PCCPCOMMUNICATION

Cite this: Phys. Chem. Chem. Phys.,
2019, 21, 7256
Received 21st December 2018,
Accepted 28th January 2019

Surface-mediated spin dynamics probed
by optical-pump–probe scanning tunnelingmicroscopy†
Zi-Han Wang, Cheul-Hyun Yoon, Shoji Yoshida, Yusuke Arashida, Osamu Takeuchi,
Yuzo Ohno and Hidemi Shigekawa *DOI: 10.1039/c8cp07786j
rsc.li/pccp

In current materials science and technologies, surface eﬀects on
carrier and spin dynamics in functional materials and devices are ofgreat importance. In this paper, we present the surface-sensitive
probing of electron spin dynamics, performed by optical-pump–probe scanning tunneling microscopy (OPP-STM). Time-resolved
spin lifetime information on a manganese (Mn)-deposited GaAs(110)
surface was successfully obtained for the first time. With increasing
Mn density via in situ evaporation, a nonlinear change in the spin
lifetime in the picosecond range was clearly observed, while
directly confirming the Mn density by STM. In comparison withthe results obtained by the conventional OPP method, we have also
demonstrated that the observed nonlinear spin lifetime behavior
was surface-mediated, which can be characterized using only thesurface-sensitive OPP-STM technique.

The use of functions realized in low dimensional materials
such as topological insulators (TIs)1 and mono-to-multi layer
two-dimensional (2D) transition metal dichalcogenides (TMDCs)2
has been attracting considerable attention. In particular, nanoscale spin dynamics and spin transport properties on surfaces
and/or at interfaces are of great interest,3–6 and the applicationof their characteristics is being actively pursued7,8 Therefore, asuitable measurement technique for the surface-sensitive characterization of spin-related properties with simultaneous hightemporal and spatial resolutions is highly desirable. However, inconventional methods, information on spin-related properties is
spatially averaged over a region including the bulk and generally
is not surface-sensitive. For example, optical-pump–probe reflectivity (OPPR) has an excellent time resolution9 but its signal isgenerally averaged over the bulk in the light spot area. On theother hand, scanning tunneling microscopy (STM) is a promisingtechnique to realize the investigation of the local electronic state
while confirming atomic-level structures, and it has been

Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki,305-8573, Japan. E-mail: Hidemi@ims.tsukuba.ac.jp† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8cp07786j

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successfully used to investigate local electronic structures in
TIs10,11 and various types of TMDCs.12,13 However, its time
resolution has generally been limited to the millisecond or submillisecond range.14–16
Recently, by combining the ultrafast OPP technique with
STM, OPP-STM has been developed to obtain new insights into
ultrafast dynamics at the nanoscale, which is far beyond thespatial resolution of conventional OPPR measurement while
simultaneously surpassing the preamplifier-bandwidth-limited
temporal resolution of conventional STM.17 Single-atomic-levelmeasurement has been performed using this OPP-STM
technique.18,19 Furthermore, the electron spin lifetime on GaAs
and its nanostructures such as quantum wells, as well as spin
precession dynamics under an external magnetic field,20 has
also been determined. Since OPP-STM probes the tunnel
current, in addition to its high spatial resolution, high surfacesensitivity is also expected.
Here, we present the results of observing the electron spin
dynamics on a manganese (Mn)-deposited GaAs (110) surface
by OPP-STM as an example to demonstrate surface eﬀects on
spin dynamics. While confirming the structure of the surface,
we attempted to reveal the eﬀect of the surface impurities on
the spin dynamics. In comparison with the results obtained by
the conventional OPP method, we have demonstrated that theobserved nonlinear spin lifetime behaviour was surface-mediatedand can be characterized using only the surface-sensitive OPP-STM
technique.
The experimental setup for OPPR and OPP-STM measurements is shown in Fig. 1. As is well known, the electron spin insemiconductors such as GaAs can be optically oriented bycircularly polarized light.21 The OPP-STM system used in thiswork employs circularly polarized femtosecond laser pulsesthat couple to the STM tip–sample junction in a typical pump–probe scheme by using a novel laser polarization modulation
technique while allowing the simultaneous observation of thesurface by STM (see the ESI† for more details).
After confirming the quality of a GaAs(110) sample surface
prepared by cleavage in a vacuum, Mn atoms were evaporated

