arXiv subjects

Mathias S. Scheurer

Publications and source records attributed to Mathias S. Scheurer.

At least 19 recordsLinked to original sources

arXiv2026

Altermagnetic spin textures coupled to superconductors: Domain wall spin-triplet superconductivity and supercurrent-induced torques

Motivated by the absence of sizable stray fields and the recently discovered highly non-trivial impact of altermagnetic textures on itinerant electrons, we here study the form of Cooper pairs in spatially varying altermagnets coupled to conventional $s$-wave superconductors. As a consequence of the detrimental impact of altermagnetism on spin-singlet pairing and the local symmetry reduction caused by textures in the magnetic order parameter, we show that superconductivity predominantly impacts the regions between altermagnetic domains. Focusing on a planar radial domain wall for concreteness, we show that emergent Zeeman and spin-orbit fields create spatially separated triplet hotspots and transitions between nodal and fully gapped superconducting regions, whose structure is set by both the domain wall and the altermagnetic order parameter. We also identify a reciprocal effect, where a supercurrent generates a quasiparticle-mediated quadrupolar torque that inherits the symmetry of the altermagnetic order. Our results show that accounting for spatial inhomogeneities in the altermagnetic order parameter is essential for an understanding of the superconducting proximity effect and suggest that hybrid systems of altermagnetic textures and superconductors offer unique opportunities for local engineering of Cooper pairs and for detecting altermagnetic order.

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arXiv2026

Anisotropic vacancy-induced magnetization textures in altermagnets

We study magnetic textures induced by vacancies in altermagnets using microscopic simulations and low-energy field theory. We show that a vacancy generically produces a real-space anisotropic distortion of the magnetic order, whose structure encodes the symmetry of the underlying altermagnetic state. This impurity response offers a direct route to detecting altermagnetic order with locally resolved probes. We demonstrate this for both classical altermagnets, where vacancies generate anisotropic magnetization textures in a transverse magnetic field, and quantum models, where fluctuations induce longitudinal power-law decaying magnetic distortions even at zero field.

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arXiv2026

In-plane and out-of-plane magnetic field driven Josephson diode effect in magic-angle twisted four-layer graphene

The superconducting diode effect offers a powerful probe into the fundamental symmetries of quantum materials. Recent studies on twisted graphene diodes have predominantly focused on bilayer or trilayer systems under out-of-plane magnetic fields. Here, we demonstrate both out-of-plane and in-plane driven Josephson diode effects in a magic-angle twisted four-layer graphene junction, i.e., an even number of layers. We observe the emergence of a diode effect at zero out-of-plane field, tuned by an increasing in-plane magnetic field. This result points to the presence of strong in-plane orbital coupling, which is highly sensitive to the specific layer parity of the structure. Our findings provide experimental insights into the symmetry-breaking mechanisms of even-layer twisted graphene, establishing in-plane magnetic fields as a vital tool for unravelling their microscopic properties.

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arXiv2026

Intervalley coherence and flavor polarization in three-valley moir\'e systems

We investigate interaction-induced symmetry breaking in moir\'e superlattices created by twisting two identical materials where the electronic low-energy degrees of freedom reside in the vicinity of the $M$ points. Based on general symmetry arguments, we identify and classify the possible candidate instabilities that, besides flavor polarized states, also involve a variety of intervalley-coherent (IVC) orders. This complexity is related primarily to the presence of three valleys, instead of the well-studied scenario of two, e.g., in graphene: IVC states can couple all three valleys identically, with a non-trivial sign structure, or even with different magnitudes. We study the energetics using an analytical strong-coupling framework and unrestricted Hartree-Fock applied to the full continuum model, with very good agreement between the two approaches. Interestingly, depending on stacking, IVC instabilities not only appear due to superexchange at moderate bandwidths, but also deep in the strong-coupling regime as a result of deviations from the flat-metric condition. Our work demonstrates that twisted $M$-point materials provide a rich playground for complex correlated physics and highlights differences and similarities to twisted multilayer graphene.

