d-electron superconductors, such as cuprates and iron arsenides, exhibited various unique superconductivities that have attracted attention in condensed matter physics. The d electrons of transition metals play a major role in the electrical properties of them and the cooperation between various features of d electrons, such as strong electron correlation, strong spin-orbit coupling, and strong spin and orbital fluctuations, results in the emergence of unique unconventional superconductivities. However, such d-electron superconductivities appeared only in materials with the specific combination of transition metal elements and crystal structure, which prevents complete understanding of them based on the systematic experimental studies.

Here we report the discovery of bulk superconductivity in Sc6MTe2, where M is Fe, Co, Ni, Ru, Rh, Os, and Ir [1]. They crystallize in the hexagonal Zr6CoAl2 type with the P-62m space group without inversion symmetry. The critical temperatures for M = 3d elements were higher than those for M = 4d and 5d ones, as shown in the Figure, reaching the highest Tc = 4.7 K for M = Fe. The first principles calculation results indicated the Fe 3d electrons have considerable contribution to the electronic state at the Fermi energy. The upper critical field Hc2 also has a pronounced element dependence, where Sc6MTe2 with M = Os and Ir showed the high Hc2(0) violating the Pauli limit. These results clearly show that Sc6MTe2 is a unique d-electron superconductor family that showed superconductivity in all 3d, 4d, and 5d cases, where the electron correlation and spin-orbit coupling of d electrons might play roles.

[1] Y. Shinoda et al., arXiv:2304.01444.

The discovery of the quasi-2D antiferromagnet CrSBr has stimulated much interest in the van der Waals (vdW) magnet because of its simultaneously tunable magnetic and electric properties. In this work, we present the low-energy magnon excitations in bulk CrSBr obtained by microwave absorption technique. The magnetic-field dependence of two resonance modes is measured in the three main crystallographic directions and up to well above the saturation fields, revealing magnetic anisotropies, magnetic transitions, and interlayer exchange energy. To account for the observed results, we formulate a microscopic spin model with biaxial single-ion anisotropy and interplane exchange, which gives an excellent description of the full magnon spectra and allows us to precisely determine the microscopic interaction parameters for CrSBr.

The RM3(BO3)4 series, comprising R = rare-earth metals or Y and M = Al, Ga, and Fe, represents a novel multiferroic system characterized by its non-centrosymmetric trigonal structure. This system has a diverse range of physical properties arising from the interactions between rare-earth 4f electrons and metal 3d electrons, such as an antiferromagnetic (AFM) transition in RFe3(BO3)4 at low temperatures. Neutron diffraction experiments have confirmed the presence of the AFM transition, and Neel temperatures (TN) of RFe3(BO3)4 compounds fall within a narrow range of 30 K to 40 K, suggesting that Fe plays a dominant role in the AFM behavior. However, there are several AFM structures observed, including easy-axis and easy-plane, which highlight the significance of the R ions in determining the AFM behavior. [1-3] The use of linearly polarized light, specifically X-ray linear dichroism (XLD), allows us to investigate orbital anisotropy arising from both structural distortion and magnetic interactions. To this end, we conducted XLD experiments on single crystals of TbFe3(BO3)4 and GdFe3(BO3)4. Our results revealed that the AFM phase transition caused modifications in the Fe-L2,3 spectra across TN. Moreover, the Fe XLD of TbFe3(BO3)4 differs from that of GdFe3(BO3)4, consistent with their distinct AFM structures. Additionally, the enhancement in the magnetization of TbFe3(BO3)4 was investigated via X-ray magnetic circular dichroism, and the orientation of Fe moment as well as Tb moment was determined through the spectra combined with configuration-interaction cluster calculations.

[1] Y. Hinatsu et al., J. Solid State Chem. 172, 438-445 (2003).

[2] E. A. Popova et al., Phys. Rev. B 75, 224413 (2007).

[3] F. Yen et al., Phys. Rev. B 73, 054435 (2006).

Symmetry-protected band degeneracy, coupled with magnetic order, is the key to realizing novel magnetoelectric phenomena in topological magnets. Ferrimagnetic semiconductor Mn3Si2Te6 is recently reported to show colossal angular magnetoresistance (CAMR), which is related to the insulator-to-metal transition driven by the tunable spin-orbit coupling gap with spin orientation [1]. In this case, the spin-polarized nodal states have been identified to be responsible for the extremely-sensitive electronic responses to the magnetic states. Here, taking external pressure as a control knob, we show that an insulator-metal transition, a spin reorientation transition, and a structural modification can be induced concomitantly at Pc ~ 14 GPa, when the nodal-line state approaches the Fermi level as the band gap closes in a ferrimagnetic semiconductor Mn3Si2Te6. These unique pressure-driven magnetic and electronic transitions, which originate from the interplay between spin-orbit coupling of the nodal-line state and magnetic frustration of localized spins also led to the unusual dome-shaped Tc variation with pressure, with Tc reaching up to nearly room temperature at Pc. Our findings highlight that the nodal-line states, isolated from other trivial states, can facilitate strongly tunable magnetic properties in topological magnets.

