arXiv subjects

Hiroaki Ikeda

Publications and source records attributed to Hiroaki Ikeda.

At least 19 recordsLinked to original sources

arXiv2026

Altermagnetic spin-split Fermi surfaces in CrSb revealed by quantum oscillation measurements

We report a comprehensive quantum oscillation study of the prototypical altermagnet CrSb, combining high-field magnetotransport and torque measurements with DFT + $U$ calculations including spin-orbit coupling. Multiple quantum oscillation frequencies were observed and tracked over wide angular ranges. The measured frequency branches are consistently explained by the spin-split Fermi surfaces arising from the altermagnetic electronic structure. Our determined Fermi surface reveals that bands 1 and 2 form closed pockets centered at the A point, rather than the tubular $c$-axis-open sheets or $Γ$-centered closed pockets proposed in previous studies. Our findings establish the Fermi-surface topology of CrSb and provide a firm basis for exploring emergent phenomena in altermagnetic materials.

cond-mat.mtrl-sci↗

arXiv2026

Electron chirality and hydrodynamic helicity: Analysis in the atomic limit

Electron chirality has been proposed as a microscopic quantity that characterizes electronic handedness, yet its underlying control parameter has not been clearly identified. Furthermore, its applicability is limited to systems with spin-orbit coupling, which motivates the need for alternative measures of chirality. In this work, we explore two complementary measures of chirality: electron chirality and hydrodynamic helicity. By analyzing a minimal atomic model under chiral crystal fields, we clarify how the interplay among crystal fields, spin-orbit coupling, and electron correlation gives rise to non-zero values of chirality measures. Although electron chirality increases with both spin-orbit coupling and chiral crystal field strength, the dependence on these two factors is highly non-trivial. Particularly, when the chiral crystal field is varied continuously and the energy levels approach quasidegenerate points, the electron chirality is insensitive to spin-orbit coupling, resulting in a remarkable enhancement of chirality. In contrast, the hydrodynamic helicity, defined as a two-body pseudoscalar quantity, remains non-zero even without spin-orbit coupling, originating from electron-electron interactions. Perturbative analysis reveals distinct symmetry selection rules governing the two quantities. Our results provide fundamental insight into the origin of chiralities in electronic systems.

cond-mat.mtrl-sci↗

arXiv2025

In-plane and interlayer magnetoresistance in FeSe

We report measurements of the in-plane and interlayer magnetoresistances of FeSe. The in-plane magnetoresistance $Δρ_{ab}/ρ_{ab}(0)$ for $B \parallel c$ is positive below $T_s$ and increases with decreasing temperature, exceeding 2.5 at $T$ = 10 K and $B$ = 14 T. The field-direction dependence indicates that the in-plane magnetoresistance is basically determined by the $c$-axis component of the magnetic field. The interlayer magnetoresistance $Δρ_{c}/ρ_{c}(0)$ is negative below $T_s$ but turns positive below $\sim$18 K, which is probably due to the contamination by the large in-plane magnetoresistance. The field-direction dependence of the interlayer magnetoresistance can approximately be described by a standard formula for quasi-two-dimensional electron systems except near $B \parallel ab$. The experimental magnetoresistance near $B \parallel ab$ is larger than the formula, which can be attributed to the so-called interlayer coherence peak. The large width of the peak indicates the correspondingly large interlayer transfer energy.

cond-mat.supr-con↗

arXiv2025

Possibility of BCS-BEC crossover in $κ$-type organic superconductors

The realization of BCS-BEC crossover in superconductors, which smoothly connects Bardeen-Cooper-Schrieffer (BCS) theory with Bose-Einstein Condensation (BEC) in fermion systems, is an intriguing recent topic in strongly correlated electron systems. The organic superconductor $κ$-(BEDT-TTF)$_4$Hg$_{2.89}$Br$_8$ ($κ$-HgBr) under pressure is one of the leading candidates, owing to its unique metallic spin-liquid nature and tunable electron correlation. We theoretically investigate the extended Hubbard model for $κ$-HgBr and discuss the possibility of the BCS-BEC crossover by systematically calculating superconducting correlation function, coherence length, superfluid weight, and chemical potential. Our findings show that the BCS-BEC crossover can be observed when competing phases, such as Mott insulators and charge and/or spin orders, are suppressed by appropriate hole doping. $κ$-HgBr is just the case because both the Mott insulating phase and magnetic orders are absent due to its nonstoichiometric Hg composition and geometrical frustration. We further propose that other $κ$-type organic superconductors could serve as potential candidates of the BCS-BEC crossover if their band fillings and degree of geometrical frustration are systematically tuned.

