Tuning gaps and phases of a two-subband system in a quantizing magnetic field

In this work we study the properties of a two-subband quasi-two-dimensional electron system in a strong magnetic field when the electron filling factor is equal to four. When the cyclotron energy is close to the intersubband splitting the system can be mapped onto a four-level electron system with an effective filling factor of two. The ground state is either a ferromagnetic state or a spin-singlet state, depending on the values of the inter-level splitting and Zeeman energy. The boundaries between these phases are strongly influenced by the inter-electron interaction. A significant exchange-mediated enhancement of the excitation gap results in the suppression of the electron-phonon interaction. The rate of absorption of non-equilibrium phonons is calculated as a function of Zeeman energy and inter-subband splitting. The phonon absorption rate has two peaks as a function of intersubband splitting and has a step-like structure as a function of Zeeman energy

Donor states in modulation-doped Si/SiGe heterostructures

We present a unified approach for calculating the properties of shallow donors inside or outside heterostructure quantum wells. The method allows us to obtain not only the binding energies of all localized states of any symmetry, but also the energy width of the resonant states which may appear when a localized state becomes degenerate with the continuous quantum well subbands. The approach is non-variational, and we are therefore also able to evaluate the wave functions. This is used to calculate the optical absorption spectrum, which is strongly non-isotropic due to the selection rules. The results obtained from calculations for Si/Si$_{1-x}$Ge$_x$ quantum wells allow us to present the general behavior of the impurity states, as the donor position is varied from the center of the well to deep inside the barrier. The influence on the donor ground state from both the central-cell effect and the strain arising from the lattice mismatch is carefully considered.Comment: 17 pages, 10 figure

Magneto-optics and magneto-capacitance studies of voltage-tuneable GaAs/AlGaAs quantum dots

Contains fulltext : 112792.pdf (publisher's version ) (Open Access

Carbon dioxide laser saturation spectroscopy at kHz linewidths

Consideration is given to factors limiting the resolution in laser saturation experiments. At linewidths in the range 100 kHz to 1 MHz the most important factors arise from the residence time of the absorbing molecule within the radiation field and from saturation broadening. These problems are circumvented by using beams of large diameters and low powers. Two experiments yielding spectra at linewidths around 20 kHz (full-width half-maximum) are described in outline, each employing a CO<sub>2</sub> laser beam of diameter several centimetres. In one experiment 10 µm saturation spectra are obtained directly using an etalon technique; in the other radio-frequency transitions are obtained in a double-resonance experiment

Far-infrared cyclotron resonance study of the effect of strain and localisation in Si/SiGe two dimensional electron gases

Far infrared cyclotron resonance measurements have been used to investigate the effective mass (m*) in the strained silicon channel of modulation-doped, two dimensional electron gases grown on relaxed Si1−xGex. Samples with germanium fractions from 24% to 31% were measured to investigate the influence of strain on m*. Little variation as a function of strain was observed, but for one sample, the resonance position was shifted up in frequency due to localisation effects. This persisted up to higher Landau level filling factors than has been observed previously in other materials systems and was accompanied by a large enhancement in the quantum lifetime