I11-High Resolution Powder Diffraction
|
Henrik
Jacobsen
,
Hai L.
Feng
,
Andrew J.
Princep
,
Marein C.
Rahn
,
Yanfeng
Guo
,
Jie
Chen
,
Yoshitaka
Matsushita
,
Yoshihiro
Tsujimoto
,
Masahiro
Nagao
,
Dmitry
Khalyavin
,
Pascal
Manuel
,
Claire A.
Murray
,
Christian
Donnerer
,
James G.
Vale
,
Marco
Moretti Sala
,
Kazunari
Yamaura
,
Andrew T.
Boothroyd
Diamond Proposal Number(s):
[9839]
Abstract: We report on the structural, magnetic, and electronic properties of two new double-perovskites synthesized under high pressure,
Pb
2
CaOsO
6
and
Pb
2
ZnOsO
6
. Upon cooling below 80 K,
Pb
2
CaOsO
6
simultaneously undergoes a metal-to-insulator transition and develops antiferromagnetic order.
Pb
2
ZnOsO
6
, on the other hand, remains a paramagnetic metal down to 2 K. The key difference between the two compounds lies in their crystal structures. The Os atoms in
Pb
2
ZnOsO
6
are arranged on an approximately face-centered cubic lattice with strong antiferromagnetic nearest-neighbor exchange couplings. The geometrical frustration inherent to this lattice prevents magnetic order from forming down to the lowest temperatures. In contrast, the unit cell of
Pb
2
CaOsO
6
is heavily distorted up to at least 500 K including antiferroelectriclike displacements of the Pb and O atoms despite metallic conductivity above 80 K. This distortion relieves the magnetic frustration, facilitating magnetic order which, in turn, drives the metal-insulator transition. Our results suggest that the phase transition in
Pb
2
CaOsO
6
is spin driven and could be a rare example of a Slater transition.
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Dec 2020
|
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I05-ARPES
I11-High Resolution Powder Diffraction
|
J.-R.
Soh
,
F.
De Juan
,
M. G.
Vergniory
,
N. B. M.
Schröter
,
M. C.
Rahn
,
D. Y.
Yan
,
J.
Jiang
,
M.
Bristow
,
P. A.
Reiss
,
J. N.
Blandy
,
Y. F.
Guo
,
Y. G.
Shi
,
T. K.
Kim
,
A.
Mccollam
,
S. H.
Simon
,
Y.
Chen
,
A. I.
Coldea
,
A. T.
Boothroyd
Diamond Proposal Number(s):
[19234, 18786]
Abstract: We report theoretical and experimental evidence that
EuCd
2
As
2
in magnetic fields greater than 1.6 T applied along the
c
axis is a Weyl semimetal with a single pair of Weyl nodes. Ab initio electronic structure calculations, verified at zero field by angle-resolved photoemission spectra, predict Weyl nodes with wave vectors
k
=
(
0
,
0
,
±
0.03
)
×
2
π
/
c
at the Fermi level when the Eu spins are fully aligned along the
c
axis. Shubnikov–de Haas oscillations measured in fields parallel to
c
reveal a cyclotron effective mass of
m
∗
c
=
0.08
m
e
and a Fermi surface of extremal area
A
ext
=
0.24
nm
−
2
, corresponding to 0.1% of the area of the Brillouin zone. The small values of
m
∗
c
and
A
ext
are consistent with quasiparticles near a Weyl node. The identification of
EuCd
2
As
2
as a model Weyl semimetal opens the door to fundamental tests of Weyl physics.
|
Nov 2019
|
|
B18-Core EXAFS
|
J.
Sannigrahi
,
D. T.
Adroja
,
C.
Ritter
,
W.
Kockelmann
,
A. D.
Hillier
,
K. S.
Knight
,
A. T.
Boothroyd
,
M.
Wakeshima
,
Y.
Hinatsu
,
J. F. W.
Mosselmans
,
S.
Ramos
Diamond Proposal Number(s):
[63810]
Abstract: Bulk studies have revealed a first-order valence phase transition in
Ba
2
PrRu
1
−
x
Ir
x
O
6
(
0.10
≤
x
≤
0.25
), which is absent in the parent compounds with
x
=
0
(
Pr
3
+
) and
x
=
1
(
Pr
4
+
), which exhibit antiferromagnetic order with transition temperatures
T
N
=
120
and 72 K, respectively. In the present study, we have used magnetization, heat capacity, neutron diffraction, inelastic neutron scattering, and x-ray absorption measurements to investigate the nature of the Pr ion in
x
=
0.1
. The magnetic susceptibility and heat capacity of
x
=
0.1
show a clear sign of the first-order valence phase transition below 175 K, where the Pr valence changes from 3+ to 4+. Neutron diffraction analysis reveals that
x
=
0.1
crystallizes in a monoclinic structure with space group
P
2
1
/
n
at 300 K, but below 175 K two phases coexist, the monoclinic having the Pr ion in a 3+ valence state and a cubic one (
F
m
¯
3
m
) having the Pr ion in a 4+ valence state. Clear evidence of an antiferromagnetic ordering of the Pr and Ru moments is found in the monoclinic phase of the
x
=
0.1
compound below 110 K in the neutron diffraction measurements. Meanwhile, the cubic phase remains paramagnetic down to 2 K, a temperature below which heat capacity and susceptibility measurements reveal a ferromagnetic ordering. High energy inelastic neutron scattering data reveal well-defined high-energy magnetic excitations near 264 meV at temperatures below the valence transition. Low energy INS data show a broad magnetic excitation centered at 50 meV above the valence transition, but four well-defined magnetic excitations at 7 K. The high energy excitations are assigned to the
Pr
4
+
ions in the cubic phase and the low energy excitations to the
Pr
3
+
ions in the monoclinic phase. Further direct evidence of the Pr valence transition has been obtained from the x-ray absorption spectroscopy. The results on the
x
=
0.1
compound are compared with those for
x
=
0
and 1.