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arXiv2026

Probing pairing symmetries through quasiparticle interference in chiral Bloch bands

Recent experiments in van der Waals multi-layer systems have demonstrated that superconductivity can emerge from symmetry-reduced, chiral normal states. We here provide a theory for quasiparticle interference (QPI) of superconductors with chiral Bloch bands. Our analysis reveals how the non-trivial quantum geometry of the Bloch states crucially affects the interference pattern even in the normal state, inducing significant sublattice dependence. In the superconducting state, the behavior becomes more complex due to the interplay of the quantum geometry of the Bogoliubov quasiparticles with the momentum-dependent phase of the order parameter. We reveal how the spatial dependence of the local spectral function around impurities can be used to distinguish between different candidate pairing states, both with zero and finite center-of-mass momentum. Our work thus provides guidance to interpreting QPI patterns in materials with chiral bands, which may be useful when probing the rich physics of pairing in such systems.

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arXiv2026

Noise spectroscopy of insulating and itinerant altermagnets

One of the central goals in the emergent field of altermagnetism is the unambiguous experimental identification and characterization of altermagnetic order across a variety of compounds. This motivates exploring tools that can clearly distinguish altermagnets from antiferromagnets, based on symmetry signatures, and offer access to the dominant orbital character (e.g., $d$-wave vs. $g$-wave) of the magnetic order parameter. In this work, we theoretically explore the potential of noise magnetometry for this task, studying contributions from both magnons and itinerant electrons in different regimes and scenarios. While altermagnetism and antiferromagnetism also lead to different noise spectra for magnons, we find the most striking and symmetry-sensitive signatures in the charge fluctuations of itinerant altermagnets. Both for the homogeneous bulk case and in the presence of strain and/or around domain walls, we identify noise contributions that are only permitted by symmetry in the altermagnet and, thus, provide a unique signature of altermagnetism. Furthermore, the angular dependence of noise around domain walls also offers access to the orbital character of the altermagnet. On a more technical note, we discuss the role and relevance of lattice effects related to the dipole tensor. We hope that our work will help pave the way towards the clear experimental identification of altermagnetism across a wide range of candidate materials.

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arXiv2026

Hatsugai-Kohmoto-like Models for Altermagnets and Odd-Parity Magnets

We introduce a generalized Hatsugai-Kohmoto multi-orbital model and study its phase diagram and physical properties in the additional presence of perturbations that lift any extensive ground-state degeneracies. The unperturbed, exactly solvable model already displays a rich set of spectral functions, including regimes reminiscent of unconventional magnets. We map the first-order study of additional spatially local multi-orbital Hubbard interactions to a Heisenberg model in momentum space, which leads to symmetry-breaking instabilities already at weak coupling. Interestingly, translational-symmetry breaking orders, such as antiferromagnetism, are excluded. Instead, in addition to ferromagnetism, unconventional $p$-wave and $d$-wave magnets occur, characterized by spin order on the bonds of the underlying square lattice. Adding another type of momentum-space interaction, which still allows to solve the model exactly, is shown to stabilize a non-degenerate singlet ground state that retains the spin splitting characteristic of unconventional magnets. We discuss its impact on the spin structure factor. Taken together, our findings show that Hatsugai-Kohmoto-like models provide a rich playground for unconventional magnetism.

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arXiv2026

High-harmonic generation in systems with chiral Bloch states: application to rhombohedral graphene

Nonlinear light-matter interaction and, in particular, high-harmonic generation (HHG) are fundamentally interesting and frequently discussed as versatile probes of quantum materials with potential for optical information processing applications. Meanwhile, there has also been significant progress in graphene-based multilayer systems to engineer interesting band structures and boost correlation effects. Motivated by the successful demonstration of HHG in graphene, we here study this effect in rhombohedral stacks of $n$ layers of graphene, a recent very prominent representative of correlated multilayer graphene systems. We show how the chiral Bloch states of the valleys of this system crucially affect the HHG. The "winding" of the Bloch states scales linearly with $n$, just like the dominant harmonic order. The location of the strongest quantum geometry in momentum space on a ring of finite radius is shown to be imprinted on the time-dependent momentum distribution at the beginning of the strong laser pulse. We further demonstrate that the presence of an interaction-induced splitting of the two valleys leads to a complex interplay of the opposite chiralities of the two valleys, directly visible in the $n$ dependence of the circular dichroism. We also analyze the impact of doping and identify a quantity that tracks the net chirality of the occupied states. Our findings show that rhombohedral graphene constitutes a promising platform for exploring rich nonlinear optical phenomena.

