ELFT HÍRADÓ

[Janos Asboth <asboth.janos AT ttk.bme.hu>]

Kedves Mind!

A mistake in my previous email's main text: the seminar by Thiering Gergő
will take place tomorrow,

Feb 14  péntek 10h15,
1111 Bp., Budafoki út 8., BME FIII. magasföldszint 1, szemináriumi szoba

Thiering Gergő (HUN-REN Wigner Research Centre for Physics, Budapest,
Hungary)
Ab-initio theory of orbital and phonon driven relaxation pathways in
quantum defects of semiconductors

not on the twentyfourth of February.

Minden érdeklődőt szeretettel várunk.
Asbóth János,
 szemináriumi koordinátor

On Thu, Feb 13, 2025 at 8:30 AM Janos Asboth <asboth.janos AT ttk.bme.hu>
wrote:

> Meghívó
>
> BME Elméleti Fizika Szeminárium,
>
> feb. 24. péntek 10h15,
> 1111 Bp., Budafoki út 8., BME FIII. magasföldszint 1, szemináriumi szoba
>
> Thiering Gergő (HUN-REN Wigner Research Centre for Physics, Budapest,
> Hungary)
> Ab-initio theory of orbital and phonon driven relaxation pathways in
> quantum defects of semiconductors
>
> 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
>
> Minden érdeklődőt szeretettel várunk.
> Asbóth János,
>  szemináriumi koordinátor
>