Electronic aspects of the ferromagnetic transition in manganese perovskites

Atomic replacement effects on the band structure of doped perovskite thin films

The potential applications of perovskite manganite R_1-xA_xMnO₃ (R = rare earth element; A = Sr, Ca) thin films have been continuously explored due to their multi-functional properties. In particular, the optimally hole-doped La_0.67Ca_0.33MnO₃ thin film demonstrates a colossal magneto-resistance that is beneficial to the performance of spintronic devices. To understand the effect of R and A ions on the material properties, we systematically measure the resistivity, magnetization, and electronic energy states for three optimally hole-doped R_0.67A_0.33MnO₃ thin films with R = La, Sm and A = Sr, Ca. Various energy parameters are derived based on the X-ray absorption and X-ray photoelectron spectra, including the band gap, the charge frustration energy and the magnetic exchange energy. It is interesting to find that the replacement of La with Sm is more effective than that of Sr with Ca in terms of tuning the electrical property, the Curie temperature, and the band gap. The strain-induced reduction of the O 2p- Mn 3d hybridization and the interplay of R/A site disorder and strain effect are discussed. The results of this study provide useful information for the band design of perovskite oxide films.

Identifying the Chemical Origin of Oxygen Redox Activity in Li-Rich Anti-Fluorite Lithium Iron Oxide by Experimental and Theoretical X-ray Absorption Spectroscopy

Harnessing oxygen redox reactions is an intriguing route to increasing capacity in Li-ion batteries (LIBs). Despite numerous experimental and theoretical attempts to unravel the mechanism of oxygen redox behavior, the electronic origin of oxygen activities in energy storage of Li-rich LIB materials remains under intense debate. In this work, the onset of oxygen activity was examined using a Li-rich material that has been reported to exhibit oxygen redox, namely, Li₅FeO₄. By comparing experimental measurements and first-principles Bethe-Salpeter equation calculations of oxygen K-edge X-ray absorption spectra (XAS), it was found that experimentally-observed changes in XAS originate from the nonbonding oxygen states in cation-disordered delithiated Li₅FeO₄, and the spectral features of oxygen dimers were also determined. This combined experimental and theoretical study offers an effective approach to disentangle the intertwined signals in XAS and can be further utilized in broader contexts for characterizing other energy storage and conversion materials.

Electronic structure of Pr2MnNiO6 from x-ray photoemission, absorption and density functional theory

The electronic structure of double perovskite Pr₂MnNiO₆ was studied using core x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. The 2p x-ray absorption spectra show that Mn and Ni are in 4+  and 2+  states respectively. Based on charge transfer multiplet analysis of the 2p XPS spectra of both ions, we find charge transfer energies [Formula: see text] of 3.5 and 2.5 eV for Ni and Mn respectively. The ground state of Ni²⁺ and Mn⁴⁺ ions reveal a higher d electron count of 8.21 and 3.38 respectively as compared to the ionic values. The partial density of states clearly show a charge transfer character of the system for U  -  J [Formula: see text] 2 eV. The O 1s edge absorption spectra reveal a band gap of 0.9 eV, which is close to the value estimated from analysis of Ni and Mn 2p photoemission and absorption spectra. The combined analysis of nature of spectroscopic data and first principles calculations reveal that the material is a p  -  d type charge transfer insulator with an intermediate covalent character according to the Zannen-Sawatzy-Allen phase diagram.

Interface-Induced Enhancement of Ferromagnetism in Insulating LaMnO3 Ultrathin Films

Engineering ferromagnetism, by modulating its magnitude or anisotropy, is an important topic in the field of magnetism and spintronics. Among different types of magnetic materials, ferromagnetic insulators, in which magnetic moment unusually coexists with localized electrons, are of particular interest. Here, we report a remarkable interfacial enhancement of the ferromagnetism by adding one unit-cell LaAlO₃ adjacent to an insulating LaMnO₃ ultrathin film. The enhancement of ferromagnetism is explained in terms of charge transfer at the interface, as evidenced by X-ray absorption spectroscopy and ab initio calculations. This study demonstrates an effective and dramatic approach to modulate the functionality of ferromagnetic insulators, contributing to the arsenal of engineering techniques for future spintronics.

