781 research outputs found
Charge noise at Cooper-pair resonances
We analyze the charge dynamics of a superconducting single-electron
transistor (SSET) in the regime where charge transport occurs via Cooper-pair
resonances. Using an approximate description of the system Hamiltonian, in
terms of a series of resonant doublets, we derive a Born-Markov master equation
describing the dynamics of the SSET. The average current displays sharp peaks
at the Cooper-pair resonances and we find that the charge noise spectrum has a
characteristic structure which consists of a series of asymmetric triplets of
peaks. The strongest feature in the charge noise spectrum is the triplet of
peaks centered at zero frequency which has a peak spacing equal to the level
separation within the doublets and is similar to the triplet in the spectrum of
a driven, damped, two-level system. We also explore the back-action that the
SSET charge noise would have on an oscillator coupled to the island charge,
measurement of which provides a way of probing the charge noise spectrum.Comment: 14 pages, 7 figure
Frequency dispersion of photon-assisted shot noise in mesoscopic conductors
We calculate the low-frequency current noise for AC biased mesoscopic chaotic
cavities and diffusive wires. Contrary to what happens for the admittance, the
frequency dispersion is not dominated by the electric response time (the "RC"
time of the circuit), but by the time that electrons need to diffuse through
the structure (dwell time or diffusion time). Frequency dispersion of noise
stems from fluctuations of the Fermi distribution function that preserve charge
neutrality. Our predictions can be verified with present experimental
technology.Comment: 5 pages, 3 Figure
Euler buckling instability and enhanced current blockade in suspended single-electron transistors
Single-electron transistors embedded in a suspended nanobeam or carbon
nanotube may exhibit effects originating from the coupling of the electronic
degrees of freedom to the mechanical oscillations of the suspended structure.
Here, we investigate theoretically the consequences of a capacitive
electromechanical interaction when the supporting beam is brought close to the
Euler buckling instability by a lateral compressive strain. Our central result
is that the low-bias current blockade, originating from the electromechanical
coupling for the classical resonator, is strongly enhanced near the Euler
instability. We predict that the bias voltage below which transport is blocked
increases by orders of magnitude for typical parameters. This mechanism may
make the otherwise elusive classical current blockade experimentally
observable.Comment: 15 pages, 10 figures, 1 table; published versio
The charge shuttle as a nanomechanical ratchet
We consider the charge shuttle proposed by Gorelik {\em et al.} driven by a
time-dependent voltage bias. In the case of asymmetric setup, the system
behaves as a rachet. For pure AC drive, the rectified current shows a complex
frequency dependent response characterized by frequency locking at fracional
values of the external frequency. Due to the non-linear dynamics of the
shuttle, the rachet effect is present also for very low frequencies.Comment: 4 pages, 4 figure
Large current noise in nanoelectromechanical systems close to continuous mechanical instabilities
We investigate the current noise of nanoelectromechanical systems close to a
continuous mechanical instability. In the vicinity of the latter, the
vibrational frequency of the nanomechanical system vanishes, rendering the
system very sensitive to charge fluctuations and, hence, resulting in very
large (super-Poissonian) current noise. Specifically, we consider a suspended
single-electron transistor close to the Euler buckling instability. We show
that such a system exhibits an exponential enhancement of the current noise
when approaching the Euler instability which we explain in terms of telegraph
noise.Comment: 11 pages, 12 figures; v2: minor changes, published versio
U-J Synergy Effect for the High Tc Superconductors
Using renormalization group and exact diagonalization of small clusters we
investigate the ground state phase diagram of a two-dimensional extended
Hubbard model with nearest-neighbor exchange interaction J, in addition to the
local Coulomb repulsion U. The main instabilities are antiferromagnetism close
to half-filling and d-wave superconductivity in the doped system. Our results
suggest that the combined action of J and U interactions provide a remarkably
efficient mechanism to enhance both d-wave superconducting and
antiferromagnetic correlations.Comment: Final version, to appear in PR
Discontinuous Euler instability in nanoelectromechanical systems
We investigate nanoelectromechanical systems near mechanical instabilities.
We show that quite generally, the interaction between the electronic and the
vibronic degrees of freedom can be accounted for essentially exactly when the
instability is continuous. We apply our general framework to the Euler buckling
instability and find that the interaction between electronic and vibronic
degrees of freedom qualitatively affects the mechanical instability, turning it
into a discontinuous one in close analogy with tricritical points in the Landau
theory of phase transitions.Comment: 4+ pages, 3 figures, published versio
Perfusion lung scintigraphy for the diagnosis of pulmonary embolism: a reappraisal and review of the Prospective Investigative Study of Acute Pulmonary Embolism Diagnosis methods.
The paroxysmal event and its deposits
The 5 April 2003 eruption of Stromboli volcano (Italy) was the most violent in the past 50 years. It was also the best documented due to the accurate geophysical monitoring of the ongoing effusive eruption. Detailed field studies carried out a few hours to a few months after the event provided further information that were coupled with visual documentation to reconstruct the explosive dynamics. The eruption consisted of an 8-min-long explosive event preceded by a short-lived precursory activity that evolved into the impulsive ejection of gas and pyroclasts.
Meter-sized ballistic blocks were launched to altitudes of up to 1400 m above the craters falling on the volcano flanks and on the village of Ginostra, about 2 km far from the vent. The vertical jet of gas and pyroclasts above the craters fed a convective plume that reached a height of 4 km. The calculated erupted mass yielded values of 1.1–1.4 × 108 kg. Later explosions generated a scoria flow deposit, with an estimated mass of 1.0–1.3 × 107 kg. Final, waning ash explosions closet the event. The juvenile fraction consisted of an almost aphyric, highly vesicular pumice mingled with a shallow-derived, crystal-rich, moderately vesicular scoria.
Resuming of the lava emission a few hours after the paroxysm indicate that the shallow magmatic system was not significantly modified during the explosions. Combination of volume data with duration of eruptive phases allowed us to estimate the eruptive intensity: during the climactic explosive event, the mass discharge rate was between 106 and 107 kg/s, whereas during the pyroclastic flow activity, it was 2.8–3.6 × 105 kg/s. Strong similarities with other historical paroxysms at Stromboli suggest similar explosion dynamics
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