pith. sign in

arxiv: 2511.22608 · v3 · pith:YOCI2RAInew · submitted 2025-11-27 · 🪐 quant-ph

High-yield engineering and identification of oxygen-related modified divacancies in 4H-SiC

Qi-Cheng Hu , Ji-Yang Zhou , Shuo Ren , Zhen-Xuan He , Zhi-He Hao , Rui-Jian Liang , Wu-Xi Lin , Xiangru Han
show 4 more authors
Adam Gali Jin-Shi Xu Chuan-Feng Li Guang-Can Guo
This is my paper

Pith reviewed 2026-05-22 12:01 UTC · model grok-4.3

classification 🪐 quant-ph
keywords modifieddivacanciescentersengineeringfourh-sichigh-yieldidentification
0
0 comments X

The pith

Oxygen-ion implantation in 4H-SiC yields four types of oxygen-vacancy complexes that comprise over 90% of defects, show superior optical and spin properties, and are identified by isotope-resolved hyperfine interactions.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The research focuses on creating special defects called modified divacancies in silicon carbide, a material used in electronics. These defects can act like tiny quantum bits because they have stable spins that can be controlled with light and magnetic fields at room temperature. Normally, making these defects is inefficient, and scientists weren't sure exactly what they looked like at the atomic level. The team used a technique called oxygen-ion implantation, where they shoot oxygen atoms into the silicon carbide crystal. This creates oxygen-vacancy complexes, which are the modified divacancies. By looking at how light interacts with these defects and how their spins respond to microwaves, they found four different versions. Using a special isotope of oxygen, they measured interactions that confirmed these are indeed oxygen combined with vacancies in four possible arrangements in the crystal lattice. Importantly, most of the defects created this way are these oxygen-vacancy ones, over 90 percent, and they work better than defects made with other methods like carbon implantation. They have clearer light emission lines and longer spin coherence times. The researchers also studied how these properties change with temperature and optimized the implantation and heating process to get lots of these defects. They even saw patterns in how the spins oscillate that match different orientations of the defects. This work provides a reliable way to produce these quantum-friendly defects in large numbers, which is key for building practical quantum devices.

Core claim

By measuring isotope-resolved 17O hyperfine interactions, we identify them as the four crystallographic configurations of oxygen-vacancy (OV) complexes. Remarkably, single OV centers account for over 90% of the total defect population and exhibit superior optical properties and spin coherence compared with defects created by conventional carbon or nitrogen implantation.

Load-bearing premise

The distinct optical and spin-resonance characteristics combined with 17O hyperfine interactions uniquely identify the observed centers as the four OV configurations, with minimal contribution from other defect species or alternative structural assignments.

Figures

Figures reproduced from arXiv: 2511.22608 by Adam Gali, Chuan-Feng Li, Guang-Can Guo, Jin-Shi Xu, Ji-Yang Zhou, Qi-Cheng Hu, Rui-Jian Liang, Shuo Ren, Wu-Xi Lin, Xiangru Han, Zhen-Xuan He, Zhi-He Hao.

Figure 1
Figure 1. Figure 1: FIG. 1. Single modified divacancies in oxygen ion–implanted samples. (a) Photoluminescence (PL) map of a 10 [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Spin coherent control of single PL5 and PL6 centers. (a) Ramsey measurement of the inhomogeneous dephasing time [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Zero-phonon line (ZPL) and zero-field splitting (ZFS) of single modified divacancy defects. (a-d) Measured zero-phonon [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Optical and spin properties of ensemble samples. (a) Optimization of implantation dose and annealing temperature for [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
read the original abstract

