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Register a new userElectric-dipole active two-magnon excitation in {textit{ab}} spiral spin phase of a ferroelectric magnet Gd$_{textbf{0.7}}$Tb$_{textbf{0.3}}$MnO$_{textbf 3}$
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A broad continuum-like spin excitation (1--10 meV) with a peak structure around 2.4 meV has been observed in the ferroelectric $ab$ spiral spin phase of Gd$_{0.7}$Tb$_{0.3}$MnO$_3$ by using terahertz (THz) time-domain spectroscopy. Based on a complete set of light-polarization measurements, we identify the spin excitation active for the light $E$ vector only along the a-axis, which grows in intensity with lowering temperature even from above the magnetic ordering temperature but disappears upon the transition to the $A$-type antiferromagnetic phase. Such an electric-dipole active spin excitation as observed at THz frequencies can be ascribed to the two-magnon excitation in terms of the unique polarization selection rule in a variety of the magnetically ordered phases.
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We report structural and physical properties of the single crystalline ${mathrm{Ca}}{mathrm{Mn}}_{2}{mathrm{P}}_{2}$. The X-ray diffraction(XRD) results show that ${mathrm{Ca}}{mathrm{Mn}}_{2}{mathrm{P}}_{2}$ adopts the trigonal ${mathrm{Ca}}{mathrm{Al}}_{2}{mathrm{Si}}_{2}$-type structure. Temperature dependent electrical resistivity $rho(T)$ measurements indicate an insulating ground state for ${mathrm{Ca}}{mathrm{Mn}}_{2}{mathrm{P}}_{2}$ with activation energies of 40 meV and 0.64 meV for two distinct regions, respectively. Magnetization measurements show no apparent magnetic phase transition under 400 K. Different from other ${mathrm{A}}{mathrm{Mn}}_{2}{mathrm{Pn}}_{2}$ (A = Ca, Sr, and Ba, and Pn = P, As, and Sb) compounds with the same structure, heat capacity $C_{mathrm{p}}(T)$ and $rho(T)$ reveal that ${mathrm{Ca}}{mathrm{Mn}}_{2}{mathrm{P}}_{2}$ has a first-order transition at $T$ = 69.5 K and the transition temperature shifts to high temperature upon increasing pressure. The emergence of plenty of new Raman modes below the transition, clearly suggests a change in symmetry accompanying the transition. The combination of the structural, transport, thermal and magnetic measurements, points to an unusual origin of the transition.
We observed the atomic $1s$ and $2p$ states of $pi^-$ bound to ${}^{121}{rm Sn}$ nuclei as distinct peak structures in the missing mass spectra of the ${}^{122}{rm Sn}(d,{}^3{rm He})$ nuclear reaction. A very intense deuteron beam and a spectrometer with a large angular acceptance let us achieve potential of discovery, which includes capability of determining the angle-dependent cross sections with high statistics. The $2p$ state in a Sn nucleus was observed for the first time. The binding energies and widths of the pionic states are determined and found to be consistent with previous experimental results of other Sn isotopes. The spectrum is measured at finite reaction angles for the first time. The formation cross sections at the reaction angles between 0 and $2^circ$ are determined. The observed reaction-angle dependence of each state is reproduced by theoretical calculations. However, the quantitative comparison with our high-precision data reveals a significant discrepancy between the measured and calculated formation cross sections of the pionic $1s$ state.
Using data from the NEMO-3 experiment, we have measured the two-neutrino double beta decay ($2 ubetabeta$) half-life of $^{82}$Se as $T_{1/2}^{2 u} = left[ 9.39 pm 0.17,left(mbox{stat}right) pm 0.58,left(mbox{syst}right)right] times 10^{19}$ y under the single-state dominance hypothesis for this nuclear transition. The corresponding nuclear matrix element is $left|M^{2 u}right| = 0.0498 pm 0.0016$. In addition, a search for neutrinoless double beta decay ($0 ubetabeta$) using 0.93 kg of $^{82}$Se observed for a total of 5.25 y has been conducted and no evidence for a signal has been found. The resulting half-life limit of $T_{1/2}^{0 u} > 2.5 times 10^{23} ,mbox{y} ,(90%,mbox{C.L.})$ for the light neutrino exchange mechanism leads to a constraint on the effective Majorana neutrino mass of $langle m_{ u} rangle < left(1.2 - 3.0right) ,mbox{eV}$, where the range reflects $0 ubetabeta$ nuclear matrix element values from different calculations. Furthermore, constraints on lepton number violating parameters for other $0 ubetabeta$ mechanisms, such as right-handed currents, majoron emission and R-parity violating supersymmetry modes have been set.
Exclusive measurements of high energy $gamma$-rays are performed in $rm ^{124}Ba$ and $rm ^{136}Ba$ at the same excitation energy ($sim$ 49 MeV), to study properties of the giant dipole resonance (GDR) over a wider $N/Z$ range. The high energy $gamma$-rays are measured in coincidence with the multiplicity of low energy $gamma$-rays to disentangle the effect of temperature ($T$) and angular momentum ($J$). The GDR parameters are extracted employing a simulated Monte Carlo statistical model analysis. The observed $gamma$-ray spectra of $rm ^{124}Ba$ can be explained with prolate deformation, whereas a single component Lorentzian function which corresponds to a spherical shape could explain the $gamma$-ray spectra from $rm ^{136}Ba$. The observed GDR width in $rm ^{136}Ba$ is narrower compared to that of $rm ^{124}Ba$. The statistical model best fit GDR cross sections are found to be in good agreement with the thermal shape fluctuation model (TSFM) calculations. Further, it is shown that the variation of GDR width with $T$ is well reproduced by the TSFM calculations over the temperature range of 1.1--1.7MeV.
Electronic orderings of charges, orbitals and spins are observed in many strongly correlated electron materials, and revealing their dynamics is a critical step toward understanding the underlying physics of important emergent phenomena. Here we use time-resolved resonant soft x-ray scattering spectroscopy to probe the dynamics of antiferromagnetic spin ordering in the manganite Pr$_{0.7}$Ca$_{0.3}$MnO$_3$ following ultrafast photo-exitation. Our studies reveal a glass-like recovery of the spin ordering and a crossover in the dimensionality of the restoring interaction from quasi-1D at low pump fluence to 3D at high pump fluence. This behavior arises from the metastable state created by photo-excitation, a state characterized by spin disordered metallic droplets within the larger charge- and spin-ordered insulating domains. Comparison with time-resolved resistivity measurements suggests that the collapse of spin ordering is correlated with the insulator-to-metal transition, but the recovery of the insulating phase does not depend on the re-establishment of the spin ordering.
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