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arXiv2026

Discovery of Quasi One Dimensional Superconductivity in PtPb3Bi

Quasi one dimensional materials provide a compelling platform where reduced dimensionality stabilizes intertwined topological and superconducting phases. Here we report superconductivity in a new Bi based quasi 1D compound, PtPb3Bi, which hosts a nontrivial electronic structure. It exhibits type II superconductivity below 3.01(1) K. Heat capacity and transverse field muon spin rotation relaxation (muSR) measurements demonstrate a fully gapped isotropic s wave state with moderate electron phonon coupling, while zero field muSR confirms the preservation of time reversal symmetry (TRS). Transport measurements reveal low carrier mobility with diffusive normal state transport. Electronic structure calculations show strong dispersion along the quasi 1D direction and relatively flatter bands in the transverse plane, giving rise to pronounced Fermi surface nesting in the kx-ky plane. Consistent with this, the compound undergoes a charge density wave transition at 280(1) K. The flow of Wannier charge centers, together with surface state dispersion, establishes nontrivial band topology. These results identify PtPb3Bi as a new quasi 1D superconductor with nontrivial electronic structure and a promising candidate for topological superconductivity.

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arXiv2026

Tuning correlated states of twisted mono-bilayer graphene with proximity-induced spin-orbit coupling

We study the correlated ground states of twisted mono-bilayer graphene with and without proximity-induced spin-orbit coupling (SOC) from a transition-metal dichalcogenide layer placed on top. We perform self-consistent Hartree-Fock calculations that allow the variational space to include multi-$Q$ translational symmetry broken states for all integer and half-integer fillings of the conduction bands, where signatures of correlated, topological states have been reported experimentally. We find interaction-induced insulators that retain moir\'e translational symmetry at integer fillings, but that break this symmetry at half-integer fillings. We argue that translational symmetry breaking arises from half-filled polarized bands, even when SOC is present. Yet, we find that small SOC can already crucially affect the spin nature of correlated states. Generally, Ising SOC favors out-of-plane spin polarization and spin-valley locking, while Rashba SOC favors in-plane spin order. If only one of these two terms is present, we find that, depending on the type of SOC, it drives a transition from a tetrahedal antiferromagnet to either a coplanar, non-coplanar, or collinear spin-density wave state for half-integer fillings. The frustration associated with the simultaneous presence of both types of SOC can induce chiral, non-coplanar order in parameter ranges where the ground state in the absence of SOC is collinear.

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arXiv2026

Altermagnetic spin textures: Emergent electrodynamics, quantum geometry, and probes

Emergent electrodynamics arising from spatially and temporally varying magnetic textures provides a framework for spin control in quantum materials. While this principle is established for ferromagnetic and antiferromagnetic textures, its consequences for altermagnets -- magnetic orders with vanishing net magnetization but finite spin splitting -- remain largely unexplored. In this work, we develop an effective low-energy theory of itinerant electrons coupled to smoothly varying altermagnetic spin textures. In the adiabatic regime, we show that altermagnetic textures generate additional emergent electromagnetic fields and quantum-geometric effects that are absent in conventional magnetic systems. These effects include emergent Zeeman fields that encode the structure of the altermagnetic order parameter, enabling local spin manipulation and a way to distinguish different altermagnetic orders. Moreover, we demonstrate a quantum-metric-induced, spin-dependent electron lensing effect that provides a mechanism for spin filtering, and discuss the local admixture of effective odd-parity magnetic components. Our results suggest that textured altermagnets could serve as a versatile resource for spintronics functionalities and a probe of altermagnetism.

