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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.


Knobs control Dirac energy in topological insulators

Four knobs - bulk stoichiometry, surface decoration, temperature, and photon exposure - are shown to control the energy of Dirac surface states in topological insulators, and can be used to determine the true 'flat band' energy band alignment at the surface of binary, ternary or quaternary 3D TI's.

Four different knobs can be used to control the Dirac energy in topological insulators, in this way one can tune the surface electronic structure of 3D TI's. This work is just out in PRB.


Top: the surfaces of all five TI's are n-type after band bending. Bottom: using the illumination 'knob', flat band conditions are restored and the true Fermi level position emerges.

Top: the surfaces of all five TI's are n-type after band bending.
Bottom: using the illumination 'knob', flat band conditions are restored and the true Fermi level position emerges.

 


Interfacing 3D topological insulator BSTS with Ag, Fe and Nb published in PRB

PRB now out on the electronic structure of interfaces between the bulk insulating 3D topological insulator BSTS and a commonly used contact metal (Ag), a magnetic metal (Fe) and a superconductor (Nb) metal on the bulk TI BSTS.

Interfacing 3D topological insulator BSTS. Nick de Jong's paper is now out in PRB on the electronic structure of interfaces between the bulk insulating 3D topological insulator BSTS and a commonly used contact metal (Ag), a magnetic metal (Fe) and a superconductor (Nb) metal on the bulk TI BSTS.

Binding energy of the Dirac point in BSTS for deposition of Ag, Fe and Nb at 40K. First n-type and then p-type behaviour is seen.

Binding energy of the Dirac point in BSTS for deposition of Ag, Fe and Nb at 40K. First n-type and then p-type behaviour is seen.


Nick found that for all coverages at room temperature and <0.2 ML coverages at 40K that all three metals shift the chemical potential up, increasing the binding energy of the Dirac point. For deposition and measurement at low T, after the initial increase in Dirac point binding energy (upward shift of chemical potential, donor behaviour of the metal overlayer), the behaviour reverses and the chemical potential them moves downward, reducing the Dirac point energy (acceptor behaviour of the metals). For silver this switch goes together with formation of substitutional defects (AgTe,Se), but for the less mobile Fe and Nb it is linked to clustering of the adatoms on the surface.


Pick up the paper in the publications section of the webpage, or click here.

The experiments were carried at the 1^2 end-station at BESSY-II/HZB in Berlin.


This research is part of the FOM/NWO-N Programme 134 'Topological Insulators', which combines the forces of groups from Leiden, Delft, Amsterdam & Twente, and is led by the Mark Golden in Amsterdam.