Room temperature superconducting technology has been an aspiration of physicists and engineers for many decades. In conventional BCS superconductivity, the Landau quasiparticles of a metal form Cooper pairs with binding energy D. For such superconductors, if D >100meV the superconducting critical temperature Tc would be well above room temperature. This could revolutionize technologies of transport, power transmission, particle acceleration, computing and information technology, energy conservation and much more. Our best hopes for achieving this goal have been the hole-doped copper-oxide Mott insulators (MI) because they exhibit the highest Tc of any known material (165K). But the Tc of copper-oxides falls to zero as D increases to exceed 100meV with decreasing hole-density p - a frustrating disappointment and a profound mystery.
During 2007 we have made several advances towards understanding the explanation. The key point is that when a copper-oxide MI is converted into a high temperature superconductor, the MI states localized in real-space must evolve into momentum-space eigenstates. The unique capability of quasiparticle interference (QPI) imaging to determine the electronic structure simultaneously in r-space and k-space makes it ideal for studying such effects. We use superconducting QPI , to study them as p_0 in Bi2Sr2CaCu2O8+d. We find that the Bogoliubov quasiparticle states are confined to a k-space ‘Bogoliubov Arc’ that shrinks rapidly with p. The end points of the arc lie near the diagonal lines connecting k=(0,±_p/a0) and k=(±p/a0,0). The quasiparticle interferences disappear at a doping-dependent ‘extinction’ energy E=D0(p). Remarkably, for E>D0 the electronic excitations evolve from k-space quasiparticles into to r-space quasi-localized states where the translational and rotational symmetries are broken locally. From tunneling asymmetry contrast, we show that that the r-space character of these states is most pronounce at the ‘pseudogap’ energy. The extinction of the very k-space states required for Cooper pairing with the coincident transfer of spectral weight to r-space, may represent the Achilles’ heel of copper-oxide high-Tc superconductivity.
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