Experimental Physics II

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Research

Ultrafast Carrier Dynamics in Low Dimensional Heterostructures

 

B. Arnoldi

Ultrafast Magnetization Dynamics        

 

© University of Augsburg

Spin Functionalities in Molecular Hybrid Structures

 

B. Arnoldi

 

 

 

Tuning topological electronic states by external pressure

 

© University of Augsburg

 

 

 

Pressure-induced phase transitions in 2D van der Waals materials   

 

© University of Augsburg

 

 

 

Synthesis and characterization of materials with strong spin-orbit coupling

 

© University of Augsburg

 

Ultrafast Carrier Dynamics in Low Dimensional Heterostructures

The ultrafast dynamics of charge and spin carriers in solids plays a crucial role in many fundamental and device-relevant phenomena, ranging from light harvesting in molecular and inorganic semiconductors to spin filtering at functional interfaces. In our research, we study the essential energy and (angular) momentum dissipation process of optically excited charge and spin carriers in energy and momentum space on their intrinsic femtosecond to picosecond timescale. This will build the foundation for shaping the spin-dependent materials properties on ultrafast timescales by optical excitation. Currently, we focus on

 
  • the evolution and interconversion of charge transfer and Frenkel excitons in molecular materials
  • intersite and interlayer charge and spin transfer processes in molecular and inorganic 2D heterostructures
  • momentum dependent lifetimes of quantum-confined electrons in 2D metal-organic porous network structures

For our research, we employ time-, spin- and momentum-resolved photoemission spectroscopy and momentum microscopy. In this pump-probe study, a first femtosecond laser pulse optically excites electrons in the excited state region, while the second probe pulse photoexcites carriers into the vacuum with a well-defined time delay, where they are detected by state-of-the-art photoemission detector systems. For molecular systems, we additionally use the framework of photoemission orbital tomography to gain insight into the spatial distribution of the emitted carriers through their characteristic spectroscopic signatures in momentum space.

Schematic illustration and example momentum microscopy data of the ultrafast carrier scattering in a material with simple parabolic bands.

 

Optical excitation of low dimensional materials can lead to the formation of various excitons ranging from charge transfer and Frenkel excitations in molecular materials to intra- and interlayer excitons in 2D heterostructures.

 

Selected Publications:

  • Revealing hidden spin polarization in centrosymmetric van der Waals materials on ultrafast timescales
    B. Arnoldi, S. L. Zachritz, S. Hedwig, M. Aeschlimann, O. L. A. Monti, and B. Stadtmüller
    Nat. Commun. 15, 3573, (2024)
  • Disentangling the multiorbital contributions of excitons by photoemission exciton tomography
    W. Bennecke, A. Windischbacher, D. Schmitt, J. P. Bange, R. Hemm, C. S. Kern, G. D’Avino, X. Blase, D. Steil, S. Steil, M. Aeschlimann, B. Stadtmüller, M. Reutzel, P. Puschnig, G. S. M. Jansen, and S. Mathias
    Nat. Commun. 15, 1804 (2024)
  • Coherent response of the electronic system driven by non-interfering laser pulses
    T. Eul, E. Prinz, M. Hartelt, B. Frisch, M. Aeschlimann, and B. Stadtmüller
    Nat. Commun. 13, 3324 (2022)
  • Time-resolved two-photon momentum microscopy—A new approach to study hot carrier lifetimes in momentum space
    F. Haag, T. Eul, P. Thielen, N. Haag, B. Stadtmüller, and M. Aeschlimann
    Review of Scientific Instruments 90, 103104 (2019)
  • Strong modification of the transport level alignment in organic materials after optical excitation
    B. Stadtmüller, S. Emmerich, D. Jungkenn, N. Haag, M. Rollinger, S. Eich, M. Maniraj, M. Aeschlimann, M. Cinchetti, and S. Mathias
    Nat. Commun. 10, 1470 (2019)

 

 

Ultrafast Magnetization Dynamics

Controlling spin states of condensed matter is a crucial prerequisite for the realization of novel information technology concepts. In our research, we lay the foundation for such efforts by focusing on the ultrafast magnetization dynamics of magnetic materials after optical excitation with femtosecond light pulses. We aim to reveal the microscopic processes governing the ultrafast magnetization dynamics and identify new ways to control the generation of spin-polarized carriers in these materials.

Currently, we focus on

  • optically induced intersite and interlayer spin transfer in ferromagnetic alloys and metal-organic hybrid structures
  • ultrafast magnetization dynamics in compensated magnets, i.e., in antiferromagnets and altermagnets

For our research, we combine time-, spin- and momentum-resolved photoemission spectroscopy with ultrafast magneto-optics experiments to obtain complementary views on the ultrafast spin-dependent material response of the magnets after optical excitation with fs-light pulses.

