arXiv subjects

J. F. Mitchell

Publications and source records attributed to J. F. Mitchell.

At least 91 records · Page 5Linked to original sources

arXiv2011

Response of acoustic phonons to charge and orbital order in the 50\% doped bilayer manganite LaSr$_2$Mn$_2$O$_7$

We report an inelastic neutron scattering study of acoustic phonons in the charge and orbitally ordered bilayer manganite LaSr$_2$Mn$_2$O$_7$. For excitation energies less than $15\,\rm{meV}$, we observe an abrupt increase (decrease) of the phonon energies (linewidths) of a transverse acoustic phonon branch at $\mathbf{q} = (h, h, 0)$, $h\le0.3$, upon entering the low temperature charge and orbital ordered state ($T_{COO} = 225\,\rm{K}$). This indicates a reduced electron-phonon coupling due to a decrease of electronic states at the Fermi level leading to a partial removal of the Fermi surface below $T_{COO}$ and provides direct experimental evidence for a link between electron-phonon coupling and charge order in manganites.

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arXiv2011

Isospin Dynamics in Sr2IrO4 : Forging Links to Cuprate Superconductivity

We used resonant inelastic x-ray scattering to reveal the nature of magnetic interactions in Sr2IrO4, a 5d transition-metal oxide with a spin-orbit entangled ground state and Jeff=1/2 magnetic momemts, referred to as 'isospins'. The magnon dispersion in Sr2IrO4 is well described by an antiferromagnetic Heisenberg model with isospin one-half moments on a square lattice, which renders the low-energy effective physics of Sr2IrO4 much akin to that in superconducting cuprates. This is further supported by the observation of exciton modes in Sr2IrO4 whose dispersion is strongly renormalized by magnons, which can be understood by analogy to the hole propagation in the background of antiferromagnetically ordered spins in the cuprates.

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arXiv2010

Surface Melting of Electronic Order

We report temperature-dependent surface x-ray scattering studies of the orbital ordered surface in La$_{0.5}$Sr$_{1.5}$MnO$_4$. We find that the interfacial width of the electronic order grows as the bulk ordering temperature is approached from below, though the bulk correlation length remains unchanged. Close to the transition, the surface is so rough that there is no well-defined electronic surface, despite the presence of bulk electronic order, that is the electronic surface has melted. Above the bulk transition, finite-sized isotropic fluctuations of orbital order are observed, with a correlation length equal to that of the electronic surfaces' in-plane correlation length at the transition temperature.

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arXiv2010

Spin Correlations in the Geometrically Frustrated RBaCo4O7 Antiferromagnets: Mean-Field Approach and Monte-Carlo Simulations

Spin correlations in the geometrically frustrated RBaCo4O7 compounds, usually described as an alternating stacking of Kagome and triangular layers on a hexagonal lattice, have been studied by mean-field approach and by Monte-Carlo simulations. The behavior of the system was modelled with an isotropic Heisenberg Hamiltonian as a function of the relevant parameter Jout/Jin, representing the ratio between exchange integrals inside the Kagome layers, Jin, and between Kagome and triangular layers, Jout. This ratio can be varied in real systems by appropriate chemical substitutions. At the mean-field level, long-range magnetic order with the wave vector at the K-point of symmetry (k=a*/3+b*/3) has been found for Jout/Jin>0.7. Below this value, the dominant Fourier modes are completely degenerate in the entire Brillouin zone. The Monte-Carlo simulations revealed that the long-range ordered configuration found in the mean-field calculations becomes the ground state of the system for Jout/Jin>1.5. Below this critical ratio, quasi one-dimensional magnetic ordering along the c-axis, involving spins of the triangular sublattice was observed. The correlations in the (ab) plane were found to have a short-range 120o character with correlation length dependent on Jout/Jin.

cond-mat.mtrl-sci↗

arXiv2010

Spin-ordering and magnetoelastic coupling in the extended Kagome system YBaCo4O7

Low temperature magnetic and structural behavior of the extended Kagome system YBaCo4O7 has been studied by single crystal neutron diffraction and high-resolution powder X-ray diffraction. Long-range magnetic ordering associated with a structural transition from orthorhombic Pbn2_1 to monoclinic P2_1 symmetry has been found at T_1 ~ 100 K. The interplay between the structural and magnetic degrees of freedom testifies that the degeneracy of the magnetic ground state, present in the orthorhombic phase, is lifted through a strong magnetoelastic coupling, as observed in other frustrated systems. At T_2 ~ 60 K, an additional magnetic transition is observed, though iso-symmetric. Models for the magnetic structures below T_1 and T_2 are presented, based on refinements using a large number of independent reflections. The results obtained are compared with previous single crystal and powder diffraction studies on this and related compositions.

