Local Spin Fluctuations Research Articles

Nonlinear spin dynamics in nuclear magnets

A method is developed for solving nonlinear systems of differential, or integrodifferential, equations with stochastic fields. The method makes it possible to give an accurate solution for an interesting physical problem: What are the peculiarities of nonlinear spin dynamics in nonequilibrium nuclear magnets coupled with a resonator? Evolution equations for nuclear spins are derived basing on a Hamiltonian with dipole interactions. The ensemble of spins is coupled with a resonator electric circuit. Seven types of main relaxation regimes are found: free induction, collective induction, free relaxation, collective relaxation, weak superradiance, pure superradiance, and triggered superradiance. The initial motion of spins can be originated by two reasons, either by an imposed initial coherence or by local spin fluctuations due to nonsecular dipole interactions. The relaxation regimes caused by the second reason cannot be described by the Bloch equations. Numerical estimates show good agreement with experiment.

Comparison of the high-frequency magnetic fluctuations in insulating and superconducting La2-xSrxCuO4.

Inelastic neutron scattering performed at a spallation source is used to make absolute measurements of the dynamic susceptibility of insulating La2CuO4 and superconducting La2-xSrxCuO4 over the energy range 15<EN<350 meV. The effect of Sr doping on the magnetic excitations is to cause a large broadening in wavevector and a substantial change in the spectrum of the local spin fluctuations. Comparison of the two compositions reveals a new energy scale of 22 meV in La1.86Sr0.14CuO4.

Fast polarization reversal of proton spins in solid-state targets

The method of polarization reversal of proton spins in solid-state targets, by using the effect of spin superradiance is analysed theoretically. The main aim is to find out the most accurate way of calculating the time of reversal and the final reversal polarization. Three approaches are compared: one, based on the standard Bloch equations; another, using numerical simulations for finite-particle model; and an approach based on effective equations taking into account local spin fluctuations. The latter method is shown to be the most accurate. The minimal reversal time can be of the order of 10 −5 – 10 −4 s. The maximal final polarization can reach 80–90%, but can never be 100% complete. The proposed method only works from negative to positive polarization.

Identification of local spin fluctuations by weak localization.

While some $3d$ impurities in a metal host possess a magnetic moment, the majority of $d$ impurities are nonmagnetic at low temperatures. This can be due to the Kondo effect or local spin fluctuations (LSF's). We present two experimental examples for LSF's and show that weak localization is an excellent tool to investigate LSF's. The LSF's are enhanced by the Stoner factor due to the exchange interaction. This enhancement of the LSF's causes a dephasing of the conduction electrons. The dephasing is proportional to the temperature and can be measured by weak localization.

Origin of pure spin superradiance.

The question addressed here is: What originates pure spin superradiance in a polarized spin system placed inside a resonantor? The term ``pure'' means that no initial coherence is imposed on spins, and its appearance manifests a purely self-organized collective effect. An accurate solution of evolution equations for a microscopic model is given. The results show that the resonator Nyquist noise does not play any role in starting spin superradiance, but the emergence of the latter is initiated by local spin fluctuations due to nonsecular dipole interactions.

Low-temperature thermopower of Al-based dilute alloys

We report systematic calculations of the low-temperature diffusion thermopower of Al-based dilute alloys with 3d and 4sp impurities, by solving self-consistently the linearized Boltzmann equation. The impurity scattering is described by the phase shifts obtained from self-consistent local-density-functional impurity-in-jellium calculations. Moreover, the influence of the full anisotropy of the Al Fermi surface on the scattering process is taken into account within the on-Fermi-sphere approximation. Our results explain successfully the experimentally measured variations in the thermoelectric power, except for Mn impurities. In this case, the presence of a narrow many-body resonance at the Fermi level in the localized spin-fluctuations regime seems to be responsible for the large negative value of thermopower observed.

Itinerant-Localized Duality Picture of Spin Fluctuations in High-TcCuprates. I. Nested Spin-Fluctuation Theory

The mode-mode coupling theory of spin fluctuations in the normal state of high- T c cuprates is developed on the basis of the itinerant-localized duality picture of technically nested two-dimensional metals. This offers a unified description of the anomalies observed in the normal state of high- T c cuprates. The so-called “spin-gap” behaviors of the resistivity, the specific heat, the uniform spin susceptibility, and the optical conductivity are characterized by the energy scale T * of the crossover between the itinerant and the localized spin fluctuations, while those of the Hall coefficient, the thermo-electric power, the longitudinal NMR relaxation rate, and the neutron scattering intensity, are characterized by different energy scales less than T * . The integrated weight of the nested spin fluctuation decreases when decreasing the temperature or approaching the perfect nesting, leading to the “spin-gap” behavior.

