arXiv subjects

Han-Yong Choi

Publications and source records attributed to Han-Yong Choi.

At least 37 records · Page 2Linked to original sources

arXiv2009

Impurity suppression of the critical temperature in the iron-based superconductors

We study the impurity suppression of the critical temperature $T_c$ of the FeAs superconductors theoretically based on the the $\pm$s-wave pairing state of a two band model. The effects of non-magnetic and magnetic impurities are studied with the $\mathcal{T}$-matrix approximation, which can continuously treat impurity scattering from weak to strong coupling limit. We found that both magnetic and non-magnetic impurities suppress $T_c$ with a rate that is practically indistinguishable from the standard d-wave case despite a possibly large difference of the positive and negative s-wave order parameter (OP) magnitudes. This is because the density of states enters together with the OP magnitude for the scattering process.

cond-mat.supr-con↗

arXiv2009

Resonant impurity scattering in the $\pm$s-gap state of the Fe-based superconductors

We study the impurity scattering on the $\pm$s-wave superconductor, with realistic parameters for the Fe-pnictide superconductors. Using $\mathcal{T}$-matrix method, generalized for the two bands, we found that impurity scattering of the unitary limit forms off-centered bound states inside of the superconducting gap, which modifies, surprisingly, the density of states (DOS) of a fully opened gap to a V-shaped one as in the case of a d-wave superconductor. This behavior provides coherent explanations to the several conflicting experimental issues of the Fe-pnictide superconductors: the V-shaped DOS but with an isotropic gap observed in the photoemission and tunneling experiments; the power law behavior of the nuclear spin-lattice relaxation rate ($1/T_1 \approx T^α$ ; $α\approx 3$), down to very low temperatures.

cond-mat.supr-con↗

arXiv2008

Tunneling spectroscopy of $sπ$ pairing state as a model for FeAs superconductors

We present the self-consistent Bogoliubov-de Gennes calculations of an $sπ$ pairing state of two band superconductivity as a model for the FeAs superconductors. The $sπ$ state is an s-wave pairing state with an internal $π$ phase, that is, nodeless gaps on each band but with the opposite sign. The novel features of this state are investigated by calculating the local density of states of the $π$ phase superconductor/normal metal bilayers. Because of the sign reversal between the two condensates, the zero bias conductance peak appears as observed in tunneling spectroscopy experiments on FeAs superconductors. This eliminates the major obstacle to establish the $sπ$ state as the pairing symmetry of the FeAs superconductors.

cond-mat.supr-con↗

arXiv2008

Possible pairing states of the Fe-based superconductors

We consider the minimal two-band model for the Fe-based superconductors with a phenomenological pairing interaction which mimics short-range antiferromagnetic (AFM) fluctuations. Two superconducting (SC) gap solutions are found to exist with the model: sign-changing s-wave gap ($\pm$s-wave) and double d-wave gap states. Both solutions hold the approximate relation $Δ_{h} ^{max} N_h \approx Δ_{e} ^{max} N_e$, a generic feature of two band model with a dominant interband pairing interaction. We carried out the calculations of the SC properties of the both SC states such as the density of states, temperature dependencies of spin-lattice relaxation rate $1/T_1$, Knight shift, and penetration depth, particularly taking into account of the interband coherence factors. The results are discussed in comparison with the currently available experimental data.

cond-mat.str-el↗

arXiv2007

Superconducting critical temperature and singlet and triplet pair functions of superconductor/normal-metal/ferromagnet trilayers

