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Magnetic structures of the noncentrosymmetric magnet Sr$_2$MnSi$_2$O$_7$ were examined through neutron diffraction for powder and single-crystalline samples, as well as magnetometry measurements. All allowed magnetic structures under the space group $P\bar{4}2_1 m$ with the magnetic wavevector $\vec{q}_{\rm m}=(0,0,1/2)$~r.l.u., were analysed via irreducible representation and magnetic space group approaches. The compound is refined to have in-plane magnetic moments within the magnetic space group $Cmc2_1.1^{\prime}_c$ (\#36.177) under zero field, which can be altered to $P2_1 2_1 2_1.1^{\prime}_c$ (\#19.28) above $\mu_0 H=0.067(5)$~T to align induced weak-ferromagnetic components within one layer on the $ab$-plane. All refined parameters are provided following the recent framework based upon the magnetic space group, which better conveys when exchanging crystallographic information for commensurate magnetic structures., Comment: 7 pages, 4 figures

Muon spin rotation and resonant soft X-ray scattering experiments on prototype multiferroics RMn2O5 (R = Y, Sm) are used to demonstrate that the local electric displacements are driven by the spin-current (SC) mechanism. Small local electric displacements were evaluated by observing spin polarization at ligand O ions, for which implanted muons served as an extremely sensitive probe. Our results for YMn2O5 provide evidence that the spin polarization of O ions forming a spin cycloid chain with Mn spins increases in proportion to the vector spin chirality (Si x Sj ) of the Mn ions. This relationship strongly indicates that the charge transfer between O and Mn ions is driven by the SC mechanism, which leads to the ferroelectricity accompanying O spin polarization.

The proton-proton momentum correlation functions ($C_{pp}(q)$) for kinematically complete decay channels of $^{23}$Al $\rightarrow$ p + p + $^{21}$Na and $^{22}$Mg $\rightarrow$ p + p + $^{20}$Ne have been measured at the RIKEN RI Beam Factory. From the very different correlation strength of $C_{pp}(q)$ for $^{23}$Al and $^{22}$Mg, the source size and emission time information were extracted from the $C_{pp}(q)$ data by assuming a Gaussian source profile in the correlation function calculation code (CRAB). The results indicated that the mechanism of two-proton emission from $^{23}$Al was mainly sequential emission, while that of $^{22}$Mg was mainly three-body simultaneous emission. By combining our earlier results of the two-proton relative momentum and the opening angle, it is pointed out that the mechanism of two-proton emission could be distinguished clearly., Comment: 6 pages, 4 figures; accepted by Phys. Rev. C

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., Comment: 7 pages, 7 figures

Two-proton relative momentum ($q_{pp}$) and opening angle ($\theta_{pp}$) distributions from the three-body decay of two excited proton-rich nuclei, namely $^{23}$Al $\rightarrow$ p + p + $^{21}$Na and $^{22}$Mg $\rightarrow$ p + p + $^{20}$Ne, have been measured with the projectile fragment separator (RIPS) at the RIKEN RI Beam Factory. An evident peak at $q_{pp}\sim20$ MeV/c as well as a peak in $\theta_{pp}$ around 30$^\circ$ are seen in the two-proton break-up channel from a highly-excited $^{22}$Mg. In contrast, such peaks are absent for the $^{23}$Al case. It is concluded that the two-proton emission mechanism of excited $^{22}$Mg is quite different from the $^{23}$Al case, with the former having a favorable diproton emission component at a highly excited state and the latter dominated by the sequential decay process.

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., Comment: 5 pages, 5 figures

How the magnetoelectric coupling actually occurs on a microscopic level in multiferroic BiFeO3 is not well known. By using the high-resolution single crystal neutron diffraction techniques, we have determined the electric polarization of each individual elements of BiFeO3, and concluded that the magnetostrictive coupling suppresses the electric polarization at the Fe site below TN. This negative magnetoelectric coupling appears to outweigh the spin current contributions arising from the cycloid spin structure, which should produce a positive magnetoelectric coupling., Comment: 10 pages, 4 figures, Accepted in Phys. Rev. B(Rapid Communications)

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 overdoped 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., Comment: 7 pages, 7 figures

