Doped high-Tc cuprate superconductors elucidated in the light of zeros and poles of electronic Green's function

We study electronic structure of hole- and electron-doped Mott insulators in the two-dimensional Hubbard model to reach a unified picture for the normal state of cuprate high-Tc superconductors. By using a cluster extension of the dynamical mean-field theory, we demonstrate that structure of coexisting zeros and poles of the single-particle Green's function holds the key to understand Mott physics in the underdoped region. We show evidence for the emergence of non-Fermi-liquid phase caused by the topological quantum phase transition of Fermi surface by analyzing low-energy charge dynamics. The spectra calculated in a wide range of energy and momentum reproduce various anomalous properties observed in experiments for the high-Tc cuprates. Our results reveal that the pseudogap in hole-doped cuprates has a d-wave-like structure only below the Fermi level, while it retains non-d-wave structure with a fully opened gap above the Fermi energy even in the nodal direction due to a zero surface extending over the entire Brillouin zone. In addition to the non-d-wave pseudogap, the present comprehensive identifications of the spectral asymmetry as to the Fermi energy, the Fermi arc, and the back-bending behavior of the dispersion, waterfall, and low-energy kink, in agreement with the experimental anomalies of the cuprates, do not support that these originate from (the precursors of) symmetry breakings such as the preformed pairing and the d-density wave fluctuations, but support that they are direct consequences of the proximity to the Mott insulator. Several possible experiments are further proposed to prove or disprove our zero mechanism.Comment: 17 pages, 15 figure

Screening of Coulomb interactions in transition metals

We discuss different methods of calculation of the screened Coulomb interaction $U$ in transition metals and compare the constraint local-density approximation (LDA) with the GW approach. We clarify that they offer complementary methods of treating the screening and should serve for different purposes. In the GW method, the renormalization of bare on-site Coulomb interactions between 3d electrons occurs mainly through the screening by the same 3d electrons, treated in the random phase approximation (RPA). The basic difference of the constraint-LDA method is that it deals with the neutral processes, where the Coulomb interactions are additionally screened by the ``excited'' electron, since it continues to stay in the system. This is the main channel of screening by the itinerant ($4sp$) electrons, which is especially strong in the case of transition metals and missing in the GW approach, although the details of this screening may be affected by additional approximations, which typically supplement these two methods. The major drawback of the conventional constraint-LDA method is that it does not allow to treat the energy-dependence of $U$. We propose a promising approximation based on the combination of these two methods. First, we take into account the screening of Coulomb interactions in the 3d-electron-line bands located near the Fermi level by the states from the subspace being orthogonal to these bands, using the constraint-LDA methods. The obtained interactions are further renormalized within the bands near the Fermi level in RPA. This allows the energy-dependent screening by electrons near the Fermi level including the same 3d electrons.Comment: 25 pages, 5 figures, 2 table

A revised checklist of Hawaiian mosses

A revised and updated literature-based checklist of Hawaiian mosses is presented. Geographic coverage includes the eight main Hawaiian Islands; the Northwestern Hawaiian Islands are excluded. The checklist is alphabetically ordered by scientific names; the family is noted for each genus. Synonyms and misapplied names are cross-referenced to the accepted names. A bibliography of supporting references is included

Superconductivity from Flat Dispersion Designed in Doped Mott Insulators

Routes to enhance superconducting instability are explored for doped Mott insulators. With the help of insights for criticalities of metal-insulator transitions, geometrical design of lattice structure is proposed to control the instability. A guideline is to explicitly make flat band dispersions near the Fermi level without suppressing two-particle channels. In a one-dimensional model, numerical studies show that our prescription with finite-ranged hoppings realizes large enhancement of spin-gap and pairing dominant regions. We also propose several multi-band systems, where the pairing is driven by intersite Coulomb repulsion.Comment: 4 pages, to be published in Phys. Rev. Let

Development and validation of the Wesleyan Intercultural Competence Scale (WICS): A tool for measuring the impact of study abroad experiences.

As globalization becomes commonplace and the world becomes increasingly interconnected, institutions of higher education have begun to prioritize the development of intercultural competence in their students. A recent review of university statements of essential learning outcomes revealed that 85% of top-ranked National Universities in the U.S. and 68% of top-ranked Liberal Arts colleges in the U.S. stated that the development of intercultural competence is one of their primary objectives (Stemler, 2012). For National Universities this was the most frequently cited objective across the entire sample, edging out other priorities such as writing, quantitative reasoning, and information literacy. The importance of developing the intercultural competence of students has been emphasized by private-sector businesses as well. A 2010 study commissioned by the Association of American Colleges and Universities found that 67% of the 302 employers interviewed felt that the ability to understand the global context of situations and decisions was one of the most important aims of higher education (AAC&U, 2010)

Fate of Quasiparticle at Mott Transition and Interplay with Lifshitz Transition Studied by Correlator Projection Method

