Jing-Zhi Yu

First-principles study of the electronic structures of icosahedral TiN (N=13,19,43,55) clusters

We have studied the electronic structures of icosahedral Ti(N) clusters (N=13, 19, 43, and 55) by using a real-space first-principles cluster method with generalized gradient approximation for exchange-correlation potential. The hexagonal close-packed and fcc close-packed clusters have been studied additionally for comparisons. It is found that the icosahedral structures are the most stable ones except for Ti(43), where fcc close-packed structure is favorable in energy. We present and discuss the variation of bond length, the features of the highest occupied molecular orbitals and the lowest unoccupied molecular orbital, the evolution of density of states, and the magnetic moment in detail. The results are in good agreement with the predictions from the collision-induced dissociation and size-selected anion photoelectron spectroscopy experiments.

Exact solutions of two electrons in a quantum dot

Making use of the expansion in a power series, the exact eigensolutions of two electrons in a parabolic quantum dot are obtained. The quantum-size effects on the energy spectra of two electrons are shown for the first time.

Calculations on the magnetic properties of rhodium clusters

The electronic structures of rhodium clusters with sizes of 6, 9, 13, 19, and 43 are studied by first-principles spin-polarized calculations within the local-density-functional formalism. The bond lengths of all clusters are optimized by minimizing the binding energies. The magnetic moments of the clusters are presented and compared with experiments. The electronic structure of the Rh43 cluster has almost the same features as bulk rhodium.

Resonant tunneling in step-barrier structures under an applied electric field

Resonant tunneling in step-barrier structures is investigated by using the transfer-matrix technique. The formulas for the transmission coefficient and the current density are derived when taking into account the coupling between components of the motion of an electron in directions parallel and perpendicular to the interfaces. By making a detailed comparison of resonant tunneling among single square-barrier structures, asymmetric double-barrier structures, and step-barrier structures, the tunneling properties in step-barrier structures are revealed. It is shown that the global behavior of step-barrier structures obtained resembles that of asymmetric double-barrier structures, and step-barrier structures are superior to both single- and double-barrier structures in many aspects. In comparison to asymmetric double-barrier structures, step-barrier structures have several features, such as a wider negative-differential resistance region, easier fabrication, high-speed response, and a relatively lower transmission coefficient and current peak-to-valley ratios. Moreover, higher resonant bias is required in order to obtain optimal transmission resonances in the step-barrier structure. The results shown in this work not only shed new light on the physics of resonant tunneling in electric-barrier structures but are also helpful in designing quantum devices based on step-barrier tunneling structures.

Finite size effects in carbon nanotubes

The low-energy theory for finite long carbon nanotube is derived and numerically examined. It shows that the electronic structure is dominated by the quantum confining, which determines the profile of wave functions as well as the eigen energies; while the details of the wave functions are resolved by the structure of the nanotubes. This behavior is attributed to the peculiar electronic structure of the nanotubes. Because of the slow variation of the profile of electron wave functions, the measured conductance is NOT independent of the position to measure it, which is evident in the multiprobe experiment.

Resonance splitting effect and wave-vector filtering effect in magnetic superlattices

The resonance splitting and wave-vector filtering for electron tunneling through magnetic superlattices are investigated theoretically. Two kinds of magnetic superlattices are examined. One is a periodic arrangement of identical magnetic barriers while the other is periodically juxtaposed with two different magnetic barriers. In general, one resonant domain in the former splits into two resonant domains in the latter. It is confirmed that both the resonance splitting and wave-vector filtering strongly depend on the structure of the magnetic superlattices. The numerical results indicate that the magnetic superlattice, which is a periodic arrangement of two different magnetic barriers, possesses stronger wave-vector filtering.

ELECTRONIC STRUCTURE OF C60 SIMPLE CUBIC PHASE BY FULL-POTENTIAL MIXED-BASIS BAND CALCULATION

An electronic structure calculation with full-potential and all-electron ab initio formalism is performed for hypothetical low-temperature simple cubic phases ( and space groups) of C60 molecular crystal, in which every molecule is orientationally freezed. The degeneracies at special k-points, the magnitudes of energy gaps, and the widths of bands have been found to be sensitive to the orientation of the fullerenes in the crystal. Our result suggests that the structure with space group with –22° rotation around [111] directions, which David et al. proposed, is more stable compared to the structure with space group with the same rotation angle.

Electronic structures of C70 crystalline phases

Abstract By means of an all-electron mixed basis approach, band structures of the C 70 crystalline phases are calculated for the first time for fcc, hcp ( c / a = 1.633) and also sc structures with at most four molecules inside a unit cell. The resulting densities of states are compared with our photoemission and inverse photoemission data of C 70 powder study and the best agreeemnt between the experiment and the present theory is found for the hcp symmetry.

Spin-dependent electronic states and magnetoconductance in a magnetic quantum antidot

We investigate the electron spin effect on quantum states and magnetoconductance in a magnetic quantum antidot with inhomogeneous distribution of the magnetic field. It is shown that the interaction of the electron spin with the inhomogeneous magnetic field results in further splitting of the energy levels. The predicted value of the splitting is closely related to the angular momentum quantum number and the parameters of the magnetic quantum antidot. Spin-dependent magnetoconductance is very different from that in the case where the spin effect is not included. More and shallower dips appear in the spin-dependent magnetoconductance spectra.

Transmission through a mesoscopic ring with a quantum dot

Abstract The transmission through a mesoscopic ring with a quantum dot embedded in one of its arms is studied with a one-dimensional scattering model. The quantum dot is approached by a quantum well scatterer. With the use of a scattering matrix describing the junctions between the leads and the ring, it is analytically shown that the quantum interference and the resonant tunneling dominate the transmission. When the state of the dot is far from a resonance, the system acts as a quantum wire with two separated stubs at both ends. However, when a resonant tunneling through the dot occurs, an extra phase shift may be introduced to the wave through the dot and then the quantum interference effect may be flipped. The dependence of the total transmission coefficient on the properties of the quantum dot is also presented.