Discovering the Elusive Quantum Spin Liquid State

The Science

Scientists may have discovered a quantum phase where magnetic moments of electrons (the strength and orientation of a magnet) inherently change over time and never become ordered even at absolute zero temperature. This exotic state of matter is called a quantum spin liquid.  Scientists discovered evidence of it in sodium ytterbium oxide (NaYbO2). Even at temperatures close to absolute zero, the magnetic moments in this state never stay stable due to quantum fluctuations. However, the scientists found they could align the magnetic moments by applying a magnetic field to the material. Using magnetic fields provides scientists a unique way to understand how this novel quantum spin liquid state forms.

The Impact

Researchers predicted decades ago that quantum spin liquids should exist. While they have looked for them experimentally, it remains controversial if they have been seen before. Previously, disorder on the atomic scale clouded researchers’ ability to see these quantum spin states in real materials. This disorder generally arises from chemical imperfections, such as atoms missing or not located on expected sites in a crystal. Researchers in this study chose a material with minimal disorder, making it easier for them to observe signatures of the quantum spin liquid. Using a magnetic field to manipulate this new quantum spin liquid state could be important in the growing field of quantum information science.

Summary

Quantum spin liquids are an intensively sought magnetic phase of matter of broad interest to scientists. They are an exotic new phase of matter and may have potential use as quantum bits in quantum information-based applications. In addition, these may help scientists understand high temperature superconductivity and entangled correlated electrons.  Finding materials that realize this state has been a challenge.  One predicted way to form a quantum spin liquid state is via decorating highly frustrated quantum moments on an antiferromagnetically-coupled triangular atomic lattice. In this work, scientists discovered NaYbO2 had such a lattice and minimal chemical disorder.  Researchers observed the magnetic moments (spin ½) continued to fluctuate and remain disordered consistent within a quantum spin liquid state down to 50 mK.  Crucially, scientists also observed the emergence of an antiferromagnetically ordered state in this material when they applied an external magnetic field and lifted the underlying magnetic frustration on a triangular lattice. This field-induced ordered state strongly suggests that extrinsic, chemical disorder is not the origin of the disordered ground state, and that instead quantum fluctuations drive the formation of a true quantum disordered magnetic phase.  Studying the boundary between quantum order and disorder in NaYbO2 presents a promising new frontier in the study of quantum spin liquids. It may provide a foundation to develop these materials for applications in quantum information science as topologically protected qubits (quantum bits), robust against local disorder and perturbations.=

Funding

DOE, Office of Science, Basic Energy Sciences, for both experimental and theoretical work. Neutron characterization was performed at the NIST Center for Neutron Research.  Partial support for a co-author was through the National Science Foundation Graduate Research Fellowship Program. 

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