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arXiv2010

d-d Excitations in Bilayer Manganites Probed by Resonant Inelastic X-ray Scattering

We report a high resolution resonant inelastic x-ray scattering investigation of the bilayer manganites La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ with $x = 0.36$ and $0.5$. The momentum dependence along the crystallographic $(110)$ direction for energy losses $1\,\rm{eV} \leq ΔE \leq 15\,\rm{eV}$ has been measured in detail with the data analysis focusing on the energy loss region $1\,\rm{eV} \leq ΔE \leq 5\,\rm{eV}$, which includes a strong peak located at $ΔE \approx 2\,\rm{eV}$. We observe a clear dispersion of up to $0.5\,\rm{eV}$ in the measured $\textbf{q}$ range, which is direct evidence of the non-local character of this excitation. Further, we found that the intensity in this low energy region strongly depends on both the reduced wave vector $\textbf{q} = (h, h, 0)$, $h = 0.1 - 0.5$, and temperature, i.e. different ordered phases. Results can be explained via an intersite $d-d$ charge transfer excitation, proposed for pseudo-cubic manganites, where the hopping rate is strongly increased (decreased) by ferromagnetic (antiferromagnetic) alignment of neighboring in-plane Mn ion core spins.

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arXiv2010

Signature of checkerboard fluctuations in the phonon spectra of a possible polaronic metal La1.2Sr1.8Mn2O7

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 from this low-temperature metallic phase to a high temperature insulating phase. Broad charge order peaks due to large polarons observed by neutron and x-ray scattering in the insulating phase disappear when La2-2xSr1+2xMn2O7 becomes metallic. We report results of inelastic neutron scattering measurements showing that polarons remain inside the metallic phase as fluctuations that strongly broaden and soften certain phonons near the wave vectors where the charge order peaks appeared in the insulating phase. Our findings imply that polaronic signatures in metals may generally come from a competing insulating charge-ordered phase. It is highly relevant to cuprate superconductors with both a pseudogap, and a similar phonon effect associated with a competing stripe order.

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arXiv2010

High-pressure synthesis, crystal and electronic structures of a new scandium tungstate, Sc0.67WO4

Negative thermal expansion (NTE) materials possess a low-density, open structure which can respond to high pressure conditions, leading to new compounds and/or different physical properties. Here we report that one such NTE material -- white, insulating, orthorhombic Sc2W3O12 -- transforms into a black compound when treated at 4 GPa and 1400 oC. The high pressure phase, Sc0.67WO4, crystallizes in a defect-rich wolframite-type structure, a dense, monoclinic structure (space group P2/c) containing 1-D chains of edge-sharing WO6 octahedra. The chemical bonding of Sc0.67WO4 vis-a-vis the ambient pressure Sc2W3O12 phase can be understood on the basis of the Sc defect structure. Magnetic susceptibility, resistivity, thermoelectric power and IR spectroscopic measurements reveal that Sc0.67WO4 is a paramagnet whose conductivity is that of a metal in the presence of weak localization and electron-electron interactions. Oxygen vacancies are suggested as a potential mechanism for generating the carriers in this defective wolframite material.

cond-mat.mtrl-sci↗

arXiv2009

Bilayer splitting and c-axis coupling in bilayer manganites showing colossal magnetoresistance

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 the c-axis between planes within the bilayer is lost, resulting in reduced interplane coupling. These results suggest that interplane coupling plays a large role in the CMR transition.

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arXiv2009

Non-monotonic Fermi surface evolution and its correlation with stripe ordering in bilayer manganites

In correlated electron systems such as cuprate superconductors and colossal magnetoresistive (CMR) oxides there is often a tendency for a nanoscale self-organization of electrons that can give rise to exotic properties and to extreme non-linear responses. The driving mechanisms for this self-organization are highly debated, especially in the CMR oxides in which two types of self-organized stripes of charge and orbital order coexist with each other. By utilizing angle-resolved photoemission spectroscopy measurements over a wide doping range, we show that one type of stripe is exclusively linked to long flat portions of nested Fermi surface, while the other type prefers to be commensurate with the real space lattice but also may be driven away from this by the Fermi surface. Complementarily, the Fermi surface also appears to be driven away from its non-interacting value at certain doping levels, giving rise to a host of unusual electronic properties.