Modified divacancies in the 4H polytype of silicon carbide (SiC) exhibit enhanced charge stability and spin addressability at room temperature, making them attractive for quantum applications. However, their low formation yield and lack of direct structural identification have hindered progress. Here, we demonstrate a controllable method for high-yield engineering and identification of oxygen-related modified divacancy color centers in 4H-SiC via oxygen-ion implantation. Based on their distinct optical and spin-resonance characteristics, we experimentally resolve four types of modified divacancies. Furthermore, by measuring isotope-resolved 17O hyperfine interactions, we identify them as the four crystallographic configurations of oxygen-vacancy (OV) complexes. Remarkably, single OV centers account for over 90% of the total defect population and exhibit superior optical properties and spin coherence compared with defects created by conventional carbon or nitrogen implantation. We characterize the zero-phonon lines of these OV centers and reveal distinct temperature-dependent behavior in spin-readout contrast. By optimizing implantation dose and annealing temperature, we achieve high-density ensembles and observe Rabi-oscillation beating patterns associated with different orientations of basal-type defects. These results establish a high-yield route for scalable engineering of these four oxygen-related modified divacancies in 4H-SiC and clarify their atomic structure, opening new opportunities for solid-state quantum technologies.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript reports a controllable high-yield method for engineering oxygen-related modified divacancies in 4H-SiC via oxygen-ion implantation. It experimentally resolves four types based on distinct optical and spin-resonance characteristics, identifies them as the four crystallographic configurations of oxygen-vacancy (OV) complexes through isotope-resolved 17O hyperfine interactions, and claims that single OV centers comprise over 90% of the total defect population while exhibiting superior optical properties and spin coherence relative to defects from conventional carbon or nitrogen implantation. The work also characterizes zero-phonon lines, temperature-dependent spin-readout contrast, and Rabi-oscillation patterns after optimizing dose and annealing.

Significance. If the identification of the four OV configurations and the >90% yield claim are substantiated, the results would establish a scalable route for producing high-density, high-coherence spin defects in SiC with improved charge stability and addressability, directly addressing the low formation yield that has limited prior modified-divacancy work and opening clearer pathways for quantum-technology applications.

major comments (2)
  1. [Abstract / Results on population quantification] Abstract and results on defect population: The claim that 'single OV centers account for over 90% of the total defect population' rests on an untested completeness assumption. Implantation can generate additional non-luminescent, non-paramagnetic, or differently charged defects (isolated vacancies, interstitial clusters, or other oxygen complexes) whose densities are not quantified by the reported ZPL or hyperfine spectra; an independent measure of total defect density (e.g., via Rutherford backscattering or total integrated EPR intensity) is required to support the percentage.
  2. [Hyperfine measurements and assignment] Section on hyperfine identification: The assertion that the observed 17O hyperfine interactions uniquely identify the four centers as the crystallographic OV configurations requires explicit exclusion criteria for alternative structural assignments. Without tabulated hyperfine tensors, fitting residuals, or comparison to DFT-predicted spectra for competing models (e.g., other oxygen-vacancy or divacancy variants), the uniqueness of the assignment remains moderate.
minor comments (2)
  1. [Figures and Methods] Figure captions and methods: Spectra, fitting details, error bars on hyperfine constants, and the precise criteria used to assign the four configurations should be provided or referenced to allow independent verification of the optical and EPR distinctions.
  2. [Introduction] Notation: The term 'modified divacancies' is used interchangeably with 'oxygen-vacancy (OV) complexes'; a brief clarification of the structural relationship between these descriptions would improve readability.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive and detailed comments. We address each major point below and outline the revisions we will make to strengthen the manuscript.

read point-by-point responses

  1. Referee: [Abstract / Results on population quantification] Abstract and results on defect population: The claim that 'single OV centers account for over 90% of the total defect population' rests on an untested completeness assumption. Implantation can generate additional non-luminescent, non-paramagnetic, or differently charged defects (isolated vacancies, interstitial clusters, or other oxygen complexes) whose densities are not quantified by the reported ZPL or hyperfine spectra; an independent measure of total defect density (e.g., via Rutherford backscattering or total integrated EPR intensity) is required to support the percentage.

