I05-ARPES
|
R. C.
Vidal
,
H.
Bentmann
,
J. i.
Facio
,
T.
Heider
,
P.
Kagerer
,
C. I.
Fornari
,
T. R. F.
Peixoto
,
T.
Figgemeier
,
S.
Jung
,
Cephise
Cacho
,
B.
Büchner
,
J.
Van Den Brink
,
C. M.
Schneider
,
L.
Plucinski
,
E. F.
Schwier
,
K.
Shimada
,
M.
Richter
,
A.
Isaeva
,
F.
Reinert
Diamond Proposal Number(s):
[22468]
Abstract: Using angle-resolved photoelectron spectroscopy (ARPES), we investigate the surface electronic structure of the magnetic van der Waals compounds
MnBi
4
Te
7
and
MnBi
6
Te
10
, the
n
=
1
and 2 members of a modular
(
Bi
2
Te
3
)
n
(
MnBi
2
Te
4
)
series, which have attracted recent interest as intrinsic magnetic topological insulators. Combining circular dichroic, spin-resolved and photon-energy-dependent ARPES measurements with calculations based on density functional theory, we unveil complex momentum-dependent orbital and spin textures in the surface electronic structure and disentangle topological from trivial surface bands. We find that the Dirac-cone dispersion of the topologial surface state is strongly perturbed by hybridization with valence-band states for
Bi
2
Te
3
-terminated surfaces but remains preserved for
MnBi
2
Te
4
-terminated surfaces. Our results firmly establish the topologically nontrivial nature of these magnetic van der Waals materials and indicate that the possibility of realizing a quantized anomalous Hall conductivity depends on surface termination.
|
Apr 2021
|
|
I10-Beamline for Advanced Dichroism
|
A.
Tcakaev
,
V. B.
Zabolotnyy
,
C. I.
Fornari
,
P.
Rüßmann
,
T. R. F.
Peixoto
,
F.
Stier
,
M.
Dettbarn
,
P.
Kagerer
,
E.
Weschke
,
E.
Schierle
,
P.
Bencok
,
P. H. O.
Rappl
,
E.
Abramof
,
H.
Bentmann
,
E.
Goering
,
F.
Reinert
,
V.
Hinkov
Diamond Proposal Number(s):
[19994]
Abstract: Rare-earth ions typically exhibit larger magnetic moments than transition-metal ions and thus promise the opening of a wider exchange gap in the Dirac surface states of topological insulators. Yet in a recent photoemission study of Eu-doped
Bi
2
Te
3
films, the spectra remained gapless down to
T
=
20
K. Here we scrutinize whether the conditions for a substantial gap formation in this system are present by combining spectroscopic and bulk characterization methods with theoretical calculations. For all studied Eu doping concentrations, our atomic multiplet analysis of the
M
4
,
5
x-ray absorption and magnetic circular dichroism spectra reveals a
Eu
2
+
valence and confirms a large magnetic moment, consistent with a
4
f
7
8
S
7
/
2
ground state. At temperatures below 10 K, bulk magnetometry indicates the onset of antiferromagnetic (AFM) ordering. This is in good agreement with density functional theory, which predicts AFM interactions between the Eu impurities. Our results support the notion that antiferromagnetism can coexist with topological surface states in rare-earth-doped
Bi
2
Te
3
and call for spectroscopic studies in the Kelvin range to look for novel quantum phenomena such as the quantum anomalous Hall effect.
|
Nov 2020
|
|
I09-Surface and Interface Structural Analysis
|
Celso I.
Fornari
,
Hendrik
Bentmann
,
Sergio L.
Morelhao
,
Thiago R. F.
Peixoto
,
Paulo
Rappl
,
Abdul
Tcakaev
,
Volodymyr
Zabolotnyy
,
Martin
Kamp
,
Tien-Lin
Lee
,
Chul-Hee
Min
,
Philipp
Kagerer
,
Raphael
Vidal
,
Anna
Isaeva
,
Michael
Ruck
,
Vladimir
Hinkov
,
Friedrich
Reinert
,
Eduardo
Abramof
Abstract: In the field of topological materials, the interaction between band topology and magnetism remains a current frontier for the advance of new topological states and spintronic functionalities. Doping with rare-earth elements with large magnetic moment is a current approach to exploit the phenomenology of such interaction. However, dopant solubility into the main matrix plays a major role. In this sense, the present work is focused on elucidating how Eu incorporates into Bi2Te3 lattice as a function of doping. This work reports a systematic investigation of the structural and electronic properties of bismuth telluride epitaxial layers doped with Eu. Bi2Te3 films were grown by molecular beam epitaxy (MBE) on (111) BaF2 substrates with nominal Eu doping ranging from 0 % up to 9 %. X-ray diffraction (XRD) analysis and scanning transmission electron microscopy (TEM) reveal that Eu atoms enter substitutionally on Bi sites up to 4 % of Eu doping. In contrast, the 9 % Eu-doped sample contains epitaxially oriented nanoclusters of EuTe. X ray photoelectron (XPS) and absorption (XAS) spectroscopies show that Eu atoms enter into the Bi2Te3 crystal matrix in the divalent Eu2+ state for all Eu concentrations. Angle resolved photoemission (ARPES) experiments indicate that the topological surface state is preserved in the presence of the local magnetic moments introduced by the Eu impurities.
|
Jun 2020
|
|
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
|
Raphael C.
