I05-ARPES
|
Mihir
Date
,
Francesco
Petocchi
,
Yun
Yen
,
Jonas A.
Krieger
,
Banabir
Pal
,
Vicky
Hasse
,
Emily C.
Mcfarlane
,
Chris
Körner
,
Jiho
Yoon
,
Matthew D.
Watson
,
Vladimir N.
Strocov
,
Yuanfeng
Xu
,
Ilya
Kostanovski
,
Mazhar N.
Ali
,
Sailong
Ju
,
Nicholas C.
Plumb
,
Michael A.
Sentef
,
Georg
Woltersdorf
,
Michael
Schüler
,
Philipp
Werner
,
Claudia
Felser
,
Stuart S. P.
Parkin
,
Niels B. M.
Schröter
Diamond Proposal Number(s):
[29240]
Open Access
Abstract: Crystalline solids can become band insulators due to fully filled bands, or Mott insulators due to strong electronic correlations. While Mott insulators can theoretically occur in systems with an even number of electrons per unit cell, distinguishing them from band insulators experimentally has remained a longstanding challenge. In this work, we present a unique momentum-resolved signature of a dimerized Mott-insulating phase in the experimental spectral function of Nb3Br8: the top of the highest occupied band along the out-of-plane direction kz has a momentum-space separation Δkz = 2π/d, whereas that of a band insulator is less than π/d, where d is the average interlayer spacing. Identifying Nb3Br8 as a Mott insulator is crucial to understand its role in the field-free Josephson diode effect. Moreover, our method could be extended to other van der Waals systems where tuning interlayer coupling and Coulomb interactions can drive a band- to Mott-insulating transition.
|
Apr 2025
|
|
I05-ARPES
|
Jiabao
Yang
,
Mihir
Date
,
Irián Sánchez
Ramírez
,
Vicky
Hasse
,
Deepnarayan
Biswas
,
Stuart S. P.
Parkin
,
Maia G.
Vergniory
,
Fernando
De Juan
,
Claudia
Felser
,
Matthew D.
Watson
,
Niels B. M.
Schroeter
Diamond Proposal Number(s):
[33319]
Open Access
Abstract: Recent work suggests that crystal structures with two sublattice pairs per primitive cell can host “dark states”, electronic states that barely interact with light due to destructive interference, which makes them invisible in photoemission. In practice, however, dark states are only approximately dark, arising from near but imperfect translation symmetries. Here, we demonstrate a practical consequence of this in the semiconductor (NbSe4)3I: Although its band structure indicates an almost direct gap, the material behaves optically like an indirect-gap semiconductor. Angle-resolved photoemission spectroscopy uncovers weak spectral-weight bands folded from a larger Brillouin zone, reflecting approximate intra-unit-cell symmetry. These states form a small direct band gap consistent with transport data but exhibit very low optical transition probability. Instead, optical absorption is dominated by higher-energy transitions involving bands with stronger spectral weight, effectively enlarging the observed optical gap. Our results show that dark states are approximate phenomena with significant consequences for optoelectronic properties.
|
Jan 2025
|
|
I05-ARPES
|
Diamond Proposal Number(s):
[31067, 31222, 31407]
Open Access
Abstract: We show that in some transition metal dichalcogenides, minority regions of the cleaved sample surfaces show—unexpectedly and anomalously—a finite number of 2D electronic states instead of the expected 3D valence bands. In the case of NbS2, in addition to the typical spectrum associated with bulk 2Ha stacking, we also find minority regions with electronic structures consistent with few layers of 3R stacking. In MoS2, we find areas of both bulk 2Hc and 3R stackings, and regions exhibiting finite-layer quantization of both types. We further find evidence for a more exotic 4Ha stacking of MoS2, in which the valence band maximum is quasi-2D. The results highlight how variation of the interlayer stacking of van der Waals materials beyond the commonly reported bulk polytypes can yield novel electronic structures.
|
Sep 2024
|
|
I05-ARPES
|
Jonas A.
Krieger
,
Samuel
Stolz
,
Iñigo
Robredo
,
Kaustuv
Manna
,
Emily C.
Mcfarlane
,
Mihir
Date
,
Banabir
Pal
,
Jiabao
Yang
,
Eduardo
B. Guedes
,
J. Hugo
Dil
,
Craig M.
Polley
,
Mats
Leandersson
,
Chandra
Shekhar
,
Horst
Borrmann
,
Qun
Yang
,
Mao
Lin
,
Vladimir N.
