On-line SPICE-SPIN+X Seminars

On-line Seminar: 08.04.2026 - 15:00 CET

TBA

Rafael Fernandes, UIUC

TBA

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 14.01.2026 - 15:00 CET

Magnetic excitations beyond the single- and double- magnons

Hebatalla Elnaggar , Sorbonne University

Conventional wisdom suggests that one photon that carries one unit of angular momentum (1h) can change the spin angular momentum of a magnetic site with one unit (ΔM = ±1h) at most following the selection rules. This implies that a two-photon process such as 23 resonant inelastic X-ray scattering (RIXS – see Fig. 1a-c) can change the spin angular momentum of a magnetic system with a maximum of two units (ΔM = ± 2h) [1]. Herein we describe a triple-magnon excitation in the altermagnetic system, -Fe2O3, which contradicts this conventional wisdom that only 1- and 2-magnon excitations are possible in a resonant inelastic X-ray scattering experiment [2].

Figure 1: Schematic of Resonant Inelastic X-ray Scattering (RIXS). (a) The initial state of a 3d transition metal plus a photon with energy ℏin, wave-vector kin. (b) The intermediate state where a 2p electron is excited to the empty 3d states leaving a core-hole that exists for few fs. (c) The final state where a valence 3d electron fills the core-hole and a photon with energy ℏout, wave-vector kout is emitted. The energy and momentum transfer are given by ℏ(in - out) and ℏ(kin-kout), respectively. (d) Fe 2p3d RIXS measured in -Fe2O3 single crystal where we observed multi-magnons.

We observe an excitation at exactly three times the magnon energy, along with additional excitations at four and five times the magnon energy, suggesting the presence of quadruple and quintuple magnons as well (see Fig. 1d). Guided by theoretical calculations, we reveal how a two-photon scattering process can create exotic higher-rank magnons and the relevance of these quasiparticles for understanding spin non-conserving interactions where the lattice degree of freedom acts as a reservoir of angular momentum.

References:
[1]- A. Nag, et. al., Many-body physics of single and double spin-flip excitations in NiO, Phys. Rev. Lett., 124, 067202 (2020).
[2]- H. Elnaggar, et. al., Magnetic excitations beyond the single- and double-magnons, Nat. Commun. 14, 2749 (2023).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 25.02.2025 - 15:00 CET

Emerging Altermagnetism and Polar States in Strained Metallic RuO2 Films

Bharat Jalan , University of Minnesota

Department of Chemical Engineering and Materials Science, University of Minnesota, Twin Cities

RuO2, a rutile 4d-transition metal oxide, exhibits a unique crystal structure with both edge- and corner-sharing octahedra. This intrinsic anisotropy, when combined with strain engineering, provides a powerful avenue for tuning anisotropic electronic and optical properties. However, from a synthesis perspective, challenges such as variable Ru valence states, Ru/O stoichiometry control, anisotropic strain states, and structural defects can make it difficult to distinguish intrinsic properties from extrinsic effects in RuO2 thin films – a classic trick in the pursuit of novel functionalities in quantum materials.

In this talk, I will highlight our group’s efforts in overcoming these synthesis challenges while demonstrating metallicity in epitaxial RuO2 films down to the unit cell scale. Through a combination of advanced X-ray scattering, X-ray absorption spectroscopy, transmission electron microscopy, temperature-dependent transport, magneto-optical measurements, and density functional theory (DFT) calculations, we uncover robust magnetism in epitaxially strained RuO2, consistent with an altermagnetic metallic phase [1-4]. Additionally, we reveal a novel polar phase in strained films with significant implications for electrical transport – an unexpected treat in the realm of functional oxides. I will discuss these findings in detail, emphasizing their sensitivity to material defects and structure – key ingredients that are often overlooked but crucial in determining emergent quantum phenomena.

Reference:

1. S. G. Jeong†, I. H. Choi†, S. Nair, L Buiarelli, B. Pourbahari, J. Y. Oh, N. Bassim, A. Seo, W. S. Choi, R. M. Fernandes, T. Birol, L. Zhao, J. S. Lee, and B. Jalan, Altermagnetic polar metallic phase in ultra-thin epitaxially-strained RuO2 films, (under review) (2025) [arxiv] †Equal contribution
2. S. G. Jeong, I. H. Choi, S. Lee, J. Y. Oh, S. Nair, J. H. Lee, C. Kim, A. Seo, W. S. Choi, T. Low, J. S. Lee, and B. Jalan, Anisotropic Strain Relaxation-Induced Directional Ultrafast Carrier Dynamics in RuO2 Films, Sci. Adv. 11, eadw7125 (2025)
3. S. G. Jeong, S. Lee, B. Lin, Z. Yang, I. H. Choi, J. Y Oh, S. Song, S. W. Lee, S. Nair, R. Choudhary, J. Parikh, S. Park, W. S. Choi, J. S. Lee, J. M. LeBeau, T. Low, and B. Jalan, Metallicity and Anomalous Hall Effect in Epitaxially-Strained, Atomically-thin RuO2 Films, PNAS 122(24) e2500831122
4. S. G. Jeong, B. Y. X. Lin, M. Jin, I. H. Choi, S. Lee, Z. Yang, S. Nair, R. Choudhary, J. Parikh, A. Santhosh, M. Neurock, K. A. Stoerzinger, J. S. Lee, T. Low, Q. Tu, J. M. LeBeau, and B. Jalan, Strain-Stabilized Interfacial Polarization Tunes Work Function Over 1 eV in RuO2/TiO2 Heterostructures, under review (2025) [arxiv]

