Colloquium @ Division of Physics, Hokkaido University

A Colloquium @ Division of Physics, Hokkaido University Colloquium 2001 2000 1999 1998 1997

Colloquium 2001

以下のように物理コロキュウムが予定されています。皆さん奮って御参加下さい。

Dissipation and Quantum Fluctuations
in 1D- and 2D-Array of Small Josephson Junctions

大塚　洋一氏
(筑波大学物理学系教授)

2002年1月10日（木曜日）16:00-
物理学専攻大学院講義室（2-211）にて

要旨：A small Josephson junction array is a model system which shows quantum phase transitions. It has been suggested that the competition between the Josephson coupling, the charging effect, and the dissipation leads to a superconductor-insulator transition at zero temperature: When the charging energy E_C exceeds the Josephson coupling energy E_J, quantum fluctuations destroy the global order of the superconducting phases, and the array becomes an insulator. The dissipation, which is caused by Ohmic resistors (R_S) shunting the junctions or quasiparticle tunneling, also plays an important role since the quantum fluctuation of the phases strongly depends on the strength of the dissipation.
　We have investigated the zero-temperature phase diagram of 1-dimensional (1D) and 2D arrays of small Josephson junctions experimentally. To control the ratio E_J/E_C and the strength of the dissipation independently, we fabricated arrays in which each junction (Al-AlOx-Al) is shunted by a normal-conducting Cr thin film. We observe the insulating behavior of the arrays when both E_J/E_C and R_Q/R_S are small (R_Q=h/e^2=6.45kOhm). The critical values of R_Q/R_S for small E_J/E_C are close to 1/d (d: dimensionality), which is in good agreement with the theories of the dissipation-driven SI transition.

物理コロキウム世話人:
　　　　北　孝文 (x 2687) <kita@phys.sci.hokudai.ac.jp>
　　　　末廣一彦(x 2686) <suehiro@particle.sci.hokudai.ac.jp>