1,405 research outputs found
Electron Correlation and Charge Transfer Instability in Bilayered Two Dimensional Electron Gas
We prove that the predicted charge transfer state in symmetric bilayers of
two dimensional electron gases is always unstable at zero bias voltage, due to
interlayer correlation and/or tunneling. This is most easily seen by resorting
to a pseudospin formalism and considering coherent states obtained from the
charge transfer state through rotations of the pseudospins. Evidently, the
charge transfer state is stabilized by a sufficiently strong gate voltage, as
found in recent experiments. We show that a simple model, in which the layers
are strictly two dimensional, is able to account quantitatively for such
experimental findings, when correlation is properly included.Comment: 5 pages, 3 figures. Subm. to Europhys. Let
Cross-spectral modelling of the black hole X-ray binary XTEJ1550-564: challenges to the propagating fluctuations paradigm
Timing properties of black hole X-ray binaries in outburst can be modeled
with mass accretion rate fluctuations propagating towards the black hole. Such
models predict time lags between energy bands due to propagation delays. First
application of a propagating fluctuations model to black hole power spectra
showed good agreement with the data. Indeed, hard lags observed from these
systems appear to be in agreement with this generic prediction. Our PROPFLUC
code allows to simultaneously predict power spectra, time lags, and coherence
of the variability as a function of energy. This was successfully applied to
Swift data on the black hole MAXIJ1659-152, fitting jointly the power spectra
in two energy bands and the cross-spectrum between these two bands. In the
current work, we attempt to to model two high signal to noise Rossi X-ray
Timing Explorer (RXTE) observations of the black hole XTE J1550-564. We find
that neither observation can be adequately explained by the model even when
considering, additionally to previous PROPFLUC versions, different propagation
speeds of the fluctuations. After extensive exploration of model extensions, we
tentatively conclude that the quantitative and qualitative discrepancy between
model predictions and data is generic to the propagating fluctuations paradigm.
This result encourages further investigation of the fundamental hypotheses of
the propagating fluctuations model. We discuss some of these hypotheses with an
eye to future works.Comment: 14 pages, 12 figures, accepted for publication in MNRA
Evolution of the hot flow of MAXI J1543-564
We present a spectral and timing analysis of the black hole candidate MAXI
J1543-564 during its 2011 outburst. As shown in previous work, the source
follows the standard evolution of a black hole outburst. During the rising
phase of the outburst we detect an abrupt change in timing behavior associated
with the occurrence of a type-B quasi-periodic oscillation (QPO). This QPO and
the simultaneously detected radio emission mark the transition between hard and
soft intermediate state. We fit power spectra from the rising phase of the
outburst using the recently proposed model propfluc. This assumes a truncated
disc / hot inner flow geometry, with mass accretion rate fluctuations
propagating through a precessing inner flow. We link the propfluc physical
parameters to the phenomenological multi-Lorentzian fit parameters. The
physical parameter dominating the QPO frequency is the truncation radius, while
broad band noise characteristics are also influenced by the radial surface
density and emissivity profiles of the flow. In the outburst rise we found that
the truncation radius decreases from to , and the surface
density increases faster than the mass accretion rate, as previously reported
for XTE J1550-564. Two soft intermediate state observations could not be fitted
with propfluc, and we suggest that they are coincident with the ejection of
material from the inner regions of the flow in a jet or accretion of these
regions into the BH horizon, explaining the drop in QPO frequency and
suppression of broad band variability preferentially at high energy bands
coincident with a radio flare.Comment: 13 pages, 11 figures, 2 table
Effects of thickness on the spin susceptibility of the 2D electron gas
Using available quantum Monte Carlo predictions for a strictly 2D electron
gas, we have estimated the spin susceptibility of electrons in actual devices
taking into account the effect of the finite transverse thickness and finding a
very good agreement with experiments. A weak disorder, as found in very clean
devices and/or at densities not too low, just brings about a minor enhancement
of the susceptibility.Comment: 4 pages, 3 figure
Chaos in the thermodynamic limit
We study chaos in the Hamiltonian Mean Field model (HMF), a system with many
degrees of freedom in which classical rotators are fully coupled. We review
the most important results on the dynamics and the thermodynamics of the HMF,
and in particular we focus on the chaotic properties.We study the Lyapunov
exponents and the Kolmogorov--Sinai entropy, namely their dependence on the
number of degrees of freedom and on energy density, both for the ferromagnetic
and the antiferromagnetic case.Comment: 10 pages, Latex, 4 figures included, invited talk to the Int.
school/Conf. on "Let's face Chaos Through Nonlinear Dynamics" Maribor
(Slovenia) 27 june - 11 july 1999, submitted to Prog. Theor. Physics supp
High precision locations of long-period events at La Fossa Crater (Vulcano Island, Italy)
Since the last eruption in 1888-90, the volcanic activity on Vulcano Island (Aeolian Archipelago, Italy) has been limited to fumarolic degassing. Fumaroles are mainly concentred at the active cone of La Fossa in the northern sector of the island and are periodically characterized by increases in the temperature as well as in the amount of both CO2 and He .Seismic background activity at Vulcano is dominated by micro-seismicity originating at shallow depth (<1-1.5 km) under La Fossa cone. This seismicity is related to geothermal system processes and comprises long period (LP) events. LPs are generally considered as the resonance of a fluid-filled volume in response to a trigger. We analyzed LP events recorded during an anomalous degassing period (August-October 2006) applying a high precision technique to define the shape of the trigger source. Absolute and high precision locations suggest that LP events recorded at Vulcano, during 2006, were produced by a shallow focal zone ca. 200 m long, 40 m wide and N30-40E oriented. Their occurrence is linked to magmatic fluid inputs that by modifying the hydrothermal system cause excitation of a fluid-filled cavity
Riprogettazione e ristrutturazione della stazione video di Stromboli e nuovo sistema di acquisizione e trasferimento immagini da Vulcano
La sorveglianza visiva dei vulcani attivi siciliani è una delle attività dell’unità funzionale monitoraggio Sistema Poseidon dell’INGV di Catania e consiste nel monitoraggio video continuo di detti vulcani mediante stazioni permanenti.
In questo lavoro è stata rappresentata l’opera di riprogettazione e ristrutturazione della stazione video di Stromboli posta in località “Il Pizzo Sopra la Fossa” in cima al vulcano Stromboli. Tale ristrutturazione è stata necessaria poiché il materiale della stazione video, ormai obsoleto e mal funzionante, non garantiva un servizio continuo, affidabile e qualitativamente accettabile.
Contemporaneamente è stato progettato e messo in opera al Centro Marcello Carapezza di Vulcano un nuovo sistema automatico di acquisizione, digitalizzazione, salvataggio su memoria di massa e trasferimento in internet ed in intranet delle immagini dei vulcani Stromboli e Vulcano. Ciò è stato effettuato utilizzando un software appositamente realizzato ed una linea trasmissione dati dedicata che hanno permesso di superare i continui inconvenienti nel trasferimento di tali immagini da Lipari
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