Emergent layer stacking arrangements in c-axis confined MoTe2.

The layer stacking order in 2D materials strongly affects functional properties and holds promise for next-generation electronic devices. In bulk, octahedral MoTe2 possesses two stacking arrangements, the ferroelectric Weyl semimetal Td phase and the higher-order topological insulator 1T' phase. However, in thin flakes of MoTe2, it is unclear if the layer stacking follows the Td, 1T', or an alternative stacking sequence. Here, we use atomic-resolution scanning transmission electron microscopy to directly visualize the MoTe2 layer stacking. In thin flakes, we observe highly disordered stacking, with nanoscale 1T' and Td domains, as well as alternative stacking arrangements not found in the bulk. We attribute these findings to intrinsic confinement effects on the MoTe2 stacking-dependent free energy. Our results are important for the understanding of exotic physics displayed in MoTe2 flakes. More broadly, this work suggests c-axis confinement as a method to influence layer stacking in other 2D materials.

SEEMS: A Single Event Effects and Muon Spectroscopy facility at the Spallation Neutron Source.

This study outlines a concept that would leverage the existing proton accelerator at the Spallation Neutron Source (SNS) of Oak Ridge National Laboratory to enable transformative science via one world-class facility serving two missions: Single Event Effects (SEE) and Muon Spectroscopy (µSR). The µSR portion would deliver the world's highest flux and highest resolution pulsed muon beams for material characterization purposes, with precision and capabilities well beyond comparable facilities. The SEE capabilities deliver neutron, proton, and muon beams for aerospace industries that are facing an impending challenge to certify equipment for safe and reliable behavior under bombardment from atmospheric radiation originating from cosmic and solar rays. With negligible impact on the primary neutron scattering mission of the SNS, the proposed facility will have enormous benefits for both science and industry. We have designated this facility "SEEMS."

Defect-driven extreme magnetoresistance in an I-Mn-V semiconductor.

The search for appropriate materials for technological applications is challenging, as real materials are subject to uncontrolled doping and thermal effects. Tetragonal NaMnBi of the I-Mn-V class of antiferromagnetic semiconductors with a Néel transition (TN), above room temperature, can exhibit an extreme magnetoresistance (MR), greater than 10000% at 2K and 600% at room temperature and 9T by quenching disorder into the system. Coupled with the large MR is a re-orientation of the magnetic moment, from a collinear spin arrangement along c to a canted one along the (011) crystallographic axis. The extreme MR is observed in samples with about 15% of Bi vacancies which in turn effectively introduces charge carriers into the lattice, leading to a drastic change in the electronic transport, from semiconducting to metallic, and to the very large MR under the magnetic field. In the absence of Bi defects, the MR is severely suppressed, suggesting that the hybridization of the Mn and Bi orbitals may be key to the field induced large MR. This is the only material of its class that exhibits the extreme MR and may potentially find use in microelectronic devices.

Elastic and electronic tuning of magnetoresistance in MoTe2.

Quasi-two-dimensional transition metal dichalcogenides exhibit dramatic properties that may transform electronic and photonic devices. We report on how the anomalously large magnetoresistance (MR) observed under high magnetic field in MoTe2, a type II Weyl semimetal, can be reversibly controlled under tensile strain. The MR is enhanced by as much as ~30% at low temperatures and high magnetic fields when uniaxial strain is applied along the a crystallographic direction and reduced by about the same amount when strain is applied along the b direction. We show that the large in-plane electric anisotropy is coupled with the structural transition from the 1T' monoclinic to the Td orthorhombic Weyl phase. A shift of the Td-1T' phase boundary is achieved by minimal tensile strain. The sensitivity of the MR to tensile strain suggests the possibility of a nontrivial spin-orbital texture of the electron and hole pockets in the vicinity of Weyl points. Our ab initio calculations show a significant orbital mixing on the Fermi surface, which is modified by the tensile strains.

The magnetic and crystal structures of Sr1-δFeO2-xFx, a new oxyfluoride.

A new quasi-two-dimensional oxyfluoride, Sr1-δFeO2-xFx, has been successfully synthesized by combining topotactic fluoridation and CaH2 reduction. The introduction of F through this synthesis provides a new route to introducing charge carriers into the square layered lattice through the formation of Fe(1+) ions. While the average crystal symmetry and magnetic structure remain the same as in the parent compound, the addition of F results in an enhanced buckling of the Fe(O/F)2 square plaquettes that is most likely topologically driven.

Suppression of magnetic coupling by in-plane buckling in SrFeO2.

