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The magnetic excitations of the square-lattice spin-1/2 antiferromagnet and high-Tc parent La2CuO4 are determined using high-resolution inelastic neutron scattering. Sharp spin waves with absolute intensities in agreement with theory including quantum corrections are found throughout the Brillouin zone. The observed dispersion relation shows evidence for substantial interactions beyond the nearest-neighbor Heisenberg term, which can be understood in terms of a cyclic or ring exchange due to the strong hybridization path around the Cu4O4 square plaquettes., Comment: 5 pages revtex, 3 ps figures (fig(1) color), updated figures 2&3 and added analysis details to include referees' comments

Mesoscopic (500-2000 Angstrom) and microscopic (5-20 Angstrom) phase segregation with temperature and magnetic field was studied in the model manganite Pr0.7Ca0.3MnO3 by high-resolution neutron diffraction and inelastic neutron scattering. Intra-granular strain-driven mesoscopic segregation between two insulating phases, one of which is charge ordered (CO), sets in below the CO temperature in zero field. The CO phase orders antiferromagnetically, while the other insulating phase shows spin-glass behavior. After field-induced metallization, the CO phase coexists with a ferromagnetic metallic phase., Comment: 4 figures

High temperature (T) studies of La_{1-x}Ca_{x}MnO_{3} show that charge and phonon transport is significantly suppressed in a narrow doping region of x~1/2 and in a wide T range up to 900 K, which accompanies anomalous broadening of x-ray Bragg peaks. Contrarily, ferromagnetic (FM) correlation above Curie T is sharply enhanced at x~3/8. All these observations indicate the presence of short-range charge ordering (CO) correlation at high T, possibly in the form of a FM "zigzag", a small segment of the CE-type CO state. The decoupling and coupling of the FM zigzags for x~3/8 and 1/2, respectively, is consistent with our results., Comment: 8 pages with 4 eps figures, submitted to PRL

We report low temperature specific heat measurements of Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ ($0.3\leq x \leq 0.5$) and La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ with and without applied magnetic field. An excess specific heat, $C^{\prime}(T)$, of non-magnetic origin associated with charge ordering is found for all the samples. A magnetic field sufficient to induce the transition from the charge-ordered state to the ferromagnetic metallic state does not completely remove the $C^{\prime}$ contribution. This suggests that the charge ordering is not completely destroyed by a "melting" magnetic field. In addition, the specific heat of the Pr$_{1-x}$Ca$_{x}$MnO$_{3}$ compounds exhibit a large contribution linear in temperature ($\gamma T$) originating from magnetic and charge disorder., Comment: submitted to PRL 5 pages, 3 figures included

We report an inelastic light scattering study of the effects of charge-ordering on the spin-, charge-, and lattice-dynamics in ${\rm Bi}_{1-x}{\rm Ca}_{x}{\rm MnO}_3$ $(x>0.5)$. We find that charge-ordering results in anomalous phonon behavior, such as the appearance of `activated' modes. More significantly, however, the transition to the CO phase results in the appearance of a quasielastic scattering response with the symmetry of the spin chirality operator ($T_{1g}$); this scattering response is thus indicative of magnetic or chiral spin fluctuations in the AFM charge-ordered phase., Comment: to be published in Phys. Rev. Lett. (Oct. 2000)

We measured thermal conductivity, k, thermoelectric power, S, and dc electric conductivity, sigma, of La_{5/8-x}Pr_{x}Ca_{3/8}MnO_{3}, showing an intricate interplay between metallic ferromagnetism (FM) and charge ordering (CO) instability. The change of k, S and sigma with temperature (T) and x agrees well with the effective medium theories for binary metal-insulator mixtures. This agreement clearly demonstrates that with the variation of T as well as x, the relative volumes of FM and CO phases drastically change and percolative metal-insulator transition occurs in the mixture of FM and CO domains., Comment: 8 pages, 4 eps figures included, to appear in Phys. Rev. Lett

We report polarized and unpolarized neutron diffraction measurements on La2-xSrxNiO4 (x=0.275 and 1/3). The data for the spin ordered states are consistent with a collinear spin model in which spins with S=1 are in the NiO2 plane and are uniformly rotated from the charge and spin stripe direction. The deviation angle is larger for x=1/3 than for x=0.275. Furthermore, for x=1/3, the ordered spins reorient below 50 K, which suggests a further lock-in of the doped holes and their magnetic interactions with the S=1 spins., Comment: 4 pages, 5 figures, submitted to phys. rev. lett

