Branching spin chain dynamics

We study the dynamics of various branched spin chain systems. In such systems entanglement can be generated and distributed, providing an essential resource for teleportation or distributed quantum processing. We show in detail how simple operations can be employed at chosen times to change the subsequent dynamics of the branched spin chains, rendering the distributed entanglement more accessible.     

Hydrodynamics of reactive systems: Chemical reactions near the critical points

The rate of a chemical reaction is expressed in terms of the correlation functions which are found from the full system of hydrodynamic equations in reactive systems. In the limiting cases one obtains all three types of behavior of reaction rates near the critical points found in experiments (slowing-down, speeding-up, insensitivity to criticality). This behavior depends on the number of reactive and inert components in a system, proximity and path of approach to the critical point.     

Characterization of complex networks by higher order neighborhood properties

A concept of higher order neighborhood in complex networks, introduced previously [Phys. Rev. E 73, 046101 (2006)], is systematically explored to investigate larger scale structures in complex networks. The basic idea is to consider each higher order neighborhood as a network in itself, represented by a corresponding adjacency matrix, and to settle a plenty of new parameters in order to obtain a best characterization of the whole network. Usual network indices are then used to evaluate the properties of each neighborhood. The identification of high order neighborhoods is also regarded as intermediary step towards the evaluation of global network properties, like the diameter, average shortest path between node, and network fractal dimension. Results for a large number of typical networks are presented and discussed.     

Traffic dynamics in scale-free networks with limited packet-delivering capacity

We propose a limited packet-delivering capacity model for traffic dynamics in scale-free networks. In this model, the total node’s packet-delivering capacity is fixed, and the allocation of packet-delivering capacity on node i is proportional to , where k_i is the degree of node i and ? is a adjustable parameter. We have applied this model on the shortest path routing strategy as well as the local routing strategy, and found that there exists an optimal value of parameter ? leading to the maximal network capacity under both routing strategies. We provide some explanations for the emergence of optimal ?.     

Electronic structure and g factors of narrow-gap zinc-blende nanowires and nanorods

The Hamiltonian in the framework of eight-band effective-mass approximation of the zinc-blende nanowires and nanorods in the presence of external homogeneous magnetic field is given in the cylindrical coordinate. The electronic structure, optical properties, magnetic energy levels, and g factors of the nanowires and nanorods are calculated. It is found that the electron states consist of many hole-state components, due to the coupling of the conduction band and valence band. For the normal bands which are monotone functions of |k_z|, long nanorods can be modeled by the nanowires, the energy levels of the nanorods approximately equal the values of the energy band E(k_z) of the nanowires with the same radius at a special k_z, where k_z is the wave vector in the wire direction. Due to the coupling of the states, some of the hole energy bands of the nanowires have their highest points at k_z≠0. Especially, the highest hole state of the InSb nanowires is not at the k_z=0 point. It is an indirect band gap. For these abnormal bands, nanorods can not be modeled by the nanowires. The energy levels of the nanorods show an interesting plait-like pattern. The linear polarization factor is zero, when the aspect ratio L/2R is smaller than 1, and increases as the length increases. The g_z and g_x factors as functions of the k_z, radius R and length L are calculated for the wires and rods, respectively. For the wires, the g_z of the electron ground state increases, and the g_z of the hole ground state decreases first, then increases with the k_z increasing. For the rods, the g_z and g_x of the electron ground state decrease as the R or the L increases. The g_x of the hole ground state decreases, the g_z of the hole ground state increases with the L increasing. The variation of the g_z of the wires with the k_z is in agreement with the variation of the g_z of the rods with the L.     

Random field Ising model and community structure in complex networks

We propose a method to determine the community structure of a complex network. In this method the ground state problem of a ferromagnetic random field Ising model is considered on the network with the magnetic field B_s = +∞, B_t = -∞, and B_i≠s,t=0 for a node pair s and t. The ground state problem is equivalent to the so-called maximum flow problem, which can be solved exactly numerically with the help of a combinatorial optimization algorithm. The community structure is then identified from the ground state Ising spin domains for all pairs of s and t. Our method provides a criterion for the existence of the community structure, and is applicable equally well to unweighted and weighted networks. We demonstrate the performance of the method by applying it to the Barabási-Albert network, Zachary karate club network, the scientific collaboration network, and the stock price correlation network. (Ising, Potts, etc.)     

Finite size effects on the optical transitions in quantum rings under a magnetic field

We present a theoretical study of the energy spectrum of single electron and hole states in quantum dots of annular geometry under a high magnetic field along the ring axis in the frame of uncorrelated electron-hole theory. We predict the periodic disappearance of the optical emission of the electron-hole pair as the magnetic field increases, as a consequence of the finite height of the barriers. The model has been applied to semiconductor rings of various internal and external radii, giving as limiting cases the disk and antidot.     

Random pseudofractal scale-free networks with small-world effect

A random pseudofractal network (RPN) is generated by a recursive growing rule. The RPN is of the scale-free feature and small-world effect. We obtain the theoretical results of power-law exponent γ=3, clustering coefficient C=3π²-19≈ 0.74, and a proof that the mean distance increases no faster than ln N, where N is the network size. These results agree with the numerical simulation very well. In particular, we explain the property of growth and preferential attachment in RPNs. And the properties of a class of general RPNs are discussed in the end.     

The carrier “antibinding” in quantum dots: a charge separation effect

We show that the carrier “antibinding” observed recently in semiconductor quantum dots, i.e., the fact that the ground state energy of two electron-hole pairs goes above twice the ground-state energy of one pair, can entirely be assigned to a charge separation effect, whatever its origin. In the absence of external electric field, this charge separation comes from different “spreading-out” of the electron and hole wavefunctions linked to the finite height of the barriers. When the dot size shrinks, the two-pair energy always stays below when the barriers are infinite. On the opposite, because barriers are less efficient for small dots, the energy of two-pairs in a dot with finite barriers, ends by behaving like the one in bulk, i.e., by going above twice the one-pair energy when the pairs get too close. For a full understanding of this “antibinding” effect, we have also reconsidered the case of one pair plus one carrier. We find that, while the carriers just have to spread out of the dot differently for the “antibinding” of two-pairs to appear, this “antibinding” for one pair plus one carrier only appears if this carrier is the one which spreads out the less. In addition a remarkable sum rule exists between the “binding energies” of two pairs and of one pair plus one carrier.     

Effect of annealing temperature on density of ZnO quantum dots

ZnO quantum dots (QDs) were fabricated on Si (001) substrates by pulsed laser deposition (PLD) and subsequent thermal annealing. X-ray diffraction and transmission electron microscopy analyses revealed that the ZnO QDs had polycrystalline hexagonal wurtzite structure. The size and density of ZnO QDs were investigated by atomic force microscopy. It has been found that the density decreased while the size increased with increasing annealing temperature. The analysis of size distribution of the dots shows an obvious bimodal mode according to scaling theory. The Raman spectrum shows a typical resonant multi-phonon form for the ZnO QDs. The collapse from the top of the dots was observed firstly after the samples were exposed in air for 30 days.