arXiv subjects

Adam A. Aczel

Publications and source records attributed to Adam A. Aczel.

At least 19 recordsLinked to original sources

arXiv2026

Magnetoresistive Memory in the Paramagnetic Phase of Eu$_5$In$_2$As$_6$

Magnetoresistive materials that respond sensitively to applied fields are central to modern data storage technologies. Here we unveil a novel Magnetoresistive Memory (MRM) in Eu$_5$In$_2$As$_6$, where the electrical resistivity depends not only on the magnitude but also on the history of the applied magnetic field. Such an effect has been reported in only two classes of strongly correlated electron systems: perovskite manganites and pyrochlore iridates. In both cases, the effect has been observed in the magnetically ordered phase. It has been attributed to metastable magnetic states in manganites and conducting domain walls in iridates. Remarkably, the MRM in Eu$_5$In$_2$As$_6$ onsets at twice the antiferromagnetic transition temperature, well within the paramagnetic phase. The temperature, field, and time dependence of resistivity suggest that either a hidden order or a fluctuating phase with short-range correlations underlies this effect. Our results offer MRM as a new platform for quantum sensing and memory technologies, and encourage searching for MRM in related materials.

cond-mat.str-el↗

arXiv2026

Competing ferromagnetic and antiferromagnetic interactions in non-altermagnetic Ru$_{1-x}$Cr$_{x}$O$_{2}$

We investigate the proposal of hole doping RuO$_{2}$ via alloying with Cr ions to induce altermagnetism. Thin film samples of Ru$_{1-x}$Cr$_{x}$O$_{2}$ ($0 \leq x \leq 0.28$) were prepared by reactive magnetron co-sputtering on TiO$_{2}$ substrates, with epitaxial nature verified by X-ray diffraction and composition depth profiles determined by X-ray photoelectron spectroscopy combined with Ar etching. Neutron diffraction measurements of samples with $x = 0$ and $x = 0.23$ at low temperatures show no evidence of long-range altermagnetic order with spins oriented along the $c$-axis or within the $ab$-plane. In contrast, temperature dependent susceptibility measurements in samples with $x \geq 0.16$ show a gradual downturn below $T\approx20~$K, suggestive of antiferromagnetic interactions, although this coexists with ferromagnetic hysteresis and remnant magnetization at $4~$K for samples with $x \geq 0.23$. Together, our magnetometry and neutron scattering measurements suggest the coexistence of antiferromagnetically coupled ferromagnetic CrO\textsubscript{2} clusters. This indicates that Cr ions do not hole dope Ru bands to induce an altermagnet state, but rather the holes remain localized to the Cr ions.

cond-mat.mtrl-sci↗

arXiv2025

Dynamical moment fragmentation in the all-in all-out pyrochlore Nd2Sn2O7

We report single-crystal neutron spectroscopy and bulk characterization on hydrothermally grown Nd2Sn2O7, revealing a dynamical moment fragmentation embedded within the all-in-all-out ordered state. The spectra show a nearly flat band with pinch-point-like momentum dependence, accompanied by dispersive branches that generate half-moon features across multiple Brillouin zones. These defining signatures are captured quantitatively by a minimal dipolar-octupolar spin Hamiltonian, demonstrating excellent agreement between experiment and theory. The higher flat-mode energy helps account for the absence of dynamical interference in prior muon spin relaxation (muSR) studies, while the lack of any photon-like excitation imposes strict constraints on the proposed Coulombic antiferromagnet scenario. Our results extend dynamical moment fragmentation to Nd2Sn2O7 and identify it as a clean, tractable platform for quantitative exploration of emergent gauge-field physics and multipolar spin-wave dynamics in frustrated magnets.

cond-mat.str-el↗

arXiv2025

Vanishing ordered moment in the frustrated triangular lattice antiferromagnet CuNdO$_2$

