Hyperuniformity in two-dimensional periodic and quasiperiodic point patterns.

We study hyperuniform properties in various two-dimensional periodic and quasiperiodic point patterns. Using the histogram of the two-point distances, we develop an efficient method to calculate the hyperuniformity order metric, which quantifies the regularity of the hyperuniform point patterns. The results are compared with those calculated with the conventional running average method. To discuss how the lattice symmetry affects the order metric, we treat the trellis and Shastry-Sutherland lattices with the same point density as examples of periodic lattices, and Stampfli hexagonal and dodecagonal quasiperiodic tilings with the same point density as examples of quasiperiodic tilings. It is found that the order metric for the Shastry-Sutherland lattice (Stampfli dodecagonal tilings) is smaller than the other in the periodic (quasiperiodic) tiling, meaning that the order metric is deeply related to the lattice symmetry. Namely, the point pattern with higher symmetry is characterized by the smaller order metric when their point densities are identical. Order metrics for several other quasiperiodic tilings are also calculated.

Emergent zero-field anomalous Hall effect in a reconstructed rutile antiferromagnetic metal.

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. RuO2 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, Ru0.8Cr0.2O2. The magnetization, transport and density functional theory calculations indicate that appropriate doping of Cr at Ru sites reconstructs the collinear antiferromagnetism in RuO2, 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 Ru0.8Cr0.2O2 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.

Unveiling phase diagram of the lightly doped high-Tc cuprate superconductors with disorder removed.

The currently established electronic phase diagram of cuprates is based on a study of single- and double-layered compounds. These CuO2 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 Ba2Ca5Cu6O12(F,O)2 with inner CuO2 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.

Fermi Surface Expansion above Critical Temperature in a Hund Ferromagnet.

Using a cluster extension of the dynamical mean-field theory, we show that strongly correlated metals subject to Hund's physics exhibit significant electronic structure modulations above magnetic transition temperatures. In particular, in a ferromagnet having a large local moment due to Hund's coupling (Hund's ferromagnet), the Fermi surface expands even above the Curie temperature (T_{C}) as if a spin polarization occurred. Behind this phenomenon, effective "Hund's physics" works in momentum space, originating from ferromagnetic fluctuations in the strong-coupling regime. The resulting significantly momentum-dependent (spatially nonlocal) electron correlations induce an electronic structure reconstruction involving a Fermi surface volume change and a redistribution of the momentum-space occupation. Our finding will give a deeper insight into the physics of Hund's ferromagnets above T_{C}.

Observation of small Fermi pockets protected by clean CuO2 sheets of a high-T c superconductor.

In cuprate superconductors with high critical transition temperature (T c), 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 Ba2Ca4Cu5O10(F,O)2, which has inner copper oxide (CuO2) 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 CuO2 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.

Giant thermoelectric power factor in ultrathin FeSe superconductor.

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-Tc superconductor (Tc; superconducting transition temperature), exhibit superior thermoelectric responses. With decreasing thickness d, the electrical conductivity increases accompanying the emergence of high-Tc 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 Tc superconductors indicates the high potential of two-dimensional layered materials towards multi-functionalization.

Correlation-Driven Lifshitz Transition at the Emergence of the Pseudogap Phase in the Two-Dimensional Hubbard Model.

We study the relationship between the pseudogap and Fermi-surface topology in the two-dimensional Hubbard model by means of the cellular dynamical mean-field theory. We find two possible mean-field metallic solutions on a broad range of interactions, doping, and frustration: a conventional renormalized metal and an unconventional pseudogap metal. At half filling, the conventional metal is more stable and displays an interaction-driven Mott metal-insulator transition. However, for large interactions and small doping, a region that is relevant for cuprates, the pseudogap phase becomes the ground state. By increasing doping, we show that a first-order transition from the pseudogap to the conventional metal is tied to a change of the Fermi surface from hole- to electronlike, unveiling a correlation-driven mechanism for a Lifshitz transition. This explains the puzzling link between the pseudogap phase and Fermi surface topology that has been pointed out in recent experiments.

Observation of Bogoliubov Band Hybridization in the Optimally Doped Trilayer Bi_{2}Sr_{2}Ca_{2}Cu_{3}O_{10+δ}.

