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):
[20709]
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.
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Dec 2020
|
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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
,
Kejin
Zhou
,
Giacomo
Ghiringhelli
,
Marco Moretti
Sala
Diamond Proposal Number(s):
[23413]
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.
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Dec 2020
|
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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.
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Oct 2020
|
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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.
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Aug 2020
|
|
I21-Resonant Inelastic X-ray Scattering (RIXS)
|
Soonmin
Kang
,
Kangwon
Kim
,
Beom Hyun
Kim
,
Jonghyeon
Kim
,
Kyung Ik
Sim
,
Jae-ung
Lee
,
Sungmin
Lee
,
Kisoo
Park
,
Seokhwan
Yun
,
Taehun
Kim
,
Abhishek
Nag
,
Andrew
Walters
,
Mirian
Garcia-fernandez
,
Jiemin
Li
,
Laurent
Chapon
,
Kejin
Zhou
,
Young-woo
Son
,
Jae Hoon
Kim
,
Hyeonsik
Cheong
,
Je-geun
Park
Diamond Proposal Number(s):
[18503, 18906]
Abstract: An exciton is the bosonic quasiparticle of electron–hole pairs bound by the Coulomb interaction. Bose–Einstein condensation of this exciton state has long been the subject of speculation in various model systems and examples have been found more recently in optical lattices and two-dimensional materials. Unlike these conventional excitons formed from extended Bloch states, excitonic bound states from intrinsically many-body localized states are rare. Here we show that a spin–orbit-entangled exciton state appears below the Néel temperature of 150 kelvin in NiPS3, an antiferromagnetic van der Waals material. It arises intrinsically from the archetypal many-body states of the Zhang–Rice singlet, and reaches a coherent state assisted by the antiferromagnetic order. Using configuration-interaction theory, we determine the origin of the coherent excitonic excitation to be a transition from a Zhang–Rice triplet to a Zhang–Rice singlet. We combine three spectroscopic tools—resonant inelastic X-ray scattering, photoluminescence and optical absorption—to characterize the exciton and to demonstrate an extremely narrow excitonic linewidth below 50 kelvin. The discovery of the spin–orbit-entangled exciton in antiferromagnetic NiPS3 introduces van der Waals magnets as a platform to study coherent many-body excitons.
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Jul 2020
|
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I21-Resonant Inelastic X-ray Scattering (RIXS)
|
Jiemin
Li
,
Abhishek
Nag
,
Jonathan
Pelliciari
,
Hannah
Robarts
,
Andrew
Walters
,
Mirian
Garcia-fernandez
,
Hiroshi
Eisaki
,
Dongjoon
Song
,
Hong
Ding
,
Steven
Johnston
,
Riccardo
Comin
,
Kejin
Zhou
Diamond Proposal Number(s):
[19886, 21184, 21277]
Open Access
Abstract: Charge-density waves (CDWs) are ubiquitous in underdoped cuprate superconductors. As a modulation of the valence electron density, CDWs in hole-doped cuprates possess both Cu-3d and O-2p orbital character owing to the strong hybridization of these orbitals near the Fermi level. Here, we investigate underdoped Bi2Sr1.4La0.6CuO6+δ using resonant inelastic X-ray scattering (RIXS) and find that a short-range CDW exists at both Cu and O sublattices in the copper-oxide (CuO2) planes with a comparable periodicity and correlation length. Furthermore, we uncover bond-stretching and bond-buckling phonon anomalies concomitant to the CDWs. Comparing to slightly overdoped Bi2Sr1.8La0.2CuO6+δ, where neither CDWs nor phonon anomalies appear, we highlight that a sharp intensity anomaly is induced in the proximity of the CDW wavevector (QCDW) for the bond-buckling phonon, in concert with the diffused intensity enhancement of the bond-stretching phonon at wavevectors much greater than QCDW. Our results provide a comprehensive picture of the quasistatic CDWs, their dispersive excitations, and associated electron-phonon anomalies, which are key for understanding the competing electronic instabilities in cuprates.
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Jun 2020
|
|
I21-Resonant Inelastic X-ray Scattering (RIXS)
|
J. Q.
Lin
,
H.
Miao
,
D. G.
Mazzone
,
G. D.
Gu
,
A.
Nag
,
A. C.
Walters
,
M.
Garcia-fernandez
,
A.
Barbour
,
J.
Pelliciari
,
I.
Jarrige
,
M.
Oda
,
K.
Kurosawa
,
N.
Momono
,
K.
Zhou
,
V.
Bisogni
,
X.
Liu
,
M. P. M.
Dean
Diamond Proposal Number(s):
[22261]
Abstract: The discovery of charge-density-wave-related effects in the resonant inelastic x-ray scattering spectra of cuprates holds the tantalizing promise of clarifying the interactions that stabilize the electronic order. Here, we report a comprehensive resonant inelastic x-ray scattering study of
La
2
−
x
Sr
x
CuO
4
finding that charge-density wave effects persist up to a remarkably high doping level of
x
=
0.21
before disappearing at
x
=
0.25
. The inelastic excitation spectra remain essentially unchanged with doping despite crossing a topological transition in the Fermi surface. This indicates that the spectra contain little or no direct coupling to electronic excitations near the Fermi surface, rather they are dominated by the resonant cross section for phonons and charge-density-wave-induced phonon softening. We interpret our results in terms of a charge-density wave that is generated by strong correlations and a phonon response that is driven by the charge-density-wave-induced modification of the lattice.
