In space-time inversion symmetric systems, one can choose a basis where both the Bloch wavefunctions and the Hamiltonian are real-valued functions of momentum. This real structure gives rise to Euler topology, a nontrivial multiband topology characterized by the Euler class. In this poster, I will introduce the concept of Euler topology and its role in stabilizing band degeneracies and enabling non-Abelian braiding phenomena. I will then focus on bilayer honeycomb lattices and demonestrate they are possible platform that can hosts nontrivial Euler topology. Importantly, the effective two-layer structure can emerge not only from physical bilayers, but also from spin and electron-hole sectors, broadening the scope of realizations in condensed matter systems.

1. C. Mondal*, R. Ghadimi*, and B. J. Yang, Non-Abelian charge conversion in bilayer binary honeycomb lattice systems, 

arXiv preprint arXiv:2411.06724 (2024).

2. R. Ghadimi*, C. Mondal*, S. Kim, and B. J. Yang, Quantum Valley Hall Effect without Berry Curvature, Physical Review Letters 133, 196603 (2024).

Flat bands have been proposed in various geometrically frustrated lattices, yet their experimental realization has been limited only to the two-dimensional (2D) kagome and three-dimensional (3D) pyrochlore lattices. While synthesizing bulk crystals with desired lattice geometry is generally challenging, the surfaces of 3D bulk materials sometimes naturally host 2D lattice structures hardly achievable in 2D bulk crystals. Thus, the electrons localized on such 2D surfaces can be a new platform for exotic flat bands residing on unexplored lattice geometry. Here, we report the first experimental observation of singular flat bands with band crossings in the 2D pentagonal lattice on the surface of the 3D ferromagnetic topological semimetal CoS2. We demonstrate that the coupling between localized surface flat bands and extended bulk topological bands gives rise to the surface non-Fermi liquid behavior characterized by the quasiparticle scattering rate with linear temperature dependence and quasiparticle line width far exceeding the Planckian limit. This study highlights CoS2 as an ideal platform for exploring the exotic properties of spin-polarized singular flat bands localized on the surface pentagonal lattice, which provides a fresh perspective on investigating correlated electron phenomena in pentagon-based materials. 

[1] Yuting Qian et al., Submitted (2025).

Coherent optical driving has recently emerged as a promising approach to dynamically control quantum materials. When driven at frequencies significantly higher than the material’s intrinsic electronic energy scales, the system is often described theoretically by an effective Floquet Hamiltonian that solely captures cycle-averaged interactions [1]. However, this framework overlooks fast electron dynamics occurring on sub-cycle timescales—dubbed Floquet micromotion—which remains underexplored, particularly in strongly correlated electron systems [2]. Here, we demonstrate how Floquet micromotion in correlated materials can be detected through time-resolved Raman spectroscopy. The micromotion can also be manipulated to selectively break symmetries and disrupt magnetic ordering in the low-frequency limit. Broader insights into the light-driven control of correlated materials will also be introduced.

[1] M. Bukov et al., Advances in Physics 64, 139-226 (2015).

[2] S. Ito et al., Nature 616, 696-701 (2023).

Recently, FeTe₀.₅₅Se₀.₄₅ has emerged as a potential candidate for a topological superconductor. Its topologically non-trivial characteristics are expected to host Majorana bound states, making it an attractive platform for implementing fault-tolerant quantum computing. In this study, we fabricated FeTe₀.₅₅Se₀.₄₅-based van der Waals Josephson junctions using a micro-cleaving and stacking technique on a Peltier-cooled transfer stage. This fabrication ensures the high quality of the junction interfaces, confirmed by transmission electron microscopy. We identified zero-field Fiske steps in the FeTe₀.₅₅Se₀.₄₅ van der Waals Josephson junctions, indicating the presence of randomly misaligned vortices between the junction interfaces. Additionally, we observed the first odd-step missing of Shapiro steps, a signature of the fractional Josephson effect, which arises from a 4π-periodic supercurrent between Majorana zero-modes. Numerical calculations show good agreement with the experimental results, providing a quantitative estimate of the 4π-periodic supercurrent.

