Electron correlation and Fermi surface topology of Nax_xCoO2_2

The electronic structure of Na$_x$CoO$_2$ revealed by recent photoemission experiments shows important deviations from band theory predictions. The six small Fermi surface pockets predicted by LDA calculations have not been observed as the associated $e_g^\prime$ band fails to cross the Fermi level for a wide range of sodium doping concentration $x$. In addition, significant bandwidth renormalizations of the $t_{2g}$ complex have been observed. We show that these discrepancies are due to strong electronic correlations by studying the multi-orbital Hubbard model in the Hartree-Fock and strong-coupling Gutzwiller approximation. The quasiparticle dispersion and the Fermi surface topology obtained in the presence of strong local Coulomb repulsion are in good agreement with experiments.Comment: 5 pages, 4 figures, revtex4; minor changes, to be published in Phys. Rev. Let

Coexistence of competing orders with two energy gaps in real and momentum space in high-Tc superconductor Bi2Sr2-xLaxCuO6+delta

The superconducting phase of the high-Tc cuprates has been thought to be described by a single d-wave pairing order parameter. Recently, there has been growing evidence suggesting that another form of order, possibly inherited from the pseudogap phase above Tc, may coexist with superconductivity in the underdoped regime. Through a combined study of scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we report the observation of two distinct gaps (a small-gap and a large-gap) that coexist both in real space and in the anti-nodal region of momentum space in the superconducting phase of Bi2Sr2-xLaxCuO6+delta. We show that the small-gap is associated with superconductivity. The large-gap persists to temperatures above the transition temperature Tc and is found to be linked to short-range charge ordering. Remarkably, we find a strong, short-ranged correlation between the local small- and large- gap magnitudes suggesting that the superconductivity and charge ordering are affected by similar physical processes.Comment: 19 pages, 4 figure

Inhomogeneous d-wave superconducting state of a doped Mott insulator

Recent scanning tunneling microscope (STM) measurements discovered remarkable electronic inhomogeneity, i.e. nano-scale spatial variations of the local density of states (LDOS) and the superconducting energy gap, in the high-Tc superconductor BSCCO. Based on the experimental findings we conjectured that the inhomogeneity arises from variations in local oxygen doping level and may be generic of doped Mott insulators which behave rather unconventionally in screening the dopant ionic potentials at atomic scales comparable to the short coherence length. Here, we provide theoretical support for this picture. We study a doped Mott insulator within a generalized t-J model, where doping is accompanied by ionic Coulomb potentials centered in the BiO plane. We calculate the LDOS spectrum, the integrated LDOS, and the local superconducting gap, make detailed comparisons to experiments, and find remarkable agreement with the experimental data. We emphasize the unconventional screening in a doped Mott insulator and show that nonlinear screening dominates at nano-meter scales which is the origin of the electronic inhomogeneity. It leads to strong inhomogeneous redistribution of the local hole density and promotes the notion of a local doping concentration. We find that the inhomogeneity structure manifests itself at all energy scales in the STM tunneling differential conductance, and elucidate the similarity and the differences between the data obtained in the constant tunneling current mode and the same data normalized to reflect constant tip-to-sample distance. We also discuss the underdoped case where nonlinear screening of the ionic potential turns the spatial electronic structure into a percolative mixture of patches with smaller pairing gaps embedded in a background with larger gaps to single particle excitations.Comment: 19 pages, final versio

A Dual-Band Dual-Antenna System with Common-Metal Rim for Smartphone Applications

A dual-antenna system operating at WIFI and GPS bands is proposed for common-metal rimmed smartphones applications. This dual-antenna system, which is horizontally placed on a ground plane of 4.5 × 75 mm2, consists of two folded inverted-F antennas (IFAs) sharing the same metal rim. Each IFA contains part of the metal radiating arm, and both IFAs own approximately one-quarter free space wavelengths at 2.44 GHz. A matching network is embedded in the feeding line of the left IFA to provide a resonant frequency at 1.575 GHz. By adjusting the positions of the shorting branch and feeding line, good impedance matching is obtained. Two gaps in the metal frame and a center shorting branch between two IFAs are adopted to improve the isolation. The isolations of better than 22 dB and 19 dB in GPS and WIFI bands are attained, respectively. The proposed antenna is fabricated, and the measured results regarding S-parameters, radiation efficiency, gain, as well as diversity performances are presented

Quantitative Characteristics of the Current Multi-Source Precipitation Products over Zhejiang Province, in Summer, 2019

Precipitation data with fine quality plays vital roles in hydrological-related applications. In this study, we choose the high-quality China Merged Precipitation Analysis data (CMPA) as the benchmark for evaluating four satellite-based precipitation products (PERSIANN-CCS, FY4A QPE, GSMap_Gauge, IMERG-Final) and one model-based precipitation product (ERA5-Land), respectively, at 0.1°, hourly scales over the Zhejiang province, China, in summer, from June to August 2019. The main conclusions were as follows—(1) all other products demonstrate similar patterns with CMPA (~325.60 mm/h, std ~0.07 mm/h), except FY4A QPE (~281.79 mm/h, std ~0.18 mm/h), while, overall, the PERSIANN-CCS underestimates the precipitation against CMPA with a mean value around 236.29 mm/h (std ~0.06 mm/h), and the ERA5-Land, GSMap_Guage, and IMERG-Final generally overestimate the precipitation with a mean value around 370.00 mm/h (std ~0.06 mm/h). (2) The GSMap_Gauge outperforms IMERG-Final against CMPA with CC ~0.50 and RMSE ~1.51 mm/h, and CC ~0.48 and RMSE ~1.64 mm/h, respectively. (3) The PERSIANN-CCS significantly underestimates the precipitation (CC ~0.26, bias ~−35.03%, RMSE ~1.81 mm/h, probability of detection, POD, ~0.33, false alarm ratio, FAR, ~0.47), potentially due to its weak abilities to capture precipitation events and estimate the precipitation. (4) Though ERA5-Land has the best ability to capture precipitation events (POD ~0.78), the largest misjudgments (FAR ~0.54) result in its great uncertainties with CC ~ 0.39, which performs worse than those of GSMap_Gauge and IMERG-Final. (5) The ranking of precipitation products, in terms of the general evaluation metrics, over Zhejiang province is GSMap_Gauge, IMERG-Final, ERA5-Land, PERSIANN-CCS, and FY4A QPE, which provides valuable recommendations for applying these products in various related application fields