High resolution topographic images obtained by scanning tunneling microscope in the insulating state of Pr0.68Pb0.32MnO3 single crystals showed regular stripe-like or zigzag patterns on a width scale of 0.4 - 0.5 nm confirming a high temperature polaronic state. Spectroscopic studies revealed inhomogeneous maps of zero-bias conductance with small patches of metallic clusters on length scale of 2 - 3 nm only within a narrow temperature range close to the metal-insulator transition. The results give a direct observation of polarons in the insulating state, phase separation of nanometer-scale metallic clusters in the paramagnetic metallic state, and a homogeneous ferromagnetic state.
Direct electronic structure measurements of a variety of the colossal magnetoresistive oxides show the presence of a pseudogap at the Fermi energy E_F which drastically suppresses the electron spectral function at E_F. The pseudogap is a strong function of the layer number of the samples (sample dimensionality) and is strongly temperature dependent, with the changes beginning at the ferromagnetic transition temperature T_c. These trends are consistent with the major transport trends of the CMR oxides, implying a direct relationship between the pseudogap and transport, including the "colossal" …
The temperature dependence of the electronic and atomic structure of the colossal magnetoresistive oxides $La_{1-x}Sr_{x}MnO_{3}$ (x = 0.3, 0.4) has been studied using core and valence level photoemission, x-ray absorption and emission, and extended x-ray absorption fine structure spectroscopy. A dramatic and reversible change of the electronic structure is observed on crossing the Curie temperature, including charge localization and spin moment increase of Mn, together with Jahn-Teller distortions, both signatures of polaron formation. Our data are also consistent with a phase-separation scen…
It has been observed that the low temperature magnetoresistance behaviour in polycrystalline colossal magnetoresistive (CMR) manganites differ significantly from the single crystals. The polycrystalline samples show large magnetoresistance at temperatures much below the ferromagnetic transition temperature where the magnetoresistance of single crystals is very small. This has conventionally been attributed to spin polarised tunnelling at the grain boundaries in polycrystalline samples. In this paper we show the existence of a variable range hopping behaviour in resistivity at low temperatures …
Neutron spectroscopy measurements reveal dynamic spin correlations throughout the Brillouin zone in the colossal magnetoresistive material La0.7Ca0.3MnO3 at 265 K (approximately 1.03 Tc). The long-wavelength behavior is consistent with spin diffusion, yet an additional and unexpected component of the scattering is also observed in low-energy constant-E measurements, which takes the form of ridges of strong quasielastic scattering running along (H 0 0) and equivalent directions. Well-defined Q-space correlations are observed in constant-E scans at energies up to at least 28 meV, suggesting robu…
We used high resolution angle-resolved photoemission spectroscopy to reveal the Fermi surface and key transport parameters of the metallic state of the layered Colossal Magnetoresistive (CMR) oxide La1.2Sr1.8Mn2O7. With these parameters the calculated in-plane conductivity is nearly one order of magnitude larger than the measured DC conductivity. This discrepancy can be accounted for by including the pseudogap which removes at least 90% of the spectral weight at the Fermi energy. Key to the pseudogap and many other properties are the parallel straight Fermi surface sections which are highly su…
Taking into account both the Jahn-Teller lattice distortion and the on-site electronic potential fluctuations in the orbital-degenerated double-exchange model, in which both the core-spin and the lattice distortion are treated classically, we investigate theoretically the metal-insulator transition (MIT) in manganites by considering the electronic localization effect. An inverse matrix method is developed for calculation in which we use the inverse of the transfer matrix to obtain the localization length. We find that within reasonable range of parameters, both the lattice effect and the poten…
