arXiv subjects

Niclas Heinsdorf

Publications and source records attributed to Niclas Heinsdorf.

At least 19 recordsLinked to original sources

arXiv2026

Measuring higher-order bulk topology in a programmable quantum simulator

Topological insulators lie outside the standard Landau paradigm: Instead of being characterized by local order parameters and spontaneous symmetry breaking, their defining properties are encoded in the global structure of their quantum states and manifested as robust boundary states. Crystalline higher-order topological insulators can extend this principle by hosting localized corner modes in two and higher dimensions. Although these boundary modes can be detected spectroscopically through the local density of states, determining the underlying bulk topology, for example through nested Wilson loops, remains an outstanding experimental challenge. Here we use up to 728 qubits in a superconducting analogue-digital quantum simulator to realize and probe a hierarchy of one-, two-, and three-dimensional topological insulators governed by effective tight-binding Hamiltonians. By reconstructing complex propagators from real-space measurements, we obtain complementary boundary and bulk characterizations: local densities of states reveal localized boundary modes, whereas experimentally reconstructed band projectors yield bulk topological invariants, culminating in a three-dimensional octupole moment extracted through third nested Wilson loops. These results establish a route to realizing, characterizing, and manipulating topologically protected states in programmable three-dimensional quantum matter.

quant-ph↗

arXiv2026

Reversible field-free superconducting diode effect controlled by an antiferromagnet

The semiconductor diode, which allows current to flow preferentially in one direction, is a fundamental building block of numerous modern electronic circuits. Its superconducting analogue---the superconducting diode effect---enables directional dissipationless current flow and may provide similar functionality in future superconducting quantum circuits. Realization of such a nonreciprocal supercurrent requires broken time-reversal symmetry. At zero applied field, this has been typically associated either with intrinsic unconventional superconductivity or extrinsic spin-split electronic states induced by magnetic proximity. Here we demonstrate a field-free superconducting diode effect in conventional superconducting Al proximitized by collinear antiferromagnetic CuMnAs, whose electronic structure breaks time-reversal symmetry without generating spin splitting. By tuning the proximity effect through an insulating AlAs interlayer and correlating the reversal of the diode polarity with the reversal of the remanent Néel state, we establish that the antiferromagnet controls the superconducting diode effect. Our results show that neither spin-split bands nor net magnetization is required for a magnetically controlled field-free superconducting diode effect, extending superconducting nonreciprocity to a broader class of collinear compensated magnets.

cond-mat.supr-con↗

arXiv2026

Topological superconductivity in an altermagnet-superconductor heterostructure

Fully gapped spin-triplet superconductors offer a natural setting for topological edge transport and Majorana excitations. Here we propose a heterostructure that interfaces altermagnetic monolayer V$_2$Se$_2$O with a conventional $s$-wave superconductor. Starting from an ab-initio description of V$_2$Se$_2$O, we construct a low-energy model of its spin-split Fermi surface and study the superconducting instability of the coupled stack. We find that the momentum-dependent exchange splitting of the altermagnet strongly favors equal-spin pairing and converts the proximity-induced order into a fully gapped $p$-wave state. Over a broad range of interface parameters, the resulting Bogoliubov-de Gennes bands carry a nonzero Chern number, and slab spectra exhibit chiral Majorana edge modes traversing the bulk gap. We further quantify the inverse proximity effect and show how the induced gap evolves across a finite multilayer stack. These results establish a route to topological triplet superconductivity by interfacing a conventional superconductor with an experimentally available altermagnetic semiconductor, and provide practical guidance for interface fabrication and experimental detection.

