2,939 research outputs found
Doping-Induced Spectral Shifts in Two Dimensional Metal Oxides
Doping of strongly layered ionic oxides is an established paradigm for
creating novel electronic behavior. This is nowhere more apparent than in
superconductivity, where doping gives rise to high temperature
superconductivity in cuprates (hole-doped) and to surprisingly high Tc in HfNCl
(Tc=25.5K, electron-doped). First principles calculations of hole-doping of the
layered delafossite CuAlO2 reveal unexpectedly large doping-induced shifts in
spectral density, strongly in opposition to the rigid band picture that is
widely used as an accepted guideline. These spectral shifts, of similar origin
as the charge transfer used to produce negative electron affinity surfaces and
adjust Schottky barrier heights, drastically alter the character of the Fermi
level carriers, leading in this material to an O-Cu-O molecule-based carrier
(or polaron, at low doping) rather than a nearly pure-Cu hole as in a rigid
band picture. First principles linear response electron-phonon coupling (EPC)
calculations reveal, as a consequence, net weak EPC and no superconductivity
rather than the high Tc obtained previously using rigid band expectations.
These specifically two-dimensional dipole-layer driven spectral shifts provides
new insights into materials design in layered materials foe functionalities
besides superconductivity.Comment: 6 pages, 8 figures,1 tabl
Charge and Spin Ordering in Insulating NaCoO: Effects of Correlation and Symmetry
Ab initio band theory including correlations due to intra-atomic repulsion is
applied to study charge disproportionation and charge- and spin-ordering in
insulating NaCoO. Various ordering patterns (zigzag and two
striped) for four-Co supercells are analyzed before focusing on the observed
"out-of-phase stripe" pattern of antiferromagnetic Co spins along
charge-ordered stripes. This pattern relieves frustration and shows distinct
analogies with the cuprate layers: a bipartite lattice of antialigned spins,
with axes at 90 degree angles. Substantial distinctions with cuprates are also
discussed, including the tiny gap of a new variant of "charge transfer" type
within the Co 3d system.Comment: 5 pages, 3 figure
Compensated Half-metallicity in the Trigonally Distorted Perovskite-type NiCrO
Using first principles calculations, we investigate the electronic and
magnetic properties of the trigonally distorted (R-3c) perovskite-derived
NiCrO. Within the local spin density approximation (LSDA), our calculations
show that this system is an exactly compensated half-metal (CHM). The local
spin moments of Cr 2.04, and antialigned Ni -1.41 and three oxygens -0.63 (in
the units of ), indicate high spin S=3/2 Cr and S=3/2
(NiO) units. Considering reasonable values of the on-site Coulomb
repulsion U on both Ni and Cr ions with LDA+U approach, this system becomes an
insulator (as reported by Chamberland and Cloud) having a narrow gap in the
spin-up channel, whereas the other channel has a large gap of ~3 eV. Although
inclusion of U seemingly leads to the transition Ni^{3+}_3$ and provide another route to search for
CHM, which is a property sought by many.Comment: 5 pages, 5 embedded figures, (To be published in PRB rapid Commun.
Half Semimetallic Antiferromagnetism in the SrCrTO System, T=Os, Ru
Double perovskite SrCrOsO is (or is very close to) a realization of a
spin-asymmetric semimetallic compensated ferrimagnet, according to first
principles calculations. This type of near-half metallic antiferromagnet is an
unusual occurrence, and more so in this compound because the zero gap is
accidental rather than being symmetry determined. The large spin-orbit coupling
(SOC) of osmium upsets the spin balance (no net spin moment without SOC): it
reduces the Os spin moment by 0.27 and induces an Os orbital moment of
0.17 in the opposite direction. The effects combine (with small oxygen
contributions) to give a net total moment of 0.54 per cell in \scoo,
reflecting a large impact of SOC in this compound. This value is in moderately
good agreement with the measured saturation moment of 0.75 . The value
of the net moment on the Os ion obtained from neutron diffraction (0.73
at low temperature) differs from the calculated value (1.14 ). Rather
surprisingly, in isovalent SrCrRuO the smaller SOC-induced spin changes
and orbital moments (mostly on Ru) almost exactly cancel. This makes
SrCrRuO a "half (semi)metallic antiferromagnet" (practically vanishing
net total moment) even when SOC is included, with the metallic channel being a
small-band-overlap semimetal. Fixed spin moment (FSM) calculations are
presented for each compound, illustrating how they provide different
information than in the case of a nonmagnetic material. These FSM results
indicate that the Cr moment is an order of magnitude stiffer against
longitudinal fluctuations than is the Os moment.Comment: 6 page
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