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Photothermoelectric materials have important applications in many fields. Here, we joined a silver nanostructure film (AgNSF) and a carbon nanotube film (CNTF) by van der Waals force to form a AgNSF/CNTF heterojunction, which shows excellent photothermal and photoelectric conversion properties. The local temperature difference and the output photovoltage increase rapidly when the heterojunction is irradiated by lasers with wavelengths ranging from ultraviolet to terahertz. The maximum of the local temperature difference reaches 205.9 K, which is significantly higher than that of other photothermoelectric materials reported in literatures. The photothermal and photoelectric responsivity depend on the wavelength of lasers, which are 175-601 K/W and 9.35-40.4 mV/W, respectively. We demonstrate that light absorption of the carbon nanotube is enhanced by local surface plasmons, and the output photovoltage is dominated by Seebeck effect. The AgNSF/CNTF heterostructure can be used as high-efficiency sensitive photothermal materials or as ultra-wideband fast-response photoelectric material., Comment: 11 figures

We develop a quantum noise approach to study quantum transport through nanostructures. The nanostructures, such as quantum dots, are regarded as artificial atoms, subject to quasi-equilibrium fermionic reservoirs of electrons in biased leads. Noise operators characterizing the quantum fluctuation in the reservoirs are related to the damping and fluctuation of the artificial atoms through the quantum Langevin equation. The average current and current noise are derived in terms of the reservoir noise correlations. In the white-noise limit, we show that the current and current noise can be exactly calculated by the quantum noise approach, even in the presence of interaction such as Coulomb blockade. As a typical application, the average current and current noise through a single quantum dot are studied., Comment: 12 pages, 4 figures

The ground-state energy and fine-structure splitting of ionized shallow donor impurity-exciton complex in quantum dots are investigated. It is found that fine-structure splitting could be largely reduced by the off-center ionized impurities since the anisotropic shape of exciton envelope function is significantly changed. Anomalous Stark shifts of the ground-state energy and efficient tuning of the fine-structure splitting by the external electric field due to the local electric field produced by the ionized impurities are discussed. The scheme may be useful for the design of the quantum dots-based entangled-photon source., Comment: 4 pages, 3 figures

Exciton levels and fine-structure splitting in laterally-coupled quantum dot molecules are studied. The electron and hole tunneling energies as well as the direct Coulomb interaction are essential for the exciton levels. It is found that fine-structure splitting of the two-lowest exciton levels is contributed from the intra- and inter-dot exchange interactions, both of which are largely influenced by the symmetry and tunnel-coupling between the two dots. As the inter-dot separation is reduced, fine-structure splitting of the exciton ground state is largely increased while those of the excited states are decreased. Moreover, the dependence of the fine-structure splitting in quantum dot molecules on the Coulomb correlation is clearly clarified., Comment: 8 pages, 8 figures

We propose and study a spin-orbit interaction based mechanism to actively cool down the torsional vibration of a nanomechanical resonator made by semiconductor materials. We show that the spin-orbit interactions of electrons can induce a coherent coupling between the electron spins and the torsional modes of nanomechanical vibration. This coherent coupling leads to an active cooling for the torsional modes via the dynamical thermalization of the resonator and the spin ensemble., Comment: 4 pages, 3 figures

The polarization splitting of the exciton ground state in two laterally coupled quantum dots under an in-plane electric field is investigated and its effective tuning is designed. It is found that there are significant Stark effect and anticrossing in energy levels. Due to coupling between inter- and intra-dot states, the absolute value of polarization splitting is significantly reduced, and it could be tuned to zero by the electric field for proper inter-dot separations. Our scheme is interesting for the research on the quantum dots-based entangled-photon source., Comment: 4 pages, 2 figures, to appear in Appl. Phys. Lett

By introducing a normal self-energy to incorporate the effects of magnetic impurities, Kondo effects in single-walled metallic carbon nanotubes are investigated within the Anderson model and Landauer formula. Magnetic impurities induce Kondo resonance and bring a valley of conductance unction near the Fermi level. The conductance of the nanotube increases with the temperature in the low temperature range. Thermoelectric power induced by magnetic impurities is gotten from Mott relation, and the calculations indicate its dependence on the temperature could interpret the experiment well., Comment: 5 pages, 5 figures