We report the discovery of mesoscale regions with distinctive magnetic properties in epitaxial La$_{1-x}$Sr$_{x}$MnO$_{3}$ films which exhibit tunneling-like magnetoresistance across grain boundaries. By using temperature-dependent magnetic force microscopy we observe that the mesoscale regions are formed near the grain boundaries and have a different Curie temperature (up to 20 K {\it higher}) than the grain interiors. Our images provide direct evidence for previous speculations that the grain boundaries in thin films are not magnetically and electronically sharp interfaces. The size of the m…
In view of recent experiments, indicating the spatial coexistence of conducting and insulating regions in the ferromagnetic metallic phase of doped manganites, we propose a refined mixed-phase description. The model is based on the competition of a double-exchange driven metallic component and a polaronic insulating component, whose volume fractions and carrier concentrations are determined self-consistently by requiring equal pressure and chemical potential. The resulting phase diagram as well as the order of the phase transition are in very good agreement with measured data. In addition, mod…
The spin correlations of the bilayer manganite $La_{1.2}Sr_{1.8}Mn_{2}O_{7}$ have been studied using neutron scattering. On cooling within the paramagnetic state, we observe purely two-dimensional behavior with a crossover to three-dimensional scaling close to the ferromagnetic transition. Below $T_C$, an effective finite size behavior is observed. The quantitative agreement of these observations with the conventional quasi two-dimensional Kosterlitz-Thouless model indicates that the phase transition is driven by the growth of magnetic correlations, which are only weakly coupled to polarons ab…
Neutron spectroscopy is used to probe transverse acoustic phonons near the (2, 2, 0) Bragg position in colossal magnetoresistive La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3. Upon warming to temperatures near Tc = 257 K the phonon peaks in La0.7Ca0.3MnO3 soften and damp significantly with the phonon half width at half maximum approaching 2.5 meV for phonons at a reduced wave vector of q = (0.2, 0.2, 0). Concurrently a quasielastic component develops that dominates the spectrum near the polaron position at high temperatures. This quasielastic scattering is ~5 times more intense near Tc than in La0.7Sr0.3M…
We report a significant decrease in the low-temperature resistance induced by the application of an electric current on the $ab$-plane in the paramagnetic insulating (PMI) state of (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$. A colossal electroresistance effect attaining -95% is observed at lower temperatures. A colossal magnetoresistive step appears near 5T at low temperatures below 10K, accompanied by an ultrasharp width of the insulator-metal transition. Injection of higher currents to the crystal causes a disappearance of the steplike transition. These findings have a close rel…
We have studied the bandwidth-temperature-magnetic field phase diagram of RE(0.55)Sr(0.45)MnO(3) colossal magnetoresistance manganites with ferromagnetic metallic (FM) ground state. The bandwidth (or equivalently the double exchange interaction) was controlled both via chemical substitution and hydrostatic pressure with a focus on the vicinity of the critical pressure p* where the character of the zero-field FM transition changes from first to second order. Below p* the first-order FM transition extends up to a critical magnetic field, H_cr. It is suppressed by pressure and approaches zero on …
The "colossal magnetoresistive" (CMR) manganites are highly correlated systems with a strong coupling between spin, charge, orbital, and lattice degrees of freedom, which leads to complex phase diagrams and to the coexistence of various forms of ordering. For example, nanoscale charge/orbital ordering (CO) fluctuations appear to cooperate with Jahn-Teller (JT) distortions of the MnO$_{6} octahedra in CMR manganites and compete with the electron itinerancy favored by double exchange. However, access to the ordered dynamical state has been challenging, mostly due to intrinsic experimental diffic…
A new minimal model is constructed for the doped manganese oxides which exhibit colossal magnetoresistance (CMR), involving broad spin-majority conduction band as well as nearly localised spin-minority electron states. A simple mean field analysis yields a temperature-dependent hybridised band structure with suppressed carrier weight at the Fermi level. Spin stiffness is complex, indicating unusually strong spin wave damping. Experimental and theoretical investigations are needed to further verify the relevance of the proposed model.
