|
|
Paulina
Majchrzak
,
Klara
Volckaert
,
Antonija Grubišić
Čabo
,
Deepnarayan
Biswas
,
Marco
Bianchi
,
Sanjoy K.
Mahatha
,
Maciej
Dendzik
,
Federico
Andreatta
,
Signe S.
Grønborg
,
Igor
Markovic
,
Jonathon M.
Riley
,
Jens C.
Johannsen
,
Daniel
Lizzit
,
Luca
Bignardi
,
Silvano
Lizzit
,
Cephise
Cacho
,
Oliver
Alexander
,
Dan
Matselyukh
,
Adam S.
Wyatt
,
Richard T.
Chapman
,
Emma
Springate
,
Jeppe V.
Lauritsen
,
Phil D. C.
King
,
Charlotte
Sanders
,
Jill A.
Miwa
,
Philip
Hofmann
,
Soeren
Ulstrup
Open Access
Abstract: The quasiparticle spectra of atomically thin semiconducting transition metal dichalcogenides (TMDCs) and their response to an ultrafast optical excitation critically depend on interactions with the underlying substrate. Here, we present a comparative time- and angle-resolved photoemission spectroscopy (TR-ARPES) study of the transient electronic structure and ultrafast carrier dynamics in the single- and bilayer TMDCs MoS2 and WS2 on three different substrates: Au(111), Ag(111) and graphene/SiC. The photoexcited quasiparticle bandgaps are observed to vary over the range of 1.9–2.5 eV between our systems. The transient conduction band signals decay on a sub-50 fs timescale on the metals, signifying an efficient removal of photoinduced carriers into the bulk metallic states. On graphene, we instead observe a fast timescale on the order of 170 fs, followed by a slow dynamics for the conduction band decay in MoS
. These timescales are explained by Auger recombination involving MoS
and in-gap defect states. In bilayer TMDCs on metals we observe a complex redistribution of excited holes along the valence band that is substantially affected by interactions with the continuum of bulk metallic states.
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Jul 2021
|
|
I05-ARPES
|
D.
Pei
,
Y.-Y.
Lv
,
Y. Y. Y.
Xia
,
Y. W.
Li
,
J. Y.
Liu
,
N. B. M.
Schröter
,
C.
Cacho
,
Z. K.
Liu
,
L. X.
Yang
,
G.
Li
,
Y. B.
Chen
,
Y.
Chen
Diamond Proposal Number(s):
[20617, 25135]
Abstract: We investigate transport properties and the electronic structure of BiCuSO, a thermoelectric material which was predicted as a possible parent compound for unconventional superconductivity. For the
p
-type BiCuSO samples studied in this paper, our transport measurements show metallic behavior down to 2 K, and we observe an increase of saturated magnetic moment around 4 K, which could be due to the reorientation of the magnetic easy axis. Using angle-resolved photoemission spectroscopy measurements, we acquired the comprehensive electronic structure of BiCuSO including the predicted flat band, and by carrying out photon polarization dependent measurements we further investigated the orbital characters of bands near the Fermi level. Compared with its sister compound, BiCuSeO, we find the flat band could be the origin of high figure of merit
(
Z
T
)
value in the
BiCu
C
h
O
(
C
h
=
S
,
Se
)
family.
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Jun 2021
|
|
I05-ARPES
|
D. F.
Liu
,
L. Y.
Wei
,
C. C.
Le
,
H. Y.
Wang
,
X.
Zhang
,
N.
Kumar
,
C.
Shekhar
,
N. B. M.
Schröter
,
Y. W.
Li
,
D.
Pei
,
L. X.
Xu
,
P.
Dudin
,
T. K.
Kim
,
C.
Cacho
,
J.
Fujii
,
I.
Vobornik
,
M. X.
Wang
,
L. X.
Yang
,
Z. K.
Liu
,
Y. F.
Guo
,
J. P.
Hu
,
C.