Strocov
,
Marco
Caputo
,
Matthew D.
Watson
,
Timur K.
Kim
,
Cephise
Cacho
,
Federico
Mazzola
,
Jun
Fujii
,
Ivana
Vobornik
,
Stuart S. P.
Parkin
,
Barry
Bradlyn
,
Claudia
Felser
,
Maia G.
Vergniory
,
Niels B. M.
Schröter
Diamond Proposal Number(s):
[20617, 26098, 24703]
Open Access
Abstract: Spin-orbit coupling in noncentrosymmetric crystals leads to spin-momentum locking – a directional relationship between an electron’s spin angular momentum and its linear momentum. Isotropic orthogonal Rashba spin-momentum locking has been studied for decades, while its counterpart, isotropic parallel Weyl spin-momentum locking has remained elusive in experiments. Theory predicts that Weyl spin-momentum locking can only be realized in structurally chiral cubic crystals in the vicinity of Kramers-Weyl or multifold fermions. Here, we use spin- and angle-resolved photoemission spectroscopy to evidence Weyl spin-momentum locking of multifold fermions in the chiral topological semimetal PtGa. We find that the electron spin of the Fermi arc surface states is orthogonal to their Fermi surface contour for momenta close to the projection of the bulk multifold fermion at the Γ point, which is consistent with Weyl spin-momentum locking of the latter. The direct measurement of the bulk spin texture of the multifold fermion at the R point also displays Weyl spin-momentum locking. The discovery of Weyl spin-momentum locking may lead to energy-efficient memory devices and Josephson diodes based on chiral topological semimetals.
|
May 2024
|
|
I05-ARPES
|
Diamond Proposal Number(s):
[22375]
Abstract: Type-II topological Dirac semimetals are topological quantum materials hosting Lorentz-symmetry breaking type-II Dirac fermions, which are tilted Dirac cones with various exotic physical properties, such as anisotropic chiral anomalies and novel quantum oscillations. Until now, only limited material systems have been confirmed by theory and experiments with the type-II Dirac fermions. Here, we investigated the electronic structure of a new type-II Dirac semimetal VAl3 with angle-resolved photoelectron spectroscopy. The measured band dispersions are consistent with the theoretical prediction, which suggests the Dirac points are located close to (at about 100 meV above) the Fermi level. Our work demonstrates a new type-II Dirac semimetal candidate system with different Dirac node configurations and application potentials.
|
Mar 2022
|
|
I05-ARPES
|
Niels B. M.
Schroeter
,
Iñigo
Robredo
,
Sebastian
Klemenz
,
Robert J.
Kirby
,
Jonas A.
Krieger
,
Ding
Pei
,
Tianlun
Yu
,
Samuel
Stolz
,
Thorsten
Schmitt
,
Pavel
Dudin
,
Timur K.
Kim
,
Cephise
Cacho
,
Andreas
Schnyder
,
Aitor
Bergara
,
Vladimir N.
Strocov
,
Fernando
De Juan
,
Maia G.
Vergniory
,
Leslie M.
Schoop
Diamond Proposal Number(s):
[26098, 20617]
Open Access
Abstract: Magnetic Weyl semimetals are a newly discovered class of topological materials that may serve as a platform for exotic phenomena, such as axion insulators or the quantum anomalous Hall effect. Here, we use angle-resolved photoelectron spectroscopy and ab initio calculations to discover Weyl cones in CoS2, a ferromagnet with pyrite structure that has been long studied as a candidate for half-metallicity, which makes it an attractive material for spintronic devices. We directly observe the topological Fermi arc surface states that link the Weyl nodes, which will influence the performance of CoS2 as a spin injector by modifying its spin polarization at interfaces. In addition, we directly observe a minority-spin bulk electron pocket in the corner of the Brillouin zone, which proves that CoS2 cannot be a true half-metal.
|
Dec 2020
|
|
I05-ARPES
|
Niels B. M.
Schroeter
,
Samuel
Stolz
,
Kaustuv
Manna
,
Fernando
De Juan
,
Maia G.
Vergniory
,
Jonas A.
Krieger
,
Ding
Pei
,
Thorsten
Schmitt
,
Pavel
Dudin
,
Timur K.
Kim
,
Cephise
Cacho
,
Barry
Bradlyn
,
Horst
Borrmann
,
Marcus
Schmidt
,
Roland
Widmer
,
Vladimir N.
