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All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Unusual first-order magnetic phase transition and large magnetocaloric effect in Nd 2 In

A large magnetocaloric effect with its maximum near the boiling point of natural gas occurs in a rare-earth intermetallic compound Nd 2 In. While behaviors of physical properties indicate that paramagnetic-ferromagnetic transformation supporting the large magnetocaloric effect is firstorder in nature, temperature dependent crystallographic study reveals no changes in lattice symmetry and lack of discontinuities either in phase volume or lattice parameters. In this work we discuss how the borderline first-order nature of phase transformation in Nd 2 In is markedly different from conventional firstorder magnetic transitions occurring in other members of the family – isostructural Pr 2 In and non-isostructural Eu 2 In.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Insulating band gaps both below and above the Néel temperature in d -electron LaTi O 3 , LaV O 3 , SrMn O 3 , and LaMn O 3 perovskites as a symmetry-breaking event

Metal d-electron oxides having an odd number of electrons per cell should exhibit band degeneracy at the Fermi energy, making them, in band theory, formally metallic. In many cases, however, these are “false metals”, as evidenced by the observation that many ABO 3 oxide perovskites with a magnetic 3d B-atom are observed to be insulators both below and above the Néel temperature. These inconsistencies between experimental observation and expectation have historically been resolved by invoking degeneracy-breaking physics, based largely on -pure electron effects, such as strong interelectronic correlation for d-electron compounds (the Mott mechanism). Such explanations generally consider microscopic lattice or magnetic degrees of freedom (m-DOF) as largely passive spectators, not a cause of the formally metallic being an insulator. Yet, it has long been known that ABO 3 perovskites can manifest an arrangement of m-DOFs in the form of octahedral tilting, bond dimerization, Jahn-Teller distortions, and ordering of local magnetic moments. It appears reasonable that such structural and magnetic local degrees of freedom need to be allowed to compete with purely electronic strong correlation. To answer this question, we explored a range of d-electron oxide perovskites exemplified by the archetypes LaTiO 3 , LaVO 3 , SrMnO 3 , and LaMnO 3 with 1, 2, 3, or 4 d -electrons, respectively. Using a mean-field-like electronic structure method (here, density functional theory), we find that a combination of magnetic symmetry breaking with structural distortions can account for insulating band gaps in this series while at the same time correctly predicting for the control case an intrinsic paramagnetic metal in SrVO 3 as symmetry-breaking is insufficiently strong to remove the degeneracy. Finally, this indicates that calculating quantitatively local magnetic and positional symmetry-breaking motifs in unit cells that avoid averaging at the outset over the low symmetry motifs can provide consistent trends in a Mott transition without Mott U.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Exploring the $A$-V-Sb landscape beyond $A$$\mathrm{V_3}$ $\mathrm{Sb_5}$: A case study on the $\mathrm{KV_6}$ $\mathrm{Sb_6}$ Kagome compound

Kagome compounds have garnered attention in the past few years for their intriguing magnetic properties arising from spin frustration dictated by the geometry of the Kagome sublattice. In this paper, we highlight the success of the unconventional hydride route for the fast and easy synthesis of the Kagome compound $\mathrm{KV_6}$ $\mathrm{Sb_6}$. Here, high-temperature in situ powder x-ray diffraction (PXRD) studies proved to be useful in hinting at the existence of $\mathrm{KV_6}$ $\mathrm{Sb_6}$, identifying its synthesis conditions, and understanding the reaction mechanism. The crystal structure for $\mathrm{KV_6}$ $\mathrm{Sb_6}$ was determined from high-resolution PXRD data. The compound has a layered structure [$R\overline{3}m, a = 5.5318$ (9) Å, $c = 34.23$ (3)Å 3 $Z = 3$] at room temperature] and features a Kagome bilayer of V atoms. $\mathrm{KV_6}$ $\mathrm{Sb_6}$ is isostructural to the previously reported $\mathrm{RbV_6}$ $\mathrm{Sb_6}$ and $\mathrm{CsV_6}$ $\mathrm{Sb_6}$ compounds. $\mathrm{KV_6}$ $\mathrm{Sb_6}$ is thermally stable in vacuum up to 1173 K, as evident from the high-temperature in situ PXRD and differential scanning calorimetric analysis. Investigation of magnetic properties for $\mathrm{KV_6}$ $\mathrm{Sb_6}$ between 2 and 300 K reveals temperature-independent paramagnetism and an absence of superconductivity, like the Rb and Cs analogs. Furthermore, we compare the magnetic properties of K V 3 Sb 5 , another ternary Kagome compound, synthesized via two different methods: the hydride route and the traditional route from elements. Low-temperature transport property measurements of $\mathrm{KV_6}$ $\mathrm{Sb_6}$ indicate metallic behavior and an intrinsically low thermal conductivity of $\mathrm{1.0 W K^-1}$ $\mathrm{m^-1}$ at 300 K. The layered structure of $\mathrm{KV_6}$ $\mathrm{Sb_6}$ makes it an attractive candidate for deintercalation and doping studies to tune both magnetic and transport properties, laying a foundation for further studies.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Ultrahigh supercurrent density in a two-dimensional topological material

