Guangyong Xu

Quantum magnetic excitations from stripes in copper oxide superconductors

In the copper oxide parent compounds of the high-transition-temperature superconductors the valence electrons are localized—one per copper site—by strong intra-atomic Coulomb repulsion. A symptom of this localization is antiferromagnetism, where the spins of localized electrons alternate between up and down. Superconductivity appears when mobile ‘holes’ are doped into this insulating state, and it coexists with antiferromagnetic fluctuations. In one approach to describing the coexistence, the holes are believed to self-organize into ‘stripes’ that alternate with antiferromagnetic (insulating) regions within copper oxide planes, which would necessitate an unconventional mechanism of superconductivity. There is an apparent problem with this picture, however: measurements of magnetic excitations in superconducting YBa2Cu3O6+x near optimum doping are incompatible with the naive expectations for a material with stripes. Here we report neutron scattering measurements on stripe-ordered La1.875Ba0.125CuO4. We show that the measured excitations are, surprisingly, quite similar to those in YBa2Cu3O6+x (refs 9, 10) (that is, the predicted spectrum of magnetic excitations is wrong). We find instead that the observed spectrum can be understood within a stripe model by taking account of quantum excitations. Our results support the concept that stripe correlations are essential to high-transition-temperature superconductivity.

X-ray and neutron diffraction investigations of the structural phase transformation sequence under electric field in 0.7Pb(Mg1∕3Nb2∕3)-0.3PbTiO3 crystal

The structural phase transformations of 0.7Pb(Mg1∕3Nb2∕3)O3-0.3PbTiO3 (PMN-30%PT) have been studied using x-ray diffraction (XRD) and neutron scattering as a function of temperature and electric field. We observe the phase transformational sequence (i) cubic (C)→ tetragonal (T)→ rhombohedral (R) in the zero-field-cooled (ZFC) condition; (ii) C→T→ monoclinic (MC)→ monoclinic (MA) in the field-cooled (FC) condition; and (iii) R→MA→MC→T with increasing field at fixed temperature beginning from the ZFC condition. Upon removal of the field, the MA phase is stable at room temperature in the FC condition, and also in the ZFC condition with increasing field. Several subtleties of our findings are discussed based on results from thermal expansion and dielectric measurements, including (i) the stability of the MA phase, (ii) a difference in lattice parameters between inside bulk and outside layer regions, and (iii) a difference in the phase transition temperature between XRD and dielectric data.The structural phase transformations of 0.7Pb(Mg1∕3Nb2∕3)O3-0.3PbTiO3 (PMN-30%PT) have been studied using x-ray diffraction (XRD) and neutron scattering as a function of temperature and electric field. We observe the phase transformational sequence (i) cubic (C)→ tetragonal (T)→ rhombohedral (R) in the zero-field-cooled (ZFC) condition; (ii) C→T→ monoclinic (MC)→ monoclinic (MA) in the field-cooled (FC) condition; and (iii) R→MA→MC→T with increasing field at fixed temperature beginning from the ZFC condition. Upon removal of the field, the MA phase is stable at room temperature in the FC condition, and also in the ZFC condition with increasing field. Several subtleties of our findings are discussed based on results from thermal expansion and dielectric measurements, including (i) the stability of the MA phase, (ii) a difference in lattice parameters between inside bulk and outside layer regions, and (iii) a difference in the phase transition temperature between XRD and dielectric data.

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.

Low symmetry phase in (001) BiFeO3 epitaxial constrained thin films

The lattice of (001)-oriented BiFeO3 epitaxial thin film has been identified by synchrotron x-ray diffraction. By choosing proper scattering zones containing the fixed (001) reflection, we have shown that low-symmetry phases similar to a MA phase exist in the thin film at room temperature. These results demonstrate a change in phase stability from rhombohedral in bulk single crystals, to a modified monoclinic structure in epitaxial thin films.

A Neutron Elastic Diffuse Scattering Study of PMN

We have performed elastic diffuse neutron-scattering studies on the relaxor Pb(Mg 1 / 3 Nb 2 / 3 )O 3 . The measured intensity distribution near a (100) Bragg peak in the (hk0) scattering plane assumes the shape of a butterfly with extended intensity in the (110) and (110) directions. The temperature dependence of the diffuse scattering shows that both the size of the polar nanoregions and the integrated diffuse intensity increase with cooling even for temperatures below the Curie temperature T C ∼213 K.

Dual structures in (1−x)Pb(Zn1/3Nb2/3)O3−xPbTiO3 ferroelectric relaxors

We performed x-ray diffraction studies on a series of (1−x)Pb(Zn1/3Nb2/3)O3−xPbTiO3 single crystals with different incident photon energies, and therefore different penetration depths. Our results show that outer layers of ∼10–50 μm thick are present in all samples. The structure of those outer layers is different from that of the inside of the crystals, by having much greater (rhombohedral) distortions. With increasing x, rhombohedral-type lattice distortions develop, both in the outer layer and on the inside.We performed x-ray diffraction studies on a series of (1-

Fragile phase stability in ( 1 − x ) Pb ( Mg 1 ∕ 3 Nb 2 ∕ 3 O 3 ) − x Pb Ti O 3 crystals: A comparison of [001] and [110] field-cooled phase diagrams

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Investigation of the Spin-glass Regime Between the Antiferromagnetic and Superconducting Phases in Fe1+ySexTe1−x

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

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Evidence for unusual superconducting correlations coexisting with stripe order in La 1.875 Ba 0.125 CuO 4

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