Theoretical predictions and recent experimental observations of electron orbital angular momentum dynamics draw growing interest in orbitronics. A representative application of the orbital current is the orbital torque. However, the present setup for the orbital torque utilizes the orbital-to-spin conversion in ferromagnets, which lowers the torque efficiency. This motivates one to search for orbital magnets, whose net magnetization originates primarily from orbital magnetic moments. When conventional spin magnets in the present orbital torque setup are replaced by orbital magnets, the orbital-to-spin conversion process may be bypassed, opening a path to enhance the torque efficiency. Here, we propose a design strategy for orbital magnets. We demonstrate the strategy for double perovskites via first-principles calculations and identify seven promising candidate materials for orbital magnets.

With increasing temperature (T), a rare-earth element, cerium (Ce), exhibits intricate structural phase transitions from fcc-αto dhcp-β, fcc-γ, and bcc-δphases before reaching the liquid phase. Interestingly, Ce exhibits abnormal thermal-expansion behavior (either positive or negative) with respect to the temperature change, which is associated with each structural transition. Focusing on the interplay betweenT-dependent electronic and structural degrees of freedom, we have investigated the underlying mechanism of peculiar thermal-expansion properties of Ce, which are quite distinct from those of La, Pr, and Nd. We have also explored the unusual melting physics in Ce, including the intriguingV-shaped melting curve as well as the negative thermal-expansion phenomenon upon melting. We propose two possible scenarios for the melting mechanism, Mott- and promotional-model-based ones, both of which highlight the essential role of thefelectrons in Ce in realizing the anomalous thermal-expansion behaviors across the different solid and liquid phases at highT.

Installing well-defined high density single-atomic catalyst (SAC) sites is highly desired for the synergistic cooperative effect in efficient chemical synthesis but elusive to synthesize due to unavoidable atomic segregation into clusters or particulates. We implemented a 2D-nanoconfined SAC grafting strategy. Inside a bilayer silica envelope, single metal hydroxide layer, pre-loaded with the catalytic metal (Pt, Pd, Ir) precursors, underwent controlled thermal conversion to the homogeneously embedded SACs decorated onto the in situ generated 1 nm-thin transition metal oxide (TMO) nanosheet. Ultranarrow confined slit-space exclusively availed laterally emerging cation vacancies of TMO host, as atomic scale sockets to capture SACs within 2D-plane while vertically restricting the atomic aggregation into nanoparticulates. High density SACs stably decorated on isolated TMO nanosheet, could be obtained in a controllable manner. Well-defined SACs on TMO were conveniently employed for acceptorless dehydrogenation of a variety of alcohols with quite high activities outperforming reported catalysts. In-depth, mechanistic investigation revealed the dramatic enhancement in acceptorless dehydrogenation reaction rates by following distinct reaction pathways (concerted or stepwise) depending on the density of SACs cooperativity effect. Such advantageous effect is distinct from the reactions on isolated metal sites in low-density SACs or nanoparticle surface.

Rare-earth tetraborides (RB4) have attracted a lot of recent attention due to their intriguing electronic, magnetic, and topological properties. We have theoretically investigated the topological properties of PrB4, which is unique among the RB4 family due to its ferromagnetic ground state. We have discovered that PrB4 is an intrinsic magnetic Weyl system possessing multiple topological band crossings with various chiral charges. Density-functional-theory band calculations combined with a tight-binding band analysis reveal large Fermi-arc surface states, which are characteristic fingerprints of Weyl fermions. Anomalous Hall conductivity is estimated to be very large, ranging from 500 to 1000 (Q cm)-1 near the Fermi level, which also demonstrates the topological Weyl character of ferromagnetic PrB4. These findings suggest that PrB4, being a potential candidate of a magnetic Weyl system, would be a promising rare-earth topological system for applications to next-generation spintronic and photonic devices.

In this work, we develop a non-equilibrium steady-state non-crossing approximation (NESS-NCA) impurity solver applicable to general impurity problems. The choice of the NCA as the impurity solver enables both a more accurate description of correlation effects with larger Coulomb interaction and scalability to multi-orbital systems. Based on this development, we investigate strongly correlated non-equilibrium states of a dissipative lattice system under constant electric fields. Both the electronic Coulomb interaction and the electric field are treated non-perturbatively using dynamical mean-field theory in its non-equilibrium steady-state form (NESS-DMFT) with the NESS-NCA impurity solver. We validate our implementation using a half-filled single-band Hubbard model attached to a fictitious free Fermion reservoir, which prevents temperature divergence. As a result, we identify metallic and insulating phases as functions of the electric field and the Coulomb interaction along with a phase coexistence region amid the metal-to-insulator transition (MIT). We find that the MIT driven by the electric field is qualitatively similar to the equilibrium MIT as a function of temperature, differing from results in previous studies using the iterative perturbation theory (IPT) impurity solver. Finally, we highlight the importance of the morphology of a correlated system under the influence of an electric field.

