arXiv subjects

Daniel Pajerowski

Publications and source records attributed to Daniel Pajerowski.

9 recordsLinked to original sources

arXiv2026

Magnetic order, magnons, and crystal fields in van der Waals CeSiI

We report neutron, X-ray absorption, and resonant X-ray spectroscopy of magnetic excitations in the new heavy-fermion van-der-Waals superconductor CeSiI. We determined effective Hamiltonians and ground states of crystal electric fields and magnons. Isotropic Heisenberg interactions on a quasi two dimensional lattice, including ferromagnetic nearest-neighbor exchange as the dominant interaction, provide an excellent account to the low-energy measured dynamics and stabilize a co-rotating spin cycloid. Our study provides the basis to model CeSiI from first principles, thereby laying the ground for microscopic understanding of heavy-fermion physics, their unconventional superconductivity, and quantum criticality.

cond-mat.str-el↗

arXiv2026

Universal Magnetic Structure Prediction from Atomic Coordinates with Near-Experimental Accuracy

Magnetic order is a fundamental property of materials, governing collective behavior and enabling a broad range of functionalities. Yet magnetic structure remains difficult to determine: experiments are costly and specialized, while first-principles methods often struggle with the noncollinear and incommensurate orders found in real materials. Here we introduce magnetic structure network (MSN), an E(3) equivariant graph neural network that predicts both collinear and non-collinear magnetic structures directly from atomic crystal structures, trained directly on experimentally determined structures from MAGNDATA. By proposing the primitive modulated structure representation (PMSR), we are able to encode commensurate and incommensurate structures in a unified way without symmetry assumptions. The model achieves strong performance across all modulation components and reconstructs experimental magnetic structures with high fidelity. Our approach provides a scalable framework for rapid magnetic structure prediction and opens a route to data-driven discovery of magnetic materials.

cond-mat.mtrl-sci↗

arXiv2026

Probing Non-Equilibrium Grain Boundary Dynamics with XPCS and Domain-Adaptive Machine Learning

Grain-boundary (GB) dynamics control the stability, mechanical, and functional response of nanocrystalline materials, but direct experimental access to their slow non-equilibrium motion has been limited. Here we establish X-ray photon correlation spectroscopy (XPCS), combined with domain-adaptive machine learning, as a quantitative probe of GB dynamics. Temperature- and grain-size-dependent two-time XPCS measurements in nanocrystalline silicon reveal pronounced departures from time-translation invariance, showing that GB relaxation can remain far from equilibrium over experimental timescales. However, direct extraction of quantitative physical information from these high-dimensional, noisy fluctuation maps faces a significant challenge. To overcome this barrier, we develop a semi-supervised learning framework that transfers physical parameter labels from continuum simulations to unlabeled experimental XPCS maps through domain-adaptive representation alignment. This AI-augmented approach enables the extraction of key kinetic parameters, including bulk diffusivity, GB stiffness, and effective GB concentration, directly from experimental XPCS measurements. Our results show how machine learning can transform indirect fluctuation signals into quantitative materials dynamics, providing a general route to study non-equilibrium defect motion in solids.

cond-mat.mtrl-sci↗

arXiv2026

Exotic Spin Excitation Continuum in a Weakly Coupled Quantum Chainsaw Antiferromagnet

Collective motions in strongly interacting magnets involve many spins and are often described in terms of integer-spin excitations. However, in certain cases, the collective motion can behave as if these integer excitations break apart into smaller, particle-like entities with unusual properties. Such fractionalized excitations in quantum magnets are commonly associated either with topological order in two dimensions or with criticality in one dimension. It remains unclear how these distinct mechanisms are connected across a dimensional crossover. Here we investigate the Ti-based quantum antiferromagnet, $Cs_{8}LiNa_{3}Ti_{12}F_{48}$, in which $Ti^{3+}$ ($3d^{1}$, $S=1/2$) ions interact antiferromagnetically within distorted kagome planes. Our inelastic neutron scattering study on a single crystal reveals a frustrated network of weakly coupled spin-$1/2$ chainsaws, realizing a regime of dimensional frustration in which interchain couplings fail to establish coherent two-dimensional order. The magnetic excitation spectrum exhibits a strong continuum spanning the full measured momentum and energy phase space. In addition, the dynamic spin correlation function displays rod-like scattering in momentum space, indicating a quasi-one-dimensional nature of the magnetic correlations. These results point to fractionalized excitations with intrinsically directional character, demonstrating that signatures of one-dimensional criticality can persist within a two-dimensional lattice. Our findings establish anisotropic fractionalization as a distinct organizing principle for quantum-disordered states.

cond-mat.str-el↗

arXiv2026

Coupled sawtooth chain exchange network in olivine Mn$_2$GeO$_4$

Sawtooth chain magnets have been a subject of historical interest in the field of frustrated magnetism, with classical olivine family $M_2TX_4$, ($M$ - 3d, $T$ - 4p, $X$ - chalcogen elements) typically realizing simple $\mathbf{k} = (000)$ states. The magnetism of the Mn$_2$GeO$_4$ olivine is surprisingly complex, proceeding from commensurate states to a multiferroic commensurate + incommensurate phase. Here we report inelastic neutron scattering results from a Mn$_2$GeO$_4$ single crystal and develop an effective Hamiltonian including long-distance bilinear and dipolar interactions. The magnetic interactions are predominantly antiferromagnetic and span a three-dimensional exchange network consisting of coupled sawtooth chains. Based on the determined strength of the couplings, the dominant sawtooth chains appear at third- and fourth- rather than next-nearest-neighbor. However the next-nearest-neighbor interaction is, along with a modest Dzyaloshinskii-Moriya interaction, important for modeling the observed incommensurability. We use the best-fit Hamiltonian as the basis for Langevin dynamics simulations and Luttinger-Tisza calculations of the high-temperature commensurate transition.

