酒井 志朗

<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>

<jats:p>In cuprate superconductors with high critical transition temperature (<jats:italic>T</jats:italic><jats:sub>c</jats:sub>), light hole-doping to the parent compound, which is an antiferromagnetic Mott insulator, has been predicted to lead to the formation of small Fermi pockets. These pockets, however, have not been observed. Here, we investigate the electronic structure of the five-layered Ba<jats:sub>2</jats:sub>Ca<jats:sub>4</jats:sub>Cu<jats:sub>5</jats:sub>O<jats:sub>10</jats:sub>(F,O)<jats:sub>2</jats:sub>, which has inner copper oxide (CuO<jats:sub>2</jats:sub>) planes with extremely low disorder, and find small Fermi pockets centered at (π/2, π/2) of the Brillouin zone by angle-resolved photoemission spectroscopy and quantum oscillation measurements. The d-wave superconducting gap opens along the pocket, revealing the coexistence between superconductivity and antiferromagnetic ordering in the same CuO<jats:sub>2</jats:sub> sheet. These data further indicate that superconductivity can occur without contribution from the antinodal region around (π, 0), which is shared by other competing excitations.</jats:p>

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.

<jats:p>Alkali-doped fullerides<jats:italic>A</jats:italic><jats:sub>3</jats:sub>C<jats:sub>60</jats:sub>(<jats:italic>A</jats:italic>= K, Rb, Cs) are surprising materials where conventional phonon-mediated superconductivity and unconventional Mott physics meet, leading to a remarkable phase diagram as a function of volume per C<jats:sub>60</jats:sub>molecule. We address these materials with a state-of-the-art calculation, where we construct a realistic low-energy model from first principles without using a priori information other than the crystal structure and solve it with an accurate many-body theory. Remarkably, our scheme comprehensively reproduces the experimental phase diagram including the low-spin Mott-insulating phase next to the superconducting phase. More remarkably, the critical temperatures<jats:italic>T</jats:italic><jats:sub>c</jats:sub>’s calculated from first principles quantitatively reproduce the experimental values. The driving force behind the surprising phase diagram of<jats:italic>A</jats:italic><jats:sub>3</jats:sub>C<jats:sub>60</jats:sub>is a subtle competition between Hund’s coupling and Jahn-Teller phonons, which leads to an effectively inverted Hund’s coupling. Our results establish that the fullerides are the first members of a novel class of molecular superconductors in which the multiorbital electronic correlations and phonons cooperate to reach high<jats:italic>T</jats:italic><jats:sub>c</jats:sub><jats:italic>s</jats:italic>-wave superconductivity.</jats:p>