Symmetry Relaxation Research Articles

Gold Network Structures in Rhombohedral and Monoclinic Sr2Au6(Au,T)3 (T = Zn, Ga). A Transition via Relaxation

Quantitative syntheses, structure determinations and interpretations, and band calculations are reported for the nonstoichiometric rhombohedral (R3c) and monoclinic (C2/c) Sr2Au6(Au(3-x)T(x)) (T = Zn, Ga) compounds. Several different compositions of the two Sr phases were similarly refined from single crystal X-ray diffraction data as R3c: a ≈ 8.43 Å, c ≈ 21.85 Å, Z = 6 and C2/c: a ≈ 14.70 Å, b ≈ 8.47 Å, c ≈ 8.70 Å, β ≈ 123.2°, Z = 4. The R3c Zn phase is stable in the composition region x ∼ 2.5-2.9 whereas its C2/c neighbor is the major product at x ∼ 2.2-2.3. Gallium versions of both were also identified. Both R3c and C2/c structural types contain hexagonal-diamond-like gold superlattices stuffed with strings of interstitial Sr and disordered triangular (Au,T)3 units. The latter space group is a maximal, nonisomorphic subgroup of the former, and the decrease in interstitial radius from Ba to Sr (∼0.08 Å experimentally) evidently drives the symmetry reduction, relaxation, and small distortions, principally around the Sr sites. Au-Au bonding among the Au hexagons in the host lattices and with gold components in the triangular interstitials is dominant and reflected in their tight packing and short interatomic separations.

Nuclear Spin Symmetry Conservation and Relaxation in Water (1H216O) Studied by Cavity Ring-Down (CRD) Spectroscopy of Supersonic Jets

We report high resolution near-infrared laser spectra of water seeded in a supersonic jet expansion of argon probed by cavity ring-down spectroscopy (CRDS) in the R branch of the 2ν3 band (above 7500 cm(-1)) at several effective temperatures T < 30 K. Our goal is to study nuclear spin symmetry conservation and relaxation. For low mole fractions of water in the gas mixture, we obtained the lowest rotational temperatures and observed nuclear spin symmetry conservation, in agreement with theoretical expectation for inelastic collisions of isolated H2O molecules with Ar and similar to a previous series of experiments with other small molecules in supersonic jet expansions. However, for the highest mole fractions of water, which we used (xH2O < 1.6%), we obtained slightly higher rotational temperatures and observed nuclear spin symmetry relaxation, which cannot be explained by the intramolecular quantum relaxation mechanism in the monomer H2O. The nuclear spin symmetry relaxation observed is, indeed, seen to be related to the formation of water clusters at the early stage of the supersonic jet expansion. Under these conditions, two mechanisms can contribute to nuclear spin symmetry relaxation. The results are discussed in relation to claims of the stability of nuclear spin isomers of H2O in the condensed phase and briefly also to astrophysical spectroscopy.

Pairing Symmetry and Magnetic Relaxation in Topological Superconductor CuxBi2Se3

Topological insulators are materials with a bulk-insulating gap, exhibiting a quantum-Hall-effect-like behavior in the absence of a magnetic field. The experimental as well as theoretical study show Bi2Se3 has a single surface Dirac cone associated with the topologically protected surface state against time reversal symmetry. CuxBi2Se3 is of particular interest because of the signature of superconductivity found at low temperatures. Here we report the growth and the observation of bulk superconductivity from dc magnetization measurements in a cylindrical single crystal of CuxBi2Se3. The magnitude of the magnetization in the Meissner state is very small and the magnetic field dependence of the magnetization just above the lower critical field Hc1 is very different from those of usual type II superconductors. We believe that superconductivity observed in CuxBi2Se3 is consistent with the spin-triplet pairing superconductivity with odd parity. We also observed a rapid relaxation phenomenon of the diamagnetic magnetization, indicating the flexible motion of the vortices in that temperature and field regime.