cond-mat.supr-con↗

arXiv2026

Altermagnetic Instabilities from Quantum Geometry

Altermagnets are a newly identified type of collinear anti-ferromagnetism with vanishing net magnetic moment, characterized by lifted Kramers' degeneracy in parts of the Brillouin zone. Their time-reversal symmetry broken band structure has been observed experimentally and is theoretically well-understood. On the contrary, altermagnetic fluctuations and the formation of the corresponding instabilities remains largely unexplored. We establish a correspondence between the quantum metric of normal and the altermagnetic spin-splitting of ordered phases. We analytically derive a criterion for the formation of instabilities and show that the quantum metric favors altermagnetism. We recover the expression for conventional q=0 instabilities where the spin-splitting terms of the normal state model are locally absent. As an example, we construct an effective model of MnTe and illustrate the relationship between quantum geometry and altermagnetic fluctuations by explicitly computing the quantum metric and the generalized magnetic susceptibility.

cond-mat.str-el↗

arXiv2026

Magnetic Field Control of the Néel Vector and Magnon Visibility in Altermagnetic MnTe

Altermagnetic order gives rise to momentum-dependent spin splitting of electronic and magnonic excitations even in the absence of a net magnetization. Here, we investigate the magnetic field dependence of the spin-wave spectrum of altermagnetic $α$-MnTe using inelastic neutron scattering and linear spin-wave theory. An in-plane magnetic field continuously reorients the Néel vector by overcoming the weak crystalline anisotropy, while remaining small compared with the dominant exchange scale. We find that this reorientation leaves the magnon energies and line widths essentially unchanged, but strongly modifies the measured spectral intensity through the transverse-momentum projection. Our results demonstrate a clear separation between the soft orientational degree of freedom of the antiferromagnetic order and the robust exchange-dominated chiral magnon spectrum. This combination establishes $α$-MnTe as a platform for reconfigurable magnon coupling, in which external fields tune how excitations interact with polarized probes without substantially altering their frequency or coherence.

cond-mat.str-el↗

arXiv2026

Higher Berry curvature, second Chern numbers and magnetoelectric coupling in crystalline insulators

We rewrite a lattice model of the four-dimensional Chern insulator as a family of translationally-invariant infinite chains over the three-dimensional Brillouin zone and compute its higher three-form Berry curvature using infinite matrix product states (iMPS). We calculate the topological phase diagram of the associated Dixmier--Douady--Kapustin--Spodyneiko (DDKS) number as a function of the model's mass term, and show that it is exactly congruent to the phase diagram in terms of the second Chern number, the analytic expression of which is known for this particular model. This agreement demonstrates that higher Berry curvature can be used to compute second Chern numbers in a manifestly quantized manner. Motivated by the connection between the second Chern form and the Chern--Simons axion coupling, we study magnetoelectric coupling in three dimensions and its relation to higher Berry phases.

cond-mat.str-el↗

arXiv2026

Coherent high-velocity chiral magnons in the metallic altermagnet CrSb

We report the collective magnetism of the metallic altermagnet CrSb. Magnetic susceptibility and polarized neutron diffraction measurements show that CrSb is a perfectly compensated Ising altermagnet below a Néel temperature of $T_N = 733(4)$ K. Inelastic neutron scattering experiments reveal coherent and highly-dispersive antiferromagnetic spin waves with large group velocities of 61(2) km s$^{-1}$ and 58(2) km s$^{-1}$ along the in-plane and out-of-plane directions, respectively. The observed magnon dispersions along high-symmetry directions of the Brillouin zone are well described by a minimal Heisenberg model up to third nearest neighbors of alternating antiferromagnetic and ferromagnetic character, $J_1 = 23(4)$ meV, $J_2 = -5.4(8)$ meV, $J_3 = 5.2(8)$ meV, and an Ising single-ion anisotropy term $D = 0.15(4)$ meV. We observe clear momentum space signatures of chiral spin splitting along the low-symmetry $Γ$-$L$ direction, characteristic of higher-order altermagnetic exchange interactions, the first such observation in a metallic altermagnet. These findings identify CrSb as a singular material: a metallic altermagnet in which coherent spin-split magnons persist well above room temperature, providing a compelling platform for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Intrinsic defect intolerance in the ultra-pure metal PtSn$_4$

