Normal-state transport in superconducting NbN films on r-cut sapphire

High-quality thin NbN films are very crucial for realizing quantum devices. Here, we investigated electrical transport and noise properties of a series of thin NbN films of various thicknesses grown on r-cut sapphire substrate using a DC magnetron sputtering technique. The films exhibit non-uniform thickness dependences for superconducting transition temperature (Te ) and normal-state resistivity. Morphological characterization of NbN samples of various thicknesses reveals uniform structure in thin films and granular structure in thick films. By measuring transport and noise properties in a normal state, we observe that the granular structure of NbN films does not have a strong effect on resistivity and does not cause an additional source of current noise.

Influence of disorder strength on the superconducting mechanism of MgB2

We investigate the effect of disorder on the superconducting mechanism of MgB2 thin films using low-energy ion irradiation. The c-axis lattice constant and T c of MgB2 thin films change systematically as the magnitude of disorder, which corresponds to the value of average displacements per atom (dpaavg ), increases. Here, dpaavg is controlled by the amount of irradiated ions. The dpaavg dependence of the electron-phonon coupling constants (λ) is estimated using the McMillan equation. For dpaavg ≤ 0.049, λ is linearly proportional to dpaavg . On the other hand, for dpaavg > 0.049, the T c of the disordered MgB2 deviates from the linear fitting curve, and insulating behavior is observed in the normal state resistivity. These results indicate that the superconducting mechanism of MgB2 can be changed by the electronic system caused by disorder strength affecting the electron-phonon coupling constant λ.

Irreversibility line and thermally activated flux flow in Micro-bridges of high-temperature superconductor

We have studied the electrical resistivity as a function of temperature in Micro-bridges of YBa2Cu3O7-δ deposited by laser ablation on the SrTiO3 substrate face, around superconducting transition region in different magnetic fields. The activation energy U0 was determined and discussed using the Arrhenius relation. The irreversibility line Birr and the upper critical field Bc2were obtained using 10% and 90% criteria of the normal-state resistivity value from ρ (T) curves. A phase diagram of the studied sample is constructed showing the Tg glass line and a very broad vortex-liquid phase regime.

Correlation between scale-invariant normal-state resistivity and superconductivity in an electron-doped cuprate

An understanding of the normal state in the high-temperature superconducting cuprates is crucial to the ultimate understanding of the long-standing problem of the origin of the superconductivity itself. This so-called “strange metal” state is thought to be associated with a quantum critical point (QCP) hidden beneath the superconductivity. In electron-doped cuprates—in contrast to hole-doped cuprates—it is possible to access the normal state at very low temperatures and low magnetic fields to study this putative QCP and to probe theT➔ 0 K state of these materials. We report measurements of the low-temperature normal-state magnetoresistance (MR) of the n-type cuprate system La2−xCexCuO4and find that it is characterized by a linear-in-field behavior, which follows a scaling relation with applied field and temperature, for doping (x) above the putative QCP (x= 0.14). The magnitude of the unconventional linear MR decreases asTcdecreases and goes to zero at the end of the superconducting dome (x~ 0.175) above which a conventional quadratic MR is found. These results show that there is a strong correlation between the quantum critical excitations of the strange metal state and the high-Tcsuperconductivity.