Dimitrios G. Xenikos

ac susceptibility apparatus for measuring the transition temperature of high‐Tc crystals, sintered samples, and films

We describe a simple ac magnetic susceptibility system which can be used to observe the superconducting transition temperature in the range 4.2–300 K. The apparatus is extremely flexible and has been tested using high‐quality YBa2Cu3O7−x single crystals with volume as small as 2×10−5 cm3, much larger sintered polycrystalline samples of the same compound, and finally thin films deposited over a surface area of 1 cm2. The system measures resonant frequency variations of a parallel LC circuit with the superconductor close to the inductor. Typical frequencies and fields are in the ranges 0.2–10 MHz and 1–2 mG ac in the inductor.

Resistivity and Tc measurements on YBa2(Cu1−xMx)3O7−δ films (M=Ni,Al)

YBa2(Cu1−xMx)3O7−δ films (M=Ni and Al) with up to 11 at. % dopant are fabricated on (001) SrTiO3 substrates by codeposition of Y, BaF2, Cu, and M, and then post‐annealing in moist oxygen. All films are 300 nm thick. They are highly oriented with the c axis perpendicular to the substrate, and their surfaces are as smooth as for undoped YBa2Cu3O7−δ films. Tc(x) decreases with x consistent with bulk ceramic samples. The resistivity is metallic and ranges from 100 μΩ cm to 2.5 mΩ cm at 100 K. The residual resistivity ρab(0) at T=0 K, extrapolated from ρab(T) above Tc is approximately proportional to x up to 7 at. % Al films and 11 at. % Ni.

Correlation length and time in the vortex-glass critical regime in YBa2Cu3O7−x films

Abstract We report on the complex conductivity of a YBa2Cu3O7−x microbridge in the presence of magnetic field 0.5 ≤B≤6.5 T applied parallel to the c-axis. A broad range of frequences 10 ≤ f ≤ 2 × 106 Hz for the transport current passing through the bridge allows us to explore the ac dynamical response of the vortices where conflicting results have been reported, in addition to the static characteristics of the vortex system (dc limit). In consistency with the concept of a correlation length and a correlation time that diverge at the transition temperature Tg(B), we demostrate that a single value for Tg(B) can be identified in both the static and dynamical measurements, within an experimental uncertainty of 1 K. In conflict to previous reports, the real and imaginary components of the ac-linear voltage at T=Tg(B) were found to follow the same power law Vi ∝ Vo ∝ fα, with α=0.82±0.05. Finally, as T→Tg+, the critical scaling laws break down when the diverging vortex correlation length matches the film thickness of our sample and the correlation time exceeds 10 μs.

The effect of Al doping on the critical scaling of YBa2Cu3O7 films, in high magnetic fields

Abstract We present current-voltage characteristics (I–V) in YBa 2 (Cu 1−x Al x ) 3 O 7 films (0≤x≤0.03), in magnetic fields 0.5 ≤ B ≤ 6 T, which are consistent with the existence of a phase transition with critical exponents and scaling functions independent of Al concentration. We study the role of Al doping in flux pinning by analyzing the non-universal characteristics of the transition such as the width of the critical regime, the characteristic length for vortex correlations, and the temperature and the resistivity that correspond to the onset of the critical response.

Vortex pinning in YBa2Cu3O7−x films

Evidence that pinning on linear or planar defects dominates the vortex dynamics in YBa2Cu3O7−x (YBCO) films is provided by complex impedance measurements at temperature 8 K<T<Tc and magnetic field 0<B<6 T. Below the vortex lattice melting transition Bg(T) but above a threshold field Bp≈8(1-T/Tc) T, the inductance of vortices increases as B2, much less rapidly than predicted for collective pinning of vortices by point defects. Above the vortex melting line, critical scaling persists over the region Bg(T<B<B*(T) where the vortex correlation length ξ exceeds a characteristic length scale ξ*≡ξ(B=B*)≈450Å. The value of ξ* is not sensitive to Al-doping in the Cu sites in the lattice and is close to the size of twin domains in the film. The nature of the observed crossovers is discussed in terms of available theoretical models for a glass-liquid transition at Bg.