Boron Carbide exhibits a broad composition range, implying a degree of intrinsic substitutional disorder. While the observed phase has rhombohedral symmetry (space group R3(bar)m), the enthalpy ...minimizing structure has lower, monoclinic, symmetry (space group Cm). The crystallographic primitive cell consists of a 12-atom icosahedron placed at the vertex of a rhombohedral lattice, together with a 3-atom chain along the 3-fold axis. In the limit of high carbon content, approaching 20% carbon, the icosahedra are usually of type B11Cp, where the p indicates the carbon resides on a polar site, while the chains are of type C-B-C. We establish an atomic interaction model for this composition limit, fit to density functional theory total energies, that allows us to investigate the substitutional disorder using Monte Carlo simulations augmented by multiple histogram analysis. We find that the low temperature monoclinic Cm structure disorders through a pair of phase transitions, first via a 3-state Potts-like transition to space group R3m, then via an Ising-like transition to the experimentally observed R3(bar)m symmetry. The R3m and Cm phases are electrically polarized, while the high temperature R3(bar)m phase is nonpolar.
During the analysis of the La (SS-B) autoantigen for catalytic activities an ATP-dependent doublestranded RNA unwinding activity was detected. Both native and recombinant La proteins from different ...species displayed this activity, which could be inhibited by monospecific anti-La antibodies. La protein was able to melt dsRNA substrates with either two 3′-overhangs or a single 3′- and a 5′-overhang. Double-stranded RNAs with two 5′-overhangs were not unwound, indicating that at least one 3′-overhang is required for unwinding. Sequence elements of the La protein that might be involved in dsRNA unwinding, such as an evolutionarily conserved putative ATP-binding motif and an element that is homologous to the double-stranded RNA binding protein kinase PKR, are discussed.
We are interested in the average behavior of interior-point methods (IPMs) for linear programming problems (LPs). We use the rotation-symmetry-model as the probabilistic model for the average case ...analysis. This model had been used by Borgwardt in his average case analysis of the simplex-method. IPMs solve LPs in three phases. First, one has to find an appropriate starting point, then a sequence of interior points is generated, which converges to the optimal face. Finally, the optimum has to be calculated, as it is not an interior point. We present upper bounds on the average number of iterations in the first and the third phase by looking at random figures of the underlying polyhedron. These bounds show, that IPMs solve LPs in strongly polynomial time in the average case, so only the dimension parameters and not the encoding length of the problem determine the average behavior of IPMs.
Recently, the I2–II–IV–VI4 (I = Cu, Ag; II = Ba, Sr; IV = Ge, Sn; VI = S, Se) materials family was identified as a promising source of potential new photovoltaic (PV) and photoelectrochemical (PEC) ...absorbers. These materials avoid the pitfalls of the successful photovoltaic semiconductors Cu(In,Ga)(S,Se)2 and CdTe, as they do not contain scarce (In, Te) or toxic (Cd) elements. Furthermore, ionic sizes and coordination preferences are very different for the I, II, and IV cations in the I2–II–IV–VI4 family, providing an intriguing avenue to avoid intrinsic antisite disordering that limits efficiency improvement in Cu2ZnSn(S,Se)4 (where Cu and Zn can easily substitute for one another). Here, we experimentally and computationally explore alloys Cu2BaGe1–x Sn x Se4 (CBGTSe, 0 ≤ x ≤ 1) to fine-tune the structural, optical, and electronic properties for the relatively large band gap (E g = 1.91(5) eV) unalloyed compound Cu2BaGeSe4 (CBGSe). We show that CBGTSe maintains the P31 crystal structure type of the parent CBGSe up to x ≤ 0.70. A minimum band gap value of 1.57(5) eV can be reached at x = 0.70 before the structure transforms to the Ama2 structure type. The experimental and theoretical investigations demonstrate the potential of CBGTSe for thin-film PV and PEC absorbers.
More than 10,000-fold enhanced magnetic resonance signals with >20-min signal lifetimes on universal biomolecular markers.
Conventional magnetic resonance (MR) faces serious sensitivity limitations ...which can be overcome by hyperpolarization methods, but the most common method (dynamic nuclear polarization) is complex and expensive, and applications are limited by short spin lifetimes (typically seconds) of biologically relevant molecules. We use a recently developed method, SABRE-SHEATH, to directly hyperpolarize
15
N
2
magnetization and long-lived
15
N
2
singlet spin order, with signal decay time constants of 5.8 and 23 minutes, respectively. We find >10,000-fold enhancements generating detectable nuclear MR signals that last for over an hour.
15
N
2
-diazirines represent a class of particularly promising and versatile molecular tags, and can be incorporated into a wide range of biomolecules without significantly altering molecular function.
Conventional magnetic resonance (MR) faces serious sensitivity limitations which can be overcome by hyperpolarization methods, but the most common method (dynamic nuclear polarization) is complex and ...expensive, and applications are limited by short spin lifetimes (typically seconds) of biologically relevant molecules. We use a recently developed method, SABRE-SHEATH, to directly hyperpolarize (15)N2 magnetization and long-lived (15)N2 singlet spin order, with signal decay time constants of 5.8 and 23 minutes, respectively. We find >10,000-fold enhancements generating detectable nuclear MR signals that last for over an hour. (15)N2-diazirines represent a class of particularly promising and versatile molecular tags, and can be incorporated into a wide range of biomolecules without significantly altering molecular function.
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