Anisotropic dielectric function of cuprates

For valence band excitations, however, a different method has to be used to study the anisotropic dielectric properties due to the smaller q ( rv- 0.02 A 1 for 20 eV energy loss with 100 kV incoming beam energy), which means that the momentum transfer in the low loss region is mostly due to q. This requires the sample to be oriented in a certain direction relative to the direction of the electron beam to study the dielectric response in a particular orientation. Because of the small beam size available in cFEG TEM, a specific orientation of a thin section with millimeter size area can be easily obtained by ion beam thinning techniques to directly measure the anisotropic properties of the material. 

It is well known that for all Cu—O high Tc compounds, the electrical conductivity is much higher within the ab plane than out of it. For example, by 

For anisotropy measurements, the YBa2Cu40g (Y124) crystal was oriented in such a way that either the a- or Z -axis was parallel to the electron beam 

2 Confinement of the Hubbard transition in the Cu02 plane in the infinite-layer compound 

Unlike other copper oxide superconductors which have higher cja ratio, the cj a ratio in the non-defective infinite-layer compound is close to 1. However, because of a lack of apical oxygen, the electronic structure is expected to be more anisotropic than all other high oxides, despite the similar c and a lattice constants of the unit cell. This supposition was recently confirmed by X-ray absorption spectroscopy (XAS) measurements [8.27], in which an anisotropic upper Hubbard band was reported. In this section, we will discuss the question of whether the electronic transitions near the Fermi surface behave anisotropically with respect to the a6-plane and the c-axis. 

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