Engineering topics

Upton, Mary H.

Publications and source records attributed to Upton, Mary H..

7 records

DOE OSTI2024

Chiral spin-liquid-like state in pyrochlore iridate thin films

The pyrochlore iridates have become ideal platforms to unravel fascinating correlated and topological phenomena that stem from the intricate interplay among strong spin-orbit coupling, electronic correlations, lattice with geometric frustration, and itinerancy of the 5d electrons. The all-in-all-out antiferromagnetic state, commonly considered as the magnetic ground state, can be dramatically altered in reduced dimensionality, leading to exotic or hidden quantum states inaccessible in bulk. Here, by means of magnetotransport, resonant elastic and inelastic x-ray scattering experiments, we discover an emergent quantum disordered state in (111) Y 2 Ir 2 O 7 thin films (thickness ≤30 nm) persisting down to 5 K, characterized by dispersionless magnetic excitations. The anomalous Hall effect observed below an onset temperature near 125 K corroborates the presence of chiral short-range spin configurations expressed in non-zero scalar spin chirality, breaking the macroscopic time-reversal symmetry. The origin of this chiral state is ascribed to the restoration of magnetic frustration on the pyrochlore lattice in lower dimensionality, where the competing exchange interactions together with enhanced quantum fluctuations suppress any long-range order and trigger spin-liquid-like behavior with degenerate ground-state manifold.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2024

Spin-Orbit-Lattice Entangled State in A 2 ⁢MgReO 6 (A = Ca, Sr, Ba) Revealed by Resonant Inelastic X-Ray Scattering

The 5d 1 ordered double perovskites present an exotic playground for studying novel multipolar physics due to large spin-orbit coupling. Here, we present Re L 3 edge resonant inelastic Xray scattering (RIXS) results that reveal the presence of the dynamic Jahn-Teller effect in the A 2 MgReO 6 (A = Ca, Sr, Ba) family of 5d 1 double perovskites. The spin-orbit excitations in these materials show a strongly asymmetric lineshape and exhibit substantial temperature dependence, indicating that they are dressed with lattice vibrations. Our experimental results are explained quantitatively through a RIXS calculation based on a spin-orbit-lattice entangled electronic ground state with the dynamic Jahn-Teller effect taken into consideration. We find that the spin-orbitlattice entangled state is robust against magnetic and structural phase transitions as well as against significant static Jahn-Teller distortions. Our results illustrate the importance of including vibroniccoupling for a complete description of the ground state physics of 5d 1 double perovskites.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2024

Local site behavior of the 5$d$ and 4$f$ ions in the frustrated pyrochlore Ho 2 Os 2 O 7

The pyrochlore osmate Ho 2 Os 2 O 7 is a candidate material for a fragile J=0 local singlet ground state, however little is known regarding the single-ion behavior of either the Os or Ho ions. To address this we present polarized neutron powder diffraction (PNPD) and resonant inelastic x-ray scattering (RIXS) measurements that separately probe the local site behavior of the Os and Ho ions. The PNPD results are dominated by Ho 3+ scattering and the analysis reveals local site susceptibility behavior consistent with spin ice materials. Complimentary unpolarized neutron powder diffraction show an ordered spin ice ground state in an applied magnetic field. To isolate the Os 4+ single-ion behavior we present resonant inelastic x-ray scattering (RIXS) measurements at the osmium L-edge. Analysis of the RIXS spectra parameterize the spin-orbit coupling (0.35 eV), Hund’s coupling (0.27 eV) and trigonal distortion (-0.17 eV). Here, the results are considered within the context of a J=0 model and possible departures from this through structural distortions, excitonic interactions and 5d-4f interactions between the Os ion and the surrounding Ho lattice. The experimental methodology employed highlights the complimentary information available in rare earth based 5d pyrochlores from distinct neutron and x-ray scattering techniques that allow for the isolation and determination of the behavior of the different ions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Relationship between A-site cation and magnetic structure in 3d–5d–4f double perovskite iridates Ln 2 NiIrO 6 ( Ln = La, Pr, Nd)