Using a two-band double-exchange model with Jahn-Teller lattice distortions and super-exchange interactions, supplemented by quenched disorder, at electron density $n=0.65$, we explicitly demonstrate the coexistence of the $n$ = 1/2-type ($π, π$) charge-ordered and the ferromagnetic nanoclusters above the ferromagnetic transition temperature $T_{\rm c}$ in colossal magnetoresistive (CMR) manganites. The resistivity increases due to the enhancement of the volume fraction of the charge-ordered and the ferromagnetic nanoclusters with decreasing the temperature down to $T_{\rm c}$. The ferromagnet…
We address the issue of massive phase separation (PS) in manganite family of doped Mott insulators through ac conductivity measurements on La$_{0.625-y}$Pr$_{y}$Ca$_{0.375}$MnO$_{3}$ (0.375 $\leq$ y $\leq$ 0.275), and establish applicability of the scaling theory of percolation in the critical regime of the PS. Measurements of dc resistivity, magnetization (M(T)) and electron diffraction show incomplete growth of a ferromagnetic (FM) metallic component on cooling the high temperature charge ordered (CO) phase well below Curie temperature. The impedance $\mid$Z(T,f)$\mid$ measured over a freque…
Manganites are famous mostly for the colossal magnetoresistive effect, which involves the phase separation between ferromagnetic phase and charge-ordered CE-type antiferromagnetic phases. Furthermore, manganites contain some typical magnetic ferroelectrics, e.g. E-type antiferromagnetic $o$-HoMnO$_3$. Here we re-examined these zigzag-winding antiferromagnetic phases (CE-type and E-type antiferromagnets) from the topological perspective. Our theoretical analysis proved that the E-type phase is a weak topological insulator belonging to the $\mathbb{Z}$ class. In momentum space, we classify the s…
While the spin dynamics of La$_{0.7}$Ca$_{0.3}$MnO$_3$ in the ferromagnetic phase are known to be unconventional, previous measurements have yielded contradictory results regarding the damping of spin wave excitations. Neutron spectroscopy measurements on a sample with a transition temperature of $T_c$=257 K, higher than most single crystals, unambiguously reveal an anomalous increase in spin wave damping for excitations approaching the Brillouin zone boundary along the [$100$] direction that cannot be explained as an artifact due to a noninteracting phonon branch. Spin waves throughout the ($…
We present temperature-dependent scanning tunneling spectroscopy measurements on $La_{1-x}Ca_{x}MO_{3}$ ($x\sim0.33$) films with different degrees of biaxial strain. A depletion in normalized conductance around the Fermi level is observed both above and below the insulator-to-metal transition temperature $T_{MI}$, for weakly as well as highly-strained films. This pseudogap-like depletion globally narrows on cooling. The zero-bias conductance decreases on cooling in the insulating phase, reaches a minimum close to $T_{MI}$ and increases on cooling in the metallic phase, following the trend of m…
The temperature dependence of the O K-edge pre-edge structure in the x-ray absorption spectra of the perovskites La(1-x)A(x)MnO(3), (A = Ca, Sr; x = 0.3, 0.4) reveals a correlation between the disappearance of the splitting in the pre-edge region and the presence of Jahn-Teller distortions. The different magnitudes of the distortions for different compounds is proposed to explain some dissimilarity in the line shape of the spectra taken above the Curie temperature.
The origin of the effect of colossal magneto-resistance (CMR) remains still unexplained. In this work we revisit the spin dynamics of the pseudo-cubic La1-xSrxMnO3 along the Mn-O-Mn bond direction at four x doping values (x < 0.5) and various temperatures and report a new lattice dynamics study at x0=0.2, representative of the optimal doping for CMR. We propose an interpretation of the spin dynamics in terms of orbital polarons. This picture is supported by the observation of a discrete magnetic energy spectrum Enmag (q) characteristic of the internal excitations of "orbital polarons" defined …
Spatially resolved tunneling spectroscopic studies of colossal magnetoresistive (CMR) manganite $\rm La_{0.7}Ca_{0.3}MnO_3$ (LCMO) epitaxial films on $\rm (LaAlO_3)_{0.3}(Sr_2AlTaO_6)_{0.7}$ substrate are investigated as functions of temperature, magnetic field and spin polarization by means of scanning tunneling spectroscopy. Systematic surveys of the tunneling spectra taken with Pt/Ir tips reveal spatial variations on the length scale of a few hundred nanometers in the ferromagnetic state, which may be attributed to the intrinsic heterogeneity of the manganites due to their tendency towards …
Mixed-valence perovskite manganites (Re$_{1-x}$A$_{x}$MnO$_{3}$ where Re=rare earth, A=alkaline earth) provide a unique opportunity to study the relationships between the structure and the magnetotransport properties due to an interplay among charge carriers, magnetic coupling, orbital ordering and structural distortion. This makes these compounds very exciting from both the basic research and from the technological view point. As the technology pursued with these materials requires film growth, extensive studies have been made on materials synthesis, structural and physical characterization a…
