Controlling Superconductivity Using Spin Currents

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A KAIST research team led by Professor Jhinhwan Lee of the Department of Physics has discovered a method to flip between superconducting and non-superconducting states within an iron-based superconductor using a type of electron microscopy. The team applied spin-polarized and non-polarized currents to locally change the magnetic order in the sample.

The team identified a basic physical principle required to develop transistors that control superconductivity and to implement novel magnetic memory at the atomic level. This study is the first report of a direct real-space observation of this type of control. In addition, this is the first direct atomic-scale demonstration of the correlation between magnetism and superconductivity.

The team controlled and observed the magnetic and electronic properties with a spin-polarized scanning tunneling microscope (SPSTM), a device that passes an atomically-sharp metal tip over the surface of a sample.  The team introduced new ways to perform SPSTM using an antiferromagnetic chromium tip. An antiferromagnet is a material in which the magnetic fields of its atoms are ordered in an alternating up-down pattern such that it has a minimal stray magnetic field that can inadvertently kill the local superconductivity of the sample when used as an SPSTM tip.

To study the connection between the C4 magnetic order and the suppression of superconductivity, the team performed high-resolution SPSTM scans of the C4 state with chromium tips and compared them with simulations. The results led them to suggest that the low-energy spin fluctuations in the C4 state cannot mediate pairing between electrons in the typical FeAs band structure. This is critical because this paring of electrons, defying their natural urge to repel each other, leads to superconductivity.

Professor Lee said, “Our findings may be extended to future studies where magnetism and superconductivity are manipulated using spin-polarized and unpolarized currents, leading to novel antiferromagnetic memory devices and transistors controlling superconductivity.”

This study was published in Physical Review Letters (PRL) on November 27 as the Editor’s Suggestion. It was also featured in Viewpoint in Physics, in which the top 3% of PRL papers are presented with a commentary. It was also featured on Phys.org, which is a science news website led by the US national research institutes. Furthermore, the equipment designed and manufactured by Professor Lee’s team and used for the research was selected for the cover of Review of Scientific Instruments (RSI) in the October 2017 issue. 

Professor Lee said, “When designing the experiment, we attempted to implement some decisive features. For instance, we included a spin control function using an antiferromagnetic probe, wide range variable temperature functions that were thought to be impossible in high-magnetic field structures, and multiple sample storage functions at low temperatures for systematic spin control experiments, rather than using simpler scanning probe microscopes with well-known principles or commercial microscopes. As a result, we were able to conduct systematic experiments on controlling magnetism and superconductivity, which competing groups would take years to replicate.”

He continued, “There were some minor difficulties in the basic science research environment such as the lack of a shared helium liquefier on campus and insufficient university-scale appreciation for large scale physics that inevitably takes time. We will do our best to lead the advancement of cutting-edge science through research projects expanding on this achievement in physical knowledge to practical devices and various technological innovations in measurements.” This research was funded by National Research Foundation of Korea.

Figure 1. Research concept illustration

The spin-polarized chromium (Cr) tip being scanned over the pristine superconducting area of the C2 magnetic order, represented in the background with electron pairs shown as coupled red spheres. The spin current through the tip induces the C4 magnetic order (yellow and blue plaquettes) with suppressed superconductivity in the sample because its spin fluctuations cannot mediate electron pairing, represented as decoupled red spheres in the plaquette area.