We observed a dramatic peak in the 1/f noise at the metal-insulator transition (MIT) in low-Tc manganites. This many-orders-of-magnitude noise enhancement is observed for both polycrystalline and single-crystal samples of La{5/8-y}Pr{y}Ca{3/8}MnO{3} (y=0.35-0.4) in zero magnetic field and Pr{1-x}Ca{x}MnO{3} (x=0.35-0.5) in magnetic field. This observation strongly suggests that the microscopic phase separation in the low-Tc manganites causes formation of a percolation network, and that the observed MIT is a percolation threshold., Comment: 10 pages, 5 figures

We discovered an unprecedented magnitude of the 1/f noise near the Curie temperature Tc in low-Tc manganites. The scaling behavior of the 1/f noise and the resistance provides strong evidence of the percolation nature of the ferromagnetic transition in the polycrystalline samples. The step-like changes of the resistance with temperature, observed for single crystals, suggest that the size of the ferromagnetic domains depends on the size of crystallites., Comment: 4 pages, 3 figures

We report new zero-field muon spin relaxation and neutron spin echo measurements in ferromagnetic (FM) (La,Ca)MnO3 which taken together suggest two spatially separated regions in close proximity possessing very different Mn-ion spin dynamics. One region corresponds to an extended cluster which displays 'critical slowing down' near Tc and an increasing volume fraction below Tc. The second region possesses more slowly fluctuating spins and a decreasing volume fraction below Tc. These data are discussed in terms of the growth of small polarons into overlapping regions of correlated spins below Tc, resulting in a microscopically inhomogeneous FM transition., Comment: 10 pages, 3 figures

Using inelastic magnetic neutron scattering we have discovered a localized spin resonance at 4.5 meV in the ordered phase of the geometrically frustrated cubic antiferromagnet $\rm ZnCr_2O_4$. The resonance develops abruptly from quantum critical fluctuations upon cooling through a first order transition to a co-planar antiferromagnet at $T_c=12.5(5)$ K. We argue that this transition is a three dimensional analogue of the spin-Peierls transition., Comment: 4 figures, revised and accepted in Phys. Rev. Lett

Inelastic neutron scattering was used to systematically investigate the spin-wave excitations (magnons) in ferromagnetic manganese perovskites. In spite of the large differences in the Curie temperatures ($T_C$s) of different manganites, their low-temperature spin waves were found to have very similar dispersions with the zone boundary magnon softening. From the wavevector dependence of the magnon lifetime effects and its correlation with the dispersions of the optical phonon modes, we argue that a strong magneto-elastic coupling is responsible for the observed low temperature anomalous spin dynamical behavior of the manganites., Comment: 11 pages, 4 figures

We investigated temperature dependent optical conductivity spectra of Nd0.7Sr0.3MnO3 single crystal. We found that polishing and surface scattering effects on the Nd0.7Sr0.3MnO3 crystal surfaces could significantly distort the optical responses, especially in the mid-infrared region. However, oxygen annealing and gold normalization processes seemed to remedy the distortions. We found that the spectral weight of Nd0.7Sr0.3MnO3 in the metallic state might be composed of a Drude carrier term and a strong incoherent mid-infrared absorptions. The temperature dependence of the spectral weight suggests that the electron-lattice coupling should be important in optical properties of colossal magnetoresistance manganites., Comment: 18 pages, 8 figures, submitted to PRB

The crystallographic (charge-ordered) and magnetic superstructures of La0.333Ca0.667MnO3 were studied by high-resolution synchrotron x-ray and neutron powder diffraction. In the antiferromagnetic structure, which was refined using a non-collinear model, the a lattice parameter is tripled and the c lattice parameter is doubled with respect to the average crystallographic unit cell (Pnma setting). The crystallographic structure below the charge-ordering temperature (TCO ~ 260 K) is characterized by ordering of the dz2 orbitals of the Jahn-Teller-distorted Mn3+O6 octahedra in the orthorhombic ac plane, and the appearance of superlattice peaks in the x-ray patterns corresponding to a tripling of the a axis lattice parameter. The intensities of the superlattice peaks can be accounted for satisfactorily in terms of ordering of the Mn3+ cations in sites as far apart as possible in the ac plane ("Wigner crystal" model) and transverse displacements of the Mn4+O6 octahedra in the c direction. These results are not consistent with a recently proposed model based on transmission electron microscopy (TEM) data in which the Mn3+O6 octahedra are arranged in a stripe pattern ("stripe" model). In particular, the large longitudinal stripe modulation revealed by TEM is not observed, suggesting that the TEM data may not be representative of the bulk sample. Within the framework of the "Wigner crystal" model, the magnetic structure at 1.5 K and the charge-ordered superstructure at 160 K and 1.5 K were refined from the neutron data., Comment: 22 pages, 9 figures, submitted to Physical Review B