Superconductivity in the Hubbard Model

Anisotropic superconductivity of strongly correlated electrons is studied in the 1/ d expansion, with d being the spatial dimensionality. Kondo-effect-type local spin fluctuations are of the leading order in 1/ d . They are responsible for the formation of Gutzwiller's heavy electrons. Superconductivity is of higher order in 1/ d . The superexchange interaction causes superconductivity of s or d γ wave, while a three-site exchange interaction causes that of s or p wave. Within the mean-field approximation which is one of the simplest schemes of the 1/ d expansion, the highest critical temperature is obtained for d γ wave, two dimensions, and almost half filling where the effective mass of heavy electrons is large and the three-site exchange interaction is relatively ineffective through the enhancement of vertex functions for spin channels. An argument is given in connection with high- T c superconductivity in the cuprate oxides.

Local Kondo Temperature of Strongly Correlated Electron Liquids

The Hubbard model is studied for its paramagnetic phase. The leading-order effects in 1/ d are Kondo-effect-type local spin fluctuations, with d being the spatial dimensionality. For d →+∞ , a “local” Kondo temperature is defined to show an energy scale of the local spin fluctuations or the strength of the Kondo effect to quench magnetic moments. Even for finite d , the local Kondo temperature is still unambiguously defined through the mapping to the Anderson model. It is so renormalized as to increase by intersite and critical spin fluctuations, which are of higher order in 1/ d . The renormalization of the local Kondo temperature can play crucial roles in the suppression of magnetism, the occurrence of high-temperature superconductivity, and the occurrence of metamagnetism in the presence of magnetic fields. The above results generally apply to strongly correlated electron liquids.

Electron correlation in the periodic Anderson model in d=+

The periodic Anderson model with strong repulsion is studied. The leading-order effects with respect to 1/d, d being the dimensionality of the system, are Kondo-effect-type local spin fluctuations and mean-field-approximation-type magnetic orders. The competition between the Kondo effect and the Ruderman-Kittel-Kasuya-Yosida exchange interaction determines whether the ground state is magnetic or paramagnetic. A single-site approximation (SSA) is rigorous for paramagnetic states in d\ensuremath{\rightarrow}+\ensuremath{\infty} dimensions. The SSA reduces the problem to solving the Anderson model. A ``local'' Kondo temperature is defined to show an energy scale of the local spin fluctuations. On the other hand, an SSA including magnetic mean fields is rigorous for magnetic states in d\ensuremath{\rightarrow}+\ensuremath{\infty} dimensions. All the other effects are of higher order: Critical and intersite spin fluctuations are O(1/ \ensuremath{\surd}d ); and paramagnetic orders are at most O(1/d). The 1/d expansion is one of the most useful methods for examining lower-temperature phases in real dimensions.

Electron Correlation in the Hubbard Model ind=+∞Dimension: Heavy Electrons in the Mott-Transition Region

The Hubbard model is studied for the Mott-transition region in the limit of high dimension d →+∞. Leading-order effects with respect to 1/ d are local spin fluctuations, all of which are fully included in the single-site approximation (SSA). Because the SSA is reduced to solving the Anderson model, it is argued that a normal heavy-electron liquid forms through the Kondo effect. The Fermi-liquid relations are proven within the SSA. The other leading-order effects are mean fields of magnetism caused by exchange interaction. Therefore it depends on the competition between the Kondo effect and the exchange interaction which the ground state is, magnetic or paramagnetic. However, any other effect is a higher-order effect. The 1/ d expansion from the heavy-electron liquid in the SSA is one of the most useful methods in examining lower-temperature phases in the real dimension.

Magnetism in Heavy-Electron Liquids

One of the most characteristic features of heavy-electron liquids is interplay between local and intersite spin fluctuations. Recent developments in including both spin fluctuations are reviewed. Local spin fluctuations, which are responsible for the formation of heavy electrons, are completely included in the single-site approximation (SSA). The SSA is reduced to solving the Anderson model selfconsistently. Intersite spin fluctuations are perturbatively included. Such a perturbative scheme is nothing but the 1/d expansion from the d= +oo dimension. The mean-field approximation based on the SSA is rigorous for d = +oo. Applications of the 1/d expansion to paramagnons, metamagnetism, tiny-moment magnetism, and other topics are presented.

Paramagnons in heavy-electron systems

Paramagnons in heavy-electron systems are theoretically examined. First, local spin fluctuations are fully included in the best single-site approximation. A local Kondo temperature is uniquely defined to show a characteristic energy scale of local spin fluctuations. Secondly, paramagnons are included perturbatively. Such a theory is applied to analyze the specific-heat data of heavy-electron compounds such as ${\mathrm{UAl}}_{2}$, ${\mathrm{UPt}}_{3}$, and ${\mathrm{CeSi}}_{1.85}$, where anomalous ${\mathit{T}}^{3}$lnT terms are observed at low temperatures T10 K. It is argued that not only paramagnons but also local spin fluctuations are responsible for the low-temperature specific heat and that paramagnons are responsible for the anomalous ${\mathit{T}}^{3}$lnT terms.

