|
|
Emily C.
Mcfarlane
,
Antonio
Sanna
,
Matthew J.
Gilbert
,
Jonas A.
Krieger
,
Mihir
Date
,
Gabriele
Domaine
,
Banabir
Pal
,
Anirban
Chakraborty
,
Pranava K.
Sivakumar
,
Procopios C.
Constantinou
,
Anna
Hartl
,
Enrico G.
Della Valle
,
Camilla
Pellegrini
,
Vladimir N.
Strocov
,
Stuart S. P.
Parkin
,
Niels B. M.
Schroeter
Open Access
Abstract: Superconductivity in the transition-metal dichalcogenide PdTe2 has been attributed to the proximity of a three-dimensional Van Hove singularity to the Fermi level. In isostructural NiTe2, recently used as the weak link in a Josephson diode, a similar Van Hove singularity has been predicted to occur, but superconductivity is mysteriously absent. Using bulk-sensitive soft x-ray angle-resolved photoemission spectroscopy, we reveal that this Van Hove singularity lies even closer to the Fermi level in NiTe2 than in PdTe2. To explain the lack of superconductivity in NiTe2, we perform ab initio calculations incorporating the Kukkonen Overhauser interaction, showing that an incipient magnetic instability suppresses superconductivity at an unprecedented scale. Finally, we present a tight-binding model that links the Van Hove singularity to a sign change in the Josephson diode effect at small magnetic fields, suggesting a new mechanism for Josephson diodes.
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Feb 2026
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I05-ARPES
|
Diamond Proposal Number(s):
[36633]
Open Access
Abstract: The 4Hb polytype of TaS2 is a natural heterostructure of H and T-type layers. Intriguing recent evidence points towards a possibly chiral superconducting ground state, unlike the superconductivity found in other polytypes where the T layers are absent, requiring understanding of the possible contributions of electrons from the T layers. Here we use micro-focused angle resolved photoemission spectroscopy to reveal that the T termination of the 4Hb structure is metallic, but a subsurface T layer - seen below an H termination and thus more representative of the bulk case - is gapped. The results imply a complete charge transfer of 1 electron per 13 Ta from the T to adjacent H layers in the bulk, but an incomplete charge transfer at the T termination, yielding a metallic Fermi surface with a planar-chiral character. A similar metallic state is found in an anomalous region with likely T-H-H’ stacking at the surface. Our results exclude cluster Mott localisation in either the bulk or surface of 4Hb-TaS2 and point to a scenario of superconductivity arising from Josephson-like tunneling between the H layers.
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Jan 2026
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|
I05-ARPES
|
Gabriele
Domaine
,
Moritz M.
Hirschmann
,
Kirill
Parshukov
,
Mihir
Date
,
Holger L.
Meyerheim
,
Matthew D.
Watson
,
Katayoon
Mohseni
,
Sydney K. Y.
Dufresne
,
Shigemi
Terakawa
,
Marcin
Rosmus
,
Natalia
Olszowska
,
Stuart S. P.
Parkin
,
Andreas P.
Schnyder
,
Niels B. M.
Schroeter
Diamond Proposal Number(s):
[39232]
Open Access
Abstract: Kramers nodal lines are doubly degenerate band crossings in achiral non-centrosymmetric crystals, arising from spin-orbit coupling and connecting time-reversal invariant momenta. When intersecting the Fermi level, they generate exotic three-dimensional Fermi surfaces, in some cases described by two-dimensional massless Dirac fermions, enabling enhanced graphene-like physics such as quantized optical conductivity and large anomalous Hall effects. However, no experimental realization of such materials has been reported. Here, we identify Kramers nodal line metals beyond the case of Fermi surfaces enclosing a single time-reversal invariant momentum. Using angle-resolved photoemission spectroscopy and first-principles calculations, we show that 3R-TaS2 and 3R-NbS2 host open Octdong and Spindle-torus Fermi surfaces, respectively. We observe a filling-controlled transition between these configurations and evidence of size quantization in 3R-TaS2 inclusions within 2H-TaS2. We further predict a strain- or pressure-driven transition to a conventional metal. Our results establish 3R transition-metal dichalcogenides as a tunable platform for Kramers nodal line physics.
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Dec 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.
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Nov 2025
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|
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.
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Apr 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.
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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
|
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