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Research interests

Current: Novel High-Temperature-Superconductors: Iron pnictides

Figure: LiFeAs: Fermi surfage, Gap
Sketch of the anisotropic order parameter in LiFeAs (ARPES measurement)
(Borisenko, et al., Symmetry 4, 251 (2012))
Although the phenomena of superconductivity has been discovered experimentally more than 100 years ago by Kammerlingh Onnes in 1911, it is still subject to recent research in experiment and theory. A possible reason is that it appears in many different materials and a physical understanding hasn't been achieved in a universal theory yet. The effect of superconductivity which simply speaking manifests itself by complete expulsion of a magnetic field (Meißner effect) and conductance of electric current without loss is due to pairing of electrons to the so-called Cooper pairs. In conventional superconductors (which become superconducting at quite low temperatures below 23K) the attractive interaction is believed to be mediated by lattice vibrations and the essential physics has been understood quite some time ago in terms of the BCS theory.
Figure: Fermi surface KFe2-ySe2
The Fermi surface of KFe2-ySe2 consists in 2 patches centered around the X and Y point (color code: red: dxz orbital, green: dyz orbital, blue: dxy orbital).
However, the whole understanding got screwed up by two experimental discoveries: In 1986 Bednorz and Müller discovered superconductivity in cuprate materials at such high temperatures where theoretically it shouldn't be possible within the known mechanisms. Up to now the microscopic origin of superconductivity in cuprates is still under debate whereas the materials easily entered into application due to the high transition temperatures above 100K. In 2008 Kamihara and collaborators reported superconductivity in doped iron pnictide materials which added another class of materials that are superconducting at high temperatures up to 55 K. Recently, active research has been carried out in this field because the materials show a rich phase diagram that contains an interplay between superconductivity, magnetism and crystal structure. Among the many known materials there are three compounds which are particularly interesting because they don't fit into the standard paradigm: LiFeAs, KFe2-ySe2 and FeSe (under pressure).
The main questions that we address here are the symmetry of the order parameter and how parameters make a superconductor gaped or gapless. Starting point is the so-called spin fluctuation theory of pairing where the glue of the Cooper pairs is mediated by quantum fluctuations of the electron spins.

Impurities in superconductors
Figure: spectra close to strong impurity
Spatial evolution of the spectra close to a strong impurity in the material NaCCOC
(Choubey, et al., Phys. Rev. B 96, 174523 (2017))
Impurities are often an unwanted property of crystalline systems because they destroy the strict discrete translation symmetry of a crystal. In the case of superconductors, they can however be used to probe the properties of the system. Impurities can be introduced in a controlled way by exposing the crystal to electron irradiation such that the properties of the superconducting order parameter changes as a function of impurity concentration. Another way to observe the implications on impurities is to grow crystals of iron-based superconductors in presence of small concentrations of atoms that substitute for iron atoms and then observe this on the atomic scale with the scanning tunneling microscopy technique.
Superconductivity is a phenomenon that emerges due to the quantum mechanical properties of the electrons which are described by a wavefunction. Within a interference experiment of these waves, one can reveal the structure of this state. Impurities can be used as scattering centers for these waves and the resulting interference phenomena can be observed on the microscopic scale. This method called quasiparticle interference (QPI) delivers information about the electronic system in the normal (non-superconducting) state and the superconducting state with high energetic resolution.

Investigations of the gap structure
Proposals for the pairing mechanism that leads to superconductivity, i.e. the glue that binds together electrons to form the Cooper pairs, need to be tested towards expected structures of the order parameter. Theoretically, one starts from a microscopic model for the pairing interaction and calculates for example the excitation gap in the superconducting state. This can be measured directly with spectroscopic probes, or has imprints on thermodynamic quantities as for example specific heat. Comparison to experimental results finally can decide whether the theoretical proposal is compatible with the observations or has to be rejected.

Theory for STM
Figure: tunneling conductange close to Zn impurity
Theoretically expected tunneling conductance close to a strong impurity in Cuprates within different approximations compared to experimental observations
(Kreisel, et al., Phys. Rev. Lett. 114, 217002 (2015))
The experimental tool of scanning tunneling microscopy delivers spectacular images of conducting surfaces on the atomic scale. It has been used to examine the normal state and the superconducting state of high-Tc materials. However, these materials are layered such that the tunneling out of the surface needs to take into account the wave function of the electronic state when calculating tunneling with sub-atomic resolution.

