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.
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Jul 2019
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I05-ARPES
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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.
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Jul 2019
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I05-ARPES
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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.
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Jul 2019
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I05-ARPES
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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
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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.
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May 2019
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I05-ARPES
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Diamond Proposal Number(s):
[21083]
Abstract: Identifying the electron dynamics in CuO chains has been elusive in Y-Ba-Cu-O (YBCO) cuprate systems when using standard angle-resolved photoemission spectroscopy (ARPES); a cleaved sample exhibits areas terminated by both CuO chains or BaO layers, and the size of a typical beam results in ARPES signals that are superposed from both terminations. Here, we employ spatially resolved ARPES with a submicrometric beam (nano-ARPES) to reveal the surface-termination-dependent electronic structures of the double CuO chains in
YBa
2
Cu
4
O
8
. We present an observation of sharp metallic dispersions and Fermi surfaces of the double CuO chains buried underneath the
CuO
2
-plane block on a BaO-terminated surface. While the observed Fermi surfaces of the CuO chains are highly one dimensional, the electrons in the CuO chains do not undergo significant electron correlations and no signature of a Tomonaga-Luttinger liquid or a marginal Fermi liquid is found. Our work represents an important experimental step toward understanding the charge dynamics and provides a starting basis for modeling the high-
T
c
superconductivity in YBCO cuprate systems.
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Apr 2019
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I05-ARPES
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D.
Sutter
,
M.
Kim
,
C. E.
Matt
,
M.
Horio
,
R.
Fittipaldi
,
A.
Vecchione
,
V.
Granata
,
K.
Hauser
,
Y.
Sassa
,
G.
Gatti
,
M.
Grioni
,
M.
Hoesch
,
T. K.
Kim
,
E.
Rienks
,
N. C.
Plumb
,
M.
Shi
,
T.
Neupert
,
A.
Georges
,
J.
Chang
Diamond Proposal Number(s):
[15296]
Abstract: We present a comprehensive angle-resolved photoemission spectroscopy study of
Ca
1.8
Sr
0.2
RuO
4
. Four distinct bands are revealed and along the Ru-O bond direction their orbital characters are identified through a light polarization analysis and comparison to dynamical mean-field theory calculations. Bands assigned to
d
x
z
,
d
y
z
orbitals display Fermi liquid behavior with fourfold quasiparticle mass renormalization. Extremely heavy fermions—associated with a predominantly
d
x
y
band character—are shown to display non-Fermi-liquid behavior. We thus demonstrate that
Ca
1.8
Sr
0.2
RuO
4
is a hybrid metal with an orbitally selective Fermi liquid quasiparticle breakdown.
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Mar 2019
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I05-ARPES
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O. J.
Clark
,
F.
Mazzola
,
I.
Markovic
,
J. M.
Riley
,
J.
Feng
,
B.-j.
Yang
,
K.
Sumida
,
T.
Okuda
,
J.
Fujii
,
I.
Vobornik
,
T. K.
Kim
,
K.
Okawa
,
T.
Sasagawa
,
M. S.
Bahramy
,
P. D. C.
King
Diamond Proposal Number(s):
[14927, 16262]
Open Access
Abstract: The band inversions that generate the topologically non-trivial band gaps of topological insulators and the isolated Dirac touching points of three-dimensional Dirac semimetals generally arise from the crossings of electronic states derived from different orbital manifolds. Recently, the concept of single orbital-manifold band inversions occurring along high-symmetry lines has been demonstrated, stabilising multiple bulk and surface Dirac fermions. Here, we discuss the underlying ingredients necessary to achieve such phases, and discuss their existence within the family of transition metal dichalcogenides. We show how their three-dimensional band structures naturally produce only small k z projected band gaps, and demonstrate how these play a significant role in shaping the surface electronic structure of these materials. We demonstrate, through spin- and angle-resolved photoemission and density functional theory calculations, how the surface electronic structures of the group-X TMDs PtSe2 and PdTe2 are host to up to five distinct surface states, each with complex band dispersions and spin textures. Finally, we discuss how the origin of several recently-realised instances of topological phenomena in systems outside of the TMDs, including the iron-based superconductors, can be understood as a consequence of the same underlying mechanism driving k z -mediated band inversions in the TMDs.
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Mar 2019
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I05-ARPES
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Abstract: 2D electron gases (2DEGs) in oxides show great potential for the discovery of new physical phenomena and at the same time hold promise for electronic applications. In this work, angle‐resolved photoemission is used to determine the electronic structure of a 2DEG stabilized in the (111)‐oriented surface of the strong spin–orbit coupling material KTaO3. The measurements reveal multiple sub‐bands that emerge as a consequence of quantum confinement and form a sixfold symmetric Fermi surface. This electronic structure is well reproduced by self‐consistent tight‐binding supercell calculations. Based on these calculations, the spin and orbital texture of the 2DEG is determined. It is found that the 2DEG Fermi surface is derived from bulk J = 3/2 states and exhibits an unconventional anisotropic Rashba‐like lifting of the spin‐degeneracy. Spin‐momentum locking holds only for high‐symmetry directions and a strong out‐of‐plane spin component renders the spin texture threefold symmetric. It is found that the average spin‐splitting on the Fermi surface is an order of magnitude larger than in SrTiO3, which should translate into an enhancement in the spin–orbitronic response of (111)‐KTaO3 2DEG‐based devices.
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Mar 2019
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I05-ARPES
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Ryo
Noguchi
,
T.
Takahashi
,
K.
Kuroda
,
M.
Ochi
,
T.
Shirasawa
,
M.
Sakano
,
C.
Bareille
,
M.
Nakayama
,
M. D.
Watson
,
K.
Yaji
,
A.
Harasawa
,
H.
Iwasawa
,
P.
Dudin
,
T. K.
Kim
,
M.
Hoesch
,
V.
Kandyba
,
A.
Giampietri
,
A.
Barinov
,
S.
Shin
,
R.
Arita
,
T.
Sasagawa
,
Takeshi
Kondo
Diamond Proposal Number(s):
[15095, 16161]
Abstract: The major breakthroughs in understanding of topological materials over the past decade were all triggered by the discovery of the Z2-type topological insulator—a type of material that is insulating in its interior but allows electron flow on its surface. In three dimensions, a topological insulator is classified as either ‘strong’ or ‘weak’ and experimental confirmations of the strong topological insulator rapidly followed theoretical predictions. By contrast, the weak topological insulator (WTI) has so far eluded experimental verification, because the topological surface states emerge only on particular side surfaces, which are typically undetectable in real three-dimensional crystals. Here we provide experimental evidence for the WTI state in a bismuth iodide, β-Bi4I4. Notably, the crystal has naturally cleavable top and side planes—stacked via van der Waals forces—which have long been desirable for the experimental realization of the WTI state. As a definitive signature of this state, we find a quasi-one-dimensional Dirac topological surface state at the side surface (the (100) plane), while the top surface (the (001) plane) is topologically dark with an absence of topological surface states. We also find that a crystal transition from the β-phase to the α-phase drives a topological phase transition from a nontrivial WTI to a normal insulator at roughly room temperature. The weak topological phase—viewed as quantum spin Hall insulators stacked three-dimensionally—will lay a foundation for technology that benefits from highly directional, dense spin currents that are protected against backscattering.
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Feb 2019
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