6,114 research outputs found
Persistence of the valence bond glass state in the double perovskites Ba2-xSrxYMoO6
Peer reviewedPublisher PD
Ferromagnetism below 10 K in Mn doped BiTe
Ferromagnetism is observed below 10 K in [Bi0.75Te0.125Mn0.125]Te. This
material has the BiTe structure, which is made from the stacking of two
Te-Bi-Te-Bi-Te blocks and one Bi-Bi block per unit cell. Crystal structure
analysis shows that Mn is localized in the Bi2 blocks, and is accompanied by an
equal amount of TeBi anti-site occupancy in the Bi2Te3 blocks. These TeBi
anti-site defects greatly enhance the Mn solubility. This is demonstrated by
comparison of the [Bi1-xMnx]Te and [Bi1-2xTexMnx]Te series; in the former, the
solubility is limited to x = 0.067, while the latter has xmax = 0.125. The
magnetism in [Bi1-xMnx]Te changes little with x, while that for
[Bi1-2xTexMnx]Te shows a clear variation, leading to ferromagnetism for x >
0.067. Magnetic hysteresis and the anomalous Hall Effect are observed for the
ferromagnetic samples.Comment: Accepted for publication in Phys. Rev.
Valence bond glass on an fcc lattice in the double perovskite Ba2YMoO6
Peer reviewedPublisher PD
Evidence for hard and soft substructures in thermoelectric SnSe
SnSe is a topical thermoelectric material with a low thermal conductivity
which is linked to its unique crystal structure. We use low-temperature heat
capacity measurements to demonstrate the presence of two characteristic
vibrational energy scales in SnSe with Debye temperatures thetaD1 = 345(9) K
and thetaD2 = 154(2) K. These hard and soft substructures are quantitatively
linked to the strong and weak Sn-Se bonds in the crystal structure. The heat
capacity model predicts the temperature evolution of the unit cell volume,
confirming that this two-substructure model captures the basic thermal
properties. Comparison with phonon calculations reveals that the soft
substructure is associated with the low energy phonon modes that are
responsible for the thermal transport. This suggests that searching for
materials containing highly divergent bond distances should be a fruitful route
for discovering low thermal conductivity materials.Comment: Accepted by Applied Physics Letter
Direct evidence for the magnetic ordering of Nd ions in NdFeAsO by high resolution inelastic neutron scattering
We investigated the low energy excitations in the parent compound NdFeAsO of
the Fe-pnictide superconductor in the eV range by a back scattering
neutron spectrometer. The energy scans on a powder NdFeAsO sample revealed
inelastic peaks at E = 1.600 eV at T = 0.055 K on both energy
gain and energy loss sides. The inelastic peaks move gradually towards lower
energy with increasing temperature and finally merge with the elastic peak at
about 6 K. We interpret the inelastic peaks to be due to the transition between
hyperfine-split nuclear level of the Nd and Nd isotopes with
spin . The hyperfine field is produced by the ordering of the
electronic magnetic moment of Nd at low temperature and thus the present
investigation gives direct evidence of the ordering of the Nd magnetic
sublattice of NdFeAsO at low temperature
Magnetism and structure of LixCoO2 and comparison to NaxCoO2
The magnetic properties and structure of LixCoO2 for x between 0.5 and 1.0
are reported. Co4+ is found to be high-spin in LixCoO2 for x between 0.94 and
1.0 and low-spin for x between 0.50 and 0.78. Weak antiferromagnetic coupling
is observed, increasing in strength as more Co4+ is introduced. At an x value
of about 0.65, the temperature-independent contribution to the magnetic
susceptibility and the electronic contribution to the specific heat are
largest. Neutron diffraction analysis reveals that the lithium oxide layer
expands perpendicular to the basal plane and the Li ions displace from their
ideal octahedral sites with decreasing x. A comparison of the structures of the
NaxCoO2 and LixCoO2 systems reveals that the CoO2 layer changes substantially
with alkali content in the former but is relatively rigid in the latter.
Further, the CoO6 octahedra in LixCoO2 are less distorted than those in
NaxCoO2. We postulate that these structural differences strongly influence the
physical properties in the two systems
Iron spin-reorientation transition in NdFeAsO
The low-temperature magnetic structure of NdFeAsO has been revisited using
neutron powder diffraction and symmetry analysis using the Sarah
representational analysis program. Four magnetic models with one magnetic
variable for each of the Nd and Fe sublattices were tested. The best fit was
obtained using a model with Fe moments pointing along the c-direction, and Nd
moments along the a-direction. This signals a significant interplay between
rare-earth and transition metal magnetism, which results in a
spin-reorientation of the Fe sublattice upon ordering of the Nd moments. All
models that fit the data well, including collinear models with more than one
magnetic variable per sublattice, were found to have an Fe moment of 0.5 BM and
a Nd moment of 0.9 BM, demonstrating that the low-temperature Fe moment is not
substantially enhanced compared to the spin-density wave (SDW) state.Comment: accepted to J. Phys.: Cond. Ma
Theoretical investigation of magnetic order in ReOFeAs, Re = Ce, Pr
Density functional theory (DFT) calculations are carried out on ReOFeAs, Re =
Ce, Pr, the parent compounds of the high-T superconductors
ReOFFeAs, in order to determine the magnetic order of the ground
state. It is found that the magnetic moments on the Fe sites adopt a collinear
antiferromagnetic order, similar to the case of LaOFeAs. Within the generalized
gradient approximation along with Coulomb onsite repulsion (GGA+U), we show
that the Re magnetic moments also adopt an antiferromagnetic order for which,
within the ReO layer, same spin Re sites lie along a zigzag line perpendicular
to the Fe spin stripes. While within GGA the Re 4f band crosses the Fermi
level, upon inclusion of onsite Coulomb interaction the 4f band splits and
moves away from the Fermi level, making ReOFeAs a Mott insulator.Comment: 5 pages, 4 figure
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