Dissertation: Spin-wave calculations for Heisenberg magnets with reduced symmetry

Figure: dispersion of quasiparticles
Kinematics determines the possibility of quasiparticle decay: stable quasi particles (left), unstable quasiparticles (right) upon inclusion of 3 magnon scattering
(Dissertation)
The phenomenon of magnetism is a pure quantum effect and has been studied since the beginning of civilization. The practical use of magnetic materials for technical purposes was well established in the 19th century; still nowadays there is no lack of new high-tech applications based on magnetism for example in information technology to store and process data.
Figure: dispersion of magnons
Spin-wave dispersion of the lowest magnon mode in a YIG film: A Bose condensate evolves at the minimum of the dispersion and imposes additional condensates at odd multiples of the momentum at the minimum
(Dissertation)
This thesis does not focus on the development of new applications of magnetism in technology, nor enhancement of known fields of application. Instead, the intention is to use a quantum theory of magnetism for obtaining new insights on physical effects that accompany the phenomenon of magnetism. Therefore three different model systems, each of which are believed to describe a class of real compounds, are considered. Starting from the idea that magnetism can be understood by use of the so-called Heisenberg model that microscopically characterizes the interaction between localized magnetic moments, we restrict ourselves to the case where a long-range magnetic order is present. In order to deduce consequences resulting from this microscopic picture we use the spin-wave theory that is introduced in the first chapter. Central objects of this theory are the magnons which are elementary quantum excitations in ordered magnets.
An application of these mathematical techniques to a model that describes an antiferromagnet in an external magnetic field is presented in the second chapter. Quantities like the spin-wave velocity and the damping of magnons are calculated using a Hermitian operator approach in the framework of spin-wave theory. A strong renormalization of the magnetic excitations arises because the symmetry of the system is reduced due to the external magnetic field.
Figure: classical ground state
Classical ground state of a frustrated anti-ferromagnet with anisotropy (model for Cesium-Copper-cloride)
(Dissertation)
In the second model system, that describes thin films of a ferromagnet, concepts of classical physics meet quantum physics: The magnetic dipole-dipole interaction that is also known in everyday life from the magnetic forces between magnets and was initially formulated in the theory of electromagnetism, is included in the microscopic model. Having a special compound in mind where the magnetic excitations are directly accessible in experiments, the energy dispersions of magnon modes in thin-film ferromagnets are deduced. Our approach is essentially a basis for further investigations beyond this thesis to describe strong correlations and condensation of magnons. A recent realization of data processing devices with spin waves puts the understanding of physical processes in these ferromagnetic films in the focus of upcoming research.
Figure: Pulse-echo technique
Measurement of the sound velocity and damping in a crystal using the pulse-echo technique
(Disputation)
The third model system brings in the so-called frustration where the interactions between the spins are such that the total energy cannot be minimized by an appropriate alignment of the magnetic moments in the classical picture. In the simplest case this appears because the antiferromagnetically coupled spins are located on a triangular lattice. This situation will lead to strong quantum fluctuations which make this model system interesting. Finally the overall symmetry is reduced by inclusion of spin anisotropies and an external magnetic field. Instead of focusing on the properties of the magnetic excitations, the effect of the magnetic field on the properties of the lattice vibrations is subject to the investigation. This is interesting because the characteristics of lattice vibrations can be measured experimentally using the supersonic technique.

