Doping dependence of density response and bond-stretching phonons in cuprates

We explain the anomalous doping dependence of zone boundary $(\pi,0)$ and $(\pi,\pi)$ bond-stretching phonons in the high-temperature superconductor La$_{2-\delta}$Sr$_{\delta}$CuO$_4$ in the doping range $0<\delta<0.35$. Our calculations are based on a theory for the density response of doped Mott-Hubbard insulators.Comment: 2 pages, 3 figures, SCES conference, Karlsruhe 200

Christmas Is All About Receiving

Luke 2:10-11

From print to \u27image media\u27: Neil Postman\u27s The disappearance of childhood

Reviewed Book: Postman, Neil. The Disappearance of Childhood. New York: Dell, 1982

Did you have a good Christmas

Luke 2:29-3

On the electron-energy loss spectra and plasmon resonance in cuprates

The consequences of the non-Drude charge response in the normal state of cuprates and the effect of the layered structure on electron-energy loss spectra are investigated, both for experiments in the transmission and the reflection mode. It is shown that in the intermediate doping regime the plasmon resonance has to be nearly critically damped as a result of the anomalous frequency dependence of the relaxation rate. This also implies an unusual low-energy dependence of the loss function. Both facts are consistent with experiments in cuprates. Our study based on the t-J model shows good agreement with measured plasmon frequencies.Comment: LaTeX, 4 pages with 2 figures. submitted to PR

Charge dynamics of t-J model and anomalous bond-stretching phonons in cuprates

The density response of a doped Mott-Hubbard insulator is discussed starting from the t-J model in a slave boson 1/N representation. In leading order O(1) the density fluctuation spectra $N({\bf q},\omega)$ are determined by an undamped collective mode at large momentum transfer, in striking disagreement with results obtained by exact diagonalization, which reveal a very broad dispersive peak, reminescent of strong spin-charge coupling. The 1/N corrections introduce the polaron character of the bosonic holes moving in a uniform RVB background. The resulting $N({\bf q},\omega)$ captures all features observed in diagonalization studies, fulfills the appropriate sum rules, and apart from the broadening of the collective mode shows a new low energy feature at the energy $\chi J+\delta t$ related to the polaron motion in the spinon background. It is further shown that the low energy structure, which is particularly pronounced in $(\pi,0)$ direction, describes the strong renormalization and anomalous damping of the highest bond-stretching phonons in La$_{2-x}$Sr$_x$CuO$_4$.Comment: Presented at the meeting "Highlights in Condensed Matter Physics" in honor of the 60th birthday of Prof. Ferdinando Mancini, May 9-11, 2003, Salerno, Ital