    Authors: We agree that the >90% claim is based on the relative integrated intensities of the four OV centers in the EPR and PL spectra, which dominate under the reported implantation and annealing conditions. No other paramagnetic or luminescent species are observed at comparable levels. We will revise the text to clarify that this percentage refers to the fraction of defects detected by our spectroscopic methods and to explicitly note the possibility of undetected non-luminescent or non-paramagnetic defects as a limitation of the current quantification. We will also add a brief comparison of total spin density estimates derived from the EPR data. revision: partial

  2. Referee: [Hyperfine measurements and assignment] Section on hyperfine identification: The assertion that the observed 17O hyperfine interactions uniquely identify the four centers as the crystallographic OV configurations requires explicit exclusion criteria for alternative structural assignments. Without tabulated hyperfine tensors, fitting residuals, or comparison to DFT-predicted spectra for competing models (e.g., other oxygen-vacancy or divacancy variants), the uniqueness of the assignment remains moderate.

    Authors: The current manuscript reports the measured 17O hyperfine splittings and their consistency with the expected number and symmetry of oxygen nuclei in the four OV configurations. To address the request for stronger evidence, we will add a supplementary table listing the fitted hyperfine tensors, the associated fitting residuals, and a direct comparison to DFT-predicted spectra for alternative models (including bare divacancies and other oxygen complexes). This addition will include explicit exclusion criteria based on mismatches in tensor anisotropy and the number of interacting nuclei. revision: yes

Circularity Check

0 steps flagged

No circularity: experimental identification via direct measurements

full rationale

The paper reports experimental results from oxygen-ion implantation in 4H-SiC, followed by optical spectroscopy, spin-resonance measurements, and isotope-resolved 17O hyperfine interactions to resolve and assign four OV configurations. The >90% population claim is stated as arising from the relative intensities of the observed signals compared to total defect response, without any derivation, equation, or model that reduces by construction to fitted inputs, self-citations, or ansatzes internal to the paper. No load-bearing self-citation chains, uniqueness theorems, or renamings of known results appear in the presented chain; the work is self-contained against external benchmarks through direct spectroscopic identification.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claims rest on standard assumptions about 4H-SiC crystallography and defect spectroscopy rather than new free parameters or invented entities.

axioms (1)
  • domain assumption 4H-SiC possesses four distinct crystallographic configurations for oxygen-vacancy complexes that produce distinguishable optical and spin signatures.
    Invoked when mapping the four observed defect types to specific OV arrangements.

pith-pipeline@v0.9.0 · 5822 in / 1417 out tokens · 71935 ms · 2026-05-22T12:01:12.142046+00:00 · methodology

Review history (2 revisions) →

discussion (0)

Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.

Reference graph

Works this paper leans on

37 extracted references · 37 canonical work pages

  1. [1]

    (b) Spin coherence time (T2) measurement of the left branch of PL5

    for PL5 at 0 G. (b) Spin coherence time (T2) measurement of the left branch of PL5. (c) Spin-lattice relaxation time (T1) measurement of PL5 at 0 G. (d) Ramsey measurement of the inhomogeneous dephasing time (T∗

    work page

  2. [2]

    (e) Spin coherence time (T2) measurement of the left branch of PL6 at 180 G

    for PL6 at 0 G. (e) Spin coherence time (T2) measurement of the left branch of PL6 at 180 G. (f) Spin-lattice relaxation time (T1) measurement of PL6 at 180 G. A. The single-photon nature of the emitters was veri- fied by measuring the second-order autocorrelation func- tiong (2)(τ)as a function of time delayτ(Fig. 1b). The experimental raw data under low...

    work page 1947

  3. [3]

    F., Buckley, B

    Koehl, W. F., Buckley, B. B., Heremans, F. J., Calusine, G. & Awschalom, D. D. Room temperature coherent con- trol of defect spin qubits in silicon carbide.Nature479, 84–87 (2011)

    work page 2011

  4. [4]

    J., Falk, A

    Christle, D. J., Falk, A. L., Andrich, P., Klimov, P. V., Hassan, J. U., Son, N. T., Janzén, E., Ohshima, T. & Awschalom, D. D. Isolated electron spins in silicon car- bide with millisecond coherence times.Nat. Mater.14, 160–163 (2015)

    work page 2015

  5. [5]

    T., Fed- der, H., Paik, S., Yang, L.-P., Zhao, N., Yang, S., Booker, I., et al

    Widmann, M., Lee, S.-Y., Rendler, T., Son, N. T., Fed- der, H., Paik, S., Yang, L.-P., Zhao, N., Yang, S., Booker, I., et al. Coherent control of single spins in silicon carbide at room temperature.Nat. Mater.14, 164–168 (2015)

    work page 2015

  6. [6]