Ongoing advances in superconductors continue to revolutionize technology thanks to the increasingly versatile and robust availability of lossless supercurrents. In particular, high supercurrent density can lead to more efficient and compact power transmission lines, high-field magnets, as well as high-performance nanoscale radiation detectors and superconducting spintronics. In this work, we report the discovery of an unprecedentedly high superconducting critical current density (17 MA / cm 2 at 0 T and 7 MA / cm 2 at 8 T) in 1T'–WS 2 , exceeding those of all reported two-dimensional superconductors to date. 1T'–WS 2 features a strongly anisotropic (both in- and out-of-plane) superconducting state that violates the Pauli paramagnetic limit signaling the presence of unconventional superconductivity. Spectroscopic imaging of the vortices further substantiates the anisotropic nature of the superconducting state. More intriguingly, the normal state of 1T'–WS 2 carries topological properties. The band structure obtained via angle-resolved photoemission spectroscopy and first-principles calculations points to a Z 2 topological invariant. The concomitance of topology and superconductivity in 1T'–WS 2 establishes it as a topological superconductor candidate, which is promising for the development of quantum computing technology.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Local probe investigation of the spin dynamics in the kagome and interlayers of orthorhombic barlowite $\mathrm{Cu4(OD)_6FBr; ^{79}Br}$ and $\mathrm{^{63}Cu}$ NQR study

Here we report 79 Br and 63 Cu nuclear quadrupole resonance (NQR) in the paramagnetic state above T N =15 K of the antiferromagnetic orthorhombic phase of barlowite Cu 4 (OD) 6 FBr consisting of a layered kagome structure. The divergent behavior of the longitudinal 79 (1/T 1 ) and transverse 79 (1/T 2 ) relaxation rates observed at 79Br sites evidences that critical slowing down of Cu spin fluctuations sets in below ~20 K. This means that one or more Cu sites, most likely at the interlayer Cu(3,4,5) sites between the kagome planes, undergo the antiferromagnetic phase transition in a fairly conventional way. On the other hand, the 63 Cu NQR signal intensity is gradually wiped out below ~30 K, pointing toward gradual spin freezing of the kagome layers instead. These contrasting findings suggest significant roles played by magnetic frustration effects within the kagome layers.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Antiferromagnetic V 2 ⁢O 3 based exchange coupling

Vanadium sesquioxide (V 2 ⁢O 3 ) is a strongly correlated electronic material that famously undergoes a triple coupled first-order transition where it transitions from a paramagnetic metal with a rhombohedral structure at high temperature to an antiferromagnetic insulator with a monoclinic structure. While several studies have used one of both electronic and structural transitions to control the properties of a heterostructure, evidence of magnetic coupling has notoriously yet to be found. In this paper, we report on a robust magnetic coupling between the antiferromagnetic (AFM) V 2 ⁢O 3 and ferromagnetic (FM) Permalloy (Py) layers that results in a significant exchange bias and strain-induced coercivity enhancement. We provide a temperature and angle-dependent study of magnetic properties, which clearly indicates exchange bias at the AFM/FM interface that appears at the onset of the metal-insulator transition. The magnitude of the exchange bias is strongest when the field is applied along the [001] V 2 ⁢O 3 crystallographic orientation which corresponds to an AFM spin configuration on the [110] surface. Here, this opens the door to designing and implementing novel functionalities in transition metal oxide-based computing using the connection between magnetism and the metal-insulator transition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Complex fermiology and electronic structure of antiferromagnet EuSnP

