arXiv subjects

T. S. Monteiro

Publications and source records attributed to T. S. Monteiro.

60 records · Page 4Linked to original sources

arXiv1999

Semiclassical Gaussian matrix elements for chaotic quantum wells

We derive semiclassical expressions for spectra, weighted by matrix elements of a Gaussian observable, relevant to a range of molecular and mesoscopic systems. We apply the formalism to the particular example of the resonant tunneling diode (RTD) in tilted fields. The RTD is a new experimental realization of a mesoscopic system exhibiting a transition to chaos. It has generated much interest and several different semiclassical theories for the RTD have been proposed recently.Our formalism clarifies the relationship between the different approaches and to previous work on semiclassical theories of matrix elements. We introduce three possible levels of approximation in the application of the stationary phase approximation, depending on typical length scales of oscillations of the semiclassical Green's function, relative to the degree of localization of the observable. Different types of trajectories (periodic, normal, closed and saddle orbits) are shown to arise from such considerations. We propose a new type of trajectory (``minimal orbits'')and show they provide the best real approximation to the complex saddle points of the stationary phase approximation. We test the semiclassical formulae on quantum calculations and experimental data. We show that the new types of trajectories (saddle and minimal orbits) provide accurate results. We discuss a divergence of the contribution of saddle orbits, which suggests the existence of bifurcation-type phenomena affecting the complex and non-periodic saddle orbits.

chao-dyn↗

arXiv1999

Spectral properties of Atoms in Fields: A Semiclassical Analysis

We develop a semiclassical theory for the spectral rigidity of non-hydrogenic Rydberg atoms in electric fields and evaluate the significant deviations from the well-known Poissonian behaviour in the hydrogenic case. The resulting formula is shown to be in excellent agreement with the exact quantal result. We also investigate diamagnetic atoms; we find that, in contrast to the classically integrable atoms, diffraction has a small effect on the spectral rigidity in the classically chaotic atom. We show our predictions can also be of use in the mixed phase space regime.

chao-dyn↗

arXiv1998

Quantum chaos with non-periodic, complex orbits in the Resonant Tunneling Diode

We show that a special type of orbits, which are non-periodic and complex, 'Saddle Orbits' (SOs) describe accurately the quantal and experimental current oscillations in the Resonant Tunneling Diode in tilted fields. This is the first demonstration that one needs to abandon the periodic orbits (POs) paradigm in favour of this new and peculiar type of orbit. The SOs solve the puzzle of broad regions of experimental oscillations where we find no real or complex PO that can explain the data. The SOs succeeds in regimes involving several non-isolated POs, where PO formulas fail.

chao-dyn↗

arXiv1998

Periodic Orbit theory for Resonant Tunneling Diodes : comparison with quantum and experimental results

We investigate whether the quantal and experimental amplitudes of current oscillations of Resonant Tunneling Diodes in tilted fields are obtainable from Periodic Orbit (PO) theories by considering recently proposed PO approaches. We show, for the first time, that accurate amplitudes and frequency shifts for the current oscillations (typically to within a few %) can be obtained from a simple analytical formula both in the stable (torus-quantization) limit and the unstable regimes of the experiments which are dominated by isolated PO's. But we find that the PO approach does not describe quantitatively the dynamically interesting intermediate experimental regimes which appear to be dominated by contributions from complex orbits and multiple non-isolated PO's. We conclude that these regimes will not easily be described by the usual PO approach, even with simple normal forms.

chao-dyn↗

arXiv1998

Atoms in static fields: Chaos or Diffraction?

A brief review of the manifestations of classical chaos observed in atomic systems is presented. Particular attention is paid to the analysis of atomic spectra by periodic orbit-type theories. For diamagnetic non-hydrogenic Rydberg atoms, the dynamical explanation for observed spectral features has been disputed. By building on our previous work on the photoabsorption spectrum, we show how, by the addition of diffractive terms, the spectral fluctuations in the energy level spectrum of general Rydberg atoms can be obtained with remarkable precision from the Gutzwiller trace formula. This provides further evidence that non-hydrogenic systems are most naturally described in terms of diffraction rather than classical chaos.

physics.atom-ph↗

arXiv1998

Periodic Orbit Theory for Rydberg Atoms in External Fields

Although hydrogen in external fields is a paradigm for the application of periodic orbits and the Gutzwiller trace formula to a real system, the trace formula has never been applied successfully to other Rydberg atoms. We show that spectral fluctuations of general Rydberg atoms are given with remarkable precision by the addition of diffractive terms. Previously unknown features in atomic spectra are exposed: there are new modulations that are neither periodic orbits nor combinations of periodic orbits; `core-shadowing' generally decreases primitive periodic orbit amplitudes but can also lead to increases.

chao-dyn↗