arXiv subjects

Emilia Morosan

Publications and source records attributed to Emilia Morosan.

At least 19 recordsLinked to original sources

arXiv2026

A Strategy Toward Room Temperature Topological Hall Effect via Local Moment Magnetism

Topological spin textures in local moment systems hold great promise for technological applications due to their large magnetic moments, strong spin-orbit coupling (SOC), and high tunability. Finding new spin textures that are stable near room temperature is paramount to maximizing their potential for applications. Here, we provide a strategy for realizing topological spin textures at high temperatures by identifying rare earth ($R$) magnets ordering at or near room temperature. We demonstrate the feasibility of this strategy in one of these magnets, hexagonal Gd$_5$Pb$_3$, which orders at $T_C$ = 285 K. The indication for topological spin textures comes from topological Hall effect (THE), which, in Gd$_5$Pb$_3$, occurs between $T$ = 100 - 200 K, an order of magnitude higher temperature than in other reported $R$-based systems. Our results present an opportunity to explore the role of SOC, anisotropic exchange, geometric frustration, and magnetic interactions in stabilizing topological spin textures, and provide a pathway toward realizing them near room temperature in $R$-based magnets.

cond-mat.mtrl-sci↗

arXiv2026

Competing collinear and non-collinear spin textures imaged by spatially-resolved REXS in Eu(Al0.4Ga0.6)4

Here, we use resonant elastic x-ray scattering (REXS) to investigate the inhomogeneity of the zero-field magnetic spin texture of Eu(Al$_{0.4}$Ga$_{0.6}$)$_4$. By using spatially-resolved REXS, we show that the two magnetic transitions at T$_{N1}$ = 17 K and T$_{N2}$ = 14 K originate from two nearly degenerate, yet distinct, orthogonal pairs of q-vectors. The corresponding phases are segregated spatially, such that one fraction of the sample comprises coexisting orthogonal spin-density-wave domains, while another fraction hosts coexisting orthogonal helical domains. Additionally, the helical state forms inversion domains indicating that the inversion symmetry is not broken prior the magnetic transition. Our results suggest that the magnetic state is single-q and revealed a large variation of spin textures across a 0.8 - 1 mm area of the sample surface. These results demonstrate clear differences between the locally and globally probed magnetic textures, typically assumed to be representative of the system as a whole, highlighting the importance of spatially-resolved probes for accurately describing the magnetic behavior of this class of materials.

cond-mat.str-el↗

arXiv2026

Quantum geometry and critical temperature enhancement in MgB$_2$ superconductivity

MgB$_2$, a phonon-mediated superconductor with record-high critical temperature $T_c\simeq 39$ K, is revisited to obtain a comprehensive theory of electrons, phonons, and their coupling with minimal ab initio input. We construct compact analytic models for the electronic structure, phonons, and electron-phonon coupling (EPC) of MgB$_2$. We show that strong in-plane B $sp^2$ bonding realizes an obstructed band structure whose natural description is a bond-centered kagome lattice, yielding small quasi-2D $σ$-band Fermi-surface cylinders and pronounced quantum-geometric effects. The phonon spectrum is found to closely track that of a graphene-like boron layer, but the heavy intercalated Mg atoms dominate the three acoustic branches and rigidly lift the boron modes into the optical sector, while the in-plane B-B bond-stretching mode exhibits a pronounced softening along $Γ$-A. By symmetry, this $Γ$-point bond-stretching mode is the only $Γ$ phonon that can couple to the $σ$ Fermi surface, explaining its dominant contribution to the EPC. Upon electron doping toward the doubly degenerate band edge of the $σ$ sheets, we find that a reduced density of states competes with enhanced EPC matrix elements. At light electron doping, ab initio calculations show that the EPC enhancement dominates, leading to an increase in $T_c$ (within the clean doping limit without disorder effects). Using the Gaussian approximation for the EPC tensor, we further show that this enhancement is overwhelmingly quantum geometric in origin, arising from a geometric EPC contribution of the small $σ$ Fermi surface peaked at $Γ$. Overall, our results provide a transparent, symmetry-based account of superconductivity in MgB$_2$ and suggest that quantum-geometric effects can be essential for shaping doping trends in phonon-mediated superconductors.

