arXiv subjects

Kei Yamamoto

Publications and source records attributed to Kei Yamamoto.

At least 55 records · Page 3Linked to original sources

arXiv2018

Spin colossal magnetoresistance in an antiferromagnetic insulator

Colossal magnetoresistance (CMR) refers to a large change in electrical conductivity induced by a magnetic field in the vicinity of a metal-insulator transition and has inspired extensive studies for decades\cite{Ramirez1997, Tokura2006}. Here we demonstrate an analogous spin effect near the Néel temperature $T_{\rm{N}}$=296 K of the antiferromagnetic insulator \CrO. Using a yttrium iron garnet \YIG/\CrO/Pt trilayer, we injected a spin current from the YIG into the \CrO layer, and collected via the inverse spin Hall effect the signal transmitted in the heavy metal Pt. We observed a change by two orders of magnitude in the transmitted spin current within 14 K of the Néel temperature. This transition between spin conducting and nonconducting states could be also modulated by a magnetic field in isothermal conditions. This effect, that we term spin colossal magnetoresistance (SCMR), has the potential to simplify the design of fundamental spintronics components, for instance enabling the realization of spin current switches or spin-current based memories.

cond-mat.mtrl-sci↗

arXiv2018

Gluino-mediated electroweak penguin with flavor-violating trilinear couplings

In light of a discrepancy of the direct $CP$ violation in $K\toππ$ decays, $\varepsilon'/\varepsilon_K$, we investigate gluino contributions to the electroweak penguin, where flavor violations are induced by squark trilinear couplings. Top-Yukawa contributions to $ΔS = 2$ observables are taken into account, and vacuum stability conditions are evaluated in detail. It is found that this scenario can explain the discrepancy of $\varepsilon'/\varepsilon_K$ for the squark mass smaller than 5.6 TeV. We also show that the gluino contributions can amplify $\mathcal{B}(K \to πν\overlineν)$, $\mathcal{B}(K_S \to μ^+ μ^-)_{\rm eff}$ and $ΔA_{\rm CP}(b\to sγ)$. Such large effects could be measured in future experiments.

hep-ph↗

arXiv2018

Spin caloric effects in antiferromagnets assisted by an external spin current

Searching for novel spin caloric effects in antiferromagnets we study the properties of thermally activated magnons in the presence of an external spin current and temperature gradient. We predict the spin Peltier effect -- generation of a heat flux by spin accumulation -- in an antiferromagnetic insulator with cubic or uniaxial magnetic symmetry. This effect is related with spin-current induced splitting of the relaxation times of the magnons with opposite spin direction. We show that the Peltier effect can trigger antiferromagnetic domain wall motion with a force whose value grows with the temperature of a sample. At a temperature, larger than the energy of the low-frequency magnons, this force is much larger than the force caused by direct spin transfer between the spin current and the domain wall. We also demonstrate that the external spin current can induce the magnon spin Seebeck effect. The corresponding Seebeck coefficient is controlled by the current density. These spin-current assisted caloric effects open new ways for the manipulation of the magnetic states in antiferromagnets.

cond-mat.mes-hall↗

arXiv2017

Revisiting Kaon Physics in General $Z$ Scenario

New physics contributions to the $Z$ penguin are revisited in the light of the recently-reported discrepancy of the direct CP violation in $K\toππ$. Interference effects between the standard model and new physics contributions to $ΔS = 2$ observables are taken into account. Although the effects are overlooked in the literature, they make experimental bounds significantly severer. It is shown that the new physics contributions must be tuned to enhance $\mathcal{B}(K_L \to π^{0} ν\barν)$, if the discrepancy of the direct CP violation is explained with satisfying the experimental constraints. The branching ratio can be as large as $6 \times 10^{-10}$ when the contributions are tuned at the 10 % level.