    Room-temperature coherent manipulation of single-spin qubits in silicon carbide with a high readout contrast

    Li, Q., Wang, J.-F., Yan, F.-F., Zhou, J.-Y., Wang, H.-F., Liu, H., Guo, L.-P., Zhou, X., Gali, A., Liu, Z.-H., et al. Room-temperature coherent manipulation of single-spin qubits in silicon carbide with a high readout contrast. Nat. Sci. Rev.9, nwab122 (2022)

    work page 2022

  7. [7]

    & Guo, G.-C

    He, Z.-X., Zhou, J.-Y., Li, Q., Lin, W.-X., Liang, R.-J., Wang, J.-F., Wen, X.-L., Hao, Z.-H., Liu, W., Ren, S., Li, H., You, L.-X., Zhang, R.-J., Zhang, F., Tang, J.-S., Xu, J.-S., Li, C.-F. & Guo, G.-C. Robust single mod- ified divacancy color centers in 4H-SiC under resonant excitation.Nat. Commun.15, 10146 (2024)

    work page 2024

  8. [8]

    Room- temperature coherent control of implanted defect spins in silicon carbide.npj Quantum Inf.6, 38 (2020)

    Yan, F.-F., Yi, A.-L., Wang, J.-F., Li, Q., Yu, P., Zhang, J.-X., Gali, A., Wang, Y., Xu, J.-S., Ou, X., et al. Room- temperature coherent control of implanted defect spins in silicon carbide.npj Quantum Inf.6, 38 (2020)

    work page 2020

  9. [9]

    M., Dory, C., Guidry, M

    Lukin, D. M., Dory, C., Guidry, M. A., Yang, K. Y., Mishra, S. D., Trivedi, R., Radulaski, M., Sun, S., Ver- cruysse, D., Ahn, G. H., et al. 4H-silicon-carbide-on- insulator for integrated quantum and nonlinear photon- ics.Nat. Photonics14, 330–334 (2020)

    work page 2020

  10. [10]

    Fabrication and nanophotonic waveguide integration of silicon carbide colour centres with preserved spin-optical coherence.Nat

    Babin, C., Stöhr, R., Morioka, N., Linkewitz, T., Steidl, T., Wörnle, R., Liu, D., Hesselmeier, E., Vorobyov, V., Denisenko, A., et al. Fabrication and nanophotonic waveguide integration of silicon carbide colour centres with preserved spin-optical coherence.Nat. Mater.21, 67–73 (2022)

    work page 2022

  11. [11]

    P., Miao, K

    Bourassa, A., Anderson, C. P., Miao, K. C., Onizhuk, M., Ma, H., Crook, A. L., Abe, H., Ul-Hassan, J., Ohshima, T., Son, N. T., et al. Entanglement and control of single nuclear spins in isotopically engineered silicon carbide. Nat. Mater.19, 1319–1325 (2020)

    work page 2020

  12. [12]

    Magnetic detection under high pressures using designed silicon vacancy centres in silicon carbide.Nat

    Wang, J.-F., Liu, L., Liu, X.-D., Li, Q., Cui, J.-M., Zhou, D.-F., Zhou, J.-Y., Wei, Y., Xu, H.-A., Xu, W., et al. Magnetic detection under high pressures using designed silicon vacancy centres in silicon carbide.Nat. Mater.22, 489–494 (2023)

    work page 2023

  13. [13]

    Coherent control of nitrogen-vacancy center spins in sil- icon carbide at room temperature.Phys

    Wang, J.-F., Yan, F.-F., Li, Q., Liu, Z.-H., Liu, H., Guo, G.-P., Guo, L.-P., Zhou, X., Cui, J.-M., Wang, J., et al. Coherent control of nitrogen-vacancy center spins in sil- icon carbide at room temperature.Phys. Rev. Lett.124, 223601 (2020)

    work page 2020

  14. [14]

