Evidence of precursor superconductivity as high as 180 K from infrared spectroscopy

We show that a multilayer analysis of the infrared c-axis response of RBa2Cu3O7-d (R=Y, Gd, Eu) provides important new information about the anomalous normal state properties of underdoped cuprate high temperature superconductors. Besides competing correlations which give rise to a pseudogap that depletes the low-energy electronic states below T*>>Tc, it enables us to identify the onset of a precursor superconducting state below Tons>Tc. We map out the doping phase diagram of Tons which reaches a maximum of ~180 K at strong underdoping and present magnetic field dependent data which confirm our conclusions.

Dynamical response and confinement of the electrons at the LaAlO3/SrTiO3 interface

With infrared ellipsometry and transport measurements we investigated the electrons at the interface between LaAlO3 and SrTiO3. We obtained a sheet carrier density of Ns~5-9x 10E13 cm^-2, an effective mass of m*~3m_e, and a strongly frequency dependent mobility. The latter are similar as in bulk SrTi1-xNbxO3 and therefore suggestive of polaronic correlations of the confined carriers. We also determined the vertical density profile which has a strongly asymmetric shape with a rapid initial decay over the first 2 nm and a pronounced tail that extends to about 11 nm.

Superconducting energy gap and c-axis plasma frequency of (Nd,Sm)O0.82F0.18FeAs superconductors from infrared ellipsometry

We present ellipsometric measurements of the far-infrared dielectric response of polycrystalline samples of the new pnictide superconductor RO0.82F0.18FeAs (R=Nd and Sm). We find evidence that the electronic properties are strongly anisotropic such that the optical spectra are dominated by the weakly conducting c-axis response similar as in the cuprate high-temperature superconductors (HTSC). Accordingly, we obtain an upper limit of the c-axis superconducting plasma frequency of $omega_{{rm pl},c}^{rm SC}leq 260cm$ which corresponds to a lower limit of the c-axis magnetic penetration depth of $lambda_cgeq6mum$ and an anisotropy of $lambda_c/lambda_{ab}geq 30$ as compared to $lambda_{ab}=185$ nm from muon spin rotation [A. Drew {it et al.}, arXiv:0805.1042]. Also in analogy to the cuprate HTSC, our spectra exhibit the signatures of a gap-like suppression of the conductivity in the superconducting state with a large gap magnitude of $2Deltaapprox300cm$ (37 meV) and a ratio of $2Delta/k_{rm B}tcapprox8$ that is suggestive of strong coupling.