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03.05.2021 – Dissipative Phases of Entangled Quantum Matter

Driven-dissipative quantum many body systems constitute a cross- disciplinary frontier of research encompassing condensed matter, AMO and solid state physics. Many-particle systems where quantum coherent dynamics and dissipative effects occur on the same footing, find experimental realization in cavity QED, driven open Rydberg systems, trapped ions, exciton-polariton condensates, coupled micro- cavity arrays — among the others.
These platforms offer the unique opportunity to explore extensive phases of matter which cannot be encompassed through conventional statistical mechanics. At the same time they pose a number of fundamental and technical challenges. The ubiquitous intrusive effect of dissipation in experiments, confronts researchers to optimize and enhance the role of quantum fluctuations in strongly noisy and decoherent environments. At the same time, an efficient simulation of open many-particle systems require a formidable combination of techniques and expertise ranging from advanced field theoretical methods to forefront numerical techniques, from machine learning to non-unitary versions of techniques from the field of strongly correlated systems.
These 3-days workshop will bring together a number of experts from a diverse and interdisciplinary set of fields, including condensed matter physics, cold atoms, quantum engineering, quantum optics, atomic and solid state physics, with a broad selection of experimentalists from currently active fields. Ample space will be devoted to the participation of emergent and promising young scholars with dedicated flash talks in a 'March Meeting' format. Furthermore, the workshop hosted two topical sessions to foster dialogue among researchers belonging to different sub-communities.

For videos of the talks and further information, please visit the workshop home page.

04.11.2020 – Online Young Research Leaders Group Workshop: Spin, Charge, and Heat Transport: From Symmetries to Emergent Functionalities

What once began with spin-polarized electric currents in ferromagnets and the giant magnetoresistance, today is an internationally overarching research field known as spintronics. The last two decades, in particular, saw the consolidation of spintronics into modern solid state research. This was possible in large parts thanks to the experimental confirmation of the spin Hall effect and its inverse counterpart that enables electrical detection of pure spin currents. By now, it is known that the electronic spin not only couples to magnetic but also electric fields as well as heat gradients, adding interconversion phenomena between spin, charge, and heat to the spintronic inventory, examples being the spin Seebeck, spin Nernst, and Edelstein effects. Being inspired by both the uncovering of fundamental physics as well as the vision that spin will serve as an information carrier, the spintronics community studied a broad range of material classes, including normal, topological, and magnetic metals as well as topological and magnetic insulators. Magnets, in particular, proved to contain a wealth of surprises, exemplified by topological magnons, topological (spin) Hall effects in skyrmion crystals, anomalous Hall effects in antiferromagnets, or the magnetic spin Hall effect.

This SPICE Young Research Leaders Group Workshop serves as a melting pot of ideas on how to tackle the major spintronic challenges of this decade. The program of this workshop is built around the following major questions:
(1) Relying on symmetry arguments, which transport phenomena do we expect?
(2) How does the topological nontriviality of the electronic or magnonic band structure influences spin, charge, and heat transport?
(3) Which materials show particularly large transport and why? (Can we engineer spin transport?)
(4) How do we perform clear-cut experiments to disentangle a particular (spin) transport phenomenon from others?
(5) How do we use the arsenal of spintronics as means to explore and characterize complex materials?

For videos of the talks and further information, please visit the workshop home page.

22.10.2020 – Topological Superconductivity in Quantum Materials

Topology in quantum mechanics is applied to determine if a system is trivial or topological. A condensed matter system has a topological nature if the general wave function describing it is adiabatically distinct from the atomic limit. Although nontrivial topology has been known to exist in quantum Hall systems for nearly four decades, recent years have seen a massive resurgence in the interest of topological matter stemming from a series of ground-breaking discoveries. In many cases, topological quantum mechanics is achieved in systems involving superconductors with highlights including: Majorana Fermions in nanowire devices; unconventional electron pairing in layered oxides and the decoding high temperature superconductivity; superconducting thin films of strontium ruthenate; topological superconductivity in UTe2; coupling superconductivity into chiral (topological) molecules; and topological superconductivity and magnetism in twisted bilayer graphene.

