B18-Core EXAFS
|
Dhanpal
Bairwa
,
Abhisek
Bandyopadhyay
,
Devashibhai
Adroja
,
Gavin B. G.
Stenning
,
Hubertus
Luetkens
,
Thomas J.
Hicken
,
Jonas A.
Krieger
,
Giannantonio
Cibin
,
M.
Rotter
,
Sudhindra
Rayaprol
,
P. D.
Babu
,
Suja
Elizabeth
Diamond Proposal Number(s):
[34771]
Open Access
Abstract: The rare-earth-based geometrically frustrated triangular magnets have attracted considerable attention due to the intricate interplay between strong spin-orbit coupling and the crystal electric field (CEF), which often leads to effective spin-1/2 degrees of freedom and therefore promotes strong quantum fluctuations at low temperatures, thus offering an excellent route to stabilize a quantum spin liquid (QSL) ground state. We have investigated the ground state magnetic properties of a polycrystalline sample of SmTa7O19 which we propose to have a gapless QSL ground state by employing powder x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS), DC and AC-magnetic susceptibility, 𝑀 versus 𝐻 isotherm, specific heat, and muon spin rotation/relaxation measurements (µSR) down to 30 mK. The combined structural and electronic studies reveal the formation of an edge-sharing equilateral triangular lattice of Sm3+ ions in 𝑎𝑏 plane. The DC and AC magnetic susceptibility, and heat capacity measurements reveal that SmTa7O19 does not exhibit any long-range magnetic ordering transition down to 50 mK. The zero-field (ZF)-µSR study strongly refutes the long-range magnetically ordered ground state and/or any partial spin-freezing down to at least 30 mK. The ZF-muon-spin relaxation rate is weakly temperature-dependent between 50 and 20 K, rapidly increases below ∼20K and saturates at low temperatures between 2 K and 30 mK, which has been attributed to a characteristic signature of QSL systems. Further, our longitudinal-field (LF)-µSR measurements at 0.1 K reveal a dynamic nature of the magnetic ground state. In addition, our high-field specific heat data suggest a gapless nature of spin excitations in this compound.
|
Mar 2025
|
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B18-Core EXAFS
|
A.
Bandyopadhyay
,
S.
Lee
,
D. T.
Adroja
,
M. R.
Lees
,
G. B. G.
Stenning
,
P.
Aich
,
L.
Tortora
,
C.
Meneghini
,
G.
Cibin
,
A.
Berlie
,
R. A.
Saha
,
D.
Takegami
,
A.
Meléndez-Sans
,
G.
Poelchen
,
M.
Yoshimura
,
K. D.
Tsuei
,
Z.
Hu
,
T.-S.
Chan
,
S.
Chattopadhyay
,
G. S.
Thakur
,
K.-Y.
Choi
Diamond Proposal Number(s):
[33369]
Open Access
Abstract: We present an experimental investigation of the magnetic ground state in Ba4NbIr3O12, a fractional valent trimer iridate. X-ray absorption and photoemission spectroscopy show that the Ir valence lies between 3+ and 4+ while Nb is pentavalent. Combined dc/ac magnetization, specific heat, and muon spin rotation/relaxation (µSR) measurements reveal no magnetic phase transition down to 0.05 K. Despite a significant Weiss temperature (ΘW∼−15 to −25 K) indicating antiferromagnetic correlations, a quantum spin-liquid (QSL) phase emerges and persists down to 0.1 K. This state likely arises from geometric frustration in the edge-sharing equilateral triangle Ir network. Our µSR analysis reveals a two-component depolarization, arising from the coexistence of rapidly (90%) and slowly (10%) fluctuating Ir moments. Powder x-ray diffraction and Ir-L3edge x-ray absorption fine structure spectroscopy identify 8–10% Nb/Ir site-exchange, reducing frustration within part of the Ir network, and likely leading to the faster muon spin relaxation, while the structurally ordered Ir ions remain highly geometrically frustrated, giving rise to the rapidly spin-fluctuating QSL ground state. At low temperatures, the magnetic specific heat varies as 𝛾𝑇+𝛼𝑇2, indicating gapless spinon excitations, and possible Dirac QSL features with linear spinon dispersion, respectively.
