Magnetic field, frequency and temperature dependence of complex conductance of ultrathin La1.65Sr0.45CuO4∕La2CuO4 films and the organic superconductors κ-(BEDT-TTF)2Cu[N(CN)2]Br

Abstract

We used atomic-layer molecular beam epitaxy (ALL-MBE) to synthesize bilayer films of a cuprate metal ([Formula: see text], LSCO) and a cuprate insulator ([Formula: see text], LCO), in which interface superconductivity occurs in a layer that is just one-half unit cell thick. We have studied the magnetic field and temperature dependence of the complex sheet conductance, [Formula: see text], of these films, and compared them to [Formula: see text]-(BEDT-TTF)[Formula: see text] single crystals. The magnetic field [Formula: see text] was applied both parallel and perpendicular to the 2D conducting layers. Experiments have been carried out at frequencies between 23 kHz and 50 MHz using either two-coil mutual inductance technique, or the LC resonators with spiral or rectangular coils. The real and the imaginary parts of the mutual-inductance [Formula: see text] between the coil and the sample were measured and converted to complex conductivity. For [Formula: see text] perpendicular to the conducting layers, we observed almost identical behavior in both films and [Formula: see text]-Br single crystals: (i) the transition onset in the inductive response, [Formula: see text](T) occurs at a temperature lower by 2 K than in [Formula: see text], (ii) this shift is almost constant with magnetic field up to 8 T; (iii) the vortex diffusion constant [Formula: see text] is exponential due to pinning of vortex cores. These results can be described by the extended dynamic theory of the Berezinski–Kosterlitz–Thouless (BKT) transition and dynamics of bound vortex–antivortex pairs with short separation lengths.