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Robust topological surface states in skyrmion-host magnets Eu(Ga,Al)4: evidence for dual topology
Authors:
Yuki Arai,
Kosuke Nakayama,
Takemi Kato,
Tomonori Nakamura,
Asuka Honma,
Seigo Souma,
Kenichi Ozawa,
Kiyohisa Tanaka,
Daisuke Shiga,
Hiroshi Kumigashira,
Yoshinori Okada,
Kouji Segawa,
Takafumi Sato
Abstract:
The interplay between real-space topology such as magnetic skyrmions and momentum-space topology characterized by topological surface states (TSSs) is predicted to realize novel phenomena and functionalities, yet materials hosting both topologies are scarce. Skyrmion-hosting helimagnet family EuGa$_2$Al$_2$ and EuAl$_4$ has been a prime candidate for such a dual-topology system, but conclusive evi…
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The interplay between real-space topology such as magnetic skyrmions and momentum-space topology characterized by topological surface states (TSSs) is predicted to realize novel phenomena and functionalities, yet materials hosting both topologies are scarce. Skyrmion-hosting helimagnet family EuGa$_2$Al$_2$ and EuAl$_4$ has been a prime candidate for such a dual-topology system, but conclusive evidence for its momentum-space topology has remained elusive. We provide this evidence by directly observing TSSs that stem from bulk Dirac nodal lines using high-resolution angle-resolved photoemission spectroscopy. These TSSs are exceptionally robust against various perturbations such as a 2$\times$1 surface reconstruction, a chemical change in the termination of the crystal surface, and the onset of helical antiferromagnetic order. Crucially, below the Neel temperature, we observe replica bands driven by the magnetic ordering. Moreover, we demonstrate clear surface-termination dependence of this magneto-topological coupling. Our findings establish Eu(Ga$_{1-x}$Al$_x$)$_4$ as a dual-topology material and offer a rare platform to explore and control the interaction between the two fundamental topological realms.
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Submitted 14 April, 2026;
originally announced April 2026.
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Origin of multiple skyrmion phases in EuAl4
Authors:
Y. Arai,
K. Nakayama,
A. Honma,
S. Souma,
D. Shiga,
H. Kumigashira,
T. Takahashi,
K. Segawa,
T. Sato
Abstract:
The Dzyaloshinskii-Moriya (DM) interaction has been considered essential for skyrmion formation, however, the discovery of skyrmion lattices (SkLs) in nominally centrosymmetric materials where the DM interaction is forbidden, such as Eu(Ga$_{1-x}$Al$_x$)$_4$, has challenged this established view. Recent structural investigations of Eu(Ga$_{1-x}$Al$_x$)$_4$ have further complicated this issue by re…
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The Dzyaloshinskii-Moriya (DM) interaction has been considered essential for skyrmion formation, however, the discovery of skyrmion lattices (SkLs) in nominally centrosymmetric materials where the DM interaction is forbidden, such as Eu(Ga$_{1-x}$Al$_x$)$_4$, has challenged this established view. Recent structural investigations of Eu(Ga$_{1-x}$Al$_x$)$_4$ have further complicated this issue by revealing that the charge-density wave breaks local symmetry, theoretically allowing DM interaction. This raises a fundamental question: are the complex magnetic phases driven by the DM interaction or by alternative mechanisms? Here, using soft-x-ray angle-resolved photoemission spectroscopy, we determine the three-dimensional bulk electronic structure of Eu(Ga$_{1-x}$Al$_x$)$_4$, and elucidate the electronic origins of its rich magnetic orders. We directly observe an x-dependent Lifshitz transition leading to the emergence of a Fermi-surface pocket. Importantly, multiple nesting vectors derived from this pocket match the symmetries and periodicities of the multiple SkLs. Moreover, these nesting vectors can also account for other magnetic orders, such as the zero-field helical magnetism, suggesting a common electronic origin of the complex magnetic phases. These findings suggest that competing nesting-induced Ruderman-Kittel-Kasuya-Yosida interactions and their engineering can generate and control various SkLs and related topological spin textures.
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Submitted 14 April, 2026;
originally announced April 2026.
