A quantum spin liquid is a state of matter where unpaired electrons’ spins, although entangled, do not show magnetic order even at the zero temperature. The realization of a quantum spin liquid is a long-sought goal in physics. In a paper published in the journal Nature Physics, physicists from Rice University and elsewhere show that cerium zirconium pyrochlore (Ce2Zr2O7), in its single-crystal form, has the right stuff to qualify as the first possible 3D version of a quantum spin liquid.
In 1973, Nobel laureate physicist Philip Anderson proposed the idea of quantum spin liquids based upon the realization that that geometric arrangement of atoms in some crystals could make it impossible for entangled spins to collectively orient themselves in stable arrangements.
The possibility that quantum spin liquids might explain high-temperature superconductivity spurred widespread interest among condensed matter physicists since the 1980s, and interest further increased when it was suggested that some examples of so-called topological quantum spin liquids may be amenable to building qubits for quantum computing.
“A quantum spin liquid is something that scientists define based on what they don’t see,” said lead author Dr. Pengcheng Dai, a member of Rice University’s Center for Quantum Materials.
“You don’t see long-range order in the arrangement of spins. You don’t see disorder. And various other things. It’s not this. It’s not that. There’s no conclusive positive identification.”
The team’s samples are believed to be the first of their kind: (i) pyrochlores because of their 2-to-2-to-7 ratio of cerium, zirconium and oxygen, and (ii) single crystals because the atoms inside them are arranged in a continuous, unbroken lattice.
“We’ve done every experiment that we could think of on this compound,” Dr. Dai said.
The physicists observed that the material underwent no phase transition down to 50 mK (millikelvin), and that there was no disorder in the crystal.
In muon spin relaxation experiments, they demonstrated an absence of long-range magnetic order down to 20 mK.
In diffraction experiments, they showed the sample had no oxygen vacancy or other known defects.
Finally, they did inelastic neutron scattering that showed the presence of a spin-excitation continuum — which may be a quantum spin liquid hallmark — down to 35 mK.
“Despite our efforts, it is impossible to definitively say Ce2Zr2O7 is a spin liquid, partly because physicists haven’t yet agreed on what experimental proof is necessary to make the declaration, and partly because the definition of a quantum spin liquid is a state that exists at absolute zero temperature, an ideal beyond the reach of any experiment,” the researchers said.
Bin Gao et al. Experimental signatures of a three-dimensional quantum spin liquid in effective spin-1/2 Ce2Zr2O7 pyrochlore. Nature Physics, published online July 15, 2019; doi: 10.1038/s41567-019-0577-6
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