Thermopower enhancement by encapsulating cerium in clathrate cages.

The increasing worldwide energy consumption calls for the design of more efficient energy systems. Thermoelectrics could be used to convert waste heat back to useful electric energy if only more efficient materials were available. The ideal thermoelectric material combines high electrical conductivity and thermopower with low thermal conductivity. In this regard, the intermetallic type-I clathrates show promise with their exceedingly low lattice thermal conductivities. Here we report the successful incorporation of cerium as a guest atom into the clathrate crystal structure. In many simpler intermetallic compounds, this rare earth element is known to lead, through the Kondo interaction, to strong correlation phenomena including the occurrence of giant thermopowers at low temperatures. Indeed, we observe a 50% enhancement of the thermopower compared with a rare-earth-free reference material. Importantly, this enhancement occurs at high temperatures and we suggest that a rattling-enhanced Kondo interaction underlies this effect.

[Intraepithelial phototherapeutic keratectomy and alcohol delamination for recurrent corneal erosions--two minimally invasive surgical alternatives]. / Intraepitheliale Phototherapeutische Keratektomie und Alkohol-Abrasio zur Therapie der rezidivierenden Erosio corneae - zwei minimalinvasive chirurgische Alternativen.

BACKGROUND: Phototherapeutic keratectomy (PTK) has become established as a successful therapy for recurrent corneal erosions. After epithelial debridement, Bowman's lamella and anterior stroma are ablated by the Excimer laser. We have evaluated two alternative stroma-sparing treatment options, intraepithelial PTK, and alcohol delamination. PATIENTS AND METHODS: All treatments were performed in the relapse-free period. 17 eyes with recurrent corneal erosions were treated with intraepithelial PTK: from the intact epithelium, 12 - 25 microm of tissue were ablated by the Excimer laser (group I). Alcohol delamination was performed in 13 eyes (group II). Follow-up time was between 6 months and 7 years (mean 4.2 years). RESULTS: Both methods turned out to be safe, no refractive changes were detectable. After intraepithelial PTK, we saw a cumulative recurrence rate of 12 % after 1 year, 18 % after 2 years, and 24 % after 3 years, and a temporary subepithelial scaring was seen. Alcohol delamination resulted in a recurrence rate of 15 % during the whole follow-up time (no statistically significant difference compared to intraepithelial PTK), showing no haze or scarring. CONCLUSION: Both minimally invasive, stroma-sparing methods were effective for the treatment of trauma-associated recurrent erosion. The ablation of Bowman's lamella or anterior stroma does not seem to be necessary. However, for basal membrane dystrophy, we recommend PTK after epithelial debridement for the partial ablation of Bowman's lamella.

Divergence of the grüneisen ratio at quantum critical points in heavy fermion metals.

We present low-temperature volume thermal expansion, beta, and specific heat, C, measurements on high-quality single crystals of CeNi2Ge2 and YbRh2(Si0.95Ge0.05)(2) which are located very near to quantum critical points. For both systems, beta shows a more singular temperature dependence than C, and thus the Grüneisen ratio Gamma proportional to beta/C diverges as T-->0. For CeNi2Ge2, our results are in accordance with the spin-density wave (SDW) scenario for three-dimensional critical spin fluctuations. By contrast, the observed singularity in YbRh2(Si0.95Ge0.05)(2) cannot be explained by the itinerant SDW theory but is qualitatively consistent with a locally quantum critical picture.

The break-up of heavy electrons at a quantum critical point.

