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We present magnetization and muon spin resonance ( μ SR ) data on the single layered manganite LaSrMnO4, nominally containing no holes. Magnetization data corroborate antiferromagnetic (AF) spin order but it also suggest additional static moments with a magnetic anisotropy parallel to the AF easy axis, at low temperatures. Our μ SR data confirm the existence of these statics moments and imply that they are in a metastable, frozen state. Upon heating, the metastability disappears and for the additional moments a very slow dynamic is detected.

The magnetoresistance (MR) of La 0.7 Sr 0.3 MnO 3 and La 0.7 Ca 0.3 MnO 3 epitaxial films and of a La 0.75 Sr 0.25 MnO 3 single crystal were measured in pulsed magnetic fields up to 50 T over a wide temperature range. MR data have been analysed utilizing the recently suggested Brillouin scaling model. The model applies well in the paramagnetic state, where results prove the presence of superparamagnetic clusters. Near the ferromagnetic ordering temperature, systematic deviations in particular in larger fields indicate that the simple model fails there. The temperature-dependent cluster size parameter is similar for our samples, but differs essentially from that obtained earlier for half-doped manganites.

We present high-resolution measurements of the thermal expansion coefficient α along the a -axis and the c -axis of La 1 - x Sr 1 + x MnO 4 ( 0 ⩽ x ⩽ 0.5 ) . For x ⩽ 0.125 , where a long-range spin ordered ground state is present, the antiferromagnetic ordering transition causes pronounced anomalies of the lattice constants a and c . In addition, even at T ⪢ T N anomalous contributions to α a , c are present. The data suggest a change of the orbital occupation of the e g -levels, i.e. an increase of the in-plane d x 2 - y 2 -character of the electrons, driven by temperature. At higher doping 1 4 ⩽ x ⩽ 1 2 no anomalous behavior is observed.

We report results on the heat transport in the spin chain system Sr 1 - x Ca x CuO 2 . The thermal conductivity in this compound is strongly anisotropic indicating excess thermal conductivity in the chain direction. Upon Ca-doping the thermal conductivity due to acoustic phonons is effectively suppressed, allowing a better qualitative and quantitative analysis of the excess thermal conductivity.

In the phase diagram of La 1− x Sr x MnO 3 the compound with x =0.125, exhibits below T C =181 K, a ferromagnetic and metallic phase (FM). At T O′O″ =159 K, a magneto-structural transition occurs towards a ferromagnetic and insulating (FI) phase with amazing properties. New superstructure Bragg peaks appear. At q =(000.5), the spin wave energies split into two levels and, along [0 0 1], they lock successively on the values of phonon modes. This suggests a complex ground state for this (FI) phase with mixed magnetic and phononic excitations. These results are discussed in the frame of existing theories.

The growth of single crystals of the single layered manganites with compositions La1−xSr1+xMnO4 (0⩽x⩽0.6) by the floating zone technique is reported. Several neutron and X-ray scattering techniques were used to characterize the samples. Based on these data, the growth parameters were optimized with regard to structural (i.e. mosaic spread) and chemical homogeneity of the crystals. In addition, the neutron data show strong changes of the antiferromagnetic ordering as a function of the doping level x, i.e. upon increasing the mean Mn valence in La1−xSr1+xMnO4.

Zero-field muon–spin-rotation investigations have been performed on single crystals of La 1− x Sr 1+ x MnO 4 with 0⩽ x ⩽0.5. In the undoped sample spontaneous precession frequencies ω μ have been observed signaling a long-range-ordered magnetic ground state. In contrast at intermediate doping x =0.25 and 0.4 exhibit only short-range order. For the half-doped ( x =0.5) sample long-range magnetic order reappears.

The electrical transport properties of grain boundaries in epitaxial thin films of the perovskite manganites have been studied as a function of temperature and applied magnetic field. Below the Curie temperature T C an additional grain boundary resistance, highly nonlinear current–voltage curves, and a large magnetoresistive effect in the whole temperature regime below T C are found. The results can be explained consistently by the presence of a disordered, a few nm wide paramagnetic grain boundary layer that is depleted below T C due to an increase of the workfunction of the ferromagnetic grain material adjacent to this layer. The related band bending and space charge effects are important for the physics of grain boundaries in the manganites.

