Effects of Rashba-spin–orbit coupling on superconducting boron-doped nanocrystalline diamond films - evidence of interfacial triplet superconductivity
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2020-08-17 |
| Journal | New Journal of Physics |
| Authors | Somnath Bhattacharyya, Davie Mtsuko, Christopher Allen, Christopher Coleman, Somnath Bhattacharyya |
| Institutions | University of Oxford, National University of Science and Technology |
| Citations | 9 |
| Analysis | Full AI Review Included |
Effects of Rashba-spin-orbit coupling on superconducting boron-doped nanocrystalline diamond films
Section titled “Effects of Rashba-spin-orbit coupling on superconducting boron-doped nanocrystalline diamond films”Executive Summary
Section titled “Executive Summary”- Core Achievement: Experimental evidence confirms the existence of interfacial triplet superconductivity in granular Boron-doped Nanocrystalline Diamond (B-NCD) films.
- Mechanism Identified: The triplet component is attributed to Rashba-type Spin-Orbit Coupling (RSOC), which is induced by the breaking of inversion symmetry and asymmetric confinement potential at the diamond grain boundaries.
- Microstructural Confirmation: Ultra-High-Resolution Transmission Electron Microscopy (UHRTEM) reveals sharp, layered grain boundaries with stacking faults, confirming the material behaves as an array of Superconductor-Insulator-Superconductor (S-I-S) junctions.
- Spin-Orbit Coupling (SOC) Signature: The Weak Anti-Localization (WAL) effect was observed in low-field magnetoresistance, a hallmark feature of significant SOC in a two-dimensional (2D) system.
- Triplet State Hallmark: A pronounced Zero Bias Conductance Peak (ZBCP) was measured in the differential conductance (dI/dV). This peak is robust against applied magnetic fields and shows no splitting, confirming the signature of a spin-triplet superconducting state.
- Lifetime Correlation: Both the extracted spin coherence lifetime (τSO) from WAL analysis and the triplet resonance lifetime (τ) from ZBCP analysis are found to be in the picosecond (ps) regime, establishing an intimate link between the interfacial SOC and the formation of the triplet condensate.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Film Thickness | ~100 | nm | Columnar growth of nanocrystalline grains |
| Applied Magnetic Field (ADMR) | 0.1 | T | Field used for angle-dependent magnetoresistance measurements |
| Upper Critical Field (Bc2) | ~1.5 | T | Magnetic field required to fully suppress the ZBCP |
| Rashba Spin-Orbit Splitting (ΔSO) | 2.5 to 4.0 | meV | Extracted from HLN fitting (temperature dependent) |
| Phase Coherence Length (Lφ) | 20 to 120 | nm | Temperature dependent, derived from WAL fitting |
| Spin Coherence Length (LSO) | ~40 | nm | Nearly temperature independent, derived from WAL fitting |
| Spin Coherence Lifetime (τSO) | ps regime | s | Derived from spin coherence field (BSO) |
| Triplet Resonance Lifetime (τ) | ps regime | s | Derived from ZBCP Full Width Half Maximum (FWHM) |
| Grain Boundary Interface Thickness | Few | nm | Observed in UHRTEM imaging |
| Pre-factor (a) in HLN fitting | 0.5 to 1 | N/A | Used to determine overall validity of quantum transport model |
Key Methodologies
Section titled “Key Methodologies”The study utilized a combination of advanced structural characterization and low-temperature quantum transport measurements to confirm the presence and origin of the triplet superconducting component.
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Sample Growth and Structure:
- Boron-doped diamond films were grown via Chemical Vapor Deposition (CVD).
- Films exhibited nanocrystalline, columnar growth, oriented close to the [110] zone axis.
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Microstructural Analysis:
- Ultra-High-Resolution Transmission Electron Microscopy (UHRTEM): Used to image the complex microstructure, specifically focusing on grain boundaries.
- HAADF-STEM Imaging: Confirmed sharp boundary regions, stacking faults, and crystal twinning, supporting the S-I-S junction array model.
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Quantum Transport Measurements:
- Magnetoresistance (MR) Measurement: Conducted at low temperatures (below 1 K) to observe the transition from Weak Anti-Localization (WAL) to Weak Localization (WL).
- HLN Formalism Fitting: The magnetoconductance data was fitted using the Hikami-Larkin-Nagaoka (HLN) equation (and a modified version for hole carriers) to extract Lφ, LSO, and the Rashba splitting (ΔSO).
- Angle Dependent Magnetoresistance (ADMR): Used to confirm the significant influence of grain boundaries on charge transport anisotropy.
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Spectroscopic Measurement:
- Differential Conductance (dI/dV) Measurement: Four-probe voltage-biased transport used to measure dI/dV as a function of voltage (V) and temperature (T).
- Zero Bias Conductance Peak (ZBCP) Analysis: The ZBCP FWHM was analyzed to determine the lifetime (τ) of the mid-gap bound states, linking it to the spin coherence lifetime (τSO).
Commercial Applications
Section titled “Commercial Applications”The confirmation of robust, interfacial triplet superconductivity in a material like diamond—known for its hardness, chemical inertness, and high thermal conductivity—opens several avenues for advanced engineering applications.
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Topological Quantum Computing:
- Triplet superconductors are necessary for realizing p-wave pairing, which is theorized to host non-Abelian quasiparticles (Majorana zero modes). These modes are the foundation for fault-tolerant quantum computation.
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Spintronics and Spin-Active Devices:
- The demonstrated Rashba-type SOC at the grain interfaces provides a mechanism for efficient spin-to-charge conversion and spin mixing, critical for developing highly efficient spin-active interfaces, spin valves, and superconducting spintronic logic circuits.
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Extreme Environment Electronics:
- Diamond’s inherent robustness makes B-NCD films ideal for superconducting devices that must operate under high radiation, high temperature, or chemically aggressive conditions where conventional superconductors fail.
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Superconducting Heterostructures:
- The granular nature and the ability to engineer spin-active boundaries suggest B-NCD could be used as a component in complex superconducting heterostructures, allowing for precise control over the proximity effect and the induction of unconventional pairing states.
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High-Frequency/High-Speed Devices:
- The picosecond (ps) lifetimes measured for the spin coherence and triplet resonance indicate potential for high-speed operation in superconducting quantum interference devices (SQUIDs) or resonators.
View Original Abstract
Abstract Among the many remarkable properties of diamond, the ability to superconduct when heavily doped with boron has attracted much interest in the carbon community. When considering the nanocrystalline boron doped system, the reduced dimensionality and confinement effects have led to several intriguing observations most notably, signatures of a mixed superconducting phase. Here we present ultra-high-resolution transmission electron microscopy imaging of the grain boundary and demonstrate how the complex microstructure leads to enhanced carrier correlations. We observe hallmark features of spin-orbit coupling (SOC) manifested as the weak anti-localization effect. The enhanced SOC is believed to result from a combination of inversion symmetry breaking at the grain boundary interfaces along with antisymmetric confinement potential between grains, inducing a Rashba-type SOC. From a pronounced zero bias peak in the differential conductance, we demonstrate signatures of a triplet component believed to result from spin mixing caused by tunneling of singlet Cooper pairs through such Rashba-SOC grain boundary junctions.