Showing papers in "Journal of the Physical Society of Japan in 2022"

Multiple Skyrmion Crystal Phases by Itinerant Frustration in Centrosymmetric Tetragonal Magnets

Abstract: A skyrmion crystal (SkX) expressed as a multiple number of spiral modulations manifests itself not only in its peculiar magnetic texture but also in nontrivial transport properties originating from an emergent magnetic field. We here report our numerical results for multiple SkXs in a centrosymmetric tetragonal crystal system. By performing simulated annealing for an effective spin model for itinerant magnets, we find that three types of the SkXs with the skyrmion numbers of one and two, which are characterized by different superpositions of helices, are stabilized in the ground state, and are transformed by an external magnetic field. The essence of the emergent multiple SkXs lies in itinerant frustration where exchange interactions are competed in momentum space due to the nature of itinerant electrons.

First Observation of the de Haas–van Alphen Effect and Fermi Surfaces in the Unconventional Superconductor UTe₂

Abstract: We report the first observation of the de Haas–van Alphen (dHvA) effect in the novel spin-triplet superconductor UTe2 using high quality single crystals with a high residual resistivity ratio (RRR) over 200. The dHvA frequencies, named α and β, are detected for the field directions between c- and a-axes. The frequency of branch β increases rapidly with the field angle tilted from c- to a-axis, while branch α splits, owing to the maximal and minimal cross-sectional areas from the same Fermi surface. Both dHvA branches, α and β reveal two kinds of cylindrical Fermi surfaces with a strong corrugation at least for branch α. The angular dependence of the dHvA frequencies is in very good agreement with that calculated by the generalized gradient approximation (GGA) method taking into account the on-site Coulomb repulsion of U = 2 eV. It indicates that the main Fermi surfaces are experimentally detected. The observed cyclotron effective masses are large in the range from 32 to 57 m0. They are approximately 10–20 times lager than the corresponding band masses, consistent with the mass enhancement obtained from the Sommerfeld coefficient, γ and the calculated density of states at the Fermi level. The local density approximation (LDA) calculations of ThTe2 assuming U4+ with the 5f2 localized model are in less agreement with our experimental results, in spite of the prediction for two cylindrical Fermi surfaces, suggesting a mixed valence states of U4+ and U3+ in UTe2.

Systematic Analysis Method for Nonlinear Response Tensors

Abstract: We propose a systematic analysis method for identifying essential parameters in various linear and nonlinear response tensors without which they vanish. By using the Keldysh formalism and the Chebyshev polynomial expansion method, the response tensors are decomposed into the model-independent and dependent parts, in which the latter is utilized to extract the essential parameters. An application of the method is demonstrated by analyzing the nonlinear Hall effect in the ferroelectric SnTe monolayer for example. It is shown that in this example the second-neighbor hopping is essential for the nonlinear Hall effect whereas the spin–orbit coupling is unnecessary. Moreover, by analyzing terms contributing to the essential parameters in the lowest order, the appearance of the nonlinear Hall effect can be interpreted by the subsequent two processes: the orbital magneto-current effect and the linear anomalous Hall effect by the induced orbital magnetization. In this way, the present method provides a microscopic picture of responses. By combining with computational analysis, it stimulates further discoveries of anomalous responses by filling in a missing link among hidden degrees of freedom in a wide variety of materials.

Superconducting Order Parameter in UTe₂ Determined by Knight Shift Measurement

Abstract: This study investigates the spin susceptibility in U-based superconductor UTe$_2$ in the superconducting (SC) state by using Knight shift measurements for a magnetic field $H$ along the $a$ axis, which is the magnetic easy axis of UTe$_2$. Although a tiny anomaly ascribed to the SC diamagnetic effect was observed just below the SC transition temperature $T_{\rm c}$, the $a$-axis Knight shift in the SC state shows no significant decrease, following the extrapolation from the normal-state temperature dependence. This indicates that the spin susceptibility is nearly unchanged below $T_{\rm c}$. Considering the previous Knight shift results for $H \parallel b$ and $H \parallel c$, the dominant SC state is determined to be $B_{\rm 3u}$ in the spin-triplet pairing, which is consistent with the spin anisotropy in the normal state. The present result shows that UTe$_2$ is a spin-triplet superconductor with spin degrees of freedom.

