Spin order due to orbital fluctuations: cubic vanadates

We investigate the highly frustrated spin and orbital superexchange interactions in cubic vanadates. The fluctuations of t(2g) orbitals trigger a novel mechanism of ferromagnetic interactions between spins S = 1 of V3+ ions along one of the cubic directions which operates already in the absence of Hund's rule exchange J(H), and leads to the C-type antiferromagnetic phase in LaVO3. The Jahn-Teller effect can stabilize the orbital ordering and the G-type antiferromagnetic phase at low temperatures, but large entropy due to orbital fluctuations favors again the C phase at higher temperatures, as observed in YVO (3).     

Characterizing R-duality in Banach spaces

R-duals of certain sequences in Hilbert spaces were introduced by Casazza, Kutyniok and Lammers in 2004 and later generalized to Banach spaces by Xiao and Zhu. In this paper we provide some characterizations of R-dual sequences in Banach spaces.     

Preclinical Testing of Radiopharmaceuticals for the Detection and Characterization of Osteomyelitis: Experiences from a Porcine Model

The development of new and better radioactive tracers capable of detecting and characterizing osteomyelitis is an ongoing process, mainly because available tracers lack selectivity towards osteomyelitis. An integrated part of developing new tracers is the performance of in vivo tests using appropriate animal models. The available animal models for osteomyelitis are also far from ideal. Therefore, developing improved animal osteomyelitis models is as important as developing new radioactive tracers. We recently published a review on radioactive tracers. In this review, we only present and discuss osteomyelitis models. Three ethical aspects (3R) are essential when exposing experimental animals to infections. Thus, we should perform experiments in vitro rather than in vivo (Replacement), use as few animals as possible (Reduction), and impose as little pain on the animal as possible (Refinement). The gain for humans should by far exceed the disadvantages for the individual experimental animal. To this end, the translational value of animal experiments is crucial. We therefore need a robust and well-characterized animal model to evaluate new osteomyelitis tracers to be sure that unpredicted variation in the animal model does not lead to a misinterpretation of the tracer behavior. In this review, we focus on how the development of radioactive tracers relies heavily on the selection of a reliable animal model, and we base the discussions on our own experience with a porcine model.     

Terahertz direct-contact refractometer/spectrometer

A method for terahertz (THz) reflection spectroscopy is presented that utilizes direct contact between the backside of the pulsed emitter and the sample. Changes in the complex reflectivity of the emitter-sample interface are used to measure the spectrum. This method is especially useful for spectroscopy of liquid samples and soft materials. It simplifies the optical apparatus, has good sensitivity, and has capability for use in the construction of miniaturized THz probes. The problem of nonplanar wave fronts inside the emitter is discussed.     

Reorganization energy of the CuA center in purple azurin: impact of the mixed valence-to-trapped valence state transition

Mixed valence (MV) coordination compounds play important roles in redox reactions in chemistry and biology. Details of the contribution of a mixed valence state to protein electron transfer (ET) reactivity such as reorganization energy, however, have not been experimentally defined. Herein we report measurements of reorganization energies of a binuclear CuA center engineered into Pseudomonas aeruginosa azurin that exhibits a reversible transition between a totally delocalized MV state at pH 8.0 and a trapped valence (TV) state at pH 4.0. The reorganization energy of a His120Ala variant of CuA azurin that displays a TV state at both the above pH values has also been determined. We found that the MV-to-TV state transition increases the reorganization energy by 0.18 eV, providing evidence that the MV state of the CuA center has lower reorganization energy than its TV counterpart. We have also shown that lowering the pH from 8.0 to 4.0 results in a similar (approximately 0.4 eV) decrease in reorganization energy for both blue (type 1) and purple (CuA) azurins, even though the reorganization energies of the two different copper centers are different at a given pH. These results suggest that the MV state plays only a secondary role in modulation of the ET reactivity via the reorganization energy, as compared to that of the driving force.     

Rectangular loop-gap resonator with the light access to the sample

A modified rectangular loop-gap resonator for X-band electron paramagnetic resonance (EPR) studies of aqueous samples, enabling the light access, is described. Changes introduced into rectangular resonator geometry, previously presented in Piasecki et al. (1998) [1], and redesigned coupling structure lead to the better thermal and mechanical stability. The modified structure makes provision for the controlled light access to the sample placed in a flat cell during an EPR experiment. The sensitivity of the resonator for aqueous samples as well as an experimentally tested microwave magnetic field homogeneity are presented. Results of simulations and experimental tests indicate that the presence of light access holes in the resonator's front side does not disturb the uniformity of microwave magnetic field distribution in the nodal plane. The optimal flat cell thickness for unsaturable and saturable aqueous samples has been calculated for this new structure. A modified rectangular geometry of the loop-gap resonator ensures a good performance for aqueous samples allowing its convenient and efficient light illumination during EPR signal recording .     

A q-rious positivity

The q-binomial coefficients ${\genfrac{[}{]}{0pt}{}{n}{m}= \prod_{i=1}^m (1-q^{n-m+i})/(1-q^i)}$ , for integers 0??? m??? n, are known to be polynomials with non-negative integer coefficients. This readily follows from the q-binomial theorem, or the many combinatorial interpretations of ${\genfrac{[}{]}{0pt}{}{n}{m}}$ . In this note we conjecture an arithmetically motivated generalisation of the non-negativity property for products of ratios of q-factorials that happen to be polynomials.     

NVU dynamics. I. Geodesic motion on the constant-potential-energy hypersurface

An algorithm is derived for computer simulation of geodesics on the constant-potential-energy hypersurface of a system of N classical particles. First, a basic time-reversible geodesic algorithm is derived by discretizing the geodesic stationarity condition and implementing the constant-potential-energy constraint via standard Lagrangian multipliers. The basic NVU algorithm is tested by single-precision computer simulations of the Lennard-Jones liquid. Excellent numerical stability is obtained if the force cutoff is smoothed and the two initial configurations have identical potential energy within machine precision. Nevertheless, just as for NVE algorithms, stabilizers are needed for very long runs in order to compensate for the accumulation of numerical errors that eventually lead to "entropic drift" of the potential energy towards higher values. A modification of the basic NVU algorithm is introduced that ensures potential-energy and step-length conservation; center-of-mass drift is also eliminated. Analytical arguments confirmed by simulations demonstrate that the modified NVU algorithm is absolutely stable. Finally, we present simulations showing that the NVU algorithm and the standard leap-frog NVE algorithm have identical radial distribution functions for the Lennard-Jones liquid.