Kondo conduction has been observed in a quantum dot with an even number of electrons at the triplet-singlet degeneracy point produced by applying a small magnetic field B orthogonal to the dot plane. At a much larger field B similar toB*, orbital effects induce the reversed transition from the singlet to the triplet state. We study the newly proposed Kondo behavior at this point. Here the Zeeman spin splitting cannot be neglected, which changes the nature of the Kondo coupling. On the grounds of exact diagonalization results in a dot with cylindrical symmetry, we show that, at odds with what happens at the other crossing point, close to B*, orbital and spin degrees of freedom are "locked together," so that the Kondo coupling involves a fictitious spin 1/2 only, which is fully compensated for by conduction electrons under suitable conditions. In this sense, spin at the dot is fractionalized. We derive the scaling equation of the system by means of a nonperturbative variational approach. The approach is extended to the B not equal B* case and the residual magnetization on the dot is discussed.

We have investigated the electron density of the delta -doped conventional AlGaAs/GaAs and pseudomorphic AlGaAs/InGaAs/GaAs heterojunctions. Shubnikov-de Hass (SdH) and quantum Hall effect measurements at 4.2 K have been carried out to investigate the existence of a quasi-two-dimensional gas (2DEG) in delta Si-doped AlGaAs/GaAs heterostructure. Cases have been also examined where the g-doping is placed in a narrow quantum well within the AlGaAs barrier with an Al concentration in the well (x(Al)(well)) less than in the barrier (x(Al)(Barr)). This is intended to increase the electron density. Secondary-ion mass spectroscopy (SIMS) measurements were also performed on one of these structures in order to demonstrate the spreading of the dopants. To improve the electron density, we have combined in only one structure the delta Si, in a narrow quantum well, and the InGaAs layer introduced at the heterointerface between the AlGaAs and the GaAs. A comparison between the theoretical and experimental results was performed. The variation trends of calculated electron density are in good agreement with the experimental ones. (C) 2001 Published by Elsevier Science B.V.

We have found that the local density of state fluctuations (LDOSF) in a disordered metal, detected using an impurity in the barrier as a spectrometer, undergo enhanced (with respect to Shubnikov-de Haas and de Haas-van Alphen effects) oscillations in strong magnetic fields, omega(c)tau greater than or equal to 1. We attribute this to the dominant role of the states near the bottom of Landau bands which give the major contribution to the LDOSF and are most strongly affected by disorder. We also demonstrate that in intermediate fields the LDOSF increase with field B in accordance with the results obtained in the diffusion approximation.

We study the magneto-transport in vertical quantum dots both theoretically and experimentally. We focus our attention on the non-linear transport regime. We demonstrate that the peak amplitudes in the I-V characteristics show a dramatically different behaviour as a function of the magnetic field, depending on the value of the angular momemtum of the dot state through which tunnelling occurs. The investigation allows us to probe details of the quantum dot wavefunctions and to distinguish tunnelling through localised states related to impurities or to quantum dots.

We have studied the effects of magnetic field on the fluctuations of the electron density of states in a disordered conductor, a contact in the resonant tunnelling structure GaAlAs/GaAs/GaAlAs. The local density of states (LDOS) in the contact is detected by an impurity which acts as a 'spectrometer'. In the regime of weak magnetic field, omega(c)tau less than or equal to 1, a linear increase of the fluctuation magnitude with B-2 is seen, due to the decrease of the volume where the LDOS is formed and a decrease in the statistical averaging. In the regime omega(c)tau greater than or equal to 1, large oscillations of the fluctuation magnitude have been observed. We suggest an explanation of this new effect, de Haas-van Alphen type oscillations in the fluctuation magnitude. The behaviour of impurity spectrometers in a magnetic field and images given by a complex spectrometer are also discussed.

We have calculated the Linear magnetoconductance across a vertical parabolic quantum dot with a magnetic field in the direction of the current. Gate voltage and magnetic field an tuned at the degeneracy point between the occupancies N=2 and N=3, close to the singlet-triplet transition for N=2. We find that the conductance is enhanced prior to the transition by nearby crossings of the levels of the three-particle dot. Immediately after it is depressed by roughly 1/3, as long as the total spin S of the three-electron ground state doesn't change from S=1/2 to S=3/2, due to spin selection rule. At low temperature this dip is very sharp, but the peak is recovered by increasing the temperature.

A study of magnetotransport through quantum dots is presented. The model allows one to analyze tunneling both from bulk-like contacts and from 2D accumulation layers. The fine features in the I-V characteristics due to the quantum dot states are known to be shifted to different voltages depending upon the value of the magnetic field. While this effect is also well reproduced by our calculations, in this work we concentrate on the amplitude of each current resonance as a function of the magnetic field. Such amplitudes show oscillations reflecting the variation of the density of states at the Fermi energy in the emitter. Furthermore the amplitude increases as a function of the magnetic field for certain features while it decreases for others. In particular, we demonstrate that the behavior of the amplitude of the current resonances is linked to the value of the angular momentum of each dot level through which tunneling occurs. We show that a selection rule on the angular momentum must be satisfied. As a consequence, tunneling through specific dot states is strongly suppressed and sometimes prohibited altogether by the presence of the magnetic field. This will allow to extract from the experimental curves detailed information on the nature of the quantum-dot wave functions involved in the electronic transport. Furthermore, when tunneling occurs from a two-dimensional accumulation layer to the quantum dot, the presence of a magnetic field hugely increases the strength of some resonant features. This effect is predicted by our model and, to the best of our knowledge, has never been observed. [S0163-1829(99)04007-2].