A characteristic feature of the copper oxide high-temperature superconductors is the dichotomy between the electronic excitations along the nodal (diagonal) and antinodal (parallel to the Cu-O bonds) directions in momentum space, generally assumed to be linked to the "d-wave" symmetry of the superconducting state. Angle-resolved photoemission measurements in the superconducting state have revealed a quasiparticle spectrum with a d-wave gap structure that exhibits a maximum along the antinodal direction and vanishes along the nodal direction. Subsequent measurements have shown that, at low dopi…
We present angle-resolved photoemission studies of (La1-zPrz)2-2xSr1+2xMn2O7 with x=0.4 and z=0.1,0.2 and 0.4 along with density functional theory calculations and x-ray scattering data. Our results show that the bilayer splitting in the ferromagnetic metallic phase of these materials is small, if not completely absent. The charge carriers are therefore confined to a single MnO2-layer, which in turn results in a strongly nested Fermi surface. In addition to this, the spectral function also displays clear signatures of an electronic ordering instability well below the Fermi level. The increase …
By using laboratory x-ray photoemission spectroscopy (XPS) and hard x-ray photoemission spectroscopy (HX-PES) at a synchrotron facility, we report an empirical semi-quantitative relationship between the valence/core-level x-ray photoemission spectral weight and electrical conductivity in La_{1-x}Sr_{x}MnO_{3} as a function of x. In the Mn 2p_{3/2} HX-PES spectra, we observed the shoulder structure due to the Mn^{3+} well-screened state. However, the intensity at x=0.8 was too small to explain its higher electrical conductivity than x=0.0, which confirms our recent analysis on the Mn 2p_{3/2} X…
Neutron scattering measurements on a magnetoresistive manganite La$_{0.75}$(Ca$_{0.45}$Sr$_{0.55}$)$_{0.25}$MnO$_3$ show that uncorrelated dynamic polaronic lattice distortions are present in both the orthorhombic (O) and rhombohedral (R) paramagnetic phases. The uncorrelated distortions do not exhibit any significant anomaly at the O-to-R transition. Thus, both the paramagnetic phases are inhomogeneous on the nanometer scale, as confirmed further by strong damping of the acoustic phonons and by the anomalous Debye-Waller factors in these phases. In contrast, recent x-ray measurements and our …
Considerable progress has been recently made in the theoretical understanding of the colossal magnetoresistance (CMR) effect in manganites. The analysis of simple models with two competing states and a resistor network approximation to calculate conductances has confirmed that CMR effects can be theoretically reproduced using non-uniform clustered states. In this paper, the recently proposed Truncated Polynomial Expansion method (TPEM) for spin-fermion systems is tested using the double-exchange one-band, with finite Hund coupling $J_{\rm H}$, and two-band, with infinite $J_{\rm H}$, models. T…
The persistent proximity of insulating and metallic phases, a puzzling characterestic of manganites, is argued to arise from the self organization of the twofold degenerate e_g orbitals of Mn into localized Jahn-Teller(JT) polaronic levels and broad band states due to the large electron - JT phonon coupling present in them. We describe a new two band model with strong correlations and a dynamical mean-field theory calculation of equilibrium and transport properties. These explain the insulator metal transition and colossal magnetoresistance quantitatively, as well as other consequences of two …
Among colossal magnetoresistive manganites the prototypical ferromagnetic manganite La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ has a relatively small magnetoresistance, and has been long assumed to have only weak electron-lattice coupling. Here we report that La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ has strong electron-phonon coupling: Our neutron and x-ray scattering experiments show strong softening and broadening of transverse acoustic phonons on heating through the Curie temperature T$_C$ = 350 K. Simultaneously, we observe two phases where metallic resistivity and polarons coexist. The ferromagnetic polaronic me…
The large, so-called colossal, magnetoresistivity of doped manganese oxides based on LaMnO3 has attracted considerable attention, but only one unusual feature of the ferromagnetic transition in these compounds. We examine in this paper the progression of magnetic and thermodynamic behavior as the transition temperature is made to vary from 360 K to 218 K by changing the divalent dopant. Single crystals of La_{0.7}Sr_{0.3}MnO3, as is well known, show modest magnetoresistivity and conventional critical behavior. La$_{0.7}$Pb$_{0.3}$ Mn$O_3$, and to an even greater extent, La$_{0.7}$Ca$_{0.3}$MnO…
We have studied the low-frequency dielectric properties of the phase-separated manganite Pr0.7Ca0.3MnO3 as a function of applied magnetic field in the low temperature phase-separated state. The dielectric constant is strongly field dependent and also depends on the magnetic field history of the sample. The dielectric behavior appears to be associated with the hopping of polaronic charge carriers, and we can derive the field dependent hopping energy barrier from the frequency dependence of the dielectric constant. This analysis allows us to associate the metal-insulator transition observed in t…
In the colossal magnetoresistance manganites polarons have been proposed as the charge carrier state which localizes across the metal-insulator transition. The character of the polarons is still under debate. We present an assessment of measurements which identify polarons in the metallic state of La{2/3}Sr{1/3}MnO{3} (LSMO) and La{2/3}Ca{1/3}MnO{3} (LCMO) thin films. We focus on optical spectroscopy in these films which displays a pronounced resonance in the mid-infrared. The temperature dependent resonance has been previously assigned to polaron excitations. These polaronic resonances are qu…
