Quantum phase transition inside the superconducting dome of Ba(Fe1−xCox)2As2 from diamond-based optical magnetometry
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2020-04-01 |
| Journal | New Journal of Physics |
| Authors | K R Joshi, N. M. Nusran, M.A. Tanatar, K.Cho, S.L. Bud′ko |
| Institutions | University of Wisconsin–Madison, Ames National Laboratory |
| Citations | 23 |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: Diamond-Based Optical Magnetometry for Quantum Phase Transitions
Section titled “Technical Documentation & Analysis: Diamond-Based Optical Magnetometry for Quantum Phase Transitions”This document analyzes the research paper “Quantum phase transition inside the superconducting dome of Ba(Fe${1-x}$Co${x}$)$_2$As$_2$ from diamond-based optical magnetometry” to highlight the critical role of high-quality MPCVD diamond and to propose specific material solutions available from 6CCVD.
Executive Summary
Section titled “Executive Summary”The research successfully employed Nitrogen-Vacancy (NV) centers in Single Crystal Diamond (SCD) to probe fundamental physics in iron-based superconductors. This application directly validates the need for ultra-high purity, custom-engineered diamond materials.
- Core Achievement: Measured the absolute value of the London penetration depth ($\lambda$) in Ba(Fe${1-x}$Co${x}$)$_2$As$_2$ at $T = 4.5$ K using minimally-invasive NV-center optical magnetometry.
- Key Finding: Discovery of a sharp, anomalous peak in the penetration depth ($\lambda$) at optimal doping ($x=0.057$), coinciding precisely with the extrapolated antiferromagnetic Quantum Phase Transition (QPT).
- Scientific Significance: Suggests that the sharp $\lambda$ peak is a universal manifestation of magnetic quantum fluctuations in iron-based superconductors, even in disordered, fully-gapped systems.
- Material Requirement: The experiment relied on a 40 µm thick, electronic-grade SCD plate with NV centers activated approximately 20 nm from the surface.
- 6CCVD Value Proposition: 6CCVD provides the necessary Optical Grade SCD with precise thickness control (0.1 µm to 500 µm) and superior surface polishing (Ra < 1 nm) essential for high-coherence NV-center quantum sensing applications.
- Customization: 6CCVD offers custom dimensions (up to 125 mm PCD) and specialized metalization services required for integrating diamond sensors into complex cryogenic and microwave setups.
Technical Specifications
Section titled “Technical Specifications”The following hard data points were extracted from the research paper, focusing on the material and measurement parameters critical for replication or extension of the study.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Diamond Sensor Material | Single Crystal Diamond (SCD) | N/A | Electronic-grade, [100] orientation |
| Diamond Sensor Thickness | 40 | µm | Required for sensor integration |
| NV Center Activation Depth | ~20 | nm | Proximity to sample surface for high sensitivity |
| Measurement Temperature | 4.5 | K | Fixed temperature for $\lambda(x)$ sweep |
| Superconductor Material | Ba(Fe${1-x}$Co${x}$)$_2$As$_2$ (Co-Ba122) | N/A | Electron-doped iron pnictide |
| Superconductor Sample Geometry | Cuboid | N/A | Required for accurate demagnetization factor calculation |
| Superconductor Sample Thickness | Typically 50 | µm | Cleaved plates |
| Optimal Doping Concentration ($x$) | 0.057 | N/A | Location of the sharp $\lambda$ peak |
| Peak London Penetration Depth ($\lambda$) | ~0.3 | µm | Measured at $x=0.057$ |
| Polishing Requirement (Inferred) | Ra < 1 nm | N/A | Required for direct, intimate contact between SCD and sample |
Key Methodologies
Section titled “Key Methodologies”The experiment relies on precise material synthesis, meticulous sample preparation, and advanced quantum sensing techniques.
- Crystal Growth: High-quality Ba(Fe${1-x}$Co${x}$)$_2$As$_2$ single crystals were grown using a self-flux solution technique.
- Doping Verification: Cobalt concentration ($x$) was accurately determined using Wavelength Dispersive Spectroscopy (WDS).
- Sample Shaping: Crystals were cleaved into thin plates (typically 50 µm thick) and further cleaved into cuboid shapes along the (100) and (010) tetragonal directions to ensure well-defined edges for $H_{c1}$ measurement.
- NV Diamond Sensor Preparation: A 40 µm thick electronic-grade SCD plate with a [100] surface was used. NV centers were activated only on the side intended for contact, approximately 20 nm deep.
- Cryogenic Setup: The SCD sensor was placed in direct contact with the superconducting sample surface within a low-temperature Attocube AFM/CFM system.
- Magnetometry: Magnetic induction was measured by monitoring the Zeeman splitting in Optically Detected Magnetic Resonance (ODMR).