Localized-versus-extended spin fluctuations in quantum antiferromagnets.

A recently devised trial wave function for the Heisenberg-Ising model is tested against linear spin waves in the whole anisotropic region. The above wave function is asymptotically exact in the limit of high anisotropy, and is constructed using localized spin fluctuations over N\'eel states. The structure of this trial state is compared with the one presented by the zero-point state of the linear-spin-wave theory. Striking similarities are found between both, despite that spin waves represent fluctuations that encompass the whole system. A crossover between both approaches near the isotropic point is attributed to a delocalization of spin fluctuations. We present results for one and two dimensions, and compare with exact results or numerical simulations.

The electronic structure of the mixed 5f system UC x N1?x

We present results of electronic structure calculations for UC x N1−x obtained with the relativistic Korringa-Kohn-Rostocker Greens function (RKKR-GF) method. While on the anion sites the disorder is treated within the Coherent Potential Approximation (CPA) the small change in the cation potential upon alloying is accounted for in the averaget-matrix approximation (ATA). The presence of strong local spin fluctuations forx≦0.6 restricts our treatment to the carbon rich phase, and even there we find that the observed linear specific heat coefficient is much enhanced over the bandstructure value. The computed X-ray photoemission spectra vary smoothly with composition the main change consisting in the “melting away” of the side peak at −3 eV binding energy seen in pure UC with the adjunction of nitrogen.

Magneto-volume effects in weakly ferromagnetic metals

Magneto-volume effects of weakly ferromagnetic metals are discussed, based on the assumption that the local spin fluctuation amplitude is almost constant. The author's result on the thermal expansion agrees with the result of Moriya and Usami (1980). He derives the universal relation of the ratio of thermal expansion above the Curie temperature and the spontaneous magnetostriction. Based on a simple thermodynamic argument, he also derives a relation connecting the magneto-volume coupling coefficient with the pressure dependence of the Curie temperature, which is slightly different from the Stoner-Wohlfarth theory.

Quantum Phenomenology for Heavy-Fermion Systems. I. Formulation of the Duality Model

A quantum mechanical phenomenology for heavy-fermion systems is formulated. The effective Lagrangian for low-energy excitations contains a localized spin-fluctuation part which is coupled with an itinerant fermion part. Each part represents the dual nature of strongly correlated f electrons. The coupling parameter is fixed by comparison with the Fermi-liquid theory for the Anderson model. The comparison also fixes the localized spin-fluctuation spectrum. As an advantage over the Fermi-liquid description the present model takes into account the RKKY interaction and non-linear effects of spin polarization. A formula for the magnetic equation of state is derived which is applicable to the metamagnetism and weak antiferromagnetism in heavy-fermion systems.

Electron correlations in narrow-band systems at finite temperatures: Gutzwiller-type variational approach

A theory is presented for finite-temperature band magnetism which takes into account the effect of electron correlations using the Gutzwiller-type variational approach. The functional-integral method recently proposed by Kotliar and Ruckenstein (1986), is combined with the alloy-analogy approximation to include the effect of local spin fluctuations at finite temperatures. The theory at T=0 K is equivalent to Gutzwiller's approximation for the correlated ground state, while in the high-temperature limit it reduces to the local spin-fluctuation theory developed previously by Hasegawa and Hubbard. Numerical calculations for the half-filled simple cubic Hubbard model demonstrate that both electron correlations and local spin fluctuations play important roles at finite temperatures. The Brinkman-Rice-type metal-insulator transition is critically discussed on the basis of the model calculation.

Anisotropic nuclear relaxation in YBa2Cu3O7.

Data are presented for the magnetic susceptibility of Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and for the anisotropy of the copper nuclear spin-lattice relaxation time ${T}_{1}$ in the normal state for planar [Cu(2)] sites in this system. Using the Cu NMR shift to partition the susceptibility and estimate the density of states, we analyze the conventional enhanced Korringa model of ${T}_{1}$ contributions, finding it incapable of accounting for the anisotropy of the observed relaxation times. A localized spin-fluctuation picture is also discussed briefly.

Dynamical effect of spin fluctuations in itinerant electron magnetism

The authors present a method which takes account of the quantum effect of spin fluctuations on itinerant electron ferromagnets in the functional integral formalism and in the gaussian approximation. Using a similar approximation to that introduced by Hertz and Klenin (1977) and the results obtained in the authors' previous work they obtain the dynamical spin susceptibility. Dynamics of spin fluctuations are taken into account through the amplitudes of the local spin fluctuations which are the most fundamental parameters dominating the thermodynamical and magnetic properties of the system. It is shown by simple numerical examples that a remarkable improvement is established for the temperature dependence of the spontaneous magnetisation and the paramagnetic susceptibility as compared with the results obtained in the so-called static approximation.