C.D. Frost

Spin waves and electronic interactions in La2CuO4

The magnetic excitations of the square-lattice spin-1/2 antiferromagnet and high- T(c) parent compound La2CuO4 are determined using high-resolution inelastic neutron scattering. Sharp spin waves with absolute intensities in agreement with theory including quantum corrections are found throughout the Brillouin zone. The observed dispersion relation shows evidence for substantial interactions beyond the nearest-neighbor Heisenberg term which can be understood in terms of a cyclic or ring exchange due to the strong hybridization path around the Cu4O4 square plaquettes.

Two energy scales in the spin excitations of the high-temperature superconductor La2-xSrxCuO4

The excitations responsible for producing high-temperature superconductivity in the copper oxides have yet to be identified. Two promising candidates are collective spin excitations and phonons(1). A recent argument against spin excitations is based on their inability to explain structures observed in electronic spectroscopies such as photoemission(2-5) and optical conductivity(6,7). Here, we use inelastic neutron scattering to demonstrate that collective spin excitations in optimally doped La2-xSrxCuO4 are more structured than previously thought. The excitations have a two-component structure with a low-frequency component strongest around 18 meV and a broader component peaking near 40 - 70 meV. The second component carries most of the spectral weight and its energy matches structures observed in photoemission(2-5) in the range 50 - 90 meV. Our results demonstrate that collective spin excitations can explain features of electronic spectroscopies and are therefore likely to be strongly coupled to the electron quasiparticles.

Ferromagnetic In-Plane Spin Fluctuations in Na~xCoO~2 Observed by Neutron Inelastic Scattering

We present neutron scattering spectra taken from a single crystal of Na0.75CoO2, the precursor to a novel cobalt-oxide superconductor. The data contain a prominent inelastic signal at low energies ( approximately 10 meV), which is localized in wave vector about the origin of two-dimensional reciprocal space. The signal is highly dispersive, and decreases in intensity with increasing temperature. We interpret these observations as direct evidence for the existence of ferromagnetic spin fluctuations within the cobalt-oxygen layers.

Disappearance of antiferromagnetic spin excitations in overdoped La2-xSrxCuO4.

Magnetic excitations for energies up to approximately 100 meV are studied for overdoped La(2-x)Sr(x)CuO(4) with x=0.25 and 0.30, using time-of-flight neutron spectroscopy. Comparison of spectra integrated over the width of an antiferromagnetic Brillouin zone demonstrates that the magnetic scattering at intermediate energies, 20 <or= omega <or= 100 meV, progressively decreases with overdoping. This strongly suggests that the magnetism is not related to Fermi surface nesting, but rather is associated with a decreasing volume fraction of (probably fluctuating) antiferromagnetic bubbles.

Emergence of coherent magnetic excitations in the high temperature underdoped La2-xSrxCuO4 superconductor at low temperatures.

We use inelastic neutron scattering to measure the magnetic excitations in the underdoped superconductor La2−xSrxCuO4 (x=0.085, Tc=22 K) over energy and temperatures ranges 5 < E < 200 meV and 5 < T < 300 K respectively. At high temperature (T = 300 K), we observe strongly damped excitations with a characteristic energy scale Γ ≈ 50 meV. As the temperature is lowered to T = 30 K, and we move into the pseudogap state, the magnetic excitations become highly structured in energy and momentum below about 60 meV. This change appears to be associated with the development of the pseudogap in the electronic excitations.

Magnetic order and dynamics of the charge-ordered antiferromagnet La 1.5 Sr 0.5 CoO 4

We describe neutron-scattering experiments performed to investigate the magnetic order and dynamics of half-doped

Effect of the pseudogap on suppressing high energy inelastic neutron scattering in superconducting YBa2Cu3O6.5

{\text{La}}_{1.5}{\text{Sr}}_{0.5}{\text{CoO}}_{4}

Localized excitation in the hybridization gap in YbAl3

. This layered perovskite exhibits a near-ideal checkerboard pattern of

Magnetic spectrum of the two-dimensional antiferromagnet La 2 CoO 4 studied by inelastic neutron scattering

{\text{Co}}^{2+}/{\text{Co}}^{3+}

Neutron scattering investigation of the d - d excitations below the Mott gap of CoO

charge order at temperatures below