This book presents modern theory of nonstationary and nonequilibrium superconductivity. It deals with superconductors in external fields varying in time and studies transport phenomena in superconductors. The book provides the microscopic theory based on the Green function formalism within the Bardeen, Cooper, and Schrieffer (BCS) theory. The method of quasiclassical Green functions is formulated for both stationary and nonequilibrium problems in the theory of superconductivity. Chapters 1 to 4 give an introduction to the Green function formalism in the BCS theory for clean materials and alloys. In next two chapters, the quasiclassical approximation is introduced and applied to some generic stationary problems such as the Ginzburg–Landau (GL) equations, critical magnetic fields, gapless superconductivity, d-wave superconductivity, bound states in the vortex core. Chapter 7 describes the quasiclassical method for layered superconductors. In Chapter 8 the nonstationary theory is formulated using both the method of analytical continuation and the Keldysh diagram technique. Next two chapters are devoted to the quasiclassical approximation and to generalized kinetic equations in nonstationary situations. Chapter 11 demonstrates how the GL model can be extended to nonstationary problems. A considerable part of the book is devoted to the vortex dynamics, which treats behaviour of type II superconductors when they carry electric currents in presence of a magnetic field. Chapters 12 to 15 deal with the dynamics of vortices. In Chapter 12, the time-dependent GL model is used to calculate the resistivity in the flux flow regime. Chapter 13 derives the forces acting on a moving vortex using the Green function formalism and applies the microscopic theory to the vortex dynamics in superconducting alloys. In Chapters 14 and 15 the vortex dynamics in clean superconductors is considered and the flux-flow conductivity, the vortex Hall effect, and the vortex mass are calculated.

We show that the topologically protected flat band emerging on a surface of a nodal fermionic system promotes the surface superconductivity due to an infinitely large density of states associated with the flat band. The critical temperature depends linearly on the pairing interaction and can be thus considerably higher than the exponentially small bulk critical temperature. We discuss an example of surface superconductivity in multilayered graphene with rhombohedral stacking.

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Topological media are systems whose properties are protected by topology and thus are robust to deformations of the system. In topological insulators and superconductors the bulk-surface and bulk-vortex correspondence gives rise to the gapless Weyl, Dirac or Majorana fermions on the surface of the system and inside vortex cores. Here we show that in gapless topological media, the bulk-surface and bulk-vortex correspondence is more effective: it produces topologically protected gapless fermions without dispersion -- the flat band. Fermion zero modes forming the flat band are localized on the surface of topological media with protected nodal lines and in the vortex core in systems with topologically protected Fermi points (Weyl points). Flat band has an extremely singular density of states, and we show that this property may give rise in particular to surface superconductivity which could exist even at room temperature.Comment: 9 pages, 5 figures, version to appear in JETP Letter