Scientists discover a material promising for future quantum computation
Scientists discover a material promising for future quantum computation
A research team led by Dr. Tien-Ming Chuang and Dr. Chia-Seng Chang at the Institute of Physics, Academia Sinica (AS), in collaboration with the researchers from National Taiwan University (NTU) and National Tsing Hua University (NTHU), has discovered the superconducting topological surface states in a layered material, PbTaSe2. Their findings provide a great platform for the study of topological superconductivity toward the future application for the fault-tolerant quantum computing. Their results were published in the American Association for the Advancement of Science (AAAS) open access online magazine Science Advances on Nov 23, 2016.
The search of novel states of matter has always been of fundamental and technical importance. For example, the study of spin order in magnetic materials has led to the giant magnetoresistance (Nobel Prize in 2007) and the miniaturization of hard drives. The discovery of the quantum Hall effect in 1980 (Nobel Prize in 1985) and the subsequent development of topological band theory (Nobel Prize this year) have open a new field to the study of topological phases. The key development is the theoretical prediction and the experimental confirmation of topological insulators (TIs). A TI, like an ordinary insulator, has a bulk energy gap separating the highest valence band from the lowest conduction band. However, at the boundary, a TI has protected conducting states that are immune to impurities and useful for high performance electronics.
When a TI combined with superconductivity, it can theoretically lead to another important class of materials: topological superconductors (TSCs). TSCs are characterized by a full superconducting gap in the bulk and topologically protected gapless surface states. In a TSC, Majorana fermion, a hypothetical particle is its own anti-particle, is bound to a topological defect such as a superconducting vortex. Such combined objects are predicted to exhibit non-Abelian statistics and to form the basis of the fault-tolerant quantum computation. The simplest way to introduce superconductivity in a TI is by making it superconducting by doping or by coating a layer of s-wave superconductor. However, both approaches are technically challenging because it is difficult to make a homogenous doped material or a heterostructure with a clean interface.
The solution is to find a stoichiometric material that exhibits topological surface states at Fermi level in the normal state combined with fully gapped bulk superconductivity below TC. So far, no such a bulk material is reported. Dr. Peng-Jen Chen from AS and Prof. Horng-Tay Jeng from NTHU found such requirements for a TSC are satisfied in a layered material, PbTaSe2 when studying its electronic structures by density functional theory. High quality single crystals are then synthesized by Dr. Raman Sankar and Dr. Fangcheng Chou at Center for Condensed Matter Sciences, NTU and the detailed crystal structure is confirmed by Dr. Ming-Wen Chu by using scanning transmission electron microscopy. By visualizing the electron wavefunction at atomic scale with the state-of-the-art home-built scanning tunneling microscopes, PhD student Syu-You Guan from NTU, Dr. Chia-Seng Chang and Dr. Tien-Ming Chuang from AS confirm the spectroscopic signature of the calculated topological band structures and superconducting properties in PbTaSe2. The fully gapped superconducting topological surface state is reported for the first time in a stoichiometric bulk material. Their work shows PbTaSe2 is a promising TSC candidate. The next step is to understand the nature of the superconducting topological surface state and to explore the novel topological superconducting phases towards future applications such as topological quantum computation.
This research was supported by Academia Sinica, Ministry of Science and Technology, and National Taiwan University. Dr. Tien-Ming Chuang also received the support from Kenda Foundation.
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Journal Links: http://dx.doi.org/10.1126/sciadv.1600894