Optical studies of structural phase transition in the vanadium-based kagome metal ScV6Sn6

Hu, Tianchen; Xu, Shuxiang; Li, Yue; Wu, Qiong; Liu, Qiaomei; Zhang, Sijie; Li, Rongsheng; Zhou, Xinyu; Yuan, Jiayu; Wang, Dong; Dong, Ting; Wang, Naiyan

doi:10.48550/arxiv.2211.03412

“…Thirdly, our findings explain the substantial drop of resistivity below T CDW [Fig. 1(c)]: the large EPC and extended phonon softening revealed in this work both enhance electron scattering above T CDW , and the removal of these effects below T CDW strongly reduces electron scattering, consistent with optical conductivity measurements [50].…”

Section: Discussionsupporting

confidence: 87%

“…4(c)]. Most importantly, in the q z = 1 3 plane, no peak is present at q s = ( 1 3 , 1 3 , 1 3 ), suggesting that Fermi surface nesting is completely irrelevant in the formation of q s -CDW, consistent with previous findings [50,58]. In the q z = 1 2 plane, hot spots are found around ( 16 , 1 6 , 1 2 ) and q * = ( 1 3 , 1 3 , 1 2 ), indicating a possible contribution of nesting towards q * -CDW.…”

Section: First-principles Calculationssupporting

confidence: 89%

“…Furthermore, whereas the CDW in AV 3 Sb 5 is dominated by in-plane displacements of the V atoms [43] and hosts a superconducting ground state, the CDW in ScV 6 Sn 6 is mostly driven by displacements of the Sc and Sn atoms along the c-axis [48], and no superconductivity is observed up to pressures of 11 GPa [49]. Optical reflectivity measurements and electronic structure calculations indicate that the CDW in ScV 6 Sn 6 is unlikely to result from Fermi-surface nesting, and the CDW does not exhibit a prominent charge gap formation [50], distinct from AV 3 Sb 5 [51,52]. This view is reinforced by electronic structure measurements, which in addition identify the lattice or a Lifshitz transition as instrumental for the CDW in ScV 6 Sn 6 [53,54].…”

Section: Introductionmentioning

confidence: 96%

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Competing charge-density wave instabilities in the kagome metal ScV6Sn6

Song

¹

,

Cao

²

,

Xu

³

et al. 2023

Preprint

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Owing to its unique geometry, the kagome lattice hosts various many-body quantum states including frustrated magnetism, superconductivity, charge-density waves (CDWs), and topologically nontrivial phases of matter. Kagome metals with 2×2 CDWs, corresponding to the nesting of van Hove saddle points, are found to exhibit a number of exotic electronic phases, and have been the focus of much recent research. More recently, a √3×√3 CDW was discovered in the kagome metal ScV6Sn6 below TCDW≈91 K, whose underlying mechanism and formation process remain unclear, particularly in comparison with the 2×2 kagome CDWs. Using inelastic X-ray scattering, we discover a short-range √3×√3×2 CDW that is dominant in ScV6Sn6 well above TCDW, distinct from the √3×√3×3 CDW below TCDW. The short-range CDW grows upon cooling, and is accompanied by the softening of phonons, indicative of its dynamic nature. As the √3×√3×3 CDW appears, the short-range CDW becomes suppressed, revealing a competition between these CDW instabilities. The competing CDW instabilities in ScV6Sn6 lead to an unusual CDW formation process, with the most pronounced phonon softening and the static CDW occurring at different wavevectors. Our first-principles calculations indicate that the √3×√3×2 CDW is energetically favored, consistent with experimental observations at high temperatures. However, the √3×√3×3 CDW is selected as the ground state due to a large wavevector-dependent electron-phonon coupling (EPC), which also accounts for the enhanced electron scattering above TCDW. Our findings reveal the dominance of EPC-driven correlated many-body physics in ScV6Sn6, making it an ideal system to study emergent quantum matter in the strong EPC regime.

