171 research outputs found
Novel Electronic Structure of Nitrogen-Doped Lutetium Hydrides
First-principles density functional theory (DFT) calculations of Lu-H-N
compounds reveal low-energy configurations of Fmm
LuHN structures that exhibit novel electronic properties such as
flat bands, sharply peaked densities of states (van Hove singularities, vHs),
and intersecting Dirac cones near the Fermi energy (E). These N-doped
LuH-based structures also exhibit an interconnected metallic hydrogen
network, which is a common feature of high-T hydride superconductors.
Electronic property systematics give estimates of T for optimally ordered
structures that are well above the critical temperatures predicted for
structures considered previously. The vHs and flat bands near E are
enhanced in DFT+U calculations, implying strong correlation physics should also
be considered for first-principles studies of these materials. These results
provide a basis for understanding the novel electronic properties observed for
nitrogen-doped lutetium hydride.Comment: To be submitted. 4 main figures, 15 SI Figures, 1 table. 5 pages (13
pgs SI
High-Pressure Reentrant Ferroelectricity in PbTiO Revisited
We study ferroelectricity in the classic perovskite ferroelectric PbTiO
to very high pressures with density functional theory (DFT) and experimental
diamond anvil techniques. We use second harmonic generation (SHG) spectroscopy
to detect lack of inversion symmetry, if present. Consistent with early
understanding and experiments, we find that ferroelectricity disappears at
moderate pressures. However, the DFT calculations show that the disappearance
arises from the overtaking of zone boundaries instabilities, and not to the
squeezing out of the off-centering ferroelectric displacements with pressure,
as previously thought. Our computations also predict a dramatic double
reentrance of ferroelectricity at higher pressures, not yet seen in
experiments
Superconductivity in the Chalcogens up to Multimegabar Pressures
Highly sensitive magnetic susceptibility techniques were used to measure the
superconducting transition temperatures in S up to 231(5) GPa. S
transforms to a superconductor with T of 10 K and has a discontinuity in
T_c dependence at 160 GPa corresponding to bco to beta-Po phase transition.
Above this pressure T_c in S has a maximum reaching about 17.3(+/-0.5) K at 200
GPa and then slowly decreases with pressure to 15 K at 230 GPa.
This trend in the pressure dependence parallels the behavior of the heavier
members Se and Te. Superconductivity in Se was also observed from 15 to 25 GPa
with T_c changing from 4 to 6 K and above 150 GPa with T_c of 8 K.
Similiarities in the T_c dependences for S, Se, and Te, and the implications
for oxygen are discussed.Comment: 4 pages, 10 figure
Doming Modes and Dynamics of Model Heme Compounds
Synchrotron far-IR spectroscopy and density-functional calculations are used to characterize the low-frequency dynamics of model heme FeCO compounds. The “doming” vibrational mode in which the iron atom moves out of the porphyrin plane while the periphery of this ring moves in the opposite direction determines the reactivity of oxygen with this type of molecule in biological systems. Calculations of frequencies and absorption intensities and the measured pressure dependence of vibrational modes in the model compounds are used to identify the doming and related normal modes
- …