Bose Einstein Condensation of Magnons in BaCuSi${}_2$O${}_6$: An experimental perspective

Han Purple, a pigment first obtained in ancient China, is one of the earliest known synthetic pigments. Also naturally occurring, as a mineral, it is known as colinowensite (Cwn). Its chemical formula is BaCuSi${}_2$O${}_6$, and its structure is a layered cyclosilicate in which magnetic Cu${}^{2+}$ ions (S = 1/2) form dimers arranged on a square lattice, making it also the first known synthetic metal dimer compound. Most interesting magnetic properties arise from a strong intradimer spin coupling, accompanied by weaker interdimer interactions within and between Cu-dimer layers. In zero or small magnetic fields, BaCuSi${}_2$O${}_6$ remains a quantum paramagnet. However, under high magnetic fields between 23 and 49 Tesla – about a million times stronger than Earth’s magnetic field – it undergoes magnetic ordering at subliquid Helium temperatures into an almost ideal easy-plane (XY) antiferromagnetic state regarded as a realization of a Bose-Einstein condensate of magnons. Within this experimentally accessible field range, BaCuSi${}_2$O${}_6$ serves as an extraordinary playground for testing predictions of quantum many-body physics.