We investigate the magnetic ground state of CuNdO$_2$, which is a delafossite with a triangular lattice of magnetic Nd$^{3+}$ ions that are well separated by non-magnetic Cu spacer layers. From inelastic neutron scattering measurements of the crystal electric field, we determine the strong Ising character of the pseudo-spin 1/2 Nd$^{3+}$ moments. Magnetic susceptibility and heat capacity measurements reveal the onset of long-range antiferromagnetic order at $T_N=0.78$ K. While the magnetic transition is definitively observed with muon spin relaxation, accompanied by the formation of a weakly dispersing spin wave excitation, no dipole-ordered moment is detected with neutron diffraction. We show that the apparent absence of a dipolar ordered moment is a consequence of the dominant Ising character of the antiferromagnetically coupled Nd$^{3+}$ moments, which experience extreme frustration on the triangular lattice. Consequently, the frustration in CuNdO$_2$ is relieved through in-plane ordering of the substantially smaller perpendicular component of the Nd$^{3+}$ moments into a 120\textdegree\ structure, with a nearly vanishing ordered moment.

cond-mat.str-el↗

arXiv2025

Disorder-induced proximate quantum spin ice phase in Pr2Sn2O7

Magnetic pyrochlores with non-Kramers rare-earth ions provide a platform for exploring emergent gauge physics and quantum spin-ice behavior, yet the influence of structural disorder on their ground states remains insufficiently understood. Here we combine bulk characterization and single-crystal neutron-scattering measurements to investigate the non-Kramers pyrochlore Pr2Sn2O7. At temperatures below ~1 K, the system exhibits key hallmarks of quantum spin-ice physics, including anisotropic spin-ice correlations and two distinct dynamical timescales. Upon further cooling, however, we observe a complete spin-freezing transition at T_f ~ 0.15 K, accompanied by recovery of the full nuclear Schottky anomaly, the emergence of a gapped magnetic excitation, and the development of incipient (100) magnetic correlations. Comparison with related Pr-based pyrochlores places Pr2Sn2O7 near the spin-frozen boundary of a disorder-perturbed phase diagram. These results establish a disorder-driven framework for the evolution of quantum spin-ice behavior into frozen ground states, revealing how signatures of a proximate quantum spin liquid can persist despite disorder-induced spin freezing in non-Kramers pyrochlores.

cond-mat.str-el↗

arXiv2025

Pressure tuning of competing interactions on a honeycomb lattice

Magnetic exchange interactions are mediated via orbital overlaps across chemical bonds. Thus, modifying the bond angles by physical pressure or strain can tune the relative strength of competing interactions. Here we present a remarkable case of such tuning between the Heisenberg (J) and Kitaev (K) exchange, which respectively establish magnetically ordered and spin liquid phases on a honeycomb lattice. We observe a rapid suppression of the Neel temperature (TN) with pressure in Ag3LiRh2O6, a spin-1/2 honeycomb lattice with both J and K couplings. Using a combined analysis of x-ray data and first-principles calculations, we find that pressure modifies the bond angles in a way that increases the |K/J| ratio and thereby suppresses TN. Consistent with this picture, we observe a spontaneous onset of muon spin relaxation (muSR) oscillations below TN at low pressure, whereas in the high-pressure phase, oscillations appear only when T < TN/2. Unlike other candidate Kitaev materials, Ag3LiRh2O6 is tuned toward a quantum critical point by pressure while avoiding a structural dimerization in the relevant pressure range.

cond-mat.str-el↗

arXiv2025

Universal dynamics of a pair condensate

Pair condensates appear in multiple branches of physics, always introducing exotic phenomena. The pair condensate in quantum magnetism is the spin nematic, whose static (quadrupolar) order is difficult to access, favoring dynamical probes. Here, we perform high-resolution neutron spectroscopy to obtain direct evidence for the presence of two spin-nematic phases induced in the triangular-lattice antiferromagnet Na2BaNi(PO4)2 by controlling the applied magnetic field. By combining precise experiments with quantitative theoretical and numerical analysis, we identify universal dynamics arising from the pair condensate. We show explicitly how the gapless Goldstone mode influences the dispersion and induces Cherenkov-like velocity-selective decay of the gapped single-quasiparticle band. These common spectral features shed new light on spin-nematic dynamics and underline the universal phenomenology shared by pair condensates across different physical systems.

