Visualizing Molecular-Scale 3D Distributions of Ionic Liquids in Electric Double-Layer Capacitor by 3D Scanning Force Microscopy with Variable Tip/Sample Bias Voltages.

Atomic force microscopy (AFM) imaging of ionic liquid (IL) distribution in electric double-layer (EDL) devices has been actively explored to understand the origin of their excellent performance. However, this has been impeded by insufficient resolution or a poor understanding of the mechanisms of 3D IL imaging. Here, we overcome these difficulties using 3D scanning force microscopy (3D-SFM) with variable tip/sample bias voltages for visualizing 3D ,-diethyl--methyl--(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI) distributions on a Au electrode in EDL capacitors.

Misfit layered superconductor (PbSe)(NbSe) with possible layer-selective FFLO state.

Two-dimensional (2D) superconductors are known for their novel emergent phenomena, however, lack of experimental probes beyond resistivity has hindered further exploration of diverse superconducting states. Bulk 2D superconductors, with superconducting layers separated by non-superconducting layers, offer a unique opportunity to break this limit. Here, we synthesized a single crystal of misfit layered compound (PbSe)(NbSe), composed of alternately stacked tri-layer NbSe and non-superconducting block layers with incompatible unit cells.

Research on Quantum Materials and Quantum Technology at RIKEN.

RIKEN covers fundamental research on physics, chemistry, biology, life and medical science, information and mathematical science, and engineering. Here, we outline research activities on quantum materials and quantum technology that include topological and correlated materials, spintronics, nanoscale materials and structures, atomic and quantum optics, and quantum computing.

Molecularly-anchored single PbS quantum dots as resonant tunnelling transistors.

The growing need for high-performance computing continues to drive improvement in circuit and device technologies, particularly with respect to speed and power efficiency. Device scaling remains the most effective strategy for meeting circuit performance requirements while reducing power consumption. Thanks to their solution processability, colloidal semiconductor quantum dots (QDs) are highly suitable for device miniaturisation as quantum information science platforms.

Gate-Controlled Potassium Intercalation and Superconductivity in Molybdenum Disulfide.

Intercalation of guest ions into a van der Waals (vdW) gap in layered materials is a powerful route to create novel material phases and functionalities. Ionic gating is a technique to control the motions and configuration of ions for both intercalation and surface electrostatic doping. The advance of ionic gating enables the probe of dynamics of ion diffusion, carrier doping, and transport properties.

Superconducting diode effect under time-reversal symmetry.

In noncentrosymmetric superconductors, superconducting and normal conductions can interchange on the basis of the current flow direction. This effect is termed a superconducting diode effect (SDE), which is a focal point of recent research. The broken inversion and time-reversal symmetry is believed to be the requirements of SDE, but their intrinsic role has remained elusive.

High Volumetric Energy Density Supercapacitor of Additive-Free Quantum Dot Hierarchical Nanopore Structure.

The high surface-area-to-volume ratio of colloidal quantum dots (QDs) positions them as promising materials for high-performance supercapacitor electrodes. However, the challenge lies in achieving a highly accessible surface area, while maintaining good electrical conductivity. An efficient supercapacitor demands a dense yet highly porous structure that facilitates efficient ion-surface interactions and supports fast charge mobility.

Giant Modulation of the Second Harmonic Generation by Magnetoelectricity in Two-Dimensional Multiferroic CuCrPS.

Multiferroic materials have attracted considerable attention owing to their unique magnetoelectric or magnetooptical properties. The recent discovery of few-layer van der Waals multiferroic crystals provides a new research direction for controlling the multiferroic properties in the atomic layer limit. However, research on few-layer multiferroic crystals is limited and the effect of thickness-dependent symmetries on those properties is less explored.

Single PbS colloidal quantum dot transistors.

Colloidal quantum dots are sub-10 nm semiconductors treated with liquid processes, rendering them attractive candidates for single-electron transistors operating at high temperatures. However, there have been few reports on single-electron transistors using colloidal quantum dots due to the difficulty in fabrication. In this work, we fabricated single-electron transistors using single oleic acid-capped PbS quantum dot coupled to nanogap metal electrodes and measured single-electron tunneling.

An anisotropic van der Waals dielectric for symmetry engineering in functionalized heterointerfaces.

Van der Waals dielectrics are fundamental materials for condensed matter physics and advanced electronic applications. Most dielectrics host isotropic structures in crystalline or amorphous forms, and only a few studies have considered the role of anisotropic crystal symmetry in dielectrics as a delicate way to tune electronic properties of channel materials. Here, we demonstrate a layered anisotropic dielectric, SiP, with non-symmorphic twofold-rotational C symmetry as a gate medium which can break the original threefold-rotational C symmetry of MoS to achieve unexpected linearly-polarized photoluminescence and anisotropic second harmonic generation at SiP/MoS interfaces.

Valley-dimensionality locking of superconductivity in cubic phosphides.

Two-dimensional superconductivity is primarily realized in atomically thin layers through extreme exfoliation, epitaxial growth, or interfacial gating. Apart from their technical challenges, these approaches lack sufficient control over the Fermiology of superconducting systems. Here, we offer a Fermiology-engineering approach, allowing us to desirably tune the coherence length of Cooper pairs and the dimensionality of superconducting states in arsenic phosphides AsP under hydrostatic pressure.

Berry curvature dipole generation and helicity-to-spin conversion at symmetry-mismatched heterointerfaces.