|
Jul 2024
|
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B18-Core EXAFS
|
Diamond Proposal Number(s):
[17752]
Open Access
Abstract: We investigate the magnetoelectric coupling in
CaMn
7
O
12
(CMO) through a comprehensive spectroscopic analysis combining inelastic neutron scattering (INS), x-ray absorption spectroscopy (XAS), and synchrotron-based powder x-ray diffraction (PXRD). CMO's intricate interplay between magnetism and ferroelectricity is dissected to uncover its underlying mechanisms. XAS reveals a mixed valency of Mn ions in CMO, reflecting the presence of
Mn
3
+
and
Mn
4
+
ions, which contributes to its magnetoelectric properties. The double structure in Mn-
K
near-edge absorption spectra reinforces multiple Mn sites as well as the mixed valency of the compound. Synchrotron-based PXRD experiments conducted over a range of temperatures unveil structural distortions near the magnetic transition temperatures of CMO. These distortions coincide with anomalies in lattice parameters and bond lengths, shedding light on the link between structural modulations and magnetoelectric behavior. Furthermore, INS measurements identify specific energy bands (
E
1
,
E
2
, and
E
3
) associated with distinct exchange interactions between Mn ions. This work advances our understanding of magnetoelectric coupling mechanisms and showcases the potential of multifunctional materials for applications in spintronics and related fields.
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Feb 2024
|
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B18-Core EXAFS
|
Manjil
Das
,
Sayantika
Bhowal
,
Jhuma
Sannigrahi
,
Abhisek
Bandyopadhyay
,
Anupam
Banerjee
,
Giannantonio
Cibin
,
Dmitry
Khalyavin
,
Niladri
Banerjee
,
Devashibhai
Adroja
,
Indra
Dasgupta
,
Subham
Majumdar
Diamond Proposal Number(s):
[17752]
Abstract: We address the concomitant metal-insulator transition (MIT) and antiferromagnetic ordering in the novel pyrochlore iridate
Eu
2
Ir
2
O
7
by combining x-ray absorption spectroscopy, x-ray and neutron diffractions, and density functional theory (DFT)-based calculations. The temperature dependent powder x-ray diffraction clearly rules out any change in the lattice symmetry below the MIT, nevertheless a clear anomaly in the Ir-O-Ir bond angle and Ir-O bond length is evident at the onset of MIT. From the x-ray absorption near edge structure (XANES) spectroscopic study of Ir-
L
3
and
L
2
edges, the effective spin-orbit coupling is found to be intermediate, at least quite far from the strong atomic spin-orbit coupling limit. Powder neutron diffraction measurement is in line with an all-in-all-out magnetic structure of the Ir-tetrahedra in this compound, which is quite common among rare-earth pyrochlore iridates. The sharp change in the Ir-O-Ir bond angle around the MIT possibly arises from the exchange striction mechanism, which favors an enhanced electron correlation via weakening of Ir-Ir orbital overlap and an insulating phase below
T
M
I
. The theoretical calculations indicate an insulating state for shorter bond angle validating the experimental observation. Our DFT calculations show a possibility of intriguing topological phase below a critical value of the Ir-O distance, which is shorter than the experimentally observed bond length. Therefore, a topological state may be realized in bulk
Eu
2
Ir
2
O
7
sample if the Ir-O bond length can be reduced by the application of sufficient external pressure.
|
Apr 2022
|
|
B18-Core EXAFS
|
Rajesh
Tripathi
,
D. T.
Adroja
,
M. R.
Lees
,
A.
Sundaresan
,
S.
Langridge
,
A.
Bhattacharyya
,
V. K.
Anand
,
D. D.
Khalyavin
,
J.
Sannigrahi
,
G.
Cibin
,
A. D.
Hillier
,
R. I.
Smith
,
H. C.
Walker
,
Y.
Muro
,
T.
Takabatake
Diamond Proposal Number(s):
[17953]
Abstract: We report a systematic study of the
5
d
-electron-doped system
Ce
(
Fe
1
−
x
Ir
x
)
2
Al
10
(
0
≤
x
≤
0.15
)
. With increasing
x
, the orthorhombic
b
axis decreases slightly while accompanying changes in
a
and
c
leave the unit cell volume almost unchanged. Inelastic neutron scattering, along with thermal and transport measurements, reveal that for the Kondo semiconductor
CeFe
2
Al
10
, the low-temperature energy gap, which is proposed to be a consequence of strong
c
−
f
hybridization, is suppressed by a small amount of Ir substitution for Fe and that the system adopts a metallic ground state with an increase in the density of states at the Fermi level. The charge or transport gap collapses (at
x
=
0.04
) faster than the spin gap with Ir substitution. Magnetic susceptibility, heat capacity, and muon spin relaxation measurements demonstrate that the system undergoes long-range antiferromagnetic order below a Néel temperature
T
N
of 3.1(2) K for
x
=
0.15
. The ordered moment is estimated to be smaller than 0.07(1)
μ
B
/Ce, although the trivalent state of Ce is confirmed by Ce
L
3
-edge x-ray absorption near edge spectroscopy. It is suggested that the
c
−
f
hybridization gap, which plays an important role in the unusually high ordering temperatures observed in
Ce
T
2
Al
10
(
T
= Ru and Os), may not be necessary for the onset of magnetic order with a low
T
N
seen here in
Ce
(
Fe
1
−
x
Ir
x
)
2
Al
10
.
|
Oct 2021
|
|
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
|
|
I05-ARPES
|
Matthew D.