Intriguing many-body entanglement phenomena, including fractionalized excitations of quantum spin liquids, appear in Mott insulators. Systematic and comprehensive analysis is limited by their intrinsic complexity despite long history of quantum many-body systems. In this work, we propose one concrete strategy to investigate a class of Mott insulators, spin liquid candidate materials, by exploiting their symmetry and topological properties. Application of the strategy to Mott insulators under strain effects allows us to construct a fully generalized spin model, and in particular, we obtain the ε-KJΓΓ’ model for the spin liquid candidate materials such as α-RuCl3. The symbol ε emphasizes the emergence of additional spin interactions under strain effects in contrast to the conventional KJΓΓ’ in literature. With strain effects, we estimate all coupling constants and their empirical values. Remarkably, under 3% strain, the emergent coupling constants become comparable in magnitude to the original ones. Furthermore, we uncover that symmetry manipulation via strain can serve as a powerful tool for controlling topological quantum phase transitions of Kitaev quantum spin liquids. Our findings suggest that various quantum phase transitions between competing phases may be driven by strain effects in experiments of Kitaev spin liquid candidate materials.

The study of quantum correction to the conductivity δg due to disorder can lead to the classification of the system. In particular, investigating the sign and magnitude of δg yields one of the three Wigner-Dyson classes. Motivated by the nontrivial properties of the Euler band model having nonzero total bulk vorticity, specific realization of fragile topological insulator, we study this model in two different ways, symmetry analysis and quantum diagrammatic calculation. Analyzing possible symmetries of the system shows that a finite correction to the conductivity is possible even when the system lacks physical time-reversal symmetry and quantitative calculation using the diagrammatic approach supports this. This unconventional behavior is achieved by effective time-reversal symmetry arising from the product of existing symmetries. Furthermore, comparing our result with the well-known system having trivial total vorticity, we conclude that relative vorticity between two nodes does not affect δg. Instead, time-reversal symmetry determines the class to which the system belongs.

Structural imperfections can be a promising testbed to engineer the symmetries and topological states of solid-state platforms. Here, we present direct evidence of hierarchical transitions of zero- (0D) and one-dimensional (1D) topological states in symmetry-enforced grain boundaries (GB) in 1T′–MoTe2. Using a scanning tunneling microscope tip press-and-pulse procedure, we construct two distinct types of GBs, which are differentiated by the underlying symmorphic and nonsymmorphic symmetries. The GBs with the non-symmorphic rotation symmetry harbor first-order topological edge states protected by a nonsymmorphic band degeneracy. On the other hand, the edge state of the symmorphic GBs attains a band gap. More interestingly, the gapped edge state realizes a hierarchical topological phase, evidenced by the additional 0D boundary states at the GB ends. We anticipate our experiments will pioneer the material platform for the hierarchical realization of first-order and higher-order topology.

Mixed-state quantum phases, namely quantum phases in noisy and thermal environments, are highlighted recently [1-5]. One interesting discovery is that there exists a non-trivial mixed-state quantum phase with the finite correlation length in one dimension [6], in contrast to the fact that there is no topological order in one dimension [7]. We observe that this 1D mixed-state topological order has Hermiticity symmetry-protected topological state in its irreducible form, namely the canonical decomposition into short-range entangled states [8], after mapping the mixed state to a pure state via Choi–Jamiołkowski isomorphism. We turn this observation into a general principle that the matrix product density operator cannot permit a short-range entangled local purification if its irreducible form non-repeatedly includes Hermiticity SPT state dominating in the thermodynamic limit. 

We further prove a new no-go theorem ruling out the class that a short-range entangled local purification exists only in the thermodynamic limit. This class is identical to the mixed-state trivial phase except that translational invariance of the purification is also required to be in this class.

[1] Ruochen Ma et al., Phys. Rev. X 13, 031016 (2023).

[2] Ruihua Fan et al., PRX Quantum 5, 020343 (2024).

[3] Zijian Wang et al., PRX Quantum 6, 010314 (2025).

[4] Ramanjit Sohal et al., PRX Quantum 6, 010313 (2025).

[5] Tyler D. Ellison et al., PRX Quantum 6, 010315 (2025).

[6] Leonardo A. Lessa et al., Phys. Rev. X 15, 011069 (2025).

[7] Xie Chen et al., Phys. Rev. B 83, 035107 (2011).

