Results 1 - 10 of 4704
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[en] The adsorption process of hydrogen atom on nitrogen-doped carbon nanotube (CNT) and its effects on the electronic properties are investigated using the first-principles density-functional methods. As possible hydrogen adsorption sites, three different positions are considered to discuss adsorption energies of the hydrogen atom on N-doped (10,0) CNTs. It is found that the favorable hydrogen adsorption site on N-doped (10,0) CNT is not on top of the nitrogen atom but on top of carbon atoms next to the nitrogen atom. Interestingly, it is found that the impurity state induced by doping with nitrogen atom is shifted from conduction-band minimum to valence-band maximum by the adsorption of the hydrogen atom.
[en] We discuss the relationship between the fractional quantum Hall effect in the vicinity of the thin-torus, a.k.a. Tao-Thouless (TT), limit and quantum spin chains. We argue that the energetics of fractional quantum Hall states in Jain sequence at filling fraction ν = p/(2p + 1) (and ν = 1 - p/(2p + 1)) in the lowest Landau level is captured by S = 1 spin chains with p spins in the unit cell. These spin chains naturally arise at sub-leading order in e-2π2 / L21 which serves as an expansion parameter away from the TT limit (L1 → 0). We also corroborate earlier results on the smooth Fermi surface deformation of the gapless state at ν = 1/2, interpolating between a state described by a critical S = 1/2 chain and the bulk.
[en] Effects of substitutional doping of nitrogen to TiO2 are studied within the framework of the density-functional theory (DFT). We compare the total energy and the electronic structure of pristine TiO2 with those of nitrogen-doped TiO2. It is found that the smaller the concentration of doped nitrogen is, the more energy is required for the substitutional doping of the nitrogen atom in both rutile and anatase phases. This result suggests that the doped nitrogen atoms tend to be clustered. A sizable reduction of the photoexcitation energy is also suggested in the rutile structure.
[en] We study errors of quantum annealing and simulated annealing to highlight better performance of quantum annealing over simulated annealing. Quantum annealing and simulated annealing perform optimization through a quantum adiabatic evolution and a quasi-static evolution respectively. In both methods, dynamics across a phase transition plays a crucial role. The Kibble-Zurek mechanism is known as an underlying physics of defect formation during a time-evolution across a phase transition. We apply an argument for the Kibble-Zurek mechanism to the error generation of quantum annealing and simulated annealing. We show that, for the disordered Ising chain, the kink density and residual energy per spin of quantum annealing decay as (ln τ)-2 and (ln τ)-4 respectively, whereas those of simulated annealing decay as (ln τ)-1 and (ln τ)-2 with the annealing rate -1/τ. These results imply better performance of quantum annealing. We also develop our theory for a two-dimensional spin-glass model.
[en] Quantum annealing is a generic solver of classical optimization problems that makes full use of quantum fluctuations. We consider work statistics given by a repetition of quantum annealing processes by employing the Jarzynski equality proposed in nonequilibrium statistical physics. In particular, we analyze a nonequilibrium average of the exponentiated work performed by a transverse field. A special symmetry, gauge symmetry, leads to a non-trivial relationship between quantum annealing toward different targets in the theory of spin glasses. We believe that our results will be a step toward an alternative realization of efficient quantum computation as well as our better understanding of nonequilibrium behavior of systems under quantum control.
[en] An Electric Dipole Moment (EDM) of the elementary particle is a good prove to observe the phenomena beyond the Standard Model. A non-zero EDM shows the violation of the time reversal symmetry, and under the CPT invariance it means the CP violation. In paramagnetic atoms, an electron EDM results in an atomic EDM enhanced by the factor of the 3rd power of the charge of the nucleus due the relativistic effects. A heaviest alkali element francium (Fr), which is the radioactive atom, has the largest enhancement factor K ∼ 895. Then, we are developing a high intensity laser cooled Fr factory at Cyclotron and Radioisotope Center (CYRIC), Tohoku University to perform the search for the EDM of Fr with the accuracy of 10-29 e · cm. The important points to overcome the current accuracy limit of the EDM are to realize the high intensity Fr source and to reduce the systematic error due to the motional magnetic field and inhomogeneous applied field. To reduce the dominant component of the systematic errors mentioned above, we will confine the Fr atoms in the small region with the Magneto-Optical Trap and optical lattice using the laser cooling and trapping techniques. The construction of the experimental apparatus is making progress, and the new thermal ionizer already produces the Fr of ∼106 ions/s with the primary beam intensity 200 nA. The developments of the laser system and optical equipments are in progress, and the present status and future plan of the experimental project is reported.
[en] As a strangeness S = -1 and baryon number B = 2 system, the two-body bound state of Λ* = Λ(1405) and a nucleon is studied. To solve the Λ* N system, we construct the Λ* N potential by extending the Juelich model with couplings estimated in the chiral unitary approach. We have the Λ* N quasi-bound state with the mass, MΛ*N ∼ 2364MeV which is shallowly bound about 9.5 MeV from the K-bar NN threshold. Decay width of the fall apart process, where the Λ* N resonance decays to πΣN with a nucleon being as a spectator, is estimated to be ΓF.A ∼ 49MeV.
[en] We show that there is an entire gauge symmetry of a novel kind which interpolates between an infinity of formulations of the laws of gravity, ranging from Einsteins pure curvature formulation to a pure torsion formulation in a teleparallel geometry. As a consequence, torsion and curvature are not independent and that torsion is an alternative description of curvature in gravity.
[en] The paper describes a solution to the problem of quantum measurement that has been proposed recently. The literal understanding of the basic rule of quantum mechanics on identical particles violates the cluster separation principle and so leads to difficulties. A proposal due to Peres of how such difficulties could be removed is reformulated and extended. Cluster separability leads to a locality requirement on observables and to the key notion of separation status. Separation status of a microsystem is shown to change in preparation and registration processes. The indispensability of detectors plays an important role. Changes of separation status are alterations of kinematic description rather than some parts of dynamical trajectories and so more radical than 'collapse of the wave function'. Textbook quantum mechanics does not provide any information of how separation status changes run, hence new rules must be formulated. This enables to satisfy the objectification requirement for registrations. To show how the ideas work, a simplified model of registration apparatus is constructed.
[en] In the context of quantum gravity theories, several researchers have proposed causal sets as appropriate discrete models of spacetime. We investigate families of causal sets obtained from two simple models of computation - 2D Turing machines and network mobile automata - that operate on 'high-dimensional' supports, namely 2D arrays of cells and planar graphs, respectively. We study a number of quantitative and qualitative emergent properties of these causal sets, including dimension, curvature and localized structures, or 'particles'. We show how the possibility to detect and separate particles from background space depends on the choice between a global or local view at the causal set. Finally, we spot very rare cases of pseudo-randomness, or deterministic chaos; these exhibit a spontaneous phenomenon of 'causal compartmentation' that appears as a prerequisite for the occurrence of anything of physical interest in the evolution of spacetime.