arXiv subjects

Kei Yamamoto

Publications and source records attributed to Kei Yamamoto.

At least 19 recordsLinked to original sources

arXiv2026

Textures of dimension-six operators in the SMEFT with non-invertible selection rules

We investigate the flavor structures of dimension-six operators in the Standard Model Effective Field Theory (SMEFT) subject to non-invertible selection rules. In particular, we classify the flavor textures of all baryon-number-conserving dimension-six SMEFT operators and determine the resulting constraints on their Wilson coefficients. The selection rules determine not only the texture zeros but also the allowed tensor structures of the Wilson coefficients, which can be expressed analytically in terms of a reduced number of independent parameters. We also find that the flavor structures of higher-dimensional operators are not necessarily aligned with those of the Yukawa couplings, in contrast to the Minimal Flavor Violation hypothesis, in which the Yukawa couplings govern the flavor structure of higher-dimensional operators. It turns out that the resulting flavor and chirality patterns differ from those typically obtained in SMEFT with conventional flavor symmetries, providing characteristic predictions for $B$-meson observables and charged-lepton-flavor-violating radiative decays.

hep-ph↗

arXiv2026

Nonreciprocity reversal of magnetoacoustic attenuation in NiFe alloy thin films

Nonreciprocity, the asymmetry of transport, underlies technologies from the diode to the microwave isolator. In a ferromagnet, a surface acoustic wave generates an elliptical effective field with propagation-locked handedness, breaking the reciprocity of its propagation. Despite decades of study on this phenomenon, a method for controlling the sign of the nonreciprocity has remained elusive. Here we observe a sign reversal in Ni$_x$Fe$_{100-x}$ films. A 0.8 at.% change across Permalloy's zero-magnetostriction composition, where the magnetoelastic coefficient $b$ changes sign, reverses the handedness of the elliptical effective field and thereby the nonreciprocity, from 78.6% to -61.8%. Angle-dependent measurements and spin-wave-ellipticity modelling show that reversing the sign of $b$ reverses the handedness of the elliptically polarized effective field. Aided by cubic frequency scaling, we resolve the sign of $b$ down to -0.05 MPa in a 10-nm film, establishing nonreciprocity as a nanoscale probe of magnetoelastic coupling.

cond-mat.mtrl-sci↗

arXiv2026

Magnetoelastic Waves in Ferromagnetic Thin Films Mediated by Dipolar Interactions

Magnetoelastic coupling mediated by magnetic dipolar interactions is theoretically investigated in ferromagnetic thin films under an in-plane magnetic field. We develop a theoretical description that incorporates dipolar fields derived from Maxwell's equations in the presence of elastic deformations. The resulting coupled equations of motion predict hybridization between magnetostatic and Lamb waves. Numerical calculations for a yttrium iron garnet (YIG) film reveal anti-crossings in the dispersion relations, with hybridization gaps ranging from $0.1$ to several MHz.

cond-mat.mtrl-sci↗

arXiv2026

The Orbital Angular Momentum of Azimuthal Spin-Waves

In the context of a growing interdisciplinary interest in the angular momentum of wave fields, the spin-wave case has yet to be fully explored, with the extensively studied notion of spin transport being only part of the broader picture. Here we report experimental evidence for magnon orbital angular momentum, demonstrating that the mode exhibits rotation rather than remaining stationary. This conclusion is drawn from observations of the lifted degeneracy of waves with counter-rotating wave fronts. This requires an unambiguous formulation of spin and orbital angular momenta for spin waves, which we provide in full generality based on a systematic application of quantum field theory techniques. The results unequivocally establish magnetic dipole-dipole interactions as a magnetic-field controllable spin-orbit interaction for magnons. Our findings open a new research direction, leveraging the spectroscopic readability of angular momentum for azimuthal spin waves and other related systems.