[8] J. Ignacio Cirac et al., Rev. Mod. Phys. 93, 045003 (2021).

We investigate the spin-dependent magnetic properties of Mn₃Si₂Te₆, a magnetic nodal-line semiconductor that exhibits colossal angular magnetoresistance (CAMR) [1]. CAMR, observed in this material, arises from spin-polarized nodal-line degeneracy and leads to an exceptionally large magnetoresistance modulation. Despite its significance, the magnetic ground state remains debated. Using torque magnetometry and ferromagnet resonance (FMR), we identify an unusual magnetic structure below Tc, where spins tilt about 10° toward the c-axis from the basal plane. This magnetic field and temperature dependent spin canting behavior can be explained through the free energy model including higher-order anisotropy terms. Our findings provide insight into the spin configuration underpinning CAMR and highlight spin reorientation as a key control parameter in magnetic topological semiconductors.

[1] Junho Seo, Chandan De, Hyunsoo Ha, Ji Eun Lee, Sungyu Park, Joonbum Park, Yurii Skourski, Eun Sang Choi, Bongjae Kim, Gil Young Cho, Han Woong Yeom, Sang-Wook Cheong, Jae Hoon Kim, Bohm-Jung Yang, Kyoo Kim, Jun Sung Kim, Nature 599, 576-581 (2021).

Triangular lattice antiferromagnets are fertile grounds for discovering exotic magnetic states and associated transport phenomena. Among them, Co₁/₃TaS₂ has emerged as a promising candidate due to its unusual spin configurations and metallic behavior. While early studies suggested a coplanar 120° spin arrangement, recent neutron scattering experiments have identified a more complex, non-coplanar tetrahedral triple-Q magnetic order. This configuration gives rise to a spontaneous Hall effect, driven by the topological nature of spin chirality and the associated emergent gauge fields [1,2,3,4].

Despite these advances, spatially resolving such intricate spin textures at the atomic scale remains a significant challenge. In this work, we employ spin-polarized scanning tunneling microscopy (SP-STM) at 7 K in ultra-high vacuum and under magnetic field to directly probe the surface magnetism of Co₁/₃TaS₂. Our measurements reveal modulations in spin contrast consistent with a spin density wave-like signature, supporting the presence of triple-Q magnetic ordering. Furthermore, variations in tunneling current with tip-sample separation indicate a nontrivial spin-dependent interaction, reflecting the influence of the local spin environment on tunneling dynamics.

These results underscore the capability of SP-STM to visualize chiral magnetic structures and provide deeper insight into the physics of frustrated magnetic systems at the atomic scale.

Keywords: Spin Polarized STM, Magnetoresistance, Antiferromagnet, 

References

1. P. Park, et. al., arxiv prep. 2410, 02180 (2024). 

2. P. Park, et. al., Nat. Comm. 14(1), 8346 (2023).

3. H. Takagi, et. al., Nat. Phy. 19(7), 961-968 (2023).

4. Z. Nussinov, et. al. Phys. Rev. B 68, 085402 (2003).

Spatially non-local quantum low-density parity-check (LDPC) codes are promising candidates for efficient quantum memories due to their high encoding rates and favorable code distance scaling. In this work, we investigate hypergraph product (HGP) and lifted product (LP) codes, which feature recently proposed hardware-efficient implementation schemes [1]. We compute the upper bounds of fundamental error threshold—independent of any decoding algorithm—as 14.8% and 14.2%, respectively. This is achieved by mapping quantum codes under incoherent noise to classical spin models and extracting their critical temperatures [2]. We also observe discontinuous phase transitions in the spin models associated with both HGP and LP codes. This behavior is in stark contrast to previously reported results for the 2D toric code, a geometrically local code with a vanishing encoding rate in the thermodynamic limit. For any HGP codes and good quantum codes, we rigorously prove that such discontinuities always occur based on their code parameters [[N,K,D]]. This result offers insight into the explicit construction of good quantum codes via gauging, since the ungauged model coincides with the spin model derived from the Rényi n=2 mapping. Finally, we analyze the behavior of the quantum relative entropy across the decoding transition using perturbation theory. This, in turn, enables a characterization of the finite-temperature phase structure of arbitrary classical LDPC models by identifying scaling laws for multi-spin correlations, where conventional distance is replaced by the qubit cost required to complete the corresponding Pauli strings into support graphs allowed by the code structure.

