Visualization of spatial inhomogeneity in the superconducting gap using micro-ARPES.

Electronic inhomogeneity arises ubiquitously as a consequence of adjacent and/or competing multiple phases or orders in strongly correlated electron systems. Gap inhomogeneity in high- cuprate superconductors has been widely observed using scanning tunneling microscopy/spectroscopy. However, it has yet to be evaluated by angle-resolved photoemission spectroscopy (ARPES) due to the difficulty in achieving both high energy and spatial resolutions.

New three-dimensional flat band candidate materials PbAsO and PbSnO.

Energy dispersion of electrons is the most fundamental property of the solid state physics. In models of electrons on a lattice with strong geometric frustration, the band dispersion of electrons can disappear due to the quantum destructive interference of the wavefunction. This is called a flat band, and it is known to be the stage for the emergence of various fascinating physical properties.

Accelerated Lanthanide Intercalation into Graphite Catalyzed by Na.

Lanthanides () are notoriously difficult to intercalate into graphite. We investigated the possibility of using Na to catalyze the formation of -intercalated graphite and successfully synthesized C ( = Sm, Eu, and Yb) significantly rapidly in high yields. The synthesis process involves the formation of the reaction intermediate NaC, through the mixing of Na and C, which subsequently reacts with upon heating to form C.

Optically Probing Unconventional Superconductivity in Atomically Thin BiSrCaYCuO.

Atomically thin cuprates exhibiting a superconducting phase transition temperature similar to that of the bulk have recently been realized, although the device fabrication remains a challenge and limits the potential for many novel studies and applications. Here, we use an optical pump-probe approach to noninvasively study the unconventional superconductivity in atomically thin BiSrCaYCuO (Y-Bi2212). Apart from finding an optical response due to the superconducting phase transition that is similar to that of bulk Y-Bi2212, we observe that the sign and amplitude of the pump-probe signal in atomically thin flakes vary significantly in different dielectric environments depending on the nature of the optical excitation.

Discovery of orbital ordering in BiSrCaCuO.

The primordial ingredient of cuprate superconductivity is the CuO unit cell. Theories usually concentrate on the intra-atom Coulombic interactions dominating the 3d and 3d configurations of each copper ion. However, if Coulombic interactions also occur between electrons of the 2p orbitals of each planar oxygen atom, spontaneous orbital ordering may split their energy levels.

Universal Stripe Symmetry of Short-Range Charge Density Waves in Cuprate Superconductors.

The omnipresence of charge density waves (CDWs) across almost all cuprate families underpins a common organizing principle. However, a longstanding debate of whether its spatial symmetry is stripe or checkerboard remains unresolved. While CDWs in lanthanum- and yttrium-based cuprates possess a stripe symmetry, distinguishing these two scenarios is challenging for the short-range CDW in bismuth-based cuprates.

Exploring spin-polarization in Bi-based high-T cuprates.

The role of spin-orbit interaction has been recently reconsidered in high-[Formula: see text] cuprates, stimulated by the recent experimental observations of spin-polarized electronic states. However, due to the complexity of the spin texture reported, the origin of the spin polarization in high-[Formula: see text] cuprates remains unclear. Here, we present the spin- and angle-resolved photoemission spectroscopy (ARPES) data on the facing momentum points that are symmetric with respect to the [Formula: see text] point, to ensure the intrinsic spin nature related to the initial state.

Smectic pair-density-wave order in EuRbFeAs.

The pair density wave (PDW) is a superconducting state in which Cooper pairs carry centre-of-mass momentum in equilibrium, leading to the breaking of translational symmetry. Experimental evidence for such a state exists in high magnetic field and in some materials that feature density-wave orders that explicitly break translational symmetry. However, evidence for a zero-field PDW state that exists independent of other spatially ordered states has so far been elusive.

Dramatically Accelerated Formation of Graphite Intercalation Compounds Catalyzed by Sodium.