Filling-control metal-insulator transition on the two-dimensional Hubbard model is investigated by using the correlator projection method, which takes into account momentum dependence of the free energy beyond the dynamical mean-field theory. The phase diagram of metals and Mott insulators is analyzed. Lifshitz transitions occur simultaneously with metal-insulator transitions at large Coulomb repulsion. On the other hand, they are separated each other for lower Coulomb repulsion, where the phase sandwiched by the Lifshitz and metal-insulator transitions appears to show violation of the Luttinger sum rule. Through the metal-insulator transition, quasiparticles retain nonzero renormalization factor and finite quasi-particle weight in the both sides of the transition. This supports that the metal-insulator transition is caused not by the vanishing renormalization factor but by the relative shift of the Fermi level into the Mott gap away from the quasiparticle band, in sharp contrast with the original dynamical mean-field theory. Charge compressibility diverges at the critical end point of the first-order Lifshitz transition at finite temperatures. The origin of the divergence is ascribed to singular momentum dependence of the quasiparticle dispersion.Comment: 24 pages including 10 figure

Insulator-Metal Transition in the One and Two-Dimensional Hubbard Models

We use Quantum Monte Carlo methods to determine $T=0$ Green functions, $G(\vec{r}, \omega)$, on lattices up to $16 \times 16$ for the 2D Hubbard model at $U/t =4$. For chemical potentials, $\mu$, within the Hubbard gap, $|\mu | < \mu_c$, and at {\it long} distances, $\vec{r}$, $G(\vec{r}, \omega = \mu) \sim e^{ -|\vec{r}|/\xi_l}$ with critical behavior: $\xi_l \sim | \mu - \mu_c |^{-\nu}$, $\nu = 0.26 \pm 0.05$. This result stands in agreement with the assumption of hyperscaling with correlation exponent $\nu = 1/4$ and dynamical exponent $z = 4$. In contrast, the generic band insulator as well as the metal-insulator transition in the 1D Hubbard model are characterized by $\nu = 1/2$ and $z = 2$.Comment: 9 pages (latex) and 5 postscript figures. Submitted for publication in Phys. Rev. Let

Quantum Transition between an Antiferromagnetic Mott Insulator and dx2−y2d_{x^2 - y^2} Superconductor in Two Dimensions

We consider a Hubbard model on a square lattice with an additional interaction, $W$, which depends upon the square of a near-neighbor hopping. At half-filling and a constant value of the Hubbard repulsion, increasing the strength of the interaction $W$ drives the system from an antiferromagnetic Mott insulator to a $d_{x^2 -y^2}$ superconductor. This conclusion is reached on the basis of zero temperature quantum Monte Carlo simulations on lattice sizes up to $16 \times 16$.Comment: 4 pages (latex) and 4 postscript figure

Absence of Translational Symmetry Breaking in Nonmagnetic Insulator Phase on Two-Dimensional Lattice with Geometrical Frustration

The ground-state properties of the two-dimensional Hubbard model with nearest-neighbor and next-nearest-neighbor hoppings at half filling are studied by the path-integral-renormalization-group method. The nonmagnetic-insulator phase sandwiched by the the paramagnetic-metal phase and the antiferromagnetic-insulator phase shows evidence against translational symmetry breaking of the dimerized state, plaquette singlet state, staggered flux state, and charge ordered state. These results support that the genuine Mott insulator which cannot be adiabatically continued to the band insulator is realized generically by Umklapp scattering through the effects of geometrical frustration and quantum fluctuation in the two-dimensional system.Comment: 4 pages and 7 figure

Magnetic and Metal-Insulator Transitions through Bandwidth Control in Two-Dimensional Hubbard Models with Nearest and Next-Nearest Neighbor Transfers

Numerical studies on Mott transitions caused by the control of the ratio between bandwidth and electron-electron interaction ($U$) are reported. By using the recently proposed path-integral renormalization group(PIRG) algorithm, physical properties near the transitions in the ground state of two-dimensional half-filled models with the nearest and the next-nearest neighbor transfers ($-t$ and $t'$, respectively) are studied as a prototype of geometrically frustrated system. The nature of the bandwidth-control transitions shows sharp contrast with that of the filling-control transitions: First, the metal-insulator and magnetic transitions are separated each other and the metal-insulator (MI) transition occurs at smaller $U$, although the both transition interactions $U$ increase with increasing $t'$. Both transitions do not contradict the first-order transitions for smaller $t'/t$ while the MI transitions become continuous type accompanied by emergence of {\it unusual metallic phase} near the transition for large $t'/t$. A nonmagnetic insulator phase is stabilized between MI and AF transitions. The region of the nonmagnetic insulator becomes wider with increasing $t'/t$. The phase diagram naturally connects two qualitatively different limits, namely the Hartree-Fock results at small $t'/t$ and speculations in the strong coupling Heisenberg limit.Comment: 30 pages including 20 figure