Throughout this work, we studied the electronic structure of a layered antiferromagnetic metal, EuSnP, in the paramagnetic and in the antiferromagnetic phase using angle-resolved photoemission spectroscopy (ARPES) alongside density-functional theory (DFT)-based first-principles calculations. The temperature dependence of the magnetic susceptibility measurements exhibits an antiferromagnetic transition at a Néel temperature of 21 K. Employing high-resolution ARPES, the valence-band structure was measured at several temperatures above and below the Néel temperature, which produced identical spectra independent of temperature. Through analysis of the ARPES results presented here, we attribute the temperature-independent spectra to the weak coupling of the Sn and P conduction electrons with Eu 4⁢ƒ states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Exploring the impact of Cr-doping on the crystallographic and magnetic structure of Mn 5 Si 3 antiferromagnetic alloy

Here, the role of Cr-doping on the structural and magnetic ground states of Mn 5 Si 3 alloy has been investigated through temperature-dependent neutron powder diffraction (NPD) and X-ray absorption fine structure (XAFS) techniques. All the Cr-doped alloys of nominal composition Mn 5-x Cr x Si 3 (for x = 0.05, 0.1, and 0.2) undergo two first-order magneto-structural phase transitions from hexagonal (space group: P6 3 /mcm) paramagnetic → orthorhombic (space group: Ccmm) collinear antiferromagnetic → orthorhombic (space group: Cc2m) non-collinear antiferromagnetic phase on cooling from room temperature. NPD studies at different constant temperatures indicate that both anti ferromagnetic phases are commensurate in nature and can be represented by q = (0,1,0) magnetic propagation vector for all the Cr-doped alloys. Such doping at the Mn-site results in a significant modification of the non-collinear antiferromagnetic structure (both moment size and orientation) and hence affects the unusual magnetic properties, like inverted hysteresis loop, thermomagnetic irreversibility, etc. The XAFS measurements were performed to interpret the local environment of doped Cr atoms in detail, which is critical for a microscopic understanding of the unusual properties of this class of Cr-doped Mn 5 Si 3 alloys. The analysis confirms the elemental state of Cr in the doped alloys and indicates a high degree of preservation of local crystallographic structure with varying Cr concentration and sample temperature. Doping induces intriguing changes in XAFS patterns, elucidated through different types of scattering mechanisms associated with the central absorbing Cr atom.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Influence of Nb alloying on Nb recrystallization and the upper critical field of Nb 3 ⁢Sn

Nb 3 Sn conductors are important candidates for high-field magnets for particle accelerators, and they continue to be widely used for many laboratory and NMR magnets. However, the critical current density, J c , of present Nb 3 Sn conductors declines swiftly above 12-15 T. State-of-the-art Ta- and Ti-doped strands exhibit upper critical field, H c2 , values of ~ 24-26.5 T (4.2 K) and do not reach the FCC target J c , which serves as the present stretch target for Nb 3 Sn development. As recently demonstrated, to meet this goal requires enhanced vortex pinning but an independent and supplementary approach is to significantly enhance H c2 . In this study, we have arc-melted multiple Nb alloys with added Hf, Zr, Ta and Ti and drawn them successfully into monofilament wires to investigate the possibilities of H c2 enhancement through alloying. H c2 (T) was measured for all samples in fields up to 16 T and some up to 31 T. We have found that all alloys show good agreement with the standard Werthamer, Helfand, and Hohenberg (WHH) fitting procedure without the need to adjust the paramagnetic limitation parameter (α) and spin-orbit scattering parameter (λ so ). The evaluation of dH c2 /dT near T c , which is proportional to the electronic specific heat coefficient γ and the normal state resistivity ρn, allows a better understanding of the induced disorder introduced by alloying in the A15 phase. So far, we have observed that Hf alloying of pure Nb can enhance H c2 (0) by 3-4 T to ~28 T, while adding just 1 at. %Hf or Zr into a Nb4Ta base alloy can raise H c2 (0) to ~31 T. Very importantly we find that Hf and Zr raise the alloy recrystallization temperature above the usual A15 reaction temperature range of 650°C – 750°C, thus ensuring denser A15 phase nucleation in the Nb alloy grain boundaries, possibly leading to a more homogeneous A15 phase Sn content and refined A15 grain size. Furthermore, the potential for further advancements in Nb 3 Sn properties is explored in relation to the recrystallization of the Nb alloy and the factors controlling the upper critical field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Tuning the interlayer coupling in La 0.7 Sr 0.3 Mn 0.95 Ru 0.05 O 3 / LaNiO 3 multilayers with perpendicular magnetic anisotropy

