Jinming Dong

Electronic properties of peapods: effects of fullerene rotation and different types of tube

Effects of encapsulated fullerene rotation on peapod electronic properties are studied by using Slater–Koster tight-binding calculations. We found that the encapsulated fullerenes can rotate freely in the space of a (10, 10) tube at room temperature, and the rotation of fullerenes will affect C60@(10,10) peapod electronic properties significantly; generally, orientational disorder will remove the sharp features of the average density of states (DOS). However, the rotation of fullerenes cannot induce a metal–insulator transition. Electronic properties of peapods formed by fullerenes and different types of single-walled carbon nanotube (SWNT) are studied too. Percentage calculations suggest that, unlike the multicarrier metallic C60@(10,10) peapod, the C60@(17,0) peapod is a semiconductor, and the effects of the encapsulated fullerenes on tube valence bands and conduction bands are asymmetrical.

Analytical approach to dimerized and frustrated Heisenberg chains

We present a self-consistent quantum-theoretical analysis of the ground-state energy and low-energy excitation spectrum of a dimerized and frustrated Heisenberg chain by the bosonization representation in the continuum-limit approach. The frustration effects on the ground-state energy and energy gap are investigated for α>αc where α is the frustration parameter and αc is its critical value, above which a frustration-induced energy gap is opened. It is shown that as α increases, the ground-state energy decreases and the energy gap increases. We find that the dimerization dependence of the ground-state energy and energy gap for small dimerization obeys the Cross-Fisher power law only at α = αc, but departs significantly from it when α>αc. The present results are in good agreement with numerical analyses from the density-matrix renormalization-group and exact diagonalization methods. The relevance of our calculated results to recent experiments on the spin-Peierls compounds CuGeO3 is also discussed.

Injection of spin-polarized carriers in ferromagnet/superconductor tunnel junctions

We apply a quantum-mechanical approach to study critical current suppression in a superconductor (SC) due to injection of spin-polarized carriers from a ferromagnet (FM) through a FM/SC tunnel junction. It is found that for a given bias voltage, the superconductivity suppression depends strongly on the polarization of the injection current, on the spin-diffusion length in the SC, and on the insulating barrier strength at the FM/SC interface.

Simulation of the vortex motion in high-Tc superconductors

1D and 2D simulations of the single-vortex dynamics in the presence of a random pinning potential and a periodic potential have been carried out. It is shown that the randomness of the pinning site distribution does not have much effect on the transport properties such as the I-V characteristics of the high-Tc superconductors, which has been widely discussed in the approximation of a periodic pinning potential using an analytical method. The randomness effect probably reduces greatly the vortex diffusing mobility only below the depinning current value; this is more obvious at low temperatures.

Magnetic ground states of a layered manganite crystal

We extend the model Hamiltonian of cubic perovskite manganites to a layered manganite system by considering the anisotropy in charge and spin couplings. By minimizing the total free energy of the system, we have obtained the magnetic ground state of the layered manganite crystal as a function of the anisotropic ratio of charge coupling, . It is found that while each bilayer has a ferromagnetic ground state, the interplane magnetic configuration changes from a ferromagnetic order to an antiferromagnetic order with decreasing . This result can account qualitatively for pressure-enhanced interplane magnetoresistance in layered manganite crystals.

Effects of interface roughness and exchange splitting on shot noise in ferromagnet/superconductor junctions

The current fluctuations in ferromagnet/superconductor (FM/SC) junctions are studied for s- and d-wave pairing by taking into account the roughness of the interfacial barrier and exchange splitting in the FM. It is shown that the ferromagnetic exchange splitting gives rise to a decrease in the average current and the shot noise power; the noise power-to-current ratio is increased for eV Δ0 in FM/s-wave SC junctions (V being the bias voltage and Δ0 the energy gap), while the ratio is increased rapidly with the exchange splitting at low voltages and tends towards the same value for high voltages in FM/d-wave SC junctions. The interface roughness is found to lead to a decrease in the average current and an increase in the noise power-to-current ratio.

Another Possible Phenomenological Model of the Normal State of the High-Tc Oxide Superconductors*

A new phenomenological model has been proposed to account for the anomalous normal state properties of the high-Tc, oxide superconductors. Comparison with the assumed one in the marginal Fermi liquid theory is made. Both of them are similar, but also different. It is shown that in a small frequency region of ω T the marginal Fermi liquid theory is appropriate.

Mixed s+d symmetry states induced by orthorhombic distortion effect

Abstract In the Van Hove scenario including orthorhombic distortion effect, we show that there is no stable mixed s + d symmetry state in a tetragonal system. However, a small orthorhombic distortion can induce s + d mixed symmetry superconductivity.

Calculation of the Binding Energy in the One-Dimensional Hubbard Model by the VMC Method

Using variational Monte-Carlo method and choosing an antiferromagnetic trial wave function, we have calculated the binding energies just off half-filling in one-dimensional Hubbard chains. We found the binding for the lattice length N being less than 40 in the degenerate case, but no binding at all in the non-degenerate case. However, with lattice length increasing gradually, difference between the two cases becomes smaller and smaller. Finally, there is no binding at all no matter whether or not the chains have a degeneracy at the Fermi level in the non-interacting limit. It may be an indication of the independence of the final calculation results on the boundary conditions for the large enough system or in thermodynamic limit.

Evaluation of the effective holon-holon interaction by the slave-fermion method

The effective holon-holon interaction by exchanging the spinons in the resonating-valence bond model is derived by the slave-fermion method. The authors found that the interaction is attractive for the holon Cooper pair. Complexity due to Bose condensation of the spinons at T=0 has been discussed.