29,560 research outputs found
Above-room-temperature ferromagnetism in half-metallic Heusler compounds NiCrP, NiCrSe, NiCrTe and NiVAs: A first-principles study
We study the interatomic exchange interactions and Curie temperatures in
half-metallic semi Heusler compounds NiCrZ (Z=P, Se, Te) and NiVAs. The study
is performed within the framework of density functional theory. The calculation
of exchange parameters is based on the frozen-magnon approach. It is shown that
the exchange interactions in NiCrZ vary strongly depending on the Z
constituent. The Curie temperature, Tc, is calculated within the mean field and
random phase approximations. The difference between two estimations is related
to the properties of the exchange interactions. The predicted Curie
temperatures of all four systems are considerably higher than room temperature.
The relation between the half-metallicity and the value of the Curie
temperature is discussed. The combination of a high spin-polarization of charge
carriers and a high Curie temperature makes these Heusler alloys interesting
candidates for spintronics applications.Comment: 6 pages, 3 figure
Stability of ferromagnetism in the half-metallic pnictides and similar compounds: A first-principles study
Based on first-principles electron structure calculations and employing the
frozen-magnon approximation we study the exchange interactions in a series of
transition-metal binary alloys crystallizing in the zinc-blende structure and
calculate the Curie temperature within both the mean-field approximation (MFA)
and random-phase approximation (RPA). We study two Cr compounds, CrAs and CrSe,
and four Mn compounds: MnSi, MnGe, MnAs and MnC. MnC, MnSi and MnGe are
isovalent to CrAs and MnAs is isoelectronic with CrSe. Ferromagnetism is
particular stable for CrAs, MnSi and MnGe: All three compounds show Curie
temperatures around 1000 K. On the other hand, CrSe and MnAs show a tendency to
antiferromagnetism when compressing the lattice. In MnC the half-metallic gap
is located in the majority-spin channel contrary to the other five compounds.
The large half-metallic gaps, very high Curie temperatures, the stability of
the ferromagnetism with respect to the variation of the lattice parameter and a
coherent growth on semiconductors make MnSi and CrAs most promising candidates
for the use in spintronics devises.Comment: 17 pages, 6 figure
Sampling-Based Query Re-Optimization
Despite of decades of work, query optimizers still make mistakes on
"difficult" queries because of bad cardinality estimates, often due to the
interaction of multiple predicates and correlations in the data. In this paper,
we propose a low-cost post-processing step that can take a plan produced by the
optimizer, detect when it is likely to have made such a mistake, and take steps
to fix it. Specifically, our solution is a sampling-based iterative procedure
that requires almost no changes to the original query optimizer or query
evaluation mechanism of the system. We show that this indeed imposes low
overhead and catches cases where three widely used optimizers (PostgreSQL and
two commercial systems) make large errors.Comment: This is the extended version of a paper with the same title and
authors that appears in the Proceedings of the ACM SIGMOD International
Conference on Management of Data (SIGMOD 2016
Propagation of Correlations in Quantum Lattice Systems
We provide a simple proof of the Lieb-Robinson bound and use it to prove the
existence of the dynamics for interactions with polynomial decay. We then use
our results to demonstrate that there is an upper bound on the rate at which
correlations between observables with separated support can accumulate as a
consequence of the dynamics.Comment: 10 page
Temperature and frequency dependent optical properties of ultra-thin Au films
While the optical properties of thin metal films are well understood in the
visible and near-infrared range, little has been done in the mid- and
far-infrared region. Here we investigate ultra-thin gold films prepared on
Si(111)(7 x 7) in UHV by measuring in the frequency range between 500 cm-1 and
7000 cm-1 and for temperatures between 300 K and 5 K. The nominal thickness of
the gold layers ranges between one monolayer and 9 nm. The frequency and
temperature dependences of the thicker films can be well described by the Drude
model of a metal, when taking into account classical size effects due to
surface scattering. The films below the percolation threshold exhibit a
non-metallic behavior: the reflection increases with frequency and decreases
with temperature. The frequency dependence can partly be described by a
generalized Drude model. The temperature dependence does not follow a simple
activation process. For monolayers we observe a transition between surface
states around 1100 cm-1.Comment: 7 pages, 10 figure
Distillation by repeated measurements: continuous spectrum case
Repeated measurements on a part of a bipartite system strongly affect the
other part not measured, whose dynamics is regulated by an effective contracted
evolution operator. When the spectrum of this operator is discrete, the latter
system is driven into a pure state irrespective of the initial state, provided
the spectrum satisfies certain conditions. We here show that even in the case
of continuous spectrum an effective distillation can occur under rather general
conditions. We confirm it by applying our formalism to a simple model.Comment: 4 pages, 2 figure
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