We have performed neutron inelastic scattering and resonant inelastic X-ray scattering (RIXS) at the Cu-$L_3$ edge to study high-energy magnetic excitations at energy transfers of more than 100 meV for overdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x=0.
25$ ($T_c=15$ K) and $x=0.30$ (non-superconducting) using identical single crystal samples for the two techniques. From constant-energy slices of neutron scattering cross-sections, we have identified magnetic excitations up to ~250 meV for $x=0.25$. Although the width in the momentum direction is large, the peak positions along the (pi, pi) direction agree with the dispersion relation of the spin-wave in the non-doped La$_{2}$CuO$_{4}$ (LCO), which is consistent with the previous RIXS results of cuprate superconductors. Using RIXS at the Cu-$L_3$ edge, we have measured the dispersion relations of the so-called paramagnon mode along both (pi, pi) and (pi, 0) directions. Although in both directions the neutron and RIXS data connect with each other and the paramagnon along (pi, 0) agrees well with the LCO spin-wave dispersion, the paramagnon in the (pi, pi) direction probed by RIXS appears to be less dispersive and the excitation energy is lower than the spin-wave of LCO near (pi/2, pi/2). Thus, our results indicate consistency between neutron inelastic scattering and RIXS, and elucidate the entire magnetic excitation in the (pi, pi) direction by the complementary use of two probes. The polarization dependence of the RIXS profiles indicates that appreciable charge excitations exist in the same energy range of magnetic excitations, reflecting the itinerant character of the overdoped sample. A possible anisotropy in the charge excitation intensity might explain the apparent differences in the paramagnon dispersion in the (pi, pi) direction as detected by the X-ray scattering.
We have performed neutron diffraction measurement on a single crystal of parent compound of iron-based superconductor, BaFe$_2$As$_2$ at 12~K. In order to investigate in-plane anisotropy of magnetic form factor in the antiferromagnetic phase, the det
winned single crystal is used in the measurement. The magnetic structure factor and magnetic form factor are well explained by the spin densities consisting of $3d_{yz}$ electrons with a fraction of about 40~% and the electrons in the other four $3d$ orbitals with each fraction of about 15~%. Such anisotropic magnetic form factor is qualitatively consistent with the anisotropic magnetic behaviors observed in the antiferromagnetic phase of the parent compound of iron-based superconductor.
We have performed simultaneous measurements of magnetic chirality by using polarized neutrons and electric polarization along the b-axis of single crystals of YMn$^{4+}$(Mn$_{1-x}$Ga$_{x}$)$^{3+}$O$_{5}$ with $x=0.047$ and 0.12, in which nonmagnetic
Ga-ions dilute Mn$^{3+}$ spins. The $x=0.047$ sample exhibits high-temperature incommensurate (HT-ICM), commensurate (CM), and low-temperature incommensurate (LT-ICM) magnetic phases in order of decreasing temperature, whereas the $x=0.12$ sample exhibits only HT-ICM and LT-ICM phases. Here, the CM and LT-ICM phases are ferroelectric and weak-ferroelectric, respectively. Measurements conducted under zero field heating after various field-cooling conditions evidence that the microscopic mechanisms of the spin-driven ferroelectricity in the CM and LT-ICM phases are different: the magnetic chirality of Mn$^{4+}$ cycloidal spins plays a dominant role in the LT-ICM phase, whereas the magnetic exchange striction by the Mn$^{4+}$-Mn$^{3+}$ chain plays a dominant role in the CM phase. The polarization of YMn$_{2}$O$_{5}$ flips upon CM to LT-ICM phase transition because the ferroelectricity driven by the magnetic chirality and the exchange striction provides opposite directions of polarization.
We have performed resonant inelastic x-ray scattering (RIXS) near the Cu-K edge on cuprate superconductors La(2-x)Sr(x)CuO(4), La(2-x)Ba(x)CuO(4), La(2-x)Sr(x)Cu(1-y)Fe(y)O(4) and Bi(1.76)Pb(0.35)Sr(1.89)CuO(6+d), covering underdoped to heavily overd
oped regime and focusing on charge excitations inside the charge-transfer gap. RIXS measurements of the 214 systems with Ei = 8.993 keV have revealed that the RIXS intensity at 1 eV energy transfer has a minimum at (0,0) and maxima at (0.4pi, 0) and $(0, 0.4pi) for all doping points regardless of the stripe ordered state, suggesting that the corresponding structure is not directly related to stripe order. Measurements with Ei = 9.003 keV on metallic La(1.7)Sr(0.3)CuO(4) and Bi(1.76)Pb(0.35)Sr(1.89)CuO(6+d) exhibit a dispersive intra-band excitation below 4 eV, similar to that observed in the electron-doped Nd(1.85)Ce(0.15)CuO(4). This is the first observation of a dispersive intra-band excitation in a hole doped system, evidencing that both electron and hole doped systems have a similar dynamical charge correlation function.
