Pairing in cuprates from high-energy electronic states

Abstract

The in-plane optical conductivity of Bi 2 Sr 2 CaCu 2 O 8+δ thin films with small carrier density (underdoped) up to large carrier density (overdoped) is analyzed with unprecedented accuracy. Integrating the conductivity up to increasingly higher energies points to the energy scale involved when the superfluid condensate builds up. In the underdoped sample, states extending up to 2 eV contribute to the superfluid. This anomalously large energy scale may be assigned to a change of in-plane kinetic energy at the superconducting transition, and is compatible with an electronic pairing mechanism. 74.25.Gz, 74.72.Hs Typeset using REVT E X

On the break in the single-particle energy dispersions and the ‘universal’ nodal Fermi velocity in the high-temperature copper oxide superconductors

“…We thus observe that the interpretation as put forward in the present paper of the experimental observations in [3][4][5][6][7][8] also provides a natural explanation concerning the experimental observations of the excess 'kinetic' energy in the normal states of the doped cuprate compounds [11,12] in comparison with the 'kinetic' energies of the GSs of conventional metals; in arriving at the latter statement we have relied on the supposition that the gapless excitations along the nodal directions of the 1BZ (below the pertinent superconducting transition temperatures) are akin to the gapless excitations of the normal states of the investigated cuprate compounds. Experimental observations, at temperatures elevated above the pertinent superconducting transition temperatures, along various directions of the 1BZ [5] (concerning change in the spectral widths see [8]), are in conformity with the abovementioned supposition.…”

“…11 11 With reference to our remark in footnote 9, we note that for cases where S − σ (ε) has the more general form as presented…”

Optical sum increase due to electron undressing

“…A natural interpretation of the sum rule experiments [1,2,4,8] is to infer a novel mechanism for superconductivity that is accompanied by a kinetic energy decrease (absolute value), in contrast to the usual BCS case. Here we have adopted an approach which in this sense is conventional; the deviation from the usual BCS case instead arises because of a change in the boson characteristics at and below the superconducting transition.…”

Concepts in High Temperature Superconductivity

“…Pairing is suppressed for t ′ < 0 and enhanced for t ′ > 0. 28 It is not clear whether the complete elimination of stripes or only a slight destabilization is more favorable to pairing correlations. Fig.…”

“…Nor is it clear what such No smoking gun an experiment would be. There are ways to falsify the conjecture that stripes produce pairing, such as a demonstration that stripes are not in some sense 55 The brilliantly conceived high precision measurements of the optical conductivity of van der Marel and collaborators [27], and more recently by Bontemps and collaborators [28], are highly suggestive in this regard. In optimally doped BSCCO, they observe a strongly temperature dependent change in the optical spectral weight integrated up to frequencies two orders of magnitude greater than Tcif interpreted in terms of the single band sum rule, this observation implies a decrease in the kinetic energy upon entering the superconducting state of a magnitude comparable to reasonable estimates of the condensation energy.…”