The Josephson effects provide a unique principle to excite high-frequency electromagnetic (EM) waves. Much effort has been made to stimulate powerful radiations by using intrinsic Josephson junctions (IJJs) provided by cuprate high-Tc superconductors of layered structures. The junctions are homogeneous at the atomic scale guaranteed by the high quality of single crystals, and the superconductivity gap is large, typically of tens of meV, which in principle permits the frequency cover the whole range of the terahertz (THz) band, a very useful regime of EM waves still lacking of compact solid-state generators.
Motivated by an experimental breakthrough in 2007 where coherent and strong THz radiations were confirmed from a mesa-shape Bi2Sr2CaCu2O8+�� (BSCCO) single crystal under dc bias, we analyzed the sine-Gordon equations with large inductive coupling. A new dynamic state of superconductivity phase difference was then found for IJJs stack under dc bias. The system creates alternative ���� phase kinks in phase differences when they rotate totally with the angle velocity determined by the bias voltage according to the ac Josephson relation. The phase kinks couples the lateral cavity modes of the transverse Josephson plasma to the dc bias, and excites the Josephson plasma oscillation in an efficient way when the bias voltage is tuned to the value corresponding to the cavity frequency of the Josephson plasma. Due to the high nonlinearity of the dc Josephson effect, a large current is injected into the system, and a part of the energy is radiated as THz EM waves from the sides of mesa into space.
The superconductivity at nano-meter scale is crucial for this novel �� phase kink state since it is stable only when the large inductive coupling among Josephson junctions exists, which is proportional to the ratio between the London penetration depth ��ab=400nm and the period of IJJs s=1.5nm.
Future activities are highly anticipated for revealing deeply the nonlinear properties of the new state. The outputs would not only be very important for science of superconductivity, but also be essential for development of THz generator as well as detectors and amplifiers. New ways to control superconductivity phase may be formulated during the course of the study, which is then useful for quantum information technologies.
Figure: �� phase kink in a rectangular Josephson junction.
Rotation of a pair of �� kinks in two adjacent layers.