[1] Qian Xu. et al. Nat. Phys. 20, 1084–1090 (2024)

[2] Ruihua Fan. et al. PRX Quantum 5, 020343 (2024)

Here, we report on imaging the spin texture of triple-Q magnetic order of Co1/3TaS2. The sample was measured with a low temperature STM (T=6K) under ultra-high vacuum with normal and spin-polarized tips. The STM images with the normal tip show the triangular lattice of the sample. The spin-polarized (SP) tip shows an additional symmetry related to the triple-Q ordering. In addition to that, the SP STM images revealed different spin textures with respect to the tip-spin orientation. The analysis suggests the presence of a phase difference between the tip and the triple-Q ordering of the sample. This work gives a new insight into the exploration of chiral magnetic ordering with topological Hall effect using scanning probe microscopy. 

Electron spin resonance (ESR) spectroscopy is a powerful technique that detects the resonant absorption of microwaves by unpaired electron spins under an external magnetic field. While ESR has been widely applied to study paramagnetic defects in solids, extending this technique to atoms and molecules on surfaces and proposing them as potential quantum bits for quantum computation opens new avenues in surface science [1]. Particularly, surface-ensemble ESR enables the study of suitable new surface spin systems that are difficult to access through single-spin techniques such as ESR combined scanning tunnelling microscopy (ESR-STM), which often suffer from environmental noise and extreme operating conditions.

To address these limitations and enable high-sensitivity ESR measurements from spins on surfaces, we developed a custom X-band ESR spectrometer operating in UHV and integrated a planar microstrip line resonator with an atomically clean Cu(111)/Al2O3  surface finish. However, in order to have a versatile platform to grow different types of molecular structures, further research to design new crystalline-type resonators that accommodate alternative metals and surface symmetries is needed.

In this work, we have extended our study to Ag(111) and Au(111) resonators epitaxially deposited on Cu(111)/Al2O3. Their structural quality was characterized by atomic force microscopy (AFM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). Both substrates show low roughness and a high level of crystallinity, evinced from the sharp spots in the LEED characterization. Furthermore, we deposited YPc₂ molecules on the Au(111) surface and characterized their spin properties using ESR, evincing differences with respect to the same molecules deposited on Cu(111) and on ZnPc/Cu(111). In parallel, we developed a fabrication method for Ag resonators with cubic (100) crystallographic orientation grown on MgO substrate. We systematically optimized the growth conditions, and evaluated the resulting ESR performance to identify the substrate offering the highest crystallinity and the lowest background signal. Our study extends the capabilities of surface-ensemble ESR platforms for exploring molecule-based spin qubit candidates on different metallic substrates.

[1] Chen, Y., Bae, Y., & Heinrich, A. J. Advanced Materials, 35(27), 2107534 (2023).

[2] Cho, F. H., Park, J., Oh, S., Yu, J., Jeong, Y., Colazzo, L., ... & Donati, F. Review of Scientific Instruments, 95(6) (2024)

Two-dimensional metal–organic structures that form ordered magnetic lattices on a superconducting substrate provide a unique platform to study the interplay between magnetic ordering and superconductivity. In this work, we investigate Fe- and Ni-based dicyanoanthracene structures on NbSe₂ to explore the magnetic coupling in this system and to understand how it is influenced by both the molecular linkers and the superconducting substrate. We realized a YSR lattice in this system, and it is a promising route to topological superconductivity.

Using STM, XAS, and XMCD, we observed negligible magnetic anisotropy and antiferromagnetic interactions between Ni centers in NiDCA₃, which result from competing contributions: superexchange via the molecular linkers and RKKY interactions mediated by the conduction electrons of the substrate. Conversely, in the isostructural FeDCA₃, we find negligible magnetic coupling, while we identified strong out-of-plane anisotropy, indicating that the effect of the interaction between the Fe 3d orbitals and the molecular ligand is mainly to lift the degeneracy of the spin multiplet. Our work reveals the dual role of organic coordination and substrate coupling in determining the overall magnetic behavior of metal–organic structures on superconductors. The magnetism we have found is exotic, underscoring a ground state that is both rare and potentially significant for exploring unconventional superconductivity.

