Soft Mode Anomalies in the Perovskite Relaxor Pb(Mg1/3Nb2/3)O3

Neutron inelastic scattering measurements of the polar TO phonon mode in the cubic relaxor Pb(Mg1/3Nb2/3)O3, at room temperature, reveal anomalous behavior similar to that recently observed in the Pb(Zn1/3Nb2/3)_{0.92}Ti_{0.08}O3 system in which the optic branch appears to drop precipitously into the acoustic branch at a finite value of the momentum transfer q = 0.20 1/Angstroms, measured from the zone center. By contrast, a recent neutron study showed that PMN exhibits a normal TO phonon dispersion at 800 K. We speculate this behavior is common to all relaxor materials and is the result of the presence of nanometer-scale polarized domains in the crystal that form below a temperature Td, which effectively prevent the propagation of long wavelength (q = 0) phonons.Comment: 5 pages, 4 figures To appear as an AIP Conference Proceedings Volume for the Aspen 2000 Winter Conference on the Fundamental Physics of Ferroelectric

The impact of reward value on early sensory processing and its interaction with selective attention

Reward value affects the earliest stages of sensory perception. Whereas a host of previous studies have investigated the underlying mechanisms of reward-driven modulation of visual perception, reward effects in other sensory modalities have remained underexplored. Specifically, it has remained unknown how reward signals should be coordinated and communicated across sensory modalities. The current PhD thesis aimed to gain insight into the underlying mechanisms of reward-driven modulation of perception and its interaction with attention across sensory modalities. To this end, three experiments were conducted to identify the behavioral and electrophysiological correlates of reward effects. In Study 1, we found that high reward, task-irrelevant visual cues (intra-modal) elicited an early suppression of visual event-related potentials (ERPs). High reward auditory cues (cross-modal), on the other hand, led to a later modulation of visual ERPs and facilitated behavioral performance. Study 2 tested the dependence of reward effects on the spatial and temporal arrangement of intra-modal and cross-modal cues relative to the target, and showed that each reward cue maximally exerts its effect under a specific size of attentional focus. Study 3 explicitly manipulated the spatial attention and tested how reward associations of an audiovisual stimulus influence the allocation of attention. We found that auditory rewards enhanced the attentional modulation of both visual and auditory ERPs. Interestingly, although visual rewards did not lead to a distinguishable ERP modulation, they led to strong modulations when they were combined with auditory rewards, suggesting that integration across modalities boosts the reward effects. Taken together, the current PhD thesis identified the behavioral and neural signatures of reward-driven modulation of perception under different modes of reward signaling and different degrees of attentional engagement. Our findings inspire a two-stage model of reward processing, with local, intra-modal reward effects occurring at an early stage and long-range, multimodal reward effects arising at a later stage. Cross-modal reward signals have important ramifications for clinical applications where the impaired function of one sense can be rehabilitated by motivational signals conveyed through another sensory modality.2021-11-2

Neutron Diffuse Scattering from Polar Nanoregions in the Relaxor Pb(Mg1/3Nb2/3)O3

We have studied the neutron diffuse scattering in the relaxor PMN. The diffuse scattering appears around the Burns temperature (~620K), indicating its origin from the polar nanoregions (PNR). While the relative diffuse intensities are consistent with previous reports, they are entirely different from those of the lowest-energy TO phonon. Because of that, it has been considered that this TO mode could not be the ferroelectric soft mode. Recently, a neutron scattering study has unambiguously shown that the TO mode does soften on cooling. If the diffuse scattering in PMN originates from the soft mode condensation, then the atomic displacements must satisfy the center of mass condition. But, the atomic displacements determined from diffuse scattering intensities do not fulfill this condition. To resolve this contradiction, we propose a simple model in which the total atomic displacement consists of two components: $\delta_{CM}$ is created by the soft mode condensation, satisfying the center of mass condition, and, $\delta_{shift}$ represents a uniform displacement of the PNR along their polar direction relative to the surrounding (unpolarized) cubic matrix. Within this framework, we can successfully describe the neutron diffuse scattering intensities observed in PMN.Comment: 7 pages, 7 figures (Revised: 11-16-2001

Diffuse neutron scattering in relaxor ferroelectric PbMg1/3Nb2/3O3

High energy resolution neutron spin-echo spectroscopy has been used to measure intrinsic width of diffuse scattering discovered earlier in relaxor ferroelectric crystals. The anisotropic and transverse components of the scattering have been observed in different Brillouin zones. Both components are found to be elastic within experimental accuracy of 1 eV. Possible physical origin of the static-like behavior is discussed for each diffuse scattering contribution.Comment: Submitted to the "Physical Chemistry and Chemical Physics" (Proceedings of the QENA2004

Direct evidence of soft mode behavior near the Burns' temperature in PbMg1/3_{1 / 3}Nb2/3_{2 / 3}O3_{3} (PMN) relaxor ferroectric

Inelastic neutron scattering measurements of the relaxor ferroelectric PbMg$_{1 / 3}$Nb$_{2 / 3}$O$_{3}$ (PMN) in the temperature range 490~K$<$T$<$880~K directly observe the soft mode (SM) associated with the Curie-Weiss behavior of the dielectric constant $\varepsilon$(T). The results are treated within the framework of the coupled SM and transverse optic (TO1) mode and the temperature dependence of the SM frequency at q=0.075 a* is determined. The parameters of the SM are consistent with the earlier estimates and the frequency exhibits a minimum near the Burns temperature ($\approx$ 650K)Comment: 6 figure

Interplay between static and dynamic polar correlations in relaxor Pb(Mg_{1/3}Nb_{2/3})O_{3}

We have characterized the dynamics of the polar nanoregions in Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$ (PMN) through high-resolution neutron backscattering and spin-echo measurements of the diffuse scattering cross section. We find that the diffuse scattering intensity consists of \emph{both} static and dynamic components. The static component first appears at the Curie temperature $\Theta \sim 400$ K, while the dynamic component freezes completely at the temperature T$_{f} \sim 200$ K; together, these components account for all of the observed spectral weight contributing to the diffuse scattering cross section. The integrated intensity of the dynamic component peaks near the temperature at which the frequency-dependent dielectric constant reaches a maximum (T$_{max}$) when measured at 1 GHz, i. e. on a timescale of $\sim 1$ ns. Our neutron scattering results can thus be directly related to dielectric and infra-red measurements of the polar nanoregions. Finally, the global temperature dependence of the diffuse scattering can be understood in terms of just two temperature scales, which is consistent with random field models.Comment: (8 pages, 5 figures, submitted to Phys. Rev. B