arXiv subjects

Samuel Awelewa

Publications and source records attributed to Samuel Awelewa.

6 recordsLinked to original sources

arXiv2026

Finite-momentum coupling of Higgs and Bardasis--Schrieffer modes in superconductors with competing pairing channels

In superconductors with competing pairing channels, two well defined excitations exist below the pair-breaking edge: the Higgs mode of the condensed $s$-wave channel and the Bardasis--Schrieffer (BS) exciton of the subdominant $d$-wave channel. Their mixing is doubly forbidden --- by point-group symmetry at zero momentum and because the two reside in the amplitude and phase sectors of the order parameter respectively, by particle--hole symmetry at every momentum. Working in a Nambu--Keldysh quasiclassical framework extended to leading $1/\varepsilon_F$ corrections and including the self-consistently screened Coulomb potential, we show that finite momentum combined with particle--hole asymmetry generates a direct coupling which we obtain in closed form. Whether this coupling produces an avoided crossing is decided, however, not by its magnitude but by kinematics. In the clean limit the Higgs is not a sub-gap pole but a resonance pinned to the pair-breaking edge, which disperses with coefficient unity in $(v_Fq)^2$, while the bound BS mode disperses more slowly: the two branches therefore separate rather than converge and never become degenerate. The obstruction is specific to the clean limit: exact dirty-limit results show that disorder detaches the amplitude resonance from the edge and reverses its dispersion, which can result in an avoided crossing with the BS mode at intermediate scattering. In that regime, the coupling computed here would set the splitting between the hybridized branches. We discuss the experimental implications of these results.

cond-mat.supr-con↗

arXiv2026

Quasiclassical theory of nonlinear response in d-wave superconductors

We use a self-consistent Keldysh--Nambu quasiclassical theory to study two related nonlinear phenomena in clean d-wave superconductors: the photo-induced static correction to the order parameter -- the Eliashberg effect -- and third-harmonic generation. Both follow from a systematic perturbative solution of the out-of-equilibrium Eilenberger equation for the Keldysh propagator. For the steady-state correction to the pairing amplitude we find that at temperatures close to the critical temperature and to leading order in the gap magnitude $\Delta$, the photo-induced change of the order parameter is zero at all drive frequencies: in contrast to s-wave superconductors, a clean d-wave superconductor exhibits no Eliashberg enhancement at this order. The gap-enhancing quasiparticle-redistribution channel that drives the effect in the s-wave case is suppressed by an additional power of $\Delta$ in the d-wave case. For third-harmonic generation we find that the charge-density-fluctuation (particle--hole) channel significantly dominates the Schmid--Higgs amplitude-mode contribution over a broad frequency range, the two becoming comparable only in a narrow window near the resonance frequency $\omega\approx 2\sqrt{2}\,\Delta$ if one neglects the diamagnetic part of the current in the normal state. We trace this to the nonequilibrium dynamics of nodal quasiparticles, which must be retained explicitly and which also makes the response sensitive to the orientation of the driving field.

cond-mat.supr-con↗

arXiv2026

Spatially resolved collective modes in d-wave superconductors

We analyze the dispersion of collective modes in a superconductor with $d-$wave symmetry of the order parameter in the presence of long-range Coulomb interaction. We use diagrammatic technique and quasiclassical theory in Keldysh-Nambu formalism to compute longitudinal and transverse pair susceptibilities and extract from them the dispersion of the longitudinal and transverse collective mode. We show that at T=0, the dispersion of the transverse (plasma) mode is the same as in an s-wave superconductor, but at a finite temperature it is softer and has a much larger decay rate due to the partial screening of the Coulomb potential by nodal quasiparticles. We show that the dispersion of the longitudinal mode depends on the direction of momentum with respect to the positions of the nodes of the d-wave gap, while the decay rate of this mode does not depend on momentum. We discuss experimental implications of our results.

cond-mat.supr-con↗

arXiv2025

Dynamics of the Schmid-Higgs Mode in $d$-wave superconductors

We study the dynamics of the longitudinal collective mode in an unconventional superconductor. For concreteness, we assume that the superconductor is described by a $d$-wave order parameter with $d_{x^2-y^2}$ symmetry. After the superconductor has been suddenly subjected to a perturbation at time $t=0$, the order parameter exhibits a peculiar oscillatory behavior, with the amplitude of the oscillations slowly decaying with time in a power-law fashion. Assuming that the initial perturbation is weak, we use a formalism based on quasi-classical approach to superconductivity to determine both the frequency of the oscillations as well as how fast these oscillations decay with time by evaluating the time dependence of the pairing susceptibility. We find that the frequency of the oscillations is given by twice the value of the pairing amplitude in the anti-nodal direction and its amplitude decays as $1/t^2$. The results are also verified by a direct calculation of the order parameter dynamics by numerically solving the equations of motion for the Anderson pseudospins.

cond-mat.supr-con↗

arXiv2024

Magnetic impurities in a strongly coupled superconductor

We revisit certain aspects of a problem concerning the influence of carrier scattering induced by magnetic impurities in metals on their superconducting properties. Superconductivity is assumed to be driven by strong electron-phonon interaction. We use the self-consistent solution of the Nagaoka equations for the scattering matrix together with the Migdal-Eliashberg theory of superconductivity to compute the energy of the in-gap bound states, superconducting critical temperature and tunneling density of states for a wide range of values of the Kondo temperature and impurity concentrations. It is found that similar to the case of the weak coupling (BCS) superconductors there is only one pair of the bound states inside the gap as well as re-entrant superconductivity for the case of antiferromagnetic exchange coupling between the conduction electrons and magnetic impurities. In agreement with the earlier studies we find that the gapless superconductivity can be realized which in the case of antiferromagnetic exchange requires much smaller impurity concentration. Surprisingly, in contrast with the weakly coupled superconductors we find that superconducting transition exhibits two critical temperatures for the ferromagnetic exchange coupling.

cond-mat.supr-con↗

arXiv2024

Migdal-Eliashberg superconductivity in a Kondo lattice

We apply the Migdal-Eliashberg theory of superconductivity to heavy-fermion and mixed valence materials. Specifically, we extend the Anderson lattice model to a case when there exists a strong coupling between itinerant electrons and lattice vibrations. Using the saddle-point approximation, we derive a set of coupled nonlinear equations which describe competition between the crossover to a heavy-fermion or mixed-valence regimes and conventional superconductivity. We find that superconductivity at strong coupling emerges on par with the development of the many-body coherence in a Kondo lattice. Superconductivity is gradually suppressed with the onset of the Kondo screening and for strong electron-phonon coupling the Kondo screening exhibits a characteristic re-entrant behavior. Even though for both weak and strong coupling limits the suppression of superconductivity is weaker in the mixed-valence regime compared to the local moment one, superconducting critical temperature still remains nonzero. In the weak coupling limit the onset of the many body coherence develops gradually, in the strong coupling limit it emerges abruptly in the mixed valence regime while in the local moment regime the $f$-electrons remain effectively decoupled from the conduction electrons. Possibility of experimental realization of these effects in Ce-based compounds is also discussed.

cond-mat.supr-con↗