First Observation of the de Haas–van Alphen Effect and Fermi Surfaces in the Unconventional Superconductor UTe2

Abstract: 1IMR, Tohoku University, Oarai, Ibaraki 311-1313, Japan 2Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan 3Faculty of Engineering, Niigata University, Ikarashi, Niigata 950-2181, Japan 4Department of Physics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan 5Graduate School of Science, Kobe University, Kobe 657-8501, Japan 6Univ. Grenoble Alpes, CEA, Grenoble INP, IRIG, PHELIQS, F-38000 Grenoble, France

Developments in the Tensor Network — from Statistical Mechanics to Quantum Entanglement

Abstract: Tensor networks (TNs) have become one of the most essential building blocks for various fields of theoretical physics such as condensed matter theory, statistical mechanics, quantum information, and quantum gravity. This review provides a unified description of a series of developments in the TN from the statistical mechanics side. In particular, we begin with the variational principle for the transfer matrix of the 2D Ising model, which naturally leads us to the matrix product state (MPS) and the corner transfer matrix (CTM). We then explain how the CTM can be evolved to such MPS-based approaches as density matrix renormalization group (DMRG) and infinite time-evolved block decimation. We also elucidate that the finite-size DMRG played an intrinsic role for incorporating various quantum information concepts in subsequent developments in the TN. After surveying higher-dimensional generalizations like tensor product states or projected entangled pair states, we describe tensor renormalization groups (TRGs), which are a fusion of TNs and Kadanoff–Wilson type real-space renormalization groups, focusing on their fixed point structures. We then discuss how the difficulty in TRGs for critical systems can be overcome in the tensor network renormalization and the multi-scale entanglement renormalization ansatz.

Persistent Homology Analysis for Materials Research and Persistent Homology Software: HomCloud

Abstract: This paper introduces persistent homology, which is a powerful tool to characterize the shape of data using the mathematical concept of topology. We explain the fundamental idea of persistent homology from scratch using some examples. We also review some applications of persistent homology to materials researches and software for persistent homology data analysis. HomCloud, one of persistent homology software, is especially featured in this paper.

Superconducting Order Parameter in UTe2 Determined by Knight Shift Measurement

Abstract: This study investigates the spin susceptibility in U-based superconductor UTe2 in the superconducting (SC) state by using Knight shift measurements for a magnetic field H along the a axis, which is the magnetic easy axis of UTe2. Although a tiny anomaly ascribed to the SC diamagnetic effect was observed just below the SC transition temperature Tc, the a-axis Knight shift in the SC state shows no significant decrease, following the extrapolation from the normalstate temperature dependence. This indicates that the spin susceptibility is nearly unchanged below Tc. Considering the previous Knight shift results for H ‖ b and H ‖ c, the dominant SC state is determined to be B3u in the spin-triplet pairing, which is consistent with the spin anisotropy in the normal state. The present result shows that UTe2 is a spintriplet superconductor with spin degrees of freedom.

Slow Electronic Dynamics in the Paramagnetic State of UTe₂

Abstract: 125Te NMR experiments in field (H) applied along the easy magnetization axis (the a-axis) revealed slow electronic dynamics developing in the paramagnetic state of UTe2. The observed slow fluctuations are concerned with a successive growth of long-range electronic correlations below 30–40 K, where the spin susceptibility along the hard magnetization axis (the b-axis) shows a broad maximum. The experiments also imply that tiny amounts of disorder or defects locally disturb the long-range electronic correlations and develop an inhomogeneous electronic state at low temperatures, leading to a low temperature upturn observed in the bulk-susceptibility in H || a. We suggest that UTe2 would be located on the paramagnetic side near an electronic phase boundary, where either magnetic or Fermi-surface instability would be the origin of the characteristic fluctuations.