Vidal
,
Alexander
Zeugner
,
Jorge I.
Facio
,
Rajyavardhan
Ray
,
M. Hossein
Haghighi
,
Anja U. b.
Wolter
,
Laura T.
Corredor Bohorquez
,
Federico
Caglieris
,
Simon
Moser
,
Tim
Figgemeier
,
Thiago R. F.
Peixoto
,
Hari Babu
Vasili
,
Manuel
Valvidares
,
Sungwon
Jung
,
Cephise
Cacho
,
Alexey
Alfonsov
,
Kavita
Mehlawat
,
Vladislav
Kataev
,
Christian
Hess
,
Manuel
Richter
,
Bernd
Büchner
,
Jeroen
Van Den Brink
,
Michael
Ruck
,
Friedrich
Reinert
,
Hendrik
Bentmann
,
Anna
Isaeva
Diamond Proposal Number(s):
[22468]
Open Access
Abstract: Combinations of nontrivial band topology and long-range magnetic order hold promise for realizations of novel spintronic phenomena, such as the quantum anomalous Hall effect and the topological magnetoelectric effect. Following theoretical advances, material candidates are emerging. Yet, so far a compound that combines a band-inverted electronic structure with an intrinsic net magnetization remains unrealized.
MnBi
2
Te
4
has been established as the first antiferromagnetic topological insulator and constitutes the progenitor of a modular
(
Bi
2
Te
3
)
n
(
MnBi
2
Te
4
)
series. Here, for
n
=
1
, we confirm a nonstoichiometric composition proximate to
MnBi
4
Te
7
. We establish an antiferromagnetic state below 13 K followed by a state with a net magnetization and ferromagnetic-like hysteresis below 5 K. Angle-resolved photoemission experiments and density-functional calculations reveal a topologically nontrivial surface state on the
MnBi
4
Te
7
(
0001
)
surface, analogous to the nonmagnetic parent compound
Bi
2
Te
3
. Our results establish
MnBi
4
Te
7
as the first band-inverted compound with intrinsic net magnetization providing a versatile platform for the realization of magnetic topological states of matter.
|
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
|
|
I09-Surface and Interface Structural Analysis
|
V. B.
Zabolotnyy
,
K.
Fürsich
,
R. J.
Green
,
P.
Lutz
,
K.
Treiber
,
Chul-Hee
Min
,
A. V.
Dukhnenko
,
N. Y.
Shitsevalova
,
V. B.
Filipov
,
B. Y.
Kang
,
B. K.
Cho
,
R.
Sutarto
,
Feizhou
He
,
Friedrich
Reinert
,
D. S.
Inosov
,
V.
Hinkov
Diamond Proposal Number(s):
[14732]
Abstract: Samarium hexaboride (SmB6), a Kondo insulator with mixed valence, has recently attracted much attention as a possible host for correlated topological surface states. Here, we use a combination of x-ray absorption and reflectometry techniques, backed up with a theoretical model for the resonant M4,5 absorption edge of Sm and photoemission data, to establish laterally averaged chemical and valence depth profiles at the surface of SmB6. We show that upon cleaving, the highly polar (001) surface of SmB6 undergoes substantial chemical and valence reconstruction, resulting in boron termination and a Sm3+ dominated subsurface region. Whereas at room temperature, the reconstruction occurs on a timescale of less than 2 h, it takes about 24 h below 50 K. The boron termination is eventually established, irrespective of the initial termination. Our findings reconcile earlier depth resolved photoemission and scanning tunneling spectroscopy studies performed at different temperatures and are important for better control of surface states in this system.
|
May 2018
|
|
I09-Surface and Interface Structural Analysis
|
Diamond Proposal Number(s):
[11952]
Abstract: Samarium hexaboride (SmB6), which lies in themixed valence regime
in the Anderson model, has been predicted to possess topologically
protected surface states. The intensive investigations on SmB6have
brought up the long-standing questions about the discrepancy
between the surface and bulk electronic properties in rare earth
compounds in general. Here, we investigate and eventually clarify
this discrepancy in the particular case of SmB6by the photoemission
core-level spectra. We focus on the change in both Sm and B
states depending on time, temperature, probing depth and surface
termination on the cleaved (1 0 0) surface. Our spectra show that the
unusual time-dependent change in the Sm valence occurs within a
period of hours, which is not related to the adsorption of residual
gases. Moreover, we observe a reduction of the surface feature in
the B and Sm states on the same timescale accompanied by the
formation of a subsurface region. Thus, it indicates the relatively slow
charge redistribution between the surface and subsurface regions.
Our findings demonstrate that the f states is strongly involved in the
surface relaxation.
|
Jun 2016
|
|
