32 research outputs found
Crossover from itinerant to localized magnetic excitations through the metal-insulator transition in NaOsO
NaOsO undergoes a metal-insulator transition (MIT) at 410 K,
concomitant with the onset of antiferromagnetic order. The excitation spectra
have been investigated through the MIT by resonant inelastic x-ray scattering
(RIXS) at the Os L edge. Low resolution ( 300 meV)
measurements over a wide range of energies reveal that local electronic
excitations do not change appreciably through the MIT. This is consistent with
a picture in which structural distortions do not drive the MIT. In contrast,
high resolution ( 56 meV) measurements show that the
well-defined, low energy magnons in the insulating state weaken and dampen upon
approaching the metallic state. Concomitantly, a broad continuum of excitations
develops which is well described by the magnetic fluctuations of a nearly
antiferromagnetic Fermi liquid. By revealing the continuous evolution of the
magnetic quasiparticle spectrum as it changes its character from itinerant to
localized, our results provide unprecedented insight into the nature of the MIT
in \naoso. In particular, the presence of weak correlations in the paramagnetic
phase implies a degree of departure from the ideal Slater limit.Comment: Joint submission with Physical Review Letters [Phys. Rev. Lett. 120,
227203 (2018), accepted version at arXiv:1805.03176]. This article includes
further discussion about the calculations performed, models used, and so o
Strongly Gapped Spin-Wave Excitation in the Insulating Phase of NaOsO3
NaOsO3 hosts a rare manifestation of a metal-insulator transition driven by
magnetic correlations, placing the magnetic exchange interactions in a central
role. We use resonant inelastic x-ray scattering to directly probe these
magnetic exchange interactions. A dispersive and strongly gapped (58 meV)
excitation is observed indicating appreciable spin-orbit coupling in this 5d3
system. The excitation is well described within a minimal model Hamiltonian
with strong anisotropy and Heisenberg exchange (J1=J2=13.9 meV). The observed
behavior places NaOsO3 on the boundary between localized and itinerant
magnetism
Evolution of the magnetic excitations in NaOsO through its metal-insulator transition
The temperature dependence of the excitation spectrum in NaOsO
through its metal-to-insulator transition (MIT) at 410 K has been investigated
using resonant inelastic X-ray scattering (RIXS) at the Os L edge.
High resolution ( 56 meV) measurements show that the
well-defined, low energy magnons in the insulating state weaken and dampen upon
approaching the metallic state. Concomitantly, a broad continuum of excitations
develops which is well described by the magnetic fluctuations of a nearly
antiferromagnetic Fermi liquid. By revealing the continuous evolution of the
magnetic quasiparticle spectrum as it changes its character from itinerant to
localized, our results provide unprecedented insight into the nature of the MIT
in NaOsO.Comment: Accepted in Physical Review Letters, part of a joint submission to
Physical Review B. Supersedes arXiv:1707.0555
All-in all-out magnetic order and propagating spin-waves in Sm2Ir2O7
Using resonant magnetic x-ray scattering we address the unresolved nature of the magnetic
groundstate and the low-energy effective Hamiltonian of Sm2Ir2O7, a prototypical pyrochlore iridate
with a finite temperature metal-insulator transition. Through a combination of elastic and inelastic
measurements, we show that the magnetic ground state is an all-in all-out (AIAO) antiferromagnet.
The magnon dispersion indicates significant electronic correlations and can be well-described by a
minimal Hamiltonian that includes Heisenberg exchange (J = 27:3(6) meV) and Dzyaloshinskii-
Moriya interaction (D = 4:9(3) meV), which provides a consistent description of the magnetic
order and excitations. In establishing that Sm2Ir2O7 has the requisite inversion symmetry preserv-
ing AIAO magnetic groundstate, our results support the notion that pyrochlore iridates may host
correlated Weyl semimetals
Tuning of the Ru4+ ground-state orbital population in the 4d(4) Mott insulator Ca2RuO4 achieved by La doping
The ground-state orbital occupancy of the Ru4+ ion in Ca2−xLaxRuO4[x = 0, 0.05(1), 0.07(1), and 0.12(1)]
was investigated by performing x-ray absorption spectroscopy (XAS) in the vicinity of the O K edge as a function
of the angle between the incident beam and the surface of the single-crystal samples. A minimal model of
the hybridization between the O 2p states probed at the K edge and the Ru 4d orbitals was used to analyze
the XAS data, allowing the ratio of hole occupancies nxy/nyz,zx to be determined as a function of doping and
temperature. For the samples displaying a low-temperature insulating ground state (x 0.07), nxy/nyz,zx is found
to increase significantly with increasing doping, with increasing temperature acting to further enhance nxy/nyz,zx .
