Neutron Diffraction Study of Field Cooling Effects on Relaxor Ferroelectrics Pb[(Zn_{1/3} Nb_{2/3})_{0.92} Ti_{0.08}] O_{3}

High-temperature (T) and high-electric-field (E) effects on Pb[(Zn_{1/3} Nb_{2/3})_{0.92} Ti_{0.08}]O_3 (PZN-8%PT) were studied comprehensively by neutron diffraction in the ranges 300 <= T <= 550 K and 0 <= E <= 15 kV/cm. We have focused on how phase transitions depend on preceding thermal and electrical sequences. In the field cooling process (FC, E parallel [001] >= 0.5 kV/cm), a successive cubic (C) --> tetragonal (T) --> monoclinic (M_C) transition was observed. In the zero field cooling process (ZFC), however, we have found that the system does not transform to the rhombohedral (R) phase as widely believed, but to a new, unidentified phase, which we call X. X gives a Bragg peak profile similar to that expected for R, but the c-axis is always slightly shorter than the a-axis. As for field effects on the X phase, we found an irreversible X --> M_C transition via another monoclinic phase (M_A) as expected from a previous report [Noheda et al. Phys. Rev. Lett. 86, 3891 (2001)]. At a higher electric field, we confirmed a c-axis jump associated with the field-induced M_C --> T transition, which was observed by strain and x-ray diffraction measurements.Comment: 8 pages, 9 figures, revise

Monte Carlo Study of Relaxor Systems: A Minimum Model for Pb(In1/2_{1/2}Nb1/2_{1/2})O3_3}

We examine a simple model for Pb(In$_{1/2}$Nb$_{1/2}$)O$_3$ (PIN), which includes both long-range dipole-dipole interaction and random local anisotropy. A improved algorithm optimized for long-range interaction has been applied for efficient large-scale Monte Carlo simulation. We demonstrate that the phase diagram of PIN is qualitatively reproduced by this minimum model. Some properties characteristic of relaxors such as nano-scale domain formation, slow dynamics and dispersive dielectric responses are also examined.Comment: 5 pages, 4 figure

A continuum model of gas flows with localized density variations

We discuss the kinetic representation of gases and the derivation of macroscopic equations governing the thermomechanical behavior of a dilute gas viewed at the macroscopic level as a continuous medium. We introduce an approach to kinetic theory where spatial distributions of the molecules are incorporated through a mean-free-volume argument. The new kinetic equation derived contains an extra term involving the evolution of this volume, which we attribute to changes in the thermodynamic properties of the medium. Our kinetic equation leads to a macroscopic set of continuum equations in which the gradients of thermodynamic properties, in particular density gradients, impact on diffusive fluxes. New transport terms bearing both convective and diffusive natures arise and are interpreted as purely macroscopic expansion or compression. Our new model is useful for describing gas flows that display non-local-thermodynamic-equilibrium (rarefied gas flows), flows with relatively large variations of macroscopic properties, and/or highly compressible fluid flows

Polarization-analyzed resonant inelastic x-ray scattering of the orbital excitations in KCuF3

We report a Cu K-edge resonant inelastic x-ray scattering (RIXS) study of orbital excitations in KCuF3 . By performing the polarization analysis of the scattered photons, we disclose that the excitation between the eg orbitals and the excitations from t2g to eg exhibit distinct polarization dependence. The polarization dependence of the respective excitations is interpreted based on a phenomenological consideration of the symmetry of the RIXS process that yields a necessary condition for observing the excitations. In addition, we show that the orbital excitations are dispersionless within our experimental resolution.Comment: 5 pages, 3 figure

Symmetry of high-piezoelectric Pb-based complex perovskites at the morphotropic phase boundary I. Neutron diffraction study on Pb(Zn1/3Nb2/3)O3 -9%PbTiO3

The symmetry was examined using neutron diffraction method on Pb(Zn1/3Nb2/3)O3 -9%PbTiO3 (PZN/9PT) which has a composition at the morphotropic phase boundary (MPB) between Pb(Zn1/3Nb2/3)O3 and PbTiO3. The results were compared with those of other specimens with same composition but with different prehistory. The equilibrium state of all examined specimens is not the mixture of rhombohedral and tetragonal phases of the end members but exists in a new polarization rotation line Mc# (orthorhombic-monoclinic line). Among examined specimens, one exhibited tetragonal symmetry at room temperature but recovered monoclinic phase after a cooling and heating cycle

Ordering in Two-Dimensional Ising Models with Competing Interactions

We study the 2D Ising model on a square lattice with additional non-equal diagonal next-nearest neighbor interactions. The cases of classical and quantum (transverse) models are considered. Possible phases and their locations in the space of three Ising couplings are analyzed. In particular, incommensurate phases occurring only at non-equal diagonal couplings, are predicted. We also analyze a spin-pseudospin model comprised of the quantum Ising model coupled to XY spin chains in a particular region of interactions, corresponding to the Ising sector's super-antiferromagnetic (SAF) ground state. The spin-SAF transition in the coupled Ising-XY model into a phase with co-existent SAF Ising (pseudospin) long-range order and a spin gap is considered. Along with destruction of the quantum critical point of the Ising sector, the phase digram of the Ising-XY model can also demonstrate a re-entrance of the spin-SAF phase. A detailed study of the latter is presented. The mechanism of the re-entrance, due to interplay of interactions in the coupled model, and the conditions of its appearance are established. Applications of the spin-SAF theory for the transition in the quarter-filled ladder compound NaV2O5 are discussed.Comment: Minor revisions and refs. added; published version of the invited paper in a special issue of "Low Temp. Physics

Low-Temperature Structure of the Quarter-Filled Ladder Compound alpha'-NaV2O5

The low-temperature (LT) superstructure of $\alpha'$-NaV$_2$O$_5$ was determined by synchrotron radiation x-ray diffraction. Below the phase transition temperature associated with atomic displacement and charge ordering at 34K, we observed the Bragg peak splittings, which evidence that the LT structure is monoclinic. It was determined that the LT structure is $(a-b)\times 2b \times 4c$ with the space group $A112$ where $a, b$ and $c$ represent the high temperature orthorhombic unit cell. The valence estimation of V ions according to the bond valence sum method shows that the V sites are clearly separated into two groups of V$^{4+}$ and V$^{5+}$ with a $zigzag$ charge ordering pattern. This LT structure is consistent with resonant x-ray and NMR measurements, and strikingly contrasts to the LT structure previously reported, which includes V$^{4.5+}$ sites.Comment: 4 pages, 3 figures, 1 tabl