Dynamical splayed ferromagnetic ground state in the quantum spin ice Yb(2)Sn(2)O(7).

From magnetic, specific heat, (170)Yb Mössbauer effect, neutron diffraction, and muon spin relaxation measurements on polycrystalline Yb(2)Sn(2)O(7), we show that below the first order transition at 0.15 K all of the Yb(3+) ions are long-range magnetically ordered and each has a moment of 1.1 µ(B) which lies at ≃ 10° to a common fourfold cubic axis. The four sublattice moments have four different directions away from this axis and are therefore noncoplanar. We term this arrangement splayed ferromagnetism. This ground state has a dynamical component with a fluctuation rate in the megahertz range. The net ferromagnetic exchange interaction has an anisotropy that favors the local threefold axis. We discuss our results in terms of the phase diagram proposed by Savary and Balents [Phys. Rev. Lett. 108, 037202 (2012)] for a pyrochlore lattice of Kramers 1/2 effective spins.

Magnetic frustration in the disordered pyrochlore Yb(2)GaSbO(7).

In the pyrochlore Yb(2)GaSbO(7), the Yb(3+) sublattice forms a network of corner sharing tetrahedra and the second sublattice is made up of disordered, non-magnetic Ga(3+) and Sb(5+) ions. We have examined this compound using magnetic susceptibility, (170)Yb Mössbauer spectroscopy (down to 0.03 K) and muon spin relaxation (µSR) (down to 0.02 K) measurements. We establish the size of the Yb(3+) magnetic moments and that of the Yb(3+)-Yb(3+) coupling. At low temperatures, the correlated moments fluctuate between directions that are well tilted relative to the local [111] axis. The lattice disorder does not quench the frustration induced low temperature spin fluctuations but it does remove the first order dynamic transition that is present in the crystallographically ordered counterpart Yb(2)Ti(2)O(7). Below 1.0 K, the fluctuation rate of the correlated moments decreases progressively as the temperature is reduced and the moments remain dynamic down to 0.02 K where the rate is 7 × 10(7) s(-1). Magnetic frustration is operative in Yb(2)GaSbO(7) where the Yb(3+)-Yb(3+) interaction is antiferromagnetic as it is in Yb(2)Ti(2)O(7) where the interaction is ferromagnetic.

Magnetic density of states at low energy in geometrically frustrated systems.

Using muon-spin-relaxation measurements we show that the pyrochlore compound Gd(2)Ti(2)O(7), in its magnetically ordered phase below approximately 1 K, displays persistent spin dynamics down to temperatures as low as 20 mK. The characteristics of the induced muon relaxation can be accounted for by a scattering process involving two magnetic excitations, with a density of states characterized by an upturn at low energy and a small gap depending linearly on the temperature. We propose that such a density of states is a generic feature of geometrically frustrated magnetic materials.

Planar spin fluctuations with a quadratic thermal dependence rate in spin liquid Gd3Ga5O12.

The spin liquid properties of Gd3Ga5O12 have been examined using 155Gd Mössbauer spectroscopy down to 0.027 K. Information has been obtained concerning both the directional properties of the short range correlated moments and the thermal dependence of their spin fluctuation rates. Each Gd3+ spin (S=7/2) is found to be confined to a plane and its fluctuation rate decreases from approximately 2.8 x 10(9) s(-1) at 0.4 K to approximately 0.03 x 10(9) s(-1) at 0.09 K following a close-to-quadratic thermal dependence.

Comparison of the amniotic fluid index with gray-scale and color Doppler ultrasound.

OBJECTIVES: This study was undertaken to compare the amniotic fluid index (AFI) obtained with gray-scale ultrasound and color Doppler. STUDY DESIGN: We examined 77 patients ranging from 22 to 41 weeks' gestation with two of five sonographers obtaining two measures of the AFI utilizing gray-scale and color Doppler. RESULTS: Of the measurements of AFI, 96% showed the gray scale measurement to be greater than the color measurement (p < 0.0001; mean 9.3 +/- 3.3 cm vs. 8.5 +/- 3.0 cm). At gray-scale AFI < 5 cm, color AFI was essentially the same, but at gray-scale AFI 5-10 cm, color AFI was < 5 cm, 15.2% and 7.8% of the time. At gray-scale AFI > 10 cm, no color AFI was < 5 cm. Individual (n = 5) interobserver reliability was r = 0.79 (p < 0.0001) and intraobserver reliability was r = 0.94 (p < 0.0001). CONCLUSIONS: AFI by color Doppler was always less than with gray scale. At an AFI of 5-10 cm, color demonstrated an AFI of < 5 cm in up to 16% of patients, and increased the diagnosis of oligohydramnios.

First-order transition in the spin dynamics of geometrically frustrated Yb2Ti2O7.

Using neutron diffraction, 170Yb Mössbauer and muon spin relaxation spectroscopies, we have examined the pyrochlore Yb2Ti2O7, where the Yb3+S' = 1/2 ground state has planar anisotropy. Below approximately 0.24 K, the temperature of the known specific-heat lambda transition, there is no long range magnetic order. We show that the transition corresponds to a first-order change in the fluctuation rate of the Yb3+ spins. Above the transition temperature, the rate, in the GHz range, follows a thermal excitation law, whereas below, the rate, in the MHz range, is temperature independent, indicative of a quantum fluctuation regime.

The elusive benzocyclobutenylidene: a combined computational and experimental attempt.

Ab initio and density functional theory calculations predict that benzocyclobutenylidene (1) has a singlet ground state in contrast to the parent phenylcarbene and many other simply substituted arylcarbenes. Calculations also predict that 1 should lie in a relatively deep potential well, while its triplet state is 14.5 kcal mol(-)(1) higher in energy. However, attempts to observe 1 directly by photolysis of two different nitrogenous precursors were not successful. Irradiation of diazobenzocyclobutene (7) (lambda > 534 nm or lambda > 300 nm) or azibenzocyclobutene (10) (lambda > 328 nm) in Ar matrixes at 10 K leads to the formation of the strained cycloalkyne 7-methylenecyclohepta-3,5-dien-1-yne (3). (13)C-Labeled 3 was also prepared in a similar manner. There is very good agreement between experimental IR spectra and computationally derived harmonic vibrational frequencies for 3 and [(13)C]-3 and excellent agreement between observed and calculated isotopic shifts. Prolonged short-wavelength irradiation converts 3 into benzocyclobutadiene (5). Phenylacetylene (6) and benzocyclobutadiene dimer (11) were identified as products arising from flash vacuum pyrolysis of diazirine 10 at 500 degrees C.

Aerosol extinction contribution to atmospheric attenuation in infrared wavelengths.

Atmospheric attenuation in several good transmission windows is estimated for long sea-level paths using relatively unsophisticated techniques. The estimates consist of separate estimates of gas and water vapor absorption, aerosol extinction, and aerosol liquid absorption. The contribution of aerosol extinction and aerosol liquid absorption to total atmospheric attenuation is explored. Estimated spectral aerosol extinction and absorption coefficients between the wavelengths of 2 microm and 14 micron are plotted for a wide variety of relative humidities and aerosol nucleation particle distributions. Total attenuation estimates are compared to accepted field measurement data. The agreement is sufficiently good to provide a base for estimating infrared instrument performance under similar conditions.