cond-mat.mes-hall↗

arXiv2026

Field Theory of Linear Spin-Waves in Finite Textured Ferromagnets

In the context of an ever-expanding experimental and theoretical interest in the magnetization dynamics of mesoscopic magnetic structures, both in the classical and quantum regimes, we formulate a low energy field theory for the linear spin-waves in finite and textured ferromagnets and we perform its constrained canonical quantization. The introduction of a manifestly gauge invariant Lagrangian enables a straightforward application of the Noether's theorem. Taking advantage of this in the context of a broad class of axisymmetric ferromagnets of special conceptual and experimental relevance, a general expression of the conserved and quantized spin-wave total angular momentum is rigorously derived, while separate conservation and quantization of its orbital and spin components are established for a more restricted class of uniaxial exchange ferromagnets. Further particularizing this general framework to the case of axially saturated magnetic thin disks, we develop a semi-analytic theory of the low frequency part of the exchange-dipole azimuthal spin wave spectrum, providing a powerful theoretical platform for the analysis and interpretation of magnetic resonance experiments on magnetic microdots as further demonstrated in a joint paper [arxiv The Orbital Angular Momentum of Azimuthal Spin-Waves]

cond-mat.mes-hall↗

arXiv2026

Electromagnetic evanescent field associated with surface acoustic wave: Response of metallic thin films

Surface acoustic waves (SAWs), coherent vibrational modes localized at solid surfaces, have been employed to manipulate and detect electronic and magnetic states in condensed-matter systems via strain. SAWs are commonly excited in a piezoelectric material, often the substrate. In such systems, SAWs not only generate strain but also electric field at the surface. Conventional analysis of the electric field accompanying the SAW invokes the electrostatic approximation, which may fall short in fully capturing its essential characteristics by neglecting the effect of the magnetic field. Here we study the electric and magnetic fields associated with SAWs without introducing the electrostatic approximation. The plane wave solution takes the form of an evanescent field that decays along the surface normal with a phase velocity equal to the speed of sound. If a metallic film is placed on the piezoelectric substrate, a time- and space-varying electric field permeates into the film with a decay length along the film normal defined by the skin depth and the SAW wavelength. For films with high conductivity, the phase of the electric field varies along the film normal. The emergence of the evanescent field is a direct consequence of dropping the electrostatic approximation, providing a simple but critical physical interpretation of the SAW-induced electromagnetic field.

cond-mat.mes-hall↗

arXiv2026

Feasibility Study of Lepton Number Violation in Rare $B$ and $K$ Meson Decays

We study lepton-number-violating interactions at dimension seven in the Standard Model effective field theory that contribute to the meson decays $B \to K νν$ and $K \to πνν$. Such interactions could washout the baryon asymmetry of the Universe and also contribute to the neutrinoless double beta decay, even though the interactions involve a change in down-type quark flavors. We clarify conditions under which excesses in meson decay rates over the Standard Model predictions can be successfully observed. We also show that, although these interactions contribute to neutrino masses at the two-loop level, the Weinberg operator can be introduced consistently without spoiling the scenario.

hep-ph↗

arXiv2026

Dynamical stability by spin transfer in nearly isotropic magnets

Spin transfer torques (STTs) control magnetisation by electric currents, enabling a range of nano-scale spintronic applications. They can destabilise the equilibrium magnetisation state by counteracting magnetic relaxation. Here, we maximise the STT effect through a dedicated growth-annealing protocol for CoFeB thin films, such that magnetic anisotropies originating from the interface and shape almost cancel each other. The nearly isotropic magnets enable low-current dynamical stabilisation of the magnetisation in the direction opposite to an applied magnetic field, thereby realising a spintronic analogue of the Kapitza pendulum. In an intermediate current regime, the STT drives large magnetisation vector fluctuations that cover the entire Bloch sphere. The continuous variable associated with the stochastic magnetisation direction may serve as a resource for probabilistic computing and neuromorphic hardware. Our results establish isotropic magnets as a platform to study as-yet-uncharted, far-from-equilibrium spin dynamics including anti-magnonics, with promising implications for unconventional computing paradigms.

cond-mat.mes-hall↗

arXiv2025

Magnon-polaron control in a surface magnetoacoustic wave resonator

Strong coupling between distinct quasiparticles in condensed matter systems gives rise to hybrid states with emergent properties. We demonstrate the hybridization of confined phonons and finite-wavelength magnons, forming a magnon-polaron cavity with tunable coupling strength and spatial confinement controlled by the applied magnetic field direction. Our platform consists of a low-loss, single-crystalline yttrium iron garnet (YIG) film coupled to a zinc oxide (ZnO)-based surface acoustic wave (SAW) resonator. This heterostructure enables exceptionally low magnon-polaron dissipation rates below $κ/ 2π< 1.5\;$MHz. The observed mode hybridization is well described by a phenomenological model incorporating the spatial profiles of magnon and phonon modes. Furthermore, we report the first observation of Rabi-like oscillations in a coupled SAW-spin wave system, revealing the dynamical formation of magnon-polarons in the time domain. These results establish a platform for engineering hybrid spin-acoustic excitations in extended magnetic systems and enable time-resolved studies of magnon-polaron states.

