Monitoring Ultrafast Lattice Dynamics in 2D NbTe2

Abstract

The discovery and control of emergent phenomena in strongly-correlated materials is a cornerstone of modern condensed matter physics and materials science. Among these phenomena, charge density waves (CDWs) represent a striking example of how the coupling between electrons and the atomic lattice can give rise to new properties. Understanding the microscopic mechanisms behind CDW formation and their dynamical evolution is crucial not only for fundamental science, but also for the development of ultrafast, energy-efficient electronic and quantum devices. The idea behind controlling such phenomena has been propelled by the advent of ultrafast lasers which enables investigation of electron-lattice interactions and has lead to the realization of many phase transitions. In this thesis, the ultrafast lattice dynamics of the layered quasi-two-dimensional material niobium ditelluride (NbTe2) are explored, a system known to host a robust CDW phase. By employing both time-resolved transient reflectivity (TR) and ultrafast electron diffrac- tion (UED), the femtosecond response is revealed from two different perspectives. These techniques enable direct observations of the dynamical structural distortion and coherent phonon generation with sub-picosecond temporal resolution. These findings reveal a rapid, photoinduced suppression of the CDW order within 200 femtoseconds, followed by coherent lattice oscillations that reflect the material’s transient structural state. UED measurements quantify a transient 1.3% CDW order suppression, while TR data show fluence-dependent modulations of phonon frequencies and lifetimes, highlighting the complex nature of the lattice response. At high fluence, the CDW order of NbTe2 approaches a complete melting along with an irreversible tellurium crystallization on the sample surface—a phenomenon characterized by Raman spectroscopy and interpreted through density functional theory (DFT)-based calculations.

Beyond characterizing the behavior of NbTe2, this thesis establishes a broader experimental framework for investigating symmetry-breaking transitions and metastable states in low- dimensional quantum materials. The work highlights the power of ultrafast techniques for unveiling non-equilibrium phenomena and offers insights into how light can be used to engineer and manipulate material properties on demand.