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New states of order may arise in quantum magnetic materials under the influence of magnetic fields. Now an international team has gained new insights into these special states
New states of order may arise in quantum magnetic materials under the influence of magnetic fields. Now an international team has gained new insights into these special states of matter, thanks to experiments at Berlin’s BER II neutron source and its high-field magnet. BER II served science until the end of 2019 and has since been closed. The results of the BER II data are currently being published.
Dr. “We carried out the measurements in November 2019, our experiment was one of the last experiments performed on a high-field magnet at BER,” says Ellen Fogg. The physicist leads a group at the Quantum Magnetism Laboratory of the École Polytechnique Fédérale de Lausanne (EPFL) and has now published intriguing new insights into quantum materials obtained in collaboration with colleagues in Japan, Qatar and Switzerland.
“Many effects in matter only become visible under extreme conditions, that is, at temperatures near zero Kelvin and magnetic fields above 20 Tesla,” he says. The ideal place to study these effects was the BER II neutron source, where the HZB team deployed a unique high-field magnet reaching almost 26 Tesla.
“The evaluation took a long time,” he says. This is because neutron scattering data does not automatically provide a picture but must be interpreted. This requires convincing theoretical models. “We played table tennis with a team of theorists, but now we have some very interesting results.”
Fogg and his team analyzed SrCu samples2 (BO3)2 is a system model for ideal frustration in a two-dimensional (2D) rotation system. It consists of pairs of spins arranged perpendicularly on a square lattice and affecting each other in different ways. This “fully frustrated” geometry gives rise to many unusual effects described in terms of entangled quantum states and their excitations (magnons). The magnetic order in such materials is often described as a Bose-Einstein Condensation (BEC) of magnons.
“We wanted to find out whether this magnon BEC also occurs in strong magnetic fields in our model system or whether there is an alternative mechanism,” says Fogg. The neutron scattering experiment in the BER II high-field magnet was ideally suited for this purpose: “We were able to measure the spin excitations of SrCu2 (BO3)2 up to 25.9 T and reproduce experimental spectra with high accuracy using theoretical models.” The experiments were carried out at ambient pressure and temperatures close to absolute zero, 200 millielvin.
Analysis and interpretation of the measurement data show that a spin-nematic phase is formed under these extremely strong magnetic fields. Instead of single magnons, these are pairs of bonded magnons that condense at this stage. There is even an analogy with superconductivity that suggests the spin-nematic phase in SrCu.2 (BO3)2 It is best understood as a condensation of bosonic Cooper pairs.
The results show that neutron scattering experiments in extremely strong magnetic fields can be used to probe previously unknown regions of matter, including associated phases of many-body systems. Fogg concludes: “Many new situations and orders can still be found in conditions of intense frustration and controlled excess.”
Source: Port Altele
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