The Berry curvature dipole (BCD) is a key parameter that describes the geometric nature of energy bands in solids. It defines the dipole-like distribution of Berry curvature in the band structure and plays a key role in emergent nonlinear phenomena. The theoretical rationale is that the BCD can be generated at certain symmetry-mismatched van der Waals heterointerfaces even though each material has no BCD in its band structure.

Enabling metallic behaviour in two-dimensional superlattice of semiconductor colloidal quantum dots.

Semiconducting colloidal quantum dots and their assemblies exhibit superior optical properties owing to the quantum confinement effect. Thus, they are attracting tremendous interest from fundamental research to commercial applications. However, the electrical conducting properties remain detrimental predominantly due to the orientational disorder of quantum dots in the assembly.

Layer-Number-Independent Two-Dimensional Ferromagnetism in CrTe.

In a conventional magnetic material, a long-range magnetic order develops in three dimensions, and reducing a layer number weakens its magnetism. Here we demonstrate anomalous layer-number-independent ferromagnetism down to the two-dimensional (2D) limit in a metastable phase of CrTe. We fabricated CrTe thin films by molecular-beam epitaxy and found that CrTe could host two distinct ferromagnetic phases characterized with different Curie temperatures ().

Giant bulk piezophotovoltaic effect in 3R-MoS.

Given its innate coupling with wavefunction geometry in solids and its potential to boost the solar energy conversion efficiency, the bulk photovoltaic effect (BPVE) has been of considerable interest in the past decade. Initially discovered and developed in ferroelectric oxide materials, the BPVE has now been explored in a wide range of emerging materials, such as Weyl semimetals, van der Waals nanomaterials, oxide superlattices, halide perovskites, organics, bulk Rashba semiconductors and others. However, a feasible experimental approach to optimize the photovoltaic performance is lacking.

Vortex dynamics in the two-dimensional BCS-BEC crossover.

The Bardeen-Cooper-Schrieffer (BCS) condensation and Bose-Einstein condensation (BEC) are the two limiting ground states of paired Fermion systems, and the crossover between these two limits has been a source of excitement for both fields of high temperature superconductivity and cold atom superfluidity. For superconductors, ultra-low doping systems like graphene and LiZrNCl successfully approached the crossover starting from the BCS-side. These superconductors offer new opportunities to clarify the nature of charged-particles transport towards the BEC regime.

Spontaneous spin-valley polarization in NbSe at a van der Waals interface.

A proximity effect at a van der Waals (vdW) interface enables creation of an emergent quantum electronic ground state. Here we demonstrate that an originally superconducting two-dimensional (2D) NbSe forms a ferromagnetic ground state with spontaneous spin polarization at a vdW interface with a 2D ferromagnet VSe. We investigated the anomalous Hall effect (AHE) of the NbSe/VSe magnetic vdW heterostructures, and found that the sign of the AHE was reversed as the number of the VSe layer was thinned down to the monolayer limit.

Magnetic Anisotropy Control with Curie Temperature above 400 K in a van der Waals Ferromagnet for Spintronic Device.

The technological appeal of van der Waals ferromagnetic materials is the ability to control magnetism under external fields with desired thickness toward novel spintronic applications. For practically useful devices, ferromagnetism above room temperature or tunable magnetic anisotropy is highly demanded but remains challenging. To date, only a few layered materials exhibit unambiguous ferromagnetic ordering at room temperature via gating techniques or interface engineering.

Giant second harmonic transport under time-reversal symmetry in a trigonal superconductor.

Nonreciprocal or even-order nonlinear responses in symmetry-broken systems are powerful probes of emergent properties in quantum materials, including superconductors, magnets, and topological materials. Recently, vortex matter has been recognized as a key ingredient of giant nonlinear responses in superconductors with broken inversion symmetry. However, nonlinear effects have been probed as excess voltage only under broken time-reversal symmetry.

Evidence of band filling in PbS colloidal quantum dot square superstructures.

PbS square superstructures are formed by the oriented assembly of PbS quantum dots (QDs), reflecting the facet structures of each QD. In the square assembly, the quantum dots are highly oriented, in sharp contrast to the conventional hexagonal QD assemblies, in which the orientation of QDs is highly disordered, and each QD is connected through ligand molecules. Here, we measured the transport properties of the oriented assembly of PbS square superstructures.

Ferromagnetism and giant magnetoresistance in zinc-blende FeAs monolayers embedded in semiconductor structures.

Material structures containing tetrahedral FeAs bonds, depending on their density and geometrical distribution, can host several competing quantum ground states ranging from superconductivity to ferromagnetism. Here we examine structures of quasi two-dimensional (2D) layers of tetrahedral Fe-As bonds embedded with a regular interval in a semiconductor InAs matrix, which resembles the crystal structure of Fe-based superconductors. Contrary to the case of Fe-based pnictides, these FeAs/InAs superlattices (SLs) exhibit ferromagnetism, whose Curie temperature (T) increases rapidly with decreasing the InAs interval thickness t (T ∝ t), and an extremely large magnetoresistance up to 500% that is tunable by a gate voltage.

Probing the Chiral Domains and Excitonic States in Individual WS Tubes by Second-Harmonic Generation.

Distinct from carbon nanotubes, transition-metal dichalcogenide (TMD) nanotubes are noncentrosymmetric and polar and can exhibit some intriguing phenomena such as nonreciprocal superconductivity, chiral shift current, bulk photovoltaic effect, and exciton-polaritons. However, basic characterizations of individual TMD nanotubes are still quite limited, and much remains unclear about their structural chirality and electronic properties. Here we report an optical second-harmonic generation (SHG) study on multiwalled WS nanotubes on a single-tube level.