I05-ARPES
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Y.
Morita
,
K.
Nakayama
,
N.
Ito
,
T.
Kato
,
T.
Nakamura
,
H.
Zhang
,
X.
Tang
,
T.
Lin
,
K.
Yanagizawa
,
S.
Souma
,
S.
Masaki
,
T.
Ikushima
,
Y.
Moriyasu
,
K.
Hagiwara
,
F.
Matsui
,
K.
Tanaka
,
K.
Ozawa
,
D.
Shiga
,
H.
Kumigashira
,
M.
Maeda
,
Y.
Niimi
,
T.
Kida
,
M.
Hagiwara
,
T. K.
Kim
,
C.
Cacho
,
T.
Takahashi
,
Y.
Okada
,
S.
Zhou
,
T.
Koretsune
,
K.
Kudo
,
T.
Sato
Diamond Proposal Number(s):
[38107]
Open Access
Abstract: Exploiting electronic singularities to drive emergent quantum phenomena is a key basis in condensed matter physics, yet this powerful strategy has been largely limited to two dimensions, leaving the potential of three-dimensional materials often inaccessible. A “saddle-loop” singularity—a one-dimensional ring of saddle points—has been proposed as a way to extend the concept of van Hove singularities to three-dimensional electron systems, but has remained experimentally elusive. Here, we provide the direct experimental evidence for the saddle‑loop singularity in Pt(Bi1−xSex)2, using angle‑resolved photoemission spectroscopy. We also show that the saddle loop is positioned closer to the Fermi level at low x values where the superconducting critical temperature is higher. Furthermore, our advanced theoretical calculations reveal that the saddle loop originates from inter-orbital hybridization, establishing a generalizable design concept. Our findings demonstrate a viable mechanism for realizing singular electronic states in three-dimensional electron systems, providing a platform to investigate correlation-driven phenomena in three dimensions.
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Jun 2026
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I05-ARPES
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Junhyeok
Jeong
,
Yamato
Enomoto
,
Yoshimitsu
Kohama
,
Tomotaka
Nakayama
,
Kotaro
Ando
,
Kifu
Kurokawa
,
Soonsang
Huh
,
Zhuo
Yang
,
Toshihiro
Nomura
,
Matthew D.
Watson
,
Timur K.
Kim
,
Cephise
Cacho
,
Chun
Lin
,
Makoto
Hashimoto
,
Donghui
Lu
,
Shiro
Sakai
,
Takami
Tohyama
,
Kazuyasu
Tokiwa
,
Takeshi
Kondo
Diamond Proposal Number(s):
[36822, 30646, 28930, 25416]
Open Access
Abstract: Fermi arcs observed in underdoped cuprates have sparked debate over whether they represent segments of a large Fermi surface or small Fermi pockets. This ambiguity has long hindered their classification as either the conventional Bardeen-Cooper-Schrieffer (BCS) regime or the strongly coupled Bose-Einstein condensation (BEC) crossover limit. Here, using angle-resolved photoemission spectroscopy and quantum oscillations, we demonstrate the coexistence of a small Fermi pocket and a large superconducting gap in the clean inner CuO2 layers of the four-layer cuprate Ba2Ca3Cu4O8(F,O)2. This coexistence constitutes a hallmark of the BCS-BEC crossover and has remained elusive for decades. Despite the presence of antiferromagnetic (AF) order, the superconducting gap in the small pocket is remarkably large, yielding a gap-to-Fermi energy ratio (Δpocket/εF ~ 0.6) and a critical-to-Fermi temperature ratio (Tc/TF ~ 0.13) that reach the theoretical upper bound for two-dimensional superconductivity. Unexpectedly, this BCS-BEC crossover emerges not as the carrier density decreases but as it increases, abruptly within a narrow doping range of less than 1%. These results provide a long-sought microscopic foundation for the d-wave pairing mechanism in doped AF-Mott insulators.
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Jun 2026
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I05-ARPES
|
Chan-Young
Lim
,
Francesc
Ballester
,
Arunava
Kar
,
Manex
Alkorta
,
David
Subires
,
Ji
Dai
,
Massimo
Tallarida
,
Elio
Vescovo
,
Timur K.
Kim
,
Cephise
Cacho
,
Changjiang
Yi
,
Subhajit
Roychowdhury
,
Avdhesh Kumar
Sharma
,
Yongseong
Choi
,
Gilberto
Fabbris
,
Joerg
Strempfer
,
Pierluigi
Gargiani
,
Chandra
Shekhar
,
Claudia
Felser
,
Ion
Errea
,
Maia G.
