arXiv subjects

Radosław Szczȩśniak

Publications and source records attributed to Radosław Szczȩśniak.

3 recordsLinked to original sources

arXiv2021

The phonon-mediated superconductivity in bismuthates by non-adiabatic pairing

In the present paper, the impact of small Fermi energy on the selected parameters of the superconducting state in Ba$_{1-x}$K$_{x}$BiO$_{3}$ (BKBO) is studied at $x \in (0.3, 0.4, 0.5)$. This is done by employing the adiabatic and non-adiabatic Eliashberg equations in context of the available experimental data. It is found that the retardation, strong-coupling and the non-adiabatic effects notably influence superconducting phase in BKBO. In particular, the electron-electron interaction, approximated here by the Coulomb pseudopotential, is argued to be reduced by the non-adiabatic effects that supplement retardation and allow for the phonon-mediated superconductivity. These findings are reinforced by further analysis of the isotope effect showing reduction of the isotope coefficient with respect to the canonical Bardeen-Cooper-Schrieffer (BCS) level, as caused by the interplay of all effects mentioned above. Although physics behind the isotope effect appears to be complex, its resulting behavior comply with the scenario for the conventional superconductors. In summary, obtained results confirm recent theoretical and experimental studies that suggest phonon-mediated mechanism of superconductivity in BKBO. However, they also point out that this phase cannot be properly described with the BCS theory due to the existence of somewhat unusual effects.

cond-mat.supr-con↗

arXiv2019

Signatures of non-adiabatic superconductivity in lithium-decorated graphene

Recent experimental studies confirmed that a long searched conventional superconducting phase in graphene can be induced via lithium deposition. However, the canonical character of the lithium-decorated graphene (LiC$_{6}$) superconductor postulated therein, as defined by the Bardeen-Cooper-Schrieffer (BCS) theory, is ambiguous due to the moderate electron-phonon coupling constant and low Fermi energy in this material. Herein, this issue is addressed within the Migdal-Eliashberg formalism and beyond it via the first-order electron-phonon vertex corrections, to account for the potentially pivotal effects, hitherto not captured. The conducted analysis yields similar value of the metal-superconductor transition temperature as in the previous studies ($T_{C} \sim 6$ K), yet for a weaker depairing electron correlations, which magnitude in the non-adiabatic regime approaches predictions of the Morel-Anderson model. Moreover, the characteristic ratio of the pairing gap and the inverse temperature is found to notably exceeds estimates of the BCS theory. In a results, it is argued that the superconducting state in LiC$_{6}$ may have non-canonical character strongly influenced by the non-adiabatic and retardation effects. Moreover, discussed phase appears as a vital case study, which suggests that other two-dimensional superconductors of hexagonal structure may exhibit similar non-canonical behavior as LiC$_{6}$.

cond-mat.supr-con↗

arXiv2018

Unbalanced superconductivity induced by the constant electron-phonon coupling on a square lattice

In the present paper, we analyze the properties of the unbalanced superconducting state on a square lattice with the constant value of the electron-phonon coupling function. We conduct our analysis in the framework of the Eliashberg formalism, explicitly considering {\bf k}-dependence of the electron and phonon dispersion relations. It is found that the balanced superconducting state does not induce itself in the system due to the high value of the electron effective mass. However, in the unbalanced case the thermodynamic properties of the superconducting condensate can be distinctly different from the predictions of the Bardeen-Cooper-Schieffer theory; when the coupling constant value in the diagonal channel of the self-energy is diminished comparing to the non-diagonal channel. This is due to the reduced dimensionality of the tested system and the strong-coupling effects included in the Eliashberg formalism.

cond-mat.supr-con↗