Our research
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We experimentally investigate quantum properties and functionality of nanomaterials, expecially from the viewpoint of symmetry. In particular, two-dimensional materials such as graphene and molybdenum disulfide (MoS2) are a group of materials that cover a very wide range of physical phenomena. In addition, the existence of crystal polymorphism and the high degree of freedom in creating artificial structures make it possible to investigate the pure effect of symmetry. This is in startk contract to bulk materials, whose crystal structure and symmetry are largely determined by the constituent elements.
Among the many methods for evaluating physical properties, our laboratory places great importance on microscopic optical measurements, because "seeing is believing." It is, however, technically difficult to microscopically observe nanomaterials in environments such as extremely low temperatures and high magnetic fields. Our laboratory has set up a unique facility that performs quantum transport measurements and optical spectroscopy measurements while performing microscopic observations at temperatures down to 1.8 K and magnetic fields up to 9 T. We pursue the quantum properties of nanomaterials from a new perspective that is different from conventional methods, and continue to develop novel functionality.
Nonlinear optical response
Nonlinear responses, for instance, second harmonic generation (SHG) and bulk photovoltaic effect (BPVE), are typical phenomena that are greatly influenced by symmetry. In particular, BPVE has potential for solar energy harvesting. In our group, we study BPVE in quasi-one-dimensional systems [1] in which two-dimensional materials are "rolled up", and in van der Waals layered systems [2].
Since BPVE is the name of a phenomenon, there is not just one mechanism. BPVE can occur due to various mechanisms, such as injection current, ballistic current, and even shift current due to the influence of the Berry connection. We not only try to detect BPVE in various materials but also aiming at clarifying its mechanism.
We are also interested in the synergy between multiple quantum phenomena. For example, WS2 nanotubes are highly doped with electrons, they undergo a superconducting transition at temperatures below 10K, and at that time, they exhibit Little-perks oscillations due to their quasi-one-dimensional structure [3]. We would like to clarify the nonlinear optical response in such a unique electronic state.
[1] YJZ et al. Nature 570, 349-353 (2019).
[2] YJZ et al. Appl. Phys. Lett. 120, 013103 (2022).
[3] Qin et al. Nature. Commun. 8, 14465 (2017).
Visualization of low-energy physics
When a superconducting transition or a charge density wave (CDW) is formed, an energy gap occurs in the electronic state. The gap is very small, and when converted to the frequency of light, it corresponds to THz range. Hence, it has been difficult to evaluate the electronic gap in nanomaterials due to the diffraction limit.
We are attempting to detect electronic gaps using low-wavenumber Raman measurements with visible light. By using light, we can optically approach non-surface electronic states such as electric-field-induced superconductivity [4] that cannot be evaluated by angle-resolved photoemission spectroscopy (ARPES) or scanning probe techniques.
[4] Ye, YJZ et al. Science 338, 1193-1196 (2012).
System: YJZ et al. Appl. Phys. Lett. 120, 053106 (2022).
Intrinsic one-dimensional moiré superlattice
Moiré superlattices appear in (twisted) van der Waals structures of two-dimensional materials. For structures composed of the same materials (homo-structures"), it has been considered that the moiré superlattice appears only at small twist angle and is two-dimensional. Recently, we discovered a conceptually new class of moiré superlattice: the intrinsic one-dimensional moiré superlattice, which appears in large-angle twisted homostructures and the moiré superlattice is matehmatically one-dimensional [5]. We will continue investigation of one-dimensional moiré superlattices and pursue its pysical properties.
[5] Yang, YJZ et al. ACS Nano 19, 13007-13015 (2025).
In-situ characterization of van der Waals assembly
The van der Waals assembly technique is capable to create infinite number of structures by varying material combination and twist angle. Although the magic angle twisted bilayer graphene was theoretically predicted [6] before experimental study, it is in general very hard to theoretically predict an appearance of fantastic physical properties in random combination and twist angle.
The quantum twisting microscope [7] is one of the potential techniques potentially overcome this difficulty. This technique enables study of transport properties of samples at the surface. In contrast, our aim is to investigate optical properties during vdW assembly, whose targets are not limited to surface systems.
[6] Bistritzer & MacDonaldo Proc. Natl. Acad. Sci. USA 108, 12233-12237 (2011).
[7] Inbar et al. Nature 614, 682-687 (2023).
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