cond-mat.supr-con↗

arXiv2026

Modification of Charge and Spin Textures by Light Chemical Substitution in Eu(Al$_{1-x}$Ga$_{x}$)$_4$ ($x=0.1$)

We present the results of a resonant X-ray diffraction experiment, resolving both charge and spin textures in the intermetallic topological magnet Eu(Al$_{1-x}$Ga$_{x}$)$_4$, $x$ = 0.1. Below $\approx$ 75 K the system develops a charge density wave (CDW) with propagation vector kCDW ~ (0, 0, 0.18). The CDW order parameter grows monotonically on cooling until ~ 15 K, when a sudden decrease in the CDW amplitude occurs. Pairs of magnetic satellites of the (0, 0, 8) Bragg reflection corresponding to two distinct domains, k = ($\pm δ_\text{m}$, 0, 0), k2 = (0, $\pmδ_\text{m}$, 0), $δ_\text{m} = 0.2002(4)$ were studied at the Eu L3 edge, appearing below TN = 14.8 K. Our measurement of TN is exactly coincident with the sudden drop in the CDW amplitude, which suggests strong coupling between the charge and spin orders, as observed in other compounds of the Eu(Al$_{1-x}$Ga$_{x}$)$_4$ series. Azimuthal measurements revealed a single helical spin arrangement with an elliptical envelope of $μ_\text{Y}/μ_\text{Z}$ = 1.19(6) for the k2 domain, and a tilted helical (helicoidal) spin arrangement for the k1 domain, with $μ_\text{Y}/μ_\text{Z}$ = 1.14(4) and $μ_\text{X}/μ_\text{Z}$ = 0.20(2) that may be hidden for the k2 domain due to multiple subdomains. Temperature evolution of the magnetic satellite intensities in linear and circularly polarised light found the respective ratio to be invariant with temperature, suggesting a single magnetic phase below TN. This behaviour is unlike the x = 0 material, in which a spin density wave forms first, transitioning to a helical ground state on cooling through intermediate phases. Future theoretical work on the Eu electronic ground state, supported by related experiments, will help understand the effects of Ga substitution on the evolution of the magnetic structure.

cond-mat.str-el↗

arXiv2026

Evidence for itinerant electron-local moment interaction in Li-doped $α$-MnTe

We use inelastic neutron scattering (INS) and angle-resolved photoemission spectroscopy (ARPES) to study the impact of Li doping on the semiconducting altermagnet $α$-MnTe. Introducing Li results in a spin reorientation from in-plane to out-of-plane direction and increases the density of itinerant carriers. While our ARPES measurements do not indicate any notable doping-induced changes in the electronic band structure or the magnitude of the altermagnetic band splitting, our INS measurements reveal an abrupt carrier-induced decrease in the spin wave lifetime near the zone boundary at high energies. This finding is consistent with a new magnon decay channel driven by doping-induced subtle changes in the band structure and enhanced interactions between Mn$^{2+}$ local moments and itinerant electrons. By extracting the local dynamic susceptibility from INS spectra and applying the total moment sum rule, we find that both undoped and Li-doped $α$-MnTe exhibit the full expected Mn$^{2+}$ local moment of $\approx5.9~μ_\mathrm{B}$ with $S=5/2$. These findings suggest that $α$-MnTe hosts robust local-moment altermagnetism which shows a breakdown at high energies upon addition of itinerant carriers, highlighting the importance of carrier-spin coupling in magneto-transport and spin dynamic properties of altermagnets even in the dilute-carrier limit.

cond-mat.str-el↗

arXiv2026

Quasiparticle properties below coherence onset in YbAl3 nanostructures

Mesoscopic transport measurements are underexplored as probes of quasiparticles and their properties in correlated metals. The mixed valence compound YbAl$_3$ exhibits a single-ion Kondo temperature of 670 K, while thermodynamic and transport properties (probed with specific heat, magnetic susceptibility, Hall effect, and resistivity) imply the onset of coherence of heavy fermion quasiparticles at T$* \approx$ 37 K. To characterize these quasiparticles, we utilize mesoscopic techniques familiar from weakly correlated conductors. In lithographically-defined nanowires etched from epitaxial films, we observe weak antilocalization magnetoresistance and universal conductance fluctuations, consistent with electronic coherence lengths of tens of nanometers. Additionally, analysis of Johnson-Nyquist noise measurements as a function of bias current reveal, within the context of a range of accepted models, a significant electron-phonon energy loss that increases with decreasing temperature, a finding that we contextualize within the broader properties of YbAl$_3$.

