95,877 research outputs found

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

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    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

    Metallic Continuum Quantum Ferromagnets at Finite Temperature

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    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

    Symplectic Quantization for Reducible Systems

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    We study an extension of the symplectic formalism in order to quantize reducible systems. We show that a procedure like {\it ghost-of-ghost} of the BFV method can be applied in terms of Lagrange multipliers. We use the developed formalism to quantize the antisymmetric Abelian gauge fields.Comment: 12 pages, IF-UFRJ-22/9

    Magnetic quantum phase transitions of the antiferromagnetic J_{1}-J_{2} Heisenberg model

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    We obtain the complete phase diagram of the antiferromagnetic J1J_{1}-J2J_{2} model, 0≤α=J2/J1≤10\leq \alpha = J_2/J1 \leq 1, within the framework of the O(N)O(N) nonlinear sigma model. We find two magnetically ordered phases, one with N\' eel order, for α≤0.4\alpha \leq 0.4, and another with collinear order, for α≥0.6\alpha\geq 0.6, separated by a nonmagnetic region, for 0.4≤α≤0.60.4\leq \alpha \leq 0.6, where a gapped spin liquid is found. The transition at α=0.4\alpha=0.4 is of the second order while the one at α=0.6\alpha=0.6 is of the first order and the spin gaps cross at α=0.5\alpha=0.5. Our results are exact at N→∞N\rightarrow\infty and agree with numerical results from different methods.Comment: 4 pages, 5 figure

    Strong evidences for a nonextensive behavior of the rotation period in Open Clusters

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    Time-dependent nonextensivity in a stellar astrophysical scenario combines nonextensive entropic indices qKq_{K} derived from the modified Kawaler's parametrization, and qq, obtained from rotational velocity distribution. These qq's are related through a heuristic single relation given by q≈q0(1−Δt/qK)q\approx q_{0}(1-\Delta t/q_{K}), where tt is the cluster age. In a nonextensive scenario, these indices are quantities that measure the degree of nonextensivity present in the system. Recent studies reveal that the index qq is correlated to the formation rate of high-energy tails present in the distribution of rotation velocity. On the other hand, the index qKq_{K} is determined by the stellar rotation-age relationship. This depends on the magnetic field configuration through the expression qK=1+4aN/3q_{K}=1+4aN/3, where aa and NN denote the saturation level of the star magnetic field and its topology, respectively. In the present study, we show that the connection q−qKq-q_{K} is also consistent with 548 rotation period data for single main-sequence stars in 11 Open Clusters aged less than 1 Gyr. The value of qK∼q_{K}\sim 2.5 from our unsaturated model shows that the mean magnetic field topology of these stars is slightly more complex than a purely radial field. Our results also suggest that stellar rotational braking behavior affects the degree of anti-correlation between qq and cluster age tt. Finally, we suggest that stellar magnetic braking can be scaled by the entropic index qq.Comment: 6 pages and 2 figures, accepted to EPL on October 17, 201
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