Theoretical investigations of bistability and decoherence of toroidal magnetic molecules

• Applicant: Professor Dr. Jürgen Schnack
• Subject Area: Theoretical Condensed Matter Physics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 563482790

In view of the possibly great potential of toroidal magnetic molecules both for applications in magnetic storage and quantum computing it is of utmost importance to obtain a systematic overview of properties in terms of typical structural motives, i.e., triangles, squares, hexagons, as well as in terms of variations of the spin quantum number and typical terms in the Hamiltonian. The goal is thus not to consider specific molecules, but a large multi-dimensional parameter space of potential molecules in order to identify promising regions in such a parameter space. We therefore want to pursue the following research objectives structured as work packages 1-4. WP1: A systematic investigation of the tunneling gap for integer total spin as well as the transition matrix element for transitions induced by transverse fields shall be conducted for geometric polygon structures of sizes N=2, 3, 4 and 6 as well as for hourglass structures as function of the spin quantum number s. The Hamiltonian at this stage shall be of minimal complexity, i.e., contain Heisenberg exchange, single-ion easy axis anisotropy as well as a Zeeman term. WP2: In a second step, the impact of anisotropic Hamiltonians on the figures of merit, tunneling gap and transition matrix elements, shall be investigated. Anisotropic contributions are typically of dipolar or Dzyaloshinskii-Moriya type. Such terms also break the apparent rotational symmetries of the aforementioned Hamiltonian. WP3: It is so far not clear how perfect toroidal structures can be manipulated experimentally. We therefore want to estimate the chances to use tunneling currents of scanning tunneling microscopes, spin transfer torque, again employing scanning tunneling microscopes as well as local time-dependent electromagnetic fields for the manipulation of toroidal magnetic states. WP4: Finally, we plan to address decoherence. Again, a systematic investigation of decoherence properties of toroidal magnetic molecules with respect to the parameters mentioned above shall be executed. We are particularly interested in the unusually long decoherence times found for certain superpositions. As part of this objective we need to estimate the feasibility in EPR experiments, i.e., whether the desired superpositions can be generated by dipole transitions at all. We hope that our investigations can stimulate experimental investigations of toroidal magnetic molecules for the envisioned applications in quantum devices. To this end, we have an agreement to study suited magnetic molecules according to our theoretical predictions.

• DFG Programme: Research Grants