A Rather Illuminating Experiment Using Ultrafast Lasers

Historically, in condensed matter physics, there have been generally two experimental strategies: (i) scattering/spectroscopy experiments such as angle-resolved photoemission or X-ray scattering, and (ii) experiments involving macroscopic variables such as specific heat, resistivity, or magnetization. In the past few decades, a qualitatively new frontier opened up. This consisted of experiments that involved kicking a system out of equilibrium (usually with a pulsed femtosecond laser) and monitoring its relaxation back to equilibrium.

There are by now tons of papers using this technique, and I’ve also blogged previously about a particularly elegant use of these methods in a slightly different context than the one I’ll be discussing hereThe paper I’m concerned with today uses ultrafast lasers in another rather illuminating context. It involved apparently melting the electronic order in 1T-TiSe2, while the lattice distortion remained in tact.

The importance of this experiment requires a little background. There has been debate for a couple decades now in the literature as to whether excitonic correlations are driving the charge density wave transition in 1T-TiSe2. This experiment claims that one can non-thermally melt (with the ultrafast laser) the excitonic order while the lattice remains distorted. This is done by monitoring the optical response of the sample at time intervals after the intense pulsed laser hits the sample: zone-folded phonons are monitored as evidence of the lattice distortion while the plasmon peak energy is monitored as evidence of excitonic order. The conclusion that the authors come to is that it cannot be purely an excitonic mechanism that is responsible for the charge density wave in this material as the plasmon peak energy is drastically affected by the laser pulse, while the zone-folded phonons do not react.

There is one caveat in this otherwise quite solid piece of work, however. The authors have equated the shift in the plasmon peak frequency (immediately following the arrival of the ultrafast laser pulse on the sample) with the melting of excitonic order. While this interpretation is plausible, it is not necessarily correct considering that the laser is photo-exciting a large number of charge carriers.

Regardless of this last point, the paper is definitely worth the read and highlights the kinds of experiments that can be conducted with these techniques. To my mind, this is one of the more illuminating experiments conducted on 1T-TiSe2 as many other experiments have been quite inconclusive about the mechanism behind the CDW in this material. Despite the aforementioned caveat, this experiment quite definitively demonstrates that one cannot ignore the role that electron-phonon coupling plays in the formation of the CDW in 1T-TiSe2.

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