Spin-Wave Lifetimes Throughout the Brillouin Zone

We use a neutron spin-echo method with $\mu$eV resolution to determine the lifetimes of spin waves in the prototypical antiferromagnet MnF$_2$ over the entire Brillouin zone. A theory based on the interaction of magnons with longitudinal spin fluctuations provides an excellent, parameter-free description of the data, except at the lowest momenta and temperatures. This is surprising, given the prominence of alternative theories based on magnon-magnon interactions in the literature. The results and technique open up a new avenue for the investigation of fundamental concepts in magnetism. The technique also allows measurement of the lifetimes of other elementary excitations (such as lattice vibrations) throughout the Brillouin zone.Comment: 12 pages, 4 figure

Bond Randomness Induced Magnon Decoherence in a Spin-1/2 Ladder Compound

We have used a combination of neutron resonant spin-echo and triple-axis spectroscopies to determine the energy and linewidth of the magnon resonance in IPA-Cu(Cl$_{0.95}$Br$_{0.05}$)$_3$, a model spin-1/2 ladder antiferromagnet where Br substitution induces bond randomness. We find that the bond defects induce a blueshift, $\delta \Delta$, and broadening, $\delta \Gamma$, of the magnon gap excitation compared to the pure compound. At temperatures exceeding the energy scale of the inter-ladder exchange interactions, $\delta \Delta$ and $\delta \Gamma$ are temperature independent within the experimental error, in agreement with Matthiessen's rule according to which magnon-defect scattering yields a temperature independent contribution to the magnon mean free path. Upon cooling, $\delta \Delta$ and $\delta \Gamma$ become temperature dependent and saturate at values lower than those observed at higher temperature, consistent with the crossover from one-dimensional to two-dimensional spin correlations with decreasing temperature previously observed in pure IPA-CuCl$_3$. These results indicate limitations in the applicability of Matthiessen's rule for magnon scattering in low-dimensional magnets.Comment: 4 pages, 3 figure

Large magnetic field-induced spectral weight enhancement of high-energy spin excitations in La1.88Sr0.12CuO4La_{1.88}Sr_{0.12}CuO_{4}

We report electronic Raman scattering experiments on a superconducting ${\rm La_{1.88}Sr_{0.12}CuO_{4}}$ single crystal in a magnetic field. At low temperatures, the spectral weight of the high-energy two-magnon peak increases linearly with field and is amplified by a factor of more than two at 14 T. The effect disappears at elevated temperatures and is not present in undoped ${\rm La_{2}CuO_{4}}$. This observation is discussed in terms of an electronically inhomogeneous state in which the field enhances the volume fraction of a phase with local antiferromagnetic order at the expense of the superconducting phase.Comment: to appear in PR

Magnetic structure of the edge-sharing copper oxide chain compound NaCu2O2

Single-crystal neutron diffraction has been used to determine the incommensurate magnetic structure of NaCu2O2, a compound built up of chains of edge-sharing CuO4 plaquettes. Magnetic structures compatible with the lattice symmetry were identified by a group-theoretical analysis, and their magnetic structure factors were compared to the experimentally observed Bragg intensities. In conjunction with other experimental data, this analysis yields an elliptical helix structure in which both the helicity and the polarization plane alternate among copper-oxide chains. This magnetic ground state is discussed in the context of the recently reported multiferroic properties of other copper-oxide chain compounds

Spin-orbital excitation continuum and anomalous electron-phonon interaction in the Mott insulator LaTiO3_3

Raman scattering experiments on stoichiometric, Mott-insulating LaTiO$_3$ over a wide range of excitation energies reveal a broad electronic continuum which is featureless in the paramagnetic state, but develops a gap of $\sim 800$ cm$^{-1}$ upon cooling below the N\'eel temperature $T_N = 146$ K. In the antiferromagnetic state, the spectral weight below the gap is transferred to well-defined spectral features due to spin and orbital excitations. Low-energy phonons exhibit pronounced Fano anomalies indicative of strong interaction with the electron system for $T > T_N$, but become sharp and symmetric for $T < T_N$. The electronic continuum and the marked renormalization of the phonon lifetime by the onset of magnetic order are highly unusual for Mott insulators and indicate liquid-like correlations between spins and orbitals.Comment: to appear in Phys. Rev. Let

Coherent Modulation of the YBa2Cu3O6+x Atomic Structure by Displacive Stimulated Ionic Raman Scattering

We discuss the mechanism of coherent phonon generation by Stimulated Ionic Raman Scattering, a process different from conventional excitation with near visible optical pulses. Ionic Raman scattering is driven by anharmonic coupling between a directly excited infrared-active phonon mode and other Raman modes. We experimentally study the response of YBa2Cu3O6+x to the resonant excitation of apical oxygen motions at 20 THz by mid-infrared pulses, which has been shown in the past to enhance the interlayer superconducting coupling. We find coherent oscillations of four totally symmetric (Ag) Raman modes and make a critical assessment of the role of these oscillatory motions in the enhancement of superconductivity.Comment: 12 pages, 4 figure

Magnetic Resonant excitations in High-{Tc\rm T_c} superconductors

The observation of an unusual spin resonant excitation in the superconducting state of various High-Tc ~copper oxides by inelastic neutron scattering measurements is reviewed. This magnetic mode % (that does not exist in conventional superconductors) is discussed in light of a few theoretical models and likely corresponds to a spin-1 collective mode.Comment: 4 figures, Proceedings conference MSM'03 (september 2003) in Monastir (Tunisia) to be published in Phys. Stat. Solid

Magnetic order in lightly doped La_{2-x}Sr_{x}CuO_{4}

We study long wavelength magnetic excitations in lightly doped La_{2-x}Sr_{x}CuO_{4} (x < 0.03) detwinned crystals. The lowest energy magnetic anisotropy induced gap can be understood in terms of the antisymmetric spin interaction inside the antiferromagnetic (AF) phase. The second magnetic resonace, analyzed in terms of in-plane spin anisotropy, shows unconventional behavior within the AF state and led to the discovery of collective spin excitations pertaining to a field induced magnetically ordered state. This state persists in a 9 T field to more than 100 K above the N\'{e}el temperature in x = 0.01.Comment: 5 pages, 5 figure