Inversion phenomenon and phase diagram of the S=1/2S=1/2 distorted diamond chain with the XXZXXZ interaction anisotropy

We discuss the anisotropies of the Hamiltonian and the wave-function in an $S=1/2$ distorted diamond chain. The ground-state phase diagram of this model is investigated using the degenerate perturbation theory up to the first order and the level spectroscopy analysis of the numerical diagonalization data. In some regions of the obtained phase diagram, the anisotropy of the Hamiltonian and that of the wave-function are inverted, which we call inversion phenomenon; the N\'{e}el phase appears for the XY-like anisotropy and the spin-fluid phase appears for the Ising-like anisotropy. Three key words are important for this nature, which are frustration, the trimer nature, and the $XXZ$ anisotropy.Comment: 4pages, 10 figures, for proceedings of SPQS 200

Microscopic Inhomogeneity and Superconducting Properties of a Two-dimensional Hubbard Model for High-TcT_c Cuprate

Recent scanning tunneling microscopy measurements on cuprate superconductors have revealed remarkable spatial inhomogeneities in the single-particle energy gap. Using cellular dynamical mean-field theory, we study the zero temperature superconducting properties of a single-band Hubbard model with a spatial modulation of the electron density. We find that the inhomogeneity in the electronic structure results in a substantial spatial variation in the superconducting order parameter and single-particle energy gap, reminiscent of the experimental results. In particular, we find that the order parameter and gap amplitudes in the hole-rich regions are significantly enhanced over the corresponding quantities in a uniform system, if the hole-rich regions are embedded in regions with smaller hole density.Comment: 6 pages, 4 figures, and 2 table

Enhanced Superconductivity in Superlattices of High-TcT_c Cuprates

The electronic properties of multilayers of strongly correlated models for cuprate superconductors are investigated using cluster dynamical mean-field techniques. We focus on combinations of underdoped and overdoped layers and find that the superconducting order parameter in the overdoped layers is enhanced by the proximity effect of the strong pairing scale originating from the underdoped layers. The enhanced order parameter can even exceed the maximum value in uniform systems. This behavior is well reproduced in slave-boson mean-field calculations which also find higher transition temperatures than in the uniform system.Comment: 4 pages, 4 figure

Metal-Insulator Transition and Spin Degree of Freedom in Silicon 2D Electron Systems

Magnetotransport in 2DES's formed in Si-MOSFET's and Si/SiGe quantum wells at low temperatures is reported. Metallic temperature dependence of resistivity is observed for the n-Si/SiGe sample even in a parallel magnetic field of 9T, where the spins of electrons are expected to be polarized completely. Correlation between the spin polarization and minima in the diagonal resistivity observed by rotating the samples for various total strength of the magnetic field is also investigated.Comment: 3 pages, RevTeX, 4 eps-figures, conference paper (EP2DS-13

Painleve equations from Darboux chains - Part 1: P3-P5

We show that the Painleve equations P3-P5 can be derived (in a unified way) from a periodic sequence of Darboux transformations for a Schrodinger problem with quadratic eigenvalue dependency. The general problem naturally divides into three different branches, each described by an infinite chain of equations. The Painleve equations are obtained by closing the chain periodically at the lowest nontrivial level(s). The chains provide ``symmetric forms'' for the Painleve equations, from which Hirota bilinear forms and Lax pairs are derived. In this paper (Part 1) we analyze in detail the cases P3-P5, while P6 will be studied in Part 2.Comment: 23 pages, 1 reference added + minor change

Irregular conformal blocks, with an application to the fifth and fourth Painlev\'e equations

We develop the theory of irregular conformal blocks of the Virasoro algebra. In previous studies, expansions of irregular conformal blocks at regular singular points were obtained as degeneration limits of regular conformal blocks; however, such expansions at irregular singular points were not clearly understood. This is because precise definitions of irregular vertex operators had not been provided previously. In this paper, we present precise definitions of irregular vertex operators of two types and we prove that one of our vertex operators exists uniquely. Then, we define irregular conformal blocks with at most two irregular singular points as expectation values of given irregular vertex operators. Our definitions provide an understanding of expansions of irregular conformal blocks and enable us to obtain expansions at irregular singular points. As an application, we propose conjectural formulas of series expansions of the tau functions of the fifth and fourth Painlev\'e equations, using expansions of irregular conformal blocks at an irregular singular point.Comment: 26 page

Spontaneous fourfold-symmetry breaking driven by electron-lattice coupling and strong correlations in high-TcT_c cuprates

Using dynamical-mean-field theory for clusters, we study the two-dimensional Hubbard model in which electrons are coupled with the orthorhombic lattice distortions through the modulation in the hopping matrix. Instability towards spontaneous symmetry breaking from a tetragonal symmetric phase to an orthorhombic distorted phase is examined as a function of doping and interaction strength. A very strong instability is found in the underdoped pseudogap regime when the interaction strength is large enough to yield the Mott insulating phase at half filling. The symmetry breaking accompanies the recovery of quasiparticle weights along one of the two antinodal directions, leading to the characteristic Fermi arc reconnection. We discuss the implications of our results to the fourfold symmetry breaking reported in systems where the underlying crystal does not have any structural anisotropy.Comment: 6 pages with 4 figure