845 research outputs found
New Solutions of the T-Matrix Theory of the Attractive Hubbard Model
This short paper summarizes a calculational method for obtaining the
dynamical properties of many-body theories formulated in terms of
(unrenormalized) bare propagators (and more generally, in terms of meromorphic
functions, or convolutions over meromorphic functions) to a very high accuracy.
We demonstrate the method by applying it to a T-matrix theory of the attractive
Hubbard model in two dimensions. We expand the pair propagator using a partial
fraction decomposition, and then solve for the residues and pole locations of
such a decomposition using a computer algebra system to an arbitrarily high
accuracy (we used MapleV and obtained these quantities to a relative error of
10^(-80)). Thus, this method allows us to bypass all inaccuracies associated
with the traditional analytical continuation problem. Our results for the
density of states make clear the pronounced development of a pseudogap as the
temperature is lowered in this formulation of the attractive Hubbard model.Comment: 2 pages, 2 figure
Two mechanisms of pseudogap formation in Bi-2201: Evidence from the c-axis magnetoresistance
Measurements of the c-axis resistivity and magnetoresistance have been used
to investigate the pseudogap (PG) behavior in Bi_{2+z}Sr_{2-x-z}La_xCuO_y
(Bi-2201) crystals at various hole densities. While the PG opening temperature
T* increases with decreasing hole doping, the magnetic-field sensitivity of the
PG is found to have a very different trend: it appears at lower temperatures in
more underdoped samples and vanishes in non-superconducting samples. These data
suggest that besides the field-insensitive pseudogap emerging at T*, a distinct
one is formed above T_c as a precursor to superconductivity.Comment: 7 pages, 6 figures, accepted for publication in Europhysics Letters
(initially submitted to PRL on 14 June 2000
Magnetron Sputter deposition of a 48-member cuprate superconductor library: Bi2Sr2YxCa1-xCu2Oy (0.5 <= x <= 1) linearly varying in steps of 0.01
Using magnetron sputtering, a spatial composition spread approach was applied
successfully to obtain 48-member libraries of the Bi2Sr2YxCa1-xCu2Oy (0.5<= x
<=1)cuprate superconducting system. The libraries of each system were deposited
onto (100) single crystal MgO, mounted on a water cooled rotating table, using
two targets: the antiferromagnetic insulator Bi2Sr2YCu2Oy (P=98 W RF) and the
hole doped superconductor Bi2Sr2CaCu2Oy (P=44 W DC). A low chamber pressure of
0.81 mTorr argon is used to reduce scattering by the process gas. To minimize
oxygen resputtering a substrate bias of -20 V was used as well as a process gas
free of oxygen. A rapid thermal processor is used to post-anneal the amorphous
deposited films. A step annealing regime was used, with a ramp rate of 5
degrees C/s for heating and cooling, with a first plateau at 780 C held for 200
s, and a second at 875 C held for 480 s. X-ray diffraction reveals that the
films develop crystalline order with the c-axis lattice parameter contracting
linearly from 30.55 Angstroms (x=0.5) to 30.24 Angstroms (x=1.0) with
increasing Y-content, consistent with bulk values. The crystallized films are
polycrystalline, developing preferred orientation (c-axis parallel to the
substrate) for thinner members of the library. There is a change of 0.01 in
doping per library member which will enable further studies to densely map
phase space.Comment: 4 pages, 6 figures, submitted Jan. 31, 2007: Applied Surface Science
- Proceedings of the 4th International Workshop on Combinatorial Materials
Science & Technology, San Juan, Puerto Ric
Crossover from a pseudogap state to a superconducting state
On the basis of our calculation we deduce that the particular electronic
structure of cuprate superconductors confines Cooper pairs to be firstly formed
in the antinodal region which is far from the Fermi surface, and these pairs
are incoherent and result in the pseudogap state. With the change of doping or
temperature, some pairs are formed in the nodal region which locates the Fermi
surface, and these pairs are coherent and lead to superconductivity. Thus the
coexistence of the pseudogap and the superconducting gap is explained when the
two kinds of gaps are not all on the Fermi surface. It is also shown that the
symmetry of the pseudogap and the superconducting gap are determined by the
electronic structure, and non-s wave symmetry gap favors the high-temperature
superconductivity. Why the high-temperature superconductivity occurs in the
metal region near the Mott metal-insulator transition is also explained.Comment: 7 pages, 2 figure
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