Watson
,
Yu
Feng
,
Christopher
Nicholson
,
Claude
Monney
,
Jonathon
Riley
,
Hideaki
Iwasawa
,
Keith
Refson
,
Vincent
Sacksteder
,
Devashibhai
Adroja
,
Jun
Zhao
,
Moritz
Hoesch
Diamond Proposal Number(s):
[13797, 14572]
Open Access
Abstract: We present angle-resolved photoemission spectroscopy measurements of the quasi-one-dimensional superconductor K2Cr3As3. We find that the Fermi surface contains two Fermi surface sheets, with linearly dispersing bands not displaying any significant band renormalizations. The one-dimensional band dispersions display a suppression of spectral intensity approaching the Fermi level according to a linear power law, over an energy range of ∼200 meV. This is interpreted as a signature of Tomonoga-Luttinger liquid physics, which provides a new perspective on the possibly unconventional superconductivity in this family of compounds.
|
Mar 2017
|
|
I11-High Resolution Powder Diffraction
|
H.
Sim
,
S.
Lee
,
K.-P.
Hong
,
J.
Jeong
,
J. R.
Zhang
,
T.
Kamiyama
,
D. T.
Adroja
,
C. A.
Murray
,
S. P.
Thompson
,
F.
Iga
,
S.
Ji
,
D.
Khomskii
,
J.-G.
Park
Diamond Proposal Number(s):
[12470]
Abstract: DyB4 has a two-dimensional Shastry-Sutherland (Sh-S) lattice with strong Ising character of the Dy ions. Despite the intrinsic frustrations, it undergoes two successive transitions: a magnetic ordering at TN=20K and a quadrupole ordering at TQ=12.5K. From high-resolution neutron and synchrotron x-ray powder diffraction studies, we have obtained full structural information on this material in all phases and demonstrate that structural modifications occurring at quadrupolar transition lead to the lifting of frustrations inherent in the Sh-S model. Our paper thus provides a complete experimental picture of how the intrinsic frustration of the Sh-S lattice can be lifted by the coupling to quadrupole moments. We show that two other factors, i.e., strong spin-orbit coupling and long-range Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in metallic DyB4, play an important role in this behavior.
|
Nov 2016
|
|
B18-Core EXAFS
|
D. T.
Adroja
,
A. D.
Hillier
,
Y.
Muro
,
J.
Kajino
,
T.
Takabatake
,
P.
Peratheepan
,
A. M.
Strydom
,
P. P.
Deen
,
F.
Demmel
,
J. R.
Stewart
,
J. W.
Taylor
,
R. I.
Smith
,
S.
Ramos
,
M. A.
Adams
Abstract: We have carried out muon spin relaxation (μSR), neutron diffraction, and inelastic neutron scattering (INS) investigations on polycrystalline samples of Ce(Ru1−xFex)2Al10 (x=0, 0.3, 0.5, 0.8, and 1) to investigate the nature of the ground state (magnetic ordered versus paramagnetic) and the origin of the spin-gap formation as evident from the bulk measurements in the end members. Our zero-field μSR spectra clearly reveal coherent two-frequency oscillations at low temperature in x=0, 0.3, and 0.5 samples, which confirm the long-range magnetic ordering of the Ce moment with Nèel temperature TN=27, 26, and 21 K, respectively. On the other hand, the μSR spectra of x=0.8 and x=1 down to 1.4 K and 0.045 K, respectively, exhibit a temperature-independent Kubo-Toyabe term, confirming a paramagnetic ground state. The long-range magnetic ordering in x=0.5 below 21 K has been confirmed through the neutron diffraction study. INS measurements of x=0 clearly reveal the presence of a sharp inelastic excitation near 8 meV between 5 K and 26 K, due to an opening of a gap in the spin excitation spectrum, which transforms into a broad response at and above 30 K. Interestingly, at 4.5 K, the spin-gap excitation broadens in x=0.3 and exhibits two clear peaks at 8.4(3) and 12.0(5) meV in x=0.5. In the x=0.8 sample, which remains paramagnetic down to 1.2 K, there is a clear signature of a spin gap of 10–12 meV at 7 K, with a strong wave-vector–dependent intensity. Evidence of a spin gap of 12.5(5) meV has also been found in x=1. The observation of a spin gap in the paramagnetic samples (x=0.8 and 1) is an interesting finding in this study, and it challenges our understanding of the origin of the semiconducting gap in CeT2Al10 (T = Ru and Os) compounds in terms of a hybridization gap opening only a small part of the Fermi surface, gapped spin waves, or a spin-dimer gap.
|
Jun 2013
|
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