In noncentrosymmetric materials, higher-order nonlinear responses are generally expected. We focus on the second-order nonlinear transport and investigate the nonreciprocal charge transport and bulk photovoltaic effect, which are the intrinsic rectification effects reflecting the symmetry breaking. As well as investigating the quantum mechanical origins of the intrinsic rectification effect, we are also exploring the novel nonlinear transport phenomena.

T. Ideue, et al. Nature Physics, 13, 578-583 (2017).
Y.J. Zhang, et al. Nature, 570, 349-353 (2019).
Y. Itahashi et al. Science Advances, 6, eaay9120 (2020)

Two dimensional materials, which can be exfoliated from three dimensional layered compounds by scotch tape method, can be stacked in any order and stacking angle. In such van der Waals interfaces, characteristic nanostructures or symmetries such as moiré pattern or polar quai-periodicity can emerge which affect the interfacial physical properties. We are investigating emergent quantum properties and functionalities originating from unique nanostructures in van der Waals interfaces.

M. Onga et al. Nano Letters, 20, 4625(2020).
T. Akamatsu et al. Science, 372, 68 (2021)

Electronics, infrastructure technology of modern society, constructs logic circuits by controlling charge degree of freedom. Recently, there has been considerable interest in “valleytronics”, which is a new challenge integrating “valley”, a quantum mechanical degree of freedom, into electronics. Valley degree of freedom is available when multi and equivalent bottoms of conduction bands can be geometrically distinguished. Because it couples with spin degree of freedom through the spin-orbit interaction, new quantum phenomena and new types of devices are expected to be realized.

R. Suzuki, M. Sakano et al., Nature Nanotechnology 9, 611 (2014)
Y. J. Zhang et al., Science 344, 725 (2014)
M. Onga et al., Nature Materials, 16, 1193 (2017)

After the discovery of copper oxide high temperature superconductors in the last century, doping carriers into semiconductors and insulators has been played a very important role in materials research. We have focused on electrostatic carrier doping using the electric-double-layer-transistor (EDLT) structure and electrochemical carrier doping by intercalation, which are powerful to search for novel superconductors. By these methods, we can control not only the carrier density but also broken inversion symmetry and dimensionality. We have been investigating detailed and characteristic superconducting properties by measuring the thickness dependence, vortex dynamics, and superconducting gap structure etc.

Y. Saito et al., Science 350, 409 (2015).
Y. Saito et al., Nature Physics 12, 114 (2016)
Y. Nakagawa et al. Science, 372, 6538, 190-195 (2021).