The point at absolute zero where matter becomes unstable to new forms of order is called a quantum critical point (QCP). The quantum fluctuations between order and disorder that develop at this point induce profound transformations in the finite temperature electronic properties of the material. Magnetic fields are ideal for tuning a material as close as possible to a QCP, where the most intense effects of criticality can be studied. A previous study on the heavy-electron material YbRh2Si2 found that near a field-induced QCP electrons move ever more slowly and scatter off one another with ever increasing probability, as indicated by a divergence to infinity of the electron effective mass and scattering cross-section. But these studies could not shed light on whether these properties were an artefact of the applied field, or a more general feature of field-free QCPs. Here we report that, when germanium-doped YbRh2Si2 is tuned away from a chemically induced QCP by magnetic fields, there is a universal behaviour in the temperature dependence of the specific heat and resistivity: the characteristic kinetic energy of electrons is directly proportional to the strength of the applied field. We infer that all ballistic motion of electrons vanishes at a QCP, forming a new class of conductor in which individual electrons decay into collective current-carrying motions of the electron fluid.

Magnetic-field induced quantum critical point in YbRh(2)Si(2).

We report low-temperature calorimetric, magnetic, and resistivity measurements on the antiferromagnetic (AF) heavy-fermion metal YbRh(2)Si(2) ( T(N)=70 mK) as a function of magnetic field B. While for fields exceeding the critical value B(c0) at which T(N)-->0 the low-temperature resistivity shows an AT2 dependence, a 1/(B-B(c0)) divergence of A(B) upon reducing B to B(c0) suggests singular scattering at the whole Fermi surface and a divergence of the heavy quasiparticle mass. The observations are interpreted in terms of a new type of quantum critical point separating a weakly AF ordered from a weakly polarized heavy Landau-Fermi liquid state.

Microinterferometry: Three-Dimensional Reconstruction of Surface Microtopography for Thin-Film and Wetting Studies by Reflection Interference Contrast Microscopy (RICM).

We present an improved theory of image formation by reflection interference contrast microscopy (RICM) for structural studies of stratified films on planar substrates and propose a new theoretical approach to analyzing the surface profile of nonplanar films. We demonstrate the validity of the new approach by analyzing the fringe patterns of RICM images from wedge-shaped liquid films and spherical probes. By simulation of various scenarios, we study the effect of finite-aperture illumination and the shape of the nonplanar interface on the interference fringe pattern of RICM images. We show how the reconstruction of the microscopic topography of the sample from the fringe spacing is corrected by angular and curvature correction terms. We discuss the variation of the mean intensity of the fringe patterns and the decay in the fringe amplitude with increasing fringe order that is caused by nonplanar interfaces of different slope.

Engineered fusion molecules at chelator lipid interfaces imaged by reflection interference contrast microscopy (RICM).

In molecular biology, biotechnology, and protein-engineering, the expression of histidine fusion proteins is a very powerful technique for the identification and one-step purification based on the interaction of the histidine stretch with immobilized metal complexes. By synthesis of a novel class of chelator lipids, this technique was combined with the concept of self-assembly leading to interfaces for immobilization and orientation of histidine-tagged biomolecules (Schmitt et al., 1994). Here, the chelator lipid layers were transferred onto solid substrate by vesicle fusion and Langmuir-Blodgett-techniques. Specific binding of a peptide containing an oligohistidine sequence to these functionalized interfaces was demonstrated by reflection interference contrast microscopy (RICM). Due to the phase separation behaviour of lipid mixtures, the chelator lipid interface could be further structured in two dimensions. Binding and organization of histidine-tagged molecules at these two-dimensional recognition arrays was imaged by RICM with a layer thickness resolution of 0.2 nm, and 0.5 microm laterally. Specific docking can be triggered by adding nickel ions and disrupted by EDTA. This concept opens up possibilities for reversible immobilization, enrichment and organization of histidine fusion proteins at interfaces and their application in biosensing.

[Rapid production of non-porous shielding blocks in radiotherapy]. / Schnelle Anfertigung von lunkerfreien Strahlausblendungsblöcken in der Strahlentherapie.

The production of shielding blocks nearly free from air inclusions (pipes), as they are necessary for example in case of mantle field irradiations, is often not possible in a satisfactory manner by means of conventional techniques; furthermore it is time-consuming. The authors present a new method for a fast production of shielding blocks without pipes for radiotherapy.