- Penetration Depth Determination: The lower critical field ($H_{c1}$) was measured at the sample edge by detecting the onset of first Abrikosov vortex penetration ($H_p$). The penetration depth ($\lambda$) was then deduced from $H_{c1}$ using calculated effective demagnetization factors specific to the cuboid geometry.
6CCVD Solutions & Capabilities
Section titled “6CCVD Solutions & Capabilities”6CCVD is uniquely positioned to supply the advanced MPCVD diamond materials necessary to replicate, scale, and extend this critical quantum sensing research.
Applicable Materials
Section titled “Applicable Materials”To achieve the high sensitivity and coherence required for NV-center magnetometry, the material must be ultra-low strain and possess extremely low nitrogen content.
- Recommended Material: Optical Grade Single Crystal Diamond (SCD).
- Justification: This material offers the highest purity and lowest defect density, ensuring maximum NV-center coherence time ($T_2$) and stability, which is essential for high-sensitivity magnetic induction measurements at cryogenic temperatures.
Customization Potential
Section titled “Customization Potential”The success of this experiment hinges on precise material dimensions and surface quality, areas where 6CCVD excels.
| Research Requirement | 6CCVD Capability | Technical Advantage |
|---|---|---|
| Thin SCD Plates (40 µm) | Custom Thickness Control: SCD available from 0.1 µm up to 500 µm. | We can supply the exact 40 µm thickness required, minimizing material waste and processing time for the end user. |
| High Surface Quality | Precision Polishing: Ra < 1 nm (SCD). | Ensures intimate, direct contact between the NV layer and the superconducting sample, maximizing magnetic coupling and measurement fidelity. |
| Crystallographic Orientation | Standard [100] and Custom Orientations. | We guarantee the required [100] orientation necessary for consistent NV-center alignment and optimal ODMR performance. |
| Device Integration | Custom Metalization: Internal capability for Au, Pt, Pd, Ti, W, Cu. | For future device iterations requiring integrated microwave antennas or contact pads on the diamond surface, 6CCVD offers full in-house metalization services. |
| Scaling Potential | Large Area PCD: Plates/wafers up to 125 mm. | While SCD was used here, 6CCVD can provide large-area Polycrystalline Diamond (PCD) substrates (up to 10 mm thick) for supporting larger experimental setups or for applications where thermal management is critical. |
Engineering Support
Section titled “Engineering Support”The interplay between quantum critical fluctuations, superconductivity, and disorder requires specialized material knowledge.
- Expert Consultation: 6CCVD’s in-house PhD team provides authoritative engineering support for projects involving NV-center quantum sensing and cryogenic material integration.
- Application Focus: We assist researchers in optimizing material selection (e.g., nitrogen concentration, isotopic purity, and surface termination) specifically for low-temperature physics and superconductivity studies similar to this London penetration depth ($\lambda$) measurement.
- Global Logistics: We offer reliable global shipping (DDU default, DDP available) to ensure timely delivery of sensitive materials to research facilities worldwide.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
Abstract Unconventional superconductivity often emerges in close proximity to a magnetic instability. Upon suppressing the magnetic transition down to zero temperature by tuning the carrier concentration, pressure, or disorder, the superconducting transition temperature T c acquires its maximum value. A major challenge is the elucidation of the relationship between the superconducting phase and the strong quantum fluctuations expected near a quantum phase transition (QPT) that is either second order (i.e. a quantum critical point) or weakly first order. While unusual normal state properties, such as non-Fermi liquid behavior of the resistivity, are commonly associated with strong quantum fluctuations, evidence for its presence inside the superconducting dome are much scarcer. In this paper, we use sensitive and minimally invasive optical magnetometry based on NV-centers in diamond to probe the doping evolution of the T = 0 penetration depth in the electron-doped iron-based superconductor Ba(Fe 1− x Co x ) 2 As 2 . A non-monotonic evolution with a pronounced peak in the vicinity of the putative magnetic QPT is found. This behavior is reminiscent to that previously seen in isovalently-substituted BaFe 2 (As 1− x P x ) 2 compounds, despite the notable differences between these two systems. Whereas the latter is a very clean system that displays nodal superconductivity and a single simultaneous first-order nematic-magnetic transition, the former is a charge-doped and significantly dirtier system with fully gapped superconductivity and split second-order nematic and magnetic transitions. Thus, our observation of a sharp peak in λ ( x ) near optimal doping, combined with the theoretical result that a QPT alone does not mandate the appearance of such peak, unveils a puzzling and seemingly universal manifestation of magnetic quantum fluctuations in iron-based superconductors and unusually robust quantum phase transition under the dome of superconductivity.