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Journal Articles

Emergence of inductance and capacitance from topological electromagnetism

Araki, Yasufumi; Ieda, Junichi

Journal of the Physical Society of Japan, 92(7), p.074705_1 - 074705_9, 2023/06

 Times Cited Count:0 Percentile:0(Physics, Multidisciplinary)

Topological electromagnetism owing to nontrivial momentum-space topology of electrons in insulators gives rise to diverse anomalous magnetoelectric responses. While conventional inductors and capacitors are based on classical electromagnetism described by Maxwell's equations, here we show that topological electromagnetism in combination with spin dynamics in magnets also generates an inductance or a capacitance. We build a generic framework to extract the complex impedance on the basis of topological field theory, and demonstrate the emergence of an inductance or a capacitance in several heterostructure setups. In comparison with the previously-studied emergent inductances in metallic magnets, insulators highly suppress the power loss, because of the absence of Joule heating. We show that the inductance from topological electromagnetism is achieved at low current and high frequency, and is also advantageous in its power efficiency, as characterized by the high quality factor (Q-factor).

Journal Articles

Magnetic orderings from spin-orbit coupled electrons on kagome lattice

Watanabe, Jin*; Araki, Yasufumi; Kobayashi, Koji*; Ozawa, Akihiro*; Nomura, Kentaro*

Journal of the Physical Society of Japan, 91(8), p.083702_1 - 083702_5, 2022/08

 Times Cited Count:1 Percentile:29.72(Physics, Multidisciplinary)

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

Journal Articles

Heavy-quark spin polarization induced by the Kondo effect in a magnetic field

Suenaga, Daiki*; Araki, Yasufumi; Suzuki, Kei; Yasui, Shigehiro*

Physical Review D, 105(7), p.074028_1 - 074028_19, 2022/04

 Times Cited Count:0 Percentile:21.8(Astronomy & Astrophysics)

We propose a new mechanism of the heavy-quark spin polarization (HQSP) in quark matter induced by the Kondo effect under an external magnetic field. The Kondo effect is caused by a condensate between a heavy and a light quark called the Kondo condensate leading to a mixing of the heavy- and light-quark spins. Thus, the HQSP is driven through the Kondo effect from light quarks coupling with the magnetic field in quark matter. For demonstration, we employ the Nambu-Jona-Lasinio type model under a magnetic field and investigate the HQSP within the linear response theory with vertex corrections required by the $$U(1)_{rm EM}$$ electromagnetic gauge invariance. As a result, we find that the HQSP arises significantly with the appearance of the Kondo effect. Our findings are testable in future sign-problem-free lattice simulations.

Journal Articles

Observation of topological Hall torque exerted on a domain wall in the ferromagnetic oxide SrRuO$$_3$$

Yamanouchi, Michihiko*; Araki, Yasufumi; Sakai, Takaki*; Uemura, Tetsuya*; Ota, Hiromichi*; Ieda, Junichi

Science Advances (Internet), 8(15), p.eabl6192_1 - eabl6192_6, 2022/04

 Times Cited Count:5 Percentile:75.68(Multidisciplinary Sciences)

In a ferromagnetic Weyl metal SrRuO$$_3$$, a large effective magnetic field $$H_mathrm{eff}$$ exerted on a magnetic domain wall (DW) by current has been reported. We show that the ratio of $$H_mathrm{eff}$$ to current density exhibits nonmonotonic temperature dependence and surpasses those of conventional spin-transfer torques and spin-orbit torques. This enhancement is described well by topological Hall torque (THT), which is exerted on a DW by Weyl electrons emerging around Weyl points when an electric field is applied across the DW. The ratio of the $$H_mathrm{eff}$$ arising from the THT to current density is over one order of magnitude higher than that originating from spin-transfer torques and spin-orbit torques reported in metallic systems, showing that the THT may provide a better way for energy-efficient manipulation of magnetization in spintronics devices.

