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Angle resolved photoemission spectroscopy combined with isotope substitution (O→O) sample preparation method is used to probe the effect of the lattice degrees of freedom on the electron dynamics of optimally doped BiSrCaCO high temperature superconductors, as a function of momentum and temperature. Our data show that the lattice dynamics strongly renormalizes the electron dispersion and the photoemission line shapes. The renormalization is enhanced near the anti-nodal region and in the superconducting state, i.e. as the superconducting gap opens up. This unusual behavior is direct evidence that the electron-phonon interaction is correlated with the electron pairing in the high temperature superconductivity.

Direct ARPES studies on grown strained LSCO-214 films show striking strain effects on the electronic structure. The Fermi surface (FS) of LSCO-214 evolves with doping, yet changes even more significantly with strain. The compressive strain changes the Fermi surface topology from hole-like to electron-like and causes band dispersion along k and the Fermi level crossing before the Brillouin zone boundary, in sharp contrast to the ‘usual’ flat band remaining ≈30meV below E measured on unstrained samples. The associated reduction of the density of states does not diminish the superconductivity; rather, T is strongly enhanced (up to factor of two) in compressively strained thin films.

We report the first observation of polar angular magnetoresistance oscillations in the high- cuprate TlBaCuO. These measurements establish the existence of a coherent three-dimensional Fermi surface on the overdoped, superconducting side of the cuprate phase diagram, even in materials with extreme electrical anisotropy. Detailed analysis of the oscillations reveals that -axis dispersion vanishes at specific symmetry points. This observation has implications for our understanding of a wide range of unusual metallic and superconducting properties of cuprates.

From very high accuracy reflectivity spectra, we have derived the optical conductivity and estimated the spectral weight up to various cut-off frequencies in underdoped BiSrCaCuO (Bi-2212). We show that, when evaluating the optical spectral weight over the full conduction band (1 eV), the kinetic energy decreases in the superconducting state, unlike in conventional BCS superconductors. As a consequence, the Ferrell-Glover-Tinkham sum rule is not satisfied up to this energy scale. This stands as a very unconventional behavior, contrasted with the overdoped Bi-2212 sample.

We studied the optical conductivity of electron doped PrCeCuO from the underdoped to the overdoped regime. The observation of low to high frequency spectral weight transfer reveals the presence of a gap, except in the overdoped regime. A Drude peak at all temperatures shows the partial nature of this gap. The close proximity of the doping at which the gap vanishes to the antiferromagnetic phase boundary leads us to assign this partial gap to a spin density wave.

In this article, we focus on the dip structure of tunneling spectra in overdoped BiSrCaCuO (Bi2212), measured in Bi2212/vaccum/Bi2212 junctions, which are of the superconductor/insulator/superconductor (SIS) type below . It is confirmed that the dip structure appears at voltages || ∼ (2Δ+)/ in the SIS type tunneling spectra, where 2Δ is twice the d-wave superconducting (SC) gap amplitude and the energy of an antiferromagnetic (AF) resonance mode. Furthermore, it is demonstrated that the temperature evolution of the dip structure is in good agreement with that of the AF resonance mode, which also develops below . These results suggest that the tunneling dip structure will be associated with the development of AF resonance mode in accordance with the SC transition.

Andreev — Saint James reflections have been observed at normal metal contacts and tunnel junctions with YBCO, LSCO, BSCCO and other High Temperature Superconductors. The characteristic feature of these reflections is an enhanced conductance at low bias. Its observation implies the existence of extended quasiparticle arcs around the nodal directions. The enhancement persists up to Tc and so far has not been observed above it. Return to the normal state conductance occurs at an energy that scales with Tc, including in the pseudogap regime. In YBCO, the detailed shape of the conductance at low bias is consistent with a pure d-wave symmetry of the order parameter in the optimally doped and underdoped regimes, and with a mixed symmetry in the overdoped one.

The combined bicrystal, artificial cross-whisker, and natural cross-whisk-axis twist experiments on BiSrCaCuO (Bi2212) provide strong evidence that the superconducting order parameter is predominantly _wave for ≤ , and that the -axis tunneling is strongly incoherent.

Muon spin rotation (SR) results are reported on single-crystal YBaCuO, having a transition temperature of T = 91.3 K with ΔT < 0.5 K (in zero applied field). Flux motion and de-pinning can mask intrinsic properties such as the true underlying pairing state, and has led to misinterpretations of data. The present data exhibit a non-monotonic behavior for the second moment of the internal field distribution as a function of field as T→0, which rules out any single pairing state explanation of the data, without including other effects. The data are, however, consistent with -wave (or extended -wave) pairing, provided that field-dependent and temperature-activated vortex de-pinning are first taken into account. Applying a self-consistent vortex de-pinning model, the data are found to be best described by an underlying two-fluid model, yielding a London penetration depth value of λ(T=0,H=0) = 127.6 nm. Attempts to fit the data using BCS theory with varying coupling strengths and a -wave model produced much poorer fits. In fact, the probability that the -wave model gives a better fit than the two-fluid model is less that 4×10. This work reveals that the -wave interpretations are incorrect, confirms earlier work (Refs. 1–3) which first established -wave pairing in YBaCuO powders and (heavily twinned) crystals, and re-establishes -wave pairing for superconductivity in this material.

Existing data on Cu-nuclear spin relaxation reveal two independent relaxation processes: the one that is temperature independent we link to incommensurate peaks seen by neutrons, while the “universal” temperature dependent contribution coincides with 1/() for two-chain YBCO 124. We argue that this new result substitutes for a “pseudogap” regime in a broad class of high- cuprates and stems from the 1st order phase transition that starts well above the superconductivity but becomes frustrated because of broken electroneutrality in the CuO plane.