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 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.

When a system is thermally coupled to only a small part of a larger bath, statistical fluctuations of the temperature (more precisely, the internal energy) of this sub-bath around the mean temperature defined by the larger bath can become significant. We show that these temperature fluctuations generally give rise to 1/f-like noise power spectral density from even a single two-level system. We extend these results to a distribution of fluctuators, finding the corresponding modification to the Dutta-Horn relation. Then we consider the specific situation of charge noise in silicon quantum dot qubits and show that recent experimental data [E. J. Connors et al., Phys. Rev. B 100, 165305 (2019)] can be modeled as arising from as few as two two-level fluctuators, and accounting for sub-bath size improves the quality of the fit.

We study the intrinsic superconductivity in a dissipative Floquet electronic system in the presence of attractive interactions. Based on the functional Keldysh theory beyond the mean-field treatment, we find that the system shows a time-periodic bosonic condensation and reaches an intrinsic dissipative Floquet superconducting (SC) phase. Due to the interplay between dissipations and periodic modulations, the Floquet SC gap becomes soft and contains the diffusive fermionic modes with finite lifetimes. However, bosonic modes of the bosonic condensation are still propagating even in the presence of dissipations.

We develop a framework to evaluate the time-dependent resonant inelastic x-ray scattering (RIXS) signal with the use of nonequilibrium dynamical mean-field theory simulations. The approach is based on the solution of a time-dependent impurity model which explicitly incorporates the probe pulse. It avoids the need to compute four-point correlation functions and can in principle be combined with different impurity solvers. This opens a path to study time-resolved RIXS processes in multiorbital systems. The approach is exemplified with a study of the RIXS signal of a melting Mott antiferromagnet.

We report on an experimentally observed doubly split spectrum and its split-width fluctuation due to charge fluctuation in an ultrastrongly coupled superconducting qubit and resonator. From an analysis of the circuit model Hamiltonian, we found that the doubly split spectrum and split-width fluctuation are caused by discrete charge hops due to quasiparticle tunneling and a continuous background charge fluctuation in islands of a flux qubit. During 70 h of spectrum measurement, the split width fluctuated but the middle frequency of the split was constant. This observation indicates that the quasiparticles in our device mainly tunnel in one particular junction, as expected from the energy difference between quasiparticle states, during this 70 h. The background offset charge obtained from the split width has the 1/f noise characteristic.