Ion storage rings have been serving as powerful tools in atomic and molecular physics since the high-energy magnetic ion storage ring were developed in the 1980s. In the 21st century, the technologies were inherited by the ‘electrostatic’ ion storage rings and brought a new opportunity of experiments with large molecular ions. Based on a number of advantages demonstrated in the electrostatic storage rings, new-generation cryogenic electrostatic rings were developed at Stockholm University, Max Planck Institute, and RIKEN. In this article, we review the historical background and technical developments of the RIKEN Cryogenic Electrostatic storage ring (RICE). We also overview the recent experiments with cold molecular ions at the cryogenic storage rings.

We report the recent progress in the new physics search using isotope shifts. We introduce the King linearity between isotope shifts of two transitions in an element and its generalization to three or more transitions. The effect of the Yukawa potential caused by the exchange of a novel particle between an electron and a neutron is explored by looking for nonlinearity in the King plot and its generalization. We present a new result of systematic isotope-shift measurement in a narrow optical transition, (6s)^{2 1}S₀-6s6p ³P₀ (578 nm), of neutral ytterbium (Yb) atom. We have achieved an unprecedented accuracy of a few Hz in our experiment in which we employed a three-dimensional magic-wavelength optical lattice to trap isolated Yb atoms. Combining these data with those of Yb ion, we have shown that there are at least two sources of nonlinearity in the data. We have derived a bound on new physics according to this observation.

A fundamental problem posed from the study of correlated electron compounds is the need to understand the physics of states near a quantum critical point (QCP). In the prototype systems of heavy-fermion compounds, the pertinent issue is when the f-electrons become itinerant. The most influential proposal has been the so-called Kondo breakdown scenario where the localized-to-itinerant transition takes place right at the antiferromagnetic QCP. Here, we review a recent work by pressure-dependent ¹¹⁵In nuclear quadrupole resonance (NQR) measurements on heavy-fermion antiferromagnetic superconductor CeRh_0.5Ir_0.5In₅ (T_N=3.0 K, T_c=0.9 K). The experiments reveal an antiferromagnetic (AF) QCP at P_c^AF=1.2 GPa where a dome of superconductivity reaches a maximum transition temperature T_c^max=1.4 K. Preceding P_c^AF, however, the NQR frequency ν_Q undergoes an abrupt increase at P_c*=0.8 GPa in the zero-temperature limit, indicating a change from localized to itinerant character of cerium’s f-electron and associated small-to-large change in the Fermi surface. These findings are at odds with the Kondo breakdown scenario. Furthermore, an unusually large fraction of gapless excitations was observed well below T_c even though the superconductivity is optimized there. This implicates spin-singlet, odd-frequency pairing symmetry which can be understood as a direct consequence of a large Fermi surface at the QCP.

Antiferromagnets and ferrimagnets have negligible net magnetization. Yet, microscopic magnetic moments on each magnetic sublattice can exhibit their dynamic modes as a small fluctuation of the net magnetization. Control and investigation of them has been limited by technical difficulty dealing with a very high frequency and small signals. However, recent experimental efforts reveal intriguing aspects of the antiferro- and ferri- magnetic dynamics. Here, we show our recent experiments of magnetization dynamics in antiferromagnets and ferrimagnets and their unique properties.

Superconducting parametrons in the single-photon Kerr regime, called KPOs (Kerr Parametric Oscillators), have been attracting attention in terms of their applications to quantum annealing machines and universal quantum computers. We present the basic properties of KPOs and review recent theoretical studies on their measurement and control. In particular, we review the theory of reflection spectroscopy for KPOs which provides useful information of KPOs, such as energy level structure, and we give a brief overview of study on controls of a KPO under the effect of an intrinsic source of error, non-resonant rapidly oscillating terms, coming from the violation of the rotating wave approximation.

Since in most of functional materials or strongly correlated electron systems, the properties can be controlled by foreign elements doping. Thus, to understand the doping effects, atomic imaging around the dopant must be indispensable. White neutron holography, which was developed by the authors, is a novel probe for atomic imaging of light elements around a dopant in materials. Using this method, the authors group have succeeded in visualizing local atomic images around dopants in B doped Si, B doped Mg₂X (X: Si, Sn), Sm doped RB₆(R: Yb, La) and so on. The authors believe that white neutron holography in J-PARC will be a novel and important probe for materials science.