9,351 research outputs found

    Nonequilibrium Green's Function Approach to Phonon Transport in Defective Carbon Nanotubes

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    We have developed a new theoretical formalism for phonon transport in nanostructures using the nonequilibrium phonon Green's function technique and have applied it to thermal conduction in defective carbon nanotubes. The universal quantization of low-temperature thermal conductance in carbon nanotubes can be observed even in the presence of local structural defects such as vacancies and Stone-Wales defects, since the long wavelength acoustic phonons are not scattered by local defects. At room temperature, however, thermal conductance is critically affected by defect scattering since incident phonons are scattered by localized phonons around the defects. We find a remarkable change from quantum to classical features for the thermal transport through defective CNTs with increasing temperature.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. Let

    Linear Programming Relaxations for Goldreich's Generators over Non-Binary Alphabets

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    Goldreich suggested candidates of one-way functions and pseudorandom generators included in NC0\mathsf{NC}^0. It is known that randomly generated Goldreich's generator using (rβˆ’1)(r-1)-wise independent predicates with nn input variables and m=Cnr/2m=C n^{r/2} output variables is not pseudorandom generator with high probability for sufficiently large constant CC. Most of the previous works assume that the alphabet is binary and use techniques available only for the binary alphabet. In this paper, we deal with non-binary generalization of Goldreich's generator and derives the tight threshold for linear programming relaxation attack using local marginal polytope for randomly generated Goldreich's generators. We assume that u(n)βˆˆΟ‰(1)∩o(n)u(n)\in \omega(1)\cap o(n) input variables are known. In that case, we show that when rβ‰₯3r\ge 3, there is an exact threshold ΞΌc(k,r):=(kr)βˆ’1(rβˆ’2)rβˆ’2r(rβˆ’1)rβˆ’1\mu_\mathrm{c}(k,r):=\binom{k}{r}^{-1}\frac{(r-2)^{r-2}}{r(r-1)^{r-1}} such that for m=ΞΌnrβˆ’1u(n)rβˆ’2m=\mu\frac{n^{r-1}}{u(n)^{r-2}}, the LP relaxation can determine linearly many input variables of Goldreich's generator if ΞΌ>ΞΌc(k,r)\mu>\mu_\mathrm{c}(k,r), and that the LP relaxation cannot determine 1rβˆ’2u(n)\frac1{r-2} u(n) input variables of Goldreich's generator if ΞΌ<ΞΌc(k,r)\mu<\mu_\mathrm{c}(k,r). This paper uses characterization of LP solutions by combinatorial structures called stopping sets on a bipartite graph, which is related to a simple algorithm called peeling algorithm.Comment: 14 pages, 1 figur

    Electronic Transport in Fullerene C20 Bridge Assisted by Molecular Vibrations

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    The effect of molecular vibrations on electronic transport is investigated with the smallest fullerene C20 bridge, utilizing the Keldysh nonequilibrium Green's function techniques combined with the tight-binding molecular-dynamics method. Large discontinuous steps appear in the differential conductance when the applied bias-voltage matches particular vibrational energies. The magnitude of the step is found to vary considerably with the vibrational mode and to depend on the local electronic states besides the strength of electron-vibration coupling. On the basis of this finding, a novel way to control the molecular motion by adjusting the gate voltage is proposed.Comment: 9 pages, 4 figures, accepted for publication in Phys. Rev. Let

    Fermi Surface and Magnetism in the Kondo lattice: A Continuum Field Theory Approach

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    We consider the Fermi surface inside the antiferromagnetic ordered region of a Kondo lattice system in an arbitrary dimension higher than one. We establish the existence of AFS{\rm AF_S}, an antiferromagnetic phase whose Fermi surface is ``small,'' in the sense that the local moments do not participate in the Fermi-surface formation. This is in contrast to the ``large'' Fermi surface that is typically assumed for heavy fermion metals. We extend our earlier work to the case that the Fermi surface of the conduction electrons intersects the antiferromagnetic Brillouin zone boundary. Our results provide a new perspective on local quantum criticality. In addition, our results imply that, for the AFS{\rm AF_S} phase, it is important to keep track of the dynamical screening processes; we suggest that this effect is not captured in a recent variational Monte-Carlo study of the Kondo lattice.Comment: 2 pages, 1 embedded eps figure, proceedings of SCES'0

    Dβˆ—D^{\ast} polarization vs. RD(βˆ—)R_{D^{(\ast)}} anomalies in the leptoquark models

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    Polarization measurements in BΛ‰β†’D(βˆ—)τν‾\bar{B} \to D^{(\ast)} \tau \overline{\nu} are useful to check consistency in new physics explanations for the RDR_{D} and RDβˆ—R_{D^{\ast}} anomalies. In this paper, we investigate the Dβˆ—D^{\ast} and Ο„\tau polarizations and focus on the new physics contributions to the fraction of a longitudinal Dβˆ—D^{\ast} polarization (FLDβˆ—F_{L}^{D^{\ast}}), which is recently measured by the Belle collaboration FLDβˆ—=0.60Β±0.09F_{L}^{D^{\ast}} = 0.60 \pm 0.09, in model-independent manner and in each single leptoquark model (R2{\rm R}_2, S1{\rm S}_1 and U1{\rm U}_1) that can naturally explain the RD(βˆ—)R_{D^{(\ast)}} anomalies. It is found that B(Bc+β†’Ο„+Ξ½)\mathcal{B}(B_c^{+} \to \tau^{+} \nu) severely restricts deviation from the Standard Model (SM) prediction of FL,SMDβˆ—=0.46Β±0.04F_{L, \textrm{SM}}^{D^{\ast}} = 0.46 \pm 0.04 in the leptoquark models: [0.43, 0.44], [0.42, 0.48], and [0.43, 0.47] are predicted as a range of FLDβˆ—F_{L}^{D^{\ast}} for the R2{\rm R}_2, S1{\rm S}_1, and U1{\rm U}_1 leptoquark models, respectively, where the current data of RD(βˆ—)R_{D^{(\ast)}} is satisfied at 1 σ1\,\sigma level. It is also shown that the Ο„\tau polarization observables can much deviate from the SM predictions. The Belle II experiment, therefore, can check such correlations between RD(βˆ—)R_{D^{(\ast)}} and the polarization observables, and discriminate among the leptoquark models.Comment: 24 pages, 3 figures, 1 table; references added, version published in JHE
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