Topological Edge States and Bulk-edge Correspondence in Dimerized Toda Lattice

Abstract: The Toda lattice is a model of nonlinear wave equations allowing exact soliton solutions. It is realized by an electric circuit made of a transmission line with inductors and variable capacitance diodes. It has been generalized to the dimerized Toda lattice by introducing alternating bondings specified by a certain parameter λ so that it is reduced to the Toda lattice at λ = 0. In this paper we investigate the topological property of this nonlinear system. It is intriguing that the system contains the Su–Schrieffer–Heeger (SSH) Hamiltonian as an essential term. We make an analytical study of the condition when an isolated zero-mode state emerges at the edge of a transmission line. It is identified with the topological edge state well known in the SSH model. It is argued that the system contains the topological and trivial phases with the phase transition point given by the original Toda lattice (λ = 0). To confirm these observations, we investigate numerically the quench dynamics of the voltage propagation along a finite transmission line, when a voltage is injected instantaneously at one edge. The topological phase transition is observable by a significant difference between the dynamics of voltages along the transmission line in the two phases, which is explained by the emergence of the topological edge state. This is a bulk-edge correspondence in nonlinear systems.

Electric Ferro-Axial Moment as Nanometric Rotator and Source of Longitudinal Spin Current

Abstract: An electric ferro-axial moment, which is characterized by a nonzero expectation value of a time-reversal-even axial vector, exhibits distinct spatial-inversion and time-reversal properties from conventional ferroelectric, ferromagnetic, and ferro-magnetoelectric orders. Nevertheless, physical properties characteristic of the electric ferro-axial moment have been obscure owing to the absence of its conjugate electromagnetic fields. We theoretically investigate consequences of the presence of the ferro-axial moment on the basis of the symmetry and microscopic model analyses. We show that atomic-scale electric toroidal multipoles are the heart of the ferro-axial moment, which act as a nanometric rotator against external stimuli. Furthermore, we propose an intrinsic generation of a spin current parallel to an applied electric field in both metals and insulators. Our results not only provide a deep microscopic understanding of the role of the ferro-axial moment but also stimulate a new development for functional materials with use of the electric toroidal moment.

Nonreciprocal Transport in Noncoplanar Magnetic Systems without Spin–Orbit Coupling, Net Scalar Chirality, or Magnetization

Abstract: We propose a new mechanism of nonlinear nonreciprocal transport in magnetic systems. By considering a noncoplanar magnetic ordering on a bilayer triangular lattice, we clarify that a local scalar chirality degree of freedom is a source of nonreciprocal transport, which does not require any relativistic spin-orbit coupling, net scalar chirality, and net magnetization. We show a close relationship between the asymmetric band modulation and the nonreciprocal transport under the noncoplanar magnetic ordering based on the model-parameter dependences in the real-space picture.

Nonlinear Pressure Waves in Bubbly Flows with Drag Force: Theoretical and Numerical Comparison of Acoustic and Thermal and Drag Force Dissipations

Abstract: Weakly nonlinear propagation of plane pressure waves in flowing compressible water containing many spherical microbubbles is theoretically investigated. Special focus is placed on the thermal conduction inside the bubble and drag force acting on translational bubbles. From the method of multiple scales, the Korteweg–de Vries–Burgers equation is derived and two types of dissipation terms appear: a term with a second-order partial derivative owing to the liquid compressibility and a term without differentiation owing to the drag force and thermal conduction. Finally, we numerically found that the dissipation effect due to the thermal conduction is the largest, followed by that due to the acoustic radiation and drag force.