Journal Articles

Intrinsic torques emerging from anomalous velocity in magnetic textures

Araki, Yasufumi; Ieda, Junichi

Physical Review Letters, 127(21), p.277205_1 - 277205_7, 2021/12

 Times Cited Count:3 Percentile:42.26(Physics, Multidisciplinary)

Momentum-space topology of electrons under strong spin-orbit coupling contributes to the electrically induced torques exerting on magnetic textures insensitively to disorder or thermal fluctuation. We present a direct connection between band topology and the torques by classifying the whole torques phenomenologically. As well as the intrinsic anomalous Hall effect, the torques also emerge intrinsically from the anomalous velocity of electrons regardless of a nonequilibrium transport current. We especially point out the intrinsic contribution arising exclusively in magnetic textures, which we call the "topological Hall torque (THT)". The THT emerges in bulk crystals without any interface or surface structures. We numerically demonstrate the enhancement of the THT in comparison with the conventional spin-transfer torque in the bulk metallic ferromagnet, which accounts for the giant current-induced torque measured in ferromagnetic $$mathrm{SrRuO_3}$$.

Journal Articles

Nodal lines and boundary modes in topological Dirac semimetals with magnetism

Araki, Yasufumi; Watanabe, Jin*; Nomura, Kentaro*

Journal of the Physical Society of Japan, 90(9), p.094702_1 - 094702_9, 2021/09

 Times Cited Count:1 Percentile:17.22(Physics, Multidisciplinary)

While nodal-line semimetals with magnetism have been theoretically predicted and experimentally observed in a few compounds, idea on the relation between the magnetic order and the electronic structure is still limited. We theoretically explore the electronic structure in bulk and boundary of such a magnetic nodal-line state by introducing magnetism in topological Dirac semimetal (TDSM). TDSMs, such as $$mathrm{Cd_3 As_2}$$ and $$mathrm{Na_3 Bi}$$,are characterized by a pair of spin-degenerate Dirac points protected by rotational symmetries of crystals. By introducing local magnetic moments coupled to the electron spins in the lattice model of TDSM, we show that the TDSM can turn into either a Weyl semimetal or a nodal-line semimetal, which is determined by the orbital dependence in the exchange coupling and the direction of magnetization formed by the magnetic moments. In this magnetic nodal-line semimetal state, we find zero modes with drumhead-like band structure at the boundary that are characterized by the topological number of $$mathbb{Z}$$. Those zero modes are numerically demonstrated by introducing magnetic domain walls in the lattice model.

Journal Articles

Microscopic theory of electrically induced spin torques in magnetic Weyl semimetals

Kurebayashi, Daichi*; Araki, Yasufumi; Nomura, Kentaro*

Journal of the Physical Society of Japan, 90(8), p.084702_1 - 084702_9, 2021/08

 Times Cited Count:5 Percentile:62.6(Physics, Multidisciplinary)

We theoretically study current- and charge-induced spin torque in magnetic Weyl semimetals. In magnetic Weyl semimetals (WSM), topologically nontrivial band structure mediates anomalous coupling between magnetization and transport, making WSMs preferable for spintronics systems. In this paper, we determine all current-induced spin torques including spin-orbit torque and spin-transfer torque, up to first order with respect to spatial and temporal derivatives and electrical currents. We also calculate the charge-induced spin torque microscopically. We find the charge-induced spin torque originates from the chiral anomaly due to the correspondence between spin operators and axial current operators in our model.

