C.M. Lee

Low-lying energy spectrum of a magnetopolaron bound to a Coulomb impurity in a quantum dot

Abstract By using an exact numerical diagonalization scheme, we studied the low-lying spectrum of a magnetopolaron quantum dot with a confined parabolic potential in the presence of a Coulomb impurity at the center. Continuous ground-state energy transitions induced by the magnetic field are reported. Furthermore, the effect of different electron–LO phonon interaction strengths on the energy spectrum has been predicted.

Polaron effect on energy spectrum in two-electron quantum dot under magnetic field

Abstract Numerical results for the low-lying spectra of parabolic quantum dots in which two electrons interact with each other through both coulomb repulsion and longitudinal-optical (LO) phonon under a magnetic field are presented. With typical semiconducting GaAs-based materials, the orbital angular momentum L-transition and/or spin angular momentum S-transition takes place in the ground state, which is qualitatively the same as that without the electron–LO phonon interaction. The effect of different electron–phonon coupling strengths on low-lying spectra are also predicted.

Can the effective pinning potential of MgB2 behave logarithmically in the high temperature range

Abstract The pinning potential barrier U o ( T , H ) is the main parameter describing the flux pinning of MgB 2 . Comparing the experimental results published recently by Thompson et al. and Bygoslavsky et al., one can prove theoretically that in the high temperature range, U o ( T , H ) is of logarithmic form similar to that suggested for high T c superconductors by Zeldov et al.. Meanwhile, the major pinning centers of MgB 2 employ the planar pinning mechanism. Furthermore, it is found that U o ( T , H ) depends on the temperature linearly and also on the magnetic field with the form H −1∼−2 at high temperature range.

Temperature dependence of strongly coupled surface polaron in potassium iodide semi-infinite polar crystal

Temperature dependences of the effective Hamiltonian and the renormalized mass of surface polaron, for which the electron is strongly coupled with the surface-optical (SO) phonons but weakly or intermediately coupled with the bulk longitudinal-optical (LO) phonons, are derived. Temperature dependences of the self-energy and the renormalized mass of the polaron for the polar crystal of potassium iodide are evaluated as a function of the depth from the crystal surface. Results of our calculation indicate that the self-trapping energy increases for strong electron-SO-phonon coupling but decreases for weak electron-LO-phonon coupling with temperature.

AC response studying on MgB2 superconductor by Maley’s method

Abstract We have measured the imaginary susceptibility of MgB 2 at different frequencies and at various dc applied magnetic fields up to 4 T. The method proposed firstly by Maley et al. on fixed applied magnetic field relaxation measurement was used to analyze the data. The information of the flux activation energy so extracted is found to be a nonlinear function of the current density, indicating a logarithmic relation U ( H , J )= U 0 ( H )ln( J 0 / J ) of the current density in this material. The dependence of the activation energy on temperature is determined by a power law U ( T )∝(1−( T / T irr ) 2 ). The functional dependence U 0 ( H )∝ H −0.8 on the applied magnetic field is determined to be in the sample.

Pinning characteristics of MgB2 near the melting curve

A less controversial method is used to analyse the imaginary part of the ac susceptibility measurement of MgB2. The pinning potential barrier Uo(T, H) is obtained near the melting curve. It is found that Uo(T, H) depends linearly on both the temperature and the applied field.

Vortex-line shape and second peak effect in high-Tc superconductors

The origin of the second peak effect in melt-textured YBa2Cu3O7-δ was investigated using the sweeping rate dependence of magnetic hysteresis loops. An anomalous change of the vortex-line shape was found around the second peak position. This phenomenon was ascribed to the competition between elastic energy Eel and pinning energy Epin of the flux system. By comparing it with the results of crystal samples such as YBCO, NCCO and Pb-doped BSCCO, we concluded that this characteristic is uniform for these high-Tc superconductors and should be responsible for the widely observed second peak effect in magnetization measurements.

AC response and current density relaxation

Abstract The current density relaxation equation on account for the derivative of the ac external field has been derived. The effective attempt time for ac response, t eff ac , differs from that obtained under the condition of the static magnetic field by Blatter et al. . The current relaxation equations under different models of the effective pinning potential have been obtained and discussed. In very short time window, say 10 −5 – 10 −3 s, we have found that the current density of the Hg 0.69 Pb 0.31 Ba 2 Ca 3 O 8+δ superconductor has nonlogarithmic relaxation characteristic behaviour. This characteristic nicely agrees with the collective pinning model. Furthermore, we also introduce and consider the dependence of t eff ac in the current relaxation study, and the effect of the t eff ac on the study of current relaxation has been discussed.

Investigating E–J characteristics of melt-textured YBa2Cu3O7−δ

Abstract We studied the E – J characteristics of a melt-textured YBa 2 Cu 3 O 7− δ sample by measuring its sweeping rate dependence of magnetization with a vibrating sample magnetometer. Firstly, we derived the relationship between the electric field and sweeping rates from flux conservation equation, and furthermore developed a method of obtaining ln E –ln J curves and thermal excitation potential U 0 ( T , B ) from magnetization measurement. Then, curves of ln E –ln J and U 0 ( T , B ) were deduced from the magnetic measurements on a melt-textured YBa 2 Cu 3 O 7− δ sample prepared by MTG method, and thereby different dynamic behaviors were studied in accordance with our experimental data.

Roles of LO-phonon and SO-phonon induced potentials for a bipolaron in the superconductor

Abstract The bipolaronic system of an infinite quantum well, in which two electrons or holes are weakly or intermediately coupled with both longitudinal-optical (LO) and surface-optical (SO) phonons, may be applied on the layer-type high- T c superconductivity. Using the Lee-Low-Pines method, we have solved the Hamiltonian for the above bipolaron system. The numerical results show that, when the thickness of polar slab increases, the binding energy increases for LO phonon contribution but decreases for SO phonon contribution. Taking the Coulombic repulsion into account, the effective interaction potential energy between the two holes that form the stable bipolaron in a relatively thin polar slab of YBCO has a distinct equilibrium position at its minimum.