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 26.11.2025 - 15:00 CET

Novel glimpse into ground states of quantum matter

Vesna Mitrović, Brown University

In this talk I would describe novel in-situ ``interferometry'' technique that is employed to probe ground state properties of the complex materials. Examples of the power of this nuclear magnetic resonance inspired technique will be illustrated on magnetic and frustrated materials.
Specifically, I will show how this technique can be used to sense changes in quantum mechanical ground state wavefunction through a high temperature magnetic phase transition.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 12.11.2025 - 15:00 CET

Superconducting spintronics with magnetically compensated materials

Jacob Wüsthoff Linder, NTNU

Altermagnets have emerged as intriguing materials supporting strongly spin-polarized
currents despite the lack of a net magnetization. We demonstrate that altermagnets
enable several promising functionalities when merged with conventional superconductors.
We predict that altermagnets act as spin-filters for triplet Cooper pairs
and that they can function as cryogenic spin-valves acting as a memory device without stray fields, offering
high storage densities. Finally, we discuss the interplay between p-wave magnetism
and superconductivity both intrinsically in a material and via the proximity effect in a bilayer.
We show that p-wave magnets induce a charge-to-spin conversion in combination with
superconductors that, unexpectedly, is even larger than in the normal state.

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 15.10.2025 - 15:00 CEST

Superconductivity in altermagnets

Annica Black-Schaffer, Uppsala University

Altermagnets break spin-degeneracy, as in a ferromagnet, but with a momentum dependent spin splitting resulting in zero net magnetization, as in antiferromagnets. Due to this unique magnetization, altermagnets also produce intriguing possibilities for other ordered phases of matter. Magnetism and superconductivity are two of the most celebrated quantum phases of matter and usually have a ‘friend-foe’ dichotomous relation and combining superconductivity with altermagnetism turns out to open for new exceptional possibilities. In this talk I will show several novel effects occurring when superconductivity appears in altermagnets, including finite momentum pairing, field-induced superconductivity [1], and a perfect superconducting diode effect [2], as well as demonstrate constraints on the superconducting pairing [3]. If time permits, I will also demonstrate the possibility for orbital-selective altermagnetism in the unconventional superconductor Sr2RuO4 [4].

[1] D. Chakraborty and A. M. Black-Schaffer, Zero-field finite-momentum and field-induced superconductivity in altermagnets, Phys. Rev. B110, L060508 (2024).
[2] D. Chakraborty and A. M. Black-Schaffer, Perfect superconducting diode effect in altermagnets, Phys. Rev. Lett. 135, 026001 (2025)
[3] D. Chakraborty and A. M. Black-Schaffer, Constraints on superconducting pairing in altermagnets, Phys. Rev. B 112, 014516 (2025)
[4] C. Autieri, G. Cuono, D. Chakraborty, P. Gentile, and A. M. Black-Schaffer, Conditions for orbital-selective altermagnetism in Sr2RuO4: Tight-binding model, similarities with cuprates, and implications for superconductivity, Phys. Rev. B 112, 014412 (2025)

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 06.08.2025 - 15:00 CEST

Chiral phonons for spintronics

Ulrich Nowak, University of Konstanz

The discovery and investigation of chiral phonons uncovered a variety of novel physical effects which are related to the chiral phonon's angular momentum as well as its associated magnetic field. Their interaction with the electron spin offers opportunities for an exploitation in spintronics, where they can be functionalized as additional carriers for the transport of anular momentum. However, the microscopic understanding of the coupled dynamics of the electronic angular momenta and the lattice degrees of freedom [1] is still incomplete.

This new research field calls for new modelling approaches and numerical tools. In this talk we report on recent developments in the microscopic understanding of the coupling between spin and lattice degrees of freedom with an emphasis on the exchange of angular momentum between these two subsystems. Specifically, we discuss a framework for spin-molecular dynamics that connects, on the one hand, to ab initio calculations of spin-lattice coupling parameters [2,3] and, on the other hand, to the magneto-elastic continuum theory. This framework allows for multi-scale modeling approaches including the development of material-specific atomistic models for the coupled spin and lattice degrees of freedom, the calculation and investigation of magnon-phonon dispersion relations [4], and the development and use of modelling tools for coupled spin-lattice dynamics.