hep-ph↗

arXiv2017

Tunable sign change of spin Hall magnetoresistance in Pt/NiO/YIG structures

Spin Hall magnetoresistance (SMR) has been investigated in Pt/NiO/YIG structures in a wide range of temperature and NiO thickness. The SMR shows a negative sign below a temperature which increases with the NiO thickness. This is contrary to a conventional SMR theory picture applied to Pt/YIG bilayer which always predicts a positive SMR. The negative SMR is found to persist even when NiO blocks the spin transmission between Pt and YIG, indicating it is governed by the spin current response of NiO layer. We explain the negative SMR by the NiO 'spin-flop' coupled with YIG, which can be overridden at higher temperatures by positive SMR contribution from YIG. This highlights the role of magnetic structure in antiferromagnets for transport of pure spin current in multilayers.

cond-mat.mes-hall↗

arXiv2016

Probing the SUSY with $10$ TeV stop mass in rare decays and CP violation of Kaon

We probe the SUSY at the $10 \ {\rm TeV}$ scale in the rare decays and the CP violation of the kaon. We focus on the processes of $K_L \to π^0 ν{\bar ν}$ and $K^+ \to π^+ ν{\bar ν}$ combined with the CP violating parameters $ε_K$ and $ε_K^\prime/ε_K$. The Z-penguin mediated by the chargino loop cannot enhance $K_L \to π^0 ν{\bar ν}$ and $K^+ \to π^+ ν{\bar ν}$ because the left-right mixing of the stop is constrained by the $125$ GeV Higgs mass. On the other hand, the Z-penguin mediated by the gluino loop can enhance the branching ratios of both $K_L \to π^0 ν{\bar ν}$ and $K^+ \to π^+ ν{\bar ν}$. The former increases up to more than $1.0 \times 10^{-10}$, which is much larger than the SM prediction even if the constraint of $ε_K$ is imposed. It is remarkable that the Z-penguin mediated by the gluino loop can enhance simultaneously $ε^\prime_K/ε_K$ and the branching ratio of $K_L \to π^0 ν{\bar ν}$, which increases up to $1.0 \times 10^{-10}$. We also study the decay rates of $K_L\rightarrow μ^+μ^-$, $B^0\rightarrow μ^+μ^-$ and $B_s\rightarrow μ^+μ^-$, which correlate with the $K_L \to π^0 ν{\bar ν}$ decay through the Z penguin. It is important to examine the $B^0\rightarrow μ^+μ^-$ process since we expect the enough sensitivity of this decay mode to the SUSY at LHCb.

hep-ph↗

arXiv2016

Chargino contributions in light of recent $ε'/ε$

Recently, the standard model prediction of $ε'/ε$ was improved, and a discrepancy from the experimental results was reported at the $2.9σ$ level. We study the chargino contributions to $Z$ penguin especially with the vacuum stability constraint. The vacuum decay rate is investigated, and it is shown that the discrepancy can be explained if superparticles are lighter than 4-6 TeV. Correlations with $\mathcal{B}(K_L\toπ^0ν\barν)$ and other experimental constraints are also discussed.

hep-ph↗

arXiv2015

Anomalous Hall effect driven by dipolar spin waves in uniform ferromagnets

A new type of anomalous Hall effect is shown to arise from the interaction of conduction electrons with dipolar spin waves in ferromagnets. This effect exists even in homogeneous ferromagnets without relativistic spin-orbit coupling. The leading contribution to the Hall conductivity is proportional to the chiral spin correlation of dynamical spin textures and is physically understood in terms of the skew scattering by dipolar magnons.

cond-mat.mtrl-sci↗

arXiv2015

Sensitivity of High-Scale SUSY in Low Energy Hadronic FCNC

We discuss the sensitivity of the high-scale SUSY at $10$-$1000$ TeV in $B^0$, $B_s$, $K^0$ and $D$ meson systems together with the neutron EDM and the mercury EDM. In order to estimate the contribution of the squark flavor mixing to these FCNCs,we calculate the squark mass spectrum, which is consistent with the recent Higgs discovery. The SUSY contribution in $ε_K$ could be large, around 40% in the region of the SUSY scale $10$-$100$ TeV. The neutron EDM and the mercury EDM are also sensitive to the SUSY contribution induced by the gluino-squark interaction. The predicted EDMs are roughly proportional to $|ε_K^{\rm SUSY}|$. If the SUSY contribution is the level of O(10%) for epsilon_K, the neutron EDM is expected to be discovered in the region of $10^{-28}$-$10^{-26}$ecm. The mercury EDM also gives a strong constraint for the gluino-squark interaction. The SUSY contribution of $ΔM_D$ is also discussed.

