Scientists have achieved a first in studying lanthanum superhydrides, a class of materials that could help unlock superconductivity at much higher temperatures. The dream of transmitting electricity without energy loss has driven decades of superconductivity research. Some of the most promising candidates yet are superhydrides, hydrogen-rich materials that, under immense pressures, have exhibited superconducting behavior [...]

Pristine iron telluride is a superconductor, with the natural material’s superconductivity suppressed by excess iron in the crystal lattice, researchers in the US have shown. This resolves a long-standing puzzle about why, when other materials with similar structures showed superconductivity at low temperatures, iron telluride had always retained an antiferromagnetic order. The results provide a secure platform for further exploration of iron-based superconductivity, and could open the door to the study of interesting physics such as potential topological superconductivity in iron telluride itself.

Much like the cuprates, iron-based superconductors such as chalcogenides like iron selenide often exhibit complex phase diagrams in which antiferromagnetic ground states compete with superconducting ones. Although tellurium sits directly underneath selenium in the periodic table, superconductivity has never been observed in pure iron telluride. It can behave as a “parent compound” for inducing superconductivity via chemical substitution with selenium, for example.

“One thing that’s always been a puzzle in the field is that the magnetic structure of iron telluride is fundamentally different from that of all other iron-based superconductors,” says condensed matter physicist Pengcheng Dai of Rice University in Texas; “People say ‘Oh, it’s more correlated’ – but the problem with that is that when you dope it with selenium and it does become superconducting, all the electric and magnetic properties occur at the exact same wave vector as other iron-based superconductors.”

Barely discussed

Condensed matter experimentalist Cui-Zu Chang of Pennysylvania State University in the US and colleagues had conducted multiple experiments involving the growth of tellurium compounds on iron telluride substrates, and reliably found that these produced supercondivity. Nevertheless, says Chang, the possibility that iron telluride itself might have a superconducting state was barely discussed by theorists.

Following Chang’s philosophy that “for superconductivity, if you follow theory and try to do something, 99% of the time you will fail,” the researchers set out to ascertain the state of pristine iron telluride experimentally. They bombarded a strontium titanate substrate with high purity beams of gaseous iron and tellurium atoms to produce 40-layer-thick films of iron tellurium. When they examined these using a scanning tunnelling microscope, they found that the films showed antiferromagnetic order. However, electron microscopy showed that the structures contained excess iron atoms clustered together periodically.

The researchers therefore performed multiple cycles of post-growth annealing, bombarding the structure with pure tellurium. These reacted with the interstitial iron, removing it from the structure by forming more iron telluride on the surface. The researchers monitored the electrical behaviour of the sample in tandem with its structural evolution, finding that, as regions approached stoichiometric FeTe, the antiferromagnetic order disappeared. After five cycles of annealing, the material was pure iron telluride, and the researchers showed that it behaved as a robust superconductor with a critical temperature of around 13.5 K. They confirmed this with the observation of the Josephson effect, Cooper-pair tunnelling and other related phenomena.

The researchers now intend to study the specific properties of stoichiometric iron telluride in more detail: “Because tellurium is heavier than selenium you have stronger spin-orbit coupling, so iron telluride should be a topological insulator at the same time as it’s a superconductor,” says Chang;  “We call these topological superconductors.” Such topological superconductors – the first of which was uranium ditelluride – are of great interest in quantum computing thanks to their potential to host protected Majorana qubits. More broadly, the researchers believe it is important to study whether other materials may host “hidden” superconducting states suppressed by disorder.

Dai, who was not involved in the research, is impressed: “It’s surprising, in the sense that it solves a fundamental puzzle that’s been in the field for some time,” he says. He notes that definitive proof is not achieved because the material is on a substrate, so techniques such as neutron diffraction traditionally used to probe the magnetic structure of bulk materials are impossible. It is also possible to question whether the substrate is influencing the material. Nevertheless, he is persuaded: “At least to me, it really unifies the picture that the magnetism is probably universal for all the iron-based superconductors,” he concludes; “In the same way that in the cuprates, the parent compounds are basically Mott insulators, from this experiment we can basically say that in iron-based superconductors the parent compounds are basically simple stripes, and this oddball is because of the excess iron that stabilizes the particular structure.”