Journal Articles

Long-range spin transport on the surface of topological Dirac semimetal

Araki, Yasufumi; Misawa, Takahiro*; Nomura, Kentaro*

Physical Review Research (Internet), 3(2), p.023219_1 - 023219_15, 2021/06

We theoretically propose the long-range spin transport mediated by the gapless surface states of topological Dirac semimetal (TDSM). Low-dissipation spin current is a building block of next-generation spintronics devices. While conduction electrons in metals and spin waves in ferromagnetic insulators (FMIs) are the major carriers of spin current, their propagation length is inevitably limited due to the Joule heating or the Gilbert damping. In order to suppress dissipation and realize long-range spin transport, we here make use of the spin-helical surface states of TDSMs, such as Cd$$_3$$As$$_2$$ and Na$$_3$$Bi, which are robust against disorder. Based on a junction of two FMIs connected by a TDSM, we demonstrate that the magnetization dynamics in one FMI induces a spin current on the TDSM surface flowing to the other FMI. By both the analytical transport theory on the surface and the numerical simulation of real-time evolution in the bulk, we find that the induced spin current takes a universal semi-quantized value that is insensitive to the microscopic coupling structure between the FMI and the TDSM. We show that this surface spin current is robust against disorder over a long range, which indicates that the TDSM surface serves as a promising system for realizing spintronics devices.

Journal Articles

Spin-orbital magnetic response of relativistic fermions with band hybridization

Araki, Yasufumi; Suenaga, Daiki*; Suzuki, Kei; Yasui, Shigehiro*

Physical Review Research (Internet), 3(2), p.023098_1 - 023098_17, 2021/05

Spins of relativistic fermions are related to their orbital degrees of freedom. In order to quantify the effect of hybridization between relativistic and nonrelativistic degrees of freedom on spin-orbit coupling, we focus on the spin-orbital (SO) crossed susceptibility arising from spin-orbit coupling. The SO crossed susceptibility is defined as the response function of their spin polarization to the "orbital" magnetic field, namely the effect of magnetic field on the orbital motion of particles as the vector potential. Once relativistic and nonrelativistic fermions are hybridized, their SO crossed susceptibility gets modified at the Fermi energy around the band hybridization point, leading to spin polarization of nonrelativistic fermions as well. These effects are enhanced under a dynamical magnetic field that violates thermal equilibrium, arising from the interband process permitted by the band hybridization. Its experimental realization is discussed for Dirac electrons in solids with slight breaking of crystalline symmetry or doping, and also for quark matter including dilute heavy quarks strongly hybridized with light quarks, arising in a relativistic heavy-ion collision process.

Journal Articles

Magnetization energy current in the axial magnetic effect

Shitade, Atsuo*; Araki, Yasufumi

Physical Review B, 103(15), p.155202_1 - 155202_8, 2021/04

 Times Cited Count:2 Percentile:18.63(Materials Science, Multidisciplinary)

The axial magnetic effect (AME) is one of the anomalous transport phenomena in which the energy current is induced by an axial magnetic field. Here we numerically study the AME for the relativistic Wilson fermion in the axial magnetic field and a twisted Dirac semimetal. The AME current density inside the bulk is nonzero, and particularly in the low energy regime for the former model, it is explained by the field-theoretical results without any fitting parameter. However, for both models, the average AME current density vanishes owing to the surface contribution. The axial gauge field is regarded as the spatially modulated (effective) Zeeman field and induces the spatially modulated energy magnetization. The AME is attributed to the magnetization energy current and hence cannot be observed in transport experiments.

Journal Articles

Chiral separation effect catalyzed by heavy impurities

Suenaga, Daiki*; Araki, Yasufumi; Suzuki, Kei; Yasui, Shigehiro*

Physical Review D, 103(5), p.054041_1 - 054041_17, 2021/03

 Times Cited Count:3 Percentile:39.22(Astronomy & Astrophysics)

We investigate the influence of the Kondo effect, namely, the nonperturbative effect induced by heavy impurities, on the chiral separation effect (CSE) in quark matter. We employ a simple effective model incorporating the Kondo condensate made of a light quark and a heavy quark, and compute the response function of the axial current to the magnetic field in the static and dynamical limits. As a result, we find that the Kondo effect catalyzes the CSE in both of the limits, and in particular the CSE in the dynamical limit can be enhanced by a factor of approximately 3. Our findings clearly show that the presence of heavy impurities in quark matter can play an important role in the transport phenomena of light quarks induced by a magnetic field.