cond-mat.str-el↗

arXiv2026

Strain-tunable anomalous Hall effect in hexagonal MnTe

The ability to control and manipulate time-reversal ($T$) symmetry-breaking phases with near-zero net magnetization is a sought-after goal in spintronic devices. The recently discovered hexagonal altermagnet manganese telluride ($α$-MnTe) is a prime example. It has a compensated altermagnetic ground state where the magnetic moments are aligned in each layer and stacked antiparallel along the $c$ axis, yet it exhibits a spontaneous anomalous Hall effect (AHE) that breaks the $T$-symmetry with a vanishingly small $c$-axis ferromagnetic (FM) moment. However, the presence of three 120$^\circ$ separated in-plane magnetic domains presents a challenge in understanding the origin of the AHE and the effective control of the altermagnetic state. Here we use neutron scattering to show that symmetry breaking anisotropic strain, induced by compressive uniaxial pressure along the nearest-neighbor (NN) Mn-Mn bond directions, detwins $α$-MnTe into a single in-plane magnetic domain. This control over in-plane domains allows us to unambiguously establish that the in-plane moments are aligned along the NNN Mn-Mn bond direction, irrespective of the applied strain directions. Mounting the sample on a piezoelectric strain cell along both NN and NNN directions can drive the sample into a single-domain state that significantly sharpens the AHE hysteresis loop and extends the AHE to lower temperatures. Furthermore, tuning the uniaxial strain reverses the sign of the AHE near room temperature. Remarkably, this is achieved without altering the altermagnetic phase-transition temperature or substantially changing the small $c$-axis FM moment. Combined with our phenomenological model, we argue that these effects result from the modification of the electronic Berry curvature by a combination of both spin-orbit coupling and strain. (See the full abstract in the PDF.)

cond-mat.str-el↗

arXiv2026

Uniaxial strain tuned magnetism of the altermagnet candidate h-FeS

Altermagnets are collinear magnetic materials with 'alter'nating local crystalline environments, characterized by joint spin and crystalline symmetries that enable ferromagnetic-like transport properties but with vanishing net magnetization. Hexagonal FeS (h-FeS) is a recently identified altermagnet candidate that shows a spontaneous anomalous Hall effect (AHE) accompanied by a tiny net magnetization. Here, we show that both the spontaneous AHE and magnetization can be effectively suppressed by an in-plane compressive strain. Since neutron diffraction measurements show that the applied uniaxial strain only modifies the in-plane domain population but does not affect the in-plane magnetic structure, the major effect of the applied strain is to tune the small $c$-axis ferromagnetic moment. Our results demonstrate a strong correlation between the tiny net magnetization and the spontaneous AHE in h-FeS, and show that uniaxial strain provides an effective knob to tune both properties in this altermagnet candidate for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2026

Atomically-sharp magnetic soliton in the square-net lattice EuRhAl$_{4}$Si$_{2}$

Topological spin textures are hallmark manifestations of competing interactions in magnetic matter. Their effective description by nonlinear field theories reflects an energetic frustration that destabilizes uniform order while selecting finite-size, topologically nontrivial configurations as stationary states. Among the most extreme realizations are atomically-sharp domain wall excitations, namely one-dimensional (1D) magnetic solitons, which represent the ultimate scaling limit of magnetic textures. Such solitons may emerge in magnetic systems where effective exchange interactions compete directly with uniaxial magnetic anisotropy. Here we show that the square-net rare earth compound EuRhAl$_{4}$Si$_{2}$ realizes a very susceptible regime where the magnetic anisotropy competes with highly frustrated exchange interactions stabilizing a rare ferrimagnetic $\uparrow\uparrow\downarrow$ state that, under applied magnetic field, supports the formation of atomically-sharp soliton defects. We confirm the bulk response of the 1D magnetic solitons via magnetization and electrical transport measurements. We establish both the zero- and in-field $\uparrow\uparrow\downarrow$ order via neutron diffraction, while magnetic force microscopy visualizes its real-space evolution into a stripe-like array. To elucidate the microscopic origin of the soliton, we relate the Ruderman-Kittel-Kasuya-Yosida (RKKY)-driven exchange interactions and the magnetic anisotropy through density functional theory, and we construct an effective 1D $J_{1}$-$J_{2}$-$K$ model whose atomistic spin dynamics simulations reproduce the observed soliton states as a function of external field. Our results demonstrate that EuRhAl$_{4}$Si$_{2}$ hosts atomically-sharp, field-driven 1D magnetic solitons, providing a new platform for studying 1D topological excitations at the atomic length scale.

