Language：English   Publishing type：Research paper (scientific journal)  

The effective tight-binding model with compensated ferrimagnetic inverse-Heusler lattice Ti2MnAl, candidate material of magnetic Weyl semimetal, is proposed. The energy spectrum near the Fermi level, the configurations of the Weyl points, and the anomalous Hall conductivity are calculated. We found that the orbital magnetization is finite, while the total spin magnetization vanishes, at the energy of the Weyl points. The magnetic moments at each site are correlated with the orbital magnetization, and can be controlled by the external magnetic field.

DOI： https://doi.org/10.7566/JPSJ.93.034703

Publisher：Physical Review Applied  

We theoretically study the spin Hall effect in a simple tight-binding model of the stacked kagome Weyl semimetal Co3Sn2S2 with ferromagnetic ordering. We focus on the two types of spin Hall current: one flowing in the in-plane direction with respect to the kagome lattice (in-plane spin Hall current), and the other flowing in the stacking direction (out-of-plane spin Hall current). We show that the spin Hall conductivities for those spin currents drastically change depending on the direction of the magnetic moment. In particular, the out-of-plane spin Hall current may induce surface spin accumulations, which are useful for magnetization switching via spin-orbit torque.

DOI： 10.1103/PhysRevApplied.21.014041

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Language：English   Publishing type：Research paper (scientific journal)  

We theoretically study the spin Hall effect in a simple tight-binding model of stacked-kagome Weyl semimetal Co3Sn2S2 with ferromagnetic ordering. We focus on the two types of the spin Hall current: one flowing in the in-plane direction with respect to the kagome lattice (in-plane spin Hall current), and one flowing in the stacking direction (out-of-plane spin Hall current). We show the spin Hall conductivities for those spin currents drastically change depending on the direction of the magnetic moment. Especially, the out-of-plane spin Hall current may induce surface spin accumulation, which are useful for the perpendicular magnetization switching via spin-orbit torque.

DOI： https://doi.org/10.1103/PhysRevApplied.21.014041

Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

Higher-order topological phases are gapped phases of matter that host gapless corner or hinge modes. For the case of superconductors, corner or hinge modes are gapless Majorana modes or Majorana zero modes. To construct three-dimensional higher-order topological superconductors, we consider a topological-insulator/superconductor multilayer under in-plane Zeeman coupling. We found three different types of higher-order topological superconductor phases, a second-order topological superconductor phase with Majorana hinge flat bands, a second-order Dirac superconductor phase with surface Majorana cones and Majorana hinge arcs, and nodal-line superconductor phases with drumhead surface states and Majorana hinge arcs.

DOI： 10.1103/PhysRevB.108.184517

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.108.184517/fulltext

Language：English   Publishing type：Research paper (scientific journal)  

Higher-order topological phases are gapped phases of matter that host gapless corner or hinge modes. For the case of superconductors, corner or hinge modes are gapless Majorana modes or Majorana zero modes. To construct 3d higher-order topological superconductors, we consider a topological-insulator/superconductor multilayer under in-plane Zeeman coupling. We found three different types of higher-order topological superconductor phases, a second-order topological superconductor phase with Majorana hinge flat bands, a second-order Dirac superconductor phase with surface Majorana cones and Majorana hinge arcs, and nodal-line superconductor phases with drumhead surface states and Majorana hinge arcs.

DOI： https://doi.org/10.1103/PhysRevB.108.184517

Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

Magnetic Weyl semimetals (mWSMs) are characterized by linearly dispersive bands with chiral Weyl node pairs associated with broken time-reversal symmetry. One of the hallmarks of mWSMs is the emergence of large intrinsic anomalous Hall effect. On heating the mWSM above its Curie temperature, the magnetism vanishes while exchange-split Weyl point pairs collapse into doubly degenerate gapped Dirac states. Here, we reveal the attractive potential of these Dirac nodes in paramagnetic state for efficient spin current generation at room temperature via the spin Hall effect. Ni and In are introduced to separately substitute Co and Sn in a prototypal mWSM Co3Sn2S2 shandite film and tune the Fermi level. Composition dependence of spin Hall conductivity for paramagnetic shandite at room temperature resembles that of anomalous Hall conductivity for ferromagnetic shandite at low temperature; exhibiting peak-like dependence centering around the Ni-substituted Co2Ni1Sn2S2 and undoped Co3Sn2S2 compositions, respectively. The observed spin Hall and anomalous Hall conductivity maxima at different compositions reflect optimum Fermi-level positioning relative to the paramagnetic Dirac and magnetic Weyl states, suggesting the common origin and intercorrelation between the two Hall effects. Our findings highlight a strategy for the quest of spin Hall materials, guided by the abundant experimental anomalous Hall-effect data of ferromagnets in the literature.

