We propose a two-dimensional (2D) band structure calculation for alcalineearth-substituted La2Cu04 in the tetragonal phase. We find a degenerate logarithmic singularity in the electronic density of states, as usual in 2D systems. This leads to an orthorhombic-to-tetragonal structural phase transition (SPT). Using the BCS theory, we calculate the superconducting critical temperature T, as a function of the position of the Fermi level (i.e. Cu+++/Cu++ ratio). This model explains the high T:s observed experimentally and the relation between superconductivity and SPT.
We describe a novel single-coil mutual inductance technique for measuring the magnetic penetration depth of superconductors at 2-4 MHz as a function of temperature in the 4-100 K range. We combine a single-coil configuration with a high-stability marginal oscillator; this enables us to measure the absolute value of on both bulk samples and thin films with very high resolution (␦ϭ10 pm͒ and a precision of 30 nm. As example of application, we report measurements on NbTi bulk samples and Nb films. This contactless technique is suited for probing the superconducting properties of samples over large surfaces.
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