Melanie Mueller

I am a group leader at the Fritz Haber Institute of the Max Planck Society in Berlin. My research focuses on the development and application of ultrafast scanning tunnelling microscopy approaches to study photoinduced and nonequilibrium processes at surfaces with atomic spatial resolution. Beyond advancing ultrafast STM techniques, my team and I are particularly interested in applying these techniques to explore complex quantum phases in quantum materials under nonequilibrium conditions. We aim to develop a better understanding of how the local response and properties of such phases shape global material properties, and ultimately, to use light to coherently control quantum phases and visualize their behaviour at the atomic scale. Our long-term goal is to further develop ultrafast STM as a tool for exploring fundamental quantum phenomena and nonequilibrium dynamics in novel material systems.

Abstract

Ultrafast Optical and Terahertz STM of Light-Induced Processes at Solid Surfaces

The integration of broadband optical and terahertz (THz) radiation with low-temperature scanning tunneling microscopy (STM) has opened up new possibilities for probing ultrafast processes and the optical response of surfaces at the atomic scale [1]. I will introduce two modes of ultrafast STM - optical photon-driven STM (ph-STM) and THz-lightwave-driven STM (THz-STM) - which allow for the probing of light-induced and ultrafast processes at surfaces with simultaneous femtosecond temporal and nanometer to angstrom spatial resolution. As a first example, I will discuss the capabilities of ph-STM for the local detection of coherent phonon (CP) dynamics in ultrathin ZnO films on Ag(111) [2]. Here, the optical resonance of the ZnO/Ag(111) interface plays a crucial role in the efficient detection of CPs via the photoinduced tunneling current, which we further explore using the recently developed two-color pump-probe ph-STM. In another study, we use THz-STM to probe the photoinduced dynamics of the charge density wave (CDW) insulator 1T-TaS2 [3]. Upon photoexcitation, we observe a periodic modulation of the tunneling current at a frequency of 2.4 THz, corresponding to the CDW amplitude mode in 1T-TaS2, which can be locally probed by the THz-lightwave-driven tunneling current. Finally, I will briefly outline the next steps and challenges in this vibrant field of STM towards imaging and spectroscopy of light-induced phenomena in complex quantum materials at the atomic scale.

References:
[1] M. Müller, Prog. Surf. Sci. 99, 1 (2024)
[2] S. Liu et al., Sci. Adv. 8, 42 (2022)
[3] L. Parra Lopéz et al. (in preparation)

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