cond-mat.str-el↗

arXiv2026

Machine Learning-Guided Discovery of Kagome Superconductors YRu3B2 and LuRu3B2

We report the experimental discovery of bulk superconductivity in two kagome lattice compounds, YRu$_3$B$_2$ and LuRu$_3$B$_2$, which were predicted through machine learning-accelerated high-throughput screening combined with first principles calculations. These materials crystallize in the hexagonal CeCo$_3$B$_2$-type structure with planar kagome networks formed by Ru atoms. We observe superconducting critical temperatures of $T_{c} = 0.81$~K for YRu$_3$B$_2$ and $T_{c} = 0.95$~K for LuRu$_3$B$_2$, confirmed through magnetization and specific heat measurements. Both compounds exhibit nearly 100\% superconducting volume fractions, demonstrating bulk superconductivity. Compared with LaRu$_3$Si$_2$, YRu$_3$B$_2$ and LuRu$_3$B$_2$ show a more dispersive Ru local $d_{x^2-y^2}$ quasi-flat band (and thus a reduced DOS at $E_F$) together with an overall hardening of the phonon spectrum, both of which lower the electron-phonon coupling (EPC) constant $λ$. Meanwhile, the dominant real-space EPC between Ru local $d_{x^2-y^2}$ states and the low-frequency Ru in-plane local $x$ branch remains nearly unchanged, indicating that the reduction of $λ$ originates from the $d_{x^2-y^2}$ DOS reduction and the overall phonon hardening. Superfluid weight calculations show that conventional contributions dominate over quantum geometric effects due to the dispersive nature of bands near the Fermi level. This work demonstrates the effectiveness of integrating machine learning screening, first principles theory, and experimental synthesis for accelerating the discovery of new superconducting materials.

cond-mat.supr-con↗

arXiv2025

Quantum oscillations and anisotropic magnetoresistance in the quasi-two-dimensional Dirac nodal line superconductor $\mathrm{YbSb_2}$

Recent interest in quantum materials has focused on systems exhibiting both superconductivity and non-trivial band topology as material candidates to realize topological or unconventional superconducting states. So far, superconductivity in most topological materials has been identified as type II. In this work, we present magnetotransport studies on the quasi-two-dimensional type I superconductor $\mathrm{YbSb_2}$. Combined ab initio DFT calculations and quantum oscillation measurements confirm that $\mathrm{YbSb_2}$ is a Dirac nodal line semimetal in the normal state. The complex Fermi surface morphology is evidenced by the non-monotonic angular dependence of both the quantum oscillation amplitude and the magnetoresistance. Our results establish $\mathrm{YbSb_2}$ as a candidate material platform for exploring the interplay between band topology and superconductivity.

cond-mat.supr-con↗

arXiv2025

Fermi surface and Berry phase analysis for Dirac nodal line semimetals: cautionary tale to SrGa$_2$ and BaGa$_2$

A Berry phase of odd multiples of $π$ inferred from quantum oscillations (QOs) has often been treated as evidence for nontrivial reciprocal space topology. However, disentangling the Berry phase values from the Zeeman effect and the orbital magnetic moment is often challenging. In centrosymmetric compounds, the case is simpler as the orbital magnetic moment contribution is negligible. Although the Zeeman effect can be significant, it is usually overlooked in most studies of QOs in centrosymmetric compounds. Here, we present a detailed study on the non-magnetic centrosymmetric $\mathrm{SrGa_2}$ and $\mathrm{BaGa_2}$, which are predicted to be Dirac nodal line semimetals (DNLSs) based on density functional theory (DFT) calculations. Evidence of the nontrivial topology is found in magnetotransport measurements. The Fermi surface topology and band structure are carefully studied through a combination of angle-dependent QOs, angle-resolved photoemission spectroscopy (ARPES), and DFT calculations, where the nodal line is observed in the vicinity of the Fermi level. Strong de Haas-van Alphen fundamental oscillations associated with higher harmonics are observed in both compounds, which are well-fitted by the Lifshitz-Kosevich (LK) formula. However, even with the inclusion of higher harmonics in the fitting, we found that the Berry phases cannot be unambiguously determined when the Zeeman effect is included. We revisit the LK formula and analyze the phenomena and outcomes that were associated with the Zeeman effect in previous studies. Our experimental results confirm that $\mathrm{SrGa_2}$ and $\mathrm{BaGa_2}$ are Dirac nodal line semimetals. Additionally, we highlight the often overlooked role of spin-damping terms in Berry phase analysis.

