Chris Stock

Phase instability induced by polar nanoregions in a relaxor ferroelectric system

Relaxor ferroelectrics are a special class of material that exhibit an enormous electromechanical response and are easily polarized with an external field. These properties make them attractive for applications as sensors and actuators. Local clusters of randomly oriented polarization, known as polar nanoregions (PNRs), are specific to relaxor ferroelectrics and play a key role in governing their dielectric properties. Here, we show through neutron inelastic scattering experiments that the PNRs can also significantly affect the structural properties of the relaxor ferroelectric Pb(Zn(1/3)Nb(2/3))O(3)-4.5%PbTiO(3) (PZN-4.5%PT). A strong interaction is found between the PNRs and the propagation of acoustic phonons. A comparison between acoustic phonons propagating along different directions reveals a large asymmetry in the lattice dynamics that is induced by the PNRs. We suggest that a phase instability induced by this PNR-phonon interaction may contribute to the ultrahigh piezoelectric response of this and related relaxor ferroelectric materials. Our results naturally explain the emergence of the various observed monoclinic phases in these systems.

Spin waves and revised crystal structure of honeycomb iridate Na2IrO3.

We report inelastic neutron scattering measurements on Na2IrO3, a candidate for the Kitaev spin model on the honeycomb lattice. We observe spin-wave excitations below 5 meV with a dispersion that can be accounted for by including substantial further-neighbor exchanges that stabilize zigzag magnetic order. The onset of long-range magnetic order below T(N)=15.3  K is confirmed via the observation of oscillations in zero-field muon-spin rotation experiments. Combining single-crystal diffraction and density functional calculations we propose a revised crystal structure model with significant departures from the ideal 90° Ir-O-Ir bonds required for dominant Kitaev exchange.

Spin resonance in the d-Wave superconductor CeCoIn5

Neutron scattering is used to probe antiferromagnetic spin fluctuations in the d-wave heavy fermion superconductor CeCoIn5 (T_(c)=2.3 K). Superconductivity develops from a state with slow (variant Plancks over 2piGamma=0.3+/-0.15 meV) commensurate [Q_(0)=(1/2,1/2,1/2)] antiferromagnetic spin fluctuations and nearly isotropic spin correlations. The characteristic wave vector in CeCoIn5 is the same as CeIn3 but differs from the incommensurate wave vector measured in antiferromagnetically ordered CeRhIn5. A sharp spin resonance (variant Plancks over 2piGamma<0.07 meV) at variant Plancks over 2piomega=0.60+/-0.03 meV develops in the superconducting state removing spectral weight from low-energy transfers. The presence of a resonance peak is indicative of strong coupling between f-electron magnetism and superconductivity and consistent with a d-wave gap order parameter satisfying Delta(q+Q0)=-Delta(q).

Ferroelectric ordering in the relaxor Pb(Mg1/3Nb2/3)O-3 as evidenced by low-temperature phonon anomalies

Neutron scattering measurements of the lowest-energy TO phonons in the relaxor Pb(Mg 1 / 3 Nb 2 / 3 )O 3 (PMN) are reported for 10≤T7≤50 K. The soft mode, which is overdamped by the polar nanoregions below the Burns temperature T d =620 K, surprisingly recovers below 220 K. The square of the soft-mode energy (∞ω 0 ) 2 increases linearly with decreasing temperature and is consistent with the behavior of a ferroelectric soft mode. At 10 K, ∞ω 0 reaches II meV, the same value observed in ferroelectric Pb(Zn 1 / 3 Nb 2 / 3 )O 3 at low T. An unusual broadening of the TA phonon starts at T d and disappears at 220 K, coincident with the recovery of the TO mode. These dynamics suggest that a well-developed ferroelectric state is established below 220 K.

Magnetic neutron scattering in hole-doped cuprate superconductors

A review is presented of the static and dynamic magnetic properties of hole-doped cuprate superconductors measured with neutron scattering. A wide variety of experiments are described with emphasis...