Ultra-pure materials are highly valued as model systems for the study of intrinsic physics. Frequently, however, the crystal growth of such pristine samples requires significant optimization. PtSn$_4$ is a rare example of a material that naturally forms with a very low concentration of crystalline defects. Here, we investigate the origin of its low defect levels using a combination of electrical resistivity measurements, computational modeling, and scanning tunneling microscopy imaging. While typical flux-grown crystals of PtSn$_4$ can have residual resistivity ratios (RRRs) that exceed 1000, we show that even at the most extreme formation speeds, the RRR cannot be suppressed below 100. This aversion to defect formation extends to both the Pt and Sn sublattices, which contribute with equal weight to the conduction properties. Direct local imaging with scanning tunneling microscopy further substantiates the rarity of point defects, while the prohibitive energetic cost of forming a defect is demonstrated through density functional theory calculations. Taken together, our results establish PtSn$_4$ as an intrinsically defect-intolerant material, making it an ideal platform to study other properties of interest, including extreme magnetoresistance and topology.

cond-mat.mtrl-sci↗

arXiv2025

Non-Reciprocal Zone Boundary Magnon Propagation in Cu$_2$OSeO$_3$

Inelastic neutron scattering in the chiral magnet Cu$_2$OSeO$_3$ reveals strong non-reciprocal effects on magnon propagation at the boundary of the nuclear Brillouin zone. The non-reciprocal response is strongest at a central position between the zone corner and edge mid-point. We explain these results using an effective linear spin-wave model. While directional effects in chiral magnets have so far only been known to exist at low momenta close to the center of the Brillouin zone, the present study shows that non-reciprocity persists at the highest possible reduced momenta. The observed magnons show very little damping within the limits of our experimental resolution, making them of great interest for the fundamental research on compact, high-frequency magnonic applications.

cond-mat.str-el↗

arXiv2025

Beyond-classical computation in quantum simulation

Quantum computers hold the promise of solving certain problems that lie beyond the reach of conventional computers. However, establishing this capability, especially for impactful and meaningful problems, remains a central challenge. Here, we show that superconducting quantum annealing processors can rapidly generate samples in close agreement with solutions of the Schrödinger equation. We demonstrate area-law scaling of entanglement in the model quench dynamics of two-, three-, and infinite-dimensional spin glasses, supporting the observed stretched-exponential scaling of effort for matrix-product-state approaches. We show that several leading approximate methods based on tensor networks and neural networks cannot achieve the same accuracy as the quantum annealer within a reasonable time frame. Thus, quantum annealers can answer questions of practical importance that may remain out of reach for classical computation.

quant-ph↗

arXiv2025

Proximitizing altermagnets with conventional superconductors

Recent theoretical work highlighted unique properties of superconducting altermagnets, including the wealth of topologically non-trivial phases as well as their potential uses in spintronic applications. Given that no intrinsically superconducting altermagnets have yet been discovered, we study here the possibility of superconducting order induced by proximity effect from a conventional s-wave superconductor. Through symmetry analysis and microscopic modeling we find that interesting superconducting phases can indeed be proximity-induced in a thin altermagnetic film provided that weak Rashba spin-orbit coupling is present at the interface. Surprisingly, the resulting superconductor is generically nodal with a mixed singlet/triplet order parameter and, importantly for applications, capable of generating spin-polarized persistent current. We propose a set of candidate heterostructures with low lattice mismatch suitable to probe these effects experimentally.

