Hidden Local Symmetry Breaking in a Kagome-Lattice Magnetic Weyl Semimetal.

Exploring the relationship between intriguing physical properties and structural complexity is a central topic in studying modern functional materials. Co3Sn2S2, a newly discovered kagome-lattice magnetic Weyl semimetal, has triggered intense interest owing to the intimate coupling between topological semimetallic states and peculiar magnetic properties. However, the origins of the magnetic phase separation and spin glass state below TC in this ordered compound are two unresolved yet important puzzles in understanding its magnetism. Here, we report the discovery of local symmetry breaking surprisingly co-emerges with the onset of ferromagnetic order in Co3Sn2S2, by a combined use of neutron total scattering and half-polarized neutron diffraction. An anisotropic distortion of the cobalt kagome lattice at the atomic/nano level is also found, with distinct distortion directions among the two Co1 and four Co2 atoms. The mismatch of local and average symmetries occurs below TC, indicating that Co3Sn2S2 evolves to an intrinsically lattice disordered system when the ferromagnetic order is established. The local symmetry breaking with intrinsic lattice disorder provides new understanding of the puzzling magnetic properties. Our density functional theory (DFT) calculation indicates that the local symmetry breaking is expected to reorient local ferromagnetic moments, unveiling the existence of the ferromagnetic instability associated with the lattice instability. Furthermore, DFT calculation unveils that the local symmetry breaking could affect the Weyl property by breaking the mirror plane. Our findings highlight the fundamentally important role that the local symmetry breaking plays in advancing our understanding on the magnetic and topological properties in Co3Sn2S2, which may draw attention to explore the overlooked local symmetry breaking in Co3Sn2S2, its derivatives and more broadly in other topological Dirac/Weyl semimetals and kagome-lattice magnets.

SrV0.3Fe0.7O2.8: A Vacancy-Ordered Fe-Based Perovskite Exhibiting Room-Temperature Magnetoresistance.

We report room-temperature (RT) magnetoresistance (MR) in a novel Fe-based perovskite, SrV0.3Fe0.7O2.8. This compound contains ordered oxygen vacancies in every fifth primitive perovskite (111)p plane, leading to a layered structure consisting of triple-octahedral and double-tetrahedral layers. Along with the oxygen vacancies, the transition-metal ions are also ordered: the octahedral sites are occupied by 100% of Fe ions, while the tetrahedral sites are occupied by 25% of Fe ions and 75% of V ions. As a result, SrV0.3Fe0.7O2.8 forms a magnetically striped lattice in which the octahedral layers with 100% of magnetic Fe ions are separated by the diluted magnetic layer. The compound exhibits weak ferromagnetism and shows a large negative MR (-5% at 3 T) at RT, despite the small saturation moment (0.4 µB/Fe atom). Thus, this type of layered compound is promising for further large MR by an increase of magnetization through chemical substitution.

Topological Singularity Induced Chiral Kohn Anomaly in a Weyl Semimetal.

The electron-phonon interaction (EPI) is instrumental in a wide variety of phenomena in solid-state physics, such as electrical resistivity in metals, carrier mobility, optical transition, and polaron effects in semiconductors, lifetime of hot carriers, transition temperature in BCS superconductors, and even spin relaxation in diamond nitrogen-vacancy centers for quantum information processing. However, due to the weak EPI strength, most phenomena have focused on electronic properties rather than on phonon properties. One prominent exception is the Kohn anomaly, where phonon softening can emerge when the phonon wave vector nests the Fermi surface of metals. Here we report a new class of Kohn anomaly in a topological Weyl semimetal (WSM), predicted by field-theoretical calculations, and experimentally observed through inelastic x-ray and neutron scattering on WSM tantalum phosphide. Compared to the conventional Kohn anomaly, the Fermi surface in a WSM exhibits multiple topological singularities of Weyl nodes, leading to a distinct nesting condition with chiral selection, a power-law divergence, and non-negligible dynamical effects. Our work brings the concept of the Kohn anomaly into WSMs and sheds light on elucidating the EPI mechanism in emergent topological materials.

Helimagnetic Structure and Heavy-Fermion-Like Behavior in the Vicinity of the Quantum Critical Point in Mn_{3}P.

