1,436 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
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.
Resonant tunneling in a schematic model
Tunneling of an harmonically bound two-body system through an external
Gaussian barrier is studied in a schematic model which allows for a better
understanding of intricate quantum phenomena. The role of finite size and
internal structure is investigated in a consistent treatment. The excitation of
internal degrees of freedom gives rise to a peaked structure in the penetration
factor. The model results indicate that for soft systems the adiabatic limit is
not necessarily reached although often assumed in fusion of nuclei and in
electron screening effects at astrophysical energies.Comment: 7 pages, 7 figure
Magnetization Process of Nanoscale Iron Cluster
Low-temperature magnetization process of the nanoscale iron cluster in
linearly sweeped fields is investigated by a numerical analysis of
time-dependent Schrdinger equation and the quantum master
equation. We introduce an effective basis method extracting important states,
by which we can obtain the magnetization process effectively. We investigate
the structure of the field derivative of the magnetization. We find out that
the antisymmetric interaction determined from the lattice structure reproduces
well the experimental results of the iron magnets and that this interaction
plays an important role in the iron cluster. Deviations from the adiabatic
process are also studied. In the fast sweeping case, our calculations indicate
that the nonadiabatic transition dominantly occurs at the level crossing for
the lowest field. In slow sweeping case, due to the influence of the thermal
environment to the spin system, the field derivative of the magnetization shows
an asymmetric behavior, the magnetic Fhn effect, which explains
the substructure of the experimental results in the pulsed field.Comment: 5 pages of text and 2 pages of 6 figures. To appear in J. Phys. Soc.
Jp
Probing neutrino mass hierarchies and with supernova neutrinos
We investigate the feasibility of probing the neutrino mass hierarchy and the
mixing angle with the neutrino burst from a future supernova. An
inverse power-law density with varying is adopted in the
analysis as the density profile of a typical core-collapse supernova. The
survival probabilities of and are shown to reduce to
two-dimensional functions of and . It is found that in the
parameter space, the 3D plots of the probability
functions exhibit highly non-trivial structures that are sensitive to the mass
hierarchy, the mixing angle , and the value of . The conditions
that lead to observable differences in the 3D plots are established. With the
uncertainty of considered, a qualitative analysis of the Earth matter
effect is also included.Comment: 16 pages, 3 figures. Ref [11] added, and some typos correcte
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
Neutrino oscillations in matter of varying density
We consider two-family neutrino oscillations in a medium of
continuously-varying density as a limit of the process in a series of
constant-density layers. We construct analytic expressions for the conversion
amplitude at high energies within a medium with a density profile that is
piecewise linear. We compare some cases to understand the type of effects that
depend on the order of the material traversed by a neutrino beam.Comment: 10 page
Observation of quantum oscillations between a Josephson phase qubit and a microscopic resonator using fast readout
We have detected coherent quantum oscillations between Josephson phase qubits
and microscopic critical-current fluctuators by implementing a new state
readout technique that is an order of magnitude faster than previous methods.
The period of the oscillations is consistent with the spectroscopic splittings
observed in the qubit's resonant frequency. The results point to a possible
mechanism for decoherence and reduced measurement fidelity in superconducting
qubits and demonstrate the means to measure two-qubit interactions in the time
domain
On the nature of ferromagnetism in diluted magnetic semiconductors: GaAs:Mn, GaP:Mn
A microscopic Hamiltonian for interacting manganese impurities in diluted
magnetic semiconductors (DMS) is derived. It is shown that in p -type III-V DMS
the indirect exchange between Mn impurities has similarities with the Zener
mechanism in transition metal oxides. Here the mobile holes and localized
states near the top of the valence band play the role of unoccupied oxygen
orbitals which induce ferromagnetism. The Curie temperature estimated from the
proposed kinematic exchange agrees with recent experiments on GaAs:Mn. The
model is also applicable to the GaP:Mn system.Comment: 10 pages, 3 figures. Submitted to Europhysics Letters, June 25, 200
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