ANHARMONICITY OF FLUX LATTICES AND THERMAL FLUCTUATIONS IN LAYERED SUPERCONDUCTORS

We study elasticity of a perpendicular flux lattice in a layered super conductor with Josephson coupling between layers. We find that for flu x displacements rho the energy contains rho(2) In rho terms, so that e lastic constants cannot be strictly defined. Instead we define effecti ve elastic constants by a thermal average. The tilt moduli have terms similar to ln T which for lambda(J)much less than a, where lambda(J) i s the Josephson length and a is the flux-line spacing, read to [rho(2) ]similar to T / \ln T\. The expansion parameter indicates that the dom inant low-temperature phase transition is either layer decoupling at h igh fields (lambda(J)much greater than a) or melting at low fields (la mbda(J)much less than a). [S0163-1829(98)09237-6].