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Festkörperkolloquium Wintersemester 2014/15

Donnerstag, 16-18 Uhr, Raum 3701-268 (Appelstraße 2)


TerminRedner/inEinladenderThema
20.11.
15 Uhr
Antonio Tejeda
(CNRS, Orsay, Frankreich)
Christoph TegenkampAtomic and electronic structure of graphene sidewall nano ribbons: an ARPES, STM and TEM study
04.12.
16 Uhr
Wolfgang Häusler
(Universität Augsburg)
Rolf HaugFlat bands and long range Coulomb interactions: conducting or insulating?
11.12.
16 Uhr
Steven C. Erwin
(NRL, Washington, D.C.)
Herbert PfnürIntrinsic Magnetism at Silicon Surfaces
08.01.
16 Uhr
Mark Bieler
(PTB Braunschweig)
Jens HübnerAll-optically induced ultrafast photocurrents in semiconductors
22.01.
16 Uhr
Paulo Santos
(Paul Drude Institut, Berlin)
Jens HübnerTransport and manipulation of quantum wells excitons and spins

 

 

20.11.2014 - Atomic and electronic structure of graphene sidewall nano ribbons: ARPES, STM and TEM study

Antonio Tejeda

Laboratoire de Physique des Solides, CNRS, Orsay, France

 

Methods for producing semiconducting–metallic graphene networks suffer from scalability problems, stringent lithographic demands and process-induced disorder in the graphene. These problems can be overcome by taking advantage of graphene grown on patterned SiC steps [1], without relying on chemical functionalization or finite-size patterning.

This scalable bottom-up approach produces graphene in the form of a strip, at the edge of which, a semiconductor, is bonded to metallic graphene. This semiconductor ribbon is only a few nanometers wide and the semiconducting graphene strip width is defined to within a few graphene lattice constants, a level of precision beyond modern lithographic limits, and which is robust enough that there is little variation in the electronic band structure across thousands of ribbons. The energy gap is greater than 0.5 eV in an otherwise continuous metallic graphene sheet along the short direction of the ribbon [2] while it exhibits ballistic transport along the long direction of the ribbon [3]. One of our aims is to address the unknown origin of this gap, which can be driven for instance by an sp3 hybridization or by strain at the curved edge of the ribbon. Our current structural studies by STM and cross-sectional TEM allow us to shed light on the electronic properties, especially on the origin of the gap.

[1] M. Sprinkle et al., Nature Nanotech 5, 727 (2010).
[2] J. Hicks et al., Nature Phys. 9, 49 (2012).
[3] J. Baringhaus et al., Nature 506, 349 (2014).

 

 

04.12.2014 - Flat bands and long range Coulomb interactions: conducting or insulating?

Structure of the T3-lattice: A and B sites are connected to only 3 nearest H sites, while conversely, H-sites are 6-fold coordinated. The structure implies equal numbers of lattice sites.

Wolfgang Häusler

Institut für Physik, Universität Augsburg

 

Albeit discovered already in 1986 by Bill Sutherland dispersionless ("flat") electronic bands enjoy increasing attention since recently. These bands support localization through destructive quantum interference by the local topology of a periodic lattice, free of disorder. The phenomenon has been denoted as "caging" of carriers and is accompanied with insulating behavior at partial band filling and zero temperature. One may ask whether long range Coulomb interactions would alter this situation to cause metallicity. As answer we find "it depends". In the absence of any kinetic energy, flat band carriers tend to Wigner crystallize. Here, this general observation is analyzed for the two-dimensional case, specifically for the Sutherland or T3-lattice where a conductivity is found, depending non-trivially on the carrier density at small flat band fillings.

11.12.2014 - Intrinsic Magnetism at Silicon Surfaces

Ground state structure and lowest energy spin configuration of two magnetic silicon surfaces.

Steven C. Erwin

Center for Computational Materials Science, Naval Research Laboratory, Washington, DC, USA

 

It has been a long-standing goal to create magnetism in a nonmagnetic material by manipulating its structure at the nanometer scale.  I describe our recent theoretical prediction that certain stepped silicon surfaces stabilized by adsorbed gold realize this goal by self-assembly, creating linear chains of fully polarized electron spins with virtually perfect structural order.  The spins are localized at the silicon step edges, which have the form of graphitic hexagonal ribbons. The predicted magnetic state is indirectly supported by recent experimental observations, such as the coexistence of double- and triple-period distortions.  More direct evidence from two-photon photoemission and scanning tunneling spectroscopy confirms our prediction of an unoccupied silicon state that arises from strong exchange splitting.  Ordered arrays of spins at a surface offer access to local probes with single spin sensitivity, such as spin-polarized scanning tunneling microscopy.  This integration of structural and magnetic order suggests a possible new avenue toward technologies involving spin-based computation and storage at the atomic level.

 

 

08.01.2015 - All-optically induced ultrafast photocurrents in semiconductors

Mark Bieler

Physikalisch-Technische Bundesanstalt, Braunschweig

 

Optical excitation of interband transitions in semiconductors typically induces very complex carrier dynamics. Depending on the polarization of the optical excitation and the point group symmetry of the semiconductor different microscopic (spin dependent and spin independent) effects might lead to a macroscopic current response. The talk will discuss the generation of such photocurrents in GaAs (bulk and quantum wells). Particular focus will be laid on the excitation of exciton resonances with polarization-shaped optical femtosecond pulses revealing the existence of new types of photocurrents. The studies not only contribute to a better understanding of light-matter interaction but might even proof important for novel THz applications.

 

 

22.01.2015 - Transport and manipulation of quantum wells excitons and spins

Paulo Ventura Santos

Paul-Drude-Institut für Festkörperelektronik, Berlin

 

Excitons mediate the interaction between photons and electronic excitations in a semiconductor: this property makes them good candidates for the storage and processing of photonic information in the solid-state. For that purpose,information bits encoded in photons are first converted into excitons in a semiconductor quantum well (QW), which are then manipulated using external fields and subsequently reconverted to photons for further transmission. Particularly interesting for this application are spatially indirect excitons (IXs), where the electron and hole constituents are stored in two closely spaced QWs of a double quantum well (DQW) structure subjected to a transverse electric field. The latter provides electrical control of the IX recombination lifetime and, therefore, of the photon to IX inter-conversion process. In this talk, I present a novel concept for information processing based on the transport and manipulation of IXs by GHz surface acoustic waves (SAWs). These SAWs are elastic vibrations with micrometer wavelengths propagating along a surface. The moving band gap modulation created by the SAW strain on a DQW structure can trap IXs and transport them over several hundreds of micrometers.[1,2] The IX flow can be controlled by electrostatic gates along the SAW path [2,3] and efficiently transferred between crossing SAW beams. This feature is exploited to demonstrate an acoustic IX multiplexer, a device capable of interconnecting a scalable number of IXs storage sites.[4] Finally, IXs can be transported over tens of µm while maintaining their spin polarization. The controlled acoustic transport of IXs thus provide a pathway for the realization of scalable IX structures for spintronics applications.

[1] J. Rudolph, R. Hey and P. V Santos, Phys. Rev. Lett. 99, 047602[4] (2007)
[2] A. Violante, K. Cohen, S. Laziç, R. Hey, R. Rapaport, and P. V. Santos, New J. Phys. 16, 033035 (2014)
[3] A. A. High et al., Science 321, 229-231 (2008)
[4] S. Lazic, A. Violante, K. Cohen, R. Hey, R. Rapaport, and P. V. Santos, Phys. Rev. B89, 085313 (2014)