We describe a method of white-beam inelastic neutron scattering for improved measurement efficiency. The method consists of matrix inversion and selective extraction. The former is to resolve each incident energy component from the white-beam data, and the latter eliminates contamination by elastic components, which produce strong backgrounds that otherwise obfuscate the inelastic scattering components. In this method, the optimal experimental condition to obtain high efficiency will strongly depend on the specific aim of the individual experiments., Comment: 6 pages, 3 figures

The most successful model of comet dust presents comet particles as aggregates of submicron grains. It qualitatively explains the spectral and angular change in the comet brightness and polarization and is consistent with the thermal infrared data and composition of the comet dust obtained {\it in situ} for comet 1P/Halley. However, it experiences some difficulties in providing a quantitative fit to the observational data. Here we present a model that considers comet dust as a mixture of aggregates and compact particles. The model is based on the Giotto and Stardust mission findings that both aggregates (made mainly of organics, silicates, and carbon) and solid silicate particles are present in the comet dust. We simulate aggregates as {\bf Ballistic Cluster-Cluster Aggregates (BCCA)} and compact particles as polydisperse spheroids with some distribution of the aspect ratio. The particles follow a power-law size distribution with the power -3 that is close to the one obtained for comet dust {\it in situ}, at studies of the Stardust returned samples, and the results of ground-based observations of comets. The model provides a good fit to the angular polarization curve. It also reproduces the positive spectral gradient of polarization, red color of the dust, and {\bf low albedo. It also has the ratio of compact to fluffy particles close to the one found {\it in situ} for comet 1P/Halley} and the mass ratio of silicate to carbonaceous materials equal to unity that is in accordance with the elemental abundances of Halley's dust found by Giotto mission., Comment: "Earth and Planetary Science" (Japan), in press

We have studied the magnetic excitations in impurity doped La$_{1.85}$Sr$_{0.15}$Cu$_{1-y}$A$_{y}$O$_{4}$ (A=Ni or Zn) by neutron scattering. The dispersion for Zn:$y=0.017$ is similar to that for the impurity free sample: incommensurate peaks with the incommensurability $\delta=0.12\pm0.01$ (rlu) do not change their positions up to 21 meV. On the other hand, for Ni:$y=0.029$, two incommensurate peaks observed at low energies suddenly change into a broad commensurate peak at $E_\mathrm{cross}=15$ meV. Compared to the impurity free sample with a similar Sr-concentration $x=0.16$, [B. Vignolle {\it et al.} Nature Physics {\bf 3} (2007) 163], $E_\mathrm{cross}$ for Ni:$y=0.029$ is decreased by nearly the same factor for the reduction in $T_{c}$. This is very similar to the shift of the resonance energy ($E_\mathrm{res}$) in Ni-doped YBa$_{2}$Cu$_{3}$O$_{7}$.[Y. Sidis {\it et al.}: Phys. Rev. Lett. {\bf 84} (2000) 5900]. These common impurity effects on the shift of $E_\mathrm{cross}$ and $E_\mathrm{res}$ suggest the same magnetic origin for the resonance peak in YBa$_{2}$Cu$_{3}$O$_{\delta}$ and that for a crossing point of upward and downward dispersions in the La$_{2-x}$Sr$_{x}$CuO$_{4}$. We propose that the sudden change in the dispersion is better described by a crossover from incommensurate spin fluctuations to a gapped spin wave rather than a hourglass-like dispersion., Comment: 11 pages, 5 figures

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 former 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., Comment: 4 pages, 2 figures. Proceedings of PNCMI2008/QuBS2008

We investigated the infrared optical spectra of an organic dimer Mott insulator $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl, which was irradiated with X-rays. We observed that the irradiation caused a large spectral weight transfer from the mid-infrared region, where interband transitions in the dimer and Mott-Hubbard bands take place, to a Drude part in a low-energy region; this caused the Mott gap to collapse. The increase of the Drude part indicates a carrier doping into the Mott insulator due to irradiation defects. The strong redistribution of the spectral weight demonstrates that the organic Mott insulator is very close to the phase border of the bandwidth-controlled Mott transition., Comment: 4 pages, 4 figures

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 energy 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., Comment: 5 pages, 4 figures

By combining neutron four-circle diffraction and polarized neutron diffraction techniques we have determined the complex spin structures of a multiferroic, YMn2O5, that exhibits two ferroelectric phases at low temperatures. The obtained magnetic structure has spiral components in both the low temperature ferroelectric phases that are magnetically commensurate and incommensurate, respectively. Among proposed microscopic theories for the magneto-electric coupling, our results are consistent with both the spin-current model that requires a magnetic transverse (cycloidal) spiral structure in order to induce a spontaneous electric polarization and the magneto-restriction model. Our results also explain why the electric polarization changes at the commensurate-to-incommensurate phase transition.

