Kenji Ishida

We investigated the quantum size effect (QSE) in bimetallic ${\mathrm{Pt}}_{1\ensuremath{-}x}{\mathrm{Pd}}_{x}$ and ${\mathrm{Pt}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}$ nanoparticles, using $^{195}\mathrm{Pt}$ nuclear magnetic resonance measurements. The temperature and size dependencies of the anomaly in the nuclear spin-lattice relaxation rate divided by temperature $1/{T}_{1}T$ in the ${\mathrm{Pt}}_{1\ensuremath{-}x}{\mathrm{Pd}}_{x}$ nanoparticles suggest similar electron states between Pt and Pd atoms and are well understood by the QSE. The temperature and composition variations of $1/{T}_{1}T$ and the Knight shift reveal a systematic increase in the density of states and a reduction of the characteristic energy scale ${T}^{*}$ with increasing Ni content, consistent with the Kubo gap ${\ensuremath{\delta}}_{\mathrm{Kubo}}$. In contrast to the ${\mathrm{Pt}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}$ nanoparticles where the QSE is suppressed, the ${\mathrm{Pt}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}$ nanoparticles exhibit clear signatures of quantum energy discretization. This discrepancy highlights the essential role of $d$ electrons in the manifestation of the QSE. Furthermore, analysis of the modified Korringa parameter $K(\ensuremath{\alpha})$ suggests enhanced ferromagnetic correlations with increasing Ni concentration, approaching a ferromagnetic quantum critical regime. These results provide experimental evidence that $d$-electron density of states plays a crucial role in the manifestation of the QSE in the nanoparticles formed by the metallic $d$-electron atoms.

Spin-triplet superconductivity is an intriguing quantum coherent state with both spin and orbital degrees of freedom, which holds significant potential for future applications in quantum technology. However, how the spin of the triplet pairs responds to an external magnetic field remains poorly understood. This is mainly due to the absence of suitable spin-triplet superconductors. Here, we report results of Knight-shift and ac-susceptibility measurements on UTe$_2$. We demonstrate that the spin susceptibility, which slightly decreases compared to the normal-state value below the superconducting (SC) transition temperature $T_{\rm c}$, is rapidly restored and nearly recovers to the normal-state values around 5 T, well below the SC upper critical field $H_{c2}$ when the magnetic field is applied along the $c$ axis ($H \parallel c$). In addition, we found that $H_{\rm c2}$ of superconductivity becomes larger when the SC spin aligns with the magnetic field. By considering the results on $H \parallel b$, our results suggest the presence of a close relationship between the spin configuration of the triplet pair and $H_{\rm c2}$, as well as the anisotropic pinning interaction acting on the triplet pairs. These phenomena, which have never been observed in spin-singlet superconductors, represent characteristic features unique to spin-triplet superconductors. We discuss the similarities between superconductivity in UTe$_2$ and superfluid $^3$He, focusing on their spin-triplet pairing states.