We reported two-magnon Raman scattering from La_{1.9}Sr_{0.1}CuO_4, which has a suppressed Tc=12 K, as the temperature is lowered below 37 K and an ordered spin phase is formed. The two-magnon Raman intensity increases with decreasing temperature. The magnetic scattering in La_{1.9}Sr_{0.1}CuO_4 is totally different from that reported in the parent compound La_2CuO_4. We analyze the line shape of the two-magnon scattering within the traditional Loudon-Fleury theory and find the superexchange constant J=1052 cm^{-1}. The calculation of the frequency moment suggests that the quantum fluctuations are very weak in the system. The room temperature Raman scattering from La_2CuO_4 is also measured. Strong features appear in the one-phonon spectrum at the frequencies of the longitudinal optical (LO) infrared modes which we suggest become Raman active through a Fr\"{o}hlich-interaction., Comment: 6 pages, 4 eps figures, submitted to PRB

The infrared absorption of charge density waves coupled to a magnetic background is first observed in two manganites La{1-x}Ca{x}MnO{3} with x = 0.5 and x = 0.67. In both cases a BCS-like gap 2 Delta (T), which for x=0.5 follows the hysteretic ferro-antiferromagnetic transition, fully opens at a finite T{0} < T{Neel}, with 2 Delta(T{0})/kT{c} close to 5. These results may also explain the unusual coexistence of charge ordering and ferromagnetism in La{0.5}Ca{0.5}MnO{3}., Comment: File revtex + 3 figs. in epsf. To appear on Phys. Rev. Lett

We report resonant inelastic x-ray measurements of insulating La$_2$CuO$_4$ and Sr$_2$CuO$_2$Cl$_2$ taken with the incident energy tuned near the Cu K absorption edge. We show that the spectra are well described in a shakeup picture in 3rd order perturbation theory which exhibits both incoming and outgoing resonances, and demonstrate how to extract a spectral function from the raw data. We conclude by showing {\bf q}-dependent measurements of the charge transfer gap., Comment: minor notational changes, discussion of anderson impurity model fixed, references added; accepted by PRL

The infrared reflectivity of a $\rm La_{0.67}Ca_{0.33}MnO_3$ single crystal is studied over a broad range of temperatures (78-340 K), magnetic fields (0-16 T), and wavenumbers (20-9000 cm$^{-1}$). The optical conductivity gradually changes from a Drude-like behavior to a broad peak feature near 5000 cm$^{-1}$ in the ferromagnetic state below the Curie temperature $T_C=307 K$. Various features of the optical conductivity bear striking resemblance to recent theoretical predictions based on the interplay between the double exchange interaction and the Jahn-Teller electron-phonon coupling. A large optical magnetoconductivity is observed near $T_C$., Comment: 4 pages, 4 figures, Latex, PostScript; The 7th Joint MMM-Intermag Conference,San Francisco, January 6-9, 1998; The Int. Conf. on Strongly Correlated Electron Systems, Paris, July 15-18,1998

^139La nuclear magnetic resonance studies reveal markedly different magnetic properties of the two sites created by the charged domain wall formation in La_(5/3)Sr_(1/3)NiO_4. NMR is slow compared to neutron scattering; we observe a 30 K suppression in magnetic ordering temperature indicating glassy behavior. Applied magnetic field reorients the in-plane ordered moments with respect to the lattice, but the relative orientation of the spins amongst themselves is stiff and broadly distributed., Comment: 4 pages, LaTeX, 3 .eps figures; version accepted for publication in Physical Review Letters; minor revisions

For La_(1 2/3)Sr_(1/3)NiO_4 -- a commensurately doped Mott-Hubbard system -- charge- and spin-ordering in a stripe phase has been investigated by phononic and magnetic Raman scattering. Formation of a superlattice and an opening of a pseudo-gap in the electron-hole excitation spectra as well as two types of double-spin excitations -- within the antiferromagnetic domain and across the domain wall -- are observed below the charge-ordering transition. The temperature dependence suggests that the spin ordering is driven by charge ordering and that fluctuating stripes persist above the ordering transition., Comment: 5 pages, 4 EPS figures; to appear in Phys. Rev. Lett

