I05-ARPES
|
Georg
Poelchen
,
Igor P.
Rusinov
,
Susanne
Schulz
,
Monika
Guttler
,
Max
Mende
,
Alexander
Generalov
,
Dmitry Yu.
Usachov
,
Steffen
Danzenbacher
,
Johannes
Hellwig
,
Marius
Peters
,
Kristin
Kliemt
,
Yuri
Kucherenko
,
Victor N.
Antonov
,
Clemens
Laubschat
,
Evgueni V.
Chulkov
,
Arthur
Ernst
,
Kurt
Kummer
,
Cornelius
Krellner
,
Denis V.
Vyalikh
Diamond Proposal Number(s):
[24339]
Abstract: The f-driven temperature scales at the surfaces of strongly correlated materials have increasingly come into the focus of research efforts. Here, we unveil the emergence of a two-dimensional Ce Kondo lattice, which couples ferromagnetically to the ordered Co lattice below the P-terminated surface of the antiferromagnet CeCo2P2. In its bulk, Ce is passive and behaves tetravalently. However, because of symmetry breaking and an effective magnetic field caused by an uncompensated ferromagnetic Co layer, the Ce 4f states become partially occupied and spin-polarized near the surface. The momentum-resolved photoemission measurements indicate a strong admixture of the Ce 4f states to the itinerant bands near the Fermi level including surface states that are split by exchange interaction with Co. The temperature-dependent measurements reveal strong changes of the 4f intensity at the Fermi level in accordance with the Kondo scenario. Our findings show how rich and diverse the f-driven properties can be at the surface of materials without f-physics in the bulk.
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Feb 2022
|
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Open Access
Abstract: Hall effect and quantum oscillation measurements on high temperature cuprate superconductors show that underdoped compositions have small Fermi surface pockets whereas when heavily overdoped, a single much larger pocket is found. The origin of this change in electronic structure has been unclear, but may be related to the high temperature superconductivity. Here we show that the clean overdoped single-layer cuprate Tl2Ba2CuO6+δ (Tl2201) displays CDW order with a remarkably long correlation length ξ ≈ 200 Å which disappears above a hole doping of pCDW ≈ 0.265. We show that the evolution of the electronic properties of Tl2201 as the doping is lowered may be explained by a Fermi surface reconstruction which accompanies the emergence of the CDW below pCDW. Our results demonstrate importance of CDW correlations in understanding the electronic properties of overdoped cuprates.
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Jan 2022
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I05-ARPES
|
T. K.
Kim
,
K. S.
Pervakov
,
D. V.
Evtushinsky
,
S. W.
Jung
,
G.
Poelchen
,
K.
Kummer
,
V. A.
Vlasenko
,
A. V.
Sadakov
,
A. S.
Usoltsev
,
V. M.
Pudalov
,
D.
Roditchev
,
V. S.
Stolyarov
,
D. V.
Vyalikh
,
V.
Borisov
,
R.
Valentí
,
A.
Ernst
,
S. V.
Eremeev
,
E. V.
Chulkov
Diamond Proposal Number(s):
[19041, 22192]
Open Access
Abstract: In the novel stoichiometric iron-based material
RbEuFe
4
As
4
, superconductivity coexists with a peculiar long-range magnetic order of Eu 4f states. Using angle-resolved photoemission spectroscopy, we reveal a complex three-dimensional electronic structure and compare it with density functional theory calculations. Multiple superconducting gaps were measured on various sheets of the Fermi surface. High-resolution resonant photoemission spectroscopy reveals magnetic order of the Eu 4f states deep into the superconducting phase. Both the absolute values and the anisotropy of the superconducting gaps are remarkably similar to the sibling compound without Eu, indicating that Eu magnetism does not affect the pairing of electrons. A complete decoupling between Fe- and Eu-derived states was established from their evolution with temperature, thus unambiguously demonstrating that superconducting and a long-range magnetic orders exist independently from each other. The established electronic structure of
RbEuFe
4
As
4
opens opportunities for the future studies of the highly unorthodox electron pairing and phase competition in this family of iron-based superconductors with doping.
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May 2021
|
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I05-ARPES
|
Georg
Poelchen
,
Susanne
Schulz
,
Max
Mende
,
Monika
Guttler
,
Alexander
Generalov
,
Alexander V.
Fedorov
,
Nubia
Caroca-Canales
,
Christoph
Geibel
,
Kristin
Kliemt
,
Cornelius
Krellner
,
Steffen
Danzenbacher
,
Dmitry Y.
Usachov
,
Pavel
Dudin
,
Victor N.
Antonov
,
James W.
Allen
,
Clemens
Laubschat
,
Kurt
Kummer
,
Yuri
Kucherenko
,
Denis V.
