1,049 research outputs found
Warm turbulence in the Boltzmann equation
We study the single-particle distributions of three-dimensional hard sphere
gas described by the Boltzmann equation. We focus on the steady homogeneous
isotropic solutions in thermodynamically open conditions, i.e. in the presence
of forcing and dissipation. We observe nonequilibrium steady state solution
characterized by a warm turbulence, that is an energy and particle cascade
superimposed on the Maxwell-Boltzmann distribution. We use a dimensional
analysis approach to relate the thermodynamic quantities of the steady state
with the characteristics of the forcing and dissipation terms. In particular,
we present an analytical prediction for the temperature of the system which we
show to be dependent only on the forcing and dissipative scales. Numerical
simulations of the Boltzmann equation support our analytical predictions.Comment: 4 pages, 5 figure
Suppression of electron-electron repulsion and superconductivity in Ultra Small Carbon Nanotubes
Recently, ultra-small-diameter Single Wall Nano Tubes with diameter of have been produced and many unusual properties were observed, such as
superconductivity, leading to a transition temperature , much
larger than that observed in the bundles of larger diameter tubes.
By a comparison between two different approaches, we discuss the issue
whether a superconducting behavior in these carbon nanotubes can arise by a
purely electronic mechanism. The first approach is based on the Luttinger Model
while the second one, which emphasizes the role of the lattice and short range
interaction, is developed starting from the Hubbard Hamiltonian. By using the
latter model we predict a transition temperature of the same order of magnitude
as the measured one.Comment: 7 pages, 3 figures, to appear in J. Phys.-Cond. Ma
Walk-off induced dissipative breathers and dissipative breather gas in microresonators
Dissipative solitons in optical microcavities have attracted significant
attention in recent years due to their direct association with the generation
of optical frequency combs. Here, we address the problem of dissipative soliton
breathers in a microresonator with second-order nonlinearity, operating at the
exact phase-matching for efficient second-harmonic generation. We elucidate the
vital role played by the group velocity difference between the first and second
harmonic pulses for the breather existence. We report the dissipative breather
gas phenomenon, when multiple breathers propagate randomly in the resonator and
collide nearly elastically. Finally, when the breather gas reaches an
out-of-equilibrium statistical stationarity, we show how the velocity locking
between first and second harmonic is still preserved, naming such phenomena
turbulence locking.Comment: 10 pages, 10 figure
Modeling extreme wave heights from laboratory experiments with the nonlinear Schrödinger equation
Spatial variation of nonlinear wave groups with different initial envelope
shapes is theoretically studied first, confirming that the simplest nonlinear
theoretical model is capable of describing the evolution of propagating wave
packets in deep water. Moreover, three groups of laboratory experiments run
in the wave basin of CEHIPAR (Canal de Experiencias Hidrodinámicas de El
Pardo, known also as El Pardo Model Basin) was founded in 1928 by the Spanish
Navy. are systematically compared with the numerical simulations of the
nonlinear Schrödinger equation. Although a little overestimation is
detected, especially in the set of experiments characterized by higher
initial wave steepness, the numerical simulation still displays a high degree
of agreement with the laboratory experiments. Therefore, the nonlinear
Schrödinger equation catches the essential characteristics of the extreme
waves and provides an important physical insight into their generation. The
modulation instability, resulting from the quasi-resonant four-wave
interaction in a unidirectional sea state, can be indicated by the
coefficient of kurtosis, which shows an appreciable correlation with the
extreme wave height and hence is used in the modified Edgeworth–Rayleigh
distribution. Finally, some statistical properties on the maximum wave
heights in different sea states have been related with the initial
Benjamin–Feir index
Integer Spin Hall Effect in Ballistic Quantum Wires
We investigate the ballistic electron transport in a two dimensional Quantum
Wire under the action of an electric field (). We demonstrate how the
presence of a Spin Orbit coupling, due to the uniform electric confinement
field gives a non-commutative effect as in the presence of a transverse
magnetic field.
We discuss how the non commutation implies an edge localization of the
currents depending on the electron spins also giving a semi-classical spin
dependent Hall current.
We also discuss how it is possible obtain a quantized Spin Hall conductance
in the ballistic transport regime by developing the Landauer formalism and show
the coupling between the spin magnetic momentum and the orbital one due to the
presence of a circulating current.Comment: 7 pages, 5 figures, accepted for publication in Phys. Rev. B, PACS:
72.25.-b, 72.10.-d, 72.15.Rn, 73.23.-b, 71.10.P
Modelling of the temporal and spatial evolutions of weakly nonlinear random directional waves with the modified nonlinear Schrödinger equations
Spin-orbit coupling in a Quantum Dot at high magnetic field
We describe the simultaneous effects of the spin-orbit (SO) perturbation and
a magnetic field on a disk shaped quantum dot (QD). {As it is known the}
combination of electrostatic forces among the electrons confined in the QD
and the Pauli principle can induce a spin polarization when (applied in the
direction orthogonal to the QD) is above a threshold value. In the presence of
an electric field parallel to , coupled to the spin by a Rashba term,
we demonstrate that a symmetry breaking takes place: we can observe it by
analyzing the splitting of the levels belonging to an unperturbed multiplet. We
also discuss the competitive effects of the magnetic field, the SO perturbation
and the electron electron interaction, in order to define the hierarchy of the
states belonging to a multiplet. We demonstrate how this hierarchy depends on
the QD's size. We show the spin texture due to the combined effects of the
Rashba effect and the interaction responsible for the polarization.Comment: 8 pages, 3 figures, PACS: 73.21.La,71.15.Mb,75.75.+
Quenching of Spin Hall Effect in Ballistic nano-junctions
We show that a nanometric four-probe ballistic junction can be used to check
the presence of a transverse spin Hall current in a system with a Spin Orbit
coupling not of the Rashba type, but rather due to the in-plane electric field.
Indeed, the spin Hall effect is due to the presence of an effective small
transverse magnetic field corresponding to the Spin Orbit coupling generated by
the confining potential. The strength of the field and the junction shape
characterize the quenching Hall regime, usually studied by applying
semi-classical approaches. We discuss how a quantum mechanical relativistic
effect, such as the Spin Orbit one, can be observed in a low energy system and
explained by using classical mechanics techniques.Comment: 5 pages, 4 figures, PACS: 72.25.-b, 72.20.My, 73.50.Jt, to appear in
Phys. Rev.
Single Wall Nanotubes: Atomic Like Behaviour and Microscopic Approach
Recent experiments about the low temperature behaviour of a Single Wall
Carbon Nanotube (SWCNT) showed typical Coulomb Blockade (CB) peaks in the zero
bias conductance and allowed us to investigate the energy levels of interacting
electrons. Other experiments confirmed the theoretical prediction about the
crucial role which the long range nature of the Coulomb interaction plays in
the correlated electronic transport through a SWCNT with two intramolecular
tunneling barriers. In order to investigate the effects on low dimensional
electron systems due to the range of electron electron repulsion, we introduce
a model for the interaction which interpolates well between short and long
range regimes. Our results could be compared with experimental data obtained in
SWCNTs and with those obtained for an ideal vertical Quantum Dot (QD).
For a better understanding of some experimental results we also discuss how
defects and doping can break some symmetries of the bandstructure of a SWCNT.Comment: 8 pages, 4 figure
- …