We explain the observed specific heat anomaly (and hence entropy change) in the colossal magnetoresistive manganite Sm0.55Sr0.45MnO3, by introducing phase separation into the current carrier density collapse theory via the notion of the ferromagnetic volume fraction. Within the same framework, we have also been able to explain the observed electrical resistivity of Sm0.55Sr0.45MnO3 by using appropriate expressions governing the scattering mechanisms far away from the transition. Fitting specific heat and resistivity results has allowed us to separate the hopping activation energy of polaronic …
The exchange interaction of polaronic carriers with localized spins leads to a ferromagnetic/paramagnetic transition in doped charge-transfer insulators with strong electron-phonon coupling. The relative strength of the exchange and electron-phonon interactions determines whether the transition is first or second order. A giant drop in the number of current carriers during the transition, which is a consequence of local bound pair (bipolaron) formation in the paramagnetic phase, is extremely sensitive to an external magnetic field. Below the critical temperature of the transition, $T_c$, the b…
Inspired by the strong experimental evidence for the coexistence of localized and itinerant charge carriers close to the metal-insulator transition in the ferromagnetic phase of colossal magnetoresistive manganese perovskites, for a theoretical description of the CMR transition we propose a two-phase scenario with percolative characteristics between equal-density polaron and Zener band-electron states. We find that the subtle balance between these two states with distinctly different electronic properties can be readily influenced by varying physical parameters, producing various ``colossal'' …
Charge carriers in low-doped semiconductors may distort the atomic lattice and trap themselves forming so-called small polarons. High carrier concentrations can lead to short range ordered polarons (large polarons) and even to long range charge and orbital order. Both systems should be insulating with a large electrical resistivity, which decreases with increasing temperature. However, photoemission measurements recently found a polaronic pseudogap, in a metallic phase of La2-2xSr1+2xMn2O7. This layered manganite is famous for colossal magnetoresistance (CMR) associated with a phase transition…
We study the spin polarised tunnelling mechanism through magnetisation and magnetoresistance in the granular polycrystalline colossal magnetoresistive manganite, La0.55Ho0.15Sr0.3MnO3. This system has a ferromagnetic transition temperature (Tc) of 255 K associated with a metal-insulator transition around the same temperature. We have investigated dependence of the magnetoresistance due to spin polarised tunnelling on temperature and reduced magnetisation of the sample. We discuss the significance of our results within the realm of a model recently proposed by us to explain the spin polarised t…
A novel ferromagnetic transition, accompanied by carrier density collapse, is found in doped charge-transfer insulators with strong electron-phonon coupling. The transition is driven by an exchange interaction of polaronic carriers with localized spins; the strength of the interaction determines whether the transition is first or second order. A giant drop in the number of current carriers during the transition, which is a consequence of bound pairs formation in the paramagnetic phase close to the transition, is extremely sensitive to an external magnetic field. This carrier density collapse d…
By performing angle-resolved photoemission spectroscopy of the bilayer colossal magnetoresistive (CMR) manganite, $La_{2-2x}Sr_{1+2x}Mn_{2}O_{7}$, we provide the complete mapping of the Fermi level spectral weight topology. Clear and unambiguous bilayer splitting of the in-plane 3d$_{x^2-y^2}$ band, mapped throughout the Brillouin zone, and the full mapping of the 3d$_{3z^2-r^2}$ band are reported. Peculiar doping and temperature dependencies of these bands imply that as transition from the ferromagnetic metallic phase approaches, either as a function of doping or temperature, coherence along …
We report on a X-ray absorption spectroscopic (XAS) study on a thin film of La0.7Ce0.3MnO3, a manganite which was previously only speculated to be an electron doped system. The measurements clearly show that the cerium is in the Ce(IV) valence state and that the manganese is present in a mixture of Mn2+ and Mn3+ valence states. These data unambiguously demonstrate that La0.7Ce0.3MnO3 is an electron doped colossal magnetoresistive manganite, a finding that may open up new opportunities both for device applications as well as for further basic research towards a better modelling of the colossal …
Angle-resolved photoemission spectroscopy data for the bilayer manganite La1.2Sr1.8Mn2O7 show that, upon lowering the temperature below the Curie point, a coherent polaronic metallic groundstate emerges very rapidly with well defined quasiparticles which track remarkably well the electrical conductivity, consistent with macroscopic transport properties. Our data suggest that the mechanism leading to the insulator-to-metal transition in La1.2Sr1.8Mn2O7 can be regarded as a polaron coherence condensation process acting in concert with the Double Exchange interaction.
