Shiro Sakai

Abstract Fermi arcs observed in underdoped cuprates have sparked debate over whether they represent segments of a large Fermi surface or small Fermi pockets. This ambiguity has long hindered their classification as either the conventional Bardeen-Cooper-Schrieffer (BCS) regime or the strongly coupled Bose-Einstein condensation (BEC) crossover limit. Here, using angle-resolved photoemission spectroscopy and quantum oscillations, we demonstrate the coexistence of a small Fermi pocket and a large superconducting gap in the clean inner CuO ₂ layers of the four-layer cuprate Ba ₂ Ca ₃ Cu ₄ O ₈ (F,O) ₂ . This coexistence constitutes a hallmark of the BCS-BEC crossover and has remained elusive for decades. Despite the presence of antiferromagnetic (AF) order, the superconducting gap in the small pocket is remarkably large, yielding a gap-to-Fermi energy ratio (Δ _pocket / ε _F  ~ 0.6) and a critical-to-Fermi temperature ratio ( T _c / T _F  ~ 0.13) that reach the theoretical upper bound for two-dimensional superconductivity. Unexpectedly, this BCS-BEC crossover emerges not as the carrier density decreases but as it increases, abruptly within a narrow doping range of less than 1%. These results provide a long-sought microscopic foundation for the d -wave pairing mechanism in doped AF-Mott insulators.

Motivated by recent advances in the realization of Truchet-tiling structures in molecular networks and metal-organic frameworks, we investigate the wave localization issue in this kind of structure. We introduce an electron model based on random Truchet tilings, square lattices with randomly oriented diagonal links, and uncover a rich interplay between spectral and localization phenomena. By varying the strength of diagonal couplings, we explore successive transitions from an extended phase, through a regime with a mobility edge, to a fully localized phase. The energy-resolved fractal dimension analysis captures the emergence and disappearance of mobility edges, while an anomalous shift and asymmetry in the Van Hove singularity are identified as key signatures of the underlying disordered Truchet-tiling structure. Notably, by using the finite-size scaling of level spacing statistics, we clarify that the transition occurs at a finite level of disorder even in the two-dimensional system. Our findings position Truchet-tiled electron systems as a versatile platform for engineering disorder-driven localization and interaction effects in amorphous quantum materials and photonic architectures.

<jats:p>We explore the superconducting properties of the bilayer Hubbard model, which exhibits a high transition temperature <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mo>(</a:mo><a:msub><a:mi>T</a:mi><a:mi>c</a:mi></a:msub><a:mo>)</a:mo></a:math> for an <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>s</b:mi><b:mo>±</b:mo></b:msub></b:math> pairing, using a cluster extension of the dynamical mean-field theory. Unlike the single-layer Hubbard model, where the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mi>d</c:mi></c:math>-wave superconductivity emerges by doping the Mott insulator, the parent state of the bilayer system is a correlated band insulator. Above <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:msub><d:mi>T</d:mi><d:mi>c</d:mi></d:msub></d:math>, slight hole (electron) doping introduces a striking dichotomy between electron and hole pockets: The electron (hole) pocket develops a pseudogap while the other becomes a nearly incipient band. We reveal that the superconductivity is driven by kinetic (potential) energy gain in the underdoped (overdoped) region. We also find a very short coherence length, for which we argue the relevance to multiorbital physics. Our Letter offers crucial insights into the superconductivity in the bilayer Hubbard model potentially relevant to <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:msub><e:mi>La</e:mi><e:mn>3</e:mn></e:msub><e:msub><e:mi>Ni</e:mi><e:mn>2</e:mn></e:msub><e:msub><e:mi mathvariant="normal">O</e:mi><e:mn>7</e:mn></e:msub></e:mrow></e:math>.</jats:p>

