Abstract
Measurements of the magnetic relaxation rates S(T) and transport characteristics have been performed on Y 1Ba 2Cu 3O 7−δ (YBCO) films with thickness between 2.4 and 200nm. Strong quantum fluctuations of the vortex movements which occur at temperatures as high as 8.5∼15 K are detected in all ultra-thin films (2.4∼40nm), while the thick sample (200nm) does not show any quantum fluctuations at the experimental temperatures down to 4.2K. The experimental results are discussed with the theory based on dissipative tunneling of collectively pinned vortex. It is suggested that the strong quantum fluctuations in the ultra-thin films are due to the higher zero temperature normal state resistivity and/or the higher active energy of them compared to those of the thick sample.
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