Comets can be divided into two groups: type I, characterized by high gas/dust ratio, low polarization, and a weak or absent 10 micron silicate feature, and type II, for which a low gas/dust ratio, high polarization, and strong silicate feature are typical. We show that the low polarization is the apparent result of depolarization by gas contamination at low dust concentration, which, in turn, results from the dust in type I comets being concentrated near the nucleus. The simulations of thermal emission show that for more porous particles (BCCA), the silicate feature is more pronounced than more compact ones (BPCA), for which it even vanishes as the particles become larger. We also show that in both types of comets the main contribution to light scattering and emission comes from particles larger than 10 micron. Conclusions: .The strength of the silicate feature in the cometary infrared spectra suggests that the dust in type II comets consists of high-porosity aggregates, whereas the dust of type I comets contains low-porosity ones. This is consistent with the polarimetric features of these comets, which indicate that the dust in type I comets tends to concentrate near the nucleus. This may result from the predominance of highly processed particles in type I comets, whereas in type II comets we see pristine or slightly-processed dust. This conclusion is in accordance with the orbital characteristics of the comets. We have found that the strength of the silicate feature correlates with the semi-major axis of periodic comets and, for short-period comets, with the perihelion distance. Thus, the silicate feature weakens due to compaction of aggregate particles if a comet spends more time in the vicinity of the Sun, which allows the comet to accumulate a mantle on the surface of its nucleus.

Wide-field (~8' x 8') and deep near-infrared (JHKs bands) polarization images of the Orion nebulae (IRNe) around young stellar objects (YSOs), both massive and low-mass. We found the IRNe around both IRc2 and BN to be very extensive, suggesting that there might be two extended (>0.7 pc) bipolar/monopolar IRNe in these sources. We discovered at least 13 smaller-scale (~0.01-0.1 pc) IRNe around less-massive YSOs including the famous source theta^2 Ori C. We also suggest the presence of many unresolved (<690 AU) systems around low-mass YSOs and young brown dwarfs showing possible intrinsic polarizations. Wide-field infrared polarimetry is thus demonstrated to be a powerful technique in revealing IRNe and hence potential disk/outflow systems among high-mass to substellar YSOs., Comment: 6 pages, 3 figures. ApJ, in press

Neutron diffraction measurements under high magnetic fields have been performed for the multiferroic compound HoMn$_{2}$O$_{5}$. At zero field, high-temperature incommensurate magnetic (HT-ICM) -- commensurate magnetic (CM) -- low-temperature incommensurate magnetic (LT-ICM) orders occur with decreasing temperature, where ferroelectric polarization arises only in the CM phase. Upon applying a magnetic field, the LT-ICM phase completely disappears and the CM phase is induced at the lowest temperature. This field-induced CM state is completely associated with the field-induced electric polarization in this material [Higashiyama {\it et al}., Phys. Rev. B {\bf 72}, 064421 (2005).], strongly indicating that the commensurate spin state is essential to the ferroelectricity in the multiferroic $R$Mn$_{2}$O$_{5}$ system., Comment: 4 pages, 5 figures, submitted to Phys. Rev. Lett

Lattice modulation and magnetic structures in magnetoelectric compounds Tb1-xDyxMnO3 have been studied around the ferroelectric (FE) Curie temperature T_C by x-ray and neutron diffraction. Temperature-independent modulation vectors through T_C are observed for the compounds with 0.50< x < 0.68. This indicates that ferroelectricity with a polarization (P) along the c axis in the RMnO3 series cannot be ascribed to such an incommensurate-commensurate transition of an antiferromagnetic order as was previously anticipated. Neutron diffraction study of a single crystal with x=0.59 shows that the FE transition is accompanied by the transformation of the Mn-spin alignment from sinusoidal (collinear) antiferromagnetism into a transverse spiral structure. The observed spiral structure below T_C is expected to produce P along the c axis with the `inverse' Dzialoshinski-Moriya interaction, which is consistent with the observation., Comment: 9 pages, 4 figures

