Spin wave excitations: The main source of the temperature dependence of Interlayer exchange coupling in nanostructures

Quantum mechanical calculations based on an extended Heisenberg model are compared with ferromagnetic resonance (FMR) experiments on prototype trilayer systems Ni_7/Cu_n/Co_2/Cu(001) in order to determine and separate for the first time quantitatively the sources of the temperature dependence of interlayer exchange coupling. Magnon excitations are responsible for about 75% of the reduction of the coupling strength from zero to room temperature. The remaining 25% are due to temperature effects in the effective quantum well and the spacer/magnet interfaces.Comment: accepted for publication in PR

Influence of Membrane-Fusogen Distance on the Secondary Structure of Fusogenic Coiled Coil Peptides

Liposomal membrane fusion is an important tool to study complex biological fusion mechanisms. We use lipidated derivatives of the specific heterodimeric coiled coil pair E: (EIAALEK)3 and K: (KIAALKE)3 to study and control the fusion of liposomes. In this model system, peptides are tethered to their liposomes via a poly(ethylene glycol) (PEG) spacer and a lipid anchor. The efficiency of the fusion mechanism and function of the peptides is highly affected by the PEG-spacer length and the lipid anchor type. Here, the influence of membrane–fusogen distance on the peptide–membrane interactions and the peptide secondary structures is studied with Langmuir film balance and infrared reflection absorption spectroscopy. We found that the introduction of a spacer to monolayer-tethered peptide E changes its conformation from solvated random coils to homo-oligomers. In contrast, the described peptide–monolayer interaction of peptide K is not affected by the PEG-spacer length. Furthermore, the coexistence of different conformations when both lipopeptides E and K are present at the membrane surface is demonstrated empirically, which has many implications for the design of effective fusogenic recognition units and the field of artificial membrane fusion.Article / Letter to editorLIC/CB/Supramolecular & Biomat. Chem

Non-collapsing renormalized QRPA with proton-neutron pairing for neutrinoless double beta decay

Using the renormalized quasiparticle random phase approximation (RQRPA), we calculate the light neutrino mass mediated mode of neutrinoless double beta decay of Ge76, Mo100, Te128 and Te130. Our results indicate that the simple quasiboson approximation is not good enough to study the neutrinoless double beta decay, because its solutions collapse for physical values of g_pp. We find that extension of the Hilbert space and inclusion of the Pauli Principle in the QRPA with proton-neutron pairing, allows us to extend our calculations beyond the point of collapse, for physical values of the nuclear force strength. As a consequence one might be able to extract more accurate values on the effective neutrino mass by using the best available experimental limits on the half-life of neutrinoless double beta decay.Comment: 15 pages, RevTex, 2 Postscript figures, to appear in Phys. Lett.

X-ray absorption spectroscopy on layered cobaltates Na_xCoO_2

Measurements of polarization and temperature dependent soft x-ray absorption have been performed on Na_xCoO_2 single crystals with x=0.4 and x=0.6. They show a deviation of the local trigonal symmetry of the CoO_6 octahedra, which is temperature independent in a temperature range between 25 K and 372 K. This deviation was found to be different for Co^{3+} and Co^{4+} sites. With the help of a cluster calculation we are able to interpret the Co L_{23}-edge absorption spectrum and find a doping dependent energy splitting between the t_{2g} and the e_g levels (10Dq) in Na_xCoO_2.Comment: 7 pages, 8 figure

Correlation and surface effects in Vanadium Oxides

Recent photoemission experiments have shown strong surface modifications in the spectra from vanadium oxides as (V,Cr)_2O_3 or (Sr,Ca)VO_3. The effective mass is enhanced at the surface and the coherent part of the surface spectrum is narrowed as compared to the bulk. The quasiparticle weight is more sensitive at the surface than in the bulk against bandwidth variations. We investigate these effects theoretically considering the single-band Hubbard model for a film geometry. A simplified dynamical mean-field scheme is used to calculate the main features of the interacting layer-dependent spectral function. It turns out that the experimentally confirmed effects are inherent properties of a system of strongly correlated electrons. The reduction of the weight and the variance of the coherent part of the surface spectrum can be traced back to the reduced surface coordination number. Surface correlation effects can be strongly amplified by changes of the hopping integrals at the surface.Comment: to appear in PRB; 8 pages, 6 figure

A large Hilbert space QRPA and RQRPA calculation of neutrinoless double beta decay

A large Hilbert space is used for the calculation of the nuclear matrix elements governing the light neutrino mass mediated mode of neutrinoless double beta decay of Ge76, Mo100, Cd116, Te128 and Xe136 within the proton-neutron quasiparticle random phase approximation (pn-QRPA) and the renormalized QRPA with proton-neutron pairing (full-RQRPA) methods. We have found that the nuclear matrix elements obtained with the standard pn-QRPA for several nuclear transitions are extremely sensitive to the renormalization of the particle-particle component of the residual interaction of the nuclear hamiltonian. Therefore the standard pn-QRPA does not guarantee the necessary accuracy to allow us to extract a reliable limit on the effective neutrino mass. This behaviour, already known from the calculation of the two-neutrino double beta decay matrix elements, manifests itself in the neutrinoless double-beta decay but only if a large model space is used. The full-RQRPA, which takes into account proton-neutron pairing and considers the Pauli principle in an approximate way, offers a stable solution in the physically acceptable region of the particle-particle strength. In this way more accurate values on the effective neutrino mass have been deduced from the experimental lower limits of the half-lifes of neutrinoless double beta decay.Comment: 19 pages, RevTex, 1 Postscript figur

Combined 3D characterization of porous zeolites by STEM and FIB tomography

German Research Foundation Priority Program 1570German Research Foundation Cluster of Excellence EXC 315 “Engineering of Advanced Materials

Hierarchical ZSM‐5 catalysts: The effect of different intracrystalline pore dimensions on catalyst deactivation behaviour in the MTO reaction

We present the effect of different combinations of intracrystalline pore systems in hierarchical ZSM‐5 zeolites on their performance as MTO catalysts. We prepared ZSM‐5 zeolites with additional intracrystalline mesoporous, intracrystalline macropores and a novel ZSM‐5 type zeolite with intracrystalline meso and macropores. The catalytic results showed that both used catalysts with mesopores and macropores exhibited three times longer catalyst lifetime compared to a conventional catalyst. However, TGA analysis of the deactivated catalysts showed much larger coke content in the mesoporous catalyst than in the macroporous catalyst. Consequently, macropores predominantly led to reduced coke formation rate while additional mesopores predominantly enhanced the resistance against deactivation by coke. Combining both intracrystalline meso and macropores in one catalyst lead to a tenfold increase in catalyst lifetime. Besides the effect on the catalyst lifetime there was also a strong effect of the additional pore systems on the selectivity of the catalysts. The catalysts containing mesopores showed reduced selectivity to short chain olefins and increased selectivity to larger hydrocarbons in comparison to the catalysts without a mesopores system