Fulltext available

Diploma thesis: Spin-wave interactions in quantum antiferromagnets

Figure: staggered structure factor
Staggered structure factor of an anti-ferromagnet in arbitrary dimension
(Diploma thesis)
Magnetic properties of materials are well known since the days of antiquity when the Greeks described the ion ore. These materials called ferromagnets are widely used in our everyday life for example in electric apparatus. In ferromagnets the magnetic moments of the atoms involved are aligned in a parallel way. Therefore one can observe an macroscopic magnetization outside the solid.
Figure: self energy
Diagrammatic visualization of the corrections to the self energy in Hermitean parametrization
(Diploma thesis)
In the case of the anti-ferromagnet this is different: An anti parallel alignment leads to a cancellation of the total magnetic moment and it is not possible to directly observe magnetic forces. These materials are nevertheless featuring properties due to the underlying magnetism. I calculated some of these properties in my diploma thesis.
content (German only)
diploma thesis (German only)

Research Stays Abroad


  • 09/2008-10/2008, 10/2009-11/2009 International Center for Condensed Matter Physics - ICCMP at Universidade de Brasília in Brasília, Brazil.
  • 09/2010-10/2010, 12/2011 International Institute of Physics - IIP at Universidade Federal do Rio Grande do Norte in Natal, Brazil.
  • 11/2014 Kavli Institute for Theoretical Physics KITP at University of California Santa Barbara, CA, USA.


  • Publications

    Andreas Kreisel, Nils Hasselmann, and Peter Kopietz
    Phys. Rev. Lett. 98, 067203 (2007)
    Probing Anomalous Longitudinal Fluctuations of the Interacting Bose Gas via Bose-Einstein Condensation of Magnons

    Andreas Kreisel, Francesca Sauli, Nils Hasselmann, and Peter Kopietz
    Phys. Rev. B 78, 035127 (2008)
    Quantum Heisenberg antiferromagnets in a uniform magnetic field: Nonanalytic magnetic field dependence of the magnon spectrum

    Andreas Kreisel, Francesca Sauli, Lorenz Bartosch, and Peter Kopietz
    European Physical Journal B 71, 59 (2009)
    Microscopic spin-wave theory for yttrium-iron garnet films

    Andreas Kreisel, Francesca Sauli, Lorenz Bartosch, and Peter Kopietz
    Europhysics News 40/6, 18 (2009)
    Spin-waves in Yttrium-iron garnet

    Thomas Kloss, Andreas Kreisel, and Peter Kopietz
    Phys. Rev. B 81, 104308 (2010)
    Parametric pumping and kinetics of magnons in dipolar ferromagnets

    Christian Sandweg, Benjamin Jungfleisch, Vitaliy Vasyucka, Alexander Serga, Peter Clausen, Helmut Schultheiss, Burkard Hillebrands, Andreas Kreisel, and Peter Kopietz
    Rev. Sci. Instrum. 81, 073902 (2010)
    Wide-range wavevector selectivity of magnon gases in Brillouin light scattering spectroscopy

    Johannes Hick, Francesca Sauli, Andreas Kreisel, and Peter Kopietz
    European Physical Journal B 78, 429 (2010)
    Bose-Einstein condensation at finite momentum and magnon condensation in thin film ferromagnets

    Andreas Kreisel, Peter Kopietz, Pham Thanh Cong, Bernd Wolf, and Michael Lang
    Phys. Rev. B 84, 024414 (2011)
    Elastic constants and ultrasonic attenuation in the cone state of the frustrated antiferromagnet Cs2CuCl4

    Aldo Isidori, Annika Ruppel, Andreas Kreisel, Peter Kopietz, Alexander Mai, and Reinhard M. Noack
    Phys. Rev. B 84, 184417 (2011)
    Quantum criticality of dipolar spin chains

    Aldo Isidori and Andreas Kreisel
    AIP Conf. Proc. 1485, 291 (2012)
    Functional renormalization group study of a dipolar spin chain in a transverse magnetic field

    Andreas Rückriegel, Andreas Kreisel, and Peter Kopietz
    Phys. Rev. B 85, 054422 (2012)
    Time-dependent spin-wave theory

    A. A. Serga, C. W. Sandweg, V. I. Vasyuchka, M. B. Jungfleisch, B. Hillebrands, A. Kreisel, P. Kopietz, and M. P. Kostylev
    Phys. Rev. B 86, 134403 (2012)
    Brillouin light scattering spectroscopy of parametrically excited dipole-exchange magnons

    Philipp Lange, Peter Kopietz, and Andreas Kreisel
    European Physical Journal B 85, 370 (2012)
    Damping of phase fluctuations in superfluid Bose gases