|
May 2019
|
|
I16-Materials and Magnetism
|
Diamond Proposal Number(s):
[14024]
Open Access
Abstract: We study the magnetic structure of the “stuffed” (Tb-rich) pyrochlore iridate Tb2+xIr2−xO7−y, using resonant elastic x-ray scattering (REXS). In order to disentangle contributions from Tb and Ir magnetic sublattices, experiments were performed at the Ir L3 and Tb M5 edges, which provide selective sensitivity to Ir 5d and Tb 4f magnetic moments, respectively. At the Ir L3 edge, we found the onset of long-range k = 0 magnetic order below TIr N ∼71K, consistent with the expected signal of all-in all-out (AIAO) magnetic order. Using a single-ion model to calculate REXS crosssections, we estimate an ordered magnetic moment of µIr 5d ≈ 0.34(3)µB at 5K. At the Tb M5 edge,long-range k = 0 magnetic order appeared below ∼ 40K, also consistent with an AIAO magnetic structure on the Tb site. Additional insight into the magnetism of the Tb sublattice is gleaned from measurements at the M5 edge in applied magnetic fields up to 6T, which is found to completely suppress the Tb AIAO magnetic order. In zero applied field, the observed gradual onset of the Tb sublattice magnetisation with temperature suggests that it is induced by the magnetic order on the Ir site. The persistence of AIAO magnetic order, despite the greatly reduced ordering temperature and moment size compared to stoichiometric Tb2Ir2O7, for which TIr N = 130K and µIr 5d = 0.56µB, indicates that stuffing could be a viable means of tuning the strength of electronic correlations, thereby potentially offering a new strategy to achieve topologically non-trivial band crossings in pyrochlore iridates.
|
May 2019
|
|
|
D. F.
Bowman
,
E.
Cemal
,
T.
Lehner
,
A. R.
Wildes
,
L.
Mangin-Thro
,
G. J.
Nilsen
,
M. J.
Gutmann
,
D. J.
Voneshen
,
D.
Prabhakaran
,
A. T.
Boothroyd
,
D. G.
Porter
,
C.
Castelnovo
,
K.
Refson
,
J. P.
Goff
Open Access
Abstract: Pyrochlore systems are ideally suited to the exploration of geometrical frustration in three dimensions, and their rich phenomenology encompasses topological order and fractional excitations. Classical spin ices provide the first context in which it is possible to control emergent magnetic monopoles, and anisotropic exchange leads to even richer behaviour associated with large quantum fluctuations. Whether the magnetic ground state of Yb2Ti2O7 is a quantum spin liquid or a ferromagnetic phase induced by a Higgs transition appears to be sample dependent. Here we have determined the role of structural defects on the magnetic ground state via the diffuse scattering of neutrons. We find that oxygen vacancies stabilise the spin liquid phase and the stuffing of Ti sites by Yb suppresses it. Samples in which the oxygen vacancies have been eliminated by annealing in oxygen exhibit a transition to a ferromagnetic phase, and this is the true magnetic ground state.
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Feb 2019
|
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I10-Beamline for Advanced Dichroism
|
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.
|
Feb 2019
|
|
I16-Materials and Magnetism
|
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.
|
Jul 2018
|
|
I11-High Resolution Powder Diffraction
|
Diamond Proposal Number(s):
[13284]
Abstract: We have performed inelastic neutron-scattering measurements on a powder sample of the superconductor lithium iron selenide hydroxide Li 1−x Fe x ODFe 1−y Se(x≃0.16,y≃0.02,T c =41K) . The spectrum shows an enhanced intensity below T c over an energy range of 0.64×2Δ
|
Oct 2016
|
|
I19-Small Molecule Single Crystal Diffraction
|
N. R.
Davies
,
R. D.
Johnson
,
A. J
Princep
,
L. A.
Gannon
,
J.-Z.
Ma
,
T.
Qian
,
P.
Richard
,
H.
Li
,
M.
Shi
,
H.
Nowell
,
P. J.
Baker
,
Y. G.
Shi
,
H.
Ding
,
J.
Luo
,
Y. F.
Guo
,
A. T.
Boothroyd
Diamond Proposal Number(s):
[10108]
Abstract: We report single-crystal x-ray-diffraction measurements on Na 2 Ti 2 Pn 2 O(Pn=As,Sb) which reveal a charge superstructure that appears below the density wave transitions previously observed in bulk data. From symmetry-constrained structure refinements we establish that the associated distortion mode can be described by two propagation vectors q 1 =(1/2,0,l) and q 2 =(0,1/2,l) with l=0 (Sb) or l=1/2 (As) and primarily involves in-plane displacements of the Ti atoms perpendicular to the Ti-O bonds. We also present angle-resolved photoemission spectroscopy measurements, which show band folding and backbending consistent with a density wave with the same wave-vectors q 1 and q 2 associated with Fermi-surface nesting, and muon-spin relaxation data, which show no indication of spin density wave order. The results provide direct evidence for phonon-assisted charge density wave order in Na 2 Ti 2 Pn 2 O and fully characterize a proximate ordered phase that could compete with superconductivity in doped BaTi 2 Sb 2 O.
|
Sep 2016
|
|
|
Abstract: 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 Sm2Ir2 O7 has the requisite inversion symmetry preserving AIAO magnetic ground state, our results support the notion that pyrochlore iridates may host correlated Weyl semimetals.
|
Jul 2016
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