2020 Abstracts Coherent Order

Magic Angle Bilayer Graphene – Superconductors, Orbital Magnets, Correlated States and beyond

Dmitri Efetov

When twisted close to a magic relative orientation angle near 1 degree, bilayer graphene has flat moire superlattice minibands that have emerged as a rich and highly tunable source of strong correlation physics, notably the appearance of superconductivity close to interaction-induced insulating states. Here we report on the fabrication of bilayer graphene devices with exceptionally uniform twist angles. We show that the reduction in twist angle disorder reveals insulating states at all integer occupancies of the four-fold spin/valley degenerate flat conduction and valence bands, i.e. at moire band filling factors nu = 0, +(-) 1, +(-) 2, +(-) 3, and reveals new superconductivity regions below critical temperatures as high as 3 K close to - 2 filling. In addition we find novel orbital magnetic states with non-zero Chern numbers. Our study shows that symmetry-broken states, interaction driven insulators, and superconducting domes are common across the entire moire flat bands, including near charge neutrality. We further will discuss recent experiments including screened interactions, fragile topology and the first applications of this amazing new materials platform.

Real-space visualization of Majorana edge modes on the Nano-scale magnet-superconductor hybrid system

Alexandra Palacio Morales

Hybrid magnetic-superconducting systems have attracted widespread interest for their promising potential in topological quantum computation. Nano-fabrication techniques combined with local probe microscopies revealed the emergence of zero-energy modes on ferromagnetic chains [1] and ferromagnetic nanoislands [2] on s-wave superconductors. However, the observation of Majorana Fermion (MF) modes in these hybrid systems is still a subject of debate and has raised questions about experimental considerations that must be accounted. Lack of well-defined structures, MF spatial distribution and evolution inside the hybrid structures are among the reasons of these concerns. Nevertheless, advances in nano-fabrication techniques such as atom manipulation combined with local probe microscopies will paved the way to overcome them.

Here, we report on the evolution of Yu-Shiba-Rusinov bands into MF by atomic length manipulation of Fe magnetic chain on Re(0001) surface [3]. Moreover, we report on the first unambiguous experimental detection and visualization of chiral Majorana edge states in a monolayer topological superconductor, a prototypical magnet-superconductor hybrid system comprised of nano-scale Fe islands of monoatomic height on a Re(0001)-O(2×1) surface [4, 5].

[1] S. Nadj-Perge et al., Science 346, 602 (2014)
[2] G. C. Ménard et al., Nat. Comm. 8, 2040 (2017)
[3] H. Kim et al., Science Adv. 4, eaar5251 (2018)
[4] A. Palacio-Morales et al., Nano Lett. 16, 6252-6256 (2016)
[5] A. Palacio-Morales et al., Science Adv. 5, eaav6600 (2019)

Probing Triplet Superconductivity by Scanning Tunneling Spectroscopy

Elke Scheer

In this talk we will address the superconducting proximity effect between a superconductor (S) and a normal metal (N) linked by a spin-active interface. With the help of a low-temperature scanning tunneling microscope we study the local density of states of trilayer systems consisting of Al (S), the ferromagnetic insulator EuS, and the noble metal Ag (N). In several recent studies it has been shown that EuS acts as ferromagnetic insulator with well-defined magnetic properties down to very low thickness [1]. We observe pronounced subgap structures that either reveal a zero-bias peak (ZBP) or an additional zero-bias splitting (ZBS) and that can be tuned by a magnetic field. We interpret our findings in the light of recent theories of odd-triplet contributions created by the spin-active interface [2,3]. In particular, we discuss that the ZBS is a hallmark for spin-polarized triplet pairs, able to carry long-ranged supercurrents in to F, while the ZBP is a signature for short-ranged, mixed-spin triplet pairs.

[1] S. Diesch et al., Nature Commun. 9, 5248 (2018)
[2] B. Li et al., Phys. Rev. Lett. 110, 09700 (2013)
[3] A. Cottet et al., Phys. Rev. B 80, 184511 (2009)

Solitons and topological superconductivity in antiferromagnet-superconductor interfaces

Jose Lado

The interplay of magnetism and superconductivity provides one of the most fertile platforms to engineer unconventional quantum matter, with the paradigmatic example of Majorana excitations in artificial topological superconductors. In particular, the potential of Majorana excitations for topological quantum computing has motivated outstanding efforts for their engineering by combining ferromagnetism, strong spin-orbit coupling, and conventional superconductivity. Here we introduce a platform alternative to those mechanisms that exploit the emergence of solitonic excitations between antiferromagnetic insulators and a conventional superconductor. First, we show that solitons at interfaces between three-dimensional antiferromagnets and superconductors can be used to engineer a two-dimensional topological superconductor, whose topological gap stems from intrinsic spin-orbit coupling [1]. Second, we show that at interfaces between two-dimensional antiferromagnetic insulators and superconductors, topological superconductivity emerges from solitons with a purely interaction- driven topological gap, requiring no spin-orbit coupling effects [2]. Ultimately, we demonstrate that many-body solitons emerge even at interfaces between quantum entangled antiferromagnets and superconductors, providing a stepping stone towards exploring emergent excitations in quantum-spin liquid superconductor junctions [3]. Our findings exemplify the potential of solitonic excitation in antiferromagnet-superconductor interfaces to engineer topological superconductivity and exotic quantum many-body states.

[1] Jose L. Lado and Manfred Sigrist, Phys. Rev. Lett. 121, 037002 (2018)
[2] Senna Luntama, Jose L. Lado and Päivi Törmä, in preparation (2020)
[3] Jose L. Lado and Manfred Sigrist, Phys. Rev. Research 2, 023347 (2020)