Manganite based hetero-junction structure of La0.7Sr(0.7-x)CaxMnO3 and CaMnO(3-δ) for cross-point arrays

Resistive random access memory and the corresponding cross-point array (CPA) structure have received a great deal of attention for high-density next generation non-volatile memory. However, the cross-talk issue of CPA structure by sneak current should be overcome to realize the highest density integration. To accomplish this, the sneak current can be minimized by high, nonlinear characteristic behaviors of resistive switching (RS). Therefore this study fabricated pnp bipolar hetero-junction structure using the perovskite manganite family, such as La0.7Sr(0.3-x)CaxMnO3 (LSCMO) and CaMnO(3-δ) (CMO), to obtain nonlinear RS behavior. The pnp structure not only shows nonlinear characteristics, but also a tunable characteristic with Ca substitution.

Fermi level shifting, charge transfer and induced magnetic coupling at La0.7Ca0.3MnO3/LaNiO3 interface

A large magnetic coupling has been observed at the La_0.7Ca_0.3MnO₃/LaNiO₃ (LCMO/LNO) interface. The x-ray photoelectron spectroscopy (XPS) study results show that Fermi level continuously shifted across the LCMO/LNO interface in the interface region. In addition, the charge transfer between Mn and Ni ions of the type Mn³⁺ − Ni³⁺ → Mn⁴⁺ − Ni²⁺ with the oxygen vacancies are observed in the interface region. The intrinsic interfacial charge transfer can give rise to itinerant electrons, which results in a “shoulder feature” observed at the low binding energy in the Mn 2p core level spectra. Meanwhile, the orbital reconstruction can be mapped according to the Fermi level position and the charge transfer mode. It can be considered that the ferromagnetic interaction between Ni²⁺ and Mn⁴⁺ gives rise to magnetic regions that pin the ferromagnetic LCMO and cause magnetic coupling at the LCMO/LNO interface.

Maximally localized Wannier functions in LaMnO3 within PBE + U, hybrid functionals and partially self-consistent GW: an efficient route to construct ab initio tight-binding parameters for eg perovskites

Using the newly developed VASP2WANNIER90 interface we have constructed maximally localized Wannier functions (MLWFs) for the e(g) states of the prototypical Jahn-Teller magnetic perovskite LaMnO(3) at different levels of approximation for the exchange-correlation kernel. These include conventional density functional theory (DFT) with and without the additional on-site Hubbard U term, hybrid DFT and partially self-consistent GW. By suitably mapping the MLWFs onto an effective e(g) tight-binding (TB) Hamiltonian we have computed a complete set of TB parameters which should serve as guidance for more elaborate treatments of correlation effects in effective Hamiltonian-based approaches. The method-dependent changes of the calculated TB parameters and their interplay with the electron-electron (el-el) interaction term are discussed and interpreted. We discuss two alternative model parameterizations: one in which the effects of the el-el interaction are implicitly incorporated in the otherwise 'noninteracting' TB parameters and a second where we include an explicit mean-field el-el interaction term in the TB Hamiltonian. Both models yield a set of tabulated TB parameters which provide the band dispersion in excellent agreement with the underlying ab initio and MLWF bands.

Origin of the pre-peak features in the oxygen K-edge x-ray absorption spectra of LaFeO₃ and LaMnO₃ studied by Ga substitution of the transition metal ion

We report on experimental oxygen K-edge x-ray absorption near edge structure (XANES) spectra of the LaFe(1 - x)Ga(x)O(3) and LaMn(1 - x)Ga(x)O(3) series. Transition metal substitution by the 3d full shell Ga atom is mainly reflected in a systematic decrease of the pre-edge structures in the XANES spectra of the two series. This result shows that the associated states originate from the hybridization of oxygen 2p and unoccupied Fe (or Mn) 3d states. In order to gain insight into the states associated with the pre-edge spectral features, we have performed ab initio theoretical calculations based on multiple scattering theory. Simulations with variable cluster size and composition around the absorber oxygen in the LaFeO(3) and LaMnO(3) crystal structures were carried out. We find that the low-energy pre-peak is reproduced once the absorbing oxygen and the two nearest neighbour Fe (or Mn) ions are considered in the cluster. Conversely, higher energy pre-peaks only arise when the full oxygen coordination geometry around the two metal sites is taken into account, implying that their energy distance is a reflection of the strength of the oxygen ligand field. Substitutions of the two nearest neighbours by Ga atoms in the cluster of calculation lead to changes in the theoretical spectra that reasonably agree with the evolution of the pre-peaks in the experimental XANES spectra of both the series.