Antiferromagnet Mn_{3}P with Neel temperature T_{N}=30 K is composed of Mn tetrahedrons and zigzag chains formed by three inequivalent Mn sites. Due to the nearly frustrated lattice with many short Mn-Mn bonds, competition of the exchange interactions is expected. We here investigate the magnetic structure and physical properties including pressure effect in single crystals of this material, and reveal a complex yet well-ordered helimagnetic structure. The itinerant character of this materials is strong, and the ordered state with small magnetic moments is easily suppressed under pressure, exhibiting a quantum critical point at ∼1.6 GPa. The remarkable mass renormalization, even in the ordered state, and an incoherent-coherent crossover in the low-temperature region, characterize an unusual electronic state in Mn_{3}P, which is most likely effected by the underlying frustration effect.

Stabilized Charge, Spin, and Orbital Ordering by the 6s2 Lone Pair in Bi0.5Pb0.5MnO3.

Bi and Pb ions with charge degree of freedom depending on 6s2 and 6s0 electronic configurations were combined with the Mn ion in a perovskite oxide. Comprehensive theoretical and experimental investigations revealed the Bi3+0.5Pb2+0.5Mn3+0.5Mn4+0.5O3 charge ordered state with CE-type spin and dz2 orbital orderings as observed in La0.5Ca0.5MnO3, Nd0.5Sr0.5MnO3, and Bi0.5Sr0.5MnO3. The charge and orbital orderings were preserved above 500 K owing to the stereochemical activity of Bi3+ and Pb2+ ions which stabilized the structural distortion.

Coexistence of Ferromagnetic and Stripe Antiferromagnetic Spin Fluctuations in SrCo_{2}As_{2}.

We use inelastic neutron scattering to study energy and wave vector dependence of spin fluctuations in SrCo_{2}As_{2}, derived from SrFe_{2-x}Co_{x}As_{2} iron pnictide superconductors. Our data reveal the coexistence of antiferromagnetic (AF) and ferromagnetic (FM) spin fluctuations at wave vectors Q_{AF}=(1,0) and Q_{FM}=(0,0)/(2,0), respectively. By comparing neutron scattering results with those of dynamic mean field theory calculation and angle-resolved photoemission spectroscopy experiments, we conclude that both AF and FM spin fluctuations in SrCo_{2}As_{2} are closely associated with a flatband of the e_{g} orbitals near the Fermi level, different from the t_{2g} orbitals in superconducting SrFe_{2-x}Co_{x}As_{2}. Therefore, Co substitution in SrFe_{2-x}Co_{x}As_{2} induces a t_{2g} to e_{g} orbital switching, and is responsible for FM spin fluctuations detrimental to the singlet pairing superconductivity.

Dual Nature of Magnetism in a Uranium Heavy-Fermion System.

The duality between the localized and itinerant nature of magnetism in 5f-electron systems has been a long-standing puzzle. Here, we report inelastic neutron scattering measurements, which reveal both local and itinerant aspects of magnetism in a single-crystalline system of UPt_{2}Si_{2}. In the antiferromagnetic state, we observe a broad continuum of diffuse magnetic scattering with a resonancelike gap of ≈7 meV and the surprising absence of coherent spin waves, suggestive of itinerant magnetism. While the gap closes above the Néel temperature, strong dynamic spin correlations persist to a high temperature. Nevertheless, the size and temperature dependence of the total magnetic spectral weight can be well described by a local moment with J=4. Furthermore, polarized neutron measurements reveal that the magnetic fluctuations are mostly transverse, with little or none of the longitudinal component expected for itinerant moments. These results suggest that a dual description of local and itinerant magnetism is required to understand UPt_{2}Si_{2} and, by extension, other 5f systems, in general.

Melting of Pb Charge Glass and Simultaneous Pb-Cr Charge Transfer in PbCrO3 as the Origin of Volume Collapse.

A metal to insulator transition in integer or half integer charge systems can be regarded as crystallization of charges. The insulating state tends to have a glassy nature when randomness or geometrical frustration exists. We report that the charge glass state is realized in a perovskite compound PbCrO3, which has been known for almost 50 years, without any obvious inhomogeneity or triangular arrangement in the charge system. PbCrO3 has a valence state of Pb(2+)(0.5)Pb(4+)(0.5)Cr(3+)O3 with Pb(2+)-Pb(4+) correlation length of three lattice-spacings at ambient condition. A pressure induced melting of charge glass and simultaneous Pb-Cr charge transfer causes an insulator to metal transition and ∼10% volume collapse.

