https://repository.library.northeastern.edu/files/neu:331186/fulltext.pdf

A survey of the Van Hove scenario for high-tc superconductivity with special emphasis on pseudogaps and striped phases

Optical conductivity spectra of conducting oxides including high-TC cuprates - compared to non-superconducting nickelates in particular - deduced from infrared reflectivity spectroscopy in polarized light and their doping and temperature dependence, are discussed with emphasis on the signatures of intrinsic inhomogeneities of charge carrier concentration. The model of disordered metal is shown to be compatible with the general loss of spectral weight in the optical conductivity upon decreasing frequency, instead of a maximum conductivity at zero frequency as found within a Drude description. In high-TC multilayer cuprates, the discussion focuses upon the c-axis response and the inhomogeneity of the electronic concentration related to the conducting planes alternating with insulating sheets. The onset of stripes that are intrinsic inhomogeneities of the charge carrier distribution is discussed in the example of nickelates.

Optical conductivity of oxides

Publisher Summary This chapter discusses the basic theory of neutron scattering and examples of applications, with the aim of elucidating the wide variety of problems in condensed-matter research that can be studied with this technique The neutron is a subatomic particle with zero charge, mass m = 10087 atomic mass units, spin 1/2, and magnetic moment μn = - 19132 nuclear magnetons These four properties combine to make the neutron a highly effective probe of condensed matter The zero charge means that its interactions with matter are confined to the short-ranged nuclear and magnetic interactions, which in turn has two important consequences: the interaction probability is small, so the neutron can usually penetrate into the bulk of a sample of condensed matter and, as it can also be described in terms of the first Born approximation and thus given in explicit form by quite simple formulas Different kinds of atomic and molecular excitations in condensed matter are discussed The chapter discusses the three special applications of nuclear scattering: quasi-elastic scattering, small-angle scattering, and diffuse scattering These applications are characterized by the nature of the scattering in terms of the distribution of the scattering function over the wave vector and energy-transfer variables, rather than by specific scientific areas to which they are being applied In fact, each of these applications covers a variety of scientific areas The chapter also discusses the nuclear spin ordering

1. Introduction to Neutron Scattering

We present a detailed room-temperature x-ray powder diffraction study on ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Ni}{\mathrm{O}}_{4+\ensuremath{\delta}}$ with $0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.12$ and $0\ensuremath{\leqslant}\ensuremath{\delta}\ensuremath{\leqslant}0.13$. For $x=0.02$, 0.04, and 0.06 the oxygen content phase diagrams of the Sr-doped samples show a similar sequence of pure phases and miscibility gaps as for pure ${\mathrm{La}}_{2}\mathrm{Ni}{\mathrm{O}}_{4+\ensuremath{\delta}}$. We find a weak Sr-doping dependence of the $\ensuremath{\delta}$ range for the pure low-temperature orthorhombic (LTO), low-temperature tetragonal, and high-temperature tetragonal (HTT) phases, but overall, the $\ensuremath{\delta}$ ranges of the different phases do not vary strongly for $x\ensuremath{\leqslant}0.06$. Drastic changes are observed for $x=0.08$ and 0.12, where miscibility gaps successively disappear. For $x=0.12$ all oxygen-doped samples are in the HTT phase. The mechanism responsible for the suppression of the phase separation seems to involves multiple factors, including the Coulomb interaction between Sr impurities and interstitial oxygens as well as the reduction of the $\mathrm{Ni}{\mathrm{O}}_{6}$ octahedral tilt angle. The doping dependence of the lattice parameters shows clear differences for pure Sr and pure O doping. With the exception of the LTO phase, the in-plane lattice parameters explicitly depend on the type of dopant, rather than the net hole content $p=x+2\ensuremath{\delta}$. In contrast, the orthorhombic strain in the LTO phase as well as the $c$-axis length appears to depend only on $p$, however, in the case of the $c$-axis length this ``universal'' behavior turns out to be coincidental. Our results also show that the chemical pressure of La-site dopants is highly anisotropic, whereas that of O interstitials appears to be more isotropic. In general, this study reveals that for an investigation of the intrinsic properties of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Ni}{\mathrm{O}}_{4}$ with $x\ensuremath{\lesssim}0.12$, samples have to be annealed carefully to achieve $\ensuremath{\delta}=0$, since already an excess oxygen content as small as $\ensuremath{\delta}=0.01$ leads to phase separation.

Oxygen and strontium codoping of La 2 Ni O 4 : Room-temperature phase diagrams

We present a detailed NMR study of the expected A and anomalous B Cu lines on a single crystal of ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$. Our results in conjunction with those of neutron scattering and La NMR lead us to conclude that oxygen ordering occurs on a microscopic scale in this compound. The ordered interstitial oxygen gives rise to a distribution of tilts of the ${\mathrm{CuO}}_{6}$ octahedra in the B domains. A sawtooth modulation of the octahedral tilt, in which the sign of the tilt changes when the tilt reaches a maximum value, is deduced. In addition we show that macroscopic phase separation is inhibited in the tetragonal phase where the ${\mathrm{CuO}}_{6}$ octahedra are not tilted.

Oxygen ordering and phase separation in La2CuO4+ delta.

The miscibility gap of ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$ was investigated by studying three electrochemically oxygenated samples with different oxygen contents, using variable-temperature neutron powder diffraction. The low-temperature lattice parameters of the two coexisting orthorhombic phases (antiferromagnetic, Bmab and superconducting, Fmmm) were found to be in good agreement with those determined for high-pressure annealed samples, indicating that the two techniques yield essentially identical samples, at least in this concentration range. A sample with \ensuremath{\delta}\ensuremath{\approxeq}0.032 remained phase separated up to 415 K and, above this temperature, was found to be single-phase tetragonal (F4/mmm). The phase separation temperature and the orthorhombic-to-tetragonal phase transition temperature were obtained for all samples, allowing the shape of the miscibility gap to be determined.

Miscibility gap in electrochemically oxygenated La2CuO4+ delta.

Neutron-transmission measurements were made to determine the proton polarization in a single crystal of yttrium ethylsulfate with the aim of exploring its utilization as a spin filter for thermal and epithermal neutrons. The proton polarization is coupled to that of a paramagnetic ytterbium impurity simply by rotating the extremely anisotropic crystal in a magnetic field (spin refrigeration). A proton polarization of 41% was obtained in a helium bath near 1.2 K by rotating a large single crystal at a frequency of 150 sec/sup -1/ in a field of 14.6 kOe. The extrapolation of the results indicates that a proton polarization p/sub H/approx. =65% can be achieved with relatively simple technology. For this value of p/sub H/ the spin refrigerator becomes a competitive neutron spin filter.

Test of a new polarized hydrogen target (spin refrigerator) as neutron polarizer

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Miscibility gap in electrochemically oxygenated La2CuO4+ delta.

Test of a new polarized hydrogen target (spin refrigerator) as neutron polarizer