arXiv subjects

Hae-Young Kee

Publications and source records attributed to Hae-Young Kee.

At least 181 records · Page 10Linked to original sources

arXiv2001

Specific heat anomaly in the d-density wave state and emergence of incommensurate orbital antiferromagnetic order

We study the effect of finite chemical potential on the d-density wave state that has been proposed to explain the pseudogap phenomena in underdoped cuprates. We find that the specific heat anomaly at the transition temperature, below which the d-density wave state forms, gets weaker when finite chemical potential is introduced. This provides a useful ground for the proper interpretation of the specific heat measurement in regard to the existence of the d-density wave state below the pseudogap temperature. Further increase of the chemical potential leads to an incommensurate orbital antiferromagnetic state before the system eventually turns into the normal state. This is an inhomogeneous state characterized by novel charge ordering and an analog of the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state in superconductivity.

cond-mat.str-el↗

arXiv2001

What the resonance peak cannot do

In certain cuprates, a spin 1 resonance mode is prominent in the magnetic structure measured by neutron scattering. It has been proposed that this mode is responsible for significant features seen in other spectroscopies, such as photoemission and optical absorption, which are sensitive to the charge dynamics, and even that this mode is the boson responsibile for ``mediating'' the superconducting pairing. We show that its small (measured) intensity and weak coupling to electron-hole pairs (as deduced from the measured lifetime) disqualifies the resonant mode from either proposed role.

cond-mat.str-el↗

arXiv2001

Neutron Scattering Signature of d-density Wave Order in the Cuprates

An ordered $d$-density wave (DDW) state has been proposed as an explanation of the pseudogap phase in underdoped high-temperature superconductors. The staggered currents associated with this order have signatures which are qualitatively different from those of ordered spins. We apply the order parameter theory to an orthorhombic bilayer system and show that the expected magnitude as well as the momentum, energy, and polarization dependence of the consequent neutron scattering is consistent with the findings of a recent experiment.

cond-mat.supr-con↗

arXiv2001

Spin and Current Correlation Functions in the \bf d-density Wave State of the Cuprates

We calculate the spin-spin and current-current correlation functions in states exhibiting d_{{x^2}-{y^2}}-density wave (DDW) order, d_{{x^2}-{y^2}} superconducting order (DSC), or both types of order. The spin-spin correlation functions in a state with both DDW and DSC order and in a state with DDW order alone, respectively, illuminate the resonant peak seen in the superconducting state of the underdoped cuprates and the corresponding feature seen in the pseudogap regime. The current-current correlation function in a state with both DDW and DSC order evinces a superfluid density with doping dependence which is consistent with that of the underdoped cuprates. These calculations strengthen the identification of the pseudogap with DDW order and of the underdoped cuprates with a state with both DDW and DSC order.

cond-mat.supr-con↗

arXiv2000

Effect of Doping-induced Disorder on Transition Temperature in High $T_c$ Cuprates

We study the effect of disorder induced by doping on the transition temperature in high $T_c$ cuprates. Since the impurity lies between the CuO$_2$ planes, the momentum transfer is restricted by small angle with a range of order of $1/(d k_F)$, where $k_F$ is the Fermi momentum and $d$ is the distance between the plane and the impurity. We find that the leading correction on the transition temperature in this case is in cubic order of $1/(d k_F)$, while the single particle scattering rate is linearly proportional to $1/(d k_F)$. Therefore, we conclude that the disorder induced by doping has rather small effect on $T_c$, while it could give large single particle scattering rate. We find that the reduction of $T_c$ is about $3-10 %$, using the single particle scattering rate ($150-450 K$) observed in the recent angle-resolved- photoemission-spectroscopy data on optimally doped Bi2212. The consequences of the small angle scattering in other physical quantities such as transport are also discussed.

cond-mat.supr-con↗

arXiv2000

Spin waves in a two dimensional p-wave superconductor: Sr$_2$RuO$_4$

We study spin excitations in a two dimensional p-wave superconductor with ${\vec Δ} = {\hat d}({\hat k}_1 \pm i {\hat k}_2)$ symmetry in the context of the newly discovered superconducting Sr$_2$RuO$_4$. The polarization and spectrum of spin wave excitations are identified and their experimental consequences are discussed.

