RESEARCH

08.12.2021: A paper coauthored by TIMO HYART has been published in PRX Quantum [Yu-Shiba-Rusinov Qubit, PRX Quantum 2, 040347 (2021)].

Chains of magnetic adatoms deposited on superconductors provide a viable platform for realizing a topological quantum computer based on Majorana zero modes. However, manipulating coherently quantum degrees of freedom in these systems remains an open challenge, and novel methodology needs to be developed for controlling such quantum states. To meet this challenge, we introduced a novel type of quantum bit, the Yu-Shiba-Rusinov qubit, stemming from two nearby magnetic adatoms coupled to a superconductor. We demonstrated that the coherent manipulation of the qubit states is possible via the well-established scanning tunneling microscopy-electron spin resonance techniques. Moreover, Yu-Shiba-Rusinov qubits can be integrated with Majorana qubits, allowing the possibility to transfer quantum information coherently between them, facilitating the implementation of a universal set of quantum gates.

23.07.2021: A paper coauthored by ALEXANDER LAU and TIMO HYART has been published in Physical Review X [Designing Three-Dimensional Flat Bands in Nodal-Line Semimetals, Phys. Rev. X 11, 031017 (2021)].

The recent discovery of unconventional superconductivity in a twisted pair of graphene sheets is a famous consequence of a flat energy band, which gives rise to enhanced correlation effects and exotic phases of matter. So far, the study of these intriguing effects has focused on 2D materials due to the lack of realistic proposals for their 3D counterparts. In this article, we present a viable approach to fill this gap and to lift the study of flat-band physics into the third dimension through strain engineering in topological nodal-line semimetals.

15.07.2021: A paper coauthored by TIMO HYART [Tunable topological states hosted by unconventional superconductors with adatoms, Phys. Rev. Research 3, 033049 (2021)] proposes new methods for manipulating Majorana zero modes in adatom chains.

16.06.2021: A paper coauthored by WOJCIECH BRZEZICKI and TIMO HYART [Signatures of dephasing by mirror-symmetry breaking in weak-antilocalization magnetoresistance across the topological transition in Pb1−xSnxSe, Phys. Rev. B 103, 245307 (2021)] reports a combined experimental and theoretical study of the effects of mirror symmetry breaking on the weak antilocalization magnetoresistance in Pb1−xSnxSe.

10.05.2021: A paper coauthored by VICTOR FERNÁNDEZ BECERRA and TIMO HYART [Topological charge, spin, and heat transistor, Phys. Rev. B 103, 205410 (2021)] proposes a charge, spin and heat transistor, which is based on topological effects in quantum spin Hall insulator-ferromagnetic insulator-superconductor heterostructures.

02.04.2021: A paper coauthored by TIMO HYART [Many-body Majorana-like zero modes without gauge symmetry breaking, Phys. Rev. Research 3, 023002 (2021)] reports numerical evidence of many-body zero-energy excitations. These excitations share many commonalities with Majorana zero modes but interestingly they can appear in interacting one-dimensional systems without gauge symmetry breaking.

04.12.2020: A paper coauthored by GRZEGORZ MAZUR and TIMO HYART [Topological valley currents via ballistic edge modes in graphene superlattices near the primary Dirac point, Communications Physics 3, 224 (2020)] reports a combined experimental and theoretical study of nonlocal resistance in graphene superlattices, which is interpreted to arise due to quantum valley Hall state. The systematic multiterminal transport measurements provide compelling experimental evidence of (quasi-)ballistic edge modes.

25.06.2020: A paper coauthored by WOJCIECH BRZEZICKI [Berry phase engineering at oxide interfaces, Phys. Rev. Research 2, 023404 (2020)] reports an investigation of the anomalous Hall effect at the oxide interfaces.

05.06.2020: A paper coauthored by WOJCIECH BRZEZICKI and TIMO HYART [Non-Hermitian Topological End-Mode Lasing in Polariton Systems, Phys. Rev. Research 2, 022051(R) (2020)] reports a prediction of the existence of non-Hermitian topologically protected end states in a one-dimensional exciton-polariton condensate lattice. A new type of topological phase transition and bulk-boundary correspondence is established based on enforced exceptional points, which can be predicted directly from the bulk Bloch wave functions.

03.06.2020: A paper by WOJCIECH BRZEZICKI and TIMO HYART [Topological domain wall states in a non-symmorphic chiral chain, Phys. Rev. B 101, 235113 (2020)] reports robust experimental signatures of the non-trivial topology in non-symmorphic systems: The domain walls carry fractional charges and the energy of topological domain wall states scales to zero with increasing width of the domain wall faster than for non-topological states.