Vyalikh
Diamond Proposal Number(s):
[17761, 24339]
Open Access
Abstract: Ultra-violet angle-resolved photoemission spectroscopy (UV-ARPES) was used to explore the temperature dependence of the Ce-4f spectral responses for surface and bulk in the antiferromagnetic Kondo lattice CeRh2Si2. Spectra were taken from Ce- and Si-terminated surfaces in a wide temperature range, and reveal characteristic 4f patterns for weakly (surface) and strongly (bulk) hybridized Ce, respectively. The temperature dependence of the Fermi level peak differs strongly for both cases implying that the effective Kondo temperature at the surface and bulk can be rather distinct. The greatly reduced crystal–electric-field (CEF) splitting at the surface gives reason to believe that the surface may exhibit a larger effective Kondo temperature because of a higher local-moment effective degeneracy. Further, the hybridization processes could strongly affect the 4f peak intensity at the Fermi level. We derived the k-resolved dispersion of the Kondo peak which is also found to be distinct due to different sets of itinerant bands to which the 4f states of surface and bulk Ce are coupled. Overall our study brings into reach the ultimate goal of quantitatively testing many-body theories that link spectroscopy and transport properties, for both the bulk and the surface, separately. It also allows for a direct insight into the broader problem of Kondo lattices with two different local-moment sublattices, providing some understanding of why the cross-talking between the two Kondo effects is weak.
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Oct 2020
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I05-ARPES
|
D. Y.
Usachov
,
M.
Guttler
,
S.
Schulz
,
G.
Poelchen
,
S.
Seiro
,
K.
Kliemt
,
K.
Kummer
,
C.
Krellner
,
C.
Laubschat
,
E. V.
Chulkov
,
D. V.
Vyalikh
Diamond Proposal Number(s):
[18844, 17761]
Abstract: The emergence of ferromagnetism in Rashba systems, where the evolving exchange interaction enters into competition with spin-orbit coupling, leads to a nontrivial spin-polarized electronic landscape with an intricate momentum-dependent spin structure, which is challenging to unveil. Here, we show a way to disentangle the contributions from the effective spin-orbit and exchange fields and thus to gain knowledge of the spin structure in ferromagnetic Rashba materials, which is required for spintronic applications. Our approach is based exclusively on spin-integrated photoemission measurements combined with a two-band modeling. As an example, we consider the mixed-valent material
EuIr
2
Si
2
which, while being nonmagnetic in the bulk, reveals strong ferromagnetism at the iridium-silicide surface where both spin-orbit and exchange magnetic interactions coexist. The combined effect of these interactions causes a complex band dispersion of the surface state which can be observed in photoemission experiments. Our method allows us to comprehensively unravel the surface-state spin structure driven by spin-orbit coupling at the ferromagnetic surface. This approach opens up opportunities to characterize the spin structure of ferromagnetic Rashba materials, especially where dedicated spin-resolved measurements remain challenging.
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Jun 2020
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I05-ARPES
|
D. y.
Usachov
,
I. a.
Nechaev
,
G.
Poelchen
,
M.
Guttler
,
E. e.
Krasovskii
,
S.
Schulz
,
A.
Generalov
,
K.
Kliemt
,
A.
Kraiker
,
C.
Krellner
,
K.
Kummer
,
S.
Danzenbächer
,
C.
Laubschat
,
A. p.
Weber
,
J.
Sánchez-Barriga
,
E. v.
Chulkov
,
A. f.
Santander-Syro
,
T.
Imai
,
K.
Miyamoto
,
T.
Okuda
,
D. V.
Vyalikh
Diamond Proposal Number(s):
[18844, 17761]
Abstract: Spin-orbit interaction and structure inversion asymmetry in combination with magnetic ordering is a promising route to novel materials with highly mobile spin-polarized carriers at the surface. Spin-resolved measurements of the photoemission current from the Si-terminated surface of the antiferromagnet
TbRh
2
Si
2
and their analysis within an ab initio one-step theory unveil an unusual triple winding of the electron spin along the fourfold-symmetric constant energy contours of the surface states. A two-band
k
⋅
p
model is presented that yields the triple winding as a cubic Rashba effect. The curious in-plane spin-momentum locking is remarkably robust and remains intact across a paramagnetic-antiferromagnetic transition in spite of spin-orbit interaction on Rh atoms being considerably weaker than the out-of-plane exchange field due to the Tb
4
f
moments.
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Jun 2020
|
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I21-Resonant Inelastic X-ray Scattering (RIXS)
|
Q.
Wang
,
M.
Horio
,
K.
Von Arx
,
Y.
Shen
,
D.
John Mukkattukavil
,
Y.
Sassa
,
O.
Ivashko
,
C. E.
Matt
,
S.
Pyon
,
T.
Takayama
,
H.
Takagi
,
T.
Kurosawa
,
N.
Momono
,
M.
Oda
,
T.
Adachi
,
S. M.
Haidar
,
Y.
Koike
,
Y.
Tseng
,
W.
Zhang
,
J.
Zhao
,
K.
Kummer
,
M.
Garcia-Fernandez
,
K.
Zhou
,
N. B.
Christensen
,
H. M.
Ronnow
,
T.
