his 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!

Programme

14:00 - 14:30 Welcome coffee and  tea

14:30 - 15:15  Anne de Visser (UvA)

Nematic superconductivity in topological materials.

15:15 - 16:00, Artem Ivashko (Leiden)

Chiral magnetic effect in Weyl semimetals: surface states revisited.

16:00 - 16:30 Coffee, Tea

16:30 - 17:15 Guido van Miert (Utrecht)

A Berry phase approach to bound states and excess charges around defects.

17:15 - 18:30 Drinks at Academische kwartier (bar downstairs of the Science Park 904)

Abstracts

Anne de Visser

Nematic superconductivity in topological materials.

Topological insulators have generated a wide research interest, because they offer access to novel quantum states with unprecedented properties. Most interestingly, the concept of topological insulators can be transferred to superconductors, where the superconducting gap plays the role of the band gap of the insulator. Topological superconductors are predicted to have an exotic Cooper-pair state in the bulk and gapless Andreev bound states at the surface (Majorana modes), and therefore offer new, challenging routes to test theories of unconventional superconductivity. In this presentation I will focus on the recent developments in superconducting Bi2Se3-based crystals, where field-angle dependent measurements of the transport, thermal and magnetic properties have revealed a spontaneous breaking of the rotational symmetry [1,2,3]. The rotational symmetry breaking in the macroscopic superconducting properties is explained in terms of nematic superconductivity, that is associated with a two-component superconducting order parameter (Eu representation) [4]. The experimental results provide solid evidence for unconventional superconductivity with an odd-parity spin-polarized triplet Cooper-pair state (Δ4-pairing), that was recently proposed for rhombohedral topological superconductors [5].

[1] Y. Pan et al., Sci. Rep. 6, 28632 (2016).

[2] S. Yonezawa et al., Nature Phys. 13, 123 (2017).

[3] T. Asaba et al., Phys. Rev. X 7, 011009 (2017).

[4] J. Venderbos et al., Phys. Rev. B 94, 180504R (2016).

[5] L. Fu, Phys. Rev. B 90, 100509R (2014).

Artem Ivashko

Chiral magnetic effect in Weyl semimetals: surface states revisited.

Weyl semimetals provide an outstanding platform to probe the properties of three-dimensional relativistic massless electrons in condensed-matter setup. One of the theoretical predictions for such excitations is the so-called Chiral Magnetic Effect, which lies in appearance of electric current proportional to the external magnetic field, but the current is known to vanish in thermal equilibrium. A simple and yet experimentally accessible way to probe the effect is to drive the system out of equilibrium by making the magnetic field time-dependent. We attempt to resolve the controversy present in the literature regarding the value of the coefficient entering the expression for the current, and to clarify the contributions of the bulk and the surface states therein. Apart from confirming the recent finding of Baireuther et al. (2016) that the boundary states contribute significantly to the current, we point out that such a surface contribution may become the only source of the Chiral Magnetic Effect in the limit of large sample sizes and large magnetic fields.

Guido van Miert

A Berry phase approach to bound states and excess charges around defects.

We show that the excess charge in crystalline insulators can be expressed as a geometric phase. However, this geometric phase is different from the Zak phase that is used within the modern theory of polarization. In fact, this provides a new way to probe geometric phases using only local measurements. We then apply these result to derive the response of 2D Dirac semimetals to an inversion-symmetry breaking perturbation. Moreover, we show that one can calculate excess charge surrounding an edge dislocations. This allows one to probe the Berry phase in a Chern insulator. In the presence of additional symmetries, like inversion and or time-reversal symmetry, the excess charge is quantized. In addition,  this implies the existence of bound states around dislocations and edges.

Location

Room C4.174, Science Park 904, Amsterdam

Organisers

Cristiane Morais Smith (Utrecht)

Vadim Cheianov (Leiden)

Vladimir Gritsev (Amsterdam)