Vergniory
,
Santiago
Blanco-Canosa
Diamond Proposal Number(s):
[36505]
Abstract: Kagome materials are known for hosting emergent quantum phenomena driven by the interaction between different lattice, charge, and spin orders. Here, we present a detailed angle-resolved photoemission (ARPES), density functional theory (DFT), and x-ray magnetic circular dichroism (XMCD) study of the electronic and magnetic structure of 𝑅Ti3Bi4 (𝑅=Nd, Sm, Gd). ARPES and DFT demonstrate that the bulk electronic band structure is dominated by the hybridization of the Ti bands, and the weak electron-like pocket at Γ is identified as a surface state. The isotropic XAS profile of the 𝑀4,5 edge of the rare earth is consistent with the presence of the 𝑅3+ oxidation state. Using the XMCD sum rules, backed by the atomic-multiplet-theory calculations, we obtain the spin and orbital magnetic moments. The Ti 𝐿2,3-edge XMCD reveals the presence of a small magnetic moment in GdTi3Bi4, presumably driven by the proximity of the Ti kagome layers to the zigzag chains of Gd, while the total magnetic moment of Gd is shared by the 𝑓 and 𝑑 electrons. Our combined XMCD, ARPES, and DFT study provides an important piece of information to understand the spin-flip transitions and anomalous Hall effect observed in the 𝑅Ti3Bi4 kagome metals.
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Jun 2026
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I05-ARPES
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Z. W.
Riedel
,
P. A. E.
Murgatroyd
,
C. S.
Kengle
,
P. M. T.
Trocado Vianez
,
A.
Schmidt
,
X.
Du
,
K.
Allen
,
Ti. K.
Kim
,
C.
Lane
,
Y. W.
Li
,
J.-X.
Zhu
,
J. D.
Thompson
,
F.
Ronning
,
S. M.
Thomas
,
P. F. S.
Rosa
,
E. D.
Bauer
Diamond Proposal Number(s):
[41855]
Abstract: The chemical flexibility of the 𝑅𝑀6𝑋6 stoichiometry, where an 𝑓-block element is intercalated in the CoSn structure type, allows for the tuning of flatbands associated with kagome lattices to the Fermi level and for emergent phenomena due to interactions between the 𝑓- and 𝑑-electron lattices. Yet, 5𝑓 members of the “166” compounds are underrepresented compared with 4𝑓 members. Here, we report single-crystal growth of UCr6Ge6, which crystallizes in a monoclinically distorted Y0.5Co3Ge3-type structure. The real-space character of the modulation, which is unique within the 𝑅𝑀6𝑋6 family, is approximated by a 3×1×2 supercell of the average monoclinic cell. The compound has kagome-lattice flatbands near the Fermi level and a moderately enhanced electronic heat capacity, as evidenced by its low-temperature Sommerfeld coefficient (𝛾=86.5 mJ mol−1 K−2) paired with band structure calculations. The small, isotropic magnetization and featureless resistivity of UCr6Ge6 suggest itinerant uranium 5𝑓 electrons and Pauli paramagnetism. Angle-resolved photoemission spectroscopy results provide evidence for uranium 5𝑓 weight at the Fermi level and for a flatband near the Fermi level associated with the chromium 3𝑑 kagome lattice. The isotropic magnetic behavior of the uranium 5𝑓 electrons starkly contrasts with localized behavior in other uranium 166 compounds, highlighting the high tunability of the magnetic ground state across the material family.
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May 2026
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I05-ARPES
|
Diamond Proposal Number(s):
[28484, 31262]
Open Access
Abstract: We report pressure-induced metallization in BaMn2P2 based on resistivity measurements in a diamond anvil cell. At ambient pressure, the temperature-dependent resistivity is well described by a two-gap Arrhenius model, yielding intrinsic and extrinsic activation energies of approximately 0.2 and 0.04 eV, respectively. Angle-resolved photoemission spectroscopy (ARPES) shows no detectable spectral weight at the Fermi level within the measured momentum window. The growth of spectral weight at higher binding energies is consistent with the energy scales inferred from transport measurements. Under pressure, the temperature dependence of the resistivity evolves from insulatinglike to mixed-slope behavior and becomes metallic above Pc ≈ 7 GPa, with no low-temperature upturn. The resistivity ratio R(P) = ρ(100 K)/ρ(300 K) also drops abruptly near Pc. A baseline transport model combining Bloch-Grüneisen phonon scattering with a thermally activated carrier density fails to reproduce this sharp change for any smoothly varying activation energy. Temperature- and pressure-dependent x-ray diffraction shows smooth evolution of V with no symmetry change and no resolvable discontinuity at Pc . Taken together, these results indicate an abrupt pressure-driven metallization near Pc, with no evidence for a structural phase transition within our experimental resolution.
