Nitrogen-Related High-Spin Vacancy Defects in Bulk (SiC) and 2D (hBN) Crystals - Comparative Magnetic Resonance (EPR and ENDOR) Study

At a Glance

Metadata	Details
Publication Date	2024-06-14
Journal	Quantum Reports
Authors	Лариса Латыпова, Fadis F. Murzakhanov, G. V. Mamin, Margarita A. Sadovnikova, H. J. von Bardeleben
Institutions	Kazan Federal University, Sorbonne Université
Citations	3
Analysis	Full AI Review Included

Executive Summary

This study provides a comparative analysis of two high-spin (S=1) vacancy defects—the Nitrogen Vacancy (NV^-) in bulk 4H-SiC and the Boron Vacancy (V_B^-) in 2D hexagonal Boron Nitride (hBN)—to assess their suitability as solid-state qubits.

Zero-Field Splitting (ZFS) Difference: The V_B^- center in 2D hBN exhibits a ZFS value (D = 3.6 GHz) 2.77 times greater than the NV^- center in 3D 4H-SiC (D = 1.3 GHz), reflecting the influence of the material matrix dimensionality on electron density distribution.
Superior Coherence in SiC: At 10 K, the NV^- center in SiC demonstrated a transverse relaxation time (T₂) of 50 µs, significantly longer than the V_B^- center in hBN (T₂ = 15 µs), due to SiC’s magnetically dilute nuclear environment (fewer magnetic isotopes).
Room Temperature Robustness: NV^- in SiC retains optical spin polarization and allows Electron-Nuclear Double Resonance (ENDOR) readout at 297 K (T₁ = 100 µs), confirming its potential as a robust room-temperature qubit platform.
Nuclear Interaction Resolution: The ENDOR absorption linewidth for V_B^- in hBN (270 kHz) is approximately 60 times broader than for NV^- in SiC (4.5 kHz), complicating highly selective excitation necessary for multi-qubit quantum registers in hBN.
Relaxation Mechanism: V_B^- in hBN shows strong coupling and nuclear spin diffusion effects (ESEEM modulation present), whereas NV^- in SiC exhibits weak coupling and single exponential decay (no ESEEM modulation).

Technical Specifications

Spin Hamiltonian Parameters (150 K / 50 K)

Parameter	Defect	Value	Unit	Context
Zero-Field Splitting (D)	V_B^- (hBN)	3.6	GHz	Strong coupling, 2D matrix
Zero-Field Splitting (D)	NV^- (SiC)	1.3	GHz	Weak coupling, 3D matrix
Hyperfine Interaction (A_zz)	V_B^- (hBN)	85	MHz	Strong coupling to ¹⁴N nuclei
Hyperfine Interaction (A_zz)	NV^- (SiC)	1.1	MHz	Weak coupling to ¹⁴N nuclei
Quadrupole Interaction (C_Q)	V_B^- (hBN)	2.11	MHz	¹⁴N nuclear quadrupole splitting
Quadrupole Interaction (C_Q)	NV^- (SiC)	2.53	MHz	¹⁴N nuclear quadrupole splitting
ENDOR Linewidth (Δν)	NV^- (SiC)	4.5	kHz	Highly resolved, narrow lines
ENDOR Linewidth (Δν)	V_B^- (hBN)	270	kHz	Broad, inhomogeneous broadening

Dynamic Characteristics (Relaxation Times)

Parameter	Defect	10 K Value	297 K Value	Unit	Context
Spin-Lattice Time (T₁)	NV^- (SiC)	500	100	ms / µs	Longitudinal relaxation
Spin-Lattice Time (T₁)	V_B^- (hBN)	3.52	20	ms / µs	Longitudinal relaxation
Phase Coherence Time (T₂)	NV^- (SiC)	50	25	µs	Transverse relaxation
Phase Coherence Time (T₂)	V_B^- (hBN)	15	4	µs	Transverse relaxation (limited by nuclear spin diffusion)
Rabi Damping Time (T_R)	NV^- (SiC)	2.2	N/A	µs	Limited by B₁ field inhomogeneity
Rabi Damping Time (T_R)	V_B^- (hBN)	5.5	N/A	µs	Limited by B₁ field inhomogeneity

Experimental Setup Parameters

Parameter	Value	Unit	Context
EPR Frequency	94	GHz	W-band operation
Magnetic Field (B₀)	3.4	T	High field, pure Zeeman interaction
Optical Excitation (λ)	532	nm	Green laser for spin polarization
Optical Power (P)	200	mW	Maximum power used for photoinduced EPR
SiC Irradiation	12 MeV protons, 1 x 10¹⁶ cm^-2	N/A	Defect creation
hBN Irradiation	2 MeV electrons, 6 x 10¹⁸ cm^-2	N/A	Defect creation
SiC Annealing Temp.	900	°C	To form V_SiN_C complexes (NV^-)

Key Methodologies

The study utilized advanced pulsed magnetic resonance techniques (EPR and ENDOR) at high frequency (W-band, 94 GHz) combined with optical excitation.

