Abstracts - QuMat 2023 Pillar meeting
Junior PI Presentations
Pillar 1 speaker: Malte Rösner
Title: Many-Body Engineering in Correlated Layered Heterostructures
Abstract: Due to confinement and a lack of screening layered materials host enhanced local and non-local Coulomb and electron-phonon interactions. These strong interactions are responsible for a plethora of pronounced many-body effects, ranging from strongly-bound excitons or gap-less plasmons to superconductivity or magnetism. At the same time, these fundamental interactions interplay with each other and can be controlled by the material’s environment. This allows for precise many-body engineering of correlated states of matter in layered heterostructures.
In this talk, I will show examples of this many-body engineering concept applied to magnetic and superconducting layered heterostructures. Focusing on the latter I will discuss how electron-phonon, electron-plasmon, and phonon-plasmon coupling can be tailored to trigger a crossover from conventional phonon-mediated superconductivity to unconventional plasmon-mediated pairing in layered superconducting heterostructures.
Pillar 2 speaker: Jagoda Sławińska
Title:Computational materials design of materials for spintronics
Abstract: Materials that exhibit spin-orbit-related phenomena are promising candidates for designing alternative memory and computing devices that move beyond the limitations of the von Neumann paradigm. One interesting approach towards realizing all-electric energy-efficient electronic devices is to design spintronics materials based on their symmetries. In this talk, I will explore the properties of several recently (re-)discovered materials that reveal spin-orbit-related phenomena, as well as different methods for controlling spins. Specifically, I will focus on the use of symmetries in material design, which has shown to be an efficient way to control spins and generate unconventional configurations of charge-to-spin conversion; they can be used, for example, to generate efficient spin-orbit torques. Second, I will focus on chiral crystals that exhibit collinear charge-to-spin conversion, similar to the chirality-induced spin selectivity observed in molecules. Trigonal Te, Se, and TaSi2 are examples of materials that manifest this effect, with good efficiency of charge-to-spin conversion and the presence of the so-called persistent spin texture that may yield very long spin lifetimes. Although the exact role of the persistent spin texture in protecting spin transport still needs to be explained, chiral materials with large spin-orbit coupling show potential in solving one of the important trade-offs in spintronics.
 A. Roy, M. Guimarães, J. Sławińska, Physical Review Materials 6, 045004 (2022)
 A. Roy, F. Cerasoli, A. Jayaraj, K. Tenzin, M. Buongiorno Nardelli, J. Sławińska, npj Computational Materials 8 (1), 243 (2022)
 H. Jafari, A. Roy, J. Sławińska, Physical Review Materials 6, L091404 (2022)
 K. Tenzin, A. Roy, H. Jafari, B. Banas, F. T. Cerasoli, A. Jayaraj, M. Buongiorno Nardelli, J. Sławińska, Physical Review B 107, 165140 (2023)
 K. Tenzin, A. Roy, F. Cerasoli, A. Jayaraj, M. Buongiorno Nardelli, J. Sławińska, arXiv:2304.05287 (2023)
Pillar 3 speaker: Puhua Wan
Title: Orbital Fulde-Ferrell-Larkin-Ovchinnikov state in an Ising superconductor
Abstract: In a conventional superconductor, when spatial inversion and time-reversal symmetries are respected, electrons form Cooper pairs with opposite momenta and spins as described by the Bardeen–Cooper–Schrieffer (BCS) theory. In superconductors possessing both time and inversion symmetries, the Zeeman effect of an external magnetic field can break the time-reversal symmetry, forming a conventional Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state characterized by Cooper pairings with finite momentum[1,2]. In superconductors lacking (local) inversion symmetry, the Zeeman effect may still act as the underlying mechanism of FFLO states by interacting with spin-orbit coupling (SOC). Specifically, the interplay between the Zeeman effect and Rashba SOC can lead to the formation of more accessible Rashba FFLO states that cover broader regions in the phase diagram[3–5]. However, when the Zeeman effect is suppressed due to the spin-locking in the presence of Ising-type SOC, the conventional FFLO scenarios are no longer effective. Instead, an unconventional FFLO state is formed by coupling the orbital effect of magnetic fields with SOC, providing an alternative mechanism in superconductors with broken inversion symmetries[6–8]. Here we show the evidence of such an orbital FFLO state in multilayer Ising superconductor 2H-NbSe2. Transport measurements show that the translational and rotational symmetries are broken in the orbital FFLO state, providing the hallmark signatures of finite momentum cooper pairings. We establish the entire orbital FFLO phase diagram, consisting of normal metal, uniform Ising superconducting phase, and a six-fold orbital FFLO state. This study highlights an alternative route to finite-momentum superconductivity and provides a universal mechanism to prepare orbital FFLO states in similar materials with broken inversion symmetries.
1. Fulde, P. & Ferrell, R. A. Superconductivity in a strong spin-exchange field. Phys. Rev. 135, (1964).
2. Larkin, A. I. & Ovchinnikov, Y. N. Nonuniform state of superconductors. Sov. Phys. JETP 47, 1136 (1964).
3. Barzykin, V. & Gor’kov, L. P. Inhomogeneous Stripe Phase Revisited for Surface Superconductivity. Phys. Rev. Lett. 89, 1–4 (2002).
4. Zheng, Z. et al. FFLO superfluids in 2D spin-orbit coupled fermi gases. Sci. Rep. 4, 1–7 (2014).
5. Sigrist, M. et al. Superconductors with staggered non-centrosymmetricity. J. Phys. Soc. Japan 83, 1–8 (2014).
6. Liu, C.-X. Unconventional Superconductivity in Bilayer Transition Metal Dichalcogenides. Phys. Rev. Lett. 118, 087001 (2017).
