Discovery of Superconductivity in Hard Hexagonal ε-NbN

At a Glance

Metadata	Details
Publication Date	2016-02-29
Journal	Scientific Reports
Authors	Yongtao Zou, Xintong Qi, Cheng Zhang, Shuailing Ma, Wei Zhang
Institutions	Brookhaven National Laboratory, State Key Laboratory of Superhard Materials
Citations	47
Analysis	Full AI Review Included

Technical Documentation & Analysis: Hard Superconductivity in Hexagonal epsilon-NbN

This document analyzes the research paper “Discovery of Superconductivity in Hard Hexagonal $\epsilon$-NbN” (Sci. Rep. 6, 22330, 2016) to provide key technical specifications and demonstrate how 6CCVD’s advanced MPCVD diamond materials and processing capabilities can support and extend this research field, particularly in hard superconducting materials and extreme environment applications.

Executive Summary

The discovery of superconductivity in hexagonal $\epsilon$-NbN synthesized under high pressure and high temperature (HPHT) establishes a new class of hard superconductors with exceptional mechanical properties.

Superior $T_c$ vs. BDD: The critical temperature ($T_c$) of $\epsilon$-NbN is measured at $\approx 11.6$ K, significantly higher than the $\approx 4$ K typically observed in Boron-Doped Diamond (BDD).
Extreme Mechanical Performance: $\epsilon$-NbN exhibits ultra-hardness ($H_v$ 22-30 GPa), ultra-incompressibility (Bulk Modulus $B \approx 373$ GPa), and high shear rigidity ($G \approx 201$ GPa), rivaling materials like sapphire and $\gamma$-B.
Stability in Extreme Conditions: The superconducting phase and structure remain stable under pressures up to 20.5 GPa, making it highly attractive for high-field and extreme environment applications.
Synthesis Method: Bulk polycrystalline $\epsilon$-NbN was synthesized using HPHT techniques (10 GPa, 1100-1200 °C).
Electronic Mechanism: The relatively lower $T_c$ compared to cubic $\delta$-NbN ($T_c \approx 17.5$ K) is attributed to weaker bonding in the Nb-N network and weaker electron-phonon coupling ($\lambda \approx 0.57-0.63$).
6CCVD Relevance: Although $\epsilon$-NbN shows a higher $T_c$, 6CCVD’s Boron-Doped Diamond (BDD) remains critical for applications requiring diamond’s superior thermal management, high purity, and integration into quantum devices, offering a complementary platform for hard superconductivity research.

Technical Specifications

Parameter	Value	Unit	Context
Superconducting $T_c$ (Major Phase)	11.6	K	Hexagonal $\epsilon$-NbN
Superconducting $T_c$ (Minor Phase)	17.5	K	Cubic $\delta$-NbN (Rock-salt structure)
Zero Resistivity $T_c$	10.5	K	Hexagonal $\epsilon$-NbN
Vickers Hardness ($H_v$)	22 - 30	GPa	Polycrystalline $\epsilon$-NbN (Load dependent)
Shear Modulus ($G$)	201 (±1)	GPa	Ambient condition
Bulk Modulus ($B$)	373 (±2)	GPa	Ambient condition (Ultra-incompressible)
Pressure Stability	Up to 20.5	GPa	Superconductivity and structure stable
Synthesis Pressure	10	GPa	Multi-anvil apparatus
Synthesis Temperature	1100 - 1200	°C	For 1.5 hour duration
Electron-Phonon Coupling ($\lambda$)	0.57 - 0.63	Dimensionless	Estimated for $\epsilon$-NbN
Debye Temperature ($\Theta_D$)	738	K	Experimentally measured for $\epsilon$-NbN
Average Grain Size	1 - 2	µm	Synthesized polycrystalline specimen
Lattice Constant (a)	2.9599(4)	Å	Hexagonal $\epsilon$-NbN (Space group P6₃/mmc)
Lattice Constant (c)	11.2497(22)	Å	Hexagonal $\epsilon$-NbN (Space group P6₃/mmc)

Key Methodologies

The research utilized a combination of HPHT synthesis and advanced characterization techniques to confirm the structural, mechanical, and superconducting properties of the novel material.

High-Pressure/High-Temperature (HPHT) Synthesis:
- Starting Material: Niobium nitride powder (99% purity).
- Equipment: High-pressure multi-anvil apparatus.
- Recipe Parameters: 10 GPa pressure, 1100-1200 °C temperature, held for 1.5 hours.
Superconductivity Characterization:
- Magnetization: Superconducting Quantum Interference Device (SQUID) magnetometer used for Zero-Field Cooling (ZFC) and Field Cooling (FC) measurements under a 3 mT magnetic field.
- Electrical Resistivity: Physical Property Measurement System (PPMS) utilizing the standard four-probe method.
Structural and Mechanical Characterization:
- In situ Synchrotron X-ray Diffraction (XRD): Performed using a Diamond Anvil Cell (DAC) up to 20.5 GPa to confirm phase stability and measure crystal-axis compression anisotropy.
- Microstructure Analysis: Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDX) for grain size and composition (Nb_0.98(2)N_0.96(5)O_0.06(4)).
- High-Resolution TEM (HRTEM) & SAED: Used to confirm the perfect crystalline form and hexagonal structure (P6₃/mmc).
- Hardness Measurement: Vickers indentation method using a pyramidal diamond indenter (loads 2.94 N to 9.8 N).
- Elastic Properties: In situ ultrasonic measurements used to derive Bulk ($B$) and Shear ($G$) moduli.
Theoretical Modeling:
- First-principles calculations (DFT using CASTEP code) were used to model structural stability, electronic properties (TDOS/PDOS), and predict elastic constants.