Journal Articles

Two relativistic Kondo effects; Classification with particle and antiparticle impurities

Araki, Yasufumi; Suenaga, Daiki*; Suzuki, Kei; Yasui, Shigehiro*

Physical Review Research (Internet), 3(1), p.013233_1 - 013233_12, 2021/03

We investigate two different types of relativistic Kondo effects, distinguished by heavy-impurity degrees of freedom, by focusing on the energy-momentum dispersion relations of the ground state with condensates composed of a light Dirac fermion and a nonrelativistic impurity fermion. Heavy fermion degrees of freedom are introduced in terms of two types of heavy-fermion effective theories, in other words, two heavy-fermion limits for the heavy Dirac fermion, which is known as the heavy-quark effective theories (HQETs) in high-energy physics. While the first one includes only the heavy-particle component, the second one contains both the heavy-particle and heavy-antiparticle components, which are opposite in their parity. From these theories, we obtain two types of Kondo effects, in which the dispersions near the Fermi surface are very similar, but they differ in the structure at low momentum. We also classify the possible forms of condensates in the two limits. The two Kondo effects will be examined by experiments with Dirac/Weyl semimetals or quark matter, lattice simulations, and cold-atom simulations.

Journal Articles

Kondo effect driven by chirality imbalance

Suenaga, Daiki*; Suzuki, Kei; Araki, Yasufumi; Yasui, Shigehiro*

Physical Review Research (Internet), 2(2), p.023312_1 - 023312_13, 2020/06

The Kondo effect is induced by the interaction between light fermions near the Fermi surface and heavy impurities, and it affects electric/thermal/transport properties of matter. The chirality (right-handed or left-handed) is one of the unique properties of relativistic (Dirac or Weyl) fermions. In normal matter, the numbers of right- and left-handed particles are equivalent to each other, but environments with a chirality imbalance can also be realized. In this paper, we theoretically propose the Kondo effect driven by a chirality imbalance (or chiral chemical potential) of relativistic light fermions. This effect is caused by the mixing between a right-handed (or left-handed) fermion and a heavy impurity in the chirality imbalanced matter. This is different from the usual Kondo effect induced by finite density (or chemical potential) for light fermions. We construct an effective model with an interaction between a relativistic fermion and a heavy impurity, and we derive the realization of the Kondo effect from both a perturbative calculation and a nonperturbative mean-field approach. We also discuss the temperature dependence, the coupling constant dependence, the susceptibilities, and the order of the phase transition for the Kondo effect. Such a Kondo effect will be tested by future lattice simulations.

Journal Articles

Dynamical spin-to-charge conversion on the edge of quantum spin Hall insulator

Araki, Yasufumi; Misawa, Takahiro*; Nomura, Kentaro*

Physical Review Research (Internet), 2(2), p.023195_1 - 023195_11, 2020/05

Journal Articles

Magnetic textures and dynamics in magnetic Weyl semimetals

Araki, Yasufumi

Annalen der Physik, 532(2), p.1900287_1 - 1900287_16, 2020/02

 Times Cited Count:30 Percentile:88.12(Physics, Multidisciplinary)

Journal Articles

High heat load test of molybdenum

Tanabe, Tetsuro*; Fujine, Michihiko*; Noguchi, Hiroshi*; Yagi, Yasufumi*; Hirano, Yoichi*; Shimizu, Hajime*; Akiba, Masato; Araki, Masanori; Kubota, Yusuke*; Miyahara, Akira*

Journal of Nuclear Materials, 200(1), p.120 - 127, 1993/03

 Times Cited Count:9 Percentile:66.96(Materials Science, Multidisciplinary)

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Oral presentation

Dissipative spin pumping in two-dimensional quantum spin Hall insulator

Araki, Yasufumi; Misawa, Takahiro*; Nomura, Kentaro*

no journal, , 

Oral presentation

Dissipative spin pumping into two-dimensional quantum spin Hall insulator

Araki, Yasufumi; Misawa, Takahiro*; Nomura, Kentaro*

no journal, , 

42 (Records 1-20 displayed on this page)