Comment on ``Loss of Superconducting Phase Coherence in YBa_2Cu_3O_7   Films: Vortex-Loop Unbinding and Kosterlitz-Thouless Phenomena''
  Recently, Kotzler et al. measured the frequency-dependent conductance for YBa_2Cu_3O_7 and interpreted their results as evidences that the decay of the superfluid density is caused by a 3D vortex loop proliferation mechanism and a dimensional crossover when the correlation length $\xi_c$ along the c axis becomes comparable to the sample thickness $d$ [PRL 87, 127005(2001)]. In this Comment, we show that the complex conductance data presented by Kotzler et al. have characteristic key features not compatible with their analysis, which are instead described by the existing phenomenology of 2D vortex fluctuation associated with a partial decoupling of CuO_2-planes. 
Kateryna Medvedyeva, Beom Jun Kim, Petter Minnhagen