Infrared absorption spectra of SrTiO$_3$ have been measured under above-band-gap photoexcitations to study the properties of photogenerated carriers, which should play important roles in previously reported photoinduced phenomena in SrTiO$_3$. A broad absorption band appears over the entire mid-infrared region under photoexcitation. Detailed energy, temperature, and excitation power dependences of the photoinduced absorption are reported. This photo-induced absorption is attributed to the intragap excitations of the photogenerated carriers. The data show the existence of a high density of in-gap states for the photocarriers, which extends over a wide energy range starting from the conduction and valence band edges., Comment: 5 pages, 5 figures, submitted to J. Phys. Soc. Jpn

Partial suppression of superconductivity by fast cooling has been observed in the organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br by two means: a marked sample size effect on the magnetic susceptibility and direct imaging of insulating regions by scanning microregion infrared reflectance spectroscopy. Macroscopic insulating regions are found in the vicinity of the crystalline surface after fast cooling, with diameters of 50--100 $\mu$m and depths of a few $\mu$m. The very large in-plane penetration depth reported to date ($\sim$ 24--100 $\mu$m) can be explained by the existence of the insulating regions., Comment: Several rhetoric alternations to avoid misleadings. 6 pages, 3 figures. to be publihsed in Phys. Rev. B

Far-infrared spectroscopy has been performed on [Fe(2-picolylamine)3]Cl2EtOH (Fe-pic) single crystals, to probe changes in the molecular vibrations upon the photoinduced and temperature-induced spin crossovers. Synchrotron radiation has been used as the farinfrared source to overcome the strong absorption and the small sizes of the samples. Absorption lines due to FeN6 cluster vibrations, observed below 400 cm-1, show strong intensity variations upon the crossover due to the deformation of FeN6 between high-spin and low-spin states. However, they remain almost unchanged between the photo- and temperature-induced high-spin states. This is in sharp contrast to the lines at 500-700 cm-1 due to intramolecular vibrations of the picolylamine ligands, which show marked variations between the two high-spin states. It is concluded that the most important microscopic difference between the two high-spin states arises from the ligands, which is likely to reflect different states of intermolecular bonding between them., Comment: 10 pages, 4 figures

Soft phonon behavior associated with a structural phase transition from the low-temperature-orthorhombic (LTO) phase ($Bmab$ symmetry) to the low-temperature-tetragonal (LTT) phase ($P4_{2}/ncm$ symmetry) was investigated in La$_{1.875}$Ba$_{0.125}$CuO$_{4}$ using neutron scattering. As temperature decreases, the TO-mode at $Z$-point softens and approaches to zero energy around $T_{\rm d2}=62$ K, where the LTO -- LTT transition occurs. Below $T_{\rm d2}$, the phonon hardens quite rapidly and it's energy almost saturates below 50 K. At $T_{\rm d2}$, the energy dispersion of the soft phonon along in-plane direction significantly changes while the dispersion along out-of-plane direction is almost temperature independent. Coexistence between the LTO phase and the LTT phase, seen in both the soft phonon spectra and the peak profiles of Bragg reflection, is discussed in context of the order of structural phase transitions., Comment: 6 pages, 8 figures

