Ultrafast electron calorimetry uncovers a new long-lived metastable state in 1
            <i>T</i>
            -TaSe
            <sub>2</sub>
            mediated by mode-selective electron-phonon coupling

Shi, Xun; You, Wenjing; Zhang, Yingchao; Tao, Zhensheng; Oppeneer, Peter M.; Wu, Xianxin; Thomale, Ronny; Roßnagel, Kai; Bauer, Michael; Kapteyn, Henry C.; Murnane, Margaret M.

doi:10.1126/sciadv.aav4449

Cited by 56 publications

(38 citation statements)

References 53 publications

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“…2(b), although on a completely different scale of fluences. In 1T-TaSe2, even after the pump induced band shift has reached a plateau, the coherent phonon oscillations related to the PLD are seen, uncovering a new metastable state mediated by selective electron-phonon coupling [49]. What is remarkable, is that the damping time of those oscillations shows similarities to our observations of Fig.…”

Section: Model and Discussionsupporting

confidence: 82%

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Excitonic and lattice contributions to the charge density wave in 1T−TiSe2 revealed by a phonon bottleneck

Hedayat

Sayers

Bugini

et al. 2019

Phys. Rev. Research

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Understanding collective electronic states such as superconductivity and charge density waves is pivotal for fundamental science and applications. The layered transition metal dichalcogenide 1T-TiSe2 hosts a unique charge density wave (CDW) phase transition whose origins are still not fully understood. Here, we present ultrafast time-and angleresolved photoemission spectroscopy (TR-ARPES) measurements complemented by time-resolved reflectivity (TRR) which allows us to establish the contribution of excitonic and electron-phonon interactions to the CDW. We monitor the energy shift of the valence band (VB) and coupling to coherent phonons as a function of laser fluence. The VB shift, directly related to the CDW gap closure, exhibits a markedly slower recovery dynamics at fluences above Fth = 60 J cm -2 . This observation coincides with a shift in the relative weight of coherently coupled phonons to higher frequency modes in time-resolved reflectivity (TRR), suggesting a phonon bottleneck. Using a rate equation model, the emergence of a high-fluence bottleneck is attributed to an abrupt reduction in coupled phonon damping and an increase in exciton dissociation rate linked to the loss of CDW superlattice phonons. Thus, our work establishes the important role of both excitonic and phononic interactions in the CDW phase transition and the advantage of combining complementary femtosecond techniques to understand the complex interactions in quantum materials. Corresponding authorCorrespondence to Enrico Da Como edc25@bath.ac.uk APPENDIX A: TIME-DEPENDENCE OF FREE CARRIER POPULATION AND LINEARITY WITH PUMP FLUENCEIn order to check the linearity of the laser pumping effect in our TR-ARPES experiment, we have analysed the total intensity above the Fermi level, EF, for all spectra, which provides an indication of the transient free carrier population induced by the pump pulse. For this, we used the normalised spectra in Fig. 6 at each pump-probe delay and integrated across the high energy tail to intensity ≤ 0.25 (just below the nodal point of all spectra in the normal phase) as shown

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Section: Model and Discussionsupporting

confidence: 82%

“…To generalize the discussion and extend it to other TMD materials exhibiting a CDW transition, it is interesting to compare our results with the very recent work of Shi et al on 1T-TaSe2 [49].…”

Section: Model and Discussionmentioning

confidence: 58%

Excitonic and lattice contributions to the charge density wave in 1T−TiSe2 revealed by a phonon bottleneck

Hedayat

Sayers

Bugini

et al. 2019

Phys. Rev. Research

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“…The elucidation of the specific underlying lattice dynamics, however, is beyond the capabilities of trARPES and calls for a direct probing of phonon couplings and populations by ultrafast diffuse scattering techniques [55]. We speculate that the rapid atomic disordering due to mode-selective emission of (high-frequency) CDW-coupled phonons by hot electrons, as uncovered here, may be a more generic phenomenon [12,56] than the recently demonstrated collective launch of incoherent phonons driven by an ultrafast change to a highly anisotropic, flat lattice potential [30].…”

