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Yan Li

Publications and source records attributed to Yan Li.

711 records · Page 40Linked to original sources
arXiv2009

Electrical Detection of Spin Precession in Single Layer Graphene Spin Valves with Transparent Contacts

Spin accumulation and spin precession in single-layer graphene are studied by non-local spin valve measurements at room temperature. The dependence of the non-local magnetoresistance on electrode spacing is investigated and the results indicate a spin diffusion length of ~1.6 microns and a spin injection/detection efficiency of 0.013. Electrical detection of the spin precession confirms that the non-local signal originates from spin injection and transport. Fitting of the Hanle spin precession data yields a spin relaxation time of ~84 ps and a spin diffusion length of ~1.5 microns, which is consistent with the value obtained through the spacing dependence.

cond-mat.mes-hall↗
arXiv2009

Electron-Hole Asymmetry of Spin Injection and Transport in Single-Layer Graphene

Spin-dependent properties of single-layer graphene (SLG) have been studied by non-local spin valve measurements at room temperature. Gate voltage dependence shows that the non-local magnetoresistance (MR) is proportional to the conductivity of the SLG, which is the predicted behavior for transparent ferromagnetic/nonmagnetic contacts. While the electron and hole bands in SLG are symmetric, gate voltage and bias dependence of the non-local MR reveal an electron-hole asymmetry in which the non-local MR is roughly independent of bias for electrons, but varies significantly with bias for holes.

cond-mat.mes-hall↗
arXiv2006

Simultaneous creations of discrete-variable entangle state and single-photon-added coherent state

The single-photon-added coherent state (SPACS), as an intermediate classical-to-purely-quantum state, was first realized recently by Zavatta \emph{$et al.$} (Science 306, 660 (2004)). We show here that the success probability of their SPACS generation can be enhanced by a simple method which leads to simultaneous creations of a discrete-variable entangled state and a SPACS or even a hybrid-variable entangled SPACS in two different channels. The impacts of the input thermal noise are also analyzed.

quant-ph↗
arXiv2006

Scheme for conditional generation of photon-added coherent state and optical entangled $W$ state

We propose a simple scheme to generate an arbitrary photon-added coherent state of a travelling optical field by using only a set of degenerate parametric amplifiers and single-photon detectors. Particularly, when the single-photon-added coherent state (SPACS) is observed by following, e.g., the novel technique of Zavatta \emph{$et al.$} (Science 306, 660 (2004)), we also obtain the generalized optical entangled $W$ state. Finally, a qualitative analysis of possible losses in our scheme is given.

quant-ph↗
arXiv2005

Metal-Semiconductor Transition in Armchair Carbon Nanotubes by Symmetry Breaking

The electronic band structure of armchair carbon nanotubes may be considerably modified by potentials with angular dependence. Different angular modes V_q ~ cos(q*theta) have been studied within a tight-binding scheme. Using symmetry arguments, we demonstrate a bandgap opening in these metallic nanotubes when certain selection rules are satisfied for both potential and nanotube structure. We estimate the bandgap opening as a function of both the external potential strength and the nanotube radius and suggest an effective mechanism of metal-semiconductor transition by combination of different forms of perturbations.

cond-mat.mtrl-sci↗
arXiv2005

Metal-Semiconductor Transition and Fermi Velocity Renormalization in Metallic Carbon Nanotubes

Angular perturbations modify the band structure of armchair (and other metallic) carbon nanotubes by breaking the tube symmetry and may induce a metal-semiconductor transition when certain selection rules are satisfied. The symmetry requirements apply for both the nanotube and the perturbation potential, as studied within a nonorthogonal $π$-orbital tight-binding method. Perturbations of two categories are considered: an on-site electrostatic potential and a lattice deformation which changes the off-site hopping integrals. Armchair nanotubes are proved to be robust against the metal-semiconductor transition in second-order perturbation theory due to their high symmetry, but can develop a nonzero gap by extending the perturbation series to higher orders or by combining potentials of different types. An assumption of orthogonality between $π$ orbitals is shown to lead to an accidental electron-hole symmetry and extra selection rules that are weakly broken in the nonorthogonal theory. These results are further generalized to metallic nanotubes of arbitrary chirality.

cond-mat.mes-hall↗
arXiv2005

Optical generation of hybrid entangled state via entangling single-photon-added coherent state

We propose a feasible scheme to realize the optical entanglement of single-photon-added coherent state (SPACS) and show that, besides the Sanders entangled coherent state, the entangled SPACS also leads to new forms of hybrid entanglement of quantum Fock state and classical coherent state. We probe the essential difference of two types of hybrid entangled state (HES). This HES provides a novel link between the discrete- and the continuous-variable entanglement in a natural way.

quant-ph↗
arXiv2002

Electronic response and bandstructure modulation of carbon nanotubes in a transverse electrical field

The electronic properties of carbon nanotubes in a uniform transverse field are investigated within a single orbital tight-binding model. For doped nanotubes, the dielectric function is found to depend not only on symmetry of the tube, but also on radius and Fermi level position. Bandgap opening/closing is predicted for zigzag tubes, while it is found that armchair tubes always remain metallic, which is explained by the symmetry in their configuration. The bandstructures for both types are considerably modified when the field strength is large enough to mix neighboring subbands.

cond-mat.mtrl-sci↗