Signature of Superfluid Density in the Single-Particle Excitation Spectrum of Bi ₂ Sr ₂ CaCu ₂ O _8+δ

Abstract

We report that the doping and temperature dependence of photoemission spectra near the Brillouin zone boundary of Bi 2 Sr 2 CaCu 2 O 8+δ δ exhibit unexpected sensitivity to the superfluid density. In the superconducting state, the photoemission peak intensity as a function of doping scales with the superfluid density and the condensation energy. As a function of temperature, the peak intensity shows an abrupt behavior near the superconducting phase transition temperature where phase coherence sets in, rather than near the temperature where the gap opens. This anomalous manifestation of collective effects in single-particle spectroscopy raises important questions concerning the mechanism of high-temperature superconductivity.The collective nature of superconductivity manifests itself contrastingly in different techniques. Microwave and muon spin relaxation measurements are inherently sensitive to the collective motion of the condensate, whereas single-electron tunneling spectroscopy and photoemission mainly probe single-particle excitations of the condensate. Hence, these two types of spectroscopies can be used to measure two essential but distinct ingredients of superconductivity: the superfluid density, which characterizes the phase coherence of the Cooper pairs, and the superconducting energy gap, which reflects the strength of the pairing. We report a pronounced departure from

Electronic structure of cuprate superconductors in a full charge-spin recombination scheme

“…However, one of the most striking dilemmas is that the SC coherence of the low-energy quasiparticle excitations in cuprate superconductors seems to be described by the standard Bardeen-Cooper-Schrieffer (BCS) formalism [15][16][17][18][19] , although the normal-state is undoubtedly not the standard Landau Fermi-liquid on which the conventional electron-phonon theory is based. Angle-resolved photoemission spectroscopy (ARPES) experiments reveal sharp spectral peaks in the singleparticle excitation spectrum 5,6,[15][16][17][18][19][20][21][22][23] , indicating the presence of quasiparticle-like states, which is also consistent with the long lifetime of the electronic state as it has been determined by the conductivity measurements 10,11 . In particular, as a direct method for probing the electron Fermi surface, the early ARPES measurements indicate that in the entire doping range, the underlying electron Fermi surface satisfies Luttinger's theorem [24][25][26][27] , i.e., the electron Fermi surface with the area is proportional to 1 − δ.…”

“…Later, the ARPES experimental studies show that in the underdoped and optimally doped regimes, although the antinodal region of the electron Fermi surface is gapped out, leading to the notion that only part of the electron Fermi surface survives as the disconnected Fermi arcs around the nodes [28][29][30][31][32] , the underlying electron Fermi surface determined from the low-energy spectral weight still fulfills Luttinger's theorem in the entire doping range 32 . These ARPES experimental facts [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] on the other hand provide strong evidences supporting the notion of the charge-spin recombination 13,14 . Since the electron Fermi surface is a fundamental property of interacting electron systems, the study of the nature of the electron Fermi surface should be crucial for understanding the electronic structure of cuprate superconductors.…”

Pebbles in the nodal pond

“…excitations, but so far the evidence is rather indirect 7 . The capacity of the STM to probe quantum coherence is unique, and I anticipate spectacular results in this area in the near future.…”

“…Inappropriate activation of Wnt signalling leads to colorectal and other cancers in humans 5 . Moreover, Wnts regulate the formation of connections between neurons and their targets (other neurons or muscle) 6,7 . The signalling pathways through which Wnts control cell fate and tissue polarity have been studied in depth, but how they exert their effects on neuronal projections has largely been a mystery.…”

“…Although the nodal waves might seem at first sight to be quite conventional, there is evidence suggesting that they are much more fragile than the electron waves in normal metals. It appears that they behave like quantum waves only when the system as a whole is in a macroscopic quantum statethat is, when it is superconducting 7 .…”