    Yan Wang, Andreas Kreisel, Peter J. Hirschfeld, Vivek Mishra
    Phys. Rev. B 87, 094504 (2013)
    Using controlled disorder to distinguish s_+- and s_++ gap structure in Fe-based superconductors

    Andreas Kreisel, Yan Wang, Thomas A. Maier, Peter J. Hirschfeld, Douglas J. Scalapino
    Phys. Rev. B 88, 094522 (2013)
    Spin fluctuations and superconductivity in KxFe2-ySe2

    Y. Wang, A. Kreisel, V. B. Zabolotnyy, S. V. Borisenko, B. Büchner, T. A. Maier, P. J. Hirschfeld, D. J. Scalapino
    Phys. Rev. B 88, 174516 (2013)
    Superconducting gap in LiFeAs from three dimensional spin fluctuation pairing calculations

    Peayush Choubey, T. Berlijn, A. Kreisel, C. Cao, P. J. Hirschfeld
    Phys. Rev. B 90, 134520 (2014)
    Visualization of atomic-scale phenomena in superconductors: application to FeSe

    Andreas Kreisel, Michael Peter, Peter Kopietz
    Phys. Rev. B 90, 075130 (2014)
    Singular spin-wave theory and scattering continua in the cone state of Cs2CuCl4

    Y. Mizukami, M. Konczykowski, Y. Kawamoto, S. Kurata, S. Kasahara, K. Hashimoto, V. Mishra, A. Kreisel, Y. Wang, P. J. Hirschfeld, Y. Matsuda, T. Shibauchi
    Nat. Commun. 5, 5657 (2014)
    Disorder-induced topological change of the superconducting gap structure in iron pnictides

    A. Kreisel, Peayush Choubey, T. Berlijn, W. Ku, B. M. Andersen, P. J. Hirschfeld
    Phys. Rev. Lett. 114, 217002 (2015)
    Interpretation of scanning tunneling quasiparticle interference and impurity states in cuprates

    Shantanu Mukherjee, A. Kreisel, P. J. Hirschfeld, Brian M. Andersen
    Phys. Rev. Lett. 115, 026402 (2015)
    Model of Electronic Structure and Superconductivity in Orbitally Ordered FeSe

    A. T. Rømer, A. Kreisel, I. Eremin, M. A. Malakhov, T. A. Maier, P. J. Hirschfeld, B. M. Andersen
    Phys. Rev. B 92, 104505 (2015)
    Pairing symmetry of the one-band Hubbard model in the paramagnetic weak-coupling limit: a numerical RPA study

    A. Kreisel, Shantanu Mukherjee, P. J. Hirschfeld, Brian M. Andersen
    Phys. Rev. B 92, 224515 (2015)
    Spin excitations in a model of FeSe with orbital ordering

    A. Kreisel, A. T. Rømer, P. J. Hirschfeld, B. M. Andersen
    J. Supercond. Nov. Magn. 30, 85 (2017)
    Superconducting phase diagram of the paramagnetic one-band Hubbard model Publishers version

    Shun Chi, Ramakrishna Aluru, Udai Raj Singh, Ruixing Liang, Walter N. Hardy, D. A. Bonn, A. Kreisel, Brian M. Andersen, R. Nelson, T. Berlijn, W. Ku, P. J. Hirschfeld, Peter Wahl
    Phys. Rev. B 94, 134515 (2016)
    Impact of Iron-site defects on Superconductivity in LiFeAs

    A. Kreisel, R. Nelson, T. Berlijn, W. Ku, Ramakrishna Aluru, Shun Chi, Haibiao Zhou, Udai Raj Singh, Peter Wahl, Ruixing Liang, Walter N. Hardy, D. A. Bonn, P. J. Hirschfeld, Brian M. Andersen
    Phys. Rev. B 94, 224518 (2016)
    Towards a quantitative description of tunneling conductance of superconductors: application to LiFeAs

    Peter O. Sprau, Andrey Kostin, Andreas Kreisel, Anna E. Böhmer, Valentin Taufour, Paul C. Canfield, Shantanu Mukherjee, Peter J. Hirschfeld, Brian M. Andersen, J.C. Séamus Davis
    Science, 357, 75 (2017)
    Discovery of Orbital-Selective Cooper Pairing in FeSe, Preprint Version