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arXiv2009

Dynamics of bi-stripes and a colossal metal-insulator transition in the bi-layer manganite La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ (x~0.59)

In correlated electron materials, electrons often self-organize and form a variety of patterns with potential ordering of charges, spins, and orbitals, which are believed to be closely connected to many novel properties of these materials including superconductivity, metal-insulator transitions, and the CMR effect. How these real-space patterns affect the conductivity and other properties of materials (which are usually described in momentum space) is one of the major challenges of modern condensed matter physics. Moreover, although the presence of static stripes is indisputable, the existence (and potential impacts) of fluctuating stripes in such compounds is a subject of great debate. Here we present the electronic excitations of La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ (x ~ 0.59) probed by angle-resolved photoemission (ARPES), from which we demonstrate that a novel type of ordering, termed bi-stripes, can exhibit either static or fluctuating order as a function of temperature. We found that the static bi-stripe order is especially damaging to electrical conductivity, completely localizing the electrons in the bi-stripe regions, while the fluctuating stripes can coexist with mobile carriers. This physics drives a novel phase transition with colossal conductivity changes as a function of temperature. Our finding suggests that quantum stripes can give rise to electronic properties significantly different from their static counterparts. Inducing transition between them can turn on remarkable electronic phenomena, enriching our understanding of correlated electron systems as well as opening a window for potential applications in electronic devices.

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arXiv2009

Continuous metal-insulator transition of the antiferromagnetic perovskite NaOsO3

Newly synthesized perovskite NaOsO3 shows Curie-Weiss metallic nature at high temperature and suddenly goes into an antiferromagnetically insulating state at 410 K on cooling. Electronic specific heat at the low temperature limit is absent, indicating that the band gap fully opens. In situ observation in electron microscopy undetected any lattice anomalies in the vicinity of the transition temperature. It is most likely that the antiferromagnetic correlation plays an essential role of the gap opening.

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arXiv2009

Coexistence of Weak Ferromagnetism and Ferroelectricity in the High Pressure LiNbO3-type Phase of FeTiO3

We report the magnetic and electrical characteristics of a polycrystalline specimen of FeTiO3 synthesized at high pressure that is isostructural with acentric LiNbO3 (LBO). Piezoresponse force microscopy, optical second harmonic generation, and magnetometry demonstrate that FeTiO3-II is ferroelectric at and below room temperature and weakly ferromagnetic below ~120 K. These results validate symmetry-based materials design criteria and first principles calculations of coexistence between ferroelectricity and weak ferromagnetism in a series of transition metal titanates crystallizing in the LBO structure. The high-pressure form of FeTiO3 stands out as a rare example of a ferroelectric exhibiting weak ferromagnetism generated by a Dzyaloshinskii-Moriya interaction.

cond-mat.mtrl-sci↗

arXiv2009

Magnetic correlations in YBaCo4O7 probed by single-crystal neutron scattering

We have studied the frustrated system YBaCo4O7 generally described as an alternating stacking of Kagome and triangular layers of magnetic ions on a trigonal lattice, by single crystal neutron diffraction experiments above the Neel ordering transition. Experimental data reveals pronounced magnetic diffuse scattering, which is successfully modeled by direct Monte-Carlo simulations. Long-range magnetic correlations are found along the c-axis, due to the presence of corner-sharing bipyramids, creating quasi one-dimensional order at finite temperature. In contrast, in the Kagome layers ab-plane, the spin-spin correlation function -displaying a short-range 120 degrees configuration- decays rapidly as typically found in spin-liquids. YBaCo4O7 experimentally realizes a new class of two-dimensional frustrated systems where the strong out-of-plane coupling does not lift the in-plane degeneracy, but instead act as an external "field".

cond-mat.mtrl-sci↗

arXiv2009

Local Structure of La1-xSrxCoO3 determined from EXAFS and neutron PDF studies

The combined local structure techniques, extended x-ray absorption fine structure (EXAFS) and neutron pair distribution function analysis, have been used for temperatures 4 <= T <= 330 K to rule out a large Jahn-Teller (JT) distortion of the Co-O bond in La1-xSrxCoO3 for a significant fraction of Co sites (x <= 0.35), indicating few, if any, JT-active, singly occupied e_g Co sites exist.

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arXiv2007

Polaron Coherence Condensation as the Mechanism for Colossal Magnetoresistance in Layered Manganites

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.

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arXiv2007

Characteristic Energy of the Coulomb Interactions and the Pileup of States

Tunneling data on $\mathrm{La_{1.28}Sr_{1.72}Mn_2O_7}$ crystals confirm Coulomb interaction effects through the $\sqrt{\mathrm{E}}$ dependence of the density of states. Importantly, the data and analysis at high energy, E, show a pileup of states: most of the states removed from near the Fermi level are found between ~40 and 130 meV, from which we infer the possibility of universal behavior. The agreement of our tunneling data with recent photoemission results further confirms our analysis.

cond-mat.str-el↗

arXiv2007

Surface effects on the orbital order in the single layered manganite La0.5Sr1.5MnO4

We report the first observation of `orbital truncation rods' -- the scattering arising from the termination of bulk orbital order at the surface of a crystal. The x-ray measurements, performed on a cleaved, single-layered perovskite, La0.5Sr1.5MnO4, reveal that while the crystallographic surface is atomically smooth, the orbital `surface' is much rougher, with an r.m.s. deviation from the average `surface' of ~0.7nm. The temperature dependence of this scattering shows evidence of a surface-induced second order transition.

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