It’s not every day that we discover a new state of matter in quantum physics, the scientific field devoted to describing the behavior of atomic and subatomic particles in order to elucidate their properties.
Yet this is what an international team of researchers did, including Andrea Bianchi, professor of physics at the University of Montreal and researcher at the Quebec Regrouping on Advanced Materials, and his students Avner Fitterman and Jérémi Dudemaine.
In a recent article published in the scientific journal Physical examination Xresearchers document a “quantum spin liquid ground state” in a magnetic material created in Bianchi’s lab: This2Zr2O7a compound composed of cerium, zirconium and oxygen.
Like a liquid enclosed in an extremely cold solid
In quantum physics, spin is an internal property of electrons related to their rotation. It is the spin that gives the material of a magnet its magnetic properties.
In some materials, spin results in a disorganized structure similar to that of molecules in a liquid, hence the term “spin liquid”.
In general, a material becomes more disorganized as its temperature increases. This is the case, for example, when water turns into steam. But the main characteristic of spin liquids is that they remain disorganized even when cooled to absolute zero (–273°C).
Spin liquids remain disorganized because the direction of the spin continues to fluctuate as the material cools instead of stabilizing in the solid state, as is the case in a classical magnet, in which all the spins are aligned .
The art of “frustrating” electrons
Imagine an electron as a small compass pointing up or down. In conventional magnets, the spins of the electrons are all pointing in the same direction, up or down, creating what is called a “ferromagnetic phase”. It’s what keeps photos and notes pinned to your fridge.
But in quantum-spin liquids, the electrons are positioned in a triangular lattice and form a “threesome” characterized by intense turbulence that interferes with their order. The result is an entangled wave function and no magnetic order.
“When a third electron is added, the spins of the electrons cannot align because the two neighboring electrons must always have opposite spins, creating what we call magnetic frustration,” Bianchi explained. “This generates excitations that maintain spin disorder and therefore the liquid state, even at very low temperatures.”
So how did they add a third electron and cause such frustration?
Creating a threesome
Enter the frustrated magnet This2Zr2O7 created by Bianchi in his laboratory. To his already long list of achievements in the development of advanced materials such as superconductors, we can now add “master in the art of frustrating magnets”.
This2Zr2O7 is a cerium-based material with magnetic properties. “The existence of this compound was known,” Bianchi said. “Our breakthrough was to create it in a particularly pure form. We used samples melted in an optical furnace to produce a near-perfect triangular arrangement of atoms, then verified the quantum state.”
It is this almost perfect triangle that allowed Bianchi and his UdeM team to create a magnetic frustration in Ce2Zr2O7. Together with researchers from McMaster and Colorado State Universities, Los Alamos National Laboratory, and the Max Planck Institute for Complex Systems Physics in Dresden, Germany, they measured the compound’s magnetic scattering.
“Our measurements showed a function of overlapping particles – so no Bragg peaks – a clear sign of the absence of classical magnetic ordering,” Bianchi said. “We also observed a spin distribution with continuously fluctuating directions, which is characteristic of spin liquids and magnetic frustration. This indicates that the material we created behaves like a true spin liquid at low temperatures. .”
From dream to reality
After corroborating these observations with computer simulations, the team concluded that they were indeed witnessing a quantum state never seen before.
“Identifying a new quantum state of matter is a dream come true for any physicist,” Bianchi said. “Our material is groundbreaking because we are the first to show that it can actually be in the form of a spin liquid. This discovery could open the door to new approaches in the design of quantum computers.”
Frustrated magnets in a nutshell
Magnetism is a collective phenomenon in which the electrons of a material all rotate in the same direction. A common example is ferromagnets, which owe their magnetic properties to the alignment of spins. Neighboring electrons can also spin in opposite directions. In this case, the spins still have well-defined directions but there is no magnetization. Frustrated magnets are frustrated because neighboring electrons try to orient their spins in opposite directions, and when they end up in a triangular lattice, they can no longer settle on a common, stable arrangement. The result: a frustrated magnet.
Computer survey confirms first 3D quantum spin liquid
EM Smith et al, Case for a U(1)π Quantum Spin Liquid Ground State in the Dipole-Octupole Pyrochlore Ce2Zr2O7,Physical examination X (2022). DOI: 10.1103/PhysRevX.12.021015
Provided by the University of Montreal
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