Elasticity of Gaussian and nearly-Gaussian phantom networks

We study the elastic properties of phantom networks of Gaussian and nearly-Gaussian springs. We show that the stress tensor of a Gaussian network coincides with the conductivity tensor of an equivalent resistor network, while its elastic constants vanish. We use a perturbation theory to analyze the elastic behavior of networks of slightly non-Gaussian springs. We show that the elastic constants of phantom percolation networks of nearly-Gaussian springs have a power low dependence on the distance of the system from the percolation threshold, and derive bounds on the exponents.Comment: submitted to Phys. Rev. E, 10 pages, 1 figur

Critical behaviour of the Rouse model for gelling polymers

It is shown that the traditionally accepted "Rouse values" for the critical exponents at the gelation transition do not arise from the Rouse model for gelling polymers. The true critical behaviour of the Rouse model for gelling polymers is obtained from spectral properties of the connectivity matrix of the fractal clusters that are formed by the molecules. The required spectral properties are related to the return probability of a "blind ant"-random walk on the critical percolating cluster. The resulting scaling relations express the critical exponents of the shear-stress-relaxation function, and hence those of the shear viscosity and of the first normal stress coefficient, in terms of the spectral dimension $d_{s}$ of the critical percolating cluster and the exponents $\sigma$ and $\tau$ of the cluster-size distribution.Comment: 9 pages, slightly extended version, to appear in J. Phys.

Electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ

We study the electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ by means of density-functional band theory, Hubbard model calculations, and angle-resolved photoelectron spectroscopy (ARPES). The experimental spectra reveal significant quantitative and qualitative discrepancies to band theory. We demonstrate that the dispersive behavior as well as the temperature-dependence of the spectra can be consistently explained by the finite-energy physics of the one-dimensional Hubbard model at metallic doping. The model description can even be made quantitative, if one accounts for an enhanced hopping integral at the surface, most likely caused by a relaxation of the topmost molecular layer. Within this interpretation the ARPES data provide spectroscopic evidence for the existence of spin-charge separation on an energy scale of the conduction band width. The failure of the one-dimensional Hubbard model for the {\it low-energy} spectral behavior is attributed to interchain coupling and the additional effect of electron-phonon interaction.Comment: 18 pages, 9 figure

93Nb NMR STUDY OF NbSe3

We present a NMR study of the anisotropic metal NbSe3. 93Nb NMR lines have been recorded by conventional pulsed NMR techniques and field sweeping around 65 and ,140 kG. Measurements were made at room temperature, 77 and 4.2 K on a powder sample and on a sample made of crystals aligned along the b axis (chain axis). At room temperature, spectra with the magnetic field along b allow us to distinguish three inequivalent sites with different Knight shifts and quadrupolar frequencies. At low temperature, one site remains unaffected by the occurrence of the charge density waves at 144 and 59 K. For the two others, the quadrupolar structures disappear as a consequence of the modulation of the local electric field by the charge density waves, and the Knight shifts are changed because of the partial destruction of the Fermi surface. An interpretation is proposed in terms of band structure : the site which is not affected by the charge density waves would belong to an insulating chain with filled d subband, while the two other sites would correspond to the metallic chains with nearly one-quarter-filled bands, which are involved in the two successive charge density wave transitions. A decomposition of the paramagnetic susceptibility in orbital and spin contributions is given

93Nb NMR study of NbSe3

We present a NMR study of the anisotropic metal NbSe3. 93Nb NMR lines have been recorded by conventional pulsed NMR techniques and field sweeping around 65 and ,140 kG. Measurements were made at room temperature, 77 and 4.2 K on a powder sample and on a sample made of crystals aligned along the b axis (chain axis). At room temperature, spectra with the magnetic field along b allow us to distinguish three inequivalent sites with different Knight shifts and quadrupolar frequencies. At low temperature, one site remains unaffected by the occurrence of the charge density waves at 144 and 59 K. For the two others, the quadrupolar structures disappear as a consequence of the modulation of the local electric field by the charge density waves, and the Knight shifts are changed because of the partial destruction of the Fermi surface. An interpretation is proposed in terms of band structure : the site which is not affected by the charge density waves would belong to an insulating chain with filled d subband, while the two other sites would correspond to the metallic chains with nearly one-quarter-filled bands, which are involved in the two successive charge density wave transitions. A decomposition of the paramagnetic susceptibility in orbital and spin contributions is given.Nous présentons une étude par RMN du métal anisotrope NbSe3. La raie de résonance de 93Nb a été enregistrée en utilisant des méthodes de RMN pulsée et en balayant le champ magnétique autour de 65 et de 140 kG. Des mesures ont été effectuées à température ambiante, 77 et 4,2 K sur un échantillon en poudre et sur un échantillon composé de cristaux alignés le long de l'axe b (axe de la chaîne). A température ambiante, les spectres obtenus pour la direction du champ magnétique parallèle à b permettent de distinguer trois sites non équivalents avec des déplacements paramagnétiques et des fréquences quadrupolaires différents. A basse température, un des sites n'est pas affecté par l'apparition des ondes de densité de charge à 144 et 59 K. Pour les deux autres, la structure quadrupolaire disparaît du fait de la modulation du champ électrique local par les ondes de densité de charge; de plus, le déplacement paramagnétique est modifié par suite de la destruction partielle de la surface de Fermi. Une interprétation en termes de structure de bande est proposée : le site qui n'est pas affecté par les ondes de densité de charge appartiendrait à une chaîne isolante possédant une sous-bande d pleine, tandis que les deux autres sites correspondraient aux chaînes métalliques qui, avec des bandes quart-pleines, participent aux deux ondes de densité de charge successives. Une décomposition de la susceptibilité paramagnétique en une partie orbitale et une partie relative au spin est donnée

Solitons in Ising-like quantum spin chains in a magnetic field : a second quantization approach

We propose a second quantization formalism for the soliton modes in the s =112 Ising-like antiferromagnetic chain. We give the exact expressions, within the one-soliton approximation, for the dynamical structure factors in presence of a magnetic field, either parallel or perpendicular to the chain. We show that the field gives rise to a splitting of the soliton modes. We also derive the spin correlation function relative to the antiferromagnetic mode for finite and infinite chains.Nous proposons un formalisme de seconde quantification pour les modes de solitons dans les chaînes antiferromagnétiques d'Ising de spin s =1/2. Nous donnons les expressions exactes — dans le cadre d'une approximation à un soliton — des facteurs de structure dynamique en présence d'un champ magnétique, parallèle ou perpendiculaire à la chaîne. Nous montrons que le champ magnétique donne lieu à un dédoublement des modes de solitons. Nous calculons aussi la fonction de corrélation de spin relative au mode antiferromagnétique dans le cas de chaines finies ou infinies