Spin and lattice excitations of a BiFeO3 thin film and ceramics

We present a comprehensive study of polar and magnetic excitations in BiFeO3 ceramics and a thin film epitaxially grown on an orthorhombic (110) TbScO3 substrate. Infrared reflectivity spectroscopy was performed at temperatures from 5 to 900 K for the ceramics and below room temperature for the thin film. All 13 polar phonons allowed by the factor-group analysis were observed in theceramic samples. The thin-film spectra revealed 12 phonon modes only and an additional weak excitation, probably of spin origin. On heating towards the ferroelectric phase transition near 1100 K, some phonons soften, leading to an increase in the static permittivity. In the ceramics, terahertz transmission spectra show five low-energy magnetic excitations including two which were not previously known to be infrared active; at 5 K, their frequencies are 53 and 56 cm-1. Heating induces softening of all magnetic modes. At a temperature of 5 K, applying an external magnetic field of up to 7 T irreversibly alters the intensities of some of these modes. The frequencies of the observed spin excitations provide support for the recently developed complex model of magnetic interactions in BiFeO3 (R.S. Fishman, Phys. Rev. B 87, 224419 (2013)). The simultaneous infrared and Raman activity of the spin excitations is consistent with their assignment to electromagnons

High-temperature phase transitions in SrBi_2Ta_2O_9 film: a study by THz spectroscopy

Time-domain THz transmission experiment was performed on a $\rm SrBi_2Ta_2O_9$ film deposited on sapphire substrate. Temperatures between 300 and 923 K were investigated and complex permittivity spectra of the film were determined. The lowest frequency optic phonon near 28 cm$^{-1}$ reveals a slow monotonic decrease in frequency on heating with no significant anomaly near the phase transitions. We show that the dielectric anomaly near the ferroelectric phase transition can be explained by slowing down of a relaxational mode, observed in the THz spectra. A second harmonic generation signal observed in a single crystal confirms a loss of center of symmetry in the ferroelectric phase and a presence of polar clusters in the intermediate ferroelastic phase.Comment: subm. to J. Phys.: Condens. Matte

Terahertz and infrared spectroscopic evidence of phonon-paramagnon coupling in hexagonal piezomagnetic YMnO3

Terahertz and far-infrared electric and magnetic responses of hexagonal piezomagnetic YMnO3 single crystals are investigated. Antiferromagnetic resonance is observed in the spectra of magnetic permeability mu_a [H(omega) oriented within the hexagonal plane] below the Neel temperature T_N. This excitation softens from 41 to 32 cm-1 on heating and finally disappears above T_N. An additional weak and heavily-damped excitation is seen in the spectra of complex dielectric permittivity epsilon_c within the same frequency range. This excitation contributes to the dielectric spectra in both antiferromagnetic and paramagnetic phases. Its oscillator strength significantly increases on heating towards room temperature thus providing evidence of piezomagnetic or higher-order couplings to polar phonons. Other heavily-damped dielectric excitations are detected near 100 cm-1 in the paramagnetic phase in both epsilon_c and epsilon_a spectra and they exhibit similar temperature behavior. These excitations appearing in the frequency range of magnon branches well below polar phonons could remind electromagnons; however, their temperature dependence is quite different. We have used density functional theory for calculating phonon dispersion branches in the whole Brillouin zone. A detailed analysis of these results and of previously published magnon dispersion branches brought us to the conclusion that the observed absorption bands stem from phonon-phonon and phonon- paramagnon differential absorption processes. The latter is enabled by a strong short-range in-plane spin correlations in the paramagnetic phase.Comment: subm. to PR

Equilibrium-limited periodic separating reactors

A new unit operation is presented that utilizes a rapid feed pressure swing cycle in a bed packed with catalyst and adsorbent to effect both reaction and separation. This hybrid device combines features of a cyclic-steady-state pressure swing adsorber with those of a flow-forced catalytic reactor. Feed sequences for the periodic separating reactor (PSR) are those of rapid, single-bed pressure swing adsorbers (PSA). Only the case of extremely fast reactions is considered here. A perturbed reaction-sorption equilibrium model is formulated and solved for isothermal operation for different equilibrium constants and reaction stoichiometries. The capacity and separation performance for an equilibrium-limited PSR (EPSR) can be of the same order of magnitude as PSA alone. For reactions involving a single reactant or single product, the principal component in a particular exit stream depends upon both the reaction stoichiometry and feed fraction of the process cycle. The pressure dependency of the reaction equilibrium expression is the cause of separation reversals as parameters are varied.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/37397/1/690330810_ftp.pd

