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SCANNING PROBE MICROSCOPY

Close-up on spin coherence

Ultrafast, coherent spin dynamics in semiconductor heterostructures can be measured with a scanning tunnelling
microscope by using femtosecond pulses of circularly polarized light.

Sebastian Loth, Jacob A. J. Burgess and Shichao Yan

Spins are the basic buildings blocksof magnetism, and exploring theirdynamics and interactions is central
to understanding technologically relevantmaterials ranging from magnetic nanodots
to high-temperature superconductors.
The elementary processes that govern
the dynamics of spins typically occur onlength scales of ångströms to nanometres
and with characteristic timescales of
femto- to nanoseconds. So far, thiscombination of time and spatial resolution
has been an experimental blind spot.
Far-field techniques offer sufficient time
resolution1,2 but, despite intensive effort,
have not yet reached a resolution inreal space better than 10 nm. Scanningprobe techniques, on the other hand,readily resolve magnetic structures withsubnanometre spatial resolution, but are
typically limited to slow, millisecond speed
and their use restricted to investigations ofquasi-static surface magnetism. Writing in
Nature Nanotechnology, Hidemi Shigekawa
and colleagues at Tsukuba University now
show that ultrafast coherent magnetization
dynamics are within the reach of thescanning tunnelling microscope3.
Recently, the time resolution of scanningtunnelling microscopy (STM) has beensignificantly improved by implementing
pump–probe measurement schemes tomeasure the lifetime of photoexcited
carriers4,5 and nanosecond spin relaxationin few-atom nanomagnets6. Shigekawa andco-workers interface ultrafast optics in a

Non-local
excitation

Local detectionLΔtIRÅµm

Figure 1 | Spin-sensitive pump–probe spectroscopy in the scanning tunnelling microscope. Ultrafast laserpulses with right-circular (R) and left-circular (L) polarization excite spin-polarized carriers close to thesurface of a GaAs sample. The tip of a scanning tunnelling microscope is positioned in the micrometresized laser focus and locally measures the population of photogenerated carriers with significantly higher
spatial resolution of a few ångströms. Because the spin polarization (imbalance of up and down spins)
diminishes with the characteristic spin-relaxation time, the tunnel current magnitude, I, depends on thetime delay between the pump and probe pulses, Δt.
574

pump–probe scheme with cross-sectional
STM to resolve coherent spin dynamics on
the picosecond timescale within a quantum
well only 6 nm in width (Fig. 1). They
measured the spin relaxation of electrons
in GaAs/AlGaAs quantum wells on apicosecond timescale and the quantum
beat of the electron spin in a magnetic field.
This marks the first time that coherent
spin dynamics have been captured with ascanning tunnelling microscope, adding
laser-combined STM to the list of ultrafast
magnetization probes.
The researchers cleaved a piece of a
GaAs-based heterostructure wafer invacuum and approached the freshly
exposed surface with the microscope’s tip.
Buried interfaces and heterostructures
can be accessed in this geometry and theteam exploited this capability to makemeasurements across individual quantum
wells. Ultrafast pulses of circularly polarized
light from Ti:sapphire lasers were focused
onto the sample around the tip causing
electron–hole pairs to be photoexcited.
The holes quickly relaxed and were notmeasured. The hole relaxation leaves anaccumulation of electrons that have a net
spin polarization, due to the selection rules
for photoexcitation with circularly polarized
light. The spins lose their orientation by
scattering with impurities or other freecarriers over a characteristic time called thespin-relaxation time.
To detect this electron-spin relaxation
time the researchers make use of saturation
effects in the excitation of free carriers to
the conduction band. An initial laser pulse(the pump pulse) creates spin-polarized
electrons. The second laser pulse (the probepulse) does the same, but with an efficiency
that depends on the persistence of thepopulation of spin-polarized electrons fromthe pump pulse. If the probe pulse occurs at
the same time as the pump pulse, or before
the spin-polarized electrons have fullyrelaxed, fewer electrons will be excited. The
tunnel current cannot be measured at thespeed of the optical pulses, but depends on
the average excitation efficiency of the probepulse. Hence, the ultrafast spin relaxation
can be recorded by monitoring the change of

