95 research outputs found
Microalloying effect in ternary Al-Sm-X (X=Ag, Au, Cu) metallic glasses studied by ab initio molecular dynamics
The icosahedral-like polyhedral fraction (ICO-like fraction) has been studied
as a criterion for predicting the glass-forming ability of bulk ternary
metallic glasses, Al90Sm8X2 (X = Al (binary), Cu, Ag, Au), using ab initio
molecular dynamics (AIMD) simulations. We found that the ICO-like fraction can
be determined with adequate precision to explore correlations with AIMD
simulations. We then demonstrated that ICO-like fraction correlates with the
critical cooling rate, which is a widely used intrinsic measure of glass
forming ability. These results suggest that the ICO-like fraction from AIMD
simulations may offer a useful guide for searching and screening for good glass
formers
Spin instabilities and quantum phase transitions in integral and fractional quantum Hall states
The inter-Landau-level spin excitations of quantum Hall states at filling
factors nu=2 and 4/3 are investigated by exact numerical diagonalization for
the situation in which the cyclotron (hbar*omega_c) and Zeeman (E_Z) splittings
are comparable. The relevant quasiparticles and their interactions are studied,
including stable spin wave and skyrmion bound states. For nu=2, a spin
instability at a finite value of epsilon=hbar*omega_c-E_Z leads to an abrupt
paramagnetic to ferromagnetic transition, in agreement with the mean-field
approximation. However, for nu=4/3 a new and unexpected quantum phase
transition is found which involves a gradual change from paramagnetic to
ferromagnetic occupancy of the partially filled Landau level as epsilon is
decreased.Comment: 4 pages, 5 figures, submitted to Phys.Rev.Let
Resonant Enhancement of Inelastic Light Scattering in the Fractional Quantum Hall Regime at
Strong resonant enhancements of inelastic light scattering from the long
wavelength inter-Landau level magnetoplasmon and the intra-Landau level spin
wave excitations are seen for the fractional quantum Hall state at .
The energies of the sharp peaks (FWHM ) in the profiles of
resonant enhancement of inelastic light scattering intensities coincide with
the energies of photoluminescence bands assigned to negatively charged exciton
recombination. To interpret the observed enhancement profiles, we propose
three-step light scattering mechanisms in which the intermediate resonant
transitions are to states with charged excitonic excitations.Comment: 5 pages, 5 figure
Charged exctions in the fractional quantum Hall regime
We study the photoluminescence spectrum of a low density ()
two-dimensional electron gas at high magnetic fields and low temperatures. We
find that the spectrum in the fractional quantum Hall regime can be understood
in terms of singlet and triplet charged-excitons. We show that these spectral
lines are sensitive probes for the electrons compressibility. We identify the
dark triplet charged-exciton and show that it is visible at the spectrum at
K. We find that its binding energy scales like , where is
the magnetic length, and it crosses the singlet slightly above 15 T.Comment: 10 pages, 5 figure
Fractional Quantum Hall States of Clustered Composite Fermions
The energy spectra and wavefunctions of up to 14 interacting quasielectrons
(QE's) in the Laughlin nu=1/3 fractional quantum Hall (FQH) state are
investigated using exact numerical diagonalization. It is shown that at
sufficiently high density the QE's form pairs or larger clusters. This
behavior, opposite to Laughlin correlations, invalidates the (sometimes
invoked) reapplication of the composite fermion picture to the individual QE's.
The series of finite-size incompressible ground states are identified at the QE
filling factors nu_QE=1/2, 1/3, 2/3, corresponding to the electron fillings
nu=3/8, 4/11, 5/13. The equivalent quasihole (QH) states occur at nu_QH=1/4,
1/5, 2/7, corresponding to nu=3/10, 4/13, 5/17. All these six novel FQH states
were recently discovered experimentally. Detailed analysis indicates that QE or
QH correlations in these states are different from those of well-known FQH
electron states (e.g., Laughlin or Moore-Read states), leaving the origin of
their incompressibility uncertain. Halperin's idea of Laughlin states of QP
pairs is also explored, but is does not seem adequate.Comment: 14 pages, 9 figures; revision: 1 new figure, some new references,
some new data, title chang
Reverse Hall-Petch effect in ultra nanocrystalline diamond
We present atomistic simulations for the mechanical response of ultra
nanocrystalline diamond, a polycrystalline form of diamond with grain diameters
of the order of a few nm. We consider fully three-dimensional model structures,
having several grains of random sizes and orientations, and employ
state-of-the-art Monte Carlo simulations. We calculate structural properties,
elastic constants and the hardness of the material; our results compare well
with experimental observations for this material. Moreover, we verify that this
material becomes softer at small grain sizes, in analogy to the observed
reversal of the Hall-Petch effect in various nanocrystalline metals. The effect
is attributed to the large concentration of grain boundary atoms at smaller
grain sizes. Our analysis yields scaling relations for the elastic constants as
a function of the average grain size.Comment: Proceedings of the IUTAM Symposium on Modelling Nanomaterials and
Nanosystems, Aalborg, Denmark, May 19-22 2008; to be published in the IUTAM
Bookseries by Springe
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