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SFB 689:  Spin Phenomena in Reduced Dimensions

Subject Area Physics
Term from 2006 to 2017
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 14086190
 
Final Report Year 2018

Final Report Abstract

Within the Collaborative Research Center (SFB 689) we explored „Spin-phenomena in reduced dimensions” involving the characterization, control and manipulation of the spin degree of freedom in low dimensional systems like carbon nanotubes (cylindrical carbon nanostructures), graphene (a single atomic layer of carbon), semiconductor films and two-dimensional electron systems, or individual molecules and arrangements of individual atoms on surfaces. The spin is a quantum mechanical property of elementary particles like electrons and behaves like an angular momentum. In case of an electron the spin has angular momentum ±½ħ with the reduced Planck constant ħ. The magnetic moment associated with the spin thus has only two orientations, parallel and anti-parallel to an applied magnetic field. Within SFB 689, or more generally in the field of spintronics, one tries utilizing the electron’s spin in addition to its elementary charge -e to realize new functionalities which could be used in electronics or in quantum computation. An important quantity which characterizes the spin is its relaxation time, i.e. the time it takes a spin population with specific orientation to decay. A characteristic device which uses the spin of the electrons in addition to its charge is the spin-transistor which was proposed nearly three decades ago but still awaits realization. Even molecules could show transistor functionality if spin is involved. These are only a few examples of the issues addressed in the consortium. In the project period many important discoveries were made and concepts developed which contribute to the cutting edge of research in the field. One example is a new theory for spin relaxation in graphene which is based on the concept of resonant scattering and is in excellent agreement with experiments. We also found that carbon nanotubes in combination with superconductors can be used to generate quantum mechanically entangled electrons. Entanglement effects might be used in communication and computation. Another vivid result is resolution of the spin orientation of neighboring atoms in a NiO single crystal. By using atomic force microscopy we could directly visualize the antiferromagnetic order in NiO crystals. The “holy grail” of spintronics is efficient all electrical spin injection and detection which has been identified as one of the central goals of SFB 689. In these experiments one injects spins from a ferromag- netic injector into a nonmagnetic material like a two-dimensional electron gas in graphene or in a semiconductor heterostructures. Using sort of inverse process, the accumulated spins can be probed elec- trically. The resulting spin dependent resistance changes were typically below 1%. By using ferromagnetic semiconductors (grown in house), spin-valve geometry and epitaxial interfaces we were able to reach spin dependent resistance changes of up to 80%. Even a spin solar cell could be realized in which light generates a spin current. One effect which has been thoroughly studied, both theoretically and experimentally, is the spin Hall effect. For the first time the so called acSHE could be observed and parasitic voltages mimicking a dc inverse spin Hall effect could be avoided by using a proper measurement geometry. A valuable asset of the SFB was the close cooperation between theory and experiment resulting, e.g., in novel spin transistor concepts and novel insights regarding the so call persistent spin helix state. Ab-initio calculations provided a very effective tool to tackle complex problems from a fundamental quantum mechanical level and delivered dependencies which could be compared with experiment. Last but not least our expertise gained within the consortium has enabled us to quickly move into new emerging fields like the (spin) physics of 2D-materials, e.g., dichalcogenides. Also there we have made scientific contributions at the cutting edge of current research.

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