Reconstructing the bulk Fermi surface and the superconducting gap properties from Neutron Scattering experiments

We develop an analytical tool to extract bulk electronic properties of unconventional superconductors through inelastic neutron scattering (INS) spectra. Since the spin excitation spectrum in the superconducting (SC) state originates from Bogoliubov quasiparticle scattering associated with Fermi surface nesting, its energy-momentum relation--the so called `hour-glass' feature--can be inverted to reveal the Fermi momentum dispersion of the single-particle spectrum as well as the corresponding SC gap function. The inversion procedure is analogous to the quasiparticle interference (QPI) effect in scanning tunneling microscopy (STM). Whereas angle-resolved photoemission spectroscopy (ARPES) and STM provide surface sensitive information, our inversion procedure provides bulk electronic properties. The technique is essentially model independent and can be applied to a wide variety of materials.Comment: 8 pages, 4 figure

Nonmonotonic dx2−y2d_{x^2-y^2} superconducting gap in electron-doped Pr0.89_{0.89}LaCe0.11_{0.11}CuO4_4: Evidence of coexisting antiferromagnetism and superconductivity?

Recent experiments on Pr$_{0.89}$LaCe$_{0.11}$CuO$_4$ observe an anisotropic spin-correlation gap and a nonmonotonic superconducting (SC) gap, which we analyze within the framework of a $t-t^{\prime}-t^{\prime\prime}-t^{\prime\prime\prime}-t^{iv}-U$ model with a $d_{x^2-y^2}$ pairing interaction including a third harmonic contribution. By introducing a realistic broadening of the quasiparticle spectrum to reflect small-angle scattering, our computations explain the experimental observations, especially the presence of a maximum in the leading edge gap in the vicinity of the hot-spots. Our analysis suggests that the material behaves like a {\it two-band} superconductor with the d-wave third harmonic acting as the {\it interband pairing gap}, and that the anti-ferromagnetic (AFM) and SC orders co-exist in a uniform phase

Failure of t-J models in describing doping evolution of spectral weight in x-ray scattering, optical and photoemission spectra of the cuprates

We have analyzed experimental evidence for an anomalous transfer of spectral weight from high to low energy scales in both electron and hole doped cuprates as a function of doping. X-ray scattering, optical and photoemission spectra are all found to show that the high energy spectral weight decreases with increasing doping at a rate much faster than predictions of the large $U-$limit calculations. The observed doping evolution is however well-described by an intermediate coupling scenario where the effective Hubbard $U$ is comparable to the bandwidth. The experimental spectra across various spectroscopies are inconsistent with fixed-$U$ exact diagonalization or quantum Monte Carlo calculations, and suggest a significant doping dependence of the effective $U$ in the cuprates.Comment: Accepted for Phys. Rev. B (2010). 7 pages, 4 figure

Visualizing electron pockets in cuprate superconductors

Fingerprint of the electron-pocket in cuprates has been obtained only in numerous magneto-transport measurements, but its absence in spectroscopic observations pose a long-standing mystery. We develop a theoretical tool to provide ways to detect electron-pockets via numerous spectroscopies including scanning tunneling microscopy (STM) spectra, inelastic neutron scattering (INS), and angle-resolved photoemission spectroscopy (ARPES). We show that the quasiparticle-interference (QPI) pattern, measured by STM, shows additional 7 ${\bm q}$ vectors associated with the scattering on the electron-pocket, than that on the hole-pocket. Furthermore, the Bogolyubov quasiparticle scatterings of the electron pocket may lead to a second magnetic resonance mode in the INS spectra at a higher resonance energy. Finally, we reanalyze some STM, INS, and ARPES experimental data of several cuprate compounds which dictates the direct fingerprints of electron pockets in these systems.Comment: 10 pages, 6 figures, submitte

Remnant Fermi Surfaces in Photoemission

Recent experiments have introduced a new concept for analyzing the photoemission spectra of correlated electrons -- the remnant Fermi surface (rFs), which can be measured even in systems which lack a conventional Fermi surface. Here, we analyze the rFs in a number of interacting electron models, and find that the results fall into two classes. For systems with pairing instabilities, the rFs is an accurate replica of the true Fermi surface. In the presence of nesting instabilities, the rFs is a map of the resulting superlattice Brillouin zone. The results suggest that the gap in Ca_2CuO_2Cl_2 is of nesting origin.Comment: 4 pages LaTex, 3 ps figure

Evolution of Mid-gap States and Residual 3-Dimensionality in La2−x_{2-x}Srx_xCuO4_4

We have carried out extensive first principles doping-dependent computations of angle-resolved photoemission (ARPES) intensities in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) over a wide range of binding energies. Intercell hopping and the associated 3-dimensionality, which is usually neglected in discussing cuprate physics, is shown to play a key role in shaping the ARPES spectra. Despite the obvious importance of strong coupling effects (e.g. the presence of a lower Hubbard band coexisting with mid-gap states in the doped insulator), we show that a number of salient features of the experimental ARPES spectra are captured to a surprisingly large extent when effects of $k_z$-dispersion are properly included in the analysis.Comment: 5 pages, 4 figure