Magnetic excitations in antiferromagnetic Bi<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

Magnetic excitations in the antiferromagnetic Bi₂CuO₄ (T _N =42K) are investigated on the basis of anisotropic exchange interaction between spins of Cu²⁺ ions. We calculate the dispersion curves and evaluate the intensity of the inelastic neutron scattering by spin wave excitations. Spin contraction at OK and the effect of spin wave interaction are studied.     

Quantum spin dynamics of the bilayer ferromagnet La Sr Mn O

We construct a theory of spin wave excitations in the bilayer manganite La_1.2Sr_1.8Mn₂O₇ based on the simplest possible double-exchange model, but including leading quantum corrections to the spin wave dispersion and damping. Comparison is made with recent inelastic neutron scattering experiments. We find that quantum effects account for some part of the measured damping of spin waves, but cannot by themselves explain the observed softening of spin waves at the zone boundary. Furthermore a doping dependence of the total spin wave dispersion and the optical spin wave gap is predicted. Received 15 January 2002 Published online 6 June 2002     

Low-energy Ce spin excitations in CeFeAsO and CeFeAsO0.84F0.16

We use inelastic neutron scattering to study the low-energy spin excitations of polycrystalline samples of nonsuperconducting CeFeAsO and superconducting CeFeAsO_0.84F_0.16. Two sharp dispersionless modes are found at 0.85 and 1.16 meV in CeFeAsO below the Ce antiferromagnetic (AF) ordering temperature of T _N ^Ce ˜ 4 K. On warming to above T _N ^Ce ˜ 4 K, these two modes become one broad dispersionless mode that disappears just above the Fe ordering temperature T _N ^Fe ˜ 140 K. For superconducting CeFeAsO_0.84F_0.16, where Fe static AF order is suppressed, we find a weakly dispersive mode center at 0.4 meV that may arise from short-range Ce-Ce exchange interactions. Using a Heisenberg model, we simulate powder-averaged Ce spin wave excitations. Our results show that we need both Ce spin wave and crystal electric field excitations to account for the whole spectra of low-energy spin excitations.     

Ferromagnetism in the Hubbard model: Spin waves and instability of the Nagaoka state

We discuss two single spin flip variational wave functions describing spin wave excitations which were proposed earlier by Shastry, Krishnamurthy and Anderson (SKA) and by Basile and Elser (BE), respectively, in order to investigate the instability of the fully polarized ferromagnetic state (Nagaoka state) in the infinite U Hubbard model. We calculate the energy of these variational states for the square lattice and for multiple chains. At the zone boundary in the vicinity of the point (0, π) the spin wave energy is reduced substantially by the binding of the spin up hole to the flipped down spin. For the square lattice this leads to a critical hole density of δ_cr = 0.407 for the SKA spin wave and of δ_cr = 0.322 for the BE spin wave which implies remarkable improvements in comparison to the corresponding scattering states investigated previously.     

Infrared conductivity from spin and charge density waves: (TMTSF) 2X

Infrared conductivity from an incommensurate spin density wave occurs due to even-order charge density wave harmonics which interact with the host lattice. Phonon states within the density-wave-induced energy gap for single-particle excitations lead to conductivity much different from that of an incommensurate charge density wave including counter-ion ordering. The conductivity expected for relaxed and quenched states of (TMTSF)₂ClO₄ is discussed.     

Low temperature thermodynamical properties of the organic chain conductor (TMTSF)AsF

We report on specific heat measurements of the quasi-one-dimensional organic salt (TMTSF)₂AsF₆ in its spin density wave state between 75 mK and 7 K. Similarly to (TMTSF)₂PF₆, we find discontinuities in the lattice contribution at 1.9 K an d 3.5 K ascribed to sub-spin density wave phases. Time-dependent effects due to dynamics of low-energy excitations in metastable states occur only below 0.2 K which yields an activation energy for the equilibrium energy relaxation process of 0.34 K, 4-5 times smaller than found for (TMTSF)₂PF₆. Finally the reduction of the low-energy excitations contribution to the specific heat in comparison to PF₆ reveals an intermediate cubic-like regime between 0.25 and 0.5 K that we tentatively describe as the phason contribution of the incommensurate spin density wave modulation. Received: 17 March 1998 / Revised: 27 July 1998 / Accepted: 22 September 1998     

Low-energy Ce spin excitations in CeFeAsO and CeFeAsO<Subscript>0.84</Subscript>F<Subscript>0.16</Subscript>

We use inelastic neutron scattering to study the low-energy spin excitations of polycrystalline samples of nonsuperconducting CeFeAsO and superconducting CeFeAsO_0.84F_0.16. Two sharp dispersionless modes are found at 0.85 and 1.16 meV in CeFeAsO below the Ce antiferromagnetic (AF) ordering temperature of T _N ^Ce ? 4 K. On warming to above T _N ^Ce ? 4 K, these two modes become one broad dispersionless mode that disappears just above the Fe ordering temperature T _N ^Fe ? 140 K. For superconducting CeFeAsO_0.84F_0.16, where Fe static AF order is suppressed, we find a weakly dispersive mode center at 0.4 meV that may arise from short-range Ce-Ce exchange interactions. Using a Heisenberg model, we simulate powder-averaged Ce spin wave excitations. Our results show that we need both Ce spin wave and crystal electric field excitations to account for the whole spectra of low-energy spin excitations.     