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arXiv2026

Altermagnetic superconducting diode effect from non-collinear compensated magnetism in Mn$_3$Pt

Altermagnets have recently emerged as a distinct class of magnetic systems that exhibit spin splitting of electronic bands while retaining zero net magnetization. This unique combination makes them a promising platform for time-reversal symmetry-breaking superconducting phenomena, although identifying concrete material platforms remains an important open challenge. Here, we develop a theory for the superconducting diode effect observed experimentally in a Mn$_3$Pt-superconductor heterostructure. Using both a symmetry analysis and model calculations on the breathing kagome lattice, we show how the altermagnetic spin textures in Mn$_3$Pt generate a spin splitting of the electronic bands that remains magnetization-free even in the presence of spin-orbit coupling and, upon taking into account the proximity coupling across the interface, produces a superconducting diode effect. We also demonstrate that the angular dependence of the critical current provides a probe of the magnetic order. We hope that our work will contribute to the understanding and further discovery of candidate materials for novel altermagnet-superconductor hybrid devices.

cond-mat.mes-hall↗

arXiv2026

Altermagnetic Superconducting Diode Effect in Mn$_{3}$Pt/Nb Heterostructures

Compensated magnetic orders that can split the spin-degeneracy of electronic bands have become a very active field of research. As opposed to spin-orbit coupling, the splitting resulting from these "altermagnets" is not a small relativistic correction and, in contrast to ferromagnets, not accompanied by a net magnetization and large stray fields. In particular, the theoretical analysis of the interplay of altermagnetism and superconductivity has taken center stage, while experimental investigations of their coexistence remain in their infancy. We here study heterostructures consisting of Nb thins films interfaced with the $T_1$ and $T_2$ phases of Mn$_3$Pt. These non-collinear magnetic states can be thought of as descendants from the same altermagnetic order in the absence of spin-orbit coupling. We demonstrate the non-trivial impact on the superconducting state of Nb, which exhibits a zero-field superconducting diode effect, despite the compensated ($T_2$) and nearly-compensated ($T_1$) magnetic order; the diode efficiencies can reach large values (up to 50$\%$). The diode effect is found to be highly sensitive to the form of the magnetic order, illustrating its potential as a symmetry probe. The complex magnetic field and temperature dependence hint at a rich interplay of multiple contributing mechanisms. Our results define a new materials paradigm for dissipationless spintronics and magnetization-free diode functionality, while motivating further exploration of non-collinear altermagnetic superconductors.

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arXiv2025

Exactly Solvable Models Hosting Altermagnetic Quantum Spin Liquids

We construct spin-$3/2$ and spin-$7/2$ models on the square-octagon and checkerboard lattices that are exactly solvable with Majorana representations. They give rise to spin-liquid phases with full spin-rotation and lattice-translational symmetries but broken time-reversal symmetry. Although non-zero on elementary plaquettes, the net orbital magnetic moment is guaranteed to vanish as a result of point symmetries; due to the analogy to long-range ordered altermagnets, these types of phases were dubbed altermagnetic spin liquids in [Phys. Rev. Research 7, 023152 (2025)]. For the spin-$3/2$ model, we find that a $g$-wave altermagnetic spin liquid emerges as the unique ground state. In contrast, the spin-7/2 model exhibits a significantly richer phase diagram, involving different types of chiral spin liquids competing with a $d$-wave altermagnetic spin liquid. Finally, we identify and characterize the topological and non-topological excitations, illustrating the rich physics of altermagnetic spin liquids resulting from the interplay of non-trivial topological and symmetry aspects of this novel phase of matter.

