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On-line SPICE-SPIN+X Seminars

On-line Seminar: 08.11.2023 - 15:00 CET

Probabilistic Computing with p-bits: Optimization, Machine Learning and Quantum Simulation

Kerem Çamsarı, University of California

The slowing down of Moore's era has coincided with escalating computational demands from Machine Learning and Artificial Intelligence. An emerging trend in computing involves building physics-inspired computers that leverage the intrinsic properties of physical systems for specific domains of applications. Probabilistic computing with p-bits, or probabilistic bits, has emerged as a promising candidate in this area, offering an energy-efficient approach to probabilistic algorithms and applications.

Several implementations of p-bits, ranging from standard CMOS technology to nanodevices, have been demonstrated. Among these, the most promising p-bits appear to be based on stochastic magnetic tunnel junctions (sMTJ). sMTJs harness the natural randomness observed in low barrier nanomagnets to create energy-efficient and fast fluctuations, up to GHz frequencies. In this talk, I will discuss how magnetic p-bits can be combined with conventional CMOS to create hybrid probabilistic-classical computers for various applications. I will provide recent examples of how p-bits are naturally applicable to combinatorial optimization, such as solving the Boolean satisfiability problem, energy-based generative machine learning models like deep Boltzmann machines, and quantum simulation for investigating many-body quantum systems.

Through experimentally-informed projections for scaled p-computers using sMTJs, I will demonstrate how physics-inspired probabilistic computing can lead to GPU-like success stories for a sustainable future in computing.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 24.01.2024 - 15:00 CET

Orbital Angular Momentum for Spintronics

Mathias Kläui, JGU Mainz

Novel spintronic devices can play a role in the quest for GreenIT if they are stable and can transport and manipulate spin with low power. Devices have been proposed, where switching by energy-efficient approaches is used to manipulate topological spin structures [1,2].
We combine ultimate stability of topological states due to chiral interactions [3,4] with ultra-efficient manipulation using novel spin torques [3-5]. In particular orbital torques [6] increase the switching efficiency by more than a factor 10. We probe the materials dependence of the orbital current generation and conversion and show that the switching current density is strongly reduced compared to state-of-the-art spin-orbitronic devices.
We use skyrmion dynamics for non-conventional stochastic computing applications, where we developed skyrmion reshuffler devices [7] based on skyrmion diffusion, which also reveals the origin of skyrmion pinning [7]. We go beyond simple ferromagnets and study multilayers with antiferromagnetic coupling termed synthetic antiferromagnets. We find that the diffusion dynamics is drastically enhanced due to the topology and efficient dynamics can be induced by spin torques [8]. Finally, we find novel topological spin structures, such as bi-merons that are stabilized in synthetic antiferromagnets [9].

References
[1] G. Finocchio et al., J. Phys. D: Appl. Phys., vol. 49, no. 42, 423001, 2016.
[2] K. Everschor-Sitte et al., J. Appl. Phys., vol. 124, no. 24, 240901, 2018.
[3] S. Woo et al., Nature Mater., vol. 15, no. 5, pp. 501–506, 2016.
[4] K. Litzius et al., Nature Phys., vol. 13, no. 2, pp. 170–175, 2017.
[5] K. Litzius et al., Nature Electron., vol. 3, no. 1, pp. 30–36, 2020.
[6] S. Ding et al. Phys. Rev. Lett. 125, 177201, 2020; Phys. Rev. Lett. 128, 067201, 2022.
[7] J. Zázvorka et al., Nature Nanotechnol., vol. 14, no. 7, pp. 658–661, 2019;
R. Gruber et al., Nature Commun. vol. 13, pp. 3144, 2022.
[9] T. Dohi et al., Nature Commun. vol. 14, pp. 5424, 2023.
[9] M. Bhukta et al., arxiv:2303.14853 (Nature Commun. in press 2024)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 25.10.2023 - 15:00 CEST

