Research

Topological Quantum Phenomena and Epitaxial Thin-Film Synthesis

The motion of electron and quasiparticle in solids is fundamentally dictated by the principles of quantum mechanics and statistical mechanics. Utilizing these principles has led to remarkable advancements of condensed matter physics in 20th century , most notably in semiconductor technologies and high-temperature superconductivity. More recently, the mathematical frameworks of symmetry and topology have been recognized as pivotal in dictating an even broader range of material properties. Of particular importance are materials whose band or magnetic structures exhibit nontrivial topology, commonly referred to as topological quantum materials, which are expected to be candidate platforms for next-generation electronic applications.

In our laboratory, we leverage molecular beam epitaxy (MBE) to synthesize thin film samples of these topological quantum materials. By systematically depositing atoms on a substrate, we are able to produce low-defect, high-quality crystalline thin films. Furthermore, by engineering heterointerfaces of two different solid materials, we can design and manipulate two-dimensional systems at interfaces. In addition, we explore the application of external electric or magnetic fields to modulate and control emergent quantum phenomena, with the ultimate aim of translating these effects into novel functional device technologies.

Magnetic Proximity Heterojunctions in Topological Insulators

Topological insulators (TIs) are a representative class of topological quantum materials characterized by an electrically insulating bulk and conducting Dirac electron states confined to their surfaces. When ferromagnetic interactions are introduced into the surface Dirac states, time-reversal symmetry is broken, leading to the manifestation of the quantum anomalous Hall effect, which is a hallmark quantum transport phenomenon. In our laboratory, we have pioneered a method to induce the magnetic proximity effect by forming heterojunctions between TIs and ferromagnetic insulators, enabling the first observation of the quantum anomalous Hall effect via this approach. Beyond this achievement, we are expanding our efforts to leverage heterointerface engineering as a central strategy for discovering and controlling novel functionalities in topological quantum materials.

Exploring Functional Properties in Magnetic Rashba Semiconductors

In electronic systems lacking spatial inversion symmetry, spin degeneracy in the band structure can be lifted. While a two-dimensional electron gas at semiconductor heterointerfaces is a well-known example of this phenomenon, recent attention has focused on Rashba-type dispersion arising from bulk inversion asymmetry (polarity). To observing a macroscopic electromagnetic stemming from spatial broken inversion symmetry, high-quality and highly oriented samples that can be fabricated by advanced thin film synthesis techniques are essential.

In our laboratory, we introduce magnetic dopants into Rashba semiconductors to induce ferromagnetism, thereby enabling the exploration of various functional physical properties. For instance, we have successfully observed current-induced magnetization reversal, in which the orientation of localized magnetic moments can be switched by an applied current, as well as a nonreciprocal resistivity (a kind of “diode effect”), where the electrical resistance changes depending on the direction of current flow.

For those who want to join our lab

Beyond the topics introduced above, our lab pursues a wide range of research on the physics of topological quantum materials.

We welcome undergraduate students from the Department of Applied Physics, School of Engineering, The University of Tokyo, for their graduation research. Prospective graduate students should apply to the Department of Advanced Materials Science at the Graduate School of Frontier Sciences, The University of Tokyo (website for entrance examination). We welcome applicants not only for the Master’s course but also the Doctoral course.

Please feel free to email Prof. Yoshimi (Member) for further details or to arrange a visit.

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