Study Reveals Large Tunable Drag Response Between Normal Conductor And Superconductor

Study Reveals Large Tunable Drag Response Between Normal Conductor And Superconductor

Study Reveals Large Tunable Drag Response Between Normal Conductor And Superconductor

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

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

Usually this phenomenon was investigated using neighboring circuits made of conductive materials, or electric conductors. These are essentially materials through which electricity can easily flow.

Researchers at the University of Science and Technology of China have recently explored what happens when a circuit is based on one conductor and a neighboring circuit is based on a superconductor (that is, materials that offer no resistance to electric current). Their findings, published in physics of natureshow that in these cases the drag response is significantly greater than previously observed in studies using two normal conductors.

“The drag experiment between two electrically insulated conductors has been an effective approach to detect elemental excitations and reveal phase coherence between layers,” Changgan Zeng, one of the researchers who carried out the study, told “Replacing one of the conductors with a superconductor may open up opportunities to examine superconductivity and fluctuation effects, as well as explore new techniques for manipulating superconducting circuits.”

The first drag experiments with conductors and superconductors were performed in the 1990s. However, the devices used at the time were based on conventional metal superconducting double films, such as Au/Ti-AlOX

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

“Thanks to the new two-dimensional (2D) materials, we were able to review the problem, since the electronic properties there are highly adjustable, and ultra-small interlayer gap can also be filed,” said Lin Li, who designed and supervised this work together with Zeng.

“Our experimental group at USTC led by Professor Zeng has long experience in device fabrication and in investigating the transport properties of 2D materials. Naturally, we designed the unique Graphene-LaAlO3/SrTiO3 heterostructure to study the drag effect at the 2D ultimate boundary”.

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

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

“The optimal passive to active ratio (PAR) is much higher than the typical drag signal between two normal conductors, as well as 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 new drag mechanism is hidden behind our observations.”

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

“Finally, we reveal that the observed drag phenomenon can be attributed to dynamic coupling between the quantum fluctuations of the SC phases of a Josephson junction matrix superconductor and the charge densities in the normal conductor, which we term Josephson-Colulomb (JC ) drag effect,” Zeng said. “The revealed JC drag effect creates a new category in drag physics and manifests the unique role of quantum fluctuations in the domain of interlayer processes.”

The recent work of this team of researchers shows that the entrainment response between a driver and a superconductor can be much larger than the one between two normal conductors. This finding could have significant implications for both physical research and technological development.

The JC drag presented by the researchers could prove particularly promising for the creation of new electronic products. Specifically, it could contribute to the creation of superconductor-based components that could function as current or voltage transformers.

“In our next works, we would first like to carry out drag experiments between two 2D superconductors,” Zeng added. “In addition, we are planning to investigate the emerging interlayer coupling between broader 2D systems that exhibit various quantum phases through parameter tuning, i.e., 2D semimetal/topological insulator and 2D ferromagnet. Our goal is to discover new many-body effects due to the strong interlayer coupling between various elementary excitations”.

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

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