This study alleviates the low operating temperature constraint of Si qubits. A qubit is a key element for quantum sensors, memories, and computers. Electron spin in Si is a promising qubit, as it allows both long coherence times and potential compatibility with current silicon technology. Si qubits have been implemented using gate-defined quantum dots or shallow impurities. However, operation of Si qubits has been restricted to milli-Kelvin temperatures, thus limiting the application of the quantum technology. In this study, we addressed a single deep impurity, having strong electron confinement of up to 0.3 eV, using single-electron tunnelling transport. We also achieved qubit operation at 5–10 K through a spin-blockade effect based on the tunnelling transport via two impurities. The deep impurity was implemented by tunnel field-effect transistors (TFETs) instead of conventional FETs.
Hadronic molecules are the new form of matter induced by the strong interaction. However, identification of the hadronic molecules involves several subtle difficulties such as the model dependence and the interpretation of the resonance wave function. To overcome these difficulties, we use the compositeness to characterize the internal structure of hadrons, and generalize the weak-binding relation for unstable resonances. It is quantitatively shown that the structure of the Λ(1405) resonance is dominated by the molecular state of an antikaon and a nucleon.
Lighter isotopes normally diffuse faster than heavier ones; however, this is not necessarily the case for H. That is one of the reasons why the prediction of the kinetics of H-isotope transport and reaction in and on crystals remains a fundamental challenge in materials physics. Actually, the peculiar isotope effect experimentally observed on H diffusivities in face-centered cubic (fcc) metals has been a long-standing unsolved problem. Using a state-of-the-art theoretical approach to exploring the quantum mechanical nature of both electrons and nuclei, we succeeded in predicting the H-isotope diffusivities in fcc Pd over a wide temperature range. We found that the temperature dependence of the diffusivities has an unusual “reversed S” shape on Arrhenius plots. This irregular behavior, which stems from the competition between different nuclear quantum effects with different temperature dependencies, unravels the mechanism of anomalous crossovers between the normal and reversed isotope effects. Our approach is broadly applicable to assessing the quantum behavior of H isotopes in a range of materials.
This article describes recent experimental findings of correlated insulating and superconducting states in twisted bilayer graphene and related theoretical progress. Moiré patterns formed by two graphene layers create a long-wavelength periodic structure, giving rise to narrow low-energy bands. The electron correlation energy is comparable or exceeds the kinetic energy owing to the narrow bandwidth, and thus intriguing correlated phases appear. We explain the reconstruction of the low-energy bands, the experimental observation of the correlated phases, and theoretical attempts to comprehend the correlation physics. In addition to twisted bilayer graphene, the formation of moiré patterns and resulting correlated phenomena appear more widely in superstructures of van der Waals materials including graphene and transition metal dichalcogenides. Thanks to their flexibility and tunability, moiré materials invigorate countless studies. We conclude the article by mentioning the challenges and prospects of moiré materials.
X-ray absorption spectroscopy (XAS) is an element specific method to investigate the excitation of a specified core electron to unoccupied states. Since the soft X-ray region covers K-edges of C, N, and O, it is applicable to local structural analyses of molecular liquids. The measurement condition of XAS in the soft X-ray region is not so simple, because soft X-rays are strongly absorbed by air and liquid itself. We have established a precise thickness control method of liquid sample from 20 to 2,000 nm to obtain reliable liquid XAS spectra in transmission mode with optimized absorbance for wide concentration regions. We have investigated local structures of several liquid samples, such as unexpected temperature-dependent structural changes in liquid benzene and concentration dependence of aqueous pyridine solutions, from precise energy shift measurements and inner-shell quantum chemical calculations. The present method will be applicable to local structural analyses of several physical, chemical, and biological phenomena in solution.