arXiv subjects

Ai Yamakage

Publications and source records attributed to Ai Yamakage.

At least 37 records · Page 2Linked to original sources

arXiv2024

Theory of Nonlinear Hall Effect Induced by Electric Field and Temperature Gradient in 3D Chiral Magnetic Textures

We investigate the current proportional to the cross product of an electric field and a temperature gradient, which we call the Nonlinear Chiral Thermo-Electric (NCTE) Hall effect, in 3D chiral magnetic structures. We analyze both discrete and continuous magnetization cases. From the analysis of discrete magnetization, we clarify the conditions for the emergence of the NCTE Hall effect caused by magnetic textures. In the continuous magnetization case, we show that the NCTE Hall conductivity is proportional to the density of magnetic monopoles. This study discovers unconventional phenomena unique to 3D magnetic textures, and has significant implications for spintronics and quantum transport.

cond-mat.mes-hall↗

arXiv2024

Nonlinear charge and thermal transport properties induced by orbital magnetic moment in chiral crystal cobalt monosilicide

The existence of exotic singularities in momentum space, such as spin-1 excitations and Rarita-Schwinger-Weyl (RSW) fermions, has been discussed so far to explore unique phenomena in the nonmagnetic B20-type compounds. Meanwhile, the Nonlinear Thermo-Electric (NCTE) charge and thermal Hall effect, a response proportional to the cross product of the electric field and temperature gradient, is expected in this chiral material, yet remains unexplored in B20-type compounds. Here, based on $ab \ initio$ calculations and symmetry analysis, we quantitatively analyze the NCTE charge and thermal Hall effects in cobalt monosilicide, obtaining experimentally measurable values of NCTE charge and thermal Hall current along [111] direction, which is not expected for second-order current responses to the DC electric field. Furthermore, we demonstrate that these significant responses are enhanced around RSW fermions and spin-1 excitations. Additionally, we clarify that the NCTE Hall effect is solely governed by orbital magnetic moments due to the cancellation of Berry curvature contributions in cubic chiral crystals.

cond-mat.str-el↗

arXiv2024

Nonlinear charge transport properties in chiral tellurium

Extensive research has focused on phenomena arising from the chirality of crystalline or magnetic structures. Recently, we have proposed the "Nonlinear Chiral Thermo-Electric (NCTE) Hall effect," in which current flows in the direction of the cross product of the electric field and the temperature gradient in a material with a chiral structure. Despite its importance for device applications such as heat flow sensors and logic circuits, there are no quantitative demonstrations in actual materials. We investigate the NCTE Hall effect and the second-order response to the electric field in Te, which hosts the chiral crystal structure, based on $ab \ initio$ calculations and symmetry argument. We clarify that the NCTE Hall current can flow parallel to the chiral axis, whereas such behavior is absent for the second-order response to the DC electric field. Quantitative estimation predicts the large NCTE Hall current which is sufficiently detectable in experiments, and the orbital magnetic moment will play an important role in the vicinity of the top of the valence band.

cond-mat.str-el↗

arXiv2024

Dynamical Majorana Ising spin response in a topological superconductor-magnet hybrid by microwave irradiation

We study a dynamical spin response of surface Majorana modes in a topological superconductor-magnet hybrid under microwave irradiation. We find a method to toggle between dissipative and non-dissipative Majorana Ising spin dynamics by adjusting the external magnetic field angle and the microwave frequency. This reflects the topological nature of the Majorana modes, enhancing the Gilbert damping of the magnet, thereby, providing a detection method for the Majorana Ising spins. Our findings illuminate a magnetic probe for Majorana modes, paving the path to innovative spin devices.

cond-mat.mes-hall↗

arXiv2023

Electrical conductivity and screening effect of spin-1 chiral fermions scattered by charged impurities

We theoretically study the quantum transport in a three-dimensional spin-1 chiral fermion system in the presence of coulomb impurities based on the self-consistent Born approximation. We find that the flat-band states anomalously enhance the screening effect, and the electrical conductivity is increased in the low-energy region. It is also found that reducing the screening length leads to an increase in the forward scattering contribution and, thus, an increase in the vertex correction in the high-energy region.

cond-mat.mes-hall↗

arXiv2023

Quantum Transport in Spin-1 Chiral Fermion: Self-Consistent Born Approximation

Quantum transport for a spin-1 chiral fermion is studied within the self-consistent Born approximation. We find characteristic properties around zero energy, i.e., the peak structure of the density of states and significant suppression of electrical conductivity. These structures originate from the flat band and its interband effect.

