Ferroelectrics have been used in a wide variety of applications such as capacitors, memories, and sensor materials. Many of ferroelectric materials are solid solutions with complex compositions, and their structures are averaged. In particular, in order to clarify the polarization mechanism of pseudo-cubic structures such as relaxor ferroelectrics, not only average structural analysis assuming a conventional periodic structure, but also local structural analysis must be performed. The pair distribution function method is a local structural analysis technique in which the radial distribution function is derived from powder X-ray diffraction and model fitting is performed in real space without assuming a periodic structure. I would like to discuss the necessity of local structural analysis in ferroelectrics.

When device scaling reaches the limit imposed by atoms, technology based on atomically precise structures is expected to emerge, where the assembly of building blocks with identified atomic arrangements without contamination plays a key role. We propose a transfer technique for deterministic fabrication of carbon-nanotube based optical devices. By using single-crystalline anthracene as a medium, which can form large-area thin films, clean nanotubes are placed on a wide range of substrates. Under in-situ optical monitoring, nanotubes of desired chirality can be placed onto the desired location with sub-micron accuracy. This paper introduces the details of the transfer technique, followed by a few examples of the deterministic construction of heterostructures consisting of nanotubes with defined atomic arrangements and other nanomaterials/nanostructures.

Moiré lattice systems of two-dimensional (2D) materials, which are superlattices based on the moiré interference of crystal lattices, have attracted much attention as a new platform for studying many-body physics of electrons and excitons. Transition metal dichalcogenides (TMDs), which are 2D semiconductor materials, are good candidates to host correlated electrons in moiré lattice due to their relatively heavy mass. While excitonic properties of TMDs have been extensively studied by optical measurements, electronic properties of moiré lattice systems have not been fully explored due to the difficulty of transport measurements. In this paper, I present our recent discovery of strongly correlated electronic states in semiconductor TMD moiré lattice systems by using exciton resonance in optical spectroscopy.

Thermoelectric effects have been applied to power generators and temperature sensors that convert waste heat into electricity. The effects, however, have been limited to electrons to occur, and inevitably disappear at low temperatures due to electronic entropy quenching. Here, we report thermoelectric generation driven by nuclear spins in a solid: nuclear-spin Seebeck effect. The sample is a magnetically ordered material MnCO₃ having a large nuclear spin (I=5/2) of ⁵⁵Mn nuclei, with a Pt contact. In the system, we observe low-temperature thermoelectric signals down to 100 mK due to nuclear-spin excitation. Our theoretical calculation shows that an interfacial Korringa process plays an important role. The nuclear thermoelectric effect demonstrated here offers a way for exploring thermoelectric science and technologies at ultralow temperatures.

Ferroelectrics can act as nonvolatile memory thanks to the switchable polarization by an electric field. In conventional ferroelectrics, however, it is difficult to maintain ferroelectricity down to ultrathin thicknesses, which needs to be solved from a materials science perspective. To overcome this problem, we artificially created a two-dimensional ferroelectric material utilizing a method called van der Waals assembly. We successfully transformed boron nitride, a layered van der Waals compound without ferroelectricity, into a ferroelectric material by artificially changing its stacking order. The resulting ferroelectrics are stable up to room temperature despite their sub-nanometer thickness, enabling nonvolatile memory applications. We further demonstrated the versatility of the design principle by converting semiconducting transition metal dichalcogenides to ferroelectrics in a similar manner.

In our world, there are many situations where generated heat must be removed actively such as in engines, turbines, CPUs in datacenters, power semiconductors, and batteries. Existing solid-state thermoelectric conversion mostly harvest heat that had been ejected to outside the system but still have some utility value. However, an integration of active cooling and thermoelectric power generation was unexplored. Since 2015, we have been developing a new technology that integrates liquid-based thermoelectric conversion into forced convection cooling. This article describes the concept of this technology and summarizes our recent results.

In this paper, some important points and tips of nanoindentation testing, one of the ultra-small testing technologies, are explained to evaluate mechanical strength in a local area. The correlation between nanoindentation hardness and Vickers hardness in Fe-Cr alloys is discussed with a consideration of contact models. Examples of micro-pillar compression tests are also introduced.