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Habilitation lectureThiering GergőHUN-REN Wigner RCP SZFIAb-initio theory of 
orbital and phonon driven relaxation pathways in quantum defects of 
semiconductorsTuesday, 11 March 2025, 10:00, KFKI Campus, Bldg. 1, 2nd floor, 
Conference RoomIn 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  Minden érdeklődőt szívesen 
látunk! - Everyone is welcome to attend.Attila Nagysem-admin AT szfki.hu