The incredible progress made in materials research over the past decade and half has been central to the rapid development of unconventional superconductivity in topological quantum materials. These include the development of atomically-controlled crystals, thin films and interfaces, and the manipulation of pristine two-dimensional materials and superlattices. The widespread interest and progress in unconventional superconductivity and topology in such advanced materials continues to accelerate; however, a targeted, interdisciplinary, approach is required in order to achieve full understanding and the discovery of new science. This workshop brings together world-leading scientists from a broad range of disciplines working on overlapping themes involving correlated electrons and superconductivity in topological systems. These communities had an opportunity to appreciate how these areas are interlinked thereby stimulating further understanding and new collaborations.

For videos of the talks and further information, please visit the workshop home page.

On-line SPICE-SPIN+X Seminars

On-line Seminar: 03.05.2023 - 15:00 CEST

Spin-orbit coupling: an endless source of exotic phenomena in 2D magnets

Silvia Picozzi, D'Annunzio University

During the last decades, the spin-orbit coupling (SOC) has played an increasingly crucial role in condensed matter physics, thanks to its relevance as a rich microscopic mechanism from the fundamental point of view and as a driving force for innovative spintronic applications on the technological side. Combined with the global thrust towards miniaturization and with the ubiquitous research in two-dimensional (2D) materials, the talk will focus on the modelling of 2D magnets with emphasis on SOC-induced effects. In particular, I will focus on the magnetic and ferroelectric properties of transition-metal monolayers (mostly halides) and discuss the role of SOC in the magnetoelectric coupling. The reports of multiferroicity in NiI2 layers [1], obtained via a joint theory-experiments approach down to the single-layer limit, show the potentiality of cross-coupling phenomena in van der Waals magnets. If time permits, other recent examples – such as SOC-induced effects in CrSBr monolayers - will be discussed.

[1] Song, Q., Occhialini, C.A., Ergecen, E., Ilyas, B., Amoroso, D., Barone, P., Kapeghian, J., Watanabe, K., Taniguchi, T., Botana, A. S., Picozzi, S., Gedik, N., Comin, R., Evidence for a single-layer van der Waals multiferroic, Nature 602, 601 (2022)

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PDF file of the talk available here

On-line SPICE-SPIN+X Seminars

On-line Seminar: 08.02.2023 - 15:00 German Time

Machine learning as a tool to accelerate magnetic materials discovery

Stefano Sanvito, Trinity College Dublin

The process of finding new materials, optimal for a given application, is lengthy, often unpredictable, and has a low throughput. Here I will describe a collection of numerical methods, merging advanced electronic structure theory and machine learning, for the discovery of novel compounds, which demonstrates an unprecedented throughput and discovery speed. This is applied here to magnetism, but it can be used for any materials class and potential application.
Firstly, I will discuss a machine-learning scheme for predicting the Curie temperature of ferromagnets, which uses solely the chemical composition of a compound as feature and experimental data as target[1]. In particular, I will discuss how to develop meaningful feature attributes for magnetism and how these can be informed by experimental and theoretical results.
Then, I will describe how an accurate description of the structure of materials, which is amenable to be used with machine learning, can offer a quantum-chemistry-accurate description of local properties at virtually no computational costs. The method is not just suitable for building energy models[2], namely force fields to used across a broad spectrum of conditions[3], but also for any other local electronic quantity. These models may then be employed to design new materials, as demonstrated here for magnetic molecules with enhanced uniaxial anisotropy[4].
Finally, I will present a novel rotationally invariant representation for generic vector fields. This can be used to generate linear and non-linear machine-learning models, where the total energy depends both on the atomic position and the vector field direction[5]. The scheme will be put to the test against a hierarchy of simple spin models, demonstrating an impressive ability to extrapolate away from the training region of the data. Application to complex potential energy surfaces, as those extracted from DFT are then envisioned.

[1] J. Nelson and S. Sanvito, Predicting the Curie temperature of ferromagnets using machine learning, Phys. Rev. Mat. 3, 104405 (2019)
[2] Alessandro Lunghi and Stefano Sanvito, A unified picture of the covalent bond within quantum-accurate force fields: from simple organic molecules to metallic complexes reactivity, Science Advances 5, eaaw2210 (2019).
[3] Yanhui Zhang, Alessandro Lunghi and Stefano Sanvito, Pushing the limits of atomistic simulations towards ultra-high temperature: a machine-learning force field for ZrB2, Acta Materialia 186, 467 (2020).
[4] Alessandro Lunghi and Stefano Sanvito, Surfing multiple conformation-property landscapes via machine learning: Designing magnetic anisotropy, J. Phys. Chem. C 124, 5802 (2019).
[5] Michelangelo Domina, Matteo Cobelli and Stefano Sanvito, Spectral neighbor representation for vector fields: Machine learning potentials including spin, Phys. Rev. B 105, 214439 (2022).