Impurity effects of Zn and Ni on the low-energy spin excitations were systematically studied in optimally doped La1.85Sr0.15Cu1-yAyO4 (A=Zn, Ni) by neutron scattering. Impurity-free La1.85Sr0.15CuO4 shows a spin gap of 4meV below Tc in the antiferromagnetic(AF) incommensurate spin excitation. In Zn:y=0.004, the spin excitation shows a spin gap of 3meV below Tc. In Zn:y=0.008 and Zn:y=0.011, however, the magnetic signals at 3meV decrease below Tc and increase again at lower temperature, indicating an in-gap state. In Zn:y=0.017, the low-energy spin state remains unchanged with decreasing temperature, and elastic magnetic peaks appear below 20K then exponentially increase. As for Ni:y=0.009 and Ni:y=0.018, the low-energy excitations below 3meV and 2meV disappear below Tc. The temperature dependence at 3meV, however, shows no upturn in constrast with Zn:y=0.008 and Zn:y=0.011, indicating the absence of in-gap state. In Ni:y=0.029, the magnetic signals were observed also at 0meV. Thus the spin gap closes with increasing Ni. Furthermore, as omega increases, the magnetic peak width broadens and the peak position, i.e. incommensurability, shifts toward the magnetic zone center (pi pi). We interpret the impurity effects as follows: Zn locally makes a non-superconducting island exhibiting the in-gap state in the superconducting sea with the spin gap. Zn reduces the superconducting volume fraction, thus suppressing Tc. On the other hand, Ni primarily affects the superconducting sea, and the spin excitations become more dispersive and broaden with increasing energy, which is recognized as a consequence of the reduction of energy scale of spin excitations. We believe that the reduction of energy scale is relevant to the suppression of Tc., Comment: 13pages, 14figures; submitted to Phys. Rev. B

Systematic investigation of the electronic phase separation on macroscopic scale is reported in the organic Mott system $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br. Real space imaging of the phase separation is obtained by means of scanning micro-region infrared spectroscopy using the synchrotron radiation. The phase separation appears near the Mott boundary and changes its metal-insulator fraction with the substitution ratio $x$ in $\kappa$-[($h$-BEDT-TTF)$_{1-x}$($d$-BEDT-TTF)$_{x}$]$_{2}$Cu[N(CN)$_{2}$]Br, of which band width is controlled by the substitution ratio $x$ between the hydrogenated BEDT-TTF molecule ($h$-BEDT-TTF) and the deuterated one ($d$-BEDT-TTF). The phase separation phenomenon observed in this class of organics is considered on the basis of the strongly correlated electronic phase diagram with the first order Mott transition., Comment: 10 pages, 8 figures

The nature of charge stripe order and its relationship with structural phase transitions were studied using synchrotron X-ray diffraction in La$_{1.875}$Ba$_{0.125-x}$Sr$_{x}$CuO$_{4}$ ($0.05\leq x\leq 0.10$). For $x=0.05$, as temperature increased, incommensurate superlattice peaks associated with the charge order disappeared just at the structural phase transition temperature, $T_{\rm d2}$. However, for $x=0.075$ and 0.09, the superlattice peaks still existed as a short range correlation even above $T_{\rm d2}$, indicating a precursor of charge ordering. Furthermore, temperature dependences of the superlattice peak intensity, correlation length, and incommensurability for $x=0.05$ are different from those for $x=0.075$ and 0.09. These results suggest that the transition process into the charge stripe order strongly correlates with the order of the structural phase transitions. A quantitative comparison of the structure factor associated with the charge order have been also made for all the samples., Comment: 8 pages, 9 figures, Accepted for publication in PRB

Electronic phase separation consisting of the metallic and insulating domains with 50 -- 100 $\mu$m in diameter is found in the organic Mott system $\kappa$-[($h$8-BEDT-TTF)$_{1-x}$($d$8-BEDT-TTF)$_{x}$]$_{2}$Cu[N(CN)$_{2}$]Br by means of scanning micro-region infrared spectroscopy using the synchrotron radiation. The phase separation appears below the critical end temperature 35 -- 40 K of the first order Mott transition. The observation of the macroscopic size of the domains indicates a different class of the intrinsic electronic inhomogeneity from the nano-scale one reported in the inorganic Mott systems such as High-$T_{c}$ copper and manganese oxides., Comment: 4 pages, 3 figures

Lattice distortions associated with charge stripe order in 1/8 hole-doped La$_{1.875}$Ba$_{0.125-x}$Sr$_{x}$CuO$_{4}$ are studied using synchrotron X-ray diffraction for $x=0.05$ and $x=0.075$. The propagation wave vector and charge order correlation lengths are determined with a high accuracy, revealing that the oblique charge stripes in orthorhombic $x=0.075$ crystal are more disordered than the aligned stripes in tetragonal $x=0.05$ crystal. The twofold periodicity of lattice modulations along the c-axis is explained by long-range Coulomb interactions between holes on neighboring CuO$_{2}$ planes., Comment: 4pages, 4figures, Submitted to PRB