[1] Vano, V., Reale, S., Silveria, J. O., Longo, D., Amini, M., Kelai, M., Lee, J., Martikainen, A., Kezilebieke, S., Foster, A. S., Lado, J. L., Donati, F., Liljeroth, P., Yan, L. (2024). Emergence of Exotic Spin Texture in Supramolecular Metal Complexes on a 2D Superconductor. Phys. Rev. Lett. 133, 236203.

We report the magnetic and electrical transport properties of single-crystalline Zr₃Mn₃Sn₄Ga, a bulk compound featuring two distinct kagome lattices: a non-magnetic breathing Zr₃Sn₄ lattice and a magnetic intact Mn₃Ga lattice [1]. The material undergoes an antiferromagnetic transition at TN = 87 K, with neutron diffraction confirming a commensurate magnetic ordering characterized by k = (1/3,1/3,0). Transport measurements reveal metallic behavior, a resistivity anomaly near TN, and 12% magnetoresistance at 2 K under 9 T. Notable deviations from conventional power-law behavior, along with negative magnetoresistance and nonlinear Hall slope variations near TN, indicate strong magneto-electronic coupling. Resonant photoemission spectroscopy reveals that Zr 4d and Mn 3d orbitals dominate the valence band, linking the material’s distinct electronic features to its kagome lattice structure. Zr₃Mn₃Sn₄Ga provides a rare platform for probing magnetism–band topology interplay in hetero-kagome systems without the need for thin-film growth techniques.

[1] J. Park et al., Scripta Materialia 264, 116701 (2025).

In collinear antiferromagnets, weak ferromagnetism is primarily attributed to antisymmetric  exchange interactions, commonly 

referred to as the Dzyaloshinskii-Moriya interaction (DMI). However, in a metallic triangular antiferromagnet, Co1/3TaS2, exhibiting a non-coplanar tetrahedral triple-Q magnetic ordering with four-spin sublattices, as revealed by neutron diffraction [1] and scattering [2] experiments, the origin of its weak ferromagnetic moment remains elusive. Spontaneous magnetization is only measured by the longitudinal  magnetization measurement using SQUID-MPMS along the c-axis, whereas magnetic torque measurement confirms that the moment along c-axis is dipole-like moment generated by magnetic cluster, leaving clear rotational hysteresis by sin(𝜃 ). The conditions for rotational hysteresis are 

 i)       non-zero ms (spontaneous magnetization), hr (reversal field, coercivity), 

ii)      odd-fold symmetry upon rotating one-circle around, and 

iii)     strong anisotropy field with Kondorsky-type domain wall.

Since any m(h) curves of magnetic responses (linear, quadratic, cubic, …) are crossing the zero point, resulting in even-fold symmetric torque signals without the hysteresis. Therefore, rotational hysteresis sole corresponds to spontaneous magnetization. Furthermore, the second condition suggests that 3-fold symmetric dipole-like moment can also make rotational hysteresis, which can be generated by cluster-octupole moment [3,4]. The elucidation of the inplane rotational hysteresis with 6-peaked magnetic torque signals of Co1/3TaS2 possibly takes place.

[1] P. Park et al., Nat. Comms. 14, 8346 (2023).

[2] H. Takagi et al., Nat. Phys. 19, 961 (2023).

[3] H. Kusunose et al., J. Phys.: Condens. Matter 34, 464002 (2022).

[4] T. Ishitobi et al., Phys. Rev. B 104, L241110 (2021).