DOI： 10.1103/PhysRevB.108.064429

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.108.064429/fulltext

Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

Co3Sn2S2 (CSS) is one of the shandite compounds and becomes a magnetic Weyl semimetal candidate below the ferromagnetic phase transition temperature (TC). In this paper, we investigate the temperature (T) dependence of conversion between charge current and spin current for the CSS thin film by measuring the spin-torque ferromagnetic resonance (ST-FMR) for the trilayer consisting of CSS/Cu/CoFeB. Above TC∼170 K, the CSS/Cu/CoFeB trilayer exhibits the clear ST-FMR signal coming from the spin Hall effect in the paramagnetic CSS and the anisotropic magnetoresistance (AMR) of CoFeB. Below TC, on the other hand, it is found that the ST-FMR signal involves the dc voltages (Vdc) not only through the AMR but also through the giant magnetoresistance (GMR). Thus, the resistance changes coming from both AMR and GMR should be taken into account to correctly understand the characteristic field angular dependence of Vdc. The spin Hall torque generated from the ferromagnetic CSS, which possesses the same symmetry as that for spin Hall effect, dominantly acts on the magnetization of CoFeB. A definite increase in the spin-charge conversion efficiency is observed at T<TC, indicating that the phase transition to the ferromagnetic CSS promotes the highly efficient spin-charge conversion. In addition, our theoretical calculation shows the increase in spin Hall conductivity with the emergence of magnetic moment at T<TC, which is consistent with the experimental observation.

DOI： 10.1103/PhysRevResearch.5.013222

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevResearch.5.013222/fulltext

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

The efficiency of spintronic devices can be improved by generating higher effective magnetic fields with lower working currents. Spin-transfer torques can drive magnetic domain wall motion in a device composed of a single material, but a high threshold current density is typically required to move the domain wall and improving the effective magnetic field in common itinerant ferromagnets is difficult. Here we report magnetism modulation in Co3Sn2S2—a magnetic Weyl semimetal—via spin-transfer-torque-driven domain wall motion. We examine the effect of d.c. current on magnetic reversal using anomalous Hall resistance measurements and domain wall motion using time-of-flight measurements. At 160 K, the threshold current density for driving domain wall motion is less than 5.1 × 105A cm−2 at zero external field and less than 1.5 × 105A cm−2 at a moderate external field (0.2 kOe). The spin-transfer-torque effective field can reach as high as 2.4–5.6 kOe MA−1 cm2 at 150 K.

DOI： 10.1038/s41928-022-00879-8

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Other Link： https://www.nature.com/articles/s41928-022-00879-8

Language：Japanese   Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Temperature (T) dependence of conversion from charge current to spin current was investigated for the Co3Sn2S2 (CSS) thin film by measuring the spin-torque ferromagnetic resonance (ST-FMR) for the sample with CSS / Cu / CoFeB. Above the ferromagnetic phase transition temperature (TC), the sample exhibited the clear ST-FMR signal coming from the spin Hall effect in the paramagnetic CSS and the anisotropic magnetoresistance (AMR) of CoFeB. Below TC, on the other hand, it is found that the ST-FMR signal involves the dc voltages not only through the AMR but also through the giant magnetoresistance. In addition, the spin Hall torque generated from the ferromagnetic CSS dominantly acts on the CoFeB magnetization. An increase in the spin-charge conversion efficiency is observed at T < TC, indicating that the phase transition from the paramagnetic CSS to the ferromagnetic CSS promotes the spin-charge conversion.

DOI： 10.1109/INTERMAGShortPapers58606.2023.10228604

Scopus

Publishing type：Research paper (scientific journal)   Publisher：Journal of the Physical Society of Japan  

We studied the tunnel transport between the edge of a Pfaffian fractional quantum Hall state and that of an integer quantum Hall state. Based on the duality argument between the strong and weak tunnelings, we found that an Andreev-like reflection appeared in the strong tunneling regime. We calculated the charge conductance in the weak and strong tunneling regimes for the low-voltage limit. In the weak tunneling limit, dI=dV was proportional to V1=ν with bias voltage V and ν = 1=2. By contrast, in the strong tunneling limit, dI=dV was expressed by (e2=h)2ν=(1 + ν) with a correction term. We expect that this condition can be realized experimentally at the point contact between a fractional quantum Hall state with ν = 5=2 and an integer quantum Hall state with ν = 3.

