arXiv subjects

Akito Daido

Publications and source records attributed to Akito Daido.

At least 19 recordsLinked to original sources

arXiv2026

Quantum Theory of Current-Generating Local Orbital Magnetization

Local orbital magnetization is the field whose rotation generates the equilibrium current density. Unlike spin magnetization, a quantum-mechanical local formula consistent with both this current relation and the modern theory of bulk orbital magnetization has been missing. In this work, we derive a quantum-mechanical formula for the local orbital magnetization for non-interacting electrons by considering local-flux response of the grand potential. The local-flux response fixes the formula uniquely in two dimensions, whereas in three dimensions it selects a natural representative within a longitudinal ambiguity. Furthermore, coarse graining yields a natural local marker that generates the current to third-derivative order, and its site-position moment equals the orbital magnetic quadrupole moment of finite-size systems. We illustrate the obtained results with the Haldane model.

cond-mat.mes-hall↗

arXiv2026

Quantum-metric-nematicity induced Kerr-like polarization rotation without time-reversal symmetry breaking

The magneto-optic Kerr effect (MOKE), which describes the rotation and ellipticity of linearly polarized light upon reflection, is conventionally associated with time-reversal symmetry breaking. Here, we theoretically demonstrate that a Kerr-like polarization rotation can emerge even in nonmagnetic systems with time-reversal symmetry, owing to the nontrivial quantum metric of electronic bands. We show that the nematicity of the quantum metric, which captures the anisotropy of the quantum metric tensor due to the breaking of $n$-fold (with $n \ge 3$) rotational symmetry, gives rise to an incident-polarization-dependent reflected-polarization rotation. Notably, this mechanism requires neither magnetic order nor spin-orbit coupling, which are conventionally considered essential for MOKE. We illustrate the effect using a minimal tight-binding model and a model for strained MoS$_2$. This work reveals a quantum-geometric origin of the polarization rotation effects beyond conventional MOKE and suggests a new experimental approach to detect quantum metric nematicity.

cond-mat.mes-hall↗

arXiv2025

Quantum geometry in correlated electron phases: from flat band to dispersive band

Quantum geometry, describing the geometric properties of the Bloch wave function in momentum space, has recently been recognized as a fundamental concept in condensed matter physics. The flat-band system offers the paradigmatic platform where quantum geometry plays the essential role in correlated electron phases. However, systems that suffer from significant effects of quantum geometry are not limited to flat-band systems; dispersive-band systems also exhibit quantum condensed phases driven by quantum geometry. In this perspective, we provide a transparent account of quantum geometry and its role in correlated electron phases, throughout flat-band and dispersive-band systems.

cond-mat.str-el↗

arXiv2025

Magnetic phase transitions driven by quantum geometry

We explore how the quantum geometric properties of the Bloch wave function, characterized by the Hilbert-Schmidt quantum distance, impact magnetic phases in solid-state systems. To this end, we investigate the spin susceptibility within the random phase approximation, considering the onsite Coulomb interaction. We demonstrate that spin susceptibility can be decomposed into a trivial part, dependent solely on the band dispersion, and a geometric part, where the quantum distance plays a crucial role. Focusing on a model of a quadratic band-touching semimetal, we show that a magnetic phase transition between ferromagnetic and antiferromagnetic order can be induced solely by tuning the wavefunction geometry, even while the energy spectrum is held constant. This highlights the versatility of quantum geometry as a mechanism for tuning magnetic properties independent of the energy spectrum. Applying our framework to the Fe-pnictide and kagome lattice models, we further show that the geometric contribution is decisive in stabilizing their known antiferromagnetic and ferromagnetic states, respectively. Our work sheds light on the hidden quantum geometric aspects necessary for understanding and engineering magnetic order in quantum materials.