Structural Transition in the Hidden Ordered Phase of CeCoSi

Abstract: We have performed X-ray diffraction experiments on a single crystalline CeCoSi to investigate the unresolved ordered phase below $T_0 \sim 12$ K. We have discovered that a triclinic lattice distortion takes place below $T_0$, which is further modified in the subsequent antiferromagnetic ordered phase. The structural domains can be selected by applying a magnetic field, indicating that some electronic ordering exists behind and affects the magnetic anisotropy in the hidden ordered phase below $T_0$. The transition at $T_0$, although the order parameter is still unknown, is associated with the maximum in the $c$-axis lattice parameter. In magnetic fields along $[1, 0, 0]$, the structural transition temperature, named as $T_{\text{s1}}$, deviates from $T_0$ and decreases with increasing the field, whereas $T_0$ increases. This shows that the hidden ordered phase without triclinic distortion exists between $T_{\text{s1}}$ and $T_0$. The results for $H \parallel [1, 1, 0]$ are also reported.

The Onsager–Machlup Integral for Non-Reciprocal Systems with Odd Elasticity

Abstract: The variational principle of the Onsager–Machlup integral is used to describe the stochastic dynamics of a micromachine, such as an enzyme, characterized by odd elasticity. The obtained most probable path is found to become non-reciprocal in the presence of odd elasticity and is further related to the entropy production.

Evidence for a Phase Transition in the Quantum Spin Liquid State of a Kitaev Candidate α-RuCl3

Abstract: The Kitaev quantum spin liquid (QSL) on the two-dimensional honeycomb lattice epitomizes an entangled topological state, where the spins fractionalize into Majorana fermions. This state has aroused tremendous interest because it harbors non-Abelian anyons. The half-integer quantized thermal Hall (HIQTH) conductance observed in α-RuCl3 is a key signature of the non-Abelian topological order. However, the fate of this topologically nontrivial state at intense fields remains largely elusive. Here, we report the thermal conductivity κ and specific heat C of α-RuCl3 with in-plane magnetic fields H. For the field direction perpendicular to the Ru–Ru bond, where the HIQTH effect is observed, we find clear anomalies in κ(H) and C(H) at μ0H* ≈ 11 T, indicating the emergence of a phase transition between low- and high-field spin liquid phases. We point out that this phase transition is likely to be weak first-order. We also find that even well above H*, κ(H) is highly anisotropic with respect to H-direction. This indicates that the high field phase is not in a simple spin-polarized state and spin-fractionalization may be retained. Intriguingly, the thermal Hall conductance of the same crystal deviates from the half-integer plateau near H*, suggesting that the topological properties are affected by this transition.

Magnetovolume Effect on the First-Order Metamagnetic Transition in UTe₂

Abstract: The link between the metamagnetic transition and novel spin-triplet superconductivity of UTe$_2$ was discussed thermodynamically through magnetostriction measurements in a pulsed-magnetic field. We revealed a discontinuous magnetostriction across the metamagnetic transition at $\mu_0H_{\rm m}\approx 35$~T for the applied magnetic fields along the crystallographic $b$ axis in the orthorhombic structure. The resultanting volume magnetostriction of $\Delta V/V \approx-5.9\times 10^{-4}$ gives the initial pressure dependence of $H_{\rm m}$ by employing the Clausius-Clapeyron's equation, which agrees with previous pressure experiments. Further, significant anisotropic magnetostriction (AMS), derived by subtracting the averaged linear magnetostriction, was revealed. Contrary to the weakly field-dependent AMS along the $a$ axis, those along the $b$ and $c$ axes show strong field dependences with a similar magnitude but with opposite signs, indicating its lattice instability. The relationship between characteristic energy scales of magnetic fields and temperatures was discussed in terms of the Gr\"uneisen parameters compared to the other $f$-electron systems. The volume shrinkage in UTe$_2$ at $H_{\rm m}$, contrary to the volume expansion in typical heavy fermion metamagnets, pushes to invoke the link with the valence instability related to the itinerant-localized dual nature of the U magnetism.