<jats:p> The pseudogap phenomena have been a long-standing mystery of the cuprate high-temperature superconductors. The pseudogap in the electron-doped cuprates has been attributed to band folding due to antiferromagnetic (AFM) long-range order or short-range correlation. We performed an angle-resolved photoemission spectroscopy study of the electron-doped cuprates Pr <jats:sub> 1.3− <jats:italic>x</jats:italic> </jats:sub> La <jats:sub>0.7</jats:sub> Ce <jats:sub> <jats:italic>x</jats:italic> </jats:sub> CuO <jats:sub>4</jats:sub> showing spin-glass, disordered AFM behaviors, and superconductivity at low temperatures and, by measurements with fine momentum cuts, found that the gap opens on the unfolded Fermi surface rather than the AFM Brillouin zone boundary. The gap did not show a node, following the full symmetry of the Brillouin zone, and its magnitude decreased from the zone-diagonal to ( <jats:italic>π</jats:italic> ,0) directions, opposite to the hole-doped case. These observations were reproduced by cluster dynamical-mean-field-theory calculation, which took into account electron correlation precisely within a (CuO <jats:sub>2</jats:sub> ) <jats:sub>4</jats:sub> cluster. The present experimental and theoretical results are consistent with the mechanism that electron or hole doping into a Mott insulator creates an in-gap band that is separated from the upper or lower Hubbard band by the pseudogap. </jats:p>

<jats:p>A-site-ordered and -disordered GdBaCo<jats:sub>2</jats:sub>O<jats:sub>6</jats:sub> films were synthesized through topotactic oxidation. These films exhibit distinct magnetic and conductive properties, attributed to variations in Co spin states driven by changes in ionic arrangement.</jats:p>

<jats:title>Abstract</jats:title><jats:p>The anomalous Hall effect (AHE) that emerges in antiferromagnetic metals shows intriguing physics and offers numerous potential applications. Magnets with a rutile crystal structure have recently received attention as a possible platform for a collinear-antiferromagnetism-induced AHE. RuO<jats:sub>2</jats:sub> is a prototypical candidate material, however the AHE is prohibited at zero field by symmetry because of the high-symmetry [001] direction of the Néel vector at the ground state. Here, we show AHE at zero field in Cr-doped rutile, Ru<jats:sub>0.8</jats:sub>Cr<jats:sub>0.2</jats:sub>O<jats:sub>2</jats:sub>. The magnetization, transport and density functional theory calculations indicate that appropriate doping of Cr at Ru sites reconstructs the collinear antiferromagnetism in RuO<jats:sub>2</jats:sub>, resulting in a rotation of the Néel vector from [001] to [110] while maintaining a collinear antiferromagnetic state. The AHE with vanishing net moment in the Ru<jats:sub>0.8</jats:sub>Cr<jats:sub>0.2</jats:sub>O<jats:sub>2</jats:sub> exhibits an orientation dependence consistent with the [110]-oriented Hall vector. These results demonstrate that material engineering by doping is a useful approach to manipulate AHE in antiferromagnetic metals.</jats:p>