[1] S. R. Tauchert, M. Volkov, D. Ehberger, D. Kazenwadel, M. Evers, H. Lange, A. Donges, A. Book, W. Kreuzpaintner, U. Nowak, P. Baum: Polarized phonons carry angular momentum in ultrafast demagnetization, Nature 602, 73 (2022)
[2] S. Mankovsky, S. Polesya, H. Lange, M. Weißenhofer, U. Nowak, and H. Ebert:
Angula Momentum Transfer via Relativistic Spin-Lattice Coupling from First Principles, Phys. Rev. Lett. 129, 067202 (2022)
[3] M. Weißenhofer, H. Lange, A. Kamra, S. Mankovsky, S. Polesya, H. Ebert, and U. Nowak: Rotationally invariant formulation of spin-lattice coupling in multi-scale modeling, Phys. Rev. B 108, L060404 (2023)
[4] M. Weißenhofer, P. Rieger, M.S. Mrudul, L. Mikadze, U. Nowak, and P. M. Oppeneer: Truly chiral phonons arising fromm chirality selective Magnon-Phonon Coupling, arXiv:2411.03879v1
[5] 2022

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 25.06.2025 - 15:00 CEST

Magnetism in Moiré Materials

Allan MacDonald , University of Texas

Two-dimensional van der Waals crystals that are overlaid with a difference in lattice constant or a relative twist form a moiré pattern. In semiconductors and semimetals, the low-energy electronic properties of these systems are accurately described by Hamiltonians that have the periodicity of the moiré pattern creating artificial crystals with lattice constants on the 10 nm scale. Recent progress in fabricating two-dimensional material devices has made it possible to use moiré patterns to design quantum metamaterials in which electrons exhibit strongly-correlated and topologically non-trivial properties that are rare in naturally occuring crystals. Since the miniband widths in both graphene and TMD moiré materials can be made small compared to interaction energy scales (by mechanisms [1,2] that differ), these materials can be used both for quantum simulation and for quantum design. An important property of moiré materials is that their band filling factors can be tuned over large ranges without introducing chemical dopants, simply by using electrical gates.
In this talk I will focus on magnetism in moiré materials, which is sometimes similar to that found in atomic scale crystals and sometimes unusual. In many cases the magnetic order is purely orbital – opening the door to electrical manipulation of magnetic states. Orbital magnetic order combined with non-trivial topology in single-particle bands [3] helps to make quantum anomalous Hall effects common and gives rise to the fractional quantum anomalous Hall effect. The role of band topology is natural in graphene moirés, where it derives from the interesting band topology of graphene monolayers, but has been an unexpected bonus [3] in the case of TMD moires where it derives from the layer degree of freedom.
[1] R. Bistritzer, and A.H.MacDonald, Proceedings of the National Academy of Sciences 26, 12233 ( 2011).
[2] F. Wu, T. Lovorn, E. Tutuc, and A.H.MacDonald, Phys. Rev. Lett. 121, 026402 (2018).
[3] F. Wu, T. Lovorn, E. Tutuc, I. Martin, and A.H.MacDonald, Phys. Rev. Lett. 122, 086402 (2019).

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On-line SPICE-SPIN+X Seminars

On-line Seminar: 16.07.2025 - 15:00 CEST

Superconductivity at interfaces of the quantum paraelectric KTaO3

Anand Bhattacharya, Argonne National Laboratory

In this talk I will discuss the recently discovered two-dimensional superconductivity found at interfaces of the incipient ferroelectric KTaO3 (KTO). In its pristine insulating state, KTO is believed to be a ‘quantum paraelectric’, where the onset of ferroelectricity at low temperatures is thwarted by quantum fluctuations. A metallic electron gas can be obtained at interfaces of KTO by depositing a variety of insulating metal-oxide overlayers. Electron microscopy studies reveal the presence of both oxygen vacancies near the interface of KTO and diffusion of cations into KTO from the oxide overlayers, which dope the interfacial region of KTO with electrons. These interfacial electron gases were found to be superconducting up to temperatures as high has 2.2 K. Remarkably, the superconducting state is orientation selective, where electron gases formed at the (111) and (110) crystalline interfaces of KTO are robust two-dimensional superconductors, with Tc as high as 2.2 K and 1 K respectively, while electron gases formed at the (001) interface of KTO and oxide overlayers remain normal down to 25 mK. In this light, I will present a proposed mechanism for superconductivity at KTO interfaces where pairing involves an inter-orbital coupling mechanism mediated by the same soft phonon that is responsible for the incipient ferroelectricity in KTO. This mechanism favors superconductivity in states with maximal orbital degeneracy, and the lifting of this degeneracy due to quantum confinement effects explains the orientation selective nature of superconductivity at KTO interfaces. The broken inversion symmetry and strong spin-orbit coupling in KTO interfacial electron gases also lead to a spin-textured Fermi surface. I will outline how orbital degeneracy gives rise to a uniaxial ‘in-plane Ising’ spin texture for electron gases formed at KTO (110) interfaces, evidenced by their interaction with an insulating magnetic overlayer in both their superconducting and normal states.

References:
1. C. Liu et al., Science 371, 716 (2021).
2. C. Liu et al., Nature Communications 14, 951 (2023).
3. J. Yang et al., arXiv 2502.19599.

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