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Incoherent superconductivity well above in high- cuprates—harmonizing the spectroscopic and thermodynamic data

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Incoherent superconductivity well above in high- cuprates—harmonizing the spectroscopic and thermodynamic data
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Cuprate superconductors have long been known to exhibit an energy gap that persists high above the superconducting transition temperature (). Debate has continued now for decades as to whether it is a precursor superconducting gap or a pseudogap arising from some competing correlation. Failure to resolve this has arguably delayed explaining the origins of superconductivity in these highly complex materials. Here we effectively settle the question by calculating a variety of thermodynamic and spectroscopic properties, exploring the effect of a temperature-dependent pair-breaking term in the self-energy in the presence of pairing interactions that persist well above . We start by fitting the detailed temperature-dependence of the electronic specific heat and immediately can explain its hitherto puzzling field dependence. Taking this same combination of pairing temperature and pair-breaking scattering we are then able to simultaneously describe in detail the unusual temperature and field dependence of the superfluid density, tunneling, Raman and optical spectra, which otherwise defy explanation in terms a superconducting gap that closes conventionally at . These findings demonstrate that the gap above in the overdoped regime likely originates from incoherent superconducting correlations, and is distinct from the competing-order 'pseudogap' that appears at lower doping.
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Transcript: English(auto-generated)
Hi, I'm James Storey from the Robinson Research Institute at Victoria University of Wellington here in New Zealand, and today I'd like to introduce you to my paper, Incoherent Superconductivity Well Above TC in High-TC Coupe Rates, Harmonising the Spectroscopic
and Thermodynamic Data. This work was supported by the Marsden Fund. This paper addresses a decades-long dispute concerning the origin of a partial energy gap in copper oxide-based high-temperature superconductors.
This energy gap is known as the pseudogap. Typically, when a material is cooled below its superconducting transition temperature, or TC, an energy gap opens, and we call this gap the superconducting gap. The pseudogap, on the other hand, is unusual because it extends far above the superconducting
transition temperature. We study the pseudogap to determine its relevance to high-temperature superconductivity. Roughly speaking, there are two possibilities which can be distinguished by their temperature-doping
phase diagrams. The first is that the pseudogap is a precursor to the superconducting gap, and arises from phase-incoherent superconducting correlations. Here, the boundary of the pseudogap phase merges smoothly with the superconducting TC dome.
The alternative picture is that the pseudogap is unrelated to the superconducting gap, and coexists and competes with superconductivity. In this case, the pseudogap phase opens below a critical doping point inside the superconducting dome.
The problem we have is that there is considerable experimental support for both phase diagrams. To sort this out, I modelled a range of physical properties focusing on the slightly overdoped region near TC.
Here, any competing order gap should be absent. I found that a superconducting gap extending above TC, combined with a steep increase in pair-breaking scattering, simultaneously explains the unusual temperature dependencies observed
by five different experimental techniques. These have long defied explanation in terms of conventional mean field theories. Incorporating this result with existing evidence for a competing order gap at lower dopings, I conclude that the phase diagram is a blend of the two leading proposals, with both precursor
pairing and competing order elements. But note, at high dopings, the gap above TC originates from the superconducting gap. The superconducting transition seems to be governed by a strong increase in pair-breaking
scattering near TC. This is fundamentally different to what happens in conventional low-temperature superconductors, where the superconducting gap closes at TC and scattering is small. Strongly temperature-dependent scattering is expected to occur when superconductivity
is coupled to spin fluctuations, hinting that this might be the cause of superconductivity in these materials. I hope to look into this further in future work. Thanks for watching.