4,699 research outputs found
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
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.
Pressure-driven magnetic moment collapse in the ground state of MnO
The zero temperature Mott transition region in antiferromagnetic, spin S=5/2
MnO is probed using the correlated band theory LSDA+U method. The first
transition encountered is an insulator-insulator volume collapse within the
rocksalt structure that is characterized by an unexpected Hund's rule violating
`spin-flip' moment collapse. This spin-flip to S=1/2 takes fullest advantage of
the anisotropy of the Coulomb repulsion, allowing gain in the kinetic energy
(which increases with decreasing volume) while retaining a sizable amount of
the magnetic exchange energy. While transition pressures vary with the
interaction strength, the spin-flip state is robust over a range of interaction
strengths and for both B1 and B8 structures
Charge Disproportionation and Spin Ordering Tendencies in Na(x)CoO2
The strength and effect of Coulomb correlations in the (superconducting when
hydrated) x~1/3 and ``enhanced'' x~2/3 regimes of Na(x)CoO2 are evaluated using
the correlated band theory LDA+U method. Our results, neglecting quantum
fluctuations, are: (1) allowing only ferromagnetic order, there is a critical
U_c = 3 eV, above which charge disproportionation occurs for both x=1/3 and
x=2/3, (2) allowing antiferromagnetic order at x=1/3, U_c drops to 1 eV for
disproportionation, (3) disproportionation and gap opening occur
simultaneously, (4) in a Co(3+)-Co(4+) ordered state, antiferromagnetic
coupling is favored over ferromagnetic, while below U_c ferromagnetism is
favored. Comparison of the calculated Fermi level density of states compared to
reported linear specific heat coefficients indicates enhancement of the order
of five for x~0.7, but negligible enhancement for x~0.3. This trend is
consistent with strong magnetic behavior and local moments (Curie-Weiss
susceptibility) for x>0.5 while there no magnetic behavior or local moments
reported for x<0.5. We suggest that the phase diagram is characterized by a
crossover from effective single-band character with U >> W for x>0.5 into a
three-band regime for x U_eff <= U/\sqrt(3) ~ W and
correlation effects are substantially reduced.Comment: 10 pages, 8 figures, corrected a few typos and changed reference
Chemical Differences between K and Na in Alkali Cobaltates
KCoO shares many similarities with NaCoO, as well as some
important differences (no hydration-induced superconductivity has been
reported). At =20 K, KCoO becomes an insulator with a tiny
optical gap as happens in NaCoO at 52 K. This similarity, with a
known common structure, enables direct comparisons to be made. Using the
K-zigzag structure recently reported and the local density approximation, we
compare and contrast these cobaltates at x=0.5. Although the electronic
structures are quite similar as expected, substantial differences are observed
near the Fermi level. These differences are found to be attributable mostly to
the chemical, rather than structural difference: although Na is normally
considered to be fully ion, K has somewhat more highly ionic character than
does Na in these cobaltates.Comment: 5 paper
Boron Spectral Density and Disorder Broadening in B-doped Diamond
Comparison of periodic B dopants with a random alloy of substitional boron in
diamond is carried out using several supercells and the coherent potential
approximation (CPA) for the random alloy case. The main peak in the B local
density of states is shifted to lower binding energy compared to the
corresponding C peak in intrinsic diamond. In supercells, this shows up as
strongly B-character bands split from bulk C bands away from the zone center,in
an energy region around -1 eV. Even for a 4*4*4 supercell (BC), effects
of the dopant order are evident in the form of primarily B-character bands just
below the Fermi level at the supercell zone boundary. The bands resulting from
the CPA are of continuous mixed C-B character. They resemble virtual crystal
bands, but broadened somewhat reflecting the disorder-induced lifetime, and are
consistent with angle-resolved photoemission band maps. The B character is 1.7
times larger than for C (per atom) near the top of the valence bands for CPA,
and roughly the same for supercells. CPA results are particularly useful since
they characterize the wavevector and energy dependence of disorder broadening.Comment: 8 pages and 9 embedded figures (To appear in PRB
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