01.06.2020: A paper coauthored by GRZEGORZ MAZUR and TIMO HYART [Transition between canted antiferromagnetic and spin-polarized ferromagnetic quantum Hall states in graphene on a ferrimagnetic insulator, Phys. Rev. B 101, 241405(R) (2020)] reports a combined experimental and theoretical study of the quantum Hall states in graphene on a ferrimagnetic insulator. The magnetic exchange field induced by the ferrimagnetic insulator allows unprecedented control of the magnetic order in the graphene with the magnitude and direction of the external magnetic field. The paper was selected as Editors’ suggestion by Phys. Rev. B Rapid Communications.

30.04.2020: A paper coauthored by NGUYEN MINH NGUYEN [Hybrid-functional electronic structure of multilayer graphene, Phys. Rev. B 101, 165437 (2020)] reports density functional theory calculations of the electronic structure for rhombohedral and Bernal stacked multilayer graphene.

09.04.2020: A paper coauthored by VICTOR FERNÁNDEZ BECERRA [Spontaneous emergence of Josephson junctions in homogeneous rings of single-crystal Sr2RuO4, npj Quantum Materials 5, 21 (2020)] reports observation of intrinsic Josephson junctions in Sr2RuO4, which are interpreted to arise at the domain walls between superconducting domains of opposite chirality.

06.12.2019: A paper coauthored by TIMO HYART [Geometric and Conventional Contribution to the Superfluid Weight in Twisted Bilayer Graphene, Phys. Rev. Lett. 123, 237002 (2019)] reports a theoretical study how quantum geometry helps electrons to superconduct in twisted bilayer graphene. In twisted bilayer graphene the electrons have small kinetic energy and therefore the interactions between electrons become more important and can lead to a formation of a superconducting state. However, this lack of movement of the electrons should prevent them from carrying supercurrents. This work resolves this apparent paradox by showing that in magic angle twisted bilayer graphene the electrons can use a “quantum detour” to superconduct. The presence of such a “quantum detour” is due to the nontrivial quantum geometry of the flat bands of twisted bilayer graphene.

This finding is counterintuitive because one would think that current cannot flow if the particles do not have kinetic energy. Such physics is important for the recent observation of the superconductivity in twisted bilayer graphene, which is considered to be the most important experimental breakthrough in condensed matter physics during the recent years (see the announcements of the Physics World 2018 Breakthrough of the Year and Wolf Prize 2020).

This important finding was highlighted in Physics Viewpoint article by Laura Classen: Geometry Rescues Superconductivity in Twisted Graphene.

08.10.2019: A paper by WOJCIECH BRZEZICKI and TIMO HYART [Hidden Chern number in one-dimensional non-Hermitian chiral-symmetric systems, Phys. Rev. B 100, 161105(R) (2019)] reports a surprising discovery that the topology of one dimensional chiral-symmetric systems is described by a hidden Chern number. This was completely unexpected since so far it has been believed that the Chern number only describes the topology of two-dimensional systems. The logic behind this result is that the topology of these one-dimensional non-Hermitian Hamiltonians is described by an effective two-dimensional Hermitian Hamiltonian.

16.09.2019: Two papers coauthored by WOJCIECH BRZEZICKI, TIMO HYART [Topological properties of multilayers and surface steps in the SnTe material class, Phys. Rev. B 100, 121107(R) (2019)] and GRZEGORZ MAZUR [Experimental search for the origin of low-energy modes in topological materials, Phys. Rev. B 100, 041408(R) (2019)] report theoretical and experimental study of an appearance of robust zero-bias peak in SnTe materials. In recent experiments such zero-bias peak [S. Das et al., Appl. Phys. Lett. 109, 132601 (2016); H. Wang et al., Nat. Mater. 15, 38 (2016); L. Aggarwal et al., Nat. Mater. 15, 32 (2016)] has been interpreted as evidence of unconventional superconductivity and Majorana modes. We have shown that all the observed phenomenology can be explained without superconductivity or Majoranas. In our theory this effect is a signature of a novel non-symmorphic topological phase arising due to magnetic instability at the step defects.

13.08.2019: A viewpoint article coauthored by TIMO HYART [Moiré with flat bands is different, Europhys. News 50, 24 (2019)] argues that the recent experimental discoveries of superconductivity and other exotic electronic states in twisted bilayer graphene call for a reconsideration of our traditional theories based on the assumption of the presence of a Fermi surface. Moreover, these developments may even help us finding mechanisms of increasing the critical temperature of superconductivity towards the room temperature.

23.04.2018: A viewpoint article by TIMO HYART [A Magnetic-Field-Free Exciton Condensate, Physics 11, 39 (2018)] discusses the recent experiment suggesting that a stable condensate of electron-hole pairs can be obtained without applying a large magnetic field, simplifying future applications based on this unusual pairing phenomenon.