arXiv subjects

Kazuki Nakazawa

Publications and source records attributed to Kazuki Nakazawa.

14 recordsLinked to original sources

arXiv2026

Probing Fermi-surface spin-textures via the nonlinear Shubnikov-de Haas effect

The coupling of spin and electronic degrees of freedom via the spin-orbit interaction (SOI) is an essential ingredient for many proposed future technologies. However, probing the strength and nature of SOI is a significant challenge, especially in heterostructures. Here, we consider the nonlinear Shubnikov-de Haas (NSdH) effect, a quantum oscillatory effect that occurs under conditions similar to those of the well-known SdH effect, but is second order in the applied electric field. We demonstrate that, unlike its linear counterpart, the NSdH effect is highly sensitive to the spin textures that arise from SOI. We show that the relative phases and beating patterns of the nonlinear oscillations provide additional information that discriminates linear- and cubic-Rashba-dominated regimes in the models considered here. Our results establish NSdH as a complementary phase-resolved probe of SOI, offering a new framework for characterizing materials relevant to topology, spintronics, and solid-state quantum information technologies.

cond-mat.mes-hall↗

arXiv2025

Orbital-related gyrotropic responses in Cu$_2$WSe$_4$ and chirality indicator

In recent years, counterparts of phenomena studied in spintronics have been actively explored in the orbital sector. The relationship between orbital degrees of freedom and crystal chirality has also been intensively investigated, although the distinction from gyrotropic properties has not been fully clarified. In this work, we investigate spin and orbital Edelstein effects as well as the nonlinear responses in the ternary transition-metal chalcogenide Cu$_2$WSe$_4$, which has a gyrotropic but achiral crystal structure. We find that in the Edelstein effect, magnetization is dominated by the orbital contribution rather than the spin contribution. On the other hand, both the nonlinear chiral thermoelectric (NCTE) Hall effect--a response to the cross product of the electric field and the temperature gradient--and the nonlinear Hall effect--conventional second-order response to the electric field--are found to be dominated by the Berry curvature dipole. We further find that spin-orbit coupling plays only a minor role in these effects, whereas the orbital degrees of freedom are essential. Finally, we demonstrate that the orbital magnetic-moment contributions to both the Edelstein effect and the NCTE Hall effect are closely linked to chirality, and we discuss the possibility of using them as a chirality indicator.

cond-mat.mtrl-sci↗

arXiv2025

Observation of the nonlinear chiral thermoelectric Hall effect in tellurium

The nonlinear thermoelectric effect is a key factor for realising unconventional thermoelectric phenomena, such as heat rectification and power generation using thermal fluctuations. Recent theoretical advances have indicated that chiral materials can host a variety of exotic nonlinear thermoelectric transport arising from inversion-symmetry breaking. However, experimental demonstration has yet to be achieved. Here, we report the observation of the nonlinear chiral thermoelectric Hall effect in chiral tellurium at room temperature, where a voltage is generated as a cross product of the temperature gradient and electric field. The resulting thermoelectric Hall voltage is on the order of μV, consistent with the prediction from the ab initio calculation. Furthermore, the sign of the thermoelectric Hall voltage is reversed depending on the crystal chirality, demonstrating a novel functionality of sign control of the thermoelectric effect by the chirality degrees of freedom. Our findings reveal the potential of chiral systems as nonlinear thermoelectric materials for advanced thermal management and energy harvesting.

cond-mat.mes-hall↗

arXiv2024

Interband contributions to nonlinear transport in semiconductor nanostructures

Spin-orbit interaction (SOI) is a crucial ingredient for many potential applications of quantum devices, such as the use of semiconductor nanostructures for quantum computing. It is known that nonlinear conductivities are sensitive to the strength and type of SOI, however, many calculations of nonlinear transport coefficients are based on the semiclassical Boltzmann theory and make simplifying assumptions about scattering effects due to disorder. In this paper we develop and employ a microscopic theory based on the Keldysh formalism that goes beyond simple semiclassical approximations. This approach, for instance, naturally takes into account the effects of interband transitions, Berry curvature, and allows for a more precise treatment of impurity scattering. As a test of this formalism, we consider the nonlinear transport properties in an effective two-band model of one-dimensional nanowires (1DNWs) and two-dimensional hole gases (2DHGs) in the presence of a magnetic field causing Zeeman splittings of the spin states. We find that the small energy scales in nanostructures mean that interband contributions can be relevant, especially in the dirty limit, and therefore could modify qualitative features found using a purely semiclassical approach. Nonetheless, we find that different types of SOI (linear or cubic) still result in remarkably pronounced different in-plane field angle dependences, which survive even when interband effects are relevant. Our results provide a detailed understanding of when interband effects become important for nonlinear transport and can serve as the basis for a microscopic description to predict other nonlinear transport effects in materials and devices.

cond-mat.mes-hall↗

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

Topological transitions by magnetization rotation in kagome monolayers of ferromagnetic Weyl semimetal Co-based shandite