[1] J. Seo et al., Nature, 599, 576-581 (2021).

Spin and orbital degrees of freedom leads to rich physical phenomena such as the cross　correlation response [1]. When we perform material-based theoretical research, we need to use the parameters obtained from first-principles calculations, and treat the Coulomb interaction and　the spin-orbit coupling accurately. While the actual computation is challenging because of the huge computational cost, in the strong coupling limit where electrons are localized, it is easier to treat the both compared to itinerant systems by restoring an effective model. The localized effective model for multiorbital systems is known as Kugel-Khomskii model [2].

In our previous work, we have established the framework with the material-based Kugel-Khomskii model for arbitrary spin-orbital Mott insulators [3]. This framework allows for quantitative discussion and direct comparison with experimental results. In the present work, we apply the framework to organic Mott insulators. In these materials, it is pointed out that the spin-orbit coupling is essential to their lowenergy properties [4]. As for κ-(BEDT-TTF)2Cu[N(CN)2]Cl, which is a layered organic material, the inter-layer interaction is important to their magnetic structures in addition to the intra-layer Dzyaloshinskii-Moriya interaction [5]. In the presentation, we will report the details of the framework and the numerical results of mean-field and classical analysis for such organic Mott insulators.

[1] for example, G. Cao and P. Schlottmann, Rep. Prog. Phys. 81, 042502 (2018).

[2] K. I. Kugel and D. I. Khomskii, Zh. Eksp. Teor. Fiz. 64, 1429 (1973) [Sov. Phys. JETP 37, 725 (1973)].

[3] R. Iwazaki, H. Shinaoka, and S. Hoshino, arXiv2301.09824 (2023).

[4] S. M. Winter et al., Phys. Rev. B 95, 060404(R) (2017).

[5] R. Ishikawa et al., J. Phys. Soc. Jpn. 87, 064701 (2018).

Energy-efficient manipulation of magnetic domain wall by electrical current is one of the core subjects in the spintronic researches. Spin-transfer torque is a well-known mechanism for current-induced magnetic domain wall motion in ferromagnets. In case of non-adiabatic spin-transfer torque, effective magnetic field Heff is proportional to the current density J with a typical conversion efficiency of μ0Heff/J  < 10-13 Tm2/A. Here, we found highly efficient current-induced domain motion in Fe3GeTe2 nanoflakes by measuring the linear dependent depinning fields of a domain wall with variation of the current density. The resulting conversion efficiency is μ0Heff/J > 10-12 Tm2/A, at least an order of magnitude larger than typical values in conventional ferromagnets, and also larger than the theoretical upper bound of non-adiabatic spin-transfer torque, similar to the recently-discovered high conversion efficiency in SrRuO3[1]. Moreover, we found that the temperature-dependent conversion efficiency strongly depends on the in-plane current direction. The origin of the highly efficient and anisotropic spin transfer torque will be discussed in terms of the intrinsic spin-orbit torque due to broken inversion symmetry [2,3] or the so-called topological Hall torque, induced by Berry curvature [4].

[1] M. Yamanouchi, et al., Science Advances, 8, eabl6192 (2022).

[2] O. Johansen, et al., Phys. Rev. Lett., 122 (21), 217203 (2019). 

[3] K. Zhang, et al., Advanced Materials, 33 (4), 2004110 (2021).

[4] M. Yamanouchi, et al., Science Advances, 8, eabl6192 (2022).

Quantum spin models on the kagome lattice are prominent candidates for quantum spin liquids (QSL). For instance, the J_1-(J_2 )-J_d model serves as a typical example of chiral spin liquid with ν=1/2 fractional quantum Hall system [1,2], and the staggered scalar spin chiral term leads to a non-Fermi liquid behavior of the spinon excitation [3] (J_1 is the first-, J_2 the second-, and J_d the third-neighbor Heisenberg exchange). Such QSLs have their own characteristic gauge flux pattern on the kagome lattice. Therefore, we can naturally address the question: Can the system have other flux patterns with different physical properties? In this work, we study the competition between two models: J_1-J_2-J_d extended Heisenberg model and staggered scalar spin chiral term on the kagome lattice using a variational Monte Carlo (VMC) method. Varying J_2=J_d and the staggered scalar chiral term J_χ (J_1=1), we found that the phase diagram consists of three different QSLs: U(1)-Dirac spin liquid (U(1)-DSL), gapped chiral spin liquid (CSL), and gapless staggered CSL. For the staggered CSL case, there are three line spinon Fermi surfaces, and we show that those are protected by anti-symmetry relation between the vertical mirror plane and mean-field Hamiltonian. These line Fermi surfaces merge into one large Fermi surface as J_2=J_d comes in. Furthermore, we find that the J_χ induces a tricritical point between U(1)-DSL and gapped CSL. We construct symmetry-allowed Landau-Ginzburg theory that give rise to the tricritical point. We also discuss the static spin structure factor, the spin-spin correlation, and the longitudinal thermal conductivity of each phase as a guide for experiments.