A sharp distinction between magnetoresistance (MR) behavior for the magnetic fields applied perpendicular (H_{perp}) and parallel (H_{par}) to the film plane is found in colossal-magnetoresistance film La_{1-x}Ca_{x}MnO_{3} (x \approx 3). At increasing of H_{perp} the MR is first negative (at H_{perp} < 4 kOe), then positive (4 kOe < H_{perp} < 12 kOe), and then negative again (H_{perp} > 12 kOe). At increasing of H_{par} the MR is positive below H_{par} \simeq 6 kOe and negative above it. In both cases the magnetic field was perpendicular to the current. The anisotropic behavior of this kind …
The layered compound La2-2xSr1+2xMn2O7 (x=0.3) consists of bilayers of metallic MnO2 sheets separated by insulating material. The compound exhibits markedly anisotropic magnetoresistance at temperatures well below the three-dimensional magnetic ordering temperature TC=90 K in addition to colossal magnetoresistance around TC. We present neutron diffraction data which show that the magnetic structure of this material switches from antiferromagnetic stacking of the (ferromagnetically ordered) sheets in zero field to ferromagnetic stacking in a field of 1.5 Tesla. The data are the first to be coll…
We present a systematic optical study for a bandwidth-controlled series of nearly half doped colossal magnetoresistive manganites RE$_{0.55}$AE$_{0.45}$MnO$_3$ (RE and AE being rare earth and alkaline earth ions, respectively) under the presence of quenched disorder over a broad temperature region $T=10-800$ K. The ground state of the compounds ranges from the charge and orbital ordered insulator through the spin glass to the ferromagnetic metal. The enhanced phase fluctuations, namely the short-range charge and orbital correlations dominate the paramagnetic region of the phase diagram above a…
We have measured magnetic and transport response on the polycrystalline La$_{5/8-y}$Pr$_y$Ca$_{3/8}$MnO$_3$ ($y=0.30$, average grain size 2 microns) compound. In the temperature range where ferromagnetic metallic and insulating regions coexist we observed a persistent memory of low magnetic fields ($<$ 1 T) which is determined by the actual amount of the ferromagnetic phase. The possibility to manipulate this fraction with relatively small external perturbations is related to the phase separated nature of these manganese oxide based compounds. The colossal magnetoresistance figures obtained (a…
Electronic structure of the three-dimensional colossal magnetoresistive perovskite La1-xSrxMnO3 has been established using soft-X-ray ARPES with its intrinsically sharp definition of three-dimensional electron momentum. The experimental results show much weaker polaronic coupling compared to the bilayer manganites and are consistent with the GGA+U band structure. The experimental Fermi surface unveils the canonical topology of alternating three-dimensional electron spheres and hole cubes, with their shadow contours manifesting the rhombohedral lattice distortion. This picture has been confirme…
We have studied the [100]-[110] anisotropy of the Compton profile in the bilayer manganite. Quantitative agreement is found between theory and experiment with respect to the anisotropy in the two metallic phases (i.e. the low temperature ferromagnetic and the colossal magnetoresistant phase under a magnetic field of 7 T). Robust signatures of the metal-insulator transition are identified in the momentum density for the paramagnetic phase above the Curie temperature. We interpret our results as providing direct evidence for the transition from the metallic-like to the admixed ionic-covalent bon…
High critical-temperature superconductivity and large (colossal) magnetoresistances are two important electronic conducting phenomena found in transition metal oxides. High-Tc materials have applications such as superconducting magnets for MRI and NMR, and magnetoresistive materials may find use in magnetic sensors and spintronic devices. Here we report chemical doping studies of RuSr2(R2-xCex)Cu2O10-d ruthenocuprates which show that a single oxide system can be tuned between superconductivity at high hole dopings and varied magnetoresistive properties at low doping levels. A robust variation …