Structural and thermal properties of LaMnO3 from neutron diffraction and first principles studies

Neutron diffraction experiments have been performed on powder samples of LaMnO(3) below and above the Jahn-Teller transition temperature of 750 K. Experimental investigations are assisted by density functional theory calculations. Theoretical studies are carried out for the orbitally ordered state of LaMnO(3) which allows one to compare the behavior of the orbitally ordered and disordered structures as a function of temperature. The temperature dependences of the structural parameters characterizing the Jahn-Teller distortions are reported and discussed. A gradual departure of the experimental data from theoretical predictions is observed above 650 K. In this range of temperatures, anions surrounding the Jahn-Teller active cations perform more isotropic thermal motion. The onset of structural phase transition induces a reduction of the crystal volume by about 0.4% which follows from the structural transformations yielding more regular oxygen octahedra formed above the phase transformation. It is found that above the Jahn-Teller transition the distortions of the MnO(6) octahedra are not completely removed. The non-vanishing distortions are accompanied by the lifted degeneracy of the Mn e(g) states. Weak residual distortions can be assigned to the short-range orbital order that persists within a local scale but it seems quenched on average giving rise to a disappearance of the long-range order coherency of the Jahn-Teller effect.

Hidden magnetic configuration in epitaxial La(1-x) Sr(x) MnO3 films

We present an unreported magnetic configuration in epitaxial La(1-x) Sr(x) MnO3 (x ∼ 0.3) (LSMO) films grown on strontium titanate (STO). X-ray magnetic circular dichroism indicates that the remanent magnetic state of thick LSMO films is opposite to the direction of the applied magnetic field. Spectroscopic and scattering measurements reveal that the average Mn valence varies from mixed Mn(3+)/Mn(4+) to an enriched Mn3+ region near the STO interface, resulting in a compressive lattice along the a, b axis and a possible electronic reconstruction in the Mn e(g) orbital (d(3)z(2)-r(2). This reconstruction may provide a mechanism for coupling the Mn3+ moments antiferromagnetically along the surface normal direction, and in turn may lead to the observed reversed magnetic configuration.

Jahn-Teller distortions and the magnetic order in the perovskite manganites

We introduce an effective model for e(g) electrons to describe three-dimensional perovskite (La(1 - x)Sr(x)MnO(3) and La(1 - x)Ca(x)MnO(3)) manganites and study the magnetic and orbital order on a 4 × 4 × 4 cluster using correlated wavefunctions. The model includes the kinetic energy, and on-site Coulomb interactions for e(g) electrons, antiferromagnetic superexchange interaction between S = 3/2 core spins, and the coupling between e(g) electrons and Jahn-Teller modes. The model reproduces the experimentally observed magnetic order: (i) an A-type antiferromagnetic phase in the undoped insulator LaMnO(3), with alternating e(g) orbitals and with small Jahn-Teller distortions, changing to a conducting phase at 32 GPa pressure, and (ii) ferromagnetic order in one-eighth-doped La(7/8)Sr(1/8)MnO(3) and in quarter-doped La(3/4)Sr(1/4)MnO(3) compounds. For half-doped La(1/2)Ca(1/2)MnO(3) one finds a competition between a ferromagnetic conductor and the CE insulating phase; the latter is stabilized by the Jahn-Teller coupling being two times larger than for the strontium-doped compound. Altogether, there is a subtle balance between all Hamiltonian parameters and the phase diagram is quite sensitive to the precise values they take.

Direct space structure solution from precession electron diffraction data: Resolving heavy and light scatterers in Pb(13)Mn(9)O(25)

The crystal structure of a novel compound Pb(13)Mn(9)O(25) has been determined through a direct space structure solution with a Monte-Carlo-based global optimization using precession electron diffraction data (a=14.177(3)A, c=3.9320(7)A, SG P4/m, R(F)=0.239) and compositional information obtained from energy dispersive X-ray analysis and electron energy loss spectroscopy. This allowed to obtain a reliable structural model even despite the simultaneous presence of both heavy (Pb) and light (O) scattering elements and to validate the accuracy of the electron diffraction-based structure refinement. This provides an important benchmark for further studies of complex structural problems with electron diffraction techniques. Pb(13)Mn(9)O(25) has an anion- and cation-deficient perovskite-based structure with the A-positions filled by the Pb atoms and 9/13 of the B positions filled by the Mn atoms in an ordered manner. MnO(6) octahedra and MnO(5) tetragonal pyramids form a network by sharing common corners. Tunnels are formed in the network due to an ordered arrangement of vacancies at the B-sublattice. These tunnels provide sufficient space for localization of the lone 6s(2) electron pairs of the Pb(2+) cations, suggested as the driving force for the structural difference between Pb(13)Mn(9)O(25) and the manganites of alkali-earth elements with similar compositions.

