Charge Order

We describe two-dimensional quantum spin fluctuations in a superconducting Abrikosov flux lattice induced by a magnetic field applied to a doped Mott insulator. Complete numerical solutions of a self-consistent large N theory provide detailed information on the phase diagram and on the spatial structure of the dynamic spin spectrum. Our results apply to phases with and without longrange spin density wave order and to the magnetic quantum critical point separating these phases. We discuss the relationship of our results to a number of recent neutron scattering measurements on the cuprate superconductor in the presence of an applied field. We also compute the pinning of charge order by the vortex cores in the ¡°spin gap¡± state which preserves spin rotation invariance, and discuss its connection to recent STM measurements. We show that with a single typical set of values for the coupling constants, our model describes the field dependence of the elastic neutron scattering intensity, t...

We present the complete ground state phase diagram of the Holstein model in two and three dimension considering the phonon variables to be classical. We first establish the overall structure of the phase diagram by using exact diagonalisation based Monte Carlo (ED-MC) on small lattices and then use a new ``travelling cluster'' approximation (TCA) for annealing the phonon degrees of freedom on large lattices. The phases that emerge include a Fermi liquid (FL), with no lattice distortions, an insulating polaron liquid (PL) at strong coupling, and a charge ordered insulating (COI) phase around half- filling. The COI phase is separated from the Fermi liquid by a regime of phase coexistence whose width grows with increasing electron-phonon coupling. We provide results on the electronic density of states, the COI order parameter, and the spatial organisation of polaronic states, for arbitrary density and electron-phonon coupling. The results highlight the crucial role of spatial correlations in this strong coupling problem.

We have prepared crystalline nanowires (diameter ~ 50 nm, length ~ a few microns) of the charge ordering manganite Pr_0.5Ca_0.5Mn_O3 using a low reaction temperature hydrothermal method and characterized them using X-ray diffraction, transmission electron microscopy, SQUID magnetometry and electron magnetic resonance measurements. While the bulk sample shows a charge ordering transition at 245 K and an antiferromagnetic transition at 175 K, SQUID magnetometry and electron magnetic resonance experiments reveal that in the nanowires phase, a ferromagnetic transition occurs at ~ 105 K. Further, the antiferromagnetic transition disappears and the charge ordering transition is suppressed. This result is particularly significant since the charge order in Pr_0.5Ca_0.5MnO_3 is known to be very robust, magnetic fields as high as 27 T being needed to melt it.

ABSTRACT Thin films of La0.5Ca0.5MnO3 (LCMO) deposited on SrTiO3 (STO) with orientations (1 0 0) and (1 1 1) and on (0 0 1) SrLaGaO4 (SLGO) substrates have been studied by Raman spectroscopy. Three different thicknesses for each substrate were studied at room temperature. The energy and the width of the Ag tilting mode have shown thickness and substrate dependence, implying an effect of the lattice mismatch on the tilting angle. Low temperature measurements (80 K) have been carried out at different scattering polarizations for the thicker films (290 nm). The spectra of LCMO/STO(1 0 0) films are strongly affected by the paramagnetic to ferromagnetic (FM) or charge ordered (CO) antiferromagnetic (AF) transitions. The Jahn–Teller (JT) modes gain intensity close to the transition temperature, where a considerable softening of the tilting mode has been also observed. New bands appear below Neel temperature (TN) for LCMO/STO(1 0 0), while this transition cannot be observed on the other films. A possible explanation is the phase coexistence which is discovered at the FM to CO/AF region (210–150 K) for LCM/STO(1 0 0) films. However, a strain effect seems to destroy the FM phase on contrary to the bulk. The JT modes do not appear (partially hidden by the spectrum of the substrate) in the spectra of LCMO/STO(1 1 1) films, which show a metallic-like spectrum, consistent with the resistivity measurements. The JT distortion appears decreased in LCMO/SLGO(0 0 1) films and the transition temperatures are affected from the strains.

We present the oxygen deficiency effects on the structural, magnetic, and electrical properties in La 0.7Sr 0.3MnO 3- δ□ δ solution where □ is a vacancy and 0≤ δ≤0.15. Polycrystalline samples La 0.7Sr 0.3MnO 3- δ□ δ were synthesized by a new method. In this series of manganites the Mn 3+ content is systematically increased due to the increase in the nonstoichiometry δ. X-ray diffraction analysis shows a phase transition from a rhombohedral to an orthorhombic system at 0.075≤ δ≤0.1. The material is ferromagnetic for 0≤ δ≤0.1 and antiferromagnetic for 0.125≤ δ≤0.15. The Curie temperature TC and saturation magnetization Ms decrease with increasing δ. Resistivity measurements as a function of temperature show a remarkable behavior for the La 0.7Sr 0.3MnO 2.9 compound; it is ferromagnetic metallic for 115≤ T≤180 K and becomes ferromagnetic insulator below 115 K, where a charge ordering seems to appear. The difference in the hopping energies in our samples can be related to the existence of two crystallographic structures, one orthorhombic and the other rhombohedral.

We have synthesized Sr$_6$Co$_5$O$_{15}$, a quasi-one-dimensional oxide, measured its magnetic properties and calculated its electronic structure by ab initio techniques. We have found strong evidence for its electronic and magnetic behavior not to follow the trend of its structural series. The magnetic coupling inside the CoO$_3$ chains is not purely ferromagnetic, the long-range coupling inside them is very weak. The Co moments are slightly canted due to their large orbital angular momenta being oriented along each particular quantization axis, that is different for each Co$^{4+}$ atom in the structure. Our thermopower calculations are in agreement with the experiment, supporting our model of the magnetic ground state of the compound.

Variable temperature scanning tunneling microscopy/spectroscopy studies on (110) oriented epitaxial thin films of La$_{0.350}$Pr$_{0.275}$Ca$_{0.375}$MnO$_3$ are reported in the temperature range of 77 to 340 K. The films, grown on lattice matched NdGaO$_3$ substrates, show a hysteretic metal-insulator transition in resistivity at 170 K. The topographic STM images show step-terrace morphology while the conductance images display a nearly homogeneous surface. The normalized conductance spectra at low temperatures (T$<$150 K) show an energy gap of 0.5 eV while for T$\geq$180 K a gap of 0.16 eV is found from the activated behavior of the zero bias conductance. The presence of energy gap and the absence of phase separation on the surface over more than 2 $\mu$m$\times$2 $\mu$m area contradicts the metallic behavior seen in resistivity measurements at low temperatures. We discuss the measured energy gap in terms of the stabilization of the insulating CO phase at the film surface.

Using SQUID magnetometry and both x-ray- and neutron-diffraction techniques, we have studied the structural and magnetic ordering of a series of Fe3O4/NiO superlattices grown by MBE. X-ray diffraction reveals that the superlattices are coherent, single phase crystals with narrow interfaces. Symmetry differences between the Fe3O4 spinel and NiO rocksalt structures lead to interfacial stacking faults, manifested in some diffraction intensities.

Motivated by the co-existing charge and spin order found in strongly correlated ladder systems, we study an effective pseudospin model on a coupled two-leg ladder. A bosonisation analysis yields a rich phase diagram showing Wigner/Peierls charge order and Neel/dimer spin order. In a broad parameter regime, the orbital antiferromagnetic phase is found to be stable. An intermediate gapless phase of finite width is found to lie in between two charge-ordered gapped phases. Our work is potentially relevant for a unified description of a class of strongly correlated, quarter-filled ladder systems.