arXiv subjects

Thore Posske

Publications and source records attributed to Thore Posske.

At least 19 recordsLinked to original sources

arXiv2026

Supercurrent detection and manipulation of topological phase transitions in Shiba-Majorana hybrid systems

The non-Abelian statistics of Majorana zero modes has inspired numerous proposals for their detection and manipulation in topological superconductors. Implementations based on magnetic adatoms deposited on superconductors draw particular attention due to their capabilities for precise atomic manipulation and the control over disorder. Here, we propose a scheme for detecting changes in the ground state parity of a topologically non-trivial adatom system by passing supercurrent through their low-energy modes. We unravel characteristic discontinuities in the critical current driven by zero-energy level crossings. We apply these findings to a setup where the Majorana coupling is mediated by a single control magnetic adatom hosting a Yu-Shiba-Rusinov state, and test the robustness of our results against finite temperatures and different tunneling regimes. Our findings introduce a non-invasive approach for reading out and controlling the ground state parity of Majorana states in Shiba-Majorana hybrid systems.

cond-mat.supr-con↗

arXiv2026

Symmetry and integrability in the anyon-Hubbard model

Recent cold atom experiments have realized one-dimensional anyons and enabled the tuning of 1D~statistics between bosons and fermions. Here, we analyze the symmetries, integrability, and resulting degeneracies of the underlying anyon-Hubbard model of finite length. Our results reveal a switching between symmetry classes AI, BDI, and CI in dependence on system size, particle number, and boundary conditions, and show that two anyons with periodic boundaries are integrable, while two anyons with open boundary conditions are not. We include a comprehensive analysis of all model limits, especially of interacting bosons and pseudofermions and resolve spectral signatures. We additionally reveal an exactly solvable doublon state that hides in the continuum of scattering states and the exact solution of the nullspace of two noninteracting anyons. The uncovered symmetries shape the fundamental properties of the one-dimensional anyons at hand, and the predicted states are accessible in state-of-the-art experiments.

cond-mat.quant-gas↗

arXiv2026

Universal Topological Power Transfer with Arbitrarily Large Chern Number in Driven Quantum Spin Chains

Topological frequency converters exploit a quantized transfer of power between two driving fields in a quantum system, a phenomenon topologically protected by the Chern number of the associated fiber bundle. While realizations with few-spin systems have theoretically demonstrated this effect, the conversion factors have typically been restricted to small integer values. Here, we investigate an interacting $XXZ$ Heisenberg spin-$1/2$ chain driven adiabatically by two magnetic drives with incommensurate frequencies. The Chern number, determined by the degeneracy points enclosed by the adiabatic trajectory, increases systematically with the chain length and can be tuned through the exchange anisotropy, providing direct control of the topological pumping strength. We reveal a universal dependency of the anisotropy and magnetic field strength for odd and even chain lengths of the quantum critical points. This provides a mechanism for a topological frequency converter with an arbitrarily large, quantized conversion ratio in interacting quantum spin chains. The mechanism remains the same for arbitrary coupling regions of the drives.

cond-mat.str-el↗

arXiv2026

The toric code under antiferromagnetic isotropic Heisenberg interactions

We investigate the impact of an isotropic antiferromagnetic Heisenberg perturbation on the toric code, focusing on the resulting quantum phase transition and the nature of the phase that emerges beyond topological order. Using neural-network quantum states (NQS), we compute ground states over a wide range of Heisenberg couplings while fully respecting the exact symmetries of the model. In the weak-coupling regime, the numerical results are in excellent agreement with an effective low-energy description derived from a Schrieffer-Wolff (SW) transformation, providing analytic control over the perturbative breakdown of topological order. We show that the Heisenberg perturbation only renormalizes local operators at low orders, whereas mixing between topological sectors occurs only at a perturbative order proportional to the system size. At intermediate values of the Heisenberg interaction, the topological phase breaks down. We estimate the critical point through a combination of the fidelity susceptibility and the logarithmic susceptibility of non-contractible Wilson loops for various system sizes. Furthermore, we utilize the topological entanglement entropy to provide a comprehensive characterization of the phase transition. Beyond the transition, an antiferromagnetic $\pm X/\pm Z$ N\'eel phase emerges, characterized by a fourfold-degenerate symmetry-broken manifold, which is explicitly probed using staggered-magnetization-based diagnostics. Our results show how local two-spin interactions, which naturally arise in realistic implementations of the toric code, drive the breakdown of topological order. Moreover, we establish the SW approach as a systematic framework for analyzing such perturbations in combination with variational many-body methods.

cond-mat.str-el↗

arXiv2026

Hidden Twisted Sectors and Exponential Degeneracy in Root-of-Unity XXZ Heisenberg Chains

