Surface-mediated electron spin dynamics of Mn deposited GaAs (110) probed by
Optical Pump-probe Scanning Tunneling Microscopy
Z.H. Wang1 C.H.Yoon1, S. Yoshida1, O. Takeuchi1 and H. Shigekawa1
1

Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan

Semiconductor spin dynamics has always been a research focus both theoretically and experimentally for many
decades, as understanding quantum dynamical behaviors of electrons, holes, as well as nuclei in semiconductors arequite essential for further development on Spintronics and quantum computation. Especially for diluted magnetic
semiconductors (DMSs), in which some more intriguing spin properties had been discovered compared to
non-magnetic semiconductors, are considered to be of high potential in actual Spintronics applications. Here we
present a novel STM-based ultrafast measurement technique that our group has originally developed, to investigate
spin dynamics of an in-situ UHV cleaved GaAs (110) surface with Manganese (Mn) adatoms. Empowered by thistime-resolved ultrafast Optical Pump-probe Scanning Tunneling Microscopy (OPP-STM) technique, we are able toobserve surface-impurity-mediated spin dynamics which are influenced by Mn adatoms, at nano-scale, and exclusively
with high temporal and spatial resolution at the same time.
Namely, our OPP-STM system combines the well-known Optical Pump-probe (OPP) technique with STM, todramatically enhance the temporal resolution of STM, while the spatial resolution is hardly being compromised. Simply
speaking, laser system is set in a typical time-delayed, optical pump-probe scheme, but with our originally developed
laser modulation technique in order to get rid of the tip thermal expansion effect. In our case, both pump and probe
laser pulses are modulated in terms of their circularly polarization states using our laser modulation unit (CPLD-based
electronics with controlling algorithm), they are focused and guided collinearly to illuminate the tip-sample junction in
UHV (<10-8Pa). By making use of the Tip-induced-band-bending (TIBB), as well as light-induced, transient Surface Photo
Voltage (SPV) effect, electron spin dynamics can be resolved by measuring the lock-in-detected tunneling current.
In this experiment, we investigated electron spin dynamics in Mn-deposited, n-tpye GaAs sample at room
temperature. Instead of using MBE-grown GaxMn1-xAs sample, by evaporating Mn atoms on an UHV cleaved GaAs
surface, we are able to control the amount of Mn adatoms on GaAs (110) suface, as well as investigating the spin
lifetime dependence in terms of Mn amount. We have observed that the electron spin lifetime of GaAs has a nonlinear
behavior with respect to Mn amount, which can be explained by the Dyakonov-Perel spin relaxation mechanism. Moreinterestingly, compared with the conventional OPP method, we have found that our OPP-STM method is far more
surface sensitive for probing local spin dynamics. Details of the experimental design as well as experimental results will
be introduced in the presentation. Moreover, recent progress on newly developed nuclear-spin-sensitive OPP-STM
system, which might be enabling us to probe nuclear spin dynamics on nanoscale, will also be briefly reported.

References:[1] T. Dietl, et al. Nature. Materials 9, 965–974, 2010.
[2] S. Yoshida, et al. Nature Nanotechnology 9, 588-593, 2014.
[3] Spin Dynamics in Semiconductors, Springer, 2008.