In ferromagnetic oxide epitaxial multilayers, magnetic properties and interlayer coupling are determined by a variety of factors. Beyond the contribution of interlayer exchange coupling, strain and interfacial effects, such as structural reconstructions or charge transfer, play significant roles, resulting in complex magnetic behavior. In this study, the interlayer coupling of ferromagnetic La 0.7 ⁢Sr 0.3⁢ Mn 0.95 ⁢Ru 0.05 ⁢O 3 (LSMRO) layers (8 nm thick) was investigated, when separated by epitaxial spacers of paramagnetic metallic LaNiO 3 (LNO), in stacks exhibiting perpendicular magnetic anisotropy. By varying the thickness of the spacer, it was found that the coupling between two LSMRO layers changes from antiferromagnetic (with a 4-unit-cell-thick LNO spacer) to ferromagnetic (with a 6-unit-cell-thick LNO spacer). For multilayers comprising five LSMRO layers and a 4-unit-cell-thick LNO spacer, the antiferromagnetic coupling was preserved. However, the effective magnetic anisotropy changed, causing the magnetization to cant more towards the in-plane direction. Further, this behavior was corroborated by X-ray magnetic circular dichroism (XMCD) investigations at the Mn and Ni L 3 edges. The XMCD results indicated that the 4-unit-cell-thick LNO spacer in the multilayer become magnetically ordered, closely following the magnetization of adjacent LSMRO layers.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Tuning the spin dynamics and magnetic phase transitions of the Cantor alloy via composition and sample processing protocols: A muon spin relaxation study

CrMnFeCoNi, also called the Cantor alloy, is a well-known high-entropy alloy whose magnetic properties have recently become a focus of attention. Here, we present a detailed muon spin relaxation study of the influence of chemical composition and sample processing protocols on the magnetic phase transitions and spin dynamics of several different Cantor alloy samples. Specific samples studied include a pristine equiatomic sample, samples with deficient and excess Mn content, and equiatomic samples magnetized in a field of 9 T or plastically deformed in pressures up to 0.5 GPa. The results confirm the sensitive dependence of the transition temperature on composition and demonstrate that post-synthesis pressure treatments cause the transition to become significantly less homogeneous throughout the sample volume. In addition, we observe critical spin dynamics in the vicinity of the transition in all samples, reminiscent of canonical spin glasses and magnetic materials with ideal continuous phase transitions. Application of an external magnetic field suppresses the critical dynamics in the Mn-deficient sample, while the equiatomic and Mn-rich samples show more robust critical dynamics. The spin-flip thermal activation energy in the paramagnetic phase increases with Mn content, ranging from 3.1⁢(3) × 10 -21 J for 0% Mn to 1.2⁢(2) × 10 -20 J for 30% Mn content. These results shed light on critical magnetic behavior in environments of extreme chemical disorder and demonstrate the tunability of spin dynamics in the Cantor alloy via chemical composition and sample processing.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Synthesis and characterization of the novel breathing pyrochlore compound Ba 3 ⁢Tm 2⁢ Zn 5⁢ O 11

In this study, a novel material from the rare-earth based breathing pyrochlore family, Ba 3 ⁢Tm 2 ⁢Zn 5 ⁢O 11 , was successfully synthesized. Powder x-ray diffraction and high-resolution powder neutron diffraction confirmed phase purity and the $F\bar4$3⁢𝑚 breathing pyrochlore crystal structure, while thermogravimetric analysis revealed incongruent melting behavior compared to its counterpart, Ba 3 ⁢Yb 2⁢ Zn 5⁢ O 11 . High-quality single crystals of Ba 3 ⁢Tm 2 ⁢Zn 5 ⁢O 11 were grown using the traveling solvent floating zone technique and assessed using Laue x-ray diffraction and single crystal x-ray diffraction. Further, thermodynamic characterization indicated paramagnetic behavior down to 0.05 K, and inelastic neutron scattering measurements identified distinct dispersionless crystal electric field energy bands, with the fitted crystal electric field model predicting a single-ion singlet ground state and an energy gap of ∼9 meV separating it from the first excited (singlet) state. Additional low-energy excitation studies on single crystals revealed dispersionless bands at 0.8 and 1 meV. Computed phonon dispersions from first-principles calculations ruled out phonons as the origin of these modes, further illustrating the puzzling and unique properties of Ba 3 ⁢Tm⁢ 2 Zn 5 ⁢O 11 .