    A. Kreisel, Brian M. Andersen, Peter O. Sprau, Andrey Kostin, J.C. Séamus Davis, P. J. Hirschfeld
    Phys. Rev. B 95, 174504 (2017)
    Orbital selective pairing and gap structures of iron-based superconductors

    Johannes H. J. Martiny, Andreas Kreisel, P. J. Hirschfeld, Brian M. Andersen
    Phys. Rev. B 95, 184507 (2017)
    Robustness of Quasiparticle Interference Test for Sign-changing Gaps in Multiband Superconductors

    Shun Chi, Ramakrishna Aluru, Stephanie Grothe, A. Kreisel, Udai Raj Singh, Brian M. Andersen, W. N. Hardy, Ruixing Liang, D. A. Bonn, S. A. Burke, Peter Wahl
    Nat. Commun. 8, 15996 (2017)
    Imaging the Real Space Structure of the Spin Fluctuations in an Iron-based superconductor

    Peayush Choubey, Andreas Kreisel, T. Berlijn, Brian M. Andersen, P. J. Hirschfeld
    Phys. Rev. B 96, 174523 (2017)
    Universality of scanning tunneling microscopy in cuprate superconductors

    S. L. Holm, A. Kreisel, T. K. Schäffer, A. Bakke, M. Bertelsen, U. B. Hansen, M. Retuerto, J. Larsen, D. Prabhakaran, P. P. Deen, Z. Yamani, J. O. Birk, U. Stuhr, Ch. Niedermayer, A. L. Fennell, B. M. Andersen, K. Lefmann
    Phys. Rev. B 97, 134304 (2018)
    Magnetic ground state and magnon-phonon interaction in multiferroic h-YMnO3

    Anna E. Böhmer, Andreas Kreisel
    J. Phys.: Condens. Matter 30 023001, (2018)
    Nematicity, magnetism and superconductivity in FeSe

    Andrey Kostin, Peter O. Sprau, Andreas Kreisel, Yi Xue Chong, Anna E. Böhmer, Paul C. Canfield, Peter J. Hirschfeld, Brian M. Andersen, J.C. Séamus Davis
    Nature Materials 17, 869 (2018)
    Visualizing Orbital-selective Quasiparticle Interference in the Hund's Metal State of FeSe, arXiv version

    Sonja Holm-Dahlin, Sofie Janas, Andreas Kreisel, Ekaterina Pomjakushina, Jonathan S. White, Amy Fennell, Kim Lefmann
    Quantum Beam Sci. 2018, 2(3), 16 (2018)
    The Magnetic Phase Transition and Universality Class of h-(Y0.98 Eu0.02)MnO3 and h-YMnO3 in Zero and Applied Pressure

    Jinho Lim, Wonbae Bang, Jonathan Trossman, Andreas Kreisel, Matthias Benjamin Jungfleisch, Axel Hoffmann, C. C. Tsai, John B. Ketterson
    IEEE Transactions on Magnetics, 55, 6100504 (2019)
    Study of Surface Character of Micrometer-Scale Dipole-Exchange Spin Waves in an Yttrium Iron Garnet Film
    PDF
    A. Kreisel, Brian M. Andersen, P. J. Hirschfeld
    Phys. Rev. B 98, 214518 (2018)
    Itinerant approach to magnetic neutron scattering of FeSe: effect of orbital selectivity

    Pabitra K. Biswas, Andreas Kreisel, Qisi Wang, Devashibhai T. Adroja, Adrian D. Hillier, Jun Zhao, Rustem Khasanov, Jean-Christophe Orain, Alex Amato, Elvezio Morenzoni
    Phys. Rev. B 98, 180501 (2018)
    Evidence of nodal gap structure in the basal plane of the FeSe superconductor

    Johannes H. J. Martiny, Andreas Kreisel, Brian M. Andersen
    Phys. Rev. B99, 014509 (2019)
    Theoretical study of impurity-induced magnetism in FeSe

    Jinho Lim, Wonbae Bang, Jonathan Trossman, Andreas Kreisel, Matthias Benjamin Jungfleisch, Axel Hoffmann, C. C. Tsai, and John B. Ketterson
    Phys. Rev. B, 99, 014435 (2019)
    Direct detection of multiple backward volume modes in yttrium iron garnet at micron scale wavelengths
    PDF