Dynamic spin susceptibility of hole-doped high-temperature superconductors in a singlet-correlated conduction band model

We have derived an expression for the dynamical spin susceptibility of a hole-doped high-temperature superconductor taking into account a strong correlation between the magnetization of spins of the localized and itinerant electrons. This formula has been used to calculate the imaginary part of the susceptibility as a function of the frequency and wave vector. The results are compared to experimental data on the inelastic neutron scattering in compounds of the YBa₂Cu₃O_6+y type. A peak in the scattering intensity observed at an energy of about 40 meV in the region of wave vectors Q = (π, π) and an arc-shaped dispersion relief are interpreted as manifestations of the collective spin excitations in the system, the energy of which falls within a superconducting gap (spin exciton). The U-shaped divergent relief observed in the neutron scattering intensity is assigned to collective short-rage-order spin oscillations.     

The magnetic properties across the martensitic transition in the Co38Ni34Al28 alloy

The magnetic properties of the Co₃₈Ni₃₄Al₂₈ alloy have been studied. The alloy exhibits a first order austenite-martensite phase transition in the temperature region between 155 and 247 K. A strain of 0.07% is produced across this phase transition. The Arrott plots obtained from the isothermal magnetic field dependence of magnetization indicate the presence of spontaneous magnetization both in the austenite and martensite phases, confirming the ferromagnetic character of the alloy up to room temperature. The temperature dependence of the high field magnetization indicates the presence of spin wave excitations, spin wave excitation gap and spin wave-spin wave interactions in the martensite phase. The magnetic anisotropy energy constant for the Co₃₈Ni₃₄Al₂₈ alloy is estimated both with the help of the standard law of approach to saturation of magnetization, and also from the field dependence of magnetization using the field for technical saturation of magnetization. The temperature dependences of these energy terms are compared. The estimated values of the magnetic anisotropy constant seem to be in agreement with the magnitude of the spin wave excitation gap estimated from the temperature dependence of high field magnetization.     

Quantized spin excitations in a ferromagnetic microstrip from microwave photovoltage measurements

Quantized spin excitations in a single ferromagnetic microstrip have been measured using the microwave photovoltage technique. Several kinds of spin wave modes due to different contributions of the dipole-dipole and the exchange interactions are observed. Among them are a series of distinct dipole-exchange spin wave modes, which allow us to determine precisely the subtle spin boundary condition. A comprehensive picture for quantized spin excitations in a ferromagnet with finite size is thereby established. The dispersions of the quantized spin wave modes have two different branches separated by the saturation magnetization.     

Magnetization and magnetic excitations in HoAl2

The magnetization of HoAl₂ single crystals parallel and perpendicular to the applied field for three crystallographic directions has been measured between 4.2 and 300 K in fields up to 8 T. Further the spin wave excitations were investigated by inelastic neutron scattering at 5.2 K along <100>, <110> and <111> directions in reciprocal space. The results are interpreted consistently in terms of a cubic crystal field using a simple molecular field to calculate the magnetization and an isotropic Heisenberg exchange interaction to obtain the magnetic excitations.     

The flavour liquid state and Hund‘s rule effects in spin systems with orbital degeneracy

In order to understand the properties of the spin system with orbital degeneracy, we first generalize the linearized semiclassical spin wave method for the SU(2) generators into the SU(4) case, and then investigate the elementary excitations of the orbital-spin systems in the SU(4) limit. The results show that, due to the reduction of the dimensionality of the excitations, the ordered state of the SU(4) spin-orbital model is unstable. Secondly, we study the effects of the Hund interaction on the flavour liquid state of the system. Our mean-field results suggest that, for a small Hund interaction, the flavour liquid state is still stable against the generalized spin-density wave state, but with sufficient deviation from the SU(4) limit, the long-range order may be attained in two-dimensional systems. Finally, the implications for the experimental observations on the material LiNiO₂ are discussed.     