36 MATERIALS SCIENCE↗
DOE OSTI2025

Atomic dynamics in 𝑀⁢Cr⁢𝑋 2 (𝑀=Ag, Cu; 𝑋 = S, Se) across magnetic and superionic transitions

Here, a systematic study of atomic dynamics and thermal properties of the family of layered chalcogenide compounds 𝑀⁢Cr⁢𝑋 2 (𝑀= Ag, Cu; 𝑋 = S, Se) was performed, including neutron and x-ray scattering, thermal characterization, and first-principles simulations. In all compounds, we observe a breakdown of specific phonon modes across the superionic phase transition, for phonons whose eigenvectors exhibit large contributions of mobile ions. In particular, the nondispersive portions of transverse acoustic (TA) branches at short-wavelengths and the low-energy optical phonons with large contributions from Ag + or Cu + become severely damped in the superionic phase. However, well-defined quasiparticles persist in the superionic state for long-wavelength TA phonons. In the case of AgCrS 2 , the coupling of lattice dynamics with its antiferromagnetic transition was also investigated. The magnetic ordering couples with the monoclinic–rhombohedral structural transition, and the Cr 3+ spin arrangement strongly affects the phonon dispersions. We qualitatively reproduce the magnetic and nuclear components of the INS measurement for antiferromagnetic AgCrS 2 by combining models of spin-waves and spin-polarized first-principles phonon simulations. Quasielastic magnetic fluctuations persist in the paramagnetic phase up to high temperature, but are clearly distinguished from the nuclear component through their momentum dependence. Finally, we report measurements of the thermal properties of the selenide compounds and find good agreement with our DFT simulations.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Anomalous Hall effect emerging from field-induced Weyl nodes in SmAlSi

The intrinsic anomalous Hall effect (AHE) has been reported in numerous ferromagnetic Weyl semimetals. However, the AHE in the antiferromagnetic (AFM) or paramagnetic (PM) state of Weyl semimetals has rarely been observed experimentally. Different mechanisms have been proposed to account for the emergence of the AHE from different types of magnetic order. Here, in this Letter, we propose a new model that explains the observed AHE in both the AFM and PM states of the noncentrosymmetric Weyl semimetal SmAlSi. The newly proposed mechanism is based on magnetic-field-induced Weyl node evolution, which qualitatively explains the temperature dependence of the anomalous Hall conductivity, which displays unconventional power-law behavior in both the AFM and PM states of SmAlSi.

Gao, Yuxiang [Rice University, Houston, TX (United↗
DOE OSTI2020

Magnetic polarons and spin-glass behavior in insulating La 1-x Sr x CoO 3 (x = 0.125 and 0.15)

The evolution of magnetic polarons in Sr doped La Co O 3 ( La 1 - x Sr x Co O 3 ) single crystal and polycrystalline samples are investigated by employing dc and ac magnetic measurement and small angle neutron scattering (SANS) techniques. The effect of magnetic field and temperature on magnetic polarons is experimentally studied for La 0.875 Sr 0.125 Co O 3 and La 0.85 Sr 0.15 Co O 3 compounds that belong to the spin glass insulating regime of the broader compositional phase diagram of this system. Langevin analyses of the isothermal magnetization curves in the notional paramagnetic regime prove the existence of magnetic polarons with large moments. The dc field superimposed ac susceptibility data and the analysis of the glassy dynamics prove that the size of polarons in 15% Sr doped crystal increase as the field is increased while the field effect is not visible in the 12.5% Sr doped crystal. A polycrystalline sample of La 0.85 Sr 0.15 Co O 3 is analyzed by SANS experiments, which confirm nonzero correlation length at temperatures far above the macroscopic ordering temperature and hence the presence of magnetic polarons.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Quantum critical behavior of the hyperkagome magnet Mn 3 CoSi