Padrões físicos inadequados na performance musical de estudantes de violino

O presente artigo, uma exposição resumida da dissertação de mestrado (2008), discute a observação e avaliação dos problemas corporais que permeiam a prática de seis estudantes de violino do curso de Graduação da Escola de Música da UFMG. Dados foram colhidos através de observação, filmes e fotos de seis alunos de violino em quatro situações diferentes de performance no contexto do curso, além de avaliações fisioterápicas realizadas no consultório de Fisioterapia da pesquisadora. Estes dados foram apresentados a um painel composto por quatro observadores da área da saúde e da música. A análise destes dados revela que os seis alunos de violino apresentaram padrões físicos inadequados que poderiam prejudicar sua saúde e, consequentemente, sua performance. Além disso, foi conduzida uma pesquisa bibliográfica envolvendo estudos relacionados à Anatomia, Biomecânica e Cinesiologia. Esperamos que este artigo possa chamar a atenção dos violinistas e professores de violino para a importância de uma consciência corporal durante a performance do instrumento, que vise mais simetria e relaxamento das regiões corporais envolvidas no ato de tocar o violino.

The two-dimensional cobalt oxide (9 × 2) phase on Pd(100)

The two-dimensional (2D) Co oxide monolayer phase with (9 × 2) structure on Pd(100) has been investigated experimentally by scanning tunneling microscopy (STM) and theoretically by density functional theory (DFT). The high-resolution STM images reveal a complex pattern which on the basis of DFT calculations is interpreted in terms of a coincidence lattice, consisting of a CoO(111)-type bilayer with significant symmetry relaxation and height modulations to reduce the polarity in the overlayer. The most stable structure displays an unusual zig-zag type of antiferromagnetic ordering. The (9 × 2) Co oxide monolayer is energetically almost degenerate with the c(4 × 2) monolayer phase, which is derived from a single CoO(100)-type layer with a Co(3)O(4) vacancy structure. Under specific preparation conditions, the (9 × 2) and c(4 × 2) structures can be observed in coexistence on the Pd(100) surface and the two phases are separated by a smooth interfacial boundary line, which has been analyzed at the atomic level by STM and DFT. The here described 2D Co oxide nanolayer systems are characterized by a delicate interplay of chemical, electronic, and interfacial strain interactions and the associated complexities in the theoretical description are emphasized and discussed.

X-rays from young star clusters: a complement to optical and infrared views

AbstractIn the last few years, X-ray observational studies of young star clusters have advanced significantly, mainly thanks to the great capabilities of current X-ray observatories such as Chandra and XMM/Newton. In addition to enabling a detailed study of coronae of individual bright stars, high-spatial-resolution X-ray observations of many young clusters and star-forming regions, even massive and distant ones, have led to the detection of large populations of X-ray-bright members, often down to subsolar masses, and despite strong absorption. The peculiar ability of X-ray emission to select young, low-mass cluster stars against a crowded Galactic-plane field-star background has permitted better studies of global cluster properties, with respect to optical/infrared studies alone, including of cluster initial mass functions (across wide mass ranges), star-formation histories (with indication of age spreads—or even sequences—in many clusters) and morphologies (various degrees of symmetry and dynamical relaxation), sometimes with evidence of mass segregation. Also, the complementary availability of X-ray and optical/infrared data has enabled to place constraints on lifetimes and depletion mechanisms of pre-main-sequence circumstellar disks.

High-resolution temperature-dependent Brillouin scattering studies of ferroelectricK3Nb3O6(BO3)2

