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

On the trace anomaly and the energy-momentum conservation of quantum fields at D=2 in classical curved backgrounds

We study the conformal symmetry and the energy-momentum conservation of scalar field interacting with a curved background at D=2. We avoid to incorporate the metric determinant into the measure of the scalar field to explain the conformal anomaly and the consequent energy-momentum conservation. Contrarily, we split the scalar field in two other fields, in such a way that just one of them can be quantized. We show that the same usual geometric quantities of the anomaly are obtained, which are accompanied by terms containing the new field of the theory.Comment: 5 pages, no 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