Eliashberg's foundational theory of superconductivity is based on the application of Migdal's approximation, which states that vertex corrections to lowest-order electron-phonon scattering are negligible if the ratio between phonon and electron energy scales is small. The resulting theory incorporates the first Feynman diagrams for electron and phonon self-energies. However, the latter is most commonly neglected in numerical analyses. Here we provide an extensive study of full-bandwidth Eliashberg theory in two and three dimensions, where we include the full back reaction of electrons onto the phonon spectrum. We unravel the complex interplay between nesting properties, size of the Fermi surface, renormalized electron-phonon coupling, phonon softening, and superconductivity. We propose furthermore a scaling law for the maximally possible critical temperature T-c(max) proportional to lambda(Omega) root Omega(2)(0) - Omega(2) in two- and three-dimensional systems, which embodies both the renormalized electron-phonon coupling strength lambda(Omega) and softened phonon spectrum Omega. Also, we analyze for which electronic structure properties a maximal T-c enhancement can be achieved.

We study field-driven domain wall (DW) velocities in asymmetric multilayer stacks with perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction (DMI), both experimentally and by micromagnetic simulations. Using magneto-optical Kerr microscopy under intense and nanoseconds-long magnetic field pulses, we show that DWs in these films propagate at velocities up to hundreds of meters per second and that, instead of the expected decrease in velocity after the Walker field, a long plateau with constant velocity is observed, before breakdown. Both the maximum speed and the field extent of the velocity plateau strongly depend on the values of the spontaneous magnetization and the DMI strength, as well as on the magnetic anisotropy. Micromagnetic simulations reproduce these features in sufficiently wide strips, even for perfect samples. A physical model explaining the microscopic origin of the velocity plateau is proposed.

We study thermal transport in a two-dimensional system with coexisting s- or d-wave superconducting (SC) and spin density wave (SDW) orders. We analyze the nature of coexistence phase in a tight-binding square lattice with Q = (pi, pi) SDW ordering. The electronic thermal conductivity is computed within the framework of the Boltzmann kinetic theory, using Born approximation for the impurity scattering collision integral. We describe the influence of the Fermi surface (FS) topology, the competition between the SC and SDW order parameters, and the presence or absence of zero energy excitations in the coexistence phase, on the low temperature behavior of thermal conductivity of the various pairing states. We present qualitative analytical and fully numerical results that show that the heat transport signatures of various SC states emerging from collinear SDW order are quite distinct and depend on the symmetry properties of the SC order parameter under translation by the SDW nesting vector Q. A combination of (pi, pi)-SDW and the d(x2-y2) pairing state results in fully gapped excitations, whereas (pi, pi)-SDW coexisting with either d(xy) or s-wave pairing states may always have gapless excitations. There appear special stable Dirac nodal points that are not gapped by the SC order in the coexistence phase, resulting in finite residual heat conductivity.

YbCo2Si2 is considered to serve as a stable-valent, isoelectronic reference for the extensively studied heavy-fermion system YbRh2Si2 which is situated very close to an antiferromagnetic quantum critical point (QCP). The investigation of the Fermi surface (FS) topology of YbCo2Si2 and its comparison to YbRh2Si2 could help to unravel the strongly disputed nature of this quantum phase transition, whether it originates from a local or itinerant QCP. Here we study the electronic structure and FS of YbCo2Si2 by means of angle-resolved photoelectron spectroscopy (ARPES) and compare it to ab initio band structure calculations and FS modeling. Our approach allows the electronic structure at the surface and in the bulk to be disentangled. Identifying the bulk contribution, we demonstrate that YbCo2Si2 exhibits a small FS, confirming the formation of a large FS in YbRh2Si2. This favors an itinerant QCP, instead of the widely discussed local scenario. Our study demonstrates that ARPES is a reliable tool for the study of bulk electronic states in intermetallic Kondo lattice systems despite the complexity induced by their three-dimensional character and the presence of pronounced surface states.

We examine the spin and charge excitations in antiferromagnetic iron pnictides by mean-field calculations with a random-phase approximation in a five-band itinerant model. The calculated excitation spectra reproduce well spin-wave dispersions observed in inelastic neutron scattering with a realistic magnetic moment for CaFe(2)As(2). A particle-hole gap is found to be crucial to obtain consistent results; we predict the spin wave in LaFeAsO disappears at a lower energy than in CaFe(2)As(2). We analyze that the charge dynamics to make predictions for resonant inelastic x-ray scattering spectra.