SrFeO2 is an insulating antiferromagnet with a remarkably high transition temperature in spite of its quasi-two-dimensional crystal structure. The magnetic exchange coupling is, however, very sensitive to a local mode involving transverse displacements of O and Fe, resulting in zigzag patterns of distortion. The buckling driven by rising temperatures is enhanced just as the Fe magnetic moment is reduced, implying a strong spin-lattice coupling. It is suggested that the undulations lead to orbital disorder by distorting the three possible paths to exchange interactions.

The hybrid lattice of K(x)Fe(2-y)Se2: where superconductivity and magnetism coexist.

Much remains unknown of the microscopic origin of superconductivity in atomically disordered systems of amorphous alloys or in crystals riddled with defects. A manifestation of this conundrum is envisaged in the highly defective superconductor of K(x)Fe(2-y)Se2. How can superconductivity survive under such crude conditions that call for strong electron localization? Here, we show that the Fe sublattice is locally distorted and accommodates two kinds of Fe valence environments giving rise to a bimodal bond-distribution, with short and long Fe bonds. The bimodal bonds are present even as the system becomes superconducting in the presence of antiferromagnetism, with the weight continuously shifting from the short to the long with increasing K content. Such a hybrid state is most likely found in cuprates as well while our results point to the importance of the local atomic symmetry by which exchange interactions between local moments materialize.

Possible link of a structurally driven spin flip transition and the insulator-metal transition in the perovskite La(1-x)Ba(x)CoO3.

The nature of the magnetic ground state near the insulator-metal transition (IMT) in La(1-x)Ba(x)CoO3 was investigated via neutron scattering. Below the critical concentration, x(c)∼0.22, a commensurate antiferromagnetic (AFM) phase appears initially. Upon approaching x(c), the AFM component weakens and a ferromagnetic (FM) ordered phase sets in while in the rhombohedral lattice. At x(c), a spin flip to a new FM structure occurs at the same time as the crystal symmetry transforms to orthorhombic (Pnma). The Pnma phase may be the driving force for the IMT.

Nanomagnetic droplets and implications to orbital ordering in LA(1-x)Sr(x)CoO3.

Inelastic cold-neutron scattering on LaCoO3 provided evidence for a distinct low energy excitation at 0.6 meV coincident with the thermally induced magnetic transition. Coexisting strong ferromagnetic (FM) and weaker antiferromagnetic correlations that are dynamic follow the activation to the excited state, identified as the intermediate S = 1 spin triplet. This is indicative of dynamical orbital ordering favoring the observed magnetic interactions. With hole doping as in La(1-x)Sr(x)CoO3 , the FM correlations between Co spins become static and isotropically distributed due to the formation of FM droplets. The correlation length and condensation temperature of these droplets increase rapidly with metallicity due to the double exchange mechanism.

Spin incommensurability and two phase competition in cobaltites.

The perovskite LaCoO3 evolves from a nonmagnetic Mott insulator to a spin cluster ferromagnet (FM) with the substitution of Sr2+ for La3+ in La1-xSrxCoO3. The clusters increase in size and number with x and the charge percolation through the clusters leads to a metallic state. Using elastic neutron scattering on La1-xSrxCoO3 single crystals, we show that an incommensurate spin superstructure coexists with the FM spin clusters. The incommensurability increases continuously with x, with the intensity rising in the insulating phase and dropping in the metallic phase as it directly competes with the commensurate FM, itinerant clusters. The spin incommensurability arises from local order of Co3+-Co4+ clusters but no long-range static or dynamic spin stripes develop. The coexistence and competition of the two magnetic phases explain the residual resistivity at low temperatures in samples with metalliclike transport.

Dynamical disorder of spin-induced Jahn-Teller orbitals with the insulator-metal transition in cobaltites.

Using elastic and inelastic neutron scattering, we investigated the evolution of the local atomic structure and lattice dynamics of La(1-x)SrxCoO3 (x=0-0.5) as it crosses over with x from an insulator to a ferromagnetic metal (FMM). Our pair density function analysis indicates that, in the paramagnetic insulating phase for all x, spin activation of Co3+ ions induces local static Jahn-Teller (JT) distortions. The size of the JT lattice increases almost linearly with x. However, in the FMM phase, static JT distortions are absent for x< or =30%. This coincides with narrowing of variant Planck's constant over 2 pi omega=22 and 24 meV modes in the phonon spectrum which we argue is due to localized dynamical JT fluctuations. For x>30%, static JT distortions reappear along with broadening of the phonon modes because of weakened charge-lattice interactions.