arXiv subjects

M. C. Rahn

Publications and source records attributed to M. C. Rahn.

At least 19 recordsLinked to original sources

arXiv2026

Quasi-1D Spin Textures: From Chiral Soliton Lattice to Fan State

In most helimagnets, an applied magnetic field aligns the propagation direction of a helical spin texture with the field, resulting in a conical state and obscuring the unwinding process. Here, we access a complementary regime in the anisotropic chiral magnet Mn$_{1.4}$PtSn, where crystal symmetry constrains the propagation direction of the spin modulation. Using resonant elastic X-ray scattering in a vector magnet, we track the evolution of quasi-one-dimensional spin textures that propagate along a chiral crystallographic axis while the magnetic field is applied perpendicular to this direction. Together with micromagnetic simulations, our measurements reveal a transformation from the zero-field $\pi$-chiral soliton lattice into a fan-like state. In this state, the propagation direction remains transverse to the applied field, while the spins oscillate about the field direction. During magnetization, the modulation length decreases continuously with the field and approaches the field-polarized state. Simulations indicate that magnetostatic interactions in finite samples play a key role in stabilizing this behavior. Our results provide evidence for a fan-like regime in a chiral magnet and highlight how field orientation can be used to control one-dimensional spin textures.

cond-mat.mtrl-sci↗

arXiv2026

Anisotropy of spin waves in the field-polarized phase of Fe-doped MnSi

Chiral magnetic textures, such as skyrmions, are of great interest to the condensed matter community due to their novel transport properties. The stabilization of topologically non-trivial magnetic phases, like the skyrmion lattice in MnSi, is governed by underlying magnetic interactions which can be probed via measurements of spin-wave excitations. Here, we report high-resolution inelastic neutron scattering (INS) measurements of the spin waves in Fe-doped Mn$_{0.9}$Fe$_{0.1}$Si deep within its field-polarized ferromagnetic state. We observe non-reciprocal spin waves with a parabolic dispersion that shifts linearly with magnetic field. Crucially, the spin-wave stiffness is highly anisotropic, with values of 14.7 meV $\rm{\mathring{A}}$$^2$ parallel to the applied field and 7.6 meV $\rm{\mathring{A}}$$^2$ perpendicular to it. This pronounced anisotropy in a cubic material is inconsistent with standard theoretical models for MnSi and indicates a necessity to revise our theoretical understanding.

cond-mat.str-el↗

arXiv2026

Electron-phonon coupling in EuAl4 under hydrostatic pressure

In the intermetallid rare-earth tetragonal EuAl4 system, competing itinerant exchange mechanisms lead to a complex magnetic phase diagram, featuring a centrosymmetric skyrmion lattice. Previous inelastic x-ray scattering (IXS) experiments revealed that the incommensurate charge-density wave (CDW) transition in EuAl4 (TCDW = 142 K) is driven by momentum-dependent electron-phonon coupling (EPC). We present the results of IXS under high hydrostatic pressure induced by diaond anvils and show how the EPC in EuAl4 is renormalized and suppressed in the material's temperature-pressure phase diagram. Our findings highlight the crucial role of momentum-dependent EPC in the formation of the CDW in EuAl4 and provide further insights into how external pressure can be used to tune charge ordering in quantum materials.

cond-mat.str-el↗

arXiv2025

Field-induced condensation of $π$ to 2$π$ soliton lattices in chiral magnets

Chiral soliton lattices (CSLs) are nontrivial spin textures that emerge from the competition between Dzyaloshinskii-Moriya interaction, anisotropy, and magnetic fields. While well established in monoaxial helimagnets, their role in materials with anisotropic, direction-dependent chirality remains poorly understood. Here, we report the direct observation of a tunable transition from $π$ to 2$π$ soliton lattices in the non-centrosymmetric Heusler compound Mn1.4PtSn. Using Lorentz transmission electron microscopy, resonant elastic X-ray scattering, and micromagnetic simulations, we identify a $π$-CSL as the magnetic ground state, in contrast to the expected helical phase, which evolves into a classical 2$π$-CSL under increasing out-of-plane magnetic fields. This transition is governed by a delicate interplay between uniaxial magnetocrystalline anisotropy and magnetostatic interactions, as captured by a double sine-Gordon model. Our analysis not only reveals the microscopic mechanisms stabilizing these soliton lattices but also demonstrates their general relevance to materials with D2d, S4, Cnv, or Cn symmetries. The results establish a broadly applicable framework for understanding magnetic phase diagrams in chiral systems, with implications for soliton-based spintronic devices and topological transport phenomena.