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“…However, unlike in AVS, the Sn p-bands in SVS contribute weakly (only through the trigonal and not hexagonal Sn) to the Fermi surface, thus the two vHs at M are derived from the dorbitals of the V-sublayer. Initial ARPES data [42], supported by optical spectroscopy [43], revealed no energy gap at the Fermi level, in contrast to the 20 meV CDW gap observed by scanning tunneling spectroscopy (STM) [44]. Moreover, the comparison of different members of the 166 kagome family (some at different electron num-bers) with and without periodic modulations suggests an unconventional nature of the CDW, strongly coupled with its lattice dynamics [45].…”

mentioning

confidence: 99%

Softening of a flat phonon mode in the kagome ScV6Sn6

Blanco-Canosa

¹

,

A.

²

,

Hu

³

et al. 2023

Preprint

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The long-range electronic modulations recently discovered in the geometrically frustrated kagome lattice have opened new avenues to explore the effect of correlations in materials with topological electron flat bands. Charge density waves (CDW), magnetism and superconducting phases are thought to be - depending on the electron number - the result of either the flat bands, the multiple Dirac crossings or the van Hove singularities close to the Fermi level. Nevertheless, the observation of the lattice response to the emergent new phases of matter, a soft phonon mode, has remained elusive and the microscopic origin of CDWs is still unknown. Here, we show, for the first time, a complete melting of the ScV$_6$Sn$_6$ (166) kagome lattice. The low energy longitudinal phonon with propagation vector $\frac{1}{3} \frac{1}{3} \frac{1}{2}$ collapses at 98 K, without the emergence of long-range charge order, which, remarkably, sets in with a propagation vector $\frac{1}{3} \frac{1}{3} \frac{1}{3}$. The CDW is driven (but locks at a different vector) by the softening of an overdamped phonon flat plane at k$_z$=$\pi$, characterized by an out-of-plane vibration of the trigonal Sn atoms. We observe broad phonon anomalies in momentum space, pointing to \hhy{(1)} the existence of approximately flat phonon bands which gain some dispersion due to electron renormalization, and \hhy{(2)} the effects of the momentum-dependent electron-phonon interaction in the CDW formation. \textit{Ab initio} and analytical calculations corroborate the experimental finding to indicate that the weak leading order phonon instability is located at the wave vector $\frac{1}{3} \frac{1}{3} \frac{1}{2}$ of a rather flat collapsed mode. In particular, we analytically calculate the phonon frequency renormalization from high temperatures to the soft mode, and relate it to a peak in the orbital-resolved susceptibility of the trigonal Sn atoms, obtaining excellent match with both ab initio and experimental results, and explaining the origin of the approximately flat phonon dispersion. Our data report the first example of the collapse of a kagome bosonic mode (softening of a flat phonon plane) and promote the 166 compounds of the kagome family as primary candidates to explore correlated flat phonon-topological flat electron physics.

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Softening of a flat phonon mode in the kagome ScV6Sn6

Korshunov,

Hu,

Subires

et al. 2023

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Geometrically frustrated kagome lattices are raising as novel platforms to engineer correlated topological electron flat bands that are prominent to electronic instabilities. Here, we demonstrate a phonon softening at the kz = π plane in ScV6Sn6. The low energy longitudinal phonon collapses at ~98 K and q = $$\frac{1}{3}\frac{1}{3}\frac{1}{2}$$ 1 3 1 3 1 2 due to the electron-phonon interaction, without the emergence of long-range charge order which sets in at a different propagation vector qCDW = $$\frac{1}{3}\frac{1}{3}\frac{1}{3}$$ 1 3 1 3 1 3 . Theoretical calculations corroborate the experimental finding to indicate that the leading instability is located at $$\frac{1}{3}\frac{1}{3}\frac{1}{2}$$ 1 3 1 3 1 2 of a rather flat mode. We relate the phonon renormalization to the orbital-resolved susceptibility of the trigonal Sn atoms and explain the approximately flat phonon dispersion. Our data report the first example of the collapse of a kagome bosonic mode and promote the 166 compounds of kagomes as primary candidates to explore correlated flat phonon-topological flat electron physics.

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Optical studies of structural phase transition in the vanadium-based kagome metal ScV6Sn6

Cited by 3 publications

References 39 publications

Competing charge-density wave instabilities in the kagome metal ScV6Sn6

Competing charge-density wave instabilities in the kagome metal ScV6Sn6

Softening of a flat phonon mode in the kagome ScV6Sn6

Softening of a flat phonon mode in the kagome ScV6Sn6

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