A review of two-band superconductivity: materials and effects on the thermodynamic and reversible mixed-state properties

Abstract

Two-band superconductivity has become an important topic over the last ten years. Extensive experimental and theoretical studies started with MgB2 and are now focused on the iron-based and other new superconductors. In this review, I describe how important thermodynamic, reversible mixed-state, and other superconducting properties are changed by two-band and, for comparison, by other effects such as anisotropy in a single-band material or an energy gap structure different from the conventional s-wave symmetry. The work consists of three main parts, in which I review (i) theoretical models and what they predict for experimentally accessible properties in the two-band and other scenarios, (ii) experimental methods applied for investigating superconducting properties and the results obtained in potential two-band materials, and (iii) materials, for which two-or multi-band superconductivity has been suggested. It is shown that two-band effects appear in most of the analyzed properties and that they can be quite significant but usually fade away as interband interactions increase. Anisotropy often leads to similar modifications in single-band superconductors, which is why the distinction of two-band and anisotropy effects is usually difficult, particularly when the temperature dependence of the quantities is examined. In contrast, the field dependent effects are more often different and thus more often allow a reliable distinction between the models. CONTENTS

Multigap superconductivity in chiral noncentrosymmetric TaRh2B2

“…2). A similar behavior is seen in other two-gap superconductors including Lu 2 Fe 3 Si 5 [28,29] and MgB 2 [18,21]. In these materials, a shoulder in C el (T ) is followed by a rapid fall at lower T as the second gap opens.…”

“…Multigap superconductivity has been reported in several classes of superconductor including the iron pnictides [15][16][17] and the borides including MgB 2 [16,[18][19][20][21]. There are several nonchiral NCS superconductors that exhibit multigap behavior.…”

“…Strong coupling can increase H c2 (0). The fit to the C el (T ) below T c as well as the jump in heat capacity at T c suggest the coupling is only moderately enhanced, however, multigap behavior can sometimes reduce the jump at T c [21,29]. Spin-orbit scattering associated with the presence of heavier elements Ta and Rh could help to increase H c2 (0) in TaRh 2 B 2 (cf.…”

Multiband Superconductivity in KFe2As2: Evidence for One Isotropic and Several Lilliputian Energy Gaps

“…This explains why this indirect derivation of ρ ab (T) is only sensitive to the larger gap as found for MgB 2 . 43,44 5. Mixed-state specific heat, γ (H)…”

Distinct magnetic signatures of fractional vortex configurations in multiband superconductors

“…Multiband superconductors [1,2] present a variety of intriguing properties that are not found in their singlecomponent counterparts. Theoretical predictions have added more striking properties to that list and challenge experiments to prove them.…”