Low-energy spin excitations of La1.85Sr0.15Cu1-yZnyO4 were studied by neutron scattering. In y=0.004, the incommensurate magnetic peaks show a well defined ``spin gap'' below Tc. The magnetic signals at omega=3 meV decrease below Tc=27 K for y=0.008, also suggesting the gap opening. At lower temperatures, however, the signal increases again, implying a novel in-gap spin state. In y=0.017, the spin gap vanishes and elastic magnetic peaks appear. These results clarify that doped Zn impurities induce the novel in-gap state, which becomes larger and more static with increasing Zn., Comment: 4 pages, 4 figures

We report magnetic neutron scattering measurements of incommensurate magnetic order in a superconducting single crystal of La1.88Sr0.12CuO4. We find that the incommensurate wavevectors which describe the static magnetism do not lie along high-symmetry directions of the underlying CuO2 lattice. The positions of the elastic magnetic peaks are consistent with those found in excess-oxygen doped La2CuO4+y. This behavior differs from the precise magnetic order found in the low temperature tetragonal La1.6-xNd0.4SrxCuO4 material for which stripes of spin and charge have been observed. These observations have clear implications for any stripe model proposed to describe the static magnetism in orthorhombic La2CuO4-based superconductors., Comment: 4 pages, 3 eps figures

Comprehensive inelastic neutron-scattering measurements were performed to study the soft optical phonons in La2-xSrxCuO4 at x=0.10, 0.12 and 0.18. We found at x=0.18 that the softening of Z-point phonon, suggesting incipient structural transition from the low-temperature orthorhombic (LTO) to low-temperature tetragonal (LTT) phase, breaks at Tc, which is consistent with the previous report by Lee et al. for the optimally doped x=0.15 sample. As for x=0.10 and 0.12, on the other hand, the softening continues even below Tc. It is thus clarified that the breaking of soft phonon is characteristic of La2-xSrxCuO4 in the optimally and overdoped regions. In the course of studying the soft phonons, we discovered that a central peak remains above the LTO to high-temperature tetragonal (HTT) phase transition at Ts1 and splits into incommensurate components along the (1 1 0)HTT direction at higher temperatures. This is a common feature for both x=0.12 and 0.18 and their temperature dependences of the splitting 2d can be scaled by using a renormalized temperature T/Ts1. In the high temperature limit, d saturates around d ~ 0.12 r.l.u., which value is close to the splitting of incommensurate magnetic signals. This implies that the incipient lattice modulation starts appearing at very high temperature. Details of this modulation and its relations with other properties are, however, not yet clarified., Comment: 7 pages, 5 eps figures

A lightly doped perovskite mangantite La_{0.88}Sr_{0.12}MnO_3 exhibits a phase transition at T_{OO}=145 K from a ferromagnetic metal (T_C=172 K) to a novel ferromagnetic insulator.We identify that the key parameter in the transition is the orbital degree of freedom in e_g electrons. By utilizing the resonant x-ray scattering technique, orbital ordering is directly detected below T_{OO}, in spite of a significant diminution of the cooperative Jahn-Teller distortion. The new experimental features are well described by a theory treating the orbital degree of freedom under strong electron correlation. The present experimental and theoretical studies uncover a crucial role of the orbital degree in the metal-insulator transition in lightly doped manganites., Comment: 4 pages, 4 figures

The gigantic reduction of the electric resistivity under the applied magnetic field, CMR effect, is now widely accepted to appear in the vicinity of the insulator to metal transition of the perovskite manganites. Recently, we have discovered the first order transition from ferromagnetic metal to insulator in $\rm La_{0.88}Sr_{0.12}MnO_3$ of the CMR manganite. This phase transition induces the tremendous increase of the resistivity under the external magnetic field just near above the phase transition temperature. We report here fairly detailed results from the systematic experiments including neutron and synchrotron X-ray scattering studies., Comment: 16 pages, 4 ps-figures, to appear in the proceedings of the 7th NEC symposium 'Phase Control in Spin-Charge-Orital Complex System' on Oct.11-15,1998, (Jour.Materials Science and Engineering B (Elsevier Science))