Dynamics of a quantum phase transition in a ferromagnetic Bose-Einstein condensate

We discuss dynamics of a slow quantum phase transition in a spin-1 Bose-Einstein condensate. We analytically determine the scaling properties of the system magnetization and verify them with numerical simulations in a one dimensional model.     

Terahertz emission from collapsing field domains during switching of a gallium arsenide bipolar transistor

Broadband pulsed THz emission with peak power in the sub-mW range has been observed experimentally during avalanche switching in a gallium arsenide bipolar junction transistor at room temperature, while significantly higher total generated power is predicted in simulations. The emission is attributed to very fast oscillations in the conductivity current across the switching channels, which appear as a result of temporal evolution of the field domains generated in highly dense electron-hole plasma. This plasma is formed in turn by powerful impact ionization in multiple field domains of ultrahigh amplitude.     

Microwave frequency modulation in CW EPR at W-band using a loop-gap resonator

Loop-gap resonator (LGR) technology has been extended to W-band (94GHz). One output of a multiarm Q-band (35GHz) EPR bridge was translated to W-band for sample irradiation by mixing with 59 GHz; similarly, the EPR signal was translated back to Q-band for detection. A cavity resonant in the cylindrical TE011 mode suitable for use with 100 kHz field modulation has also been developed. Results using microwave frequency modulation (FM) at 50 kHz as an alternative to magnetic field modulation are described. FM was accomplished by modulating a varactor coupled to the 59 GHz oscillator. A spin-label study of sensitivity was performed under conditions of overmodulation and gamma2H1(2)T1T2<1. EPR spectra were obtained, both absorption and dispersion, by lock-in detection at the fundamental modulation frequency (50 kHz), and also at the second and third harmonics (100 and 150 kHz). Source noise was deleterious in first harmonic spectra, but was very low in second and third harmonic spectra. First harmonic microwave FM was transferred to microwave modulation at second and third harmonics by the spins, thus satisfying the "transfer of modulation" principle. The loaded Q-value of the LGR with sample was 90 (i.e., a bandwidth between 3 dB points of about 1 GHz), the resonator efficiency parameter was calculated to be 9.3 G at one W incident power, and the frequency deviation was 11.3 MHz p-p, which is equivalent to a field modulation amplitude of 4 G. W-band EPR using an LGR is a favorable configuration for microwave FM experiments.     

Stereochemistry of rHint1 hydrolase assisted cleavage of P-N bond in nucleoside 5'-O-phosphoramidothioates

The Hint-1 hydrolase assisted cleavage of the P-N bond in adenosine-5'-O-[N-(tryptophanylamide)]phosphoramidothioate proceeds with retention of configuration at the phosphorus atom which is consistent with the formation of a covalent enzyme-substrate complex.     

Complexes between some lysine-containing peptides and crown ethers--electrospray ionization mass spectrometric study

The complexes between lysine-containing peptides (M) and crown ethers (CEs, 18C6, 15C5, 12C4) have been studied by the electrospray ionization (ESI) mass spectrometry. The maximum number of CEs attached has been found to be the same as that of the alkyl-amino side chains of lysine and as that of the protons attached. Examination of the breakdown plots of the abundances of the ions observed against the cone voltage (CV) has shown that mass spectrometric fragmentation pathways of [M + nH + (CE)n]+n may involve a loss of a neutral CE molecule as well as protonated one. The decrease in the CE cavity (the use of 12C4 or 15C5 instead of 18C6) leads to a dramatic lowering in the stability of the complexes in the gas phase but not in solution. Attachment of a CE to peptides increases their hydrophobicity, and therefore proceeds with lower efficiency in water than in methanol.     

Unprecedented flexibility of the >Ti=Si< group for the addition of H2

We show using DFT calculations that a coordinatively unsaturated >Ti=Si< group exhibits an unprecedented flexibility for the reversible addition of dihydrogen. The H2 attachment may lead to a variety of hydrogenated species (near-equivalent in energy), which contain three-center Ti...H...Si and terminal Ti-H and Si-H bonds. The >Ti=Si< moiety also exhibits a particularly low electronic barrier for heterolytic H2 attachment (approximately 0.0 eV) and beneficial thermodynamics of this process. One important path leads to the kinetically preferred H-Ti...H...Si isomer (at -0.5 eV with respect to the substrates, denoted II), which is capable of a subsequent low-barrier H2 release. Chemically pre-engineered compounds containing the >Ti=Si< grouping might be used for efficient H2 transfer catalysis in organic and inorganic hydrogenations, and for charging/discharging of the hydrogen storage materials.     

Pair potential for helium from symmetry-adapted perturbation theory calculations and from supermolecular data

Symmetry-adapted perturbation theory (SAPT) was applied to the helium dimer for interatomic separations R from 3 to 12 bohrs. The first-order interaction energy and the bulk of the second-order contribution were obtained using Gaussian geminal basis sets and are converged to about 0.1 mK near the minimum and for larger R. The remaining second-order contributions available in the SAPT suite of codes were computed using very large orbital basis sets, up to septuple-zeta quality, augmented by diffuse and midbond functions. The accuracy reached at this level was better than 1 mK in the same region. All the remaining components of the interaction energy were computed using the full configuration interaction method in bases up to sextuple-zeta quality. The latter components, although contributing only 1% near the minimum, have the largest uncertainty of about 10 mK in this region. The total interaction energy at R=5.6 bohrs is -11.000+/-0.011 K. For R< or =6.5 bohrs, the supermolecular (SM) interaction energies computed by us recently turned out to be slightly more accurate. Therefore, we have combined the SM results for R< or =6.5 bohrs with the SAPT results from 7.0 to 12 bohrs to fit analytic functions for the potential and for its error bars. The potential fit uses the best available van der Waals constants C(6) through C(16), including C(11), C(13), and C(15), and is believed to be the best current representation of the Born-Oppenheimer (BO) potential for helium. Using these fits, we found that the BO potential for the helium dimer exhibits the well depth D(e)=11.006+/-0.004 K, the equilibrium distance R(e)=5.608+/-0.012 bohrs, and supports one bound state for (4)He(2) with the dissociation energy D(0)=1.73+/-0.04 mK, and the average interatomic separation R=45.6+/-0.5 A.     

Deep Eutectic Ionic Liquids as Epoxy Resin Curing Agents

Epoxy compositions based on low-molecular bisphenol A type resin and deep eutectic solvents from choline chloride (ChCl) and guanidine derivatives or urea were prepared and investigated. Pot life at room temperature and curing process at dynamic mode were studied using DSC and rheometry while DES basic ratio was changed in a range of 3–9 wt. part/100 wt. part of epoxy resin (phr). Thermomechanical properties (glass transition and tan δ) of the cured epoxy materials (DES content up to 20 phr) using DMTA and their thermal resistance on the basis of thermogravimetric measurements were evaluated. The curing mechanism, i.e., ratio of polyaddition reaction to catalytic polymerization, was evaluated considering changeable guanidine derivative content in relation to epoxy resin.