cond-mat.mtrl-sci↗

arXiv2025

Easy-cone state mediating the spin reorientation in topological kagome magnet Fe$_3$Sn$_2$

We investigated temperature-driven spin reorientation (SR) in the itinerant kagome magnet Fe$_3$Sn$_2$ using high-resolution synchrotron x-ray diffraction, neutron diffraction, magnetometry, and magnetic force microscopy (MFM), further supported by phenomenological analysis. Our study reveals a crossover from the state with easy-plane anisotropy to the high-temperature state with uniaxial easy-axis anisotropy taking place between $\sim40-130$~ K through an intermediate easy-cone (or tilted spin) state. This state, induced by the interplay between the anisotropy constants $K_1$ and $K_2$, is clearly manifested in the thermal evolution of the magnetic structure factor, which reveals a gradual change of the SR angle $\mathbfθ$ between $40-130$~K. We also found that the SR is accompanied by a magnetoelastic effect. Zero-field MFM images across the SR range show a transformation in surface magnetic patterns from a dendritic structure at 120~K, to domain wall dominated MFM contrast at 40~K.

cond-mat.mtrl-sci↗

arXiv2025

Microscopic Observation of Non-Ergodic States in Two-Dimensional Non-Topological Bubble Lattices

Disordered 2D lattices, including hexatic and various glassy states, are observed in a wide range of 2D systems including colloidal nanoparticle assemblies and fluxon lattices. Their disordered nature determines the stability and mobility of these systems, as well as their response to the external stimuli. Here we report on the controlled creation and characterization of a disordered 2D lattice of non-topological magnetic bubbles in the non-centrosymmetric ferrimagnetic alloy Mn$_{1.4}$PtSn. By analyzing the type and frequency of fundamental lattice defects, such as dislocations, the orientational correlation, as well as the induced motion of the lattice in an external field, a non-ergodic glassy state, stabilized by directional application of an external field, is revealed.

cond-mat.mtrl-sci↗

arXiv2025

Observation of the spiral spin liquid in a triangular-lattice material

The spiral spin liquid (SSL) is a highly degenerate state characterized by a continuous contour or surface in reciprocal space spanned by a spiral propagation vector. Although the SSL state has been predicted in a number of various theoretical models, very few materials are so far experimentally identified to host such a state. Via combined single-crystal wide-angle and small-angle neutron scattering, we report observation of the SSL in the quasi-two-dimensional delafossite AgCrSe$_2$. We show that it is a very close realization of the ideal Heisenberg $J_1$--$J_2$--$J_3$ frustrated model on the triangular lattice. By supplementing our experimental results with microscopic spin-dynamics simulations, we demonstrate how such exotic magnetic states are driven by thermal fluctuations and exchange frustration.

cond-mat.str-el↗

arXiv2024

Phonon softening and atomic modulations in EuAl$_4$

EuAl$_4$ is a rare earth intermetallic in which competing itinerant and/or indirect exchange mechanisms give rise to a complex magnetic phase diagram, including a centrosymmetric skyrmion lattice. These phenomena arise not in the tetragonal parent structure but in the presence of a charge density wave (CDW), which lowers the crystal symmetry and renormalizes the electronic structure. Microscopic knowledge of the corresponding atomic modulations and their driving mechanism is a prerequisite for a deeper understanding of the resulting equilibrium of electronic correlations and how it might be manipulated. Here, we use synchrotron single-crystal X-ray diffraction, inelastic X-ray scattering, and lattice dynamics calculations to clarify the origin of the CDW in EuAl$_4$. We observe a broad softening of a transverse acoustic phonon mode that sets in well above room temperature and, at $T_\mathrm{CDW}=142$ K, freezes out in an atomic displacement mode described by the superspace group $Immm(00γ)s00$. In the context of previous work, our observation is a clear confirmation that the CDW in EuAl$_4$ is driven by electron-phonon coupling. This result is relevant for a wider family of BaAl$_4$ and ThCr$_2$Si$_2$-type rare-earth intermetallics known to combine CDW instabilities and complex magnetism.