We present measurements of the thermal transport in single crystals of single-layered manganites La 1 - x Sr 1 + x MnO 4 with 0 ⩽ x ⩽ 0.5 . The data reveal a strong but non-monotonic suppression of the phonon thermal conductivity κ ph upon hole-doping. The suppression of κ ph originates from scattering of phonons by polaronic holes. The suppression is particularly strong when charge and orbital degrees of freedom are disordered and rather weak in the case of long-range charge and orbital ordering. Moreover, slight anomalies are found in the vicinity of antiferromagnetic phase transitions, probably due to scattering of phonons by magnetic fluctuations.

The compound La0.875Sr0.125MnO3 exhibits a ferromagnetic phase below TC=181 K. At TO’O”=159 K, a magneto-structural transition occurs with spectacular properties. Extra Bragg peaks appear and, at related q-values, the spin-wave energies split and lock successively at the values of the different phonon modes. Moreover, in the long-wavelength range, two close spin-wave branches can be measured. They are characterized by two small gaps and similar stiffness constants. They have two distinct temperature behaviors. The lowest-energy branch seems to disappear at TO’O”.

In the diagram of La 1− x Sr x MnO 3 the compound with x =0.125, exhibits, below T c =181 K, a ferromagnetic phase. At T O′O″ =159 K, a magneto-structural transition occurs with amazing properties. New superstructure Bragg peaks appear. At related q -values, the spin wave energies split and lock at the values of phonon modes. This suggests a complex ground state with mixed magneto-vibrational excitations.

We used ultrafast resonant soft x-ray diffraction to probe the picosecond dynamics of spin and orbital order in La0:5Sr1:5MnO4 after photoexcitation with a femtosecond pulse of 1.5 eV radiation. Complete melting of antiferromagnetic spin order is evidenced by the disappearance of a ð 1 ; 1 ; 1 Þ diffraction peak. On the other hand, the ð 1 ; 1 ; 0Þ diffraction peak, reflecting orbital order, is only partially reduced. We interpret the results as evidence of destabilization in the short-range exchange pattern with no significant relaxation of the long-range Jahn-Teller distortions. Cluster calculations are used to analyze different possible magnetically ordered states in the long-lived metastable phase. Nonthermal coupling between light and magnetism emerges as a primary aspect of photoinduced phase transitions in manganites.

The single-layered half-doped manganite La(0.5)Sr(1.5)MnO₄ (LSMO), was studied by means of the angle-resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), and resistivity measurements. STM revealed a smooth reconstruction-free surface; the density of states, extracted from photoemission and tunneling spectroscopy, is in agreement with transport measurements. The derived from ARPES Fermi surface (FS) nesting properties correspond to the known pattern of the charge-orbital ordering (COO), which implies that FS instability is related to the propensity to form a COO state in LSMO.

We present diffraction data on a single-layered manganite La(0.42)Sr(1.58)MnO4 with hole doping (x>0.5). Overdoped La(0.42)Sr(1.58)MnO4 exhibits a complex ordering of charges, orbitals and spins. Single crystal neutron diffraction experiments reveal three incommensurate and one commensurate order parameters to be tightly coupled. The position and the shape of the distinct superstructure scattering points to a stripe arrangement in which ferromagnetic zigzag chains are disrupted by additional Mn4+ stripes.

The electronic properties of the parent compound of the single layered manganites ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{1+x}{\mathrm{MnO}}_{4}$, namely ${\mathrm{LaSrMnO}}_{4}$ have been investigated in a detailed spectroscopic study. We apply different complementary x-ray spectroscopic techniques in the soft x-ray regime. X-ray photoelectron spectroscopy and normal-like x-ray emission spectroscopy were used to reveal a detailed picture of the total and partial densities of states in this compound. Furthermore we apply resonant x-ray emission spectroscopy to the $\mathrm{Mn}\phantom{\rule{0.3em}{0ex}}{L}_{2,3}$ edges. The spectra exhibit a rich multiplet structure. Resonant x-ray emission spectroscopy at the O $K$ edge is used to study the local partial densities of states of the in plane and out of plane oxygen atoms. The results are discussed along available band structure calculations as well as charge transfer multiplet calculations.