    & Gao, W

    Jiang, Z., Cai, H., Cernansky, R., Liu, X. & Gao, W. Quantum sensing of radio-frequency signal with NV cen- ters in SiC.Sci. Adv.9, eadg2080 (2023)

    work page 2023

  15. [15]

    & Guo, G.-C

    Wang, J.-F., Yan, F.-F., Li, Q., Liu, Z.-H., Cui, J.-M., Liu, Z.-D., Gali, A., Xu, J.-S., Li, C.-F. & Guo, G.-C. Robust coherent control of solid-state spin qubits using anti-Stokes excitation.Nat. Commun.12, 3223 (2021)

    work page 2021

  16. [16]

    Abraham, J. B. S., Gutgsell, C., Todorovski, D., Sper- ling, S., Epstein, J. E., Tien-Street, B. S., Sweeney, T. M., Wathen, J. J., Pogue, E. A., Brereton, P. G., et al. Nanotesla magnetometry with the silicon vacancy in sil- icon carbide.Phys. Rev. Appl.15, 064022 (2021)

    work page 2021

  17. [17]

    O., Zhang, X

    Bracher, D. O., Zhang, X. & Hu, E. L. Selective Purcell enhancement of two closely linked zero-phonon transi- tions of a silicon carbide color center.Proc. Natl. Acad. Sci. USA114, 4060–4065 (2017)

    work page 2017

  18. [18]

    U., Karhu, R., Ivanov, I

    Nagy, R., Niethammer, M., Widmann, M., Chen, Y.-C., Udvarhelyi, P., Bonato, C., Hassan, J. U., Karhu, R., Ivanov, I. G., Son, N. T., et al. High-fidelity spin and optical control of single silicon-vacancy centres in silicon carbide.Nat. Commun.10, 1954 (2019)

    work page 1954

  19. [19]

    T., Janzén, E

    Niethammer, M., Widmann, M., Lee, S.-Y., Stenberg, P., Kordina, O., Ohshima, T., Son, N. T., Janzén, E. & Wrachtrup, J. Vector magnetometry using silicon va- cancies in 4H-SiC under ambient conditions.Phys. Rev. Appl.6, 034001 (2016)

    work page 2016

  20. [20]

    T., Csóré, A., Gällström, A., Ohshima, T., Gali, A

    Magnusson, B., Son, N. T., Csóré, A., Gällström, A., Ohshima, T., Gali, A. & Ivanov, I. G. Excitation proper- ties of the divacancy in 4H-SiC.Phys. Rev. B98, 195202 (2018)

    work page 2018

  21. [21]

    L., Buckley, B

    Falk, A. L., Buckley, B. B., Calusine, G., Koehl, W. F., Dobrovitski, V. V., Politi, A., Zorman, C. A., Feng, P. X.-L.&Awschalom, D.D.Polytypecontrolofspinqubits in silicon carbide.Nat. Commun.4, 1819 (2013). 8

    work page 2013

  22. [22]

    & Van de Walle, C

    Gordon, L., Janotti, A. & Van de Walle, C. G. Defects as qubits in 3C- and 4H-SiC.Phys. Rev. B92, 045208 (2015)

    work page 2015

  23. [23]

    T., Gali, A

    Davidsson, J., Ivády, V., Armiento, R., Son, N. T., Gali, A. & Abrikosov, I. A. First principles predictions of magneto-optical data for semiconductor point defect identification: the case of divacancy defects in 4H-SiC. New J. Phys.20, 023035 (2018)

    work page 2018

  24. [24]

    T., Abrikosov, I

    Shafizadeh, D., Son, N. T., Abrikosov, I. A. & Ivanov, I. G. Evolution of the optically detected magnetic res- onance spectra of divacancies in 4H-SiC from liquid- helium to room temperature.Phys. Rev. B111, 165201 (2025)

    work page 2025

  25. [25]

    L., Klimov, P

    Ivády, V., Davidsson, J., Delegan, N., Falk, A. L., Klimov, P. V., Whiteley, S. J., Hruszkewycz, S. O., Holt, M. V., Heremans, F. J., Son, N. T., et al. Stabilization of point-defect spin qubits by quantum wells.Nat. Com- mun.10, 5607 (2019)

    work page 2019

  26. [26]

