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In conventional superconductors, the electron pairing that allows superconductivity is caused by exchange of virtual phonons, which are quanta of lattice vibration. For high-transition-temperature (high-T(c)) superconductors, it is far from clear that phonons are involved in the pairing at all. For example, the negligible change in T(c) of optimally doped Bi2Sr2CaCu2O8+delta (Bi2212; ref. 1) upon oxygen isotope substitution (16O --> 18O leads to T(c) decreasing from 92 to 91 K) has often been taken to mean that phonons play an insignificant role in this material. Here we provide a detailed comparison of the electron dynamics of Bi2212 samples containing different oxygen isotopes, using angle-resolved photoemission spectroscopy. Our data show definite and strong isotope effects. Surprisingly, the effects mainly appear in broad high-energy humps, commonly referred to as 'incoherent peaks'. As a function of temperature and electron momentum, the magnitude of the isotope effect closely correlates with the superconducting gap--that is, the pair binding energy. We suggest that these results can be explained in a dynamic spin-Peierls picture, where the singlet pairing of electrons and the electron-lattice coupling mutually enhance each other.

A universal high energy anomaly in the single particle spectral function is reported in three different families of high temperature superconductors by using angle-resolved photoemission spectroscopy. As we follow the dispersing peak of the spectral function from the Fermi energy to the valence band complex, we find dispersion anomalies marked by two distinctive high energy scales, E1 ≈ 0.38 eV and E2 ≈ 0.8 eV. E1 marks the energy above which the dispersion splits into two branches. One is a continuation of the near parabolic dispersion, albeit with reduced spectral weight, and reaches the bottom of the band at the Γ point at ≈ 0.5 eV. The other is given by a peak in the momentum space, nearly independent of energy between E1 and E2. Above E2, a band-like dispersion re-emerges. We conjecture that these two energies mark the disintegration of the low energy quasiparticles into a spinon and holon branch in the high Tc cuprates.Understanding how doped oxygen holes are transported in the environment of antiferromagnetically coupled copper spin is one of the most fundamental problems in the field of high temperature superconductivity. In 1988 Zhang and Rice [1] proposed that the doped holes in the oxygen 2pσ orbitals form singlets with the spins of the neighboring coppers. The resulting charge-e and spin-0 object is called the Zhang-Rice singlet (ZRS). As the ZRS moves through the CuO 2 plane, the copper spins get rearranged. As a result, the ZRS couples very strongly to the antiferromagnetic environment. Remarkably as a consequence of such strong coupling, quasiparticles emerge at low energies. This is evidenced by the sharp nodal quasiparticle peaks seen in angle-resolved photoemission (ARPES) of almost all cuprate compounds [2,3]. In simple physical terms a quasiparticle is a composite object made of a ZRS and a S=1/2 copper spins. It is widely believed that, at sufficiently low temperatures, superconducting pairing occurs between these quasiparticles giving rise to the high temperature superconducting state. Thus a microscopic understanding of the pairing mechanism of high Tc superconductors requires an in-depth understanding of how a ZRS is dressed into a quasiparticle.Here we present the first systematic study of the evolution of the ARPES spectral function from the Fermi level (E F ≡ 0) to the valence band complex (at energy ≈ 1 eV [4]) for three different families of high temperature superconductors. Our results provide a surprising new experimental understanding on the important quasiparticle formation process discussed above. Specifically, by covering a much broader energy range than typically studied [2], we have identified anomalies in the ARPES spectra occurring at two universal high energy scales, E 1 ≈ 0.38 eV and E 2 ≈ 0.8 eV from E F . We conjecture that these two energies mark the threshold for the disintegration of the low-energy quasiparticles at two different binding levels.ARPES data have been collected at the Advanced Light Source, beamlines 7.0.1, 10.0.1 and 12.0.1. for three different familie...

