Symmetric carbon tetramers forming spin qubits in hexagonal boron nitride
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2023-10-11 |
| Journal | npj Computational Materials |
| Authors | Zsolt Benedek, Rohit Babar, ĂdĂĄm Ganyecz, Tibor SzilvĂĄsi, Ărs Legeza |
| Institutions | University of Alabama, HUN-REN Wigner Research Centre for Physics |
| Citations | 23 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research proposes the neutral symmetric carbon tetramer defects (C4N and C4B) in hexagonal boron nitride (hBN) as novel spin qubits (S=1) optimized for low-dimensional quantum sensing applications.
- High Sensitivity Potential: The defects exhibit a remarkably narrow Electron Spin Resonance (ESR) linewidth (down to 8 MHz in 11BN), resulting from the spin density being localized primarily on spinless carbon atoms, minimizing environmental hyperfine coupling.
- Optically Addressable: Both defects feature a triplet ground state (3A2) and an optically allowed triplet excited state (3Eâ), enabling Optical Detection of Magnetic Resonance (ODMR). The predicted Zero Phonon Line (ZPL) for C4N is 2.18 eV (569 nm), aligning well with experimental carbon-related emitters.
- Strain-Tunable Spin Contrast: Out-of-plane distortions (buckling) significantly increase the Spin-Orbit Coupling Matrix Elements (SOCMEs) for the C4N defect (up to 31.95 GHz at h/R=0.18), enabling strain-sensitive engineering of the intersystem crossing (ISC) rates and spin readout contrast.
- Robust Identification: The defects possess a characteristic 8-peak hyperfine structure when 100% 13C is included (95.0 MHz splitting for C4N), allowing for unambiguous experimental identification.
- Thermodynamic Stability: The defects show favorable formation energies (2.5 eV for C4N, 2.9 eV for C4B in optimal growth conditions) and kinetic stability, with high energy barriers preventing complex formation with carbon interstitials up to ~630 K.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| C4N Zero-Field Splitting (D) | 820 | MHz | CASSCF-NEVPT2 calculation |
| C4B Zero-Field Splitting (D) | 660 | MHz | CASSCF-NEVPT2 calculation |
| C4N ZPL Energy (Predicted) | 2.18 | eV | 3Eâ â 3A2 transition |
| C4N Radiative Lifetime (Ď) | 80.5 | ns | At 0 K, 590 nm photon |
| C4N ESR Linewidth (Natural Abundance) | 8 (11BN) / 12 (10BN) | MHz | Homogeneous linewidth at B=0 |
| C4B ESR Linewidth (Natural Abundance) | 24 (11BN) / 31 (10BN) | MHz | Homogeneous linewidth at B=0 |
| C4N Hyperfine Coupling (Central C, Azz) | -57.6 | MHz | Strongest coupling to 13C |
| C4B Hyperfine Coupling (Central C, Azz) | -48.0 | MHz | Strongest coupling to 13C |
| C4N Formation Energy | 2.5 | eV | N-rich growth conditions |
| C4B Formation Energy | 2.9 | eV | B-rich growth conditions |
| C4N ISC SOCME (Buckled) | 31.95 | GHz | 3Eâ to 1A1 transition at h/R = 0.18 |
| C4N Annealing Temperature (Stability) | 690 to 1190 | K | Required to restore isolated defects |
| Huang-Rhys Factor (HR) | 1.8 | Dimensionless | C4N PL spectrum |
| Debye-Waller (DW) Factor | 0.165 | Dimensionless | C4N PL spectrum |
Key Methodologies
Section titled âKey MethodologiesâThe study utilized complementary first-principles methods, including periodic Density Functional Theory (DFT) and high-level Quantum Chemistry (QC), to ensure reliability and accuracy.
-
Periodic DFT Calculations (VASP):
- Model: 162-atom (monolayer) and 768-atom (bulk) supercells were used.
- Functional: HSE06 hybrid exchange-correlation functional with 0.32 exact exchange fraction (HSE(0.32)).
- Basis Set/Cutoff: Plane wave basis set of 450 eV.
- Corrections: D3 correction (Grimme et al.) was included for van der Waals interaction.
- ZPL Calculation: Calculated using the energy difference between ground and excited states obtained via spin-conserved constrained DFT.
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Quantum Chemistry Calculations (ORCA 5.0.3):
- Geometry Optimization: Performed using DFT at the PBE0/cc-pVDZ level with D3(BJ) dispersion correction.
- High-Accuracy Energies: Single-point energies determined at the CASSCF/cc-pVTZ level, incorporating dynamic correlation via second-order N-electron valence perturbation theory (NEVPT2).
- Active Space: Constructed based on results from TD-DFT and Density Matrix Renormalization Group (DMRG) calculations.
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Spin Property Calculations:
- ZFS/SOC/SSC: Calculated within the framework of the quasi-degenerate perturbation theory (QDPT) using the CASSCF roots.
- ESR Spectrum Simulation: Implemented via a fast Monte Carlo method, generating 2-10 million random spin state configurations. The simulation included the four carbon spins (if 13C) and the six closest nitrogen or boron nuclear spins.
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Excited State Dynamics:
- Decay Rates/PL Spectrum: Calculated based on the Golden Rule rate equation, utilizing transition dipole moments and harmonic vibrational modes obtained from TD-DFT.
Commercial Applications
Section titled âCommercial ApplicationsâThe unique properties of the C4N and C4B defectsâespecially their narrow linewidth and strain-tunabilityâmake them highly valuable for next-generation quantum technologies integrated into 2D materials.
- Quantum Sensing:
- High-Resolution Magnetometry: The narrow ESR linewidth (8-12 MHz) suggests high sensitivity for detecting weak magnetic fields at the nanoscale.
- Strain/Pressure Sensing: The strong dependence of the ISC rate on local strain allows the defects to function as highly sensitive strain quantum sensors.
- Temperature Sensing: Qubit properties can be used to monitor local temperature variations in 2D systems.
- Low-Dimensional Quantum Devices:
- Atomic Thin Sensors: Integration of these qubits directly into exfoliated hBN monolayers or van der Waals heterostructures allows for sensing extremely close to the target system, crucial for high spatial resolution.
- Dynamic Nuclear Polarization (DNP):
- The defects can act as efficient spin polarization sources, coupling to nuclear spins outside the hBN host. This capability is sought after for boosting the sensitivity of conventional NMR and MRI applications.
- Tailored Defect Engineering:
- The theoretical understanding supports the tailored fabrication of these carbon clusters in hBN using techniques like carbon implantation or Scanning Transmission Electron Microscope (STEM) mapping followed by controlled annealing.
View Original Abstract
Abstract Point defect quantum bits in semiconductors have the potential to revolutionize sensing at atomic scales. Currently, vacancy-related defects are at the forefront of high spatial resolution and low-dimensional sensing. On the other hand, it is expected that impurity-related defect structures may give rise to new features that could further advance quantum sensing in low dimensions. Here, we study the symmetric carbon tetramer clusters in hexagonal boron nitride and propose them as spin qubits for sensing. We utilize periodic-DFT and quantum chemistry approaches to reliably and accurately predict the electronic, optical, and spin properties of the studied defect. We show that the nitrogen-centered symmetric carbon tetramer gives rise to spin state-dependent optical signals with strain-sensitive intersystem crossing rates. Furthermore, the weak hyperfine coupling of the defect to their spin environments results in a reduced electron spin resonance linewidth that can enhance sensitivity.