<jats:title>Abstract</jats:title><jats:p>The currently established electronic phase diagram of cuprates is based on a study of single- and double-layered compounds. These CuO<jats:sub>2</jats:sub> planes, however, are directly contacted with dopant layers, thus inevitably disordered with an inhomogeneous electronic state. Here, we solve this issue by investigating a 6-layered Ba<jats:sub>2</jats:sub>Ca<jats:sub>5</jats:sub>Cu<jats:sub>6</jats:sub>O<jats:sub>12</jats:sub>(F,O)<jats:sub>2</jats:sub> with inner CuO<jats:sub>2</jats:sub> layers, which are clean with the extremely low disorder, by angle-resolved photoemission spectroscopy (ARPES) and quantum oscillation measurements. We find a tiny Fermi pocket with a doping level less than 1% to exhibit well-defined quasiparticle peaks which surprisingly lack the polaronic feature. This provides the first evidence that the slightest amount of carriers is enough to turn a Mott insulating state into a metallic state with long-lived quasiparticles. By tuning hole carriers, we also find an unexpected phase transition from the superconducting to metallic states at 4%. Our results are distinct from the nodal liquid state with polaronic features proposed as an anomaly of the heavily underdoped cuprates.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We study the distribution of supercurrent in the attractive Hubbard model on a periodic approximant of the Ammann-Beenker structure by means of the Bogoliubov-de Gennes (BdG) mean-field theory. We first investigate the spatial distributions of the electron density and superconducting order parameter and find a crossover between superconducting states with (i) extended, (ii) localized, and (iii) short-ranged Cooper pairs, similar to the previous results on the Penrose tiling. Furthermore, we formulate the expression of the supercurrent for the periodic approximant in terms of the eigenenergies and the wave functions of the Bogoliubov-de Gennes Hamiltonian and numerically investigate how the local supercurrent distribution varies by changing the direction of the applied phase gradient.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We study the electron charge distribution on a quasiperiodic tiling in terms of hyperuniformity. In an extended Hubbard model on the Ammann-Beenker tiling, the electron distribution changes significantly with the Fermi energy and electron-interaction strength. Unlike periodic systems, these changes are not characterized by translational-symmetry breaking. We show that the electron charge distribution is not characterized by multifractality, either. We find that the distribution is instead characterized by hyperuniformity of Class I.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The magnetic ground states of <jats:italic>R</jats:italic> <jats:sub>2</jats:sub>Ru<jats:sub>2</jats:sub>O<jats:sub>7</jats:sub> and <jats:italic>A</jats:italic> <jats:sub>2</jats:sub>Ru<jats:sub>2</jats:sub>O<jats:sub>7</jats:sub> with <jats:italic>R</jats:italic> = Pr, Gd, Ho, and Er, as well as <jats:italic>A</jats:italic> = Ca, Cd are predicted devising a combination of the cluster-multipole (CMP) theory and spin-density-functional theory (SDFT). The strong electronic correlation effects are estimated by the constrained-random-phase approximation (cRPA) and taken into account within the dynamical-mean-field theory (DMFT). The target compounds feature d-orbital magnetism on Ru<jats:sup>4+</jats:sup> and Ru<jats:sup>5+</jats:sup> ions for <jats:italic>R</jats:italic> and <jats:italic>A</jats:italic>, respectively, as well as f-orbital magnetism on the <jats:italic>R</jats:italic> site, which leads to an intriguing interplay of magnetic interactions in a strongly correlated system. We find CMP + SDFT is capable of describing the magnetic ground states in these compounds. The cRPA captures a difference in the screening strength between <jats:italic>R</jats:italic> <jats:sub>2</jats:sub>Ru<jats:sub>2</jats:sub>O<jats:sub>7</jats:sub> and <jats:italic>A</jats:italic> <jats:sub>2</jats:sub>Ru<jats:sub>2</jats:sub>O<jats:sub>7</jats:sub> compounds, which leads to a qualitative and quantitative understanding of the electronic properties within DMFT.</jats:p>

<jats:title>Abstract</jats:title><jats:p>The interplay between electron correlation and topology of relativistic electrons may lead to a fascinating stage of the research on quantum materials and emergent functions. The emergence of various collective electronic orderings/liquids, which are tunable by external stimuli, is a remarkable feature of correlated electron systems, but has rarely been realized in the topological semimetals with high-mobility relativistic electrons. Here, we report that the correlated Dirac electrons in perovskite CaIrO<jats:sub>3</jats:sub> show unconventional field-induced successive metal–insulator–metal crossovers in the quantum limit accompanying a giant magnetoresistance (MR) with MR ratio of 3500 % (18 T and 1.4 K). In conjunction with the numerical calculation, we propose that the insulating state originates from the collective electronic ordering such as charge/spin density wave promoted by electron correlation, whereas it turns into the quasi-one-dimensional metal at higher fields due to the field-induced reduction of chemical potential, highlighting the highly field-tunable character of correlated Dirac electrons.</jats:p>

