arXiv subjects

Wonkee Kim

Publications and source records attributed to Wonkee Kim.

26 records · Page 2Linked to original sources

arXiv2001

C-axis Response of a High-T_{c} Superconductor with D-Density Wave Order

The microscopic origin of the pseudogap state which exists in the underdoped cuprates remains unknown. The $c$-axis properties in the pseudogap regime are particularly anomalous. We use a recently proposed model of a $d$-density wave which leads to staggered currents and the doubling of the unit cell to investigate the $c$-axis kinetic energy and the optical sum rule. The density of states expected in the model is also considered.

cond-mat.supr-con↗

arXiv2001

C-axis Optical Sum Rule in Josephson Coupled Vortex State

Observed violations of the $c$-axis optical sum rule can give important information on deviations from in-plane Fermi liquid behavior and on the nature of interlayer coupling between adjacent copper oxide planes. Application of a magnetic field perpendicular to these planes is another way to probe in-plane dynamics. We find that the optical sum rule is considerably modified in the presence of the $c$-axis magnetic field. Interlayer correlation of pancake vortices is involved in the sum rule modification; however, details of the vortex distribution in the plane are less important.

cond-mat.supr-con↗

arXiv2001

Quasiparticle States around a Nonmagnetic Impurity in D-Density-Wave State of High-$T_c$ Cuprates

Recently Chakravarty {\em et al.} proposed an ordered $d$-density wave (DDW) state as an explanation of the pseudogap phase in underdoped high-temperature cuprates. We study the competition between the DDW and superconducting ordering based on an effective mean-field Hamiltonian. We are mainly concerned with the effect of the DDW ordering on the electronic state around a single nonmagnetic impurity. We find that a single subgap resonance peak appears in the local density of state around the impurity. In the unitary limit, the position of this resonance peak is always located at $E_r=-μ$ with respect to the Fermi energy. This result is dramatically different from the case of the pure superconducting state for which the impurity resonant energy is approximately pinned at the Fermi level. This can be used to probe the existence of the DDW ordering in cuprates.

cond-mat.supr-con↗

arXiv2000

Interlayer coupling and the c-axis quasiparticle transport in high-$T_{c}$ cuprates

The c-axis quasiparticle conductivity shows different behavior depending on the nature of the interlayer coupling. For coherent coupling with a constant hopping amplitude $t_{\perp}$, the conductivity at zero frequency and zero temperature $σ(0,0)$ depends on the direction of the magnetic field, but it does not for angle-dependent hopping $t(ϕ)$ which removes the contribution of the nodal quasiparticles. For incoherent coupling, the conductivity is also independent of field direction and changes only when paramagnetic effects are included. The conductivity sum rule can be used to determine the admixture of coherent to incoherent coupling. The value of $σ(0,0)$ can be dominated by $t_{\perp}$ while at the same time $t(ϕ)$ dominates the temperature dependence of the superfluid density.

cond-mat.supr-con↗

arXiv2000

Effects of an in-plane magnetic field on c-axis sum rule and superfluid density in high-$T_{c}$ cuprates

In layered cuprates, the application of an in-plane magnetic field $({\bf H})$ changes the c-axis optical sum rule and superfluid density $ρ_{s}$. For pure incoherent c-axis coupling, ${\bf H}$ has no effect on either quantities but it does if an additional coherent component is present. For the coherent contribution, different characteristic variations on ${\bf H}$ and on temperature result from the constant part $(t_{\perp})$ of the hopping matrix element and from the part $(t_ϕ)$ which has zero on the diagonal of the Brillouin zone. Only the constant part $(t_{\perp})$ leads to a dependence on the direction of ${\bf H}$ as well as on its magnitude.

cond-mat.supr-con↗

arXiv2000

Conductivity sum rule, implication for in-plane dynamics and c-axis response

Recently observed $c$-axis optical sum rule violations indicate non-Fermi liquid in-plane behavior. For coherent $c$-axis coupling, the observed flat, nearly frequency independent $c$-axis conductivity $σ_{1}(ω)$ implies a large in-plane scattering rate $Γ$ around $(0,π)$ and therefore any pseudogap that might form at low frequency in the normal state will be smeared. On the other hand incoherent $c$-axis coupling places no restriction on the value of $Γ$ and gives a more consistent picture of the observed sum rule violation which, we find in some cases, can be less than half.

cond-mat.supr-con↗

arXiv2000

Conductivity sum rule; comparison of coherent and incoherent c-axis coupling

We calculate the $c$-axis kinetic energy difference between normal and superconducting state for coherent and for incoherent interlayer coupling between CuO$_{2}$ planes. For coherent coupling the ratio of the missing conductivity spectral weight to the superfluid density is equal to one and there is no violation of the conventional sum rule, but for the incoherent case we find it is always greater than one whatever the nature of the impurity potential may be. To model more explicitly YBa$_{2}$Cu$_{3}$O$_{7-x}$ around optimum doping, which is found to obey the sum rule, we consider a plane-chain model and show that the sum rule still applies. A violation of the sum rule of either sign is found even for coherent coupling when the in-plane density of electronic states depends on energy on a scale of the order of the gap.

cond-mat.supr-con↗

arXiv1999

Competing effects of mass anisotropy and spin Zeeman coupling on the upper critical field of a mixed $d$- and s-wave superconductor

Based on the linearized Eilenberger equations, the upper critical field $(H_{c2})$ of mixed d- and s-wave superconductors has been microscopically studied with an emphasis on the competing effects of mass anisotropy and spin Zeeman coupling. We find the mass anisotropy always enhance $H_{c2}$ while the Zeeman interaction suppresses $H_{c2}$. As required by the thermodynamics, we find $H_{c2}$ is saturated at zero temperature. We compare the theoretical calculations with recent experimental data of YBa$_{2}$Cu$_{3}$O$_{7-+AFw-delta}$.

cond-mat.supr-con↗