A comparative Raman study between YbVO3 and YVO3

An orbital ordering effect is observed in YbVO3 around 170 K while the crystal structure is orthorhombic (space group pnma). A monoclinic transition has been reported below TN = 104 K, while according to recent specific heat measurements, it occurs at 170 K. The crystal structure of YVO3 at 300 K is also orthorhombic. It becomes monoclinic at Tc = 200 K and back orthorhombic at T = 77 K. Spins order into the C-type antiferromagnetic structure below TN1 = 116 K and the order changes into the G-type antiferromagnetic structure below TN2 = 77 K. Controversial interpretations of YVO3 Raman active excitations have been reported. For instance the 489 and 679 cm-1 excitations have been assigned either to phonons or orbitons in two recent studies. In this communication we present a micro-Raman study of YbVO3 and YVO3 Raman active excitations as a function of temperature in order to trace the multiple phase transitions. Also by comparing the two single crystals spectra and previous studies in rare-earth manganites, high energy Raman active excitations are tentatively assigned

Electronic Properties of Metal Doped Fullerides

Metal doped C60 compounds comprise a class of materials, which includes insulators, conductors and superconductors which exhibit record superconducting transition temperatures Tc for a molecularly based solid. The moderately high values of Tc originate from the interaction of the conduction electrons with high frequency intramolecular phonons, and from the high density of states at the Fermi level. The high density of states and narrow band width arise from the small orbital overlap between the C60 molecules. Hence, both electron-phonon and electron-electron interactions are expected to be important features of the electronic structure of metal doped fullerides. Whereas superconductivity is mediated by electron-phonon interactions in the A3C60 phases, we show that electron-electron interactions determine the low temperature transport properties. We compare the electronic properties of these materials with other classes of superconductors

Magnetoelectric coupling in MnTiO3

We give general arguments that show that the linear magnetoelectric effect in antiferromagnetic materials gives rise to a magnetocapacitance anomaly—a divergence of the dielectric constant at the magnetic ordering temperature TN that appears in an applied magnetic field. The measurement of magnetodielectric response thus provides a definitive and experimentally accessiblemethod to recognize antiferromagnetic linear magnetoelectric materials, circumventing the experimental difficulties often involved in measuring electric polarization. We confirm this result experimentally using the example of MnTiO3, which we show to exhibit the linear magnetoelectric effect. No dielectric anomaly is observed at TN in the absence of an applied magnetic field. However, a sharp peak in the dielectric constant appears here when a magnetic field is applied along the c axis, reflecting a linear coupling of the polarization P with the antiferromagnetic order parameter L. In accordance with our theoretical analysis, the dielectric constant close to TN increases with the square of the magnetic field.

Superconducting (Rb,Ba)BiO3 Thin Films Grown by Molecular Beam Epitaxy

The simple cubic perovskite (Rb,Ba)BiO3 can be grown by molecular beam epitaxy using an RF plasma oxygen source. Films with superconducting onsets in resistivity as high as 27K are obtained without annealing. The epitaxy proceeds in the normal (1 0 0) orientation on (1 0 0) SrTiO3, despite a 10% lattice mis-match. (1 1 0) epitaxy is obtained on (1 0 0) MgO substrates, despite the good lattice match for (1 0 0) growth. The films with the lowest normal state resistivity are those grown on SrTiO3 at 300±10°C and are Bi deficient despite excess Bi flux during growth. The temperature dependence of the resistivity for these films is metallic down to Tc. Hc2 for a Tc=20K film is 10±1T

Spin-valve behaviour of anti-ferromagnetic boundaries in ultrathin magnetite films

Magneto-resistance (MR) measurements on epitaxial Fe3O4 films grown on polished MgO have been performed. The measurements presented here are interpreted by a model that describes the MR behaviour as spin-polarised transport across antiferromagnetic (AF) interfaces. The Fe3O4 films consist of structural domains, separated by anti-phase boundaries where an AF coupling is present. These AF interfaces enhance the resistance of the films. Upon application of a magnetic field the AF-spins rotate towards each other and the resistance decreases. The AF interfaces are thus behaving as spin-valves. In agreement with the model, the observed magneto-resistance is negative and shows linear and quadratic field dependence up to the anisotropy field for fields applied parallel and perpendicular to the film plane respectively. Above the anisotropy field, the slopes of the two MR curves are expected to be equal, which is observed at 60 K. Above the Verwey transition, the shape of the normalised MR curves is independent of temperature. Below the Verwey transition the MR curve becomes more linear with decreasing temperature. A large difference between parallel and perpendicular MR is observed at the Verwey transition.

Band Electronic Structure of One- and Two-Dimensional Pentacene Molecular Crystals

We report EHT calculations of the band electronic structure of substituted pentacene derivatives and the polymorphs of the parent compound. The results show that there are wide disparities among the bandwidths and electronic dimensionalities of these compounds. The parent pentacene polymorphs are 2-dimensional in their band electronic structure with moderate dispersions; the bandwidths in the 14.1 Å d-spacing polymorph are noticeably larger than for the 14.5 Å d-spacing polymorph, reported by Campbell. Whereas the parent pentacene polymorphs adopt the well-known herringbone packing, the new, substituted pentacenes are noticeably different in their solid state structures and this is reflected in the band electronic structures. TMS adopts a highly 1-dimensional structure that leads to a large bandwidth along the stacking direction; TIPS also adopts a stacked structure, but because the molecules are laterally interleaved in the fashion of bricks in a wall, this compound is strongly 2-dimensional.

Site disorder induced hexagonal-orthorhombic transition in Y1- x3+Gdx3+MnO3

We show that the transition in AMnO3 from the orthorhombic perovskite phase to the hexagonal phase is promoted by inducing site disorder on the A-site. The gap between the orthorhombic and the hexagonal phase is widened for site disordered, mixed yttrium-gadolinium manganite samples. At the cost of the orthorhombic phase, a two phase region emerges. The phase separation exhibits very unusual thermodynamical behavior. We also show that high-pressure synthesis favors the orthorhombic phase. YMnO3 is formed in the orthorhombic phase at 15 kbar.