cond-mat.supr-con↗

arXiv2025

Evolution and Instability of Bogoliubov Fermi Surfaces under Zeeman Field

We theoretically investigate the evolution and instability of the Bogoliubov Fermi surface (BFS) in the spherical $j=3/2$ model under a Zeeman field. The applied field induces a pronounced expansion in the BFS with $j_z = \pm 3/2$ component. Such behavior can be detected by spectroscopic techniques such as angle-resolved photoemission spectroscopy (ARPES). Interestingly, the Pauli susceptibility exhibits behavior that appears discontinuous just below the transition temperature at zero field, even though it is a second-order transition. This is due to spontaneous magnetization. Furthermore, the analysis of the bogolon correlations in the superconducting phase suggests the possibility of the chiral $p$- or $f$-wave bogolon pairing instabilities rather than the Pomeranchuk instability. These chiral states coexist with the chiral $d$-wave superconducting state, spontaneously break inversion symmetry, and lead to the disappearance of the torus-shaped BFS structure.

cond-mat.supr-con↗

arXiv2024

Problem of nonlinear conductivity within relaxation time approximation in noncentrosymmetric insulators

With the recent advancements in laser technology, there has been increasing interest in nonlinear and nonperturbative phenomena such as nonreciprocal transport, the nonlinear Hall effect, and nonlinear optical responses. When analyzing the nonequilibrium steady state, the relaxation time approximation (RTA) in the quantum kinetic equation has been widely used. However, recent studies have highlighted problems with the use of RTA that require careful consideration. In a study published in Phys. Rev. B, $\textbf{109}$, L180302 (2024), we revealed that the RTA has a flaw in predicting finite linear conductivity even for insulators under weak electric fields, and improved the RTA based on the Redfield equation. In this paper, we further extend our approach to nonlinear responses. This approach provides a simple alternative to RTA and is expected to be useful for the study of nonlinear and nonequilibrium phenomena.

cond-mat.mes-hall↗

arXiv2024

Multi-tunneling effect of nonreciprocal Landau-Zener tunneling: Insights from DC field responses

Recent advancements in laser technology have spurred growing interest in nonlinear and nonequilibrium phenomena. Here, we investigate the geometric aspects of quantum tunneling and the nonreciprocal response, particularly focusing on the shift vector, in noncentrosymmetric insulators under a strong DC electric field. In insulators under a strong electric field, electrons undergoing Bloch oscillations interfere with each other by passing through different paths via Landau-Zener tunneling. We found that the interference effect due to multi-tunneling causes the oscillating nonreciprocal response that is significantly amplified with increasing electric field intensity. We also clarified the role of the shift vector in the interference conditions through an analysis of the nonequilibrium steady state. These results will contribute significantly to advancing a systematic understanding of quantum geometric effects in the nonperturbative regime.

cond-mat.mes-hall↗

arXiv2024

Quantification of electronic asymmetry: chirality and axiality in solids

Chiral and axial materials offer platforms for intriguing phenomena, such as cross-correlated responses and chirality-induced spin selectivity. However, quantifying the properties of such materials has generally been considered challenging. Here, we demonstrate that the spatial distribution of the electron chirality, represented by $Ψ^\dagger γ^5 Ψ$ with the four-component Dirac field $Ψ$, characterizes the chirality and axiality of materials. Furthermore, we reveal that spin-derived electric polarization can serve as an effective indicator of material polarity. We present quantitative evaluations of electron chirality distribution and spin-derived electric polarization based on first-principles calculations. Additionally, we propose that electron chirality can be directly observed via circular dichroism in photoemission spectroscopy, which measures the difference between right- and left-handed circularly polarized light. Electron chirality and spin-derived electric polarization provide a new framework for quantifying chirality, axiality, and polarity in asymmetric materials, paving the way for the exploration of novel functional materials.

cond-mat.mtrl-sci↗

arXiv2024

Dirac bilinears in condensed matter physics: Relativistic correction for observables and conjugate electromagnetic fields

Inspired by recent developments in electron chirality, we reconsider some microscopic physical quantities that have been overlooked or have received little attention in condensed matter physics, based on the non-relativistic limit of the Dirac bilinears in relativistic quantum theory. We identify the expression of physical quantities defined by the four-component Dirac field in terms of the two-component Schrödinger field, which is usually used in condensed matter physics, and clarify its conjugate electromagnetic field. This consideration bridges the fields of condensed matter physics, quantum chemistry, and particle physics, and paves the way to electromagnetic control of matter. Our findings provide a means of {\it ab initio} quantification of material characters such as chirality and axiality that are unique to low-symmetry materials, and stimulate the systematic search for useful, new functionalities.

cond-mat.mes-hall↗

arXiv2024

Unexpected linear conductivity in Landau-Zener model: limitations and improvements of the relaxation time approximation in the quantum master equation