High-resolution micro-Brillouin scattering studies of ${\mathrm{K}}_{3}{\mathrm{Nb}}_{3}{\mathrm{O}}_{6}{(\mathrm{B}{\mathrm{O}}_{3})}_{2}$, exhibiting superionic and ferroelectric properties, were performed in the temperature range of $300--870\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Strong increase in damping of longitudinal and transverse acoustic phonons corresponding to the ${c}_{33}$ and ${c}_{44}$ elastic constants was observed above $650\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This increase in damping was correlated with anomalous decrease in the ${c}_{33}$ and ${c}_{44}$ elastic constants and strong increase in central peak intensities. The observed behavior has been attributed to melting of the ${\mathrm{K}}^{+}$ sublattice as a result of the order-disorder phase transitions. The obtained results also revealed a critical slowing down of the ${E}^{\ensuremath{''}}$-symmetry relaxation mode near the phase transitions at 388 and $690\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ as well as the broad ${A}_{1}^{\ensuremath{'}}$-symmetry relaxation mode near the phase transition at $750\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This behavior indicates that the relaxation modes play an important role in the mechanism of these phase transitions. In particular, this behavior suggests that the phase transition near $690\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is induced by instability of the ${E}^{\ensuremath{''}}$-symmetry relaxation mode.

UDFT and MCSCF descriptions of the photochemical Bergman cyclization of enediynes.

Several singlet and triplet potential energy surfaces (PES) for the Bergman cyclization of cis-1,5-hexadiyne-3-ene (1a) have been computed by UDFT, CI, CASCI, CASSCF, and CASMP2 methods. It is found that the first six excited states of 1a can be qualitatively described as linear combinations of the configurations of weakly interacting ethylene and acetylene units. Although the symmetry relaxation from C2nu to C2 makes cyclization of the 13B state Woodward-Hoffmann allowed, it also increases the probability of competing cis-trans isomerization. Hydrogen atom abstraction is another plausible pathway because the terminal alkyne carbons possess a large radical character. In view of the competing processes, we conclude that the Bergman cyclization along the 13B path is unlikely despite its exothermicity (Delta = -42 kcal/mol). Calculations on cyclic analogues of 1a lead to similar conclusions. A less exothermic, but more plausible pathway for photochemical cyclization lies on the 2(1)A PES (Delta = -18 kcal/mol). Compared to the 1(1)A(1) and 1(3)B states, the 2(1)A state has less in-plane electron repulsion which may facilitate cyclization. The resulting p-benzyne intermediate has an unusual electronic structure combining singlet carbene and open-shell diradical features. Deactivation of the 2(1)A state of 1a is a competing pathway.

EPR study of Mn(II)-doped CoH 6CeMo 12O 42·12H 2O: Host site symmetry and spin-lattice relaxation time

Single crystal EPR studies of Mn(II)-doped cobalt 12-molybdocerate(IV) is studied at 300 K. The paramagnetic impurity is found to occupy the substitutional site of the cobalt atom. Angular variation of fine-structure lines shows the presence of a single site. The spin Hamiltonian parameters obtained at 300 K are D=224, E=−37, g ‖=2.013, g ⊥=2.003, A ‖=−81.5 and A ⊥=−76.7 ( A, D and E are in the units of 10 −4 cm −1). A large value of E is indicative of low symmetry for the substitutional site, in accordance with the crystal structure of the isomorphous CuH 6UMo 12O 42·12H 2O. From the line width of the dopant Mn(II), the T 1 of Co 2+ is estimated to be 2.6×10 −12 s at 300 K. The covalency of the Mn–O bond, estimated from Matamura’s plot, is 10.2%.

Calculation of the electronic factor

An effective electronic factor Δ for electron transfer (ET) reactions can be calculated as an energy splitting at an avoided crossing. We employed quantum chemical ab initio techniques including correlation and large basis sets. In particular we allowed symmetry relaxation, which gives an important correlation to Koopmans' theorem in cases of a large electron or hole polarization. We obtained encouraging agreement with experimental ET data as well as photoelectron and electron transmission data. The dependence of Δ on the distance betwwen donor and acceptor is exponential if the orbital energy of the transferring electron is well above the orbital energies of the bridge electrons. Semi-empirical spectroscopic methods give qualitatively but not quantitatively correct results.

