Ultrafast nano-imaging of the order parameter in a structural phase transition

Abstract number
754
Event
Virtual Early Career European Microscopy Congress 2020
Presentation Form
Submitted Oral
DOI
10.22443/rms.emc2020.754
Corresponding Email
[email protected]
Session
PST.7 - Fast and Ultrafast dynamics using Transmission Electron Microscopy
Authors
Thomas Danz (1), Till Domröse (1), Claus Ropers (1)
Affiliations
1. 4th Physical Institute – Solids and Nanostructures, University of Göttingen
Keywords

charge-density wave, order parameter mapping, phase transition, ultrafast dark-field imaging, ultrafast transmission electron microscopy

Abstract text

In this work, we demonstrate a new experimental approach enabling real-space phase imaging in a charge-density wave (CDW) model system with nanometer spatial and femtosecond temporal resolution. By means of a tailored dark-field (DF) approach, we obtain maps of the CDW order parameter in an ultrafast transmission electron microscope.

Ultrafast transmission electron microscopy (UTEM) is an emerging approach to study ultrafast processes in heterogeneous materials with nanometer spatial resolution [1]. Using the imaging, diffraction, and spectroscopy capabilities of such an instrument, we investigate structural, electronic, and magnetic dynamics in a laser pump/electron probe scheme. To this end, the Göttingen UTEM is equipped with a nanoscopic tip emitter, delivering highly coherent electron pulses with down to 200 fs pulse duration, 0.6 eV energy width, and sub-nm spot diameter by linear photoemission [2].

These exceptional beam properties allow for a versatile use of the Göttingen UTEM, as demonstrated in recent years [3-6]. However, some of the most interesting possibilities are connected to the investigation of various kinds of structural and electronic phase transitions in correlated materials, such as transition metal dichalcogenides, e.g., 1T-TaS2. This quasi-2D material favors a periodic modulation of the electronic density, i.e. the formation of CDWs coupled to a periodic lattice distortion (PLD) [7]. Various ultrafast electron diffraction experiments have elucidated the optically induced dynamics of transitions between several CDW/PLD phases [8-11].

Figure 1. (a) Simplified schematic of the experimental setup, and scanning electron micrograph of the tailored DF aperture array. (b) Ultrafast electron micrographs of the specimen before and after time zero, showing CDW domains of the room- (bright) and the high-temperature phase (dark). (c) Static electron micrographs of the steady-state heating experiment at two different continuous-wave laser heating intensities.

In the experiments, a free-standing, single-crystalline 1T-TaS2 thin film [12] is pumped out of the nearly commensurate CDW phase at room temperature towards the high-temperature incommensurate CDW phase using a spatially structured laser field distribution (see Fig. 1a for a schematic of the experimental setup). We employ ultrafast DF imaging using a tailored DF aperture array to follow the formation, stabilization, and relaxation of CDW domain patterns on their intrinsic femtosecond to nanosecond timescales, yielding nanoscale access to the order parameter of the structural phase transition (see Fig. 1b).

Additionally, we obtain a precise thermal characterization of our specimen structure from a steady-state heating experiment (see Fig. 1c). Based on the information obtained, we model the spatio-temporal domain evolution using the time-dependent Ginzburg-Landau framework. We demonstrate that this approach reproduces the most prominent features of the ultrafast experiment, and allows us to distinguish different regimes of the observed dynamics [13].

In conclusion, we have reported on the first ultrafast real-space imaging of CDW dynamics. In combination with static specimen characterization and theoretical modeling, ultrafast DF imaging allows for novel insights into the interplay of order parameter dynamics and thermal transport on nanometer length and femtosecond timescales.

References

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[12] Th. Danz et al., J. Phys.: Condens. Matter 28, 356002 (2016).
[13] Th. Danz et al., in preparation.