11,119 research outputs found
Half-metallic ferromagnets: From band structure to many-body effects
A review of new developments in theoretical and experimental electronic
structure investigations of half-metallic ferromagnets (HMF) is presented.
Being semiconductors for one spin projection and metals for another ones, these
substances are promising magnetic materials for applications in spintronics
(i.e., spin-dependent electronics). Classification of HMF by the peculiarities
of their electronic structure and chemical bonding is discussed. Effects of
electron-magnon interaction in HMF and their manifestations in magnetic,
spectral, thermodynamic, and transport properties are considered. Especial
attention is paid to appearance of non-quasiparticle states in the energy gap,
which provide an instructive example of essentially many-body features in the
electronic structure. State-of-art electronic calculations for correlated
-systems is discussed, and results for specific HMF (Heusler alloys,
zinc-blende structure compounds, CrO FeO) are reviewed.Comment: to be published in Reviews of Modern Physics, vol 80, issue
Observation of two-orbital spin-exchange interactions with ultracold SU(N)-symmetric fermions
We report on the direct observation of spin-exchanging interactions in a
two-orbital SU(N)-symmetric quantum gas of ytterbium in an optical lattice. The
two orbital states are represented by two different (meta-)stable electronic
configurations of fermionic Yb-173. A strong spin-exchange between particles in
the two separate orbitals is mediated by the contact interaction between atoms,
which we characterize by clock shift spectroscopy in a 3D optical lattice. We
find the system to be SU(N)-symmetric within our measurement precision and
characterize all relevant scattering channels for atom pairs in combinations of
the ground and the excited state. Elastic scattering between the orbitals is
dominated by the antisymmetric channel, which leads to the strong spin-exchange
coupling. The exchange process is directly observed, by characterizing the
dynamic equilibration of spin imbalances between two large ensembles in the two
orbital states, as well as indirectly in atom pairs via interaction shift
spectroscopy in a 3D lattice. The realization of a stable SU(N)-symmetric
two-orbital Hubbard Hamiltonian opens the route towards experimental quantum
simulation of condensed-matter models based on orbital interactions, such as
the Kondo lattice model.Comment: Correction: In the original version of this preprint the assignment
of states with symmetric electronic wavefunction (|eg+>) and with
antisymmetric electronic wavefunction (|eg->) to the observed spectral lines
was inverted. This has been corrected in the current version. The results of
the paper remain unchanged, with the exchange coupling being inverted to a
ferromagnetic exchang
Hard thermal loops with a background plasma velocity
I consider the calculation of the two and three-point functions for QED at
finite temperature in the presence of a background plasma velocity. The final
expressions are consistent with Lorentz invariance, gauge invariance and
current conservation, pointing to a straightforward generalization of the hard
thermal loop formalism to this physical situation. I also give the resulting
expression for the effective action and identify the various terms.Comment: 11 pages, no figure
Efficiency at maximum power of thermally coupled heat engines
We study the efficiency at maximum power of two coupled heat engines, using
thermoelectric generators (TEGs) as engines. Assuming that the heat and
electric charge fluxes in the TEGs are strongly coupled, we simulate
numerically the dependence of the behavior of the global system on the
electrical load resistance of each generator in order to obtain the working
condition that permits maximization of the output power. It turns out that this
condition is not unique. We derive a simple analytic expression giving the
relation between the electrical load resistance of each generator permitting
output power maximization. We then focuse on the efficiency at maximum power
(EMP) of the whole system to demonstrate that the Curzon-Ahlborn efficiency may
not always be recovered: the EMP varies with the specific working conditions of
each generator but remains in the range predicted by irreversible
thermodynamics theory. We finally discuss our results in light of non-ideal
Carnot engine behavior.Comment: 11 pages, 7 figure
Value orientations and environmental beliefs in five countries - Validity of an instrument to measure egoistic, altruistic and biospheric value orientations
Various scholars argue that egoistic, altruistic, and biospheric value orientations are important for understanding environmental beliefs and behavior. However, little empirical evidence has been provided for the distinction between altruistic and biospheric values. This study examines whether this distinction is valid across different countries (i.e., Austria, Czech Republic, Italy, the Netherlands, and Sweden) by using a new value instrument. Relationships between these value orientations and behavior-specific beliefs (i.e., awareness of environmental consequences and personal norms) are investigated to further examine the validity of the value instrument. Results provide support for the generalization of the three-way distinction. Furthermore, value orientations were related to behavior-specific beliefs in all countries. The authors conclude that the distinction between the three value orientations is valid and useful for examining environmentally relevant behavior.</p
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