cond-mat.str-el↗

arXiv2024

Two types of colossal magnetoresistance with distinct mechanisms in Eu5In2As6

Recent reports of colossal negative magnetoresistance (CMR) in a few magnetic semimetals and semiconductors have attracted attention, because these materials are devoid of the conventional mechanisms of CMR such as mixed valence, double exchange interaction, and Jahn-Teller distortion. New mechanisms have thus been proposed, including topological band structure, ferromagnetic clusters, orbital currents, and charge ordering. The CMR in these compounds has been reported in two forms: either a resistivity peak or a resistivity upturn suppressed by a magnetic field. Here we reveal both types of CMR in a single antiferromagnetic semiconductor Eu5In2As6. Using the transport and thermodynamic measurements, we demonstrate that the peak-type CMR is likely due to the percolation of magnetic polarons with increasing magnetic field, while the upturn-type CMR is proposed to result from the melting of a charge order under the magnetic field. We argue that similar mechanisms operate in other compounds, offering a unifying framework to understand CMR in seemingly different materials.

cond-mat.str-el↗

arXiv2024

Pulling order back from the brink of disorder: Observation of a nodal line spin liquid and fluctuation stabilized order in K$_2$IrCl$_6$

Competing interactions in frustrated magnets can give rise to highly degenerate ground states from which correlated liquid-like states of matter often emerge. The scaling of this degeneracy influences the ultimate ground state, with extensive degeneracies potentially yielding quantum spin liquids, while sub-extensive or smaller degeneracies yield static orders. A longstanding problem is to understand how ordered states precipitate from this degenerate manifold and what echoes of the degeneracy survive ordering. Here, we use neutron scattering to experimentally demonstrate a new "nodal line" spin liquid, where spins collectively fluctuate within a sub-extensive manifold spanning one-dimensional lines in reciprocal space. Realized in the spin-orbit coupled, face-centered cubic iridate K$_2$IrCl$_6$, we show that the sub-extensive degeneracy is robust, but remains susceptible to fluctuations or longer range interactions which cooperate to select a magnetic order at low temperatures. Proximity to the nodal line spin liquid influences the ordered state, enhancing the effects of quantum fluctuations and stabilizing it through the opening of a large spin-wave gap. Our results demonstrate quantum fluctuations can act counter-intuitively in frustrated materials: instead of destabilizing ordering, at the brink of the nodal spin liquid they can act to stabilize it and dictate its low-energy physics.

cond-mat.str-el↗

arXiv2024

Helimagnetism in CrBr$_2$ and CrIBr

In CrX$_2$ (X=Br, I), a Jahn-Teller effect distorts the octahedral configuration of the anions about Cr, resulting in a "ribbon chain" structure. We previously observed helimagnetism in CrI$_2$ propagating along the ribbon chains, with an 80-90$^{\circ}$ rotation per Cr ion. Via neutron scattering, we report that CrBr$_2$ and the solid solution CrIBr are also helimagnetic, with N\'{e}el temperatures of 17 and 12 K, respectively, and signs of intermediate magnetic transitions in both compounds. The helical angle between spins on consecutive Cr ions along the ribbon chains increases with increasing Br substitution, from 89.7$^{\circ}$ (CrI$_2$) to 116$^{\circ}$ (CrIBr) to 147$^{\circ}$ (CrBr$_2$), possibly as a result of decreasing longer-range intrachain spin interactions with the increased electron localization of the lighter anions.

