ELFT HÍRADÓ

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

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