cond-mat.supr-con↗

arXiv2025

Persistent spin currents in superconducting altermagnets

Superconductors are famously capable of supporting persistent electrical currents, that is, currents that flow without any measurable decay as long as the material is kept in the superconducting state. We introduce here a class of materials -- superconducting altermagnets -- that can both generate and carry persistent {\em spin} currents. This includes spin-polarized electrical supercurrent as well as pure spin supercurrent that facilitates spin transport in the absence of any charge transport. A key to this remarkable property is the realization that the leading superconducting instability of altermagnetic metals consists of two independent condensates formed of spin-up and spin-down electrons. In the non-relativistic limit the two condensates are decoupled and can thus naturally support persistent currents with any spin polarization, including pure spin supercurrents realized in the charge counterflow regime. We describe a novel ``spin-current dynamo effect'' that can be used to generate pure spin supercurrent in such systems by driving a charge current along certain crystallographic directions. Away from the non-relativistic limit, when spin-orbit interactions and magnetic disorder are present, we find that the spin current generically develops spatial oscillations but, importantly, no dissipation or decay. This is in stark contrast to spin currents in normal diffusive metals which tend to decay on relatively short lengthscales. We illustrate the above properties by performing model calculations relevant to two distinct classes of altermagnets and various device geometries.

cond-mat.supr-con↗

arXiv2025

Edge-Edge Correlations without Edge-States: $η$-clustering State as Ground State of the Extended Attractive SU(3) Hubbard Chain

We explore the phase diagram of the extended attractive SU($3$) Hubbard chain with two-body hopping and nearest-neighbor attraction at half-filling. In the large on-site attraction limit, we identify three different phases: phase separation (PS), Tomonaga-Luttinger liquid (TLL), and charge density wave (CDW). Our analysis reveals that the $η$-clustering state, a three-component generalization of the $η$-pairing state, becomes the ground state at the boundary between the PS and TLL phases. On an open chain, this state exhibits an edge-edge correlation, which we call boundary off-diagonal long-range order (bODLRO). Using the density matrix renormalization group (DMRG) method, we numerically study the phase diagram of the model with large but finite on-site interactions and find that the numerical results align with those obtained in the strong coupling limit.

cond-mat.str-el↗

arXiv2025

Fate of Bosonic Topological Edge Modes in the Presence of Many-Body Interactions

Many magnetic materials are predicted to exhibit bosonic topological edge modes in their excitation spectra, because of the nontrivial topology of their magnon, triplon, or other quasi-particle band structures. However, there is a discrepancy between theory prediction and experimental observation, which suggests some underlying mechanism that intrinsically suppresses the expected experimental signatures, like the thermal Hall current. Many-body interactions that are not accounted for in the non-interacting quasi-particle picture are most often identified as the reason for the absence of the topological edge modes. Here we report persistent bosonic edge modes at the boundaries of a ladder quantum paramagnet with gapped triplon excitations in the presence of the full many-body interaction. We use tensor network methods to resolve topological edge modes in the time-dependent spin-spin correlations and the dynamical structure factor, which is directly accessible experimentally. We further show that signatures of these edge modes survive even when the non-interacting quasi-particle theory breaks down, discuss the topological phase diagram of the model, demonstrate the fractionalization of its low-lying excitations, and propose potential material candidates.

cond-mat.str-el↗

arXiv2024

Nature of the current-induced insulator-to-metal transition in Ca$_2$RuO$_4$ as revealed by transport-ARPES

The Mott insulator Ca$_2$RuO$_4$ exhibits a rare insulator-to-metal transition (IMT) induced by DC current. While structural changes associated with this transition have been tracked by neutron diffraction, Raman scattering, and x-ray spectroscopy, work on elucidating the response of the electronic degrees of freedom is still in progress. Here we unveil the current-induced modifications of the electronic states of Ca$_2$RuO$_4$ by employing angle-resolved photoemission spectroscopy (ARPES) in conjunction with four-probe transport. Two main effects emerge: a clear reduction of the Mott gap and a modification in the dispersion of the Ru-bands. The changes in dispersion occur exclusively along the $XM$ high-symmetry direction, parallel to the $b$-axis where the greatest in-plane lattice change occurs. These experimental observations, together with dynamical mean-field theory (DMFT) calculations simulated from the current-induced structural distortions, indicate the intimate interplay of lattice and orbital-dependent electronic response in the current-driven IMT. Furthermore, based on a free energy analysis, we demonstrate that the current-induced phase, albeit thermodynamically equivalent, is electronically distinct from the high-temperature zero-current metallic phase. Our results provide insight into the elusive nature of the current-induced IMT of Ca$_2$RuO$_4$ and advance the challenging, yet powerful, technique of transport-ARPES.