Schmitt
,
J.
Chang
Diamond Proposal Number(s):
[20828, 24481]
Abstract: We use resonant inelastic x-ray scattering to investigate charge-stripe correlations in
La
1.675
Eu
0.2
Sr
0.125
CuO
4
. By differentiating elastic from inelastic scattering, it is demonstrated that charge-stripe correlations precede both the structural low-temperature tetragonal phase and the transport-defined pseudogap onset. The scattering peak amplitude from charge stripes decays approximately as
T
−
2
towards our detection limit. The in-plane integrated intensity, however, remains roughly temperature independent. Therefore, although the incommensurability shows a remarkably large increase at high temperature, our results are interpreted via a single scattering constituent. In fact, direct comparison to other stripe-ordered compounds (
La
1.875
Ba
0.125
CuO
4
,
La
1.475
Nd
0.4
Sr
0.125
CuO
4
, and
La
1.875
Sr
0.125
CuO
4
) suggests a roughly constant integrated scattering intensity across all these compounds. Our results therefore provide a unifying picture for the charge-stripe ordering in La-based cuprates. As charge correlations in
La
1.675
Eu
0.2
Sr
0.125
CuO
4
extend beyond the low-temperature tetragonal and pseudogap phase, their emergence heralds a spontaneous symmetry breaking in this compound.
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May 2020
|
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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.
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Dec 2019
|
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I05-ARPES
|
Susanne
Schulz
,
Ilya A.
Nechaev
,
Monika
Güttler
,
Georg
Poelchen
,
Alexander
Generalov
,
Steffen
Danzenbächer
,
Alla
Chikina
,
Silvia
Seiro
,
Kristin
Kliemt
,
Alexandra Yu.
Vyazovskaya
,
Timur K.
Kim
,
Pavel
Dudin
,
Evgueni V.
Chulkov
,
Clemens
Laubschat
,
Eugene E.
Krasovskii
,
Christoph
Geibel
,
Cornelius
Krellner
,
Kurt
Kummer
,
Denis V.
Vyalikh
Diamond Proposal Number(s):
[18844, 17761]
Open Access
Abstract: The development of materials that are non-magnetic in the bulk but exhibit two-dimensional (2D) magnetism at the surface is at the core of spintronics applications. Here, we present the valence-fluctuating material EuIr2Si2, where in contrast to its non-magnetic bulk, the Si-terminated surface reveals controllable 2D ferromagnetism. Close to the surface the Eu ions prefer a magnetic divalent configuration and their large 4f moments order below 48 K. The emerging exchange interaction modifies the spin polarization of the 2D surface electrons originally induced by the strong Rashba effect. The temperature-dependent mixed valence of the bulk allows to tune the energy and momentum size of the projected band gaps to which the 2D electrons are confined. This gives an additional degree of freedom to handle spin-polarized electrons at the surface. Our findings disclose valence-fluctuating rare-earth based materials as a very promising basis for the development of systems with controllable 2D magnetic properties which is of interest both for fundamental science and applications.
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Jun 2019
|
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I05-ARPES
|
A.
Generalov
,
J.
Falke
,
I. A.
Nechaev
,
M. M.
Otrokov
,
M.
Guttler
,
A.
Chikina
,
K.
Kliemt
,
S.
Seiro
,
K.
Kummer
,
S.
Danzenbächer
,
D.
Usachov
,
T. K.
Kim
,
P.
Dudin
,
E. V.
Chulkov
,
C.
Laubschat
,
C.
Geibel
,
C.
Krellner
,
D. V.
Vyalikh
Diamond Proposal Number(s):
[14811]
Abstract: Strong spin-orbit coupling (SOC) in combination with a lack of inversion symmetry and exchange magnetic interaction proves to be a sophisticated instrument allowing efficient control of the spin orientation, energy and trajectories of two-dimensional (2D) electrons and holes trapped at surfaces or interfaces. Exploiting Kondo-related phenomena and crystal-electric-field effects at reduced dimensionalities opens new opportunities to handle their spin-dependent properties offering novel functionalities. We consider here a 2D Kondo lattice represented by a Si-Ir-Si-Yb (SISY) surface block of the heavy-fermion material YbIr2Si2. We show that the Kondo interaction with 4f moments allows finely tuning the group velocities of the strongly spin-polarized carriers in 2D itinerant states of this noncentrosymmetric system. To unveil the peculiarities of this interaction, we used angle-resolved photoemission measurements complemented by first-principles calculations. We established that the strong SOC of the Ir atoms induces spin polarization of the 2D states in SISY block, while the 2D lattice of Yb 4f moments acts as a source for coherent f−d interplay. The strong SOC and lack of inversion symmetry turn out to lead not only to the anticipated Rashba-like splitting of the 2D states, but also to spin splitting of the 4f Kramers doublets. They couple temperature-dependently to the spin-polarized 2D states and thereby guide the properties of the latter.
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Sep 2018
|
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