Theoretical Physics
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Open Access
pending processing
Abstract: Polarization-dependent photon spectroscopy (dichroism) of the second-harmonic generation response is shown to reveal chiral and magnetic properties of a sample. Two dichroic signals are allowed with electric-dipole and electric-quadrupole scattering events, and both require circular polarization in the primary beam. Explicit expressions for the signals are derived using theoretical techniques from atomic physics.
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Dec 2019
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Abstract: The compression of liquid nickel and cobalt has been investigated up to 102 and 83 GPa, respectively, by x-ray absorption spectroscopy at the Ni and Co
K
edges, using laser-heated diamond anvil cell. A shortening of about 8%–10% of the metal bond distances is observed in the pressure range considered and the experimental results are confirmed by theoretical calculations. Ab initio quantum molecular dynamics calculations have been performed, providing a model pair distribution function for cobalt, used as a starting model in the extended x-ray absorption fine structure analysis. The specific volume of the liquid has also been estimated and compared to calculations in the literature. This work sets the basis for x-ray absorption spectroscopy studies of liquid metals under extreme conditions with potentially important implications for the understanding of Earth and planetary interiors.
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Nov 2019
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I05-ARPES
I11-High Resolution Powder Diffraction
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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.
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Nov 2019
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I15-1-X-ray Pair Distribution Function (XPDF)
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Diamond Proposal Number(s):
[17685]
Abstract: The structural dynamics of a quasi-one-dimensional (1D) mixed-metal cyanide,
C
u
1
/
3
A
g
1
/
3
A
u
1
/
3
CN
, with intriguing thermal properties is explored. All the current known related compounds with straight-chain structures, such as group 11 cyanides CuCN, AgCN, AuCN, and bimetallic cyanides
M
x
M
′
1
−
x
CN
(
M
,
M
′
=
Cu
, Ag, Au), exhibit 1D negative thermal expansion (NTE) along the chains and positive thermal expansion (PTE) perpendicular to them.
C
u
1
/
3
A
g
1
/
3
A
u
1
/
3
CN
exhibits similar PTE perpendicular to the chains, however PTE, rather than NTE, is also observed along the chains. In order to understand the origin of this unexpected behavior, inelastic neutron scattering measurements were carried out, underpinned by lattice-dynamical density-functional-theory (DFT) calculations. Synchrotron-based pair-distribution-function analysis and
13
C
solid-state nuclear-magnetic-resonance measurements were also performed to build an input structural model for the lattice dynamical study. The results indicate that transverse motions of the metal ions are responsible for the PTE perpendicular to the chains, as is the case for the related group 11 cyanides. However, NTE along the chain due to the tension effect of these transverse motions is not observed. As there are different metal-to-cyanide bond lengths in
C
u
1
/
3
A
g
1
/
3
A
u
1
/
3
CN
, the metals in neighboring chains cannot all be truly coplanar in a straight-chain model. For this system, DFT-based phonon calculations predict small PTE along the chain due to low-energy chain-slipping modes induced by a bond-rotation effect on the weak metallophilic bonds. However the observed PTE is greater than that predicted with the straight-chain model. Small bends in the chain provide an alternative explanation for thermal behavior. These would mitigate the tension effect induced by the transverse motions of the metals and, as temperature increases and the chains move further apart, a straightening could occur resulting in the observed PTE. This hypothesis is further supported by unusual evolution in the phonon spectra, which suggest small changes in local symmetry with temperature.
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Nov 2019
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I10-Beamline for Advanced Dichroism
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Diamond Proposal Number(s):
[20437]
Abstract: A detailed magnetic phase diagram of FeGe membranes with fields applied out-of-plane is determined using small-angle soft x-ray elastic scattering. In addition to the well-established skyrmion and helical phase, we identify an additional ordered phase which partly coexists with the helical phase in a pocket beneath the skyrmion phase. Furthermore, an evolution of the modulation wave vector,
q
, is observed when traversing the magnetic phase pocket. For the ordinary and the additional helical phase,
q
only varies with normalized field but not with temperature—in sharp contrast to the skyrmion phase, for which
q
is both field and temperature dependent. This study of the helical phases in thin FeGe membranes lays the foundation for their use in future magnetic storage and logic devices.
