I16-Materials and Magnetism
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C. D.
Dashwood
,
A. H.
Walker
,
M. P.
Kwasigroch
,
L. S. I.
Veiga
,
Q.
Faure
,
J. G.
Vale
,
D. G.
Porter
,
P.
Manuel
,
D. D.
Khalyavin
,
F.
Orlandi
,
C. V.
Colin
,
O.
Fabelo
,
F.
Krüger
,
R. S.
Perry
,
R. D.
Johnson
,
A. G.
Green
,
D. F.
Mcmorrow
Diamond Proposal Number(s):
[23580, 25554]
Open Access
Abstract: The layered-ruthenate family of materials possess an intricate interplay of structural, electronic and magnetic degrees of freedom that yields a plethora of delicately balanced ground states. This is exemplified by Ca3Ru2O7, which hosts a coupled transition in which the lattice parameters jump, the Fermi surface partially gaps and the spins undergo a 90∘ in-plane reorientation. Here, we show how the transition is driven by a lattice strain that tunes the electronic bandwidth. We apply uniaxial stress to single crystals of Ca3Ru2O7, using neutron and resonant x-ray scattering to simultaneously probe the structural and magnetic responses. These measurements demonstrate that the transition can be driven by externally induced strain, stimulating the development of a theoretical model in which an internal strain is generated self-consistently to lower the electronic energy. We understand the strain to act by modifying tilts and rotations of the RuO6 octahedra, which directly influences the nearest-neighbour hopping. Our results offer a blueprint for uncovering the driving force behind coupled phase transitions, as well as a route to controlling them.
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Oct 2023
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I16-Materials and Magnetism
I21-Resonant Inelastic X-ray Scattering (RIXS)
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Abstract: The burgeoning field of quantum materials concerns systems that do not adhere to the traditional theories of condensed matter physics. A key feature of these materials is a strong coupling between structural, electronic and magnetic degrees of freedom, which is especially prominent in 4d and 5d transition- metal oxides. The consequences of this coupling are wide, stabilising a range of emergent phases that are sensitive to perturbation. In this thesis, I develop novel techniques based on neutron and x-ray scattering to characterise and control electron-lattice coupling in 4d and 5d quantum materials.
I begin with Ca3Ru2O7, a 4d polar metal that hosts a spin-reorientation transition. Using neutron and resonant x-ray scattering, I reveal a new cy- cloidal magnetic phase, arising from spin-orbit coupling, that rapidly evolves with temperature to mediate the transition. I further show that the cycloid- mediated spin-reorientation can be driven by anisotropic strain, demonstrating the control enabled by coupling to the lattice.
I then turn to resonant inelastic x-ray scattering (RIXS), which has re- cently received interest as a new probe of electron-phonon coupling (EPC). Using graphite as a model system, I demonstrate the power of RIXS to probe the momentum-dependent EPC for a range of excited electronic states. Our RIXS data reveal some key deficiencies of current theoretical models of phonon excitations in RIXS, and prompt the development of a new Green’s-function– based model by our collaborators to address these issues.
Finally, I present a study of the 5d material Sr2IrO4, a famous jeff = 1/2 spin-orbit Mott insulator. I characterise the phonon spectrum with non- resonant inelastic x-ray scattering, before using RIXS to explore the phonon and magnon excitations. I find a strong EPC similar to that seen in the cuprates, and offer a new interpretation of the magnon dispersion involving coupling to spin-orbit excitons.
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Jun 2022
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I21-Resonant Inelastic X-ray Scattering (RIXS)
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C. D.
Dashwood
,
A.
Geondzhian
,
J. G.
Vale
,
A. C.
Pakpour-Tabrizi
,
C. A.
Howard
,
Q.
Faure
,
L. S. I.
Veiga
,
D.
Meyers
,
G. S.
Chiuzbaian
,
A.
Nicolaou
,
N.
Jaouen
,
R. B.
Jackman
,
A.
Nag
,
M.
Garcia-Fernandez
,
Ke-Jin
Zhou
,
A. C.
Walters
,
K.
Gilmore
,
D. F.
Mcmorrow
,
M. P. M.
Dean
Diamond Proposal Number(s):
[22695]
Open Access
Abstract: Interactions between electrons and lattice vibrations are responsible for a wide range of material properties and applications. Recently, there has been considerable interest in the development of resonant inelastic x-ray scattering (RIXS) as a tool for measuring electron-phonon (
e
-ph) interactions. Here, we demonstrate the ability of RIXS to probe the interaction between phonons and specific electronic states both near to, and away from, the Fermi level. We perform carbon
K
-edge RIXS measurements on graphite, tuning the incident x-ray energy to separately probe the interactions of the
π
∗
and
σ
∗
electronic states. Our high-resolution data reveal detailed structure in the multiphonon RIXS features that directly encodes the momentum dependence of the
e
-ph interaction strength. We develop a Green’s-function method to model this structure, which naturally accounts for the phonon and interaction-strength dispersions, as well as the mixing of phonon momenta in the intermediate state. This model shows that the differences between the spectra can be fully explained by contrasting trends of the
e
-ph interaction through the Brillouin zone, being concentrated at the
Γ
and
K
points for the
π
∗
states while being significant at all momenta for the
σ
∗
states. Our results advance the interpretation of phonon excitations in RIXS and extend its applicability as a probe of
e
-ph interactions to a new range of out-of-equilibrium situations.
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Dec 2021
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I16-Materials and Magnetism
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C. D.
Dashwood
,
L. S. I.
Veiga
,
Q.
Faure
,
J. G.
Vale
,
D. G.
Porter
,
S. P.
Collins
,
P.
Manuel
,
D. D.
Khalyavin
,
F.