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arXiv2025

Anomalous impurity-induced charge modulations in black phosphorus

We observe anomalous charge modulations induced by ionized indium impurities on the surface of the semiconductor black phosphorus by scanning tunneling microscopy (STM). When the impurities are switched into a negatively charged state by the STM tip, periodic charge modulations emerge around the impurity center, but strictly confined by the nanoscale impurity potential. These modulations form a distorted triangular pattern, whose periodicity remains unchanged in a wide range of positive bias. Furthermore, these local charge orders exhibit an anisotropy opposite to that expected based on the anisotropy of the Fermi surface, challenging a simple band-structure interpretation. Our experiment demonstrates the possibility of creating and manipulating macroscopic charge orders through impurity engineering.

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arXiv2025

Correlations between superconducting and resistive anisotropies

There are multiple possible origins of transport anisotropies in metals and superconductors. For instance, rotational symmetry can be spontaneously broken in the normal state as a result of electronic nematic order inducing anisotropies in an otherwise $s$-wave superconducting phase. Another possibility is that the dominant source of rotational symmetry breaking is the superconductor itself and its vestiges that may survive in the normal state. We here theoretically analyze the correlations of transport anisotropies in the normal and the corresponding superconducting phase for different scenarios of broken symmetry, either coming solely from the normal state, solely from the superconductor and its vestiges in the metallic regimes, or from both simultaneously. We further include both zero-momentum and finite-momentum pairing; we develop a theory of vestigial order for the latter, characterized by broken rotational and translational symmetry. Our findings reveal that the relative transport anisotropies in the normal and superconducting phases sensitively depend on the scenario, including the form of vestigial order and, in some cases, the parity of the superconducting order parameter. As such, measuring the directional dependence of the critical current and resistivity can provide strong constraints on the origin of rotational symmetry breaking. We demonstrate our findings in minimal models relevant to twisted multilayer graphene, rhombohedral graphene, and twisted transition metal dichalcogenides.

cond-mat.supr-con↗

arXiv2025

Quantum geometry and impurity sensitivity of superconductors without time-reversal symmetry: application to rhombohedral graphene and altermagnets

Analyzing the consequences of the quantum geometry induced by the momentum dependence of Bloch states has emerged as a very rich and active field in condensed matter physics. For instance, for the superfluid stiffness or the pairing mechanism, these geometric aspects can play an important role. We here demonstrate that quantum geometry can also be essential for the disorder sensitivity of a superconductor, in particular when time-reversal symmetry is broken in the normal-state Bloch Hamiltonian. We derive a general expression for the behavior of the critical temperature $T_c$ involving weighted (anti-)commutators of the superconducting order parameter and impurity matrix elements, which includes both wave-function effects and kinetic pair breaking due to broken time-reversal symmetry in the dispersion. We analyze how the former effects lead to "quantum geometric pair breaking", where any superconductor becomes susceptible to microscopically non-magnetic impurities, and formally relate it to the maximum possible localization of two-particle Wannier states. Further, in the presence of kinetic pair breaking, impurities can also enhance pairing, leading to an overall more complex, non-monotonic behavior of $T_c$ with impurity concentration. We also analyze the fate of finite-momentum pairing. Our results are directly relevant to rhombohedral graphene, twisted MoTe$_2$, and superconducting altermagnets.

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arXiv2025

Superconductivity of Incoherent Electrons near the Relativistic Mott Transition in Twisted Dirac Materials

We demonstrate that superconductivity driven by strong quantum-critical fluctuations can emerge near relativistic Mott transitions in twisted two-dimensional materials, taking on a remarkably rich character. In twisted double-bilayer WSe$_2$, all time-reversal-even, gap-opening collective modes promote pairing, whereas time-reversal-odd modes do not. In a Dirac model of twisted bilayer graphene, the Gross-Neveu transition into inter-valley-coherent insulators gives rise to a spectrum of degenerate and nearly degenerate superconducting states. More generally, we show that the richer the Dirac structure, the more readily pairs can form. A crucial ingredient of the theory is that critical fluctuations render the electronic states strongly incoherent, allowing attractive pairing channels to overcome the bare Dirac semi-metal behavior. Finally, we demonstrate a direct relation between boson-mediated pairing and the formation of charge-carrying skyrmionic excitations in the proximate insulating state.

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