cond-mat.str-el↗

arXiv2023

Unusual magnetism of the axion-insulator candidate Eu$_5$In$_2$Sb$_6$

Eu$_5$In$_2$Sb$_6$ is a member of a family of orthorhombic nonsymmorphic rare-earth intermetallics that combines large localized magnetic moments and itinerant exchange with a low carrier density and perpendicular glide planes. This may result in special topological crystalline (wallpaper fermion) or axion insulating phases. Recent studies of Eu$_5$In$_2$Sb$_6$ single crystals have revealed colossal negative magnetoresistance and multiple magnetic phase transitions. Here, we clarify this ordering process using neutron scattering, resonant elastic X-ray scattering, muon spin-rotation, and magnetometry. The nonsymmorphic and multisite character of Eu$_5$In$_2$Sb$_6$ results in coplanar noncollinear magnetic structure with an Ising-like net magnetization along the $a$ axis. A reordering transition, attributable to competing ferro- and antiferromagnetic couplings, manifests as the onset of a second commensurate Fourier component. In the absence of spatially resolved probes, the experimental evidence for this low-temperature state can be interpreted either as an unusual double-$q$ structure or in a phase separation scenario. The net magnetization produces variable anisotropic hysteretic effects which also couple to charge transport. The implied potential for functional domain physics and topological transport suggests that this structural family may be a promising platform to implement concepts of topological antiferromagnetic spintronics.

cond-mat.str-el↗

arXiv2023

Phonon topology and winding of spectral weight in graphite

The topology of electronic and phonon band structures of graphene is well studied and known to exhibit a Dirac cone at the K point of the Brillouin zone. Here, we applied inelastic x-ray scattering (IXS) along with $\textit{ab initio}$ calculations to investigate phonon topology in graphite, the 3D analogue of graphene. We identified a pair of modes that form a very weakly gapped linear anticrossing at the K point that can be essentially viewed as a Dirac cone approximant. The IXS intensity in the vicinity of the quasi-Dirac point reveals a harmonic modulation of the phonon spectral weight above and below the Dirac energy, which was previously proposed as an experimental fingerprint of the nontrivial topology. We illustrate how the topological winding of IXS intensity can be understood in terms of atomic displacements, and highlight that the intensity winding is not in fact sensitive in telling quasi- and true Dirac points apart.

cond-mat.mes-hall↗

arXiv2022

Kondo quasiparticle dynamics observed by resonant inelastic x-ray scattering

Effective models focused on pertinent low-energy degrees of freedom have substantially contributed to our qualitative understanding of quantum materials. An iconic example, the Kondo model, was key to demonstrating that the rich phase diagrams of correlated metals originate from the interplay of localized and itinerant electrons. Modern electronic structure calculations suggest that to achieve quantitative material-specific models, accurate consideration of the crystal field and spin-orbit interactions is imperative. This poses the question of how local high-energy degrees of freedom become incorporated into a collective electronic state. Here, we use resonant inelastic x-ray scattering (RIXS) on CePd$_3$ to clarify the fate of all relevant energy scales. We find that even spin-orbit excited states acquire pronounced momentum-dependence at low temperature - the telltale sign of hybridization with the underlying metallic state. Our results demonstrate how localized electronic degrees of freedom endow correlated metals with new properties, which is critical for a microscopic understanding of superconducting, electronic nematic, and topological states.

cond-mat.str-el↗

arXiv2021

Collinear antiferromagnetic order in URu$_2$Si$_{2-x}$P$_x$ revealed by neutron diffraction

