arXiv subjects

Jonas Braun

Publications and source records attributed to Jonas Braun.

5 recordsLinked to original sources

arXiv2026

Causal Local States: Scalable Simultaneous Causal Network Inference and Forecasting for Dynamical Systems

Machine learning methods predict many real-world systems with remarkable accuracy, but they are typically treated as black boxes that offer no insight into which interactions drive the dynamics. Causal discovery methods reconstruct the interaction network from observational data, but without regard to whether the inferred structure supports prediction. Existing approaches combining both tasks rely on a single global hyperparameter, such as a causal threshold or a fixed neighborhood size, which cannot recover the structure of heterogeneous systems. Here we introduce causal local states (CLS), a framework that simultaneously infers an approximate Granger-causal interaction network and forecasts the system dynamics. For each node independently, we select the smallest set of neighbors that allows a predictive model to forecast the node near-optimally, and the resulting neighborhoods are then combined for a forecast of the full system. On three benchmarks of increasing difficulty, we achieve reconstruction of the underlying networks with high fidelity and forecasts on par with a model that is supplied with the true network, providing a step toward explainable and scalable forecasting of complex systems.

cs.LG↗

arXiv2025

Finite-Temperature Toroidal Moment Amenable to Direct Observation in an Fe$_{10}$Dy$_{10}$ Molecular Ring

Single-molecule toroics (SMTs) host closed magnetic-vortex configurations that carry toroidal moments $\boldsymbol{\tau}$, whose electric-dipole symmetry enables magnetoelectric spin control. Yet, opposite toroidal chiralities are degenerate in conventional magnetic fields, making direct detection of molecular toroidal polarisation challenging. Current approaches probe molecular toroidal dynamics only indirectly through weak residual magnetism, leaving direct interrogation of toroidal polarisation an open challenge. Moreover, the survival of toroidal polarization at finite temperature, and realistic preparation-and-readout conditions, have not been quantitatively established. Here we investigate the icosanuclear $3d$--$4f$ molecular ring Fe$_{10}$Dy$_{10}$, featuring a $\sim$62-billion-dimensional low-energy manifold with pervasive toroidal character, rendered computationally tractable via an ab initio-informed transfer-matrix framework with perturbative corrections. Our model reproduces magnetic and calorimetric measurements and reveals a maximally toroidal ground doublet with robust finite-temperature toroidal response. We introduce the toroidal susceptibility $\xi$ as a finite-temperature linear-response function to quantify toroidal polarisation induced by magnetic-field curl. We then develop a preparation-and-detection protocol in which a temporally asymmetric near-infrared waveform generates a cumulative toroidal population imbalance, while an ab initio-informed magnetoelectric tensor predicts an electric-field-induced magnetic moment within $\mu$SQUID detectability. These results establish Fe$_{10}$Dy$_{10}$ as a molecular platform where toroidal polarisation can be prepared, accumulated and read out under realistic experimental conditions.

cond-mat.mes-hall↗

arXiv2018

Halo structure of $^{17}$C

$^{17}$C has three states below the $^{16}$C + $n$ threshold with quantum numbers $J^P=3/2^+,1/2^+,5/2^+$. These states have relatively small neutron separation energies compared to the neutron separation and excitation energies of $^{16}$C. This separation of scales motivates our investigation of $^{17}$C in a Halo effective field theory (Halo EFT) with a $^{16}$C core and a valence neutron as degrees of freedom. We discuss various properties of the three states such as electric radii, magnetic moments, electromagnetic transition rates and capture cross sections. In particular, we give predictions for the charge radius and the magnetic moment of the $1/2^+$ state and for neutron capture on $^{16}$C into this state. Furthermore, we discuss the predictive power of the Halo EFT approach for the $3/2^+$ and $5/2^+$ states which are described by a neutron in a $D$-wave relative to the core.

nucl-th↗

arXiv2016

Electric Properties of One-Neutron Halo Nuclei in Halo EFT

We exploit the separation of scales in weakly-bound nuclei to compute E2 transitions and electric form factors in a Halo EFT framework. The relevant degrees of freedom are the core and the halo neutron. The EFT expansion is carried out in powers of $R_{core}/R_{halo}$, where $R_{core}$ and $R_{halo}$ denote the length scales of the core and halo, respectively. We include the strong $s$-wave and $d$-wave interactions by introducing dimer fields. The dimer propagators are regulated by employing the power divergence subtraction scheme and matched to the effective range expansion in the respective channel.Electromagnetic interactions are included via minimal substitution in the Lagrangian. We demonstrate that, depending on the observable and respective partial wave, additional local gauge-invariant operators contribute in LO, NLO and higher orders.

nucl-th↗

arXiv2015

Importance-Truncated Large-Scale Shell Model

We propose an importance-truncation scheme for the large-scale nuclear shell model that extends its range of applicability to larger valence spaces and mid-shell nuclei. It is based on a perturbative measure for the importance of individual basis states that acts as an additional truncation for the many-body model space in which the eigenvalue problem of the Hamiltonian is solved numerically. Through a posteriori extrapolations of all observables to vanishing importance threshold, the full shell-model results can be recovered. In addition to simple threshold extrapolations, we explore extrapolations based on the energy variance. We apply the importance-truncated shell model for the study of 56-Ni in the pf valence space and of 60-Zn and 64-Ge in the pfg9/2 space. We demonstrate the efficiency and accuracy of the approach, which pave the way for future shell-model calculations in larger valence spaces with valence-space interactions derived in ab initio approaches.

nucl-th↗