In-Plane and Out-of-Plane Charge Dynamics of High-TcT_c Cuprates

We propose a theoretical expression for the $k$- and $\omega$-dependent dielectric function of a stack of two-dimensional layers coupled along the direction perpendicular to the layers, and discuss some of its properties. We argue that the plasma frequencies at $k=0$ should correspond to those which are experimentally obtained from optical measurements on {\em e.g.} La$_{1-x}$Sr$_{x}$CuO$_4$ via the $f$-sum rule analysis, regardless of the fact that such systems are strongly correlated. We discuss some of the ramifications due to strong anisotropy of the charge transport in these systems, and the lack of coherence for the transport in the direction perpendicular to the layers.Comment: 8 pages, postscript, uuencoded gz-compressed .tar fil

Local environment around the oxygen atoms in Y-Ba-Cu-O studied x-ray absorption fine structure (XAFS) spectroscopy

The local environment about oxygen atoms in the CuO{sub 2} plane in YBa{sub 2}Cu{sub 3}O{sub 7} single crystals has been investigated by orientation-dependent x-ray absorption fine structure (XAFS) spectroscopy at the O ls edge using bulk-sensitive fluorescence-yield-detection method. Within experimental accuracy ({plus minus}0.03 {Angstrom}) no structural change in local ordering about oxygen atoms in the CuO{sub 2} plane at 90 K has been detected. 14 refs

Local environment around the oxygen atoms in Y-Ba-Cu-O studied x-ray absorption fine structure (XAFS) spectroscopy

The local environment about oxygen atoms in the CuO{sub 2} plane in YBa{sub 2}Cu{sub 3}O{sub 7} single crystals has been investigated by orientation-dependent x-ray absorption fine structure (XAFS) spectroscopy at the O ls edge using bulk-sensitive fluorescence-yield-detection method. Within experimental accuracy ({plus_minus}0.03 {Angstrom}) no structural change in local ordering about oxygen atoms in the CuO{sub 2} plane at 90 K has been detected. 14 refs

Studies of Y-Ba-Cu-O single crystals by x-ray absorption spectroscopy

The symmetry and density of unoccupied states of YBa{sub 2}Cu{sub 3}O{sub 7} YBa{sub 2}Cu{sub 4}O{sub 8} have been investigated by orientation dependent x-ray absorption spectroscopy on the O 1s edge using a bulk-sensitive fluorescence-yield-detection method. It has been found that the O 2p holes are distributed equally between the CuO{sub 2} planes and CuO chains and that the partial density of unoccupied O 2p states in the CuO{sub 2} planes are identical in both systems investigated. The upper Hubbard band has been observed in the planes but not in the chains in both systems. 18 refs

Possible Atmospheric Diversity of Low Mass Exoplanets – Some Central Aspects

Exoplanetary science continues to excite and surprise with its rich diversity. We discuss here some key aspects potentially influencing the range of exoplanetary terrestrial-type atmospheres which could exist in nature. We are motivated by newly emerging observations, refined approaches to address data degeneracies, improved theories for key processes affecting atmospheric evolution and a new generation of atmospheric models which couple physical processes from the deep interior through to the exosphere and consider the planetary-star system as a whole. Using the Solar System as our guide we first summarize the main processes which sculpt atmospheric evolution then discuss their potential interactions in the context of exoplanetary environments. We summarize key uncertainties and consider a diverse range of atmospheric compositions discussing their potential occurrence in an exoplanetary context