Mesoscopic Physics on Surfaces
| Date/Time: | Thursday, 19 Feb 2015 from 4:10 pm to 5:00 pm |
|---|---|
| Location: | Physics 0003 |
| Phone: | 515-294-5441 |
| Channel: | College of Liberal Arts and Sciences |
| Actions: | Download iCal/vCal | Email Reminder |
Abstract: Low dimensional structures, e.g., quantum wires and films, reveal fascinating phenomena of modern condensed matter physics. Among others, 2D-superconductvity, charge density waves, Luttinger liquids and topologically protected edge states have been realized lately by atomic-sized surface structures. The surface science approach benefits from the fact that these systems can be comprehensively characterized and manipulated in view of their atomic structure and electronic bands. Even more, tuning of external fields, temperature, chemical potentials by adsorption, ordering, etc. facilitates a systematic approach to study effects of electronic correlation and mesoscopic properties and tailor the properties surface quantum systems.
In this talk I will highlight this capability by giving three recent surface science examples from seemingly different fields, e.g. mesoscopic physics, spintronic and correlated materials. In particular I will report on macro- and nanoscopic surface transport studies revealing correlated spin-polarized transport, e.g. in metal/semiconductor systems, as well as on exceptional ballistic transport behavior found in epitaxial graphene nanoribbons.
In detail, Bi(111) films grown on Si(111) (and also vicinal counterparts) can be used as prototype systems with strongly spin-polarized surface states, where backscattering is strongly suppressed and weak localization not expected like in topological insulators. Adsorption of magnetic impurities (Co,Tb,Fe,..) lifts this protection, but in addition strong doping effects, local re-hybridization of the band structure as well as diffusion processes (island formation, incorporation, etc.) become relevant and need to be considered to understand the concept of breaking time reversal symmetry [1,2].
Pb adsorbed on Si(557) leads to formation of long range ordered quantum wire ensembles. For temperatures below 78K the system undergoes a refacetting transition accompanied by a metal-insulator transition. The insulating behavior found in the direction across the wires can be explained in terms of Fermi nesting [3]. Recently, the giant Rashba-splitting in this system and its impact to magneto-transport measurements have been analyzed in more detail. The spin polarization of the Fermi surface, manifested already in the suppression of spin-orbit coupling along the wires within the 1D transport regime, has been confirmed by spin-resolved ARPES measurements. The magnitude of the Rashba splitting found in this system suggests to explain the nesting rather in terms of the formation of a spin-orbit density wave (SODW) [4,5].
The regime of mesoscopic physics has been entered with graphene nanoribbon structures (GNR) which can be grown via self-assembly on appropriately designed SiC surfaces. The systematic change of probe distances and number of (invasive and passive) probes using a 4-tip STM/SEM system have affirmed ballistic (and spin-polarized) transport channels with mean free path lengths up to 20 (mu)m at room temperature [6] most likely along edge-localized states which have been seen also in local spectroscopy [7]. Further functionalization of these ribbons structures have the potential to serve thoroughly as building blocks for future carbon-based electronics.
[1] D. Luekermann, et al. PRB 86, 195432 (2012)
[2] S. Sologub, et al. PRB 88, 115412 (2013)
[3] C. Tegenkamp, et al. PRL 95, 176804 (2005)
[4] C. tegenkamp, et al. PRL 109, 266401 (2012)
[5] C. Brand, et al. Correlated spin-orbit order in a strongly anisotropic 2DEG system submitted, submitted
[6] J. Baringhaus, et al. Nature 506, 349 (2014)
[7] J. Baringhaus, et al. JPCM, 25, 392001 (2013)
Bio: Christoph Tegenkamp works in the field of transport phenomena in correlated matter and nanoscaled structures on surface and is currently holder of a professorship for nano-electromechanical quantum systems at the Leibniz University of Hannover.
He got his PhD in 2000 where he dealt with functionalization of surfaces due adsorption of organic molecules on epitaxially grown insulating films. After his Post-Doc time at the University of Maryland he returned back to Hannover, where he introduced novel concepts of nanostructuring to surfaces. The combination of both can used flexibly to fabricate various kinds of metallic nanostructures with atomic precision and intriguing properties.
Among other topics he is works currently on atomic wires grown by self-assembly on semiconducting surfaces, graphene nanostructures and went recently into the field of topological insulators and complex oxides. Besides fundamental aspects where the role of spin-orbit coupling is the focus of research, he also works in the field of molecular electronics. The properties in these systems are investigated mostly by surface sensitive transport measurements. These measurements are complemented by synchrotron-based photoemission experiments (ALS,SLS,Bessy,ESRF).
At the moment Christoph Tegenkamp is speaker of a DFG-funded research unit dealing with correlation effects and the tunability of instabilities in atomic wires. Moreover, he is member of the editorial board of JPCM and in the review panel of CFN at Brookhaven National labs. He is author of more than 75 peer-reviewed papers in prestigious scientific journals, e.g. Nature, Physical Review Letters, NanoLetters etc.