Static Susceptibility Chi Research Articles

63,65Cu and 17O spin-echo decay and the static susceptibility chi '(q) in La1.85Sr0.15CuO4.

Data are presented and analyzed for the nuclear spin-echo decay of $^{63,65}\mathrm{Cu}$ and $^{17}\mathrm{O}$ in ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4}$ at T=100 K. The echo-decay data are compared with calculated waveforms in which time fluctuations of coupled neighbor spins are taken into account. Data for the two copper isotopes are found to show the effects of both ${\mathit{T}}_{1}$ and nuclear spin exchange fluctuations. Experimental decay rates for the $^{63,65}\mathrm{Cu}$ are in good agreement with indirect nuclear spin-spin interactions calculated using values of \ensuremath{\chi}'(q,0) inferred from neutron-scattering data for S(q,\ensuremath{\omega}) for this system. For $^{17}\mathrm{O}$, the echo decay is found to be in good agreement with an interpretation based on Cu-O nuclear dipolar couplings alone, in which the same Cu ${\mathit{T}}_{1}$ and nuclear spin exchange values are employed as for the copper echo decay. Calculated Cu-O indirect couplings of comparable magnitude to the dipole-dipole terms are therefore concluded to be absent. We further conclude that the transferred Cu-O hyperfine coupling is much smaller than that deduced from the measured $^{17}\mathrm{O}$ NMR shift. It follows that the excitations which provide the $^{17}\mathrm{O}$ NMR shift and relaxation are separate and distinct from those associated with spin fluctuations on the copper sites. Similar conclusions were reached in a recent study of $^{17}\mathrm{O}$ nuclear spin-lattice relaxation in this system.

Effects of Fermi surface anisotropy and topology on the spin susceptibility of metals

Intending to describe band-structure effects on the low-frequency and long-range quantum fluctuations in heavy-fermion metals, especially in UPt3, analytical results for the wavevector- and frequency-dependent spin susceptibility chi 0(q, omega ) of non-interacting electrons at zero temperature on the basis of an uniaxially anisotropic and nonparabolic single-band model are derived. The Fermi surface (FS) anisotropy and topology results in a qualitatively different behaviour of chi 0(q, omega ) compared with the spherical FS model. The static susceptibility chi 0(q) for the closed FS reveals a maximum at either q=0 or at the Brillouin zone boundary depending on the band filling. For the open FS various Kohn anomalies in chi 0(q) are found. The spectral density Im chi 0(q, omega ) for small omega , mod q mod , and omega / mod q mod shows pronounced structures and, for the open FS, a logarithmic divergence (Taylor anomaly).

Transport in the Frenkel-Kontorova model. I. Diffusion and single-particle motion

The author formulates the relations needed for calculating the diffusion coefficient D. The Kubo formula for D, or the closely related mobility, involves a time autocorrelation function and a static susceptibility chi . He discusses the interpretation of chi as a mean square fluctuation of interparticle distance, a thermodynamic derivative and a density of kink excitations. The single-particle motion is discussed in terms of the time-dependent mean square displacement Z(t). He shows how general arguments based only on the existence of D yield the Z approximately t1/2 behaviour at long time, going over ultimately to Z approximately t, as discussed by previous authors on the basis of kink phenomenology. The resulting formulae for the coefficients expressed as functions of D and chi are valid even when a description in terms of kink excitations is not justified.

Calculation of the static susceptibility χ(q) in orientationally disordered crystals with hard-core correlations: plastic CBr4and TBBA smectic B phase

In a previous paper (1981), a new method of calculating the static susceptibility chi (q) was developed with a view to studying the intermolecular correlations in orientationally disordered crystals. The purpose of the present paper is to apply this perturbative method to the typical case of orientationally disordered crystals with hard-core correlations as the plastic phase of CBr4 and the smectic B phase of TBBA. The calculation of the elastic diffuse neutron scattering leads to theoretical scattering patterns in good agreement with the experimental ones.

Intermolecular correlations in plastic crystals: one more step in the calculation of the static susceptibility chi (q)

The authors develop a method for calculating the static susceptibility chi (q) with a view to studying intermolecular correlations in orientationally disordered crystals. This method enables them to express chi (q) in a closed form at least to the degree of the Bethe approximation and to take account simultaneously of all the kinds of interactions whether they lead to critical fluctuations or not. The method is presented and discussed on a model of steric hindrance. This is done to calculate the elastic diffuse neutron scattering.

Critical order-parameter relaxation in EuS, a nearly isotropic ferromagnet

From high-frequency (1.6-1900 MHz) dispersion measurements between the critical isochore (H=0, T>or=Tc) and the critical isotherm (H>or=0, T=Tc) of EuS, the sample-shape-independent kinetic coefficient of the homogeneous magnetization, Gamma , is derived. Presenting Gamma (H,T) in terms of the static susceptibility chi ( omega =0; H,T), all data collapse onto a single curve. At large chi the kinetic coefficient approaches conventional behaviour, whereas near chi =1 a gradual decrease is observed, which appears to be indicative of the critical speeding-up predicted for Gamma (H=0,T) of isotropic ferromagnets.