1,616 research outputs found
Different origin of the ferromagnetic order in (Ga,Mn)As and (Ga,Mn)N
The mechanism for the ferromagnetic order of (Ga,Mn)As and (Ga,Mn)N is
extensively studied over a vast range of Mn concentrations. We calculate the
electronic structures of these materials using density functional theory in
both the local spin density approximation and the LDA+U scheme, that we have
now implemented in the code SIESTA.
For (Ga,Mn)As, the LDA+U approach leads to a hole mediated picture of the
ferromagnetism, with an exchange constant =~ -2.8 eV. This is smaller
than that obtained with LSDA, which overestimates the exchange coupling between
Mn ions and the As holes.
In contrast, the ferromagnetism in wurtzite (Ga,Mn)N is caused by the
double-exchange mechanism, since a hole of strong character is found at the
Fermi level in both the LSDA and the LDA+U approaches. In this case the
coupling between the Mn ions decays rapidly with the Mn-Mn separation. This
suggests a two phases picture of the ferromagnetic order in (Ga,Mn)N, with a
robust ferromagnetic phase at large Mn concentration coexisting with a diluted
weak ferromagnetic phase.Comment: 12 pages, 11 figure
Phase Diagram and Incommensurate Phases in Undoped Manganites
We study the existence of incommensurate phases in the phase diagram of the
two orbital double exchange model coupled with Jahn-Teller phonons and with
superexchange interactions. In agreement with experimental results, we find
that undoped manganites ( being some rare earth element) show
temperature induced commensurate-incommensurate phase transitions. In the
incommensurate phase the magnetic wave vector varies with temperature. The
incommensurate phase arises from the competition between the short range
antiferromagnetic superexchange interaction and the long range ferromagnetic
double exchange interaction
Thermoelastic Damping in Micro- and Nano-Mechanical Systems
The importance of thermoelastic damping as a fundamental dissipation
mechanism for small-scale mechanical resonators is evaluated in light of recent
efforts to design high-Q micrometer- and nanometer-scale electro-mechanical
systems (MEMS and NEMS). The equations of linear thermoelasticity are used to
give a simple derivation for thermoelastic damping of small flexural vibrations
in thin beams. It is shown that Zener's well-known approximation by a
Lorentzian with a single thermal relaxation time slightly deviates from the
exact expression.Comment: 10 pages. Submitted to Phys. Rev.
Coating thermal noise of a finite-size cylindrical mirror
Thermal noise of a mirror is one of the limiting noise sources in the high
precision measurement such as gravitational-wave detection, and the modeling of
thermal noise has been developed and refined over a decade. In this paper, we
present a derivation of coating thermal noise of a finite-size cylindrical
mirror based on the fluctuation-dissipation theorem. The result agrees to a
previous result with an infinite-size mirror in the limit of large thickness,
and also agrees to an independent result based on the mode expansion with a
thin-mirror approximation. Our study will play an important role not only to
accurately estimate the thermal-noise level of gravitational-wave detectors but
also to help analyzing thermal noise in quantum-measurement experiments with
lighter mirrors.Comment: 13 pages, 4 figure
Disorder-Induced First Order Transition and Curie Temperature Lowering in Ferromagnatic Manganites
We study the effect that size disorder in the cations surrounding manganese
ions has on the magnetic properties of manganites. This disorder is mimic with
a proper distribution of spatially disordered Manganese energies. Both, the
Curie temperature and the order of the transition are strongly affected by
disorder. For moderate disorder the Curie temperature decreases linearly with
the the variance of the distribution of the manganese site energies, and for a
disorder comparable to that present in real materials the transition becomes
first order. Our results provide a theoretical framework to understand disorder
effects on the magnetic behavior of manganites.Comment: 4 pages, three figures include
Magnetic Transition Temperature of (La,Sr)MnO
Using the Kondo lattice model with classical spins in infinite dimension,
magnetic phase transition in the perovskite-type transition-metal oxide
(La,Sr)MnO is theoretically studied. On the Bethe lattice, the
self-consistency equations are solved exactly. Curie temperatures at the region
of double-exchange ferromagnetism as well as the Neel
temperature at are well reproduced quantitatively. Pressure effect on the
Curie temperature is also discussed.Comment: 7 pages, 1 PS file with 3 figures appended at the end, LaTe
Bloch oscillations in Fermi gases
The possibility of Bloch oscillations for a degenerate and superfluid Fermi
gas of atoms in an optical lattice is considered. For a one-component
degenerate gas the oscillations are suppressed for high temperatures and band
fillings. For a two-component gas the Landau criterion is used for specifying
the regime where Bloch oscillations of the superfluid may be observed. We show
how the amplitude of Bloch oscillations varies along the BCS-BEC crossover.Comment: 4 pages, 2 figures. explanations adde
Point defect in solids: Shear dominance of the far-field energy
It is shown that the elastic energy far from a point defect in an isotropic
solid is mainly shear elastic energy. The calculation, which is based on a
standard dipole expansion, shows that no matter how large or small the bulk
modulus is compared to the shear modulus, less than 10% of the distant point
defect energy is associated with volume changes.Comment: Brief not
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