540 research outputs found
Obtaining correct orbital ground states in electron systems using a nonspherical self-interaction corrected LDA+ method
The electronic structure of lanthanide and actinide compounds is often
characterized by orbital ordering of localized -electrons.
Density-functional theory (DFT) studies of such systems using the currently
available LDA+ method are plagued by significant orbital-dependent
self-interaction, leading to erroneous orbital ground states. An alternative
scheme that modifies the exchange, not Hartree, energy is proposed as a remedy.
We show that our LDA+ approach reproduces the expected degeneracy of
and states in free ions and the correct ground states in solid PrO.
We expect our method to be useful in studying compounds of - and heavy-
elements.Comment: 11 pages, 4 figure
Modeling of the Reservoir Effect on Electromigration Lifetime
Electromigration behaviour in W-plug/metal stripe structures is different from conventional metal-strip structures because there is a blocking boundary formed by the immobile W-plug in the contact/via. Electromigration failures occur more readily close to the W-plug than in metal-strip structures because metal ions are forced away from the contacts/vias by electric current, blocking the contacts/vias area. Several works have reported electromigration lifetime of multiple level interconnects to be influenced by the presence of a reservoir around the contacts/vias. Reservoirs are metal parts that are not or are hardly conducting current that act as a source to provide atoms for the area around the blocking boundary where the atoms migrate away due to the electric current. Interconnect lifetime can be prolonged by using the reservoirs, called the Âżreservoir effectÂż. 2D simulation of the effects of reservoirs has been performed. The stress build-up during electromigration in the contact area can be simulated for several configurations, separating the effects of overlap, total reservoir area, the reservoir layout directions (vertical and horizontal), number of contacts/vias and contact/via placement. It is very useful for IC design rules to estimate which parameters are important for IC reliability. In this study, we considered the critical stress that the metal line can sustain before void formation as failure criterion. The failure time is determined by the time to reach the critical stres
High-Performance Deep SubMicron CMOS Technologies with Polycrystalline-SiGe Gates
The use of polycrystalline SiGe as the gate material for deep submicron CMOS has been investigated. A complete compatibility to standard CMOS processing is demonstrated when polycrystalline Si is substituted with SiGe (for Ge fractions below 0.5) to form the gate electrode of the transistors. Performance improvements are achieved for PMOS transistors by careful optimization of both transistor channel profile and p-type gate workfunction, the latter by changing Ge mole fraction in the gate. For the 0.18 Âżm CMOS generation we record up to 20% increase in the current drive, a 10% increase in the channel transconductance and subthreshold swing improvement from 82 mV/dec to 75 mV/dec resulting in excellent ÂżonÂż/ÂżoffÂż currents ratio. At the same time, NMOS transistor performance is not affected by gate material substitutio
Testable polarization predictions for models of CMB isotropy anomalies
Anomalies in the large-scale CMB temperature sky measured by WMAP have been
suggested as possible evidence for a violation of statistical isotropy on large
scales. In any physical model for broken isotropy, there are testable
consequences for the CMB polarization field. We develop simulation tools for
predicting the polarization field in models that break statistical isotropy
locally through a modulation field. We study two different models: dipolar
modulation, invoked to explain the asymmetry in power between northern and
southern ecliptic hemispheres, and quadrupolar modulation, posited to explain
the alignments between the quadrupole and octopole. For the dipolar case, we
show that predictions for the correlation between the first 10 multipoles of
the temperature and polarization fields can typically be tested at better than
the 98% CL. For the quadrupolar case, we show that the polarization quadrupole
and octopole should be moderately aligned. Such an alignment is a generic
prediction of explanations which involve the temperature field at recombination
and thus discriminate against explanations involving foregrounds or local
secondary anisotropy. Predicted correlations between temperature and
polarization multipoles out to l = 5 provide tests at the ~ 99% CL or stronger
for quadrupolar models that make the temperature alignment more than a few
percent likely. As predictions of anomaly models, polarization statistics move
beyond the a posteriori inferences that currently dominate the field.Comment: 17 pages, 15 figures; published in PRD; references adde
Magnetic defects promote ferromagnetism in Zn1-xCoxO
Experimental studies of Zn1-xCoxO as thin films or nanocrystals have found
ferromagnetism and Curie temperatures above room temperature and that p- or
n-type doping of Zn1-xCoxO can change its magnetic state. Bulk Zn1-xCoxO with a
low defect density and x in the range used in experimental thin film studies
exhibits ferromagnetism only at very low temperatures. Therefore defects in
thin film samples or nanocrystals may play an important role in promoting
magnetic interactions between Co ions in Zn1-xCoxO. The electronic structures
of Co substituted for Zn in ZnO, Zn and O vacancies, substituted N and
interstitial Zn in ZnO were calculated using the B3LYP hybrid density
functional in a supercell. The B3LYP functional predicts a band gap of 3.34 eV
for bulk ZnO, close to the experimental value of 3.47 eV. Occupied minority
spin Co 3d levels are at the top of the valence band and unoccupied levels lie
above the conduction band minimum. Majority spin Co 3d levels hybridize
strongly with bulk ZnO states. The neutral O vacancy and interstitial Zn are
deep and shallow donors, respectively. The Zn vacancy is a deep acceptor and
the acceptor level for substituted N is at mid gap. The possibility that p- or
n-type dopants promote exchange coupling of Co ions was investigated by
computing total energies of magnetic states of ZnO supercells containing two Co
ions and an oxygen vacancy, substituted N or interstitial Zn in various charge
states. The neutral N defect and the singly-positively charged O vacancy are
the only defects which strongly promote ferromagnetic exchange coupling of Co
ions at intermediate range.Comment: 9 pages, 11 figure
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