Photoelectron Escape Depth and Inelastic Secondaries in High Temperature Superconductors

We calculate the photoelectron escape depth in the high temperature superconductor Bi2212 by use of electron energy-loss spectroscopy data. We find that the escape depth is only 3 Ang. for photon energies typically used in angle resolved photoemission measurements. We then use this to estimate the number of inelastic secondaries, and find this to be quite small near the Fermi energy. This implies that the large background seen near the Fermi energy in photoemission measurements is of some other origin.Comment: 2 pages, revtex, 3 encapsulated postscript figure

Sum rule analysis of Umklapp processes and Coulomb energy: application to cuprate superconductivity

The third moment frequency sum rule for the density-density correlation function is rederived in the presence of Umklapp processes. Upper and lower bounds on the electron-electron Coulomb energy are derived in two-dimensional and three-dimensional media, and the Umklapp processes are shown to be crucial in determining the spectrum of the density fluctuations (especially for the two-dimensional systems). This and other standard sum rules can be used in conjunction with experimental spectroscopies (electron-energy loss spectroscopy, optical ellipsometry, etc.) to analyse changes of the electron-electron Coulomb energy at the superconducting transition in cuprates

Charge Dynamics in Cuprate Superconductors

In this lecture we present some interesting issues that arise when the dynamics of the charge carriers in the CuO$_2$ planes of the high temperature superconductors is considered. Based on the qualitative picture of doping, set by experiments and some previous calculations, we consider the strength of various inter and intra-cell charge transfer susceptibilities, the question of Coulomb screening and charge collective modes. The starting point is the usual p-d model extended by the long range Coulomb (LRC) interaction. Within this model it is possible to examine the case in which the LRC forces frustrate the electronic phase separation, the instability which is present in the model without an LRC interaction. While the static dielectric function in such systems is negative down to arbitrarily small wavevectors, the system is not unstable. We consider the dominant electronic charge susceptibilities and possible consequences for the lattice properties.Comment: 14 pages, 15 figures, latex, to be published in "From Quantum Mechanics to Technology", Lecture Notes in Physics, Springe

Charge-density wave formation in Sr_{14}Cu_{24}O_{41}

The electrodynamic response of the spin-ladder compound Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ ($x=0, 3, 9$) has been studied from radiofrequencies up to the infrared. At temperatures below 250 K a pronounced absorption peak appears around 12 cm$^{-1}$ in Sr$_{14}$Cu$_{24}$O$_{41}$ for the radiation polarized along the chains/ladders (${\bf E}\parallel {\bf c}$). In addition a strongly temperature dependent dielectric relaxation is observed in the kHz - MHz range. We explain this behavior by a charge density wave which develops in the ladders sub-system and produces a mode pinned at 12 cm$^{-1}$. With increasing Ca doping the mode shifts up in frequency and eventually disappears for $x=9$ because the dimensionality of the system crosses over from one to two dimensions, giving way to the superconducting ground state under pressure.Comment: One name added to author list 4 pages, 2 figures, email: [email protected]

ELECTRONIC STRUCTURE STUDIES OF HIGH-TC SUPERCONDUCTORS BY HIGH-ENERGY SPECTROSCOPIES

Author Institution: Kernforschungszentrum Karlsruhe, INFP, P. O. Box 3640, D-7500 Karlsruhe, FRG; $^{\ast}$ Institut f\""ur Experimentalphysik, Universit\""at KielThe electronic structure studies of the high-temperature superconductors $La_{2}-xSr_{x}CuO_{4}, YBa_{2}Cu_{3}O_{7-x}, Bi_{2}Sr_{2}Can_{-1}Cu_{n}O_{4+2n}$ and $T1_{2}Ca_{2}Ba_{2}Cu_{3}O_{10}$ by high-energy spectroscopies are reviewed. Electron energy-loss spectroscopy in transmission (EELS), angle-resolved photoemission (ARUPS) and inverse angle-resolved photoemission (ARIPES) have been used. Low energy-loss spectra $(0. 1<E\leq 50eV)$ yield the orientation dependent dielectric function over a wide energy range. Information on d-d excitations, charge-transfer excitations, charge-carrier plasmons and interband transitions is obtained. Absorption edges of the O 1s and the Cu 2p levels provide information on the symmetry and the character of holes in these materials. In addition, the electronic states of $Bi_{2}Sr_{2}CaCu_{2}O_{3}$ at the Fermi level had been studied by high resolution ARUPS and ARIPES. Temperature-dependent measurements indicate below $T_{c}$ an opening of a gap of about 60 meV yielding a ratio $\Delta(0)/k_{B}T_{c}$ about twice as large as the BCS value. Recent EELS results on the electron high-$T_{c}$ superconductor $Nd_{2-x}Ca_{x}CuO_{4-y}$ are reported