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Discovery of a Highly Anisotropic Type-II Ferromagnetic Weyl State Exhibiting a 3D Quantum Hall Effect
Authors:
Yingdong Guan,
Abhinava Chatterjee,
Trace Bivens,
Seng Huat Lee,
Asuka Honma,
Hirofumi Oka,
Jorge D Vega Bazantes,
Ruiqi Zhang,
David Graf,
Jianwei Sun,
Seigo Souma,
Takafumi Sato,
Yong P. Chen,
Yuanxi Wang,
Chaoxing Liu,
Zhiqiang Mao
Abstract:
Topological semimetals, particularly Weyl semimetals (WSMs), are crucial platforms for exploring emergent quantum phenomena due to their unique electronic structures and potential to transition into various topological phases. In this study, we report the discovery of a ferromagnetic (FM) type-II WSM in Mn(Bi1-xSbx)4Te7, which exhibits a remarkable three-dimensional (3D) quantum Hall effect (QHE).…
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Topological semimetals, particularly Weyl semimetals (WSMs), are crucial platforms for exploring emergent quantum phenomena due to their unique electronic structures and potential to transition into various topological phases. In this study, we report the discovery of a ferromagnetic (FM) type-II WSM in Mn(Bi1-xSbx)4Te7, which exhibits a remarkable three-dimensional (3D) quantum Hall effect (QHE). By precisely tuning the chemical potential through Sb doping, we obtained samples with the Fermi level near the charge neutrality point for x = ~ 0.27. This was confirmed by spectroscopy measurements (ARPES and STS), and these samples showed strong quantum oscillations along with a key transport signature of a Weyl state - chiral anomaly, and Fermi surface reconstruction driven by FM ordering. Our theoretical analysis indicates that this Weyl state evolves from a parent nodal ring state, where higher-order k-terms split the nodal line into type-II Weyl nodes. The Weyl state exhibits significant anisotropy, characterized by a pronounced reduction in Fermi velocity along the kz-axis, likely accounting for the observed 3D QHE. These results not only highlight the exceptional tunability of the Mn(Bi1-xSbx)4Te7 system, where precise control of the chemical potential and magnetic properties opens access to novel quantum phases, but also advance the understanding of FM WSMs.
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Submitted 10 March, 2025;
originally announced March 2025.
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Spin-degenerate bulk bands and topological surface states associated with Dirac nodal lines in RuO2
Authors:
T. Osumi,
K. Yamauchi,
S. Souma,
S. Paul,
A. Honma,
K. Nakayama,
K. Ozawa,
M. Kitamura,
K. Horiba,
H. Kumigashira,
C. Bigi,
F. Bertran,
T. Oguchi,
T. Takahashi,
Y. Maeno,
T. Sato
Abstract:
Altermagnets are a novel platform to realize exotic electromagnetic properties distinct from those of conventional ferromagnets and antiferromagnets. We report results of micro-focused angle-resolved photoemission spectroscopy (ARPES) on RuO2, in which its altermagnetic nature has been under fierce debate in connection with crystal-orientation-dependent spintronic functionalities. By elucidating t…
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Altermagnets are a novel platform to realize exotic electromagnetic properties distinct from those of conventional ferromagnets and antiferromagnets. We report results of micro-focused angle-resolved photoemission spectroscopy (ARPES) on RuO2, in which its altermagnetic nature has been under fierce debate in connection with crystal-orientation-dependent spintronic functionalities. By elucidating the band structure of the (100), (110) and (101) surfaces of a bulk single crystal using micro-ARPES, we found that, irrespective of the surface orientation, the experimental band structures show a good agreement with the bulk-band calculations for the nonmagnetic phase, but display a severe disagreement with those for the antiferromagnetic phase. Moreover, spin-resolved ARPES signifies a negligible spin polarization in the bulk bands, suggesting the absence of antiferromagnetism and altermagnetic spin splitting. In addition, we identified a nearly flat surface band and a dispersive one near the Fermi level at the (100)/(110) and (101) surfaces, respectively. Our first-principles calculations and analysis of Berry phase attribute these states to the topological surface bands emerging from the bulk Dirac nodal lines around the Fermi level. Our results indicate that such topological surface/interface states must be considered to understand the spintronic functionalities of RuO2, and may provide new insights into its catalytic characteristics.
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Submitted 14 January, 2026; v1 submitted 17 January, 2025;
originally announced January 2025.