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Fig. 1 Schematic illustration of the experimental setup. P.C.: ultrafast
Pockels cells; M: mirror; H.M.: half mirror; P.M.: mirror on a piezo mirror
mount; R: right-handed circularly polarized pulses, which correspond tothe blue spikes and the blue circular arrows; L: left-handed circularly
polarized pulses, which correspond to the red spikes and the red circular
arrows. The polarization states (R or L) of laser pulses were modulatedusing Pockels cells, and aligned and steered using the necessary optics,
and then the pulses were focused onto the sample surface in the UHVchamber. The slight frequency diﬀerence Df in the modulation frequency
Df = 1 MHz
0.999 MHz = 1 kHz here was used as the reference
frequency for lock-in detection. The raw tunneling current I* firstly
amplified then lock-in-detected to obtain the time-resolved tunnelingcurrent signal as a function of the delay time td between the pump andprobe pulses.

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deposited Mn. The estimated Mn density on the surface linearly
increased with the evaporation time as expected.
Next, OPP-STM measurements were carried out on Mn-deposited
GaAs(110) surfaces. Sample B was cleaved in a vacuum and Mn was
deposited on the surface by an identical procedure to that for
sample A. Fig. 3(a) shows the spin signal intensity as a functionof the delay time between the optical pump and probe pulses.
The black lines are fitting curves with a single exponentialfunction (see the ESI† for more details). Fig. 3(b) shows the spin
lifetime determined by fitting as a function of the deposition
time. Very interestingly, a nonlinear change in the spin lifetime
was clearly observed. Namely, the spin lifetime increased until
it reached a peak at a deposition time of 60 s, and then it
started to decrease.
To understand the origin of the observed nonlinear spin
lifetime behaviour, spin relaxation mechanisms are briefly
reviewed. In n-type GaAs, as used in this experiment, at room
temperature (RT = 300 K), the dominant spin relaxation process
is represented by the Dyakonov–Perel (DP) mechanism (see the
ESI,† Fig. S1(a)).23 Owing to the spin–orbit interaction, microscopic effective magnetic fields are generated and act upon

onto the surface at various deposition times, whose amounts
were determined from STM images. Since Mn acts as an
acceptor, it is possible to analyse the spin dynamics as compared with mechanisms such as electron–hole interactions.22
Two types of GaAs wafer were used: sample A to confirm the Mndensities on the surface prepared at different deposition times,
and sample B for OPP-STM and OPPR experiments (see the ESI†for more details). Fig. 2(a)–(e) show typical STM images ofsample A with different Mn densities. Fig. 2(f) shows therelationship between the evaporation time and the amount ofFig. 2 Formation of Mn-deposited GaAs(110) surfaces. (a–e) Typical STM
images of the GaAs(110) surfaces at different Mn deposition times ((a):
clean, (b): 15 s, (c): 30 s, (d): 40 s, (e): 60 s) obtained using sample A. (f)Relationship between the deposition time and the amount of Mn atoms,
which was determined by counting the number of Mn atoms, for example,
indicated by the white arrows in (b) to (e), using wider STM images. A clear
linear relationship was observed.

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Fig. 3 Electron spin lifetime at diﬀerent surface Mn densities obtained by
OPP-STM. (a) Mn-density-dependent OPP-STM spectra, obtained for the
samples shown in Fig. 2, in which the oﬀset was manually adjusted foreach spectrum. The black lines are fitting curves with a single exponentialfunction, and the lifetimes determined were, from top to bottom, 21  2,
55  3, 114  5, 167  15, 301  8, 219  11, 146  4, and 102  5 ps,
respectively. (b) Spin lifetimes, which were determined from the fitting
curves in (a), plotted as a function of the Mn deposition time. The peak
lifetime was observed at a deposition time of 60 s, corresponding to a Mn
density of B2.3  1012 cm 2. The same colors are used in (a) and (b).

