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 (λ) and Josephson (λ_J) lengths, α=λ∕λ_J. At large α the chain is composed of tilted vortices (tilted chains) and at small α it consists of a crossing array of Josephson vortices and pancake stacks (crossing chains). We studied the chain structures at intermediate α’s and found two types of behavior. (i) In the range 0.4≲α≲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≲α≲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.