cond-mat.str-el↗

arXiv2025

Layer Pseudospin Superconductivity in Twisted MoTe$_2$

Recent experiments have observed signatures of spin-valley-polarized unconventional superconductivity in twisted bilayer MoTe$_2$ (tMoTe$_2$). Here, we explore the rich physics of superconducting tMoTe$_2$, enabled by its unique layer-pseudospin structure. Within a minimal two-orbital layer-pseudospin model framework, both interlayer and intralayer Cooper pairings can be effectively visualized using a layer-space Bloch sphere representation. Remarkably, we find that interlayer pairing prevails in the spin-valley-polarized state, whereas intralayer pairing dominates in the spin-valley-unpolarized state. Strikingly, we further predict that for spin-valley-polarized intravalley superconducting state, experimentally feasible weak displacement fields can stabilize finite-momentum pairings at low temperatures. Additionally, in-plane magnetic fields, which break three-fold rotational symmetry, induce field-direction-dependent finite-momentum pairing states, leading to a versatile momentum-selection phase diagram. Our work highlights the crucial role of layer pseudospin in tMoTe$_2$'s unconventional superconductivity and demonstrates its unique tunability via external fields.

cond-mat.supr-con↗

arXiv2025

Superconducting Acoustogalvanic Effect in Twisted Transition Metal Dichalcogenides

Two-dimensional van der Waals superconductors are attracting much attention owing to their rich phase diagrams including possible unconventional superconductivity. However, they suffer from a lack of reliable methods for identifying their nontrivial pairing symmetries and quantum geometry. In this study, we propose nonlinear responses driven by surface acoustic waves as a novel probe to access exotic Bogoliubov quasiparticles in such superconductors. Our approach is particularly suitable for addressing the superconducting gap structure as the gap energies in these systems typically lie within the frequency range of surface acoustic waves, and thus paves the way toward the experimental identification of exotic superconducting states especially in low-$T_c$ superconductors.

cond-mat.supr-con↗

arXiv2025

Quantum geometric ferromagnetism by singular saddle point

We propose ferromagnetism that occurs in electrons at a saddle point with band touching, which we call the singular saddle point. At the singular saddle point, the divergent quantum metric induces ferromagnetic correlation, and the logarithmic divergence of the density of states ensures ferromagnetism within Stoner theory. This is a prototypical example of quantum geometric ferromagnetism. The two-dimensional $t_{2g}$-orbital model accommodates the ferromagnetism by this mechanism, which is continuously connected to the exactly proven flat-band ferromagnetism.

cond-mat.str-el↗

arXiv2025

Nonreciprocal Current-Induced Zero-Resistance State in Valley-Polarized Superconductors

The recently observed nonreciprocal current-induced zero-resistance state (CIZRS) in twisted trilayer graphene/WSe$_2$ heterostructure has posed a significant theoretical challenge. In the experiment, the system shows a zero-resistance state only when a sufficiently large current is applied in a particular direction, while stays in an incipient superconducting state with small resistance when the current is small or flows in the opposite direction. In this Letter, we provide a theory of CIZRS. We show that the threefold degenerate Fulde-Ferrell (FF) states are stabilized by the valley polarization and trigonal warping effects of twisted trilayer graphene/WSe$_2$ heterostructures. Moreover, a current flowing in a particular direction breaks the threefold degeneracy and favors a particular FF pairing domain. We therefore propose that the incipient superconducting state is naturally understood as a multidomain state where the interdomain supercurrent is difficult to flow due to the tiny Josephson coupling caused by the mismatch of Cooper-pair momenta between different FF domains. Nevertheless, a sufficiently large current in a particular direction can selectively populate a certain FF state and create monodomain pathways with zero resistance. Crucially, due to the threefold symmetry of the system, a current flowing in the opposite direction can fail to generate the zero-resistance pathways, thus giving rise to the observed nonreciprocity. Finally, we suggest that the long-sought-after triangular finite-momentum state can also be realized in valley-polarized superconductors.

cond-mat.supr-con↗

arXiv2024

Field-free superconducting diode effect in layered superconductor FeSe

The superconducting diode effect (SDE), where zero-resistance states appear nonreciprocally during current injection, is receiving tremendous interest in both fundamental and applied physics because the SDE is a novel manifestation of symmetry breaking and enables the creation of a novel diode. In particular, magnetic-field-free SDEs have been extensively investigated because of their potential to serve as building blocks for superconducting circuit technology. In this letter, we report the field-free SDE in a layered superconductor, FeSe. Its underlying physics is clarified by systematic controlled experiments to be an interplay of a large thermoelectric response and geometrical asymmetry in FeSe. Our findings can pave a new avenue for the construction of novel material and device platforms utilizing SDEs.

