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.
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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
|
|
|
|
Abstract: Quasiparticle interference (QPI) provides a wealth of information relating to the electronic structure of a material. However, it is often assumed that this information is constrained to two-dimensional electronic states. We show that this is not necessarily the case. For FeSe, a system dominated by surface defects, we show that it is actually all electronic states with negligible group velocity in the
z
axis that are contained within the experimental data. By using a three-dimensional tight-binding model of FeSe, fit to photoemission measurements, we directly reproduce the experimental QPI scattering dispersion, within a
T
-matrix formalism, by including both
k
z
=
0
and
k
z
=
π
electronic states. This result unifies both tunnelling based and photoemission based experiments on FeSe and highlights the importance of
k
z
within surface sensitive measurements of QPI.
|
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
|
Diamond Proposal Number(s):
[15074, 19041]
Open Access
Abstract: A fundamental part of the puzzle of unconventional superconductivity in the Fe-based superconductors is the understanding of the magnetic and nematic instabilities of the parent compounds. The issues of which of these can be considered the leading instability, and whether weak- or strong-coupling approaches are applicable, are both critical and contentious. Here, we revisit the electronic structure of BaFe2As2 using angle-resolved photoemission spectroscopy (ARPES). Our high-resolution measurements of samples “detwinned” by the application of a mechanical strain reveal a highly anisotropic 3D Fermi surface in the low-temperature antiferromagnetic phase. By comparison of the observed dispersions with ab initio calculations, we argue that overall it is magnetism, rather than orbital/nematic ordering, which is the dominant effect, reconstructing the electronic structure across the Fe 3d bandwidth. Finally, using a state-of-the-art nano-ARPES system, we reveal how the observed electronic dispersions vary in real space as the beam spot crosses domain boundaries in an unstrained sample, enabling the measurement of ARPES data from within single antiferromagnetic domains, and showing consistence with the effective mono-domain samples obtained by detwinning.
|
Jul 2019
|
|
I05-ARPES
|
Sergey
Borisenko
,
Daniil
Evtushinsky
,
Quinn
Gibson
,
Alexander
Yaresko
,
Klaus
Koepernik
,
Timur
Kim
,
Mazhar
Ali
,
Jeroen
Van Den Brink
,
Moritz
Hoesch
,
Alexander
Fedorov
,
Erik
Haubold
,
Yevhen
Kushnirenko
,
Ivan
Soldatov
,
Rudolf
Schäfer
,
Robert J.
Cava
Diamond Proposal Number(s):
[11643]
Open Access
Abstract: Spectroscopic detection of Dirac and Weyl fermions in real materials is vital for both, promising applications and fundamental bridge between high-energy and condensed-matter physics. While the presence of Dirac and noncentrosymmetric Weyl fermions is well established in many materials, the magnetic Weyl semimetals still escape direct experimental detection. In order to find a time-reversal symmetry breaking Weyl state we design two materials and present here experimental and theoretical evidence of realization of such a state in one of them, YbMnBi2. We model the time-reversal symmetry breaking observed by magnetization and magneto-optical microscopy measurements by canted antiferromagnetism and find a number of Weyl points. Using angle-resolved photoemission, we directly observe two pairs of Weyl points connected by the Fermi arcs. Our results not only provide a fundamental link between the two areas of physics, but also demonstrate the practical way to design novel materials with exotic properties.
|
Jul 2019
|
|
I05-ARPES
|
Diamond Proposal Number(s):
[13856, 15936]
Abstract: Nematicity plays an important role in the physics of iron-based superconductors (IBS). Its microscopic origin and in particular its importance for the mechanism of high-temperature superconductivity itself are highly debated. A crucial knowledge in this regard is the degree to which the nematic order influences the electronic structure of these materials. Earlier angle-resolved photoemission spectroscopy (ARPES) studies found that the effect is dramatic in three families of IBS including 11, 111, and 122 compounds: Energy splitting reaches 70 meV and Fermi surface becomes noticeably distorted. More recent experiments, however, reported significantly lower energy scale in 11 and 111 families, thus questioning the degree and universality of the impact of nematicity on the electronic structure of IBS. Here, we revisit the electronic structure of the undoped parent
BaFe
2
As
2
(122 family). Our systematic ARPES study, including the detailed temperature and photon energy dependencies, points to the significantly smaller energy scale also in this family of materials, thus establishing the universal scale of this phenomenon in IBS. Our results form a necessary quantitative basis for theories of high-temperature superconductivity focused on the nematicity.
|
Jul 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
|
Niels B. M.
Schröter
,
Ding
Pei
,
Maia G.
Vergniory
,
Yan
Sun
,
Kaustuv
Manna
,
Fernando
De Juan
,
Jonas A.
Krieger
,
Vicky
Süss
,
Marcus
Schmidt
,
Pavel
Dudin
,
Barry
Bradlyn
,
Timur K.
Kim
,
Thorsten
Schmitt
,
Cephise
Cacho
,
Claudia
Felser
,
Vladimir N.
Strocov
,
Yulin
Chen
Diamond Proposal Number(s):
[19883, 21400]
Abstract: Topological semimetals in crystals with a chiral structure (which possess a handedness due to a lack of mirror and inversion symmetries) are expected to display numerous exotic physical phenomena, including fermionic excitations with large topological charge1, long Fermi arc surface states2,3, unusual magnetotransport4 and lattice dynamics5, as well as a quantized response to circularly polarized light6. So far, all experimentally confirmed topological semimetals exist in crystals that contain mirror operations, meaning that these properties do not appear. Here, we show that AlPt is a structurally chiral topological semimetal that hosts new four-fold and six-fold fermions, which can be viewed as a higher spin generalization of Weyl fermions without equivalence in elementary particle physics. These multifold fermions are located at high symmetry points and have Chern numbers larger than those in Weyl semimetals, thus resulting in multiple Fermi arcs that span the full diagonal of the surface Brillouin zone. By imaging these long Fermi arcs, we experimentally determine the magnitude and sign of their Chern number, allowing us to relate their dispersion to the handedness of their host crystal.
|
May 2019
|
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