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.
|
May 2021
|
|
|
|
Vasily S.
Stolyarov
,
Kirill S.
Pervakov
,
Anna S.
Astrakhantseva
,
Igor A.
Golovchanskiy
,
Denis V.
Vyalikh
,
Timur K.
Kim
,
Sergey V.
Eremeev
,
Vladimir A.
Vlasenko
,
Vladimir M.
Pudalov
,
Alexander A.
Golubov
,
Eugene V.
Chulkov
,
Dimitri
Roditchev
Abstract: In pnictide RbEuFe4As4, superconductivity sets in at 36 K and coexists, below 15–19 K, with the long-range magnetic ordering of Eu 4f spins. Here we report scanning tunneling experiments performed on cold-cleaved single crystals of the compound. The data revealed the coexistence of large Rb-terminated and small Eu-terminated terraces, both manifesting 1 × 2 and 2‾√×2‾√
2
×
2
reconstructions. On 2‾√×2‾√
2
×
2
surfaces, a hidden electronic order with a period ∼5 nm was discovered. A superconducting gap of ∼7 meV was seen to be strongly filled with quasiparticle states. The tunneling spectra compared with density functional theory calculations confirmed that flat electronic bands due to Eu 4f orbitals are situated ∼1.8 eV below the Fermi level and thus do not contribute directly to Cooper pair formation.
|
Oct 2020
|
|
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.
|
Jun 2020
|
|
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.
|
Jun 2020
|
|
I05-ARPES
|
Erik
Haubold
,
Alexander
Fedorov
,
Florian
Pielnhofer
,
Igor P.
Rusinov
,
Tatiana V.
Menshchikova
,
Viola
Duppel
,
Daniel
Friedrich
,
Richard
Weihrich
,
Arno
Pfitzner
,
Alexander
Zeugner
,
Anna
Isaeva
,
Setti
Thirupathaiah
,
Yevhen
Kushnirenko
,
Emile
Rienks
,
Timur
Kim
,
Evgueni V.
Chulkov
,
Bernd
Büchner
,
Sergey
Borisenko
Diamond Proposal Number(s):
[18586]
Open Access
Abstract: We report experimental and theoretical evidence that GaGeTe is a basic Z2 topological semimetal with three types of charge carriers: bulk-originated electrons and holes as well as surface state electrons. This electronic situation is qualitatively similar to the classic 3D topological insulator Bi2Se3, but important differences account for an unprecedented transport scenario in GaGeTe. High-resolution angle-resolved photoemission spectroscopy combined with advanced band structure calculations show a small indirect energy gap caused by a peculiar band inversion at the T-point of the Brillouin zone in GaGeTe. An energy overlap of the valence and conduction bands brings both electron and holelike carriers to the Fermi level, while the momentum gap between the corresponding dispersions remains finite. We argue that peculiarities of the electronic spectrum of GaGeTe have a fundamental importance for the physics of topological matter and may boost the material’s application potential.
|
Dec 2019
|
|
I05-ARPES
|
M. M.
Otrokov
,
I. I.
Klimovskikh
,
H.
Bentmann
,
D.
Estyunin
,
A.
Zeugner
,
Z. S.
Aliev
,
S.
Gaß
,
A. U. B.
Wolter
,
A. V.
Koroleva
,
A. M.
Shikin
,
M.
Blanco-Rey
,
M.
Hoffmann
,
I. P.
Rusinov
,
A. Yu.
Vyazovskaya
,
S. V.
Eremeev
,
Yu. M.
Koroteev
,
V. M.
Kuznetsov
,
F.
Freyse
,
J.
Sánchez-Barriga
,
I. R.
Amiraslanov
,
M. B.
Babanly
,
N. T.
Mamedov
,
N. A.
Abdullayev
,
V. N.
Zverev
,
A.
Alfonsov
,
V.
Kataev
,
B.
Büchner
,
E. F.
Schwier
,
S.
Kumar
,
A.
Kimura
,
L.
Petaccia
,
G.
Di Santo
,
R. C.
Vidal
,
S.
Schatz
,
K.
Kißner
,
M.
Unzelmann
,
C. H.
Min
,
Simon
Moser
,
T. R. F.
Peixoto
,
F.
Reinert
,
A.
Ernst
,
P. M.
Echenique
,
A.