Magnetic Hedgehog Lattice in a Centrosymmetric Cubic Metal

Abstract: The hedgehog lattice (HL) is a three-dimensional topological spin texture hosting a periodic array of magnetic monopoles and antimonopoles. It has been studied theoretically for noncentrosymmetric systems with the Dzyaloshinskii–Moriya interaction, but the stability, as well as the magnetic and topological properties, remains elusive in the centrosymmetric case. We here investigate the ground state of an effective spin model with long-range bilinear and biquadratic interactions for a centrosymmetric cubic metal by simulated annealing. We show that our model stabilizes a HL composed of two pairs of left- and right-handed helices, resulting in no net scalar spin chirality, in stark contrast to the noncentrosymmetric case. We find that the HL turns into topologically-trivial conical states in an applied magnetic field. From the detailed analyses of the constituent spin helices, we clarify that the ellipticity and angles of the helical planes change gradually while increasing the magnetic field. We discuss the results in comparison with the experiments for a centrosymmetric cubic metal SrFeO3.

High-resolution Spectroscopy and Single-photon Rydberg Excitation of Reconfigurable Ytterbium Atom Tweezer Arrays Utilizing a Metastable State

Abstract: We present an experimental system for Rydberg tweezer arrays with ytterbium (Yb) atoms featuring internal state manipulation between the ground ${}^1$S$_0$ and the metastable ${}^3$P$_2$ states, and single-photon excitation from the ${}^3$P$_2$ to Rydberg states. In the experiments, single Yb atoms are trapped in two-dimensional arrays of optical tweezers and are detected by fluorescence imaging with the intercombination ${}^1$S$_0 \leftrightarrow {}^3$P$_1$ transition, and the defect-free single atom arrays are prepared by the rearrangement with the feedaback. We successfully perform high-resolution ${}^1$S$_0\leftrightarrow {}^3$P$_2$ state spectroscopy for the single atoms, demonstrating the utilities of this ultranarrow transition. We further perform single-photon excitation from the ${}^3$P$_2$ to Rydberg states for the single atoms, which is a key for the efficient Rydberg excitation. We also perform a systematic measurement of a complex energy structure of a series of D states including newly observed ${}^3$D$_3$ states. The developed system shows feasibility of future experiments towards quantum simulations and computations using single Yb atoms.

Relationship between the Electronic Polarization and the Winding Number in Non-Hermitian Systems

Abstract: We discuss an extension of the Resta's electronic polarization to non-Hermitian systems with periodic boundary conditions. We introduce the ``electronic polarization'' as an expectation value of the exponential of the position operator in terms of the biorthogonal basis. We found that there appears a finite region where the polarization is zero between two topologically distinguished regions, and there is one-to-one correspondence between the polarization and the winding number which takes half-odd integers as well as integers. We demonstrate this argument in the non-Hermitian Su-Schrieffer-Heeger model.

Effects of Electron Correlation and Geometrical Frustration on Magnetism of Icosahedral Quasicrystals and Approximants — An Attempt to Bridge the Gap between Quasicrystals and Heavy Fermions

Abstract: Icosahedral quasicrystals (QCs), which are long-range ordered materials featuring a symmetry incompatible with translational invariance, have received a revived interest since the observation of their non-Fermi liquid (FL) behaviors, owing to an interest in the strong correlation effects developing in quasiperiodic structures. Herein, we review the progress in our understanding of magnetism in QCs, with an emphasis on the comparison with approximant crystals (ACs) and heavy fermions (HFs). ACs are solids that possess similar local structures (clusters) as QCs but form a periodic array of clusters, and HFs are periodic crystals showing a strong electron correlation effect (e.g., the Kondo effect) and unconventional quantum criticality. Topics discussed in this review lie at the crossroads of two fundamental enterprises (i.e., QCs and HFs) in condensed matter physics. Therefore, the fundamentals of QC structures and the basic notions of HF physics are introduced for readers who are unfamiliar with either QCs or HFs. According to a recent cutting-edge experiment, the mean valence of Yb exhibits instability at a critical lattice parameter, independently of whether the global structure is quasiperiodic or periodic. In contrast, the QC and AC exhibit a difference in the emergence of divergence in their magnetic susceptibility under pressure, which is considered to be an effect of the quasiperiodic structure on physical properties. Using these results, we reveal novel structure–property relationships (e.g., the interrelation between quasiperiodicity and quantum critical behavior) and discuss how the quantum criticality results from quantum critical valence fluctuations. We further discuss the possible presence of the quantum critical phase, a new state of matter, in the QC. In addition, an overview of the local moment magnetism of ACs is given with a focus on geometrical-frustration effects. Throughout the review, we attempt to develop a unified view on the magnetism of these intriguing materials.

