Scanning Tunneling Spectroscopy of Suspended Single-Wall Carbon Nanotubes

We have performed low-temperature STM measurements on single-wall carbon nanotubes that are freely suspended over a trench. The nanotubes were grown by CVD on a Pt substrate with predefined trenches etched into it. Atomic resolution was obtained on the freestanding portions of the nanotubes. Spatially resolved spectroscopy on the suspended portion of both metallic and semiconducting nanotubes was also achieved, showing a Coulomb-staircase behavior superimposed on the local density of states. The spacing of the Coulomb blockade peaks changed with tip position reflecting a changing tip-tube capacitance

In an Attempt to Introduce Long-range Interactions into Small-world Networks

Distinguishing the long-range bonds with the regular ones, the critical temperature of the spin-lattice Guassian model built on two typical Small-world Networks (SWNs) is studied. The results show much difference from the classical case, and thus may induce some more accurate discussion on the critical properties of the spin-lattice systems combined with the SWNs.Comment: 4 pages, 3 figures, 18 referenc

Spin Accumulation in the Extrinsic Spin Hall Effect

The drift-diffusion formalism for spin-polarized carrier transport in semiconductors is generalized to include spin-orbit coupling. The theory is applied to treat the extrinsic spin Hall effect using realistic boundary conditions. It is shown that carrier and spin diffusion lengths are modified by the presence of spin-orbit coupling and that spin accumulation due to the extrinsic spin Hall effect is strongly and qualitatively influenced by boundary conditions. Analytical formulas for the spin-dependent carrier recombination rates and inhomogeneous spin densities and currents are presented.Comment: 5 pages, 3 figure

Correlations and fluctuations of a confined electron gas

The grand potential $\Omega$ and the response $R = - \partial \Omega /\partial x$ of a phase-coherent confined noninteracting electron gas depend sensitively on chemical potential $\mu$ or external parameter $x$. We compute their autocorrelation as a function of $\mu$, $x$ and temperature. The result is related to the short-time dynamics of the corresponding classical system, implying in general the absence of a universal regime. Chaotic, diffusive and integrable motions are investigated, and illustrated numerically. The autocorrelation of the persistent current of a disordered mesoscopic ring is also computed.Comment: 12 pages, 1 figure, to appear in Phys. Rev.

Experience versus Talent Shapes the Structure of the Web

We use sequential large-scale crawl data to empirically investigate and validate the dynamics that underlie the evolution of the structure of the web. We find that the overall structure of the web is defined by an intricate interplay between experience or entitlement of the pages (as measured by the number of inbound hyperlinks a page already has), inherent talent or fitness of the pages (as measured by the likelihood that someone visiting the page would give a hyperlink to it), and the continual high rates of birth and death of pages on the web. We find that the web is conservative in judging talent and the overall fitness distribution is exponential, showing low variability. The small variance in talent, however, is enough to lead to experience distributions with high variance: The preferential attachment mechanism amplifies these small biases and leads to heavy-tailed power-law (PL) inbound degree distributions over all pages, as well as over pages that are of the same age. The balancing act between experience and talent on the web allows newly introduced pages with novel and interesting content to grow quickly and surpass older pages. In this regard, it is much like what we observe in high-mobility and meritocratic societies: People with entitlement continue to have access to the best resources, but there is just enough screening for fitness that allows for talented winners to emerge and join the ranks of the leaders. Finally, we show that the fitness estimates have potential practical applications in ranking query results

Electronic excitation spectrum of metallic carbon nanotubes

We have studied the discrete electronic spectrum of closed metallic nanotube quantum dots. At low temperatures, the stability diagrams show a very regular four-fold pattern that allows for the determination of the electron addition and excitation energies. The measured nanotube spectra are in excellent agreement with theoretical predictions based on the nanotube band structure. Our results permit the complete identification of the electron quantum states in nanotube quantum dots.Comment: 4 pages, 3 figure

Long-term X-ray Variability Study of IC342 from XMM-Newton Observations

We presented the results of an analysis of four XMM-Newton observations of the starburst galaxy IC342 taken over a four-year span from 2001 to 2005, with an emphasis on investigating the long-term flux and spectral variability of the X-ray point sources. We detected a total of 61 X-ray sources within 35' $\times$ 30' of the galaxy down to a luminosity of (1-2)$\times$1037 erg s-1 depending on the local background. We found that 39 of the 61 detected sources showed long-term variability, in which 26 of them were classified as X-ray transients. We also found 19 sources exhibiting variations in hardness ratios or undergoing spectral transitions among observations, and were identified as spectral variables. In particular, 8 of the identified X-ray transients showed spectral variability in addition to flux variability. The diverse patterns of variability observed is indicative of a population of X-ray binaries. We used X-ray colors, flux and spectral variability, and in some cases the optical or radio counterparts to classify the detected X-ray sources into several stellar populations. We identified a total of 11 foreground stars, 1 supersoft sources (SSS), 3 quasisoft sources (QSS), and 2 supernova remnants (SNR). The identified SSS/QSS are located near or on the spiral arms, associate with young stellar populations; the 2 SNR are very close to the starburst nucleus where current star formation activities are dominated. We also discovered a spectral change in the nuclear source of IC342 for the first time by a series of X-ray spectrum analysis.Comment: 45 pages, 6 figures accepted by Ap