Multipole (IMAGE)
Caption
In this figure, the electric dipole moment is represented by a positive charge (red dot) and a negative charge (blue dot) that are spatially separated. The quadrupole, made up of two opposing dipole moments, is inherently two dimensional, and the octupole, made up of two opposing quadrupoles, is inherently three dimensional. While materials with quantized dipole moments were previously known, insulating crystals with quantized quadrupole and octupole moments are topological phases unlike any discovered to date.
Credit
Wladimir Balcazar, Department of Physics and Institute for Condensed Matter Theory, University of Illinois at Urbana-Champaign.
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