The goal of this project is to resolve spin dynamics at nanometer length and picosecond time scales. The spin dynamics in single atoms crucially depends on details in the atomic-scale environment. Relaxation times range from the picosecond (ps) to the millisecond (ms) timescale. Our project is devoted to the study of ultrafast spin dynamics in single atoms and specifically designed nanostructures on a monolayer of molybdenum disulfide (MoS₂) grown on Au(111). We will build these magnetic nanostructures with desired properties of magneticstability, anisotropy barriers, and magnetic coupling between the individual atoms. We will investigate the interaction of these magnetic structures with the spin-split electronic bands of MoS₂. These investigations demand a new experimental set-up, which combines the atomic lateral resolution of a scanning tunneling microscope (STM) with ultrafast time resolution. To achieve this, our first step will be to couple a terahertz (THz) laser system to a low-temperature STM. The coupling of the electric field of the THz pulses into the STM junction will modify the tunneling barrier and, therefore, the measured dc tunneling current. Sub-picosecond time resolution will be achieved by driving the junction with two THz pulses having variable delay.

In a second step, we will use optical pump pulses for creating excited charge carriers in the MoS₂ layer and probe their relaxation dynamics with THz pulses. We will explore the effect of magnetic adatoms and magnetic nanostructures on the trapping of charge carriers and change in relaxation properties. Using circularly polarized light, the optical pump-THz probe scheme also opens the perspective to investigate the creation of spinpolarized charge carriers.

Developing and employing the ultra-fast measurements and control of atomic-size objects, we expect to gain a fundamental understanding of the dynamics of spin states in single atoms coupled to a tailored environment.

Publications (1^st Funding Period 2018 - 2021)

• Moiré tuning of spin excitations: Individual Fe atoms on MoS₂/Au(111) 
  S. Trishin, C. Lotze, N. Bogdanoff, F. von Oppen, and K. J. Franke
  Phys. Rev. Lett. 127, 236801 (2021)  - DOI: 10.1103/PhysRevLett.127.236801

• Monolayers of MoS₂ on Ag(111) as decoupling layers for organic molecules: resolution of electronic and vibronic states of TCNQ 
  A. Yousofnejad, G. Reecht, N. Krane, C. Lotze, and K. J. Franke 
  Beilstein J. Nanotechnol. 11, 1062 (2020)  - DOI: 10.3762/bjnano.11.91

• π-Radical formation by pyrrolic H abstraction of phthalocyanine molecules on molybdenum disulfide 
  G. Reecht, N. Krane, C. Lotze, and K. J. Franke
  ACS Nano 13, 7031 (2019)  - DOI: 10.1021/acsnano.9b02117

• Mapping the perturbation potential of metallic and dipolar tips in tunneling spectroscopy on MoS₂
  N. Krane, C. Lotze, N. Bogdanoff, G. Reecht, L. Zhang, A. L. Briseno, and K. J. Franke
  Phys. Rev. B 100, 035410 (2019)  - DOI: 10.1103/PhysRevB.100.035410

• Moiré structure of MoS₂ on Au(111): Local structural and electronic properties
  N. Krane, C. Lotze, and K. J. Franke
  Surf. Sci. 678, 136 (2018)  - DOI: 10.1016/j.susc.2018.03.015

• High-resolution vibronic spectra of molecules on molybdenum disulfide allow for rotamer identification
  N. Krane, C. Lotze, G. Reecht, L. Zhang, A. L. Briseno, and K. J. Franke
  ACS Nano 12, 11698 (2018)  - DOI: 10.1021/acsnano.8b07414