We review recent measurements of the high-frequency dynamic magnetic susceptibility in the high-$T_c$ superconducting systems La$_{2-x}$Sr$_{x}$CuO$_4$ and YBa$_2$Cu$_3$O$_{6+x}$. Experiments were performed using the chopper spectrometers HET and MARI at the ISIS spallation source. We have placed our measurements on an absolute intensity scale, this allows systematic trends to be seen and comparisons with theory to be made. We find that the insulating S=1/2 antiferromagnetic parent compounds show a dramatic renormalization of the spin wave intensity. The effect of doping on the response is to cause broadenings in wave vector and large redistributions of spectral weight in the frequency spectrum., Comment: 9 pages, 7 figures (To be published in Physica B). Typos corrected

Elastic and inelastic neutron scattering was used to study two ferromagnetic manganites A$_{1-x}$B$_{x}$MnO$_3$ (x $\approx$ 0.3) with $T_c$=197.9 K and 300.9 K. The spin dynamical behavior of these is similar at low temperatures, but drastically different at temperatures around $T_c$. While the formation of spin clusters of size ($\sim20$ \AA) dominates the spin dynamics of the 197.9 K sample close to $T_c$, the paramagnetic to ferromagnetic transition for the 300.9 K sample is more conventional. These results, combined with seemingly inconsistent earlier reports, reveal clear systematics in the spin dynamics of the manganites., Comment: zipped RevTeX file (7 pages) and 4 postscript figures

We have studied the spin dynamics in Pr$_{0.63}$Sr$_{0.37}$MnO$_3$ above and below the Curie temperature $T_C=301$ K. Three distinct new features have been observed: a softening of the magnon dispersion at the zone boundary for $T

Commensurability effects for nickelates have been studied by the first neutron experiments on La5/3Sr1/3NiO4. Upon cooling, this system undergoes three successive phase transitions associated with quasi-two-dimensional (2D) commensurate charge and spin stripe ordering in the NiO$_2$ planes. The two lower temperature phases (denoted as phase II and III) are stripe lattice states with quasi-long-range in-plane charge correlation. When the lattice of 2D charge stripes melts, it goes through an intermediate glass state (phase I) before becoming a disordered liquid state. This glass state shows short-range charge order without spin order, and may be called a "stripe glass" which resembles the hexatic/nematic state in 2D melting., Comment: 10 pages, RevTex, 4 figures available on request to shl@rrdjazz.nist.gov

By comparing the optical conductivities of La_{1.67}Sr_{0.33}NiO_{4} (LSNO), Sr_{1.5}La_{0.5}MnO_4 (SLMO), Nd_2CuO_{4-y} (NCO), and Nd_{1.96}Ce_{0.04}CuO_{4} (NCCO), we have identified a peculiar behavior of polarons in this cuprate family. While in LSNO and SLMO small polarons localize into ordered structures below a transition temperature, in those cuprates the polarons appear to be large, and at low T their binding energy decreases. This reflects into an increase of the polaron radius, which may trigger coherent transport., Comment: File latex, 15 p. incl. 4 Figs. epsf, to appear on the Journal of Superconductivity - Proc. "Stripes 1996" - Roma Dec 1996

Inelastic neutron scattering performed at a spallation source is used to make absolute measurements of the dynamic susceptibility of insulating La2CuO4 and superconducting La2-xSrxCuO4 over the energy range 15

Single crystalline (Bi,Ca)MnO3 (74< %Ca <82) were studied with neutron scattering, electron diffraction and bulk magnetic measurement. We discovered dynamic ferromagnetic spin correlations at high temperatures, which are replaced by antiferromagnetic spin fluctuations at a concomitant charge ordering and structural transition. Our results indicate that thermal-activated hopping of the Jahn-Teller active e_g electrons in these insulating materials, nevertheless, induce ferromagnetic interaction through double-exchange mechanism. It is the ordering of these charges competing with the double-exchange ferromagnetic metallic state., Comment: 11 pages, 3 figures, Revtex

We report on the infrared absorption spectra of three oxides where charged superlattices have been recently observed in diffraction experiments. In La$_{1.67}$Sr$_{0.33}$NiO$_4$, polaron localization is found to suppress the low-energy conductivity through the opening of a gap and to split the $E_{2u}$-$A_{2u}$ vibrational manifold of the oxygen octahedra. Similar effects are detected in Sr$_{1.5}$La$_{0.5}$MnO$_4$ and in La$_2$NiO$_{4+y}$, with peculiar differences related to the type of charge ordering., Comment: File latex, 11 p. + 3 Figures, to appear on Phys. Rev. B (Rapid Commun.), 1 Oct. 1996. The figures will be faxed upon request. E-mail:p.calvani@caspur.it Fax: +39-6-4463158