arXiv subjects

P. M. Woodward

Publications and source records attributed to P. M. Woodward.

8 recordsLinked to original sources

arXiv2024

The origin of magnetism in a supposedly nonmagnetic osmium oxide

A supposedly nonmagnetic 5d$^1$ double perosvkite oxide is investigated by a combination of spectroscopic and theoretical methods, namely resonant inelastic X-ray scattering, X-ray absorption spectroscopy, magnetic circular dichroism, and multiplet ligand field calculations. We found that the large spin-orbit coupling admixes the 5d $t_{2g}$ and $e_g$ orbitals, covalency raises the 5d population well above the nominal value, and the local symmetry is lower than $O_h$. The obtained electronic interactions account for the finite magnetic moment of Os in this compound and, in general, of 5d$^1$ ions. Our results provide direct evidence of elusive Jahn-Teller distortions, hinting at a strong electron-lattice coupling.

cond-mat.str-el↗

arXiv2022

Effects of charge doping on Mott insulator with strong spin-orbit coupling, Ba$_2$NaOsO$_6$

The effects of doping on the electronic evolution of the Mott insulating state have been extensively studied in efforts to understand mechanisms of emergent quantum phases of materials. The study of these effects becomes ever more intriguing in the presence of entanglement between spin and orbital degrees of freedom. Here, we present a comprehensive investigation of charge doping in the double perovskite Ba$_2$NaOsO$_6$, a a complex Mott insulator where such entanglement plays an important role. We establish that the insulating magnetic ground state evolves from canted antiferromagnet (cAF)to Néel order for dopant levels exceeding ~ 10 %. Furthermore, we determine that a broken local point symmetry (BLPS) phase, precursor to the magnetically ordered state, occupies an extended portion of the (H-T) phase diagram with increased doping. This finding reveals that the breaking of the local cubic symmetry is driven by a multipolar order, most-likely of the antiferro-quadrupolar type.

cond-mat.str-el↗

arXiv2018

Origin of magnetic excitation gap in double perovskite Sr$_2$FeOsO$_6$

Sr$_2$FeOsO$_6$ is an insulating double perovskite compound which undergoes antiferromagnetic transitions at 140 K ($T_{N1}$) and 67 K ($T_{N2}$). To study the underlying electronic and magnetic interactions giving rise to this behavior we have performed inelastic neutron scattering (INS) and resonant inelastic x-ray scattering (RIXS) experiments on polycrystalline samples of Sr$_2$FeOsO$_6$. The INS data reveal that the spectrum of spin excitations remains ungapped below T$_{N1}$, however below T$_{N2}$ a gap of 6.8 meV develops. The RIXS data reveals splitting of the T$_{2g}$ multiplet consistent with that seen in other 5d$^3$ osmium based double perovskites. Together these results suggest that spin-orbit coupling is important for ground state selection in 3d-5d$^3$ double perovskite materials.

cond-mat.str-el↗

arXiv2016

Spin-orbit coupling controlled $J=3/2$ electronic ground state in 5$d^{3}$ oxides

Entanglement of spin and orbital degrees of freedom drives the formation of novel quantum and topological physical states. Discovering new spin-orbit entangled ground states and emergent phases of matter requires both experimentally probing the relevant energy scales and applying suitable theoretical models. Here we report resonant inelastic x-ray scattering measurements of the transition metal oxides Ca$_3$LiOsO$_6$ and Ba$_2$YOsO$_6$. We invoke an intermediate coupling approach that incorporates both spin-orbit coupling and electron-electron interactions on an even footing and reveal the ground state of $5d^3$ based compounds, which has remained elusive in previously applied models, is a novel spin-orbit entangled J=3/2 electronic ground state. This work reveals the hidden diversity of spin-orbit controlled ground states in 5d systems and introduces a new arena in the search for spin-orbit controlled phases of matter.

cond-mat.str-el↗

arXiv2016

Spin-orbit coupling controlled ground state in Sr$_2$ScOsO$_6$

We report neutron scattering experiments which reveal a large spin gap in the magnetic excitation spectrum of weakly-monoclinic double perovskite Sr2ScOsO6. The spin gap is demonstrative of appreciable spin-orbit-induced anisotropy, despite nominally orbitally-quenched 5d3 Os5+ ions. The system is successfully modeled including nearest neighbor interactions in a Heisenberg Hamiltonian with exchange anisotropy. We find that the presence of the spin-orbit-induced anisotropy is essential for the realization of the type I antiferromagnetic ground state. This demonstrates that physics beyond the LS or JJ coupling limits plays an active role in determining the collective properties of 4d3 and 5d3 systems, and that theoretical treatments must include spin-orbit coupling.

cond-mat.str-el↗

arXiv2015

Magnetic order and electronic structure of 5d3 double perovskite Sr2ScOsO6

The magnetic susceptibility, crystal and magnetic structures, and electronic structure of double perovskite Sr2ScOsO6 are reported. Using both neutron and x-ray powder diffraction we find that the crystal structure is monoclinic P21/n from 3.5 to 300 K. Magnetization measurements indicate an antiferromagnetic transition at TN=92K, one of the highest transition temperatures of any double perovskite hosting only one magnetic ion. Type I antiferromagnetic order is determined by neutron powder diffraction, with an Os moment of only 1.6(1) muB, close to half the spin-only value for a crystal field split 5d electron state with t2g^3 ground state. Density functional calculations show that this reduction is largely the result of strong Os-O hybridization, with spin-orbit coupling responsible for only a ~0.1 muB reduction in the moment.

cond-mat.str-el↗

arXiv2013

Probing the Spin Pumping Mechanism: Exchange Coupling with Exponential Decay in Y3Fe5O12/barrier/Pt Heterostructures

Ferromagnetic resonance driven spin pumping of pure spin currents from a ferromagnet into a nonmagnetic material promises new spin-functional devices with low energy consumption. The mechanism of spin pumping is under intense investigation and it is widely believed that exchange interaction between the ferromagnet and nonmagnetic material is responsible for this phenomenon. We observe a thousand-fold exponential decay of the spin pumping from 20-nm thick Y3Fe5O12 films to platinum across insulating barriers, from which the exponential decay lengths of 0.16 and 0.23 nm are extracted for oxide barriers with band gaps of 4.93 eV and 2.36 eV, respectively. This prototypical signature of quantum tunneling through a barrier underscores the importance of exchange coupling for spin pumping and reveals its dependence on the characteristics of the barrier material.

cond-mat.mtrl-sci↗

arXiv2003

Temperature dependent total scattering structural study of CaCu3Ti4O12

X-ray and neutron powder diffraction data as a function of temperature are analyzed for the colossal dielectric constant material CaCu3Ti4O12. The local structure is studied using atomic pair distribution function analysis. No evidence is found for enhanced oxygen displacement parameters suggesting that short-range octahedral tilt disorder is minimal. However, an unusual temperature dependence for the atomic displacement parameters of calcium and copper is observed. Temperature dependent modeling of the structure, using bond valence concepts, suggests that the calcium atoms become underbonded below approximately 260 K, which provides a rationale for the unusually high Ca displacement parameters at low temperature.

cond-mat.mtrl-sci↗