The research of Ando Laboratory focuses on growths of high-quality singe crystals and top-notch transport measurements of novel materials, such as topological insulators and topological superconductors. Our emphasis is on precise and systematic measurements of basic physical properties, which allows one to unveil the peculiar electronic states of novel materials. This is achieved by combining the expertise in solid-state physics and applied chemistry. Our goal is two-fold: Creating innovative materials for solving urgent issues of the human society, while exploring fundamental new physics in condensed matter.
Insulating Behavior in Topological Insulators
The 3D topological insulator is a novel quantum state for matter that is supposed to show insulating behavior in the bulk and spin-dependent metallic conduction on the surface. In practice, however, it is very difficult to get rid of the residual bulk conduction originating from defects in the crystals of such materials. As a result, transport studies of the topological surface state have been quite challenging. We have synthesized a new topological insulator, Bi2Te2Se, which approaches insulating behavior in the bulk with a high resistivity, thanks to the peculiar chemistry associated with its layered structure. We observed clear Shubnikov-de Haas oscillations coming from the 2D surface state and were able to determine the transport mechanism in the bulk, paving the way for exploiting the unique surface conduction properties of topological insulators.[Physical Review B 82, 241306(R) (2010)] This work was spotlighted in the APS online journal Physics.
A Big Step toward Discovering a Topological Superconductor
Soon after the discovery of topological insulators, a new class of condensed matter phase called "topological superconductor" was theoretically predicted and generated great interest. Finding its first concrete example would make an important landmark in physics. One of its prime candidates is the electron-doped topological insulator CuxBi2Se3, which was found in 2009 to superconduct below 3 K. However, this material is very difficult to synthesize, and samples with only a small fraction of superconducting volume had been available. Using a rather simple electrochemistry technique, we have managed to produce high-quality crystals of CuxBi2Se3 with a large superconducting volume fraction. Our specific-heat measurement elucidated that CuxBi2Se3 is a bulk strong-coupling superconductor with a full energy gap, which qualifies this system to be topological. This is a big step towards the goal of identifying the new topological phase of matter, whose applications include the fault-tolerant topological quantum computing. [Physical Review Letters 106, 127004 (2011)]
Synthesis of the First "Ambipolar" Cuprate
High-temperature superconductivity occurs when a sufficient number of charge carriers are doped into a parent cuprate Mott insulator. Its mechanism remains a mystery, mostly because we still do not know the appropriate description of those carriers, despite the many theoretical models considered so far (Hubbard, t-J, d-p, RVB, Stripes, etc.). To dissect a Mott insulator and examine the possible seeds of superconductivity, one would like to study what happens when a small number of electrons are either added or removed from the Mott-insulating state. Yet, until recently, there were no examples of a single Mott insulating material that could be made to conduct by both addition and removal of electrons (or equivalently, addition of holes). We have synthesized a unique cuprate material Y1-zLazBa2-xLaxCuOy to address this need, and found intriguing results at very low doping levels, where a marked difference in properties of electron- and hole-doped materials is manifest. Perhaps the most striking is the efficacy of carrier injection, that is, electrons are found to be much more effectively injected compared with holes, which probably stems from the inherent electron-hole asymmetry in "charge-transfer-type" Mott insulators. [Nature Physics 6, 579 (2010)]
This work was spotlighted in the News & Views section of Nature Physics.
Ando Laboratory is also called the Department of Quantum Functional Materials at the Institute of Scientific and Industrial Research (ISIR), Osaka University. Our materials synthesis lab, as well as our offices, are located on the 2nd floor of the 2nd ISIR Building, while our low temperature lab is located at the Low-Temperature Center just across the street. The access to ISIR can be found here.
Ando Lab is associated with the Department of Applied Chemistry, Graduate School of Engineering, where we participate in the education of graduate students. Those who wish to join Ando Lab as graduate students must pass the entrance examination of the Department of Applied Chemistry, and should contact Prof. Ando well in advance. Those who wish to enhance their careers by working in our laboratory as postdocs should feel free to contact Prof. Ando at any time, since there are various opportunities.
Prof. Yoichi Ando
Institute of Scientific and Industrial Research, Osaka University
Room 208, 2nd ISIR Building, Suita Campus
E-mail: y_ando (at-mark) sanken.osaka-u.ac.jp