Recently, product states have been identified as simple-structured eigenstates of XXZ Heisenberg spin models in arbitrary dimensions, occurring at anisotropy values corresponding to certain roots of unity. Yet, the product states typically only span parts of a larger degenerate eigenspace. Here, we classify this eigenspace in the one-dimensional periodic XXZ chain at all roots of unity $q$, where $q^2$ is an $\ell$-th primitive root of unity. For commensurate chain lengths $N$ with $q^N=1$, we prove that the minimal degeneracy is $2^{N/\ell}\ell$ using the representation theory of the affine Temperley-Lieb (aTL) algebra. For the incommensurate case, we derive analogous exponential lower bounds of $2^{2\lfloor\frac{N}{2\ell}\rfloor+1}$ if $N$ is even and $2^{2\lfloor \frac{N}{2\ell}+\frac{1}{2}\rfloor}$ if $N$ is odd and $q^\ell=1$. Our proof employs the morphisms between aTL modules discovered by Pinet and Saint-Aubin and emphasizes the importance of exact sequences and hidden twisted boundary condition sectors that mediate the degeneracy. In the case of commensurate chain lengths, we connect to the Fabricius-McCoy string construction of all Bethe roots of the degenerate subspace, which previously uncovered parts of our results. We corroborate our results numerically and demonstrate that the lower bound is saturated for chain lengths $N\leq20$. Our work demonstrates for a concrete system how the interplay of the Bethe ansatz, aTL representation theory, and twisted boundary conditions explains degeneracy connected to long-lived product states, stimulating research towards generalization to higher dimensions. Exponential degeneracy could boost applications of spin chains as quantum sensors.

cond-mat.stat-mech↗

arXiv2026

In-plane ferromagnetism-driven topological nodal-point superconductivity with tilted Weyl cones

The potential application of topological superconductivity in quantum transport and quantum information has fueled an intense investigation of hybrid materials with emergent electronic properties, including magnet-superconductor heterostructures. Here, we report evidence of a topological nodal-point superconducting phase in a one-atom-thick in-plane ferromagnet in direct proximity to a conventional $s$-wave superconductor. Low-temperature scanning tunneling spectroscopy data reveal the presence of a double-peak low-energy feature in the local density of states of the hybrid system, which is rationalized via model calculations to be an emergent topological nodal-point superconducting phase with tilted Weyl cones. Our results further establish the combination of in-plane ferromagnetism and conventional superconductivity as a route to design two-dimensional topological quantum phases.

cond-mat.supr-con↗

arXiv2025

Time-reversal symmetric topological superconductivity in Machida-Shibata lattices

Recent experiments engineered special spin-degenerate Andreev bound states in atomic cages of adatoms on superconductors, the Machida-Shibata states, a promising building block for quantum matter. Here, we investigate the formation of time-reversal symmetric bands by hybridizing multiple such states and analyzing their electronic topological properties assuming small on-site electron repulsion. The low-energy theory shows that competing emerging singlet and triplet superconducting pairings drive the formation of topologically non-trivial phases in symmetry class DIII. We, therefore, predict Kramers pairs of Majorana zero modes appear at the ends of Machida-Shibata chains, while two-dimensional lattices host helical Majorana edge modes. Additionally, we discover extended regions in the Brillouin zone with vanishing superconducting pairings, which are lifted by the repulsive electron interactions. Our findings offer new perspectives for manipulating topological superconductivity and pairings in non-magnetic adatom systems.

cond-mat.supr-con↗

arXiv2024

Non-local detection of coherent Yu-Shiba-Rusinov quantum projections

Probing spatially confined quantum states from afar - a long-sought goal to minimize external interference - has been proposed to be achievable in condensed matter systems via coherent projection. The latter can be tailored by sculpturing the eigenstates of the electron sea that surrounds the quantum state using atom-by-atom built cages, so-called quantum corrals. However, assuring the coherent nature of the projection, and manipulating its quantum composition, has remained an elusive goal. Here, we experimentally realize the coherent projection of a magnetic impurity-induced, Yu-Shiba-Rusinov quantum state using the eigenmodes of corrals on the surface of a superconductor, which enables us to manipulate the particle-hole composition of the projected state by tuning corral eigenmodes through the Fermi energy. Our results demonstrate a controlled non-local method for the detection of magnet superconductor hybrid quantum states.

cond-mat.supr-con↗

arXiv2024

Large diversity of magnetic phases in two-dimensional magnets with spin-orbit coupling and superconductivity