For the x = 0.12 sample, which has a metallic ground state, the XAS spectra are found to be independent of
temperature and not to be describable by the minimal hybridization model, while being qualitatively similar
to the spectra displayed by the x 0.07 samples above their insulating to metallic transitions. To understand
the origin of the evolution of the electronic structure of Ca2−xLaxRuO4 across its phase diagram, we have
performed theoretical calculations based on a model Hamiltonian, comprising electron-electron correlations,
crystal field , and spin-orbit coupling λ, of a Ru-O-Ru cluster, with realistic values used to parametrize the
various interactions taken from the literature. Our calculations of the Ru hole occupancy as a function of /λ
provide an excellent description of the general trends displayed by the data. In particular they establish that the
enhancement of nxy/nyz,zx is driven by significant modifications to the crystal field as the tetragonal distortion of
the RuO6 octahedral changes from compressive to tensile with La doping. We have also used our model to show
that the hole occupancy of the O 2p and Ru 4d orbitals displays the same general trend as a function of /λ,
thus validating the minimal hybridization model used to analyze the data. In essence, our results suggest that the
predominant mechanism driving the emergence of the low-temperature metallic phase in La-doped Ca2RuO4 is
the structurally induced redistribution of holes within the t2g orbitals, rather than the injection of free carriers
p130Cas and p140Cap as the Bad and Good Guys in Breast Cancer Cell Progression to an Invasive Phenotype
Persistence of antiferromagnetic order upon La substitution in the Mott insulator CaRuO
The chemical and magnetic structures of the series of compounds
CaLaRuO [, , , ] have been
investigated using neutron diffraction and resonant elastic x-ray scattering.
Upon La doping, the low temperature S-Pbca space group of the parent compound
is retained in all insulating samples [], but with significant
changes to the atomic positions within the unit cell. These changes can be
characterised in terms of the local RuO octahedral coordination: with
increasing doping the structure, crudely speaking, evolves from an orthorhombic
unit cell with compressed octahedra to a quasi-tetragonal unit cell with
elongated ones. The magnetic structure on the other hand, is found to be
robust, with the basic , -axis antiferromagnetic order of the
parent compound preserved below the critical La doping concentration of
. The only effects of La doping on the magnetic structure are to
suppress the A-centred mode, favouring the B mode instead, and to reduce the
N\'{e}el temperature somewhat. Our results are discussed with reference to
previous experimental reports on the effects of cation substitution on the
Mott insulator CaRuO, as well as with regard to theoretical
studies on the evolution of its electronic and magnetic structure. In
particular, our results rule out the presence of a proposed ferromagnetic
phase, and suggest that the structural effects associated with La substitution
play an important role in the physics of the system.Comment: 10 pages, 9 figure
All-in-all-Out Magnetic Order and Propagating Spin Waves in Sm2Ir2 O7
Using resonant magnetic x-ray scattering we address the unresolved nature of the magnetic ground state and the low-energy effective Hamiltonian of Sm2Ir2O7, a prototypical pyrochlore iridate with a finite temperature metal-insulator transition. Through a combination of elastic and inelastic measurements, we show that the magnetic ground state is an all-in-all-out (AIAO) antiferromagnet. The magnon dispersion indicates significant electronic correlations and can be well described by a minimal Hamiltonian that includes Heisenberg exchange [J=27.3(6) meV] and Dzyaloshinskii-Moriya interactions [D=4.9(3) meV], which provides a consistent description of the magnetic order and excitations. In establishing that Sm2Ir2O7 has the requisite inversion symmetry preserving AIAO magnetic ground state, our results support the notion that pyrochlore iridates may host correlated Weyl semimetals
Evolution of the Magnetic Excitations in NaOsO3 through its Metal-Insulator Transition
The temperature dependence of the excitation spectrum in
NaOsO
3
through its metal-to-insulator transition (MIT) at 410 K has been investigated using resonant inelastic x-ray scattering at the Os
L
3
edge. High-resolution (
Δ
E
∼
56
 
 
meV
) measurements show that the well-defined, low-energy magnons in the insulating state weaken and dampen upon approaching the metallic state. Concomitantly, a broad continuum of excitations develops which is well described by the magnetic fluctuations of a nearly antiferromagnetic Fermi liquid. By revealing the continuous evolution of the magnetic quasiparticle spectrum as it changes its character from itinerant to localized, our results provide unprecedented insight into the nature of the MIT in
NaOsO
3
[J. G. Vale, S. Calder, C. Donnerer, D. Pincini, Y. G. Shi, Y. Tsujimoto, K. Yamaura, M. M. Sala, J. van den Brink, A. D. Christianson, and D. F. McMorrow, Phys. Rev. B 97, 184429 (2018)]