 

 

 

 

 

The interaction of fs light pulses with magnetic materials leads to a loss of magnetic order on sub-ps timescales. This loss of magnetic order can be monitored in real-time using magneto-optical pump-probe techniques.
Optical-induced intersite and interlayer charge transfer processes in exchange coupled metal-organic hybrid structures can lead to transient changes in the magnetic order in tailored magnetic-molecular hybrid structures, but also in magnets such as antiferro- and altermagnets [right image adapted from Phys. Rev. X 12, 040501 (2022)].

 

Selected Publications and Reprints:

  • All optical excitation of spin polarization in d-wave altermagnets
    M. Weber, S. Wust, L. Haag, A. Akashdeep, K. Leckron, C. Schmitt, R. Ramos, T. Kikkawa, E. Saitoh, M. Kläui, L. Šmejkal, J. Sinova, M. Aeschlimann, G. Jakob, B. Stadtmüller, H. C. Schneider
    arXiv:2408.05187; https://doi.org/10.48550/arXiv.2408.05187
  • Observation of time-reversal symmetry breaking in the band structure of altermagnetic RuO2
    O. Fedchenko, J. Minár, A. Akashdeep, S. W. D’Souza, D. Vasilyev, O. Tkach, L. Odenbreit, Q. Nguyen, D. Kutnyakhov, N. Wind, L. Wenthaus, M. Scholz, K. Rossnagel, M. Hoesch, M. Aeschlimann, B. Stadtmüller, M. Kläui, G. Schönhense, T. Jungwirth, A. B. Hellenes, G. Jakob, L. Šmejkal, J. Sinova, H.-J. Elmers
    Sci. Adv. 10, eadj4883 (2024)
  • Control of transport phenomena in magnetic heterostructures by wavelength modulation
    C. Seibel, M. Weber, M. Stiehl, S. T. Weber, M. Aeschlimann, H. C. Schneider, B. Stadtmüller, B. Rethfeld
    Phys. Rev. B 106, L140405 (2022)
  • Ultrafast spin transfer in ferromagnetic alloys
    M. Hofherr, S. Häuser, P. Tengdin, S. Sakshath, H. T. Nembach, S. T. Weber, J. M. Shaw, T. J. Silva, H. C. Kapteyn, M. Cinchetti, B. Rethfeld, M. M. Murnane, D. Steil, B Stadtmüller, S. Sharma, M. Aeschlimann, S. Mathias
    Sci. Adv. 6 eaay8717 (2020)
  • Band structure evolution during the ultrafast ferromagnetic-paramagnetic phase transition in cobalt
    S. Eich, M. Plötzing, M. Rollinger, S. Emmerich, R. Adam, C. Chen, H. C. Kapteyn, M. M. Murnane, L. Plucinski, D. Steil, B. Stadtmüller, M. Cinchetti, M. Aeschlimann, C. M. Schneider, S. Mathias
    Sci. Adv. 3 (3), e1602094 (2017)

 

 

Spin Functionalities in Molecular Hybrid Structures

Molecular materials on surfaces are a fascinating construction kit for controlling spin functionalities such as spin filtering and spin diffusion on the nanoscale. However, tuning such spin functionalities requires an in-depth understanding of the delicate balance of the intermolecular and molecule-surface interactions in molecular-based hybrid structures typically reflected by the arrangement of the molecular superstructures. In our research, we, therefore, focus on the link between the efficiency of spin functionalities and the structural order of molecular–surface hybrid structures. Currently, we focus on

  • chirality-induced spin filtering in chiral metal-molecular hybrid structures
  • spin-polarization of quantum confined surface electrons in tailored metal-organic networks
  • spin filtering in bare and passivated magnetic metal-molecule hybrid structures

For our investigations, we combine structural characterization tools such as scanning tunneling microscopy and the synchrotron-based normal-incidence X-ray standing waves technique with spin-resolved electron spectroscopy tools to obtain a comprehensive view of the interactions and corresponding spin functionalities in molecular hybrid structures.

 

 

The chirality-induced spin filtering or CISS effect leads to spin filtering in chiral structures without magnetic materials or large spin-orbit materials.

 

 

The structural properties of molecular hybrid structures can be controlled by external parameters such as temperature, intercalation, or light.