Sample Preparation (SiC):
- Commercial N-doped 4H-SiC single crystals (2 x 10¹⁷ cm^-3) were irradiated with 12 MeV protons (1 x 10¹⁶ cm^-2) at 295 K.
- Samples were subsequently annealed at 900 °C to promote Si vacancy diffusion and form NV^- centers (V_SiN_C complexes).
Sample Preparation (hBN):
- Commercial hBN single crystals were irradiated with 2 MeV electrons (6 x 10¹⁸ cm^-2) at room temperature to create V_B^- defects. No annealing was performed.
EPR/ENDOR Spectroscopy:
- Experiments were conducted using a Bruker Elexsys E680 W-band (94 GHz) spectrometer equipped with a helium flow cryostat.
- The magnetic field was set to B₀ = 3.4 T, aligning the c-axis of the crystal parallel to B₀.
Pulse Sequences and Relaxation Measurements:
- EPR Spectra: Recorded using the primary Electron Spin Echo (ESE) sequence (π/2 - τ - π - τ - ESE).
- Phase Coherence (T₂): Measured using the standard Hahn sequence.
- Spin-Lattice Relaxation (T₁): Measured using the inversion-recovery sequence (π - T + dT - π/2 - τ - π - τ - ESE).
- Rabi Oscillations: Measured using a long microwave pulse to demonstrate quantum manipulation capability.
- ENDOR Spectra: Obtained using the Mims pulse sequence (π_µw/2 - τ - π_MW/2 - RF - π_MW/2 - τ - ESE) with a 150 kW RF generator.
Optical Excitation:
- A green laser (λ = 532 nm, up to 200 mW) was used for photoinduced EPR to achieve effective spin polarization via the inter-combination conversion mechanism.

Commercial Applications

The robust spin properties of these color centers, particularly the NV^- center in SiC, position them for integration into next-generation quantum technologies and advanced sensing platforms.

Quantum Computing and Registers:
- NV^- in SiC is suitable for creating multi-qubit quantum registers due to its narrow ENDOR linewidth (4.5 kHz) and the presence of multiple structurally non-equivalent centers, allowing selective excitation.
- The ability to read out ¹⁴N nuclear spin states at room temperature (SiC) is crucial for long-lived quantum memory.
Quantum Sensing:
- Both defects are proposed as quantum sensors for temperature, pressure, and magnetic fields.
- The V_B^- center in hBN is noted for its high “flexibility” and sensitivity to external influences, making it a strong candidate for nanoscale sensors.
Quantum Communication:
- SiC color centers emit intense luminescence in the infrared (IR) range (1.1-1.2 µm), which corresponds to the transmission band of fiber optic networks, enabling long-distance quantum communication.
Integrated Semiconductor Electronics:
- Silicon Carbide (SiC) is a mature semiconductor platform, allowing high-spin defects to be easily integrated into existing semiconductor electronics, including high-power devices.
Solid-State Hybrid Quantum Systems:
- The defects can be used to design hybrid quantum systems (e.g., coupling NV^- centers with superconducting qubits) that are more resistant to decoherence.

View Original Abstract

The distinct spin, optical, and coherence characteristics of solid-state spin defects in semiconductors have positioned them as potential qubits for quantum technologies. Both bulk and two-dimensional materials, with varying structural properties, can serve as crystalline hosts for color centers. In this study, we conduct a comparative analysis of the spin-optical, electron-nuclear, and relaxation properties of nitrogen-bound vacancy defects using electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) techniques. We examine key parameters of the spin Hamiltonian for the nitrogen vacancy (NV−) center in 4H-SiC: D = 1.3 GHz, Azz = 1.1 MHz, and CQ = 2.53 MHz, as well as for the boron vacancy (VB−) in hBN: D = 3.6 GHz, Azz = 85 MHz, and CQ = 2.11 MHz, and their dependence on the material matrix. The spin-spin relaxation times T2 (NV− center: 50 µs and VB−: 15 µs) are influenced by the local nuclear environment and spin diffusion while Rabi oscillation damping times depend on crystal size and the spatial distribution of microwave excitation. The ENDOR absorption width varies significantly among color centers due to differences in crystal structures. These findings underscore the importance of selecting an appropriate material platform for developing quantum registers based on high-spin color centers in quantum information systems.

Tech Support

Original Source

DOI: https://doi.org/10.3390/quantum6020019

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