Christian Hess

The electronic phase diagram of the LaO1|[minus]|xFxFeAs superconductor

The competition of magnetic order and superconductivity is a key element in the physics of all unconventional superconductors, for example in high-transition-temperature cuprates, heavy fermions and organic superconductors. Here superconductivity is often found close to a quantum critical point where long-range antiferromagnetic order is gradually suppressed as a function of a control parameter, for example charge-carrier doping or pressure. It is believed that dynamic spin fluctuations associated with this quantum critical behaviour are crucial for the mechanism of superconductivity. Recently, high-temperature superconductivity has been discovered in iron pnictides, providing a new class of unconventional superconductors. Similar to other unconventional superconductors, the parent compounds of the pnictides show a magnetic ground state and superconductivity is induced on charge-carrier doping. In this Letter the structural and electronic phase diagram is investigated by means of X-ray scattering, muon spin relaxation and Mössbauer spectroscopy on the series LaO(1-x)F(x)FeAs. We find a discontinuous first-order-like change of the Néel temperature, the superconducting transition temperature and the respective order parameters. Our results strongly question the relevance of quantum critical behaviour in iron pnictides and prove a strong coupling of the structural orthorhombic distortion and the magnetic order both disappearing at the phase boundary to the superconducting state.

Graphene Synthesis on Cubic SiC/Si Wafers. Perspectives for Mass Production of Graphene-Based Electronic Devices

The outstanding properties of graphene, a single graphite layer, render it a top candidate for substituting silicon in future electronic devices. The so far exploited synthesis approaches, however, require conditions typically achieved in specialized laboratories and result in graphene sheets whose electronic properties are often altered by interactions with substrate materials. The development of graphene-based technologies requires an economical fabrication method compatible with mass production. Here we demonstrate for the fist time the feasibility of graphene synthesis on commercially available cubic SiC/Si substrates of >300 mm in diameter, which result in graphene flakes electronically decoupled from the substrate. After optimization of the preparation procedure, the proposed synthesis method can represent a further big step toward graphene-based electronic technologies.

Pore structure and surface area of silica SBA-15: influence of washing and scale-up

Summary The removal of the surfactant (EO20PO70EO20) by washing before final calcination is a critical step in the synthesis of silica SBA-15. In contrast to washing with pure water or ethanol, washing with water and ethanol may, depending on the quantity of solvent used, alter the homogeneity and order of the pores, but also lead to an increase of the surface area of SBA-15. A reduction of solvent volume and a controlled washing protocol allow the synthesis of high surface area SBA-15 materials with a narrow monomodal pore size distribution. For larger batch sizes the influence of the quantity of solvent on the quality of the SBA-15 is reduced.

Magnon heat transport in(Sr,Ca,La)14Cu24O41

We have measured the thermal heat conductivity kappa of the compounds Sr_14Cu_24O_41 and Ca_9La_5Cu_24O_41 containing doped and undoped spin ladders, respectively. We find a huge anisotropy of both, the size and the temperature dependence of kappa which we interpret in terms of a very large heat conductivity due to the magnetic excitations of the one-dimensional spin ladders. This magnon heat conductivity decreases with increasing hole doping of the ladders. The magnon heat transport is analyzed theoretically using a simple kinetic model. From this analysis we determine the spin gap and the temperature dependent mean free path of the magnons which ranges by several thousand angstroms at low temperature. The relevance of several scattering channels for the magnon transport is discussed.

Desorption of CO from Ru(001) induced by near-infrared femtosecond laser pulses

Irradiation of a Ru(001) surface covered with CO using intense femtosecond laser pulses (800 nm, 130 fs) leads to desorption of CO with a nonlinear dependence of the yield on the absorbed fluence (100–380 J/m2). Two-pulse correlation measurements reveal a response time of 20 ps (FWHM). The lack of an isotope effect together with the strong rise of the phonon temperature (2500 K) and the specific electronic structure of the adsorbate–substrate system strongly indicate that coupling to phonons is dominant. The experimental findings can be well reproduced within a friction-coupled heat bath model. Yet, pronounced dynamical cooling in desorption, found in the fluence-dependence of the translational energy, and in a non-Arrhenius behavior of the desorption probability reflect pronounced deviations from thermal equilibrium during desorption taking place on such a short time scale.

Nanostructured vanadium oxide model catalysts for selective oxidation reactions.

