14,002 research outputs found
Initial pseudo-steady state & asymptotic KPZ universality in semiconductor on polymer deposition
The Kardar-Parisi-Zhang (KPZ) class is a paradigmatic example of universality
in nonequilibrium phenomena, but clear experimental evidences of asymptotic
2D-KPZ statistics are still very rare, and far less understanding stems from
its short-time behavior. We tackle such issues by analyzing surface
fluctuations of CdTe films deposited on polymeric substrates, based on a huge
spatio-temporal surface sampling acquired through atomic force microscopy. A
\textit{pseudo}-steady state (where average surface roughness and spatial
correlations stay constant in time) is observed at initial times, persisting up
to deposition of monolayers. This state results from a fine
balance between roughening and smoothening, as supported by a phenomenological
growth model. KPZ statistics arises at long times, thoroughly verified by
universal exponents, spatial covariance and several distributions. Recent
theoretical generalizations of the Family-Vicsek scaling and the emergence of
log-normal distributions during interface growth are experimentally confirmed.
These results confirm that high vacuum vapor deposition of CdTe constitutes a
genuine 2D-KPZ system, and expand our knowledge about possible
substrate-induced short-time behaviors.Comment: 13 pages, 8 figures, 2 table
Temperature effect on (2+1) experimental Kardar-Parisi-Zhang growth
We report on the effect of substrate temperature (T) on both local structure
and long-wavelength fluctuations of polycrystalline CdTe thin films deposited
on Si(001). A strong T-dependent mound evolution is observed and explained in
terms of the energy barrier to inter-grain diffusion at grain boundaries, as
corroborated by Monte Carlo simulations. This leads to transitions from
uncorrelated growth to a crossover from random-to-correlated growth and
transient anomalous scaling as T increases. Due to these finite-time effects,
we were not able to determine the universality class of the system through the
critical exponents. Nevertheless, we demonstrate that this can be circumvented
by analyzing height, roughness and maximal height distributions, which allow us
to prove that CdTe grows asymptotically according to the Kardar-Parisi-Zhang
(KPZ) equation in a broad range of T. More important, one finds positive
(negative) velocity excess in the growth at low (high) T, indicating that it is
possible to control the KPZ non-linearity by adjusting the temperature.Comment: 6 pages, 5 figure
Quantum critical point in the spin glass-antiferromagnetism competition in Kondo-lattice systems
A theory is proposed to describe the competition among antiferromagnetism
(AF), spin glass (SG) and Kondo effect. The model describes two Kondo
sublattices with an intrasite Kondo interaction strength and an
interlattice quantum Ising interaction in the presence of a transverse field
. The interlattice coupling is a random Gaussian distributed variable
(with average and variance ) while the field is
introduced as a quantum mechanism to produce spin flipping. The path integral
formalism is used to study this fermionic problem where the spin operators are
represented by bilinear combinations of Grassmann fields. The disorder is
treated within the framework of the replica trick. The free energy and the
order parameters of the problem are obtained by using the static ansatz and by
choosing both and to allow, as previously,
a better comparison with the experimental findings.
The results indicate the presence of a SG solution at low and for
temperature ( is the freezing temperature). When is
increased, a mixed phase AF+SG appears, then an AF solution and finally a Kondo
state is obtained for high values of . Moreover, the behaviors of the
freezing and Neel temperatures are also affected by the relationship between
and the transverse field . The first one presents a slight
decrease while the second one decreases towards a Quantum Critical Point (QCP).
The obtained phase diagram has the same sequence as the experimental one for
, if is assumed to increase with , and
in addition, it also shows a qualitative agreement concerning the behavior of
the freezing and the Neel temperatures.Comment: 11 pages, 3 figures, accepted for publication in J. Phys.
Characterization of the Intra-Unit-Cell magnetic order in Bi2Sr2CaCu2O8+d
As in YBa2Cu3O6+x and HgBa2CuO8+d, the pseudo-gap state in Bi2Sr2CaCu2O8+d is
characterized by the existence of an intra-unit-cell magnetic order revealed by
polarized neutron scattering technique. We report here a supplementary set of
polarized neutron scattering measurements for which the direction of the
magnetic moment is determined and the magnetic intensity is calibrated in
absolute units. The new data allow a close comparison between bilayer systems
YBa2Cu3O6+x and Bi2Sr2CaCu2O8+d and rise important questions concerning the
range of the magnetic correlations and the role of disorder around optimal
doping.Comment: 12 pages, 8 figures, submitted to physical review
Alternate islands of multiple isochronous chains in wave-particle interactions
We analyze the dynamics of a relativistic particle moving in a uniform
magnetic field and perturbed by a standing electrostatic wave. We show that a
pulsed wave produces an infinite number of perturbative terms with the same
winding number, which may generate islands in the same region of phase space.
