Investigations On Ultrafast And Terahertz Responses Of Collective Excitations Associated With Charge Density Waves In Low-dimensional Topological Systems

Many quasi-one-dimensional and two-dimensional(2D)layered materials in lowdimensional topological systems encompass a large portfolio of topological states and strongly correlated electronic structures.The interlayer van der Waals interactions further lead to a highly complex phase diagram involving intricate interactions among electronic,spin,orbital,and lattice degrees of freedom.The collective excitations in these materials are directly related to various macroscopic properties.Utilizing ultrafast optical techniques,it is possible to selectively detect and manipulate different collective excitations,as well as observe their dynamic processes and decouple different order parameters.They hence offer a wealth of opportunities to not only improve our comprehension of fundamental physics but also provide a foundation for the development of next-generation electronic devices.In this thesis,we systematacially investigate three low-dimensional quantum materials with distinct charge density wave(CDW)properties via ultrafast optical pump-probe spectroscopy,and terahertz emission spectroscopy.The main research results are listed below.(1)We investigated the the collective excitations and quasiparticle dynamics in 2D kagome semimetal material CsV3Sb5 using ultrafast optical pump-probe spectroscopy.According to our results,in addition to the CDW phase related quasiparticle and coherent phonon dynamics,we found another additional relaxation process below T*(～20 K).We speculate that this new physical process arises from other electronic excitations,related to the nematicity order in CsV3Sb5.Furthermore,we observe a divergence in the carrier relaxation time near the CDW transition temperature(TCDW～92 K),a CDW gap openning of 42 meV.Moreover,we identify a CDW-induced collective mode at 1.3 THz below TCDW,as well as two additional modes at frequencies of 3.1 THz and 3.8 THz.These two modes exhibit anomalies at 60 K,suggesting a possible connection to the breaking of C6 rotational symmetry,which is consistent with the appearance of uniaxial charge order at this temperature.These findings indicate the presence of other order parameters interacting with the CDW in CsV3Sb5 below TCDW,and the strong coupling between CDW collective excitations and lattice degrees of freedom,providing important insights for further investigations on the interplay between charge order and other order parameters in kagome lattice materials.(2)We investigated for this first time the kagome semimetal material FeGe where antiferromagnetic order coexists with CDW using ultrafast optical pump-probe technique.We find an additional relaxation process below the CDW transition temperature TCDW(～110 K),and identified its origin as possibly due to the opening of new relaxation channels by the fluctuating CDW.We observe a divergence of relaxation time near TCDW in the slow relaxation process,indicating the CDW gap of 56 meV.Meanwhile,we discovered an abnormal spin relaxation near TCDW,suggesting a possible correlation between the charge density wave and antiferromagnetic order.This is the first ultrafast measurement of a 2D antiferromagnetic-CDW kagome lattice material,and its results can be mutually corroborated with other experimental results,playing a significant role in the understanding of the origin of CDWs and the relationship between lattice and magnetism in Kagome lattice material.(3)The terahertz emission spectroscopy has been used to study the CDWs in the typical quasi-one-dimensional Weyl semimetal(TaSe4)2I.Successfully manipulating the phase of CDW collective modes through their interaction with the topological properties.Upon excitation of femtosecond optical pulses with proper photon energies,we observe broadband THz emission including the contributions from the ultrafast photocurrent and the collective modes of CDW.We discover that the collective modes due to phaseamplitude-coupled modes can be manipulated by tuning the ultrafast photocurrent via controlling the polarization of incident light.Essentially,the Weyl fermions play the key role in switching the phase of collective modes.Our findings have significant implications for manipulating CDW,provide novel insights into control of charge ordering parameters in quantum materials.