hep-ph↗

arXiv2015

$K_L \to π^0 ν{\bar ν}$ decay correlating with $ε_K$ in high-scale SUSY

We have studied the contribution of the high-scale SUSY to the K_L to pi^0 nu{bar nu} and K^+ to pi^+ nu{bar nu} processes correlating with the CP violating parameter epsilon_K. Taking account of the recent LHC results for the Higgs discovery and the SUSY searches, we consider the high-scale SUSY at the 10-50 TeV scale in the framework of the non-minimal squark (slepton) flavor mixing. The Z penguin mediated the chargino dominates the SUSY contribution for these decays. At the 10 TeV scale of the SUSY, the chargino contribution can enhance the branching ratio of K_L to pi^0 nu{bar nu} in eight times compared with the SM predictions whereas the predicted branching ratio BR(K^+ to pi^+ nu {bar nu}) increases up to three times of the SM one. The gluino box diagram dominates the SUSY contribution of epsilon_K up to 30%. If the down-squark mixing is neglected compared with the up-squark mixing, the Z penguin mediated the chargino dominates both SUSY contributions of K_L to pi^0 nu {bar nu} and epsilon_K. Then, it is found a correlation between them, but the chargino contribution to epsilon_K is at most 3%. Even if the SUSY scale is 50TeV, the chargino process still enhances the branching ratio of K_L to pi^0 nu {bar nu} from the SM prediction in the factor two, and epsilon_K is deviated from the SM prediction in O(10%) through the gluino box diagram unless the down-squark mixing is suppressed. We also discuss the chargino contribution to K_L to pi^0 e^+e^- process.

hep-ph↗

arXiv2015

$\mathcal{I}$-degenerate pseudo-Riemannian metrics

In this paper we study pseudo-Riemannian spaces with a degenerate curvature structure i.e. there exists a continuous family of metrics having identical polynomial curvature invariants. We approach this problem by utilising an idea coming from invariant theory. This involves the existence of a boost which is assumed to extend to a neighbourhood. This approach proves to be very fruitful: It produces a class of metrics containing all known examples of $\mathcal{I}$-degenerate metrics. To date, only Kundt and Walker metrics have been given, however, our study gives a plethora of examples showing that $\mathcal{I}$-degenerate metrics extend beyond the Kundt and Walker examples. The approach also gives a useful criterion for a metric to be $\mathcal{I}$-degenerate. Specifically, we use this to study the subclass of VSI and CSI metrics (i.e., spaces where polynomial curvature invariants are all vanishing or constants, respectively).

math-ph↗

arXiv2015

Linking Leptonic CP violation to Quark Unitarity Triangle

We study the linking between the CP violating phase of the lepton sectors and the unitarity triangle of the B0 meson system. Antusch, King, Malinsky and Spinrath have shown that the quark mass matrices with the negligible 1-3 mixing give an interesting relation between the phase of the quark mixing matrices and CP violating measure phi_2(alpha). This approach is extended by considering the SO(10) GUT including the Pati-Salam symmetry, which links the Yukawa matrices of the quark sector to the one of the lepton sector. We discuss the relation of the CP violating phases between both quark and lepton sectors as well as the mixing angles. Then, the leptonic CP violating phase is predicted in terms of the angle of the unitarity triangle of the B0 meson system. The leptonic CP violating phase deltaPMNS is predicted in the region -74 to -89, which is the consistent with the recent T2K results. Our predicted phase is sensitive to phi_2(alpha) and phi_3(gamma). These predictions will be clearly tested in the future neutrino experiments as well as the Belle-II experiment.

