I05-ARPES
|
V. A.
Rogalev
,
F.
Reis
,
F.
Adler
,
M.
Bauernfeind
,
J.
Erhardt
,
A.
Kowalewski
,
M. R.
Scholz
,
L.
Dudy
,
L. B.
Duffy
,
T.
Hesjedal
,
M.
Hoesch
,
G.
Bihlmayer
,
J.
Schäfer
,
R.
Claessen
Diamond Proposal Number(s):
[10244, 10289, 12892, 15285]
Abstract: We report on the electronic structure of
α
-Sn films in the very low thickness regime grown on InSb(111)A. High-resolution low photon energy angle-resolved photoemission spectroscopy allows for the direct observation of the linearly dispersing two-dimensional (2D) topological surface state (TSS) that exists between the second valence band and the conduction band. The Dirac point of this TSS was found to be 200 meV below the Fermi level in 10-nm-thick films, which enables the observation of the hybridization gap opening at the Dirac point of the TSS for thinner films. The crossover to a quasi-2D electronic structure is accompanied by a full gap opening at the Brillouin-zone center, in agreement with our density functional theory calculations. We further identify the thickness regime of
α
-Sn films where the hybridization gap in the TSS coexists with the topologically nontrivial electronic structure and one can expect the presence of a one-dimensional helical edge state.
|
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
|
Veronika
Sunko
,
Edgar
Abarca Morales
,
Igor
Markovic
,
Mark E.
Barber
,
Dijana
Milosavljević
,
Federico
Mazzola
,
Dmitry A.
Sokolov
,
Naoki
Kikugawa
,
Cephise
Cacho
,
Pavel
Dudin
,
Helge
Rosner
,
Clifford
Hicks
,
Philip D. C.
King
,
Andrew P.
Mackenzie
Diamond Proposal Number(s):
[20427]
Open Access
Abstract: Pressure represents a clean tuning parameter for traversing the complex phase diagrams of interacting electron systems, and as such has proved of key importance in the study of quantum materials. Application of controlled uniaxial pressure has recently been shown to more than double the transition temperature of the unconventional superconductor Sr2RuO4, leading to a pronounced peak in Tc versus strain whose origin is still under active debate. Here we develop a simple and compact method to passively apply large uniaxial pressures in restricted sample environments, and utilise this to study the evolution of the electronic structure of Sr2RuO4 using angle-resolved photoemission. We directly visualise how uniaxial stress drives a Lifshitz transition of the γ-band Fermi surface, pointing to the key role of strain-tuning its associated van Hove singularity to the Fermi level in mediating the peak in Tc. Our measurements provide stringent constraints for theoretical models of the strain-tuned electronic structure evolution of Sr2RuO4. More generally, our experimental approach opens the door to future studies of strain-tuned phase transitions not only using photoemission but also other experimental techniques where large pressure cells or piezoelectric-based devices may be difficult to implement.
|
Dec 2019
|
|
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]
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
I11-High Resolution Powder Diffraction
|
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.
|
Nov 2019
|
|
I05-ARPES
|
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.
|
Nov 2019
|
|
I05-ARPES
|
Y. J.
Chen
,
L. X.
Xu
,
J. H.
Li
,
Y. W.
Li
,
H. Y.
Wang
,
C. F.
Zhang
,
H.
Li
,
Y.
Wu
,
A. J.
Liang
,
C.
Chen
,
S. W.
Jung
,
C.
Cacho
,
Y. H.
Mao
,
S.
Liu
,
M. X.
Wang
,
Y. F.
Guo
,
Y.
Xu
,
Z. K.
Liu
,
L. X.
Yang
,
Y. L.
Chen
Diamond Proposal Number(s):
[23648, 24827]
Open Access
Abstract: The intrinsic magnetic topological insulator
MnBi
2
Te
4
exhibits rich topological effects such as quantum anomalous Hall effect and axion electrodynamics. Here, by combining the use of synchrotron and laser light sources, we carry out comprehensive and high-resolution angle-resolved photoemission spectroscopy studies on
MnBi
2
Te
4
and clearly identify its topological electronic structure. In contrast to theoretical predictions and previous studies, we observe topological surface states with diminished gap forming a characteristic Dirac cone. We argue that the topological surface states are mediated by multidomains of different magnetization orientations. In addition, the temperature evolution of the energy bands clearly reveals their interplay with the magnetic phase transition by showing interesting differences between the bulk and surface states, respectively. The investigation of the detailed electronic structure of
MnBi
2
Te
4
and its temperature evolution provides important insight into not only the exotic properties of
MnBi
2
Te
4
, but also the generic understanding of the interplay between magnetism and topological electronic structure in magnetic topological quantum materials.
|
Nov 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
|
D. F.
Liu
,
A. J.
Liang
,
E. K.
Liu
,
Q. N.
Xu
,
Y. W.
Li
,
C.
Chen
,
D.
Pei
,
W. J.
Shi
,
S. K.
Mo
,
P.
Dudin
,
T.
Kim
,
C.
Cacho
,
G.
Li
,
Y.
Sun
,
L. X.
Yang
,
Z. K.
Liu
,
S. S. P.
Parkin
,
C.
Felser
,
Y. L.
Chen
Diamond Proposal Number(s):
[22367, 20683]
Abstract: Weyl semimetals are crystalline solids that host emergent relativistic Weyl fermions and have characteristic surface Fermi-arcs in their electronic structure. Weyl semimetals with broken time reversal symmetry are difficult to identify unambiguously. In this work, using angle-resolved photoemission spectroscopy, we visualized the electronic structure of the ferromagnetic crystal Co3Sn2S2 and discovered its characteristic surface Fermi-arcs and linear bulk band dispersions across the Weyl points. These results establish Co3Sn2S2 as a magnetic Weyl semimetal that may serve as a platform for realizing phenomena such as chiral magnetic effects, unusually large anomalous Hall effect and quantum anomalous Hall effect.
|
Sep 2019
|
|
I05-ARPES
|
H. F.
Yang
,
L. X.
Yang
,
Z. K.
Liu
,
Y.
Sun
,
C.
Chen
,
H.
Peng
,
M.
Schmidt
,
D.
Prabhakaran
,
B. A.
Bernevig
,
C.
Felser
,
B. H.
Yan
,
Y. L.
Chen
Diamond Proposal Number(s):
[19310, 18005]
Open Access
Abstract: Surface Fermi arcs (SFAs), the unique open Fermi-surfaces (FSs) discovered recently in topological Weyl semimetals (TWSs), are unlike closed FSs in conventional materials and can give rise to many exotic phenomena, such as anomalous SFA-mediated quantum oscillations, chiral magnetic effects, three-dimensional quantum Hall effect, non-local voltage generation and anomalous electromagnetic wave transmission. Here, by using in-situ surface decoration, we demonstrate successful manipulation of the shape, size and even the connections of SFAs in a model TWS, NbAs, and observe their evolution that leads to an unusual topological Lifshitz transition not caused by the change of the carrier concentration. The phase transition teleports the SFAs between different parts of the surface Brillouin zone. Despite the dramatic surface evolution, the existence of SFAs is robust and each SFA remains tied to a pair of Weyl points of opposite chirality, as dictated by the bulk topology.
|
Aug 2019
|
|
