Spin chirality fluctuation in two-dimensional ferromagnets with perpendicular anisotropy

Non-coplanar spin textures with scalar spin chirality can generate effective magnetic field that deflects the motion of charge carriers, resulting in topological Hall effect (THE), a powerful probe of the ground state and low-energy excitations of correlated systems. However, spin chirality fluctuation in two-dimensional ferromagnets with perpendicular anisotropy has not been considered in prior studies. Herein, we report direct evidence of universal spin chirality fluctuation by probing the THE above the transition temperatures in two different ferromagnetic ultra-thin films, SrRuO$_3$ and V doped Sb$_2$Te$_3$. The temperature, magnetic field, thickness, and carrier type dependences of the THE signal, along with our Monte-Carlo simulations, unambiguously demonstrate that the spin chirality fluctuation is a universal phenomenon in two-dimensional Ising ferromagnets. Our discovery opens a new paradigm of exploring the spin chirality with topological Hall transport in two-dimensional magnets and beyondComment: accepted by nature material

A Highly Overmoded Structure for Hundred-Kilowatt-Class <italic>Ka</italic>-Band Extended Interaction Klystron

Here, we report on the development of highly overmoded structure for a millimeter-wave (MMW) extended interaction klystron (EIK). To enhance electron beam loading, a new method for designing an oversized beam tunnel in a large cavity by concentrating the axial field is demonstrated. The transmission and oscillation characteristics of the interaction circuit operating in the quasi-TM04 mode are tested by the developed mode conversion circuit. Results suggest that a five-cavity EIK based on this highly overmoded structure can achieve an output power of 289 kW at 32.92 GHz with a saturated gain of 51.6 dB by injecting a 3.3-mm-diameter electron beam with a current of 18 A. The output power exceeds 100 kW at a bandwidth of 100 MHz.</p

Study of an overmoded structure for megawatt Ka-band extended interaction klystron

For most applications in the millimeter wave band, corresponding to Ka and higher-frequency bands, relatively high atmospheric absorption necessitates the use of high-power sources. Here, a new approach for projecting an oversized beam tunnel in an overmoded structure by concentrating the axial field is demonstrated to meet the high-frequency and high-power demands of compact devices. Due to the enhanced intense beam loading capability of the interaction circuit, a six-cavity Ka-band extended interaction klystron with a four-coupling-hole disk-loaded structure is designed that can stably obtain high output power. An analysis of optimization tradeoffs from introducing high order modes for allowing the application of more powerful beams to improving high order mode field distribution for enhancing the electron-wave coupling and suppressing mode competition is reported. 3D particle-in-cell simulations show attainable output powers of 1.11 MW at 32.94 GHz with a saturated gain of 57 dB by injecting a 3.3 mm diameter electron beam with a current of 24 A.</p

Search for hidden-charm tetraquark with strangeness in e+e−→K+Ds∗−D∗0+c.c.e^{+}e^{-}\rightarrow K^+ D_{s}^{*-} D^{*0}+c.c.

We report a search for a heavier partner of the recently observed $Z_{cs}(3985)^{-}$ state, denoted as $Z_{cs}^{\prime -}$, in the process $e^{+} e^{-}\rightarrow K^{+}D_{s}^{*-}D^{* 0}+c.c.$, based on $e^+e^-$ collision data collected at the center-of-mass energies of $\sqrt{s}=4.661$, 4.682 and 4.699 GeV with the BESIII detector. The $Z_{cs}^{\prime -}$ is of interest as it is expected to be a candidate for a hidden-charm and open-strange tetraquark. A partial-reconstruction technique is used to isolate $K^+$ recoil-mass spectra, which are probed for a potential contribution from $Z_{cs}^{\prime -}\to D_{s}^{*-}D^{* 0}$ ($c.c.$). We find an excess of $Z_{cs}^{\prime -}\rightarrow D_{s}^{*-}D^{*0}$ ($c.c.$) candidates with a significance of $2.9\sigma$, after considering systematic uncertainties, at a mass of $(4123.5 \pm 0.7_{\mathrm{stat.}} \pm 1.1_{\mathrm{syst.}}) \mathrm{MeV}/c^{2}$. As the data set is limited in size, the upper limits are evaluated at the 90% confidence level on the product of the Born cross section and the branching fraction of $Z_{cs}^{\prime-}\rightarrow D_{s}^{*-}D^{* 0}$, $\sigma^{\rm Born}\cdot\mathcal{B}$ at the three energy points, under different assumptions of the $Z_{cs}^{\prime -}$ mass from 4.120 to 4.140 MeV and of the width from 10 to 50 MeV. Under various mass and width assumptions, the upper limits of $\sigma^{\rm Born}\cdot\mathcal{B}$ are found to lie in the range of $2\sim6$, $3\sim7$ and $3\sim6$ pb at $\sqrt{s}=4.661$, 4.682 and 4.699 GeV, respectively. The larger data samples that will be collected in the coming years will allow a clearer picture to emerge concerning the existence and nature of the $Z_{cs}^{\prime -}$ state.Comment: 17 pages, 7 figure

Production of doubly-charged Δ\Delta baryon in e+e−e^{+}e^{-} annihilation at energies from 2.3094 to 2.6464 GeV