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High-T c cuprate superconductors are characterized by a strong momentum-dependent anisotropy between the low energy excitations along the Brillouin zone diagonal (nodal direction) and those along the Brillouin zone face (antinodal direction). Most obvious is the d-wave superconducting gap, with the largest magnitude found in the antinodal direction and no gap in the nodal direction. Additionally, while antinodal quasiparticle excitations appear only below T c , superconductivity is thought to be indifferent to nodal excitations as they are regarded robust and insensitive to T c . Here we reveal an unexpected tie between nodal quasiparticles and superconductivity using high resolution time-and angle-resolved photoemission on optimally doped Bi 2 Sr 2 CaCu 2 O 8+δ . We observe a suppression of the nodal quasiparticle spectral weight following pump laser excitation and measure its recovery dynamics. This suppression is dramatically enhanced in the superconducting state. These results reduce the nodal-antinodal dichotomy and challenge the conventional view of nodal excitation neutrality in superconductivity.The electronic structures of high-T c cuprates are strongly momentum-dependent. This is one reason why the momentum-resolved technique of angle-resolved photoemission spectroscopy (ARPES) has been a central tool in the field of high-temperature superconductivity. 1 For example, coherent low energy excitations with momenta near the Brillouin zone face, or antinodal quasiparticles (QPs), are only observed below T c and have been linked to superfluid density. 2,3 They have therefore been the primary focus of ARPES studies. In contrast, nodal QPs, with momenta along the Brillouin zone diagonal, have received less attention and are usually regarded as largely immune to the superconducting transition because they seem insensitive to perturbations such as disorder, 4-6 doping, 6-8 isotope exchange, 9 charge ordering, 6,10,11 and temperature. 12-15 Clearly, finding any strong dependencies of the nodal QPs will alter the conventional view and enrich our understanding of high temperature superconductivity.Time resolution through pump-and-probe techniques adds a new dimension to ARPES by directly measuring how the electronic structure of a material responds to perturbations on femtosecond time scales. Here we report a unique ultrafast time-resolved ARPES study of a high-T c cuprate superconductor. Compared to previous time-resolved studies, 16-18 the primary advantage of this work is an unprecedented momentum (angular) resolution (∆k ∼0.003 vs. 0.05Å −1 ), on par with that of stateof-the-art ARPES. This has allowed the time-resolved measurement of significantly sharper QP spectral peaks with strikingly larger peak-to-background ratios than previously reported. 16 Additionally, a lower pump fluence is used ( 40µJ/cm 2 vs. ∼100µJ/cm 2 ), which reduces pump-induced sample temperature increase and related thermal smearing of spectral features. This allows us to uncover a surprising meltdown of nodal QP spectral weight following...

We report the first measurement of the Cu-O bond stretching phonon dispersion in optimally doped Bi2Sr1.6La0.4Cu2O6+delta using inelastic x-ray scattering. We found a softening of this phonon at q=( approximately 0.25,0,0) from 76 to 60 meV, similar to the one reported in other cuprates. A comparison with angle-resolved photoemission data on the same sample revealed an excellent agreement in terms of energy and momentum between the angle-resolved photoemission nodal kink and the soft part of the bond stretching phonon. Indeed, we find that the momentum space where a 63+/-5 meV kink is observed can be connected with a vector q=(xi,0,0) with xi > or =0.22, corresponding exactly to the soft part of the bond stretching phonon.

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The joint density of states of Bi2Sr2CaCu2O(8+delta) is calculated by evaluating the autocorrelation of the single particle spectral function A(k, omega) measured from angle resolved photoemission spectroscopy (ARPES). These results are compared with Fourier transformed (FT) conductance modulations measured by scanning tunneling microscopy (STM). Good agreement between the two experimental probes is found for two different doping values examined. In addition, by comparing the FT-STM results to the autocorrelated ARPES spectra with different photon polarization, new insight on the form of the STM matrix elements is obtained. This shines new light on unsolved mysteries in the tunneling data.