cond-mat.mtrl-sci↗

arXiv2024

Reciprocal and nonreciprocal paraconductivity in bilayer multiphase superconductors

Thin-film multiphase superconductors are attracting much attention, and it is important to propose how to detect them in experiments. In this work, we study the reciprocal and nonreciprocal paraconductivity of a bilayer model with staggered Rashba-type spin-orbit coupling with and without the potential gradient and Zeeman field. This model shows the Bardeen-Cooper-Schrieffer phase, the pair-density-wave phase, and the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase, and we focus on how their properties are encoded to the charge transport. We show that the reciprocal paraconductivity has a peak associated with the phase transitions between different superconducting states due to the degeneracy of the transition temperatures as well as the paramagnetic depairing effect. We also show that the FFLO superconducting state shows a sizable nonreciprocal paraconductivity once the degeneracy of Cooper pairs is lifted by applying the potential gradient. Observation of the peaked reciprocal and nonreciprocal paraconductivity can be used as a probe of multiphase superconductivity.

cond-mat.supr-con↗

arXiv2024

Generalized Pitaevskii relation between rectifying and linear responses: its application to reciprocal magnetization induction

Nonlinear optics has regained attention in recent years, especially in the context of optospintronics and topological materials. Nonlinear responses involved in various degrees of freedom manifest their intricacy more pronounced than linear responses. However, for a certain class of nonlinear responses, a connection can be established with linear-response coefficients, enabling the exploration of diverse nonlinear-response functionality in terms of the linear-response counterpart. Our study quantum-mechanically elucidates the relation between such nonlinear and linear responses we call the Pitevskii relation and identifies the condition for the relation to hold. Following the obtained general formulation, we systematically identify the Pitaevskii relations such as the inverse magnetoelectric effect and inverse natural optical activity unique to systems manifesting the space-inversion-symmetry breaking. These results provide a systematic understanding of intricate nonlinear responses and may offer further implications to ultrafast spintronics.

cond-mat.mtrl-sci↗

arXiv2023

Quantum geometry encoded to pair potentials

Bloch wave functions of electrons have properties called quantum geometry, which has recently attracted much attention as the origin of intriguing physical phenomena. In this paper, we introduce the notion of the quantum-geometric pair potentials (QGPP) based on the generalized band representation and thereby clarify how the quantum geometry of electrons is transferred to the Cooper pairs they form. QGPP quantifies the deviation of multiband superconductors from an assembly of single-band superconductors and has a direct connection to the quantum-geometric corrections to thermodynamic coefficients. We also discuss their potential ability to emulate exotic pair potentials and engineer intriguing superconducting phenomena including topological superconductivity.

cond-mat.supr-con↗

arXiv2023

Unidirectional superconductivity and superconducting diode effect induced by dissipation

A general principle of condensed matter physics prohibits the electric current in equilibrium. This prevents a zero-resistance state realized solely under a finite electric current, namely unidirectional superconductivity. In this paper, we propose a setup to realize the unidirectional superconductivity as a nonequilibrium steady state. We focus on the in-plane transport of atomically thin bilayer superconductors lacking the in-plane inversion symmetry and introduce dissipation by applying the out-of-plane electric field and current. By analyzing the time-dependent Ginzburg-Landau equations, we show that locally stable steady-state solutions appear only under the in-plane supercurrent when the out-of-plane electric field exceeds a threshold value. Our system also realizes the dissipation-induced superconducting diode effect up to 100% efficiency by purely electric means.

cond-mat.supr-con↗

arXiv2023

Orbital effect on intrinsic superconducting diode effect

Much ink has recently been spilled on nonreciprocal phenomena in superconductors, especially the superconducting diode effect (SDE) characterized by the nonreciprocity of the critical current $\Delta J_c$. Contrary to the fundamental and practical significance of the SDE, the precise underlying mechanism remains unclear. In this paper, we investigate the impact of an orbital effect on the intrinsic SDE in a bilayer superconductor with Rashba spin-orbit coupling and an in-plane magnetic field. We show that a small orbital effect leads to the sign reversal of $\Delta J_c$ and a crossover of the helical superconducting state at a lower magnetic field than the monolayer superconductor. On the other hand, a large orbital effect induces a decoupling transition, stabilizing a finite momentum Cooper pairing state called the orbital Fulde-Ferrell-Larkin-Ovchinnikov state, and results in the drastic change of the SDE. Owing to the orbital effect, the field dependence of the SDE may show oscillations several times. The results shed light on the mechanism of the SDE in atomically-thin multilayer superconductors.

