研究目的
To measure the lower critical magnetic field Hc1 of superconductors using nitrogen-vacancy centers in diamond optical magnetometry, and to determine the London penetration depth λ.
研究成果
The N-V center-based optical magnetometry provides a noninvasive and sensitive method to measure Hc1 and λ in superconductors. Results for Ba(Fe1?xCox)2As2, CaKFe4As4, and YBCO show good agreement with literature values from other techniques, validating the approach. The vector field capability confirms Brandt's model of flux penetration via corner cutting at approximately 45 degrees.
研究不足
The spatial resolution is limited by the optical diffraction limit to approximately 500 nm. Temperature fluctuations due to microwave excitation can occur, though controlled to within 10 mK. Integration times per data point are 5-10 minutes, which could be optimized with adaptive protocols or lock-in detection. The method requires careful alignment of sample edges with diamond crystal axes.
1：Experimental Design and Method Selection:
The experiment uses optical magnetometry based on nitrogen-vacancy (N-V) centers in diamond to measure the vector magnetic field with high spatial resolution. The optically detected magnetic resonance (ODMR) technique is employed to detect Zeeman splittings proportional to local magnetic fields.
2：Sample Selection and Data Sources:
Three superconducting samples are used: Ba(Fe1?xCox)2As2 (x=
3：07), CaKFe4As4, and YBa2Cu3O7?δ. Samples are precharacterized by thermodynamic and transport techniques and imaged by scanning electron microscopy to ensure well-defined surfaces and edges. List of Experimental Equipment and Materials:
A diamond plate with embedded N-V centers (
4：5 × 1 × 04 mm3), an Attocube attoAFM/CFM system, a confocal microscope, a helium cryostat, a temperature-controlled cold stage, a 50× confocal-microscope objective, a green laser for excitation, a red fluorescence detection system, a microwave antenna (single-turn 50-μm-diameter silver wire), and a superconducting magnet. Experimental Procedures and Operational Workflow:
The sample is cooled to below Tc in zero field (ZFC). A small magnetic field is applied, and ODMR signals are recorded at points near the sample edge. The magnetic field is incremented in steps, and deviations from linear behavior in ODMR splittings are detected to identify Hp (the onset of flux penetration). Hc1 and λ are calculated using equations for demagnetization factors and the relationship between Hc1 and λ.
5：Data Analysis Methods:
ODMR spectra are analyzed to extract Zeeman splittings, which are converted to magnetic field components. The onset of nonlinearity in ZS (smaller splitting) is used to determine Hp. Statistical errors are estimated from standard errors in fitting parameters.
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diamond plate
N-V embedded
Used as a magneto-optical indicator with nitrogen-vacancy centers for sensing local magnetic fields.
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Attocube attoAFM/CFM system
attoAFM/CFM
Attocube
Base system for the experimental setup, including confocal microscopy.
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confocal microscope
Used for excitation and detection in the N-V magnetometry setup.
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helium cryostat
Provides low-temperature environment for experiments.
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temperature-controlled cold stage
Maintains stable temperature during measurements.
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microwave antenna
single-turn 50-μm-diameter silver wire
Applies microwave radiation for ODMR measurements.
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superconducting magnet
Generates the external magnetic field applied to the samples.
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green laser
Excites the N-V centers in the diamond.
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red fluorescence detection system
Detects fluorescence from N-V centers for ODMR measurements.
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