# Chiral topological semimetals: From multifold fermions with long fermi-arcs to correlation induced topological phase transitions

### Max Planck Quantum Matter Seminar

- Date: May 19, 2022
- Time: 15:00 - 16:00
- Speaker: Niels Schröter
- MPI for Microstructure Physics, Halle
- Location: CFEL (Bldg. 99)
- Room: Seminar Room IV, O1.111, and on Zoom
- Host: Michael Sentef

An object that cannot be superimposed with its mirror image is said to be chiral, a concept first proposed by Lord Kelvin that has become ubiquitous across the modern sciences, ranging from high-energy physics to biology. In condensed matter physics, two important notions of chirality are (1) the structural chirality of a crystal or magnetic structure due to the (often helical) arrangement of atoms or spins in real space; and (2) the electronic chirality of quasiparticles formed by band crossings in reciprocal space, which can act as quantized monopoles of Berry curvature and gives a handedness to their wave functions.

In this talk, I will introduce chiral topological semimetals (CTSs), a new class of quantum materials that combine both of these notions of chirality by realizing Berry curvature monopoles in momentum space in structurally chiral crystals that possess neither inversion nor mirror-symmetries. These materials have been predicted to realize various new phenomena, such as multifold fermions with large Chern numbers (1), long fermi-arc surface states (2, 3), unusual magnetotransport (4) and lattice dynamics (5), topological superconductivity even with negligible spin-orbit coupling (6), and a quantized response to circularly polarized light (7). However, until recently, all known topological semimetals hosted mirror symmetries, which means that the aforementioned phenomena must vanish.

We have now identified a family of intermetallic compounds crystallizing in the cubic B20 as chiral topological semimetals and visualized their multifold fermions and long fermi-arcs directly with angle-resolved photoemission (ARPES) (8). By resolving a band splitting in the fermi-arc surface states, we showed that these quasiparticles carry the largest Chern number possible for quasiparticles in metals. We were also able to show experimentally that there is a direct relationship between the handedness of the crystal structure and the electronic chirality (i.e., the Chern number sign) of these new fermions, which indicates that structural chirality can be used as a control parameter to manipulate phenomena that are sensitive to the electronic chirality, such as the direction of topological photocurrents (9). I will then also present our latest results on a magnetic chiral topological semimetal where strong electron correlations may induce a topological phase transition.

Finally, if there is time, I will also present our latest contribution towards identifying finite momentum Cooper pairing as the origin of a Josephson Diode effect in a topological semimetal heterostructure (10).

1. B. Bradlyn et al., Science. 353, aaf5037 (2016).

2. P. Tang, Q. Zhou, S.-C. Zhang, Phys. Rev. Lett. 119, 206402 (2017).

3. G. Chang et al., Phys. Rev. Lett. 119, 206401 (2017).

4. S. Zhong, J. E. Moore, I. Souza, Phys. Rev. Lett. 116, 077201 (2016).

5. P. Rinkel, P. L. S. Lopes, I. Garate, Phys. Rev. Lett. 119, 107401 (2017).

6. J. Z. S. Gao et al., ArXiv201211287 Cond-Mat (2021).

7. F. de Juan, A. G. Grushin, T. Morimoto, J. E. Moore, Nat. Commun. 8, 1–7 (2017).

8. N. B. M. Schröter et al., Nat. Phys. 15, 759–765 (2019).

9. N. B. M. Schröter et al., Science. 369, 179–183 (2020).

10. B. Pal et al., ArXiv211211285 Cond-Mat (2021), to appear in Nature Physics (2022).