I21-Resonant Inelastic X-ray Scattering (RIXS)
|
J.
Li
,
L.
Xu
,
M.
Garcia-Fernandez
,
A.
Nag
,
H. C.
Robarts
,
A. C.
Walters
,
X.
Liu
,
J.
Zhou
,
K.
Wohlfeld
,
J.
Van Den Brink
,
H.
Ding
,
K.-J.
Zhou
Open Access
Abstract: We explore the existence of the collective orbital excitations, orbitons, in the canonical orbital system
KCuF
3
using the Cu
L
3
-edge resonant inelastic x-ray scattering. We show that the nondispersive high-energy peaks result from the
Cu
2
+
d
d
orbital excitations. These high-energy modes display good agreement with the ab initio quantum chemistry calculation, indicating that the
d
d
excitations are highly localized. At the same time, the low-energy excitations present clear dispersion. They match extremely well with the two-spinon continuum following the comparison with Müller ansatz calculations. The localized
d
d
excitations and the observation of the strongly dispersive magnetic excitations suggest that the orbiton dispersion is below the resolution detection limit. Our results can reconcile with the strong local Jahn-Teller effect in
KCuF
3
, which predominantly drives orbital ordering.
|
Mar 2021
|
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I21-Resonant Inelastic X-ray Scattering (RIXS)
|
J. q.
Lin
,
P.
Villar Arribi
,
G.
Fabbris
,
A. s.
Botana
,
D.
Meyers
,
H.
Miao
,
Y.
Shen
,
D. G.
Mazzone
,
J.
Feng
,
G. S.
Chiuzbaian
,
A.
Nag
,
A. C.
Walters
,
M.
Garcia-Fernandez
,
K.-J.
Zhou
,
J.
Pelliciari
,
I.
Jarrige
,
J. w.
Freeland
,
J.
Zhang
,
J. f.
Mitchell
,
V.
Bisogni
,
X.
Liu
,
M. r.
Norman
,
M. p. m.
Dean
Diamond Proposal Number(s):
[18519]
Abstract: The discovery of superconductivity in a
d
9
−
δ
nickelate has inspired disparate theoretical perspectives regarding the essential physics of this class of materials. A key issue is the magnitude of the magnetic superexchange, which relates to whether cuprate-like high-temperature nickelate superconductivity could be realized. We address this question using Ni
L
-edge and O
K
-edge spectroscopy of the reduced
d
9
−
1
/
3
trilayer nickelates
R
4
Ni
3
O
8
(where
R
=
La
, Pr) and associated theoretical modeling. A magnon energy scale of
∼
80
meV
resulting from a nearest-neighbor magnetic exchange of
J
=
69
(
4
)
meV
is observed, proving that
d
9
−
δ
nickelates can host a large superexchange. This value, along with that of the Ni-O hybridization estimated from our O
K
-edge data, implies that trilayer nickelates represent an intermediate case between the infinite-layer nickelates and the cuprates. Layered nickelates thus provide a route to testing the relevance of superexchange to nickelate superconductivity.
|
Feb 2021
|
|
B18-Core EXAFS
I21-Resonant Inelastic X-ray Scattering (RIXS)
|
Diamond Proposal Number(s):
[20363, 23889]
Open Access
Abstract: In the search for high energy density cathodes for next-generation lithium-ion batteries, the disordered rocksalt oxyfluorides are receiving significant attention due to their high capacity and lower voltage hysteresis compared with ordered Li-rich layered compounds. However, a deep understanding of these phenomena and their redox chemistry remains incomplete. Using the archetypal oxyfluoride, Li2MnO2F, we show that the oxygen redox process in such materials involves the formation of molecular O2 trapped in the bulk structure of the charged cathode, which is reduced on discharge. The molecular O2 is trapped rigidly within vacancy clusters and exhibits minimal mobility unlike free gaseous O2, making it more characteristic of a solid-like environment. The Mn redox process occurs between octahedral Mn3+ and Mn4+ with no evidence of tetrahedral Mn5+ or Mn7+. We furthermore derive the relationship between local coordination environment and redox potential; this gives rise to the observed overlap in Mn and O redox couples and reveals that the onset potential of oxide ion oxidation is determined by the degree of ionicity around oxygen, which extends models based on linear Li–O–Li configurations. This study advances our fundamental understanding of redox mechanisms in disordered rocksalt oxyfluorides, highlighting their promise as high capacity cathodes.
|
Dec 2020
|
|
I21-Resonant Inelastic X-ray Scattering (RIXS)
|
Roberto
Fumagalli
,
Abhishek
Nag
,
Stefano
Agrestini
,
Mirian
Garcia-Fernandez
,
Andrew C.
