09:00  09:35

Jennifer Cano
(Stony Brook University)
Designer Meron Lattice on the Surface of a Topological Insulator

09:35  10:10

Hoi Chun (Adrian) Po
(The Hong Kong University of Science and Technology)
Symmetric JordanWigner transformation in higher dimensions (virtual)
The JordanWigner transformation provides a manifestly local representation of a lattice fermionic system using bosonic degrees of freedom. The bosonic description could provide an alternative starting point for tackling problems concerning correlated electrons in both numerical studies and, possibly, nearterm intermediatescale quantum simulators. However, a direct application of the original JordanWigner transformation to higher than one spatial dimension suffers from nonlocality. Local generalizations of the transformation to higher dimensions exist, but they either lead to unnatural symmetry representations, or require an excessive number of auxiliary degrees of freedom. In this talk, we discuss how one could perform JordanWigner transformation in higher dimensions which respect keep the locality and symmetries manifest.

10:10  10:20

Aleksandra Nelson
(Physics Institute of the University of Zurich)
Symmetryprotected delicate topology
We supplement the notions of stable and fragile topology by introducing delicate topological insulators: band structures possessing topological invariants that can be trivialized through an addition of a trivial band to either valence or conduction subspace. In my talk I will present a family of delicate topological insulators protected by rotational symmetry. I will discuss their unusual topological classification given by complementary topological invariants, which are at the same time related to each other modulo the protecting rotation symmetry. I will also introduce bulkboundary correspondence of the studied delicate models, which differs from bulkboundary correspondence known for stable topological insulators and manifests in anomalous symmetry eigenvalues of the surface states. These anomalous eigenvalues lead to conducting states at sharply terminated boundaries of delicate topological insulators.

10:20  10:30

Mario Moda Piva
(Max Planck Institute for Chemical Physics of Solids (MPICPfS))
Pressure effects on the Weyl semimetal CeAlSi
Nontrivial topological phases are much pursued by the researchers in condensed matter physics due to its novel transport properties, which may enable the development of future technologies in the field of spintronics. Furthermore, the addition of Ce ions in the compounds leads to other complex properties, such as crystalline electrical field effects, magnetism, and the Kondo effect. The interaction of these effects with topological phases are not fully understood yet and might enhance the observation of unusual transport properties, for instance, the Kondo effect may pin band crossings close to the Fermi energy favoring nontrivial topological properties [1].
In particular, the family of compounds $Ln$Al$X$ ($Ln =$ lanthanides, $X =$ Ge, Si) is promising to host nontrivial topological phases. Its members crystallize in the noncentrosymmetric structure ($I4_{1}md$), which breaks the spaceinversion symmetry, and many present magnetic orders, that break timereversal symmetry. The breaking of these symmetries are the main ingredients to the formation of magnetic Weyl semimetals. Here we focus on CeAlSi, in which a noncollinear ferromagnetic order takes place below 8.2~K [2] and chiral domain walls were recently observed [3,4]. We investigated the nontrivial topological properties of CeAlSi by combining electrical resistivity, Hall effect, and magnetization measurements with pressure tuning and DFT calculations [5]. A large contribution of domain wall scattering to the anomalous Hall effect and an atypical temperature response of the quantum oscillations amplitude were observed. Applying external pressure suppresses both behaviors; whereas it enhances $T_{C}$ to 9.4~K at 2.7~GPa. Magnetization measurements show no evidence of changes in the magnetic structure as a function of pressure. Finally, DFT calculations revealed a negligible effect of external pressure on the position of the Weyl nodes. Therefore, we suppose that the suppression of the anomalous responses of the Hall effect and the quantum oscillations amplitude are related to changes in the domain wall landscape of CeAlSi.
[1] S. E. Grefe, H.H. Lai, S. Paschen, and Q. Si, Phys. Rev. B, 101, 075138 (2020).
[2] H.Y. Yang et al., Phys. Rev. B, 103, 115143 (2021).
[3] B. Xu, J. Franklin, A. Jayakody, H.Y. Yang, F. Tafti, I. Sochnikov, Adv. Quantum Technol., 4, 2000101 (2021).
[4] Y. Sun, C. Lee, H.Y. Yang, D. H. Torchinsky, F. Tafti, J. Orenstein, Phys. Rev. B, 104, 235119 (2021).
[5] M. M. Piva, J. C. Souza, V. BrousseauCouture, K. R. Pakuszewski, Janas K. John, C. Adriano, M. Côté, P. G. Pagliuso, M. Nicklas, arXiv:2111.05742v1 (2021).

10:30  10:40

Miguel Ángel Sánchez Martínez
(Centre National de la Recherche Scientifique (CNRS))
Unconventional fermions and how to find them: optical probes of chiral multifold semimetals
Physics is an empirical science. Thus, experiments play a fundamental role in what we know and understand about reality. In this brief talk, I introduce the concept of chiral multifold fermions: lowenergy quasiparticles emerging near multifold band crossings protected by the crystal symmetries of the material. I propose optical probes to detect chiral multifold fermions in real materials and provide explicit examples with the multifold semimetals RhSi and CoSi. I conclude by pointing out the current challenges in using optical probes to identify multifold fermions, both theoretically and experimentally.

