Collective Spin Modes Research Articles

Sum Rules for the spin surface response of metal clusters within the time dependent local spin density approximation

We study surface spin collective modes of simple metallic clusters using a sum rule approach. We derive analytical expressions for the energy and cubic energy-weighted moments of the time-dependent local-spin-density approximation (TDLSDA) strength function. With these two moments we obtain a mean excitation energy of multipole spin modes and study its dispersion with cluster size up to very large sizes. The crucial roles played by the surface diffuseness and by correlation and kinetic energy effects on the resonance energies are stressed.

Collective spin modes in superconducting double layers

We investigate a double layer system with tight-binding hopping, intra-layer and inter-layer interactions, as well as a Josephson like coupling. We find that an antiferromagnetic spin polarization induces additional spin-triplet pairing (with $S_z =0$) to the singlet order parameter. This causes an undamped collective mode in the superconducting state below the particle-hole threshold, which is interpreted as a Goldstone excitation.

Spin dynamics in sodium clusters

We investigate the dynamics of the electron cloud in a metal cluster from the regime of small oscillations up to high excitations in the multi-plasmon regime. Particular attention is paid to the effect of spin modes. Test cases are a spin saturated cluster, Na9+, and a strongly spin polarized cluster, the spherical isomer of Na6+. The spectral distributions are dominated by the Mie plasmon resonance and a strong collective spin mode in all cases considered. Nonlinear effects as cross talk between multi-plasmon states or chaotic pattern cannot be observed in the investigated energy range.

A sum rule approach to collective spin modes in paramagnetic clusters, cavities and shells

We study spin collective modes in paramagnetic systems using a sum rule approach. We give a simple and analytical expression for the static magnetic polarizability of finite size systems in terms of their radii and of the magnetic susceptibility of a uniform system. A systematics for the mean excitation energy of spin modes in clusters and shells is given. In metal spheres, the hydrodynamical model is solved and compared with the sum rule approach. An estimate for the mean excitation energy of the spin dipole collective state in fullerene is given.

Supersymmetric approach to heavy-fermion systems

We present a new supersymmetric approach to the Kondo lattice model in order to describe simultaneously the quasiparticle excitations and the low-energy magnetic fluctuations in heavy-Fermion systems. This approach mixes the fermionic and the bosonic representation of the spin following the standard rules of superalgebra. Our results show the formation of a bosonic band within the hybridization gap reflecting the spin collective modes. The density of states at the Fermi level is strongly renormalized while the Fermi surface sum rule includes $n_{c}+1$ states. The dynamical susceptibility is made of a Fermi liquid superimposed on a localized magnetism contribution.

Two-loop scattering amplitudes for the t-J model: Evidence for a spin-exchange collective mode.

We study contributions to the spin-exchange scattering amplitude at two-loop (1/${\mathit{N}}^{2}$) order for the metallic phase of the two-dimensional t-J model, for which a well-defined 1/N expansion is possible. We find for small hole dopings and small exchange couplings (t\ensuremath{\gg}J) a positive spin-exchange Landau parameter, which suggests that the spin-density fluctuations show a well-defined collective mode near the metal-insulator filling. This behavior results from the fact that the chirality fluctuations decouple from and are suppressed relative to the charge fluctuations for large mass enhancements. The result is interpreted as paralleling the collective spin and charge modes characteristic of the Luttinger liquid for low wave vectors.

Spin mode, electrical resistivity, and thermal conductivity for the two-dimensional Hubbard model.

The spin and electronic spectra of the two-dimensional Hubbard model doped away from half-filling (n\ensuremath{\simeq}0.9) are calculated in a conserving fluctuation-exchange approximation for coupling U/t up to 7 and temperature down to 30 K. We find neither a spin-density wave nor a superconducting instability. The energy of the overdamped collective spin mode decreases as T decreases and U/t increases. The Wiedemann-Franz ratio of the thermal and electrical conductivities becomes smaller than the Sommerfeld value. We make comparison with experimental data on cuprates.

Wigner crystal in one dimension.