cond-mat.str-el↗

arXiv2024

Role of stacking defects on the magnetic behavior of CrCl$_3$

In the study of van der Waals-layered magnetic materials, the properties of CrCl$_3$ continue to attract attention. This compound is reported to undergo antiferromagnetic (AFM) ordering below $\sim$14 K, with a ferromagneticlike region proposed to exist between 14 and 17 K. Ideally, the crystal structure is rhombohedral (R) below $\sim$235 K, separated from a higher-temperature monoclinic (M) phase by a layer-sliding structural phase transition. However, the structural transition is often inhibited even in bulk single crystals, allowing M-type layer stacking, reported to have a tenfold greater interlayer magnetic coupling than R-type stacking, to be present at low temperature. To clarify the effect of stacking defects on CrCl$_3$, we report magnetization measurements on samples of varying crystalline quality. At low applied magnetic field, some crystals predominantly show the $T_N=14$ K peak, but other crystals show hysteretic behavior and a magnetization enhancement at a slightly higher temperature ($14 < T \lesssim 17$ K.) Samples with anomalous behavior exhibit a transition around $\sim$2 T in isothermal magnetization-field data, providing evidence that M-type stacking defects are the source of these anomalies. Ground powder samples are especially likely to show strongly anomalous behavior. We suggest that the anomalous behavior arises from few-layer magnetic domains that form just above $T_N$ in an environment of mixed interlayer magnetic coupling strength. We argue that the influence of M-type stacking boundaries on sublattice magnetization is already observable in reported neutron scattering data, and may be responsible for a certain feature in reported specific heat data.

cond-mat.mtrl-sci↗

arXiv2023

Monte-Carlo ray-tracing studies of multiplexed prismatic graphite analyzers for the cold-neutron triple-axis spectrometer at the High Flux Isotope Reactor

A modern cold triple-axis spectrometer to study quantum condensed matter systems is planned for the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory. Here, we describe the conceptual principles and design of a secondary spectrometer using a multiplexed, prismatic analyzer system relying on graphite crystals and inspired by the successful implementation of the Continuous Angle Multiple Energy Analysis (CAMEA) spectrometers at the Paul Scherrer Institute. This project is currently known as MANTA for Multi-analyzer Neutron Triple-Axis. We report Monte-Carlo ray-tracing simulations on a simple but realistic sample scattering kernel to further illustrate the prismatic analyzer concept's workings, calibration, and performance. Then, we introduce a new statistical analysis approach based on the prismatic analyzer concept to improve the number of final energies measured on the spectrometer. We also study possible evolutions in the CAMEA design relevant for MANTA.

physics.ins-det↗

arXiv2023

Revisiting spin ice physics in the ferromagnetic Ising pyrochlore Pr$_2$Sn$_2$O$_7$

Pyrochlore materials are characterized by their hallmark network of corner-sharing rare-earth tetrahedra, which can produce a wide array of complex magnetic ground states. Ferromagnetic Ising pyrochlores often obey the "two-in-two-out" spin ice rules, which can lead to a highly-degenerate spin structure. Large moment systems, such as Ho$_2$Ti$_2$O$_7$ and Dy$_2$Ti$_2$O$_7$, tend to host a classical spin ice state with low-temperature spin freezing and emergent magnetic monopoles. Systems with smaller effective moments, such as Pr$^{3+}$-based pyrochlores, have been proposed as excellent candidates for hosting a "quantum spin ice" characterized by entanglement and a slew of exotic quasiparticle excitations. However, experimental evidence for a quantum spin ice state has remained elusive. Here, we show that the low-temperature magnetic properties of Pr$_2$Sn$_2$O$_7$ satisfy several important criteria for continued consideration as a quantum spin ice. We find that Pr$_2$Sn$_2$O$_7$ exhibits a partially spin-frozen ground state with a large volume fraction of dynamic magnetism. Our comprehensive bulk characterization and neutron scattering measurements enable us to map out the magnetic field-temperature phase diagram, producing results consistent with expectations for a ferromagnetic Ising pyrochlore. We identify key hallmarks of spin ice physics, and show that the application of small magnetic fields ($\mu_0 H_c \sim$0.75T) suppresses the spin ice state and induces a long-range ordered magnetic structure. Together, our work clarifies the current state of Pr$_2$Sn$_2$O$_7$ and encourages future studies aimed at exploring the potential for a quantum spin ice ground state in this system.