[1] SS. Gong et al., Sci Rep 4, 6317 (2014).

[2] W.-J. Hu et al., Phys. Rev. B 91, 041124(R) (2015).

[3] B. Bauer et al., Phys. Rev. B 99, 035155 (2019).

Recently, spin liquid physics has been sought in 1T phase group V transition metal dichalcogenides (TMDs) where the electrons form large localized magnetic moments driven by electrons organized into David star (DS) charge density waves (CDWs). Based on density functional theory calculations, we discuss the possibility the electrons instead organize into inverted DS CDWs. We analyze the strong-correlation limit spin model using a combination of classical techniques, exact diagonalization, and variational Monte Carlo with wavefunctions derived from parton mean field theory. We discuss the phases that emerge and propose further experimental probes.

Two-dimensional topologically-ordered states such as fractional quantum Hall fluids host anyonic excitations, which are relevant for realizing fault-tolerant topological quantum computers. Classification and characterization of topological orders have been intensely pursued in both the condensed matter and mathematics literature. These topological orders can be bosonic or fermionic depending on whether the system hosts fundamental fermionic excitations or not. In particular, emergent topological orders in usual solid state systems are fermionic topological orders because the electron is a fermion. Recently, bosonic topological orders have been extensively completely classified up to rank 6 using representation theory. Inspired by their method, we provide in this paper a systematic method to classify the fermionic topological orders by explicitly building their modular data, which encodes the self and mutual statistics between anyons. Our construction of the modular data relies on the fact that the modular data of a fermionic topological order forms a projective representation of the Γ_θ subgroup of the modular group SL(2,Z). We carry out the classification up to rank 10 and obtain both unitary and non-unitary modular data. This includes all previously known unitary modular data, and also a new class of modular data of rank 10. We also determine the chiral central charges (mod 1/2) via a novel method which does not require the explicit computation of modular extensions.

Topological stabilizer models in 2,3D have enormous variations such as 2D toric code, and one of them has system size-dependent ground state degeneracy[1]. In this work, we introduce a series of Z_N generalization of 3D stabilizer models, whose Z_2 analogue has been studied in the name of 3D Toric code, X-cube, Sierpinski fractal spin liquid and Haah’s code. We also present an entirely new model without any Z_2 counterpart. Each model realizes new topological phases with novel types of excitations and braiding. For example, in the Z_N generalization of fractal type fracton models, the fractonic excitations, which were immobile in the Z_2 model, are now mobile and no longer "fractonic" in certain conditions. We will also work out the ground state degeneracies, which strongly depend on the system sizes.

[1] Haruki Watanabe, Meng Cheng, Yohei Fuji, arXiv:2211.00299 (2022).

We report the integer and fractional quantum Hall effects in decoupled two bilayer graphene stacks. Using a mismatch of momentum space, we suppress the single particle tunneling between two bilayer graphene even with layer separation of 0.34 nm, which brings strong interlayer coupling. We found remarkably stable Bose-Einstein condensation for the second orbital at the half-filled Landau level of each bilayer graphene sheet. Yet, they are missing in the zeroth orbital. We interpret this as effective skyrmion-anti-skyrmion interactions overwhelming the state. Asymmetric gate voltage bias by top and bottom gates governs bilayer graphene’s orbital characters, leading to orbitally mixed space that hosts even denominator fractional quantum Hall state, tot = -3/2. This observation opens unique edge construction utilizing electrons tied with chiral p-wave composite fermion.

Whereas it is well known that the intrinsic anomalous Hall conductivity (AHC) comes from the Berry curvature (BC) induced by spin-orbit gapped band anticrossing [1], the AHC values reported are usually limited in the range between 100 and 1000  [2]. The main cause of small AHC is that the BC sources in the momentum space may cancel each other [3]. In this research, the AHC of cobalt disulfides (CoS2) exhibits resonant behavior as the chemical potential being close to the middle of the gap by varying Fe-doping level. The AHC of Co0.95Fe0.05S2 shows a resonant peak of 2507 , more than four times larger than that of CoS2. According to our density functional theory and tight-binding analyses, the primary source of the large AHC was ascribed to four spin-polarized massive Dirac dispersions in the  plane of the Brillouin zone, slightly below the Fermi level. The observed colossal AHC can be resulted from the four BC sources, which have same sign and do not cancel out. Our result unveils the mechanism of the huge tunable AHC in CoS2 and sheds light on a strategy to search for the materials.

