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要旨:
Quantum materials are solids with tantalizing properties arising from special symmetry, dimensionality, topology, and many-body interactions between elementary degrees of freedom (charge, spin, orbital, and lattice). They are host to fascinating emergent phenomena such as unconventional superconductivity, Mott transitions, charge density waves (CDWs), and topologically-protected electronic states, and hold promise for revolutionizing electricity generation and distribution, (quantum) computing, and data storage. Gaining a microscopic understanding of quantum materials to experimentally realize and control many-body phases is one of the overarching goals of modern condensed-matter physics. A promising pathway to fulfilling this goal are ultrashort optical excitations. Tracking the response of a broken-symmetry state after perturbation by a light pulse grants access to the relevant many-body interactions governing the emergence of equilibrium quantum states. Additionally, the interaction of quantum materials with light can induce novel emergent phenomena by steering a system towards specific transient or metastable states, facilitating control over additional functionalities within the light-enriched phase diagram. This thesis explores the electronic structure and ultrafast dynamics of several single-layer and layered quasi-2D quantum materials using femtosecond time- and angle-resolved photoemission spectroscopy (trARPES). We first establish a novel time-of-flight-based photoelectron detector for trARPES, a momentum microscope, and benchmark its performance against the widely used hemispherical analyzers. Next, we utilize the complementary nature of both detectors to characterize the electronic nonequilibrium properties of a novel 2D topological insulator, bismuthene. We map the transiently occupied conduction band after photoexcitation, observe faint signatures of topological edge states within the large fundamental bulk band gap, and track the full relaxation pathway of hot photocarriers. Next, using trARPES in combination with a complementary time-resolved structural probe, we investigate the dynamics of a prototypical layered CDW compound, TbTe3, after optical excitation. Tracking the system's order parameter during the photoinduced CDW melting and recovery reveals a surprising reemergence of CDW order at elevated electronic temperatures far greater than the thermal critical temperature, which we attribute to strong nonequilibrium between coupled electronic and lattice degrees of freedom. Additionally, we show how changes of the CDW energy gap during the CDW-to-metal transition can lead to a transient modulation of the relaxation rates of excited high-energy photocarriers. Theoretical calculations based on a nonequilibrium Green's function formalism reveal the critical role of the phase space of electron-electron scattering and the interplay of elementary interactions and the electronic band structure. Lastly, we study the ultrafast nonthermal pathway to a long-lived metastable quantum state in bulk 1T-TaS2 after optical excitation. Utilizing a double-pulse excitation of a vibrational CDW coherence, we demonstrate a high degree of control over the phase transition, laying the basis for actively controlling macroscopic material properties on ultrafast timescales. The thesis concludes with an outlook on future research of quantum materials enabled by time-resolved momentum microscopy.
