|The electronic states at the surface of the topological insulator CuxBi2Se3, which can now be measured. The horizontal axis shows the electron energy in electron volts; the vertical axes depict the electron momentum in the 2D plane. (Copyright 2011, American Physical Society)|
Harima, Japan--Physicists at the RIKEN Spring-8 Center, along with colleagues from the University of Tokyo and several other institutes in Japan, China, and the USA, can now visualize the electronic states at the surface of a crystal, or the 2D layers within a sample, using a variant of angle-resolved photoemission spectroscopy (ARPES). This should give them insight into high-temperature superconductors.
The technique enables in-depth study of these two-dimensional electronic states for the first time (see figure). Many of the materials of greatest interest for novel electronic applications are based on the intricate properties of such electronic states, explains team member Yukiaki Ishida.
In ARPES, laser light is shone on a crystal and the pattern of the photoelectrons ejected from the crystal’s surface is recorded. Beams of differing polarization allow further details of the electronic states of the crystal to be obtained. When studying ARPES measurements made on two different crystals--niobium-doped strontium titanate (SrTiO3:Nb) and copper-intercalated bismuth selenide (CuxBi2Se3)--Ishida and colleagues discovered that, under some experimental circumstances, there is a common pattern of electron photoemission. The catch is that this pattern occurs only when the 2D electronic states probed therein are thin enough.
This technique can provide unique insight into a number of widely studied materials. High-temperature superconductors are made of thin atomic layers that are crucial to their superconductivity. A recently discovered class of materials, topological insulators (such as CuxBi2Se3), also has promising properties: the electrons of these materials can travel at their surface almost without any losses to the orientation of the electron’s magnetic property, which has potential application in new ways of computing.
Beyond the study of novel electronic materials, studying the surfaces of materials may reveal new findings, says Ishida. “How surface states change during catalytic reactions is of major scientific and commercial interest,” he explains. “For example, by monitoring the surface states we may be able to investigate how deep a chemical reaction penetrates into the crystal.”