Abstract
A stack of intrinsic Josephson junctions of high-transition -temperature superconductors is emerging as compact sources of coherent THz radiation. We present an effective microfabrication technique for accurately controlling the number of intrinsic Josephson junctions and sidewall angles of the mesa structure made of single crystalline Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> CaCu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8+δ</sub> . We investigate the effect of the number of the resistive junctions and spatial homogeneity on synchronization using rectangular, square, and disc Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> CaCu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8+δ</sub> mesas. By examining the inner regions of the multiple current-voltage characteristic branches, we found that the coherent radiation fails to occur when the number of resistive junctions is less than a certain threshold number of approximately 100-200. This indicates that the transitions of spontaneous synchronization among the stacked junctions can take place according to the frequency locking phenomenon in large populations of disordered junctions as nonlinear oscillators. This finding allows us to construct highly efficient terahertz sources based on superconductors that are able to fill the technological gap in the THz regime.
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