Origin of Jahn-Teller distortion and orbital order in LaMnO3

The origin of the cooperative Jahn-Teller distortion and orbital order in LaMnO3 is central to the physics of the manganites. The question is complicated by the simultaneous presence of tetragonal and GdFeO3-type distortions and the strong Hund's rule coupling between e{g} and t{2g} electrons. To clarify the situation we calculate the transition temperature for the Kugel-Khomskii superexchange mechanism by using the local density approximation+dynamical mean-field method, and disentangle the effects of superexchange from those of lattice distortions. We find that superexchange alone would yield T{KK} approximately 650 K. The tetragonal and GdFeO3-type distortions, however, reduce T{KK} to approximately 550 K. Thus electron-phonon coupling is essential to explain the persistence of local Jahn-Teller distortions to greater than or approximately 1150 K and to reproduce the occupied orbital deduced from neutron scattering.

Electronic correlations decimate the ferroelectric polarization of multiferroic homn2o5

We show that electronic correlations decimate the intrinsic ferroelectric polarization of multiferroic manganites RMn2O5, where R is a rare earth element. Such is manifest from ab initio band structure computations that account for the Coulomb interactions between the manganese 3d electrons--the root of magnetism in RMn2O5. Including these leads to an amplitude and direction of polarization of HoMn2O5 that agree with experiment. The decimation is caused by a near cancellation of the ionic polarization induced by the lattice and the electronic one due to valence charge redistributions.

Experimental and theoretical charge density distribution of the colossal magnetoresistive transition metal sulfide FeCr2S4

The total charge density distribution rho(r) of the colossal magnetoresistive transition metal sulfide FeCr(2)S(4) was evaluated through a multipole formalism from a set of structure factors obtained both experimentally, by means of single crystal high-quality x-ray diffraction data collected at T=23 K, and theoretically, with an extended-basis unrestricted Hartree-Fock periodic calculation on the experimental geometry. A full topological analysis, followed by the calculation of local energy density values and net atomic charges, was performed using the quantum theory of atoms in molecules. The experimental and theoretical results were compared. Good agreement was found for the topological properties of the system, as well as for the atomic net charges and the nature of the chemical bonds. An analysis of the electron density rho(r), its Laplacian nabla(2)[rho(r)], and the total energy density H(r) at the bond critical points was employed to classify all the interactions that resulted as predominantly closed shell (ionic) in nature. The topological indicators of the bonded interactions for Fe are distinct from those for Cr. The Fe-S bond distances were found to be 0.145 A shorter than the ideal values computed on the basis of Shannon's crystal radii, much shorter than the Cr-S distances with respect to their ideal Shannon lengths. Concomitantly, rho(r) and |H(r)| at the bond critical points are greater for Fe-S interactions, indicating that the local concentration of charge density in the internuclear region is larger for the tetrahedrally coordinated iron than for the octahedrally coordinated chromium. The isosurface in the real space for nabla(2)[rho(r)]=0 was plotted for both iron and chromium, pointing out the local zones of valence shell charge concentration and relating them to the partial d-orbital occupancy of the two transition metal atoms.

Bandstructure meets many-body theory: the LDA+DMFT method

Ab initio calculation of the electronic properties of materials is a major challenge for solid-state theory. Whereas 40 years' experience has proven density-functional theory (DFT) in a suitable form, e.g. local approximation (LDA), to give a satisfactory description when electronic correlations are weak, materials with strongly correlated electrons, say d- or f-electrons, remain a challenge. Such materials often exhibit 'colossal' responses to small changes of external parameters such as pressure, temperature, and magnetic field, and are therefore most interesting for technical applications. Encouraged by the success of dynamical mean-field theory (DMFT) in dealing with model Hamiltonians for strongly correlated electron systems, physicists from the bandstructure and many-body communities have joined forces and developed a combined LDA+DMFT method for treating materials with strongly correlated electrons ab initio. As a function of increasing Coulomb correlations, this new approach yields a weakly correlated metal, a strongly correlated metal, or a Mott insulator. In this paper, we introduce the LDA+DMFT method by means of an example, LaMnO(3). Results for this material, including the 'colossal' magnetoresistance of doped manganites, are presented. We also discuss the advantages and disadvantages of the LDA+DMFT approach.

