104 research outputs found
Inhomogeneous CuO_{6} Tilt Distribution and Charge/Spin Correlations in La_{2-x-y}Nd_{y}Sr_{x}CuO$_{4} around commensurate hole concentration
Phononic and magnetic Raman scattering are studied in
LaNdSrCuO with three doping concentrations: x ~
1/8, y = 0; x ~ 1/8, y = 0.4; and x = 0.01, y = 0. We observe strong disorder
in the tilt pattern of the CuO_{6} octahedra in both the orthorhombic and
tetragonal phases which persist down to 10 K and are coupled to bond disorder
in the cation layers around 1/8 doping independent of Nd concentration. The
weak magnitude of existing charge/spin modulations in the Nd doped structure
does not allow us to detect the specific Raman signatures on lattice dynamics
or two-magnon scattering around 2200 cm-1.Comment: to be published in Phys. Rev.
A 2:1 cocrystal of 6,13-dihydropentacene and pentacene
6,13-Dihydropentacene and pentacene cocrystallize in a ratio of 2:1, i.e. C22H16·0.5C22H14, during vapour transport of commercial pentacene in a gas flow. The crystal structure is monoclinic, space group P21/n, and contains one dihydropentacene molecule and half a pentacene molecule in the asymmetric unit.
Competing covalent and ionic bonding in Ge-Sb-Te phase change materials
Ge2Sb2Te5 and related phase change materials are highly unusual in that they
can be readily transformed between amorphous and crystalline states using very
fast melt, quench, anneal cycles, although the resulting states are extremely
long lived at ambient temperature. These states have remarkably different
physical properties including very different optical constants in the visible
in strong contrast to common glass formers such as silicates or phosphates.
This behavior has been described in terms of resonant bonding, but puzzles
remain, particularly regarding different physical properties of crystalline and
amorphous phases. Here we show that there is a strong competition between ionic
and covalent bonding in cubic phase providing a link between the chemical basis
of phase change memory property and origins of giant responses of piezoelectric
materials (PbTiO3, BiFeO3). This has important consequences for dynamical
behavior in particular leading to a simultaneous hardening of acoustic modes
and softening of high frequency optic modes in crystalline phase relative to
amorphous. This different bonding in amorphous and crystalline phases provides
a direct explanation for different physical properties and understanding of the
combination of long time stability and rapid switching and may be useful in
finding new phase change compositions with superior properties
- …