Conventional electronics is based on semiconductor materials such as silicon, germanium, or gallium arsenide, which are increasingly reaching their performance limits. One key approach to make these electronic devices faster and more efficient is spintronics, which requires new materials. Scientists have had their sights on oxides such as strontium titanate (SrTiO3) as an alternative to the well-established semiconductors. A thin conductive layer forms on the pure surface of SrTiO3—a two-dimensional electron gas (2DEG), where electrons can virtually move freely, like gas particles. Milan Radović and his colleague Nicholas Plumb at Paul Scherrer Institute have now measured the properties of the electrons in this 2DEG, providing the clearest description of the electronic structure of the metallic surface state on SrTiO3 to date. It is characterized by a band structure, which can be imagined as a multilane motorway for electrons. On each lane, the electrons possess certain properties, such as a specific spin direction or certain energy levels.
To study the spin of the 2D electrons in more detail, spin and angular-resolution photoemission measurements (SARPES) were performed with colleagues from EPFL Switzerland and CSNSM, Université Paris-Sud, France. The results showed that these 2D electrons are located in two subbands and that the majority of the electron spins are aligned parallel to the surface in both bands. In one band, however, their orientation rotates clockwise, in the other counterclockwise.
While the researchers had expected this helical spin structure, they were surprised to find two separate subbands with spins oriented in opposing directions. They were also surprised to find that a relatively large amount of energy (100 meV) is required to allow the electron transition from one band to the other. The researchers refer to a sizeable bandgap, which is around 10 times larger than in other known systems up to now.
So far, however, this effect has only been observed under ultrahigh vacuum conditions. Whether it can be achieved on the same scale under practical conditions remains to be seen.
The scientists published their results in the August 18 issue of Physical Review Letters (DOI: 10.1103/PhysRevLett.113.086801) and the October 12 issue of Nature Materials (DOI: 10.1038/NMAT4107).
