Séminaire DQMP: Topological superconducting phases in 3D and 2D materials

09.01.2018 13:00 – 14:00

The mathematical concept of topology has recently been used extensively to describe systems of condensed matter in which the symmetries of electronic wave functions differ from the atomic limit. Of particular interest is the topological superconductor, which features the pairing gap in its bulk, but with topologically protected gapless edge states that support Majorana bound states in the form of quasiparticles as low energy excitations. These Majorana states are of great interest for fault tolerant applications in quantum computation. In addition, topological superconductors are of fundamental interest because of their unconventional pairing symmetries. In my talk, I will review our recent results on various topological superconducting systems.
The topological insulator Bi2Se3 can be driven superconducting upon doping with Cu, Sr or Nb ions1-3. It was predicted that an odd-parity pairing topological superconducting state with Majorana surface states would occur with two possible topological representations4; a nematic state which spontaneously breaks the trifold crystal symmetry, or a chiral state. For Cu and Sr intercalated Bi2Se3, it has been shown that the nematic state is formed, but for Nb intercalated Bi2Se3 controversial results were presented3,5. NbxBi2Se3 is special because the Nb ions have a magnetic moment, and it was predicted that the phase space of the chiral state would be enlarged5,6. I will present our recent findings that help to clarify this issue.
When an AIII-class topological insulator is driven superconducting via the proximity effect, a BDI class topological superconductor with chiral Majorana edge states is expected. It was predicted that it could have two distinct topological superconducting phases with either two (N=2) or one (N=1) Majorana edge modes7. Such a material can be realized in a quantum anomalous Hall insulator / superconductor heterostructure. The N = 2 phase appears when the quantum anomalous Hall insulator is fully magnetized while the N = 1 phase is formed in a narrow field region upon magnetization reversal. In a recent experiment it has been shown that quantized half integer conductance plateaus with a value 0.5e2/h are formed in a device in which a narrow superconducting strip is deposited on a quantum anomalous Hall insulator, which have been interpreted as due to the N = 1 phase8. However, these results are controversially debated. We used a quantum point contact at the edge of a quantum anomalous Hall insulator / superconductor heterostructure to probe the low-energy excitations. We find a dip-like structure that is framed by two peaks, which agrees perfectly with the theoretical prediction of the N = 2 state. As the field is gradually increased, thus inducing magnetization reversal in the quantum anomalous Hall insulator, the dip transforms in a narrow field region into a quantized plateau with conductance of exactly 2e2/h as expected when the N = 1 state is formed. These results confirm the presence of these two distinct topological superconducting phases, which differ dramatically in their electronic transport properties.

[1] Y. S. Hor et al, Phys. Rev. Lett. 104, 057001 (2010).
[2] Z. Liu et al, J. Am. Chem. Soc. 137, 10512 (2015).
[3] Y. Qiu et al, arXiv:1512.03519.
[4] L. Fu, Phys. Rev. B 90, 100509(R) (2016).
[5] T. Asaba et al, Phys. Rev. X 7, 011009 (2017).
[5] J. W. F. Venderbos et al, Phys. Rev. B 94, 180504(R) (2016).
[6] N. F. Q. Yuan et al, Phys. Rev. B. 95, 201109 (2017).
[7] J. J. He et al., Nat. Commun. 5:3232 (2014).
[8] Q. L. He et al, Science 357, 294 (2017).

Lieu

Bâtiment: Ecole de Physique

Auditoire Stückelberg

entrée libre