Spintronics with low-symmetry materials

Felix Casanova, CIC nanoGUNE

Two-dimensional materials are an exciting new material family in which the proximity effect is especially important and opens ways to transfer useful spintronic properties from one 2D material into another. For instance, transition metal dichalcogenides (TMD) can be used to enhance the spin-orbit coupling of graphene. The spin-orbit proximity in such graphene/TMD van der Waals heterostructures leads to spin-to-charge conversion (SCC) of out-of-plane spins due to spin Hall effect (SHE), first observed by our group using MoS2 as the TMD [1]. The combination of long-distance spin transport and SHE in the same material gives rise to an unprecedented figure of merit (product of spin Hall angle and spin diffusion length) of 40 nm in graphene proximitized with WSe2, which is also gate tunable [2].
The low symmetry present in many of these low-dimensional materials allows the creation of spin polarizations in unconventional directions and enables new fundamental effects and configurations for devices. In this regard, chiral systems are the ultimate expression of broken symmetry, lacking inversion and mirror symmetry. One way to achieve this is by twisting a graphene/TMD heterostructure. We use twisted graphene/WSe2 to observe SCC arising from Rashba-Edelstein effect (REE) from spins not only perpendicular to the current (conventional configuration), but also parallel to the current (unconventional configuration) [3]. Furthermore, we can tune the twist angle between graphene and WSe2 to control the helicity of the Rashba spin texture, which even changes sign, in excellent agreement with theoretical predictions [4].
Another way to exploit chirality is by directly using materials with a chiral crystal structure, such as elemental tellurium (Te), a 1D van der Waals material. We have recently demonstrated a gate-tunable chirality-dependent charge-to-spin conversion in Te, [5], detected by recording a large unidirectional magnetoresistance (up to 7%). The orientation of the electrically generated spin polarization is determined by the crystal handedness, while its magnitude can be manipulated by an electrostatic gate.
Our results pave the way for the development of chirality-based spintronic devices.

References
[1] C. K. Safeer, FC et al., Nano Lett. 19, 1074 (2019).
[2] F. Herling, FC et al. APL Mater. 8, 071103 (2020).
[3] H. Yang, FC et al. submitted.
[4] S. Lee, FC et al. Phys. Rev. B 106, 165420 (2022).
[5] F. Calavalle, FC et al., Nat. Mater. 21, 526 (2022).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 18.10.2023 - 15:00 CEST

Spin and Charge Pumping in the Presence of Spin-Orbit Coupling in THz Spintronics with Antiferromagnets

Branislav K. Nikolić, University of Delaware

The interaction of fs light pulses with magnetic materials has been intensely studied for more than two decades to understand ultrafast demagnetization in single magnetic layers or THz emission from their bilayers with nonmagnetic spin-orbit (SO) materials. Despite long history, microscopic understanding of ultrafast-light-driven magnets is incomplete due to numerous competing effects and with virtually no study reporting calculation of output THz radiation. This talk presents a recently developed [1] multiscale quantum-classical formalism where conduction electrons are described by quantum master equation of the Lindblad type; classical dynamics of local magnetization is described by the Landau-Lifshitz-Gilbert (LLG) equation; and incoming light is described by classical vector potential while outgoing electromagnetic radiation is computed using the Jefimenko equations for retarded electric and magnetic fields. We illustrate it by application to a bilayer of Weyl antiferromagnet Mn3Sn with noncollinear local magnetization in contact with SO-coupled nonmagnetic material, revealing new mechanisms of THz radiation due to direct charge pumping by local magnetization dynamics of Mn3Sn in the presence of its strong intrinsic SO coupling. I also discuss how to modify this approach when LLG equation becomes inapplicable and local magnetization must be treated by quantum many-body techniques including dissipation [2], as is the case of strongly correlated antiferromagnet NiO. Finally, the simplest example of dynamics of magnetization leading to spin and charge pumping is that of microwave-driven uniformly precessing local magnetization within ferro- or antiferromagnets, where we show how to handle the presence of SO coupling using the Floquet-Keldysh formalism [3] with possible first-principles Hamiltonian as an input [4]. This yields a new prediction of high harmonics [3] in pumped spin and charge currents due to peculiar motion of flowing electron spins generated by the SO coupling.

