Nitrogen Vacancy Center Optical Magnetometry of a Barium-Iron-Cobalt Superconductor

At a Glance

Metadata	Details
Publication Date	2020-01-01
Journal	Digital Commons at Macalester (Macalester College)
Authors	William Setterberg
Analysis	Full AI Review Included

Executive Summary

Method Validation: Nitrogen Vacancy (NV) center optical magnetometry was successfully utilized for the first time to measure the absolute, temperature-dependent London penetration depth (λ) in a superconductor.
Material Characterization: The intrinsic superconducting properties (λ and lower critical field H_c1) of the iron-based pnictide Ba(Fe_0.926Co_0.074)₂As₂ (BaCo122) were quantified across a temperature range of 5 K to 17 K.
Minimally-Invasive Probing: NV magnetometry offers a non-invasive, local probe technique, enabling the measurement of magnetic fields near ideal, defect-free regions of the crystal surface.
Key Findings: The critical temperature (T_c) was confirmed at 22.2 ± 1 K. Measured H_c1 values decreased from 181 Oe at 5 K to 90 Oe at 17 K, consistent with thermodynamic stability decreasing near T_c.
Technique Comparison: Local NV results for λ align well with other local probes (MFM, µSR) but diverge significantly from bulk techniques (TDR, SQUID) at temperatures > 10 K, suggesting that bulk measurements are strongly influenced by crystalline imperfections.
Future Potential: The technique shows promise for advanced applications, including pulsed-wave NV magnetometry for enhanced signal-to-noise and single-NV probes for direct imaging of Abrikosov vortices.

Technical Specifications

Parameter	Value	Unit	Context
Superconductor	Ba(Fe_1-xCo_x)₂As₂	N/A	Cobalt doping x = 7.4% (BaCo122)
Critical Temperature (T_c)	22.2 ± 1	K	Measured via NV magnetometry transition
Base Measurement Temp	4.2	K	Boiling point of liquid Helium
NV Center Depth	~20	nm	Ensemble NV layer embedded in diamond
NV Center Spin System	Spin 1	N/A	Nitrogen Vacancy (NV^-) defect
Zero-Field Splitting (D)	2.87	GHz	Magnetic dipole transition frequency
London Penetration Depth (λ) @ 5 K	209 ± 9.8	nm	Absolute value measured by NV ODMR
Lower Critical Field (H_c1) @ 5 K	181 ± 15	Oe	Calculated from penetrative field H_p
London Penetration Depth (λ) @ 17 K	288 ± 27.5	nm	Absolute value measured by NV ODMR
Lower Critical Field (H_c1) @ 17 K	90 ± 15	Oe	Calculated from penetrative field H_p
Sample Dimensions (a, b, c)	720 ± 5, 810 ± 5, 36 ± 5	nm	Used for geometric demagnetization factor (N)
NV Excitation Wavelength	Green	N/A	Used for optical pumping
NV Fluorescence Wavelength	637	nm	Red photons used for readout

Key Methodologies

Sample Preparation and Alignment:
- BaCo122 single crystal was cleaved to ~1 mm² and mounted onto a diamond substrate containing an ensemble NV layer.
- Sample edges were verified for sharpness using Scanning Electron Microscopy (SEM) to ensure well-defined geometry for magnetic field calculations.
- The assembly was placed on a piezoelectric stage (0.1 nm resolution) inside an attoAFM/CFM cryogenic system.
Superconducting Transition Measurement (T_c):
- The sample was heated above T_c (30 K) in zero magnetic field, then cooled to 4.2 K to ensure dissipation of all Abrikosov vortices (zero-field cooling).
- A constant, weak magnetic field (H = 10² Oersted) was applied perpendicular to the surface.
- The Zeeman splitting (Z) was recorded as temperature increased; the transition (T_c) was identified by a dramatic increase in Z, indicating magnetic field penetration.
NV Optical Magnetometry (ODMR):
- The NV centers were excited using green laser light and simultaneously radiated with continuous-wave microwaves near the 2.87 GHz zero-field splitting frequency.
- Red fluorescence intensity was measured as a function of microwave frequency (ODMR spectrum). Resonance between the m_s = 0 and m_s = ±1 states causes a reduction in fluorescence.
- The difference between the two resulting spin-resonant peaks (Z_d) is proportional to the magnetic field projection along the NV axis.
Lower Critical Field (H_c1) Determination:
- Measurements were performed at fixed temperatures (5 K, 10 K, 15 K, 17 K) with the confocal volume focused on the center of a sharp sample edge.
- The applied magnetic field (H) was incrementally increased until the Zeeman splitting (Z_d) began to widen, indicating the onset of magnetic flux leakage (vortex penetration). This field is defined as the penetrative field (H_p).
- H_p was determined by fitting the ODMR data with a double Lorentzian function and averaging results from four physical points along the edge.
Calculation of H_c1 and λ:
- The measured H_p values were numerically solved using established equations (Prozorov and Kogan) that incorporate the geometric demagnetization factor (N) and the superconducting coherence length (ξ) to yield the absolute values of H_c1 and λ.

Commercial Applications

Quantum Computing and Sensing: NV centers are foundational components for solid-state quantum processors and highly sensitive magnetometers, enabling precise control and readout of quantum states.
Superconducting Materials Development: The ability to locally measure intrinsic parameters (λ, H_c1) helps accelerate the discovery and optimization of novel high-temperature superconductors, reducing reliance on expensive cryogenic coolants.
Non-Invasive Diagnostics: NV magnetometry provides a powerful tool for characterizing magnetic phenomena in thin films and interfaces where traditional bulk techniques are unsuitable or overly perturbative.
Cryogenic Instrumentation: Data on the temperature dependence of λ and H_c1 is critical for engineering robust superconducting components used in advanced scientific instruments and high-field magnets (e.g., specialized MRI or particle accelerators).
Fundamental Physics Research: The technique is essential for investigating complex quantum phenomena, such as the symmetry of the superconducting wavefunction (e.g., s±-wave symmetry in pnictides) and the effects of localized crystalline disorder.

View Original Abstract

Experimentally probing the intrinsic properties of superconductors—such as the London penetration depth λ and the critical fields Hc1 and Hc2—poses a difficult task. Various sample- and measurement-related factors can impact the efficacy of results obtained for λ or Hc1, such as perturbations to the magnetic properties of a superconducting sample or crystalline defects. One measurement technique that can minimize the impact of both of these issues is known as Nitrogen Vacancy (NV) center magnetometry. In this work, we use NV center magnetometry to perform minimally-invasive measurements of the lower critical field Hc1 and the London penetration depth λ on a sample of Ba(Fe1−xCox)2As2, x = 7.4% (BaCo122).

Tech Support

Original Source

DOI: None