Spring Meeting 2013 in Regensburg
Long-wavelength helimangetic order and skyrmion lattice phase
Mon 16:00-16:15 H19, Session TT 17.5
Alfonso Chacon1,2, •Tim Adams1, Michael Wagner1, Andreas Bauer1,
Georg Brandl1,2, Bjoern Pedersen2, Helmuth Berger3, Peter Lemmens4, and
Christian Pfleiderer1 — 1Technische Universität München, Physik-Department E21,
D-85748 Garching, Germany — 2Forschungsneutronenquelle Heinz Maier Leibnitz (FRM
II), Lichtenbergstr. 1, 85748 Garching, Germany — 3Ecole Polytechnique Federale
Lausanne, CH-1015 Lausanne, Switzerland — 4Institute for Condensed Matter
Physics, TU Braunschweig, D-38106 Braunschweig, Germany
We report a long-wavelength helimagnetic superstructure in bulk samples of the ferrimagnetic insulator Cu2OSeO3. The magnetic phase diagram associated with the helimagnetic modulation inferred from small-angle neutron scattering and magnetization measurements includes a skyrmion lattice phase and is strongly reminiscent of MnSi, FeGe, and Fe1−xCoxSi, i.e., binary isostructural siblings of Cu2OSeO3 that order helimagnetically. The temperature dependence of the specific heat of Cu2OSeO3 is characteristic of nearly critical spin fluctuations at the helimagnetic transition. This provides putative evidence for effective spin currents as the origin of enhancements of the magnetodielectric response instead of atomic displacements considered so far.
Coherent/decoherent coupling of excitations in
Mon, Mar 11 2013, 16:00-16:15, H31, Session: O 19.1
•Stephanie Cheylan1, Hongdan Yan1, Peter Lemmens1,
Meinhard Schilling2, Andrey Bakin3, Johannes Ahrens4, and Martin Bröring4 —
1IPKM, TU-BS, Braunschweig — 2EMG, TU-BS, Braunschweig — 3IHT, TU-BS,
Braunschweig — 4IAAC, TU-BS, Braunschweig
We study the coherent coupling between plasmons of Au nanowire arrays and excitons of moleculare components, given by BODIPY and its derivative aza-BODIPY. The exciton energy of these two classes of molecules is tuned to match with different plasmonic modes. Different interactions are investigated depending on the morphology of the nanowire array and the energy of molecular excitons. Work supported by DFG, B-IGSM and NTH School for Contacts in Nanosystems.
Low versus high energy excitations in the Skyrmion
lattice system Cu2OSeO3,
Tue, Mar 12 2013, 9:30-9:45, H3, Session: MA 15.1, HL32.1
•Dirk Wulferding1, Peter Lemmens1, Vladimir
Gnezdilov2, Yurii Pashkevich3, Christian Pfleiderer4, and Helmuth Berger5 —
1IPKM, TU-BS, Braunschweig — 2ILTPE, Kharkov, Ukraine — 3DonFTI, Donetsk,
Ukraine — 4Physikdepartment, TU München — 5EPFL, Lausanne, Switzerland
Cu2OSeO3, a ferromagnetic insulator with a skyrmion lattice phase previously known from intermetallics, shows low as well as high energy excitations that strongly depend on small applied magnetic fields. In particular, the low energy anomalies are discussed in relation to the Skyrmion lattice. Work supported by DFG, B-IGSM and NTH School for Contacts in Nanosystems.
Interplay between lattice and spin degrees of
freedom in FeSe superconductors
Wed, Mar 13 2013, 9:30-9:45, H18, Session: TT 36.1
Vladimir Gnezdilov1, •Peter Lemmens2, Yurii
Pashkevich3, Tatiana Shevtsova3, Alexander Gusev3, Dirk Wulferding2, Dmitriy
Chareev4, and Alexander Vasiliev5 — 1ILTPE, Kharkov, Ukraine — 2IPKM, TU-BS,
Braunschweig — 3DonFTI, Donetsk, Ukraine — 4IEM, RAS, Moscow, Russia — 5Moscow
State Univ., Moscow, Russia
Recently grown high quality FeSe single crystals show pronounced anomalies in the temperature dependent Raman spectra. An anomalous hardening of one phonon mode upon decreasing temperature is related to local fluctuations of the Fe orbital occupation, described by the Fe spin state. The enhancement of the low-frequency spectral weight above the structural phase transition temperature TS and its change below TS is discussed in connection with the opening of a spin state gap which results in the absence of magnetic order in FeSe. Work supported by DFG, B-IGSM and NTH School for Contacts in Nanosystems.