cond-mat.supr-con↗

arXiv2000

Collective modes and sound propagation in a p-wave superconductor: Sr$_2$RuO$_4$

There are five distinct collective modes in the recently discovered p-wave superconductor Sr$_2$RuO$_4$; phase and amplitude modes of the order parameter, clapping mode (real and imaginary), and spin wave. The first two modes also exist in the ordinary s-wave superconductors, while the clapping mode with the energy $\sqrt{2} Δ(T)$ is unique to Sr$_2$RuO$_4$ and couples to the sound wave. Here we report a theoretical study of the sound propagation in a two dimensional p-wave superconductor. We identified the clapping mode and study its effects on the longitudinal and transverse sound velocities in the superconducting state. In contrast to the case of $^3$He, there is no resonance absorption associated with the collective mode, since in metals $ω/(v_F |{\bf q}|) \ll 1$, where $v_F$ is the Fermi velocity, {\bf q} is the wave vector, and $ω$ is the frequency of the sound wave. However, the velocity change in the collisionless limit gets modified by the contribution from the coupling to the clapping mode. We compute this contribution and comment on the visibility of the effect. In the diffusive limit, the contribution from the collective mode turns out to be negligible. The behaviors of the sound velocity change and the attenuation coefficient near $T_c$ in the diffusive limit are calculated and compared with the existing experimental data wherever it is possible. We also present the results for the attenuation coefficients in both of the collisionless and diffusive limits at finite temperatures.

cond-mat.supr-con↗

arXiv2000

Half-quantum vortex and d-soliton in Sr$_2$RuO$_4$

Assuming that the superconductivity in Sr$_2$RuO$_4$ is described by a planar p-wave order parameter, we consider possible topological defects in Sr$_2$RuO$_4$. In particular, it is shown that both of the ${\hat d}$-soliton and half-quantum vortex can be created in the presence of the magnetic field parallel to the $a$-$b$ plane. We discuss how one can detect the ${\hat d}$-soliton and half-quantum vortex experimentally.

cond-mat.supr-con↗

arXiv2000

On Measuring Condensate Fraction in Superconductors

An analysis of off-diagonal long-range order in superconductors shows that the spin-spin correlation function is significantly influenced by the order if the order parameter is anisotropic on a microscopic scale. Thus, magnetic neutron scattering can provide a direct measurement of the condensate fraction of a superconductor. It is also argued that recent measurements in high temperature superconductors come very close to achieving this goal.

cond-mat.supr-con↗

arXiv1999

Probing Pairing Symmetry in Sr$_2$RuO$_4$

We study the spin dynamics in a p-wave superconductor at the nesting vector associated with $α$ and $β$ bands in Sr$_2$RuO$_4$. We find a collective mode at the nesting vector in the superconducting phase identified as the odd-parity pairing state which breaks time reversal symmetry. This mode in the spin channel only exists in the p-wave superconductor, not in s- or d-wave superconductors. We propose that probing this mode would clarify the pairing symmetry in Sr$_2$RuO$_4$. The possibility of second superconducting phase transition is also discussed.

cond-mat.supr-con↗

arXiv1998

Frustrated kinetic energy, the optical sum rule, and the mechanism of superconductivity

The theory that the change of the electronic kinetic energy in a direction perpendicular to the CuO-planes in high-temperature superconductors is a substantial fraction of the condensation energy is examined. It is argued that the consequences of this theory based on a rigorous $c$-axis conductivity sum rule are consistent with recent optical and penetration depth measurements.

cond-mat.supr-con↗

arXiv1998

Incommensurate Charge and Spin Fluctuations in d-wave Superconductors

We show analytic results for the irreducible charge and spin susceptibilities, $χ_0 (ω, {\bf Q})$, where ${\bf Q}$ is the momentum transfer between the nodes in d-wave superconductors. Using the BCS theory and a circular Fermi surface, we find that the singular behavior of the irreducible charge susceptibility leads to the dynamic incommensurate charge collective modes. The peaks in the charge structure factor occur at a set of wave vectors which form an ellipse around ${\bf Q}_π=(π,π)$ and ${\bf Q}_0=(0,0)$ in momentum space with momentum dependent spectral weight. It is also found that, due to the non-singular irreducible spin susceptibility, an extremely strong interaction via random phase approximation is required to support the magnetic peaks near ${\bf Q}_π$. Under certain conditions, the peaks in the magnetic structure factor occur near ${\bf Q}=(π,π(1 \pm δ))$ and $(π(1 \pm δ),π)$.