As a consequence, the number of isochronous island chains varies as a function
of the wave parameters. We observe that in all the resonances, the number of
chains is related to the amplitude of the various resonant terms. We determine
analytically the position of the periodic points and the number of island
chains as a function of the wave number and wave period. Such information is
very important when one is concerned with regular particle acceleration, since
it is necessary to adjust the initial conditions of the particle to obtain the
maximum acceleration.Comment: Submitte
Are Brazilian innovation systems innovative? Regional and sectorial decompositions of triple-helix synergies
A knowledge-based economy adds innovation as another dynamic to a political economy. Whereas a political economy is institutionalized-for example, in a nation state-the knowledge base is volatile although it leaves footprints behind by transforming the institutional layers. This transformation into new options can be measured as redundancy using the Tri-ple-Helix indicator. The balance between historical entropy generation and the knowledge-based generation of options can be measured in terms of positive and negative contributions to the prevailing uncertainty. At what scale and in which sectors is synergy among geographical, technological, and organizational distributions of firms evident? Using mutual information in the three dimensions as an indicator, we analyse a dataset of more than 16 million firms in Brazil and compute synergy within and across states and sectors in this country in terms of bits of information. The results suggest that no synergy is generated at the national level. The political economy of the country has not (yet) been transformed into a national innovation system. At state level, synergies vary according to geographical levels and sectors due to the specifics of the states. Above-average values were found for some states in the South and Southeast Regions. Also, the political capital, Brasilia, has resulted to have no impact in the innovation system of Brazil
Living bacteria rheology: population growth, aggregation patterns and cooperative behaviour under different shear flows
The activity of growing living bacteria was investigated using real-time and
in situ rheology -- in stationary and oscillatory shear. Two different strains
of the human pathogen Staphylococcus aureus -- strain COL and its isogenic cell
wall autolysis mutant -- were considered in this work. For low bacteria
density, strain COL forms small clusters, while the mutant, presenting
deficient cell separation, forms irregular larger aggregates. In the early
stages of growth, when subjected to a stationary shear, the viscosity of both
strains increases with the population of cells. As the bacteria reach the
exponential phase of growth, the viscosity of the two strains follow different
and rich behaviours, with no counterpart in the optical density or in the
population's colony forming units measurements. While the viscosity of strain
COL keeps increasing during the exponential phase and returns close to its
initial value for the late phase of growth, where the population stabilizes,
the viscosity of the mutant strain decreases steeply, still in the exponential
phase, remains constant for some time and increases again, reaching a constant
plateau at a maximum value for the late phase of growth. These complex
viscoelastic behaviours, which were observed to be shear stress dependent, are
a consequence of two coupled effects: the cell density continuous increase and
its changing interacting properties. The viscous and elastic moduli of strain
COL, obtained with oscillatory shear, exhibit power-law behaviours whose
exponent are dependent on the bacteria growth stage. The viscous and elastic
moduli of the mutant have complex behaviours, emerging from the different
relaxation times that are associated with the large molecules of the medium and
the self-organized structures of bacteria. These behaviours reflect
nevertheless the bacteria growth stage.Comment: 9 pages, 10 figure
Nanometric pitch in modulated structures of twist-bend nematic liquid crystals
The extended Frank elastic energy density is used to investigate the
existence of a stable periodically modulate structure that appears as a ground
state exhibiting a twist-bend molecular arrangement. For an unbounded sample,
we show that the twist-bend nematic phase is characterized by a
heliconical structure with a pitch in the nano-metric range, in agreement with
experimental results. For a sample of finite thickness, we show that the wave
vector of the stable periodic structure depends not only on the elastic
parameters but also on the anchoring energy, easy axis direction, and the
thickness of the sample.Comment: 11 page
Tetragonal tungsten bronze compounds: relaxor vs mixed ferroelectric - dipole glass behavior
We demonstrate that recent experimental data (E. Castel et al J.Phys. Cond.
Mat. {\bf 21} (2009), 452201) on tungsten bronze compound (TBC)
BaPrNdFeNbO can be well explained in our model
predicting a crossover from ferroelectric () to orientational (dipole)
glass (), rather then relaxor, behavior. We show, that since a "classical"
perovskite relaxor like Pb(Mn Nb)O is never a
ferroelectric, the presence of ferroelectric hysteresis loops in TBC shows that
this substance actually transits from ferroelectric to orientational glass
phase with growth. To describe the above crossover theoretically, we use
the simple replica-symmetric solution for disordered Ising model.Comment: 5 two-column pages, 4 figure
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