Nonradiative Recombination of Excitons in Carbon Nanotubes Mediated by Free Charge Carriers

Free electrons or holes can mediate the nonradiative recombination of excitons in carbon nanotubes. Kinematic constraints arising from the quasi one-dimensional nature of excitons and charge carriers lead to a thermal activation barrier for the process. However, a model calculation suggests that the rate for recombination mediated by a free electron is the same order of magnitude as that of two-exciton recombination. Small amounts of doping may contribute to the short exciton lifetimes and low quantum yields observed in carbon nanotubes.Comment: 18 pages, 4 figures. Submitted to Physical Review

An explanation of the Δ5/2−(1930)\Delta_{5/2^{-}}(1930) as a ρΔ\rho\Delta bound state

We use the $\rho\Delta$ interaction in the hidden gauge formalism to dynamically generate $N^{\ast}$ and $\Delta^{\ast}$ resonances. We show, through a comparison of the results from this analysis and from a quark model study with data, that the $\Delta_{5/2^{-}}(1930),$ $\Delta_{3/2^{-}}(1940)$ and $\Delta_{1/2^{-}}(1900)$ resonances can be assigned to $\rho\Delta$ bound states. More precisely the $\Delta_{5/2^{-}}(1930)$ can be interpreted as a $\rho\Delta$ bound state whereas the $\Delta_{3/2^{-}}(1940)$ and $\Delta_{1/2^{-}}(1900)$ may contain an important $\rho\Delta$ component. This interpretation allows for a solution of a long-standing puzzle concerning the description of these resonances in constituent quark models. In addition we also obtain degenerate $J^{P}=1/2^{-},3/2^{-},5/2^{-}$ $N^{*}$ states but their assignment to experimental resonances is more uncertain.Comment: 19 pags, 8 fig

Couplings in coupled channels versus wave functions in the case of resonances: application to the two Λ(1405)\Lambda(1405) states

In this paper we develop a formalism to evaluate wave functions in momentum and coordinate space for the resonant states dynamically generated in a unitary coupled channel approach. The on shell approach for the scattering matrix, commonly used, is also obtained in Quantum Mechanics with a separable potential, which allows one to write wave functions in a trivial way. We develop useful relationships among the couplings of the dynamically generated resonances to the different channels and the wave functions at the origin. The formalism provides an intuitive picture of the resonances in the coupled channel approach, as bound states of one bound channel, which decays into open ones. It also provides an insight and practical rules for evaluating couplings of the resonances to external sources and how to deal with final state interaction in production processes. As an application of the formalism we evaluate the wave functions of the two $\Lambda(1405)$ states in the $\pi \Sigma$, $\bar{K} N$ and other coupled channels.Comment: 23 pages, 3 figures. v2: Added a section to calculate form factor

Kˉ∗\bar K^* meson in dense matter

We study the properties of $\bar K^*$ mesons in nuclear matter using a unitary approach in coupled channels within the framework of the local hidden gauge formalism and incorporating the $\bar K \pi$ decay channel in matter. The in-medium $\bar K^* N$ interaction accounts for Pauli blocking effects and incorporates the $\bar K^*$ self-energy in a self-consistent manner. We also obtain the $\bar K^*$ (off-shell) spectral function and analyze its behaviour at finite density and momentum. At normal nuclear matter density, the $\bar K^*$ meson feels a moderately attractive potential while the $\bar K^*$ width becomes five times larger than in free space. We estimate the transparency ratio of the $\gamma A \to K^+ K^{* -} A^\prime$ reaction, which we propose as a feasible scenario at present facilities to detect the changes of the properties of the $\bar K^*$ meson in the nuclear medium.Comment: 26 pages, 9 figures, one new section added, version published in Phys. ReV. C, http://link.aps.org/doi/10.1103/PhysRevC.82.04521

A new interpretation for the Ds2∗(2573)D^*_{s2}(2573) and the prediction of novel exotic charmed mesons

In this manuscript we study the vector - vector interaction within the hidden gauge formalism in a coupled channel unitary approach. In the sector $C=1,S=1,J=2$ we get a pole in the T-matrix around $2572$ MeV that we identify with the $D^*_{s2}(2573)$, coupling strongly to the $D^*K^*$($D^*_s\phi$($\omega$)) channels. In addition we obtain resonances in other exotic sectors which have not been studied before such as $C=1,S=-1$, $C=2,S=0$ and $C=2,S=1$. This 'flavor-exotic' states are interpreted as $D^*\bar{K^*}$, $D^*D^*$ and $D^*_sD^*$ molecular states but have not been observed yet. In total we obtain nine states with different spin, isospin, charm and strangeness of non $C=0,S=0$ and $C=1,S=0$ character, which have been reported before

Effects of Velocity-Dependent Dark Matter Annihilation on the Energy Spectrum of the Extragalactic Gamma-ray Background

