Tuning materials properties on demand is at the heart of solid-state physics [1]. Charge transport and magnetism are strongly linked to the overlap of electronic wave functions and can be, thus, manipulated by varying the electronic bandwidth through chemical substitution or physical pressure. Yet, a controlled modification of geometrical frustration of transfer integrals and exchange interactions remained inaccessible so far. Here, we tune kagome frustration in two distinct directions by pressure and strain in the clean quantum spin system Y3Cu9(OH)19)Cl8 without disorder [2]. As we break the hexagonal symmetry in a continuous manner through in-plane uniaxial strain – in situ within one single crystal – we revive antiferromagnetic order through a controlled release of frustration [3]. Vice versa, we increase frustration by applying hydrostatic pressure which suppresses antiferromagnetic order entirely – establishing a major step forward towards ultimately stabilizing a real quantum spin liquid [4]. Based on our comprehensive optical characterizations of charge excitations and magnetoelastic coupling [5,6], we reveal the absence of any structural transition by probing optical phonon modes under pressure [4]. Our pioneering endeavors [3,4] demonstrate spectroscopy under pressure and strain as powerful tools to tweak interacting electrons on frustrated lattices – with the prospect of tuning frustration-induced topological flat bands in kagome metals and other exotic phenomena. References [1] D. N. Basov, R. D. Averitt, and D. Hsieh, Nat. Mater. 16, 1077–1088 (2017). [2] P. Puphal, M. Bolte, D. Sheptyakov, A. Pustogow, K. Kliemt, M. Dressel, M. Baenitz, and C. Krellner, J. Mater. Chem. C 5, 2629 (2017). [3] Jierong Wang, Y.-S. Su, M. Spitaler, K.M. Zoch, C. Krellner, P. Puphal, S.E. Brown, and A. Pustogow, Phys. Rev. Lett. 131, 256501 (2023). [4] D. Chatterjee, P. Doležal, F. Abbruciati, T. Biesner, K. M. Zoch, R. Khasanov, S. Sohel Islam, G. Kaur, S. Roh, F. Capitani, G. Garbarino, C. Krellner, P. Mendels, E. Kermarrec, M. Dressel, B. Wehinger, A. Pustogow, F. Bert, and P. Puphal, arXiv:2502.09733. [5] A. Pustogow, Ying Li, I. Voloshenko, P. Puphal, C. Krellner, I. I. Mazin, M. Dressel, and R. Valentí, Phys. Rev. B 96, 241114(R) (2017). [6] P. Doležal, T. Biesner, Y. Li, R. Mathew Roy, S. Roh, R. Valentí, M. Dressel, P. Puphal, and A. Pustogow, Phys. Rev. B 110, 174445 (2024).