The spin-dependent trial wave functions with rotational symmetry are introduced to describe rotating Wigner molecular states with spin degree of freedom in four- and five-electron quantum dots under magnetic fields. The functions are constructed with unrestricted Hartree-Fock orbits and projection technique in long-range interaction limit. They highly overlap with the exact-diagonalized ones and give the accurate energies in strong fields. The zero points, i.e. vortices of the functions have straightforward relations to the angular momenta of the states. The functions with different total spins automatically satisfy the angular momentum transition rules with the increase of magnetic fields and explicitly show magnetic couplings and characteristic oscillations with respect to the angular momenta. Based on the functions, it is demonstrated that the entanglement entropies of electrons depend on the z-component of total spin and rise with the increase of angular momenta.

The vortex structures and formations of the few-electron states in quantum dots without the Zeeman splitting are investigated. With spin degree of freedom, it is noticed that both the choices of probe electron and the ways to fix the other electrons in conditional single-particle wave functions affect the display of the vortex structures and behaviors. Then the vortex transitions in magnetic fields for the lowest states with different spins are studied. When the field is not very strong, with the increase of the field, the vortex number is monotone non-decreasing, and there are absent states although their angular momenta are in accordance with transition rules given by the theory of electron molecules. Different behaviors of the vortices with the change of interaction range reveal the respective analogies to the vortices of electrons and quasi-particles in fractional quantum Hall system. The separated vortices keep apart from the electrons even when the interaction is screened and such behavior can give an understanding of the absences of the angular momenta in the transition sequences.

We explore the correlations and entanglements of exact-diagonalized few-electron wave functions in a quantum dot in magnetic fields without the Zeeman splitting. With the increase of the field, the lowest states with different spins gradually form a narrow band and the electronic states undergo a transition from liquids to rotating Wigner molecules which are accompanied by different characters of charge correlations. For both the liquid and crystal states, the spin conditional probability densities show magnetic couplings between the particles which depend on the particle numbers, the total spins and the angular momenta of the states. The von Neumann entropies show the spin-dependent entanglements between electrons. The regular magnetic-coupling oscillations and converging entanglement entropies emerge in the rotating Wigner molecular states.

Quantum transport through the junction between two metallic carbon nanotubes connected by intertube links has been studied within the TB method and Landauer formula. It is found that the conductance oscillates with both of the coupling strength and length. The corresponding local density of states (LDOS) is clearly shown and can be used to explain the reason why there are such kinds of oscillations of the conductances, which should be noted in the design of nanotube-based devices., Comment: 6 pages, 4 figures

We present a scheme for charge qubit implementation in a double-barrier nanoring. The logical states of the qubit are encoded in the spatial wavefunctions of the two lowest energy states of the system. The Aharonov-Bohm phase introduced by magnetic flux, instead of tunable tunnelings, along with electric fields can be used for implementing the quantum gate operations. During the operations, the external fields should be switched smoothly enough to avoid the errors caused by the transition to higher-lying states. The structure and field effects on the validity of the qubit are also studied., Comment: 6 pages, 7 figures

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 modes bosons interacting with two spins. The lower excitation 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 modes of bosons, serve as a quantized data bus to exchange the quantum information between two electrons. Then a nontrivial two-qubit logic gate is realized and spin entanglement between the two electrons is created by virtue of spin-orbit coupling., Comment: 7 pages, 5 figures

The tunneling of a giant spin at excited levels is studied theoretically in mesoscopic magnets with a magnetic field at an arbitrary angle in the easy plane. Different structures of the tunneling barriers can be generated by the magnetocrystalline anisotropy, the magnitude and the orientation of the field. By calculating the nonvacuum instanton solution explicitly, we obtain the tunnel splittings and the tunneling rates for different angle ranges of the external magnetic field ($\theta_{H}=\pi/2$ and $\pi/2<\theta_{H}<\pi$). The temperature dependences of the decay rates are clearly shown for each case. It is found that the tunneling rate and the crossover temperature depend on the orientation of the external magnetic field. This feature can be tested with the use of existing experimental techniques., Comment: 27 pages, 4 figures, accepted by Euro. Phys. J. B