Selected Publications:

  • Vectorial Electron Spin Filtering by an All-Chiral Metal–Molecule Heterostructure
    C. B. Viswanatha, J. Stöckl, B. Arnoldi, S. Becker, M. Aeschlimann, B. Stadtmüller
    J. Phys. Chem. Lett. 2022, 13, 6244–6249
  • Observation of optical coherence in a disordered metal-molecule interface by coherent optical two-dimensional photoelectron spectroscopy
    M. Aeschlimann, T. Brixner, M. Cinchetti, M. Feidt, N. Haag, M. Hensen, B. Huber, T. Kenneweg, J. Kollamana, C. Kramer, W. Pfeiffer, S. Ponzoni, B. Stadtmüller, P. Thielen
    Phys. Rev. B 105, 205415 (2022)
  • Thermal-Driven formation of 2D Nanoporous Networks on Metal Surfaces
    L. Lyu, M. Mahalingam, S. Mousavion, S. Becker, H. Huang, M. Aeschlimann, B. Stadtmüller
    J. Phys. Chem. C 123, 26263-26271 (2019)
  • Spin- and Angle-Resolved Photoemission Study of the Alq3/Co Interface
    J. Stöckl, A. Jurenkow, N. Großmann, M. Cinchetti, B. Stadtmüller, M. Aeschlimann
    J. Phys. Chem. C 122 (12), 6585–6592 (2018)
  • Modifying the Surface of a Rashba-Split Pb-Ag Alloy Using Tailored Metal-Organic Bonds
    B. Stadtmüller, J. Seidel, N. Haag, L. Grad, C. Tusche, G. van Straaten, M. Franke, J. Kirschner, C. Kumpf, M. Cinchetti, M. Aeschlimann
    Phys. Rev. Lett. 117, 096805 (2016)

 

Tuning topological electronic states by external pressure

to be added

 

Pressure-induced phase transitions in 2D van der Waals materials

to be added

 

Synthesis and characterization of materials with strong spin-orbit coupling

The pyrochlore iridates R2Ir2O7 (R = rare-earths, Y and Bi), being the three-dimensional analogue of the kagome structure, have been predicted to host novel topological phases. The corner-sharing tetrahedral network of the pyrochlore lattice naturally gives rise to strong geometrical frustration, which suppresses conventional magnetic ordering and favors the emergence of unconventional ground states. Generally, iridates have been extensively investigated in the last years because of the interplay of Coulomb interaction, spin-orbit coupling, crystal field splitting, and electronic bandwidth, all with comparable energy scales. Their delicate balance induces novel phases and promotes the possible realization of the quantum spin-liquid ground state. In particular, a Weyl semimetal phase has been proposed for the magnetic pyrochlores A2Ir2O7 (A = Y, Eu, Sm and Nd) in their antiferromagnetic state with all-in/all-out spin order, which preserves inversion symmetry but breaks time-reversal symmetry.

 

We employ the flux-growth method to obtain high-quality single crystals of pyrochlore iridates. The crystals are characterized using energy-dispersive X-ray spectroscopy, dc electrical transport, magnetic susceptibility, heat capacity, Raman spectroscopy, and infrared spectroscopy. Crystal synthesis is carried out in close collaboration with Anton Jesche and Philipp Gegenwart, Experimental Physics VI.

 

 

Experimental phase diagram of pyrochlore iridates R2Ir2O7 derived from transport and magnetic measurements and Cubic pyrochlore structure of R2Ir2O7 showing the corner-shared R4O′ (cyan) and Ir4O (orange) tetrahedra.

 

 

Images of pyrochlore iridates single crystals R2Ir2O7 along with the other iridates R3IrO7 and IrO2.

Selected publications:

  • Mott physics and band topology in materials with strong spin-orbit interaction
    D. Pesin, L. Balents
    Nat. Phys. 6, 376–381 (2010).
  • Correlated quantum phenomena in the strong spin-orbit regime
    W. Witczak-Krempa, G. Chen, Y. Baek Kim, L. Balents
    Annu, Rev. Condens. Matter Phys. 5, 57-82 (2014)
  • Topological semimetal and Fermi-arc surface states in the electronic structure of pyrochlore iridates
    X. Wan, A. M. Turner, A. Vishwanath, S. Y. Savrasov
    Phys. Rev. B 83, 205101 (2011)
  • Magnetic and transport properties in pyrochlore iridates (Y1-xPrx)2Ir2O7: The role of f-d exchange interaction and d-p orbital hybridization
    H. Kumar, K. C. Kharkwal, K. Kumar, K. Asokan, A. Banerjee, A. K. Pramanik
    Phys. Rev. B 101, 064405 (2020)
  • Optical Conductivity of metallic Pyrochlore Iridate Pr2Ir2O7: Influence of Spin-Orbit and Electron-Electron Interactions on the electronic structure
    H. Kumar, M. Köpf, P. Telang, N. Bura, A. Jesche, P. Gegenwart, C. A. Kuntscher
    Phys. Rev. B 110, 035140 (2024)

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