β -Mn-type family alloys Mn 3 T X ( T = Co , Rh, and Ir; X = Si and Ge) have a three-dimensional antiferromagnetic (AF) corner-shared triangular network, i.e., the hyperkagome lattice. The antiferromagnet Mn 3 RhSi shows magnetic short-range order over a wide temperature range of approximately 500 K above the Néel temperature T N of 190 K. In this family of compounds, as the lattice parameter decreases, the long-range magnetic ordering temperature decreases. Mn 3 CoSi has the smallest lattice parameter and the lowest T N in the family. The quantum critical point (QCP) from AF to the quantum paramagnetic state is expected near a cubic lattice parameter of 6.15 Å . Although the Néel temperature of Mn 3 CoSi is only 140 K, the emergence of the quantum critical behavior in Mn 3 CoSi is discussed. We study how the magnetic short-range order appears in Mn 3 CoSi by using neutron scattering, μ SR , and bulk characterization such as specific heat capacity. According to the results, the neutron scattering intensity of the magnetic short-range order in Mn 3 CoSi does not change much at low temperatures from that of Mn 3 RhSi , although the μ SR short-range order temperature of Mn 3 CoSi is largely suppressed to 240 K from that of Mn 3 RhSi . Correspondingly, the volume fraction of the magnetic short-range order regions, as shown by the initial asymmetry drop ratio of μ SR above T N , also becomes small. Instead, the electronic-specific heat coefficient γ of Mn 3 CoSi is the largest in this Mn 3 T Si system, possibly due to the low-energy spin fluctuation near the quantum critical point. Published by the American Physical Society 2024

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Quantum and classical spin dynamics across temperature scales in the S = 1 / 2 Heisenberg antiferromagnet

Using the framework of semiclassical Landau-Lifshitz dynamics (LLD), we conduct a systematic investigation of the temperature-dependent spin dynamics in the S = 1 / 2 Heisenberg square-lattice antiferromagnet (SAFM). By performing inelastic neutron scattering measurements on Zn 2 VO ( PO 4 ) 2 (ZVPO) and corresponding finite-temperature spin dynamics simulations based on LLD, we present a comprehensive analysis that bridges quantum and classical spin dynamics over a broad temperature range. First, remarkable agreement between experimental data and LLD simulations is found in the paramagnetic phase of ZVPO, demonstrating the capability of LLD in accurately determining the spin Hamiltonian of S = 1 / 2 systems and capturing the quantum-to-classical crossover of their spin dynamics. Second, by analyzing the discrepancies between the experimental data and the LLD simulations at lower temperatures, we determine the experimental temperature dependence of the quantum effects in the excitation spectrum of the S = 1 / 2 SAFM: the quantum renormalization factor for the magnon energies and the quantum continuum above the one-magnon bands. Notably, the emergence of each quantum effect is found to correlate with the formation of three-dimensional long-range order. This work demonstrates the utility of LLD in gaining experimental insights into the temperature-induced modifications of quantum spin dynamics and their convergence towards classical expectations at higher temperatures. This motivates further applications to more challenging quantum antiferromagnets dominated by stronger quantum fluctuations. Published by the American Physical Society 2024

36 MATERIALS SCIENCE↗
DOE OSTI2024

Symmetry Breaking and Ascending in the Magnetic Kagome Metal FeGe

Spontaneous symmetry breaking—the phenomenon in which an infinitesimal perturbation can cause the system to break the underlying symmetry—is a cornerstone concept in the understanding of interacting solid-state systems. In a typical series of temperature-driven phase transitions, higher-temperature phases are more symmetric due to the stabilizing effect of entropy that becomes dominant as the temperature is increased. However, the opposite is rare but possible when there are multiple degrees of freedom in the system. Here, we present such an example of a symmetry-ascending phenomenon upon cooling in a magnetic kagome metal FeGe by utilizing neutron Larmor diffraction and Raman spectroscopy. FeGe has a kagome lattice structure with simple A-type antiferromagnetic order below Néel temperature T N ≈ 400 K and a charge density wave (CDW) transition at T CDW ≈ 110 K , followed by a spin-canting transition at around 60 K. In the paramagnetic state at 460 K, we confirm that the crystal structure is indeed a hexagonal kagome lattice. On cooling to around T N , the crystal structure changes from hexagonal to monoclinic with in-plane lattice distortions on the order of 10 − 4 and the associated splitting of the double-degenerate phonon mode of the pristine kagome lattice. Upon further cooling to T CDW , the kagome lattice shows a small negative thermal expansion, and the crystal structure gradually becomes more symmetric upon further cooling. A tendency of increasing the crystalline symmetry upon cooling is unusual; it originates from an extremely weak structural instability that coexists and competes with the CDW and magnetic orders. These observations are against the expectations for a simple model with a single order parameter and hence can only be explained by a Landau free energy expansion that takes into account multiple lattice, charge, and spin degrees of freedom. Thus, the determination of the crystalline lattice symmetry as well as the unusual spin-lattice coupling is a first step towards understanding the rich electronic and magnetic properties of the system, and it sheds new light on intertwined orders where the lattice degree of freedom is no longer dominant. Published by the American Physical Society 2024

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