The exact nature of the low temperature electronic phase of the manganite materials family, and hence the origin of their colossal magnetoresistant (CMR) effect, is still under heavy debate. By combining new photoemission and tunneling data, we show that in La{2-2x}Sr{1+2x}Mn2O7 the polaronic degrees of freedom win out across the CMR region of the phase diagram. This means that the generic ground state is that of a system in which strong electron-lattice interactions result in vanishing coherent quasi-particle spectral weight at the Fermi level for all locations in k-space. The incoherence of …
We address the issue of endemic electronic inhomogeneities in manganites using extensive simulations on a new model with Coulomb interactions amongst two electronic fluids, one localized (polaronic), the other extended (band-like), and dopant ions. The long range Coulomb interactions frustrate phase separation induced by the strong on site repulsion between the fluids. A single quantum phase ensues which is intrinsically and strongly inhomogeneous at a nano-scale, but homogeneous on meso-scales, with many characteristics (including colossal responses)that agree with experiments. This, we argue…
A tight binding parametrization of local spin density functional band theory is combined with a dynamical mean field treatment of correlations to obtain a theory of the magnetic transition temperature, optical conductivity and T=0 spinwave stiffness of a minimal model for the pseudocubic metallic $CMR$ manganites such a $La_{1-X}Sr_{x}MnO_{3}$. The results indicate that previous estimates of $T_{c}$ obtained by one of us (Phys. Rev. \textbf{B61} 10738-49 (2000)) are in error, that in fact the materials are characterized by Hunds coupling $J\approx 1.5eV$, and that magnetic-order driven changes…
The two-dimensional, colossal magnetoresistive system GdI_2 develops an unusual metallic state below its ferromagnetic transition and becomes insulating at low temperatures. It is argued that this geometrically frustrated, correlated poor metal is a possible candidate for a ferromagnetic excitonic liquid. The renormalized Fermi surface supports a further breaking of symmetry to a charge ordered, excitonic solid ground state at lower temperatures via order by disorder mechanism. Several experimental predictions are made to investigate this unique orbitally correlated ground state.
Neutron spectroscopy measurements reveal short-range spin correlations near and above the ferromagnetic-paramagnetic phase transition in manganite materials of the form La$_{1-x}A_{x}$MnO$_{3}$, including samples with an insulating ground state as well as colossal magnetoresistive samples with a metallic ground state. Quasielastic magnetic scattering is revealed that forms clear ridges running along the [100]-type directions in momentum space. A simple model consisting of a conduction electron hopping between spin polarized Mn ions that becomes self-trapped after a few hops captures the essent…
The magnetoresistance (MR) in polycrystalline colossal magnetoresistive compounds follows a behavior different from single crystals below the ferromagnetic transition temperature. This difference is usually attributed to spin polarized tunneling at the grain boundaries of the polycrystalline sample. Here we derive a theoretical expression for the contribution of spin polarized tunneling to the magnetoresistance in granular ferromagnetic systems under the mean field approximation. We apply this model to our experimental data on the half metallic ferromagnet La0.7Sr0.3MnO3, and find that the the…
A ferromagnetic phase, characterized by electron carriers and a high temperature colossal magnetoresistance (HTCMR) dependent on the magnetic moment, and a semiconducting phase, characterized by hole carriers and a low temperature CMR (LTCMR), are observed in La$_{1-x}$Ca$_{x}$MnO$_3$ thin films by the van der Pauw method. The LTCMR is much more sensitive to the magnetic field than the HTCMR. In the ferromagnetic phase for films with anisotropic moments in two dimensions, a remnant resistivity of the order of 10$^{-8} \~Ωm$ is observed up to 100 K and increases exponentially with both a temper…
