Vortex-chain phases in layered superconductors

Layered superconductors in tilted magnetic field have a very rich spectrum of vortex lattice configurations. In the presence of in-plane magnetic field, a small c-axis field penetrates in the form of isolated vortex chains. The structure of a single chain is mainly determined by the ratio of the London (lambda) and Josephson (lambda(J)) lengths, alpha=lambda/lambda(J). At large a the chain is composed of tilted vortices (tilted chains) and at small a it consists of a crossing array of Josephson vortices and pancake stacks (crossing chains). We studied the chain structures at intermediate a's and found two types of behavior. (i) In the range 0.4 less than or similar to alpha less than or similar to 0.5 a c-axis field first penetrates in the form of pancake-stack chains located on Josephson vortices. Due to attractive coupling between deformed stacks, their density jumps from zero to a finite value. With further increase of the c-axis field the chain structure smoothly evolves into modulated tilted vortices and then transforms via a second-order phase transition into the tilted straight vortices. (ii) In the range 0.5 less than or similar to alpha less than or similar to 0.65 a c-axis field first penetrates in the form of kinks creating kinked tilted vortices. With increasing the c-axis field this structure is replaced via a first-order phase transition by the strongly deformed crossing chain. This transition is accompanied by a large jump of pancake density. Further evolution of the chain structure is similar to the higher anisotropy scenario: it smoothly transforms back into the tilted straight vortices.