We report on the pressure effects on the orbital polaron lattice in the lightly doped manganites $\mathrm{La_{1-x}Sr_xMnO_{3}}$, with $x\sim 1/8$. The dependence of the orbital polaron lattice on $negative$ chemical pressure is studied by substituting Pr for La in $\mathrm{(La_{1-y}Pr_y)_{7/8}Sr_{1/8}MnO_{3}}$. In addition, we have studied its hydrostatic pressure dependence in $\mathrm{(La_{0.9}Pr_{0.1})_{7/8}Sr_{1/8}MnO_{3}}$. Our results strongly indicate that the hopping $t$ significantly contributes to the stabilization of the orbital polaron lattice and that the orbital polarons are ferromagnetic objects which get stabilized by local double exchange processes. The analysis of short range orbital correlations and the verification of the Grueneisen scaling by hard x-ray, specific heat and thermal expansion data reinforces our conclusions.

The crystal structure of ${\mathrm{Ca}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{RuO}}_{4}$ with $0.2\ensuremath{\le}x\ensuremath{\le}1.0$ has been studied by diffraction techniques and by high resolution capacitance dilatometry as a function of temperature and magnetic field. Upon cooling in zero magnetic field, the crystal structure and the octahedra shrink along the $c$ direction and elongate in the $a$ and $b$ planes, whereas the opposite occurs upon cooling at high field ($x=0.2$ and 0.5). These findings yield evidence for an orbital rearrangement driven by temperature and magnetic field, which accompanies the metamagnetic transition at low temperature. The temperature and magnetic-field dependencies are found to be governed by the same energy scale.

We report on Raman scattering measurements of single crystalline ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ ($x=0$, 0.06, 0.09, and 0.125), focusing on the high-frequency regime. We observe multiphonon scattering processes up to fourth order which show distinct features: (i) anomalies in peak energy and its relative intensity and (ii) a pronounced temperature, polarization, and doping dependence. These features suggest a mixed orbiton-phonon nature of the observed multiphonon Raman spectra.

We present an inelastic light scattering study of single crystalline ${({\mathrm{La}}_{1\ensuremath{-}y}{\mathrm{Pr}}_{y})}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ ($0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.14$, $y=0$ and $x=1∕8$, $0\ensuremath{\leqslant}y\ensuremath{\leqslant}0.5$). A giant softening up to $20\char21{}30\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ of the $\mathrm{Mn}\ensuremath{-}\mathrm{O}$ breathing mode has been observed only for the ferromagnetic insulating (FMI) samples $(0.11\ensuremath{\leqslant}x\ensuremath{\leqslant}0.14)$ upon cooling below the Curie temperature $({T}_{C})$. With increasing Pr-doping the giant softening is strongly suppressed. This is attributed to a coupling of the breathing mode to orbital polarons, which are present in the FMI state. Moreover, the softening that persists to lowest temperature without saturation suggests a continuous, temperature-dependent evolution of orbital polarons below ${T}_{C}$.

The crystal and magnetic structure of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{1+x}\mathrm{Mn}{\mathrm{O}}_{4}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.6)$ has been studied by diffraction techniques and high resolution capacitance dilatometry. There is no evidence for a structural phase transition related to octahedron tilting like those found in isostructural cuprates or nickelates, but there are significant structural changes induced by the variation of temperature and doping which we attribute to a rearrangement of the orbital occupation.