Dynamic stripes and resonance in the superconducting and normal phases of YBa2Cu3O6.5 ortho-II superconductor

We describe the relation between spin fluctuations and superconductivity in a highly ordered sample of YBa 2 Cu 3 O 6 , 5 using both polarized and unpolarized neutron inelastic scattering. The spin susceptibility in the superconducting phase exhibits one-dimensional incommensurate modulations at low energies, consistent with hydrodynamic stripes. With increasing energy the susceptibility curves upward to a commensurate, intense, well-defined, and asymmetric resonance at 33 meV with a precipitous high-energy cutoff. In the normal phase, which we show is gapless, the resonance remains surprisingly strong and persists clearly in Q scans and energy scans. Its similar asymmetric spectral form above T c =59 K suggests that incoherent superconducting pairing fluctuations are present in the normal state. On cooling, the resonance and the stripe modulations grow in well above T c below a temperature that is comparable to the pseudogap temperature where suppression occurs in local and low-momentum properties. The spectral weight that accrues to the resonance is largely acquired by transfer from suppressed low-energy fluctuations. We find the resonance to be isotropically polarized, consistent with a triplet carrying ∼2.6% of the total spectral weight of the Cu spins in the planes.

Universal static and dynamic properties of the structural transition in Pb(Zn1/3Nb2/3)O-3

The relaxors Pb(Zn 1 / 3 Nb 2 / 3 )O 3 (PZN) and Pb(Mg 1 / 3 Nb 2 / 3 )O 3 (PMN) have very similar properties based on the dielectric response around the critical temperature T c (defined by the structural transition under the application of an electric field). It has been widely believed that these materials are quite different below T c with the unit cell of PMN remaining cubic while in PZN the low-temperature unit cell is rhombohedral in shape. However, this has been clarified by recent high-energy x-ray studies which have shown that PZN is rhombohedral only in the skin while the shape of the unit cell in the bulk is nearly cubic. In this study we have performed both neutron elastic and inelastic scattering to show that the temperature dependence of both the diffuse and phonon scattering in PZN and PMN are very similar. Both compounds show a nearly identical recovery of the soft optic mode and a broadening of the acoustic mode below T c . The diffuse scattering in PZN is suggestive of an onset at the high-temperature Burns temperature similar to that in PMN. In contrast to PMN, we observe a broadening of the Bragg peaks in both the longitudinal and transverse directions below T c . We reconcile this additional broadening, not observed in PMN, in terms of structural inhomogeneity in PZN. Based on the strong similarities between PMN and PZN. we suggest that both materials belong to the same universality class and discuss the relaxor transition in terms of the three-dimensional Heisenberg model with cubic anisotropy in a random field.

Mode coupling and polar nanoregions in the relaxor ferroelectric Pb(Mg1/3Nb2/3)O-3

We present a quantitative analysis of the phonon line shapes obtained by neutron inelastic scattering methods in the relaxor ferroelectric Pb(Mg 1 / 3 Nb 2 / 3 )O 3 (PMN). Differences in the shapes and apparent positions ofthe transverse acoustic- (TA) and transverse optic- (TO) phonon peaks measured in the (300) and (200) Brillouin zones at 690 K are well described by a simple model that couples the TA and soft TO modes in which the primary parameter is the wave vector and temperature-dependent TO linewidth Γ(q,T). This mode-coupling picture provides a natural explanation for the uniform displacements of the polar nanoregions (PNRs), discovered by Hirota et al. as the PNR result from the condensation of a soft TO mode that also contains a large acoustic component.

Magnetic-crystallographic phase diagram of the superconducting parent compound Fe1+xTe

Through neutron diffraction experiments, including spin-polarized measurements, we find a collinear incommensurate spin-density wave with propagation vector k= [0.4481(4)012] at base temperature in the superconducting parent compound Fe1+xTe. This critical concentration of interstitial iron corresponds to x?12% and leads to crystallographic phase separation at base temperature. The spin-density wave is short-range ordered with a correlation length of 22(3) A, and as the ordering temperature is approached its propagation vector decreases linearly in the H direction and becomes long-range ordered. Upon further populating the interstitial iron site, the spin-density wave gives way to an incommensurate helical ordering with propagation vector k= [0.3855(2)012] at base temperature. For a sample with x?9(1)%, we also find an incommensurate spin-density wave that competes with the bicollinear commensurate ordering close to the Neel point. The shifting of spectral weight between competing magnetic orderings observed in several samples is supporting evidence for the phase separation being electronic in nature, and hence leads to crystallographic phase separation around the critical interstitial iron concentration of 12%. With results from both powder and single crystal samples, we construct a magnetic-crystallographic phase diagram of Fe1+xTe for 5%

GROUND STATE OF THE RELAXOR FERROELECTRIC PB(ZN1/3NB2/3)O3

High energy x-ray diffraction measurements on