I05ARPES

Diamond Proposal Number(s):
[21083]
Abstract: Identifying the electron dynamics in CuO chains has been elusive in YBaCuO (YBCO) cuprate systems when using standard angleresolved 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 (nanoARPES) to reveal the surfaceterminationdependent 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 BaOterminated 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 TomonagaLuttinger 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.

Apr 2019


I05ARPES

Diamond Proposal Number(s):
[16764, 14488]
Open Access
Abstract: This article reports on the fabrication and testing of dedicated Fresnel zone plates for use at the nanoARPES branch of the I05ARPES beamline of Diamond Light Source to perform angleresolved photoelectron spectroscopy with submicrometre resolution in real space. The aim of the design was to provide high photon flux combined with submicrometre spot sizes. The focusing lenses were tested with respect to efficiency and spatial resolution in the extreme ultraviolet between 50 eV and 90 eV. The experimentally determined diffraction efficiencies of the zone plates are as high as 8.6% at 80 eV, and a realspace resolution of 0.4 µm was demonstrated. Using the zoneplatebased setup, monolayer flakes of the twodimensional semiconductor WS2 were investigated. This work demonstrates that the local electronic structure can be obtained from an area of a few micrometres across a twodimensional heterostructure.

Mar 2019


I05ARPES

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 nontrivial band gaps of topological insulators and the isolated Dirac touching points of threedimensional Dirac semimetals generally arise from the crossings of electronic states derived from different orbital manifolds. Recently, the concept of single orbitalmanifold band inversions occurring along highsymmetry 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 threedimensional 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 angleresolved photoemission and density functional theory calculations, how the surface electronic structures of the groupX 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 recentlyrealised instances of topological phenomena in systems outside of the TMDs, including the ironbased superconductors, can be understood as a consequence of the same underlying mechanism driving k z mediated band inversions in the TMDs.

Mar 2019


I05ARPES

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.

Mar 2019


I05ARPES

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 angleresolved photoemission spectroscopy study of
Ca
1.8
Sr
0.2
RuO
4
. Four distinct bands are revealed and along the RuO bond direction their orbital characters are identified through a light polarization analysis and comparison to dynamical meanfield 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 nonFermiliquid 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.

Mar 2019


I05ARPES

Binbin
Fu
,
Changjiang
Yi
,
Zhijun
Wang
,
Meng
Yang
,
Baiqing
Lv
,
Xin
Gao
,
Man
Li
,
Yaobo
Huang
,
Hongming
Weng
,
Youguo
Shi
,
Tian
Qian
,
Hong
Ding
Abstract: Topological Dirac semimetals (DSMs) present a kind of topologically nontrivial quantum state of matter, which has massless Dirac fermions in the bulk and topologically protected states on certain surfaces. In superconducting DSMs, the effects of their nontrivial topology on superconducting pairing could realize topological superconductivity in the bulk or on the surface. As superconducting pairing takes place at the Fermi level E F, to make the effects possible, the Dirac points should lie in the vicinity of E F so that the topological electronic states can participate in the superconducting paring. Here, we show using angleresolved photoelectron spectroscopy that in a series of (Ir1−x Pt x )Te2 compounds, the typeII Dirac points reside around E F in the superconducting region, in which the bulk superconductivity has a maximum T c of ~ 3 K. The realization of the coexistence of bulk superconductivity and lowenergy Dirac fermions in (Ir1−x Pt x )Te2 paves the way for studying the effects of the nontrivial topology in DSMs on the superconducting state.

Mar 2019


I05ARPES

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 Z2type 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 threedimensional crystals. Here we provide experimental evidence for the WTI state in a bismuth iodide, βBi4I4. 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 quasionedimensional 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 βphase to the αphase 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 threedimensionally—will lay a foundation for technology that benefits from highly directional, dense spin currents that are protected against backscattering.

Feb 2019


I05ARPES

Diamond Proposal Number(s):
[18586]
Abstract: Using angleresolved photoemission spectroscopy and density functional theory (DFT) we study the electronic structure of layered BaZnBi2. Our experimental results show no evidence of Dirac states in BaZnBi2 originated either from the bulk or the surface. The calculated band structure without spinorbit interaction shows linear band dispersions at X along the X−M highsymmetry line. In addition, the calculations suggest a gapless band crossing point along the Γ−M highsymmetry line. However, as soon as the spinorbit interaction is turned on, the band crossing point is significantly gapped out. These observations suggest that the Dirac fermions in BaZnBi2 are trivial similar to the Dirac states observed in grapheme. The experimental observations are in good agreement with the DFT calculations.

Feb 2019


I05ARPES

Q.
Yao
,
D.
Kaczorowski
,
P.
Swatek
,
D.
Gnida
,
C. H. P.
Wen
,
X. H.
Niu
,
R.
Peng
,
H. C.
Xu
,
P.
Dudin
,
S.
Kirchner
,
Q. Y.
Chen
,
D. W.
Shen
,
D. L.
Feng
Diamond Proposal Number(s):
[16345]
Abstract: The localizedtoitinerant transition of f electrons lies at the heart of heavyfermion physics, but has only been directly observed in singlelayer Cebased materials. Here, we report a comprehensive study on the electronic structure and nature of the Ce 4f electrons in the heavyfermion superconductor Ce2PdIn8, a typical n=2 CenMmIn3n+2m compound, using highresolution and 4d−4f resonant photoemission spectroscopies. The electronic structure of this material has been studied over a wide temperature range, and hybridization between f and conduction electrons can be clearly observed to form a Kondo resonance near the Fermi level at low temperatures. The characteristic temperature of the localizedtoitinerant transition is around 120 K, which is much higher than its coherence temperature Tcoh∼30K.

Feb 2019


I05ARPES

Diamond Proposal Number(s):
[19771, 18555, 16262]
Abstract: We revisit the enduring problem of the 2 × 2 × 2 charge density wave (CDW) order in TiSe2, utilizing photon energydependent angleresolved photoemission spectroscopy to probe the full threedimensional high and lowtemperature electronic structure. Our measurements demonstrate how a mismatch of dimensionality between the 3D conduction bands and the quasi2D valence bands in this system leads to a hybridization that is strongly kz dependent. While such a momentumselective coupling can provide the energy gain required to form the CDW, we show how additional “passenger” states remain, which couple only weakly to the CDW and thus dominate the lowenergy physics in the ordered phase of TiSe2.

Feb 2019

