The room temperature fine structure of the NV −

3

E level

The first observation of the room temperature ³E fine structure was made by Epstein et al [160] in their ODMR study of single centres and ensembles.

Using precise magnetic field alignment, Epstein et al detected a LAC at an axial magnetic field of ∼500 G, which they assigned to ³E. This assignment was later confirmed by direct ODMR detection of the room temperature fine structure using single centres [79, 80]. These direct observations were made at similar times to the first direct observations of the low temperature fine structure [84, 85]. However, the observations [9, 79] incorrectly iden-tified the room temperature fine structure as the upper branch of the low temperature fine structure. The misidentification was justifiably made for two reasons. The first, was that there were only three fine structure lev-els observed at room temperature, which is consistent with one of the low temperature branches. The second reason was that the upper branch was known to undergo bright spin-conserving transitions in contrast to the lower branch. The lower branch was thus concluded to be unobservable at room temperature due to the presence of spin-flip transitions and increased non-radiative decay at room temperature. The room temperature fine structure was correctly identified as being the result of phonon mediated averaging of both low temperature fine structure branches by Rogers et al [87].

Rogers et al observed the temperature dependence of their CW ODMR detection of the excited state LAC in a NV⁻ ensemble. For temperatures down to 25 K, there existed a single LAC at ∼ 510 G that was consistent with the room temperature fine structure observed in the single centre stud-ies [79, 80]. Below 25 K, Rogers et al observed an additional LAC feature at ∼ 250 G and that this feature varied with applied stress. By modeling the strain dependence of the low temperature fine structure, Rogers et al showed that the behaviour of the LAC features at ∼ 250 G and ∼ 510 G in response to applied stress was consistent with the low temperature fine structure. Hence, Rogers et al had demonstrated the transition between the low and room temperature ³E fine structures. In order to explain their ob-servation, Rogers et al proposed that phonon transitions between the low temperature fine structure levels resulted in an average fine structure that varied with temperature, such that at low temperature the averaging effect was minimal and at room temperature the averaging resulted in the observed room temperature fine structure. Specifically, the averaging process is be-lieved to occur via the redistribution of population optically excited to a given

3E low temperature fine structure level to all the other fine structure levels with the same spin-projection through spin-conserving phonon transitions.

The degree of population redistribution that occurs within the lifetime of

3E depends on the rates of the phonon transitions, which in turn depend on temperature. At sufficiently high temperature, the phonon transition rates will greatly exceed the ³E decay rate and the population will be equally dis-tributed between the low temperature fine structure levels of the occupied

spin-projection. Therefore, at sufficiently high temperatures the energy of the centre’s state is the average of the low temperature fine structure lev-els of the occupied spin-projection. The equivalence of the average of the low temperature fine structure and the room temperature fine structure was confirmed by the single centre experiments of Batalov et al [84]. A detailed model of the averaging process is yet to be presented and a thorough anal-ysis of the transition between the low and room temperature fine structures remains an outstanding issue to be resolved.

The observed room temperature³E fine and hyperfine structures can be described by a spin-Hamiltonian that is equivalent to that used to describe the ³A2 ground state

Hˆ_es^RT = D_es^{k h}Sˆ_z²− S(S + 1)/3ⁱ+ A^k_esSˆzIˆz+ A^⊥_es^hSˆxIˆx+ ˆSyIˆy

i

+Pes

hIˆ_z²− I(I + 1)/3ⁱ (5)

where the fine and hyperfine parameters are tabulated in table 4. Note that the hyperfine structure has only been observed at room temperature and no model has yet been developed for the low temperature hyperfine structure.

Comparing the above with ˆH_es^LT (3), it is evident that the averaging processes can be described by a partial trace over the orbital states ˆH_es^RT = ¹₂trσˆ{ ˆH_es^LT}, which is conceptually consistent.

The influence of static electric ~E, magnetic ~B and strain ~δ fields on the room temperature ³E fine structure has been observed to be well described by the addition of the following potential to the above spin-Hamiltonian (5) Vˆ_es^RT = d^k_es(Ez + δz) + g_es^RTS · ~~ B + ξ( ˆS_y²− ˆS_x²) (6) where ξ is a parameter that has been related to strain [79, 80, 84], and as previously discussed, g^RT_es = 2.01 ± 01 [79, 80] is the measured isotropic room temperature electronic g-factor of ³E. Comparing the above with the low temperature potential (4) and extending the notion of the room temperature averaging being consistent with a partial trace over the orbital states, it is not clear where the strain related ‘ξ’ term originates from, since all of the non-axial strain related terms in (4) are traceless. Fuchs et al [79] measured ξ ∼ 70 ± 30 MHz for one of the centres they investigated and related the parameter to strain because the strain was very large in that particular centre (as indicated by a ground state strain splitting of ∼6 MHz). Neumann et al [80] only observed ξ > 0 for < 20% of the centres they surveyed in bulk diamond. However, Neumann et al did observe a splitting consistent with the ‘ξ’ term in a single centre within a nanodiamond. As internal strains are typically large in nanodiamonds, this observation is consistent with Fuchs

Table 4: Experimental measurements and ab initio calculations of the room temperature NV^{− 3}E fine and hyperfine structure parameters tabulated by reference and nitrogen isotope. Measurement uncertainty ranges are indi-cated if provided by source reference. Note that the ¹⁵N nucleus does not have a quadrupole moment.

Reference Iso. D_es^k (MHz) A^⊥_es (MHz) A^k_es (MHz) Pes

Steiner^e [98] ¹⁴N 1420 (±)40 (±)40

Fuchs^e [79] ¹⁵N 1425±3 (±)61±6 (±)61±6 n/a

Neumann^e [80] ¹⁵N 1423±10 n/a

Gali^a [63] ¹⁵N -39.2 -57.8 n/a

eExperiment, ^aab initio

et al’s conclusion that ξ is related to strain. Consequently, it appears that the averaging process is not complete for centres with very large strains, leading to an additional splitting being observed in their room temperature fine structures that is otherwise absent in centres with smaller strains. Hence, the objective of any attempt at a detailed model of the averaging process should be the description of the conditions required to observe the strain related splitting of the room temperature ³E fine structure.

Comparing the room temperature ³E magnetic hyperfine parameters to those of the³A₂ ground state (refer to table 1), it is evident that the magnetic hyperfine interaction is much larger in ³E. This difference can be explained by once again returning to the simple molecular model of Loubser and van Wyk [44, 45]. The unpaired spin density of ³E arises from the unpaired electrons occupying both the A1 (a1) and E (ex, ey) MOs in the first excited configuration a1e³. Unlike the E MOs, the A1 MO has a contribution from the dangling N sp³ atomic orbital and thus contributes to both the Fermi contact and dipolar components of the magnetic hyperfine interaction [79].

Furthermore, since these contributions arise from unpaired valence electrons and not core polarisation, the contributions will be opposite in sign to the core polarisation contributions in the ground state [63]. Thus, the magnetic hyperfine parameters are expected to change sign, as supported by the ab initio calculation of Gali [63].