I05-ARPES
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Hongyun
Zhang
,
Jinxi
Lu
,
Kai
Liu
,
Yijie
Wang
,
Size
Wu
,
Wanying
Chen
,
Xuanxi
Cai
,
Kenji
Watanabe
,
Takashi
Taniguchi
,
Jose
Avila
,
Pavel
Dudin
,
Matthew D.
Watson
,
Alex
Louat
,
Takafumi
Sato
,
Pu
Yu
,
Wenhui
Duan
,
Zhida
Song
,
Guorui
Chen
,
Shuyun
Zhou
Diamond Proposal Number(s):
[37939]
Abstract: The fractional quantum anomalous Hall effect (FQAHE) is a fascinating emergent quantum state characterized by fractionally charged excitations in the absence of a magnetic field. Recently, the FQAHE has been observed in aligned rhombohedral pentalayer graphene on BN (aligned R5G/BN)1 with moiré potential. Intriguingly, the FQAHE preferably emerges when carriers are displaced away from the moiré interface1,2,3, raising debates about the role of moiré potential4,5,6,7,8,9,10,11,12,13,14,15,16,17. Here, by performing nanospot angle-resolved photoemission spectroscopy, we directly visualize the topological flat band in both aligned and non-aligned R5G/BN. The moiré potential in the aligned sample generates moiré bands and enhances the topological flat band as compared to non-aligned sample. Combined with theoretical calculations, we propose that the moiré bands on the top surface arise through the interlayer Coulomb interaction with the moiré-modulated bottom layer. Our results provide direct experimental evidence for the role of moiré potential in aligned rhombohedral graphene, and establish a foundation for understanding its emergent quantum phenomena.
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Nov 2025
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I05-ARPES
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Diamond Proposal Number(s):
[34479, 33317, 35210]
Open Access
Abstract: Understanding the interface between metals and two-dimensional materials is critical for their application in electronics and for the development of metal-mediated exfoliation of large area monolayers. Studying the intricate interactions at the interface requires model systems that enable control of the roughness, purity, and crystallinity of the metal surface. Here, we use angle-resolved photoemission spectroscopy to investigate the layer-dependent electronic structure of WSe\textsubscript{2}~on template-stripped gold substrates fabricated using both silicon and mica templates, giving crystallographically disordered and Au(111) ordered surfaces, respectively, and contrast these findings with \textit{ab initio} predictions. We observe strong hybridization around the Brillouin zone centre at $\overline{\Gamma}$, indicating a covalent admixture in the gold-WSe\textsubscript{2}~interaction, and band shifts that suggest charge rearrangement at the Au(111) / WSe\textsubscript{2}~interface. Core-level spectroscopy shows a single chemical environment for the interfacial WSe\textsubscript{2}~layer on the template-stripped gold, distinct from the subsequent layers. These results reveal a mixture of van der Waals and covalent interactions, best described as a covalent-like quasi-bonding with intermediate interaction strength.
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Oct 2025
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I05-ARPES
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Diamond Proposal Number(s):
[28445]
Open Access
Abstract: The metallic delafossites host ultrahigh-mobility carriers in the bulk, while at their polar surfaces, intrinsic electronic reconstructions stabilize markedly distinct electronic phases, from charge-disproportionated insulators to Rashba-split heavy-hole gases and ferromagnetic metals. The understanding of these phases has been strongly informed by surface spectroscopic measurements, but previous studies have been complicated by the presence of spatially varying terminations of the material surface. Here, we demonstrate the potential of microscopic-area angle-resolved photoemission to overcome these challenges. Our measurements of the model compound PdCoO2 yield extremely high quality spectra of the electronic structure, which allows us to place stringent experimental constraints on the weak electron-phonon coupling in the bulk of PdCoO2, while revealing much stronger interactions at its surfaces. While the CoO2-terminated surface exhibits a conventional weak-coupling behavior, our measurements reveal surprising spectroscopic signatures of polaron formation at the Pd-terminated surface, despite its pronounced metallicity. Together, our findings reveal how mode- and symmetry-selective couplings can markedly tune the electron-phonon interactions in a single host material, here opening routes to stabilize surprisingly persistent polaronic quasiparticles.
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Aug 2025
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Open Access
Abstract: TiSe2 is a narrow-gap insulator with a rich array of unique properties. In addition to being a superconductor under certain modifications, it is commonly thought to be a rare realisation of an excitonic insulator. Below 200 K, TiSe2 undergoes a transition from a high-symmetry () phase to a low-symmetry () charge density wave (CDW). Here we establish that it is indeed an insulator in both and phases. However, the insulating state is driven not by excitonic effects but by symmetry-breaking. In the CDW phase it is static. At high temperature, thermally driven instantaneous deviations from break the symmetry on the characteristic time scale of a phonon. Even though the time-averaged lattice structure assumes symmetry, the time-averaged energy band structure is closer to the CDW phase – a rare instance of a metal-insulator transition induced by dynamical symmetry breaking. We establish these conclusions from quasiparticle self-consistent GW (QSGW) and many-body calculations (QS), in combination with molecular dynamics simulations to capture the effects of thermal disorder. The many-body theory includes explicitly ladder diagrams in the polarizability, which incorporates excitonic effects in an ab initio manner. We find that the excitonic modification to the potential is weak, ruling out the possibility that TiSe2 is an excitonic insulator.