cond-mat.str-el↗

arXiv2025

Sunny.jl: A Julia Package for Spin Dynamics

Sunny is a Julia package designed to serve the needs of the quantum magnetism community. It supports the specification of a very broad class of spin models and a diverse suite of numerical solvers. These include powerful methods for simulating spin dynamics both in and out of equilibrium. Uniquely, it features a broad generalization of classical and semiclassical approaches to SU(N) coherent states, which is useful for studying systems exhibiting strong spin-orbit coupling or local entanglement effects. Sunny also offers a well-developed framework for calculating the dynamical spin structure factor, enabling direct comparison with scattering experiments. Ease of use is a priority, with tools for symmetry-guided modeling and interactive visualization.

quant-ph↗

arXiv2023

Coexistence of symmetry-protected topological order and Neel order in the spin-1/2 ladder antiferromagnet C9H18N2CuBr4

Topological phases of matter are beyond the paradigm of Landau's symmetry breaking and have challenged our understanding of condensed matter systems. Here we report a new type of symmetry-protected topological phase of matter in the spin-1/2 coupled two-leg ladder antiferromagnet C9H18N2CuBr4, DLCB for short. In this two-sublattice antiferromagnet with a weak easy-axis anisotropy, we find no evidence of a conventional spin-flop transition in the magnetization with the magnetic field applied parallel to the easy axis at T=0.4 K, well below TN=2.0 K. Moreover, the temperature dependence of the gapped transverse excitations across TN indicates that they are not the conventional S=1 magnons associated with explicit symmetry breaking. Instead, the thermal renormalization of the gap energy shows a remarkable agreement with a calculation for the three-dimensional O(3) nonlinear sigma model. Accordingly, the spin gap in DLCB is not due to the spin anisotropy but to the separation between a spin singlet state and a triplet excited state. Since an antiferromagnetic spin-1/2 ladder systems can be mapped onto the spin-1 chain, the notion of the Haldane gap is proposed to explain the opening of the spin gap in DLCB. Therefore, the ground state of DLCB is best described as a quantum superposition of a Haldane phase and a Neel-ordered phase, which resembles the quantum state of a qubit in quantum computing. Our results indicate the presence of a symmetry-protected topological order coexisting with an antiferromagnetic order in this material.

cond-mat.str-el↗

arXiv2019

Coexistence of ferromagnetic and stripe antiferromagnetic spin fluctuations in SrCo$_2$As$_2$

We use inelastic neutron scattering to study energy and wave vector dependence of spin fluctuations in SrCo$_2$As$_2$, derived from SrFe$_{2-x}$Co$_x$As$_2$ iron pnictide superconductors. Our data reveals the coexistence of antiferromagnetic (AF) and ferromagnetic (FM) spin fluctuations at wave vectors $\textbf{Q}_{\rm AF}$=(1,0) and $\textbf{Q}_{\rm FM}$=(0,0)/(2,0), respectively. By comparing neutron scattering results with those of dynamic mean field theory calculation and angle-resolved photoemission spectroscopy experiments, we conclude that both AF and FM spin fluctuations in SrCo$_2$As$_2$ are closely associated with a flat band of the $e_g$ orbitals near the Fermi level, different from the $t_{2g}$ orbitals in superconducting SrFe$_{2-x}$Co$_x$As$_2$. Therefore, Co-substitution in SrFe$_{2-x}$Co$_x$As$_2$ induces a $t_{2g}$ to $e_g$ orbital switching, and is responsible for FM spin fluctuations detrimental to the singlet pairing superconductivity.

cond-mat.str-el↗

arXiv2015

Spin Jam: a quantum-fluctuation-induced glassy state of a frustrated magnet

Since the discovery of spin glasses in dilute magnetic systems, their study has been largely focused on understanding randomness and defects as the driving mechanism. The same paradigm has also been applied to explain glassy states found in dense frustrated systems. Recently, however, it has been theoretically suggested that different mechanisms, such as quantum fluctuations and topological features, may induce glassy states in defect-free spin systems, far from the conventional dilute limit. Here we report experimental evidence for the existence of a glassy state, that we call a spin jam, in the vicinity of the clean limit of a frustrated magnet, which is insensitive to a low concentration of defects. We have studied the effect of impurities on SrCr9pGa12-9pO19 (SCGO(p)), a highly frustrated magnet, in which the magnetic Cr3+ (s=3/2) ions form a quasi-two-dimensional triangular system of bi-pyramids. Our experimental data shows that as the nonmagnetic Ga3+ impurity concentration is changed, there are two distinct phases of glassiness: a distinct exotic glassy state, which we call a "spin jam", for high magnetic concentration region (p>0.8) and a cluster spin glass for lower magnetic concentration, (p<0.8). This observation indicates that a spin jam is a unique vantage point from which the class of glassy states in dense frustrated magnets can be understood.

cond-mat.str-el↗