in terms of first and second order quadrupolar interaction. The anisotropic shift component $K_{\mathrm{ab}}$ and $K_{\mathrm{c}}$ could be derived from the $^{75}$As powder spectra. Towards lower temperature a strong shift anisotropy was found. Nonetheless $K_{\mathrm{iso}}$ tracks the bulk susceptibility down to $T=$50 K very well. Furthermore the presence of weak correlations among the Ce ions in the ferromagnetic state is discussed. The observed increase of $C/T$ towards lower temperatures supports this interpretation.

cond-mat.str-el↗

arXiv2010

The trap DOS in small molecule organic semiconductors: A quantitative comparison of thin-film transistors with single crystals

We show that it is possible to reach one of the ultimate goals of organic electronics: producing organic field-effect transistors with trap densities as low as in the bulk of single crystals. We studied the spectral density of localized states in the band gap (trap DOS) of small molecule organic semiconductors as derived from electrical characteristics of organic field-effect transistors or from space-charge-limited-current measurements. This was done by comparing data from a large number of samples including thin-film transistors (TFT's), single crystal field-effect transistors (SC-FET's) and bulk samples. The compilation of all data strongly suggests that structural defects associated with grain boundaries are the main cause of "fast" hole traps in TFT's made with vacuum-evaporated pentacene. For high-performance transistors made with small molecule semiconductors such as rubrene it is essential to reduce the dipolar disorder caused by water adsorbed on the gate dielectric surface. In samples with very low trap densities, we sometimes observe a steep increase of the trap DOS very close (<0.15 eV) to the mobility edge with a characteristic slope of 10-20 meV. It is discussed to what degree band broadening due to the thermal fluctuation of the intermolecular transfer integral is reflected in this steep increase of the trap DOS. Moreover, we show that the trap DOS in TFT's with small molecule semiconductors is very similar to the trap DOS in hydrogenated amorphous silicon even though polycrystalline films of small molecules with van der Waals-type interaction on the one hand are compared with covalently bound amorphous silicon on the other hand.

cond-mat.mtrl-sci↗

arXiv2009

Quantum criticality in Yb(Rh0.97Co0.03)2Si2 probed by low-temperature resistivity

Quantum criticality in Yb(Rh0.97Co0.03)2Si2 is investigated by means of resistivity and magnetoresistance. The partial substitution of Co leads to a stabilization of the magnetism as expected according to the application of chemical pressure for Yb systems. However, the signature of the Kondo-breakdown remains at the same position in the temperature-magnetic field phase diagram compared to stoichiometric YbRh2Si2. As a consequence, the Kondo-breakdown is situated within the antiferromagnetic phase. These results fit well within the global phase diagram under chemical pressure [1].

cond-mat.str-el↗

arXiv2008

Relevance of ferromagnetic correlations for the Electron Spin Resonance in Kondo lattice systems

Electron Spin Resonance (ESR) measurements of the ferromagnetic Kondo lattice system CeRuPO show a well defined ESR signal which is related to the magnetic properties of the Ce3+ moment. In contrast, no ESR signal could be observed in the antiferromagnetic homologue CeOsPO. Additionally, we detect an ESR signal in a further ferromagnetic Yb compound, YbRh, while it was absent in a number of Ce or Yb intermetallic compounds with dominant antiferromagnetic exchange, independently of the presence of a strong Kondo interaction or the proximity to a (quantum) critical point. Thus, the observation of an ESR signal in a Kondo lattice is neither specific to Yb nor to the proximity of a quantum critical point, but seems to be connected to the presence of ferromagnetic fluctuations. These conclusions not only provide a basic concept to understand the ESR in Kondo lattice systems even well below the Kondo temperature as observed in the heavy fermion metal YbRh2Si2 but point out ESR as a prime method to investigate directly the spin dynamics of the Kondo ion.

cond-mat.str-el↗