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In their quest to explore and characterize high-temperature superconductors, physicists have mostly focused on a material that is not the absolute highest. That's because that crystal is much easier to split into uniform, easily measurable samples. But in 2024, researchers found a way to grow good crystals that are very similar to the highest temperature superconductor.

Now, many from the same group have analyzed these new crystals and determined why the highest temperature superconductor is indeed higher and what details were missed by looking at the more popular crystal. Their work is in Â鶹ÒùÔºical Review Letters.

The cuprate Bi2223, which at ambient pressure (about 100,000 pascals) superconducts at 110 Kelvin (-163°C), has proven easier to study and specify, even though the similar cuprate Hg1223 superconducts at 134 K.

Cuprates have crystal structures that crucially contain copper-oxygen (CuO₂) planes and are often ceramics. The latter cuprate is a mercury-based cuprate with the chemical formula HgBa₂Ca₂Cu₃O_8+δ, where the delta symbol δ represents a variable amount of excess oxygen in the material's crystalline structure.

Both Hg1223 and Bi2223 are trilayer cuprates, meaning the unit cell consists of three layers of CuO₂ planes. In the first, mercury (Hg) is inside the unit cell (plus several other atoms), while in the second it's bismuth (Bi).

In 2024, a Japanese group with many of the same co-authors an (Hg,Re)1223 cuprate with the same structure as Hg1223 but with some mercury atoms replaced by rhenium (Re) atoms, which helped stabilize the crystal. It had a critical superconducting temperature of 130 K.

It is the latter cuprate that lead author Masafumi Horio at the University of Tokyo and colleagues analyzed. To do so, they used angle-resolved photoemission spectroscopy (ARPES), a popular experimental method for investigating superconductors.

ARPES is a way to map a material's electronic energy levels as a function of momentum, which in turn provides key properties such as energy gaps, electron interactions and the relationship between electron energies and their in the material's electronic band structure.

The technique involves shining ultraviolet or X-ray light of a single wavelength onto the material and measuring the energy and momentum (emission angle) of the electrons emitted by the photoelectric effect.

These ejected electrons are captured and analyzed using a high-resolution electron spectrometer, with their binding energy then determined from their kinetic energy relative to the incoming photon energies and the material's —how much energy it takes an electron to escape the material.

With the binding energy from the ARPES technique, the binding energy of the electron pairs ("Cooper pairs") can be determined. They carry the superconductor's supercurrent, which travels without electrical resistance. These pairings create a gap in the spectrum of allowed electron energy states, called the superconducting gap, and all excitations of the material must be at least as high as the top of this gap—they must possess a minimum amount of energy determined by the gap.

The gap leads to superconductivity because smaller excitations, such as electron scattering, are forbidden. The wider the energy gap, the higher the critical temperature for the superconductivity in a nearly linear relationship.

Horio and colleagues used ARPES to understand (Hg,Re)1223. They found that the superconducting gap of (Hg,Re)1223's inner CuO₂ layer was quite similar to Bi2223's inner layer—63 to 62 millielectronvolts (meV).

It was already known that Bi2223's inner CuO₂ layer had a much wider superconducting gap than its two outer layers—apparently the inner layer is responsible for its high superconducting temperature of 110 K.

However, the outer layers of (Hg,Re)1223 were found to have a much higher superconducting gap than the outer layers of Bi2223, 57 meV to 43 meV. This pushed up the critical temperature of (Hg,Re)1223 to 130 K, and presumably the same is true for the original Hg1223.

"While strong pairing in the [inner plane] has been highlighted as the key element of trilayer cuprates," the group writes in their paper, "the present results suggest that the enhanced pairing energy in the [outer plane] is essential for the highest Tc [critical temperature] at ambient pressure realized in the Hg-based trilayer cuprate."

Having demonstrated the feasibility of such ARPES studies, expect more thorough investigations into the finer features of the emitted electrons and thus the superconductor parameters. This will allow precise quantification of the various couplings in the cuprate interlayers as well as the intralayer ones ().

"Such efforts will pave the way for narrowing down the essential ingredients for the high T_c of the Hg-based trilayer cuprates, and eventually for exploring a path to unlocking the current limitation of T_c at ambient pressure."