Oxide 2D electron gas shows high resolution quantum oscillations

Researchers from the HFML and the University of Twente have measured quantum oscillations from the 2D electron gas which forms at the interface between LaAlO₃ and SrTiO₃ - materials which are leading the drive for multifunctional, oxide-based electronic devices. Quantum oscillations give unique information about conduction bandstructure and other fundamental properties of mobile electrons in the 2D electron gas, and these new results show that multiple conduction bands are present in LaAlO₃/SrTiO₃, with a closely-spaced structure which is quite different to that predicted by initial theories for this system. The work was published in APL Materials on 4th February, 2014.

A two-dimensional electron gas is a collection of electrons which are confined to a plane surface or interface, but which are highly mobile within that plane. Until quite recently, these rather special electronic systems were only found in semiconducting materials, in some organic salts or on the surface of liquid helium. Now a new class of 2D electron gas has been discovered at the interface between insulating oxide materials such as LaAlO₃ and SrTiO₃. Under certain conditions, these new systems show properties such as magnetism and superconductivity, which have never been observed in ‘conventional', semiconducting 2D electron gases and which have enormous potential for advanced electronics applications, if they can be understood and controlled.

Quantum oscillations in the resistivity as a function of magnetic field, shown at several temperatures.

By tracking quantum oscillations as a function of temperature, magnetic field strength and field orientation, the HFML experiments reveal, firstly, that a concentration of ~ 10¹³ cm^-2 highly mobile electrons occupy at least four conduction bands, or sub-bands; secondly that the sub-bands are separated by only a few millielectronvolts in energy(see figures); and, finally, that the effective masses and quantum mobilities of the electrons differ by, at most, a factor 2-3 between sub-bands. Early theories of how the electron gas is formed in LaAlO₃/SrTiO₃ considered an ‘electronic reconstruction' scenario which predicts carrier concentrations, sub-band spacing and variation in effective mass that are an order of magnitude larger than we observe. In contrast, our experimental results show close correspondence to more recent theoretical work by van Heeringen et al. (see second reference below), and indicate a promising direction for future research and for our detailed understanding of these oxide systems.

Energies of the 4 measured sub-bands relative to the Fermi energy, as a function of the density of states.

Reference
A. McCollam, S. Wenderich, M.K. Kruize, V.K. Guduru, H.J.A. Molegraaf, M. Huijben, G. Koster, D.H.A. Blank, G. Reijnders, A. Brinkman, H. Hilgenkamp, U. Zeitler and J.C. Maan, APL Materials 2, 022102 (February 2014)

See also: L. van Heeringen, G.A. de Wijs, A. McCollam, J.C. Maan and A. Fasolino, Phys. Rev. B 88, 205140(November 2013)