Photoluminescence from an individual double-walled carbon nanotube

We report direct and unambiguous evidence of the existence of inner semiconducting tube (ISCT) photoluminescence (PL) from measurements performed on four individual freestanding index-identified double-walled carbon nanotubes (DWNTs). Based on thorough Rayleigh scattering, Raman scattering, and PL experiments, we are able to demonstrate that the ISCT PL is observed with a quantum yield estimated to be a few 10-6 independent of the semiconducting or metallic nature of the outer tube. This result is mainly attributed to ultrafast exciton transfer from the inner to outer tube. Furthermore, by carrying out PL excitation experiments on the (14, 1)@(15, 12) DWNT, we show that the ISCT PL can be detected through the optical excitation of the outer tube, indicating that the exciton transfer can also occur in the opposite way

Thermopower and thermal conductivity of superconducting perovskite MgCNi3MgCNi_3

The thermopower and thermal conductivity of superconducting perovskite $MgCNi_3$ ($T_c \approx$ 8 K) have been studied. The thermopower is negative from room temperature to 10 K. Combining with the negative Hall coefficient reported previously, the negative thermopower definetly indicates that the carrier in $MgCNi_3$ is electron-type. The nonlinear temperature dependence of thermopower below 150 K is explained by the electron-phonon interaction renormalization effects. The thermal conductivity is of the order for intermetallics, larger than that of borocarbides and smaller than $MgB_2$. In the normal state, the electronic contribution to the total thermal conductivity is slightly larger than the lattice contribution. The transverse magnetoresistance of $MgCNi_3$ is also measured. It is found that the classical Kohler's rule is valid above 50 K. An electronic crossover occures at $T^* \sim 50 K$, resulting in the abnormal behavior of resistivity, thermopower, and magnetoresistance below 50 K.Comment: Revised on 12 September 2001, Phys. Rev. B in pres

Self-induced and induced transparencies of two-dimensional and three- dimensional superlattices

The phenomenon of transparency in two-dimensional and three-dimensional superlattices is analyzed on the basis of the Boltzmann equation with a collision term encompassing three distinct scattering mechanisms (elastic, inelastic and electron-electron) in terms of three corresponding distinct relaxation times. On this basis, we show that electron heating in the plane perpendicular to the current direction drastically changes the conditions for the occurrence of self-induced transparency in the superlattice. In particular, it leads to an additional modulation of the current amplitudes excited by an applied biharmonic electric field with harmonic components polarized in orthogonal directions. Furthermore, we show that self-induced transparency and dynamic localization are different phenomena with different physical origins, displaced in time from each other, and, in general, they arise at different electric fields.Comment: to appear in Physical Review

Towards a global analysis of polarized parton distributions

We present a technique for implementing in a fast way, and without any approximations, higher-order calculations of partonic cross sections into global analyses of parton distribution functions. The approach, which is set up in Mellin-moment space, is particularly suited for analyses of future data from polarized proton-proton collisions, but not limited to this case. The usefulness and practicability of this method is demonstrated for the semi-inclusive production of hadrons in deep-inelastic scattering and the transverse momentum distribution of ``prompt'' photons in pp collisions, and a case study for a future global analysis of polarized parton densities is presented.Comment: 20 pages, LaTeX, 6 eps figures, final version to appear in PRD (minor changes

Keldysh Green's function approach to coherence in a non-equilibrium steady state: connecting Bose-Einstein condensation and lasing

Solid state quantum condensates often differ from previous examples of condensates (such as Helium, ultra-cold atomic gases, and superconductors) in that the quasiparticles condensing have relatively short lifetimes, and so as for lasers, external pumping is required to maintain a steady state. On the other hand, compared to lasers, the quasiparticles are generally more strongly interacting, and therefore better able to thermalise. This leads to questions of how to describe such non-equilibrium condensates, and their relation to equilibrium condensates and lasers. This chapter discusses in detail how the non-equilibrium Green's function approach can be applied to the description of such a non-equilibrium condensate, in particular, a system of microcavity polaritons, driven out of equilibrium by coupling to multiple baths. By considering the steady states, and fluctuations about them, it is possible to provide a description that relates both to equilibrium condensation and to lasing, while at the same time, making clear the differences from simple lasers

Normal-superconducting transition induced by high current densities in YBa2Cu3O7-d melt-textured samples and thin films: Similarities and differences

Current-voltage characteristics of top seeded melt-textured YBa2Cu3O7-d are presented. The samples were cut out of centimetric monoliths. Films characteristics were also measured on microbridges patterned on thin films grown by dc sputtering. For both types of samples, a quasi-discontinuity or quenching was observed for a current density J* several times the critical current density Jc. Though films and bulks much differ in their magnitude of both Jc and J*, a proposal is made as to a common intrinsic origin of the quenching phenomenon. The unique temperature dependence observed for the ratio J*/Jc, as well as the explanation of the pre-quenching regime in terms of a single dissipation model lend support to our proposal.Comment: 10 pages, 10 figures, submitted to Physical Review

Model-independent measurement of t\boldsymbol{t}-channel single top quark production in ppˉ\boldsymbol{p\bar{p}} collisions at s=1.96\boldsymbol{\sqrt{s}=1.96} TeV

We present a model-independent measurement of $t$-channel electroweak production of single top quarks in \ppbar collisions at $\sqrt{s}=1.96\;\rm TeV$. Using $5.4\;\rm fb^{-1}$ of integrated luminosity collected by the D0 detector at the Fermilab Tevatron Collider, and selecting events containing an isolated electron or muon, missing transverse energy and one or two jets originating from the fragmentation of $b$ quarks, we measure a cross section \sigma({\ppbar}{\rargap}tqb+X) = 2.90 \pm 0.59\;\rm (stat+syst)\; pb for a top quark mass of $172.5\;\rm GeV$. The probability of the background to fluctuate and produce a signal as large as the one observed is $1.6\times10^{-8}$, corresponding to a significance of 5.5 standard deviations.Comment: 8 pages, 4 figures, submitted to Phys. Lett.