Researchers from Augsburg, Oxford, and Nanjing report in Nature Communications on a neutron experiment exposing experimental signatures of a low-temperature state predicted 44 years ago
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 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.
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.
Yuesheng Li, Devashibhai Adroja, David Voneshen, Robert I. Bewley, Qingming Zhang, Alexander A. Tsirlin, and Philipp Gegenwart, Nearest-neighbor resonating valence bonds in YbMgGaO4, Nat. Commun. 8 (2017), 15814.
Prof. Dr. Philipp Gegenwart and Dr. Alexander Tsirlin
Chair of Experimental Physics VI / EKM
Institute of Physics / Center of Electronic Correlations and Magnetism
University of Augsburg
Klaus P. Prem | idw - Informationsdienst Wissenschaft
First evidence on the source of extragalactic particles
13.07.2018 | Technische Universität München
Simpler interferometer can fine tune even the quickest pulses of light
12.07.2018 | University of Rochester
For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.
To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...
For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...
Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...
Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....
Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.
Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...
13.07.2018 | Event News
12.07.2018 | Event News
03.07.2018 | Event News
13.07.2018 | Event News
13.07.2018 | Materials Sciences
13.07.2018 | Life Sciences