    Chandan Setty, Shinibali Bhattacharyya, Yifu Cao, Andreas Kreisel, Peter Hirschfeld
    Nat. Commun. 11, 523 (2020)
    Ultranodal pair states in iron-based superconductors

    Tong Chen, Youzhe Chen, Andreas Kreisel, Xingye Lu, Astrid Schneidewind, Yiming Qiu, J. T. Park, Toby G. Perring, J Ross Stewart, Huibo Cao, Rui Zhang, Yu Li, Yan Rong, Yuan Wei, Brian M. Andersen, P. J. Hirschfeld, Collin Broholm, Pengcheng Dai
    Nat. Mater. 18, 709 (2019)
    Anisotropic spin fluctuations in detwinned FeSe
    PDF Supplement

    Astrid T. Rømer, Thomas A. Maier, Andreas Kreisel, Ilya Eremin, P. J. Hirschfeld, Brian M. Andersen
    Phys. Rev. Research 2, 013108 (2020)
    Pairing in the Two-Dimensional Hubbard Model from Weak to Strong Coupling

    Sang Yong Song, J. H. J. Martiny, A. Kreisel, B. M. Andersen, Jungpil Seo
    Phys. Rev. Lett. 124, 117001 (2020)
    Visualization of local magnetic moments emerging from impurities in the Hund's metal states of FeSe

    Jia-Xin Yin, Songtian S. Zhang, Guangyang Dai, Yuanyuan Zhao, Andreas Kreisel, Gennevieve Macam, Xianxin Wu, Hu Miao, Zhi-Quan Huang, Johannes H. J. Martiny, Brian M. Andersen, Nana Shumiya, Daniel Multer, Maksim Litskevich, Zijia Cheng, Xian Yang, Tyler A. Cochran, Guoqing Chang, Ilya Belopolski, Lingyi Xing, Xiancheng Wang, Yi Gao, Feng-Chuan Chuang, Hsin Lin, Ziqiang Wang, Changqing Jin, Yunkyu Bang, M. Zahid Hasan
    Phys. Rev. Lett. 123, 217004 (2019)
    Quantum phase transition of correlated iron-based superconductivity in LiFe1−xCoxAs

    Shinibali Bhattacharyya, Kristofer Björnson, Karim Zantout, Daniel Steffensen, Laura Fanfarillo, Andreas Kreisel, Roser Valentí, Brian M. Andersen, P. J. Hirschfeld
    Phys. Rev. B 102, 035109 (2020)
    Non-local correlations in Iron Pnictides and Chalcogenides

    Astrid T. Rømer, Andreas Kreisel, Marvin A. Müller, P. J. Hirschfeld, Ilya M. Eremin, Brian M. Andersen
    Phys. Rev. B 102, 054506
    Theory of Strain-Induced Magnetic Order and Splitting of Tc and TTRSB in Sr2RuO4

    Chandan Setty, Yifu Cao, Andreas Kreisel, Shinibali Bhattacharyya, P. J. Hirschfeld
    Phys. Rev. B 102, 064504 (2020)
    Bogoliubov Fermi Surfaces in Spin-1/2 Systems: Model Hamiltonians and Experimental Consequences

    Eric Lee-Wong, Ruolan Xue, Feiyang Ye, Andreas Kreisel, Toeno van der Sar, Amir Yacoby, Chunhui Rita Du
    Nano Lett. 20, 3284-3290 (2020)
    Nanoscale Detection of Magnon Excitations with Variable Wavevectors Through a Quantum Spin Sensor

    Rahul Sharma, Andreas Kreisel, Miguel Antonio Sulangi, Jakob Böker, Andrey Kostin, Milan P. Allan, H. Eisaki, Anna E. Böhmer, Paul C. Canfield, Ilya Eremin, J.C. Séamus Davis, P.J. Hirschfeld, Peter O. Sprau
    arXiv:2005.05140
    Multi-Atom Quasiparticle Scattering Interference for Superconductor Energy-Gap Symmetry Determination