Orbital-occupancy versus charge ordering and the strength of electron correlations in electron-doped CaMnO3

The structural, electronic, and magnetic properties of mixed-valence compounds are believed to be governed by strong electron correlations. Here we report benchmark density-functional calculations in the spin-polarized generalized-gradient approximation (GGA) for the ground-state properties of doped CaMnO(3). We find excellent agreement with all available data, while inclusion of strong correlations in the GGA+U scheme impairs this agreement. We demonstrate that formal oxidation states reflect only orbital occupancies, not charge transfer, and resolve outstanding controversies about charge ordering.

Interplay between the orbital and magnetic order in monolayer La(1-x)Sr(1+x)MnO(4) manganites

A two-dimensional model which describes e(g) electrons in a monolayer of an undoped and half doped manganite La(1-x)Sr(1+x)MnO(4) is studied using correlated wavefunctions. The effective Hamiltonian takes into account the kinetic energy, the crystal field splitting between x(2)-y(2) and 3z(2)-r(2) orbitals, and on-site Coulomb interactions for e(g) electrons. They interact with S = 3/2 spins due to t(2g) electrons, which are treated as frozen core spins. Furthermore, the model includes antiferromagnetic superexchange interaction between core spins, and the coupling between e(g) electrons and Jahn-Teller modes. The model reproduces the antiferromagnetic order in the undoped LaSrMnO(4) compound, with occupied 3z(2)-r(2) orbitals and elongated MnO(6) octahedra along the direction perpendicular to the Mn-O plane. In half doped La(0.5)Sr(1.5)MnO(4) manganite one finds robust chequerboard-like charge order using realistic parameters. However, the experimentally observed CE phase is more difficult to stabilize, and we discuss the necessary conditions to obtain it within the present model. Altogether, we conclude that the Jahn-Teller effect plays a crucial role in the entire regime of doping.

Pressure-induced metal-insulator transition in LaMnO3 is not of Mott-Hubbard type

Calculations employing the local density approximation combined with static and dynamical mean field theories (LDA+U and LDA+DMFT) indicate that the metal-insulator transition observed at 32 GPa in paramagnetic LaMnO3 at room temperature is not a Mott-Hubbard transition, but is caused by orbital splitting of the majority-spin eg bands. For LaMnO3 to be insulating at pressures below 32 GPa, both on-site Coulomb repulsion and Jahn-Teller distortion are needed.

Electronic Structure of A- and B-Site Doped Lanthanum Manganites: A Combined X-ray Spectroscopic Study

The electronic properties of a series of colossal magnetoresistance (CMR) compounds, namely LaMnO3, La(1-x)Ba(x)(MnO3 (0.2 < or = x < or = 0.55), La(0.76)Ba(0.24)Mn(0.84)Co(0.16)O3, and La(0.76)Ba(0.24)Mn(0.78)Ni(0.22)O3, have been investigated in a detailed spectroscopic study. A combination of X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy (XES), X-ray absorption spectroscopy (XAS), and resonant inelastic X-ray scattering (RIXS) was used to reveal a detailed picture of the electronic structure in the presence of Ba, Co, and Ni doping in different concentrations. The results are compared with available theory. The valence band of La(1-x)()Ba(x)MnO3 (0 < or = x < or = 0.55) is dominated by La 5p, Mn 3d, and O 2p states, and strong hybridization between Mn 3d and O 2p states is present over the whole range of Ba concentrations. Co-doping at the Mn site leads to an increased occupancy of the e(g) states near the Fermi energy and an increase in the XPS valence band intensity between 0.5 and 5 eV, whereas the Ni-doped sample shows a lower density of occupied states near the Fermi energy. The Ni d states are located in a band spanning the energy range of 1.5-5 eV. XAS spectra indicate that the hole doping leads to mixed Mn 3d-O 2p states. Furthermore, RIXS at the Mn L edge has been used to probe d-d transitions and charge-transfer excitations in La(1-x)Ba(x)MnO3.

Nature of the well screened state in hard X-ray Mn 2p core-level photoemission measurements of La1-xSrxMnO3 films

Using hard x-ray (HX; hnu=5.95 keV) synchrotron photoemission spectroscopy (PES), we study the intrinsic electronic structure of La(1-x)Sr(x)MnO(3) (LSMO) thin films. Comparison of Mn 2p core-levels with soft x-ray (SX; hnu approximately 1000 eV) PES shows a clear additional well-screened feature only in HX PES. Takeoff-angle dependent data indicate its bulk (> or =20 A) character. The doping and temperature dependence track the ferromagnetism and metallicity of the LSMO series. Cluster model calculations including charge transfer from doping-induced states show good agreement, confirming this picture of bulk properties reflected in Mn 2p core-levels using HX PES.

