Fate of the Peak Effect in a Type-II Superconductor: Multicriticality in the Bragg-Glass Transition

Abstract

We have used small-angle neutron scattering (SANS) and ac magnetic susceptibility to investigate the global magnetic field H vs temperature T phase diagram of a Nb single crystal in which a first-order transition of Bragg-glass melting (disordering), a peak effect, and surface superconductivity are all observable. It was found that the disappearance of the peak effect is directly related to a multicritical behavior in the Bragg-glass transition. Four characteristic phase boundary lines have been identified on the H-T plane: a first-order line at high fields, a mean-field-like continuous transition line at low fields, and two continuous transition lines associated with the onset of surface and bulk superconductivity. All four lines are found to meet at a multicritical point.

Comparative analysis of tunneling magnetoresistance in low-TcNb/Al-AlOx/Nb and high-TcBiKrasnov1, Motzkau2, Golod3 et al. 2011Phys. Rev. B236500View full textAdd to dashboardCite

“…At H c2 < H < H c3 superconductivity exists only in surface layers. A similar behavior has been reported in clean Nb, [73][74][75] as well as in MgB 2 . 76 From the inset in Fig.…”

“…A similar behavior was reported for clean Nb and discussed in terms of a tricritical point. 73 However, it should be noted that our Nb films are in the dirty limit and are affected by the proximity effect with Al. The columnar structure of sputtered Nb films with a large effective surfaceto-volume ratio and columns orientation perpendicular to the film may also enhance the role of surface superconductivity in the out-of-plane magnetic field.…”

Surface superconductivity as the primary cause of broadening of superconducting transition in Nb films at high magnetic fields

“…Thus the full width of the transition is dominated by SSC. Although SSC is well known for carefully polished single crystals [7,9], it is usually considered to be insignificant for disordered, rough or inhomogeneous superconducting films because of its assumed fragility and sensitivity to surface conditions [1,6,8,10]. Therefore, observation of a very robust SSC in our strongly disordered polycrystalline films is rather surprising, especially for field perpendicular to the film.…”

“…1.69H c2 than bulk superconductivity [1], which can also smear the superconducting transition. Although SSC is quite profound in polished clean superconductors [6][7][8][9], it is usually ignored for disordered, polycrysstaline films because SSC is considered to be very sensitive to the quality of the surface (e.g., surface passivation [8] and order parameter suppression [1]), surface roughness [6,8] and surface scattering [10].…”

Unusual two-dimensional behavior of iron-based superconductors with low anisotropy

“…However, recently it has been demonstrated that the SSC is very robust and exists even in polycrystalline films and in perpendicular magnetic fields [21]. Indeed, SSC has been observed in a variety of superconductors, including conventional low-T c [21,[24][25][26], MgB 2 [27,28], and some iron pnictides [29].…”