Understanding the origin of distinct charge density wave (CDW) instabilities in layered R Te n (n n = 2, 3) compounds (R, R , rare earth element) has been an important issue. In this research update, we have investigated the electronic structures of PrTen n (n n = 2, 3) and ErTe3 3 layered CDW compounds employing angle-resolved photoemission spectroscopy (ARPES) and soft x-ray absorption spectroscopy (XAS). The trivalent valency of R 3 + ions is confirmed for PrTen n (n n = 2, 3) and ErTe3, 3 , supporting that R-Te slabs serve as charge reservoirs and that the CDW instability occurs in the partially filled Te sheets. Both R 4d d -> 4 f resonant photoemission spectroscopy and photon-energy map measurements provide evidence that R 4 f electrons do not contribute directly to the CDW formation but that the indirect contribution from Pr 4 f electrons through the Pr 4 f - Te 5p p hybridization is feasible in PrTen n (n n = 2, 3). Circular and linear dichroism ARPES measurements indicate that the chirality of the Te 5p p orbitals certainly plays a role in the CDW formation of R Te 3 (R R = Pr, Er) while it is relatively weak in PrTe2, 2 , and that the E F-crossing orbitals, responsible for the CDW formation, are ordered in plane (in the ac plane) in all of them. Different CDW-induced Fermi surface reconstructions between R Te 3 and R Te 2 are due to (i) the existence of two Te sheets and one Te sheet per unit cell in R Te 3 and R Te 2 , respectively, so as to produce different numbers of hole carriers, and (ii) the different lattice parameters of Te sheets in R Te n , leading to the different densities of states at E F .

We have investigated the origin of the negative-thermal-expansion (NTE) phenomena observed in various strongly correlated f-electron systems, including Ce, Sm, and Yb rare-earth compounds and actinide element Pu. We have thoroughly surveyed existing nonmagnetic f-electron NTE materials and identified that the temperature (T)-induced valence transition plays an essential role in realizing the NTE in Sm-and Yb-based mixed-valence (MV) systems. We have discussed the contrasting thermal-expansion behaviors between (Sm, Yb)-based MV systems and their electron-hole symmetric counterparts (Ce, Eu)-based MV systems that exhibit positive-thermal-expansion (PTE) phenomena. We have also clarified the origin of intriguing thermal expansion behavior of Pu, which, upon cooling, reveals not only the NTE, but also the strong PTE. Fi-nally, we have predicted the possible existence of an additional NTE feature in topological-Kondo-insulator (TKI) candidates of SmB6, g-SmS, and YbB12 in the low-T regime where the TKI nature is expected to emerge.

We investigated CeCoIn5 and LaCoIn5 single crystals, which have the same HoCoGa5-type tetragonal crystal structure, using infrared spectroscopy. However, while CeCoIn5 has 4f electrons, LaCoIn5 does not. By comparing these two material systems, we extracted the temperature-dependent electronic evolution of the f electrons of CeCoIn5. We observed that the differences caused by the f electrons are more obvious in low-energy optical spectra at low temperatures. We introduced a complex optical resistivity and obtained a magnetic optical resistivity from the difference in the optical resistivity spectra of the two material systems. From the temperature-dependent average magnetic resistivity, we found that the onset temperature of the Kondo effect is much higher than the known onset temperature of Kondo scattering (≃200 K) of CeCoIn5. Based on momentum-dependent hybridization, the periodic Anderson model, and a maximum entropy approach, we obtained the hybridization gap distribution function of CeCoIn5 and found that the resulting gap distribution function of CeCoIn5 was mainly composed of two (small and large) components (or gaps). We assigned the small and large gaps to the in-plane and out-of-plane hybridization gaps, respectively. We expect that our results will provide useful information for understanding the temperature-dependent electronic evolution of f-electron systems near Fermi level.