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Universal Role of Combined Symmetry for the Protection of the Dirac Cone in Antiferromagnetic Topological Insulators
Authors:
Asuka Honma,
Noriyuki Kabeya,
Seigo Souma,
Yongjian Wang,
Kunihiko Yamauchi,
Kosuke Nakayama,
Daichi Takane,
Kenichi Ozawa,
Miho Kitamura,
Koji Horiba,
Hiroshi Kumigashira,
Tamio Oguchi,
Takashi Takahashi,
Noriaki Kimura,
Yoichi Ando,
Takafumi Sato
Abstract:
Antiferromagnetic topological insulators (AF TIs) are predicted to exhibit exotic physical properties such as gigantic optical and topological magnetoelectric responses. While a key to achieving such phenomena relies on how to break the symmetry protecting the Dirac-cone surface state (SS) and acquire the mass of Dirac fermions, the mechanism has yet to be clarified. To address this issue, we carr…
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Antiferromagnetic topological insulators (AF TIs) are predicted to exhibit exotic physical properties such as gigantic optical and topological magnetoelectric responses. While a key to achieving such phenomena relies on how to break the symmetry protecting the Dirac-cone surface state (SS) and acquire the mass of Dirac fermions, the mechanism has yet to be clarified. To address this issue, we carried out micro-focused angle-resolved photoemission spectroscopy for GdBi hosting the type-II AF order, and uncovered the stripe-type 2$\times$1 reconstruction of the Fermi surface associated with the AF band folding. Intriguingly, in contrast to NdBi with the type-I AF order displaying the surface-selective Dirac-fermion mass, GdBi shows massless behavior irrespective of AF domains due to the robust topological protection. These results strongly suggest a crucial role of the ThetaTD (time-reversal and translational) symmetry to create the Dirac-fermion mass in AF TIs.
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Submitted 9 October, 2024; v1 submitted 19 June, 2024;
originally announced June 2024.
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Antiferromagnetic topological insulator with selectively gapped Dirac cones
Authors:
A. Honma,
D. Takane,
S. Souma,
K. Yamauchi,
Y. Wang,
K. Nakayama,
K. Sugawara,
M. Kitamura,
K. Horiba,
H. Kumigashira,
K. Tanaka,
T. K. Kim,
C. Cacho,
T. Oguchi,
T. Takahashi,
Yoichi Ando,
T. Sato
Abstract:
Antiferromagnetic (AF) topological materials offer a fertile ground to explore a variety of quantum phenomena such as axion magnetoelectric dynamics and chiral Majorana fermions. To realize such intriguing states, it is essential to establish a direct link between electronic states and topology in the AF phase, whereas this has been challenging because of the lack of a suitable materials platform.…
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Antiferromagnetic (AF) topological materials offer a fertile ground to explore a variety of quantum phenomena such as axion magnetoelectric dynamics and chiral Majorana fermions. To realize such intriguing states, it is essential to establish a direct link between electronic states and topology in the AF phase, whereas this has been challenging because of the lack of a suitable materials platform. Here we report the experimental realization of the AF topological-insulator phase in NdBi. By using micro-focused angle-resolved photoemission spectroscopy, we discovered contrasting surface electronic states for two types of AF domains; the surface having the out-of-plane component in the AF-ordering vector displays Dirac-cone states with a gigantic energy gap, whereas the surface parallel to the AF-ordering vector hosts gapless Dirac states despite the time-reversal-symmetry breaking. The present results establish an essential role of combined symmetry to protect massless Dirac fermions under the presence of AF order and widen opportunities to realize exotic phenomena utilizing AF topological materials.
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Submitted 20 November, 2023;
originally announced November 2023.
