Ultrafast dynamics of charge-density waves

Kai Rossnagel, Christian-Albrechts-Universität zu Kiel

Abstract:

Studying condensed-matter systems out of equilibrium, by impulsively pumping them at one time and stroboscopically probing them at femtosecond and picosecond time delays, is a fertile research field with many discoveries waiting to be made. This point of view can be motivated by the possibility to transiently drive systems into novel, hidden, or competing phases that cannot be accessed or cannot even exist in equilibrium. However, a more basic motivation is the ability to temporally resolve dynamics at the fundamental time scales of electron and lattice dynamics, where direct dynamical information and temporal dissection can be used to identify the dominant degrees of freedom of the equilibrium phase, to determine the strength of the coupling between the different degrees of freedom, and thus to gain insight into the nature of complex equilibrium phases and, possibly, the origin of phase transitions. A prime example is the chicken-and-egg question that inevitably arises when an electronic phase transition is accompanied by a lattice distortion.

In the first part of my talk, I will address this question for canonical quasi-one- and quasi-two-dimensional charge-density-wave (CDW) systems: the blue bronze Rb_0.3MoO₃ and the transition-metal dichalcogenides 1T-TiSe₂, 1T-TaS₂, and 1T-TaSe₂. Using femtosecond time- and angle-resolved photoemission spectroscopy with extreme ultraviolet radiation, we can directly resolve and compare the momentum-dependent energy-gap and spectral-weight dynamics on the first few 100 femtoseconds after impulsive near-infrared excitation [1-3]. Surprisingly, different gap melting time scales and mechanisms are found for the different materials, suggesting different natures of the CDWs with dominant electronic or dominant lattice degrees of freedom, respectively. In the second part of the talk, I will complement the results on ultrafast electron dynamics in CDW systems by time-resolved electron and x-ray diffraction results on 1T-TaS₂ [4] and thin Cr films [5], respectively, where the focus is on ultrafast structural CDW dynamics. In the end, both types of experiments provide a motivation for future combined time-resolved photoelectron spectroscopy and diffraction experiments at X-ray free-electron lasers with high repetition rates.

[1] T. Rohwer et al., Nature 471, 490 (2011).

[2] S. Hellmann et al., Nature Commun. 3, 1069 (2012).

[3] C. Sohrt et al., Faraday Discuss. 171, 243 (2014).

[4] K. Haupt et al., Phys. Rev. Lett. 116, 016402 (2016).

[5] A. Singer et al., arXiv:1511.08261.