Rotational symmetry breaking in topological superconductor

Our group, in collaboration with the Institute for Materials Science in Tsukuba (Japan), has discovered an exceptional new quantum state within a superconducting material. This exceptional quantum state is characterised by a broken rotational symmetry – in other words, if you turn the material in a magnetic field, the superconductivity isn't the same everywhere in the material.

The material in which the new quantum state was discovered is bismuth selenide, or Bi2Se3. This material is a topological isolator. This group of materials exhibits a strange quality: they don't conduct electricity on the inside, but only on their surface. What's more, the researchers are able to make the material even more exceptional – by adding a small amount of strontium to the bismuth-selenide, the material transforms into a superconductor. This means the material can conduct electricity extremely well at low temperatures, because the electrical resistance has completely disappeared.

Electron seeks mate
Superconductivity can be explained by the behaviour of electrons within the material. In a superconductor, certain electrons seek a mate and combine into pairs. These pairs, so-called Cooper pairs, can move through the material without resistance or a loss of energy.

Broken symmetry
The research team placed the material in a magnetic field that suppresses the superconducting properties of the material. Bismuth selenide has a layered crystalline structure, and the magnetic field the researchers used was directed parallel to the plane of these layers. Usually, it makes no difference in which direction the magnetic field points, because the suppression is the same in all directions. However, the researchers discovered that this isn't the case with their exceptional material. When they turned the magnetic field in the plane of the layers, they discovered that the superconductivity was suppressed to a greater and to a lesser extent, depending on the direction in which the field pointed. In other words, the material's rotational symmetry was broken.

Preferred direction
The phenomenon of broken symmetry can only be explained if the electrons in this material form special Cooper pairs, namely spin-triplet pairs, instead of the usual spin-singlet pairs. Such Cooper pairs can adopt a preferred direction within the crystal.

New lab tools
The team, including FOM phd students Yu Pan en Artem Nikitin, points out that the discovery proves that this exceptional material forms a unique laboratory tool. The superconductor will allow physicists to study the exceptional quantum effects of topological superconductivity.


Direct bulk sensitive probe of 5f symmetry in URu2Si2

URu2Si2 is a mysterious quantum material where an ordering transition was found at 17 degrees above absolute zero temperature. However, so far, there is no satisfying answer to the question of what orders and how. In an international collaboration we provide a new important piece to solve this puzzle.

Excerpt of the press release:
By now it is generally accepted that the distribution of the electronic density of the uranium atoms at low temperatures is an important key for understanding the hidden order. However, measuring these densities has so far not been possible. This is where the team of the Max-Planck Institute for Chemical Physics of Solids in Dresden in collaboration with the group of Dr. Andrea Severing from the Institute of Physics II at the University of Cologne and scientists from the Van-der-Waals-Zeeman Institute at the University of Amsterdam in the Netherlands approaches the hidden order problem. The result of this work has now been published under the title Direct bulk sensitive probe of 5f symmetry in URu2Si2 in the Proceedings of the National Academy of Sciences.

Thanks to modern inelastic scattering methods with x-rays (IXS), that are available e.g. at the European Synchrotron Radiation Facility (ESRF) in Grenoble in France, the relevant 5f electron density of uranium in URu2Si2 could be measured. The experimental results limit the number of hidden order scenarios and are therefore an important touch-stone for theoretical models.
The work is published in the Proceedings of the National Academy of Science.

YPtBi: a possible half-Heusler unconventional superconductor

We have published a new paper in Solid State Comm. From susceptibility and muon spin rotation measurements we extract an upper bound on the spontaneous magnetic field that would be produced by an odd-parity order parameter.

musr_ssc In this publication we report new results on the low-field magnetic response of the non-centrosymmetric superconductor YPtBi (Tc 0.77 K) in Solid state communications. We find a surprisingly small lower critical field. From muon spin rotation experiments we are able to deduce an upper bound of 0.04 mT on the spontaneous magnetic field associated with a possible odd-parity component in the superconducting order parameter.