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."
