arXiv subjects

Hyun-Tak Kim

Publications and source records attributed to Hyun-Tak Kim.

41 records · Page 3Linked to original sources

arXiv2002

Extended Brinkman-Rice Picture and Its Application to High-Tc Superconductors

This is a full-paper of the paper(Physica C341-348, 259(2000)) published previously. The effective charge and the Coulomb energy are justified by means of measurement. Theoretical calculations of the effective mass depending on band filling are also given by Gutzwiller's variational calculations. On the basis of the concept of measurement, the effective mass is an average of the true effective mass in the Brinkman-Rice(BR) picture for metal phase and is the effect of measurement. The true correlation strength (U/U_c=k_{BR}) in the BR picture is evaluated as 0.92<k_{BR}<1 for YBCO. High-Tc superconductivity is attributed to the true effective mass caused by the large k_{BR} value. Furthermore, the validity of the BR picture is indirectly proved through the extended BR picture.

cond-mat.str-el↗

arXiv2001

Justification of the effective fractional Coulomb energy and the extended Brinkman-Rice picture

In order to calculate the effective mass of quasiparticles for a strongly correlated metallic system in which the number of carriers, n, is less than that of atoms (or lattices), l, the metallic system is averaged by one effective charge per atom over all atomc sites. The effective charge of carriers, =(n/l)e={rho}e, and the on-site Coulomb repulsion, U={rho}^2 ={rho}^2kU_c, are defined and are justified by means of measurement. {rho} is band filling and k is the correlation strength. The effective mass, m^*/m=1/(1-k^2{rho}^4), calculated by the Gutzwiller variational theory, is regarded as the average of the true effective mass in the Brinkman-Rice (BR) picture and is the effect of measurement. The true effective mass has the same value irrespective of {rho}, and is not measured experimentally. The true correlation strength of the BR picture is evaluated as 0.90<k_{BR}<1 for Sr_{1-x}La_xTiO_3 and 0.92<k_{BR}<1 for YBCO. High-Tc superconductivity for YBCO is attributed to the true effective mass caused by the large k_{BR} value. The effective mass explains the metal-insulator transition of the first order on band filling. Furthermore, the pseudogap is predicted in this system.

cond-mat.str-el↗

arXiv2001

Temperature dependences of the surface resistance and the diamagnetic shielding susceptibility at T_c-T<<T_c for high-T_c superconductors

At T_c-T<<T_c(i.e., near T_c), in order to demonstrate the condudction mechanism and temperature dependencies of the diamagnetic-shielding susceptibility and the penetration depth, we fabricated Ba_1_xBiO_3(BKBO) thin films and measured the energy gap by tunnel effect and shielding susceptibilities which are compared with those measureed for BKBO and YBCO single crystals. The shielding susceptibilities for BKBO and YBCO single crystals well-fit χ(T)/χ(0)=1-exp(-2\triangle(T)/k_BT), while that for the BKBO film follows χ(T)/χ(0)=1-T/T_c) which may not be intrinsic. The exponential decrease of the susceptibilities near T_c indicates that the conduction mechanism is hopping. The energy gaps are observed as 2\triangle(0)=(3.5+-0.1)k_BT_c for the BKBO film by the tunnel effect, 2\triangle(0)=(3.9+-0.1)k_BT_c for the BKBO single crystal, and 2\triangle(0)=(8+-0.2)k_BT_c for the YBCO single crystal. Furthermore, for microwave device applications of superconductors, at T_c-T<<T_c, the surface resistance R_s(T) is derived from the surface impedance at ωτ_(tr)<<1, where σ_s(0) and σ_n are the conductivities of the superconducting state and the normal state, respectively, and f(T)=χ(T)/χ(0)=1-exp(-2\tringle(T)/k_BT).

cond-mat.supr-con↗

arXiv2001

Extension of the Brinkman-Rice picture and the Mott transition

In order to explain the metal-Mott-insulator transition, the Brinkman-Rice (BR) picture is extended. In the case of less than one as well as one electron per atom, the on-site Coulomb repulsion is given by U={kappa}{rho}^2U_c by averaging the electron charge per atom over all atomic sites, where {kappa} is the correlation strength of U, {rho} is the band filling factor, and U_c is the critical on-site Coulomb energy. The effective mass of a quasiparticle is found to be m*/m=1/{1-{kappa}^2{rho}^4} for 0<{kappa}{rho}^2<1 and seems to follow the heat capacity data of Sr_{1-x}La_xTiO_3 and YBa_2Cu_3O_{7-delta} at {kappa}=1 and 0<{kappa}{rho}^2<1. The Mott transition of the first order occurs at {kappa}{rho}^2=1 and a band-type metal-insulator transition takes place at {kappa}{rho}^2=0. This Mott transition is compared with that in the d=infinity Hubbard model. The effective mass for 2D-DOS instead of the vHs can be used for the mechanism of high T_c superconductivity.

cond-mat.str-el↗

arXiv2000

Observation of a linear temperature dependence of the critical current density in a Ba_{0.63}K_{0.37}BiO_3 single crystal

For a Ba_{0.63}K_{0.37}BiO_3 single crystal with T_c=31 K, H_{c1}=750 Oe at 5 K, and dimensions 3x3x1 mm^3, the temperature and field dependences of magnetic hysteresis loops have been measured within 5-25 K in magnetic fields up to 6 Tesla. The critical current density is J_c(0)=1.5 x 10^5 A/cm^2 at zero field and 1 x 10^5 A/cm^2 at 1 kOe at 5 K. J_c decreases exponentially with increasing field up to 10 kOe. A linear temperature dependence of J_c is observed below 25 K, which differs from the exponential and the power-law temperature dependences in high-Tc superconductors including the BKBO. The linear temperature dependence can be regarded as an intrinsic effect in superconductors.

cond-mat.supr-con↗