We calculate the superconducting critical temperature $T_c$, the singlet pair function $Ψ^+(x)$, and triplet pair function $Ψ^-(x)$ of superconductor/normal metal/ferromagnet (S/N/F) trilayers using the linearized Usadel equation near $T_c$. The Green's function method developed by Fominov $et al.$ for the S/F bilayers is extended to the S/N/F trilayer systems. The S of the trilayers is taken to be an s-wave singlet pairing superconductor, and the S/N and N/F interfaces are modeled in terms of the interface resistances parameterized, respectively, by $γ_b^{SN}$ and $γ_b^{NF}$. We present the $T_c$, $Ψ^+(x)$, and $Ψ^-(x)$ for typical $γ_b^{SN}$, $γ_b^{NF}$, and the exchange energy $ E_{ex}$: (a) For a small (large) $γ_b^{NF}$, $T_c$ of S/N/F trilayers, as $d_N$ is increased, increases (decreases) on the length scale of N coherence length $ξ_N$ with a discontinuity at $d_N=0$ due to a boundary condition mismatch. (b) $T_c(d_F)$ shows a non-monotonic behavior like S/F bilayers with a weakened shallow dip. (c) The odd frequency triplet component $Ψ^-(x)$, induced by $E_{ex}$ and proximity effects, has a maximum near the N/F interface and decreases on the length scale $ξ_{ex}$ in F. It also penetrates into N and S regions on the length scale $ξ_N$ and $ξ_S$, respectively. Based on these results we make comments on the experimental observation of the odd triplet components and the recent $T_c$ measurements in Nb/Au/CoFe trilayer systems.

cond-mat.supr-con↗

arXiv2005

Proximity Effect in Nb/Au/CoFe Trilayers

We have investigated the superconducting critical temperatures of Nb/Au/CoFe trilayers as a function of Au and CoFe thicknesses. Without the CoFe layer the superconducting critical temperatures of Nb/Au bilayers as a function of Au thickness follow the well-known proximity effect between a superconductor and a normal metal. The superconducting critical temperatures of Nb/Au/CoFe trilayers as a function of Au thickness exhibit a rapid initial increase in the small Au thickness region and increase slowly to a limiting value above this region, accompanied by a small oscillation of Tc. On the other hand, the superconducting critical temperatures of Nb/Au/CoFe trilayers as a function of CoFe thickness show non-monotonic behavior with a shallow dip feature. We analyzed the Tc behavior in terms of Usadel formalism and found that most features are consistent with the theory, although the small oscillation of Tc as a function of the Au thickness cannot be accounted for. We have also found quantitative values for the two interfaces: Nb/Au and Au/CoFe.

cond-mat.supr-con↗

arXiv2005

Quantitative analysis of proximity effect in $Nb/Co_{60}Fe_{40}$, Nb/Ni, and $Nb/Cu_{40}Ni_{60}$ bilayers

We have studied the behavior of the superconducting critical temperature Tc in $Nb/Co_{60}Fe_{40}$, Nb/Ni, and $Nb/Cu_{40}Ni_{60}$ bilayers as a function of the thickness of each ferromagnetic metal layer. The Tc s of three sets of bilayers exhibit non-monotonic behavior as a function of each ferromagnetic metal thickness. Employing the quantitative analysis based on Usadel formalism of the effect of the exchange energy, we observed that the Tc behavior of $Nb/Co_{60}Fe_{40}$ bilayers is in good agreement with the theoretical values over the entire range of the data. On the other hand, the Tc s of Nb/Ni and $Nb/Cu_{40}Ni_{60}$ bilayers show a higher value in the small thickness regime than the theoretical prediction obtained from the calculation, which matches the dip position and the saturation value of Tc in the large thickness limit. This discrepancy is probably due to the weakened magnetic properties of Ni and $Cu_{40}Ni_{60}$ when they are thin. We discuss the values of our fitting parameters and its implication on the validity of the current Usadel formalism of the effect of the exchange energy.

cond-mat.supr-con↗

arXiv2004

Slave-boson approach to the infinite-U Anderson-Holstein impurity model

The infinite-$U$ Anderson-Holstein impurity model is studied with a focus on the interplay between the strong electron correlation and the weak electron-phonon interaction. The slave boson method has been employed in combination with the large degeneracy expansion (1/N) technique. The charge and spin susceptibilities and the phonon propagator are obtained in the approximation scheme where the saddle point configuration and the Gaussian 1/N fluctuations are taken into account. The spin susceptibility is found not to be renormalized by electron-phonon interaction, while the charge susceptibility is renormalized. From the renormalized charge susceptibility the Kondo temperature is found to increase by the electron-phonon interaction. It turns out that the bosonic 1/N Gaussian fluctuations play a very crucial role, in particular, for the phonon propagator.

cond-mat.str-el↗

arXiv2004

Odd triplet pairing effects induced by interface spin-flip scatterings: critical temperature of superconductor/ferromagnet bilayers