The nonequilibrium steady states of quantum materials have many challenges. Here, we highlight issues with the relaxation time approximation (RTA) for the DC conductivity in insulating systems. The RTA to the quantum master equation (QME) is frequently employed as a simple method, yet this phenomenological approach is exposed as a fatal approximation, displaying metallic DC conductivity in insulating systems within the linear response regime. We find that the unexpected metallic behavior is caused by the fact that the density matrix in the RTA incompletely incorporates the first order of the external field. To solve this problem, we have derived a new calculation scheme based on the QME that ensure correct behavior in low electric fields. Our method reproduces well the overall features of the exact electric currents in the whole field region. It is not time-consuming, and its application to lattice systems is straightforward. This method will encourage progress in this research area as a simple way to more accurately describe nonequilibrium steady states.

cond-mat.mes-hall↗

arXiv2023

Impurity effect on Bogoliubov Fermi surfaces: Analysis based on iron-based superconductors

The effect of impurities on a superconductor with Bogoliubov Fermi surfaces (BFSs) is studied using a realistic tight-binding model. Based on the band structure composed of $d$-orbitals in tetragonal FeSe, whose S-doped sample is a potential material for BFS, we construct the superconducting state by introducing a time-reversal broken pair potential in terms of the band index. We further consider the effect of impurities on the BFS, where the impurity potential is defined as a local potential for the original $d$-orbitals. The self-energy is calculated using the (self-consistent) Born approximation, which shows an enhancement of the single-particle spectral weight on the Fermi surface. This is consistent with the previous phenomenological theory and is justified by the present more detailed calculation based on the FeSe-based material.

cond-mat.supr-con↗

arXiv2023

Monte Carlo study of cuprate superconductors in a four-band $d$-$p$ model: Role of orbital degrees of freedom

Understanding the complex phase diagram of cuprate superconductors is a long-standing challenging problem. Recent studies have shown that orbital degrees of freedom, both Cu $e_g$ orbitals and O $p$ orbitals, are a key ingredient for a unified understanding of cuprate superconductors, including the material dependence. Here we investigate a four-band $d$-$p$ model derived from the first-principles calculations with the variational Monte Carlo method, which allows us to elucidate competing orders on an equal footing. The obtained results can consistently explain the doping dependence of superconductivity, antiferromagnetic and stripe phases, phase separation in the underdoped region, and also novel magnetism in the heavily-overdoped region. Our four-band $d$-$p$ model with neighbouring intersite interactions is a minimal model to describe the phase diagram comprehensively. The presence of $p$ orbitals is critical to the charge-stripe features, which induce two types of stripe phases with $s'$-wave and $d$-wave bond stripe. On the other hand, the presence of $d_{z^2}$ orbital is indispensable to material dependence of the superconducting transition temperature ($T_{\mathrm{c}}$), and enhances local magnetic moment as a source of novel magnetism in the heavily-overdoped region as well. These findings beyond one-band description could provide a major step toward a full explanation of unconventional normal state and high $T_{\mathrm{c}}$ in cuprate supercondutors.

cond-mat.supr-con↗

arXiv2022

In-plane electronic anisotropy revealed by interlayer resistivity measurements on the iron-based superconductor parent compound CaFeAsF

Both cuprates and iron-based superconductors demonstrate nematicity, defined as the spontaneous breaking of rotational symmetry in electron systems. The nematic state can play a role in the high-transition-temperature superconductivity of these compounds. However, the microscopic mechanism responsible for the transport anisotropy in iron-based compounds remains debatable. Here, we investigate the electronic anisotropy of CaFeAsF by measuring its interlayer resistivity under magnetic fields with varying field directions. Counterintuitively, the interlayer resistivity was larger in the longitudinal configuration ($B \parallel I \parallel c$) than in the transverse one ($B \perp I \parallel c$). The interlayer resistivity exhibited a so-called coherence peak under in-plane fields and was highly anisotropic with respect to the in-plane field direction. At $T$ = 4 K and $B$ = 14 T, the magnetoresistance $Δρ/ρ_0$ was seven times larger in the $B \parallel b_o$ than in the $B \parallel a_o$ configuration. Our theoretical calculations of the conductivity based on the first-principles electronic band structure qualitatively reproduced the above observations but underestimated the magnitudes of the observed features. The proposed methodology can be a powerful tool for probing the nematic electronic state in various materials.

cond-mat.supr-con↗

arXiv2022

Spin current and chirality degrees of freedom inherent in localized electron orbitals