The research is described in Nature.

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Could a new pressure quenching technique help researchers move forward on the road to reaching room-temperature superconductivity? Researchers at the University of Houston are pinning their hopes on this approach and say they have already used it to achieve a record-high superconducting transition temperature (T_c) of 151 K at ambient pressure in a metastable phase of HgBa₂Ca₂Cu₃O_8+δ (or HBCCO) The phase remains stable for around at least three days when held at 77 K, although its T_c degrades when heated to above 200 K.

Achieving ambient-pressure room-temperature superconductivity remains the holy grail for scientists working in this field. This is because superconductors that work at ambient temperatures and pressures could revolutionize a host of application areas, including increasing the efficiency of electrical generators and transmission lines through lossless electricity transmission. They would also greatly simplify technologies such as magnetic resonance imaging (MRI) that rely on the generation or detection of magnetic fields.

While much progress has been made in the last decades, increasing the T_c often relies on squashing materials at extremely high pressure – usually in a device known as a diamond anvil cell (DAC). Some examples include the sulphide material H₃S, which has a T_c of 203 K when compressed to pressures of 150 GPa and the cerium hydrides, CeH₉ and CeH₁₀, which boast high-temperature superconductivity at lower pressures of about 80 GPa with a T_c of around 100 K.

HBCCO is a high-temperature superconducting cuprate that has a T_c of 133 K at ambient pressure. This can be pushed to 164 K by applying a pressure of 31 GPa to it.

High-pressure-induced metastable superconducting phase

The high T_c of HBCCO is thought to come from the high electron density of states of a possible “van Hove singularity” associated with the two-dimensional CuO₂ planes in it. In the new work, a team led by Ching-Wu Chu and Liangzi Deng of the Department of Physics and Texas Center for Superconductivity at the University of Houston decided to study a high-pressure-induced metastable superconducting phase in the material that they think might be able to form at ambient pressure as a result of this singularity (which leads to strong interactions between electrons) and/or other anomalies in the electronic energy spectrum.

To investigate further, the researchers developed a pressure-quench protocol to stabilize this metastable phase at ambient pressure. Their process involves first identifying the target phase in a DAC under high pressures of between 10–30 GPa. Next, the material is quenched (that is, the pressure is rapidly removed) at 4.2 K.

Chu and Deng confirmed that they had indeed isolated this phase and not another using synchrotron X-ray diffraction (at the 16-ID-B beamline of the Advanced Photon Source) before removing it from the cell. These measurements also show that the pressure-quenched phase at ambient pressure retains its original crystal structure, but possibly contains defects, generated under pressure and during quenching. The researchers think that these defects might help preserve the metastable high- T_c phase.

Thanks to their technique, they say they have achieved a hitherto unreported ambient-pressure T_c of 151 K.

Tiny samples

The experiments were far from easy, however, they say. The samples were extremely small (around just 50–80 microns in size), so handling them in high-pressure experiments is inherently challenging, explains Chu. Another major difficulty was preventing the electrical leads used for the resistivity measurements from breaking during the pressure-quenching process. Recovering the samples after quenching for more detailed analyses at ambient pressures was technically demanding too.

Looking ahead, the researchers say they would now like to better understand where the high T_c in HBCCO comes from – both under pressure before quenching and at ambient pressure after quenching. “We would also like to elucidate the mechanisms that lock in the high T_c phase at ambient pressure after quenching,” says Chu.

The impact of the new work, which is detailed in PNAS, might even extend beyond superconductivity, adds Deng. “Indeed, our approach could allow us stabilize quantum metastable states at ambient pressure that have enhanced or unique properties that only emerge under pressures. Based on our experimental results, using theoretical modelling and AI-driven approaches, we would like to identify different types of quantum materials that are suitable for pressure quenching.”

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