Diamond Defects Maintain Stable Light Emission under Extreme Pressure

Scientists investigate the magneto-optical properties of the neutral silicon-vacancy (SiV) center in diamond, a robust emitter with potential applications in quantum technologies. Meysam Mohseni and Gergő Thiering, both from the Wigner Research Centre for Physics, alongside Adam Gali from Budapest University of Technology and Economics, detail how this centre responds to extreme isotropic strain using first-principles density-functional theory. Their collaborative work reveals a structural instability governed by a quadratic product Jahn-Teller model, demonstrating a quantifiable link between strain and key optical properties like zero-phonon line position and vibronic gap. This research is significant because it establishes the SiV centre as a symmetry-protected, strain-tunable emitter capable of operating under pressures equivalent to multiple megabars, offering a pathway towards developing highly sensitive strain sensors and robust quantum emitters.

Scientists are unlocking new potential in quantum technology by meticulously controlling the properties of defects within diamond, focusing on the silicon-vacancy (SiV) centre, a robust quantum emitter resilient against disruptive electric fields. This research details how applying controlled pressure alters the SiV centre’s behaviour, opening avenues for precise calibration of its optical and spin characteristics. Researchers quantified the SiV centre’s response to compression and tensile strain, simulating hydrostatic pressures ranging from approximately -80 to 180 GPa, revealing a structural instability stemming from doubly degenerate electronic levels captured by a quadratic product Jahn-Teller model. Under compression, the zero-phonon line blue-shifts nearly linearly while the relevant phonon stiffens, suppressing instabilities and reducing unwanted quenching of emission. This leads to a substantial increase in excited-state spin-orbit splitting and a widening of the gap between bright and dark vibronic states. Conversely, tensile strain enhances vibronic effects and induces symmetry breaking beyond a critical point, mediated by quantum tunneling, with tunneling rates reaching 22430.0MHz at 4.00% tensile strain and decreasing to 0.9MHz at 8.00%, and barrier height decreasing from 115 meV to essentially zero. Importantly, throughout the symmetry-preserving regime, the parity of the SiV centre remains well-defined, preventing activation of a problematic dark transition. Calculations of charge-transition levels demonstrate the photostability of the emission even under intense compression, and near a deformation of approximately 100 GPa, the radiative lifetime increases despite rising energy. These findings establish a direct link between optical and spin observables and applied isotropic strain, solidifying the SiV centre’s position as a symmetry-protected, strain-tunable emitter capable of functioning in environments mimicking pressures exceeding multiple megabars, providing compact calibration relations for quantum sensing and photonic applications in extreme conditions. Analysis of the defect’s electronic structure shows the SiV centre adopts a split-vacancy configuration with D3d point group symmetry, possessing localized eu and eg orbitals near the valence band maximum, with a ground state spin triplet (S = 1) transforming as 3A2g. A 4×4×4 cubic diamond supercell, comprising 512 atoms, underpins all first-principles calculations performed within the Kohn-Sham density functional theory framework, employing the projector-augmented-wave (PAW) formalism as implemented in VASP. A plane-wave kinetic energy cutoff of 540 eV was used throughout the computations, and the screened hybrid functional HSE06 was utilised for electronic structure, total-energy, magnetic properties, and defect-level determinations. Atomic positions were relaxed until residual Hellmann-Feynman forces fell below 10−2 eV/Å, guaranteeing a stable and accurate structural configuration, with excited-state geometries obtained using the constrained ∆SCF procedure. Zero-phonon line (ZPL) energies were evaluated as the adiabatic energy difference between fully relaxed minima of the ground and excited states, denoted as EZPL = Emin exc −Emin gs. Charge transition levels (CTLs) were computed using the Freysoldt, Neugebauer, Van de Walle (FNV) finite-size correction scheme, expressed as Ecorr(q) = Eel + q ∆V, and thermodynamic transition levels were referenced to the valence band maximum (VBM) as ε(q/q′) = (Eq tot + Ecorr(q)) −(Eq′ tot + Ecorr(q′)) q′ −q −EVBM. This methodology allows for precise determination of the defect’s energy levels and its response to external stimuli, extending to the negative hydrostatic-pressure regime, investigating uniform tensile strains up to 8%, corresponding to effective pressures of several tens of gigapascals, within an experimentally realistic elastic window. Scientists have long sought robust quantum emitters, and the silicon-vacancy (SiV) centre in diamond is rapidly emerging as a leading candidate. The ability to predictably tune SiV properties via strain offers a potentially stable and compact control mechanism, contrasting with intricate electrical and magnetic fields susceptible to noise. The research demonstrates that SiV centres remain remarkably stable even under pressures equivalent to those found deep within the Earth’s mantle, opening doors to novel device architectures, such as quantum sensors operating reliably in harsh environments and integrated photonic circuits where qubit behaviour is dictated by the diamond substrate’s physical shape. However, symmetry-breaking effects observed under extreme tensile strain represent a significant challenge, requiring further investigation to map the limits of tunability and mitigate potential performance degradation. The next step involves exploring carefully engineered strain gradients and combinations with other control parameters to unlock more sophisticated quantum functionalities, ultimately realising the promise of SiV through precise sculpting of its properties.