7. Nakamura, Y. & Yanase, Y. Odd-parity superconductivity in bilayer transition metal dichalcogenides. Phys. Rev. B 96, 1–11 (2017).
8. Hu, L.-H., Liu, C.-X. & Zhang, F.-C. Topological Larkin-Ovchinnikov phase and Majorana zero mode chain in bilayer superconducting topological insulator films. Commun. Phys. 2, 25 (2019).
Pillar 4 speaker: Marcos Guimarães
Title: An Ultrafast View on the Magnetization Dynamics of Van der Waals Materials
Abstract: Van der Waals (vdW) magnets are ideal systems for the study of magnetism in low dimensions since they maintain their excellent magnetic properties down to the atomic limit. Because of their low dimensionality, these materials possess another exciting property, they are extremely sensitive to external stimuli, such as light and electric fields. In this talk, I will show how we can manipulate and measure the magnetization of vdW magnets using ultrafast (fs) laser pulses. After a brief introduction to magneto-optics and ultrafast magnetization dynamics, I will present our results on Fe3GeTe2. We observe an efficient light-induced demagnetization and a high spin-flip probability. The spin scattering is found to be strongly dependent on the magnetization direction, making it an appealing tuning knob for spintronic devices. I will also show how we can measure the magnetization dynamics of another vdW magnet, Cr2Ge2Te6, and how its magnetic properties can be electrically tuned. Our studies illustrate the potential of vdW magnets for combining optics and magnetism, making them appealing for new opto-magnetic device architectures for future integrated photonic systems.
Pillar 1 speaker: Dennis Klaassen
Title: Topological edge states in parallel armchair germanene nanoribbons
Abstract: Charge transport without dissipation at the edges of quantum spin Hall (QSH) insulators can revolutionize low-energy electronics. Unfortunately, the limited number of conductive channels and small band gaps in QSH insulators could restrict their practical use. To overcome these limitations, we have developed a method to increase the number of topologically protected channels by growing dense arrays of parallel germanene nanoribbons. These nanoribbons have a strained lattice structure, with armchair terminations and width-dependent electronic band structures. Metallic states localize at the nanoribbon edges in sufficiently wide nanoribbons (≥ 6 nm) and delocalize due to interedge interaction in narrower nanoribbons. Importantly, the edge states are topologically protected, rapidly decaying into the bulk while remaining robust to disorder. Our research has identified the topological features of quasi-1D structures and created a system with a high density of topological edge states, paving the way for new low-energy electronic devices.
Pillar 3 speaker – I: Pim Lueb
Title: Development of high quality PbTe nanowire networks and in-situ device fabrication technology using molecular beam epitaxy
Abstract: Hybrid semiconductor-superconductor nanowires are promising candidates as quantum information processing devices. The need for scalability and complex designs calls for the development of selective area growth techniques. We introduce the growth of large-scale lead telluride (PbTe) networks by molecular beam epitaxy. The group IV-VI lead-salt semiconductor is an attractive material choice due to its large dielectric constant, strong spin-orbit coupling, and high carrier mobility. A crystal reorientation process during the initial growth stages leads to single crystalline nanowire networks despite having a large lattice mismatch, different crystal structure, and diverging thermal-expansion coefficient w.r.t the indium phosphide substrate. By combining our growth platform with state-of-the-art shadow wall fabrication techniques, we are able to make complex semiconductor-superconductor structures in-situ. The high quality and scalable approach of our methods show the potential of our system as a basis for research of topological quantum devices.
Pillar 3 speaker – II: Rebecca Gharibaan
Title: Towards microwave experiments in III-V2DEGs
Abstract: Two-dimensional electron gases (2DEGs) in III-V materials offer a versatile and flexible platform to study hybrid superconductor-semiconductor devices. Recently such 2DEGs have been used to create gate-tunable superconducting qubits and are a promising platform to explore topological systems. In order to study these systems it is desirable to control them at short timescales. However, in general, these 2DEGs are not ideal for microwave frequency experiments, since they significantly lower the quality factor of on-chip microwave resonators due to dielectric losses. This limits the ability to perform fast operations and to study these hybrid systems on short timescales. Moreover, the additional fabrication steps required to create the microwave circuitry are often incompatible with the processing of hybrid devices in the 2DEGs.
Pillar 4 speaker: Oscar Eastman
Title: Finding Majorana: A Coherent guide to incoherence
Abstract: As many will know, there has been a frenzied search in recent years for elusive Majorana Bound States in condensed matter systems. Owing to its simplicity, the Kitaev chain has proven to be a particularly appealing avenue for research. Unfortunately, the zero-bias signature of these states is shared with other contenders such as Yu Shiba Rusinov and Andreev Bound States. Recent papers have suggested that Majorana states can be resolved by their noise characteristics. In this talk I will describe recent work undertaken to develop a rigorous theoretical framework which incorporates the influence of the STM tip and leads on the system.
Pillar MT speaker: Lumen Eek
Title: Topological Monomodes in non-Hermitian Systems
Abstract: We show theoretically and experimentally the existence of topological monomodes in non-Hermitian systems created by loss engineering. This challenges the idea that edge states always come in pairs in ℤ2 symmetry-protected topological systems. We theoretically show the existence of a monomode in a non-Hermitian 1D and 2D SSH models. Furthermore, we classify the systems in terms of the (non-Hermitian) symmetries that are present and calculate the corresponding topological invariant. To corroborate the theory, we present experiments in photonic lattices in which a monomode is observed in a non-Hermitian 1D SSH chain.
Dennis de Wal – Thermally and electrically generated magnon transport in quasi-two-dimensional antiferromagnetic materials