Browsing by Author "Zhao, X"
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- ItemDynamical mechanism of phase transitions in a-site ferroelectric relaxor (Na1/2Bi1/2)TiO3(APS Physics, 2014-10-13) Deng, GC; Danilkin, SA; Zhang, HW; Imperia, P; Li, XB; Zhao, X; Luo, HQThe dynamical phase transition mechanism of (Na1/2Bi1/2)TiO3 (NBT) was studied using inelastic neutron scattering. Softening was observed of multiple phonon modes in the phase transition sequence of NBT. As usual, the softening of the zone center transverse optical modes Δ5 and Σ3 was observed in the (200) and (220) zones, showing the Ti vibration instabilities in TiO6 octahedra for both cubic-tetragonal (C-T) and tetragonal-rhombohedral (T-R) phase transitions. In these two phase transitions, however, Ti4+ has different preferential displacement directions. Surprisingly, the longitudinal optic mode also softens significantly toward the zone center in the range of the transition temperature, indicating the Na+/Bi3+ vibration instability against TiO6 octahedra during the T-R phase transition. Strong inelastic diffuse scattering shows up near M(1.5, 0.5, 0) and R(1.5, 1.5, 0.5) in the tetragonal and rhombohedral phases, respectively, indicating the condensations of the M3 and R25 optic modes for the corresponding transitions. This reveals the different rotation instabilities of TiO6 in the corresponding transition temperature range. Bottleneck or waterfall features were observed in the dispersion curves at certain temperatures but did not show close correlations to the formation of polar nanoregions. Additional instabilities could be the origin of the complexity of phase transitions and crystallographic structures in NBT. © 2014 American Physical Society.
- ItemDynamical mechanism of phase transitions in a-site ferroelectric relaxor (Na1/2Bi1/2)TiO3(Australian Institute of Nuclear Science and Engineering, 2016-11-29) Deng, GC; Danilkin, SA; Zhang, HW; Imperia, P; Li, XB; Zhao, X; Luo, HQThe dynamical phase transition mechanism of (Na1/2Bi1/2)TiO3 (NBT) was studied using inelastic neutron scattering. Softening of multiple phonon modes were observed to correlate with the phase transition sequence of NBT. As usual, the softening of the zone centre transverse optic (TO) modes Δ5 and Σ3 was observed in (200) and (220) zone, showing the Ti vibration instabilities in TiO6 octahera for both cubic-tetragonal (C-T) and tetragonal rhombohedral (T-R) phase transitions. In this two phase transitions, however, Ti4+ has different preferential displacement directions. Surprisingly, the longitudinal optic (LO) mode also soften significantly toward zone centre in the vicinity range of the transition temperature, indicating the Na+/Bi3+ vibration instability against TiO6 octahera during the R-T phase transition. Strong inelastic diffuse scattering shows up near M(1.5, 0.5, 0) and R(1.5, 1.5, 0.5) in the tetragonal and rhombohedral phases, respectively, indicating the condensations of the M3 and R25 optic modes for the corresponding phase transitions. This reveals the rotation instabilities of TiO6 in the corresponding phase transition temperature range. Bottleneck or waterfall features were observed in the dispersion curves at certain temperatures, but did not show the close correlations to the formation of polar nanoregions (PNRs). Additional instabilities are the origin of the complexity of phase transitions and crystallographic structures in NBT.