    Andreas Kreisel, P.J. Hirschfeld, Brian M. Andersen
    Symmetry, 12, 1402 (2020)
    On the Remarkable Superconductivity of FeSe and its Close Cousins

    Talks

    DPG-Frühjahrstagung 2007 Regensburg
    session: TT 31.5
    Thu 29.03.2007, 18:45-19:00, H19
    Probing anomalous longitudinal fluctuations of the interacting Bose gas via Bose-Einstein condensation of magnons
    slides
    Ruperto Carola Symposion Heidelberg: Ultracold Quantum Gases
    18-20 July 2007
    Bose-Einstein condensation and spin-wave interactions in quantum antiferromagnets
    slides
    DPG-Frühjahrstagung 2008 Berlin
    session: TT 24.11
    Wed 27.02.2008, 16:45-17:00, H2053
    Hybrid approach for quantum antiferromagnets in a uniform magnetic field: Application
    slides
    DPG-Frühjahrstagung 2009 Dresden
    Session: MA 17.10
    Wed 25.03.2009, 12:30-12:45, HS04
    Microscopic theory of spin waves in ferromagnetic films
    slides
    University of São Paulo (USP)
    Thu 19.11.2009, 14:30-15:30
    Spin-Wave Theory for Magnetic Insulators
    From textbook knowledge towards BEC of magnons
    slides
    abstract
    DPG-Frühjahrstagung 2010 Regensburg
    Session: TT 30.8
    Thu 25.03.2010, 11:30-11:45, H20
    Spin-Phonon Interactions in triangular Antiferromagnets
    slides
    Workshop "Ultracold Quantum Gases Beyond Equilibrium", 2010 Natal, Rio Grande do Norte, Brasilien
    Tue 28.09.2010, 11:50-12:10
    Bose Einstein Condenation at finite momentum
    slides
    Longer Version, held on 17.09.2010 at Instituto Internacional da Física, Natal
    slides
    Korrelationstage 2011, Dresden
    Fri 04.03.2011, 9:25-9:50
    Magnon-phonon interactions in triangular antiferromagnets
    slides
    Disputation: Spin-wave calculations for Heisenberg magnets with reduced symmetry
    Thu 14.07.2011, 14:00-14:20
    Spin-wave calculations for Heisenberg magnets with reduced symmetry
    slides
    APS March Meeting 2012, Boston, MA
    Tue 28.02.2012, 15:30-15:42
    Time-dependent spin-wave theory
    slides
    APS March Meeting 2013, Baltimore, MD
    Tuesday, 19.03.2013, 12:39-12:51
    Pairing strength and gap functions in multiband superconductors: 3D effects
    slides
    Condensed matter group seminar, The Niels Bohr Institute, University of Copenhagen, Denmark
    Thursday, 24.10.2013, 13:15
    Spin-waves and BEC in thin-film ferromagnets; Spin fluctuation pairing and symmetry of the order parameter in KxFe2-ySe2
    slides
    DPG-Frühjahrstagung 2014 Dresden
    Session: TT 92.5
    Thursday 03.04.2014, 16:00-16:15, HSZ 201
    Spin fluctuations and superconductivity in KxFe2-ySe2
    slides
    Correlations & Coherence at different scales, 2014, Ustroń, Poland
    Monday 08.09.2014, 17:20-17:40
    Visualization of atomic-scale phenomena in superconductors
    slides
    APS March Meeting 2015, San Antonio, TX
    Thursday, March 5, 2015, 3:06 PM–3:18 PM
    Interpretation of scanning tunneling quasiparticle interference and impurity states
    slides
    DPG-Frühjahrstagung 2015 Berlin
    Session: DF 10.10
    Wednesday, March 18, 2015, 12:00–12:15, EB 107
    Magnon-phonon interactions in hexagonal multiferroic YMnO3
    Contribution
    20th International Conference on Magnetism, Barcelona
    Tuesday, July 7 2015, 12:30-12:45
    Magnon-phonon interactions in hexagonal multiferroic YMnO3
    Contribution
    Physics of Interfaces and Layered Structures, Stockholm
    Monday, September 7 2015, 11:00
    Visualization of atomic-scale phenomena in superconductors
    slides
    ITF-Seminar, Leibnitz Institut für Festkörper- und Werkstoffforschung, Dresden
    Monday, October 19 2015,15:00
    Interpretation of scanning tunneling quasiparticle interference and impurity states
