Uncovering origins of heterogeneous superconductivity in La$_3$Ni$_2$O$_7$ using quantum sensors

The family of nickelate superconductors have long been explored as analogs of the high temperature cuprates. Nonetheless, the recent discovery that certain stoichiometric nickelates superconduct up to high $T_c$ under pressure came as a surprise. The mechanisms underlying the superconducting state remain experimentally unclear. In addition to the practical challenges posed by working in a high pressure environment, typical samples exhibit anomalously weak diamagnetic responses, which have been conjectured to reflect inhomogeneous `filamentary’ superconducting states. We perform wide-field, high-pressure, optically detected magnetic resonance spectroscopy to image the local diamagnetic responses of as grown La$_3$Ni$_2$O$_7$ samples \emph{in situ}, using nitrogen vacancy quantum sensors embedded in the diamond anvil cell. These maps confirm significant inhomogeneity of the functional superconducting responses at the few micron scale. By spatially correlating the diamagnetic Meissner response with both the local tensorial stress environment, also imaged \emph{in situ}, and stoichiometric composition, we unravel the dominant mechanisms suppressing and enhancing superconductivity. Our wide-field technique simultaneously provides a broad view of sample behavior and excellent local sensitivity, enabling the rapid construction of multi-parameter phase diagrams from the local structure-function correlations observed at the sub-micron pixel scale.

💡 Research Summary

In this work the authors address the long‑standing puzzle of why the bulk nickelate La₃Ni₂O₇ shows highly inhomogeneous superconductivity under pressure, often described as “filamentary”. They develop a novel high‑pressure microscopy platform that embeds a shallow layer of nitrogen‑vacancy (NV) color centers inside the diamond anvils of a DAC. By performing wide‑field optically detected magnetic resonance (ODMR) on thousands of NVs simultaneously, they obtain sub‑micron maps of both the local magnetic field (through the Zeeman shift) and the full stress tensor (through the stress‑induced shift of the NV spin levels).

Three floating‑zone grown single crystals (S1, S2, S3) are loaded into separate DACs and electrically contacted for four‑probe resistance measurements. The authors apply a uniform external magnetic field along the DAC axis, cool the samples under zero‑field (ZFC) or field‑cooling protocols, and record the ratio s = B/H at each pixel. s < 1 indicates magnetic‑field suppression (Meissner screening), s > 1 indicates field line crowding at the edge of a superconducting region, and s ≈ 1 corresponds to a non‑superconducting area.

The key observations are:

1. Superconducting dome in normal stress – At average σzz ≈ 21 GPa the maps show a clear annular region of s < 1 surrounded by a thin s > 1 rim, both co‑located with the crystal. As σzz is increased further the diamagnetic signal weakens and the spatial extent shrinks, reproducing the familiar pressure‑temperature dome but now resolved in space. The local transition temperature extracted from s(T) matches the temperature at which the bulk resistance first drops, confirming that the ODMR signal is indeed the Meissner effect.

2. Shear‑stress suppression – By exploiting the NV’s sensitivity to the off‑diagonal stress components (σxz, σyz) the authors construct a map of the shear magnitude τ = √(σxz² + σyz²). They find that regions where τ exceeds ≈ 2 GPa never develop a diamagnetic response, even if σzz is locally high. Conversely, low‑τ zones become superconducting as soon as σzz passes a threshold. This establishes a three‑dimensional phase diagram T‑σzz‑τ, showing that shear stress is a decisive antagonistic parameter that cannot be captured by a scalar pressure.

3. Chemical inhomogeneity – Sample S3 deliberately exhibits sizable stoichiometric variations as measured by energy‑dispersive X‑ray spectroscopy. In this crystal, pockets of strong diamagnetism appear without any global resistance drop; these pockets coincide with regions of optimal La:Ni ratio. Thus the “filamentary” superconductivity reported in transport is simply the macroscopic averaging of a mosaic of locally optimal and sub‑optimal chemical domains.

4. Flux trapping – After field‑cooling in 150 G and turning the field off, the NV maps reveal remanent fields up to ~10 G localized exactly where the strongest s < 1 signal was observed. This is the first direct imaging of flux trapping in a nickelate and demonstrates that the superconducting regions behave like a type‑II superconductor capable of pinning vortices.

5. Methodological impact – The simultaneous acquisition of magnetic, stress, and compositional maps at sub‑micron resolution under >10 GPa pressure is unprecedented. It allows the authors to disentangle the intertwined roles of normal stress, shear stress, and chemistry, providing a quantitative explanation for the sample‑to‑sample variability that has plagued the field.

Overall, the paper delivers four major contributions: (i) a high‑pressure NV‑based imaging platform, (ii) a quantitative demonstration that normal stress promotes superconductivity while shear stress above ~2 GPa quenches it, (iii) direct evidence that local stoichiometry controls where superconductivity nucleates, and (iv) the first visualization of vortex trapping in La₃Ni₂O₇. These findings reshape the way we think about pressure‑tuned superconductivity in nickelates and open a pathway for similar multi‑parameter imaging of other quantum materials under extreme conditions.