cond-mat.str-el↗

arXiv2023

Stripe Helical Magnetism and Two Regimes of Anomalous Hall Effect in NdAlGe

We report the magnetic and electronic transport properties of the inversion and time-reversal symmetry breaking Weyl semimetal NdAlGe. This material is analogous to NdAlSi, whose helical magnetism presents a rare example of a Weyl-mediated collective phenomenon, but with a larger spin-orbit coupling. Our neutron diffraction experiments revealed that NdAlGe, similar to NdAlSi, supports an incommensurate Ising spin density wave ($T_{\text{inc}}=6.8$ K) with a small helical spin canting of 3$^\circ$ and a long-wavelength of $\sim$ 35 nm, which transitions to a commensurate ferrimagnetic state below $T_{\text{com}}=5.1$ K. Using small-angle neutron scattering, we showed that the zero-field cooled ferrimagnetic domains form stripes in real space with characteristic length scales of 18 nm and 72 nm parallel and perpendicular to the [110] direction, respectively. Interestingly, for the transport properties, NdAlSi does not exhibit an anomalous Hall effect (AHE) that is commonly observed in magnetic Weyl semimetals. In contrast to NdAlSi, we identify two different AHE regimes in NdAlGe that are respectively governed by intrinsic Berry curvature and extrinsic disorders/spin fluctuations. Our study suggests that Weyl-mediated magnetism prevails in this group of noncentrosymmetric magnetic Weyl semimetals NdAl$X$, but transport properties including AHE are affected by material-specific extrinsic effects such as disorders, despite the presence of prominent Berry curvature.

cond-mat.mtrl-sci↗

arXiv2022

Topological spiral magnetism in the Weyl semimetal SmAlSi

Weyl electrons are intensely studied due to novel charge transport phenomena such as chiral anomaly, Fermi arcs, and photogalvanic effect. Recent theoretical works suggest that Weyl electrons can also participate in magnetic interactions, and the Weyl-mediated indirect exchange coupling between local moments is proposed as a new mechanism of spiral magnetism that involves chiral electrons. Despite reports of incommensurate and non-collinear magnetic ordering in Weyl semimetals, an actual spiral order has remained hitherto undetected. Here, we present evidence of Weyl-mediated spiral magnetism in SmAlSi from neutron diffraction, transport, and thermodynamic data. We show that the spiral order in SmAlSi results from the nesting between topologically non-trivial Fermi pockets and weak magnetocrystalline anisotropy, unlike related materials (Ce,Pr,Nd)AlSi, where a strong anisotropy prevents the spins from freely rotating. We map the magnetic phase diagram of SmAlSi and reveal an A-phase where topological magnetic excitations may exist. This is corroborated by the observation of a topological Hall effect within the A-phase.

cond-mat.mtrl-sci↗

arXiv2021

Generic character of charge and spin density waves in superconducting cuprates

Understanding the nature of charge density waves (CDW) in cuprate superconductors has been complicated by material specific differences. A striking example is the opposite doping dependence of the CDW ordering wavevector in La-based and Y-based compounds, the two families where charge ordering is strongest and best characterized. Here we report a combined resonant soft X-ray scattering (RSXS) and neutron scattering study of charge and spin density waves in isotopically enriched La$_{1.8-x}$ Eu$_{0.2}$ Sr$_{x}$ CuO$_{4}$ over a range of doping $0.07 \leq x \leq 0.20$. For all dopings studied by RSXS, we find that the CDW amplitude is approximately temperature-independent and develops well above experimentally accessible temperatures. Surprisingly, the CDW ordering wavevector shows a non-monotonic temperature dependence, with a sudden change occurring at temperatures near the SDW onset temperature. We describe this behavior with a Landau-Ginzburg theory for an incommensurate CDW in a metallic system with a finite charge compressibility and CDW-SDW coupling. Our Landau-Ginzburg analysis suggests that the ordering wavevector at high temperatures decreases with increased doping. This behavior is opposite to the trend at low temperatures and highly reminiscent of the doping dependence seen in YBa$_2$ Cu$_3$ O$_{6+\delta}$ , suggesting a common origin of the CDW in hole-doped cuprate superconductors.

