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The standard interpretation of the phase diagram of type-II superconductorswas developed in 1960s and has since been considered a well-established part ofclassical superconductivity. However, upon closer examination a number offundamental issues arise that leads one to question this standard picture. Toaddress these issues we studied equilibrium properties of niobium samples nearand above the upper critical field Hc2 in parallel and perpendicular magneticfields. The samples investigated were very high quality films and singlecrystal discs with the Ginzburg-Landau parameters 0.8 and 1.3, respectively. Arange of complementary measurements have been performed, which include dcmagnetometry, electrical transport, muSR spectroscopy and scanning Hall-probemicroscopy. Contrarily to the standard scenario, we observed that asuperconducting phase is present in the sample bulk above Hc2 and the field Hc3is the same in both parallel and perpendicular fields. Our findings suggestthat above Hc2 the superconducting phase forms filaments parallel to the fieldregardless on the field orientation. Near Hc2 the filaments preserve thehexagonal structure of the preceding vortex lattice of the mixed state and thefilament density continuously falls to zero at Hc3. Our work has importantimplications for the correct interpretation of properties of type-IIsuperconductors and can also be essential for practical applications of thesematerials.

Possible existence of a filamentary state in type-II superconductors