Manipulation of the zero-damping conditions and unidirectional invisibility in cavity magnonics

In this paper, we report a cavity magnonic system consisting of a split-ring resonator coupled with a yttrium iron garnet sphere, in which both coherent and dissipative coupling strengths can be tuned continuously, thereby making the zero damping conditions (ZDCs) controllable in a frequency range up to 100 MHz. Unidirectional invisibility has been observed under the ZDCs, with the microwaves freely transmitted in the forward direction (-2.9 dB) but almost completely blocked in the backward direction (-83 dB). The effective isolation ratio reaches up to 80 dB, which has surpassed the ideal isolation ratio limited by the state-of-the-art technique, and its direction is reversible by the orientation of the external static magnetic field. As an unconventional nonreciprocal device with flexibility and sensitivity, our magnonic device may open a promising way for robust coherent and quantum information processing.