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confidence: 74%

Bypassing the Structural Bottleneck in the Ultrafast Melting of Electronic Order

et al. 2020

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Impulsive optical excitation generally results in a complex nonequilibrium electron and lattice dynamics that involves multiple processes on distinct timescales, and a common conception is that for times shorter than about 100 fs the gap in the electronic spectrum is not seriously affected by lattice vibrations. Here, however, by directly monitoring the photoinduced collapse of the spectral gap in a canonical chargedensity-wave material, the blue bronze Rb 0.3 MoO 3 , we find that ultrafast (∼60 fs) vibrational disordering due to efficient hot-electron energy dissipation quenches the gap significantly faster than the typical structural bottleneck time corresponding to one half-cycle oscillation (∼315 fs) of the coherent chargedensity-wave amplitude mode. This result not only demonstrates the importance of incoherent lattice motion in the photoinduced quenching of electronic order, but also resolves the perennial debate about the nature of the spectral gap in a coupled electron-lattice system.

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“…A detailed structural description of the change in stacking at the C-IC phase transition may contribute to the understanding of the correlation between the intralayer structure and the interlayer van der Waals interaction in TMDs. An understanding of this correlation may shed light on the processes that underpin the many intriguing fundamental properties of this class of materials such as the long-lived metastable state in 1T -TaSe 2 [23], hidden states in 1T -TaS 2 [24], and the relation between CDW and superconductivity [25].…”

Section: Introductionmentioning

confidence: 99%

Transient three-dimensional structural dynamics in 1T−TaSe2

Grånäs

Weissenrieder

2020

Phys. Rev. B

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We report on thermal and optically driven transitions between the commensurate (C) and incommensurate (IC) charge-density wave (CDW) phases of 1T-TaSe 2. Optical excitation results in suppression of the C-CDW on a subpicosecond timescale. The optically driven C to IC transition involves a short-lived (∼1 ps) unreconstructed phase. Nucleation of an IC phase stacking order is observed already at ∼4 ps following photoexcitation. The short timescales involved in establishing the stacking order implies that the nucleation of the IC phase is influenced by the local geometry of the adjacent layers such that the stacking direction of the C phase determines the stacking direction of the IC phase. From this follows that the nucleation of the IC-CDW is inherently three dimensional (3D). We observe the activation of a coherent shear mode in the optically driven transitions to the transiently stabilized unreconstructed phase. The activation mechanism starts with a rapid lifting of the periodic lattice distortions (PLD) of the Ta sublattice which results in formation of local transient velocity disparities in the Se sublattice. The local differences in Se-phonon amplitudes result in noncompensated shear forces between the layers. This is an example of a multistep coherent launching mechanism. The energy of the optically excited electronic state dissipates energy into modes of the PLD through strong electron-phonon coupling. The rapid suppression of the PLD launches the third step, a coherent vibrational shear mode with low dissipation. The results highlight the importance in considering the 3D nature of the CDWs in the analysis of both structure and dynamics in transition-metal dichalcogenides.

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Ultrafast electron calorimetry uncovers a new long-lived metastable state in 1 T -TaSe ₂ mediated by mode-selective electron-phonon coupling

Cited by 56 publications

References 53 publications

Excitonic and lattice contributions to the charge density wave in 1T−TiSe2 revealed by a phonon bottleneck

Excitonic and lattice contributions to the charge density wave in 1T−TiSe2 revealed by a phonon bottleneck

Bypassing the Structural Bottleneck in the Ultrafast Melting of Electronic Order

Transient three-dimensional structural dynamics in 1T−TaSe2

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Ultrafast electron calorimetry uncovers a new long-lived metastable state in 1 T -TaSe 2 mediated by mode-selective electron-phonon coupling

Cited by 56 publications

References 53 publications

Excitonic and lattice contributions to the charge density wave in 1T−TiSe2 revealed by a phonon bottleneck

Excitonic and lattice contributions to the charge density wave in 1T−TiSe2 revealed by a phonon bottleneck

Bypassing the Structural Bottleneck in the Ultrafast Melting of Electronic Order

Transient three-dimensional structural dynamics in 1T−TaSe2

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Product

Resources

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Ultrafast electron calorimetry uncovers a new long-lived metastable state in 1 T -TaSe ₂ mediated by mode-selective electron-phonon coupling