AC-DC coupling experiments in the spin density wave state of tetramethyltetraselenofulvalene-hexafluorophosphate [(TMTSF)2PF6]

Experiments performed on (TMTSF)₂PF₆ in the spin density wave (SDW) state in the presence of joint ac and dc excitations do not show evidence for coupling between dc and ac response. Irradiation by microwaves does not lead to strong dc nonlinearity, and attempts to resurrect the ESR signal by applied dc currents were also unsuccessful. While the strongly frequency dependent response is indicative of collective effects, the response of the SDW ground state is fundamentally different from that of a charge density wave condensate.     

ESR and ENDOR studies of solitons and polarons in conjugated polymers

Nonlinear excitations such as solitons and polarons in conjugated polymers carry spins. In this case electron spin resonance (ESR) and electron-nuclear double resonance (ENDOR) provide unique methods to determine their wave functions. In this review article, the case of solitons in polyacetylene, CH_x, and polarons in an electroluminescent polymer, poly(paraphenylene vinylene) (PPV) are discussed as typical examples. High-resolution proton ENDOR spectra, obtained with stretch-oriented samples, yield the half extension of the excitations of 18 carbon atoms and 4 phenyl rings for CH_x and PPV, respectively. These extensions are well described by the theories in the case of finite electron correlation. In addition, light-induced ESR technique is shown to be useful in obtaining site-selective information of spin distribution in the case of PPV derivatives, as well as the excitation spectra of polarons.     

Magnetic excitations in triangular lattice NaCrO2

We report the first inelastic neutron scattering measurements on the rhombohedrally stacked triangular antiferromagnet NaCrO₂ which has recently been shown via μSR and NMR measurements to exhibit an unusually broad fluctuating cross-over regime extending far below the onset of spin freezing at T_c. Our results show that strong spin fluctuations persist far below T_c and that dispersive spin wave excitations only appear near the cross-over temperature 0.75T_c.     

Resonating valence bond wave function for the two-dimensional fractional spin liquid

The unconventional low-lying spin excitations, recently observed in neutron scattering experiments on Cs2CuCl4, are explained with a spin liquid wave function. The dispersion relation as well as the wave vector of the incommensurate spin correlations are well reproduced within a projected BCS wave function with gapless and fractionalized spin-1/2 excitations around the nodes of the BCS gap function. The proposed wave function is shown to be very accurate for one-dimensional spin-1/2 systems and remains similarly accurate in the two-dimensional model corresponding to Cs2CuCl4, thus representing a good ansatz for describing spin fractionalization in two dimensions.     

Light scattering observations of spin reversal excitations in the fractional quantum Hall regime

Resonant inelastic light scattering experiments access the low lying excitations of electron liquids in the fractional quantum Hall regime in the range 2/5≥ν≥1/3. Modes associated with changes in the charge and spin degrees of freedom are measured. Spectra of spin reversed excitations at filling factor ν?1/3 and at ν?2/5 identify a structure of lowest spin-split Landau levels of composite fermions (CFs) that is similar to that of electrons. Observations of spin wave excitations enable determinations of energies required to reverse spin. The spin reversal energies obtained from the spectra illustrate the significant residual interactions of composite fermions. At ν=1/3 energies of spin reversal modes are larger but relatively close to spin conserving excitations that are linked to activated transport. Predictions of composite fermion theory are in good quantitative agreement with experimental results.     

Spin Excitations in the Field-Induced Phase of the Quasi-One-Dimensional S = 1 Heisenberg Antiferromagnet NDMAP

The S = 1 quasi-one-dimensional Heisenberg antiferromagnet [Ni(C₅H₁₄N₂)₂N₃](PF₆), abbreviated as NDMAP, has been studied by electron spin resonance in a magnetic field above the critical field (H _c). We studied angular and frequency dependences of spin excitations. The angular dependence of the spin excitations in the vicinity of H _c is explained well by a phenomenological field theory, but the agreement between the experiment and the calculation is not satisfactory above 10 T. In high magnetic fields above 15 T, we obtained some characteristic spin excitations which are well explained by conventional antiferromagnetic resonance modes. These results suggest that the spin excitations change from a quantum state to a classical one due to the suppression of quantum fluctuations by high magnetic fields.     

Fractionally charged excitations in the charge density wave state of a quarter-filled t-J chain with quantum phonons

Elementary excitations of the 4k _F charge density wave state of a quarter-filled strongly correlated electronic one-dimensional chain are investigated in the presence of dispersionless quantum optical phonons using Density Matrix Renormalization Group techniques. Such excitations are shown to be topological solitons carrying charge e/2 and spin zero. Relevance to the 4k _F charge density wave instability in (DI - DCNQI)₂ A g or recently discovered in (TMTTF)₂X ( X=PF ₆, AsF₆) is discussed. Received 30 March 2001 and Received in final form 11 May 2001