Dynamics of Nanoelectronic Systems


All-electronic Pump-Probe spectroscopy in the STM

Nanoelectronics aim to advance information processing beyond the limitations of conventional electronic devices. To achieve this goal we have to understand the energetic and dynamic properties of matter at the intrinsic length scale of the underlying interactions: the atomic scale.
A major experimental challenge arises from the largely orthogonal development of techniques that offer high spatial resolution (e.g. scanning probe microscopy or transmission electron microscopy) and techniques that have high temporal resolution (e.g. ultra-fast optics). New tools are emerging that bridge this gap. Structural analysis of nanoscale objects for example is now possible in free-electron lasers and synchrotrons.

At the IBM Almaden Research Center, we developed a new technique capable of detecting the dynamical evolution of low-energy excitations (meV-scale) in few-atom nanostructures: all-electronic pump-probe spectroscopy in a low-temperature scanning tunneling microscope. In this measurement scheme a strong voltage pulse (the pump pulse) applied to the probe tip locally excites a structure. The resultant dynamics are monitored by a time-delayed weaker voltage pulse (the probe pulse). The time resolution that can be achieved for repetitive processes is determined solely by the precision of the voltage pulses and can greatly exceed the bandwidth of the high-gain current amplifiers that detect the tunnel current in an STM.

In a first application, we measured the fast spin relaxation in individual Fe atoms. The STM-based pump‑probe technique can be applied to study the dynamics of a large number of nanoscale systems given they are excitable by tunneling electrons and evolve on accessible time scales. 
Research conducted at the IBM Almaden Research Center, San Jose, California (USA) in Andreas Heinrich’s group. Fincancial support: Office of Naval Research and Alexander von Humboldt Foundation.

Single-atom Spin Dynamics

Iron and Copper atoms on a surface

The atoms are placed on a monatomic decoupling layer of Cu2N which protect the magnetic moment of the d-electrons from the conduction electrons of the Cu(100) single crystal substrate.

Shrinking magnets to a point where they consist of only a few atoms gives rise to exciting quantum phenomena. Well-defined model systems for quantized spins that are accessible to the STM are transition metal atoms adsorbed to a monatomic decoupling layer on a metal substrate.

Tunneling electrons interact with the localized spin of the atoms. The energetic landscape of the spin states can be studied using inelastic tunneling spectroscopy. And by using a spin-polarized tip, changes in the orientation of the surface-bound spins can be detected. We found that attaching a single magnetic atom to the apex of the probe tip is sufficient to produce spin polarization of up to 50%. It becomes possible to investigate the same magnetic nanostructure with spin-polarized and non-spin-polarized electrons simply by attaching or detaching the magnetic atom from the tip.

Using this technique we discovered a quantized analogue to the conventional spin-momentum transfer that allows driving the spin into highly excited states and enabled identification of the dominant spin relaxation mechanism of the surface-bound magnetic atoms.

Measurement of Fast Electron Spin Relaxation Times with Atomic Resolution.<br />Science <strong>329</strong>, 1628–1630 (2010). Zoom Image
Measurement of Fast Electron Spin Relaxation Times with Atomic Resolution.
Science 329, 1628–1630 (2010).

To get direct access to the time-dependent evolution of the atomic spins we combined spin-polarized tunneling spectroscopy with an all-electronic pump-probe scheme (see above) and measured fast electron-spin relaxation with atomic spatial and nanosecond temporal resolution. For close-spaced dimers of a Fe and a Cu atoms that we formed through atom manipulation showed spin relaxation time above 200 ns. The addition of Cu to the Fe atom boosted the easy-axis magnetic anisotropy energy of the Fe atom to ~20 meV. This barrier height is sufficient to suppress thermally induced spin relaxation completely for temperatures below 6 K. In this regime, quantum tunneling of magnetization proves to be the dominant relaxation process.

Currently, we use this new STM-based pump-probe technique together with atom manipulation for systematic studies of the interplay of local environment and dynamical behavior of quantum magnetic systems.

Research conducted at the IBM Almaden Research Center, San Jose, California (USA) in Andreas Heinrich’s group. Fincancial support: Office of Naval Research and Alexander von Humboldt Foundation.

Dopants in semiconductors

Mn acceptors in InAs, depth related contrast. Zoom Image
Mn acceptors in InAs, depth related contrast.
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