577 research outputs found
Contribution to the Performance Determination of Microjets
Theoretical investigations are made on the performance of microjets. A description is given of experiments with micropropulsion units to correlate the results obtained in the first part of the report. Execution of performance measurements is discussed, and error calculations are presented
Lieb-Mattis ferrimagnetism in diluted magnetic semiconductors
We show the possibility of long-range ferrimagnetic ordering with a
saturation magnetisation of the order of 1 Bohr magneton per spin for
arbitrarily low concentration of magnetic impurities in semiconductors,
provided that the impurities form a superstructure satisfying the conditions of
the Lieb-Mattis theorem. Explicit examples of such superstructures are given
for the wurtzite lattice, and the temperature of ferrimagnetic transition is
estimated from a high-temperature expansion. Exact diagonalization studies show
that small fragments of the structure exhibit enhanced magnetic response and
isotropic superparamagnetism at low temperatures. A quantum transition in a
high magnetic field is considered and similar superstructures in cubic
semiconductors are discussed as well.Comment: 6 pages,4 figure
Electronic structure and Jahn-Teller effect in GaN:Mn and ZnS:Cr
We present an ab-initio and analytical study of the Jahn-Teller effect in two
diluted magnetic semiconductors (DMS) with d4 impurities, namely Mn-doped GaN
and Cr-doped ZnS. We show that only the combined treatment of Jahn-Teller
distortion and strong electron correlation in the 3d shell may lead to the
correct insulating electronic structure. Using the LSDA+U approach we obtain
the Jahn-Teller energy gain in reasonable agreement with the available
experimental data. The ab-initio results are completed by a more
phenomenological ligand field theory.Comment: 15 pages, 5 figure
Planar spin exchange in LiNiO_2
We study the planar spin exchange couplings in LiNiO2 using a perturbative
approach. We show that the inclusion of the trigonal crystal field splitting at
the Oxygen sites leads to the appearance of antiferromagnetic exchange
integrals in deviation from the Goodenough-Kanamori-Anderson rules for this 90
degree bond. That gives a microscopic foundation for the recently observed
coexistence of ferromagnetic and antiferromagnetic couplings in the
orbitally-frustrated state of LiNiO2. (F. Reynaud et al, Phys. Rev. Lett. 86,
3638 (2001))Comment: latex, revtex4, 6 pages, 3 figure
Hole motion in an arbitrary spin background: Beyond the minimal spin-polaron approximation
The motion of a single hole in an arbitrary magnetic background is
investigated for the 2D t-J model. The wavefunction of the hole is described
within a generalized string picture which leads to a modified concept of spin
polarons. We calculate the one-hole spectral function using a large string
basis for the limits of a Neel ordered and a completely disordered background.
In addition we use a simple approximation to interpolate between these cases.
For the antiferromagnetic background we reproduce the well-known quasiparticle
band. In the disordered case the shape of the spectral function is found to be
strongly momentum-dependent, the quasiparticle weight vanishes for all hole
momenta. Finally, we discuss the relevance of results for the lowest energy
eigenvalue and its dispersion obtained from calculations using a polaron of
minimal size as found in the literature.Comment: 13 pages, 8 figures, to appear in Phys. Rev.
Theory of magnetic domains in uniaxial thin films
For uniaxial easy axis films, properties of magnetic domains are usually
described within the Kittel model, which assumes that domain walls are much
thinner than the domains. In this work we present a simple model that includes
a proper description of the magnetostatic energy of domains and domain walls
and also takes into account the interaction between both surfaces of the film.
Our model describes the behavior of domain and wall widths as a function of
film thickness, and is especially well suited for the strong stripe phase. We
prove the existence of a critical value of magneto-crystalline anisotropy above
which stripe domains exist for any film thickness and justify our model by
comparison with exact results. The model is in good agreement with experimental
data for hcp cobalt.Comment: 15 pages, 7 figure
How chemical pressure affects the fundamental properties of rare-earth pnictides: an ARPES view
Angle-resolved photoelectron spectroscopy, supplemented by theoretical
calculations has been applied to study the electronic structure of
heavy-fermion material CeFePO, a homologue to the Fe-based high-temperature
superconductors, and CeFeAs_0.7P_0.3O, where the applied chemical pressure
results in a ferromagnetic order of the 4f moments. A comparative analysis
reveals characteristic differences in the Fe-derived band structure for these
materials, implying a rather different hybridization of valence electrons to
the localized 4f orbitals. In particular, our results suggest that the
ferromagnetism of Ce moments in CeFeAs_0.7P_0.3O is mediated mainly by Fe
3d_xz/yz orbitals, while the Kondo screening in CeFePO is instead due to a
strong interaction of Fe 3d_3z^2-r^2 orbitals.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. B (Rapid
CeFePO: f-d hybridization and quenching of superconductivity
Being homologue to the new, Fe-based type of high-temperature
superconductors, CeFePO exhibits magnetism, Kondo and heavy-fermion phenomena.
We experimentally studied the electronic structure of CeFePO by means of
angle-resolved photoemission spectroscopy. In particular, contributions of the
Ce 4f-derived states and their hybridization to the Fe 3d bands were explored
using both symmetry selection rules for excitation and their photoionization
cross-section variations as a function of photon energy. It was experimentally
found - and later on confirmed by LDA as well as DMFT calculations - that the
Ce 4f states hybridize to the Fe 3d states of d_{3z^2-r^2} symmetry near the
Fermi level that discloses their participation in the occurring
electron-correlation phenomena and provides insight into mechanism of
superconductivity in oxopnictides.Comment: 5 pages, 3 figure
Classical Phase Fluctuations in Incommensurate Peierls Chains
In the pseudogap regime of one-dimensional incommensurate Peierls systems,
fluctuations of the phase of the order parameter prohibit the emergence of
long-range order and generate a finite correlation length. For classical phase
fluctuations, we present exact results for the average electronic density of
states, the mean localization length, the electronic specific heat and the spin
susceptibility at low temperatures. Our results for the susceptibility give a
good fit to experimental data.Comment: 4 Revtex pages, 4 figures, submitted to Phys. Rev. Let
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