Session X44: Ultrafast Collective Dynamics of Charge Density Waves

Ultrafast electron imaging and diffraction of non-equilibrium structural dynamics in the charge-density wave system 1T-TaS2

Time-resolved electron imaging, diffraction and spectroscopy are exceptional laboratory-based tools to trace non-equilibrium phenomena in materials with a sensitivity to structural, electronic and electromagnetic degrees of freedom. The capabilities of these approaches are largely governed by the quality of the beam of electrons used.
This talk will discuss recent advances made by employing high-coherence ultrashort electron pulses from nanoscale field emitters, which substantially enhance the achievable image resolution in both real and reciprocal space. We have recently developed two complementary experimental techniques with ultimate surface sensitivity and spatial resolution, respectively, namely Ultrafast Low-Energy Electron Diffraction (ULEED) [1] and Ultrafast Transmission Electron Microscopy (UTEM) [2].
These methods are aplied to study non-equilibrium dynamics in the prominent charge-density wave (CDW) material 1T-TaS₂ after femtosecond optical excitation. Specifically, we study structural phase transitions between different CDW phases and the optical excitation of fluctuation modes. In one set of experiments, ULEED is used to investigate the transient population of phonons and phase modes (phasons). For the transition between the domain-like nearly-commensurate (NC) CDW phase and the incommensurate (IC) phase, we identify the importance of dislocation-type topological defects in the periodic lattice distortion for establishing long-range phase-order. Finally, a new contrast mechanism for UTEM is developed that allows for ultrafast real-space domain imaging upon the structural phase transition.

[1] S. Vogelgesang et al., Nature Physics 14, 184–190 (2018).
[2] A. Feist et al., Ultramicroscopy 176, 63-73 (2017).   

Evidence for Topological Defects in a Photoinduced Charge Density Wave Transition

Upon excitation with an intense laser pulse, a symmetry-broken ground state can undergo a non-equilibrium phase transition through pathways different from those in thermal equilibrium. The mechanism underlying these photoinduced phase transitions has long been researched in the study of condensed matter systems, but many details in this ultrafast, non-adiabatic regime still remain to be clarified. To this end, we investigate the light-induced melting of a unidirectional charge density wave (CDW) in LaTe₃. Using a suite of time-resolved probes, we independently track the amplitude and phase dynamics of the CDW. We find that a fast (approximately 1 picosecond) recovery of the CDW amplitude is followed by a slower re-establishment of phase coherence. This longer timescale is dictated by the presence of topological defects: long-range order is inhibited and is only restored when the defects annihilate.   

Ultrafast time-resolved x-ray scattering reveals diffusive charge order dynamics in La2-xBaxCuO4

Charge order (CO) is ubiquitous in the high-T_c cuprates but its relation to superconductivity is still unclear. While static charge order seems to anticorrelate with superconductivity, charge order fluctuations may favor or enhance Cooper pairing. Here, we investigate the collective CO dynamics in the prototypical cuprate La_2-xBa_xCuO₄ with time-resolved resonant soft x-ray scattering. By suddenly quenching the CO diffraction peak with ultrafast optical pump pulses and probing its recovery in the time domain with resonant soft x-rays, we find that CO excitations in the energy range 0.4-2.0 meV are overdamped and diffusive. At energy scales lower than 0.4 meV, the CO phase exhibits dynamical critical scaling, a universal behavior associated with the propagation of topological defects. Our observations imply that charge order is dynamic and may play a role in establishing superconducting pairing.   

Exploring ideal charge-density-wave in superconducting YBCO via innovative uses of X-ray FEL and the pulsed magnet

Increasingly compelling evidence of various forms of non-superconducting electronic orders, such as charge/spin density waves (CDW/SDW), in the cuprate high-T_c superconductors has fundamentally altered our understanding of the essential physics of these materials. However, it has been difficult to establish the nature of the quantum (zero-temperature) phases that compete and/or coexist with superconductivity. In order to overcome this difficulty, it is necessary to destabilize superconducting long-range order at low temperatures by a high magnetic field. In this context, we performed x-ray scattering at an x-ray free electron laser (FEL) in the presence of pulsed high magnetic fields. This approach provides us with the new insight to understand the intertwined phenomena between CDW and SC in YBa₂Cu₃O_x (YBCO) cuprates at magnetic fields, as well as an ideal CDW phase. More details will be touched in this presentation.