arXiv subjects

Joshua Althüser

Publications and source records attributed to Joshua Althüser.

4 recordsLinked to original sources

arXiv2026

Secondary Collective Excitations in Intermediate to Strong-Coupling Superconductors

Considering systematically derived energy-transfer-dependent effective electron-electron interactions leads to the appearance of secondary phase and amplitude modes in isotropic superconductors in the intermediate-to-strong-coupling regime. We study the implications of such interactions on Bravais lattices by computing the corresponding response functions using the iterated equations of motion (iEoM) approach. In the weak-coupling regime, we find the conventional, primary amplitude and phase modes at $ω=2Δ$ and $ω=0$, respectively. For intermediate coupling, the amplitude mode detaches from the quasiparticle continuum towards lower energies. Increasing the coupling further leads to additional, long-lived secondary collective excitations below the continuum. This phenomenon is largely independent of the underlying lattice and the specific Fermi level. The amplitude and phase modes couple if the system is not particle-hole symmetric. Additionally, we extend the method to compute eigenoperators, i.e., linear combinations of operators that excite each secondary mode specifically. We identify nodal structures in the coefficients for these eigenoperators reminiscent of wave functions in the Hydrogen problem.

cond-mat.supr-con↗

arXiv2026

Enhanced Superconductivity in Proximity to Peaks in Densities of States

For the BCS theory of superconductivity, the electron-phonon interaction is transformed to an attractive electron-electron interaction in the vicinity of the Fermi energy only. At the same time, its formal derivation using a unitary transformation reveals that the electrons attract one another whenever their energies do not differ more than the phonon energy $ω_\mathrm{D}$, independent of closeness to the Fermi energy. Consequently, the order parameter becomes finite even away from the Fermi level. Yet, for small interactions, its magnitude is usually small and can be safely ignored, justifying the BCS approximation. Intriguingly, we find that an accumulation of density-of-states at an energy $\varepsilon_\mathrm{peak}$ in proximity to the Fermi energy induces a significant order parameter magnitude around $\varepsilon_\mathrm{peak}$, which exceeds the one at $E_\mathrm{F}$ for moderate coupling strengths. This strong enhancement is heralded by the softening of an additional collective mode, which resembles a second phase transition. We predict measurable signatures in the thermodynamic and spectroscopic response of this unexpected phenomenon, guiding future experimental searches for it.

cond-mat.supr-con↗

arXiv2025

Collective modes in superconductors including Coulomb repulsion

We numerically study the collective excitations present in isotropic superconductors including a screened Coulomb interaction. By varying the screening strength, we analyze its impact on the system. We use a formulation of the effective phonon-mediated interaction between electrons that depends on the energy transfer between particles, rather than being a constant in a small energy shell around the Fermi edge. This justifies considering also rather large attractive interactions. We compute the system's Green's functions using the iterated equations of motion (iEoM) approach, which ultimately enables a quantitative analysis of collective excitations. For weak couplings, we identify the well-known amplitude (Higgs) mode at the two-particle continuum's lower edge and the phase (Anderson-Bogoliubov) mode at $ω= 0$ for a neutral system, which shifts to higher energies as the Coulomb interaction is switched on. As the phononic coupling is increased, the Higgs mode separates from the continuum, and additional phase and amplitude modes appear, persisting even in the presence Coulomb interactions.

cond-mat.supr-con↗

arXiv2024

Collective excitations in competing phases in two and three dimensions

We investigate the superconducting (SC), charge-density wave (CDW), and antiferromagnetic (AFM) phases in the extended Hubbard model at zero temperature and half-filling. We employ the iterated equations of motion approach to compute the two-particle Green's functions and their spectral densities. This renders a comprehensive analysis of the behavior of collective excitations possible as the model's parameters are tuned across phase transitions. We identify the well-known amplitude (Higgs) and phase (Anderson-Bogoliubov) modes within the superconducting phase and observe a similar excitation (cooperon) in the CDW phase which shifts towards zero energy as the system approaches the phase transition to the SC phase. In the CDW phase, close to the phase transition to the AFM phase, we find a collective mode, an exciton, that does not change significantly and another mode, a longitudinal magnon, that emerges from the two-particle continuum as the system approaches the phase transition to the AFM phase. It becomes identical with the former at the transition. In the AFM phase, their roles are reversed. Additionally, we find a transversal Goldstone magnon located at zero energy.

cond-mat.str-el↗