Narrow Results

Recent angle-resolved photoemission (ARPES) studies of the La_2-xSr_xCuO₄ (LSCO) compound provide a new piece of information on the evolution of the electronic structure in cuprate systems with doping. Our aim is to analyze that evolution by means of the spin polaron approach based on the string picture. We study the motion of a single mobile hole in the stripe phase of a doped antiferromagnet. Holes within the stripes are taken to be static, undoped antiferromagnetic domains between hole stripes are assumed to have an alternating staggered magnetization, as it has been suggested by neutron scattering experiments. The system is described by the t-t′-t′′-J model with realistic parameters and we compute the single particle spectral density. In this paper we concentrate on the filling level 1/12. Theoretical spectra and results of measurements performed at that hole concentration show reasonable agreement. In both cases we observe, along the direction (0,0)→(π,0), "splitting" of the spectrum and formation of new low energy states around (π,0). This splitting is also observed in the direction (0,π)→ (π,π). The spectra along the direction (π,0)→ (π,π) are much broader. That hinders us in assignment of any band for some range of k points in that direction. Finally, the spectra along the direction (0,0)→ (π,π) are very broad and it is also practically impossible to distinguish any sharp band.

We discuss how superconductivity can emerge from (1)quasi–1D spin-Peierls system and (2) stripe-ordered state, with an emphasis on the role played by global antiferromagnetic fluctuations. A parallelism between the two cases based on physical and symmetry considerations is noted.

We report on the calculation of the frequency-dependent conductivity of the quantum Hall stripes and bubble crystals that form in high Landau level. We use the replica and Gaussian variational methods (GVM) with a dynamical matrix obtained from the time-dependent Hartree-Fock approximation. In the stripe state, we go beyond the semiclassical approximation of the saddle point equations obtained with the GVM and demonstrate the existence of a quantum depinning transition as a function of filling factor. Below a critical filling factor, the pinned state is described by a replica symmetry breaking (RSB) solution that gives resonant peaks in the frequency-dependent conductivity in both directions, parallel and perpendicular to the stripes orientation. These peaks shift to zero frequency as the critical filling is approached. Above the critical filling, we find a depinned stripe state described by a partial replica symmetry breaking solution in which there is free sliding only along the stripe direction. The transition has a Kosterlitz-Thouless character and includes a jump in the low-frequency exponent of the dynamical conductivity. In the bubble crystals a semiclassical approximation yields a pinning peak frequency and resonance width that generally decrease with increasing filling factor, in accordance with recent microwave absorption experiments.

Our recent experimental works on the field-induced and impurity-induced magnetic order in La_2-xSr_xCuO₄ are reviewed. The muon-spin-relaxation (μSR) measurements of La_2-xSr_xCu_1-yM_yO₄ (M=Zn, Ni) have revealed that both Zn and Ni impurities develop the magnetic order around x=0.115, though the effect of Zn on the development of the magnetic order is more marked than that of Ni. The field-induced magnetic order around x=0.115 pointed out from the neutron scattering experiments has also been investigated by the thermal conductivity measurements in magnetic fields. Marked suppression of the thermal conductivity by the application of magnetic fields parallel to the c-axis has been observed in x=0.10 and x=0.13, which appears to be in good correspondence to the development of the magnetic order. In conclusion, not only impurities but also vortex cores in the CuO₂ plane seem to operate to pin the dynamical stripes of spins and holes, leading to the development of the magnetic order.

Pattern formation occurs spontaneously in endothelial cell cultures, leading to the formation of capillary networks, which eventually grow to form blood vessels. This phenomenon occurs on a time scale of a few days.

We show here that patterns can also be induced on a much shorter time scale, by using the Faraday hydrodynamic instability, resulting from an oscillatory motion of the container. Close to the threshold of instability, the patterns observed are very sharp concentric rings or stripes. The patterns can be induced only inside a very narrow time window, ~ 5 min. Cells attachment then develops, and pattern formation can no longer be induced. The time window for pattern formation was diminished by favoring cell attachment, for instance by treating culture dishes with cationic macromolecules, such as poly-L-Lysine. It was increased by cooling the cells to 18°C, or by a prolonged exposure of the cells to trypsin, which is known to digest adhesion molecules.

The Faraday instability leads to a method to characterize cell attachment. It also permits the production of heterogeneous cultures with several cell types, with a well controlled heterogeneity. This can be used to study heterotypic cell interactions in vitro.

Assume that $K_{j\times n}$ is a complete, and multipartite graph consisting of j partite sets and n vertices in each partite set. For given graphs $G_1,G_2,\dots ,G_n$, the multipartite Ramsey number (M-R-number) $m_j(G_1,G_2,\dots ,G_n)$, is the smallest integer $t$, such that for any $n$-edge-coloring $(G^1,G^2,\dots ,G^n)$ of the edges of $K_{j\times t}$, $G^i$ contains a monochromatic copy of $G_i$ for at least one $i$. The size of M-R-number $m_j(C_3,n{K}_2,)$ for $j,n\ge 2$, the size of M-R-number $m_j(n{K}_2,m{K}_2)$ for $j\ge 2$ and $n,m\ge 1$, and the size of M-R-number $m_j(C_3,C_3,n{K}_2)$ for $j\ge 2$ and $n\ge 1$ have been computed in several papers up to now. In this paper, we determine some lower bounds for the M-R-number $m_j(C_3,C_3,n{K}_2,m{K}_2)$ for each $j,n,m\ge 2$, and some values of M-R-number $m_j(C_3,C_3,n{K}_2,m{K}_2)$ for some $j\ge 2$, and each $n,m\ge 1$.

We report on the calculation of the frequency-dependent conductivity of the quantum Hall stripes and bubble crystals that form in high Landau level. We use the replica and Gaussian variational methods (GVM) with a dynamical matrix obtained from the timedependent Hartree-Fock approximation. In the stripe state, we go beyond the semiclassical approximation of the saddle point equations obtained with the GVM and demonstrate the existence of a quantum depinning transition as a function of filling factor. Below a critical filling factor, the pinned state is described by a replica symmetry breaking (RSB) solution that gives resonant peaks in the frequency-dependent conductivity in both directions, parallel and perpendicular to the stripes orientation. These peaks shift to zero frequency as the critical filling is approached. Above the critical filling, we find a depinned stripe state described by a partial replica symmetry breaking solution in which there is free sliding only along the stripe direction. The transition has a Kosterlitz-Thouless character and includes a jump in the low-frequency exponent of the dynamical conductivity. In the bubble crystals a semiclassical approximation yields a pinning peak frequency and resonance width that generally decrease with increasing filling factor, in accordance with recent microwave absorption experiments.

Charge stripes can be traced back to the 1990s or even earlier.^[1] Stripes are a special case of the general phenomenon of electronic phase separation in strongly correlated transition metal oxide systems wherein regions with a high concentration of mobile carriers are separated from those with a low concentration due to Coulomb interactions. The carrier-rich regions are generally metallic and/or ferromagnetic, while the carrier-poor regions are antiferromagnetic and insulating. Such phase separation manifests itself in the form of metallic droplets in an antiferromagnetic insulating matrix or self-organization, for example, in one dimension as stripes.^{[1, 2]} The phase-separated regions tend to be of nanometric dimensions…

Please refer to full text.