Walters
,
Davide
Betto
,
Nicholas B.
Brookes
,
Lucio
Braicovich
,
Ke-Jin
Zhou
,
Giacomo
Ghiringhelli
,
Marco Moretti
Sala
Diamond Proposal Number(s):
[23423]
Abstract: Motivated by the recent synthesis of Ba
CuO
(BCO), a high temperature superconducting cuprate with putative
ground state symmetry, we investigated its electronic structure by means of Cu
x-ray absorption (XAS) and resonant inelastic x-ray scattering (RIXS) at the Cu
edge on a polycrystalline sample. We show that the XAS profile of BCO is characterised by two peaks associated to inequivalent Cu sites, and that its RIXS response features a single, sharp peak associated to crystal-field excitations. We argue that these observations are only partially compatible with the previously proposed crystal structure of BCO. Based on our spectroscopic results and on previously published powder diffraction measurements, we propose a crystalline structure characterized by two inequivalent Cu sites located at alternated planes along the
axis: nominally trivalent Cu(1) belonging to very short Cu-O chains, and divalent Cu(2) in the oxygen deficient CuO
planes. We also analyze the low-energy region of the RIXS spectra to estimate the magnitude of the magnetic interactions in BCO and find that in-plane nearest neighbor superexchange exceeds 120 meV, similarly to that of other layered cuprates. Although these results do not support the pure
ground state scenario, they hint at a significant departure from the common quasi-2D electronic structure of superconducting cuprates of pure
symmetry.
|
Dec 2020
|
|
B18-Core EXAFS
I21-Resonant Inelastic X-ray Scattering (RIXS)
|
Diamond Proposal Number(s):
[20363, 23889]
Open Access
Abstract: In the search for high energy density cathodes for next-generation lithium-ion batteries, the disordered rocksalt oxyfluorides are receiving significant attention due to their high capacity and lower voltage hysteresis compared with ordered Li-rich layered compounds. However, a deep understanding of these phenomena and their redox chemistry remains incomplete. Using the archetypal oxyfluoride, Li2MnO2F, we show that the oxygen redox process in such materials involves the formation of molecular O2 trapped in the bulk structure of the charged cathode, which is reduced on discharge. The molecular O2 is trapped rigidly within vacancy clusters and exhibits minimal mobility unlike free gaseous O2, making it more characteristic of a solid-like environment. The Mn redox process occurs between octahedral Mn3+ and Mn4+ with no evidence of tetrahedral Mn5+ or Mn7+. We furthermore derive the relationship between local coordination environment and redox potential; this gives rise to the observed overlap in Mn and O redox couples and reveals that the onset potential of oxide ion oxidation is determined by the degree of ionicity around oxygen, which extends models based on linear Li–O–Li configurations. This study advances our fundamental understanding of redox mechanisms in disordered rocksalt oxyfluorides, highlighting their promise as high capacity cathodes.
|
Dec 2020
|
|
I21-Resonant Inelastic X-ray Scattering (RIXS)
|
A.
Nag
,
M.
Zhu
,
M.
Bejas
,
J.
Li
,
H. C.
Robarts
,
H.
Yamase
,
A. N.
Petsch
,
D.
Song
,
H.
Eisaki
,
A. C.
Walters
,
M.
Garcia-Fernandez
,
A.
Greco
,
S. M.
Hayden
,
K.
Zhou
Diamond Proposal Number(s):
[24587]
Open Access
Abstract: High
T
c
superconductors show a rich variety of phases associated with their charge degrees of freedom. Valence charges can give rise to charge ordering or acoustic plasmons in these layered cuprate superconductors. While charge ordering has been observed for both hole- and electron-doped cuprates, acoustic plasmons have only been found in electron-doped materials. Here, we use resonant inelastic x-ray scattering to observe the presence of acoustic plasmons in two families of hole-doped cuprate superconductors (
La
1.84
Sr
0.16
CuO
4
and
Bi
2
Sr
1.6
La
0.4
CuO
6
+
δ
), crucially completing the picture. Interestingly, in contrast to the quasistatic charge ordering which manifests at both Cu and O sites, the observed acoustic plasmons are predominantly associated with the O sites, revealing a unique dichotomy in the behavior of valence charges in hole-doped cuprates.