Spin Conductivity Based on Magnetic Toroidal Quadrupole Hidden in Antiferromagnets

Abstract: We report our theoretical results on spin conductivity in antiferromagnets by focusing on the role of the magnetic toroidal quadrupole (MTQ) in electron systems. The MTQ is characterized as a time-reversal-odd rank-2 polar tensor degree of freedom in electrons, which is distinct from conventional rank-1 magnetic and magnetic toroidal dipoles. Based on a microscopic sd model analysis for a tetragonal system under both collinear and noncollinear antiferromagnetic orderings, we clarify that the MTQ becomes a source of an extrinsic spin conductivity even with neither a uniform magnetization nor spin–orbit coupling. We also list all the magnetic point groups to accommodate the MTQs as a primary order parameter as well as the candidate antiferromagnetic materials.

Multiple Skyrmion Crystal Phases by Itinerant Frustration in Centrosymmetric Tetragonal Magnets

Abstract: A skyrmion crystal (SkX) expressed as a multiple number of spiral modulations manifests itself not only in its peculiar magnetic texture but also in nontrivial transport properties originating from an emergent magnetic field. We here report our numerical results for multiple SkXs in a centrosymmetric tetragonal crystal system. By performing simulated annealing for an effective spin model for itinerant magnets, we find that three types of the SkXs with the skyrmion numbers of one and two, which are characterized by different superpositions of helices, are stabilized in the ground state, and are transformed by an external magnetic field. The essence of the emergent multiple SkXs is lied in itinerant frustration where exchange interactions are competed in momentum space due to the nature of itinerant electrons.

Two-Dimensional XY-Type Magnetic Properties of Locally Noncentrosymmetric Superconductor CeRh2As2

Abstract: We performed As-NMRmeasurements to investigate the normal-state magnetic properties of CeRh2As2, a recently-discovered heavy-fermion superconductor. The magnitude and temperature dependence of the Knight shift at the As(2) site indicate easy-plane-type magnetic anisotropy in CeRh2As2. With regard to spin fluctuations, the temperature dependence of the nuclear spin-lattice relaxation rate 1/T1 arising from the 4f electrons decreases from high-temperature constant behavior on cooling at ∼ 40 K, which is a typical behavior of heavy-fermion systems. In addition, 1/T1 becomes constant at low temperatures, suggesting spatially two-dimensional antiferromagnetic fluctuations. Two-dimensional magnetic correlations in the real space are quite rare among heavy-fermion superconductors, and they may be a key factor in the unique superconducting multi phase in CeRh2As2.

Nonreciprocal Transport in Noncoplanar Magnetic Systems without Spin–Orbit Coupling, Net Scalar Chirality, or Magnetization

Abstract: We propose a new mechanism of nonlinear nonreciprocal transport in magnetic systems. By considering a noncoplanar magnetic ordering on a bilayer triangular lattice, we clarify that a local scalar chirality degree of freedom is a source of nonreciprocal transport, which does not require any relativistic spin-orbit coupling, net scalar chirality, and net magnetization. We show a close relationship between the asymmetric band modulation and the nonreciprocal transport under the noncoplanar magnetic ordering based on the model-parameter dependences in the real-space picture.