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electron spins, in which the impurity atoms included in thesample play an important role in determining the spin relaxation process. Namely, with spin-polarized electrons being
scattered off by the impurities, spin relaxation occurs during two
subsequent scattering events. Therefore, in the DP mechanism, thespin lifetime (ts) is inversely proportional to the momentum lifetime (tp), that is, ts B tp 1. In contrast, in p-type GaAs at RT,
particularly when the hole concentration is relatively high, thedominant process causing spin relaxation is the Bir–Aronov–Pikus(BAP) mechanism (see the ESI,† Fig. S1(b)),23 in which the electrons
lose their spin polarization owing to an ultrafast spin exchangeinteraction with the holes. In this case, the spin lifetime is inversely
proportional to the hole density (nh), i.e., ts B nh 1.
From the perspective described above, because the Mn
atoms at 300 K act as paramagnetic impurities in the system,
we found that the rate of scattering between the electrons andthe Mn atoms, which suppresses the spin relaxation, increases
with increasing Mn density, and the spin lifetime is initiallyextended owing to the DP mechanism. However, with a further
increase in the surface Mn density, since most of the Mn atoms
replace the Ga sites and act as acceptors,24 eﬀective (p-) dopingon the surface induces a transition from a lightly doped n-typesemiconductor to a hole-rich p-type one at and near the surface,
and the decrease in the spin lifetime after it reached the peak
position is attributed to the eﬀect of the BAP mechanism.
That is, at a relatively high surface Mn density, as the BAP
mechanism is significantly enhanced, the increased competition between the DP and BAP mechanisms leads to an overall
decrease in the spin lifetime. This eﬀect of the DP–BAP
competition upon increasing the effective p-type doping concentration has been theoretically predicted to occur in a similarp-type GaMnAs quantum well system.25,26 This is the first direct
observation of the transition of the spin relaxation process fromthe DP mechanism to the BAP mechanism.
To evaluate the z-directional (perpendicular to the sample
surface) spatial resolution of OPP-STM, conventional OPPR
measurements were carried out on identical Mn-deposited
surfaces using the same setup as that for OPP-STM with the
STM tip extracted from the sample (see the ESI† for moredetails). Fig. 4(a) shows the OPPR signal as a function of thedelay time. The black lines are fitting curves with a single
exponential function. Fig. 4(b) shows the lifetimes determinedfrom the data in Fig. 4(a), where the results obtained by
OPP-STM presented in Fig. 3(b) are shown together for comparison. As is clearly shown, the tendency of the nonlinear lifetime
observed by OPP-STM was much less pronounced in the case of
OPPR measurement. This is considered to be due to thediﬀerence in the probing depth between OPP-STM and OPPR
measurements.
In OPP-STM of a semiconductor, its signal mainly originatesfrom the area where tip-induced band bending (TIBB) occurs.27,28
The spin-related tunnel current depends on the potential height
determined by the amount of spin polarized photo-carriers excitedby the pump and probe pulses, which depends on the delay time
due to the mechanism of absorption bleaching. Namely,
OPP-STM is highly sensitive to the surface dynamics. The TIBB

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Fig. 4 Comparison of the spin lifetimes determined by the OPP-STM and
OPPR methods. (a) Mn density dependent OPPR spectra. From top tobottom, the deposition time (Mn density) increased, with the oﬀset
manually adjusted for each spectrum. The black lines are fitting curves
and the lifetimes determined were, from top to bottom, 26  1, 52 
1, 65  2, 38  1, and 30  1 ps, respectively. (b) Comparison of thedeposition-rate dependence of the spin lifetime determined by the two
methods. The results obtained by OPP-STM shown in Fig. 3b are shown
together for comparison. (c) Schematic illustration of the probing area in
OPP-STM measurement. (d) Schematic illustration of the probing area in
OPPR measurement. The measured spin lifetime was almost entirely
determined by the bulk, and the Mn-deposited surface had very little
eﬀect on the measured lifetime signal. Here, blue balls, Mn atoms; redballs, electrons; and grey balls, Si dopant atoms. For simplicity, only
electrons with down spins are shown here.

