Festkörperkolloquium Sommersemester 2015
Donnerstag, 16-18 Uhr, Raum 3701-268 (Appelstraße 2)
|21.05.||Kirsten von Bergmann|
|Christoph Tegenkamp||From Spin Spirals to Magnetic Skyrmions, studied by spin-polarized STM|
|Michael Oestreich||Quantum Photonic Engineering with Semiconductor Quantum Dots|
|Rolf Haug||Quantum Hall Effect: Where does the current flow for the quantized|
|Herbert Pfnür||Probing Bulk Properties with a Surface Sensitive Tool|
21.05.2015 - From Spin Spirals to Magnetic Skyrmions, studied by spin-polarized STM
Kirsten von Bergmann ✉
Department of Physics, University of Hamburg
Magnetism in low-dimensions is a versatile topic and broken inversion symmetry due to the presence of a surface may give rise to sizeable Dzyaloshinsky-Moriya interaction, which induces the formation of non-collinear magnetic states. In such systems the spin rotates from one atom to the next resulting for instance in spin spirals with nanometer sized magnetic periods and unique rotational sense [1,2]. An ideal tool to investigate such systems down to the atomic scale is spin- polarized scanning tunneling microscopy. In the case of the monolayer Fe on Ir(111) we find skyrmion lattices on the atomic scale [3,4]. A tuning of the magnetic properties can be realized by tiny variations of the electronic structure due to different stacking of the magnetic layer. While for fcc-Fe we find a square nanoskyrmion lattice with vanishing net magnetic moment , the ground state of hcp-Fe is a hexagonal nanoskyrmion lattice that responds to external magnetic field and is subject to thermal fluctuations at a temperature well below the critical temperature of the other stacking . By covering the fcc-Fe system with one atomic layer of Pd we obtain an ultrathin film system in which individual skyrmions can be written and deleted with local spin-polarized currents . We analyze the field-dependent size and shape of such isolated skyrmions and comparison to theory yields the relevant material parameters, i.e., exchange, Dzyaloshinsky- Moriya interaction, and magnetic anisotropy . In addition to measurements with spin-polarized currents we demonstrate that skyrmions can be detected with non-spin-polarized currents due to spin-mixing effects in the non-collinear spin texture. We employ spatially resolved tunneling spectroscopy to investigate this new magnetoresistive effect and find that it correlates with the angle between nearest neighbors. As this is expected to be a general phenomenon in non- collinear magnetic states we propose to use it as an easy and reliable scheme to detect skyrmions in future spintronics devices.
 K. von Bergmann et al., J. Phys.: Cond. Mat. 26, 394002 (2014).
 M. Bode et al., Nature 447, 190 (2007).
 S. Heinze et al., Nature Phys. 7, 713 (2011).
 K. von Bergmann et al., Nanolett. (in press) DOI: 10.1021/acs.nanolett.5b00506.
 N. Romming et al., Science 341, 636 (2013).
 N. Romming et al., Phys. Rev. Lett. (in press) arxiv.org/abs/1504.01573.
11.06.2015 - Quantum Photonic Engineering with Semiconductor Quantum Dots
Fei Ding ✉
Institute for Integrative Nanosciences, IFW Dresden
Future quantum networks involve the manipulation of stationary quantum bits in each node, as well as the transmission of information between the different nodes using flying qubits. Recent experiments have shown that the semiconductor quantum dots (QDs) are ideal candidates for both tasks, and a number of quantum hardware has been envisaged. However, a stumbling block for the exciting quantum network scenario is the lack of precise control over the electronic and optical properties of QDs. In this talk I will review several of our most recent progresses in the quantum photonic engineering with QDs, which are achieved by using a unique electro-mechanical tuning technique developed in Dresden. [1-4]
Firstly I will introduce a quantum light-emitting-diode (LED) which generates wavelength-tunable single photons on demand. This Q-LED paves the way towards the Bell-State measurement with indistinguishable photons emitted from spatially separated quantum nodes.  Secondly, I will talk about our recent report on a Q-LED that emits entangled photon pairs with a fastest operation speed ever reported. The third part of my talk focuses on a hybrid system operating at exactly 384.23 THz (780.241 nm, corresponds to Rubidium D2 transition). The Rubidium-locked single photon emission lies at the heart of a solid-state quantum memory. Finally a triggered single photon source based on light-hole exciton in QDs is introduced, which may provide a novel solution to the reversible quantum state transfer between single photons and single spins. [6,7]
 F. Ding et al., Phys. Rev. Lett. 104, 067405 (2010)
 F. Ding et al., Nano Lett. 10, 3453 (2010)
 K. Jöns et al., Phys. Rev. Lett. 107, 217402 (2011)
 R. Trotta et al., Phys. Rev. Lett. 109, 147401 (2012)
 J. Zhang et al., Nano Lett. 13, 5808 (2013)
 Y. Huo et al., Nature Phys. 10, 46 (2014)
 J. Zhang et al., Nano Lett. 15, 422 (2015)
 J. Zhang et al., arxiv. 1505.03026 (2015)
18.06.2015 - Quantum Hall Effect: Where does the current flow for the quantized Hall resistance?
Max Planck Institut für Festkörperforschung, Stuttgart
Since the discovery of the quantum Hall effect by Klaus von Klitzing in 1980, various microscopic pictures have been developed to explain the quantized Hall resistance in two-dimensional charge carrier systems. Most prominent is the edge-state picture found in many textbooks.
We have performed detailed scanning probe measurements on various quantum Hall samples over the last decade revealing the actual Hall potential and current distributions over the width of such samples under different conditions.
We have observed, the current distribution within a quantum Hall plateau varies systematically with increasing magnetic field. Based on these investigations, we can clearly state that the externally biased current is carried dissipationless in electronically incompressible regions of the sample, i.e., by electronic states under the local Fermi level. Furthermore, I will point out, disorder - leading to charge carrier localization which has been seen as inherently been important for the quantum Hall effect - is not a prerequisite for the quantized Hall resistance but the finite size of the sample might be enough. The role of contacts, the evolution towards the electrical breakdown of the quantum Hall effect and the Hall potential distribution in graphene will be discussed.
02.07.2015 - Probing Bulk Properties with a Surface Sensitive Tool
IV. Physikalisches Institut, Georg-August-Universität Göttingen
For more than 30 years now, Scanning Tunneling Microscopy (STM) and its related techniques stood for one of the utmost expertise in the art of characterizing the structural and electronic properties of surfaces down to the atomic scale. Special interest has been on surface related defects like e.g. adatoms or adsorbates. In contrast to the huge amount of STM studies in surface science, this talk focuses on recent developments transferring scanning probe techniques to bulk systems. Using low temperature scanning tunneling spectroscopy and scanning tunneling potentiometry, a few long-standing topics in solid state are addressed: What are the structural and electronic properties of a metal-semiconductor interface? What causes the electric resistivity on the atomic scale? Can we access signatures of electron correlations of magnetic impurities from inside a metal?