Periodic separating reactors: Experiments and theory

The novel combination of a pressure swing adsorber (PSA) with a periodic flow-forced packed-bed reactor is explored. The device provides integral component separation and reaction. Feed sequences studied for the periodic separating reactor (PSR) were those of rapid, single-bed pressure swing adsorption (RPSA). The experimental investigation employed CO oxidation over a packed bed of supported platinum catalyst and molecular sieve adsorbent. A reaction rate limited model is formulated and solved for a variety of irreversible and reversible reactions. The presence of irreversible chemical reaction is shown to greatly enhance the separation achievable by RPSA alone. For a wide range of inlet CO/O 2 ratios, CO 2 production could be increased up to two times over steady-state plug-flow reactor operation, while providing a recycle stream without phase change or extractive procedures. Selectivity and conversion improvements were predicted for multiple reaction systems. Other unusual features of operation, such as separation reversals, were also predicted and observed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/37406/1/690350514_ftp.pd

Positive solutions of Schr\"odinger equations and fine regularity of boundary points

Given a Lipschitz domain $\Omega$ in ${\mathbb R} ^N$ and a nonnegative potential $V$ in $\Omega$ such that $V(x)\, d(x,\partial \Omega)^2$ is bounded in $\Omega$ we study the fine regularity of boundary points with respect to the Schr\"odinger operator $L_V:= \Delta -V$ in $\Omega$. Using potential theoretic methods, several conditions equivalent to the fine regularity of $z \in \partial \Omega$ are established. The main result is a simple (explicit if $\Omega$ is smooth) necessary and sufficient condition involving the size of $V$ for $z$ to be finely regular. An essential intermediate result consists in a majorization of $\int_A | {\frac {u} {d(.,\partial \Omega)}} | ^2\, dx$ for $u$ positive harmonic in $\Omega$ and $A \subset \Omega$. Conditions for almost everywhere regularity in a subset $A$ of $\partial \Omega$ are also given as well as an extension of the main results to a notion of fine ${\mathcal L}_1 | {\mathcal L}_0$-regularity, if ${\mathcal L}_j={\mathcal L}-V_j$, $V_0,\, V_1$ being two potentials, with $V_0 \leq V_1$ and ${\mathcal L}$ a second order elliptic operator.Comment: version 1. 23 pages version 3. 28 pages. Mainly a typo in Theorem 1.1 is correcte

Relationship of Hypochaeris salzmanniana (Asteraceae, Lactuceae), an endangered species of the Iberian Peninsula, to H. radicata and H. glabra and biogeographical implications

Hypochaeris salzmanniana DC. (Asteraceae, Lactuceae) is an endangered species on the Iberian Peninsula, known from only eight coastal populations. Most authors have treated it as a variety, subspecies or simply as a synonym of H. glabra L. On the basis of morphological and cytological characters, Talavera recently separated H. salzmanniana (2n = 8) from H. glabra (2n = 10). Material of H. salzmanniana, H. glabra and H. radicata was collected from Spain, Italy, Sicily and Tunisia in order to assess taxonomic status and population relationships. Amplified Fragment Length Polymorphism (AFLP) analysis revealed three well-differentiated species. A close relationship between H. salzmanniana and H. radicata is also confirmed by AFLP analysis and chromosome number (2n = 8), morphology, and rDNA localization (FISH, fluorescence in situ hybridization). Hypochaeris salzmanniana and H. radicata share three fixed diagnostic AFLP fragments out of 348 fragments scored. The population structure of H. salzmanniana reveals distinct groups in southern Spain that are separated geographically. High differentiation among a western (Conil to Zahara), an intermediate (Punta Paloma and Los Algarbes) and an eastern (Algeciras and La Línea) group may reflect ancient separation. Population sizes and genetic compatibility differ greatly among populations and can be used to explain levels of within-population genetic diversity, together with recent documented loss of habitats resulting from tourist developments. Population structures of H. radicata and H. glabra show a similar geographical patterning: strongly differentiated populations from the Betic Cordillera and from the Iberian Massif, which are separated at present by the Guadalquivir river. Geological events at the end of the Tertiary (Tortonian-Messinian Miocene) might help explain patterns of differentiation in these three species of sect. Hypochaeris.Austrian Science Foundation FWF P-15225Ministerio de Ciencia y Tecnología PB96-1352, REN2002-04634-C05-03, REN2002-04354-C02-0