Orlandi
,
R. S.
Perry
,
R. D.
Johnson
,
D. F.
Mcmorrow
Diamond Proposal Number(s):
[23580]
Abstract: We show how complex modulated order can spontaneously emerge when magnetic interactions compete in a metal with polar lattice distortions. Combining neutron and resonant x-ray scattering with symmetry analysis, we reveal that the spin reorientation in
Ca
3
Ru
2
O
7
is mediated by a magnetic cycloid whose eccentricity evolves smoothly but rapidly with temperature. We find the cycloid to be highly sensitive to magnetic fields, which appear to continuously generate higher harmonic modulations. Our results provide a unified picture of the rich magnetic phases of this correlated, multiband polar metal.
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Nov 2020
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I21-Resonant Inelastic X-ray Scattering (RIXS)
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J. G.
Vale
,
C. D.
Dashwood
,
E.
Paris
,
L. S. I.
Veiga
,
Mirian
Garcia-Fernandez
,
A.
Nag
,
A.
Walters
,
K.
Zhou
,
I.-M.
Pietsch
,
Anton
Jesche
,
P.
Gegenwart
,
R.
Coldea
,
T.
Schmitt
,
D. F.
Mcmorrow
Diamond Proposal Number(s):
[20569]
Abstract: The excitations in honeycomb
α
−
Li
2
IrO
3
have been investigated with high-resolution resonant inelastic x-ray scattering (RIXS) at the O
K
edge. The low-energy response is dominated by a fully resolved ladder of excitations, which we interpret as being due to multiphonon processes in the presence of strong electron-phonon coupling (EPC). At higher energies, the orbital excitations are shown to be dressed by phonons. The high quality of the data permits a quantitative test of the analytical model for the RIXS cross section, which has been proposed to describe EPC in transition-metal oxides (TMOs). We find that the magnitude of the EPC is comparable to that found for a range of
3
d
TMOs. This indicates that EPC may be of equal importance in determining the phenomenology displayed by corresponding
5
d
-based systems.
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Dec 2019
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I10-Beamline for Advanced Dichroism - scattering
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Abstract: 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 Δ, 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 Δ/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 Δ/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.
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Feb 2019
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I16-Materials and Magnetism
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D.
Pincini
,
S.
Boseggia
,
R.
Perry
,
Ma. J.
Gutmann
,
S.
Ricco
,
L. S. I.
Veiga
,
C. D.
Dashwood
,
S. P.
Collins
,
G.
Nisbet
,
A.
Bombardi
,
D. G.
Porter
,
F.
Baumberger
,
A. T.
Boothroyd
,
D. F.
Mcmorrow
Diamond Proposal Number(s):
[15323, 15952, 15867, 18934]
Abstract: The chemical and magnetic structures of the series of compounds Ca2−xLaxRuO4 [x=0, 0.05(1), 0.07(1), 0.12(1)] 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 [x≤0.07(1)], but with significant changes to the atomic positions within the unit cell. These changes can be characterized in terms of the local
RuO6 octahedral coordination: with increasing doping, the structure, crudely speaking, evolves from an orthorhombic unit cell with compressed octahedra to a quasitetragonal unit cell with elongated ones. The magnetic structure on the other hand, is found to be robust, with the basic k=(0,0,0), b-axis antiferromagnetic order of the parent compound preserved below the critical La doping concentration of x≈0.11. The only effects of La doping on the magnetic structure are to suppress the A-centred mode, favoring the B mode instead, and to reduce the Néel temperature somewhat. Our results are discussed with reference to previous experimental reports on the effects of cation substitution on the d4 Mott insulator Ca2RuO4, 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.
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Jul 2018
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I07-Surface & interface diffraction
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Edward P.
Booker
,
Tudor H.
Thomas
,
Claudio
Quarti
,
Michael R.
Stanton
,
Cameron D.
Dashwood
,
Alexander J.
Gillett
,
Johannes M.
Richter
,
Andrew J.
Pearson
,
Nathaniel J. L. K.
Davis
,
Henning
Sirringhaus
,
Michael B.
Price
,
Neil C.
Greenham
,
David
Beljonne
,
Sian E.
Dutton
,
Felix
Deschler
Diamond Proposal Number(s):
[14886]
Open Access
Abstract: We investigate the origin of the broadband visible emission in layered hybrid lead-halide perovskites and its connection with structural and photophysical properties. We study <001> oriented thin films of hexylammonium (HA) lead iodide, (C6H16N)2PbI4, and dodecylammonium (DA) lead iodide, (C12H28N)2PbI4 by combining first-principles simulations with time-resolved photoluminescence, steady-state absorption and X-ray diffraction measurements on cooling from 300 K to 4 K. Ultrafast transient absorption and photoluminescence measurements are used to track the formation and recombination of emissive states. In addition to the excitonic photoluminescence near the absorption edge, we find a red-shifted, broadband (full-width at half maximum of about 0.4 eV), emission band below 200 K, similar to emission from <110> oriented bromide 2D perovskites at room temperature. The lifetime of this sub-bandgap emission exceeds that of the excitonic transition by orders of magnitude. We use X-ray diffraction measurements to study the changes in crystal lattice with temperature. We report changes in the octahedral tilt and lattice spacing in both materials, together with a phase change around 200 K in DA2PbI4. DFT simulations of the HA2PbI4 crystal structure indicate that the low-energy emission is due to interstitial iodide and related Frenkel defects. Our results demonstrate that white-light emission is not limited to <110> oriented bromide 2D perovskites but a general property of this class of system and highlight the importance of defect control for the formation of low-energy emissive sites, which can provide a pathway to design tailored white-light emitters.
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Nov 2017
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