6,014 research outputs found
Electron quantum dynamics in closed and open potentials at high magnetic fields: Quantization and lifetime effects unified by semicoherent states
We have developed a Green's function formalism based on the use of an
overcomplete semicoherent basis of vortex states, specially devoted to the
study of the Hamiltonian quantum dynamics of electrons at high magnetic fields
and in an arbitrary potential landscape smooth on the scale of the magnetic
length. This formalism is used here to derive the exact Green's function for an
arbitrary quadratic potential in the special limit where Landau level mixing
becomes negligible. This solution remarkably embraces under a unified form the
cases of confining and unconfining quadratic potentials. This property results
from the fact that the overcomplete vortex representation provides a more
general type of spectral decomposition of the Hamiltonian operator than usually
considered. Whereas confining potentials are naturally characterized by
quantization effects, lifetime effects emerge instead in the case of
saddle-point potentials. Our derivation proves that the appearance of lifetimes
has for origin the instability of the dynamics due to quantum tunneling at
saddle points of the potential landscape. In fact, the overcompleteness of the
vortex representation reveals an intrinsic microscopic irreversibility of the
states synonymous with a spontaneous breaking of the time symmetry exhibited by
the Hamiltonian dynamics.Comment: 19 pages, 4 figures ; a few typos corrected + some passages in Sec. V
rewritte
Diagrammatic Approach for the High-Temperature Regime of Quantum Hall Transitions
We use a general diagrammatic formalism based on a local conductivity
approach to compute electronic transport in continuous media with long-range
disorder, in the absence of quantum interference effects. The method allows us
then to investigate the interplay of dissipative processes and random drifting
of electronic trajectories in the high-temperature regime of quantum Hall
transitions. We obtain that the longitudinal conductance \sigma_{xx} scales
with an exponent {\kappa}=0.767\pm0.002 in agreement with the value
{\kappa}=10/13 conjectured from analogies to classical percolation. We also
derive a microscopic expression for the temperature-dependent peak value of
\sigma_{xx}, useful to extract {\kappa} from experiments.Comment: 4+epsilon pages, 5 figures, attached with Supplementary Material. A
discussion and a plot of the temperature-dependent longitudinal conductance
was added in the final versio
Localization of quantum wave packets
We study the semiclassical propagation of squeezed Gau{\ss}ian states. We do
so by considering the propagation theorem introduced by Combescure and Robert
\cite{CR97} approximating the evolution generated by the Weyl-quantization of
symbols . We examine the particular case when the Hessian
evaluated at the corresponding solution of
Hamilton's equations of motion is periodic in time. Under this assumption, we
show that the width of the wave packet can remain small up to the Ehrenfest
time. We also determine conditions for ``classical revivals'' in that case.
More generally, we may define recurrences of the initial width. Some of these
results include the case of unbounded classical motion. In the classically
unstable case we recover an exponential spreading of the wave packet as in
\cite{CR97}
Children's suggestibility in relation to their understanding about sources of knowledge
In the experiments reported here, children chose either to maintain their initial belief about an object's identity or to accept the experimenter's contradicting suggestion. Both 3– to 4–year–olds and 4– to 5–year–olds were good at accepting the suggestion only when the experimenter was better informed than they were (implicit source monitoring). They were less accurate at recalling both their own and the experimenter's information access (explicit recall of experience), though they performed well above chance. Children were least accurate at reporting whether their final belief was based on what they were told or on what they experienced directly (explicit source monitoring). Contrasting results emerged when children decided between contradictory suggestions from two differentially informed adults: Three– to 4–year–olds were more accurate at reporting the knowledge source of the adult they believed than at deciding which suggestion was reliable. Decision making in this observation task may require reflective understanding akin to that required for explicit source judgments when the child participates in the task
Hall effect in quasi one-dimensional organic conductors
We study the Hall effect in a system of weakly coupled Luttinger Liquid
chains, using a Memory function approach to compute the Hall constant in the
presence of umklapp scattering along the chains. In this approximation, the
Hall constant decomposes into two terms: a high-frequency term and a Memory
function term. For the case of zero umklapp scattering, where the Memory
function vanishes, the Hall constant is simply the band value, in agreement
with former results in a similar model with no dissipation along the chains.