Co-based shandite Co$_3$Sn$_2$S$_2$ is a ferromagnet hosting Weyl fermions in the layered Co kagome structure. The band topology as well as the magnetism is predicted to vary drastically in the atomically thin films depending on the thickness and surface termination, and as an extreme case, the quantum anomalous Hall state is expected in a monolayer of the Co kagome lattice. Given that the bulk Weyl gap depends on the magnetization direction, here we theoretically study how the topological nature and transport properties vary with the magnetization direction in the systems with kagome monolayer with both Sn and S surface terminations. By using $ab \ initio$ calculations, we find that in the Sn-end monolayer the anomalous Hall conductivity shows successive discrete changes between different quantized values by rotating the magnetization, indicating several topological transitions between the anomalous quantum Hall insulators with different Chern numbers. Notably, when the magnetization is oriented in-plane and perpendicular to the Co-Co bond, the system exhibits a planar quantized anomalous Hall effect. We clarify that these peculiar behaviors are due to topological changes in the band structures associated with gap closing of the Weyl nodes. In contrast, the S-end monolayer shows rather continuous changes in the transport properties since the system is metallic, although the band structure contains many Weyl nodes. Our results pave the way for controlling Weyl fermions in atomically thin films of Co-based shandite, where the topological nature associated with the Weyl nodes appears more clearly than the bulk.

cond-mat.str-el↗

arXiv2024

Magnetic, transport and topological properties of Co-based shandite thin films

The kagome ferromagnet, Co-based shandite Co3Sn2S2, shows a large anomalous Hall effect (AHE) associated with the Weyl nodes. A thin film with a Co kagome monolayer was predicted to exhibit the quantum AHE, which awaits the experimental realisation. However, it is challenging to precisely predict how the Weyl nodes reside in thin films where the lattice and electronic structures are in general different from the bulk. Here we report comprehensive ab initio results for thin films of Co3Sn2S2 with one, two and three Co layers with Sn or S surface terminations. We find that all the Sn-end films stabilise a ferromagnetic state similar to the bulk, and retain the large AHE down to the monolayer limit where the AHE is quantised, while the magnetic and topological properties drastically change with the number of Co layers in the S-end films. Our results would stimulate further experimental exploration of thin Weyl materials.

cond-mat.str-el↗

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

Topological spin Hall effect in antiferromagnets driven by vector Néel chirality

Spin Hall effect of spin-texture origin is explored theoretically for antiferromagnetic (AF) metals. It is found that a vector chirality formed by the Néel vector gives rise to a topological spin Hall effect. This is topological since it is proportional to the winding number counted by in-plane vector chirality along the sample edge, which can be nonvanishing for AF merons but not for AF skyrmions. The effect is enhanced when the Fermi level lies near the AF gap, and, surprisingly, at weak coupling with small AF gap. These features are confirmed numerically based on the Landauer-Büttiker formula. Important roles played by nonadiabatic processes and spin dephasing are pointed out.

cond-mat.mes-hall↗

arXiv2022

Asymmetric modulation of Majorana excitation spectra and nonreciprocal thermal transport in the Kitaev spin liquid under a staggered magnetic field

We present the theoretical results for the Majorana band structure and thermal transport properties in the Kitaev honeycomb model under a staggered magnetic field in addition to a uniform one. The Majorana band structure is asymmetrically modulated in momentum space, and the valley degree of freedom is activated and controlled by the magnitudes and directions of the uniform and staggered magnetic fields; as a result, the Majorana excitation is either gapped or gapless, the latter of which in general has Majorana Fermi surfaces, in contrast to the point nodes in the absence of the fields. We show that the asymmetric deformation of the Majorana band induces a nonlinear (nonreciprocal) thermal transport, in addition to the linear one. The field dependence of the linear thermal current is correlated with the magnitude of the Majorana gap, while that of the nonlinear thermal current is associated with the asymmetry of the Majorana excitation spectra. We show the estimates of the thermal currents and discuss that the linear response is experimentally measurable, whereas future improvement is required for the observation of the nonlinear one. We also discuss how the thermal currents are modulated by other additional effects of the magnetic field and contributions from conventional magnons, latter of which are expected in heterostructures to realize an internal staggered magnetic field.

cond-mat.str-el↗

arXiv2019

Giant topological Hall effect in correlated oxide thin films

Strong electronic correlations can produce remarkable phenomena such as metal-insulator transitions and greatly enhance superconductivity, thermoelectricity, or optical non-linearity. In correlated systems, spatially varying charge textures also amplify magnetoelectric effects or electroresistance in mesostructures. However, how spatially varying spin textures may influence electron transport in the presence of correlations remains unclear. Here we demonstrate a very large topological Hall effect (THE) in thin films of a lightly electron-doped charge-transfer insulator, (Ca, Ce)MnO3. Magnetic force microscopy reveals the presence of magnetic bubbles, whose density vs. magnetic field peaks near the THE maximum, as is expected to occur in skyrmion systems. The THE critically depends on carrier concentration and diverges at low doping, near the metal-insulator transition. We discuss the strong amplification of the THE by correlation effects and give perspectives for its non-volatile control by electric fields.

cond-mat.mtrl-sci↗

arXiv2017

Topological Hall effect from strong to weak coupling

Topological Hall effect (THE) of electrons coupled to a noncoplanar spin texture has been studied so far for the strong- and weak-coupling regimes separately; the former in terms of the Berry phase and the latter by perturbation theory. In this letter, we present a unified treatment in terms of spin gauge field by considering not only the adiabatic (Berry phase) component of the gauge field but also the nonadiabatic component. While only the adiabatic contribution is important in the strong-coupling regime, it is completely canceled by a part of the nonadiabatic contribution in the weak-coupling regime, where the THE is governed by the rest of the nonadiabatic terms. We found a new weak-coupling region that cannot be accessed by a simple perturbation theory, where the Hall conductivity is proportional to M, with 2M being the exchange splitting of the electron spectrum.

cond-mat.mes-hall↗