Sung-Sik Lee

High-pressure synthesis of the homogeneous infinite-layer superconductor Sr0.9La0.1CuO2

Abstract We synthesized the electron-doped infinite-layer superconductor Sr 0.9 La 0.1 CuO 2 in the high-pressure condition of about 4 GPa. Structural and low-field magnetization studies demonstrated that the superconducting properties of our samples were far superior to any reported results.

Holon pair condensation and phase diagram of high- T c cuprates

A possibility of holon (boson) pair condensation is explored for hole doped high T_c cuprates, by using the U(1) slave-boson representation of the t-J Hamiltonian with the inclusion of hole-hole repulsion. A phase diagram of the hole doped high T_c cuprates is deduced by allowing both the holon pairing and spinon pairing. It is shown that the spin gap size remains nearly unchanged below the holon pair condensation temperature. We find that the s-wave holon pairing under the condition of d-wave singlet pairing is preferred, thus allowing d-wave hole pairing.

Origin of the hump structure in the in-plane optical conductivity of high-TC cuprates based on a SU(2) slave-boson theory

An improved version of SU(2) slave-boson approach is applied to study the in-plane optical conductivity of the two dimensional systems of high Tc cuprates. We investigate the role of fluctuations of both the phase and amplitude of order parameters on the (Drude) peak-dip-hump structure in the in-plane conductivity as a function of hole doping concentration and temperature. The mid-infrared(MIR) hump in the in-plane optical conductivity is shown to originate from the antiferromagnetic spin fluctuations of short range(the amplitude fluctuations of spin singlet pairing order parameters), which is consistent with our previous U(1) study. However the inclusion of both the phase and amplitude fluctuations is shown to substantially improve the qualitative feature of the optical conductivity by showing substantially reduced Drude peak widths for entire doping range. Both the shift of the hump position to lower frequency and the growth of the hump peak height with increasing hole concentration is shown to be consistent with observations.

Doping and temperature dependence of superfluid weight and spectral function : Universal scaling behavior of the pseudogap

Using the improved U(1) and SU(2) slave-boson approaches of the t-J Hamiltonian that we developed recently, we study the doping and temperature dependence of superfluid weight and spectral function and discuss our finding of the universal scaling behavior of pseudogap. It is shown that at low hole doping concentrations and at low temperatures there exists a propensity of a linear decrease in the superfluid weight with temperature, and a tendency of doping independence in the linearly decreasing slopes of the superfluid weight with temperature in qualitative agreement with the experiments. It is also demonstrated that there exists the boomerang behavior, that is, both Tc and the superfluid weight increase with hole doping concentration x in the underdoped region, reaches a saturation(maximum) at a hole doping above optimal doping and decreases beyond the saturation point in the overdoped region in agreement with mu-SR measurements. We further investigate the doping and temperature dependence of spectral function and discuss our theoretical finding of a scaling behavior of pseudogap. In addition we discuss the cause of hump and quasi-particle peak in the observed spectral functions of high Tc cuprates. It is demonstrated that the sharpening of the observed quasi-particle peak below Tc is attributed to the bose condensation of holon pair in agreement with observations. From the predicted ratios of pseudogap to both the superconducting temperature Tc and the pseudogap temperature T* respectively as a function of hole doping concentration x, we find that there exists a universal scaling behavior (sample independence) of these ratios with doping, by showing a nonlinearly decreasing behavior of the former and a near doping independence of the latter.

Origin of peak-dip-hump structure in the in-plane optical conductivity of the high- T C cuprates: Role of antiferromagnetic spin fluctuations of short-range order

An improved U(1) slave-boson approach is applied to study the optical conductivity of the two-dimensional systems of antiferromagnetically correlated electrons over a wide range of hole doping and temperature. Interplay between the spin and charge degrees of freedom is discussed to explain the origin of the peak-dip-hump structure in the in-plane conductivity of high-

PHONON SCATTERING BY BREATHERS IN THE DISCRETE NONLINEAR SCHRÖDINGER CHAIN

{T}_{C}

Holon pair bose condensation based on the SU(2) slave-boson approach to the t–J Hamiltonian; instability of the normal state against holon pairing

cuprates. The role of spin fluctuations of short-range order (spin singlet pair) is investigated. It is shown that the spin fluctuations of the short-range order can cause the midinfrared hump, by exhibiting a linear increase of the hump frequency with the antiferromagnetic Heisenberg coupling strength.

Study of hole pair condensation based on the SU(2) slave-boson approach to the t}J Hamiltonian

Linear theory for phonon scattering by discrete breathers in the discrete nonlinear Schrodinger equation using the transfer matrix approach is presented. Transmission and reflection coefficients are obtained as a function of the wave vector of the input phonon. The occurrence of a nonzero transmission, which in fact becomes perfect for a symmetric breather, is shown to be connected with localized eigenmodes thresholds. In the weak-coupling limit, the perfect reflection is shown to exist, which requires two scattering channels. A necessary condition for a system to have a perfect reflection is also considered in a general context.

Holon-pair condensation of high {Tc} cuprates: Supersymmetry conditions

Abstract Based on the application of the SU(2) slave-boson theory to the Bethe–Salpeter equation for the boson (holon) sector, we derive the bose condensation temperature T c as a function of hole doping rate for high T c cuprates. It is shown that the computed results of the t -matrix yielded good agreement with the functional integral approach of the SU(2) slave-boson theory in reproducing the bose condensation temperature T c of an arch shape as a function of doping rate.

Role of Antiferromagnetic Fluctuations in High Temperature Superconductivity

Abstract We present a phase diagram of high T c cuprates involving the holon pair condensation based on the SU(2) slave-boson approach to the t – J Hamiltonian. Owing to the proper account of low-energy phase fluctuations by the SU(2) theory, we find that the predicted holon pairing temperature (the superconducting transition temperature) is in better agreement with experiments, compared to the U(1) mean field theory.