A 40-year-old prediction
In 1973, physicist Philip W. Anderson proposed a new state of matter called a quantum spin liquid. In typical magnetic materials, as temperatures drop, the electron spins—the property that gives them a magnetic moment—align with each other. They either lock into a parallel formation (ferromagnetism) or an alternating anti-parallel pattern (antiferromagnetism). Anderson's theory described a material where the spins would refuse to order, even at absolute zero. The spins would remain in a fluctuating, entangled state, like a liquid.
This behavior is possible in materials with a specific atomic arrangement known as a kagome lattice. This structure, named after a Japanese basket-weaving pattern, consists of corner-sharing triangles. If spins on a triangle try to align antiferromagnetically, where each spin points opposite to its neighbor, a problem arises. Two spins can align oppositely, but the third spin is "frustrated"—it cannot satisfy both neighbors at once. This geometric frustration prevents the system from settling into a stable, ordered magnetic state. For decades, the quantum spin liquid remained a theoretical concept because no known material perfectly exhibited these properties.
From Chilean mine to MIT lab
The search for a real-world quantum spin liquid led researchers to herbertsmithite, a green-blue mineral with the chemical formula ZnCu₃(OH)₆Cl₂. First discovered in a Chilean mine in 1972, it was named after mineralogist G. F. Herbert Smith. The important feature of herbertsmithite is that its copper ions form a nearly perfect kagome lattice. Naturally occurring herbertsmithite, however, contains impurities that interfere with its magnetic properties.
The breakthrough came from a multidisciplinary team at MIT. Led by physicist Young Lee and chemist Daniel Nocera, the researchers developed a painstaking process to grow large, pure single crystals of synthetic herbertsmithite. One crystal, just 7 millimeters long and weighing 0.2 grams, took ten months to grow.
In 2012, the MIT team, in collaboration with researchers from NIST and Johns Hopkins University, used these pure crystals to confirm the material's exotic nature. They used a technique called neutron scattering at the NIST Center for Neutron Research. By firing a beam of neutrons at the crystal, they could map the behavior of the electron spins. The results were remarkable. Instead of scattering with a single, well-defined energy, the neutrons emerged with a wide continuum of energies. This indicated the presence of fractionalized excitations known as spinons, a signature of a quantum spin liquid state and an important discovery in condensed matter physics.