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May 2025
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I05-ARPES
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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
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I05-ARPES
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Diamond Proposal Number(s):
[32858]
Open Access
Abstract: Rare-earth tellurides present unique opportunities to explore the interplay between charge density waves (CDWs) and electronic structure in quasi-two-dimensional systems. Unlike their van der Waals counterparts, the rare-earth tritellurides, ditellurides such as LaTe2 feature distinct polar surfaces and Fermi-surface nesting properties. In this paper, we investigate the electronic band structure of LaTe2, distinguishing the surface states from the bulk bands, and enabling the study of the impact of band filling on CDW stability. We uncover the intricate dependence of CDW formation on the electronic environment and highlight the robustness of the CDW order against variations in band filling. The results provide insights into the complex relationship between electronic structure and CDW formation in this class of materials.
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Feb 2025
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I05-ARPES
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Diamond Proposal Number(s):
[33694, 34335]
Open Access
Abstract: Stacking of strongly-correlated 2D materials is opening the possibility to demonstrate novel electronic or magnetic ordering phenomena. In this regard the intrinsic polytypism of tantalum dichalcogenides has emerged as a platform to generate clean and controllable material interfaces. Here, we report on the Fermi surface of 4Hb-TaSe2, a polytype which consists of alternately stacked layers with octahedral (T) and trigonal prismatic (H) coordination of tantalum in the Se-Ta-Se layers. The material is known to host a charge density wave (CDW) phase with star clusters in the T-layers, intercalated by metallic H-layers, but its momentum resolved electronic structure remains undetermined. Using selective area angle resolved photoemission spectroscopy on the T termination combined with ab initio calculations, we unveil a finely structured Fermi surface arising from band folding in the reconstructed Brillouin zone caused by the CDW star clusters. The star-shaped Fermi surface is rotated away from the high-symmetry directions of the normal phase, and exhibits pseudochirality. Theoretical analysis supports the metallic nature of the system and interlayer interactions leading to hybridization. The work provides a detailed overview on the impact of band hybridization with the CDW on the Fermi surface of a material for new phases of quantum matter.
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Feb 2025
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I05-ARPES
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Liam
Trzaska
,
Lei
Qiao
,
Matthew D.
Watson
,
Monica
Ciomaga Hatnean
,
Igor
Markovic
,
Edgar
Abarca Morales
,
Tommaso
Antonelli
,
Cephise
Cacho
,
Geetha
Balakrishnan
,
Wei
Ren
,
Silvia
Picozzi
,
Phil D. C.
King
Diamond Proposal Number(s):
[21986, 25564]
Open Access
Abstract: The recent discovery of the persistence of long-range magnetic order when van der Waals magnets are thinned towards monolayers provides a tunable platform for engineering of novel magnetic structures and devices. Here, we study the evolution of the electronic structure of CrGeTe3 as a function of surface electron doping. From angle-resolved photoemission, we observe spectroscopic fingerprints that this electron doping drives a marked increase in TC, reaching values more than double that of the undoped material, in agreement with recent studies using electrostatic gating. Together with density functional theory calculations and Monte Carlo simulations, we show that, surprisingly, the increased TC is mediated by the population of spin-minority Cr t2g states, forming a half-metallic 2D electron gas. This promotes a novel variant of double exchange, and unlocks a significant influence of Ge – which was previously thought to be electronically inert in this system – in mediating Cr-Cr exchange.
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Jan 2025
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I05-ARPES
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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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Jan 2025
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I05-ARPES
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Chun
Lin
,
Armando
Consiglio
,
Ola Kenji
Forslund
,
Julia
Kuespert
,
M. Michael
Denner
,
Hechang
Lei
,
Alex
Louat
,
Matthew D.
Watson
,
Timur K.
Kim
,
Cephise
Cacho
,
Dina
Carbone
,
Mats
Leandersson
,
Craig
Polley
,
Thiagarajan
Balasubramanian
,
Domenico
Di Sante
,
Ronny
Thomale
,
Zurab
Guguchia
,
Giorgio
Sangiovanni
,
Titus
Neupert
,
Johan
Chang
Diamond Proposal Number(s):
[30650, 33528]
Open Access
Abstract: Tunable quantum materials hold great potential for applications. Of special interest are materials in which small lattice strain induces giant electronic responses. The kagome compounds AV3Sb5 (A = K, Rb, Cs) provide a testbed for electronic tunable states. In this study, through angle-resolved photoemission spectroscopy, we provide comprehensive spectroscopic measurements of the electronic responses induced by compressive and tensile strains on the charge-density-wave (CDW) and van Hove singularity (VHS) in CsV3Sb5. We observe a tripling of the CDW gap magnitudes with ~ 1% strain. Simultaneously, changes of both energy and mass of the VHS are observed. Combined, this reveals an anticorrelation between the unconventional CDW order parameter and the mass of the VHS, and highlight the role of the latter in the superconducting pairing. The substantial electronic responses uncover a rich strain tunability of the versatile kagome system in studying quantum interplays under lattice variations.
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Dec 2024
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