Observation of variable range hopping at “natural” phonon frequencies

Hill's analysis of hopping conductivity data has been applied to ion-bombarded amorphous-silicon samples. The apparent hopping conductivity parameters derived from a standardT ^–0.25 plot undergo changes by several orders of magnitude when plotted with the exact scaling of the abscissa. A typical example is characterized by a temperature dependence ofdc conductivity according to= ₀ exp (—(T ₀/T) ^m ), withm=0.45,T ₀ =6.410⁴K and ₀=6.610^{1 –1} cm^–1. From ₀ a phonon frequency of about 3–1510¹² s^–1 is derived.     

Ion-implantation doping of crystalline 6H-SiC

The feasibility of ion implantation for p- and n-type doping of 6H-SiC has been studied. Single crystals were implanted at room temperature with 10¹⁷ ions/cm³ of B and Al, and of N and P, respectively, and step-annealed at temperatures up to 1900 K. The state of the crystal order was monitored by ion-beam-scattering techniques. After annealing at 1800 K, at a backscattering yield of about 1% in 0001-direction, maximum electrical activity of all dopants was observed within the range of 3–80% at room temperature. Impurity ionization levels were derived from conductivity measurements in the temperature range between 300–80 K, which also indicate the presence of compensating defects.     

Use of Global Symmetries in Automated Signal Class Recognition by a Bayesian Method

Automated or semiautomated pattern recognition in multidimensional NMR spectroscopy is strongly hampered by the large number of noise and artifact peaks occurring under practical conditions. A general Bayesian method which is able to assign probabilities that observed peaks are members of given signal classes (e.g., the class of true resonance peaks or the class of noise and artifact peaks) was proposed previously. The discriminative power of this approach is dependent on the choice of the properties characterizing the peaks. The automated class recognition is improved by the addition of a nonlocal feature, the similarities of peak shapes in symmetry-related positions. It turns out that this additional property strongly decreases the overlap of the multivariate probability distributions for true signals and noise and hence largely increases the discrimination of true resonance peaks from noise and artifacts.     

High-sensitivity sapphire cells for high pressure NMR spectroscopy on proteins

High pressure NMR spectroscopy is a most exciting method for studying the structural anisotropy and conformational dynamics of proteins. The restricted volume of the high pressure glass cells causes a poor signal to noise ratio which up to now renders the application of most of the multidimensional NMR experiments impossible. The method presented here using high strength single crystal sapphire cells doubles the signal-to-noise ratio and allows to perform high pressure NMR measurements more easily. As a first application the difference of partial molar volumes caused by cis-trans-isomerisation of a prolyl peptide bond in the tetrapeptide Gly-Gly-Pro-Ala could be determined as 0.25 ml mol(-1) at 305 K.     

Optical absorptivity of ion-beam irradiated silicon

Single-crystal layers of silicon on sapphire have been irradiated with Ne and Kr ions at room temperature. The concomitant changes in optical absorption have been measured as a function of photon energy. The absorptivity of the amorphized silicon is about one order of magnitude higher as compared to the crystalline state in the photon energy interval of 1.5–3 eV. This is sufficient for generating optical patterns of high contrast by irradiation of selected parts of the wafer.     

Ionographic patterns with amorphous/crystalline contrast

Ion irradiation of thin layers of crystalline semiconductors induces a phase transition to the amorphous state. The concomitant optical contrast between unirradiated, crystalline, and irradiated, amorphous, material may be used for pattern fabrication in the submicron range. This process will be explained by the example of silicon single-crystal layers on sapphire.International Patents pending     

Pulsed pressure perturbations, an extra dimension in NMR spectroscopy of proteins

The introduction of multidimensional NMR spectroscopy was a breakthrough in biological NMR methodology because it allowed the unequivocal correlation of different spin states of the system. The introduction of large pressure perturbations in the corresponding radio frequency (RF) pulse sequences adds an extra structural dimension into these experiments. We have developed a microprocessor-controlled pressure jump unit that is able to introduce fast, strong pressure changes at any point in the pulse sequences. Repetitive pressure changes of 80 MPa in the sample tube are thus feasible in less than 30 ms. Two general forms of these experiments are proposed here, the pressure perturbation transient state spectroscopy (PPTSS) and the pressure perturbation state correlation spectroscopy (PPSCS). PPTSS can be used to measure the rate constants and the activation energies and activation volumes for the transition between different conformational states including the folded and unfolded state of proteins, for polymerization-depolymerization processes, and for ligand binding at atomic resolution. PPSCS spectroscopy correlates the NMR parameters of different pressure-induced states of the system, thus allowing the measurement of properties of a given pressure induced state such as a folding intermediate in a different state, for example, the folded state. Selected examples for PPTSS and PPSCS spectroscopy are presented in this Article.     

Cu2+-cyclen as probe to identify conformational states in guanine nucleotide binding proteins

(31)P NMR spectroscopy is a suitable method for identifying conformational states in the active site of guanine nucleotide binding proteins detecting the nucleotide placed there. Because there is no labeling necessary, this method is gaining increasing interest. By (31)P NMR spectroscopy two major conformational states, namely state 1(T) and state 2(T), can be detected in active Ras protein characterized by different chemical shifts. Depending on the conformational state Ras shows clearly different physiological properties. Meanwhile analogous conformational equilibria could also be shown for other members of the Ras superfamily. It is often difficult to determine the conformational states of the proteins on the basis of chemical shift alone; therefore, direct detection would be a great advantage. With the use of Cu(2+)-cyclen which selectively interacts only with one of the major conformational states (state 1) one has a probe to distinguish between the two states, because only proteins existing in conformational state 1 interact with the Cu(2+)-cyclen at low millimolar concentrations. The suitability was proven using Ras(wt) and Ras mutants, Ras complexed with GTP, GppNHp, or GTPγS, as well as two further members of the Ras superfamily namely Arf1 and Ran.     

Structures, spectroscopic studies and solid-state thermal transformations of coordination polymers from P4Se3 and CuX (X=Cl, Br, I)

The formation of coordination polymers (CuCl)P₄Se₃ (1), (CuBr)₃(P₄Se₃)₂ (2), (CuI)₃(P₄Se₃)₂ (3) and (CuI)P₄Se₃ (4), from solutions of copper(I) halides and P₄Se₃ by diffusion methods has been studied. The new compounds were characterized by X-ray crystallography, solid-state ³¹P MAS NMR and Raman spectroscopy. Theoretical studies on the DFT level in the crystalline phase allowed the unequivocal assignment of the recorded Raman shifts between 200 and 480 cm⁻¹. The structure of 1 consists of a 2D network of castellated [CuCl]_n chains and bidentate P₄Se₃ molecules. The 3D network of 2 comprises [CuBr]_n chains, which are linked by tridentate P₄Se₃ molecules. Compound 3 is a three-dimensional polymer composed of four-membered (CuI)₂ rings and castellated [CuI]_n chains, which are linked by tridentate P₄Se₃ molecules involving two basal and the apical P atoms. Thermal conversion of 1 at 230 °C gives (CuCl)₃(P₄Se₃)₂ (5), which is isostructural with 2. The thermal conversion of (CuI)₃P₄S₃, which was studied for comparison gave at 371 °C (CuI)₃P₄S₄, Cu₃PS₄ and small amounts of Cu₆PS₅I.