The processes $e^{+}e^{-} \to \Delta^{++}\bar{\Delta}^{--}$ and $e^{+}e^{-}\to \Delta^{++} \bar{p} \pi^{-} + c.c.$ are studied for the first time with $179~{\rm pb}^{-1}$ of $e^{+}e^{-}$ annihilation data collected with the BESIII detector at center-of-mass energies from $2.3094$ GeV to $2.6464$ GeV. No significant signal for the $e^{+}e^{-}\to \Delta^{++}\bar{\Delta}^{--}$ process is observed and the upper limit of the Born cross section is estimated at each energy point. For the process $e^{+}e^{-} \to \Delta^{++} \bar{p} \pi^{-} + c.c.$, a significant signal is observed at center-of-mass energies near 2.6454 GeV and the corresponding Born cross section is reported.Comment: 10 pages, 4 figure

Search for a scalar partner of the X(3872)X(3872) via ψ(3770)\psi(3770) decays into γηη′\gamma\eta\eta' and γπ+π−J/ψ\gamma\pi^{+}\pi^{-}J/\psi

Using a data sample corresponding to an integrated luminosity of 2.93 fb$^{-1}$ collected at a center-of-mass energy of 3.773~GeV with the BESIII detector at the BEPCII collider, we search for a scalar partner of the $X(3872)$, denoted as $X(3700)$, via $\psi(3770)\to \gamma\eta\eta'$ and $\gamma\pi^{+}\pi^{-}J/\psi$ processes. No significant signals are observed and the upper limits of the product branching fractions ${\cal B}(\psi(3770)\to\gamma X(3700))\cdot {\cal B}(X(3700)\to \eta\eta')$ and ${\cal B}(\psi(3770)\to\gamma X(3700))\cdot {\cal B}(X(3700)\to\pi^{+}\pi^{-}J/\psi)$ are determined at the 90\% confidence level, for the narrow $X(3700)$ with a mass ranging from 3710 to 3740 MeV/$c^2$, which are from 0.8 to 1.8 $(\times 10^{-5})$ and 0.9 to 3.4 $(\times 10^{-5})$, respectively

Measurement of branching fractions of Λc+\Lambda_{c}^{+} decays to Σ+K+K−\Sigma^{+} K^{+} K^{-}, Σ+ϕ\Sigma^{+}\phi and Σ+K+π−(π0)\Sigma^{+} K^{+} \pi^{-}(\pi^{0})

Based on 4.5 fb$^{-1}$ data taken at seven center-of-mass energies ranging from 4.600 to 4.699 GeV with the BESIII detector at the BEPCII collider, we measure the branching fractions of $\Lambda_{c}^{+}\rightarrow\Sigma^{+}+hadrons$ relative to $\Lambda_{c}^{+}\rightarrow \Sigma^+ \pi^+ \pi^-$. Combining with the world average branching fraction of $\Lambda_{c}^{+}\rightarrow \Sigma^+ \pi^+ \pi^-$, their branching fractions are measured to be $(0.377\pm0.042\pm0.018\pm0.021)\%$ for $\Lambda_{c}^{+}\rightarrow\Sigma^{+} K^{+} K^{-}$, $(0.200\pm0.023\pm0.010\pm0.011)\%$ for $\Lambda_{c}^{+}\rightarrow\Sigma^{+} K^{+} \pi^{-}$, $(0.414\pm0.080\pm0.029\pm0.023)\%$ for $\Lambda_{c}^{+}\rightarrow\Sigma^{+}\phi$ and $(0.197\pm0.036\pm0.008\pm0.011)\%$ for $\Lambda_{c}^{+}\rightarrow\Sigma^{+}K^{+} K^{-}$(non-$\phi$). In all the above results, the first uncertainties are statistical, the second are systematic and the third are from external input of the branching fraction of $\Lambda_{c}^{+}\rightarrow \Sigma^+ \pi^+ \pi^-$. Since no signal for $\Lambda_{c}^{+}\rightarrow\Sigma^{+} K^{+} \pi^{-}\pi^{0}$ is observed, the upper limit of its branching fraction is determined to be 0.11\% at the 90$\%$ confidence level

Observation of the Singly Cabibbo-Suppressed Decay Λc+→Σ−K+π+\Lambda_{c}^{+}\to \Sigma^{-}K^{+}\pi^{+}

The singly Cabibbo-suppressed decay $\Lambda_{c}^{+}\to \Sigma^{-}K^{+}\pi^{+}$ is observed for the first time with a statistical significance of $6.4\sigma$ by using 4.5 fb$^{-1}$ of $e^+e^-$ collision data collected at center-of-mass energies between 4.600 and 4.699 GeV with the BESIII detector at BEPCII. The absolute branching fraction of $\Lambda_{c}^{+}\to \Sigma^{-}K^{+}\pi^{+}$ is measured to be $(3.8\pm1.3_{\rm stat}\pm0.2_{\rm syst})\times 10^{-4}$ in a model-independent approach. This is the first observation of a Cabibbo-suppressed $\Lambda_{c}^{+}$ decay involving $\Sigma^-$ in the final state. The ratio of branching fractions between $\Lambda_{c}^{+}\to \Sigma^{-}K^{+}\pi^{+}$ and the Cabibbo-favored decay $\Lambda_{c}^{+}\to \Sigma^- \pi^+\pi^+$ is calculated to be $(0.4 \pm 0.1)s_{c}^{2}$, where $s_{c} \equiv \sin\theta_c = 0.2248$ with $\theta_c$ the Cabibbo mixing angle. This ratio significantly deviates from $1.0s_{c}^{2}$ and provides important information for the understanding of nonfactorization contributions in $\Lambda_{c}^{+}$ decays.Comment: 8 pages, 2 figure