hep-ph↗

arXiv2014

Predicting CP violation in Deviation from Tri-bimaximal mixing of Neutrinos

We study the CP violation in the deviation from the tri-bimaximal mixing (TBM) of neutrinos. We examine non-trivial relations among the mixing angles and the CP violating Dirac phase in the typical four cases of the deviation from the TBM. The first two cases are derived by the additional rotation of the 2-3 or 1-3 generations of neutrinos in the TBM basis. The other two cases are given by the additional rotation of the 1-3 or 1-2 generations of charged leptons with the TBM neutrinos. These four cases predict different relations among three mixing angles and the CP violating Dirac phase. The rotation of the 2-3 generations of neutrinos in the TBM basis predicts $\sin ^2θ_{12}<1/3$, and the CP violating Dirac phase to be $\pm (0.09π\sim 0.76π)$ for NH ($\pm (0.15π\sim 0.73π) \ \text{for IH}$) depending on $\sin ^2θ_{23}$. The rotation of the 1-3 generations of neutrinos in the TBM basis gives $\sin ^2θ_{12}>1/3$. The CP violating Dirac phase is not constrained by the input of the present experimental data. For the case of the 1-3 and 1-2 rotations of charged leptons in the TBM basis, the CP violating Dirac phase is predicted in $\pm(0.35π\sim 0.60π)$ depending on $\sin ^2θ_{12}$ for both NH and IH cases. We also discuss the specific case that $θ_{13}$ is related with the Cabibbo angle $λ$ such as $\sinθ_{13}=λ/\sqrt{2}$, in which the maximal CP violation is preferred. The CP violating Dirac phase can distinguish the lepton flavor mixing patterns at T2K and NO$ν$A experiments in the future.

hep-ph↗

arXiv2014

Probing the high scale SUSY in CP violations of K, B0 and Bs mesons

We probe the high scale SUSY at 10-50 TeV in the CP violations of K, B0 and Bs mesons. In order to estimate the contribution of the squark flavor mixing to these CP violations, we discuss the squark mass spectrum, which is consistent with the recent Higgs discovery. Taking the universal soft parameters at the SUSY breaking scale, we obtain the squark mass spectrum at 10 TeV and 50 TeV, where the SM emerges. Then, the 6times6 mixing matrix between down-squarks and down-quarks is discussed by input of the experimental data of K, B0 and Bs mesons.It is found that epsilon_K is most sensitive to the high scale SUSY. The SUSY contributions for the time-dependent CP asymmetries S_{ J/psi K_S} and S_{ J/psi phi} are 6-8% at the SUSY scale of 10 TeV. We also discuss the SUSY contribution to the chromo-EDM of the strange quark.

hep-ph↗

arXiv2014

Searching for the squark flavor mixing in CP violations of Bs -> K+ K- and K0bar K0 decays

We study CP violations in the B_s-> K+K- and Bs->K0K0 decays in order to find the contribution of the supersymmetry, which comes from the gluino-squark mediated flavor changing current. We obtain the allowed region of the squark flavor mixing parameters by putting the experimental data, the mass difference Delta M_Bs, the CP violating phase phi_s in Bs to J/psi phi decay and the b to s gamma branching ratio. In addition to these data, we take into account the constraint from the asymmetry of B0->K+pi because the Bs->K+K- decay is related with the B0->K+pi- decay by replacing the spectator s with d. Under these constraints, we predict the magnitudes of the CP violation in the Bs->K+K- and Bs->K0K0 decays. The predicted region of the CP violation C_{K+K-} is strongly cut from the direct CP violation of barB0 to K-pi+, therefore, the deviation from the SM prediction of C_{K+K-} is not found. On the other hand, the CP violation S_{K+K-} is possibly deviated from the SM prediction considerably, in the region of 0.1- 0.5. Since the standard model predictions of C_{K0bar K0} and S_{K0bar K0} are very small, the squark contribution can be detectable in C_{K0bar K0} and S_{K0bar K0}. These magnitudes are expected in the region C_{K0bar K0}=-0.06-0.06 and S_{K0bar K0}=-0.5-0.3. More precise data of these CP violations provide us a crucial test for the gluino-squark mediated flavor changing current.