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May 2020
|
|
I21-Resonant Inelastic X-ray Scattering (RIXS)
|
A.
Nag
,
H. C.
Robarts
,
F.
Wenzel
,
J.
Li
,
H.
Elnaggar
,
R.-p.
Wang
,
A.
Walters
,
M.
Garcia-fernandez
,
F. M. F.
De Groot
,
M. W.
Haverkort
,
K.
Zhou
Open Access
Abstract: Understanding many-body physics of elementary excitations has advanced our control over material properties. Here, we study spin-flip excitations in NiO using Ni
L3-edge resonant inelastic x-ray scattering (RIXS) and present a strikingly different resonant energy behavior between single and double spin-flip excitations. Comparing our results with single-site full-multiplet ligand field theory calculations we find that the spectral weight of the double-magnon excitations originates primarily from the double spin-flip transition of the quadrupolar RIXS process within a single magnetic site. Quadrupolar spin-flip processes are among the least studied excitations, despite being important for multiferroic or spin-nematic materials due to their difficult detection. We identify intermediate state multiplets and intra-atomic core-valence exchange interactions as the key many-body factors determining the fate of such excitations. RIXS resonant energy dependence can act as a convincing proof of existence of nondipolar higher-ranked magnetic orders in systems for which, only theoretical predictions are available.
|
Feb 2020
|
|
I21-Resonant Inelastic X-ray Scattering (RIXS)
|
Robert A.
House
,
Urmimala
Maitra
,
Miguel A.
Pérez-osorio
,
Juan G.
Lozano
,
Liyu
Jin
,
James W.
Somerville
,
Laurent C.
Duda
,
Abhishek
Nag
,
Andrew
Walters
,
Kejin
Zhou
,
Matthew R.
Roberts
,
Peter G.
Bruce
Abstract: Alkali-rich transition metal oxide intercalation cathodes can increase the energy density of Li and Na-ion batteries by storing charge on the oxide and transition metal (TM) ions1–10. The high voltage associated with oxidation of O2- on the first charge is not recovered on discharge, resulting in significantly reduced energy density11. Displacement of TM ions from the TM to alkali metal layers has been proposed to explain the first cycle voltage loss9,12–16. By comparing two closely related intercalation cathodes, Na0.75[Li0.25Mn0.75]O2 and Na0.6[Li0.2Mn0.8]O2, we show that the first cycle voltage hysteresis is determined by the superstructure, specifically the local ordering of Li/TM ions in the TM layers. The honeycomb superstructure of Na0.75[Li0.25Mn0.75]O2, present in almost all O-redox compounds, is lost on charging, driven, in part, by formation of molecular O2 inside the solid. The O2 molecules are cleaved on discharge reforming O2-, but the Mn ions have migrated in the plane changing the coordination around O2- significantly lowering the voltage on discharge (hysteresis). The ribbon superstructure in Na0.6[Li0.2Mn0.8]O2 inhibits Mn disorder and hence O2 formation, suppressing hysteresis and promoting stable electron holes on O2- which are revealed by XAS. The results show that voltage hysteresis can be avoided in O-redox cathodes by forming materials with a superstructure (cation ordering) in the TM layers that suppresses TM migration.
|
Dec 2019
|
|
I21-Resonant Inelastic X-ray Scattering (RIXS)
|
Diamond Proposal Number(s):
[18469, 18512]
Abstract: We report high-resolution resonant inelastic x-ray scattering (RIXS) measurements of the collective spin fluctuations in three compositions of the superconducting cuprate system
La
2
−
x
Sr
x
CuO
4
. We have mapped out the excitations throughout much of the two-dimensional
(
h
,
k
)
Brillouin zone. The spin fluctuations in
La
2
−
x
Sr
x
CuO
4
are found to be fairly well described by a damped harmonic oscillator model, thus our data allows us to determine the full wave vector dependence of the damping parameter. This parameter increases with doping and is largest along the
(
h
,
h
) line, where it is peaked near
(
0.2
,
0.2
)
. We have used a new procedure to determine the absolute wave vector dependent susceptibility for the doped compositions
La
2
−
x
Sr
x
CuO
4
(
x
=
0.12
,
0.16
)
by normalizing our data to
La
2
CuO
4
measurements made with inelastic neutron scattering (INS). We find that the evolution with doping of the intensity of high-energy excitations measured by RIXS and INS is consistent. For the doped compositions, the wave vector dependent susceptibility is much larger at
(
1
4
,
1
4
)
than at
(
1
2
,
0
)
. It increases rapidly along the
(
h
,
h
)
line towards the antiferromagnetic wave vector of the parent compound
(
1
2
,
1
2
)
. Thus, the strongest magnetic excitations, and those predicted to favor superconductive pairing, occur towards the
(
1
2
,
1
2
)
position as observed by INS.
|
Dec 2019
|
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