We investigate the temperature ($T$)-evolution of orbital anisotropy and its effect on spectral function and optical conductivity in Ce$_{2}$IrIn$_{8}$, using a first principles dynamical mean field theory combined with density functional theory. The orbital anisotropy develops by lowering $T$ and it is intensified below a temperature corresponding to the crystalline-electric field (CEF) splitting size. Interestingly, the depopulation of CEF excited states leaves a spectroscopic signature, "shoulder", in the $T$-dependent spectral function at the Fermi level. From the two-orbital Anderson impurity model, we demonstrate that CEF splitting size is the key ingredient influencing the emergence and the position of the "shoulder". Besides the two conventional temperature scales $T_{K}$ and $T^{*}$, we introduce an additional temperature scale to deal with the orbital anisotropy in heavy fermion systems.

We have investigated the electronic structure and the topological property of a Ce Kondo system, ${\mathrm{CeFe}}_{2}{\mathrm{Al}}_{10}$, employing the first-principles density functional theory (DFT) and dynamical mean-field theory (DMFT) band calculations. Based on the DMFT band calculation, we have found that, upon cooling, Ce $4f$ electrons in ${\mathrm{CeFe}}_{2}{\mathrm{Al}}_{10}$ become coherent at $T\ensuremath{\approx}150$ K, so as to form coherent bands revealing the hybridization pseudogap near the Fermi level (${E}_{\text{F}}$). We have checked that the band structure near ${E}_{\text{F}}$ and the Fermi surface from the DMFT at low $T$ are almost identical with those from the renormalized DFT. We have explored the topological nature based on the DFT band structure and found that ${\mathrm{CeFe}}_{2}{\mathrm{Al}}_{10}$ has a topological Kondo insulating nature with ${Z}_{4}=1$. Therefore, our finding reveals that ${\mathrm{CeFe}}_{2}{\mathrm{Al}}_{10}$ would display $T$-dependent topological phase transition from a topological Kondo insulating phase at low $T$ to a trivial semimetallic phase at high $T$. The analysis of $T$-dependent local magnetic susceptibility obtained from the DMFT calculation confirms the Kondo coherence mechanism upon cooling in ${\mathrm{CeFe}}_{2}{\mathrm{Al}}_{10}$.

In order to clarify the controversial issue of the topological nature in a mixed-valent Kondo system, ${\mathrm{SmB}}_{6}$, we have explored the surface states on the nonpolar (110) surface of ${\mathrm{SmB}}_{6}$, employing both angle-resolved photoemission spectroscopy (ARPES) experiment and ab initio density-functional theory (DFT) band calculations. Based on ARPES spectroscopic fingerprints and the DFT surface band structures, we ascribe the observed spectral weights at $\overline{X}$ and $\overline{Y}$ on the (110) surface Brillouin zone to topological surface states (TSSs) of ``topological insulator (TI)'' nature and of ``topological crystalline insulator (TCI)'' nature, respectively. With varying the chemical potential, the double Dirac cones of the TCI nature exhibit a Lifshitz transition of Fermi surfaces with intriguing spin textures. We have also examined the TSSs on the nearly nonpolar (111) surface of ${\mathrm{SmB}}_{6}$ in connection with a recently reported ARPES result and proposed a way to probe the Dirac points that are buried in the bulk-projected bands.

The hitherto-studied topological nature in Ce monopnictides (CePn) has been discussed based solely on their p-d band inversion, despite the existence of f electrons. Interestingly, however, we have found that CeN, the lightest CePn, has nontrivial Z(2) topology originating from f-d band inversion. Through density-functional theory and dynamical mean-field theory calculations, we have demonstrated that, among the correlated f electron systems of CePn, the lightest CeN is a coherent narrow f-band system having an f-d band-inverted topological Kondo insulator nature, whereas the heaviest CeBi is a strongly localized f-electron system having a conventional p-d band-inverted topological insulator nature. For CeN, topological surface states (TSSs) are identified clearly on three different (001), (110), and (111) surfaces. For CeBi, however, TSSs are identified only on the (111) surface. Furthermore, intriguing topological-crystalline insulator-type TSSs are identified on the (110) surfaces of both CeN and CeBi.

