Zhejiang University: Scientists unravel electric field control of superconductivity at oxide interface

Lanthanum aluminate (LaAlO3) and potassium tantalite (KTaO3) are insulators, but when they are combined together, the interface can conduct electricity and even superconduct. This kind of “emerging” superconductivity” at the interface has sparked keen interest among scientists. Scholars from the Department of Physics at Zhejiang University and the Institute of Physics at the Chinese Academy of Sciences have found that the conductive properties of the LaAlO3/KTaO3 interface can be continuously tuned by voltage like a semiconductor device. Meanwhile, the research team has also observed many novel physical phenomena at this interface, such as the quantum metallic state.

Tuning is the most important tool and element of experimental scientific research. In this study, researchers discovered a brand-new modulation mechanism by which the LaAlO3/KTaO3 interface could be continuously tuned from superconducting into insulating states.

“At low temperatures, coupled electrons will generate superconductivity. There are many known superconducting systems, but few of them can be controlled by electric fields,” XIE Yanwu from Zhejiang University Department of Physics, co-corresponding author of the study, introduced. “Our method is, in essence, to modulate the spatial distribution of electrons so that they can move closer to or further away from the interface.”

When plenty of electrons are moving near the oxide interface, they will be affected by lattice defects, also known as disorder. “This is like driving into an obstacle,” XIE Yanwu said, “The closer this disorder is to the interface, the denser it will become. The further it is away from the interface, the sparser it will become.”

On the strength of this understanding, researchers proposed the idea of changing the spatial distribution of electrons. “If more electrons get close to the interface, they will encounter more ‘obstacles’ on the whole, which can significantly affect the motion of electrons as well as coupled superconducting Cooper pairs.”

In this experiment, researchers tested the conductivity of the interface as the gate voltage ranged from -200V to 150V. “The conductivity can be continuously tuned both above and below the superconducting transition temperature.”

“On the surface, our method is similar to the traditional one in that both apply a gate voltage, but the modulation mechanism behind is utterly new,” SUN Jirong said, “Conductivity is traditionally regulated by changing the concentration of electrons, be it a semiconductor transistor or the LaAlO3/SrTiO3 interface. This is based on a prerequisite that the concentration of electrons should be low. In contrast, the concentration of electrons at the LaAlO3/KTaO3 interface is exceedingly high and cannot satisfy the demands of the conventional modulation mechanism. Therefore, a completely new mechanism needs to be explored.”

As the experiment proceeded, a larger body of data came out. When researchers put them together, they were amazed to observe one horizontal line after another at low temperatures, which means that the resistance of the LaAlO3/KTaO3 interface was virtually constant no matter how the temperature varied between 0 and 1K. “Quantum metals are a novel quantum state featured by partial superconductivity and metallic idiosyncrasies,” ZHOU Yi said, “All known quantum metallic states exist only at a certain quantum critical point. This system, however, can be continuously tuned, and quantum metals exist as a phase of matter on the phase diagram. This discovery is really thrilling.”

“This discovery opens the door to exploring low-temperature quantum phenomena and provides new insights into the development of superconducting devices,” said XIE Yanwu.

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