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May 2026
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I05-ARPES
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Songyuan
Geng
,
Xin
Wang
,
Risi
Guo
,
Chen
Qiu
,
Fangjie
Chen
,
Qun
Wang
,
Kangjie
Li
,
Peipei
Hao
,
Hanpu
Liang
,
Yang
Huang
,
Yunbo
Wu
,
Shengtao
Cui
,
Zhe
Sun
,
Timur K.
Kim
,
Cephise
Cacho
,
Daniel S.
Dessau
,
Benjamin T.
Zhou
,
Haoxiang
Li
Diamond Proposal Number(s):
[38254]
Open Access
Abstract: Flat electronic bands, where interactions among electrons overwhelm their kinetic energies, hold the promise for exotic correlation physics. The dice lattice has long been theorized as a host of flat bands with intriguing band topology. However, to date, no material has ever been found to host the characteristic flat bands of a dice lattice. Here, using angle-resolved photoemission spectroscopy (ARPES), we discover a dice-lattice flat band at EF in the van der Waals (vdW) electride [YCl]2+: 2e-. In this system, excess valence electrons from Y deconfine from the cation framework to form an interstitial anionic electron lattice that constitutes the dice lattice. Our ARPES measurements unambiguously identify two sets of dice-lattice bands in YCl, including a nearly dispersionless band at the Fermi level. The near-EF electronic structure observed in ARPES, which consists of the flat bands and other dispersive band features, find excellent agreement with first-principles calculations and is well captured by a simple dice-lattice model. Our findings thus end the long quest of a real dice flat band material and establish vdW electride YCl as a prototype of dice metals. Our results further demonstrate the anionic electron lattice as a novel scheme for realizing lattice geometries and electronic structures rare to find in conventional crystalline systems.
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Jan 2026
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I05-ARPES
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I.
Biało
,
Qisi
Wang
,
J.
Küspert
,
X.
Hong
,
L.
Martinelli
,
O.
Gerguri
,
Y.
Chan
,
K.
Von Arx
,
O. K.
Forslund
,
W. R.
Pudełko
,
C.
Lin
,
N. C.
Plumb
,
Y.
Sassa
,
D.
Betto
,
N. B.
Brookes
,
M.
Rosmus
,
N.
Olszowska
,
Ma. D.
Watson
,
T. K.
Kim
,
C.
Cacho
,
M.
Horio
,
M.
Ishikado
,
H. M.
Rønnow
,
J.
Chang
Diamond Proposal Number(s):
[32147]
Open Access
Abstract: Strong electron correlations drive Mott insulator transitions. Yet, there exists no framework to classify Mott insulators by their degree of correlation. Cuprate superconductors, with their tunable doping and rich phase diagrams, offer a unique platform to investigate the evolution of these interactions. However, spectroscopic access to a clean half-filled Mott-insulating state is lacking in compounds with the highest superconducting onset temperature. To fill this gap, we introduce a pristine, half-filled thallium-based cuprate system, Tl2Ba5Cu4Ox. Using high-resolution resonant inelastic x-ray scattering, we probe long-lived magnon excitations and uncover a pronounced kink in the magnon dispersion, marked by a simultaneous change in group velocity and lifetime broadening. Modeling the dispersion within a Hubbard-Heisenberg approach, we extract the interaction strength and compare it with other cuprate systems. Our results establish a cuprate universal relation between electron-electron interaction and magnon zone-boundary dispersion. Superconductivity seems to be optimal at intermediate correlation strength, suggesting an optimal balance between localization and itinerancy.
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Dec 2025
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I05-ARPES
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Cong
Li
,
Yang
Wang
,
Jianfeng
Zhang
,
Hongxiong
Liu
,
Wanyu
Chen
,
Guowei
Liu
,
Hanbin
Deng
,
Timur K.
