Non-thermal separation of electronic and structural orders in a persisting charge density wave (1604.05627v1)

Abstract: The simultaneous ordering of different degrees of freedom in complex materials undergoing spontaneous symmetry-breaking transitions often involves intricate couplings that have remained elusive in phenomena as wide ranging as stripe formation, unconventional superconductivity or colossal magnetoresistance. Ultrafast optical, x-ray and electron pulses can elucidate the microscopic interplay between these orders by probing the electronic and lattice dynamics separately, but a simultaneous direct observation of multiple orders on the femtosecond scale has been challenging. Here we show that ultrabroadband terahertz pulses can simultaneously trace the ultrafast evolution of coexisting lattice and electronic orders. For the example of a charge-density-wave (CDW) in 1T-TiSe2, we demonstrate that two components of the CDW order parameter - excitonic correlations and a periodic lattice distortion (PLD) - respond very differently to 12-fs optical excitation. Even when the excitonic order of the CDW is quenched, the PLD can persist in a coherently excited state. This observation proves that excitonic correlations are not the sole driving force of the CDW transition in 1T-TiSe2, and exemplifies the sort of profound insight that disentangling strongly coupled components of order parameters in the time domain may provide for the understanding of a broad class of phase transitions.

Non-thermal Separation of Electronic and Structural Orders in a Charge Density Wave

In this paper, the authors investigate the complex interplay between electronic and structural orders in the charge density wave (CDW) of 1T-TiSe$_2$ using ultrabroadband terahertz pulses. This paper is significant for understanding phase transitions characterized by spontaneous symmetry breaking, which is prevalent in many complex materials. The experimental setup uses 12-femtosecond optical excitations to disentangle the excitonic correlations and the periodic lattice distortion (PLD) that constitute the CDW order parameter.

The authors demonstrate that these two components respond differently upon excitation; notably, the PLD persists in an excited state even when excitonic order is quenched. This finding is pivotal as it implies that excitonic correlations are not the sole drivers of the CDW transition in 1T-TiSe$_2$, challenging the prevalent singular mechanistic hypothesis. This newly observed decoupling provides insights into how strongly coupled order parameters can be separated in the time domain.

Key numerical results from the paper reveal that upon photoexcitation, the plasmon pole, associated with the electronic order, remains largely intact, indicating that charge order is maintained even when excitonic correlation is temporarily suppressed. However, an increase in pump fluence beyond a threshold leads to a significant and transient disruption of the electronic order, characterized by a reduction of the carrier scattering time, $\tau$, to that of a normal metallic phase. Importantly, under such high-fluence conditions, the PLD remains unaffected, which is a notable deviation from the behavior expected if electronic interactions were the primary drivers of the PLD.

This non-thermal approach to disentangling electronic and structural dynamics has broader implications. The persistence of the PLD in the absence of excitonic order could mean that similar phenomena may occur in other materials with complex phase transitions, such as cuprates and iron pnictides. As such, the techniques and insights provided by this research could be extended to paper a wide range of materials where multiple order parameters complicate the understanding of phase transitions.

The authors also utilize a theoretical framework to support the empirical findings, suggesting that Coulombic interactions play a subtler role than previously thought. The paper suggests a cooperative coupling between the excitonic and Jahn-Teller effects, where both mechanisms might co-contribute to the CDW state. This insight prompts a reevaluation of theoretical models for CDW phenomena, broadening the understanding of phase transitions beyond electron-phonon or purely electronic-driven models.

Moving forward, this research raises intriguing questions about other potential metastable states that could exist in different materials. The methodology outlined in this paper could be adopted to probe unconventional superconductors and various quantum materials, potentially unveiling new states of matter and highlighting the nuances of electron-phonon interactions in complex systems. The use of ultrabroadband terahertz pulses in femtosecond experiments offers a robust pathway for future investigation into the intricate dynamics governing symmetry-breaking transitions in quantum materials.