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Speakers: Katharina Franke (Freie Universität Berlin), Michel Fruchart (Leiden Uni), Frederic Piechon (Université Paris-Sud) and Neil Robinson (UvA). Location: Marinus Ruppert building, room Rood ['red'], Utrecht.

Event details of Quantum and Topological Matter meeting
Date 24 November 2017
Time 13:30 -17:00

This event is part of a regular series of meetings on Quantum and Topological Matter, sponsored by Delta ITP. The objective is to bring together the theoretical physics communities in Amsterdam, Leiden and Utrecht. We encourage researchers from different areas in theoretical physics to participate!


13:30 - 14:00 coffee/tea

14:00 - 14:45 Katharina Franke (Freie Universität Berlin) 

From single magnetic adatoms on superconductors to coupled spin chains

14:45 - 15:10 Michel Fruchart, Leiden University

Self-assembled topological metamaterials: Weyl points for light and sound

15:10 - 15:45 coffee/tea

15:45 - 16:30 Frederic Piechon (Université Paris-Sud)

Geometrical description of the orbital magnetic susceptibilit 

16:30 - 16:55 Neil Robinson, University of Amsterdam

Title TBA 

17:00 - Drinks/snacks 


Katharina Franke (Freie Universität Berlin)

From single magnetic adatoms on superconductors to coupled spin chains

Abstract: Magnetic adsorbates on conventional s-wave superconductors lead to exchange interactions that can induce bound states inside the superconducting energy gap. These states are known as Yu-Shiba-Rusinov (YSR) states and can be resolved by scanning tunneling spectroscopy as a pair of resonances at positive and negative bias voltages in the superconducting gap.
Here, we employ tunneling spectroscopy at 1.1 K to investigate magnetic atoms and chains on superconducting Pb surfaces. We show that individual Manganese (Mn) atoms give rise to a distinct number of YSR-states, depending on the crystal field imposed by the adsorption site. The spatial extension of these states directly reflects their origin as the singly occupied d-states [1].
When the atoms are brought into sufficiently close distance, the Shiba states hybridize, thus giving rise to states with bonding and anti-bonding character. It has been shown that the Pb(110) surface supports the self-assembly of Fe chains, which exhibit fingerprints of Majorana bound states [2]. Here, we show that Co chains on Pb(110) exhibit similar characteristics of ferromagnetic coupling. Despite of this similarity they do not exhibit zero-energy states. In a simple model of tight-binding calculations, we ascribe the absence of Majorana end states to an even number of d-bands crossing the Fermi level [3].

Michel Fruchart, Leiden University

Self-assembled topological metamaterials: Weyl points for light and sound 

Abstract: Soft materials such as liquid crystals, block copolymers, or colloidal particles can self-assemble into highly structured phases which replicate at the mesoscopic scale the symmetry of atomic crystals. As such, they offer an unparalleled platform to design metamaterials for light and sound. Here, we present a bottom-up approach based on self-assembly to engineer three-dimensional photonic and phononic metamaterials with topologically protected Weyl points. In addition to angular and frequency selectivity of their bulk optical response, Weyl metamaterials are endowed with topological surface states, which allows for the existence of one-way channels even in the presence of time-reversal invariance. Using a combination of group-theoretical methods and numerical simulations, we identify the general symmetry constraints that a self-assembled structure has to satisfy to host Weyl points, and describe how to achieve such constraints using a symmetry-driven pipeline for metamaterial design and discovery.

Frederic Piechon (Université Paris-Sud)

Geometrical description of the orbital magnetic susceptibility 

Abstract: The orbital magnetic susceptibility measures the equilibrium response of a spinless electronic system to an external magnetic field B. For a single band model, the orbital susceptibility is given by the Landau-Peierls (LP) formula and is entirely determined by the energy spectrum. In particular, it is diamagnetic at parabolic band edges but appears strongly paramagnetic at a Van-Hove singularity.
For multi-band systems we have recently developed a general formalism that goes beyond the LP formula. This formalism allows the study of the possible strong inter-band effects that are encoded in the geometric structure of Bloch states. In particular, for two-band models, it appears that the inter-band susceptibility is composed of essentially two distinct contributions. The first contribution is entirely determined by the Berry curvature. It is paramagnetic inside the bands and exhibits a diamagnetic plateau in a gap separating two bands. The second contribution depends on the quantum metric tensor defining the distance between Bloch states. Interestingly, this second contribution always exists even for systems with a vanishing Berry curvature. It is shown that it can be tuned to exhibits either a diamagnetic or a paramagnetic plateau in a gap. Finally it is shown that these two interband geometric contributions may lead to an important paramagnetic contribution for a flat band.

Neil Robinson, University of Amsterdam



Marinus Ruppert building, room Rood ['red'], adress: Leuvenlaan 1, Utrecht

Local Organiser

Lars Fritz (Utrecht)