166 research outputs found
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
Complete d-Band Dispersion and the Mobile Fermion Scale in NaxCoO2
We utilize fine-tuned polarization selection coupled with excitation-energy
variation of photoelectron signal to image the \textit{complete d}-band
dispersion relation in sodium cobaltates. A hybridization gap anticrossing is
observed along the Brillouin zone corner and the full quasiparticle band is
found to emerge as a many-body entity lacking a pure orbital polarization. At
low dopings, the quasiparticle bandwidth (Fermion scale, many-body
\textit{E} 0.25 eV) is found to be smaller than most known oxide
metals. The low-lying density of states is found to be in agreement with
bulk-sensitive thermodynamic measurements for nonmagnetic dopings where the 2D
Luttinger theorem is also observed to be satisfied.Comment: 4+ pages, 5 Fig
Quasiparticle coherence and the nature of the metal-insulator phase transition in NaCoO
Layered cobaltates embody novel realizations of correlated quantum matter on
a spin-1/2 triangular lattice. We report a high-resolution systematic
photoemission study of the insulating cobaltates (Na1/2CoO2 and K1/2CoO2).
Observation of single-particle gap opening and band-folding provides direct
evidence of anisotropic particle-hole instability on the Fermi surface due to
its unique topology. Kinematic overlap of the measured Fermi surface is
observed with the x cobalt charge-order Brillouin zone near
x=1/3 but not at x=1/2 where insulating transition is actually observed. Unlike
conventional density-waves, charge-stripes or band insulators, the on-set of
the gap depends on the quasiparticle's quantum coherence which is found to
occur well below the disorder-order symmetry breaking temperature of the
crystal (the first known example of its kind).Comment: 4+ pages, 5 figure
Low-lying quasiparticle states and hidden collective charge instabilities in parent cobaltate superconductors (NaxCoO2)
We report a state-of-the-art photoemission (ARPES) study of high quality
single crystals of the NaxCoO2 series focusing on the fine details of the
low-energy states. The Fermi velocity is found to be small (< 0.5 eV.A) and
only weakly anisotropic over the Fermi surface at all dopings setting the size
of the pair wavefunction to be on the order of 10-20 nanometers. In the low
doping regime the exchange inter-layer splitting vanishes and two dimensional
collective instabilities such as 120-type fluctuations become kinematically
allowed. Our results suggest that the unusually small Fermi velocity and the
unique symmetry of kinematic instabilities distinguish cobaltates from other
unconventional oxide superconductors such as the cuprates or the ruthenates.Comment: Accepted for publication in Phys. Rev. Lett. (2006
New Magnetic Materials Obtained by Ion-exchange Reactions from Non-magnetic Layered Perovskites
New layered magnetic materials, (MCl)Ca2Ta3O10 (M = Cu, Fe), have been prepared by ion-exchange reactions of non-magnetic perovskite derivatives, ACa2Ta3O10 (A = Rb, Li), in corresponding anhydrous molten salts. Powder x-ray diffraction patterns of the products are successfully indexed assuming tetragonal symmetry with cell dimensions a = 3.829 Ã… and c = 15.533 Ã… for Cu, and a = 3.822 Ã… and c = 15.672 Ã… for Fe. Being separated by the Ca2Ta3O10 triple-layer perovskite slabs, the transition-metal chloride (MCl) network provides a two-dimensional magnetic lattice. Magnetic susceptibility measurements show that (CuCl)Ca2Ta3O10 is in an antiferromagnetic state below 8 K, while (FeCl)Ca2Ta3O10 has two anomalies at 91 and 125 K, suggesting successive phase transitions due to geometrical spin frustration
Stoichiometric oxygen content in NaxCoO2
The crystal structure and oxygen stoichiometry in two-layer Na0.74CoO2 and
Na0.38CoO2 at room temperature are analyzed by powder neutron diffraction. Two
sets of diffraction data for each sample, taken at different incident neutron
wavelengths, lambda=1.1968 Angstroms and lambda=1.5403 Angstroms, are analyzed
simultaneously by the Rietveld method, allowing for the independent refinement
of all structural parameters. The fractional oxygen site occupancies are found
to be 1.01(1) for Na0.74CoO2 and 0.99(2) for Na0.38CoO2 respectively. These
results indicate that the oxygen content of these phases is stoichiometric to a
precision of 1 to 2%, and therefore the formal cobalt oxidation state is
determined solely by the sodium content. The analysis also reveals that both
types of sodium ions in the structure are in off-center distorted trigonal
prismatic geometry.Comment: 15 pages, 2 tables, 3 figure
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
Quasiparticle Dynamics in the Vicinity of Metal-Insulator Phase Transition in Na_xCoO_2
Layered cobaltates embody novel realizations of correlated matter on a spin-1/2 triangular lattice. We report a high-resolution systematic photoemission study of the insulating cobaltates. The observation of a single-particle gap opening and band folding provides direct evidence of anisotropic particle-hole instability on the Fermi surface due to its unique topology. Overlap of the measured Fermi surface is observed with the 3√× 3√ charge-order Brillouin zone near x = 1/3 but not at x = 1/2 where the insulating transition is actually observed. Unlike conventional density waves, charge stripes, or band insulators, the onset of the gap depends on the quasiparticle’s quantum coherence which is found to occur well below the disorder-order symmetry breaking temperature of the crystal (the first known example of its kind)
- …