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

NV Center Diamond Substrates for Superconductivity Research: Technical Documentation

This document analyzes the research paper, “Nitrogen Vacancy Center Optical Magnetometry of a Barium-Iron-Cobalt Superconductor,” focusing on the material science requirements and aligning them with 6CCVD’s advanced MPCVD diamond capabilities.

Executive Summary

The research successfully validates Nitrogen Vacancy (NV) center magnetometry as a powerful, minimally-invasive tool for characterizing Type II superconductors, relying critically on high-quality diamond substrates.

First-Time Measurement: Demonstrated the first use of NV center magnetometry to measure the absolute London penetration depth ($\lambda$) as a function of temperature (5 K to 17 K).
Material Foundation: The experiment relies on ensemble NV centers embedded $\sim 20$ nm deep into the surface of a flat, electronics-grade Single Crystal Diamond (SCD) substrate.
High-Resolution Probing: NV magnetometry provides diffraction-limited spatial resolution, allowing local probing of defect-free regions of the superconductor (BaCo122).
Key Discrepancy Identified: Local probe results for $\lambda$ agree with Magnetic Force Microscopy (MFM) and Muon Spin Relaxation ($\mu$SR) but diverge from bulk techniques (TDR, SQUID) at higher temperatures, attributed to the NV technique’s ability to bypass bulk crystalline imperfections.
Future Development: The work sets the stage for next-generation experiments, including pulsed wave NV magnetometry and single-NV scanning probes, requiring ultra-high purity SCD material for enhanced coherence times.
6CCVD Value Proposition: 6CCVD specializes in providing the high-purity, low-defect SCD wafers and precision fabrication necessary to meet the stringent material requirements for advanced quantum sensing applications.

Technical Specifications

The following hard data points were extracted from the experimental results and methodology:

Parameter	Value	Unit	Context
Superconductor Material	Ba(Fe${1-x}$Co${x}$)${2}$As${2}$	N/A	Cobalt doping $x = 7.4%$ (BaCo122)
Critical Temperature ($T_c$)	$22.2 \pm 1$	K	Measured via NV magnetometry
NV Center Depth	20	nm	Required depth for high-sensitivity surface probing
NV Center Type	NV[-]	N/A	Spin 1 quantum system used for magnetometry
Zero-Field Splitting ($D$)	$2.87$	GHz	Intrinsic on-axis energy of the NV center
Lowest Measured Temperature	5	K	Base temperature for $\lambda$ and $H_{c1}$ measurements
Highest Measured Temperature	17	K	Temperature near $T_c$ where $\lambda$ was measured
London Penetration Depth ($\lambda$) @ 5 K	$209 \pm 9.8$	nm	Absolute value measured by NV ODMR
London Penetration Depth ($\lambda$) @ 17 K	$288 \pm 27.5$	nm	Absolute value measured by NV ODMR
Lower Critical Field ($H_{c1}$) @ 5 K	$181 \pm 15$	Oe	Calculated from penetrative field ($H_p$)
Sample Dimension (c-axis)	$36 \pm 5$	nm	Used in demagnetization factor calculation

Key Methodologies

The NV center magnetometry procedure relies on precise material alignment, cryogenic control, and optically detected magnetic resonance (ODMR) techniques.

Sample Preparation: The BaCo122 crystal was cleaved to $\sim 1$ mm area and inspected via Scanning Electron Microscopy (SEM) to ensure sharp, well-defined edges, critical for geometric demagnetization factor calculations.
Substrate Integration: The superconductor sample was mounted onto a piezoelectric stage and placed adjacent to a flat, electronics-grade diamond substrate containing ensemble NV centers.
Cryogenic Environment: The assembly was transferred into an attoAFM/CFM cryogenic system, allowing temperature control from the base temperature (4.2 K) up to 30 K using a thermoelectric heater.
NV Excitation and Readout: NV centers were excited (“pumped”) using green laser light. Continuous-wave microwaves ($\sim 2.87$ GHz) were applied via a silver loop antenna to induce spin resonance.
Optical Detection: Red fluorescence emitted by the relaxing NV centers was collected using a confocal microscope and recorded by a photon counter.
Magnetic Field Measurement: A constant, weak magnetic field ($H \sim 5$ Oe) was applied perpendicular to the sample surface. The magnetic field projection along the NV axis was determined by measuring the Zeeman splitting ($Z_d$) of the ODMR fluorescence reduction peaks.
Penetrative Field ($H_p$) Determination: Measurements were taken at the sample edge while varying the applied field strength. $H_p$ was defined as the field strength where the Zeeman splitting began to deviate from the zero-field baseline, indicating magnetic flux penetration (Abrikosov vortex leakage).

6CCVD Solutions & Capabilities

This research highlights the critical need for high-quality diamond materials with precise defect engineering for quantum sensing applications. 6CCVD is uniquely positioned to supply the foundational materials required to replicate and advance this work.

Applicable Materials

To replicate the high-fidelity magnetometry demonstrated, the core material requirement is a low-defect, high-purity diamond substrate.

Research Requirement	6CCVD Material Recommendation	Technical Rationale
Electronics-Grade Diamond	Optical Grade Single Crystal Diamond (SCD)	SCD offers the lowest defect density and highest crystalline quality, essential for achieving long NV spin coherence times and maximizing measurement sensitivity.
Ensemble NV Centers	SCD with Controlled Nitrogen Doping	6CCVD can control nitrogen concentration during MPCVD growth to facilitate the creation of ensemble NV layers, ensuring uniform density and predictable performance.
Future: Single-NV Probes	Ultra-High Purity SCD Wafers	For single-NV applications (e.g., scanning probes), ultra-low background nitrogen is required prior to precise ion implantation. 6CCVD provides SCD with exceptional purity.

Customization Potential

The future direction of NV magnetometry requires specialized material geometries and surface modifications, all within 6CCVD’s core capabilities.

Precision NV Layer Depth: The experiment required NV centers embedded at a critical depth of $\sim 20$ nm. 6CCVD provides the necessary ultra-smooth SCD substrates (Ra < 1nm), which are prerequisite for achieving highly controlled, shallow NV implantation or delta-doping layers.
Custom Dimensions and Shaping: While the BaCo122 sample was small, future scanning NV probes may require custom-cut diamond tips or plates for integration into AFM/cryogenic systems. 6CCVD offers custom dimensions (plates/wafers up to 125mm PCD) and precision laser cutting services to meet these unique geometric demands.
Metalization for Microwave Delivery: The experiment utilized a silver loop antenna for microwave delivery. For integrated quantum devices, 6CCVD offers in-house metalization capabilities (Au, Pt, Pd, Ti, W, Cu), allowing researchers to deposit microwave striplines directly onto the diamond surface for optimized ODMR performance.

Engineering Support

The observed divergence between local (NV) and bulk (TDR/SQUID) measurements highlights the complexity of crystalline imperfections in superconductors. 6CCVD’s in-house PhD team specializes in material characterization and can assist researchers in selecting the optimal diamond specifications (purity, orientation, surface finish) for similar NV Center Magnetometry projects, ensuring the diamond substrate itself does not introduce unwanted noise or defects.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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