cond-mat.mes-hall↗

arXiv2025

Determination of Majorana type-phases from the time evolution of lepton numbers

We have investigated an approach for determining the Majorana type-phases using the time evolution of lepton family numbers. We show how the second-order time derivative of the expectation values for the lepton family numbers depends on the sum of the Majorana type-phases. Furthermore, others have connected the Majorana type-phases to the orientation of unitary triangles for the PMNS matrix, and the usual Majorana phases. Theoretically,this allows for the extraction of the orientation of the triangles and the Majorana phases from lepton family numbers. We study three example situations. First, how to extract the Majorana type-phases and the lightest neutrino mass for three massive neutrinos, and when a neutrino is massless. Second,the determination of the Majorana phase and the lightest neutrino mass for a two generation toy model. Third, simplified realizations of the type I seesaw model with two gauge singlet neutrinos and two families of lepton doublets. We calculate how the Majorana phases and Majorana type-phases are related to CP violation for leptogenesis at high energies. At first,the effective Majorana mass matrix is parametrized with real and positive diagonal elements. In this basis, the phase of the off-diagonal elements are related to the CP violating phases in the PMNS matrix. We explicitly show the relation between the single Majorana phase and the phase of the effective Majorana mass matrix for the toy model with two generations of active neutrinos. Then for the model with two gauge singlet neutrinos and two families of lepton doublets, we study the effective Majorana mass matrix generated by the seesaw model. In that model, we can show how the Majorana phase at low energy is related to the two CP violating phases of the seesaw matrix. That relation between the phases depends on the lepton number asymmetries of the heavy Majorana neutrinos decays for the toy models.

hep-ph↗

arXiv2025

Controlling spin currents with magnon interference in a canted antiferromagnet

Controlling spin current lies at the heart of spintronics and its applications. The sign of spin currents is monotonous in ferromagnets once the current direction is determined. Spin currents in antiferromagnets can possess opposite polarization, but requires enormous magnetic fields to lift the degeneracy. Controlling spin currents with different polarization is urgently demanded but remains hitherto elusive. Here, we demonstrate the control of spin currents at room temperature by magnon interference in a canted antiferromagnet, hematite recently also classified as an altermagnet. Magneto-optical characterization by Brillouin light scattering revealed that the spatial periodicity of the beating patterns was tunable via the microwave frequency. The inverse spin-Hall voltage changed sign as the frequency was scanned, i.e., a frequency-controlled switching of polarization in pure spin currents was obtained. Our work marks the use of antiferromagnetic magnon interference to control spin currents, which substantially extends the horizon for the emerging field of coherent antiferromagnetic spintronics.

cond-mat.mes-hall↗

arXiv2024

Spin current generation by acousto-electric evanescent wave

We experimentally demonstrate that a spin current can be induced by the acousto-electric evanescent wave, an electric field associated with surface acoustic waves (SAWs) that decay along the surface normal. A previous study showed that a magnetic-field-dependent dc voltage (acoustic voltage) emerges in heavy metal (HM)/ferromagnet (FM) bilayers under excitation of SAWs. The effect, referred to as the acoustic spin Hall effect, was understood by assuming a SAW-induced ac spin current rectified by the oscillation of the FM layer magnetization and the inverse spin Hall effect. However, the mechanism of the spin current generation remained unidentified. Here we measure the acoustic voltage as a function of the SAW propagation direction relative to the crystalline orientation of a LiNbO$_3$ substrate. We find that the magnetic field angle dependence of the acoustic voltage exhibits a phase shift depending on the SAW propagation direction. The result is consistently explained in terms of the acousto-electric evanescent wave generating the spin current in HM layer via the spin Hall effect, thus clarifies the origin of the acoustic spin Hall effect.

cond-mat.mes-hall↗

arXiv2024

Observation of nonreciprocal diffraction of surface acoustic wave

Rectification phenomenon caused by the simultaneous breaking of time reversal and spatial inversion symmetries has been extended to a wide range of (quasi)particles and waves; however, the nonreciprocal diffraction, which is the imbalance of upward and downward deflections, was previously observed only for photons and remained to be extended to other (quasi)particles. In this study, we present evidence of the nonreciprocal diffraction of surface acoustic wave (SAW) utilizing a magnetoelastic grating on a SAW device. Asymmetric diffraction intensities were observed when the ferromagnetic resonance was acoustically excited. Based on a theoretical model, we attribute the microscopic origin of this phenomenon to the resonant scattering involving ferromagnetic resonance excitations. The novel property may pave an avenue to further development of SAW devices for various purposes, including microwave communications and quantum engineering applications.