Controlled synthesis based on spectroscopic characterization, structure, and catalytic performance of mesoporous silica SBA-15-supported vanadium oxide model catalysts (see TEM image) are reviewed. The effect of water on the structure and dispersion of highly dispersed vanadium oxide is discussed in the light of recent results in multiple in situ spectroscopy.Characterization of the synthesis, structure, and catalytic performance of vanadium oxide catalysts supported on mesoporous silica SBA-15 for selective oxidation reactions are reviewed. Controlled synthesis on the basis of surface functionalization and ion exchange allows homogeneous deposition of vanadium oxide within the pores of SBA-15. Catalytic tests on the selective oxidation of methanol and propane demonstrate the full catalytic function of these model powder catalysts. Characterization of the dehydrated state by vibrational spectroscopy provides insight into the structure of SBA-15-supported vanadium oxide at low loadings. The effect of water on the structure and dispersion of highly dispersed vanadium oxide is discussed in light of recent results from multiple in situ spectroscopy.

The dynamics of vibrational excitations on surfaces: CO on Ru(001)

We present an experimental and theoretical study of vibrational excitation of the C–O stretch vibration of carbon monoxide adsorbed on a ruthenium Ru(001) surface with ultrashort femtosecond infrared laser pulses. After broadband excitation leading to transfer of a significant fraction of the CO molecules to their first (∼15%) and second (∼5%) vibrationally excited states, we observe a competition between vibrational energy relaxation and energy delocalization through dipole–dipole coupling. We reproduce the observed excited state spectra by solving the three-level Bloch equations and accounting for intermolecular vibrational energy transfer on a picosecond time scale. The rate of vibrational energy transfer, and its coverage-dependence, can be described by a Forster energy transfer mechanism. We discuss possibilities to optimize the degree of localized vibrational excitation of a specific bond of molecules at surfaces through chirped pulse IR excitation.

1‐(α‐Aminobenzyl)‐2‐naphthol: A New Chiral Auxiliary for the Synthesis of Enantiopure α‐Aminophosphonic Acids

A new diastereoselective synthesis of alpha-aminophosphonates has been developed, based on the reaction, in the presence of trifluoroacetic acid, of trialkyl phosphites with chiral imines derived from (R)- or (S)-1-(alpha-aminobenzyl)-2-naphthol. The reaction proceeds at room temperature in toluene with high diastereoselectivity. The major diastereomer can be separated by crystallization from an appropriate solvent. The relative configuration of both chiral centers of the major diastereomer was determined by single-crystal X-ray structure analysis. The desired alpha-aminophosphonic acids can be obtained in enantiopure form by treatment of the corresponding diastereomers with HCl.

Carboxylic Acid-Doped SBA-15 Silica as a Host for Metallo-supramolecular Coordination Polymers

The adsorption of a metallo-supramolecular coordination polymer (Fe-MEPE) in the cylindrical pores of SBA-15 silica with pure and carboxylic acid (CA) carrying pore walls has been studied. Fe-MEPE is an intrinsically stiff polycation formed by complexation of Fe(II)-acetate with an uncharged ditopic bis-terpyridine ligand. The adsorption affinity and kinetics of the Fe-MEPE chains is strongly enhanced when the pore walls are doped with CA, and when the pH of the aqueous medium or temperature is increased. The initial fast uptake is connected with a decrease of pH of the aqueous solution, indicating an ion-exchange mechanism. It is followed by a slower (presumably diffusion-controlled) further uptake. The maximum adsorbed amount of Fe-MEPE in the CA-doped material corresponds to a monolayer of Fe-MEPE chains disposed side-by-side along the pore walls. The stoichiometry of Fe-MEPE in the pores (determined by XPS) was found to be independent of the loading and similar to that of the starting material. The mean chain length of Fe-MEPE before and after embedding in the CA-doped matrix was studied by solid-state 15N NMR using partially 15N-labeled Fe-MEPE. It is shown that the average chain length of Fe-MEPE is reduced when the complex is incorporated in the pores.

Hot-band excitation of CO chemisorbed on Ru(001) studied with broadband-IR sum-frequency generation

Abstract Broadband-IR sum-frequency generation spectroscopy was used to investigate the CO-stretch vibration of CO chemisorbed on a Ru(001) surface at coverages as low as 0.001 monolayers (ML). Due to the high intensity of the broadband-IR pulses the v =1→2 hot band of the CO-stretch vibration of CO on pure and oxygen-covered Ruthenium is observed for the first time. The simultaneous detection of the fundamental and hot-band transition allows the determination of the anharmonicity constant x e ω e directly. For a coverage of 0.004 ML one obtains 13.6 cm −1 for 12 C 16 O and 14.5 cm −1 for 12 C 16 O co-adsorbed with O.