We calculate the effects of velocity-dependent dark matter annihilation cross sections on the intensity of the extragalactic gamma-ray background. Our formalism does not assume a locally thermal distribution of dark matter particles in phase space, and is valid for arbitrary velocity-dependent annihilation. As concrete examples, we calculate the effects of p-wave annihilation (with the $v$-weighted cross section of $\sigma v=a+bv^2$) on the mean intensity of extragalactic gamma rays produced in cosmological dark matter halos. This velocity variation makes the shape of the energy spectrum harder, but this change in the shape is too small to see unless b/a\agt 10^6. While we find no such models in the parameter space of the Minimal Supersymmetric Standard Model (MSSM), we show that it is possible to find b/a\agt 10^6 in the extension MSSM$\otimes U(1)_{B-L}$. However, we find that the most dominant effect of the p-wave annihilation is the suppression of the amplitude of the gamma-ray background. A non-zero $b$ at the dark matter freeze-out epoch requires a smaller value of $a$ in order for the relic density constraint to be satisfied, suppressing the amplitude by a factor as low as $10^{-6}$ for a thermal relic. Non-thermal relics will have weaker amplitude suppression. As another velocity-dependent effect, we calculate the spectrum for s-wave annihilation into fermions enhanced by the attractive Sommerfeld effect. Resonances associated with this effect result in significantly enhanced intensities, with a slightly softer energy spectrum.Comment: 18 pages, 10 figure

Iterative solution of a Dirac equation with inverse Hamiltonian method

We solve a singe-particle Dirac equation with Woods-Saxon potentials using an iterative method in the coordinate space representation. By maximizing the expectation value of the inverse of the Dirac Hamiltonian, this method avoids the variational collapse, in which an iterative solution dives into the Dirac sea. We demonstrate that this method works efficiently, reproducing the exact solutions of the Dirac equation.Comment: 4 pages, 3 figure

Lepton flavor violating decays of vector mesons

We estimate the rates of lepton flavor violating decays of the vector mesons $\rho, \omega, \phi \to e \mu$. The theoretical tools are based on an effective Lagrangian approach without referring to any specific realization of the physics beyond the standard model responsible for lepton flavor violation (\Lfv). The effective lepton-vector meson couplings are extracted from the existing experimental bounds on the nuclear $\mu^--e^-$ conversion. In particular, we derive an upper limit for the \Lfv branching ratio ${\rm Br}(\phi \to e \mu) \leq1.3 \times 10^{-21}$ which is much more stringent than the recent experimental result ${\rm Br}(\phi \to e \mu) < 2 \times 10^{-6}$ presented by the SND Collaboration. Very tiny limits on \Lfv decays of vector mesons derived in this letter make direct experimental observation of these processes unrealistic.Comment: 3 pages, 1 figure, accepted for publication in Phys. Rev.

New bounds on lepton flavor violating decays of vector mesons and the Z0 boson

We give an estimate for the upper bounds on rates of lepton flavor violating (LFV) decays M to mu(pm) + e(mp) of vector mesons M = rho0, omega, phi, J/psi, Upsilon and the Z0 boson in a model independent way, analyzing the corresponding lowest dimension effective operators. These operators also contribute to nuclear mu-e-conversion. Based on this observation and using the existing experimental limits on this LFV nuclear process, we show that the studied two-body LFV decays of vector bosons are strongly suppressed independent on the explicit realization of new physics. The upper limits on the rates of some of these decays are significantly more stringent than similar limits known in the literature. In view of these results experimental observation of the two-body LFV decays of vector bosons looks presently unrealistic.Comment: 4 pages, 1 figur

Theory of the thermoelectricity of intermetallic compounds with Ce or Yb ions

The thermoelectric properties of intermetallic compounds with Ce or Yb ions are explained by the single-impurity Anderson model which takes into account the crystal-field splitting of the 4{\it f} ground-state multiplet, and assumes a strong Coulomb repulsion which restricts the number of {\it f} electrons or {\it f} holes to $n_f\leq 1$ for Ce and $n_f^{hole}\leq 1$ for Yb ions. Using the non-crossing approximation and imposing the charge neutrality constraint on the local scattering problem at each temperature and pressure, the excitation spectrum and the transport coefficients of the model are obtained. The thermopower calculated in such a way exhibits all the characteristic features observed in Ce and Yb intermetallics. Calculating the effect of pressure on various characteristic energy scales of the model, we obtain the $(T,p)$ phase diagram which agrees with the experimental data on CeRu$_{2}$Si$_2$, CeCu$_{2}$Si$_2$, CePd$_{2}$Si$_2$, and similar compounds. The evolution of the thermopower and the electrical resistance as a function of temperature, pressure or doping is explained in terms of the crossovers between various fixed points of the model and the redistribution of the single-particle spectral weight within the Fermi window.Comment: 13 pages, 11 figure