Graphite intercalation compounds (GICs) have a variety of functions due to their rich material variations, and thus, innovative methods for their synthesis are desired for practical applications. It is discovered that Na has a catalytic property that dramatically accelerates the formation of GICs. It is demonstrated that LiC (n = 1, 2), KC , KC (n = 2, 3, 4), and NaC are synthesized simply by mixing alkali metals and graphite powder with Na at room temperature (≈25 °C), and A C (A  = Ca, Sr, Ba) are synthesized by heating Na-added reagents at 250 °C only for a few hours.

Doping-dependent superconducting physical quantities of K-doped BaFe[Formula: see text]As[Formula: see text] obtained through infrared spectroscopy.

We investigated four single crystals of K-doped BaFe[Formula: see text]As[Formula: see text] (Ba-122), Ba[Formula: see text]K[Formula: see text]Fe[Formula: see text]As[Formula: see text] with [Formula: see text] 0.29, 0.36, 0.

Hidden Structural and Superconducting Phase Induced in Antiperovskite Arsenide SrPdAs.

Enriching the material variation often contributes to the progress of materials science. We have discovered for the first time antiperovskite arsenide SrPdAs and revealed a hidden structural and superconducting phase in Sr(PdPt)As. The Pd-rich samples (0 ≤ ≤ 0.

Unconventional spectral signature of T in a pure d-wave superconductor.

In conventional superconductors, the phase transition into a zero-resistance and perfectly diamagnetic state is accompanied by a jump in the specific heat and the opening of a spectral gap. In the high-transition-temperature (high-T) cuprates, although the transport, magnetic and thermodynamic signatures of T have been known since the 1980s, the spectroscopic singularity associated with the transition remains unknown. Here we resolve this long-standing puzzle with a high-precision angle-resolved photoemission spectroscopy (ARPES) study on overdoped (Bi,Pb)SrCaCuO (Bi2212).

Intrinsic defect structures of polycrystalline CaKFeAs superconductors.

We investigated the defect structures of polycrystalline CaKFeAs (CaK1144) superconductors by scanning transmission electron microscopy (STEM). The STEM studies revealed the presence of a one-layer CaFeAs (∼1 nm size) defect along the -plane, as observed in single crystalline CaK1144. Step-like CaFeAs defects are also observed.

Superconductivity-driven ferromagnetism and spin manipulation using vortices in the magnetic superconductor EuRbFeAs.

Magnetic superconductors are specific materials exhibiting two antagonistic phenomena, superconductivity and magnetism, whose mutual interaction induces various emergent phenomena, such as the reentrant superconducting transition associated with the suppression of superconductivity around the magnetic transition temperature ( ), highlighting the impact of magnetism on superconductivity. In this study, we report the experimental observation of the ferromagnetic order induced by superconducting vortices in the high-critical-temperature (high- ) magnetic superconductor EuRbFeAs Although the ground state of the Eu moments in EuRbFeAs is helimagnetism below , neutron diffraction and magnetization experiments show a ferromagnetic hysteresis of the Eu spin alignment. We demonstrate that the direction of the Eu moments is dominated by the distribution of pinned vortices based on the critical state model.

Experimental and Computational Determination of Optimal Boron Content in Layered Superconductor ScCBC.

It is generally difficult to quantify the amount of light elements in materials because of their low X-ray-scattering power, as this means that they cannot be easily estimated via X-ray analyses. Meanwhile, the recently reported layered superconductor, ScCBC, requires a small amount of boron, which is a light element, for its structural stability. In this context, here, we quantitatively evaluate the optimal value using both experimental and computational approaches.

Structural Phase Transitions and Superconductivity Induced in Antiperovskite Phosphide CaPdP.

In this study, we succeeded in synthesizing new antiperovskite phosphides PdP (M = Ca, Sr, Ba) and discovered the appearance of a superconducting phase (0.17 ≤ ≤ 0.55) in a solid solution (CaSr)PdP.