hep-ph↗

arXiv2013

Stability analysis of inflation with an SU(2) gauge field

We study anisotropic cosmologies of a scalar field interacting with an SU(2) gauge field via a gauge-kinetic coupling. We analyze Bianchi class A models, which include Bianchi type I, II, VI0, VII0, VIII and IX. The linear stability of isotropic inflationary solution with background magnetic field is shown, which generalizes the known results for U(1) gauge fields. We also study anisotropic inflationary solutions, all of which turn out to be unstable. Then nonlinear stability for the isotropic inflationary solution is examined by numerically investigating the dependence of the late-time behaviour on the initial conditions. We present a number of novel features that may well affect physical predictions and viability of the models. First, in the absence of spatial curvature, strong initial anisotropy leads to a rapid oscillation of gauge field, thwarting convergence to the inflationary attractor. Secondly, the inclusion of spatial curvature destabilizes the oscillatory attractor and the global stability of the isotropic inflation with gauge field is restored. Finally, based on the numerical evidence combined with the knowledge of the eigenvalues for various inflationary solutions, we give a generic lower-bound for the duration of transient anisotropic inflation, which is inversely proportional to the slow-roll parameter.

gr-qc↗

arXiv2013

Sensitivity of the squark flavor mixing to the CP violation of K, B0 and Bs mesons

We study the sensitivity of the squark flavor mixing to the CP violating phenomena of $K$, $B^0$ and $B_s$ mesons in the framework of the split-family scenario, where the first and second family squarks are very heavy, ${\cal O}(10)$TeV, on the other hand, the third family squark masses are at ${\cal O}(1)$TeV. In order to constrain the gluino-sbottom-quark mixing parameters, we input the experimental data of the CP violations of $K$, $B^0$, and $B_s$ mesons, that is $ε_K$, $ϕ_d$, and $ϕ_s$. The experimental upper bound of the chromo-EDM of the strange quark is also input. In addition, we take account of the observed values $ΔM_{B^0}$, $ΔM_{B_s}$, the CKM mixing $|V_{ub}|$, and the branching ratio of $b\to sγ$. The allowed region of the mixing parameters are obtained as $|δ_{13}^{dL(dR)}|=0\sim 0.01$ and $|δ_{23}^{dL(dR)}|=0\sim 0.04$. By using these values, the deviations from the SM are estimated in the CP violations of the $B^0$ and $B_s$ decays. The deviation from the SM one is tiny in the CP asymmetries of $B^0\to ϕK_S$ and $B^0\to η'K^0$ due to the chromo-EDM of the strange quark. On the other hand, the CP asymmetries $B_s \to ϕϕ$ and $B_s \to ϕη'$ could be largely deviated from the SM predictions. We also predict the time dependent CP asymmetry of $B^0\to K^0\bar K^0$ and the semi-leptonic CP asymmetries of $B^0 \to μ^-X$ and $B_s \to μ^-X$. We expect those precise measurements at Belle II, which will provide us interesting tests for the squark flavor mixing.

hep-ph↗

arXiv2013

Primordial bispectrum from inflation with background gauge fields

We study the primordial bispectrum of curvature perturbation in the uniform- density slicing generated by the interaction between the inflaton and isotropic background gauge fields. We derive the action up to cubic order in perturbation and take into account all the relevant effects in the leading order of slow-roll expansion. We first treat the quadratic vertices perturbatively and confirm the results of past studies, while identifying their regime of validity. We then extend the analysis to include the effect of the quadratic vertices to all orders by introducing exact linear mode functions, allowing us to make accurate predictions long after horizon crossing where the features of both the power spectrum and the bispectrum are drastically different. It is shown that the spPPectra become constant and scale-invariant in the limit of large e-folding. As a result, we are able to impose reliable constraints on the parameters of our theory using the recent observational data coming from Planck.

astro-ph.CO↗