[1] Z. Fang et al., Science 302, 92 (2003).

[2] E. Liu et al., Nature Physics 14, 1125 (2018).

[3] C. Zhang et al., Science Bulletin 63, 580 (2018).

Magnetic frustration, realized in the special geometrical arrangement of localized spins, often promotes topologically nontrivial spin textures in the real space and induces significantly large unconventional Hall responses. This spin Berry curvature effect in itinerant frustrated magnets mainly works with a static spin order, limiting the effective temperature range below the magnetic transition temperature and yielding the typical anomalous Hall conductivity below ~103Ω-1cm-1. Here we show that an ultraclean triangular-lattice antiferromagnet PdCrO2 exhibits a large anomalous Hall conductivity up to ~106Ω-1cm-1 in the paramagnetic state, which is maintained far above the Neel temperature (TN) up to ~4 TN. This drastic enhancement of anomalous Hall response above TN is attributed to the skew scattering of highly mobile Pd electrons to fluctuating but locally-correlated Cr spins with a finite spin chirality. Our findings highlight a novel route to realizing high-temperature giant anomalous Hall responses, exploiting magnetic frustration in the ultraclean regime. 

Van der Waals topological materials has drawn attetion as highly efficient spin source materials [1]. However, vdW topological insulator(TI) can't be easily used to make spin device, because of its low conductivity and restriction of controlling thickness. We study SOT efficiency of vdW TI, Sn doped BSTS, in the heterostructure Sn-BSTS / FGT. Low conductivity of TI material can be overcame with lowering ferromagnet thickness to tri layers [2]. SOT efficiency is evaluated as 13.9 by second harmonic measurement and It is comparable with other TI material results. Also we investigate magnetization switching with DC pulse measurement and critical current is lower than heavy metal cases and even TI SOT device [3,4]. our results demonstrate efficient charge-spin conversion in vdW TI and provide possibility of application vdW TI spintronic device.

[1] Inseob Shin et al. Adv Mat. 34, 2101730 (2022)

[2] Deng, Y et al. Nature. 563, 94–99 (2018)

[3] L. Liu et al. Science. 336, 555 (2012)

[4] J. Han et al. PRL, 119, 077702 (2017)

Anomalous transport responses, dictated by the nontrivial band topology, are the key for application of topological materials to advanced electronics and spintronics. One promising platform is topological nodal-line semimetals due to their rich topology and exotic physical properties. However, their transport signatures have often been masked by the complexity in band crossings or the coexisting topologically trivial states. Here we show that, in slightly hole-doped SrAs3, the single-loop nodal-line states are well-isolated from the trivial states and entirely determine the transport responses. The characteristic torus-shaped Fermi surface and the associated encircling Berry flux of nodal-line fermions are clearly manifested by quantum oscillations of the magnetotransport properties and the quantum interference effect resulting in the two-dimensional behaviors of weak antilocalization. These unique quantum transport signatures make the isolated nodal-line fermions in SrAs3 desirable for novel devices based on their topological charge and spin transport.

[1] Kim, H. et al. Nat Commun 13, 7188 (2022)

The band topology in van der Waals (vdW) magnets provides a promising route to enhance and control magnetoelectronic responses, thereby enabling the identification of novel spintronic functionalities. However, such functionalities have been demonstrated below room temperature due to a lack of suitable materials possessing both topological band structure and high-Tc magnetism. Here we show that a room temperature vdW ferromagnet Fe3GaTe2 hosts double topological nodal-line states with large and tunable Berry curvature. The two orbital-driven nodal-line states with opposite spin-polarization, one flat and the other dispersive in energy, are located near the Fermi level and contribute additively to Berry curvature. The dominant contribution of the flat nodal-line state to the Berry curvature is modulated by spin orientation due to spin-orbit coupling, leading to unconventional angle-dependent anomalous Hall effect. Our results demonstrate that Fe3GaTe2 is a promising spintronic material for exploiting unique band topology and two-dimensional magnetism at room temperature. 

Kagome magnets, recent members of topological quantum materials, host nontrivial electronic and magnetic states, owing to the interplay between magnetism, topology, and electronic correlation. Topological band crossings and flat bands in Kagome magnets are closely linked to spin configurations and orientations, often leading to unprecedented magnetotransport properties. Here, taking a rare-earth Kagome magnet YMn6Sb6 as a model system, we show that complex evolution of the anomalous Hall conductivity across magnetic phase transitions is related to the field-driven changes in electronic structures and thus the Berry curvature. Also, we suggest that the angle and temperature dependent quantum oscillation are connected to the band structure near Fermi energy in ferromagnetic spin configuration. A detailed comparison on the anomalous Hall effect and electronic structure calculations will be addressed.