Abstract
Time-resolved soft-x-ray photoemission spectroscopy is used to simultaneously measure the ultrafast dynamics of core-level spectral functions and excited states upon excitation of excitons in WSe_{2}. Wepresent a many-body approximation for the Green's function, which excellently describes the transient core-hole spectral function. The relative dynamics of excited-state signal and core levels clearly show a delayed core-hole renormalization due to screening by excited quasifree carriers resulting from an excitonic Mott transition. These findings establish time-resolved core-level photoelectron spectroscopy as a sensitive probe of subtle electronic many-body interactions and ultrafast electronic phase transitions.
Highlights
The relative dynamics of excited-state signal and core levels clearly show a delayed core-hole renormalization due to screening by excited quasifree carriers resulting from an excitonic Mott transition
Optoelectronic properties of semiconductors are largely governed by two types of excitations—excitons [1], the bosonic quasiparticles comprised of an electron and a hole bound by Coulomb interaction, and quasifree carriers (QFCs) of single-particle character [2,3]
In this Letter, we show that detailed information about the dynamics of both excitons and QFCs can be deduced from the simultaneous measurement of the core-hole spectral function and the excited state population with ultrafast time-resolved x-ray photoelectron spectroscopy [6]
Summary
Higher binding energy originates from the core-hole screening by conduction electrons. In the presence of excited carriers, a semiconductor becomes partially metallic and one can expect a renormalization of the core-hole line shape This opens up the possibility of studying nonequilibrium dynamics with XPS. Technological advances of femtosecond x-ray sources [20] and photoelectron detectors [21] have made ultrafast trXPS experiments possible These include the observation of melting of charge-density wave states in Mott insulators [22,23], charge-transfer dynamics at semiconductor interfaces [24] or transient surface-photovoltage control [25]. We show a potential for trXPS studies of excitonic Mott transition, which is important for understanding the fundamental processes behind the dynamics of optically excited semiconductors This is especially relevant to 2D materials, such as transition metal dichalcogenides (TMDCs), in which excitonic effects dominate low-energy behavior due to large exciton binding energy and reduced dielectric screening. In order to understand the origin of the observed trXPS spectral changes, we propose a theoretical model to
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