It has been recently predicted that nonsymmorphic crystalline insulators can host two exotic topological surface states (TSSs). One is the "hourglass fermion", and the other is the "wallpaper Dirac fermion". For the former, a few real materials were predicted and already confirmed experimentally. For the latter, however, no bulk-insulating and experimentally accessible candidate has been identified yet. Here we show that the localized 5f-electrons in PuB4, the single crystal of which was recently synthesized and was found to exhibit Kondo-insulating nature, form a closed manifold over the Brillouin zone via the Kondo coherence effect at low temperature, and host hitherto unobserved wallpaper Dirac fermions at the nonsymmorphic symmetry-preserving (001) surface. The topological nature of TSSs in PuB4 can be described by topological invariants of two Z4 indices [(χx, χy) = (1, 1)] of double-glide symmetries of p4g wallpaper group; thus, PuB4 is a 3D nonsymmorphic topological insulator that exhibits the TSSs of peculiar 4-fold surface Dirac fermions as well as 2-fold double-glide spin-Hall and nodal-line-type fermions. On top of its interesting 5f-electron Kondo-insulating nature, the unique 4-fold wallpaper Dirac fermions in PuB4, which are quite distinct from previously reported nonsymmorphic Dirac insulator or hourglass TCI fermions, broaden our recognition of the embedded fermions in strongly correlated Kondo systems with nonsymmorphic symmetries.

The electronic structure of a possible topological Kondo insulator of CeRhAs has been investigated by employing temperature $(T)$-dependent angle-resolved photoemission spectroscopy (ARPES). Fermi surfaces (FSs) and band structures are successfully measured for three orthogonal crystallographic planes. The measured Fermi-edge states are found to have a three-dimensional (3D) character, contradictory to the proposed topological surface states of the $2\text{D}$ character. The measured FSs due to the Ce $4f$ electrons agree well with those from the density functional theory band structures unfolded into the reduced Ce-only unit cell. The theoretically predicted hourglass-type bulk bands along $\overline{X}\overline{S}\overline{X}$ are not clearly resolved, in spite of the evident existence of those band features. $T$-dependent ARPES measurements reveal that the coherent Ce $4f$ states, having two pseudogap structures of ${\mathrm{\ensuremath{\Delta}}}_{1}\ensuremath{\sim}80$ meV and ${\mathrm{\ensuremath{\Delta}}}_{2}\ensuremath{\sim}30$ meV, persist to remain above 200 K, in agreement with the high Kondo temperature of CeRhAs.

We demonstrate theoretically that the golden phase of SmS (g-SmS), a correlated mixed-valent system, exhibits nontrivial surface states with diverse topology. It turns out that this material is an ideal playground to investigate different band topologies in different surface terminations. We have explored surface states on three different (001), (111), and (110) surface terminations. The topological signature on the (001) surface is not apparent due to a hidden Dirac cone inside the bulk-projected bands. In contrast, the (111) surface shows a clear gapless Dirac cone in the gap region, demonstrating the unambiguous topological Kondo nature of g-SmS. Most interestingly, the (110) surface exhibits both topological-insulator-type and topological-crystalline-insulator (TCI)-type surface states simultaneously. Two different types of double Dirac cones, Rashba-type and TCI-type, realized on the (001) and (110) surfaces, respectively, are analyzed with the mirror eigenvalues and mirror Chern numbers obtained from the model-independent ab initio band calculations.

We have investigated the electronic structures of Ce Kondo systems, CeNiSn, CeRhAs, and CeRhSb, which have attracted recent attention as promising candidates of "Mobius Kondo insulators." In the conventional density functional theory calculations, all three systems are obtained to be semimetallic, while only CeRhAs opens a full gap in the band calculation incorporating the more elaborate (modified Becke-Jones) exchange-correlation potential. Intriguingly, for all three systems, we have obtained the hourglass-type bulk band crossings around k = S, which produce Dirac nodal-loop structures enclosing S in the [100] Brillouin zone boundary. Such topologically protected hourglass-type band structures arise from the spin-orbit coupling on top of the nonsymmorphic crystal symmetry (Pnma) of these systems. Our finding suggests that Ce Kondo insulators would display temperature (T)-dependent topological phase transitions from a potential Mobius Kondo insulator at low T to a Dirac nodal-loop semimetal at high T.

A typical f-electron Kondo lattice system Ce exhibits the well-known isostructural transition, the so-called gamma-alpha transition, accompanied by an enormous volume collapse. Most interestingly, we have discovered that a topological-phase transition also takes place in elemental Ce, concurrently with the gamma-alpha transition. Based on the dynamical mean-field theory approach combined with density functional theory, we have unravelled that the non-trivial topology in alpha-Ce is driven by the f-d band inversion, which arises from the formation of coherent 4f band around the Fermi level. We captured the formation of the 4f quasi-particle band that is responsible for the Lifshitz transition and the non-trivial Z2 topology establishment across the phase boundary. This discovery provides a concept of 'topology switch' for topological Kondo systems. The 'on' and 'off' switching knob in Ce is versatile in a sense that it is controlled by available pressure (around 1 GPa) at room temperature.