Stirring up a Quantum Spin Liquid with Disorder

The Science

Disorder is generally thought to be detrimental to creating materials with unusual magnetism or other quantum phenomena. However, a team found that weak disorder surprisingly stabilizes a rare quantum state called a quantum spin liquid. In this state, fluctuations of electronic spins persist all the way to temperatures near absolute zero. The particular material is made of praseodymium, zirconium, and oxygen (Pr₂Zr₂O₇). The material contains rare earth ions (Pr³⁺) with an even number of electrons on a weakly disordered crystalline lattice. While the crystalline lattice frustrates conventional magnetism, weak disorder promotes the rare quantum spin liquid state.

The Impact

The discovery that weak disorder can induce this rare state opens a new direction in the quest for a practical quantum spin liquid. This state of matter could be useful as building blocks for quantum computers. These computers could be orders of magnitude faster than today’s computers. This speed translates into computational power to solve problems that can’t be feasibly solved today.

Summary

A rare state of matter called a quantum spin liquid proposed over 40 years ago is being pursued for its unique physical properties. In a quantum spin liquid, electronic spins do not form a static ordered pattern as in conventional magnetic materials, but fluctuate in a coordinated (entangled) fashion even at extremely low temperatures. But can such quantum fluctuating states of matter exist in practical materials which are inevitably disordered and can quantitative information about them be extracted from neutron scattering experiments? Towards addressing such questions, a team of researchers led by Johns Hopkins University studied a pyrochlore oxide with the chemical formula Pr₂Zr₂O₇. Conventional magnetism is frustrated by the geometry of the pyrochlore lattice when magnetic ions occupy a specific location in the crystal lattice and this can lead to exotic magnetic states. Combining solid-state synthesis, high-quality single crystal growth, and advanced neutron scattering, this research unveiled the presence of a quantum spin liquid in Pr₂Zr₂O₇. The inelastic neutron scattering data show the signature for this rare state where a band of (inelastic, w>0) scattering intensity persists at temperatures close to absolute zero. This research points to a new paradigm that at appropriate levels, weak structural disorder may actually promote the formation of a quantum spin liquid with entangled electronic spins. This ability to manipulate and induce quantum magnetism in spite of disorder could pave the way for new and more practical materials with the elusive quantum spin liquid for applications including quantum computing. 

Funding

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (Institute for Quantum Matter (IQM) at Johns Hopkins University); Japan Science and Technology Agency, Japan Society for the Promotion of Science; National Institute of Standards and Technology and National Science Foundation (Center for High Resolution Neutron Scattering).

Publication

J.J. Wen, S. Koohpayeh, K. Ross, B. Trump, T.M. McQueen, K. Kimura, S. Nakatsuji, Y. Qiu, D. Pajerowski, J. Copley, and C. Broholm, “Disordered route to the Coulomb quantum spin liquid: Random transverse fields on spin ice in Pr₂Zr₂O₇.” Physical Review Letters 118, 107206 (2017). [DOI: 10.1103/PhysRevLett.118.107206]

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