Electric-Field Manipulation of Magnetic Chirality in a Homo-Ferro-Rotational Helimagnet.

Ferro-rotational (FR) materials, renowned for their distinctive material functionalities, present challenges in the growth of homo-FR crystals (i.e., single FR domain). This study explores a cost-effective approach to growing homo-FR helimagnetic RbFe(SO4)2 (RFSO) crystals by lowering the crystal growth temperature below the TFR threshold using the high-pressure hydrothermal method. Through polarized neutron diffraction experiments, it is observed that nearly 86% of RFSO crystals consist of a homo-FR domain. Notably, RFSO displays remarkable stability in the FR phase, with an exceptionally high TFR of ≈573 K. Furthermore, RFSO exhibits a chiral helical magnetic structure with switchable ferroelectric polarization below 4 K. Importantly, external electric fields can induce a single magnetic domain state and manipulate its magnetic chirality. The findings suggest that the search for new FR magnets with outstanding material properties should consider magnetic sulfates as promising candidates.

Ten-Gram-Scale Total Synthesis of the Anticancer Drug Candidate E7130 to Supply Clinical Trials.

E7130 is a novel drug candidate with an exceedingly complex chemical structure of the halichondrin class, discovered by a total synthesis approach through joint research between the Kishi group at Harvard University and Eisai. Only 18 months after completion of the initial milligram-scale synthesis, ten-gram-scale synthesis of E7130 was achieved, providing the first good manufacturing practice (GMP) batch to supply clinical trials. This paper highlights the challenges in developing ten-gram-scale synthesis from the milligram-scale synthesis.

Temperature-dependent transformation of the magnetic excitation spectrum on approaching superconductivity in Fe(1+y-x)(Ni/Cu)(x)Te(0.5)Se(0.5).

Spin excitations are one of the top candidates for mediating electron pairing in unconventional superconductors. Their coupling to superconductivity is evident in a large number of systems, by the observation of an abrupt redistribution of magnetic spectral weight at the superconducting transition temperature, T(c), for energies comparable to the superconducting gap. Here we report inelastic neutron scattering measurements on Fe-based superconductors, Fe(1+y-x)(Ni/Cu)(x)Te(0.5)Se(0.5) that emphasize an additional signature. The overall shape of the low energy magnetic dispersion changes from two incommensurate vertical columns at T≫T(c) to a distinctly different U-shaped dispersion at low temperature. Importantly, this spectral reconstruction is apparent for temperatures up to ~3T(c). If the magnetic excitations are involved in the pairing mechanism, their surprising modification on the approach to T(c) demonstrates that strong interactions are involved.

Evidence for pressure induced unconventional quantum criticality in the coupled spin ladder antiferromagnet C9H18N2CuBr4.

Quantum phase transitions in quantum matter occur at zero temperature between distinct ground states by tuning a nonthermal control parameter. Often, they can be accurately described within the Landau theory of phase transitions, similarly to conventional thermal phase transitions. However, this picture can break down under certain circumstances. Here, we present a comprehensive study of the effect of hydrostatic pressure on the magnetic structure and spin dynamics of the spin-1/2 ladder compound C9H18N2CuBr4. Single-crystal heat capacity and neutron diffraction measurements reveal that the Néel-ordered phase breaks down beyond a critical pressure of Pc ∼ 1.0 GPa through a continuous quantum phase transition. Estimates of the critical exponents suggest that this transition may fall outside the traditional Landau paradigm. The inelastic neutron scattering spectra at 1.3 GPa are characterized by two well-separated gapped modes, including one continuum-like and another resolution-limited excitation in distinct scattering channels, which further indicates an exotic quantum-disordered phase above Pc.

Anisotropic effect of a magnetic field on the neutron spin resonance in FeSe.