DOI： 10.7566/JPSJ.91.123703

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.7566/JPSJ.91.123703

Publishing type：Research paper (scientific journal)   Publisher：Journal of the Physical Society of Japan  

We investigated the magnetic orderings on the kagome lattice numerically from the tight-binding Hamiltonian of electrons, governed by the filling factor and spin-orbit coupling (SOC). We found that even a simple kagome lattice model can host both ferromagnetic and noncollinear antiferromagnetic orderings depending on the electron filling, reflecting the gap structures in the Dirac and flat bands characteristic of the kagome lattice. Kane-Mele- or Rashba-type SOC tends to stabilize noncollinear orderings, such as magnetic spirals and 120-degree antiferromagnetic orderings, because of the effective Dzyaloshinskii-Moriya interaction from SOC. The obtained phase structure helps in the qualitative understanding of magnetic orderings in various kagome-layered materials with Weyl or Dirac electrons.

DOI： 10.7566/JPSJ.91.083702

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

We develop a formulation of the coherent potential approximation (CPA) on the basis of the Wannier representation to advance a computationally efficient method for the treatment of homogeneous random alloys that is independent of the applied first-principles electronic structure code. To verify the performance of this CPA implementation within the Wannier representation, we examine the Bloch spectral function, the density of states, and the magnetic moment in Fe-based transition metal alloys Fe-X (X=V, Co, Ni, and Cu) and compare the results with those of the well-established CPA implementation based on the Korringa-Kohn-Rostoker (KKR) Green's function method. The Wannier-CPA and the KKR-CPA methods lead to very similar results. The presented Wannier-CPA method has a wide potential applicability to other physical quantities and large compound systems because of the low computational effort required.

DOI： 10.1103/PhysRevB.105.125136

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.105.125136/fulltext

Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

We study the electronic state of an inverse spinel compound MnMg₂O₄ based on first-principles calculations. The high-spin state is realized in Mn ions on the diamond lattice, resulting in that this material is found to be a half-metallic semimetal with the minority spin-gap about 3eV, and also with line nodes in the Brillouin zone. The intrinsic anomalous Hall conductivity (AHC) is also computed as a function of the chemical potential of the system assuming the rigid band structure, and is found to exhibit a peak structure with a maximum value of 200 S/cm at only 15 meV above the Fermi level. The relation between the large AHC and Berry curvature in the Brillouin zone is also discussed.

DOI： 10.1063/9.0000317

Scopus

Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

The quantum Hall system can be used to study many-body physics owing to its multiple internal electronic degrees of freedom and tunability. While quantum phase transitions have been studied intensively, research on the temperature-induced phase transitions of this system is limited. We measured the pure bulk conductivity of a quantum Hall antiferromagnetic state in bilayer graphene over a wide range of temperatures and revealed the two-step phase transition associated with the breaking of the long-range order, i.e., the Kosterlitz-Thouless transition, and short-range antiferromagnetic order. Our findings are fundamental to understanding electron correlation in quantum Hall systems.

DOI： 10.1103/physrevb.105.075427

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.105.075427/fulltext

Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

We theoretically study the carrier doping effect for magnetism in the stacked-kagome system Co3Sn2S2 based on an effective model and the Hartree-Fock method. We show the electron filling and temperature dependencies of the magnetic order parameter. The perpendicular ferromagnetic ordering is suppressed by hole doping, whereas undoped Co3Sn2S2 shows a magnetic Weyl semimetal state. Additionally, in the electron-doped regime, we find a noncollinear antiferromagnetic ordering. Especially, in the noncollinear antiferromagnetic state, by considering a certain spin-orbit coupling, the finite orbital magnetization and the anomalous Hall conductivity are obtained.

DOI： 10.1103/PhysRevMaterials.6.024202

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevMaterials.6.024202/fulltext

Publisher：Journal of the Physical Society of Japan  

We study the intrinsic and extrinsic Hall effects in disordered magnetic Weyl semimetals numerically. We show that in Weyl metals, where the Fermi energy deviates from the Weyl point, the Hall and longitudinal conductances exhibit a specific relation, which is distinguished from the well-known relation in integer quantum Hall systems. Around the Weyl point, the Hall conductance increases with increasing longitudinal conductance. This increasing behavior indicates the existence of additional contributions to the Hall conductance from the subbands of Weyl cones besides that from the bulk Berry curvature. We also show that the extrinsic anomalous Hall effect due to the spin scatterers (skew scattering) is significantly suppressed in Weyl metals.

DOI： 10.7566/JPSJ.91.013703

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