Isaeva
,
E. V.
Chulkov
Abstract: Magnetic topological insulators are narrow-gap semiconductor materials that combine non-trivial band topology and magnetic order. Unlike their nonmagnetic counterparts, magnetic topological insulators may have some of the surfaces gapped, which enables a number of exotic phenomena that have potential applications in spintronics, such as the quantum anomalous Hall effect and chiral Majorana fermions. So far, magnetic topological insulators have only been created by means of doping nonmagnetic topological insulators with 3d transition-metal elements; however, such an approach leads to strongly inhomogeneous magnetic and electronic properties of these materials, restricting the observation of important effects to very low temperatures. An intrinsic magnetic topological insulator—a stoichiometric well ordered magnetic compound—could be an ideal solution to these problems, but no such material has been observed so far. Here we predict by ab initio calculations and further confirm using various experimental techniques the realization of an antiferromagnetic topological insulator in the layered van der Waals compound MnBi2Te4. The antiferromagnetic ordering that MnBi2Te4 shows makes it invariant with respect to the combination of the time-reversal and primitive-lattice translation symmetries, giving rise to a ℤ2 topological classification; ℤ2 = 1 for MnBi2Te4, confirming its topologically nontrivial nature. Our experiments indicate that the symmetry-breaking (0001) surface of MnBi2Te4 exhibits a large bandgap in the topological surface state. We expect this property to eventually enable the observation of a number of fundamental phenomena, among them quantized magnetoelectric coupling and axion electrodynamics. Other exotic phenomena could become accessible at much higher temperatures than those reached so far, such as the quantum anomalous Hall effect and chiral Majorana fermions.
|
Dec 2019
|
|
I05-ARPES
|
R. C.
Vidal
,
H.
Bentmann
,
T. R. F.
Peixoto
,
A.
Zeugner
,
S.
Moser
,
C.-H.
Min
,
S.
Schatz
,
K.
Kissner
,
M.
Unzelmann
,
C. I.
Fornari
,
H. B.
Vasili
,
M.
Valvidares
,
K.
Sakamoto
,
D.
Mondal
,
J.
Fujii
,
I.
Vobornik
,
S.
Jung
,
C.
Cacho
,
T. K.
Kim
,
R. J.
Koch
,
C.
Jozwiak
,
A.
Bostwick
,
J. D.
Denlinger
,
E.
Rotenberg
,
J.
Buck
,
M.
Hoesch
,
F.
Diekmann
,
S.
Rohlf
,
M.
Kalläne
,
K.
Rossnagel
,
M. M.
Otrokov
,
E. V.
Chulkov
,
M.
Ruck
,
A.
Isaeva
,
F.
Reinert
Diamond Proposal Number(s):
[19278, 22468]
Abstract: The layered van der Waals antiferromagnet
MnBi
2
Te
4
has been predicted to combine the band ordering of archetypical topological insulators such as
Bi
2
Te
3
with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topological states. We have systematically investigated the surface electronic structure of
MnBi
2
Te
4
(0001) single crystals by use of spin- and angle-resolved photoelectron spectroscopy experiments. In line with theoretical predictions, the results reveal a surface state in the bulk band gap and they provide evidence for the influence of exchange interaction and spin-orbit coupling on the surface electronic structure.
|
Sep 2019
|
|
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.
|
Jun 2019
|
|
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.
|
Sep 2018
|
|
I05-ARPES
|
Alexander
Generalov
,
Mikhail M.
Otrokov
,
Alla
Chikina
,
Kristin
Kliemt
,
Kurt
Kummer
,
Marc
Höppner
,
Monika
Guttler
,
Silvia
Seiro
,
Alexander
Fedorov
,
Susanne
Schulz
,
Steffen
Danzenbächer
,
Evgueni V.
Chulkov
,
Christoph
Geibel
,
Clemens
Laubschat
,
Pavel
Dudin
,
Moritz
Hoesch
,
Timur
Kim
,
Milan
Radovic
,
Ming
Shi
,
Nicholas C.
Plumb
,
Cornelius
Krellner
,
Denis V.
Vyalikh
Diamond Proposal Number(s):
[11512]
Abstract: Finding ways to create and control the spin-dependent properties of two-dimensional electron states (2DESs) is a major challenge for the elaboration of novel spin-based devices. Spin–orbit and exchange–magnetic interactions (SOI and EMI) are two fundamental mechanisms that enable access to the tunability of spin-dependent properties of carriers. The silicon surface of HoRh2Si2 appears to be a unique model system, where concurrent SOI and EMI can be visualized and controlled by varying the temperature. The beauty and simplicity of this system lie in the 4f moments, which act as a multiple tuning instrument on the 2DESs, as the 4f projections parallel and perpendicular to the surface order at essentially different temperatures. Here we show that the SOI locks the spins of the 2DESs exclusively in the surface plane when the 4f moments are disordered: the Rashba-Bychkov effect. When the temperature is gradually lowered and the system experiences magnetic order, the rising EMI progressively competes with the SOI leading to a fundamental change in the spin-dependent properties of the 2DESs. The spins rotate and reorient toward the out-of-plane Ho 4f moments. Our findings show that the direction of the spins and the spin-splitting of the two-dimensional electrons at the surface can be manipulated in a controlled way by using only one parameter: the temperature.
|
Jan 2017
|
|
