Published in Physical Review B, Volume 71, Issue 22, June 17, 2005, pages 224506.1-224506.36.
Copyright © 2005 American Physical Society. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Physical Society. The following article appeared in Physical Review B and may be found at http://dx.doi.org/10.1103/PhysRevB.71.224506.
We study the elasticity, fluctuations, and pinning of a putative spontaneous vortex solid in ferromagnetic superconductors. Using a rigorous thermodynamic argument, we show that in the idealized case of vanishing crystalline pinning anisotropy the long-wavelength tilt modulus of such a vortex solid vanishes identically, as guaranteed by the underlying rotational invariance. The vanishing of the tilt modulus means that, to lowest order, the associated tension elasticity is replaced by the softer, curvature elasticity. The effect of this is to make the spontaneous vortex solid qualitatively more susceptible to the disordering effects of thermal fluctuations and random pinning. We study these effects, taking into account the nonlinear elasticity, that, in three dimensions, is important at sufficiently long length scales, and showing that a “columnar elastic glass” phase of vortices results. This phase is controlled by a previously unstudied zero-temperature fixed point, and it is characterized by elastic moduli that have universal strong wave-vector dependence out to arbitrarily long length scales, leading to non-Hookean elasticity. We argue that, although translationally disordered for weak disorder, the columnar elastic glass is stable against the proliferation of dislocations and is, therefore, a topologically ordered elastic glass. As a result, the phenomenology of the spontaneous vortex state of isotropic magnetic superconductors differs qualitatively from a conventional, external-field-induced mixed state. For example, for weak external fields H, the magnetic induction scales universally like B(H)∼B(0)+cHα, with α≈0.72.