    T., Anderson, C

    Son, N. T., Anderson, C. P., Bourassa, A., Miao, K. C., Babin, C., Widmann, M., Niethammer, M., Ul Hassan, J., Morioka, N., Ivanov, I. G., et al. Developing silicon carbide for quantum spintronics.Appl. Phys. Lett.116, 190501 (2020)

    work page 2020

  27. [27]

    Silicon carbide single-photon sources: challenges and prospects.Mater

    Castelletto, S. Silicon carbide single-photon sources: challenges and prospects.Mater. Quantum Technol.1, 023001 (2021)

    work page 2021

  28. [28]

    & Kobayashi, T

    Iwamoto, S., Shimura, T., Watanabe, H. & Kobayashi, T. Oxygen-related defects in 4H-SiC from first principles. Appl. Phys. Express17, 051008 (2024)

    work page 2024

  29. [29]

    & Watanabe, H

    Kobayashi, T., Shimura, T. & Watanabe, H. Oxygen- vacancy defect in 4H-SiC as a near-infrared emitter: An ab initio study.J. Appl. Phys.134, 143102 (2023)

    work page 2023

  30. [30]

    & Wu, Y.-N

    Bai, R., Liu, D., Chen, S. & Wu, Y.-N. Origin of the unidentified color centers in 4H-SiC from first principles. Phys. Rev. B111, 014106 (2025)

    work page 2025

  31. [31]

    C., Ivády, V., Stavrias, N., Umeda, T., Gali, A

    Castelletto, S., Johnson, B. C., Ivády, V., Stavrias, N., Umeda, T., Gali, A. & Ohshima, T. A silicon carbide room-temperature single-photon source.Nat. Mater.13, 151–156 (2014)

    work page 2014

  32. [32]

    & Guo, G.-C

    Wang, J.-F., Cui, J.-M., Yan, F.-F., Li, Q., Cheng, Z.- D., Liu, Z.-H., Lin, Z.-H., Xu, J.-S., Li, C.-F. & Guo, G.-C. Optimization of power broadening in optically de- tected magnetic resonance of defect spins in silicon car- bide.Phys. Rev. B101, 064102 (2020)

    work page 2020

  33. [33]

    & Gali, A

    Li, P., Zhou, J.-Y., Li, S., Udvarhelyi, P., Xu, J.-S., Li, C.-F., Huang, B., Guo, G.-C. & Gali, A. Non-invasive bioinert room-temperature quan- tum sensor from silicon carbide qubits.Nat. Mater. https://doi.org/10.1038/s41563-025-02382-9, 1 (2025)

    work page doi:10.1038/s41563-025-02382-9 2025

  34. [34]

    Nagy, R., Widmann, M., Niethammer, M., Dasari, D. B. R., Gerhardt, I., Soykal, Ö. O., Radulaski, M., Ohshima, T., Vučković, J., Son, N. T., et al. Quantum properties of dichroic silicon vacancies in silicon carbide.Phys. Rev. Appl.9, 034022 (2018)

    work page 2018

  35. [35]

    T., Shafizadeh, D., Ohshima, T

    Son, N. T., Shafizadeh, D., Ohshima, T. & Ivanov, I. G. Modified divacancies in 4H-SiC.J. Appl. Phys.132, 025701 (2022)

    work page 2022

  36. [36]

    & Duan, C.-K

    Zhao, X., Liu, M. & Duan, C.-K. Towards identifying the PL6 center in SiC: From first-principles screening to hy- perfine validation of competing defect candidates.Phys. Rev. Mater.9, 116204 (2025)

    work page 2025

  37. [37]

    & Meijer, J

    Pezzagna, S., Diziain, S., Martelock, H., Neugebauer, P., Michaelis, J., Lühmann, T. & Meijer, J. Polymorphs of 17O-Implanted ST1 Spin Centers in Diamond and Spec- troscopy of Strongly Coupled 13C Nuclear Spins.ACS Photon.11, 1969–1980 (2024)

    work page 1969