Spectral Weight Changes At The Superconducting Transition Of Bi Sub 2 Sr Sub 2 Cacu Sub 2 O Sub 8 Delta
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Total Pages | : 11 |
Release | : 1991 |
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Download Spectral Weight Changes at the Superconducting Transition of Bi Sub 2 Sr Sub 2 CaCu Sub 2 O Sub 8+. Delta Book in PDF, Epub and Kindle
An overview of our gap studies in high-{Tc} superconductors is presented for the workshop on Fermiology of high-{Tc}'s. The work is centered on the study of single crystal Bi2Sr2CaCu2O{sub 8+{delta}}. In a conventional BCS superconductor, a superconducting gap [Delta] is formed when the near Fermi edge electrons condense to form Cooper pairs at low temperatures. As the material goes superconducting the density of states is modified such that the spectral intensity in the region from the Fermi energy down to an energy [Delta] is transferred to a regions just below [Delta]. While this spectral weight transfer has in the past been studied with tunneling spectroscopy, the size of the gap as well as improvements in our instrument resolution allow us now to study it with photoelectron spectroscopy. We have found that as the sample goes superconducting, not only is there spectral weight transfer from the gap region as in BCS theory, but along the [Gamma]-M direction there is also some spectral weight transfer from higher binding energies resulting in a dspectral dip at about -90 meV relative E{sub F}. The total spectral weight decreases along the [Gamma]-M direction, but actually increases along the [Gamma]-X direction. This temperature dependent spectral transfer is discussed in terms of (1) a two to three dimensional phase transition from RVB theory; (2) a manifestation of the electron-boson interaction in the form of [alpha]2F oscillations; and (3) conformity with the theory of Van Hove singularities. The latter are particularly attractive in that there are several other observations possibly explained by them: (1) the observation that the magnitude of the gap is anisotropic in the a-b plane; (2) the observation that for overdoped samples the magnitude of D appears to fall off faster then {Tc}. 25 refs., 8 figs., 1 tab.
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Release | : 2001 |
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Download Tunneling Spectra and Superconducting Gap in Bi[sub 2]r[sub 2]CaCu[sub 2]O[sub 8+delta] and Tl[sub 2]Ba[sub 2]CuO[sub 6+delta]. Book in PDF, Epub and Kindle
Tunneling spectra are reported for Bi[sub 2]Sr[sub 2]CaCu[sub 2] ([sub 8+[delta]]) (Bi-2212) over a wide doping range using superconductor-insulator-superconductor (SIS) break junctions. The energy gap inferred from the tunneling data displays a remarkable monotonic dependence on doping, increasing to very large values in the underdoped region even as T[sub c] decreases. This leads to unphysically large values of the strong coupling ratio ([approximately]20). The tunneling spectra are qualitatively similar over the entire doping range even though the gap parameter, [Delta], changes from 12 meV to 80 meV. Each spectrum exhibits dip and hump features at high bias with characteristic energies that scale with the superconducting gap. Tunneling spectra of near optimally-doped Tl[sub 2]Ba[sub 2]CuO[sub 6+[delta]] (Tl-2201) also display a weak dip feature in superconductor-insulator-normal metal (SIN) junctions. Generated SIS spectra of Tl-2201 are compared with measured spectra on Bi-2212 and it is concluded that the dip and hump features are generic to high temperature superconductors.
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Total Pages | : 1730 |
Release | : 1992 |
Genre | : Government publications |
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Download Government Reports Annual Index Book in PDF, Epub and Kindle
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Release | : 1956 |
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Download Mary Alyce Beatty Book in PDF, Epub and Kindle
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Total Pages | : 836 |
Release | : 1992 |
Genre | : Science |
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Download Government Reports Announcements & Index Book in PDF, Epub and Kindle
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Release | : 2001 |
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Download Evolution of Magnetic and Superconducting Fluctuations with Doping of High-T[sub C] Superconductors Book in PDF, Epub and Kindle
For YBa[sub 2]Cu[sub 3]O[sub 6+[delta]] and Bi[sub 2]Sr[sub 2]CaCu[sub 2]O[sub 3[+-][delta]] superconductors, electronic Raman scattering from high- and low-energy excitations has been studied in relation to the hole doping level, temperature, and energy of the incident photons. For underdoped superconductors, it is concluded that short range antiferromagnetic (AF) correlations persist with hole doping and doped single holes are incoherent in the AF environment. Above the superconducting (SC) transition temperature T[sub c] the system exhibits a sharp Raman resonance of B[sub 1g] symmetry and about 75 meV energy and a pseudogap for electron-hole excitations below 75 meV, a manifestation of a partially coherent state forming from doped incoherent quasi-particles. The occupancy of the coherent state increases with cooling until phase ordering at T[sub c] produces a global SC state.
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Release | : 2001 |
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Download Bi[sub 2]r[sub 2]CaCu[sub 2]O[sub 8+delta] C-axis Twist Josephson Junctions Book in PDF, Epub and Kindle
Li et al. found that the critical current density J[sub c][sup J] across atomically clean c-axis twist junctions of Bi[sub 2]Sr[sub 2]CaCu[sub 2]O[sub 8+[delta]] is the same as that of the constituent single crystal, J[sub c][sup S], independent of the twist angle[phi][sub 0], even at and below T[sub c]. They investigated theoretically if a d[sub x[sup 2][minus]y[sup 2]] wave order parameter might twist by mixing in d[sub xy]-wave components, but found that such mixing cannot possibly explain the data near to T[sub c]. Combined with group theoretical arguments, they then conclude that the order parameter contains at least a substantial s-wave component, but does not contain any purported d[sub x[sup 2][minus]y[sup 2]]-wave component, except possibly below a second, unobserved phase transition. By studying tunneling models, they further conclude that the intrinsic c-axis Josephson tunneling in Bi[sub 2]Sr[sub 2]CaCu[sub 2]O[sub 8+[delta]] is likely to be mostly incoherent.
Author | : Carl Thomas Simmons |
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Total Pages | : 70 |
Release | : 1993 |
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Download High resolution photoemission study of the superconducting state of Bi 2 Sr 2 CaCu 2 0 8 + Delta Book in PDF, Epub and Kindle
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Total Pages | : 71 |
Release | : 1990 |
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Download High-resolution Angle-resolved Photoemission Studies of High Tc Superconductor Bi Sub 2 Sr Sub 2 CaCu Sub 2 O Sub 8 Book in PDF, Epub and Kindle
An angle-resolved photoemission study of the normal and superconducting states in Bi2Sr2CaCu2O was performed. Measurements in the normal state show bands dispersing through the Fermi level from at least 350 meV below E{sub F}. The Fermi level crossings are consistant with local-density band calculation, including a point calculated to be of Bi-O character. Additional measurements were made where bands crossed the Fermi level between 100 and 250K, along with measurements on an adjacent Pt foil. The Fermi edges of both materials agree to within the noise. Below the Fermi level, the spectra show correlation effects on the form of an increased effective mass. The shape of the spectra can be explained by a lifetime-broadened photohole and secondary electrons. The effective inverse photohole lifetime is linear in energy. A superconducting gap has been measured at a number of points where there is density at the Fermi level in the normal state. By proper modeling, a gap of 24 meV was obtained for all these points, including points of Cu-O and Bi-O character respectively, according to band calculation. The lack of gap anisotropy in the basal plane suggests that pinning in this material is not d-wave pairing.
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Total Pages | : 446 |
Release | : 1991 |
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Download Bibliography of Lewis Research Center Technical Publications Announced in 1990 Book in PDF, Epub and Kindle