[1] A. Suresh and B. K. Nikolić, Phys. Rev. B 107, 174421 (2023)
[2] F. Garcia-Gaitan and B. K. Nikolić, https://doi.org/10.48550/arXiv.2303.17596 (2023)
[3] J. Varela-Manjarres and B. K. Nikolić, J. Phys. Mater. 6, 045001 (2023)
[4] K. Dolui, A. Suresh, and B. K. Nikolić, J. Phys. Mater. 5, 034002 (2022)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 21.06.2023 - 15:00 CEST

All electrical magnon transport experiments in magnetically ordered insulators

Matthias Althammer, Walther-Meißner-Institut

Pure spin currents, i.e. the flow of angular momentum without an accompanying charge current represents a new paradigm in the field of spintronics. Most importantly, pure spin currents can be transported by fermions, i.e. by electrons, in electrical conductors as well as by bosons, i.e. by magnons, the quantized spin excitations in magnetically ordered systems. Interestingly, heterostructures consisting of spin-orbit coupled metals with magnetically ordered insulators allow to investigate pure spin current transport in both regimes and their interconversion at the interface. This approach enables an all electrical injection and detection scheme to study magnon transport phenomena in magnetically ordered insulators [1]. I will present recent results focusing on ferrimagnetic and antiferromagnetic insulators. First, I will present spin conductance measurements in yttrium iron garnet thin films. Here, I will highlight how charge currents can control the magnon spin conductance realizing a compensation of magnon damping via spin-orbit torques [2,3]. In the second part, I will focus on magnon spin transport in antiferromagnetic insulators. The quantized spin excitations of an ordered antiferromagnet with opposite chirality represent pairs of spin-up and -down magnons and this two-level nature can be characterized by a magnonic pseudospin. In the last years, first descriptions and observations of the associated dynamics of antiferromagnetic pseudospin have been reported [1,2,3,4]. I will introduce these magnon pseudospin dynamics and describe how they lead to the manifestation of the magnon Hanle effect in hematite thin films.

[1] M. Althammer, Phys. Stat. Sol. RRL 15, 2100130 (2021)
[2] T. Wimmer et al., Phys. Rev. Lett. 123, 257201 (2019)
[3] J. Gückelhorn et al., Phys. Rev. B 104, L140404 (2021)
[4] T. Wimmer et al., Phys. Rev. Lett. 125, 247204 (2020)
[5] A. Kamra et al., Phys. Rev. B 102, 174445 (2020)
[6] J. Gückelhorn et al., Phys. Rev. B 105, 094440 (2022)
[7] J. Gückelhorn et al., arXiv:2209.09040 (accepted in PRL 2023)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 22.03.2023 - 15:00 German Time

Strong coupling of microwaves and magnons in YIG microstructures

Georg Schmidt, Martin Luther Univ. Halle

Strong coupling between microwaves and magnons has already been demonstrated. So, magnon modes in an yttrium iron garnet (YIG) sphere of several hundred micrometer diameter were successfully coupled to the microwave modes of a large microwave cavity[1]. Also coupling of magnons in YIG to phonons[2] or to optical photons[3] has already been shown. All these experiments have in common that rather macroscopic pieces of YIG were used. This is unfavorable if the effect is to be integrated for device purposes, both in terms of size and technology.
On the other hand coupling between magnetic microstructures and superconducting resonators has been reported making use of ferromagnetic metals that can easily be patterned[4,5]. Nevertheless, the lifetime of spin waves in ferromagnetic metals is rather small and although strong coupling could be demonstrated, it would be desirable to use microscopic YIG resonators instead.
We have realized coupling between microwave photons in superconducting lumped element resonators and magnons in Permalloy and YIG nanostructures, respectively. With the metallic ferromagnet we realize an unusual coupling behavior because of the strong shape anisotropy in an elongated structure. With YIG, we are able to reach the strong coupling regime. This is possible because of an optimized lumped element resonator that concentrates the magnetic field in the magnetic microstructure.

