Accurate measurement of ^{13}C - ^{15}N distances with solid-state NMR

Solid-state NMR technique for measureing distances between hetero-nuclei in static powder samples is described. It is based on a two-dimensional single-echo scheme enhanced with adiabatic cross-polarization. As an example, the results for intra-molecular distances in $\alpha$-crystalline form of glycine are presented. The measured NMR distances ^13 C(2) - ^15 N and ^13 C(1) - ^15 N are 1.496 $\pm$ 0.002 \AA and 2.50 $\pm$ 0.02 \AA, respectively.Comment: 12 page

Electron Self-Energy of High Temperature Superconductors as Revealed by Angle Resolved Photoemission

In this paper, we review some of the work our group has done in the past few years to obtain the electron self-energy of high temperature superconductors by analysis of angle-resolved photoemission data. We focus on three examples which have revealed: (1) a d-wave superconducting gap, (2) a collective mode in the superconducting state, and (3) pairing correlations in the pseudogap phase. In each case, although a novel result is obtained which captures the essense of the data, the conventional physics used leads to an incomplete picture. This indicates that new physics needs to be developed to obtain a proper understanding of these materials.Comment: 5 pages, revtex, 3 encapsulated postscript figures, SNS97 proceeding

Uso de RMN no Estado Estacionário para Aumento de Sensibilidade de Análises Químicas.

bitstream/CNPDIA/10489/1/CT37_2000.pd

Metodologia de medida da difusividade térmica por RMN-CWFP.

bitstream/CNPDIA-2009-09/11832/1/CT72_2006.pd

Screening effects in the electron-optical phonon interaction

We show that recently reported unusual hardening of optical phonons renormalized by the electron-phonon interaction is due to the neglect of screening effects. When the electron-ion interaction is properly screened optical phonons soften in three dimension. It is important that for short-wavelength optical phonons screening is static while for long-wavelength optical phonons screening is dynamic. In two-dimensional and one-dimensional cases due to crossing of the nonperturbed optical mode with gapless plasmons the spectrum of renormalized optical phonon-plasmon mode shows split momentum dependence.Comment: 7 page

Effect of electron-phonon interaction on the shift and attenuation of optical phonons

Using the Boltzmann equation for electrons in metals, we show that the optical phonons soften and have a dispersion due to screening in agreement with the results reported recently [M. Reizer, Phys. Rev. B {\bf 61}, 40 (2000)]. Additional phonon damping and frequency shift arise when the electron--phonon interaction is properly included.Comment: 4 pages, late

Aplicações da espectroscopia de RMN (CWFP) na medida de umidade em sementes e grãos.

Neste trabalho demonstramos as vantagens e desvantagens de se usar a técnica de CWFP para medidas de óleo e umidade em sementes, em relação as técnicas normais de RMN com o uso do FID e eco.bitstream/CNPDIA/8115/1/CiT14_2002.pd

Medida Rápida e Simultânea dos Tempos de Relaxação Longitudinal e Transversal por RMN-CWFP.

bitstream/CNPDIA/10475/1/CT66_2005.pd

Nonanalytic corrections to the specific heat of a three-dimensional Fermi liquid

We revisit the issue of the leading nonanalytic corrections to the temperature dependence of the specific heat coefficient, $\gamma (T)=C(T)/T,$ for a system of interacting fermions in three dimensions. We show that the leading temperature dependence of the specific heat coefficient $\gamma (T)-\gamma (0) \propto T^3 \ln T$ comes from two physically distinct processes. The first process involves a thermal excitation of a single particle-hole pair, whose components interact via a nonanalytic dynamic vertex. The second process involves an excitation of three particle-hole pairs which interact via the analytic static fixed-point vertex. We show that the single-pair contribution is expressed via the backscattering amplitude of quasiparticles at the Fermi surface. The three-pair contribution does not have a simple expression in terms of scattering in particular directions. We clarify the relation between these results and previous literature on both 3D and 2D systems, and discuss the relation between the nonanalyticities in $\gamma$ and those in spin susceptibilities

Exchange narrowing of NMR line shapes in randomly diluted magnetic systems

An analysis of 19F NMR linewidths in the randomly diluted magnetic system KMnxMg1-xF3 is presented. It is shown that good agreement with measured linewidths can be obtained if in the usual asymptotic spin-diffusion assumption for the spin autocorrelation function 〈Siα(τ)Siα(0)〉avατ-d(x)/2, d(x) is taken to be independent of x above the percolation concentration. Experimental results in the system KNixMg1-xF3 are also presented. These data exhibit striking differences with the behavior of isostructural KMnxMg1-xF3 whose origin is discussed