The hidden order phase in URu$_2$Si$_2$ is highly sensitive to electronic doping. A special interest in silicon-to-phosphorus substitution is due to the fact that it may allow one, in part, to isolate the effects of tuning the chemical potential from the complexity of the correlated $f$ and $d$ electronic states. We investigate the new antiferromagnetic phase that is induced in URu$_2$Si$_{2-x}$P$_x$ at $x\gtrsim0.27$. Time-of-flight neutron diffraction of a single crystal ($x=0.28$) reveals $c$-axis collinear $\mathbf{q}_\mathrm{m}=(\frac12,\frac12,\frac12)$ magnetic structure with localized magnetic moments ($\approx2.1\,μ_\mathrm{B}$). This points to an unexpected analogy between the (Si,P) and (Ru,Rh) substitution series. Through further comparisons with other tuning studies of URu$_2$Si$_2$, we are able to delineate the mechanisms by which silicon-to-phosphorus substitution affects the system. In particular, both the localization of itinerant 5$f$ electrons as well as the choice of $\mathbf{q}_m$ appears to be consequences of the increase in chemical potential. Further, enhanced exchange interactions are induced by chemical pressure and lead to magnetic order, in which an increase in inter-layer spacing may play a special role.

cond-mat.str-el↗

arXiv2020

Colossal magnetoresistance in a nonsymmorphic antiferromagnetic insulator

Here we investigate antiferromagnetic Eu$_{5}$In$_{2}$Sb$_{6}$, a nonsymmorphic Zintl phase. Our electrical transport data show that Eu$_{5}$In$_{2}$Sb$_{6}$ is remarkably insulating and exhibits an exceptionally large negative magnetoresistance, which is consistent with the presence of magnetic polarons. From {\it ab initio} calculations, the paramagnetic state of Eu$_{5}$In$_{2}$Sb$_{6}$ is a topologically nontrivial semimetal within the generalized gradient approximation (GGA), whereas an insulating state with trivial topological indices is obtained using a modified Becke-Johnson potential. Notably, GGA+U calculations suggest that the antiferromagnetic phase of Eu$_{5}$In$_{2}$Sb$_{6}$ may host an axion insulating state. Our results provide important feedback for theories of topological classification and highlight the potential of realizing clean magnetic narrow-gap semiconductors in Zintl materials.

cond-mat.str-el↗

arXiv2020

Magnetic and orbital correlations in multiferroic CaMn$_7$O$_{12}$ probed by x-ray resonant elastic scattering

The quadruple perovskite CaMn$_7$O$_{12}$ is a topical multiferroic, in which the hierarchy of electronic correlations driving structural distortions, modulated magnetism, and orbital order is not well known and may vary with temperature. x-ray resonant elastic scattering (XRES) provides a momentum-resolved tool to study these phenomena, even in very small single crystals, with valuable information encoded in its polarization- and energy-dependence. We present an application of this technique to CaMn$_7$O$_{12}$. By polarization analysis, it is possible to distinguish superstructure reflections associated with magnetic order and orbital order. Given the high momentum resolution, we resolve a previously unknown splitting of an orbital order superstructure peak, associated with a distinct \textit{locked-in} phase at low temperatures. A second set of orbital order superstructure peaks can then be interpreted as a second-harmonic orbital signal. Surprisingly, the intensities of the first- and second-harmonic orbital signal show disparate temperature and polarization dependence. This orbital re-ordering may be driven by an exchange mechanism, that becomes dominant over the Jahn-Teller instability at low temperature.

cond-mat.mtrl-sci↗

arXiv2019

An ideal Weyl semimetal induced by magnetic exchange

Weyl semimetals exhibit exceptional quantum electronic transport due to the presence of topologically-protected band crossings called Weyl nodes. The nodes come in pairs with opposite chirality, but their number and location in momentum space is otherwise material specific. Following the initial discoveries there is now a need for better material realizations, ideally comprising a single pair of Weyl nodes located at or very close to the Fermi level and in an energy window free from other overlapping bands. Here we propose the layered intermetallic EuCd$_2$As$_2$ to be such a system. We show that Weyl nodes in EuCd$_2$As$_2$ are magnetically-induced via exchange coupling, emerging when the Eu spins are aligned by a small external magnetic field. The identification of EuCd$_2$As$_2$ as a model magnetic Weyl semimetal, evidenced here by ab initio calculations, photoemission spectroscopy, quantum oscillations and anomalous Hall transport measurements, opens the door to fundamental tests of Weyl physics.

