FERMI | T-ReX Strain-enhanced charge order in a kagome metal unveiled by time-resolved optical spectroscopy Kagome metals are fertile grounds for unconventional electronic behavior, where geometry-induced frustration enables the emergence of complex many-body phenomena, including superconductivity, topological states, and Charge Density Waves (CDWs). Among these systems, ScV6Sn6 stands out for hosting an unconventional CDW phase whose origin lies not in a conventional Peierls instability, but in the softening of a flat phonon band. This band involves out-of-plane vibrations of the Sc and Sn atoms and leads to a charge modulation that does not match the wavevector of the leading lattice instability, marking a striking departure from the canonical CDW behavior. In this work, researchers made use of time-resolved optical spectroscopy to gently excite the CDW phase of ScV6Sn6 and probe its Amplitude Mode (AM), namely, the mode linked to the amplitude modulation of the electronic and lattice distortions. This experiment brings information on the microscopic nature of the CDW phase. In particular, time-resolved reflectivity experiments with femtosecond time resolution were performed at the T-ReX facility at FERMI, Elettra, using the pump-probe approach. The AM brings information about the strength of the CDW order, with the quantities of interest being its frequency and linewidth. In general, the frequency of the AM softens (ie, gets smaller) as the temperature of the sample increases towards the critical temperature of the CDW phase, signalling that the strength of the CDW phase is decreased while approaching the critical temperature. Remarkably, in the present study the authors have shown that the amplitude mode (AM) of the CDW can be also controlled via uniaxial strain, externally applied to the sample. This was made possible thanks to a strain device integrated in the sample holder, that allows to precisely set the amount of desired strain. In detail, by applying mechanical deformation along the in-plane axis of the ScV6Sn6 crystal and measuring the resulting dynamics by femtosecond time-resolved optical reflectivity experiments under the presence of strain, the researchers were able to track the evolution of the AM frequency across a wide range of temperature and strain (Fig. 1). The data reveal a key insight: while the amplitude mode frequency is essentially unaffected by strain at low temperatures, where the CDW is fully developed, a marked AM frequency increase occurs close to the transition temperature (≈92 K). This behavior indicates a reduction in the phonon softening, hence a straininduced stabilization of the CDW, further supported by the observed narrowing of the mode linewidth. In other words, strain effectively enhances the CDW state by reinforcing the electron-phonon coupling that underpins it. To assess the structural impact of the strain, High-Resolution X-ray Diffraction (HR-XRD) was performed. The measurements show a measurable elongation (≈0.6%) of the c-axis lattice parameter upon strain application, confirming a uniaxial expansion of the crystal aligned with the direction of the relevant phonon modes. These observations are quantitatively consistent with first-principles Density Functional Theory (DFT) calculations, which demonstrate that the frequency of the softened mode increases with in-plane compressive strain (equivalent to out-of-plane lattice expansion), particularly when electronic smearing (simulating an increased temperature) is present. Importantly, this mode softening is second-order in character, while the overall CDW phase transition remains first-order, hence highlighting an unusual decoupling between lattice dynamics and the thermodynamic transition. Such a scenario has rarely been observed and adds depth to the understanding of bosonic instabilities in geometrically frustrated lattices. The experimental findings have been benchmarked by DFT calculations. In particular, to investigate the straininduced reduction of AM phonon softening, DFT calculations were conducted in the CDW phase using VASP code with GGA-PBE functionals. Calculations were performed varying the uniaxial strain along the a-axis and including an electronic smearing to simulate the pump-induced elevated electronic temperatures. Phonon frequencies were extracted via quadratic fits to the Born-Oppenheimer potential, accounting for effective mass and force constants. Results show that strain increases phonon frequency by enhancing the energy differences between the pristine and CDW phases, while electronic smearing destabilizes the CDW, reducing the phonon softening. A 3.2% reduction in phonon softening under strain at 70 K matches well with the time-resolved reflectivity data. The calculated CDW energy gap also reflects a similar strain-dependent trend, highlighting the interplay between strain, polarization, and CDW stability. Together, these experimental and theoretical findings illustrate a powerful strategy for controlling and enhancing collective modes in quantum materials, using mechanical strain as a tuning parameter to access or stabilize exotic phases. Moreover, this study underscores the importance of time-resolved optical spectroscopy in the femtosecond time-domain for unravelling the subtle interplay between lattice vibrations and emergent electronic orders. This work not only sheds light on the complex electron-phonon interplay in ScV6Sn6 but also paves the way for broader efforts to control unconventional ordered phases in kagome and topological materials, via strain engineering, mode-selective probing, and time-resolved spectroscopy. Elettra Highlights 2024–2025 94
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