Alberto G. Rojo

Electron-drag effects in coupled electron systems

The advances in the techniques used for fabrication and lithography of semiconductors have made it possible to study bi-layer systems made of two electronic layers separated by distances of several hundred angstrIn this situation the electrons in layer 1 are distinguishable from those in layer 2, and can communicate through the direct inter-layer Coulomb interaction. In particular, if a current is applied to one of the layers, the electrons in the second will be dragged, giving rise to a transresistance D. In this article we review recent theoretical and experimental developments in the understanding of this effect. At very low temperatures it turns out that phonons dominate the transresistance. The direct Coulomb interaction and plasmon excitations are important at temperatures T> 0 : 1 T F, with TF the Fermi temperature. If a magnetic field is applied, the transresistance is increased, in a very interesting interplay between D and Landau quantization. The non-dissipative drag is also reviewed.

Optimally squeezed spin states

We consider optimally spin-squeezed states that maximize the sensitivity of the Ramsey spectroscopy, and for which the signal-to-noise ratio scales as the number of particles N. Using the variational principle we prove that these states are eigensolutions of the Hamiltonian

Spin squeezing via atom-cavity field coupling

H(\ensuremath{\lambda})=\ensuremath{\lambda}{S}_{z}^{2}\ensuremath{-}{S}_{x},

The rolling sphere, the quantum spin, and a simple view of the Landau–Zener problem

and that, for large N, the states become equivalent to the quadrature squeezed states of the harmonic oscillator. We present numerical results that illustrate the validity of the equivalence.

ANOMALOUS DIMENSION AND SPATIAL CORRELATIONS IN A POINT-ISLAND MODEL

Spin squeezing via atom-field interactions is considered within the context of the Tavis-Cummings model. An ensemble of N two-level atoms interacts with a quantized cavity field. For all the atoms initially in their ground states, it is shown that spin squeezing of both the atoms and the field can be achieved provided the initial state of the cavity field has coherence between number states differing by 2. Most of the discussion is restricted to the case of a cavity field initially in a coherent state, but initial squeezed states for the field are also discussed. Optimal conditions for obtaining squeezing are obtained. An analytic solution is found that is valid in the limit that the number of atoms is much greater than unity and is also much larger than the average number of photons, {alpha}{sup 2}, initially in the coherent state of the cavity field. In this limit, the degree of spin squeezing increases with increasing {alpha}, even though the field more closely resembles a classical field for which no spin squeezing could be achieved.

Talbot Oscillations and Periodic Focusing in a One-Dimensional Condensate

We solve the problem of a sphere rolling on a curved surface by mapping it to the precession of a spin 1/2 in a magnetic field of variable magnitude and direction. The mapping can be instructive for discussing both rolling and spin precession. As an example, we show that the Landau–Zener problem corresponds to the rolling of a sphere on a Cornu spiral and derive the probability of a nonadiabatic transition using this correspondence. We also discuss the adiabatic limit and the vanishing of geometric phases for rolling on curved surfaces.

A dimerized Kronig–Penney model

We examine the island size distribution function and spatial correlation function of a model for island growth in the submonolayer regime in both 1 and 2 dimensions. In our model the islands do not grow in shape, and a fixed number of adatoms are added, nucleate, and are trapped at islands as they diffuse. We study the cases of various critical island sizes

Rayleigh scattering revisited: From gases to crystals

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Optical mechanical analogy and nonlinear nonholonomic constraints.

for nucleation as a function of initial coverage. We found anomalous scaling of the island size distribution for large

The Parallelometer: a mechanical device to study curvature

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