Felser
,
S. S. P.
Parkin
,
Y. L.
Chen
Diamond Proposal Number(s):
[18005]
Open Access
Abstract: Dirac semimetals are classified into different phases based on the types of Dirac fermions. Tuning the transition among different types of Dirac fermions in one system remains a challenge. Recently, KMgBi was predicted to be located at a critical state in which various types of Dirac fermions can be induced owing to the existence of a flatband. Here, we carried out systematic studies on the electronic structure of KMgBi single crystals by combining angle-resolve photoemission spectroscopy and scanning tunneling microscopy/spectroscopy. The flatband was clearly observed near the Fermi level. We also revealed a small bandgap of ∼20 meV between the flatband and the conduction band. These results demonstrate the critical states of KMgBi that transition among various types of Dirac fermions can be tuned in one system.
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Jun 2021
|
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I05-ARPES
|
Y. W.
Li
,
H. J.
Zheng
,
Y. Q.
Fang
,
D. Q.
Zhang
,
Y. J.
Chen
,
C.
Chen
,
A. J.
Liang
,
W. J.
Shi
,
D.
Pei
,
L. X.
Xu
,
S.
Liu
,
J.
Pan
,
D. H.
Lu
,
M.
Hashimoto
,
A.
Barinov
,
S. W.
Jung
,
Cephise
Cacho
,
M. X.
Wang
,
Y.
He
,
L.
Fu
,
H. J.
Zhang
,
F. Q.
Huang
,
L. X.
Yang
,
Z. K.
Liu
,
Y. L.
Chen
Diamond Proposal Number(s):
[125135]
Open Access
Abstract: Topological superconductors (TSCs) are unconventional superconductors with bulk superconducting gap and in-gap Majorana states on the boundary that may be used as topological qubits for quantum computation. Despite their importance in both fundamental research and applications, natural TSCs are very rare. Here, combining state of the art synchrotron and laser-based angle-resolved photoemission spectroscopy, we investigated a stoichiometric transition metal dichalcogenide (TMD), 2M-WS2 with a superconducting transition temperature of 8.8 K (the highest among all TMDs in the natural form up to date) and observed distinctive topological surface states (TSSs). Furthermore, in the superconducting state, we found that the TSSs acquired a nodeless superconducting gap with similar magnitude as that of the bulk states. These discoveries not only evidence 2M-WS2 as an intrinsic TSC without the need of sensitive composition tuning or sophisticated heterostructures fabrication, but also provide an ideal platform for device applications thanks to its van der Waals layered structure.
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May 2021
|
|
I05-ARPES
|
Diamond Proposal Number(s):
[26443]
Open Access
Abstract: We investigate the electronic structure of the
2
H
and
3
R
polytypes of
Nb
S
2
. The Fermi surfaces measured by angle-resolved photoemission spectroscopy show a remarkable difference in size, reflecting a significantly increased band filling in
3
R
−
Nb
1
+
x
S
2
compared to
2
H
−
Nb
S
2
, which we attribute to the presence of additional interstitial Nb, which act as electron donors. Thus, we find that the stoichiometry, rather than the stacking arrangement, is the most important factor in the difference in electronic and physical properties of the two phases. Our high resolution data on the
2
H
phase shows kinks in the spectral function that are fingerprints of the electron-phonon coupling. However, the strength of the coupling is found to be much larger for the the sections of bands with Nb
4
d
x
2
−
y
2
,
x
y
character than for the Nb
4
d
3
z
2
−
r
2
. Our results provide an experimental framework for interpreting the two-gap superconductivity and latent charge density wave in
2
H
−
Nb
S
2
.
|
Apr 2021
|
|
I05-ARPES
|
R. C.
Vidal
,
H.
Bentmann
,
J. i.
Facio
,
T.
Heider
,
P.
Kagerer
,
C. I.
Fornari
,
T. R. F.
Peixoto
,
T.
Figgemeier
,
S.
Jung
,
Cephise
Cacho
,
B.
Büchner
,
J.