Kagome magnets have emerged as a subject of intensive research owing to their intrinsic topological quantum phenomena arising from the synergistic interplay between unique electronic structure characteristics (Dirac bands, flat bands, van Hove singularities) and diverse magnetic behaviors (geometric frustration, spin-orbit coupling, and non-collinear spin textures). In this study, we investigate the modulation of Berry curvature induced by the magnetic field in the kagome-lattice magnet YMn6Sn6, which hosts a unique electronic structure characterized by Dirac points, flat bands, and van Hove singularities. We employ a synergistic approach combining angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations to probe magnetic field-induced modifications in the electronic structure. A pronounced modulation of the anomalous Hall effect (AHE) under applied magnetic fields was observed, which is attributed to field-induced reconfiguration of Berry curvature arising from spin texture evolution and topological band structure modifications in the kagome lattice. The experimental observation of anomalous Hall conductivity (AHC) exceeding theoretical predictions from DFT-based calculations is attributed to skew scattering which plays a critical role in modulating electronic and magnetic properties of YMn6Sn6 via interplay between intrinsic/extrinsic mechanisms. These findings establish a novel analytical framework for investigating electrical and magnetic properties in kagome-lattice magnets like YMn6Sn6, leveraging the interplay between intrinsic Berry curvature and extrinsic skew scattering mechanisms.

Two-dimensional honeycomb antiferromagnets have emerged as fertile grounds for investigating exotic quantum phenomena, particularly the realization of Kitaev quantum spin liquids (QSLs). These QSLs are characterized by an absence of magnetic order even at absolute zero temperature, along with fractionalized excitations—properties that stem from the underlying bond-dependent and highly anisotropic magnetic interactions. A central challenge in this field is the identification of candidate materials that exhibit tell-tale signs of proximity to a Kitaev QSL, including strong in-plane magnetic anisotropy, bond-directional exchange interactions, and the presence of a broad fluctuation regime that extends well above the magnetic ordering temperature.

In this study, we investigate the layered cobalt-based honeycomb antiferromagnet Cu3Co2SbO6, which exhibits strong coupling between spin and excitonic degrees of freedom, enabling optical probes to reveal intricate magnetic dynamics. Notably, optical spectroscopy uncovers a pronounced fluctuating regime extending well above the Néel temperature of 16 K, with spectral features that respond sensitively to light polarization. Within both the antiferromagnetic phase and the higher-temperature paramagnetic regime, we observe anisotropic spectral weight redistribution, a hallmark of bond-dependent interactions. This behavior provides compelling evidence for Kitaev-type spin exchange anisotropy, supported by persistent short-range correlations in the absence of long-range magnetic order.

Importantly, these observations suggest that light not only probes but may actively modulate the underlying spin Hamiltonian, offering new pathways for optical control of quantum magnetism. Cu3Co2SbO6 thus emerges as a promising platform for the study of bond-anisotropic exchange and the realization of Kitaev physics in solid-state systems, with implications for both fundamental research and future quantum technologies.

Anomalous transport phenomena arising from nontrivial band topology hold significant promise for spintronic and electronic applications. However, the presence of topologically trivial states often obscures these effects, highlighting the need for systems where conduction is governed exclusively by nontrivial bands. One promising approach is to engineer band crossings in direct-gap semiconductors with bands of opposite parity. Here, we demonstrate that europium doping in CaAs3-a small direct-gap semiconductor-reduces its bandgap and, at a doping level of x = 0.3, induces an insulator-to-metal transition accompanied by a structural phase change from P1 ̅ to C2/m symmetry. Within the critical doping range of x = 0.2–0.3, band crossings near the Fermi level emerge, driven by the combined effects of Zeeman splitting under magnetic fields and Eu-derived exchange interactions. Angle-resolved photoemission spectroscopy (ARPES) and Shubnikov–de Haas oscillations confirm the absence of trivial states and reveal a field-induced topological Lifshitz transition exclusively in this regime. Furthermore, the low density of states and enhanced Berry curvature yield a giant anomalous Hall angle, tan⁡θ_H=0.65, surpassing most known topological materials. Our work establishes (Ca,Eu)As3 as a prototypical topological diluted magnetic semiconductor and demonstrates its exceptional potential for realizing giant anomalous transport effects in topology-driven systems.

