A new study shows that iron telluride, or FeTe, long thought to be a simple magnetic metal, is actually a superconductor when excess iron atoms are removed. Superconductors carry electricity with no energy loss as heat, enabling technologies such as MRI machines, particle accelerators and potentially quantum computers. Researchers found that extra iron atoms hidden in the material are responsible for its magnetism and suppress superconductivity. Once those atoms are eliminated, electricity flows with zero resistance, and the material’s superconducting properties can be further tuned using layered structures and moiré effects.
The findings are detailed in two papers published back-to-back, both led by Penn State physicist Cui-Zu Chang. The first paper explains how to activate superconductivity in FeTe. The second describes a new type of “quantum dance” in which superconductivity interacts with the material’s atomic structure when a different top layer is added, allowing scientists to adjust its properties.
Mystery Behind FeTe’s Missing Superconductivity
“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 investigate this difference, the team created thin films of FeTe using molecular beam epitaxy. This method produces extremely clean, atomically thin materials by slowly depositing source elements onto a suitable surface.
When the researchers examined the samples at the atomic level using scanning tunneling microscopy, they found that the structure was not perfectly uniform. Extra iron atoms were embedded within the crystal lattice of FeTe.
Excess Iron Atoms Disrupt Superconductivity
“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.
To test this idea, the researchers developed a way to control the material’s purity by exposing the FeTe films to tellurium vapor. This process offsets the excess iron and pushes the material toward its ideal composition.
“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.”
Engineering Superconductivity with Layered Structures
In the second study, after confirming that FeTe is inherently a superconductor, the researchers investigated how its superconducting behavior could be controlled. They built layered structures by placing a thin material with a different crystal lattice on top of FeTe. Because the two materials have different atomic arrangements, they form a larger repeating pattern at their boundary, known as a moiré superlattice.
“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 allows imaging at the atomic scale, the team observed that superconductivity appears as a repeating, droplet-like pattern that follows the moiré superlattice, described by the researchers as a “quantum dance.” They also found that this pattern can be tuned by changing the material used 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.”
For more information: Nature
Image: A sample of a thin film of the compound iron telluride (FeTe)—the dark region on the clear substrate at the center of the image—created using molecular beam epitaxy. Long thought to be an ordinary magnetic metal, researchers have now shown that exposing the thin film of FeTe to tellurium vapor removes disorder created by excess iron atoms trapped in the crystal structure of the material, revealing that FeTe is a superconductor. Credit: Chang Laboratory/Penn State.