The spin-phase interference effects are studied analytically in resonant quantum tunneling of the N\'{e}el vector between degenerate excited levels in nanometer-scale single-domain antiferromagnets in the absence of an external magnetic field. We consider a model for mesoscopic antiferromagnets with uncompensated excess spins for the more general structure of magnetic anisotropy, such as biaxial, trigonal, tetragonal and hexagonal crystal symmetry. This study provides a nontrivial generalization of the Kramers degeneracy for double-well system to coherently spin tunneling at ground states as well as low-lying excited states in AFM system with $m$-fold rotational symmetry around the $\hat{z}$ axis. The energy level spectrum and the thermodynamic properties of magnetic tunneling states are found to depend significantly on the parity of the excess spins at sufficiently low temperatures. Possible relevance to experiments is also discussed., Comment: 22 pages, no figure, accepted for publication in PRB

The quantum interference effects induced by the topological phase are studied analytically in biaxial antiferromagnets with an external magnetic field at an arbitrarily angle. This study provides a nontrivial generalization of the Kramers degeneracy for equivalent double-well system to coherently spin tunneling at ground states as well as low-lying excited states for antiferromagnetic system with asymmetric twin barriers. The spin-phase interference effects are found to depend on the orientation of the magnetic field distinctly., Comment: 10 pages, 3 figures

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., Comment: 10 pages, 9 ps figures, submitted to Physcial Review B

We consider an effective mass model for an electron-hole pair in a simplified confinement potential, which is applicable to both a nanoscopic self-assembled semiconducting InAs ring and a quantum dot. The linear optical susceptibility, proportional to the absorption intensity of near-infrared transmission, is calculated as a function of the ring radius $% R_0$. Compared with the properties of the quantum dot corresponding to the model with a very small radius $R_0$, our results are in qualitative agreement with the recent experimental measurements by Pettersson {\it et al}., Comment: 4 pages, 4 figures, revised and accepted by Phys. Rev. B

The size effects of an exciton in a nano-ring are investigated theoretically by using an effective-mass Hamiltonian which can be separated in terms of the center-of-mass and relative coordinates. The binding energy and oscillator strength of the ground state are calculated for two different ring radii as a function of the ring width. The resulting linear optical susceptibility of the low-lying exciton states is also discussed., Comment: 4 pages, 4 figures, accepted by J. Phys: Condens. Matter

Macroscopic quantum coherence and spin-phase interference are studied between excited levels in single-domain ferromagnetic particles in a magnetic field along the hard anisotropy axis. The system has the general structure of magnetocrystalline anisotropy, such as biaxial, trigonal, tetragonal, and hexagonal symmetry. This study not only just yields the previous spin-phase interference results for the ground state tunneling, but also provide a generalization of the Kramers degeneracy to coherently spin tunneling at low-lying excited states. These analytical results are found to be in good agreement with the numerical diagonalization. We also discuss the transition from quantum to classical behavior and the possible relevance to experiment., Comment: 20 pages, 5 figures, revised and accepted by J. Phys.: Condens. Matter (Sep. 2000)

The phenomenon of quantum nucleation is studied in a nanometer-scale antiferromagnet with biaxial symmetry in the presence of a magnetic field at an arbitrary angle. Within the instanton approach, we calculate the dependence of the rate of quantum nucleation and the crossover temperature on the orientation and strength of the field for bulk solids and two-dimensional films of antiferromagnets, respectively. Our results show that the rate of quantum nucleation and the crossover temperature from thermal-to-quantum transitions depend on the orientation and strength of the field distinctly, which can be tested with the use of existing experimental techniques., Comment: 21 pages, 5 figures, Final version and accepted by Eur. Phys. JB

