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):
[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


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

Open Access
Abstract: Silicene, the silicon analogue of graphene, consists of an atomically buckled honeycomb lattice of silicon atoms. Theory predicts exceptional electronic properties, including Dirac fermions and a topological spin Hall insulator phase. An important obstacle impeding exploration of such properties in electronic devices is the chemical sensitivity of silicene, hampering its incorporation in layer stacks. Here we show experimentally that epitaxial silicene and hexagonal boron nitride (hBN) can be stacked without perturbing the electronic properties of silicene. Intercalated silicene underneath epitaxial hBN on ZrB2(0001) substrate films is obtained by depositing Si atoms at room temperature. Using (angle resolved) photoelectron spectroscopy (ARPES, PES) and scanning tunneling microscopy (STM) we find that the intercalated silicene exhibits the same electronic properties as epitaxial silicene on ZrB2, while it resists oxidation in air up to several hours. This is an essential step towards the development of layer stacks that allow for fabrication of devices.

Feb 2019


I05ARPES

Daichi
Takane
,
Zhiwei
Wang
,
Seigo
Souma
,
Kosuke
Nakayama
,
Takechika
Nakamura
,
Hikaru
Oinuma
,
Yuki
Nakata
,
Hideaki
Iwasawa
,
Cephise
Cacho
,
Timur
Kim
,
Koji
Horiba
,
Hiroshi
Kumigashira
,
Takashi
Takahashi
,
Yoichi
Ando
,
Takafumi
Sato
Diamond Proposal Number(s):
[18839]
Abstract: Topological semimetals materialize a new state of quantum matter where massless fermions protected by a specific crystal symmetry host exotic quantum phenomena. Distinct from wellknown Dirac and Weyl fermions, structurally chiral topological semimetals are predicted to host new types of massless fermions characterized by a large topological charge, whereas such exotic fermions are yet to be experimentally established. Here, by using angleresolved photoemission spectroscopy, we experimentally demonstrate that a transitionmetal silicide CoSi hosts two types of chiral topological fermions, a spin1 chiral fermion and a double Weyl fermion, in the center and corner of the bulk Brillouin zone, respectively. Intriguingly, we found that the bulk Fermi surfaces are purely composed of the energy bands related to these fermions. We also find the surface states connecting the Fermi surfaces associated with these fermions, suggesting the existence of the predicted Fermiarc surface states. Our result provides the first experimental evidence for the chiral topological fermions beyond Dirac and Weyl fermions in condensedmatter systems, and paves the pathway toward realizing exotic electronic properties associated with unconventional chiral fermions.

Feb 2019


I05ARPES
I19Small Molecule Single Crystal Diffraction

Diamond Proposal Number(s):
[8776, 11039, 13797, 17065]
Abstract: The charge density wave (CDW) in ZrTe3 is quenched in samples with a small amount of Te isoelectronically substituted by Se. Using angleresolved photoemission spectroscopy we observe subtle changes in the electronic band dispersions and Fermi surfaces upon Se substitution. The scattering rates are substantially increased, in particular for the large threedimensional Fermi surface sheet. The quasionedimensional band is unaffected by the substitution and still shows a gap at low temperature, which starts to open from room temperature. Longrange order is, however, absent in the electronic states as in the periodic lattice distortion. The competition between superconductivity and the CDW is thus linked to the suppression of longrange order of the CDW.

Jan 2019


I05ARPES

Diamond Proposal Number(s):
[13438, 16262, 18705]
Abstract: We investigate the electronic structure of a twodimensional electron gas created at the surface of the multivalley semimetal 1T−PtSe2. Using angleresolved photoemission and firstprinciplesbased surface spacecharge calculations, we show how the induced quantum well subband states form multiple Fermi surfaces, which exhibit highly anisotropic Rashbalike spin splittings. We further show how the presence of both electronlike and holelike bulk carriers causes the nearsurface band bending potential to develop an unusual nonmonotonic form, with spatially segregated electron accumulation and hole accumulation regions, which in turn amplifies the induced spin splitting. Our results thus demonstrate the novel environment that semimetals provide for tailoring electrostatically induced potential profiles and their corresponding quantum subband states.

Jan 2019


I05ARPES

Diamond Proposal Number(s):
[19816]
Open Access
Abstract: The discovery in 2008 of the ironbased superconducting pnictide and
chalcogenide compounds has provided an entirely new family of materials for studying
the crucial interplay between superconductivity and magnetism in unconventional
superconductors. The alkalimetalintercalated iron selenide (AxFe2ySe2, A =
alkali metal) superconductors are of particular interest owing to their relatively high
transition temperatures of 30 K and the coexistence of the superconducting state
with antiferromagnetic ordering. Intrinsic phase separation on the mesoscopic scale is
known to occur in "single crystals" of these compounds, adding to the complexity of
interpretation of bulk property measurements. In this study, we investigate the local
electronic structure and chemistry of RbxFe2ySe2 crystals using scanning microscopy
techniques. Nanofocussed angleresolved photoemission spectroscopy (NanoARPES)
has enabled the band structure of the minority superconducting phase and the non
superconducting matrix to be measured independently and linked to the surface
chemistry from the same regions using corelevel spectroscopy. Valence band mapping
reveals the characteristic microstructure of these crystals, but does not have sucient
spatial resolution to enable the precise morphology of the superconducting phase
to be elucidated. Cryogenic magnetic force microscopy (MFM) has shown that the
superconducting phase has a finescale stripey morphology that was not resolved in
the SPEM experiment. The correlation of these findings with previous microstructural
studies, bulk measurements and firstprinciples DFT calculations paves the way for
understanding the intriguing electronic and magnetic properties of these compounds.

Jan 2019


I05ARPES

Federico
Mazzola
,
Veronika
Sunko
,
Seunghyun
Khim
,
Helge
Rosner
,
Pallavi
Kushwaha
,
Oliver J.
Clark
,
Lewis
Bawden
,
Igor
Markovic
,
Timur K.
Kim
,
Moritz
Hoesch
,
Andrew P.
Mackenzie
,
Phil D. C.
King
Diamond Proposal Number(s):
[12469, 14927, 16262]
Abstract: The ability to modulate the collective properties of correlated electron systems at their interfaces and surfaces underpins the burgeoning field of “designer” quantum materials. Here, we show how an electronic reconstruction driven by surface polarity mediates a Stonerlike magnetic instability to itinerant ferromagnetism at the Pdterminated surface of the nonmagnetic delafossite oxide metal PdCoO2. Combining angleresolved photoemission spectroscopy and densityfunctional theory calculations, we show how this leads to a rich multiband surface electronic structure. We find similar surface state dispersions in PdCrO2, suggesting surface ferromagnetism persists in this sister compound despite its bulk antiferromagnetic order.

Dec 2018

