Előadó: Thiering Gergő (Wigner FK SZFI)
Előadásának címe: Ab-initio theory of orbital and phonon driven relaxation pathways in quantum defects of semiconductors
Dátum: 2025. március 11. kedd, 10 óra
Helyszín: 1-es épület tanácsterem
Összefoglaló (Habilitációs előadás):
In the past decades, various crystallographic point defects were identified in two- and three-dimensional host materials such as diamond, silicon, silicon carbide, and 2D-boron-nitride. Initially, the characterization of defects started from the materials science point of view to unravel and understand their physics in various hosts. However, within the past decades, new proposed applications have been emerged mainly for quantum applications [1,2]. However, there are various technological challenges to overcome for defect-based qubits and quantum emitters that still limit the defect qubit applications "en masse". Mainly, these challenges are related to the loss of coherence within qubits which is especially important when the qubits are entangled together as a solid-state spin register.
Therefore, in my talk I will show various processes [3,4,5,6] that can ultimately lead to relaxation of electronic orbital "L" states, electronic "S" spin or nuclear "I" spin degrees of freedom. For example, both the electronic and 14N nuclear spin of NV(-) (nitrogen vacancy) in diamond are proposed for applications as NV in general as been both measured extensively and theoretically modelled by vast number of studies in the past decades [1,2]. We modelled by ab-initio DFT (density functional theory) calculations that all SDS (zero-field), SAI (hyperfine) and IPI (quadrupolar) 3×3 tensors acting in |³E⟩ optical excited upper triplet state of NV are entangled with the 2× orbital degeneracy ("mL=±1") that of |e±⟩ electronic orbitals localized on the defect. In most studies, ¹⁴N "I" spins are usually treated devoid from any relaxation during of optical cycles. However, we show [3,4] both experimentally and theoretically that the traditional "green laser (532-nm)" optical pumping into the upper |³E⟩ spin triplet excited state leads to additional "ΔmI=±2" double jump relaxation channels for ¹⁴N via orbital coupling of the quadrupolar (Q) tensor by means of a "Q₂(L₊²I₋²+L₋²I₊²)" Hamiltonian.
Nevertheless, the lower spin triplet of NV(-) is an orbitally non-degenerate |³A2⟩ multiplet and thus exempt from orbitally assisted relaxation. However, phonons of diamond can still relax the electronic spin via the "spin-phonon" on which we developed [5] an ab-initio framework that can predict the temperature dependence of rates acting between |mS = 0⟩↔|mS = +1⟩↔|mS = -1⟩ spin states of NV(-). We find that our ab-initio tools and experimental measurements depict that two distinct quasilocal phonons centred at 68.2(17) and 167(12) meV are involved in the relaxation of "S" spin between the 9-474 K temperature range in high-purity diamond samples.
Additionally, in conjunction with experimental work [6] we develop the key elements of orbital and spin flipping processes induced by thermal phonons for the SiV(-) centre of diamond. We find that group theory considerations and selection rules are crucial to understand the observed anisotropy and thus we were able to distinguish the strength of pure orbital-phonon ("ΔmL=±2") relaxation and various other weaker diagonal and off-axis spin-orbit-phonon relaxation pathways.
In summary, in the present talk I will try to depict a general spin-orbit-phonon theory that can be used to model the processes acting in defect qubits that may highlight the limitations and caveats of quantum technology applications of point defects in solids.
[1] Wolfowicz, et al. "Quantum guidelines for solid-state spin defects." Nat Rev Mater 6, 906–925 (2021).
[2] I. B. W. Harris and D. Englund, Phys. Rev. B 109, 085414 (2024)
[3] R. Monge, T. Delord, G. Thiering, Á. Gali, and C. A. Meriles, Phys. Rev. Lett. 131, 236901 (2023)
[4] G. Thiering, Á. Gali, arXiv:2402.19418 [quant-ph] (2024)
[5] M. Cambria, ... G. Thiering, ... S. Kolkowitz, Physical Review Letters 130 (25), 256903 (2024)
[6] G. Thiering, A. Gali, F. Jelezko, K Senkalla, F. Frank, B. Koslowski (APS global meeting 2025) https://summit.aps.org/events/MAR-T19/2
[6] G. Thiering, A. Gali, F. Jelezko, K Senkalla, F. Frank, B. Koslowski (APS global meeting 2025) https://summit.aps.org/events/MAR-T19/2
