Metallic Continuum Quantum Ferromagnets at Finite Temperature

We study via renormalization group (RG) and large N methods the problem of continuum SU(N) quantum Heisenberg ferromagnets (QHF) coupled to gapless electrons. We establish the phase diagram of the dissipative problem and investigate the changes in the Curie temperature, magnetization, and magnetic correlation length due to dissipation and both thermal and quantum fluctuations. We show that the interplay between the topological term (Berry's phase) and dissipation leads to non-trivial effects for the finite temperature critical behavior.Comment: Corrected typos, new discussion of T=0 results, to appear in Europhys. Let

Charge stripe order from antiphase spin spirals in the spin-Fermion model

We revisit the ground state of the spin-Fermion model within a semiclassical approximation. We demonstrate that antiphase spin spirals, or pi-spirals, whose chirality alternates between consecutive rows (or columns) of local moments, have, for sufficiently high carrier concentration, lower energy than the traditional Shraiman and Siggia spirals. Furthermore, pi-spirals give rise to modulated hopping, anisotropic 1D transport, and charge density wave formation. Finally, we discuss the relevance of pi-spirals to the physics of charge stripe formation in cuprates, such as La(2-x)Sr(x)CuO4.Comment: 4 pages, 3 figure

Impurity susceptibility and the fate of spin-flop transitions in lightly-doped La(2)CuO(4)

We investigate the occurrence of a two-step spin-flop transition and spin reorientation when a longitudinal magnetic field is applied to lightly hole-doped La(2)CuO(4). We find that for large and strongly frustrating impurities, such as Sr in La(2-x)Sr(x)CuO(4), the huge enhancement of the longitudinal susceptibility suppresses the intermediate flop and the reorientation of spins is smooth and continuous. Contrary, for small and weakly frustrating impurities, such as O in La(2)CuO(4+y), a discontinuous spin reorientation (two-step spin-flop transition) takes place. Furthermore, we show that for La(2-x)Sr(x)CuO(4) the field dependence of the magnon gaps differs qualitatively from the La(2)CuO(4) case, a prediction to be verified with Raman spectroscopy or neutron scattering.Comment: 4 pages, 3 figures, For the connection between spin-flops and magnetoresistance, see cond-mat/061081

The specific entropy of elliptical galaxies: an explanation for profile-shape distance indicators?

Dynamical systems in equilibrium have a stationary entropy; we suggest that elliptical galaxies, as stellar systems in a stage of quasi-equilibrium, may have a unique specific entropy. This uniqueness, a priori unknown, should be reflected in correlations between the parameters describing the mass (light) distribution in galaxies. Following recent photometrical work (Caon et al. 1993; Graham & Colless 1997; Prugniel & Simien 1997), we use the Sersic law to describe the light profile of elliptical galaxies and an analytical approximation to its three dimensional deprojection. The specific entropy is calculated supposing that the galaxy behaves as a spherical, isotropic, one-component system in hydrostatic equilibrium, obeying the ideal gas state equations. We predict a relation between the 3 parameters of the Sersic, defining a surface in the parameter space, an `Entropic Plane', by analogy with the well-known Fundamental Plane. We have analysed elliptical galaxies in Coma and ABCG 85 clusters and a group of galaxies (associated with NGC 4839). We show that the galaxies in clusters follow closely a relation predicted by the constant specific entropy hypothesis with a one-sigma dispersion of 9.5% around the mean value of the specific entropy. Assuming that the specific entropy is also the same for galaxies of different clusters, we are able to derive relative distances between the studied clusters. If the errors are only due to the determination of the specific entropy (about 10%), then the error in the relative distance determination should be less than 20% for rich clusters. We suggest that the unique specific entropy may provide a physical explanation for the distance indicators based on the Sersic profile put forward by Young & Currie (1994, 1995) and discussed by Binggeli & Jerjen (1998).Comment: Submitted to MNRAS (05/05/99), 15 pages, 10 figure