An elusive pocket Superconductivity in copper oxide materials emerges by doping a special kind of correlated state called the Mott insulator. However, studying what happens when a small concentration of charge carriers—holes or electrons—is added to a Mott insulator is experimentally challenging. It has been predicted that the so-called “Fermi pockets” should become visible during experimentation, but such pockets have not been unambiguously observed. Kunisada et al. studied the unusual cuprate Ba ₂ Ca ₄ Cu ₅ O ₁₀ (F,O) ₂ , which has five copper oxide planes in a unit cell, whereas most cuprates have one or two (see the Perspective by Vishik). They observed two Fermi pockets in both photoemission and quantum oscillations data, with the innermost copper oxide planes playing a crucial role. Science , this issue p. 833 ; see also p. 775

Abstract The thermoelectric effect is attracting a renewed interest as a concept for energy harvesting technologies. Nanomaterials have been considered a key to realize efficient thermoelectric conversions owing to the low dimensional charge and phonon transports. In this regard, recently emerging two-dimensional materials could be promising candidates with novel thermoelectric functionalities. Here we report that FeSe ultrathin films, a high-T_c superconductor (T_c; superconducting transition temperature), exhibit superior thermoelectric responses. With decreasing thickness d, the electrical conductivity increases accompanying the emergence of high-T_c superconductivity; unexpectedly, the Seebeck coefficient α shows a concomitant increase as a result of the appearance of two-dimensional natures. When d is reduced down to ~1 nm, the thermoelectric power factor at 50 K and room temperature reach unprecedented values as high as 13,000 and 260 μW cm⁻¹ K⁻², respectively. The large thermoelectric effect in high T_c superconductors indicates the high potential of two-dimensional layered materials towards multi-functionalization.

Abstract Electrons in conventional metals become less mobile under the influence of electron correlation. Contrary to this empirical knowledge, we report here that electrons with the highest mobility ever found in known bulk oxide semiconductors emerge in the strong-correlation regime of the Dirac semimetal of perovskite CaIrO₃. The transport measurements reveal that the high mobility exceeding 60,000 cm²V⁻¹s⁻¹ originates from the proximity of the Fermi energy to the Dirac node (ΔE &lt; 10 meV). The calculation based on the density functional theory and the dynamical mean field theory reveals that the energy difference becomes smaller as the system approaches the Mott transition, highlighting a crucial role of correlation effects cooperating with the spin-orbit coupling. The correlation-induced self-tuning of Dirac node enables the quantum limit at a modest magnetic field with a giant magnetoresistance, thus providing an ideal platform to study the novel phenomena of correlated Dirac electron.

The pseudogap phenomena have been a long-standing mystery of the cuprate high-temperature superconductors. Unlike the pseudogap in hole-doped cuprates, however, the pseudogap in the electron-doped counterpart has been attributed to band folding due to short-range antiferromagnetic (AFM) order. We performed angle-resolved photoemission spectroscopy measurements on electron-doped cuprates showing spin-glass and disordered AFM behaviors at low temperatures, and found that the gap magnitude \textit{decreases} in the antinodal region contrary to the hole-doped case. Moreover, the gap opening position was not always on the AFM Brillouin zone boundary in contradiction with the requirement of the AFM band-folding picture. These features are consistent with cluster dynamical-mean-field-theory calculations which predict an $s$-symmetry pseudogap that shrinks in the andinodal region. The present results support the scenario that the proximity to the Mott insulator, without relying on the well-developed AFM correlation, gives rise to a momentu m-dependent pseudogap of $s$-symmetry with indirect gap commonly in the electron-doped and hole-doped cuprates, implying a universal origin of the pseudogap with a similarity to the Mott gap formation.