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Nov 2019
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I05-ARPES
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Xiaoqing
Zhou
,
Kyle N.
Gordon
,
Kyung-hwan
Jin
,
Haoxiang
Li
,
Dushyant
Narayan
,
Hengdi
Zhao
,
Hao
Zheng
,
Huaqing
Huang
,
Gang
Cao
,
Nikolai D.
Zhigadlo
,
Feng
Liu
,
Daniel
Dessau
Diamond Proposal Number(s):
[17595]
Abstract: Most topological superconductors known to date suffer from low transition temperatures (
T
c
) and/or high fragility to disorder and dopant levels, which is hampering the progress in this promising field. Here, utilizing a combination of angle-resolved photoemission spectroscopy measurements and density-functional theory calculations, we show the presence of a type of topological Dirac nodal line surface state on the [010] faces of the
T
c
=
39
K BCS superconductor
MgB
2
. This surface state should be highly tolerant against disorder and inadvertent doping variations and is expected to go superconducting via the proximity effect to the bulk superconductor that this state is intimately connected to. This would represent a form of high-temperature topological superconductivity.
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Nov 2019
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I07-Surface & interface diffraction
I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[15205, 15334]
Abstract: n this work an alternate pathway is demonstrated to form ultrathin cobalt ferrite (
Co
x
Fe
3
−
x
O
4
) films by interdiffusion of
Fe
3
O
4
/CoO bilayers. Bilayer samples with different
Fe
3
O
4
/CoO thickness ratios have been prepared by reactive molecular beam epitaxy on Nb-doped
SrTiO
3
(001) substrates to obtain cobalt ferrite films of varied stoichiometry. Subsequently, oxygen-assisted postdeposition annealing experiments for consecutive temperature steps between
300
∘
C
and
600
∘
C
have been conducted monitoring the interdiffusion process by means of high-resolution x-ray reflectivity, soft and angle-resolved hard x-ray photoelectron, and x-ray absorption spectroscopy. Magnetic properties were characterized using superconducting quantum interference device magnetometry. The interdiffusion process starts from
300
∘
C
annealing temperature and is completed for temperatures above
500
∘
C
. For completely interdiffused films with Co:Fe ratios larger than 0.84:2 a thin segregated CoO layer on top of the ferrite is formed. This CoO segregation is attributed to surface and interface effects. In addition, multiplet calculations of x-ray absorption spectra are performed to determine the occupancy of different sublattices. These results are correlated with the magnetic properties of the ferrite films. A stoichiometric
CoFe
2
O
4
film with partial inversion has been formed exhibiting homogeneously distributed
Co
2
+
and mainly
Fe
3
+
valence states if the initial Co:Fe content is 1.09:2. Thus, for the formation of stoichiometric cobalt ferrite by the proposed postdeposition annealing technique an initial Co excess has to be provided as the formation of a top CoO layer is inevitable.
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Oct 2019
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I16-Materials and Magnetism
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Diamond Proposal Number(s):
[20776]
Abstract: We examine the magnetic ordering of UN and of a closely related nitride,
U
2
N
3
, by preparing thin epitaxial films and using synchrotron x-ray techniques. The magnetic configuration and subsequent coupling to the lattice are key features of the electronic structure. The well-known antiferromagnetic (AF) ordering of UN is confirmed, but the expected accompanying distortion at
T
N
is not observed. Instead, we propose that the strong magnetoelastic interaction below
T
N
causes substantial changes in the strain in the sample being measured. These strains vary as a function of the sample form. As a consequence, the accepted AF configuration of UN may be incorrect. In the case of cubic
α
−
U
2
N
3
, no single crystals have been previously prepared, and we have determined the AF ordering wave vector. The AF
T
N
is close to that previously reported. In addition, resonant diffraction methods have identified an aspherical quadrupolar charge contribution in
U
2
N
3
involving the
5
f
electrons.