[1] e.g. Y. Tabuchi et al., Phys. Rev. Lett. 113, 083603 (2014)
[2] X. Zhang et al., Science Advances 2, e1501286
[3] A. Osada et al., Phys. Rev. Lett. 116, 223601 (2016)
[4] Y. Li et al., Phys. Rev. Lett. 123, 107701 (2019)
[5] J.T. Hou et al., Phys. Rev. Lett. 123, 107702 (2019)
 

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12.09.2022 – Elasto-Q-Mat Summer School 2022: Interplay of multiple degrees of freedom – Charge, Spin and lattice

The exotic phases arising from the complex interplay between the electron and the elementary excitation such as phonons, magnons, etc. is one of the prominent aspects of condensed matter physics. The complex interplay often results in different competing ground states with different microscopic properties and different low energy excitations. Disrupting the system by external stimuli such as changing temperature, applying pressure, or doping with different chemical elements, one can manipulate through different phases and try to understand the microscopic multiple degrees of freedom in correlated many body systems. In addition, complex systems offer a great deal of real world applications, however, sufficient understanding and knowledge of many body interactions is first necessary on a fundamental level.
In this regard, the Elasto-Q-Mat Summer School “Interplay of multiple degrees of freedom – charge, spin and lattice” is intended to bring the state of the art expertise in the field of condensed matter physics to educate our PhD student within the SFB Transregio 288 project. Thus, our students have the opportunity to become familiar with the current research both in terms of theoretical and experimental perspective in the diverse field of many body systems.

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19.07.2022 – Orbitronics: from Topological Matter to next Level Electronics


Orbitronics: from Topological Matter to next Level Electronics

This workshop aims to boost the new field of orbitronics – a next generation device technology which utilizes the orbital current as an information carrier. The orbital current is expected to be crucial in understanding physical properties of topological matters and to interact with various orders and quasi-particle excitations in nontrivial ways, which may shed lights on unresolved puzzles in correlated matters and lead to discoveries of exotic quantum phenomena. The workshop highlights the emerging concept of the orbital current from the perspective of topology and strong correlation, which are two major pillars of contemporary condensed matter physics, and seek for a novel route to achieving orbitronic devices with different materials such as van der Waals 2D materials, topological matters, oxides, surfaces and interfaces. This would not only have significant impact on next-generation of spin-torque-based memories and devices but also open a new venue for spintronics and valleytronics. The envisioned impact of the workshop is to review status-of-the-art and to discuss challenges and future directions of orbitronics by gathering both young and renowned researchers from condensed matter physics, material science, and nanotechnology.

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06.06.2022 – Spins, Orbits, Charges, and Heat in Magnets


Spins, Orbits, Charges, and Heat in Magnets

This SPICE Young Research Leaders Group Workshop serves as a melting pot of researchers to discuss recent developments in our understanding of the interplay between magnetism and spin, charge, orbital, and heat transport. 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 and heat gradients, adding interconversion phenomena between spin, charge, orbital degrees of freedom and heat to the spintronic inventory, examples being the spin Seebeck, spin Nernst, Edelstein, and orbital Hall 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 Hall effects in skyrmion crystals, anomalous Hall effects and spin splitting in antiferromagnets, and the magnetic spin Hall effect. These findings constitute the chalk with which to draw the outlines of next-generation technologies, such as antiferromagnetic and topological spintronics, (topological) magnonics, obitronics, etc.

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21.06.2022 – Non-Equilibrium Emergence in Quantum Design

Non-Equilibrium Emergence in Quantum Design

Design of quantum many body states which elude conventional thermodynamics, has nowadays become a reality in a number of experimental platforms operating in the far-from-equilibrium regime. This workshop merges experts from three different topical areas exploring non-equilibrium control and engineering, ranging from the microscopic to the macroscopic.
At SPICE we will gather scholars working on fundaments of many-body quantum correlations and frontiers of quantum simulation in closed and open systems, encompassing applications to quantum technologies.
The goal of the conference is foster dialogue at the interface of these different research sectors, focusing on three keynote themes: (1) present and future of quantum many body simulators and their expected impact in the NISQ (noisy intermediate-scale quantum) era; (2) state of art of quantum thermalization and scrambling from the standpoint of statistical mechanics, and its role in the development of a novel generation of quantum devices; (3) survival and control of quantum many-body correlations in strongly driven-open settings.
The structure of the workshop revolves around alternating sessions on these thematic areas, offering a kaleidoscope of three workshops entangled into one. Our invited speakers are equally selected between fundamental and application-oriented areas. We encourage young scientists from all over the world to join us, and we look forward engaging them at our dedicated poster sessions.

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