In solid state physics, any phase transition is commonly observed as a change in the microscopic distribution of charge, spin, or current. Here we report the nature of an exotic order parameter inherent in the localized electron orbitals that cannot be primarily captured by these three fundamental quantities. This order parameter is described as the electric toroidal multipoles connecting different total angular momenta under the spin-orbit coupling. The corresponding microscopic physical quantity is the spin current tensor on an atomic scale, which induces spin-derived electric polarization and the chirality of the Dirac equation. We stress that the chirality intrinsic to the elementary particle is the essence of electric toroidal multipoles. These findings link microscopic spin currents and chirality in the Dirac theory to the concept of multipoles and provide a new perspective for quantum states of matter.

cond-mat.str-el↗

arXiv2021

Unified description of cuprate superconductors using four-band $d$-$p$ model

In the 35 years since the discovery of cuprate superconductors, we have not yet reached a unified understanding of their properties, including their material dependence of the superconducting transition temperature $T_{\text{c}}$. The preceding theoretical and experimental studies have provided an overall picture of the phase diagram, and some important parameters for the $T_{\text{c}}$, such as the contribution of the Cu $d_{z^2}$ orbital to the Fermi surface and the site-energy difference $Δ_{dp}$ between the Cu $d_{x^2-y^2}$ and O $p$ orbitals. However, they are somewhat empirical and limited in scope, always including exceptions, and do not provide a comprehensive view of the series of cuprates. Here we propose a four-band $d$-$p$ model as a minimal model to study material dependence in cuprates. Using the variational Monte Carlo method, we theoretically investigate the phase diagram for the La$_2$CuO$_4$ and HgBa$_2$CuO$_4$ systems and the correlation between the key parameters and the superconductivity. Our results comprehensively account for the empirical correlation between $T_{\text{c}}$ and model parameters, and thus can provide a guideline for new material design. We also show that the effect of the nearest-neighbor $d$-$d$ Coulomb interaction $V_{dd}$ is actually quite important for the stability of superconductivity and phase competition.

cond-mat.supr-con↗

arXiv2019

Theoretical studies for identifying horizontal line nodes via angle-resolved density of states measurements ---Application to Sr$_2$RuO$_{4}$---

On the basis of the microscopic quasi-classical Eilenberger theory, we analyze the recent angle-resolved specific heat experiment carried out at low temperature for Sr$_2$RuO$_{4}$ to identify the superconducting gap symmetry, comprising either horizontal or vertical line nodes relative to the tetragonal crystal symmetry. Several characteristics, in particular, the landscape of the in-plane oscillation amplitude $A_4(B, T)$ with a definite sign for almost the entire $B$-$T$ plane are best explained by the horizontal line node symmetry, especially when the multiband effect and Pauli paramagnetic effect are taken into account. The present analysis of $A_4(B,T)$ with definite sign points to the presence of an anomalous field region at a lower temperature in the experimental data, whose origin is investigated. Our theory demonstrates the application and uniqueness of the field-rotating thermodynamic measurements in uncovering the precise gap structure for target materials.

cond-mat.supr-con↗

arXiv2019

Accurate determination of the Fermi surface of tetragonal FeS via quantum oscillation measurements and quasiparticle self-consistent \textit{GW} calculations

We perform de Haas-van Alphen measurements and quasiparticle self-consistent \textit{GW} (QS\textit{GW}) calculations on FeS. The calculated Fermi surface (FS) consists of two hole and two electron cylinders. We observe all the eight predicted FS cross sections experimentally. With momentum-independent band-energy adjustments of less than 0.1 eV, the maximum deviation between the calculated and observed cross sections is less than 0.2\% of the Brillouin zone area for $B \parallel c$. The carrier density is $\sim$0.5 carriers/Fe. The mass enhancements are nearly uniform across the FS cylinders and moderate, $\sim$2. The absence of a third hole cylinder with $d_{xy}$ character is favorable for the formation of a nodal superconducting gap.

cond-mat.supr-con↗

arXiv2018

Effect of electron correlations on spin excitation bandwidth in Ba$_{0.75}$K$_{0.25}$Fe$_{2}$As$_{2}$ as seen via time-of-flight inelastic neutron scattering

We use inelastic neutron scattering (INS) to investigate the effect of electron correlations on spin dynamics in the iron-based superconductor Ba$_{0.75}$K$_{0.25}$Fe$_{2}$As$_{2}$. Our INS data show a spin-wave-like dispersive feature, with a zone boundary energy of 200 meV. A first principles analysis of dynamical spin susceptibility, incorporating the mass renormalization factor of 3, as determined by angle-resolved photoemission spectroscopy, provides a reasonable description of the observed spin excitations. This analysis shows that electron correlations in the Fe-3$d$ bands yield enhanced effective electron masses, and consequently, induce substantial narrowing of the spin excitation bandwidth. Our results highlight the importance of electron correlations in an itinerant description of the spin excitations in iron-based superconductors.

cond-mat.supr-con↗