Jean-Guy Caputo

Controlling collapse in Bose-Einstein condensates by temporal modulation of the scattering length

We consider, by means of the variational approximation (VA) and direct numerical simulations of the Gross-Pitaevskii (GP) equation, the dynamics of two-dimensional (2D) and 3D condensates with a scattering length containing constant and harmonically varying parts, which can be achieved with an ac magnetic field tuned to the Feshbach resonance. For a rapid time modulation, we develop an approach based on the direct averaging of the GP equation, without using the VA. In the 2D case, both VA and direct simulations, as well as the averaging method, reveal the existence of stable self-confined condensates without an external trap, in agreement with qualitatively similar results recently reported for spatial solitons in nonlinear optics. In the 3D case, the VA again predicts the existence of a stable self-confined condensate without a trap. In this case, direct simulations demonstrate that the stability is limited in time, eventually switching into collapse, even though the constant part of the scattering length is positive (but not too large). Thus a spatially uniform ac magnetic field, resonantly tuned to control the scattering length, may play the role of an effective trap confining the condensate, and sometimes causing its collapse.

Vortex polarity switching by a spin-polarized current

The spin-transfer effect is investigated for the vortex state of a magnetic nanodot. A spin current is shown to act similarly to an effective magnetic field perpendicular to the nanodot. Then a vortex with magnetization (polarity) parallel to the current polarization is energetically favorable. Following a simple energy analysis and using direct spin-lattice simulations, we predict the polarity switching of a vortex. For magnetic storage devices, an electric current is more effective to switch the polarity of a vortex in a nanodot than the magnetic field.

Unidirectional propagation of an ultra-short electromagnetic pulse in a resonant medium with high frequency Stark shift

Abstract We consider in the unidirectional approximation the propagation of an ultra short electromagnetic pulse in a resonant medium consisting of molecules characterized by a transition operator with both diagonal and non-diagonal matrix elements. We find the zero-curvature representation of the reduced Maxwell–Bloch equations in the sharp line limit. This can be used to develop the inverse scattering transform method to solve these equations. Finally we obtain two types of exact traveling pulse solutions, one with the usual exponential decay and another with an algebraic decay.

Extremely short electromagnetic pulses in a resonant medium with a permanent dipole moment

The propagation of an extremely short (one cycle long) pulse of an electromagnetic field in a medium whose resonant transition is characterized by both diagonal and off-diagonal matrix elements of the dipole-moment operator is considered theoretically. The set of Maxwell-Bloch equations without the approximation of the slowly varying envelopes is used. Two types of solutions of this set, which are found describing the steady-state propagation of an electromagnetic pulse in such a medium.

Nonlinear energy transmission in the gap

Abstract Numerical simulations of the scattering of a linear plane wave incoming onto a nonlinear medium (sine–Gordon) reveals that: (i) nonlinearity allows energy transmission in the forbidden band, (ii) this nonlinear transmission occurs beyond an energy threshold of the incoming wave, (iii) the process begins (at the threshold) with large amplitude breathers, and then energy is generically transmitted both by kink–antikink pairs and breathers.

A Semi-Linear Elliptic Pde Model For The Static Solution Of Josephson Junctions

In this study we derive a semi-linear Elliptic Partial Differential Equation (PDE) problem that models the static (zero voltage) behavior of a Josephson window junction. Iterative methods for solving this problem are proposed and their computer implementation is discussed. The preliminary computational results that are given, show the modeling power of our approach and exhibit its computational efficiency.

Analytical solutions of jam pattern formation on a ring for a class of optimal velocity traffic models

A follow-the-leader model of traffic flow on a closed loop is considered in the framework of the extended optimal velocity (OV) model where the driver reacts to both the following and the preceding car. Periodic wave train solutions that describe the formation of traffic congestion patterns are found analytically. Their velocity and amplitude are determined from a perturbation approach based on collective coordinates with the discrete modified Korteweg–de Vries equation as the zero order equation. This contains the standard OV model as a special case. The analytical results are in excellent agreement with numerical solutions.

Stability analysis of static solutions in a Josephson junction

We present all the possible solutions of a Josephson junction with bias current and magnetic field with both inline and overlap geometry, and examine their stability. We follow the bifurcation of new solutions as we increase the junction length. The analytical results are in terms of elliptic functions for the case of inline geometry, and are in agreement with the numerical calculations, explaining also the strong hysteretic phenomena typically seen in the calculation of the maximum tunnelling current. This suggests a different experimental approach based on the use, instead of the external magnetic field, of the modulus of the elliptic function or the related quantity the total magnetic flux to avoid hysteretic behaviour and unfold the overlapping Imax (H ) curves.

Effect Of Geometry On Fluxon Width In A Josephson Junction

We investigate the electromagnetic influence of the surrounding idle (no tunneling) region on static fluxons in window Josephson junctions. We calculated the fluxon width as a function of the size of the idle region for three different window (active tunneling area) geometries, namely elongated truncated rhombus, rectangular and bow-tie and derived approximate expressions for the case of small and large idle regions. The window geometry affects both the fluxon width and the fluxon stability. One can define an effective λJ which depends on the junction width, the idle region width and the inductance ratio and has important consequences on the static and dynamic properties of window Josephson junctions. We also show the effect of the idle region on the maximum tunneling current as a function of the external magnetic field.

Practical Remarks on the Estimation of Dimension and Entropy from Experimental Data

For non-linear dissipative dynamical system, invariant sets have volume zero in phase-space, and trajectories possess the property of sensitivity to initial conditions. The geometry and dynamics can be characterised by quantities, dimensions and entropies which will be invariant under a large class of coordinate changes. To calculate these quantities from experimental data one needs to imbed the data in R n . These projections generically are diffeomorphisms for n sufficiently large [1,2]. In practice, they are done using time-delays see [3] for example or building vectors from measurements in different locations. However the geometrical structure of these projections sets will determine the values obtained in practice for the dimensions and entropies. For simple model systems these sets can be studied in a semi-quantitative way using the Grassberger-Procaccia correlation integral [4]. This yields some criterions on the choice of the distance in R n , the different parameters: the number of vectors N and averages m, the sampling period, the embedding dimension and the delay. Deterministic chaotic data are projected onto manifolds which are locally the product of cantor sets and smooth manifolds, it will be shown that the curvature of these sets caused by the divergence of trajectories becomes predominant for the determination.