cond-mat.supr-con↗

arXiv1998

Polarizability and single particle spectra of two-dimensional s- and d-wave superconductors

We present analytic results for the polarizabilities, $χ({\bf Q},ω)$, in the charge, spin, and current channels for two-dimensional s- and d-wave superconductors for large momentum transfer. A collective mode in the charge channel is predicted to exist for extremum vectors of the Fermi-surface with energy below twice the maximum superconducting gap. Such modes are directly observable through inelastic x-ray or electron scattering. Scattering of single particle excitations by these collective modes leads to several unusual features in the single particle spectrum in the superconducting state which are seen in angle resolved photoemission experiments.

cond-mat.supr-con↗

arXiv1998

Theory of Tunneling Anomaly in Superconductor above Paramagnetic Limit

We study the tunneling density of states (DoS) in the superconducting systems driven by Zeeman splitting $E_Z$ into the paramagnetic phase. We show that, even though the BCS gap disappears, superconducting fluctuations cause a strong DoS singularity in the vicinity of energies $-E^*$ for electrons polarized along the magnetic field and $E^*$ for the opposite polarization. The position of this singularity $E^*=\case{1}{2}(E_Z + \sqrt{E_Z^2- Δ^2})$ (where $Δ$ is BCS gap at $E_Z=0$) is universal. We found analytically the shape of the DoS for different dimensionality of the system. For ultra-small grains the singularity has the shape of the hard gap, while in higher dimensions it appears as a significant though finite dip. The spin-orbit scattering, and the orbital magnetic field suppress the singularity. Our results are qualitatively consistent with recent experiments in superconducting films.

cond-mat.supr-con↗

arXiv1997

Conductance fluctuations at the fractional quantum Hall plateau transitions

We obtain a ``mean field'' scaling flow of the longitudinal and the Hall conductivities in the fractional quantum Hall regime. Using the composite fermion picture and assuming that the composite fermions follow the Khmelnitskii-Pruisken scaling flow for the integer quantum Hall effect, the unstable fixed points which govern the transitions between different fractional quantum Hall states are identified. Distributions of the critical longitudinal and Hall conductivities at the unstable fixed points are obtained and implications of the results for the experiments on mesoscopic quantum Hall systems are discussed.

cond-mat.mes-hall↗

arXiv1997

Phonon Anomalies Induced by Superconductivity

We calculate the electronic polarizability in the superconducting state near extremum vectors ${\vec Q}_0$ of the Fermi surface. A pole appears in the polarizability at frequencies $ω$ near the superconducting gap $2Δ$ which leads to a sharp peak at $ω$ just below $2 Δ$ in the lattice vibrations near ${\vec Q}_0$. A second order transition to a charge density wave state below the superconducting transition is shown to be unlikely. The results are compared with the recent inelastic neutron scattering measurements in compounds, $ANi_2B_2C$, $A=Y$ or $Lu$.

cond-mat.supr-con↗

arXiv1996

Temperature and Pressure Effects on the Resistivity of the Manganese Oxides

The temperature and pressure effects on the resistivity of the manganese oxides are studied analytically via the Kondo lattice model with a strong Hund's coupling. We obtain analytically the single-particle density of states on the Bethe lattice in the large connectivity limit using the analytical variant of the dynamic Lanczos method. From the density of states for the doped system, we obtain resistivity and show resistivity drop when temperature crosses the magnetic transition point. We also demonstrate the effect of pressure both below and above the transition temperature.

cond-mat↗

arXiv1995

Strong correlation effects in the doped Hubbard model in infinite dimensions

The density of states and the optical conductivity of the doped Hubbard model on a Bethe lattice with infinite connectivities have been studied using an analytic variant of the Lanczos continued fraction method. The spectral weight of the gap states and the position of the chemical potential upon hole or electron doping have been studied. We argue that the strong correlation effects such as gap states and midinfrared band shown in two dimensions also appear in infinite dimensions.

cond-mat↗