cond-mat.mes-hall↗

arXiv2023

Hall effect of ferro/antiferromagnetic wallpaper fermions

Nonsymmorphic crystals can host characteristic double surface Dirac cones with fourfold degeneracy on the Dirac points, called wallpaper fermion, protected by wallpaper group symmetry. We clarify the charge and spin Hall effect of wallpaper fermions in the presence of the (anti)ferromagnetism.Based on a four-sublattice model, we construct the effective Hamiltonian of wallpaper fermions coupled with the ferromagnetic or antiferromagnetic moment.Both ferromagnetic and antiferromagnetic moments induce an energy gap for the wallpaper fermions, leading to quantized (spin) Hall conductivity. The ferromagnetic wallpaper fermion induces the Hall conductivity quantized into $e^2/h$, which is twice that for a single Dirac fermion on the surface of topological insulators. On the other hand, the spin Hall conductivity decays and reaches to be a finite value as the antiferromagnetic coupling increases. We also show that the results above are valid for a general model of wallpaper fermions from symmetry consideration.

cond-mat.mes-hall↗

arXiv2023

Microscopic Theory of Nonlinear Hall Effect Induced by Electric Field and Temperature Gradient

Electric current flows parallel to the outer product of an applied electric field and temperature gradient, a phenomenon we call the nonlinear chiral thermo-electric (NCTE) Hall effect. We present a general microscopic formulation of this effect and demonstrate its existence in a chiral crystal. We show that the contribution of the orbital magnetic moment, which has been previously overlooked, is just as significant as the conventional Berry curvature dipole term. Furthermore, we demonstrate a substantial NCTE Hall effect in a chiral Weyl semimetal. These findings offer new insights into nonlinear transport phenomena and have significant implications for the field of condensed matter physics.

cond-mat.mes-hall↗

arXiv2023

Anisotropic spin-current spectroscopy of ferromagnetic superconducting gap symmetries

We develop a microscopic theory of tunneling spin transport at the magnetic interface between a ferromagnetic insulator and a ferromagnetic superconductor (FSC) driven by ferromagnetic resonance. We show that the spin susceptibilities of the FSC can be extracted from the spin currents by tuning the easy axis of the FSC, and thus the spin currents can be a probe for the symmetries of the spin-triplet Cooper pairing. Our results will offer a route to exploiting the synergy of magnetism and superconductivities for spin devices.

cond-mat.supr-con↗

arXiv2023

Majorana Spin Current Generation by Dynamic Strain

Majorana fermions that emerge on the surface of topological superconductors are charge neutral but can have higher-rank electric multipoles by allowing for account time-reversal and crystalline symmetries. Applying the general classification of these multipoles, we show that the spin current of Majorana fermions is driven by spatially nonuniform dynamic strains on the (001) surface of superconducting antiperovskite Sr3SnO. We also find that the frequency dependence of the Majorana spin current reflects the energy dispersion of Majorana fermions. Our results suggest that the spin current can be a probe for Majorana fermions.

cond-mat.supr-con↗

arXiv2022

Quantum transport of a spin-1 chiral fermion

We theoretically study the quantum transport in a three-dimensional spin-1 chiral fermion system in the presence of impurity scattering. Within the self-consistent Born approximation, we find peak structure of the density of states and significant suppression of electrical conductivity around the zero energy. The zero-energy conductivity depends less on impurity concentration, unlike a Weyl fermion. These properties originate from the flat band structure of spin-1 chiral fermion.

cond-mat.mes-hall↗

arXiv2022

Anisotropic superconducting spin transport at magnetic interfaces

We present a theoretical investigation of anisotropic superconducting spin transport at a magnetic interface between a p-wave superconductor and a ferromagnetic insulator. Our formulation describes the ferromagnetic resonance modulations due to spin current generation depending on spin-triplet Cooper pair, including the frequency shift and enhanced Gilbert damping, in a unified manner. We find that the Cooper pair symmetry is detectable from the qualitative behavior of the ferromagnetic resonance modulation. Our theory paves the way toward anisotropic superconducting spintronics.

cond-mat.supr-con↗

arXiv2022

Acoustic spin current generation in superconductors

We theoretically study the generation of spin current due to a surface acoustic wave (SAW) in a superconductor. We model an s-wave superconductor as the mean-field Hamiltonian and calculate spin current generated via spin-vorticity coupling based on kinetic theory. The results suggest that the spin current can be driven in a single superconductor layer, and our estimation suggests that the detectable magnitude of the spin current can be generated in aluminum. Our proposal may contribute to the advancement of spin transport in superconductors from application and fundamental physics aspects.

