History dependent magnetoresistance in lightly doped La_{2-x}Sr_{x}CuO_{4} thin films

The in-plane magnetoresistance (MR) in atomically smooth La_{2-x}Sr_{x}CuO_{4} thin films grown by molecular-beam-epitaxy was measured in magnetic fields B up to 9 T over a wide range of temperatures T. The films, with x=0.03 and x=0.05, are insulating, and the positive MR emerges at T<4 K. The positive MR exhibits glassy features, including history dependence and memory, for all orientations of B. The results show that this behavior, which reflects the onset of glassiness in the dynamics of doped holes, is a robust feature of the insulating state.Comment: 4 pages, 4 figures, International School and Workshop on Electronic Crystals (ECRYS-2011); to appear in Physica

Superstripes and complexity in high-temperature superconductors

While for many years the lattice, electronic and magnetic complexity of high-temperature superconductors (HTS) has been considered responsible for hindering the search of the mechanism of HTS now the complexity of HTS is proposed to be essential for the quantum mechanism raising the superconducting critical temperature. The complexity is shown by the lattice heterogeneous architecture: a) heterostructures at atomic limit; b) electronic heterogeneity: multiple components in the normal phase; c) superconducting heterogeneity: multiple superconducting gaps in different points of the real space and of the momentum space. The complex phase separation forms an unconventional granular superconductor in a landscape of nanoscale superconducting striped droplets which is called the "superstripes" scenario. The interplay and competition between magnetic orbital charge and lattice fluctuations seems to be essential for the quantum mechanism that suppresses thermal decoherence effects at an optimum inhomogeneity.Comment: 20 pages, 3 figures; J. Supercon. Nov. Mag. 201

Can high-Tc superconductivity in cuprates be explained by the conventional BCS theory?

For overdoped cuprates, it is believed that the normal state behaves as an ordinary Fermi liquid while the superconducting state conforms to the BCS theory. We have put these beliefs to the test by a comprehensive experiment in which over two thousand cuprate films were synthesized by molecular beam epitaxy and studied in great detail and precision. Here, we compare our key experimental results to various proposed explanations based on BCS theory extended to dirty d-wave superconductors, including the cases of strong (unitary) and weak (Born) scattering on impurities. The discrepancies seem insurmountable, and point to the need to develop the theory further, likely beyond the canonical BCS paradigm

La2−xSrxCuO4 superconductor nanowire devices

La2−xSrxCuO4 nanowire devices have been fabricated and characterized using electrical transport measurements. Nanowires with widths down to 80 nm are patterned using high-resolution electron beam lithography. However, the narrowest nanowires show incomplete superconducting transitions with some residual resistance at T = 4 K. Here, we report on the refinement of the fabrication process to achieve narrower nanowire devices with complete superconducting transitions, opening the path to the study of novel physics arising from dimension-limited superconductivity on the nanoscale.Physic

Insights from study of high-temperature interface superconductivity

A brief overview is given of the studies of high-temperature interface superconductivity based on atomic-layer-by-layer molecular beam epitaxy (ALL-MBE). A number of difficult materials science and physics questions have been tackled, frequently at the expense of some technical tour de force, and sometimes even by introducing new techniques. ALL-MBE is especially suitable to address questions related to surface and interface physics. Using this technique, it has been demonstrated that high-temperature superconductivity can occur in a single copper oxide layer—the thinnest superconductor known. It has been shown that interface superconductivity in cuprates is a genuine electronic effect—it arises from charge transfer (electron depletion and accumulation) across the interface driven by the difference in chemical potentials rather than from cation diffusion and mixing. We have also understood the nature of the superconductor–insulator phase transition as a function of doping. However, a few important questions, such as the mechanism of interfacial enhancement of the critical temperature, are still outstanding