cond-mat.mtrl-sci↗

arXiv2025

Magnetism Enhanced Surface Bonding of O$_{2}$ on CoPt

For large-scale deployment and use of polymer electrolyte fuel cells, high-performance electrocatalysts with low platinum consumption are desirable. One promising strategy to meet this demand is to explore alternative materials that retain catalytic efficiency while introducing new mechanisms for performance enhacement. In this study, we investigate a ferromagnetic CoPt as a candidate material to accelerate oxygen reduction reactions. By using density functional theory calculations, we find the spin-polarized Co-$d$ states to enhance O$_2$ surface bonding due to local exchange splitting of Co-$d$ carriers at the Fermi level. Furthermore, O and O$_2$ adsorption and dissociation energies are found to be tuned by varying the thickness of the Pt layers. Our study gives insight into the role magnetism plays in the oxygen reduction reaction process and how magnetic ions may aid in the design of new advanced catalysts.

cond-mat.mtrl-sci↗

arXiv2025

Kramers nodal lines in intercalated TaS$_2$ superconductors

Kramers degeneracy is one fundamental embodiment of the quantum mechanical nature of particles with half-integer spin under time reversal symmetry. Under the chiral and noncentrosymmetric achiral crystalline symmetries, Kramers degeneracy emerges respectively as topological quasiparticles of Weyl fermions and Kramers nodal lines (KNLs), anchoring the Berry phase-related physics of electrons. However, an experimental demonstration for ideal KNLs well isolated at the Fermi level is lacking. Here, we establish a class of noncentrosymmetric achiral intercalated transition metal dichalcogenide superconductors with large Ising-type spin-orbit coupling, represented by In$_x$TaS$_2$, to host an ideal KNL phase. We provide evidence from angle-resolved photoemission spectroscopy with spin resolution, angle-dependent quantum oscillation measurements, and ab-initio calculations. Our work not only provides a realistic platform for realizing and tuning KNLs in layered materials, but also paves the way for exploring the interplay between KNLs and superconductivity, as well as applications pertaining to spintronics, valleytronics, and nonlinear transport.

cond-mat.supr-con↗

arXiv2025

Anomalous Electrical Transport in the Kagome Magnet YbFe$_6$Ge$_6$

Two-dimensional (2D) kagome metals offer a unique platform for exploring electron correlation phenomena derived from quantum many-body effects. Here, we report a combined study of electrical magnetotransport and neutron scattering on YbFe$_6$Ge$_6$, where the Fe moments in the 2D kagome layers exhibit an $A$-type collinear antiferromagnetic order below $T_{\rm{N}} \approx 500$ K. Interactions between the Fe ions in the layers and the localized Yb magnetic ions in between reorient the $c$-axis aligned Fe moments to the kagome plane below $T_{\rm{SR}} \approx 63$ K. Our magnetotransport measurements show an intriguing anomalous Hall effect (AHE) that emerges in the spin-reorientated collinear state, accompanied by the closing of the spin anisotropy gap as revealed from inelastic neutron scattering. The gapless spin excitations and the Yb-Fe interaction are able to support a dynamic scalar spin chirality, which explains the observed AHE. Therefore, our study demonstrates spin fluctuations may provide an additional scattering channel for the conduction electrons and give rise to AHE even in a collinear antiferromagnet.

cond-mat.str-el↗

arXiv2025

Theory of Superconductivity in LaRu$_3$Si$_2$ and Predictions of New Kagome Flat Band Superconductors

