What is the discovery?
Several kagome metals have been recently shown to host exotic electron-correlation-driven instabilities and topologically non-trivial electronic states. One such material, ScV6Sn6, exhibits charge order as a structural distortion below 92K, where the lattice distorts along the wave vector q = (1/3,1/3,1/3). The primary distortion (an out-of-plane modulation of the Sn and Sc sites) is understood to arise from strong electron-phonon coupling; however, density functional theory predicts a more energetically favorable distortion mode at q = (1/3,1/3,1/2), with a different interlayer periodicity relative to what is experimentally observed. In a recent paper appearing in the journal Physical Review Materials, a team of CHEXS users from UCSB and Cornell was able to resolve this apparent contradiction by comprehensively mapping the subtle charge correlations in ScV6Sn6 as a function of temperature at the QM2 beamline. The team, led by Prof. Stephen Wilson and first-author Postdoctoral Researcher Ganesh Pokharel, found evidence of the theoretically predicted distortion as a fluctuating, short-range-ordered state, with correlations diverging as the sample is cooled. However, the authors also show that this state is geometrically frustrated, precluding the development of long-range order. Instead, a different charge modulation is stabilized at 92K, consistent with a heuristic “3-state Potts model”.
Why is this important?
Enormous effort is currently underway worldwide to learn to (i) predict and (ii) control the unusual quantum phases of kagome metals. This paper has important consequences for each of these efforts. First, it resolves a contradiction between predictions and experiments in ScV6Sn6, by directly observing the predicted L=1/2 correlations. Second, it identifies the heuristic mechanism (geometrical frustration) that explains why one type of charge order is stabilized, while another is inhibited. Geometrical frustration has long been recognized as a tool that can be used to tune between competing quantum states in magnetic materials, but its importance in charge ordered materials has been less well explored. This research therefore identifies a new avenue for future manipulation and control of quantum states in kagome metals, by subtle adjustment of lattice geometry, for example by applying uniaxial strain.
Why did this research need CHEXS?
The QM2 beamline at CHEXS was designed to rapidly and comprehensively map subtle correlations in quantum materials, over a wide range of temperatures. This work exploited all the unique features of QM2, including high flux of high energy x-rays, access to very low temperatures, sensitivity to weak signals, and ability to process enormous experimental datasets. Analysis also made use of the unsupervised machine learning algorithm “XTEC”, which was developed at by Prof Eun-Ah Kim’s team at Cornell and is optimized for discovery of subtle temperature dependent phase changes in large x-ray datasets like those measured at QM2.
How was the work funded?
The Center for High Energy X-ray Sciences (CHEXS), NSF MPS/BIO/ENG (DMR-1829070)
Q-AMASE-i: Quantum Foundry at UCSB, NSF (DMR-1906325)
MRSEC at UCSB, NSF (DMR- 1720256)
Gordon and Betty Moore Foundation, EPiQS Initiative (GBMF10436)
Simons Fellows in Theoretical Physics (920665)
Reference
Frustrated charge order and cooperative distortions in ScV6Sn6
G Pokharel, BR Ortiz, L Kautzsch, SJ Gomez Alvarado, K Mallayya, G Wu, E-A Kim, JPC Ruff, S Sarker, and SD Wilson
Phys. Rev. Materials 7, 104201 (2023); https://doi.org/10.1103/PhysRevMaterials.7.104201