Researchers discovered a new phase of matter with β-Bi2Pd superconductor

JULY 6, 2021

The collaboration QNS – San Sebastián (DIPC and CFM) is bearing its fruits. This article summarizes a first research stay by Dr. Deung-Jang Choi from San Sebastián at QNS. She was hosted at QNS and had access to great collaborations and great equipment. Indeed, the work was performed at a new milli-Kelvin facility, permitting QNS and San Sebastián researchers to create nanostructures on the surface of a special superconductor: the β-Bi2Pd one.
Superconductivy is the only example of macroscopic quantum coherence. At low temperature, the electronic properties of a superconductor are described by a unique wave function. This is possible because electrons pair into combined particles that themselves undergo condensation into a single many-body wavefunction. The superconductor itself behaves like a single particle.

Electrons join with opposite spins, and are thus very sensitive to magnetic interactions. Indeed, superconductivity and magnetism are antagonistic. Using atomic magnetic impurities on a superconductor is a way of controlling the quantum coherence of the superconductor on the atomic scale.

In this first QNS-San Sebastián work, research from both institutions placed Cr atoms on a β-Bi2Pd material. This material was firstly synthesized by the Japanese team directed by Prof. Maeda, co- workers of the present article in the Physical Review. It has several interesting properties. Bi atoms host the largest possible spin-orbit coupling, permitting us to access special magnetic phases. And Pd is reactive, permitting us to craft atomic structures ontop of it. The work consists in creating two different types of Cr atomic chains on the surface. These chains interact with the macroscopic wave function of the superconductor and alter it. As a consequence new phases of matter can be created. The work shows that this strategy is valid to create new topological phases that host Majorana fermions.

The Majorana fermion is a particle that is its own antiparticle. When it happens on a superconductor it develops strange coherent properties that can be used in robust quantum computation. Indeed, quantum computation using Majorana particles is virtually error-free because Majorana particles are immune to decoherence and local interactions.

The joint work at QNS shows that a future of Majorana computing may be possible by using nansotructures of Cr atoms crafted on β-Bi2Pd. To achieve this, sophisticated instrumentation working at milli-kelvin temperatures is needed as this work shows. New possibilities are created thanks to the present publication and a future exciting collaboration is just starting.

Atomic manipulation of in-gap states in the β − Bi2 Pd superconductor

Cristina Mier, Jiyoon Hwang, Jinkyung Kim, Yujeong Bae, Fuyuki Nabeshima, Yoshinori Imai, Atsutaka Maeda, Nicolás Lorente, Andreas Heinrich, and Deung-Jang Choi

PHYSICAL REVIEW B 104, 045406 (2021)