We classify the magnetic ground states of a 2D lattice of localized magnetic moments which are coupled to a superconducting substrate with Rashba-spin-orbit coupling. We discover a rich magnetic phase diagram with surprisingly complex structures including 2q-spin-spirals, a 2x2-periodic pattern, and skyrmion lattices, self-consistently, using an effective classical spin Hamiltonian and show that the system hosts non-zero 4-spin interactions. Our in-depth analysis of about ten thousand magnetic configurations becomes feasible using contrastive clustering, a recent advanced unsupervised machine learning technique. This work proposes simple few-band systems for non-collinear magnetic states and stimulates further research on topological effects in their self-consistent electronic structure.

cond-mat.supr-con↗

arXiv2024

Controlling Majorana hybridization in magnetic chain-superconductor systems

We propose controlling the hybridization between Majorana zero modes at the ends of magnetic adatom chains on superconductors by an additional magnetic adatom deposited close by. By tuning the additional adatom's magnetization, position, and coupling to the superconductor, we can couple and decouple the Majorana modes as well as control the ground state parity. The scheme is independent of microscopic details in ferromagnetic and helical magnetic chains on superconductors with and without spin-orbit coupling, which we show by studying their full microscopic models and their common low-energy description. Our results show that scanning tunneling microscopy and electron spin resonance techniques are promising tools for controlling the Majorana hybridization in magnetic adatoms-superconductor setups, providing a basis for Majorana parity measurements, fusion, and braiding techniques.

cond-mat.mes-hall↗

arXiv2024

Generalized Josephson effect with arbitrary periodicity in quantum magnets

Easy-plane quantum magnets are strikingly similar to superconductors, allowing for spin supercurrent and an effective superconducting phase stemming from their $U(1)$ rotation symmetry around the $z$-axis. We uncover a generalized fractional Josephson effect with a periodicity that increases linearly with system size in one-dimensional spin-$1/2$ chains at selected anisotropies and phase-fixing boundary fields. The effect combines arbitrary integer periodicities in a single system, exceeding the $4\pi $ and $8\pi $ periodicity of superconducting Josephson effects of Majorana zero modes and other exotic quasiparticles. We reveal a universal energy-phase relation and connect the effect to the recently discovered phantom helices.

cond-mat.str-el↗

arXiv2024

Quantum skyrmion dynamics studied by neural network quantum states

We study the dynamics of quantum skyrmions under a magnetic field gradient using neural network quantum states. First, we obtain a quantum skyrmion lattice ground state using variational Monte Carlo with a restricted Boltzmann machine as the variational ansatz for a quantum Heisenberg model with Dzyaloshinskii-Moriya interaction. Then, using the time-dependent variational principle, we study the real-time evolution of quantum skyrmions after a Hamiltonian quench with an inhomogeneous external magnetic field. We show that field gradients are an effective way of manipulating and moving quantum skyrmions. Furthermore, we demonstrate that quantum skyrmions can decay when interacting with each other. This work shows that neural network quantum states offer a promising way of studying the real-time evolution of quantum magnetic systems that are outside the realm of exact diagonalization.

cond-mat.dis-nn↗

arXiv2023

All product eigenstates in Heisenberg models from a graphical construction

Recently, large degeneracy based on product eigenstates has been found in spin ladders, kagome-like lattices, and motif magnetism, connected to spin liquids, anyonic phases, and quantum scars. We unify these systems by a complete classification of product eigenstates of Heisenberg XXZ Hamiltonians with Dzyaloshinskii-Moriya interaction on general graphs in the form of Kirchhoff rules for spin supercurrent. By this, we construct spin systems with extensive degree of degeneracy linked to exotic condensates which could be studied in atomic gases and quantum spin lattices.

cond-mat.str-el↗

arXiv2023

Ground State Properties of Quantum Skyrmions described by Neural Network Quantum States

We investigate the ground state properties of quantum skyrmions in a ferromagnet using variational Monte Carlo with the neural network quantum state as variational ansatz. We study the ground states of a two-dimensional quantum Heisenberg model in the presence of the Dzyaloshinskii-Moriya interaction (DMI). We show that the ground state accommodates a quantum skyrmion for a large range of parameters, especially at large DMI. The spins in these quantum skyrmions are weakly entangled, and the entanglement increases with decreasing DMI. We also find that the central spin is completely disentangled from the rest of the lattice, establishing a non-destructive way of detecting this type of skyrmion by local magnetization measurements. While neural networks are well suited to detect weakly entangled skyrmions with large DMI, they struggle to describe skyrmions in the small DMI regime due to nearly degenerate ground states and strong entanglement. In this paper, we propose a method to identify this regime and a technique to alleviate the problem. Finally, we analyze the workings of the neural network and explore its limits by pruning. Our work shows that neural network quantum states can be efficiently used to describe the quantum magnetism of large systems exceeding the size manageable in exact diagonalization by far.