The superconducting critical temperature $T_C$ of a superconductor/ferromagnet (S/F) bilayer with spin-flip scatterings at the interface is calculated as a function of the ferromagnet thickness $d_F$ in the dirty limit employing the Usadel equation. The appropriate boundary conditions from the spin-flip scatterings at the S/F interface are derived for the Usadel equation which includes the spin triplet pairing components as well as the spin singlet one. The spin-flip processes induce the spin triplet pairing components with s-wave in momentum and odd symmetry in frequency from the s-wave singlet order parameter $Δ$ of the superconductor region. The induced triplet components alter the singlet order parameter in the superconductor through boundary conditions at the interface and, consequently, change the $T_C$ of an S/F bilayer system. The calculated $T_C(d_F)$, like the case of no spin-flips, shows non-monotonic behavior which typically decreases as $d_F$ is increased from 0 and shows a shallow minimum and then saturates slowly as $d_F$ is further increased. It is well established that as the interface resistance (parameterized in terms of $γ_b$) is increased, the $T_C$ is increased for a given $d_F$ and the non-monotonic feature in $T_C(d_F)$ is strongly suppressed. As the spin flip scattering (parameterized in terms of $γ_m$) is increased, on the other hand, the $T_C$ is also increased for a given $d_F$, but the non-monotonic feature in $T_C(d_F)$ is less suppressed or even enhanced, through the formation of the spin triplet components.

cond-mat.supr-con↗

arXiv2003

Dynamical mean-field theory of Hubbard-Holstein model at half-filling: Zero temperature metal-insulator and insulator-insulator transitions

We study the Hubbard-Holstein model, which includes both the electron-electron and electron-phonon interactions characterized by $U$ and $g$, respectively, employing the dynamical mean-field theory combined with Wilson's numerical renormalization group technique. A zero temperature phase diagram of metal-insulator and insulator-insulator transitions at half-filling is mapped out which exhibits the interplay between $U$ and $g$. As $U$ ($g$) is increased, a metal to Mott-Hubbard insulator (bipolaron insulator) transition occurs, and the two insulating states are distinct and can not be adiabatically connected. The nature of and transitions between the three states are discussed.

cond-mat.str-el↗

arXiv2003

A path integral approach to Anderson-Holstein model

The Anderson-Holstein model is studied in the framework of the semiclassical approximation. Analytic results for Kondo temperature renormalized by weak electron-phonon interaction and for phonon Green function are obtained, and they are interpreted from the viewpoint of dynamical mean field theory. Especially the isotope effect of the effective electron mass is discussed in the presence of strong electron correlation. The results are also compared with those by numerical renormalization group and other related works, and they are consistent with each other in their common domain of validity.

cond-mat.str-el↗

arXiv2002

Self-energy analysis of frequency-dependent conductivity: Application to Pb, Nb, and MgB$_2$ in normal state

We propose and demonstrate a microscopic way to analyze the frequency-dependent infrared conductivity: extraction of the electron self-energy from the inversion of experimentally measured infrared conductivity through the functional minimization and numerical iterations. The self-energy contains the full information on the coherent and incoherent parts of interacting electrons and, therefore, can describe their charge dynamics even when the quasi-particle concept is not valid. From the extracted self-energy, other physical properties such as the Raman intensity spectrum and the effective interaction between electrons can also be computed. We will first demonstrate that the self-energy analysis can be successfully implemented by fitting the frequency-dependent condcutivities of the simple metals such as Pb and Nb, and then calculating the effective interactions between electrons from the extracted self-energies and comparing them with those obtained from the tunneling experiments. We then present the self-energy analysis of the MgB$_2$ superconductors in normal state and clarify some of the controversies in their optical spectra. In particular, the small electron-phonon coupling constant obtained previously is attributed to an underestimate of the plasma frequency.

cond-mat.supr-con↗

arXiv2002

Numerical renormalization group study of the symmetric Anderson-Holstein model: phonon and electron spectral functions