We present a resonant x-ray scattering study at the manganese K edge on ${\mathrm{La}}_{7/8}{\mathrm{Sr}}_{1/8}{\mathrm{MnO}}_{3}$ at temperatures between 10 K and 310 K along with thermal expansion, magnetization, and electrical resistivity measurements. At the Mn K edge the symmetry-forbidden (300) reflection is observed in the whole temperature range under investigation and its integrated intensity is found to be directly connected to structural distortions. In particular, the intensity of the (300) reflection increases strongly in the cooperative Jahn-Teller distorted phase between 150 K and 270 K. The increase of the resonant intensity in this temperature regime, the rotation of the beam polarization from $\ensuremath{\sigma}$ to $\ensuremath{\pi},$ the ${\mathrm{sin}}^{2}$-azimuthal dependence and the anomalies displayed by the lattice constants give firm experimental evidence for an orbital ordered state between 150 K and 270 K similar to that found in ${\mathrm{LaMnO}}_{3}.$ Upon cooling, the intensity of the (300) reflection collapses at the charge ordering temperature ${T}_{\mathrm{CO}}=150\mathrm{K}$ and the resonant (1.5,1.5,3) superstructure reflection emerges. This reflection also displays a resonant behavior at the Mn K edge and a pronounced azimuthal dependence. However, a resonant signal is observed in the $\ensuremath{\sigma}\ensuremath{\sigma}$ as well as in the $\ensuremath{\sigma}\ensuremath{\pi}$ channel. The results reveal that the metal-insulator transition of ${\mathrm{La}}_{7/8}{\mathrm{Sr}}_{1/8}{\mathrm{MnO}}_{3}$ is accompanied by an orbital reordering.

Neutron scattering has been used to study static and dynamic magnetic properties of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ with $x=0.09$ and $0.125.$ The $x=0.09$ compound appears as a limit case of an inhomogeneous canted antiferromagnetic (CAF) A-type structure. Related to the mean AF medium, anisotropic superexchange integrals are determined from the measurement of a high energy spin wave branch. Coexisting with this mean state, ferromagnetic (F) hole-rich platelets spread in the basal F plane, close to the limit of percolation. Related to these F inhomogeneities, an anisotropic magnetic coupling is determined from an anisotropic low energy spin wave branch. The $x=0.125$ compound exhibits, below ${T}_{C},$ a ferromagnetic and metallic phase (FM). At ${T}_{{O}^{\ensuremath{'}}{O}^{\ensuremath{''}}},$ a magnetostructural transition occurs leading to a F and insulating phase (FI) with unusual properties: in the whole q range, a splitting of the spin wave spectrum is observed. A large gap opens at (0,0,0.5). At this point and beyond, the energy of the spin waves locks on the phonon energy values. The temperature evolution of the splitted magnetic modes and of the phonons is presented, with marked signatures of both transitions. These results are discussed in the frame of existing theories.

We present hard x-ray scattering studies of the charge and orbital ordering in ${\mathrm{Nd}}_{1/2}{\mathrm{Sr}}_{1/2}{\mathrm{MnO}}_{3}$ together with magnetization and thermal expansion measurements as a function of temperature and magnetic field. Superstructure reflections corresponding to the orbital ordering below ${T}_{\mathrm{CO}}\ensuremath{\simeq}160\mathrm{K}$ are observed in the ferromagnetic metallic phase up to 200 K. The correlation length of the observed structural modulations is found to be highly anisotropic; i.e., the correlations exist predominantly in the $\mathrm{ab}$ planes. Moreover, the periodicity of the superstructure modulations is the same above and below ${T}_{\mathrm{CO}}.$ As a consequence, the modulations found in the ferromagnetic metallic phase are incommensurate to the average lattice structure. The results from hard x-ray scattering, thermal expansion, and magnetization studies can be consistently interpreted in terms of short-range orbital ordering in the ferromagnetic metallic phase of ${\mathrm{Nd}}_{1/2}{\mathrm{Sr}}_{1/2}{\mathrm{MnO}}_{3}$.

We present measurements of the magnetic susceptibility and of the thermal expansion of a LaCoO$_3$ single crystal. Both quantities show a strongly anomalous temperature dependence. Our data are consistently described in terms of a spin-state transition of the Co$^{3+}$ ions with increasing temperature from a low-spin ground state to an intermediate-spin state without (100K - 500K) and with (>500K) orbital degeneracy. We attribute the lack of orbital degeneracy up to 500K to (probably local) Jahn-Teller distortions of the CoO$_6$ octahedra. A strong reduction or disappearance of the Jahn-Teller distortions seems to arise from the insulator-to-metal transition around 500 K., an error in the scaling factor of Eq.(4) and consequently 2 values of table I have been corrected. The conclusions of the paper remain unchanged. See also: C. Zobel et al. Phys. Rev. B 71, 019902 (2005) and J. Baier et al. Phys. Rev. B 71, 014443 (2005)

We report a drastic change of the temperature dependence of the Mn–O stretching mode at the phase boundary between canted antiferromagnetic insulating (CAF) and ferromagnetic insulating (FI) of La 1− x Sr x MnO 3 . The observation of an unusual large softening by 18 cm −1 only in the FI phase is ascribed to a change of electronic state and orbital state through CAF/FI transition.

