Studies of spin dynamics in low-dimensional systems are important from both fundamental and practical points of view. Spin-polarized scanning tunnelling microscopy allows localized spin dynamics to be characterized and plays important roles in nanoscale science and technology. However, nanoscale analysis of the ultrafast dynamics of itinerant magnetism, as well as its localized characteristics, should be pursued to advance further the investigation of quantum dynamics in functional structures of small systems. Here, we demonstrate the optical pump-probe scanning tunnelling microscopy technique, which enables the nanoscale probing of spin dynamics with the temporal resolution corresponding, in principle, to the optical pulse width. Spins are optically oriented using circularly polarized light, and their dynamics are probed by scanning tunnelling microscopy based on the optical pump-probe method. Spin relaxation in a single quantum well with a width of 6 nm was observed with a spatial resolution of ∼ 1 nm. In addition to spin relaxation dynamics, spin precession, which provides an estimation of the Landé g factor, was observed successfully.
Temporal decay characteristics of dislocation-related luminescence bands (D1–D4) were explored in strain-relieved epitaxial SiGe/Si(100). Close similarity of the decay profiles was observed not only between D1 and D2 bands but also between D3 and D4 bands. The decay transients of the D1 and D2 bands at low temperatures are characterized by long decay times, τ≳200 ns, whereas the D3 and D4 bands exhibit even sharper transients with τ<60 ns. Temperature dependence of ‘‘radiative’’ lifetimes implies a free-to-bound nature of the D1 and D2 bands, while a bound-to-bound character of the luminescence origins for the D3, D4 bands.
The electrochemical etching method by Ibe et al. [J. Vac. Sci. Technol. A 8, 3570 (1990)] to fabricate sharp tips for scanning tunneling microscopy was modified by shortening the cutoff time of the etching current after the material wire drops off at the air-electrolyte interface. The tip radius measured by field ion microscopy was successfully reduced to 8 nm when the cutoff time was shortened to 50 ns. The dependence of the field-emitting electron current from the sharpest tips was close to one expected from the Fowler–Nordheim formula with a reasonable value for the emitting area of the tip.
The hopping movements of Cl atoms on a Si(111)-(7 x 7) surface that are enhanced by an electron injection from tips of a scanning tunneling microscope (STM) exhibit a spatial spread from the electron injection point with an anisotropic distribution. The enhanced hopping effect becomes greatest at a sample bias voltage being resonant with the Si-Cl antibonding states and also exhibits an oscillatory decay with the distance from the injection point characterized by the wavelength depending on the bias voltage. All of these facts can be interpreted in terms of the coherent expansion of the electron wave packets locally formed at the STM tip.
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