When you run an electric current (say, to the right), is that current made from positive charges (moving right) or negative charges (moving left)? In some materials, that depends on which direction the current flows.
My new article in Nature News:
Conversation
By the way, I am very pleased with myself for inventing Fig. 1d: an object that looks "n-type" or "p-type" depending on which direction you look at it from.
Here is my original sketch
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