We study the symmetric Anderson-Holstein (AH) model at zero temperature with Wilson's numerical renormalization group (NRG) technique to study the interplay between the electron-electron and electron-phonon interactions. An improved method for calculating the phonon propagator using the NRG technique is presented, which turns out to be more accurate and reliable than the previous works in that it calculates the phonon renormalization explicitly and satisfies the boson sum rule better. The method is applied to calculate the renormalized phonon propagators along with the electron propagators as the onsite Coulomb repulsion $U$ and electron-phonon coupling constant $g$ are varied. As $g$ is increased, the phonon mode is successively renormalized, and for $g \gtrsim g_{co}$ crosses over to the regime where the mode splits into two components, one of which approaches back to the bare frequency and the other develops into a soft mode. The initial renormalization of the phonon mode, as $g$ is increased from 0, depends on $U$ and the hybridization $Δ$; it gets softened (hardened) for $U \gtrsim (\lesssim) U_s (Δ)$. Correlated with the emergence of the soft mode is the central peak of the electron spectral function severely suppressed. These NRG calculations will be compared with the standard Green's function results for the weak coupling regime to understand the phonon renormalization and soft mode.

cond-mat.str-el↗

arXiv2002

Nonmonotonic behavior of resistance in a superconductor-Luttinger liquid junction

Transport through a superconductor-Luttinger liquid junction is considered. When the interaction in the Luttinger liquid is repulsive, the resistance of the junction with a sufficiently clean interface shows nonmonotonic temperature- or voltage-dependence due to the competition between the superconductivity and the repulsive interaction. The result is discussed in connection with recent experiments on single-wall carbon nanotubes in contact with superconducting leads.

cond-mat.mes-hall↗

arXiv2002

Non-Fermi liquid behavior and scaling of low frequency suppression in optical conductivity spectra of CaRuO$_3$

Optical conductivity spectra $σ_1(ω)$ of paramagnetic CaRuO$_3$ are investigated at various temperatures. At T=10 K, it shows a non-Fermi liquid behavior of $σ_1(ω)\sim 1/ω^{\frac 12}$, similar to the case of a ferromagnet SrRuO$_3$. As the temperature ($T$) is increased, on the other hand, $σ_1(ω)$ in the low frequency region is progressively suppressed, deviating from the $1/ω^{\frac 12%}$-dependence. Interestingly, the suppression of $σ_1(ω)$ is found to scale with $ω/T$ at all temperatures. The origin of the $% ω/T$ scaling behavior coupled with the non-Fermi liquid behavior is discussed.

cond-mat.str-el↗

arXiv2001

Zn impurity induced moments and tunneling conductance asymmetry in cuprate superconductors

The effects of a non-magnetic Zn impurity substituting an in-plane Cu are studied by solving the Bogoliubov-de Gennes equation self-consistently which is derived from the \ttju Hamiltonian with all the allowed order parameters included. The Zn impurity, modeled in terms of a potential scatterer in unitary limit, induces local staggered magnetic moments around itself, and the calculated NMR shifts from the induced moments are in agreement with the experimental Cu NMR spectra. We also note that the experimentally observed negative slope of the tunneling conductance can result from the next-nearest hopping $t'$.

cond-mat.supr-con↗

arXiv2001

Scaling of Coulomb pseudo-potential in s-wave narrow-band superconductors

The Coulomb pseudo-potential $μ^*$ is extracted by fitting the numerically calculated transition temperature $T_c$ of the Eliashberg-Nambu equation which is extended to incorporate the narrow-band effects, that is, the vertex correction and the frequency dependence of the screened Coulomb interaction. It is shown that even for narrow-band superconductors, where the fermi energy $ ε_F$ is comparable with the phonon frequency $ ω_{ph}$, the Coulomb pseudo-potential is a pertinent parameter, and is still given by $μ^* = μ/[1+μ\ln(ε_F/ω_{ph})] $, provided $ω_{ph}$ is appropriately scaled.

cond-mat.str-el↗

arXiv2001

Theoretical study of electronic Raman scattering of Borocarbide superconductors

The electronic Raman scattering of Borocarbide superconductors is studied based on the weak coupling theory with $s+g$-wave gap symmetry. The low energy behaviors and the relative peak positions can be naturally understood, while the explanation of the detailed shape of the $B_{1g}$ peak seems to require a strong inelastic interaction not present in the weak coupling theory.

cond-mat.supr-con↗