|
Dec 2020
|
|
I21-Resonant Inelastic X-ray Scattering (RIXS)
|
Diamond Proposal Number(s):
[24583]
Open Access
Abstract: Resonant inelastic x-ray spectroscopy at the uranium $N_{4}$ absorption edge at 778 eV has been used to reveal the excitations in UO$_{2}$ up to 1 eV. The earlier (1989) studies by neutron inelastic scattering of the crystal-field states within the $^{3}H_{4}$ multiplet are confirmed. In addition, the first excited state of the $^{3}F_{2}$ multiplet at $\sim$520 meV has been established, and there is a weak signal corresponding to the next excited state at $\sim$920 meV. This represents a successful application of soft x-ray spectroscopy to an actinide sample, and resolves an open question in UO$_{2}$ that has been discussed for 50 years. The technique is described and important caveats are drawn about possible future applications.
|
Oct 2020
|
|
I21-Resonant Inelastic X-ray Scattering (RIXS)
|
Open Access
Abstract: Upon progressive refinement of energy resolution, the conventional resonant inelastic X-ray scattering (RIXS) instrumentation reaches the limit where the bandwidth of incident photons becomes insufficient to deliver an acceptable photon-count rate. Here it is shown that RIXS spectra as a function of energy loss are essentially invariant to their integration over incident energies within the core-hole lifetime. This fact permits RIXS instrumentation based on the hv2-concept to utilize incident synchrotron radiation over the whole core-hole lifetime window without any compromise on the much finer energy-loss resolution, thereby breaking the photon-count limit.
|
Sep 2020
|
|
I21-Resonant Inelastic X-ray Scattering (RIXS)
|
Diamond Proposal Number(s):
[23889]
Abstract: Li-rich cathode materials are potential candidates for next-generation Li-ion batteries. However, they exhibit a large voltage hysteresis on the first charge/discharge cycle, which involves a substantial (up to 1 V) loss of voltage and therefore energy density. For Na cathodes, for example Na0.75[Li0.25Mn0.75]O2, voltage hysteresis can be explained by the formation of molecular O2 trapped in voids within the particles. Here we show that this is also the case for Li1.2Ni0.13Co0.13Mn0.54O2. Resonant inelastic X-ray scattering and 17O magic angle spinning NMR spectroscopy show that molecular O2, rather than O22−, forms within the particles on the oxidation of O2− at 4.6 V versus Li+/Li on charge. These O2 molecules are reduced back to O2− on discharge, but at the lower voltage of 3.75 V, which explains the voltage hysteresis in Li-rich cathodes. 17O magic angle spinning NMR spectroscopy indicates a quantity of bulk O2 consistent with the O-redox charge capacity minus the small quantity of O2 loss from the surface. The implication is that O2, trapped in the bulk and lost from the surface, can explain O-redox.
|
Sep 2020
|
|
I21-Resonant Inelastic X-ray Scattering (RIXS)
|
W. S.
Lee
,
K.
Zhou
,
M.
Hepting
,
J.
Li
,
A.
Nag
,
A. C.
Walters
,
M.
Garcia-fernandez
,
H. C.
Robarts
,
M.
Hashimoto
,
H.
Lu
,
B.
Nosarzewski
,
D.
Song
,
H.
Eisaki
,
Z. X.
Shen
,
B.
Moritz
,
J.
Zaanen
,
T. P.
Devereaux
Diamond Proposal Number(s):
[18462]
Abstract: Copper oxide high-TC superconductors possess a number of exotic orders that coexist with or are proximal to superconductivity. Quantum fluctuations associated with these orders may account for the unusual characteristics of the normal state, and possibly affect the superconductivity1,2,3,4. Yet, spectroscopic evidence for such quantum fluctuations remains elusive. Here, we use resonant inelastic X-ray scattering to reveal spectroscopic evidence of fluctuations associated with a charge order5,6,7,8,9,10,11,12,13,14 in nearly optimally doped Bi2Sr2CaCu2O8+δ. In the superconducting state, while the quasielastic charge order signal decreases with temperature, the interplay between charge order fluctuations and bond-stretching phonons in the form of a Fano-like interference increases, an observation that is incompatible with expectations for competing orders. Invoking general principles, we argue that this behaviour reflects the properties of a dissipative system near an order–disorder quantum critical point, where the dissipation varies with the opening of the pseudogap and superconducting gap at low temperatures, leading to the proliferation of quantum critical fluctuations, which melt charge order.
|
Aug 2020
|
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