arXiv subjects

C. Krellner

Publications and source records attributed to C. Krellner.

81 records · Page 5Linked to original sources

arXiv2008

Magnetic and structural transitions in layered FeAs systems: AFe2As2 versus RFeAsO compounds

Resistivity, specific heat and magnetic susceptibility measurements performed on SrFe2As2 samples evidence a behavior very similar to that observed in LaFeAsO and BaFe2As2 with the difference that the formation of the SDW and the lattice deformation occur in a pronounced first order transition at T_0=205K. Comparing further data evidences that the Fe-magnetism is stronger in SrFe2As2 and in EuFe2As2 than in the other layered FeAs systems investigated up to now. Full potential LDA band structure calculations confirm the large similarity between the compounds, especially for the relevant low energy Fe 3d states. The relation between structural details and magnetic order is analyzed.

cond-mat.supr-con↗

arXiv2007

Single crystal growth and anisotropy of CeRuPO

We report on the single crystal growth of the ferromagnetic Kondo lattice system CeRuPO using a Sn flux method. Magnetic susceptibility and electrical resistivity measurements indicate strong anisotropy of this structurally layered compound. They evidence that the magnetic moments order ferromagnetically along the c-direction of the tetragonal unit cell, whereas the crystal electric field (CEF) anisotropy favors the ab-plane. Therefore, CeRuPO presents the unusual case within rare earth systems, where the anisotropy of the interionic exchange interaction overcomes the single ion anisotropy due to the CEF interaction.

cond-mat.str-el↗

arXiv2007

CeRuPO: A rare example of a Ferromagnetic Kondo lattice

We have determined the physical ground state properties of the compounds CeRuPO and CeOsPO by means of magnetic susceptibility chi(T), specific heat C(T), electrical resistivity rho(T), and thermopower S(T) measurements. chi(T) reveals a trivalent 4f1 cerium state in both compounds. For CeRuPO a pronounced decrease of rho(T) below 50K indicates the onset of coherent Kondo scattering which is confirmed by enhanced S(T). The temperature and magnetic field dependence of chi(T) and C(T) evidence ferromagnetic (FM) order at TC=15K. Thus, CeRuPO seems to be one of the rare example of a FM Kondo lattice. In contrast, CeOsPO shows antiferromagnetic order at TN=4.4K despite only minor changes in lattice parameters and electronic configuration. Additional 31P NMR results support these scenarios. LSDA+U calculations evidence a quasi two dimensional electronic band structure, reflecting a strong covalent bonding within the CeO and RuP layers and a weak ionic like bonding between the layers.

cond-mat.str-el↗

arXiv2007

Density of bulk trap states in organic semiconductor crystals: discrete levels induced by oxygen in rubrene

The density of trap states in the bandgap of semiconducting organic single crystals has been measured quantitatively and with high energy resolution by means of the experimental method of temperature-dependent space-charge-limited-current spectroscopy (TD-SCLC). This spectroscopy has been applied to study bulk rubrene single crystals, which are shown by this technique to be of high chemical and structural quality. A density of deep trap states as low as ~ 10^{15} cm^{-3} is measured in the purest crystals, and the exponentially varying shallow trap density near the band edge could be identified (1 decade in the density of states per ~25 meV). Furthermore, we have induced and spectroscopically identified an oxygen related sharp hole bulk trap state at 0.27 eV above the valence band.

cond-mat.mtrl-sci↗

arXiv2007

ESR of YbRh2Si2 and 174YbRh2Si2 : local and itinerant properties

Below the Kondo temperature the heavy Fermion compound YbRh$_{2}$Si$_{2}$ shows a well defined Electron Spin Resonance (ESR) with local Yb$^{3+}$ properties. We report a detailed analysis of the ESR intensity which gives information on the number of ESR active centers relative to the ESR of well localized Yb$^{3+}$ in YPd$_3$:Yb. The ESR lineshape is investigated regarding contributions from itinerant centers. From the ESR of monoisotopic $^{174}$YbRh$_{2}$Si$_{2}$ we could exclude unresolved hyperfine contributions to the lineshape.

cond-mat.str-el↗

arXiv2007

Multiple energy scales at a quantum critical point

We report thermodynamic measurements in a magnetic-field-driven quantum critical point of a heavy fermion metal, YbRh$_2$Si$_2$. The data provide evidence for an energy scale in the equilibrium excitation spectrum, that is in addition to the one expected from the slow fluctuations of the order parameter. Both energy scales approach zero as the quantum critical point is reached, thereby providing evidence for a new class of quantum criticality.

cond-mat.str-el↗

arXiv2006

Observation of an optical non-Fermi-liquid behavior in the heavy fermion state of YbRh$_{2}$Si$_{2}$

We report far-infrared optical properties of YbRh$_{2}$Si$_{2}$ for photon energies down to 2 meV and temperatures 0.4 -- 300 K. In the coherent heavy quasiparticle state, a linear dependence of the low-energy scattering rate on both temperature and photon energy was found. We relate this distinct dynamical behavior different from that of Fermi liquid materials to the non-Fermi liquid nature of YbRh$_{2}$Si$_{2}$ which is due to its close vicinity to an antiferromagnetic quantum critical point.

cond-mat.str-el↗

arXiv2004

Effects of polarized organosilane self-assembled monolayers on organic single-crystal field-effect transistors

The surface conductivity is measured by a four-probe technique for pentacene and rubrene single-crystals laminated on polarized and nearly unpolarized molecular monolayers with application of perpendicular electric fields. The polarization of the self-assembled monolayers (SAMs) shifts the threshold gate voltage, while maintaining a very low subthreshold swing of the single-crystal devices (0.11 V/decade). The results, excluding influences of parasitic contacts and grain boundaries, demonstrate SAM-induced nanoscale charge injection up to ~10^12 cm^-2 at the surface of the organic single crystals.

cond-mat.mtrl-sci↗

arXiv2004

Hole mobility in organic single crystals measured by a "flip-crystal" field-effect technique

We report on single crystal high mobility organic field-effect transistors (OFETs) prepared on prefabricated substrates using a "flip-crystal" approach. This method minimizes crystal handling and avoids direct processing of the crystal that may degrade the FET electrical characteristics. A chemical treatment process for the substrate ensures a reproducible device quality. With limited purification of the starting materials, hole mobilities of 10.7, 1.3, and 1.4 cm^2/Vs have been measured on rubrene, tetracene, and pentacene single crystals, respectively. Four-terminal measurements allow for the extraction of the "intrinsic" transistor channel resistance and the parasitic series contact resistances. The technique employed in this study shows potential as a general method for studying charge transport in field-accumulated carrier channels near the surface of organic single crystals.

cond-mat.other↗