cond-mat.str-el↗

arXiv2019

Selective probing of magnetic order on Tb and Ir sites in stuffed Tb$_{2+x}$Ir$_{2-x}$O$_{7-y}$ using resonant X-ray scattering

We study the magnetic structure of the "stuffed" (Tb-rich) pyrochlore iridate Tb$_{2+x}$Ir$_{2-x}$O$_{7-y}$, using resonant elastic x-ray scattering (REXS). In order to disentangle contributions from Tb and Ir magnetic sublattices, experiments were performed at the Ir $L_3$ and Tb $M_5$ edges, which provide selective sensitivity to Ir $5d$ and Tb $4f$ magnetic moments, respectively. At the Ir $L_3$ edge, we found the onset of long-range ${\bf k}={\bf 0}$ magnetic order below $T_{N}^\text{Ir}\sim$ 71 K, consistent with the expected signal of all-in all-out (AIAO) magnetic order. Using a single-ion model to calculate REXS cross-sections, we estimate an ordered magnetic moment of $μ_{5d}^{\text{Ir}} \approx 0.34(3)\,μ_B$ at 5 K. At the Tb $M_5$ edge, long-range ${\bf k}={\bf 0}$ magnetic order appeared below $\sim40$ K, also consistent with an AIAO magnetic structure on the Tb site. Additional insight into the magnetism of the Tb sublattice is gleaned from measurements at the $M_5$ edge in applied magnetic fields up to 6 T, which is found to completely suppress the Tb AIAO magnetic order. In zero applied field, the observed gradual onset of the Tb sublattice magnetisation with temperature suggests that it is induced by the magnetic order on the Ir site. The persistence of AIAO magnetic order, despite the greatly reduced ordering temperature and moment size compared to stoichiometric Tb$_{2}$Ir$_{2}$O$_{7}$, for which $T_{N}^{\text{Ir}} =130$ K and $μ_{5d}^{\text{Ir}}=0.56\,μ_B$, indicates that stuffing could be a viable means of tuning the strength of electronic correlations, thereby potentially offering a new strategy to achieve topologically non-trivial band crossings in pyrochlore iridates.

cond-mat.str-el↗

arXiv2019

Nematic state in CeAuSb$_{2}$

At ambient pressure and zero field, tetragonal CeAuSb$_{2}$ hosts stripe antiferromagnetic order at $T_{N} = 6.3$ K. Here we first show via bulk thermodynamic probes and x-ray diffraction measurements that this magnetic order is connected with a structural phase transition to a superstructure which likely breaks $C_{4}$ symmetry, thus signaling nematic order. The temperature-field-pressure phase diagram of CeAuSb$_{2}$ subsequently reveals the emergence of additional ordered states under applied pressure at a multicritical point. Our phenomenological model supports the presence of a vestigial nematic phase in CeAuSb$_{2}$ akin to iron-based high-temperature superconductors; however, superconductivity, if present, remains to be discovered.

cond-mat.str-el↗

arXiv2019

Paramagnon dispersion in $β$-FeSe observed by Fe $L$-edge resonant inelastic x-ray scattering

We report an Fe $L$-edge resonant inelastic x-ray scattering (RIXS) study of the unusual superconductor $β$-FeSe. The high energy resolution of this RIXS experiment ($\approx\,$55$\,$meV FWHM) made it possible to resolve low-energy excitations of the Fe $3d$ manifold. These include a broad peak which shows dispersive trends between 100-200$\,$meV along the $(π,0)$ and $(π,π)$ directions of the one-Fe square reciprocal lattice, and which can be attributed to paramagnon excitations. The multi-band valence state of FeSe is among the most metallic in which such excitations have been discerned by soft x-ray RIXS.

cond-mat.supr-con↗