Quantum phase interference and spin-parity effects are studied in biaxial molecular magnets in a magnetic field at an arbitrarily directed angle. The calculations of the ground-state tunnel splitting are performed on the basis of the instanton technique in the spin-coherent-state path-integral representation, and complemented by exactly numerical diagonalization. Both the Wentzel-Kramers-Brillouin exponent and the preexponential factor are obtained for the entire region of the direction of the field. Our results show that the tunnel splitting oscillates with the field for the small field angle, while for the large field angle the oscillation is completely suppressed. This distinct angular dependence, together with the dependence of the tunnel splitting on the field strengh, provide an independent test for spin-parity effects in biaxial molecular magnets. The analytical results for the molecular Fe$_{8}$ magnet, are found to be in good areement with the numerical simulations, which suggests that even the molecular magnet with total spin S=10 is large enough to be treated as a giant spin system., Comment: 19 pages, 5 figures

The effects of electron-electron interaction of a two-electron nanoring on the energy levels and far-infrared (FIR) 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 FIR 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 (2000)]., Comment: 17 pages, 6 figures, revised and accepted by Phys. Rev. B (Dec. 15)

The phenomenon of quantum nucleation is studied in a ferromagnet in the presence of a magnetic field at an arbitrary angle. We consider the magnetocrystalline anisotropy with tetragonal symmetry and that with hexagonal symmetry, respectively. By applying the instanton method in the spin-coherent-state path-integral representation, we calculate the dependence of the rate of quantum nucleation and the crossover temperature on the orientation and strength of the field for a thin film and for a bulk solid. Our results show that the rate of quantum nucleation and the crossover temperature depend on the orientation of the external magnetic field distinctly, which provides a possible experimental test for quantum nucleation in nanometer-scale ferromagnets., Comment: 19 pages and 3 figures, Final version and accepted by Phys. Rev. B (Feb. B1 2001)

The quantum tunneling of the magnetization vector are studied theoretically in single-domain ferromagnetic nanoparticles placed in an external magnetic field at an arbitrarily directed angle in the $ZX$ plane. We consider the magnetocrystalline anisotropy with trigonal and hexagonal crystal symmetry, respectively. By applying the instanton technique in the spin-coherent-state path-integral representation, we calculate the tunnel splittings, the tunneling rates and the crossover temperatures in the low barrier limit for different angle ranges of the external magnetic field ($\theta_{H}=\pi/2$, $\pi/2\ll\theta_{H}\ll\pi$, and $\theta_{H}=\pi$). Our results show that the tunnel splittings, the tunneling rates and the crossover temperatures depend on the orientation of the external magnetic field distinctly, which provides a possible experimental test for magnetic quantum tunneling in nanometer-scale single-domain ferromagnets., Comment: 31 pages, 3 figures, accepted by J. Phys.: Conden. Matter

We present an exact analytic study on the topological phase interference effect in resonant quantum tunneling of the magnetization between degenerate excited levels for biaxial ferromagnets in an arbitrarily directed magnetic field. We show that the topological phase interference effect depends on the orientation of the field distinctly. The transition from classical to quantum behavior is also discussed., Comment: 15 pages, 3 figures, rejected by Phys. Rev. Lett., and accepted by Phys. Rev. B (Nov. 2000)

The quantum tunneling of the magnetization vector between excited levels are studied theoretically in single-domain ferromagnetic nanoparticles with biaxial crystal symmetry placed in an external magnetic field at an arbitarily directed angle in the ZX plane. The temperature dependences of the tunneling frequency and the decay rate are clearly shown for each case., Comment: 19 pages, 2 figures, to be published in Phys. Rev. B (Augest 1st)

The quantum interference effects induced by the Wess-Zumino term, or Berry phase are studied theoretically in resonant quantum coherence of magnetization vector between degenerate excited states in nanometer-scale single-domain ferromagnets in the absence of an external magnetic field. By applying the periodic instanton method in the spin-coherent-state path integral, we evaluate the low-lying tunnel splittings between degenerate excited states of neighboring wells. And the low-lying energy level spectrum of m-th excited states are obtained with the help of the Bloch theorem in one-dimensional periodic potential., Comment: 23 pages, final version and accepted by Eur. Phys. J. B