We use inelastic neutron scattering to study the effect of a magnetic field on the neutron spin resonance (E r = 3.6 meV) of superconducting FeSe (T c = 9 K). While a field aligned along the in-plane direction broadens and suppresses the resonance, a c-axis aligned field does so much more efficiently, consistent with the anisotropic field-induced suppression of the superfluid density from the heat capacity measurements. These results suggest that the resonance in FeSe is associated with the superconducting electrons arising from orbital selective quasiparticle excitations between the hole and electron Fermi surfaces.

Synthesis, crystal structure, and magnetic properties of Bi3Mn4O12(NO3) oxynitrate comprising S = 3/2 honeycomb lattice.

Neutron diffraction and FT-IR analysis revealed that the novel oxynitrate Bi(3)Mn(4)O(12)(NO(3)) (space group P3, a = 4.9692(1) A, c = 13.1627(3) A) prepared by hydrothermal synthesis is of a new structural type including flat NO(3) layers alternating with blocks of two PbSb(2)O(6)-like layers. Mn(4+) (S = (3)/(2)) forms a regular honeycomb lattice, and magnetic susceptibility data indicated two-dimensional magnetism. Despite its Weiss constant of -257 K, no long-range ordering was observed down to 0.4 K because of the magnetic frustration due to the competition between the nearest and the next-nearest antiferromagnetic interactions.

Large spin-orbit torque efficiency enhanced by magnetic structure of collinear antiferromagnet IrMn.

Spin-orbit torque (SOT) offers promising approaches to developing energy-efficient memory devices by electric switching of magnetization. Compared to other SOT materials, metallic antiferromagnet (AFM) potentially allows the control of SOT through its magnetic structure. Here, combining the results from neutron diffraction and spin-torque ferromagnetic resonance experiments, we show that the magnetic structure of epitaxially grown L10-IrMn (a collinear AFM) is distinct from the widely presumed bulk one. It consists of twin domains, with the spin axes orienting toward [111] and [-111], respectively. This unconventional magnetic structure is responsible for much larger SOT efficiencies up to 0.60 ± 0.04, compared to 0.083 ± 0.002 for the polycrystalline IrMn. Furthermore, we reveal that this magnetic structure induces a large isotropic bulk contribution and a comparable anisotropic interfacial contribution to the SOT efficiency. Our findings shed light on the critical roles of bulk and interfacial antiferromagnetism to SOT generated by metallic AFM.

Next-generation diamond cell and applications to single-crystal neutron diffraction.

A diamond cell optimized for single-crystal neutron diffraction is described. It is adapted for work at several of the single-crystal diffractometers of the Spallation Neutron Source and the High Flux Isotope Reactor at the Oak Ridge National Laboratory (ORNL). A simple spring design improves portability across the facilities and affords load maintenance from offline pressurization and during temperature cycling. Compared to earlier prototypes, pressure stability of polycrystalline diamond (Versimax®) has been increased through double-conical designs and ease of use has been improved through changes to seat and piston setups. These anvils allow ∼30%-40% taller samples than possible with comparable single-crystal anvils. Hydrostaticity and the important absence of shear pressure gradients have been established with the use of glycerin as a pressure medium. Large single-crystal synthetic diamonds have also been used for the first time with such a clamp-diamond anvil cell for pressures close to 20 GPa. The cell is made from a copper beryllium alloy and sized to fit into ORNL's magnets for future ultra-low temperature and high-field studies. We show examples from the Spallation Neutron Source's SNAP and CORELLI beamlines and the High Flux Isotope Reactor's HB-3A and IMAGINE beamlines.

High resolution neutron Larmor diffraction using superconducting magnetic Wollaston prisms.

The neutron Larmor diffraction technique has been implemented using superconducting magnetic Wollaston prisms in both single-arm and double-arm configurations. Successful measurements of the coefficient of thermal expansion of a single-crystal copper sample demonstrates that the method works as expected. The experiment involves a new method of tuning by varying the magnetic field configurations in the device and the tuning results agree well with previous measurements. The difference between single-arm and double-arm configurations has been investigated experimentally. We conclude that this measurement benchmarks the applications of magnetic Wollaston prisms in Larmor diffraction and shows in principle that the setup can be used for inelastic phonon line-width measurements. The achievable resolution for Larmor diffraction is comparable to that using Neutron Resonance Spin Echo (NRSE) coils. The use of superconducting materials in the prisms allows high neutron polarization and transmission efficiency to be achieved.