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Oct 2019
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I09-Surface and Interface Structural Analysis
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X. C.
Huang
,
J. Y.
Zhang
,
M.
Wu
,
S.
Zhang
,
H. Y.
Xiao
,
W. Q.
Han
,
T.-l.
Lee
,
A.
Tadich
,
D.-c.
Qi
,
L.
Qiao
,
L.
Chen
,
K. H. L.
Zhang
Diamond Proposal Number(s):
[21432]
Abstract: This work reports a fundamental study on the electronic structure, optical properties, and defect chemistry of a series of Co-based spinel oxide (
Co
3
O
4
,
ZnCo
2
O
4
, and
CoAl
2
O
4
) epitaxial thin films using x-ray photoemission and absorption spectroscopies, optical spectroscopy, transport measurements, and density functional theory. We demonstrate that
ZnCo
2
O
4
has a fundamental bandgap of 1.3 eV, much smaller than the generally accepted values, which range from 2.26 to 2.8 eV. The valence band edge mainly consists of occupied
Co
3
d
t
6
2
g
with some hybridization with O
2
p
/Zn
3
d
, and the conduction band edge of unoccupied
e
∗
g
state. However, optical transition between the two band edges is dipole forbidden. Strong absorption occurs at photon energies above 2.6 eV, explaining the reasonable transparency of
ZnCo
2
O
4
. A detailed defect chemistry study indicates that Zn vacancies formed at high oxygen pressure are the origin of a high
p
-type conductivity of
ZnCo
2
O
4
, and the hole conduction mechanism is described by small-polaron hoping model. The high
p
-type conductivity, reasonable transparency, and large work function make
ZnCo
2
O
4
a desirable
p
-type transparent semiconductor for various optoelectronic applications. Using the same method, the bandgap of
Co
3
O
4
is further proved to be ∼0.8 eV arising from the tetrahedrally coordinated
Co
2
+
cations. Our work advances the fundamental understanding of these materials and provides significant guidance for their use in catalysis, electronic, and solar applications.
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Sep 2019
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[13284, 18786]
Abstract: We have performed transverse- and zero-field muon spin rotation/relaxation experiments, as well as magnetometry measurements, on samples of
Fe
1
−
x
V
x
Se
and their
Li
+
NH
3
intercalates
Li
0.6
(
NH
2
)
0.2
(
NH
3
)
0.8
Fe
1
−
x
V
x
Se
. We examine the low vanadium substitution regime:
x
=
0.005
, 0.01, and 0.02. The intercalation reaction significantly increases the critical temperature
(
T
c
)
and the superfluid stiffness for all
x
. The nonintercalated samples all exhibit
T
c
≈
8.5
K while the intercalated samples all show an enhanced
T
c
>
40
K. Vanadium substitution has a negligible effect on
T
c
, but seems to suppress the superfluid stiffness for the nonintercalated samples and weakly enhance it for the intercalated materials. The optimal substitution level for the intercalated samples is found to be
x
=
0.01
, with
T
c
≈
41
K
and
λ
a
b
(
0
)
≈
0.18
μ
m
. The nonintercalated samples can be modeled with either a single
d
-wave superconducting gap or with an anisotropic gap function based on recent quasiparticle imaging experiments, whereas the intercalates display multigap nodal behavior which can be fitted using
s
+
d
- or
d
+
d
-wave models. Magnetism, likely from iron impurities, appears after the intercalation reaction and coexists and competes with the superconductivity. However, it appears that the superconductivity is remarkably robust to the impurity phase, providing an avenue to stably improve the superconducting properties of transition metal substituted FeSe.
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Sep 2019
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