In 1925, Einstein predicted that an ideal gas of bosonic atoms undergoes a phase transition below a certain temperature at which the atoms suddenly begin to accumulate in the lowest quantum state. This phemomenon is now known as Bose-Einstein condensation (BEC). Until fairly recently, it could only be studied in liquid helium, but this changed dramatically in 1995 with the discovery of BEC in trapped atomic gases. This has led to a surge in experimental and theoretical activity devoted to achieving an understanding of these novel systems.

The images in the figure above were obtained by the MIT collaboration and show the spatial distribution of sodium atoms in a magnetic trap. The image on the left corresponds to a temperature above the transition temperature and shows a relatively broad, guassian-like distribution which is characteristic of a nondegenerate thermal cloud. The middle image is at a lower temperature where the formation of the Bose-Einstein condensate is becoming apparent - the sharp localized feature is due to the atoms occupying the lowest harmonic oscillator state in the trap. If the cloud is cooled further, no trace of the thermal component can be seen as all atoms now reside in the lowest quantum state. These images illustrate very graphically and beautifully the phenomenon of BEC.

During the past few years I have been collaborating with Allan Griffin and Tetsuro Nikuni on various problems dealing with BEC in trapped atomic gases [1]. The main theme has been to understand the properties of the trapped gas at finite temperatures when both a condensate and thermal cloud co-exist as in the middle image shown above. Of particular interest to us is the dynamical behaviour of the gas as revealed by its normal modes of oscillation. These oscillations are well understood at temperatures approaching absolute zero where they involve only the condensate, but many open questions remain when a thermal cloud is also present. A second problem of considerable interest has to do with the kinetics of condensate formation and growth as illustrated by the progression in the figure above. Our work [1] has provided a general theoretical formulation which can be used to describe these and other aspects of BEC in trapped atomic gases.

For further information about BEC see the very useful home page of Mark Edwards at http://amo.phy.gasou.edu/bec.html/

Selected Recent Publications

1. E. Zaremba, T. Nikuni and A. Griffin, 1999, Dynamics of Trapped Bose Gases at Finite Temperatures, J. Low Temp. Phys. 116, 277-345; preprint



2. T. Nikuni, E. Zaremba and A. Griffin, 1999, Two-Fluid Dynamics for a Bose-Einstein Condensate out of Local Equilibrium with the Noncondensate, Phys. Rev. Lett. 83, 10-13.



3. M.J. Davis, D.A.W. Hutchinson and E. Zaremba, 1999, Effects of Temperature Upon the Collapse of a Bose-Einstein Condensate in a Gas with Attractive Interactions, J. Phys. B: At. Mol. Opt. Phys. 32, 3993-9; preprint



4. D.A.W. Hutchinson and E. Zaremba, 1998, Excitations of a Bose-condensed Gas in Anisotropic Traps, Phys. Rev. A 57, 1280-5.



5. E. Zaremba, 1998, Sound Propagation in a Cylindrical Bose-condensed Gas, Phys. Rev. A 57, 518-21.



6. D.A.W. Hutchinson, E. Zaremba and A. Griffin, 1997, Finite Temperature Excitations of a Trapped Bose Gas, Phys. Rev. Lett. 78, 1842-5.

MESOSCOPIC PHYSICS

The term mesoscopic is a catch-all for things that exist on length scales between the microscopic and macroscopic. Within recent years there has been an increasing interest in such systems, partly for the novelty of the phenomena they exhibit and partly as a result of the technical advances enabling their fabrication. In the mesoscopic domain, the `graininess' of matter matters, and quantum behaviour becomes increasingly apparent. The challenge is to make these systems, to understand their physical properties and to invent useful device applications.

The starting point for many of the systems being studied is a two-dimensional electron gas at the interface between different semiconductors, which in itself is a large field of research. With the patterning capabilities provided by optical or electron beam lithography, one can, for example, produce small, localized pools of electrons. These structures are usually referred to as quantum dots, but also as artificial atoms, since they share some common characteristics with the ordinary atoms of this world. Like atoms, they can be excited by external radiation, and in the presence of a magnetic field, they exhibit a rich spectrum of excitations known as magnetoplasmons [1]. To develop a theoretical understanding of these experiments we are using density functional theory for the electronic ground state and hydrodynamics to describe the collective behaviour [4]. The theory is also being used to understand collective excitations in other structures such as electron rings [3] and antidot arrays.

A variety of other topics related to the electronic and transport properties of mesoscopic systems is also being studied [2].

Selected Recent Publications

1. B.P.van Zyl, E. Zaremba and D.A.W. Hutchinson, 1999, Magnetoplasmon Excitations in Arrays of Circular and Noncircular Quantum Dots, Phys. Rev. B, in press; preprint



2. A. Miele, R. Fletcher, E. Zaremba, Y. Feng, C.T. Foxon and J.J. Harris, 1998, Phonon-drag Thermopower and Weak Localization, Phys. Rev. B 58, 13181-90.



3. E. Zaremba, 1996, Magnetoplasma Excitations in Electron Rings, Phys. Rev. B 53, R10512-5.



4. E. Zaremba and H.C. Tso, 1994, Thomas-Fermi-Dirac-von Weizsaecker Hydrodynamics in Parabolic Wells, Phys. Rev. B 49, 8147-62.

The interaction of charged particles with matter is relevant to a wide range of disciplines, such as radiology, ion implantation, plasma confinement and the space sciences. Of primary interest is the way that a rapidly moving charge loses energy by means of electronic excitations of the material through which it is travelling. Various theoretical approaches are being used to develop a better understanding of these complex, dynamical processes. Current problems under investigation include the importance of nonlinear screening in the formation of electron wakes and the interaction of highly charged ions with solid surfaces.

Selected Recent Publications

1. A. Salin, A. Arnau, P.M. Echenique and E. Zaremba, 1999, Dynamic Nonlinear Screening of Slow Ions in an Electron Gas, Phys. Rev. B 59, 2537-48.