A one-dimensional gas of electrons interacting with long-range Coulomb forces [V(r)\ensuremath{\approxeq}1/r] is investigated. The excitation spectrum consists of separate collective charge and spin modes. For arbitrarily weak Coulomb repulsion density correlations at wave vector 4${\mathit{k}}_{\mathit{F}}$ decay extremely slowly and are best described as those of a one-dimensional Wigner crystal. Pinning of the Wigner crystal then leads to the nonlinear transport properties characteristic of charge density waves. The results allow a consistent interpretation of the plasmon and spin excitations observed in one-dimensional semiconductor structures, and suggest an interpretation of some of the observed features in terms of ``spinons.''

Nuclear spin and isospin excitations

A review is given of our present knowledge of collective spin-isospin excitations in nuclei. Most of this knowledge comes from intermediate-energy charge-exchange reactions and from inelastic electron- and proton-scattering experiments. The nuclear-spin dynamics is governed by the spin-isospin-dependent two-nucleon interaction in the medium. This interaction gives rise to collective spin modes such as the giant Gamow-Teller resonances. An interesting phenomenon is that the measured total Gamow-Teller transition strength in the resonance region is much less than a model-independent sum rule predicts. Two physically different mechanisms have been discussed to explain this so-called quenching of the total Gamow-Teller strength: coupling to subnuclear degrees of freedom in the form of $\ensuremath{\Delta}$-isobar excitation and ordinary nuclear configuration mixing. Both detailed nuclear structure calculations and extensive analyses of the scattering data suggest that the nuclear configuration mixing effect is the more important quenching mechanism, although subnuclear degrees of freedom cannot be ruled out. The quenching phenomenon occurs for nuclear-spin excitations at low excitation energies ($\ensuremath{\omega}\ensuremath{\sim}10\ensuremath{-}20$ MeV) and small-momentum transfers ($q\ensuremath{\le}0.5$ ${\mathrm{fm}}^{\ensuremath{-}1}$). A completely opposite effect is anticipated in the high ($\ensuremath{\omega}$, $q$)-transfer region ($0\ensuremath{\le}\ensuremath{\omega}\ensuremath{\le}500$ MeV, $0.5\ensuremath{\le}q\ensuremath{\le}3$ ${\mathrm{fm}}^{\ensuremath{-}1}$). The nuclear spin-isospin response might be enhanced due to the attractive pion field inside the nucleus. Charge-exchange reactions at GeV incident energies have been used to study the quasifree peak region and the $\ensuremath{\Delta}$-resonance region. An interesting result of these experiments is that the $\ensuremath{\Delta}$ excitation in the nucleus is shifted downwards in energy relative to the $\ensuremath{\Delta}$ excitation of the free proton. The physical origin of this shift is discussed, and it is shown that it may be related to the energy-dependent, attractive one-pion exchange interaction in the medium.

Spin rigidity and Landau-Ginzburg theory of chiral-spin-liquid phase.

A new kind of rigidity towards uncompensated spin in the spin-singlet ground state is considered. This spin rigidity is a generalization of the quantum-Hall-effect state incompressibility to the spin problem. The phenomenological Landau-Ginzburg theory is presented and vortexlike properties of spin quasiparticles and the spectrum of the spin collective mode are considered

Surface coupling to collective and single-particle spin modes in normal 3He

The authors propose a simple technique for probing the single-particle and collective excitations in normal /sup 3/He via a standing magnetic surface wave of arbitrary /omega/ and fixed k generated by a meander-line coil. An analytic treatment of the Landau-Sillin equation enables them to calculate the power absorption spectrum, which displays singularities associated with the 1 = 0 spin mode and a Doppler-shifted spin resonance (DSSR) of the single-particle excitations. Experimental measurement of the spectrum would determine the Landau parameters (F/sub 0//sup a/, F/sub 1//sup a/) with a spectroscopic precision. Furthermore, observation of the DSSR thresholds would provide an independent determination of the Fermi velocity v and F/sub 0//sup a/. Finally, a possibility of exciting higher, 1 /ge/ 1, spin modes is briefly mentioned in the framework of an extended Leggett-Rice equation.

Surface Coupling to Collective and Single-Particle Spin Modes in Normal 3He

A simple technique is proposed for probing the single-particle and collective excitations in normal 3He via a standing magnetic surface wave of arbitrary ω and fixed k generated by a meander-line coil. The analytically calculated power absorption spectrum for 3He displays singularities associated with the l=0 spin mode and a Doppler-Shifted Spin Resonance of the single-particle excitations. Finally a possibility of exciting higer, l≥1, spin modes is discussed.