arXiv subjects

Dakyeong Kim

Publications and source records attributed to Dakyeong Kim.

5 recordsLinked to original sources

arXiv2022

Spin Susceptibility of a J=3/2 Superconductor

We discuss the spin susceptibility of superconductors in which a Cooper pair consists of two electrons having the angular momentum J=3/2 due to strong spin-orbit interactions. The susceptibility is calculated analytically for pseudospin quintet states in a cubic superconductor within the linear response to a Zeeman field. The susceptibility for $A_{1g}$ symmetry states is isotropic in real space. For $E_g$ and $T_{2g}$ symmetry cases, the results depend sensitively on choices of order parameter. The susceptibility is isotropic for a $T_{2g}$ symmetry state, whereas it becomes anisotropic for an $E_{g} $ symmetry state. We also find in a $T_{2g}$ state that the susceptibility tensor has off-diagonal elements.

cond-mat.supr-con↗

arXiv2021

Quasiparticle on Bogoliubov Fermi Surface and Odd-Frequency Cooper Pair

We discuss a close relationship between a quasiparticle on the Bogoliubov Fermi surface and an odd-frequency Cooper pair in a superconductor in which a Cooper pair consisting of two j=3/2 electrons forms the pseudospin-quintet even-parity pair potential with breaking time-reversal symmetry. It has been established in a single-band superconductor that a low-energy quasiparticle below the superconducting gap accompanies an odd-frequency Cooper pair. In this paper, we show that an odd-frequency pair characterized by chirality coexists with a quasiparticle on the Bogoliubov Fermi surface. The symmetry of odd-frequency Cooper pairs is analyzed in detail by taking realistic pair potentials into account in a cubic superconductor.

cond-mat.supr-con↗

arXiv2021

Josephson effect of superconductors with $J=3/2$ electrons

The angular momentum of an electron is characterized well by pseudospin with $J=3/2$ in the presence of strong spin-orbit interactions. We study theoretically the Josephson effect of superconductors in which such two $J=3/2$ electrons form a Cooper pair. Within even-parity symmetry class, pseudospin-quintet pairing states with $J=2$ can exist as well as pseudospin-singlet state with $J=0$. We focus especially on the Josephson selection rule among these even-parity superconductors. We find that the selection rule between quintet states is severer than that between spin-triplet states formed by two $S=1/2$ electrons. The effects of a pseudospin-active interface on the selection rule are discussed as well as those of odd-frequency Cooper pairs generated by pseudospin dependent band structures.

cond-mat.supr-con↗

arXiv2020

Bulk-edge correspondence in nonunitary Floquet systems with chiral symmetry

We study topological phases in one-dimensional open Floquet systems driven by chiral symmetric nonunitary time evolution. We derive a procedure to calculate topological numbers from nonunitary time-evolution operators with chiral symmetry. While the procedure has been applied to open Floquet systems described by nonunitary time-evolution operators, we give the microscopic foundation and clarify its validity for the first time. We construct a model of chiral symmetric nonunitary quantum walks classified into class BDI$^\dagger$ or AIII, which is one of enlarged symmetry classes for topological phases in open systems, based on experiments of discrete-time quantum walks. Then, we confirm that the topological numbers obtained from the derived procedure give correct predictions of the emergent edge states. We also show that the model retains $\mathcal{PT}$ symmetry in certain cases and its dynamics is crucially affected by the presence or absence of $\mathcal{PT}$ symmetry.

cond-mat.mes-hall↗

arXiv2016

Explicit definition of $\mathcal{PT}$ symmetry for non-unitary quantum walks with gain and loss

$\mathcal{PT}$ symmetry, that is, a combined parity and time-reversal symmetry is a key milestone for non-Hermite systems exhibiting entirely real eigenenergy. In the present work, motivated by a recent experiment, we study $\mathcal{PT}$ symmetry of the time-evolution operator of non-unitary quantum walks. We present the explicit definition of $\mathcal{PT}$ symmetry by employing a concept of symmetry time frames. We provide a necessary and sufficient condition so that the time-evolution operator of the non-unitary quantum walk retains $\mathcal{PT}$ symmetry even when parameters of the model depend on position. It is also shown that there exist extra symmetries embedded in the time-evolution operator. Applying these results, we clarify that the non-unitary quantum walk in the experiment does have $\mathcal{PT}$ symmetry.

quant-ph↗