- ItemHidden magnetic excitation in the pseudogap phase of a high-T-c superconductor(Natural Publishing Group, 2010-11-11) Li, Y; Baledent, V; Yu, G; Barisic, N; Hradil, K; Mole, RA; Sidis, Y; Steffens, P; Zhao, X; Bourges, P; Greven, MThe elucidation of the pseudogap phenomenon of the high-transition-temperature (high-T-c) copper oxides-a set of anomalous physical properties below the characteristic temperature T* and above T-c-has been a major challenge in condensed matter physics for the past two decades(1). Following initial indications of broken time-reversal symmetry in photoemission experiments(2), recent polarized neutron diffraction work demonstrated the universal existence of an unusual magnetic order below T* (refs 3, 4). These findings have the profound implication that the pseudogap regime constitutes a genuine new phase of matter rather than a mere crossover phenomenon. They are furthermore consistent with a particular type of order involving circulating orbital currents, and with the notion that the phase diagram is controlled by a quantum critical point(5). Here we report inelastic neutron scattering results for HgBa2CuO4+delta that reveal a fundamental collective magnetic mode associated with the unusual order, and which further support this picture. The mode's intensity rises below the same temperature T* and its dispersion is weak, as expected for an Ising-like order parameter(6). Its energy of 52-56 meV renders it a new candidate for the hitherto unexplained ubiquitous electron-boson coupling features observed in spectroscopic studies(7-10). © 2011, Nature Publishing Group
- ItemMoraine dam related to late quaternary glaciation in the Yulong Mountains, southwest China, and impacts on the Jinsha River(Elsevier, 2009-12) Kong, P; Na, C; Fink, D; Zhao, X; Xiao, WThe Yulong Mountain massif is tectonically active during Quaternary and contains the southernmost glacierized mountains in China, and all of Eurasia. Past glacial remnants remain preserved on the east and west sides of the Yulong Mountains. A ridge of moraine protruded into the Jinsha River at the Daju Basin, damming the river, and forming a lake at the head of the Jinsha River. Cosmogenic 10Be and 26Al provide exposure age dates for the moraine-based fluvial terraces left behind after the dam breached, and for moraine boulders on both the eastern and western sides of the Yulong Mountains. Our results yield exposure ages for the terraces that range from 29 ka to 8 ka, and a downcutting rate of 7.6 m/ka. The preservation of the remaining dam for over 10,000 years suggests stability of the moraine dam and gradual erosion of the dam during drainage of the dammed lake. From the relationship between exposure ages and elevations of the fluvial terraces located in different walls of the Daju fault, we obtain a late Quaternary dip-slip rate of about 5.6 m/ka for the Daju fault. The exposure ages of 10.2 ka and 47 ka for moraine boulders located in the east and west sides of the Yulong Mountains, respectively, coincide with warm periods in the late Quaternary. This implies that precipitation provides the major control for glaciations on the Yulong Mountains, a domain of the southwest Asian monsoon. © 2009, Elsevier Ltd.
- ItemNeutron diffuse scattering of (1 − x)(Na0.5Bi0.5)TiO3–xBaTiO3 relaxor ferroelectric single crystals(Elsevier, 2014-09-01) Zhang, HW; Deng, GC; Studer, AJ; Li, XB; Zhao, X; Luo, HQPeculiar L-shaped diffuse streaks along <0 0 1>pc were observed in (Na0.5Bi0.5)TiO3 (NBT) below the Burns temperature, revealing the existence of polar nanoregions (PNRs). The displacement of Bi3+ cations is ∼0.26 Å along <0 0 1>pc in the PNRs, with a size of ∼13 Å in NBT. The BaTiO3 doping drives the diffuse scattering patterns to evolve from the L-shape into an ellipsoidal along <0 1 1 >pc in (Na0.5Bi0.5)TiO3–xBaTiO3. An abnormally large correlation length was observed in the morphotropic phase boundary composition with x = 0.05. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. on behalf of Acta Materialia Inc. All rights reserved.
- ItemTwo Ising-like magnetic excitations in a single-layer cuprate superconductor(Nature Publishing Group, 2012-05-01) Li, Y; Yu, G; Chan, MK; Baledent, V; Li, Y; Barisic, N; Zhao, X; Hradil, K; Mole, RA; Sidis, Y; Steffens, P; Bourges, P; Greven, MThere exists increasing evidence that the phase diagram of the high-transition temperature (T(c)) cuprate superconductors is controlled by a quantum critical point. According to one distinct theoretical proposal, on decreasing the hole-carrier concentration a transition occurs to an ordered state with two circulating orbital currents per CuO(2) square. Below the 'pseudogap' temperature T* (T* > T(c)), the theory predicts a discrete order parameter and two weakly-dispersive magnetic excitations in structurally simple compounds which should be measurable by neutron scattering. Indeed, novel magnetic order and one such excitation were recently observed. Here, we demonstrate for tetragonal HgBa(2)CuO(4+delta) the existence of a second excitation with local character, consistent with the theory. The excitations mix with conventional antiferromagnetic fluctuations, which points towards a unifying picture of magnetism in the cuprates that will probably require a multi-band description. © 2012, Nature Publishing Group.