What is the discovery?
CHEXS users have directly observed an electronic configuration that is quantum mechanically mixed between the Yb2+ and Yb3+ valence states in the material YbB4. These observations confirm an “intermediate valence” conjecture for this material, which was first hypothesized 50 years ago. They also explain why YbB4 fails to order magnetically at low temperature. These new results are reported by a team lead by Prof. Young-June Kim from the University of Toronto, in a new paper appearing in the Journal of Physics: Condensed Matter. Valence of Yb ions was measured using resonant x-ray emission spectroscopy (RXES) at the PIPOXS beamline. RXES allows high-precision determination of the Yb valence by comparing the intensity of the distinct peaks ascribed to Yb2+ and Yb3+ in the two-dimensional energy spectra (below). The intermediate valence is a signature of magnetic moments on the Yb ions interacting strongly with the spins of the conduction electrons – a process known as the Kondo effect. By monitoring the temperature evolution of the valence in this strongly-hybridized Kondo material, the authors were able to compare the temperature evolution of holes in the 4ƒ band with other similar systems.
Why is it important?
The transformational promise of quantum materials for next generation sensing, computing, and communications technology can only be realized once we learn how different microscopic interactions give rise to different quantum states. In the case of YbB4, the failure to order at low temperatures has often been suggested to arise from geometrical magnetic frustration – the Yb ions arrange themselves in a pattern called the “Shastry-Sutherland” lattice, which is known to frustrate magnetic order under certain conditions. However, this work clearly shows that it is instead a strong Kondo interaction which prohibits magnetic ordering. The level of precision with which single-ion properties can be interrogated at the PIPOXS beamline enables users to shed new light on long-standing problems in quantum magnetism.
Why did this research need CHEXS?
The PIPOXS beamline is the home of Photon-In, Photon-Out X-ray Spectroscopy at CHEXS. PIPOXS delivers high flux beams in small, focused spot sizes, using a combination of CHESS compact undulator technology, cryogenic silicon optics, and a custom-built double-focusing mirror system. PIPOXS operates several custom-built, one-of-a-kind x-ray spectrometers, including the Dual Array Valence Emission Spectrometer (DAVES) which was used for this work. PIPOXS also provides sample environments to access low temperatures, which were required for this work, and are generally essential for interrogating complex quantum states.
How was the work funded?
NSERC of Canada, Discovery Grant (RGPIN-2019-06449)
Canadian Foundation for Innovation
Ontario Research Fund
National Research Foundation of Korea (NRF-2022R1A2C1009516)
The Center for High Energy X-ray Sciences (CHEXS), NSF (DMR-1829070)
Intermediate Valence State in YbB4 Revealed by Resonant X-ray Emission Spectroscopy
Felix Frontini, Blair W Lebert, KK Cho, MS. Song, BK Cho, Christopher J Pollock and Young-June Kim
J. Phys. Condens. Matter (2022); https://doi.org/10.1088/1361-648X/ac7629