Multiorbital charge-density wave excitations and concomitant phonon anomalies in BiSrLaCuO.

Charge-density waves (CDWs) are ubiquitous in underdoped cuprate superconductors. As a modulation of the valence electron density, CDWs in hole-doped cuprates possess both Cu-3 and O-2 orbital character owing to the strong hybridization of these orbitals near the Fermi level. Here, we investigate underdoped BiSrLaCuO using resonant inelastic X-ray scattering (RIXS) and find that a short-range CDW exists at both Cu and O sublattices in the copper-oxide (CuO) planes with a comparable periodicity and correlation length.

Superconducting-Gap Anisotropy of Iron Pnictides Investigated via Combinatorial Microwave Measurements.

One of the most significant issues for superconductivity is clarifying the momentum-dependent superconducting gap Δ([Formula: see text]), which is closely related to the pairing mechanism. To elucidate the gap structure, it is essential to investigate Δ([Formula: see text]) in as many different physical quantities as possible and to crosscheck the results obtained in different methods with each other. In this paper, we report a combinatorial investigation of the superfluid density and the flux-flow resistivity of iron-pnictide superconductors; LiFeAs and BaFe(AsP) (x = 0.

Novel electronic nematicity in heavily hole-doped iron pnictide superconductors.

Electronic nematicity, a correlated state that spontaneously breaks rotational symmetry, is observed in several layered quantum materials. In contrast to their liquid-crystal counterparts, the nematic director cannot usually point in an arbitrary direction (XY nematics), but is locked by the crystal to discrete directions (Ising nematics), resulting in strongly anisotropic fluctuations above the transition. Here, we report on the observation of nearly isotropic XY-nematic fluctuations, via elastoresistance measurements, in hole-doped Ba Rb FeAs iron-based superconductors.

Incoherent strange metal sharply bounded by a critical doping in Bi2212.

In normal metals, macroscopic properties are understood using the concept of quasiparticles. In the cuprate high-temperature superconductors, the metallic state above the highest transition temperature is anomalous and is known as the "strange metal." We studied this state using angle-resolved photoemission spectroscopy.

Superconductivity in a Scandium Borocarbide with a Layered Crystal Structure.

The discovery of nearly room-temperature superconductivity in superhydrides has motivated further materials research for conventional superconductors. To realize the moderately high critical temperature () in materials containing light elements, we explored new superconducting phases in a scandium borocarbide system. Here, we report the observation of superconductivity in a new ternary Sc-B-C compound.

Evidence for a vestigial nematic state in the cuprate pseudogap phase.

The CuO antiferromagnetic insulator is transformed by hole-doping into an exotic quantum fluid usually referred to as the pseudogap (PG) phase. Its defining characteristic is a strong suppression of the electronic density-of-states () for energies || < [Formula: see text], where [Formula: see text] is the PG energy. Unanticipated broken-symmetry phases have been detected by a wide variety of techniques in the PG regime, most significantly a finite- density-wave (DW) state and a = 0 nematic (NE) state.

Superconductivity in Uncollapsed Tetragonal LaFeAs.

We report synthesis, crystal structure, and superconductivity in ThCrSi-type LaFeAs (La122). La122 was synthesized at 960 °C for 1.5 h under a pressure of 3.

Revealing hidden spin-momentum locking in a high-temperature cuprate superconductor.

Cuprate superconductors have long been thought of as having strong electronic correlations but negligible spin-orbit coupling. Using spin- and angle-resolved photoemission spectroscopy, we discovered that one of the most studied cuprate superconductors, Bi2212, has a nontrivial spin texture with a spin-momentum locking that circles the Brillouin zone center and a spin-layer locking that allows states of opposite spin to be localized in different parts of the unit cell. Our findings pose challenges for the vast majority of models of cuprates, such as the Hubbard model and its variants, where spin-orbit interaction has been mostly neglected, and open the intriguing question of how the high-temperature superconducting state emerges in the presence of this nontrivial spin texture.