arXiv subjects

K. A. Szewczyk

Publications and source records attributed to K. A. Szewczyk.

4 recordsLinked to original sources

arXiv2020

Nonadiabatic superconductivity in Li-intercalated hexagonal boron nitride bilayer

In the case of Li-intercalated hexagonal boron nitride bilayer (Li-hBN), the vertex corrections of electron-phonon interaction cannot be omitted. This is evidenced by the very high value of the ratio $λω_{D}/\varepsilon_{F}\sim 0.46$, where $λ$ is the electron-phonon coupling constant, $ω_{D}$ is the Debye frequency, and the symbol $\varepsilon_{F}$ represents the Fermi energy. Due to the nonadiabatic effects, the phonon-induced superconducting state in Li-hBN is characterized by the much lower value of critical temperature ($T^{\rm LOVC}_{C}\in\{ 19.1, 15.5, 11.8\}$ K, for $μ^{\star}\in \{0.1, 0.14, 0.2\}$), than would result from calculations not taking this effect into account: $T^{\rm ME}_{C}\in\{ 31.9, 26.9, 21\}$ K. From the technological point of view, the low value of $T_{C}$ limits the possible applications of Li-hBN superconducting properties. The calculations were carried out under the classic Migdal-Eliashberg formalism (ME) and the Eliashberg theory with the lowest-order vertex corrections (LOVC).

cond-mat.supr-con↗

arXiv2019

The unbalanced phonon-induced superconducting state on a square lattice beyond the static boundary

The paper presents our verification of induction of the superconducting state on a square lattice by the linear electron-phonon interaction for values of the unbalance parameter ($γ=λ_{D}/λ_{ND}$) less than $γ_{C}=0.42$. Symbols $λ_{D}$ and $λ_{ND}$ denote the values of the coupling constant in the diagonal and the non-diagonal channel of the self-energy. Calculations were carried out using the Eliashberg equations, in which the order parameter ($Δ_{\bf k}\left(iω_{n}\right)$) and the wave function renormalising factor ($Z_{\bf k}\left(iω_{n}\right)$) depend explicitly on the Matsubara frequency ($ω_{n}$) and the wave vector (${\bf k}$). The value of $γ_{C}$ in the static boundary ($Δ_{\bf k}\left(iω_{n}\right)\rightarrow Δ_{\bf k}\left(iω_{n=1}\right)$), equal to ($0.93$), is significantly greater than the obtained limit value of $0.42$. Values of the thermodynamic functions of the superconducting state determined for our assumptions are significantly different from the values calculated in accordance with the BCS theory. The results were obtained for the electron-phonon interaction function explicitly dependent on the momentum transfer between electron states.

cond-mat.supr-con↗

arXiv2019

Interaction of the hydrogen molecule with the environment: stability of the system

We study the stability of the hydrogen molecule interacting with the environment according to the balanced gain and loss energy scheme. We determined the properties of the molecule taking into account all electronic interactions, where the parameters of the Hamiltonian have been computed by using the variational method. The interaction of the hydrogen molecule with the environment was modeled parametrically ($γ$) with the help of the non-hermitian operator. We have shown that the hydrogen molecule is dynamically unstable. The dissociation time ($T_{D}$) decreases, if the $γ$ parameter increases (for $γ\rightarrow 0$, we get $T_{D}\rightarrow +\infty$). At the dynamic instability of the hydrogen molecule overlaps its static instability as the coupling constant $γ$ increases. We observed the decrease in the dissociation energy and the existence of the metastable state of the molecule ($γ_{MS}=0.659374$~Ry). The hydrogen molecule is statically unstable for $γ>γ_{D}=1.024638$~Ry. One can also observed the $\mathcal{PT}$ symmetry breaking effect for the electronic Hamiltonian ($γ_{\mathcal {PT}}=0.520873$~Ry). However, it does not affect the properties of the hydrogen molecule, such as: the electronic Hamiltonian parameters, the phonon and rotational energy, and the values of the electron-phonon coupling constants.

quant-ph↗

arXiv2018

Influence of strong-coupling and retardation effects on superconducting state in ${\rm YB_{6}}$ compound

In the framework of strong-coupling formalism, we have calculated the thermodynamic parameters of superconducting state in the ${\rm YB_{6}}$ compound. The values of critical temperature ($T_{C}$) are $9.5$~K and $7.9$~K for the Coulomb pseudopotential $μ^{\star}=0.1$ and $0.2$, respectively. In the paper, we have determined the low temperature values of order parameter ($Δ(0)$), specific heat jump at the critical temperature ($ΔC(T_{C})$), and thermodynamic critical field ($H_C(0)$). The dimensionless thermodynamic ratios: $R_Δ=2Δ\left(0\right)/{k_BT_C}$, $R_C=ΔC\left(T_C\right)/C^N\left(T_C\right)$, and $R_H=T_CC^N\left(T_C\right)/H_C^2\left(0\right)$ are equal to: $R_Δ\left(μ^{\star}\right)\in\lbrace 4.48,4.35\rbrace$, $R_{C}\left(μ^{\star}\right)\in\lbrace 2.62,2.55\rbrace$, and $R_{H}\left(μ^{\star}\right)\in\lbrace 0.146,0.157\rbrace$. Due to the significant strong-coupling and retardation effects ($k_{B}T_{C} / ω_{\rm ln}\sim 0.1$) those values highly deviate from the predictions of BCS theory.

cond-mat.supr-con↗