This paper gives a brief introductory overview of quantum chaology, with particular reference to recent experimental work involving the use of semiconductor heterostructures. In the presence of a tilted magnetic field, a double-barrier resonant-tunnelling device incorporating a quantum well produces a chaotic stadium for electron motion. The basic properties of this system are described. It is shown how resonant magnetotunnelling spectroscopy provides firm experimental evidence for the effect of scarred wave functions on a physically-measurable property, in this case the measured current-voltage characteristics of the device. The paper concludes with some speculations concerning for the development of this field.PACS numbers: 72.20.Μy, 73.40.Gk, 85.30.Μn, 05.45.+b
Quantum biHiards, unstable orbits and scarred wave functionsQuantum chaology has been defined by Berry [1] as the "study of semiclassical, but non-classical, behaviour of systems whose classical motion exhibits chaos". Much of the theoretical work in this field has focused on the classical motion and corresponding eigenstates of particles which are confined to move within a twodimensional stadium (also commonly referred to as a billiard) [1][2][3]. Particles are assumed to bounce elastically off the walls of the stadium, with specular reflection. The motion within the confines of the stadium is normally assumed to be free and frictionless. Particles moving in square-and circular-shaped stadia have regular motion, with one or two characteristic periods; in mathematical terms, this means that the equation of motion is separable into two one-dimensional parts. Examples of stadia which give rise to chaotic motion are the Bunimovich stadium (a square area with two semicircular areas adjoined to opposite sides; for experimental stadia see Ref. [4]) and the Sinai billiard [5] (a square with a hard-walled circular "nogo" area in the centre). The properties of unstable but periodic classical orbits are of fundamental importance for the quantum chaology of these types of billiard [1,2]. These orbits are associated with regular clustering of the quantised energy levels of the system, which gives rise to energy-periodic fluctuations in the density of states. This effect is described in terms of the well-known (609)