cond-mat.str-el↗

arXiv2024

Discovery of Superconductivity and Electron-Phonon Drag in the Non-Centrosymmetric Weyl Semimetal LaRhGe$_3$

We present an exploration of the effect of electron-phonon coupling and broken inversion symmetry on the electronic and thermal properties of the semimetal LaRhGe$_3$. Our transport measurements reveal evidence for electron-hole compensation at low temperatures, resulting in a large magnetoresistance of 3000% at 1.8 K and 14 T. The carrier concentration is on the order of $10^{21}\rm{/cm}^3$ with high carrier mobilities of $2000~\rm{cm}^2/\rm{Vs}$. When coupled to our theoretical demonstration of symmetry-protected $\textit{almost movable}$ Weyl nodal lines, we conclude that LaRhGe$_3$ supports a Weyl semimetallic state. We discover superconductivity in this compound with a $T_{\text c}$ of 0.39(1) K and $B_{\rm{c}}(0)$ of 2.2(1) mT, with evidence from specific heat and transverse-field muon spin relaxation. We find an exponential dependence in the normal state electrical resistivity below $\sim50$ K, while Seebeck coefficient and thermal conductivity measurements each reveal a prominent peak at low temperatures, indicative of strong electron-phonon interactions. To this end, we examine the temperature-dependent Raman spectra of LaRhGe$_3$ and find that the lifetime of the lowest energy $A_1$ phonon is dominated by phonon-electron scattering instead of anharmonic decay. We conclude that LaRhGe$_3$ has strong electron-phonon coupling in the normal state, while the superconductivity emerges from weak electron-phonon coupling. These results open up the investigation of electron-phonon interactions in the normal state of superconducting non-centrosymmetric Weyl semimetals.

cond-mat.supr-con↗

arXiv2023

Quantum error mitigation in quantum annealing

Quantum Error Mitigation (QEM) presents a promising near-term approach to reduce error when estimating expectation values in quantum computing. Here, we introduce QEM techniques tailored for quantum annealing, using Zero-Noise Extrapolation (ZNE). We implement ZNE through zero-temperature extrapolation as well as energy-time rescaling. We conduct experimental investigations into the quantum critical dynamics of a transverse-field Ising spin chain, demonstrating the successful mitigation of thermal noise through both of these techniques. Moreover, we show that energy-time rescaling effectively mitigates control errors in the coherent regime where the effect of thermal noise is minimal. Our ZNE results agree with exact calculations of the coherent evolution over a range of annealing times that exceeds the coherent annealing range by almost an order of magnitude.

quant-ph↗

arXiv2023

Higher Berry curvature from matrix product states

The higher Berry curvature was introduced by Kapustin and Spodyneiko as an extension of the Berry curvature in quantum mechanical systems with finite degrees of freedom to quantum many-body systems in finite spatial dimensions. In this paper, we propose an alternative formulation of the higher Berry curvature using translationally invariant matrix product states. They are the ground states of a set of gapped Hamiltonians which are evolved adiabatically through a discretized parameter space. Because matrix product states transform under a projective representation, evaluating the Berry curvature on a closed loop through parameter space is not sufficient to fix all the gauge degrees of freedom. To obtain a gauge-invariant real quantity, the higher-dimensional Berry curvature is evaluated on small tetrahedra in parameter space. Our numerical calculations confirm that the higher Berry curvature varies continuously throughout an adiabatic evolution and becomes quantized over a closed 3-dimensional parameter space.

quant-ph↗