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Our research in spintronics is aimed at understanding the basic steps
needed for spin-based information storage and processing. In spintronics
devices, the spin degree of freedom provides functionality in addition
to the charge of the electron. We are constructing devices made from
ferromagnetic metals combined with semiconductors and are investigating
the injection of spin-polarized charges, as well as the transport,
manipulation and detection of spin. In addition, the aspect of spin
coherence is investigated for possible quantum information processing
devices. |
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| Devices |
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| Our devices are made of inorganic
semiconductors, semiconductor quantum structures, and organic materials.
Planar and layered structures are fabricated and measured using in-plane
transport as well as tunneling across heterostructures. |
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| Spin injection |
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| Spin injection from a ferromagnetic
metal into a semiconductor can be achieved by electrical transport
across a tunneling barrier or a Schottky contact. The role of a tunneling
barrier is twofold. (1) It serves as a buffer layer and hence prevents
chemical reactions between metal and semiconductor layers. (2) It
provides a means to match energy levels for efficient spin-injection
efficiency. Studies of spin-injection from the ferromagnetic tip of
a scanning tunneling microscope allows us to spatially and energetically
resolve the injection efficiency. An alternative approach to spin
injection is to use optical selection rules, which relate the helicity
of photons with the spin-orientation of conductance-band electrons. |
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| Transport |
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| For device operation, spin
has to be transported between two contacts without losing its polarization.
With this respect, the coherence time and the related spin coherence
length are relevant. |
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| Manipulation |
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| Typically, spin is manipulated
by exposing it to a magnetic field. Magnetic fields can be generated
and controlled externally or in the device itself in the form of an
effective magnetic field originating from hyperfine interaction with
polarized nuclei, effective g-factors or spin-orbit interaction (Dresselhaus
and Rashba fields). |
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| Detection |
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| Various techniques are used
to detect spin-polarized charge carriers. Owing to optical selection
rules, the electroluminescence is polarized according to the spin-polarization
of the recombining charges. Kerr and Faraday rotation measurements
on semiconductors allow detecting spin polarization with sub-picosecond
time resolution. Ultimately, device operation has to rely on electrical
detection of spin polarization using spin-filtering at the interface
to a ferromagnetic metal. |
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