We present zero-field 55 Mn NMR spectra from two single crystals of La 1− x Sr 1+ x MnO 4 ( x =0 and 0.5). Both samples exhibit resonance peaks at much lower frequencies than in the pseudocubic manganites. The dependence of the peak positions on doping cannot be simply accounted for by charge transfer from Mn 3+ ions.

One challenge in condensed-matter physics is the experimental confirmation of a new kind of elementary excitation orbital waves, or orbitons, which are predicted to exist in an orbitally ordered state. Saitoh et al. have observed three peaks at 160, 144 and 126 meV in the Raman scattering of orbitally ordered lanthanum manganate (LaMnO(3)), and interpret these as evidence of orbitons. However, we find similar peaks in the optical conductivity, sigma(omega), of LaMnO(3) and point out that the direct observation of orbitons in sigma(omega) is prohibited by a selection rule. This suggests that the Raman peaks observed by Saitoh et al. arise from multiphonons, and that the existence of orbitons has yet to be experimentally confirmed.

We present a comprehensive investigation of the valence band of the single-layered manganite LaSrMnO4. X-ray photoelectron spectroscopy and x-ray emission spectroscopy were used to reveal a detailed picture of the total and partial densities of states in this compound. The results are discussed in the light of available theory.

The orbital excitations of a series of transition-metal compounds are studied by means of optical spectroscopy. Our aim was to identify signatures of collective orbital excitations by comparison with experimental and theoretical results for predominantly local crystal-field excitations. To this end, we have studied TiOCl, RTiO3 (R=La, Sm, Y), LaMnO3, Y2BaNiO5, CaCu2O3, and K4Cu4OCl10, ranging from early to late transition-metal ions, from t_2g to e_g systems, and including systems in which the exchange coupling is predominantly three-dimensional, one-dimensional or zero-dimensional. With the exception of LaMnO3, we find orbital excitations in all compounds. We discuss the competition between orbital fluctuations (for dominant exchange coupling) and crystal-field splitting (for dominant coupling to the lattice). Comparison of our experimental results with configuration-interaction cluster calculations in general yield good agreement, demonstrating that the coupling to the lattice is important for a quantitative description of the orbital excitations in these compounds. However, detailed theoretical predictions for the contribution of collective orbital modes to the optical conductivity (e.g., the line shape or the polarization dependence) are required to decide on a possible contribution of orbital fluctuations at low energies, in particular in case of the orbital excitations at about 0.25 eV in RTiO3. Further calculations are called for which take into account the exchange interactions between the orbitals and the coupling to the lattice on an equal footing., published version, discussion of TiOCl extended to low T, improved calculation of orbital excitation energies in TiOCl, figure 16 improved, references updated, 33 pages, 20 figures

At temperatures below the metal–insulator transition of La1−xSrxMnO3 with 0.1 < x < 0.15, a peculiar ferromagnetic and insulating phase is observed which has been intensively discussed over the last few years. We present a detailed investigation of this phase by means of resonant and high energy x-ray scattering along with measurements of the electrical resistivity, thermal expansion, magnetization, and specific heat. Interestingly, the data show that the metal–insulator transition of lightly doped manganites is accompanied by an orbital rearrangement. The microscopic information provided by the x-ray scattering studies together with the analysis of the macroscopic properties implies that the orbital reordering maximizes the gain of double exchange energy and, at the same time, induces an insulating behaviour. The relevance of the double-exchange mechanism for the stabilization of the ferromagnetic insulating phase is further substantiated by studies of : with increasing praseodymium content, the metal–insulator transition is dramatically suppressed which can naturally be explained by a reduction of the band width upon praseodymium doping.