cond-mat.supr-con↗

arXiv2020

Field-induced quantum critical point in the new itinerant antiferromagnet Ti$_3$Cu$_4$

New phases of matter emerge at the edge of magnetic instabilities. In local moment systems, such as heavy fermions, the magnetism can be destabilized by pressure, chemical doping, and, rarely, by magnetic field, towards a zero-temperature transition at a quantum critical point (QCP). Even more rare are instances of QCPs induced by pressure or doping in itinerant moment systems, with no known examples of analogous field-induced \textit{T} = 0 transitions. Here we report the discovery of a new itinerant antiferromagnet with no magnetic constituents, in single crystals of Ti$_3$Cu$_4$ with $T_N$ = 11.3 K. Band structure calculations point to an orbital-selective, spin density wave ground state, a consequence of the square net structural motif in Ti$_3$Cu$_4$. A small magnetic field, $H_C$ = 4.87 T, suppresses the long-range order via a continuous second-order transition, resulting in a field-induced QCP. The magnetic Gr\"uneisen ratio diverges as $H \rightarrow H_C$ and $T\rightarrow0$, with a sign change at $H_C$ and $T^{-1}$ scaling at $H~=~H_C$, providing evidence from thermodynamic measurements for quantum criticality for $H \parallel c$. Non-Fermi liquid (NFL) to Fermi liquid (FL) crossover is observed close to the QCP, as revealed by the power law behavior of the electrical resistivity.

cond-mat.str-el↗

arXiv2020

Canted antiferromagnetic order in the monoaxial chiral magnets V$_{1/3}$TaS$_2$ and V$_{1/3}$NbS$_{2}$

The Dzyaloshinskii-Moriya (DM) interaction is present in the transition metal dichalcogenides (TMDC) magnets of form $M_{1/3}T$S$_{2}$ ($M$ $=$ 3d transition metal, $T$ $\in$ {Nb, Ta}), given that the intercalants $M$ form $\sqrt{3}\times\sqrt{3}$ superlattices within the structure of the parent materials $T$S$_2$ and break the centrosymmetry. Competition between the DM and ferromagnetic exchange interactions in these systems has been shown to stabilize a topological defect known as a chiral soliton in select intercalated TMDCs, initiating interest both in terms of fundamental physics and the potential for technological applications. In the current article, we report on our study of the materials V$_{1/3}$TaS$_2$ and V$_{1/3}$NbS$_2$, using a combination of x-ray powder diffraction, magnetization and single crystal neutron diffraction. Historically identified as ferromagnets, our diffraction results instead reveal that vanadium spins in these compounds are arranged into an A-type antiferromagnetic configuration at low temperatures. Refined moments are 1.37(6)$\mu_{B}$ and 1.50(9)$\mu_{B}$ for V$_{1/3}$TaS$_2$ and V$_{1/3}$NbS$_2$, respectively. Transition temperatures $T_{c}$~$=$~32K for V$_{1/3}$TaS$_{2}$ and 50K for V$_{1/3}$NbS$_{2}$ are obtained from the magnetization and neutron diffraction results. We attribute the small net magnetization observed in the low-temperature phases to a subtle ($\sim$2$^{\circ}$) canting of XY-spins in the out-of-plane direction. These new results are indicative of dominant antiferromagnetic exchange interactions between the vanadium moments in adjacent ab-planes, likely eliminating the possibility of identifying stable chiral solitons in the current materials.

cond-mat.mtrl-sci↗