Orbital Molecules in the New Spinel GaV2O4.

The structures and properties of vanadium oxides are often related to the formation of molecule-like clusters of vanadium cations through direct V-V bonding. GaV2O4, a new vanadium spinel, was synthesized. Powder diffraction and X-ray total scattering studies, complemented by magnetization and resistivity measurements, reveal that the low-temperature phase of this material is structurally distorted and features ordered pairs of three- and four-atom vanadium clusters. These clusters persist into a disordered cubic phase above the charge-ordering transition at TCO = 415 K. Furthermore, quasi-elastic neutron scattering indicates that the disordered clusters remain well-defined and static to 1100 K.

Charge Order and Negative Thermal Expansion in V2OPO4.

The semivalent oxyphosphate V2OPO4 is found to have long-range V2+/V3+ charge ordering up to 605 K where a monoclinic to tetragonal structural transition and a switch from positive to negative thermal expansion are observed. V-V bonding within orbital polymer chains is proposed as the key factor in the novel switch of thermal expansion behavior, as loss of V-V bonding enables transverse oxygen motions to dominate the thermal expansion at high temperatures. Ferrimagnetic order of V2+ spin up and V3+ spin down states is observed below a magnetic ordering transition at 164 K, and susceptibility measurements evidence local spin pairing correlations to higher temperatures.

Dirac surface states and nature of superconductivity in Noncentrosymmetric BiPd.

In non-magnetic bulk materials, inversion symmetry protects the spin degeneracy. If the bulk crystal structure lacks a centre of inversion, however, spin-orbit interactions lift the spin degeneracy, leading to a Rashba metal whose Fermi surfaces exhibit an intricate spin texture. In superconducting Rashba metals a pairing wavefunction constructed from these complex spin structures will generally contain both singlet and triplet character. Here we examine the possible triplet components of the order parameter in noncentrosymmetric BiPd, combining for the first time in a noncentrosymmetric superconductor macroscopic characterization, atomic-scale ultra-low-temperature scanning tunnelling spectroscopy, and relativistic first-principles calculations. While the superconducting state of BiPd appears topologically trivial, consistent with Bardeen-Cooper-Schrieffer theory with an order parameter governed by a single isotropic s-wave gap, we show that the material exhibits Dirac-cone surface states with a helical spin polarization.