Topological insulators are novel materials that are insulating in the bulk but have surface states that are conducting. These surface states are topologically protected and possess several intriguing properties with the promise of potential applications.
As a result, topological insulators have attracted many theoretical and experimental studies in the last few years. More recently, the potential of interfacing topological insulators with other materials with quantum states to make hybrid structures has been recognized and a slew of new studies are underway. Professor Jian Wang from Peking University and his collaborators have reviewed the current status of research on topological insulators especially with a view towards hybrid structures. Their work, titled "Quantum transport in topological insulator hybrid structures – a combination of topological insulator and superconductor", was published in SCIENCE CHINA Physics, Mechanics and Astronomy 2012, No.(12).
The unique band structure of topological insulators gives them several fascinating properties. For example, in the metallic surface state, the electronic momentum and spin are locked preventing backscattering of electrons from any impurity that does not interact with the spin. This is akin to the edge states of the quantum Hall effect. However, the surface state of the topological insulator, unlike the quantum Hall effect, does not need an applied magnetic field. Topological insulators are also akin to graphene in that both possess Dirac cones in their band structures. However, there are significant differences from graphene too. These and other unique properties make topological insulators a hot house to explore novel physics and engineering. Many of the predicted properties for topological insulators have already been confirmed by experiments while other studies, both theoretical and experimental, are ongoing. The field is rapidly evolving and holds much promise. Therefore, there is a need to review what is known with certainty, what needs to be explored, and what the possible technological applications of topological insulators are.
The metallic surface state of topological insulators has been confirmed by angle-resolved photo-emission spectroscopy experiments. There are several ingenious transport experiments also strongly suggesting the existence of such surface states. However, detecting these states via transport experiments is challenging due to interference from the bulk. Efforts involving doping and gating to minimize such interference are underway. Transport results have shown several interesting phenomena in topological insulators like the Aharanov-Bohm effect and the Shubnikov de-Haas effect. The physics becomes even richer when topological insulators are interfaced with other materials.
In the interface of a topological insulator with a superconductor, a Majorana fermion, the only fermionic particle that is its own antiparticle is predicted to exist. Other interesting phenomena like the fractional Josephson effect have also been predicted. Specific experimental geometries to detect these phenomena have been suggested and conclusive experimental data remain to be obtained. Hybrid superconductor-topological insulator structures have yielded several surprising and interesting results.
A proximity-induced supercurrent has been observed in topological insulators. However, it is not clear whether the supercurrent is supported by the surface or the bulk channels. Several experiments aiming at exploiting the difference in properties between the bulk and the surface channels have been conducted. This channel has been confirmed to contribute to the superconducting proximity effect. However, the bulk effect has not yet been ruled out. In topological insulator thin films contacted with bulk superconducting electrodes, an upturn in resistance at the superconducting critical temperature was found. The reason for this upturn is not clearly understood yet. Unexpected features have also been found in the Josephson junction made with topological insulators. For example, the well-known relationship between critical current and normal state resistance does not hold for topological insulator Josephson junctions. These results highlight the unexplored physics of such systems and also point the way for future studies.
Experiments under a bias voltage have also been performed on topological insulator hybrid structures to probe the proximity induced gap. Multiple Andreev reflection has been observed. In addition to the conductance peak corresponding to the superconductor's gap, an additional peak has been observed. It is believed the second peak is caused by a new superconducting state existing in the topological insulator-superconductor interface. Careful controlled experiments using graphene and other two-dimensional films have been performed to rule out the possibility of the novel phenomena described being rooted in nanoscale confinement rather than the unique properties of topological insulators.
Other topological insulator hybrid structures like topological superconductors and topological semimetals are also introduced. The predicted novel properties and proposed experiments are discussed.
Promotion of this work will help in increasing understanding of the underlying physics of topological insulators at a functional level and implementation of the experiments suggested in this work will help in furthering understanding of the fascinating field of topological insulator hybrid structures.