Kim
,
Craig
Polley
,
Balasubramanian
Thiagarajan
,
Jiaxin
Yin
,
Youguo
Shi
,
Tao
Xiang
,
Oscar
Tjernberg
Diamond Proposal Number(s):
[34265, 39652]
Open Access
Abstract: For several decades, it was widely believed that a noninteracting disordered electronic system could only undergo an Anderson metal–insulator transition due to Anderson localization. However, numerous recent theoretical works have predicted the existence of a disorder-driven non-Anderson phase transition that differs from Anderson localization. The frustration lies in the fact that this non-Anderson disorder-driven transition has not yet been experimentally demonstrated in any system. Here, using angle-resolved photoemission spectroscopy, we present a case study of observing the non-Anderson disorder-driven transition by visualizing the electronic structure of the Weyl semimetal NdAlSi on surfaces with varying amounts of disorder. Our observations reveal that strong disorder can effectively suppress all surface states in the Weyl semimetal NdAlSi, including the topological surface Fermi arcs. This disappearance of surface Fermi arcs is associated with the vanishing of the topological invariant, indicating a quantum phase transition from a Weyl semimetal to a diffusive metal. These observations provide direct experimental evidence of the non-Anderson disorder-driven transition occurring in real quantum systems, a finding long anticipated by theoretical physicists.
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Oct 2025
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I05-ARPES
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Y.
Alexanian
,
A.
De La Torre
,
S.
Mckeown Walker
,
M.
Straub
,
G.
Gatti
,
A.
Hunter
,
S.
Mandloi
,
E.
Cappelli
,
S.
Riccò
,
F. Y.
Bruno
,
M.
Radovic
,
N. C.
Plumb
,
M.
Shi
,
J.
Osiecki
,
C.
Polley
,
T. K.
Kim
,
P.
Dudin
,
M.
Hoesch
,
R. S.
Perry
,
A.
Tamai
,
F.
Baumberger
Diamond Proposal Number(s):
[10348, 12404, 17381]
Open Access
Abstract: The fate of the Fermi surface in bulk electron-doped Sr2IrO4 remains elusive, as does the origin and extension of its pseudogap phase. Here, we use high-resolution angle-resolved photoelectron spectroscopy (ARPES) to investigate the electronic structure of Sr2−xLaxIrO4 up to x = 0.2, a factor of two higher than in previous work. We find that the antinodal pseudogap persists up to the highest doping level, and thus beyond the sharp increase in Hall carrier density to ≃ 1 + x recently observed above x* ≃ 0.161. This suggests that doped iridates host a unique phase of matter in which a large Hall density coexists with an anisotropic pseudogap, breaking up the Fermi surface into disconnected arcs. The temperature boundary of the pseudogap is T* ≃ 200 K for x = 0.2, comparable to cuprates and to the energy scale of short range antiferromagnetic correlations in cuprates and iridates.
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Oct 2025
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I05-ARPES
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Cong
Li
,
Mengli
Hu
,
Zhilin
Li
,
Yang
Wang
,
Wanyu
Chen
,
Balasubramanian
Thiagarajan
,
Mats
Leandersson
,
Craig
Polley
,
Timur
Kim
,
Hui
Liu
,
Cosma
Fulga
,
Maia G.
Vergniory
,
Oleg
Janson
,
Oscar
Tjernberg
,
Jeroen
Van Den Brink
Diamond Proposal Number(s):
[36464]
Open Access
Abstract: Altermagnets constitute a novel, third fundamental class of collinear magnetic ordered materials, alongside with ferro- and antiferromagnets. They share with conventional antiferromagnets the feature of a vanishing net magnetization. At the same time they show a spin-splitting of electronic bands, just as in ferromagnets, caused by the atomic exchange interaction. On the other hand, topology has recently revolutionized our understanding of condensed matter physics, introducing new phases of matter classified by intrinsic topological order. Here we connect the worlds of altermagnetism and topology, showing that the electronic structure of the altermagnet CrSb is topological. Using high-resolution angle-resolved photoemission spectroscopy, we observe the large momentum-dependent spin-splitting in CrSb that induces altermagnetic Weyl nodes. We observe the related topological Fermi-arcs, which in electronic structure calculations are spin polarized. This indicates that in altermagnets the large energy scale intrinsic to their spin-splitting creates its own realm of robust electronic topology.
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Jul 2025
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