My main research interests lie in the area of low temperature, solid state physics. This is an exciting area relating to many of the most recent advances in semiconductor physics and electronic technology. Research at liquid helium temperatures (below 4K) enables us to study aspects of crystalline behaviour which at higher temperatures are masked by the thermal vibrations of the crystal lattice, known as phonons. Understanding the properties of phonons and their interaction with other excitations in solids, such as electrons, is my major research preoccupation, and at Lancaster we use nanosecond pulse techniques to study the detailed dependences of high frequency phonon scattering.
A current project involves the new generation of semiconductor microstructures which involve layer crystals only a few atoms thick. Phonon pulse experiments are being used to study the electron-phonon interaction in these devices, which is modified from that in 3-dimensional bulk crystals. Systems currently under investigation include resonant tunnelling structures (DBRTS) and high electron mobility transistors (HEMT). Recent results include the direct observation for the first time of energy loss from the hot electrons by cascades of plasmon-optic phonon coupled modes, observation of intervalley phonon emission in GaAs, and phonon emission by phonon-assisted tunnelling.
Phonon scattering is very sensitive to tiny crystalline imperfections, and in another project we characterise the quality of crystal surfaces and interfaces by measuring the specular and diffusive phonon reflections. Surface roughness is an extremely important parameter in determining the performance of high frequency semiconductor devices, and we work with the latest silicon material through a collaboration with Kyushu Institute of Technology in Japan.
A third project is involved with studying the role of phonons in various aspects of photon detection using superconducting tunnel junctions, for example in X-ray astronomy, in a collaboration with the European Space Agency. Direct absorption of photons in a superconductor depends on the dynamics of quasiparticle creation and decay, whilst phonon-mediated detection involves phonon propagation and scattering in the detector substrate. Both areas can be studied through the use of nanosecond heat pulse experiments in which very high frequency (THz) phonons are generated to simulate the effects of the photon absorption.