Van Den Brink
,
C. M.
Schneider
,
L.
Plucinski
,
E. F.
Schwier
,
K.
Shimada
,
M.
Richter
,
A.
Isaeva
,
F.
Reinert
Diamond Proposal Number(s):
[22468]
Abstract: Using angle-resolved photoelectron spectroscopy (ARPES), we investigate the surface electronic structure of the magnetic van der Waals compounds
MnBi
4
Te
7
and
MnBi
6
Te
10
, the
n
=
1
and 2 members of a modular
(
Bi
2
Te
3
)
n
(
MnBi
2
Te
4
)
series, which have attracted recent interest as intrinsic magnetic topological insulators. Combining circular dichroic, spin-resolved and photon-energy-dependent ARPES measurements with calculations based on density functional theory, we unveil complex momentum-dependent orbital and spin textures in the surface electronic structure and disentangle topological from trivial surface bands. We find that the Dirac-cone dispersion of the topologial surface state is strongly perturbed by hybridization with valence-band states for
Bi
2
Te
3
-terminated surfaces but remains preserved for
MnBi
2
Te
4
-terminated surfaces. Our results firmly establish the topologically nontrivial nature of these magnetic van der Waals materials and indicate that the possibility of realizing a quantized anomalous Hall conductivity depends on surface termination.
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Apr 2021
|
|
I05-ARPES
|
Paulina
Majchrzak
,
Ryan
Muzzio
,
Alfred J. H.
Jones
,
Davide
Curcio
,
Klara
Volckaert
,
Deepnarayan
Biswas
,
Jacob
Gobbo
,
Simranjeet
Singh
,
Jeremy T.
Robinson
,
Kenji
Watanabe
,
Takashi
Taniguchi
,
Timur K.
Kim
,
Cephise
Cacho
,
Jill A.
Miwa
,
Philip
Hofmann
,
Jyoti
Katoch
,
Soeren
Ulstrup
Diamond Proposal Number(s):
[24072]
Open Access
Abstract: To pinpoint the electronic and structural mechanisms that affect intrinsic and extrinsic performance limits of two‐dimensional material devices, it is of critical importance to resolve the electronic properties on the mesoscopic length scale of such devices under operating conditions. The present work utilizes angle‐resolved photoemission spectroscopy with nanoscale spatial resolution (nanoARPES) to map the quasiparticle electronic structure of a twisted bilayer graphene device. The dispersion and linewidth of the Dirac cones associated with top and bottom graphene layers are determined as a function of spatial position on the device under both static and operating conditions. The analysis reveals that microscopic rotational domains in the two graphene layers establish a range of twist angles from 9.8∘ to 12.7∘. Application of current and electrostatic gating lead to strong electric fields with peak strengths of 0.75 V/μm at the rotational domain boundaries in the device. These proof‐of‐principle results demonstrate the potential of nanoARPES to link mesoscale structural variations with electronic states in operating device conditions and to disentangle such extrinsic factors from the intrinsic quasiparticle dispersion.
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Mar 2021
|
|
I05-ARPES
|
Christopher W.
Nicholson
,
Maxime
Rumo
,
Aki
Pulkkinen
,
Geoffroy
Kremer
,
Björn
Salzmann
,
Marie-Laure
Mottas
,
Baptiste
Hildebrand
,
Thomas
Jaouen
,
Timur K.
Kim
,
Saumya
Mukherjee
,
Keyuan
Ma
,
Matthias
Muntwiler
,
Fabian O.