We report a comprehensive investigation of Ln 2 NiIrO 6 (Ln=La, Pr, Nd) using thermodynamic and transport properties, neutron powder diffraction, resonant inelastic x-ray scattering, and density-functional theory (DFT) calculations to investigate the role of A-site cations on the magnetic interactions in this family of hybrid 3d–5d–4f compositions. Magnetic structure determination using neutron diffraction reveals antiferromagnetism for La 2 NiIrO 6 , a collinear ferrimagnetic Ni and Ir state that is driven to long-range antiferromagnetism upon the onset of Nd ordering in Nd 2 NiIrO 6 , and a noncollinear ferrimagnetic Ni and Ir sublattice interpenetrated by a ferromagnetic Pr lattice for Pr 2 NiIrO 6 . For Pr 2 NiIrO 6 , heat-capacity results reveal the presence of two independent magnetic sublattices, and transport resistivity indicates insulating behavior and a conduction pathway that is thermally mediated. Here, a first principles DFT calculation elucidates the existence of the two independent magnetic sublattices within Pr 2 NiIrO 6 and offers insight into the behavior in La 2 NiIrO 6 and Nd 2 NiIrO 6 . Resonant inelastic x-ray scattering is consistent with spin-orbit coupling splitting the t 2 g manifold of octahedral Ir 4+ into a J eff = 1/2 and J eff = 3/2 state for all members of the series considered.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Magnon-spinon dichotomy in the Kitaev hyperhoneycomb β-Li 2 IrO 3

The family of edge-sharing tricoordinated iridates and ruthenates has emerged in recent years as a major platform for Kitaev spin-liquid physics, where spins fractionalize into emergent magnetic fluxes and Majorana fermions with Dirac-like dispersions. While such exotic states are usually preempted by long-range magnetic order at low temperatures, signatures of Majorana fermions with long coherent times have been predicted to manifest at intermediate and higher energy scales, similar to the observation of spinons in quasi-one-dimensional spin chains. Here in this paper we present a resonant inelastic x-ray scattering study of the magnetic excitations of the hyperhoneycomb iridate β-Li 2 IrO 3 under a magnetic field with a record-high-resolution spectrometer. At low temperatures, dispersing spin waves can be resolved around the predicted intertwined incommensurate spiral and field-induced zigzag orders, whose excitation energy reaches a maximum of 16 meV. A 2 T magnetic field softens the dispersion around Q = 0. The behavior of the spin waves under magnetic field is consistent with our semiclassical calculations for the ground state and the dynamical spin structure factor, which further predicts that the ensued intertwined uniform states remain robust up to very high fields (100 T). Most saliently, the low-energy magnonlike mode is superimposed by a broad continuum of excitations, centered around 35 meV and extending up to 100 meV. This high-energy continuum survives up to at least 300 K—well above the ordering temperature of 38 K—and gives evidence for pairs of long-lived Majorana fermions of the proximate Kitaev spin liquid.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Towards cubic symmetry for Ir 4 + : Structure and magnetism of the antifluorite K 2 IrBr 6

Here, crystal structure, electronic state of Ir 4 + , and magnetic properties of the antifluorite compound K 2 IrBr 6 are studied using high-resolution synchrotron x-ray diffraction, resonant inelastic x-ray scattering (RIXS), thermodynamic and transport measurements, and ab initio calculations. The crystal symmetry is reduced from cubic at room temperature to tetragonal below 170 K and eventually to monoclinic below 122 K. These changes are tracked by the evolution of the noncubic crystal-field splitting Δ measured by RIXS. Nonmonotonic changes in Δ are ascribed to the competing effects of the tilt, rotation, and deformation of the IrBr 6 octahedra as well as tetragonal strain on the electronic levels of Ir 4 + . The Néel temperature of T N = 11.9 K exceeds that of the isostructural K 2 IrCl 6 , and the magnitude of frustration on the fcc spin lattice decreases. We argue that the replacement of Cl by Br weakens electronic correlations and enhances magnetic couplings.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2020

Strain engineering of the charge and spin-orbital interactions in Sr2IrO4

In the high spin–orbit-coupled Sr 2 IrO 4 , the high sensitivity of the ground state to the details of the local lattice structure shows a large potential for the manipulation of the functional properties by inducing local lattice distortions. We use epitaxial strain to modify the Ir–O bond geometry in Sr 2 IrO 4 and perform momentum-dependent resonant inelastic X-ray scattering (RIXS) at the metal and at the ligand sites to unveil the response of the low-energy elementary excitations. Furthermore, we observe that the pseudospin-wave dispersion for tensile-strained Sr 2 IrO 4 films displays large softening along the [h,0] direction, while along the [h,h] direction it shows hardening. This evolution reveals a renormalization of the magnetic interactions caused by a strain-driven cross-over from anisotropic to isotropic interactions between the magnetic moments. Moreover, we detect dispersive electron–hole pair excitations which shift to lower (higher) energies upon compressive (tensile) strain, manifesting a reduction (increase) in the size of the charge gap. This behavior shows an intimate coupling between charge excitations and lattice distortions in Sr 2 IrO 4 , originating from the modified hopping elements between the t 2g orbitals. Our work highlights the central role played by the lattice degrees of freedom in determining both the pseudospin and charge excitations of Sr 2 IrO 4 and provides valuable information toward the control of the ground state of complex oxides in the presence of high spin–orbit coupling.

36 MATERIALS SCIENCE↗