6CCVD Solutions & Capabilities

The discovery of hard superconductors like $\epsilon$-NbN highlights the growing demand for materials that combine extreme mechanical robustness with high-performance electronic properties. 6CCVD is uniquely positioned to support research in this domain, particularly in providing the highest quality diamond materials necessary for comparative studies, device integration, and extreme environment applications.

Applicable Materials

The paper explicitly contrasts $\epsilon$-NbN with Boron-Doped Diamond (BDD). While $\epsilon$-NbN offers a higher $T_c$ for hard superconductors, BDD offers unparalleled advantages in thermal management and purity, making it essential for high-power and quantum applications.

Boron-Doped Diamond (BDD): 6CCVD provides heavily doped PCD and SCD BDD wafers. These materials are crucial for researchers seeking to:
- Benchmark Superconductivity: Directly compare the performance of NbN heterostructures against the established BDD superconducting platform.
- High-Field Applications: Utilize BDD’s superior thermal conductivity (up to 2000 W/m·K for SCD) to manage heat dissipation in high-current or high-field superconducting devices, a critical factor where NbN’s thermal properties may be limiting.
Optical Grade Single Crystal Diamond (SCD): For high-purity substrates required in advanced electronic or quantum applications where the NbN layer must be integrated onto a defect-free, insulating platform.
Polycrystalline Diamond (PCD) Substrates: Available in large formats (up to 125mm) and thicknesses up to 10mm, ideal for use as robust, high-stiffness carriers for HPHT-synthesized materials like $\epsilon$-NbN, ensuring mechanical stability in extreme testing environments.

Customization Potential

The integration of ultra-hard materials like $\epsilon$-NbN (22-30 GPa) into functional devices requires precision processing that 6CCVD routinely delivers for diamond (up to 100 GPa).

Requirement from Research	6CCVD Capability	Technical Advantage
Device Integration	Custom Dimensions & Thickness	Plates/wafers up to 125mm (PCD) and SCD up to 500µm thick. Ideal for creating precise device geometries for SQUID or PPMS testing.
Electrical Contacts	Custom Metalization Services	Internal capability for depositing Au, Pt, Pd, Ti, W, and Cu. Essential for creating low-resistance ohmic contacts on hard materials for reliable electrical resistivity measurements.
Surface Quality	Ultra-Precision Polishing	Polishing to Ra < 1nm (SCD) and Ra < 5nm (Inch-size PCD). Critical for minimizing interface scattering losses when integrating NbN layers or for high-frequency device fabrication.
Substrate Robustness	Thick Substrates	SCD/PCD substrates available up to 10mm thickness, providing the mechanical stiffness required for high-pressure testing (e.g., DAC experiments up to 20 GPa).

Engineering Support

6CCVD’s in-house PhD team specializes in the material science of CVD diamond and its applications in extreme environments. We offer consultation services to researchers working on hard superconductors for:

Material Selection: Assisting in choosing the optimal diamond platform (SCD vs. PCD, doping level) to maximize performance in high-field or high-pressure applications, such as those involving transition-metal nitrides like $\epsilon$-NbN.
Interface Engineering: Providing expertise on surface preparation and metalization schemes necessary for integrating hard, chemically inert materials onto diamond substrates.
Thermal Management Optimization: Designing diamond heat spreaders or substrates to manage the thermal load generated by superconducting devices operating near $T_c$.

Call to Action: For custom specifications or material consultation regarding hard superconductors, BDD optimization, or extreme environment substrates, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Abstract Since the discovery of superconductivity in boron-doped diamond with a critical temperature ( T C ) near 4 K, great interest has been attracted in hard superconductors such as transition-metal nitrides and carbides. Here we report the new discovery of superconductivity in polycrystalline hexagonal ε-NbN synthesized at high pressure and high temperature. Direct magnetization and electrical resistivity measurements demonstrate that the superconductivity in bulk polycrystalline hexagonal ε-NbN is below ∼11.6 K, which is significantly higher than that for boron-doped diamond. The nature of superconductivity in hexagonal ε-NbN and the physical mechanism for the relatively lower T C have been addressed by the weaker bonding in the Nb-N network, the co-planarity of Nb-N layer as well as its relatively weaker electron-phonon coupling, as compared with the cubic δ-NbN counterpart. Moreover, the newly discovered ε-NbN superconductor remains stable at pressures up to ∼20 GPa and is significantly harder than cubic δ-NbN; it is as hard as sapphire, ultra-incompressible and has a high shear rigidity of 201 GPa to rival hard/superhard material γ-B (∼227 GPa). This exploration opens a new class of highly desirable materials combining the outstanding mechanical/elastic properties with superconductivity, which may be particularly attractive for its technological and engineering applications in extreme environments.

Tech Support

Original Source

DOI: https://doi.org/10.1038/srep22330