Von Rohr
,
Cephise
Cacho
,
Claude
Monney
Diamond Proposal Number(s):
[24880]
Open Access
Abstract: Strain is ubiquitous in solid-state materials, but despite its fundamental importance and technological relevance, leveraging externally applied strain to gain control over material properties is still in its infancy. In particular, strain control over the diverse phase transitions and topological states in two-dimensional transition metal dichalcogenides remains an open challenge. Here, we exploit uniaxial strain to stabilize the long-debated structural ground state of the 2D topological semimetal IrTe2, which is hidden in unstrained samples. Combined angle-resolved photoemission spectroscopy and scanning tunneling microscopy data reveal the strain-stabilized phase has a 6 × 1 periodicity and undergoes a Lifshitz transition, granting unprecedented spectroscopic access to previously inaccessible type-II topological Dirac states that dominate the modified inter-layer hopping. Supported by density functional theory calculations, we show that strain induces an Ir to Te charge transfer resulting in strongly weakened inter-layer Te bonds and a reshaped energetic landscape favoring the 6×1 phase. Our results highlight the potential to exploit strain-engineered properties in layered materials, particularly in the context of tuning inter-layer behavior.
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Mar 2021
|
|
I05-ARPES
|
M.
Horio
,
Q.
Wang
,
V.
Granata
,
K. P.
Kramer
,
Y.
Sassa
,
S.
Jöhr
,
D.
Sutter
,
A.
Bold
,
L.
Das
,
Y.
Xu
,
R.
Frison
,
R.
Fittipaldi
,
T. K.
Kim
,
C.
Cacho
,
J. E.
Rault
,
P. Le
Fèvre
,
F.
Bertran
,
N. C.
Plumb
,
M.
Shi
,
A.
Vecchione
,
M. H.
Fischer
,
J.
Chang
Diamond Proposal Number(s):
[20259]
Open Access
Abstract: Electronic band structures in solids stem from a periodic potential reflecting the structure of either the crystal lattice or electronic order. In the stoichiometric ruthenate Ca3Ru2O7, numerous Fermi surface-sensitive probes indicate a low-temperature electronic reconstruction. Yet, the causality and the reconstructed band structure remain unsolved. Here, we show by angle-resolved photoemission spectroscopy, how in Ca3Ru2O7 a C2-symmetric massive Dirac semimetal is realized through a Brillouin-zone preserving electronic reconstruction. This Dirac semimetal emerges in a two-stage transition upon cooling. The Dirac point and band velocities are consistent with constraints set by quantum oscillation, thermodynamic, and transport experiments, suggesting that the complete Fermi surface is resolved. The reconstructed structure—incompatible with translational-symmetry-breaking density waves—serves as an important test for band structure calculations of correlated electron systems.
|
Mar 2021
|
|
I05-ARPES
|
Simone M.
Kevy
,
Henriette E.
Lund
,
Laura
Wollesen
,
Kirstine J.
Dalgaard
,
Yu-Te
Hsu
,
Steffen
Wiedmann
,
Marco
Bianchi
,
Ann Julie Utne
Holt
,
Davide
Curcio
,
Deepnarayan
Biswas
,
Alfred J. H.
Jones
,
Klara
Volckaert
,
Cephise
Cacho
,
Pavel
Dudin
,
Philip
Hofmann
,
Martin
Bremholm
Diamond Proposal Number(s):
[20218]
Abstract: The crystal structure, electronic structure, and transport properties of crystals with the nominal composition
Nb
0.25
Bi
2
Se
3
are investigated. X-ray diffraction reveals that the as-grown crystals display phase segregation and contain major contributions of BiSe and the superconducting misfit layer compound
(
Bi
Se
)
1.1
Nb
Se
2
. The inhomogeneous character of the samples is also reflected in the electronic structure and transport properties of different single crystals. Angle-resolved photoemission spectroscopy (ARPES) reveals an electronic structure that resembles poor-quality
Bi
2
Se
3
with an ill-defined topological surface state. High-quality topological surface states are instead observed when using a highly focused beam size, i.e., nanoARPES. While the superconducting transition temperature is found to vary between 2.5 and 3.5 K, the majority of the bulk single crystals does not exhibit a zero-resistance state suggesting filamentary superconductivity in the materials. Susceptibility measurements of the system together with the temperature dependence of the coherence length extracted from the upper critical field are consistent with conventional BCS superconductivity of a type II superconductor.
|
Feb 2021
|
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