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Posted: Jun 22, 2017
Spin liquids - back to the roots
(Nanowerk News) Since 1973, Anderson's resonating valence bond model remains a paradigm for microscopic description of quantum spin liquids in frustrated magnets. It is of fundamental interest as a building unit for more complex quantum-mechanically entangled states that can be used in quantum computing.
Researchers from the Chair of Experimental Physics VI/EKM report in Nature Communications ("Nearest-neighbor resonating valence bonds in YbMgGaO4") first experimental signatures of excitations from this fundamental state exposed by a neutron-scattering study performed in collaboration with Rutherford Appleton Laboratory in Oxford and Renmin University of China.
Sketch of Anderson's resonating valence bond state formed by localized spins shown in green. The pair of opposite spins ("valence bond") is highlighted by a yellow oval. (Image: Universitšt Augsburg, EP VI/EKM)
Liquids entail haphazardly moving particles that can be correlated on the short-range scale, but lack any long-range order. In contrast to gases, liquids are only weakly compressible, because separations between their particles are small, and inter-particle interactions strong.
A liquid-like state can also form in magnets, where electron spins act as individual particles. Neighboring spins in a spin liquid strongly interact with each other, but evade long-range order, unlike, for example, in ferromagnets, where parallel alignment of spins throughout the crystal generates macroscopic magnetization that can drive rotation of the motor of an electric car or interact with Earth's magnetic field in a compass.
Spins are pairwise correlated, but remain disordered
Back in 1973 American physicist and eventual Nobel prize winner Philip W. Anderson contemplated a model, where spins are arranged on a triangular plane, and only adjacent spins (nearest neighbors) interact. These interactions trigger spins to be mutually antiparallel, but a global antiparallel (antiferromagnetic) configuration is prevented by the triangular arrangement.
The quantum-mechanical description proposed by Anderson is based on the idea of pair-wise correlations, where different pairs form, as shown in the Figure. In each pair, spins are opposite to each other forming resonating valence bonds (RVBs), the name used to emphasize close resemblance with chemical bonds between atoms in molecules and crystals.
The RVB state is quantum-mechanically entangled, it can not be represented by a simple combination of individual spins. Such entanglement opens new possibilities for high-performance calculations in a quantum computer.
Despite far-reaching implications for present-day theories, the validity of Anderson's model of the RVB state was in the meantime questioned, and signatures of the RVB state were nowhere to be seen experimentally.
New substance with the triangular spin geometry
"The formation of Anderson's RVB state requires magnetic frustration, the presence of competing interactions between the spins" explains Dr. Alexander Tsirlin, the leader of the young research group at the Center for Electronic Correlations and Magnetism at the Institute of Physics in Augsburg.
This is made possible by a new substance, YbMgGaO4, that was prepared and investigated in collaboration with Renmin University of China and Rutherford Appleton Lab in Oxford, UK. The original chemical compound features regular triangular arrangement of magnetic moments, which are localized on the ytterbium atoms (see the Figure).
Earlier work by the team confirmed that even at temperatures of several hundredths of degree above the absolute zero spins remain dynamic in the form of a spin liquid evading long-range order, a pre-condition for building the long-sought RVB state.
Magnetic excitations follow predictions of Anderson's theory
Neutrons scatter from crystals changing direction and energy, and providing researchers with a sensitive probe of correlations between the spins. Neutron-scattering experiments on YbMgGaO4 reveal two distinct regimes. At higher transfer energies, where neutrons trigger high-energy excitations, experimental observations are in perfect agreement with Anderson's RVB model.
"After several decades, signatures of the nearest-neighbor RVB state have been finally observed", explains Prof. Dr. Philipp Gegenwart, head of the Chair of Experimental Physics VI / EKM.
Less clear remains the experimental response at low energies, where Anderson's RVB picture fails. This part of the spectrum appears to be intertwined with magnetic interactions beyond Anderson's model, and may give researchers further clues as to why the RVB state has formed.