Theoretical calculations of the quartet potential energy surfaces in the NH+ + H2 system

The quartet potential energy surfaces of the NH+ (4∑−) + H2 (1∑+g) system are studied using ab initio and DIM model calculations. While there is a reaction path downhill towards H+3, the formation of NH+2 (3B2) seems to be characterized by a large activation energy barrier. In C2v configurations, the estimate for the energy barrier height was determined to be about 1.9 eV with respect to the energy of the separated species. No substantial reduction of the barrier height through symmetry relaxation could be found. A further typical feature is that the quartet states exhibit a number of crossings with low-lying doublet states, occurring mainly in the vicinity of the energy barrier.

AgBr photophysics from optical studies of quantum confined crystals.

Luminescence spectroscopy of nanocrystals (100 nm) has been used to probe the details of the low-temperature photophysics of AgBr, an indirect-gap semiconductor. Small crystallite size, moderate excitation levels, and radiative decay rates create a situation in which only one electron-hole pair exists in a given nanocrystal. These restrictions eliminate some of the decay channels and simplify the exciton emission spectrum for \ensuremath{\approxeq}50-nm edge-length crystals. The exciton recombination in 10-nm-diam crystals dominates the emission spectrum. This emission is g${10}^{4}$ times as intense as exciton emission from a macroscopic crystal. Symmetry relaxation, carrier confinement, and impurity exclusion explain this increase in exciton emission. Doping experiments confirm the assignment of one set of exciton lines (${\mathit{BX}}_{8}$) as recombination emission from an exciton bound to an ionized donor.

Superradiance and subradiance. III. Small samples

For pt.II see ibid., vol.19, p.2109 (1986). This paper is the third in a theoretical study of subradiance, i.e. the cooperative inhibition of spontaneous emission by a destructive interatomic interference, a phenomenon which has recently been observed. The paper is devoted to the problem of the cooperative spontaneous emission of atomic samples with linear dimensions much smaller than the wavelength of the emitted light. The treatment, which is based on a fully quantum mechanical description of the problem, corroborates the results obtained in a semiclassical treatment by Friedberg and Hartmann (1974). Starting from a master equation describing both the cooperative spontaneous emission and the dipole-dipole interaction, a statistical treatment of the latter is introduced and the corresponding symmetry relaxation is studied. The evolution of the atoms is described in three typical cases that correspond to different sample geometries and/or atomic distributions. In one of these cases, in particular, the symmetry is first conserved and the atoms follow Dicke's predictions for a time. A 'retarded' symmetry breaking then occurs suddenly, near the zero value of the collective energy, as in Friedberg and Hartmann's calculations for a small sphere. With regard to the further evolution of the atoms in this case, which cannot be studied in a semiclassical model, it is shown that the decay rate quickly becomes much smaller than the incoherent decay rate. An unquestionable subradiance phenomenon is thus predicted in this case.

Characterization of the NiNa 6□Cl 8 ordered phase by crystal field spectroscopy

We show that the formation of an ordered phase of transition metal impurities in ionic crystals can be monitored by means of crystal field spectroscopy of the d- d vibronic excitons. We present an optical absorption study of the Suzuki phase NiNa 6□Cl 8 in doped NaCl:Ni, in combination with ionic thermocurrent measurements and Raman spectroscopy. We find that the complex crystal field spectra are due to the coexistence of the Suzuki phase with free impurity-vacancy dipoles. The comparison with the frequencies and the absolute absorption intensities of crystal field transitions in NiCl 2 yield detailed structural information (long range order, point symmetry and anion relaxation) on the Suzuki phase, and an accurate determination of the phase concentrations.

Pulsed NMR Sulfur-33 Investigations with a Superconducting Solenoid

There is much interest in examining the structure of sulfur compounds because of the role of sulfur as a principal contributor to environmental pollution. By a pulsed NMR technique, the extremely weak free induction decay signal of 33S in several liquid compounds was detected. Using the relatively easy to detect carbon disulfide resonance as a reference model, a method is discussed for studying sulfur compounds which takes into consideration the symmetry and spin relaxation parameters ( T1 and T2). Consideration is given to the instrumental parameters which can be varied to enhance 33S pulsed NMR resonances.