Physicists at the Chinese Academy of Sciences (CAS) have used diamond-based quantum sensors to uncover what they say is the first unambiguous experimental evidence for the Meissner effect – a hallmark of superconductivity – in bilayer nickelate materials at high pressures. The discovery could spur the development of highly sensitive quantum detectors that can be operated under high-pressure conditions.
Superconductors are materials that conduct electricity without resistance when cooled to below a certain critical transition temperature Tc. Apart from a sharp drop in electrical resistance, another important sign that a material has crossed this threshold is the appearance of the Meissner effect, in which the material expels a magnetic field from its interior (diamagnetism). This expulsion creates such a strong repulsive force that a magnet placed atop the superconducting material will levitate above it.
In “conventional” superconductors such as solid mercury, the Tc is so low that the materials must be cooled with liquid helium to keep them in the superconducting state. In the late 1980s, however, physicists discovered a new class of superconductors that have a Tc above the boiling point of liquid nitrogen (77 K). These “unconventional” or high-temperature superconductors are derived not from metals but from insulators containing copper oxides (cuprates).
Since then, the search has been on for materials that superconduct at still higher temperatures, and perhaps even at room temperature. Discovering such materials would have massive implications for technologies ranging from magnetic resonance imaging machines to electricity transmission lines.
Enter nickel oxides
In 2019 researchers at Stanford University in the US identified nickel oxides (nickelates) as additional high-temperature superconductors. This created a flurry of interest in the superconductivity community because these materials appear to superconduct in a way that differs from their copper-oxide cousins.
Among the nickelates studied, La3Ni2O7-δ (where δ can range from 0 to 0.04) is considered particularly promising because in 2023, researchers led by Meng Wang of China’s Sun Yat-Sen University spotted certain signatures of superconductivity at a temperature of around 80 K. However, these signatures only appeared when crystals of the material were placed in a device called a diamond anvil cell (DAC). This device subjects samples of material to extreme pressures of more than 400 GPa (or 4 × 106 atmospheres) as it squeezes them between the flattened tips of two tiny, gem-grade diamond crystals.
The problem, explains Xiaohui Yu of the CAS’ Institute of Physics, is that it is not easy to spot the Meissner effect under such high pressures. This is because the structure of the DAC limits the available sample volume and hinders the use of highly sensitive magnetic measurement techniques such as SQUID. Another problem is that the sample used in the 2023 study contains several competing phases that could mix and degrade the signal of the La3Ni2O7-δ.
Nitrogen-vacancy centres embedded as in-situ quantum sensors
In the new work, Yu and colleagues used nitrogen-vacancy (NV) centres embedded in the DAC as in-situ quantum sensors to track and image the Meissner effect in pressurized bilayer La3Ni2O7-δ. This newly developed magnetic sensing technique boasts both high sensitivity and high spatial resolution, Yu says. What is more, it fits perfectly into the DAC high-pressure chamber.
Next, they applied a small external magnetic field of around 120 G. Under these conditions, they measured the optically detected magnetic resonance (ODMR) spectra of the NV centres point by point. They could then extract the local magnetic field from the resonance frequencies of these spectra. “We directly mapped the Meissner effect of the bilayer nickelate samples,” Yu says, noting that the team’s image of the magnetic field clearly shows both a diamagnetic region and a region where magnetic flux is concentrated.
Weak demagnetization signal
The researchers began their project in late 2023, shortly after receiving single-crystal samples of La3Ni2O7-δ from Wang. “However, after two months of collecting data, we still had no meaningful results,” Yu recalls. “From these experiments, we learnt that the demagnetization signal in La3Ni2O7-δ crystals was quite weak and that we needed to improve either the nickelate sample or the sensitivity of the quantum sensor.”
To overcome these problems, they switched to using polycrystalline samples, enhancing the quality of the nickelate samples by doping them with praseodymium to make La2PrNi2O7. This produced a sample with an almost pure bilayer structure and thus a much stronger demagnetization signal. They also used shallow NV centres implanted on the DAC cutlet (the smaller face of the two diamond tips).
“Unlike the NV centres in the original experiments, which were randomly distributed in the pressure-transmitting medium and have relatively large ODMR widths, leading to only moderate sensitivity in the measurements, these shallow centres are evenly distributed and well aligned, making it easier for us to perform magnetic imaging with increased sensitivity,” Yu explains.
These improvements enabled the team to obtain a demagnetization signal from the La2PrNi2O7 and La3Ni2O7-δ samples, he tells Physics World. “We found that the diamagnetic signal from the La2PrNi2O7 samples is about five times stronger than that from the La3Ni2O7-δ ones prepared under similar conditions – a result that is consistent with the fact that the Pr-doped samples are of a better quality.”
Physicist Jun Zhao of Fudan University, China, who was not involved in this work, says that Yu and colleagues’ measurement represents “an important step forward” in nickelate research. “Such measurements are technically very challenging, and their success demonstrates both experimental ingenuity and scientific significance,” he says. “More broadly, their result strengthens the case for pressurized nickelates as a new platform to study high-temperature superconductivity beyond the cuprates. It will certainly stimulate further efforts to unravel the microscopic pairing mechanism.”
As well as allowing for the precise sensing of magnetic fields, NV centres can also be used to accurately measure many other physical quantities that are difficult to measure under high pressure, such as strain and temperature distribution. Yu and colleagues say they are therefore looking to further expand the application of these structures for use as quantum sensors in high-pressure sensing.
They report their current work in National Science Review.
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