Ultrafast Metamorphosis of a Complex Charge-Density Wave

Haupt, K.; Eichberger, M.; Erasmus, N.; Rohwer, Andrea; Demšar, Jure; Roßnagel, Kai; Schwoerer, H.

doi:10.1103/physrevlett.116.016402

Cited by 90 publications

(121 citation statements)

References 48 publications

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“…The color scale is adapted to the intensity of the peak, which is ten times lower for the 50 ps delay. The peak at 9 ns is found to be much narrower than at 50 ps, in agreement with previous experiments [10,15].…”

Section: Resultssupporting

confidence: 92%

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Domain-size effects on the dynamics of a charge density wave in 1T−TaS2

et al. 2017

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Recent experiments have shown that the high temperature incommensurate (I) charge density wave (CDW) phase of 1T-TaS2 can be photoinduced from the lower temperature, nearly commensurate (NC) CDW state. Here we report a time-resolved x-ray diffraction study of the growth process of the photoinduced I-CDW domains. The layered nature of the material results in a marked anisotropy in the size of the photoinduced domains of the I-phase. These are found to grow self-similarly, their shape remaining unchanged throughout the growth process. The photoinduced dynamics of the newly formed I-CDW phase was probed at various stages of the growth process using a double pump scheme, where a first pump creates I-CDW domains and a second pump excites the newly formed I-CDW state. We observe larger magnitudes of the coherently excited I-CDW amplitude mode in smaller domains, which suggests that the incommensurate lattice distortion is less stable for smaller domain sizes.

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Section: Resultssupporting

confidence: 92%

“…The ratios and delay trend using these cylindrical coordinates are identical for both the radius and azimuthal angle, which is consistent with the explanation of Refs. [10,15] where the authors argue that the appearance of the I-phase arises from a highly disordered phase and not from a pure rotation.…”

Section: Resultsmentioning

confidence: 99%

Domain-size effects on the dynamics of a charge density wave in 1T−TaS2

et al. 2017

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“…A plausible scenario behind this observation could be a displacive excitation of an overdamped amplitude mode, as already proposed in Ref. 22. Figure 4a shows X-ray diffraction profiles of the I satellite peak, measured at various delays after laser excitation (F 0 = 6.8 mJ/cm 2 , T = 240 K).…”

Section: To 53 Mj/cmmentioning

confidence: 96%

“…Diffraction techniques are especially well adapted to study CDW compounds [5,10,12,14,15,22,[24][25][26][27][28][29]. Indeed, a structural modulation with wavevector q gives rise to satellite peaks located at positions ± q with respect to each regular lattice peak (Fig.…”

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confidence: 99%

Ultrafast Formation of a Charge Density Wave State in 1T−TaS2 : Observation at Nanometer Scales Using Time-Resolved X-Ray Diffraction

et al. 2017

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Femtosecond time-resolved X-ray diffraction is used to study a photo-induced phase transition between two charge density wave (CDW) states in 1T-TaS2, namely the nearly commensurate (NC) and the incommensurate (I) CDW states. Structural modulations associated with the NC-CDW order are found to disappear within 400 fs. The photo-induced I-CDW phase then develops through a nucleation/growth process which ends 100 ps after laser excitation. We demonstrate that the newly formed I-CDW phase is fragmented into several nanometric domains that are growing through a coarsening process. The coarsening dynamics is found to follow the universal Lifshitz-Allen-Cahn growth law, which describes the ordering kinetics in systems exhibiting a non-conservative order parameter.Among strongly correlated electron systems, superconductors and materials exhibiting metal-insulator transitions are usually characterized by strong electronelectron and electron-phonon couplings [1][2][3]. At thermodynamic equilibrium, the corresponding many-body interactions lead to rich phase diagrams as a function of temperature, pressure or doping. Such compounds also display fascinating out-of-equilibrium physics, in the form of ultra-fast symmetry changes known as photoinduced phase transitions [4][5][6], and occurrence of new, transient states [6][7][8].Charge density wave (CDW) states are broken symmetry states of metals arising from electron-phonon interactions. They are characterized by a periodic modulation of both atomic positions and electron density. The metal-to-CDW phase transition is characterized by the growth of a complex-valued order parameter p = A exp iΦ , which reflects both the amplitude A and the phase Φ of the periodic modulation [3]. A number of photo-induced phase transitions that have been achieved in CDW compounds correspond to a suppression of the CDW order, i.e. a transition between a CDW state and a metallic state free of any structural modulation [5,[9][10][11][12][13][14][15]. Among those, the photo-induced suppression of the CDW state in blue bronze was shown to involve a coherent motion of atoms along the normal coordinates of the CDW amplitude mode [5]. In this case, the amplitude mode allows continuous variations of the modulus of the order parameter |p|, the metallic state corresponding to |p|=0. In the present work, we focus on the photo-induced phase transition between the nearly commensurate (NC) and the incommensurate (I) CDW states in 1T-TaS 2 , which exhibit two distinct order parameters. When thermallyinduced, this first-order phase transition involves a discontinuous change of atomic positions, and a coexistence of NC and I phase domains over a 3 K range [16,17]. It is thus expected that the photo-induced I phase appears through non-coherent atomic motions, by a nucleation/growth process. We report that the photo-induced NC → I phase is completed within 100 ps after laser excitation. At this 100 ps delay, the photo-induced I-CDW phase is found divided into domains with a typical size of 150Å. Its ordering kinet...

