Study reveals large tunable drag response between normal conductor and superconductor

A giant drag effect is discovered between a graphene layer and an interfacial superconductor, which can be attributed to a unique interaction between normal electrons and dynamic fluctuations of superconducting phases mediated by static Coulomb fields. Credit: Tao et al.

Coloumb drag is a phenomenon that affects two electronic circuits, whereby a load current in one circuit induces a response current in a neighboring circuit only through so-called Coloumb interactions. They are electrostatic interactions between electric charges that follow Coulomb’s law, the key physical theory describing classical electrodynamics.

As a rule, this phenomenon has been studied using neighboring circuits made of conductive materials or electrical conductors. They are basically materials through which electricity can flow easily.

Researchers from the University of Science and Technology of China recently explored what happens when a circuit is based on a conductor and another neighbor on a superconductor (i.e. materials that offer no resistance to electric current). Their findings, published in Natural Physicsshow that in these cases the drag response is significantly greater than that previously observed in studies using two normal conductors.

“Drag experiment between two electrically insulated conductors was an effective approach to detect elemental excitations and reveal interlayer phase coherence,” said Changgan Zeng, one of the researchers who conducted the study, at Phys. org. “Replacing one of the conductors with a superconductor may open up opportunities to examine the effects of superconductivity and fluctuation as well as to explore new techniques for manipulating superconducting circuits.”

The first drag experiments using conductors and superconductors were carried out in the 1990s. The devices used at the time, however, were based on classical metal-superconductor double films, such as Au/Ti-AlOX.

The drag responses observed in these experiments were rather weak and uncontrolled. Moreover, the researchers were unable to clarify the microscopic origin of the drag effect they observed.

“Thanks to new emerging two-dimensional (2D) materials, we were able to revisit the problem, because the electronic properties in it are highly tunable, and ultra-small interlayer separation is also archivable,” said Lin Li, who designed and supervised this work. with Zeng.

“Our experimental group at USTC, led by Professor Zeng, has a long experience in device fabrication and in studying the transport properties of 2D materials. We naturally designed the unique graphene-LaAlO3/SrTiO3 heterostructure to study the drag effect in the ultimate 2D limit.”

The heterostructure that Zeng and his colleagues used in their experiments was fabricated using a layer of lanthanum aluminate (LAO) as a natural insulating spacer between conductive graphene and a 2D electron gas that formed at the interface between LAO and a layer of strontium titanate (STO). , which becomes superconducting at low temperatures.

The researchers then tuned several parameters of their system, including its temperature, magnetic field, and gate voltages. In doing so, they observed a large and tunable tailing signal in the superconducting transition regime of the LAO/STO interface.

“The optimum passive-active ratio (PAR) is much higher than the typical drag signal between two normal conductors as well as that between Au/Ti and SC AlOX obtained in existing studies,” Li said. “The giant values ​​and the anomalous temperature and carrier dependence of the PAR indicate that a novel drag mechanism is hidden behind our observations.

Dr. Hong-Yi Xie, a theoretical physicist at the Beijing Academy of Quantum Information Sciences who recently moved to the University of Oklahoma, used modern quantum many-body theory to explain observations of the ‘team. Specifically, he developed a theoretical description of what happens when a Coulomb-coupled normal conductor is paired with a superconductor.

“Finally, we revealed that the observed drag phenomenon can be attributed to the dynamic coupling between the quantum fluctuations of the SC phases of a Josephson junction-lattice superconductor and the charge densities in the normal conductor, which we called Josephson- Colulomb (JC) trail effect,” Zeng said. “The Unveiled JC Drag Effect Creates a New Category in Drag Physics and Manifests the Unique Role of Quantum Fluctuations in Dominating Interlayer Processes.”

Recent work by this team of researchers shows that the drag response between a normal conductor and a superconductor can be much greater than that between two normal conductors. This finding could have important implications for both physics research and technology development.

The JC drag unveiled by the researchers could prove particularly promising for the creation of new electronics. Specifically, it could contribute to the creation of components based on superconductors that could function as current or voltage transformers.

“In our next work, we would first like to perform drag experiments between two 2D superconductors,” Zeng added. “Additionally, we plan to study emergent interlayer coupling between larger 2D systems that exhibit various quantum phases by parameter tuning, i.e. 2D semimetal/topological insulator and 2D ferromagnet. We aim to discover new many-body effects due to strong interlayer coupling between various elemental excitations.”

More information:
Ran Tao et al, Josephson–Coulomb drag effect between graphene and a LaAlO3/SrTiO3 superconductor, Natural Physics (2023). DOI: 10.1038/s41567-022-01902-7

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