10:45  11:15

coffee break & discussion

11:15  11:50

Leslie Schoop
(Princeton University)
The chemistry of quantum Materials

11:50  12:00

Tarun Tummuru
(UZH, UBC)
Topological superconductivity in twisted bilayer cuprates
Various phenomena occur when twodimensional materials, such as graphene or transition metal dichalcogenides, are assembled into bilayers with a twist between the individual layers. As an application of this paradigm, we predict that structures composed of twomonolayerthin $d$wave superconductors with a twist angle can form a gapped topological phase with spontaneously broken timereversal symmetry and protected chiral Majorana edge modes. These structures can be realized by mechanically exfoliating van der Waalsbonded high$T_c$ cuprates, such as BSCCO. Symmetry arguments and microscopic modelling suggest that this phase will form for a range of twist angles in the vicinity of 45$^{\circ}$, and will set in at a temperature close to the bulk superconducting critical temperature of 90K. Therefore, the platform may provide a realization of a hightemperature topological superconductor.
Ref: O Can, T Tummuru, R Day, I Elfimov, A Damascelli and M Franz, ‘Hightemperature topological superconductivity in twisted doublelayer copper oxides’ Nat Phys 17, 519 (2021).

12:00  12:10

Jaime Díez Mérida
(ICFO  The Institute of Photonic Sciences)
Gate tunable magnetic Josephson Junctions in magic angle twisted bilayer graphene
Magic angle twisted bilayer graphene (MATBG) is a 2dimensional material which has emerged as a new exciting platform to study strongly correlated physics. It presents a simultaneous coexistence and gatetunability of different phases including superconductivity, magnetism and nontrivial topological orders. This opens up entirely new possibilities for the creation of complex hybrid Josephson junctions (JJ). Here we report on the creation of gatedefined, magnetic Josephson junctions in MATBG, where the weak link is gatetuned close to the correlated state at a moiré filling factor of ν = −2. A highly unconventional Fraunhofer pattern emerges, which is phaseshifted and asymmetric with respect to the current and magnetic field directions, and shows a pronounced magnetic hysteresis. We demonstrate how the combination of magnetization and current induced magnetization switching in the MATBG JJ allows us to realize a programmable zero field superconducting diode.

12:10  12:20

Piotr Majek
(Adam Mickiewicz University)
The signatures of the Majorana quasiparticles in the transport properties of strongly correlated quantum dots
The search for Majorana quasiparticles is one of the leading topics in the current solidstate physics research. The promising device to be a host for Majorana zero modes is a onedimensional topological superconducting nanowire, the idea proposed by Kitaev at the turn of the century. The signatures of the Majorana bound states may be identified within the spectral and transport properties of hybrid zero and onedimensional devices. Such a device can be a combination of a Majorana wire and a single or double quantum dot. In this communication, I want to shed some light on the electronic and thermoelectric transport properties of a double quantum dot coupled with a onedimensional topological superconducting nanowire and show, how these properties may be modified in the presence of such exotic quasiparticles. All of the results are obtained in the strongly correlated regime when the Kondo effect occurs. The work is based on the numerical renormalization group procedure, a reliable theoretical method for solving quantum impurity problems, where the interactions are present.

12:25  13:25

lunch

13:25  14:15

discussion

14:15  14:25

Yixi Su
(Forschungszentrum Jülich)
Neutron scattering on correlated topological materials
Recent theoretical predictions and experimental realizations of exotic fermions and topologically protected phases in condensed matter have led to tremendous research interests in topological quantum materials. Especially, correlated topological materials, such as magnetic Dirac and Weyl semimetals, and intrinsic magnetic topological insulators etc., in which both nontrivial topology of singleelectron band structures and electronic correlation effects are essential for the underlying physics, have emerged as an exciting platform to explore novel electronic and magnetic phenomena. In this talk, I will present a couple of the selected examples of our recent neutron scattering studies of correlated topological materials, including magnetic Dirac semimetal EuMnBi$_2$ [1], magnetic Weyl semimetal Mn$_3$Sn and topological magnon insulators in twodimensional van der Waals ferromagnets CrSiTe$_3$ and CrGeTe$_3$ [2].
[1] Fengfeng Zhu, et al., Phys. Rev. Research 2, 043100 (2020).
[2] Fengfeng Zhu, et al., Sci. Adv. 7, eabi7532 (2021).

14:25  14:35

Chris Hooley
(University of St Andrews)
Weyllike points at the surface of NbGeSb: when the orbital angular momentum winds
In this talk I present a toy model, developed together with my collaborators, of the surface electronic band structure of the material NbGeSb. This band structure shows an unusual '3+1' motif along the edge of the surface Brillouin zone, where a Rashbasplit pair of holelike bands crosses a Rashbasplit pair of electronlike ones but only one of the four crossing points appears to develop a gap. The toy model explains this feature in terms of an interesting interplay between the spin and orbital content of the bands. A result of the model is the existence of Weyllike points where it is the orbital angular momentum rather than the spin angular momentum that exhibits a nontrivial winding as the crossing point is encircled.

14:35  14:45

Andrew Huxley
(University of Edinburgh)
Nonconventional superconductivity and half quantum vortices
Halfquantum vortices (HQVs) have a Majorana bound state associated with them that mean they obey nonAbelian statistics. When moved around each other this generates a quantum entanglement that could be exploited to make a robust quantum computer.
Stable half quantum vortices (HQVs) have been observed in geometrically confined superfluid helium3 and in polariton condensates. The observation of mobile HQVs in superconductors however remains elusive. Localised halfflux quanta have been detected in unconventional superconductors at tricrystal grain boundaries in the cuprates and arguably in mesoscopic rings of Sr$_2$RuO$_4$and polycrystalline $\beta$Bi$_2$Pd. However, the noninteger flux in these cases is a consequence of the samples engineered geometry.
In this poster/talk we will explore which materials could be favourable candidates to host unconfined half quantum vortices.

14:45  17:00

poster session & coffee break

17:00  18:00

Discussion Session I

18:30  19:30

dinner