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“…[18]. The crystals were cleaved at ≈30 K in ultra-high vacuum, and ARPES data were collected on the SGM-3 end station of ASTRID2 [19].…”

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confidence: 99%

Quasi-one-dimensional metallic band dispersion in the commensurate charge density wave of 1T−TaS2

et al. 2017

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The commensurate charge density wave (CDW) in the layered compound 1T -TaS2 has hitherto mostly been treated as a quasi two-dimensional phenomenon. Recent band structure calculations have, however, predicted that the CDW coexists with a nearly one-dimensional metallic dispersion perpendicular to the crystal planes. Using synchrotron radiation based angle-resolved photoemission spectroscopy, we show that this metallic band does in fact exist. Its occupied band width is in excellent agreement with predictions for a simple τc stacking order of the CDW between adjacent layers and its periodicity in the c direction is 2π/c.The 1T polytype of TaS 2 is one of the most studied layered transition metal dichalcogenides (TMDCs). Its rich electronic phase diagram involves several charge density wave (CDW) transitions driven by strong electronic correlations and electron-phonon coupling [1,2]. Particular focus has been on the ground state below 180 K which is a commensurate CDW phase with a √ 13 × √ 13 socalled Star of David reconstruction that is rotated by 13.9• against the lattice. In this phase, the large-scale periodic lattice distortion is thought to coexist with a Mott insulating ground state arising from the single electron localized on the centre atom of the Star of David [1,3]. While the research on CDWs in layered TMDCs is more than 40 years old, renewed interest has been driven by the possibility to elucidate transitions between different CDW states using ultrafast techniques [4][5][6][7][8][9]; by the observation of metastable "hidden states" [9]; and by the experimental accessibility of metallic TMDCs as single layers [10,11].The realization that CDWs could be different in single layer TMDCs than in analogous bulk materials has drawn attention to the fact that viewing the bulk materials' electronic properties as essentially two-dimensional might be an oversimplification. While reduced dimensionality has a significant impact on electronic instabilities, due to increased electronic correlations and electron-phonon coupling, interlayer coupling also appears to be essential for a full understanding of the electronic properties of these materials [12][13][14][15][16][17]. Specifically, several calculations predict a one-dimensional metallic band formation along the Γ − A direction of the Brillouin zone in the ground state CDW of 1T -TaS 2 (i.e., perpendicular to the planes). This is found in density functional theory calculations [12,16,17], even when electronic correlations are taken into account [14,15]. While the metallic band along Γ−A is universally found in all calculations, the details of the dispersion depend on the stacking order of the CDW unit cell between adjacent planes [15,16]. Angle-resolved photoemission spectroscopy (ARPES)is an experimental technique capable of determining the three-dimensional band structure of crystalline solids, and numerous ARPES studies have been performed on 1T -TaS 2 (for a review see Ref.[2]). However, very little attention has been paid to the possibly three-dimensional ch...

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Ultrafast Metamorphosis of a Complex Charge-Density Wave

Cited by 90 publications

References 48 publications

Domain-size effects on the dynamics of a charge density wave in 1T−TaS2

Domain-size effects on the dynamics of a charge density wave in 1T−TaS2

Ultrafast Formation of a Charge Density Wave State in 1T−TaS2 : Observation at Nanometer Scales Using Time-Resolved X-Ray Diffraction

Quasi-one-dimensional metallic band dispersion in the commensurate charge density wave of 1T−TaS2

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