We present a comprehensive investigation of the flat-band kagome superconductor LaRu$_3$Si$_2$, which has recently been reported to host charge density wave (CDW) order above room temperature ($T_{CDW} \simeq 400$ K). The stable crystal structure above the CDW transition is identified via soft phonon condensation and confirmed to be harmonically stable through ab initio calculations, consistent with recent X-ray diffraction refinements. The electron-phonon coupling (EPC) in LaRu$_3$Si$_2$ is found to be mode-selective, primarily driven by strong interactions between Ru-$B_{3u}$ phonons (local $x$-direction, pointing toward the hexagon center) and Ru-$A_g$ electrons (local $d_{x^2-y^2}$ orbital) within the kagome lattice. Using a spring-ball model, we identify this mode-selective EPC as a universal feature of kagome materials. Employing the newly developed Gaussian approximation of the hopping parameters, we derive an analytical expression for the EPC and demonstrate that superconductivity in LaRu$_3$Si$_2$ is mostly driven by the coupling between the kagome $B_{3u}$ phonons and the $A_g$ electrons. The impact of doping is also investigated, revealing that light hole doping (approximately one hole per unit cell) significantly enhances the superconducting critical temperature $T_c$ by 50%, whereas heavy doping induces structural instability and ferromagnetism. Furthermore, high-throughput screening identifies 3063 stable 1:3:2 kagome materials, of which 428 are predicted to exhibit superconductivity with $T_c > 1$ K, and the highest $T_c$ reaching 15 K. These findings establish LaRu$_3$Si$_2$ and related materials as promising platforms for exploring the interplay among kagome flat bands, EPC, and superconductivity. Additionally, they may offer valuable insights into potential limitations on the $T_c$ of flat-band superconductivity in real materials.

cond-mat.supr-con↗

arXiv2025

Correlation between Complex Spin Textures and the Magnetocaloric and Hall Effects in Eu(Ga$_{1-x}$Al$_x$)$_4$ ($x$ = 0.9, 1)

Determining the electronic phase diagram of a quantum material as a function of temperature (T) and applied magnetic field (H) forms the basis for understanding the microscopic origin of transport properties, such as the anomalous Hall effect (AHE) and topological Hall effect (THE). For many magnetic quantum materials, including EuAl$_4$, a THE arises from a topologically protected magnetic skyrmion lattice with a non-zero scalar spin chirality. We identified a square skyrmion lattice (sSkL) peak in Eu(Ga$_{1-x}$Al$_x$)$_4$ ($x$ = 0.9) identical to the peak previously observed in EuAl$_4$ by performing neutron scattering measurements throughout the phase diagram. Comparing these neutron results with transport measurements, we found that in both compounds the maximal THE does not correspond to the sSkL area. Instead of the maximal THE, the maximal magnetocaloric effect (MCE) boundaries better identify the sSkL lattice phase observed by neutron scattering measurements. The maximal THE therefore arises from interactions of itinerant electrons with frustrated spin fluctuations in a topologically trivial magnetic state.

cond-mat.str-el↗

arXiv2024

2D Theoretically Twistable Material Database

The study of twisted two-dimensional (2D) materials, where twisting layers create moiré superlattices, has opened new opportunities for investigating topological phases and strongly correlated physics. While systems such as twisted bilayer graphene (TBG) and twisted transition metal dichalcogenides (TMDs) have been extensively studied, the broader potential of a seemingly infinite set of other twistable 2D materials remains largely unexplored. In this paper, we define "theoretically twistable materials" as single- or multi-layer structures that allow for the construction of simple continuum models of their moiré structures. This excludes, for example, materials with a "spaghetti" of bands or those with numerous crossing points at the Fermi level, for which theoretical moiré modeling is unfeasible. We present a high-throughput algorithm that systematically searches for theoretically twistable semimetals and insulators based on the Topological 2D Materials Database. By analyzing key electronic properties, we identify thousands of new candidate materials that could host rich topological and strongly correlated phenomena when twisted. We propose representative twistable materials for realizing different types of moiré systems, including materials with different Bravais lattices, valleys, and strength of spin-orbital coupling. We provide examples of crystal growth for several of these materials and showcase twisted bilayer band structures along with simplified twisted continuum models. Our results significantly broaden the scope of moiré heterostructures and provide a valuable resource for future experimental and theoretical studies on novel moiré systems.

cond-mat.mtrl-sci↗