    DPG-Frühjahrstagung 2016 Regensburg
    Session TT 46.7
    Wednesday, March 09 2016 12:00-12:15, H19
    Superconductivity and spin excitations in orbitally ordered FeSe
    Contribution
    Session O 81.3
    Thursday, March 10 2016 11:15-11:30, H4
    Simulating atomic-scale phenomena on surfaces of unconventional superconductors
    Contribution
    APS March Meeting 2016, Baltimore, MD
    Session P11.7
    Wednesday, March 16 2016, 16:06-16:18
    Superconductivity and spin excitations in orbitally ordered FeSe
    Contribution
    5th International Conference on Superconductivity and Magnetism 2016
    Tuesday, April 26 2016, 11:45-12:05
    Spin Excitations in a Model of FeSe with Orbital Ordering
    slides
    ITF-Seminar, Leibnitz Institut für Festkörper- und Werkstoffforschung, Dresden
    August 16 2016, 11:00-12:00
    Towards a realistic simulation of disorder in unconventional superconductors
    slides
    BSCCOfest, Department of Physics, University of Florida, Gainesville, FL
    December 16 2016, 10:30-11:00, NPB 2165
    Realistic simulation of tunneling in STM for cuprates
    slides
    Condensed Matter Theory Seminar, Institut für Theoretische Physik, Universität Frankfurt
    Februar 10 2017, 15:15
    Orbital-Selective Pairing and Gap Structures of Iron-Based Superconductors
    slides
    APS March Meeting 2017, New Orleans, LA
    Monday, March 13 2017, 14:42-14:54
    Towards a quantitative description of tunneling conductance of superconductors
    slides
    DPG-Frühjahrstagung 2017 Dresden
    Thursday, March 23 2017, 15:00–15:30, HSZ 103
    New Developments in the Theory of STM on Unconventional Superconductors
    Contribution slides
    Condensed Matter Seminar, School of Physics and Astronomy, University of St. Andrews, Scotland, April 26 2017, 12:00
    Orbital-Selective Pairing in FeSe
    Slides
    Seminar der Theorie der kondensierten Materie, RWTH Aachen, 18. July 2017, 15:00
    Orbital-Selective Pairing in iron-based superconductors
    Condensed matter seminar at KIT, Karlsruhe Institute of Technology, 20. February 2018, 11:00
    Theory of STM in Unconventional Superconductors
    Slides
    APS March Meeting 2018, Los Angeles, CA
    Monday, March 5th 2018, 11:51-12:03
    Consequences of Orbital Selectivity for Magnetism and Superconductivity in Fe-based Superconductors
    Abstract Slides
    DPG-Frühjahrstagung 2018 Berlin
    Tuesday, March 13th 2018, 11:30–11:45, A 053
    Consequences of Orbital Selectivity for Magnetism and Superconductivity in Fe-based Superconductors
    Contribution Slides
    International Workshop: New Developments in STM on Surfaces of Functional Materials, TDLI, Shanghai
    Tuesday, 28. August, 2018, 16:15-16:50
    Wannier-based Theory of Unconventional Superconductivity
    Slides
    Department of Physics, Tsinghua University, Bejiing
    Thursday, 30. August, 2018, 10:30-11:30
    Orbital-Selective Electrons in Iron-Based Superconductors
    APS March Meeting 2019, Boston, MA
    Tuesday, March 5th 2019, 8:24-8:36
    Itinerant approach to magnetic neutron scattering of FeSe: effect of orbital selectivity
    Abstract
    DPG-Frühjahrstagung 2019 Regensburg
    Monday, April 1st 2019, 10:30-10:45
    Magnetic fluctuations in FeSe: effect of orbital selectivity
    Abstract
    Monday, April 1st 2019, 17:30-18:00
    Theory of superconducting pairing in iron-based superconductors
    Abstract
    Itinerant magnetism and superconductivity 2019 - 3rd International Conference GRK 1621, Dresden
    Montag, 09. September, 2019, 10:20-10:55
    Superconducting pairing in iron-based superconductors: Beyond the s± theme

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