With umklapp scattering along the chains, we find a power-law temperature
dependance of the Hall constant. We discuss the applications to quasi 1D
organic conductors at high temperatures.Comment: Proceedings of the ISCOM conference "Sixth International Symposium on
Crystalline Organic Metals, Superconductors, and Ferromagnets", Key West,
Florida, USA (Sept. 2005), to be plublished in the Journal of Low Temperature
Physic
Quantum phase transition of dynamical resistance in a mesoscopic capacitor
We study theoretically dynamic response of a mesoscopic capacitor, which
consists of a quantum dot connected to an electron reservoir via a point
contact and capacitively coupled to a gate voltage. A quantum Hall edge state
with a filling factor nu is realized in a strong magnetic field applied
perpendicular to the two-dimensional electron gas. We discuss a noise-driven
quantum phase transition of the transport property of the edge state by taking
into account an ohmic bath connected to the gate voltage. Without the noise,
the charge relaxation for nu>1/2 is universally quantized at R_q=h/(2e^2),
while for nu<1/2, the system undergoes the Kosterlitz-Thouless transtion, which
drastically changes the nature of the dynamical resistance. The phase
transition is facilitated by the noisy gate voltage, and we see that it can
occur even for an integer quantum Hall edge at nu=1. When the dissipation by
the noise is sufficiently small, the quantized value of R_q is shifted by the
bath impedance.Comment: 5 pages, 2 figures, proceeding of the 19th International Conference
on the Application of High Magnetic Fields in Semiconductor Physics and
Nanotechnology (HMF-19
The actual annual occurrence of the green lacewings of northwestern Europe (Neuroptera: Chrysopidae)
Quantitative surveys of chrysopids from northwestern Europe were analysed. A total of thirty-five species are known within the zone although only twenty-six were recorded. Only the common green lacewings (i.e. the sibling species of the Chrysoperla carnea complex, here not differentiated) were elsewhere abundant comprising more than 3/4 of the specimens in all countries and reaching 97 % in Belgium. For the scarcer species, comments are given on their enhanced geographic range. The French fauna shows 19 species, six are exceptional (< 0.1%) such as the Atlanto-Mediterranean Dichochrysa picteti . Five species are considered rare (1<Q ≤ 5 %): Chrysopa perla , Ch. phyllochroma , Dichochrysa flavifrons, D. inornata and D. prasina. The fauna of both Great Britain and Ireland has the same faunistical richness but manifests a more balanced equitability. Chrysopa perla , Dichochrysa flavifrons and Cunctochrysa albolineata are uncommon (5 < Q ≤ 15 %), the others are at least rare. Belgium and Luxemburg gave 16 species and a very low diversity. Hypochrysa elegans, Nineta vittata, N. principiae and Chrysopa pallens are exceptional. Comments are given on some underestimated species, such as Dichochrysa mariana and Cunctochrysa bellifontensis not unanimously agreed, and D. abdominalis too recently re-instated to be identified in many collections
Dissipative phase-fluctuations in superconducting wires capacitively coupled to diffusive metals
We study the screening of the Coulomb interaction in a quasi one-dimensional
superconductor given by the presence of either a one- or a two-dimensional
non-interacting electron gas. To that end, we derive an effective low-energy
phase-only action, which amounts to treating the Coulomb and superconducting
correlations in the random-phase approximation. We concentrate on the study of
dissipation effects in the superconductor, induced by the effect of Coulomb
coupling to the diffusive density-modes in the metal, and study its
consequences on the static and dynamic conductivity. Our results point towards
the importance of the dimensionality of the screening metal in the behavior of
the superconducting plasma mode of the wire at low energies. In absence of
topological defects, and when the screening is given by a one-dimensional
electron gas, the superconducting plasma mode is completely damped in the limit
, and consequently superconductivity is lost in the wire. In contrast,
we recover a Drude-response in the conductivity when the screening is provided
by a two-dimensional electron gas.Comment: 16 pages, 8 figures, 1 table, 2 appendice
Pressure effects on the magnetic structure in La0.5Ca0.5-xSrxMnO3 (0.1 -< x -< 0.4) manganites
The effect of high pressure (0 - 8 GPa) on the magnetic structure of
polycrystalline samples of La0.5Ca0.5-xSrxMnO3 (0.1 -< x -< 0.4) manganites at
5 K is investigated using neutron diffraction technique. Application of
pressure is found to modify the previously reported magnetic structure,
observed under ambient conditions, in these compounds [I. Dhiman et al., Phys.
Rev. B 77, 094440 (2008)]. In x = 0.1 composition, at 4.6(2) GPa and beyond,
A-type antiferromagnetic structure is found to coexist with CE-type
antiferromagnetic phase, observed at ambient pressure, with TN ~ 150 K. For x =
0.3 sample, as a function of pressure the CE-type phase is fully suppressed at
2.3(1) GPa and A-type antiferromagnetic phase is favored. Further Sr doping at
x = 0.4, the A-type antiferromagnetic phase is observed at ambient pressure and
for T < TN (~ 250K). This phase is retained in the studied pressure range.
However, the magnetic moment progressively reduces with increasing pressure,
indicating the suppression of A-type antiferromagnetic phase. The present study
brings out the fragile nature of the CE-type antiferromagnetic state in half
doped manganites as a function of pressure and disorder \sigma 2. We observe
that pressure required for destabilizing the CE-type antiferromagnetic state is
reduced with increasing disorder \sigma 2. External pressure and changing
A-site ionic radii have analogous effect on the magnetic structure.Comment: 9 pages, 6 figures, 1 table, To appear in Physical Review
UV-Induced Structural Changes of Model DNA Helices Probed by Optical Spectroscopy
International audienceChemical alterations of DNA, if not repaired, may lead to carcinogenic mutations. Structural modifications of the helix around the lesion enable its recognition by repair enzymes. We have used absorption spectroscopy and mass spectrometry to detect structural changes provoked by cyclobutane thymine dimers, the major lesion induced by UV radiation. We found that formation of a cyclobutane dimer in the model duplex (dA)20 · (dT)20 destroys base stacking on the adenine strand. The physical background of this novel approach is the existence of charge-transfer states among neighboring bases, whose contribution to the hypochromism of the helix disappears following destacking
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