136,823 research outputs found
On the Reconstructed Fermi Surface in the Underdoped Cuprates
The Fermi surface topologies of underdoped samples the high-Tc superconductor
Bi2212 have been measured with angle resolved photoemission. By examining
thermally excited states above the Fermi level, we show that the Fermi surfaces
in the pseudogap phase of underdoped samples are actually composed of fully
enclosed hole pockets. The spectral weight of these pockets is vanishingly
small at the anti-ferromagnetic zone boundary, which creates the illusion of
Fermi "arcs" in standard photoemission measurements. The area of the pockets as
measured in this study is consistent with the doping level, and hence carrier
density, of the samples measured. Furthermore, the shape and area of the
pockets is well reproduced by a phenomenological model of the pseudogap phase
as a spin liquid.Comment: 4 pages, 4 figures. Submitted to Physics Review Letter
Fine Details of the Nodal Electronic Excitations in BiSrCaCuO
Very high energy resolution photoemission experiments on high quality samples
of optimally doped BiSrCaCuO show new features in the
low-energy electronic excitations. A marked change in the binding energy and
temperature dependence of the near-nodal scattering rates is observed near the
superconducting transition temperature, . The temperature slope of the
scattering rate measured at low energy shows a discontinuity at ~. In the
superconducting state, coherent excitations are found with the scattering rates
showing a cubic dependence on frequency and temperature. The superconducting
gap has a d-wave magnitude with negligible contribution from higher harmonics.
Further, the bi-layer splitting has been found to be finite at the nodal point.Comment: 5 pages, 4 figure
Matching Subsequences in Trees
Given two rooted, labeled trees and the tree path subsequence problem
is to determine which paths in are subsequences of which paths in . Here
a path begins at the root and ends at a leaf. In this paper we propose this
problem as a useful query primitive for XML data, and provide new algorithms
improving the previously best known time and space bounds.Comment: Minor correction of typos, et
4D STEM: high efficiency phase contrast imaging using a fast pixelated detector
Phase contrast imaging is widely used for imaging beam sensitive and weak phase objects in electron microscopy. In this work we demonstrate the achievement of high efficient phase contrast imaging in STEM using the pnCCD, a fast direct electron pixelated detector, which records the diffraction patterns at every probe position with a speed of 1000 to 4000 frames per second, forming a 4D STEM dataset simultaneously with the incoherent Z-contrast imaging. Ptychographic phase reconstruction has been applied and the obtained complex transmission function reveals the phase of the specimen. The results using GaN and Ti, Nd- doped BiFeO3 show that this imaging mode is especially powerful for imaging light elements in the presence of much heavier elements
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Lead halide perovskite nanowires stabilized by block copolymers for Langmuir-Blodgett assembly
The rapid development of solar cells based on lead halide perovskites (LHPs) has prompted very active research activities in other closely-related fields. Colloidal nanostructures of such materials display superior optoelectronic properties. Especially, one-dimensional (1D) LHPs nanowires show anisotropic optical properties when they are highly oriented. However, the ionic nature makes them very sensitive to external environment, limiting their large scale practical applications. Here, we introduce an amphiphilic block copolymer, polystyrene-block-poly(4-vinylpyridine) (PS-P4VP), to chemically modify the surface of colloidal CsPbBr3 nanowires. The resulting core-shell nanowires show enhanced photoluminescent emission and good colloidal stability against water. Taking advantage of the stability enhancement, we further applied a modified Langmuir-Blodgett technique to assemble monolayers of highly aligned nanowires, and studied their anisotropic optical properties. [Figure not available: see fulltext.]
Production status of GaAs/Ge solar cells and panels
GaAs/Ge solar cells with lot average efficiencies in excess of 18 percent were produced by MOCVD growth techniques. A description of the cell, its performance and the production facility are discussed. Production GaAs/Ge cells of this type were recently assembled into circuits and bonded to aluminum honeycomb panels to be used as the solar array for the British UOSAT-F program
An experimental study on (2) modular symmetry in the quantum Hall system with a small spin-splitting
Magnetic-field-induced phase transitions were studied with a two-dimensional
electron AlGaAs/GaAs system. The temperature-driven flow diagram shows the
features of the (2) modular symmetry, which includes distorted
flowlines and shiftted critical point. The deviation of the critical
conductivities is attributed to a small but resolved spin splitting, which
reduces the symmetry in Landau quantization. [B. P. Dolan, Phys. Rev. B 62,
10278.] Universal scaling is found under the reduction of the modular symmetry.
It is also shown that the Hall conductivity could still be governed by the
scaling law when the semicircle law and the scaling on the longitudinal
conductivity are invalid. *corresponding author:[email protected]: The revised manuscript has been published in J. Phys.: Condens.
Matte
Exactly solvable extended Hubbard model
In this work, we introduce long range version of the extended Hubbard model.
The system is defined on a non-uniform lattice. We show that the system is
integrable. The ground state, the ground state energies, the energy spectrum
are also found for the system. Another long range version of the extended
Hubbard model is also introduced on a uniform lattice, and this system is
proven to be integrable.Comment: 10 pages, Latex. Typoes are fixed in this revised versio
Quantum Thermalization With Couplings
We study the role of the system-bath coupling for the generalized canonical
thermalization [S. Popescu, et al., Nature Physics 2,754(2006) and S. Goldstein
et al., Phys. Rev. Lett. 96, 050403(2006)] that reduces almost all the pure
states of the "universe" [formed by a system S plus its surrounding heat bath
] to a canonical equilibrium state of S. We present an exactly solvable, but
universal model for this kinematic thermalization with an explicit
consideration about the energy shell deformation due to the interaction between
S and B. By calculating the state numbers of the "universe" and its subsystems
S and B in various deformed energy shells, it is found that, for the
overwhelming majority of the "universe" states (they are entangled at least),
the diagonal canonical typicality remains robust with respect to finite
interactions between S and B. Particularly, the kinematic decoherence is
utilized here to account for the vanishing of the off-diagonal elements of the
reduced density matrix of S. It is pointed out that the non-vanishing
off-diagonal elements due to the finiteness of bath and the stronger
system-bath interaction might offer more novelties of the quantum
thermalization.Comment: 4 pages, 2 figure
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