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PDF file of the talk available here

On-line SPICE-SPIN+X Seminars

On-line Seminar: 15.03.2023 - 15:00 German Time

Hidden magnetoelectric order

Nicola Spaldin, ETH Zurich

Most magnetic materials, phenomena and devices are well described in terms of magnetic dipoles of either spin or orbital origin. There is mounting evidence, however, that the existence and ordering of higher-order magnetic multipoles can lead to intriguing magnetic behaviors, which are often attributed to "hidden order" since they are difficult to characterize with conventional probes. In this talk I will discuss the relevance of the so-called magnetoelectric multipoles, which form the next-order term, after the magnetic dipole, in the multipolar expansion of the energy of a magnetization energy in a magnetic field. First I will describe how magnetoelectric multipoles underlie multiferroic behavior and in particular how they determine the magnetic response to applied electric fields. Then I will discuss signatures of hidden magnetoelectric multipolar order, how it can be unearthed using density functional calculations and possibilities for its direct measurement. Finally, I will show that the bulk magnetoelectric multipolization manifests at surfaces as a magnetization, and explore an analogy with the bulk electric polarization and its associated surface charge.

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PDF file of the talk available here

On-line SPICE-SPIN+X Seminars

On-line Seminar: 22.02.2023 - 15:00 German Time

Exploring spintronics at unconventional hybrid interfaces

Angela Wittmann, JGU

Controlled manipulation of a system allows for systematic investigation of the underlying interactions and phenomena. Simultaneously, tunability also enables the development of novel materials systems and devices customized for specific applications. Here, we will focus on materials systems that conventionally have not been used as active components in spintronic devices. We will explore the impact of strain on the antiferromagnetic domain structure via magneto-elastic coupling [1]. Furthermore, we will delve into hybrid molecule-magnetic interfaces. Molecules offer a unique way of controlling and varying the structure at the interface making it possible to precisely tune the spin injection and diffusion by molecular design [2]. In particular, chirality has gained recent interest in the context of the chiral-induced spin selectivity effect [3]. Here, we will explore signatures of spin filtering at a non-magnetic chiral molecule-metal interface paving the path toward novel hybrid spintronics.

[1] Wittmann, A. et al. Role of substrate clamping on anisotropy and domain structure in the canted antiferromagnet a-Fe2O3. Phys. Rev. B 106, 224419 (2022).
[2] Wittmann, A. et al. Tuning Spin Current Injection at Ferromagnet-Nonmagnet Interfaces by Molecular Design. Phys. Rev. Lett. 124, 027204 (2020).
[3] Naaman, R., Paltiel, Y. & Waldeck, D. H. Chiral molecules and the electron spin. Nat. Rev. Chem. 3, 250–260 (2019).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 01.03.2023 - 15:00 German Time

Coherent manipulation of spins in diamond via spin-wave mixing

Toeno van der Sar, TU Delft

Coherent manipulation of spins in diamond via spin-wave mixing Magnetic imaging based on nitrogen-vacancy (NV) spins in diamond enables probing condensed matter systems with nanoscale resolution[1]. In this talk I will introduce NV magnetometry as a tool for imaging spin waves – the wave-like spin excitations of magnetic materials. Using the NV sensitivity to microwave magnetic fields, we can map coherent spin waves[2] and incoherent magnon gases[3] and provide insight into their interaction and damping[4]. By using a single NV in a scanning diamond tip we gain access to spin-wave scattering at the nanoscale[5]. I will highlight how we can use spin-wave mixing to generate frequency combs that enable high-fidelity, coherent control of the NV spins even when the applied microwave drive fields are far detuned from the NV spin resonance frequency 6 (Fig. 1). These results form a basis for developing NV magnetometry into a tool for characterizing spin-wave devices and expand the control and sensing capabilities of NV spins.

