Jia-Lin Zhu

Aharonov-Bohm effect of excitons in nanorings

The magnetic field effects on excitons in an InAs nano-ring are studied theoretically. By numerically diagonalizing the effective-mass Hamiltonian of the problem, which can be separated into terms in centre-of-mass and relative coordinates, we calculate the low-lying exciton energy levels and oscillator strengths as a function of the width of the ring and the strength of the external magnetic field. The analytical results are obtained for a narrow-width nano-ring in which the radial motion is the fastest one and adiabatically decoupled from the azimuthal motions. It is shown that in the presence of Coulomb correlation, the so called Aharonov-Bohm effect of excitons exists in a finite (but small) width nano-ring. However, when the ring width becomes large, the non-simply-connected geometry of nano-rings is destroyed and in turn yields the suppression of Aharonov-Bohm effect. The conditional probability distribution calculated for the low-lying exction states allows identification of the presence of Aharonov-Bohm effect. The linear optical susceptibility is also calculated as a function of the magnetic field, to be confronted with the future measurements of optical emission experiments on InAs nano-rings.

Photoinduced currents in carbon nanotube/metal heterojunctions

Significant photoinduced currents have been explored in carbon nanotube/metal heterojunctions, i.e., heterodimension junctions containing macrolong carbon nanotubes (CNTs). The experimental measurements clearly show that the net transport direction of the photoexcited electrons is from the lower dimensional CNTs to higher dimensional CNTs or metal electrodes and the photoinduced currents dramatically depend on the laser spot position relative to the junction interface. The currents increase linearly with the increase of the laser intensity as the intensity is not very large. A prototype sensor has been constructed to illustrate applications of the heterodimension junctions in photoelectronics.

Neutral and negative donors in quantum dots

Using a method introduced earlier, the exact solutions of ground states of neutral donor (D0) centres in different quantum dots (QDs) have been obtained, and, taking the Chandrasekhar-type function as a trial function, the ground states of negative donor (D-) centres in QDs have been calculated. The dimensionality and potential-shape effects of QDs on the binding energies EB(D0) and EB(D-) of D0 and D- centres have been studied. The ratio sigma of EB(D0) to EB(D-) has clearly demonstrated the so-called freeze-out effect in different QDs. According to the value of sigma obtained, calculated results of different quantum-well structures can be checked and compared with others. It has been shown that the freeze-out effect and the electron-correlation effect are strongly dependent on the dimensionality of QDs and weakly dependent on the potential shape, and that the polarization term of the trial function not only brings in the important electron-correlation effect but also modifies the behaviour of the single-electron orbitals.

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.

Fabrication and transport properties of ZnO∕Nb-1wt%-doped SrTiO3 epitaxial heterojunctions

(110) ZnO/(001) Nb-1 wt %-doped SrTiO3 n-n type heteroepitaxial junctions were fabricated using the pulse laser deposition method. A diodelike current behavior was observed. Different from conventional p-n junctions or Schottky diodes, the diffusion voltage was found to increase with temperature. At all temperatures, the forward current was perfectly fitted on the thermionic emission model. The band bending at the interface can qualitatively explain our results, and the extracted high ideality factor at low temperatures, as well as large saturation currents, is ascribed to the deep-level-assisted tunneling current through the junction

Oxidized macroscopic-long Cu nanowire bundle photoconductor

Using a solid electrolyte RbCu4Cl3I2 thin film and a controlled constant current electric field, the authors have directly fabricated macroscopic-long Cu nanowire bundles. Their structural characters have been described at different scales. The oxidation behavior of macroscopic-long Cu nanowire bundles containing thousands of straight and dendritic Cu nanowires was studied using x-ray photoelectron spectroscopy. Benefiting from the oxidized macroscopic-long Cu nanowire bundles, they have experimentally explored photoinduced conductivity in the bundle upon illumination with a 532nm laser beam, and observed that it can be regarded as a photoconductor with high sensitivity and fast response.

Energy levels and far-infrared spectroscopy for two electrons in a nanoscopic semiconductor ring

The effects of electron-electron interaction of a two-electron nanoring on the energy levels and far-infrared spectroscopy have been investigated based on a model calculation which is performed within the exactly numerical diagonalization. It is found that the interaction changes the energy spectra dramatically, and also shows significant influence on the far-infrared spectroscopy. The crossings between the lowest spin-singlet and triplet states induced by the coulomb interaction are clearly revealed. Our results are related to the experiment recently carried out by A. Lorke et al. [Phys. Rev. Lett. 84, 2223

Negative and positive photoconductivity modulated by light wavelengths in carbon nanotube film

We explore the photoconductive properties of double-walled carbon nanotube film under laser illumination at different wavelengths in both ambient air and vacuum. In our experimental measurements, the photoconductivity of the film shows a strong dependence on the wavelength of light and can be changed from negative to positive by fully removing oxygen. We propose that competition among photoexcitation, photodesorption, and surface plasmon polaritons is responsible for these interesting phenomena. Our results are expected to provide potential applications in the field of nano-optoelectronic sensors based on carbon nanotubes.

High magnetic field annealing effect on visible photoluminescence enhancement of TiO2 nanotube arrays

TiO2 nanotube arrays with remarkable visible photoluminescence were prepared by high magnetic field annealing in air at 450 °C due to the involvement of oxygen vacancies (OVs). A field with the intensities of 0, 2, 4, 6, and 8 T were applied in the annealing processing, along the directions set at 0°, 50°, and 90° from the surface normal of the substrate. The results demonstrated that the density of oxygen vacancies in TiO2 nanotubes can be controlled by varying the intensity and direction of the magnetic field. The mechanism for the effect of the high magnetic field has been investigated. This study opens an effective way to control the oxygen vacancies in nanomaterials to improve their performance.

Spin entanglement induced by spin-orbit interactions in coupled quantum dots

We theoretically explore the possibility of creating spin quantum entanglement in a system of two electrons confined respectively in two vertically coupled quantum dots in the presence of Rashba-type spin-orbit coupling. We find that the system can be described by a generalized Jaynes-Cummings model of two-mode bosons interacting with two spins. The lower excited states of this model are calculated to reveal the underlying physics of the far-infrared absorption spectra. The analytic perturbation approach shows that an effective transverse coupling of spins can be obtained by eliminating the orbital degrees of freedom in the large-detuning limit. Here, the orbital degrees of freedom of the two electrons, which are described by two-mode bosons, serve as a quantized data bus to exchange the quantum information between the two electron spins. Then a nontrivial two-qubit logic gate is realized and spin entanglement between the two electrons is created by virtue of spin-orbit interactions.