Soft phonons in materials with competing phases

Orateur : Frank WEBER (IQMT, KIT, Karlsruhe, Germany)

It has become increasingly clear that the idealized picture of Charge Order (CO) arising from a Fermi surface nesting instability does not apply to any realistic CO material. Inst ead, CO and any related structural instability seems to generically involve a delicate interplay between momentum-dependent electron-phonon coupling (EPC) and the effects of electronic correlations on the band structure. The recognition of the central role of momentum-dependent EPC, however, also opened up a series of new questions, particularly about its influence on short-ranged ordered phases, on the competition between distinct CO phases, and on the interplay with other types of order utilizing EPC, e.g. superconductivity. Here, I will review our recent work on compounds from two seminal material families : I. the A15 superconductor V3Si and II. rare-earth tritellurides RTe3 (R = Tb, Dy, Er). The work on V3Si highlights the close interrelation between an EPC driven structural phase transition with superconductivity and the relevance of the structural soft phonon mode for mediating superconductivity has long been discussed [1]. Though our combined experimental and theoretical study clearly points to only a small impact of the soft mode on the value of the superconducting transition temperature Tc, the huge Tc-induced changes of the soft mode line shape reveals the strong phase competition and explains the absence of the structural instability if superconductivity sets in first on cooling. The family of rare-earth tritellurides RTe3 features charge-density-wave (CDW) and different members were frequently referenced as canonical Peierls-like CDW systems. What sets the heavier members of the RTe3 series apart is the observation of a second CDW transition with an orthogonal ordering wave vector of almost the same magnitude. Hence, RTe3 provide an ideal playground to study the interplay of multiple CDW phases and emerging superconductivity. We have used inelastic x-ray scattering in combination with detailed ab-initio lattice dynamical calculations to study the lattice dynamics of TbTe3 [2] and DyTe3 [3] focusing on momentum-dependent EPC and the soft phonon modes of the coexisting CO phases. Our results clearly demonstrate that momentum-dependent EPC is essential. Surprisingly, we did not find a soft phonon mode at the lower temperature CDW transition, e.g., in DyTe3, though several phonons responded strongly to onset of the high-temperature CDW phase. This indicates that the low-temperature CDW order is not just the 90° rotated analogue of the one appearing at high temperatures. This is corroborated by our ongoing investigations in PdxErTe3 [4], where strong disorder suppresses the long-range CO phases present in pristine RTe3.

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