NATURE NANOTECHNOLOGY | VOL 9 | AUGUST 2014 | www.nature.com/naturenanotechnology© 2014 Macmillan Publishers Limited. All rights reservednews & views
the tunnel current while slowly varying thetime delay between pump and probe.
As with many experiments usingnear-field probes, the key to success lies insuppressing spurious effects while boosting
the desired signal above the noise floor.
Shigekawa and colleagues demonstrate a
clever lock-in detection scheme akin toheterodyne detection in a radio receiver
to boost their sensitivity and isolate thespin-dependent signal while keeping theaverage laser power impinging on the tipconstant. Maintaining a constant power thatis absorbed by the tip is an essential stepto avoid spurious signals related to timedependent heating, which have plagued
laser-combined STM in the past 7.
An important aspect of the experimentis the non-local nature of the opticalexcitation. The micrometre-sized laser
beam presents a global excitation comparedwith the tunnel current detection, which
is localized on the ångström scale. It ispossible that many photoexcited electrons,not just those generated immediately under
the scanning tunnelling microscope tip,are detected at each pump–probe cycle.
Consequently, this measurement scheme
may also be applicable to the study of nonlocal effects, such as spin diffusion in real
space. It also opens the door to take otherestablished methods in ultrafast optics
to the nanoscale, such as the resonant
excitation of phonons8. Reliance on a global
excitation, however, also creates a challenge
for other sample systems, such as metals,
where vanishing optical penetration depthsignificantly reduces the excitation volume
and may preclude the application of lasercombined STM. Surmounting this challenge
will be important for expanding the scopeof this technique. Local enhancement of thelight intensity under the tip, as achieved, forexample, with plasmonic waveguides9, could
overcome limitations in excitation efficiency
and reach the same highly localized control
as approaches employing fast gating of thetunnel junction voltage5,6.
Perhaps the most important finding of
the work by Shigekawa and colleagues isthe measurement of spin dynamics beyondpopulation decay, showing that the scanningtunnelling microscope can detect coherent
spin precession with high fidelity. Hallmark
features are the detection of the quantum
beat of the spin polarization in a magnetic
field10 and its resonant amplification withmatched laser-pulse repetition11.
Electron spins in semiconductors canmaintain quantum–mechanical coherence
only for short periods of time. In thistime, they can, in principle, be used forspintronic applications that go further
than static magnetoresistance. The method
demonstrated by Shigekawa and colleagues
is the first that is sensitive to spin coherenceat nanometre length-scales. Decoherence
and relaxation of spins in semiconductors isgoverned by nanometre-sized interactions
with defects and disorder potentials.
Understanding these processes is of key
importance in the engineering of devices
that harness spin coherence exceeding
nanoseconds. With the local detection

mechanism established, many questions
could now be answered. For example, howspatially uniform is the spin coherence?
What role do defects play in the emissionof spin waves? What is the influence ofinterface disorder on spin injection acrossdevice layers?
Spatial heterogeneity in solids is
intricately linked to the observable dynamics
therein. Combining STM with pump–probe
measurement schemes, as exemplified by
the work of Shigekawa and colleagues,
presents a new generation of scanningprobe experiments that access elementary
processes at their intrinsic lengthand timescales.
❐Sebastian Loth, Jacob A. J. Burgess and Shichao Yan
are at the Max Planck Institute for the Structure
and Dynamics of Matter, 22761 Hamburg,
Germany, and the Max Planck Institute for Solid
State Research, 70569 Stuttgart, Germany.
e-mail: sebastian.loth@mpsd.mpg.deReferences
Sih, V. et al. Nature Phys. 1, 31–35 (2005).
Bolte, M. et al. Phys. Rev. Lett. 100, 176601 (2008).
Yoshida, S. et al. Nature Nanotech. 9, 588–593 (2014).
Terada, Y., Yoshida, S., Takeuchi, O. & Shigekawa, H.
Nature Photon. 4, 869–874 (2010).
5. Cocker, T. L. et al. Nature Photon. 7, 620–625 (2013).
6. Loth, S., Etzkorn, M., Lutz, C. P., Eigler, D. M. & Heinrich, A. J.
Science 329, 1628–1630 (2010).
7. Gerstner, V., Knoll, A., Pfeiffer, W., Thon, A. & Gerber, G.
J. Appl. Phys. 88, 4851–4859 (2000).
8. Först, M. et al. Nature Phys. 7, 854–856 (2011).
9. Neacsu, C. C. et al. Nano Lett. 10, 592–596 (2010).
10. Bar-Ad, S. & Bar-Joseph, I. Phys. Rev. Lett. 66, 2491–2494 (1991).
11. Kikkawa, J. M. & Awschalom, D. D. Phys. Rev. Lett.
80, 4313–4316 (1998).
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GRAPHENE

Electrons en masse

Massless electrons in graphene exhibit a mass when considered as collective excitations known as plasmons.

Fengnian XiaAplasmon is the quantum of collective
motion of charged carriers andtypical plasmonic materials arenoble metals such as silver and gold, whichhave many free electrons1,2. Plasmons haveattracted significant interest recently due
to their ability to confine light below thediffraction limit. In dielectric materials,
light diffraction is a result of the uncertainty
principle: limitations to the value of therefractive index of the material, n, cause
restrictions in momentum, k, space(k = nω/c, where ω is the angular frequencyof the light and c is the speed of the lightin vacuum), resulting in a spreading of
the light in real space. Plasmon waves inmetallic waveguides3 or at metal/dielectricinterfaces can overcome this fundamental
limit by leveraging the negative permittivityof metals, which arises from the collective
inertia of electrons under time-varying
electric fields, and is usually in thenear-infrared to visible frequency range.
From the perspective of circuit analysis, thisnegative but real permittivity corresponds
to an imaginary conductivity, revealing theinductive character of the metal. Such aninductance is called ‘kinetic inductance’, toNATURE NANOTECHNOLOGY | VOL 9 | AUGUST 2014 | www.nature.com/naturenanotechnology© 2014 Macmillan Publishers Limited. All rights reserved

differentiate it from the traditional ‘magneticinductance’, which results from a magnetic
field around a current-carrying conductor.
For normal metals, the kinetic inductancecan be safely ignored at microwave
frequencies because their resistancedominates the impedance at this relatively
low-frequency range. In superconductors,
the kinetic inductance can be significant
even at microwave frequencies due to theirideal conductive properties4.
Writing in Nature Nanotechnology,
Donhee Ham, Philip Kim and colleagues atHarvard University, Columbia University
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