Press Releases

2015.12.08

2D superconductor surviving in high magnetic fields over 50 Tesla :Professor Yoshihiro Iwasa, Department of Applied Physics.

Researchers at the University of Tokyo have discovered that a 2D superconductor realized on the surface of molybdenum disulfide (MoS2) can survive even in high magnetic fields over 50 Tesla, and succeeded in explaining the phenomenon by a mechanism that Cooper pairs are locked to the out-of-plane direction by the internal field originating from in-plane broken inversion symmetry. This result becomes the basis that opens the new field for the research in noncentrosymmetric superconductivity and also provides important insights into the development of superconductors stable in the strong magnetic fields.

 

A superconductor, which shows zero resistance and therefore can flow current without power consumption, is expected to be a next-generation material, and thus intensively investigated in terms of basic properties as well as potential applications all over the world. In many superconductors, two electrons with opposite electron spins form a Cooper pair (these special electron pairs make the superconducting state). So, the magnetic field, which is strong enough to align the spins of the pair, can make the superconducting state unstable, and eventually break it. Therefore, designing and then fabricating superconductors stable under strong magnetic fields is one of central issues in the world. As candidates such materials, two-dimensional superconductors in many different forms are intensively being intensively investigated.

 

The research group headed by Professor Yoshihiro Iwasa (concurrently RIKEN research group leader) and doctoral student Yu Saito from the Department of Applied Physics and Quantum-Phase Electronics Center in the University of Tokyo, has prepared an atomically thin 2D superconductor of MoS2 with minimal disorder using an electric-double-layer transistor configuration. MoS2 is a kind of an atomic layered material, which is frequently used as dry lubricant. By using the pulsed magnet system up to 55 Tesla in International MegaGauss Science Laboratory, Institute for Solid State Physics, the research group first discovered that the superconducting state in MoS2 can remain even under in-plane magnetic fields more than 50 Tesla at 1.5 Kelvin (-271.7 degrees Celsius), which is 4 times larger than the conventional theoretical limit. Here, the value of 50 Tesla is approximately 100 times larger than the commercial neodymium magnet. In addition, using first-principles band calculations followed by realistic numerical calculations, they revealed that the spins of Cooper pairs are completely locked to the out-of-plane direction by internal magnetic fields (~ 200 Tesla), originating from the locally broken inversion symmetry in MoS2 single atomic layer, which results in the dramatic enhancement of upper critical fields. These results demonstrate that a conventional semiconductor, MoS2, can be an unconventional 2D superconductor which is very robust against external magnetic fields, just by the application of an electric field.

 

Our findings indicate that, in a 2D superconductor without in-plane inversion symmetry, an unconventional superconducting state is realized, in which it can be very robust against external magnetic fields. We expect that exotic superconducting properties and new mechanism for formation of electron pairs would be revealed in the near future.

 

This research was published in the journal Nature Physics, as a paper entitled “Superconductivity protected by spin-valley locking in ion-gated MoS2”, on 7th December 2015.

 

 

Fig. Unconventional Cooper pairing protected by spin-valley locking

The spins of Cooper pairs are completely locked to the out-of-plane direction by internal magnetic fields (~200 Tesla), originating from the locally broken inversion symmetry in MoS2 single atomic layer, which results in the dramatic enhancement of upper critical fields.

 

Abstract URL : http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3580.html

 

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