cond-mat.mes-hall↗

arXiv2022

Ferromagnetic resonance modulation in $d$-wave superconductor/ferromagnetic insulator bilayer systems

We investigate ferromagnetic resonance (FMR) modulation in $d$-wave superconductor (SC)/ferromagnetic insulator (FI) bilayer systems theoretically. The modulation of the Gilbert damping in these systems reflects the existence of nodes in the $d$-wave SC and shows power-law decay characteristics within the low-temperature and low-frequency limit. Our results indicate the effectiveness of use of spin pumping as a probe technique to determine the symmetry of unconventional SCs with high sensitivity for nanoscale thin films.

cond-mat.mes-hall↗

arXiv2021

Giant Peltier Conductivity in an Uncompensated Semimetal Ta2PdSe6

Thermoelectric properties of single crystal Ta2PdSe6 is investigated by means of transport measurements, and a density functional calculation. We found a giant Peltier conductivity of 100 Acm-1K-1 at 10 K and successfully explained it by means of conventional semiconductor theory. We concluded that an uncompensated semimetal, high mobility, and heavy effective mass are responsible for the giant Peltier conductivity. Our finding opens a new ground in the field of thermoelectrics to explore much better semimetals for a new possible application such as an electric current generator for a superconducting magnet.

cond-mat.mtrl-sci↗

arXiv2021

Majorana multipole response: General theory and application to wallpaper groups

Whereas identification of Cooper pair symmetry is the first and crucial step in the investigation of unconventional superconductors, only a few have been established so far because of its own difficulties. To solve this problem, we develop a theory for identification of pairing symmetry using knowledge of topological superconductivity. Establishing the multipole theory of emergent Majorana fermions in time-reversal-invariant topological superconductors, we discover a one-to-one correspondence between the electromagnetic response of Majorana fermions and Cooper pair symmetry. The emergent Majorana fermions host magnetic structures that share the same irreducible representation with Cooper pairs under crystalline symmetry. We furthermore reveal that Majorana fermions in high-spin or nonsymmorphic superconductors may exhibit magnetic octupole responses, which give a direct evidence of these exotic superconducting states. Electric responses of multiple Majorana Kramers pairs are also clarified. Our theory provides the fundamentals for identification of unconventional Cooper pairings through surface-spin-sensitive measurements as well as that for manipulation of Majorana fermions by external electromagnetic fields.

cond-mat.supr-con↗

arXiv2021

Theory of Magnetic-Texture-Induced Anomalous Hall Effect on the Surface of Topological Insulators

The anomalous Hall effect is caused by magnetic textures such as skyrmions. We derive an analytical formula of the Hall conductivity on the surface of a topological insulator up to third order in magnetization, $\boldsymbol{M}(\boldsymbol{x})$, based on a perturbative approach. We identify the magnetic textures that contribute to the Hall conductivity up to third order in magnetization and second order in spatial differentiation. We treat magnetization as a perturbation to calculate the Hall conductivity for each magnetic texture based on the linear response theory. Furthermore, we estimate the skyrmion-induced Hall conductivity and confirm that it depends on the shape of skyrmions, such as Bloch-type or Néel-type skyrmions. The results of this study can be applied not only to conventional skyrmion systems but also to more general magnetic structures.

cond-mat.mes-hall↗

arXiv2021

Electric Multipoles of Double Majorana Kramers Pairs

A single Majorana Kramers pair hosts only one component of the magnetic multipole. This can be used to determine the bulk Cooper-pair symmetry through surface-sensitive spectroscopic measurements, either by applying a magnetic field or by using a ferromagnet/superconductor junction. This paper proposes that the electric response, which is free from the Meissner effect, can be used an alternative method to measure the bulk Cooper-pair symmetry in time-reversal-invariant superconductors with double Majorana Kramers pairs. The relationships among electric multipoles, strain tensors and superconducting symmetries under a given wallpaper group on the surfaces of topological crystalline superconductors are shown. This study also reveals that only a specific irreducible representation of a uniform strain yields a gap in the double Majorana Kramers pairs for the topological-crystalline-superconductor candidate Sr$_3$SnO. This highlights the viability of electric detection regarding Cooper-pair symmetry.

cond-mat.supr-con↗