cond-mat.mes-hall↗

arXiv2024

Electron EDM and LFV decays in the light of Muon $(g-2)_μ$ with U(2) flavor symmetry

We study the interplay of New Physics (NP) among the lepton magnetic moment, the lepton flavor violation (LFV) and the electron electric dipole moment (EDM) in light of recent data of the muon $(g-2)_μ$. The NP is discussed in the leptonic dipole operator with the $U(2)$ flavor symmetry of the charged leptons, where possible CP violating phases of the three family space are taken into account. It is remarked that the third-family contributes significantly to the LFV decay, $μ\to eγ$, and the electron EDM. The experimental upper-bound on $μ\to eγ$ decay gives a severe constraint on the parameters of the flavor model. The predicted electron EDM is rather large due to the CP violating phases in the three family space. In addition, we also study $(g-2)_{e,τ}$ of the electron and tauon, and EDMs of the muon and tauon as well as the $τ\to e γ$ and $τ\to μγ$ decays. The $τ_R \to μ_L γ$ decay is predicted to be close to the experimental upper-bound.

hep-ph↗

arXiv2023

Nonlinear enhancement of coherent magnetization dynamics

Magnets are interesting materials for classical and quantum information technologies. However, the short decoherence and dephasing times that determine the scale and speed of information networks, severely limit the appeal of employing the ferromagnetic resonance. Here we show that the lifetime and coherence of the uniform Kittel mode can be enhanced by 3-magnon interaction-induced mixing with the long-lived magnons at the minima of the dispersion relation. Analytical and numerical calculations based on this model explain recent experimental results and predict experimental signatures of quantum coherence.

cond-mat.mes-hall↗

arXiv2023

Valley-Selective Phonon-Magnon Scattering in Magnetoelastic Superlattices

Phonons and magnons are engineered by periodic potential landscapes in phononic and magnonic crystals, and their combined studies may enable valley phonon transport tunable by the magnetic field. Through nonreciprocal surface acoustic wave transmission, we demonstrate valley-selective phonon-magnon scattering in magnetoelastic superlattices. The lattice symmetry and the out-of-plane magnetization component control the sign of nonreciprocity. The phonons in the valleys play a crucial role in generating nonreciprocal transmission by inducing circularly polarized strains that couple with the magnons. The transmission spectra show a nonreciprocity peak near a transmission gap, matching the phononic band structure. Our results open the way for manipulating valley phonon transport through periodically varying magnon-phonon coupling.

cond-mat.mes-hall↗

arXiv2023

New Physics Searches at Kaon and Hyperon Factories

Rare meson decays are among the most sensitive probes of both heavy and light new physics. Among them, new physics searches using kaons benefit from their small total decay widths and the availability of very large datasets. On the other hand, useful complementary information is provided by hyperon decay measurements. We summarize the relevant phenomenological models and the status of the searches in a comprehensive list of kaon and hyperon decay channels. We identify new search strategies for under-explored signatures, and demonstrate that the improved sensitivities from current and next-generation experiments could lead to a qualitative leap in the exploration of light dark sectors.

hep-ph↗

arXiv2023

Generalized Matter Parities from Finite Modular Symmetries

We classify a supersymmetric extension of the Standard Model by discrete symmetries originating from finite modular symmetries $Γ_N$. Since all the couplings in supersymmetric theories of finite modular symmetries $Γ_N$ are described by holomorphic modular forms with even modular weights, renormalizable and non-renormalizable operators such as baryon- and/or lepton-number violating operators are severely constrained. From the modular transformation of matter multiplets with modular weight $1/M$, we find $\mathbb{Z}_{2M}$ symmetries, including the generalized baryon and lepton parities, $R$-parity, $\mathbb{Z}_3$ baryon triality and $\mathbb{Z}_6$ proton hexality. Such $\mathbb{Z}_{2M}$ symmetries are enlarged to $\mathbb{Z}_{2M} \rtimes \mathbb{Z}_2^{\text{CP}}$ symmetries together with the CP transformation.

hep-ph↗