In the vicinity of a topological quantum phase transition between direct-gap semiconducting and nodal-line semi-metallic (NLSM) phases, the interplay between band topology and electron correlations can drive instabilities toward exotic many-body states with novel electronic properties. Despite extensive theoretical predictions, experimentally identifying such correlated phases of NLSM state remains elusive. Here, we report that a direct-gap semiconductor CaAs3, close to a topological quantum phase transition, hosts unconventional two-dimensional (2D) NLSM states, exhibiting unique temperature- and electric-field-driven metal-insulator transitions. At low temperatures, where bulk conduction is fully suppressed, a 2D metallic state emerges and subsequently turns into an insulating phase. This insulating phase exhibits anomalous Shubnikov–de Haas oscillations with unusual temperature dependence and undergoes an insulator-to-metal transition at an exceptionally small electric field strength of ~10⁻³ V/cm, the lowest ever reported. These unique electronic properties are unprecedented yet consistent with the excitonic insulating phase of NLSM states, which offers a promising platform for exploring novel functionalities of correlated NLSM states near a topological quantum phase transition.

Starting from the Berry phase [1], quantum geometry has emerged as an important concept for understanding solid-state physics. Onoda demonstrated how external parameters influence polarization in a simple one-dimensional model, revealing the topological background of the ferroelectricity [2]. Meanwhile, Mitscherling showed the relationship between quantum geometry and physical observables, particularly polarization [3]. Inspired by these works, we investigate the behavior of Wannier charge center displacements under varying external conditions and extend the analysis by computing higher-order position cumulants in the given model. These explorations may lead to new insights into the role of higher-order cumulants in more general quantum systems.

[1] D. Xiao et al., Rev. Mod. Phys. 82, 1959 (2010).

[2] S. Onoda et al., Phys. Rev. Lett. 93, 167602 (2018).

[3] J. Mitscherling et al., arXiv 2412.03637 (2024).

Molecular spin qubits on surfaces are promising platforms for quantum information processing [1].  However, interactions with the supporting substrate can alter their intrinsic magnetic properties, motivating the development of techniques capable of directly probing spins at interfaces. To this extent, we developed a surface-ensemble electron spin resonance (ESR) setup based on microstrip Cu resonator. Using an ex-situ prepared resonator, we achieved 2.6 · 10¹¹ spins/G·Hz1/2 sensitivity at 15 K with a 4 nm thick yttrium bisphthalocyanine (YPc2) film. This indicates an expected signalto-noise ratio of 3.9 G·Hz1/2 from a monolayer of YPc2 molecules, enabling surface-sensitive detection. To enhance crystallinity and minimize spin noise fluctuation caused by structural disorder, we implemented an in-situ single crystalline Cu resonator fabrication method. We used this resonator to characterize from a single to dozen layers of YPc2 molecules grown directly on the resonator without exposing its surface to air. We also investigated how YPc2 molecules interact with the metallic substrate by adding a diamagnetic zinc phthalocyanine (ZnPc) decoupling layer. Our results highlight the importance of suitable decoupling layers in preventing spin quenching due to the proximity with the metal substrate. 

 In addition, pulsed ESR measurements on bulk paramagnetic samples of 1,3-Bis(diphenylene)-2-phenylallyl (BDPA) radicals diluted in polystyrene proved the capability of this setup to study spin lifetimes, which are key metrics for qubit operation [2]. Moving towards the investigation of spin coherence in thin molecular films, we demonstrate the detection of vanadyl phthalocyanine (VOPc) molecular spins diluted in various matrices, paving the way to understand such molecular spin qubit’s potential for scalable quantum architectures.

[1] Gaita-Ariño et al., Nat. Chem. 11, 301 (2019). https://doi.org/10.1038/s41557-019-0232-y

[2] Cho et al., Rev. Sci. Instrum. 95, 063904 (2024). https://doi.org/10.1063/5.0189974

Electron spin resonance combined with scanning tunneling microscopy (ESR-STM) provides a powerful tool to probe and control individual quantum states at the atomic scale [1,2]. This technique enables the study of single atoms on surfaces as qubit candidates for quantum information processing. Lanthanide atoms, with their highly localized 4f  electrons, are especially promising due to their potentially long coherence times. However, probing them directly with STM tunneling current can induce strong decoherence, limiting their use in coherent manipulation protocols. 