Using a laser-excited angle-resolved photoemission spectroscopy capable of bulk sensitive and high-energy resolution measurements, we reveal a new phenomenon of superconductors in the optimally doped trilayer Bi_{2}Sr_{2}Ca_{2}Cu_{3}O_{10+δ}. We observe a hybridization of the Bogoliubov bands derived from the inner and outer CuO_{2} planes with different magnitudes of energy gaps. Our data clearly exhibit the splitting of coherent peaks and the consequent enhancement of spectral gaps. These features are reproduced by model calculations, which indicate that the gap enhancement extends over a wide range of Fermi surface up to the antinode. The significant modulation of electron pairing uncovered here might be a crucial factor to achieve the highest critical temperature in the trilayer cuprates.

Nonempirical Calculation of Superconducting Transition Temperatures in Light-Element Superconductors.

Recent progress in the fully nonempirical calculation of the superconducting transition temperature (Tc ) is reviewed. Especially, this study focuses on three representative light-element high-Tc superconductors, i.e., elemental Li, sulfur hydrides, and alkali-doped fullerides. Here, it is discussed how crucial it is to develop the beyond Migdal-Eliashberg (ME) methods. For Li, a scheme of superconducting density functional theory for the plasmon mechanism is formulated and it is found that Tc is dramatically enhanced by considering the frequency dependence of the screened Coulomb interaction. For sulfur hydrides, it is essential to go beyond not only the static approximation for the screened Coulomb interaction, but also the constant density-of-states approximation for electrons, the harmonic approximation for phonons, and the Migdal approximation for the electron-phonon vertex, all of which have been employed in the standard ME calculation. It is also shown that the feedback effect in the self-consistent calculation of the self-energy and the zero point motion considerably affect the calculation of Tc . For alkali-doped fullerides, the interplay between electron-phonon coupling and electron correlations becomes more nontrivial. It has been demonstrated that the combination of density functional theory and dynamical mean field theory with the ab initio downfolding scheme for electron-phonon coupled systems works successfully. This study not only reproduces the experimental phase diagram but also obtains a unified view of the high-Tc superconductivity and the Mott-Hubbard transition in the fullerides. The results for these high-Tc superconductors will provide a firm ground for future materials design of new superconductors.

Exotic s-wave superconductivity in alkali-doped fullerides.

Alkali-doped fullerides (A3C60 with A = K, Rb, Cs) show a surprising phase diagram, in which a high transition-temperature (Tc) s-wave superconducting state emerges next to a Mott insulating phase as a function of the lattice spacing. This is in contrast with the common belief that Mott physics and phonon-driven s-wave superconductivity are incompatible, raising a fundamental question on the mechanism of the high-Tc superconductivity. This article reviews recent ab initio calculations, which have succeeded in reproducing comprehensively the experimental phase diagram with high accuracy and elucidated an unusual cooperation between the electron-phonon coupling and the electron-electron interactions leading to Mott localization to realize an unconventional s-wave superconductivity in the alkali-doped fullerides. A driving force behind the exotic physics is unusual intramolecular interactions, characterized by the coexistence of a strongly repulsive Coulomb interaction and a small effectively negative exchange interaction. This is realized by a subtle energy balance between the coupling with the Jahn-Teller phonons and Hund's coupling within the C60 molecule. The unusual form of the interaction leads to a formation of pairs of up- and down-spin electrons on the molecules, which enables the s-wave pairing. The emergent superconductivity crucially relies on the presence of the Jahn-Teller phonons, but surprisingly benefits from the strong correlations because the correlations suppress the kinetic energy of the electrons and help the formation of the electron pairs, in agreement with previous model calculations. This confirms that the alkali-doped fullerides are a new type of unconventional superconductors, where the unusual synergy between the phonons and Coulomb interactions drives the high-Tc superconductivity.

Hidden Fermionic Excitation Boosting High-Temperature Superconductivity in Cuprates.

The dynamics of a microscopic cuprate model, namely, the two-dimensional Hubbard model, is studied with a cluster extension of the dynamical mean-field theory. We find a nontrivial structure of the frequency-dependent self-energies, which describes an unprecedented interplay between the pseudogap and superconductivity. We show that these properties are well described by quasiparticles hybridizing with (hidden) fermionic excitations, emergent from the strong electronic correlations. The hidden fermion enhances superconductivity via a mechanism distinct from a conventional boson-mediated pairing, and originates the normal-state pseudogap. Though the hidden fermion is elusive in experiments, it can solve many experimental puzzles.

Unified understanding of superconductivity and Mott transition in alkali-doped fullerides from first principles.

Alkali-doped fullerides A 3C60 (A = 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 C60 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 T c's calculated from first principles quantitatively reproduce the experimental values. The driving force behind the surprising phase diagram of A 3C60 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 T c s-wave superconductivity.

Evolution of electronic structure of doped Mott insulators: reconstruction of poles and zeros of Green's function.