Ultrafast photoinduced reflectivity transients in doped manganite

The temperature and magnetic field dependence of ultrafast photoinduced spin and quasiparticle relaxation dynamics is reported in La(0.67)Ca(0.33)MnO(3) and LaMnO(3) single crystals and thin films. Both manganites reveal an unusually slow ( approximately 10 micros) carrier relaxation process attributed to the spin-lattice relaxation in localized states. The quasiparticle dynamics is governed by the temperature- and magnetic field-dependent pseudogap in La(0.67)Ca(0.33)MnO(3), and by the temperature-independent Jahn-Teller gap in LaMnO(3). The loss of spectral weight near the Fermi level in La(0.67)Ca(0.33)MnO(3) strongy affects the quasiparticle relaxation dynamics as temperature increases from below T(C). Our results show that the coupled dynamics of charge, spin and lattice is strongly correlated with the distinct gap structures in these manganites.

Close correlation between the magnetic moments, lattice distortions, and hybridization in LaMnO3 and La(1-x)Sr(x)MnO3+delta: doping-dependent magnetic circular X-ray dichroism study

The first observation of a magnetic circular x-ray dichroism (MCXD) at the Mn L2,3 core edges in antiferromagnetic LaMnO3 shows canted spin and orbital ( m(orb)) moments arising from lattice distortions. An L2,3-edge MCXD in ferromagnetic metals and insulators, La1-xSr(x)MnO3+delta, reveals that m(orb) of Mn strongly depends on x in the metallic regime but remains unchanged with the metal-to-insulator transition (x approximately 0.16). An O K-edge MCXD, which shows m(orb) of O caused by 2p-3d hybridization, is much larger in the ferromagnetic metal than insulator phases, sharply contrasting with m(orb) of Mn. Our findings indicate a close magnetism-lattice-hybridization coupling.

Franck-condon-broadened angle-resolved photoemission spectra predicted in LaMnO3

The sudden photohole of least energy created in the photoemission process is a vibrationally excited state of a small polaron. Therefore the photoemission spectrum in LaMnO3 is predicted to have multiple Franck-Condon vibrational sidebands. This generates an intrinsic line broadening approximately 0.5 eV. The photoemission spectral function has two peaks whose central energies disperse with bandwidth approximately 1.2 eV. Signatures of these phenomena are predicted to appear in angle-resolved photoemission spectra.

Electronic correlations in manganites

The influence of local electronic correlations on the properties of colossal magnetoresistive manganites is investigated. To this end, a ferromagnetic two-band Kondo lattice model is supplemented with the local Coulomb repulsion missing in this model and is analyzed within dynamical mean-field theory. Results for the spectral function, optical conductivity, and the paramagnetic-to-ferromagnetic phase transition show that electronic correlations have drastic effects and may explain some experimental observations.

Spatially Inhomogeneous Metal-Insulator Transition in Doped Manganites

Scanning tunneling spectroscopy was used to investigate single crystals and thin films of La(1-x)Ca(x)MnO(3) (with x of about 0.3), which exhibit colossal magnetoresistance. The different spectroscopic signatures of the insulating (paramagnetic) and metallic (ferromagnetic) phases enable their spatial extent to be imaged down to a lateral scale of the order of 10 nanometers. Above the bulk transition temperature T(c), the images show mostly insulating behavior. Below T(c), a phase separation is observed where inhomogeneous structures of metallic and more insulating areas coexist and are strongly field dependent in their size and structure. Insulating areas are found to persist far below T(c). These results suggest that the transition and the associated magnetoresistance behavior should be viewed as a percolation of metallic ferromagnetic domains.

Pseudogaps and extrinsic losses in photoemission experiments on poorly conducting solids

A photoelectron emitted from a conducting solid may suffer a substantial energy change through ohmic losses that can drastically alter the line shape on the millielectron volt scale that is now observable through improved resolution. Almost all of this energy loss occurs after the electron leaves the solid. These losses are expected to be important in isotropic materials with relatively low conductivity, such as certain colossal magnetoresistance manganates, and in very electrically anisotropic materials, such as one-dimensional conductors, and may also affect interpretation of photoemission in superconductors with high transition temperatures. The electric field of the photoelectron can penetrate the solid, and extrinsic losses of this type can mimic pseudogap effects and other peculiar features of photoemission in cubic manganates, as illustrated for La0.67Ca0.33MnO3.