We have made the first phonon absorption measurements in the fractional quantum Hall regime. Experiments have been conducted on two samples which have similar electron densities but greatly differing mobilities. The energy gaps as measured by activation studies of the longitudinal resistance differ by a factor of 2. Phonon absorption measurements give almost identical values for the energy gap, demonstrating that the gap measured in this way is rather insensitive to disorder. The value of this gap is in agreement with the activation gap measured in the high mobility sample. Values obtained at $\nu =2\mathrm{}/3$ are in good agreement with theory.

A single InAs self-assembled quantum dot is incorporated in the barrier of a tunnel diode and used as a spectroscopic probe of an adjacent two-dimensional electron system from the Fermi energy to the subband edge. We obtain quantitative information about the energy dependence of the quasiparticle lifetime. For magnetic field $B$, applied parallel to the current, we observe peaks in the current-voltage characteristics $I\left(V\right)$ corresponding to the formation of Landau levels. Close to filling factor $\nu =1\mathrm{}$ we observe directly the exchange enhancement of the Landé $g$ factor.

The application of both a strong magnetic field and magnetic field gradient to a diamagnetic body can produce a vertical force which is sufficient to counteract its weight due to gravity. By immersing the body in a paramagnetic fluid, an additional adjustable magneto-buoyancy force is generated which enhances the levitation effect. Here we show that cryogenic oxygen and oxygen–nitrogen mixtures in both gaseous and liquid form provide sufficient buoyancy to permit the levitation and flotation of a wide range of materials. These fluids may provide an alternative to synthetic ferrofluids for the separation of minerals. We also report the dynamics of corrugation instabilities on the surface of magnetized liquid oxygen.

We investigate the way in which the pattern of fringes in a coherent pair of two-dimensional Bose condensed clouds of ultracold atoms traveling in opposite directions subject to a harmonic trapping potential can seed the irreversible formation of internal excitations in the clouds, notably solitons and vortices. We identify underdamped, overdamped, and critically damped regimes in the dipole oscillations of the condensates according to the balance of internal and center-of-mass energies of the clouds. We carry out simulations of the collision of two clouds with respect to different initial phase differences in these regimes to investigate the creation of internal excitations. We distinguish the behavior of this system from previous studies of quasi-one-dimensional BECs. In particular we note that the nature of the internal excitations is only essentially sensitive to an initial phase difference between the clouds in the overdamped regime.

We investigate the effect of an applied magnetic field on resonant tunneling of electrons through the bound states of self-assembled InAs quantum dots (QDs) embedded within an (AlGa)As tunnel barrier. At low temperatures $(⩽2\mathrm{K})$, a magnetic field $\mathbf{B}$ applied either parallel or perpendicular to the direction of current flow causes a significant enhancement of the tunnel current. For the latter field configuration, we observe a strong angular anisotropy of the enhanced current when $\mathbf{B}$ is rotated in the plane of the quantum dot layer. We attribute this behavior to the effect of the lowered symmetry of the QD eigenfunctions on the electron-electron interaction.