To address this, we implemented a hybrid scheme in which a nearby spin-1/2 Ti atom, placed within ~1 nm of a single Er atom on an MgO surface, is used to drive and detect the 4f  spin states of Er via ESR-STM [3]. Using a 3-axis vector magnet, we investigated the magnetic anisotropy of the Er atom by measuring the energy of spin transitions as a function of the magnetic field direction. Taking advantage of the atomic precision in assembling the Ti–Er dimer, we tailored the magnetic interaction and fine-tuned the mixing between Er and Ti states. Through this approach, we demonstrated coherent control of the Er 4f spin and 10-fold enhancement in spin driving efficiency compared to isolated Ti atoms. We attribute this enhancement to the large magnetic moment and spin–orbit coupling of Er. These results establish a new atomic-scale platform for studying multi-photon transitions and unraveling the microscopic mechanisms allowing for coherent spin manipulation in surface atomic qubits.

[1] K. Yang et al. Science 366, 509-512 (2019).

[2] Y. Wang et al. Science 382, 87 (2023).

[3] S. Reale et al., Nat. Commun. 17, 403 (2024).

Chiral and frustrated magnetic materials have emerged as fertile ground for discovering exotic spin textures and emergent transport phenomena. Here, we present a scanning tunneling microscopy (STM) and spectroscopy study of the metallic triangular lattice antiferromagnet Co₁/₃TaS₂, which hosts a non-coplanar, triple-Q magnetic order. [1-4] With atomic-resolution STM imaging, we resolve the surface lattice of TaS₂ and detect signatures of intercalated Co atoms buried between the layers. Using spin-polarized (SP) tips, we observe the emergence of directional wave-like modulations in the topographic signal, whose orientation varies with tip magnetization—pointing to a magnetic origin. Bias-dependent dI/dV maps reveal contrasts between the Co sublattice and TaS₂ surface, offering insight into the local density of states (LDOS). First-principles DFT calculations of the projected density of states (PDOS) support the experimental observations and provide a microscopic view into the interplay of electronic structure and non-collinear magnetism. These results shed light on the atomic-scale manifestation of complex magnetic textures influenced by the underlying structural chirality and open a route toward tunneling-based detection of magnetism in layered systems.

[1] P. Park, et. al., arxiv prep. 2410, 02180 (2024).  

[2] P. Park, et. al., Nat. Comm. 14(1), 8346 (2023). 

[3] H. Takagi, et. al., Nat. Phy. 19(7), 961-968 (2023). 

[4] Z. Nussinov, et. al. Phys. Rev. B 68, 085402 (2003).

Colossal magnetoresistance (CMR), which is defined as significant reduce of electrical resistance with applying external magnetic field, arise from the complex interplay between charge, spin and orbital degrees of freedom. The typical CMR required several tesla of external magnetic field by suppressing competing charge order phase or modulating band topology by changing magnetic ground state, which was not suitable for spintronic memory application. In this study, we investigated colossal magnetoresistance with low magnetic field in quasi 1D ferromagnetic van der Waals material, CrSbSe3 nanodevice. The magnitude of CMR of 4*10^5 % was achieved with low field of 0.2 T at 10 K, which is remarkable reduce of external field compared to previous studies. The semiconductor to metal transition was accompanied by saturating ferromagnetic domain, enabling CMR can be achieved with low magnetic field. The compared semiconducting behavior in bulk CrSbSe3 with fully saturated magnetic domain indicated the effect of magnetic domain wall was only valid in quasi 1D limit. The realization of CMR with low magnetic field establish CrSbSe3 as promising platform for spintronic application.

Flat nodal-line states, characterized by one-dimensional band degeneracy in the momentum space with negligible energy dispersion, are proposed to host novel electronic properties due to enhanced electron correlation and strong Berry curvature effects. Combined with two dimensional ferromagnetism in van der Waals (vdW) structures, the magnetotransport responses of the flat nodal-line states are expected to be highly distinct from those of other topological magnets, which have remained elusive. Here, we show that a room temperature vdW ferromagnet, Fe3GaTe2, hosts orbital driven flat nodal-line states near the Fermi level, contributing dominantly to the Berry curvature over the coexisting dispersive ones with opposite spin and orbital textures. The resulting Berry curvature of the flat nodal-line state is highly tunable with spin orientation, leading to an order of magnitude enhancement of anomalous Hall conductivity at the critical spin angle. Our findings establish Fe3GaTe2 as a promising ferromagnet for exploiting topological flat band degeneracy and two-dimensional magnetism for vdW-material-based spintronic applications.