We study the evolution of metals from Mott insulators in the carrier-doped 2D Hubbard model using a cluster extension of the dynamical mean-field theory. While the conventional metal is simply characterized by the Fermi surface (pole of the Green function G), interference of the zero surfaces of G with the pole surfaces becomes crucial in the doped Mott insulators. Mutually interfering pole and zero surfaces are dramatically transferred over the Mott gap, when lightly doped holes synergetically loosen the doublon-holon binding. The heart of the Mott physics such as the pseudogap, hole pockets, Fermi arcs, in-gap states, Lifshitz transitions, and non-Fermi liquids appears as natural consequences of this global interference in the frequency space.

Itinerant ferromagnetism in the multiorbital Hubbard model: a dynamical mean-field study.

In order to resolve the long-standing issue of how itinerant ferromagnetism is affected by lattice structure and Hund's coupling, we compare various three-dimensional lattice structures in the single- and multiorbital Hubbard models with the dynamical mean-field theory with an improved quantum Monte Carlo algorithm that preserves the spin-SU(2) symmetry. The result indicates that both the lattice structure and the d-orbital degeneracy are essential for the ferromagnetism in the parameter region representing a transition metal. Specifically, (a) Hund's coupling, despite the common belief, is important, which is here identified to come from particle-hole scatterings, and (b) the ferromagnetism is a correlation effect (outside the Stoner picture) as indicated from the band-filling dependence.

Reliability of the craniomandibular index.

AIMS: To examine various dimensions of reliability of the Craniomandibular Index, a commonly used instrument for quantifying the severity of signs and symptoms of temporomandibular disorders. METHODS: Classical psychometric theory and generalizability theory were used to assess the reliability of data obtained from a calibration study of examiners participating in a multi-site clinical trial and from a random community sample. RESULTS: The reliability of aggregate scores formed by summing individual binary scored items was high, with intraclass correlations ranging from 0.81 to 0.88. When it was required that examiners recognize and agree upon a specific pattern of signs and symptoms exhibited by a patient, however, reliability dropped dramatically (multivariate kappas ranged from 0.26 to 0.32). A group of practicing examiners also showed limited ability to agree with the pattern of signs and symptoms identified by a "gold standard" examiner (multivariate kappas ranging from 0.25 to 0.32). Generalizability analysis failed to identify the specific sources of measurement error that played a major role in limiting reliability but demonstrated that generalizability of aggregate scores was very high. CONCLUSION: Methods of classical psychometric theory and generalizability theory support the conclusion that the reliability of aggregate scores is acceptably high. Individual items assessing certain aspects of jaw mobility and joint sounds are measured with poor reliability. Reliability declines when it is defined as the ability of examiners to agree among themselves upon a specific constellation of signs and symptoms or their ability to identify correctly a "correct" constellation identified by an expert examiner.

Dietary intake in edentulous subjects with good and poor quality complete dentures.

STATEMENT OF PROBLEM: Previous studies in complete denture wearers evaluated the relationship between diet and measures of chewing, yet only isolated nutrient intake was considered. This limited information makes the assessment of overall diet quality and the planning of interventions difficult. PURPOSE: This study investigated the relationship of complete denture quality to masticatory performance, perceived ability to chew, and diet quality as measured by the Healthy Eating Index (HEI), an overall diet quality index. MATERIAL AND METHOD: The study population comprised 54 complete denture wearers. Data were obtained from clinical examinations, masticatory performance measurements, and 2 non-consecutive 24-hour dietary recalls. (Masticatory performance data were not collected for 9 subjects because of time constraints, patient fatigue, or patient refusal. Statistical analysis showed no significant effect of their absence on the reported findings.) Based on a composite rating scale, subjects were divided into 3 denture quality groups described as good, medium, and poor. The outcome variables were the HEI and its components, plus selected nutrient and non-nutrient intake. Explanatory variables were quality of complete dentures, masticatory performance, and reported chewing ability. Data were analyzed with Kruskal-Wallis tests, Mann-Whitney U-tests, and Fisher exact tests. Because a large number of dietary components were examined, results were considered significant at alpha=.01. RESULTS: Masticatory performance and perceived ability to chew were unrelated to diet quality. The good quality denture group had significantly better masticatory performance than the medium and poor quality groups, but the median HEI scores and dietary intakes were not significantly different among these 3 groups. Milk, vegetable, fruit, and grain intake scores were mainly responsible for the low overall HEI scores. CONCLUSION: In the population evaluated, complete denture quality, food comminution capacity, and perceived chewing ability were not related to diet quality. The majority of subjects had deficient diets regardless of the technical quality of their dentures.