Comparing scattering processes in topological insulators and giant Rashba
Wed, Mar 13 2013, 11:45-12:00, H16, Session: HL 48.10, O38.10. TT33.10
•Peter Lemmens1, Vladimir Gnezdilov2, Dirk
Wulferding1, Patrik Recher3, Helmuth Berger4, Yoichi Ando5, Angela Möller6, R.
Sankar7, and Fang-Cheng Chou7 — 1IPKM, TU-BS, Braunschweig — 2ILTPE, Kharkov,
Ukraine — 3IMAPH, TU-BS, Braunschweig — 4EPFL, Lausanne, Switzerland — 5ISIR,
Osaka, Japan — 6Dept. of Chemistry, Univ. Houston, USA — 7CCMS, National Taiwan
Univ., Taipei, Taiwan
Using Raman scattering experiments we probe scattering processes in BiTeI and topological insulators. In the former systems the surface termination, either by Iodine - Bi or Tellur - Bi determines the low energy scattering properties. A comparison of these surface induced signals with effects seen in topological insulators leads to a considerable gain of understanding of scattering mechanisms and the respective role of symmetry. Work supported by DFG, B-IGSM and NTH School for Contacts in Nanosystems.
Verwey Transition in epitaxial Fe3O4 thin films studied by
Wed 9:30 H20, Session TT 38.1
•Mehrdad Baghaie Yazdi1, Kwang-Yong Choi2, Dirk Wulferding3,
Peter Lemmens3, and Lambert Alff1 — 1TU Darmstadt, Materialwissenschaft —
2Chung-Ang University, Department of Physics, Seoul, Korea — 3TU Braunschweig,
Institut für Festköperphysik
Magnetite has served as the model material for metal insulator transitions for more than seven decades. However, the origin and nature of the Verwey transition remains debated to this day. Recent disputes over the interpretation of measurement data [1,2] have reignited the question over the fundamental driving force behind the Verwey transition. Currently two major models try to explain the origin of this phenomenon, namely one that postulates that the transition is structurally driven  while the other sees the ordering of the t2g orbitals of the Fe2+ ions at the octahedral sites as the main cause for both charge localization and structural transformation. Using epitaxial thin films of magnetite grown on Al2O3 and MgO which exhibit different Verwey transition temperatures, 128 K and 119 K respectively, we have investigated the evolution of ordering peaks in Raman spectroscopy. These peaks have been correlated to the global magnetic and electric properties of the films measured by SQUID and four-point electrical transport measurements.
 A. Tanaka et al., Phys. Rev. Lett. 108, 227203 (2012).  S. B. Wilkins et al., Phys. Rev. B 79, 201102(R) (2009).  G. Kh. Rozenberg et al., Phys. Rev. Lett. 96 045705 (2006).
Is there a magnetic analogon of the isotope effect
Thu, Mar 14 2013, 17:30-17:45, H19, Session: TT 62.9
•Dirk Wulferding1, Peter Lemmens1, Gil Drachuck2, Meni
Shay2, Galina Bazalitsky2, Rinat Ofer2, Zaher Salman3, Alex Amato3, Christof
Niedermayer3, and Amit Keren2 — 1IPKM, TU-BS, Braunschweig — 2Technion, Haifa,
Israel — 3PSI, Villigen, Switzerland
The successful growth of large (CaxLa1−x)(Ba1.75−xLa0.25+x)Cu3Oy (CLBLCO) allows now a systematic investigation of correlations between material properties and Tc. In particular, the parameter x varies the magnetic super-exchange coupling J by controlling the Cu-O-Cu buckling angle. Using Raman scattering (and µSR) we determine J for a series of samples, and show that Tc monotonically increases with increasing J. Our results are in contrast to recent Raman data (B. Mallett, arXiv:1202.5078) showing the opposite behavior in other systems. The origin of the contradiction will be discussed. Work supported by DFG, GIF and the ESF.