depth is determined using parameters such as tip radius,
tip–sample distance, bias voltage, and doping concentration.
Considering the experimental conditions, as shown in Fig. 4(c),the effective depth was estimated to be over 10 nm (see the
ESI,† Fig. S2). Whereas in OPPR measurement, the in-planespatial resolution was determined using a laser spot size of
50 mm, and the spatial resolution perpendicular to the surface(z-direction) depends on the optical penetration depth, asshown in Fig. 4(d), which was B800 nm in the present case(see the ESI,† Fig. S4) and much larger than that of OPP-STM. In
the Mn-deposited GaAs(110) system, the Mn atoms existed only
on the surface, acting as impurities and acceptors, and Mn ion
diffusion to the bulk was suppressed during the OPP-STM
measurement because the sample was reversely biased (positive
sample bias). The influence of the screened Coulomb potential
by Mn impurity is less than 10 nm.29 Therefore, with increasing
surface Mn density, the nonlinear spin lifetime behaviour,which was determined by both the DP and BAP mechanisms,
is considered to only occur in the area close to the surface. In
contrast, in the OPPR measurement, the time-resolved spin
dynamics signal was obtained from deep inside the bulk, thus
the nonlinear contribution from the surface was significantly
reduced.
To further evaluate the observed diﬀerence between the two
methods, we carried out simple simulations of spin excitationin OPP-STM measurement. That is, although our modulation

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technique can probe only spin-related signals, the eﬀect of thenear field component should be suppressed to make band
bending sensitive to spin-related absorption bleaching. In thecase of a W tip, the tip-induced electric-field enhancement in
the z-direction is about 400 times the incident light intensity,
which sharply decreases inside the sample with increasing
distance from the surface. Therefore, the circularly polarized
light condition below the STM tip is initially disturbed aroundthe tip apex but then quickly recovers at a depth of B10 nm(see the ESI,† Fig. S3), which is comparable to the TIBB region,
enabling the spin dynamics to be easily probed by OPP-STM. By
adjusting the power of the incident laser, the depth where thecircularly polarized light condition recovers may be controlleddepending on the purpose. If one wants to observe the same
nonlinear spin lifetime behaviour in a similar system such as
GaMnAs using the conventional OPPR approach, several typesof MBE-grown samples with diﬀerent bulk Mn doping concentrations are necessary. That is, OPP-STM provides a powerfulapproach for studying electron spin dynamics, particularly
when the surface and/or interface characteristics are of interestand have a deterministic role in the desired material functionalities. Combinations of OPP with low-temperature STM shows
potential in further understanding nanoscale magnetism, thestudy of which is left as future work.

Conclusions
We have demonstrated that the dynamics of optically oriented
spins in a Mn-deposited GaAs(110) surface can be probed by
OPP-STM. With increasing Mn density via in situ evaporation, anonlinear change in the electron-spin lifetime was clearlyobserved. Namely, the spin lifetime first increased and thendecreased, with increasing density of Mn deposited on thesurface. This characteristic can be comprehensively understoodby considering the competition between two specific spin
relaxation mechanisms, i.e., the DP and BAP mechanisms. By
comparing with results obtained by the conventional OPP
method, we have also demonstrated that this nonlinear spin
lifetime behaviour involved a surface-mediated modulation,
which can be characterized exclusively by the OPP-STM technique. Further development and advancement of this technique,for example, combining with low-temperature STM, spin polarized STM,30 multiprobe STM,31 and terahertz-coupled STM,32–34
is expected to be very promising in facilitating advances in thefields of nanoscience and nanotechnology.

Conflicts of interest
There are no conflicts to declare.

Acknowledgements
H. S. acknowledges the support from the Japan Society for the
Promotion of Science (Grants-in-Aid for Scientific Research:
17H06088)

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