cond-mat.mes-hall↗

arXiv2023

The quantum skyrmion Hall effect in f electron systems

The flow of electric current through a two-dimensional material in a magnetic field gives rise to the family of Hall effects. The quantum versions of these effects accommodate robust electronic edge channels and fractional charges. Recently, the Hall effect of skyrmions, classical magnetic quasiparticles with a quantized topological charge, has been theoretically and experimentally reported, igniting ideas on a quantum version of this effect. To this end, we perform dynamical mean field theory calculations on localized $f$ electrons coupled to itinerant $c$ electrons in the presence of spin-orbit interaction and a magnetic field. Our calculations reveal localized nano quantum skyrmions that start moving transversally when a charge current in the itinerant electrons is applied. The results show the time-transient build-up of the quantum skyrmion Hall effect, accompanied by an Edelstein effect and a magnetoelectric effect that rotate the spins. This work motivates studies about the steady state of the quantum skyrmion Hall effect, looking for eventual quantum skyrmion edge channels and their transport properties.

cond-mat.str-el↗

arXiv2023

Quantum spin helices more stable than the ground state: onset of helical protection

Topological magnetic structures are promising candidates for resilient information storage. An elementary example are spin helices in one-dimensional easy-plane quantum magnets. To quantify their stability, we numerically implement the stochastic Schrödinger equation and time-dependent perturbation theory for spin chains with fluctuating local magnetic fields. We find two classes of quantum spin helices that can reach and even exceed ground-state stability: Spin-current-maximizing helices and, for fine-tuned boundary conditions, the recently discovered "phantom helices". Beyond that, we show that the helicity itself (left- or right-rotating) is even more stable. We explain these findings by separated helical sectors and connect them to topological sectors in continuous spin systems. The resulting helical protection mechanism is a promising phenomenon towards stabilizing helical quantum structures, e.g., in ultracold atoms and solid state systems. We also identify an - up to our knowledge - previously unknown new type of phantom helices.

cond-mat.str-el↗

arXiv2022

Testing the topological nature of end states in antiferromagnetic atomic chains on superconductors

Edge states forming at the boundaries of topologically non-trivial phases of matter are promising candidates for future device applications because of their stability against local perturbations. Magnetically ordered spin chains proximitized by an s-wave superconductor are predicted to enter a topologically non-trivial mini-gapped phase with zero-energy Majorana modes (MMs) localized at their ends. However, the presence of non-topological end states mimicking MM properties can spoil their unambiguous observation. Here, we report on a method to experimentally decide on the MM nature of end states observed for the first time in antiferromagnetic spin chains. Using scanning tunneling spectroscopy, we find end states at either finite or near-zero energy in Mn chains on Nb(110) or Ta(110), respectively, within a large minigap. By introducing a locally perturbing defect on one end of the chain, the end state on this side splits off from zero-energy while the one on the other side doesn't - ruling out their MM origin. A minimal model shows that, while wide trivial minigaps hosting such conventional end states are easily achieved in antiferromagnetic spin chains, unrealistically large spin-orbit couplings are required to drive the system into the topologically nontrivial phase with MMs. The methodology of perturbing chains by local defects is a powerful tool to probe the stability of future candidate topological edge modes against local disorder.

cond-mat.supr-con↗

arXiv2022

Proximity superconductivity in atom-by-atom crafted quantum dots

Gapless materials in electronic contact with superconductors acquire proximity-induced superconductivity in a region near the interface. Numerous proposals build on this addition of electron pairing to originally non-superconducting systems like ferromagnets and predict intriguing quantum phases of matter, including topological-, odd-frequency-, or nodal-point superconductivity. However, atomic-scale experimental investigations of the microscopic mechanisms leading to proximity-induced Cooper pairing in surface or interface states are missing. Here, we investigate the most miniature example of the proximity effect on only a single quantum level of a surface state confined in a quantum corral on a superconducting substrate, built atom-by-atom by a scanning tunneling microscope. Whenever an eigenmode of the corral is pitched close to the Fermi energy by adjusting the corral's size, a pair of particle-hole symmetric states enters the superconductor's gap. We identify the in-gap states as scattering resonances theoretically predicted 50 years ago by Machida and Shibata, which had so far eluded detection. We further show that the observed anticrossings of the in-gap states indicate proximity-induced pairing in the quantum corral's eigenmodes. Our results have direct consequences on the interpretation of in-gap states in unconventional or topological superconductors, corroborate concepts to induce superconductivity into a single quantum level and further pave the way towards superconducting artificial lattices.

cond-mat.supr-con↗