We report electrical resistivity measurements and neutron diffraction studies under magnetic fields of Bi$_{1.75}$Pb$_{0.35}$Sr$_{1.90}$Cu$_{0.91}$Fe$_{0.09}$O$_{6+y}$, in which hole carriers are overdoped. This compound shows short-range incommensur
ate magnetic correlation with incommensurability $delta=0.21$, whereas a Fe-free compound shows no magnetic correlation. Resistivity shows an up turn at low temperature in the form of $ln(1/T)$ and shows no superconductivity. We observe reduction of resistivity by applying magnetic fields (i.e., a negative magnetoresistive effect) at temperatures below the onset of short-range magnetic correlation. Application of magnetic fields also suppresses the Fe induced incommensurate magnetic correlation. We compare and contrast these observations with two different models: 1) stripe order, and 2) dilute magnetic moments in a metallic alloy, with associated Kondo behavior. The latter picture appears to be more relevant to the present results.
Spin correlations in the overdoped region of Bi1.75Pb0.35Sr1.90CuO6+z have been explored with Fe-doped single crystals characterized by neutron scattering, muon-spin-rotation (muSR) spectroscopy, and magnetic susceptibility measurements. Static incom
mensurate spin correlations induced by the Fe spins are revealed by elastic neutron scattering. The resultant incommensurability delta is unexpectedly large (~0.2 r.l.u.), as compared with delta ~ 1/8 in overdoped superconductor La2-xSrxCuO4. Intriguingly, the large delta in this overdoped region is close to the hole concentration p. This result is reminiscent of the delta ~ p trend observed in underdoped La2-xSrxCuO4; however, it is inconsistent with the saturation of delta in the latter compound in the overdoped regime. While our findings in Fe-doped Bi1.75Pb0.35Sr1.90CuO6+z support the commonality of incommensurate spin correlations in high-Tc cuprate superconductors, they also suggest that the magnetic response might be dominated by a distinct mechanism in the overdoped region.
Low energy spin fluctuations are studied for the electron-doped Fe-based superconductor LaFeAsO(1-x)F(x) by inelastic neutron scattering up to the energy transfer of w = 15 meV using polycrystalline samples. Superconducting samples (x=0.057, Tc=25 K
and x=0.082, Tc=29 K) show dynamical spin susceptibility chi(w) almost comparable with the parent samples. However chi(w) is almost vanished in the x=0.158 sample where the superconductivity is highly suppressed. These results are compatible with the theoretical suggestions that the spin fluctuation plays an important role for the superconductivity.
Polarized neutron diffraction experiments have been performed on multiferroic materials $R$Mn$_{2}$O$_{5}$ ($R=$Ho, Er) under electric fields in the ferroelectric commensurate (CM) and the low-temperature incommensurate (LT-ICM) phases, where the for
mer has the highest electric polarization and the latter has reduced polarization. It is found that, after cooling in electric fields down to the CM phase, the magnetic chirality is proportional to the electric polarization. Also we confirmed that the magnetic chirality can be switched by the polarity of the electric polarization in both the CM and LT-ICM phases. These facts suggest an intimate coupling between the magnetic chirality and the electric polarization. However, upon the transition from the CM to LT-ICM phase, the reduction of the electric polarization is not accompanied by any reduction of the magnetic chirality, implying that the CM and LT-ICM phases contain different mechanisms of the magnetoelectric coupling.
Charge excitations were studied for stipe-ordered 214 compounds, La$_{5/3}$Sr$_{1/3}$NiO$_{4}$ and 1/8-doped La$_{2}$(Ba, Sr)$_{x}$CuO$_{4}$ using resonant inelastic x-ray scattering in hard x-ray regime. We have observed charge excitations at the en
ergy transfer of 1 eV with the momentum transfer corresponding to the charge stripe spatial period both for the diagonal (nikelate) and parallel (cuprates) stripes. These new excitations can be interpreted as a collective stripe excitation or charge excitonic mode to a stripe-related in-gap state.
We report a high-resolution resonant inelastic x-ray scattering study of La2CuO4. A number of spectral features are identified that were not clearly visible in earlier lower-resolution data. The momentum dependence of the spectral weight and the disp
ersion of the lowest energy excitation across the insulating gap have been measured in detail. The temperature dependence of the spectral features was also examined. The observed charge transfer edge shift, along with the low dispersion of the first charge transfer excitation are attributed to the lattice motion being coupled to the electronic system. In addition, we observe a dispersionless feature at 1.8 eV, which is associated with a d-d crystal field excitation.
