[Show abstract][Hide abstract] ABSTRACT: The critical behavior associated with the cooperative Jahn-Teller phase transitions in TbV1−xAsxO4 (where x=0,0.17,1) single crystals have been studied by using high-resolution x-ray scattering. These materials undergo continuous tetragonal→orthorhombic structural phase transitions that are driven by the Jahn-Teller physics at TC=33.26(2), 30.32(2), and 27.30(2) K for x=0, 0.17, and 1, respectively. The orthorhombic strain was measured close to the phase transition and was shown to display mean field behavior in all three samples. Pronounced fluctuation effects are manifest in the longitudinal width of the Bragg scattering, which diverges as a power law, with an exponent given by x=0.45±0.04, on approaching the transition from either above or below. All samples exhibited twinning; however, the disordered x=0.17 sample showed a broad distribution of twins that were stable to relatively low temperatures, well below TC. This indicates that while the orthorhombic strain continues to develop in a conventional mean field manner in the presence of disorder, twin domains are easily pinned by quenched impurities and their associated random strains.
[Show abstract][Hide abstract] ABSTRACT: High resolution time-of-flight neutron scattering measurements on Tb(2)Ti(2)0(7) reveal a rich low temperature phase diagram in the presence of a magnetic field applied along . In zero field at T = 0.4 K, Tb(2)Ti(2)0(7) is a highly correlated cooperative paramagnet with disordered spins residing on a pyrochlore lattice of corner-sharing tetrahedra. Application of a small field condenses much of the magnetic diffuse scattering, characteristic of the disordered spins, into a new Bragg peak characteristic of a polarized paramagnet. At higher fields, a magnetically ordered phase is induced, which supports spin wave excitations indicative of continuous, rather than Ising-like, spin degrees of freedom.
[Show abstract][Hide abstract] ABSTRACT: Neutron scattering measurements on powder NaNiO2 reveal magnetic Bragg peaks and spin waves characteristic of strongly correlated s=1/2 magnetic moments arranged in ferromagnetic layers which are stacked antiferromagnetically. This structure lends itself to stacking sequence frustration in the presence of mixing between nickel and alkali metal sites, possibly providing a natural explanation for the enigmatic spin glass state of the isostructural compound, LiNiO2 .
[Show abstract][Hide abstract] ABSTRACT: Single-crystal x-ray-diffraction measurements have been carried out on Cd2Re2O7near and below the phase transition it exhibits at TC′∼195K. Cd2Re2O7 was recently discovered as the first, and to date only, superconductor that displays the cubic pyrochlore structure at room temperature. Superlattice Bragg peaks show an apparently continuous structural transition at TC′, however, their behavior is unconventional. The evolution with temperature of these Bragg intensities show anomalously strong temperature dependence at low temperatures, where it falls off as I0(1-BT2), and resolution limited critical-like scattering is seen above TC′. High-resolution measurements show that the high-temperature cubic Bragg peaks split on entering the low-temperature phase, indicating a (likely tetragonal) lowering of symmetry below TC′.
Physical Review B 10/2002; 66(13). · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: TbVO4 undergoes a Jahn-Teller induced structural phase tansition from tetragonal to a twinned-orthorhombic unit cell, T_c ~33 K. Using x-ray scattering techniques we have studied the critical behavior of this phase transition. Very high resolution measurements show critical fluctuations. Secondary twin domains are observed on cooling but not on heating through TC and near mean field behavior is seen in the temperature dependance of the order parameter.
[Show abstract][Hide abstract] ABSTRACT: We have measured the critical behaviour of TbVO_4, TbAsO4 and TbV_0.85As_0.15O_4. These systems undergo structural phase transitions induced by co-operative Jahn-Teller effects. Using x-ray scattering techniques we have directly probed the order parameter, a-b, where a and b are the lattice parameters for the orthorhombic cell, for these systems. We thus able to fully understand the nature of the phase transitions. The phase transitions are observed at T_C=33.260.02 K, T_C=27.300.02 K, and T_C=30.320.02 K for TbVO_4, TbAsO4 and TbV_0.85As_0.15O4 respectively. The order parameter for each phase tranition has been fit directly to the exact solution for mean field theory. These results indicate that the dilute system is in fact also well described by mean field theory in contrast to results proposed by previous experiments. Further more we have have made one of the first measurements of the critical flutuations associated with a tetragonal to orthorhombic phase transistion.