cond-mat.supr-con↗

arXiv2023

Spin-triplet superconductivity from quantum-geometry-induced ferromagnetic fluctuation

We show that quantum geometry induces ferromagnetic fluctuation resulting in spin-triplet superconductivity. The criterion for ferromagnetic fluctuation is clarified by analyzing contributions from the effective mass and quantum geometry. When the non-Kramers band degeneracy is present near the Fermi surface, the Fubini-Study quantum metric strongly favors ferromagnetic fluctuation. Solving the linearized gap equation with the effective interaction obtained by the random phase approximation, we show that the spin-triplet superconductivity is mediated by quantum-geometry-induced ferromagnetic fluctuation.

cond-mat.supr-con↗

arXiv2023

Rectification and nonlinear Hall effect by fluctuating finite-momentum Cooper pairs

Nonreciprocal charge transport is attracting much attention as a novel probe and functionality of noncentrosymmetric superconductors. In this work, we show that both the longitudinal and transverse nonlinear paraconductivity are hugely enhanced in helical superconductors in moderate and high magnetic fields, which can be observed by second-harmonic resistance measurements. The discussion is based on the generalized formulation of nonlinear paraconductivity in combination with the microscopically determined Ginzburg-Landau coefficients. The enhanced nonreciprocal transport would be observable even with the cyclotron motion of fluctuating Cooper pairs, which is elucidated with a Kubo-type formula of nonlinear paraconductivity. Nonreciprocal charge transport in the fluctuation regime is thereby established as a promising probe of helical superconductivity regardless of the sample dimensionality. Implications on the other finite-momentum superconducting states are briefly discussed.

cond-mat.supr-con↗

arXiv2022

Intrinsic superconducting diode effect in disordered systems

Nonreciprocal transport phenomena have attracted much attention in modern condensed matter physics. In the field of superconductivity, the superconducting diode effect (SDE) has been one of the central topics. Recent theoretical studies for the SDE in intrinsic mechanism revealed the relation between the SDE and helical superconductivity, for which experimental clarification has been awaited. In this work, we establish a microscopic theory of the intrinsic SDE in disordered systems. We show that the sign reversal of the nonreciprocal critical current is suppressed under moderate impurity concentrations. However, even in the moderately disordered region, the SDE shows a feature signaling the change in the nature of helical superconductivity. It is also found that the diode quality factor $r$ is increased by disorders and reaches 20% in the Rashba-Zeeman model.

cond-mat.supr-con↗

arXiv2022

Superconducting diode effect and nonreciprocal transition lines

Nonreciprocity in superconductors is attracting much interest owing to its fundamental importance as well as its potential applicability to engineering. In this paper, we generalize the previous theories of the intrinsic superconducting diode effect (SDE) and microscopically elucidate its relationship with the nonreciprocity of the transition lines under supercurrent. We derive a general formula for the intrinsic SDE by using the phenomenological Ginzburg-Landau theory and thereby show that the SDE is determined by the relative angle between the magnetic field and an effective anti-symmetric spin-orbit coupling defined from the Ginzburg-Landau coefficients. The obtained formula offers a convenient criterion to obtain a finite SDE. We also study the SDE and the nonreciprocal phase transitions of the $s$-wave and $d$-wave superconductors by using the mean-field theory. It is established that the sign reversal of the SDE accompanied by the crossover of the helical superconductivity is a general feature irrespective of the system details. We study the phase transition lines in the temperature-magnetic-field phase diagram under the supercurrent, and clarify that the sign reversal of the SDE generally accompanies the crossings of the transition lines under positive and negative current directions. Furthermore, the superconducting phases under the supercurrent even become re-entrant under moderate strength of the electric current, implying the current-induced first-order phase transitions. Our findings establish the electric current as the control parameter and the powerful probe to study the superconducting properties related to the finite-momentum Cooper pairs.

cond-mat.supr-con↗