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Observation of Giant Band Splitting in Altermagnetic MnTe
Authors:
T. Osumi,
S. Souma,
T. Aoyama,
K. Yamauchi,
A. Honma,
K. Nakayama,
T. Takahashi,
K. Ohgushi,
T. Sato
Abstract:
We performed angle-resolved photoemission spectroscopy (ARPES) on hexagonal MnTe, a candidate for an altermagnet with a high critical temperature (TN=307 K). By utilizing photon-energy-tunable ARPES in combination with first-principles calculations, we found that the band structure in the antiferromagnetic phase exhibits a strongly anisotropic band-splitting associated with the time-reversal-symme…
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We performed angle-resolved photoemission spectroscopy (ARPES) on hexagonal MnTe, a candidate for an altermagnet with a high critical temperature (TN=307 K). By utilizing photon-energy-tunable ARPES in combination with first-principles calculations, we found that the band structure in the antiferromagnetic phase exhibits a strongly anisotropic band-splitting associated with the time-reversal-symmetry breaking, providing the first direct experimental evidence for the altermagnetic band-splitting. The magnitude of the splitting reaches 0.8 eV at non-high-symmetry momentum points, which is much larger than the spin-orbit gap of ~0.3 eV along the GK high-symmetry cut. The present result paves the pathway toward realizing exotic physical properties associated with the altermagnetic spin-splitting.
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Submitted 10 February, 2024; v1 submitted 19 August, 2023;
originally announced August 2023.
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Unusual surface states associated with the PT-symmetry breaking and antiferromagnetic band folding in NdSb
Authors:
Asuka Honma,
Daichi Takane,
Seigo Souma,
Yongjian Wang,
Kosuke Nakayama,
Miho Kitamura,
Koji Horiba,
Hiroshi Kumigashira,
Takashi Takahashi,
Yoichi Ando,
Takafumi Sato
Abstract:
We have performed micro-focused angle-resolved photoemission spectroscopy on NdSb which exhibits the type-I antiferromagnetism below TN = 16 K. We succeeded in selectively observing the band structure for all three types of single-q antiferromagnetic (AF) domains at the surface. We found that two of the three surfaces whose AF-ordering vector lies within the surface plane commonly show twofold sym…
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We have performed micro-focused angle-resolved photoemission spectroscopy on NdSb which exhibits the type-I antiferromagnetism below TN = 16 K. We succeeded in selectively observing the band structure for all three types of single-q antiferromagnetic (AF) domains at the surface. We found that two of the three surfaces whose AF-ordering vector lies within the surface plane commonly show twofold symmetric surface states (SSs) around the bulk-band edges, whereas the other surface with an out-of-plane AF-ordering vector displays fourfold symmetric shallow electronlike SS at the Brillouin-zone center. We suggest that these SSs commonly originate from the combination of the PT (space-inversion and time-reversal) symmetry breaking at the surface and the band folding due to the AF order. The present results pave a pathway toward understanding the relationship between the symmetry and the surface electronic states in antiferromagnets.
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Submitted 13 September, 2023; v1 submitted 6 July, 2023;
originally announced July 2023.
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Development of a versatile micro-focused angle-resolved photoemission spectroscopy system with Kirkpatrick-Baez mirror optics
Authors:
Miho Kitamura,
Seigo Souma,
Asuka Honma,
Daisuke Wakabayashi,
Hirokazu Tanaka,
Akio Toyoshima,
Kenta Amemiya,
Tappei Kawakami,
Katsuaki Sugawara,
Kosuke Nakayama,
Kohei Yoshimatsu,
Hiroshi Kumigashira,
Takafumi Sato,
Koji Horiba
Abstract:
Angle-resolved photoemission spectroscopy using a micro-focused beam spot (micro-ARPES) is becoming a powerful tool to elucidate key electronic states of exotic quantum materials. We have developed a versatile micro-ARPES system based on synchrotron radiation beam focused with a Kirkpatrick-Baez mirror optics. The mirrors are monolithically installed on a stage, which is driven with five-axes moti…
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Angle-resolved photoemission spectroscopy using a micro-focused beam spot (micro-ARPES) is becoming a powerful tool to elucidate key electronic states of exotic quantum materials. We have developed a versatile micro-ARPES system based on synchrotron radiation beam focused with a Kirkpatrick-Baez mirror optics. The mirrors are monolithically installed on a stage, which is driven with five-axes motion, and are vibrationally separated from the ARPES measurement system. Spatial mapping of the Auphotolithography pattern on Si signifies the beam spot size of 10 $μ$m (horizontal) x 12 $μ$m (vertical) at the sample position, which is well suited to resolve the fine structure in local electronic states. Utilization of the micro beam and the high precision sample motion system enables the accurate spatially resolved band-structure mapping, as demonstrated by the observation of a small band anomaly associated with tiny sample bending near the edge of a cleaved topological insulator single crystal.
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Submitted 14 March, 2022;
originally announced March 2022.