Ultrafast dynamics in quantum materials probed by time-and-momentum-resolved techniques

Abstract

The interactions of quasiparticles in quantum materials give rise to intriguing phenomena, including magnetism and superconductivity. However, these interactions are often challenging to understand due to the intertwining of multiple degrees of freedom, such as charge, spin, orbital, and lattice. To fully understand such strongly correlated systems, a suite of experimental techniques that respectively probes various degrees of freedom and simultaneously resolves multiple channels, including energy, momentum, time, and space, are highly desired. This poses a significant challenge for the entire community. In this dissertation, I will focus on a series of experiments performed on quantum material systems utilizing several multi-resolution techniques. Ultrafast electron diffraction (UED) and time-and-angle-resolved photoemission spectroscopy (trARPES) are tools that I co-developed with my colleagues at MIT in the past several years. Supplemented by the time-resolved X-ray diffraction (trXRD) setup at free electron laser facilities around the world, they provide direct access to lattice (UED and trXRD) and electronic (trARPES) structures in quantum materials on an ultrafast timescale of a few hundred femtoseconds. The first part of the dissertation will briefly introduce assorted aspects of ultrafast phenomena as well as the fundamental principles and instrumentation of the several time-andmomentum-resolved techniques. Following the introduction to these time-and-momentumresolved techniques, the second part of the thesis focuses on the coherent acoustic phonons in quantum materials observed with UED. The crystalline lattice is the building block of any solid-state system and, thus, the most important aspect in condensed matter physics research. The study of coherent acoustic phonons, the fundamental coherent excitation of the lattice, could be traced back to the 1980s when solid-state ultrafast lasers were first developed. However, the knowledge about the excitation mechanism was not complete. In this part of the thesis, I will introduce a new pathway for launching coherent acoustic phonons: magnetostriction, and discuss the spin-mediated shear oscillator enabled by this mechanism in van der Waals antiferromagnet. I will further discuss the original methodology I developed that uses coherent acoustic phonon detected with UED as a picosecond timescale "lock-in" experiment that senses nano-scale mechanical motions in ultra-thin quantum materials. The last part of the dissertation will focus on charge density wave (CDW) phase transitions in quantum materials. CDWs are systems where strong interplays between electrons and phonons drive the phase transition that causes the modulation of charge density and is thus accompanied by periodic lattice distortions. In this dissertation, I will focus on systems with multiple interacting CDW orders. These systems are ideal platforms for studying the interplays among multiple order parameters. The suite of probes, including UED, trXRD, and trARPES, offers a comprehensive view of CDW systems from both phononic and electronic perspectives. This part of the thesis will examine a series of CDW materials with multiple CDW orders, including ErTe₃, EuTe₄, and, CsV₃Sb₅. Via a series of ultrafast multi-messenger experiments, I will survey various origins and behaviors of CDW interactions and answer longstanding questions about the nature of CDW ground states in these quantum materials. The overarching theme of this dissertation is to establish a paradigm of problem-solving in quantum materials research via a combination of multiple channels acquired from a suite of ultrafast momentum-resolved techniques. Coherent phonons and CDW systems are two of the richest playgrounds in the ultrafast regime. I am going to investigate various cases where an ultrafast laser pulse decodes the intertwined degrees of freedom in quantum materials. The insight developed in these case studies may be carried over to other quantum material systems with emergent quantum states, such as superconductivity and magnetic orders.