High-resolution Spectroscopy and Single-photon Rydberg Excitation of Reconfigurable Ytterbium Atom Tweezer Arrays Utilizing a Metastable State

Abstract: We present an experimental system for Rydberg tweezer arrays with ytterbium (Yb) atoms featuring internal state manipulation between the ground 1 S 0 and the metastable 3 P 2 states, and single-photon excitation from the 3 P 2 to Rydberg states. In the experiments, single Yb atoms are trapped in two-dimensional arrays of optical tweezers and are detected by fluorescence imaging with the intercombination 1 S 0 ↔ 3 P 1 transition, and the defect-free single atom arrays are prepared by the rearrangement with the feedaback. We successfully perform high-resolution 1 S 0 ↔ 3 P 2 state spectroscopy for the single atoms, demonstrating the utilities of this ultranarrow transition. We further perform single-photon excitation from the 3 P 2 to Rydberg states for the single atoms, which is a key for the efficient Rydberg excitation. We also perform a systematic measurement of a complex energy structure of a series of D states including newly observed 3 D 3 states. The developed system shows feasibility of future experiments towards quantum simulations and computations using single Yb atoms.

Zero-Field NMR for the Pressure-Induced Phase of α-Mn

Abstract: Zero field nuclear magnetic resonance under pressure was performed on the site III and the site IV of α-Mn. A decrease of the internal magnetic field was observed with increasing pressure at both the site III and the site IV. It is suggested that the magnetic moment of the site IV has disappeared above the pressure Pc1 of 1.4 GPa where the antiferromagnetic order disappears. At the site III, a sharp decrease of the internal magnetic field associated with the pressure-induced magnetic phase transition at Pc1 was observed. The internal magnetic field of the site III further decreases rapidly above Pc1. Considering the behavior of the internal magnetic fields at the site I and the site II, which were obtained previously, the magnetic structure above Pc1 is consistent with ferrimagnetic state based on irreducible representation T1.

Two-Dimensional XY-Type Magnetic Properties of Locally Noncentrosymmetric Superconductor CeRh₂As₂

Abstract: We performed $^{75}$As-NMR measurements to investigate the normal-state magnetic properties of CeRh$_2$As$_2$, a recently-discovered heavy-fermion superconductor. The magnitude and temperature dependence of the Knight shift at the As(2) site indicate easy-plane-type magnetic anisotropy in CeRh$_2$As$_2$. With regard to spin fluctuations, the temperature dependence of the nuclear spin-lattice relaxation rate $1/T_1$ arising from the 4$f$ electrons decreases from high-temperature constant behavior on cooling at $\sim$ 40~K, which is typical behavior of heavy-fermion systems. In addition, $1/T_1$ becomes constant at low temperatures, suggesting spatially two-dimensional antiferromagnetic fluctuations. Two-dimensional magnetic correlations in the real space are quite rare among heavy-fermion superconductors, and they may be a key factor in the unique superconducting multi-phase in CeRh$_2$As$_2$.

Charge and Lattice Dynamics in Excitonic Insulator Ta2NiSe5 Investigated Using Ultrafast Reflection Spectroscopy

Abstract: An excitonic insulator, in which an optical gap is formed by exciton condensation, is expected to exhibit large photoresponses. Here, we report the results of femtosecond reflection spectroscopy covering a wide infrared region on a typical excitonic insulator, Ta2NiSe5, which gives direct information not only about a change of excitonic gap but also a metallization. With a weak excitation, the excitonic gap decreases due to the weakening of excitonic effect by photocarriers. With a strong excitation, a Drude-like high-reflection band appears, showing an excitonic-insulator-to-metal transition. Analyses of spectral and temporal changes of reflectivity revealed that the metallization is accompanied by large structural changes. This demonstrates an important role of electron–lattice interaction on the stabilization of excitonic phase.

Emergence of a Giant Rotating Cluster of Fish in Three Dimensions by Local Interactions

Abstract: Schooling fish exhibit giant rotating clusters such as balls, tori, and rings, among other collective patterns. In order to account for their giantness and flexible shape change, we introduce an agent-based model that limits the number of agents that each agent can interact with (interaction capacity). Incorporating autonomous control of attractive interactions, we reproduce rotating clusters (balls, tori, and rings) that are an order of magnitude larger than the interaction range. We obtained a phase diagram of patterns including polarized schools and swarms. In our model, the scaling law between the number of agents and the projected area of the cluster is in good agreement with experimental results. The model indicates that giant rotating clusters are formed at low interaction capacity, without long-range interactions or inherent chirality of fish.