Note that this is not a 3D crystal of electrons floating in vacuum.
It's a 2D crystal of electrons that live in a thin semiconductor that is sandwiched between two layers of other semiconductor. You can't pick it with a tiny gripper.
Moreover, I don't understand all the details, but IIUC the surrounding semiconductor provides an effective force that makes the electrons attract each other when they are not too close. So in some sense, the crystal is formed by the electrons, but this does not break the expected behavior of a 100% pure negative particles in vacuum.
It's a 2D crystal of electrons that live in a thin semiconductor that is sandwiched between two layers of other semiconductor. You can't pick it with a tiny gripper.
Moreover, I don't understand all the details, but IIUC the surrounding semiconductor provides an effective force that makes the electrons attract each other when they are not too close. So in some sense, the crystal is formed by the electrons, but this does not break the expected behavior of a 100% pure negative particles in vacuum.