[Paper Review] Imaging the Meissner effect and flux trapping in a hydride superconductor at megabar pressures using a nanoscale quantum sensor

Prabudhya Bhattacharyya, Wuhao Chen+25 more|arXiv (Cornell University)|Jun 5, 2023

Diamond and Carbon-based Materials Research54 references4 citations

TL;DR

This study demonstrates sub-micron spatial resolution magnetometry in a cerium hydride superconductor (CeH₉) under megabar pressures (up to ~140 GPa) using nitrogen-vacancy (NV) color centers implanted in diamond anvil cells. The technique directly images the Meissner effect and flux trapping, revealing micron-scale inhomogeneities in superconducting regions and enabling local, simultaneous electrical and magnetic characterization of superconductivity under extreme conditions.

ABSTRACT

By directly altering microscopic interactions, pressure provides a powerful tuning knob for the exploration of condensed phases and geophysical phenomena. The megabar regime represents an exciting frontier, where recent discoveries include novel high-temperature superconductors, as well as structural and valence phase transitions. However, at such high pressures, many conventional measurement techniques fail. Here, we demonstrate the ability to perform local magnetometry inside of a diamond anvil cell with sub-micron spatial resolution at megabar pressures. Our approach utilizes a shallow layer of Nitrogen-Vacancy (NV) color centers implanted directly within the anvil; crucially, we choose a crystal cut compatible with the intrinsic symmetries of the NV center to enable functionality at megabar pressures. We apply our technique to characterize a recently discovered hydride superconductor, CeH$_9$. By performing simultaneous magnetometry and electrical transport measurements, we observe the dual signatures of superconductivity: local diamagnetism characteristic of the Meissner effect and a sharp drop of the resistance to near zero. By locally mapping the Meissner effect and flux trapping, we directly image the geometry of superconducting regions, revealing significant inhomogeneities at the micron scale. Our work brings quantum sensing to the megabar frontier and enables the closed loop optimization of superhydride materials synthesis.

Motivation & Objective

• To overcome the limitations of conventional magnetometry in high-pressure environments, particularly for small, inhomogeneous superhydride samples.
• To develop a quantum sensing platform capable of sub-micron resolution magnetometry at pressures exceeding 100 GPa.
• To directly image the spatial distribution of superconducting regions via local magnetic response, including the Meissner effect and flux trapping.
• To correlate local magnetic signatures with electrical transport measurements to validate superconductivity in CeH₉ under extreme pressure.
• To enable closed-loop optimization of superhydride synthesis by mapping spatial inhomogeneities and flux dynamics at the micron scale.

Proposed method

• Implanting shallow nitrogen-vacancy (NV) color centers into [111]-cut diamond anvils to enable high-pressure quantum sensing compatible with megabar conditions.
• Using optically detected magnetic resonance (ODMR) to measure local magnetic fields with sub-micron spatial resolution in a diamond anvil cell (DAC).
• Performing simultaneous DC and AC magnetometry alongside four-point electrical transport measurements to correlate magnetic and resistive transitions.
• Applying zero-field and field-cooling protocols to probe the Meissner effect and flux trapping behavior as a function of temperature and magnetic field history.
• Mapping spatial variations in magnetic field suppression and flux trapping to determine the geometry and size of superconducting regions.
• Using field-sweep protocols with temperature cycling to observe hysteresis in magnetization and quantify flux trapping strength.

Experimental results

Research questions

• RQ1 Can NV center-based quantum sensors achieve sub-micron spatial resolution magnetometry in diamond anvil cells under megabar pressures?
• RQ2 How do local magnetic responses—specifically the Meissner effect and flux trapping—correlate with electrical resistance in CeH₉ at high pressure?
• RQ3 What is the spatial distribution and geometry of superconducting regions in CeH₉, and how do they vary at the micron scale?
• RQ4 How does the strength and hysteresis of flux trapping depend on temperature and cooling history in CeH₉ under high pressure?
• RQ5 Can local magnetic imaging resolve inhomogeneities in superhydride materials that are obscured by global measurements?

Key findings

• The NV sensor platform successfully performed DC and AC magnetometry at pressures up to ~140 GPa, demonstrating functionality in the megabar regime.
• Local magnetic field suppression of ~20 G was observed below Tc ≈ 90 K during field cooling, confirming the Meissner effect with sub-micron spatial resolution.
• Spatial mapping revealed significant micron-scale inhomogeneities in superconducting regions, with distinct flux trapping signatures in different sample domains.
• Flux trapping was observed with a residual field of ~20 G at H_z = 0 G after field cooling to 81 K, indicating persistent currents in CeH₉.
• Hysteresis in the magnetization response was observed during field sweeps, with B_z = H_z scaling at large field amplitudes and flux trapping of ±23 G at H_z = 0 G.
• At H_z = 206 G, the magnetic transition sharpened compared to lower fields, suggesting a temperature- and field-dependent superconducting response.