[1] Casola, F., Van Der Sar, T. & Yacoby, A. Probing condensed matter physics with magnetometry based on nitrogen-vacancy centres in diamond. Nat. Rev. Mater. 3, 17088 (2018).
[2] Bertelli, I. et al. Magnetic resonance imaging of spin-wave transport and interference in a magnetic insulator. Sci. Adv. 6, eabd3556 (2020).
[3] Simon, B. G. et al. Directional Excitation of a High-Density Magnon Gas Using Coherently Driven Spin Waves. Nano Lett. 21, 8213–8219 (2021).
[4] Bertelli, I. et al. Imaging Spin‐Wave Damping Underneath Metals Using Electron Spins in Diamond. Adv. Quantum Technol. 4, 2100094 (2021).
[5] Simon, B. G. et al. Filtering and imaging of frequency-degenerate spin waves using nanopositioning of a single-spin sensor. Nano Lett. 22, 9198 (2022).
[6] Carmiggelt, J. J. et al. Broadband microwave detection using electron spins in a hybrid diamondmagnet sensor chip. Nat. Commun. 14, 490 (2022)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 29.03.2023 - 15:00 German Time

Probabilistic spintronics – Computing and Device Physics

Shunsuke Fukami, Tohoku University

Probabilistic behavior of physical system is mostly regarded as a nuisance in conventional electronics. Contrary to this perception, here I show that properly designed probabilistic systems is even useful for unconventional computers that address complex problems more efficiently than conventional computing hardware, and spintronic systems can be a prime candidate on that front, opening a new paradigm, probabilistic spintronics.
In this seminar, I will show some proof-of-concepts of the spintronic probabilistic computers and describe how the computers can be constructed from probabilistic spintronic devices and how it solves computationally hard problems [1-4]. I will also discuss the physics governing the probabilistic behavior of spintronics devices and strategy to develop the devices for high-performance probabilistic computers [5-9].

[1] K. Camsari et al., Phys. Rev. X 7, 031014 (2017).
[2] W. A. Borders et al., Nature 573, 390 (2019).
[3] J. Kaiser et al., Phys. Rev. Appl. 17, 014016 (2022).
[4] A. Grimardi et al., IEEE IEDM 2022, 22.4 (2022).
[5] S. Kanai et al., Phys. Rev. B 103, 094423 (2021).
[6] K. Hayakawa et al., Phys. Rev. Lett. 126, 117202 (2021).
[7] K. Kobayashi et al., Appl. Phys. Lett. 119, 132406 (2021).
[8] T. Funatsu et al., Nat. Comm. 13, 4079 (2022).
[9] K. Kobayashi et al., Phys. Rev. Appl. 18, 054085 (2022).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 15.02.2023 - 15:00 German Time

A stride down the quantum materials roadmap

Alberta Bonanni, Johannes Kepler University

Lately, condensed matter physics has witnessed the emergence of material systems in which quantum effects persist over a wide range of energy and length scales [1]. Such quantum materials include, among others, topological insulators, topological crystalline insulators, magnetically doped topological quantum materials, superconductors, 2-dimensional (2D). van der Waals, Kitaev and spin-orbit systems [2]. Here, the striking properties of quantum materials will be highlighted through a collection of relevant examples overarching the Rashba spin-orbit coupling in wurtzite n-GaN:Si [3,4] and the intriguing electronic properties of the magnetically doped topological crystalline insulator SnTe [5] and of the intrinsic ferromagnetic topological insulator MnSb2Te4. Striding further down the quantum materials road, the emergence of quantum chiral anomaly in 2D Weyl semimetal Td-WTe2with a record temperature of 100 K will be addressed [6]. Moreover, a bosonic island percolation model for Fe-doped superconducting NbN thin films will be presented [7]. Finally, an outlook of emergent phenomena in hybrid quantum structures with particular attention to topology, symmetry, spin-orbit coupling and superconductivity will be provided.

[1] B. Keimar et al. Nat. Phys. 13, 1045 (2017)
[2] F. Giustino et al. J. Phys. Mater. 3, 042006 (2020)
[3] W. Stefanowicz et al. Phys. Rev. B 89, 205201 (2014)
[4] R. Adhikari et al. Phys. Rev. B 94, 085205 (2016)
[5] R. Adhikari et al. Phys. Rev. B 100, 134422 (2019)
[6] R. Adhikari et al. Nanomaterials 11, 2755 (2021)
[7] R. Adhikari et al. Nanomaterials 12, 3105 (2022)

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