Superconductivity switched on in material once thought only magnetic

UNIVERSITY PARK, Pa. — Superconductivity — the ability of a material to conduct electricity without any energy loss to heat — enables highly efficient, ultra-fast electronics essential for advanced technologies such as magnetic resonance imaging (MRI) machines, particle accelerators and, potentially, quantum computers. New research has now revealed that iron telluride (FeTe), a compound composed of the chemical elements iron and tellurium and long thought to be an ordinary magnetic metal, is in fact a superconductor. The researchers found that hidden excess iron atoms induce the material’s magnetism, and removing these atoms allows electricity to flow with zero resistance.

Two papers describing the research, both led by Penn State Professor of Physics Cui-Zu Chang, published back-to-back today (April 1) in the journal Nature. The first paper focuses on how to “switch on” superconductivity in FeTe, while the second paper reveals a new kind of “quantum dance,” where superconductivity interacts with the material’s atomic structure when a different top layer is added, allowing researchers to tune its behavior.

“Unlike the well-known iron-based superconductor iron selenide (FeSe), FeTe has long been considered a magnetic metal without superconductivity, despite having an almost identical crystal structure,” Chang said. “It has remained a mystery why FeTe doesn’t share this important property.”

To explore why these two closely related compounds behave so differently, the research team grew FeTe thin films using a technique called molecular beam epitaxy. This technique creates atomically thin, exceptionally clean samples by co-evaporating source materials onto appropriate substrates. However, when the researchers looked closely at the FeTe samples they created at the atomic scale using a specialized microscope, called scanning tunneling microscopy, they saw that the material was not perfectly ideal. Extra iron atoms were embedded within the crystal lattice of FeTe.

“These excess iron atoms disrupt the ideal one-to-one ratio of iron and tellurium atoms in FeTe and upset the balance of magnetism and superconductivity,” Chang said, explaining that the researchers theorized that removing the excess atoms to make truly pure FeTe might result in a superconductor.

The team came up with a method to precisely control the purity of FeTe by exposing the FeTe films to an environment with tellurium vapor. This compensated for the excess iron atoms and drove the material towards an ideal state.

“The resulting ideal FeTe exhibits superconductivity with a critical temperature of around 13.5 Kelvin, or about negative 435 degrees Fahrenheit,” Chang said. “The excess iron atoms had disguised its superconductivity, leading to the decades-old view that FeTe was an ordinary magnetic metal. Our findings redefine the phase diagram of this class of iron containing compounds. Similar phenomena are likely to be present in other correlated materials, where hidden superconducting states or competing magnetic orders remain concealed until disorder is removed or carefully controlled. Understanding the crucial role of disorder will help us to uncover and stabilize such hidden superconducting states in other materials.”

In the second paper, having established that FeTe is intrinsically a superconductor, the team further explored how its superconducting state itself can be engineered. The team created layered structures by growing a thin material with a different lattice structure on top of FeTe. Because the two materials have different atomic arrangements, a larger repeating pattern — called a moiré superlattice — forms at their interface.

“The mismatch between the crystal structures at the interface creates what we call a moiré superlattice, which modifies the superconducting properties of FeTe,” Chang said. “In recent years, moiré superlattices in two‑dimensional materials have emerged as an important platform for discovering new quantum states.”

Using scanning tunneling microscopy, which can image materials at the atomic scale, the team directly observed that superconductivity forms a repeating, droplet-like pattern — what the researchers describe as a “quantum dance” — that follows the moiré superlattice. They also found that this pattern can be adjusted by changing the material in the top layer.

“The role of crystal lattices has often been overlooked in superconductors,” Chang said. “Our findings encourage a renewed focus on the interplay between superconductivity and lattice structure and highlight how moiré interface engineering can serve as a potentially powerful tool for tuning superconductivity and designing next‑generation quantum materials.”

In addition to Chang, the research team for the first paper, titled “Stoichiometric FeTe is a Superconductor,” included Zi-Jie Yan, Zihao Wang, Bing Xia, Stephen Paolini, Hongtao Rong, Pu Xiao, Jiatao Song and Veer Gowda from the Penn State Department of Physics; Nikalabh Dihingia, Kalana D. Halanayake and Danielle Reifsnyder Hickey from the Penn State Department of Chemistry; Ying-Ting Chan and Weida Wu from Rutgers University; and Jiabin Yu and Peter J. Hirschfeld from the University of Florida. The research was supported by the U.S. Department of Energy (DOE), with additional support from the U.S. National Science Foundation, the Office of Naval Research (ONR), the Army Research Office, the Penn State MRSEC for Nanoscale Science, the University of Florida, and the Gordon and Betty Moore Foundation’s EPiQS Initiative.

The research team for the second paper titled “Moiré engineering of Cooper-pair density modulation states,” included Chang, Zihao Wang, Bing Xia, Stephen Paolini, Zi-Jie Yan, Pu Xiao, Jiatao Song, Veer Gowda and Hongtao Rong from the Penn State Department of Physics; Di Xiao and Xiaodong Xu from the University of Washington; Weida Wu from Rutgers University; and Ziqiang Wang from Boston College. The DOE, ONR, Penn State MRSEC for Nanoscale Science, Air Force Office of Scientific Research and Gordon and Betty Moore Foundation’s EPiQS Initiative funded the research.