Strocov
,
Claudia
Felser
Diamond Proposal Number(s):
[24703, 20617]
Open Access
Abstract: Topological semimetals feature protected nodal band degeneracies characterized by a topological invariant known as the Chern number (C). Nodal band crossings with linear dispersion are expected to have at most |C|=4
|
C
|
=
4
, which sets an upper limit to the magnitude of many topological phenomena in these materials. Here, we show that the chiral crystal palladium gallium (PdGa) displays multifold band crossings, which are connected by exactly four surface Fermi arcs, thus proving that they carry the maximal Chern number magnitude of 4. By comparing two enantiomers, we observe a reversal of their Fermi-arc velocities, which demonstrates that the handedness of chiral crystals can be used to control the sign of their Chern numbers.
|
Jul 2020
|
|
I05-ARPES
|
Paolo
Sessi
,
Feng-Ren
Fan
,
Felix
Küster
,
Kaustuv
Manna
,
Niels B. M.
Schroeter
,
Jing-Rong
Ji
,
Samuel
Stolz
,
Jonas A.
Krieger
,
Ding
Pei
,
Timur K.
Kim
,
Pavel
Dudin
,
Cephise
Cacho
,
Remo N.
Widmer
,
Horst
Borrmann
,
Wujun
Shi
,
Kai
Chang
,
Yan
Sun
,
Claudia
Felser
,
Stuart S. P.
Parkin
Diamond Proposal Number(s):
[2470, 20617]
Open Access
Abstract: It has recently been proposed that combining chirality with topological band theory results in a totally new class of fermions. Understanding how these unconventional quasiparticles propagate and interact remains largely unexplored so far. Here, we use scanning tunneling microscopy to visualize the electronic properties of the prototypical chiral topological semimetal PdGa. We reveal chiral quantum interference patterns of opposite spiraling directions for the two PdGa enantiomers, a direct manifestation of the change of sign of their Chern number. Additionally, we demonstrate that PdGa remains topologically non-trivial over a large energy range, experimentally detecting Fermi arcs in an energy window of more than 1.6 eV that is symmetrically centered around the Fermi level. These results are a consequence of the deep connection between chirality in real and reciprocal space in this class of materials, and, thereby, establish PdGa as an ideal topological chiral semimetal.
|
Jul 2020
|
|
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
|
|
I05-ARPES
|
Cheng
Chen
,
Meixiao
Wang
,
Jinxiong
Wu
,
Huixia
Fu
,
Haifeng
Yang
,
Zhen
Tian
,
Teng
Tu
,
Han
Peng
,
Yan
Sun
,
Xiang
Xu
,
Juan
Jiang
,
Niels B. M.
Schroeter
,
Yiwei
Li
,
Ding
Pei
,
Shuai
Liu
,
Sandy A.
Ekahana
,
Hongtao
Yuan
,
Jiamin
Xue
,
Gang
Li
,
Jinfeng
Jia
,
Zhongkai
Liu
,
Binghai
Yan
,
Hailin
Peng
,
Yulin
Chen
Diamond Proposal Number(s):
[18005]
Open Access
Abstract: Semiconductors are essential materials that affect our everyday life in the modern world. Two-dimensional semiconductors with high mobility and moderate bandgap are particularly attractive today because of their potential application in fast, low-power, and ultrasmall/thin electronic devices. We investigate the electronic structures of a new layered air-stable oxide semiconductor, Bi2O2Se, with ultrahigh mobility (~2.8 × 105 cm2/V⋅s at 2.0 K) and moderate bandgap (~0.8 eV). Combining angle-resolved photoemission spectroscopy and scanning tunneling microscopy, we mapped out the complete band structures of Bi2O2Se with key parameters (for example, effective mass, Fermi velocity, and bandgap). The unusual spatial uniformity of the bandgap without undesired in-gap states on the sample surface with up to ~50% defects makes Bi2O2Se an ideal semiconductor for future electronic applications. In addition, the structural compatibility between Bi2O2Se and interesting perovskite oxides (for example, cuprate high–transition temperature superconductors and commonly used substrate material SrTiO3) further makes heterostructures between Bi2O2Se and these oxides possible platforms for realizing novel physical phenomena, such as topological superconductivity, Josephson junction field-effect transistor, new superconducting optoelectronics, and novel lasers.
|
Sep 2018
|
|
