A separation method includes a separation step in which a treatment target electrode including a current collector and an electrode mixture is treated with an ultrasonic wave in water (a treatment liquid) while the frequency of the ultrasonic wave is swept in order to separate the current collector and the electrode mixture from each other. A separation device includes a separation unit that treats the treatment target electrode with an ultrasonic wave in water to separate the current collector and the electrode mixture from each other and a controller that controls the separation unit such that the ultrasonic treatment is performed while the frequency of the ultrasonic wave is swept.

A composite that includes a gallium nitride layer; an intermediate layer that is formed on a surface of the gallium nitride layer and contains carbon, gallium, and oxygen; and a diamond layer that is joined to the surface of the gallium nitride layer through the intermediate layer.