Experimental proposal for accurate determination of the phase relaxation time and testing a formation of thermalized non-equilibrated matter in highly excited quantum many-body systems

We estimate how accurate the phase relaxation time of quantum many-body systems can be determined from data on forward peaking of evaporating protons from a compound nucleus. The angular range and accuracy of the data needed for a reliable determination of the phase relaxation time are evaluated. The general method is applied to analyze the inelastic scattering of 18 MeV protons from Pt for which previously measured double differential cross sections for two angles in the evaporating domain of the spectra show a strong forward peaking. A new experiment for an improved determination of the phase relaxation time is proposed. The experiment is also highly desirable for an accurate test of a formation of thermalized non-equilibrated matter in quantum many-body systems.Comment: 5 pages, 3 figure

Anderson localization in carbon nanotubes: defect density and temperature effects

The role of irradiation induced defects and temperature in the conducting properties of single-walled (10,10) carbon nanotubes has been analyzed by means of a first-principles approach. We find that di-vacancies modify strongly the energy dependence of the differential conductance, reducing also the number of contributing channels from two (ideal) to one. A small number of di-vacancies (5-9) brings up strong Anderson localization effects and a seemly universal curve for the resistance as a function of the number of defects. It is also shown that low temperatures, around 15-65 K, are enough to smooth out the fluctuations of the conductance without destroying the exponential dependence of the resistivity as a function of the tube length.Comment: 4 pages, 4 figure

Reduced nuclear helicity amplitudes for deuteron electrodisintegration and other processes

We extend the original idea of reduced nuclear amplitudes to capture individual helicity amplitudes and discuss various applications to exclusive processes involving the deuteron. Specifically, we consider deuteron form factors, structure functions, tensor polarization observables, photodisintegration, and electrodisintegration. The basic premise is that nuclear processes at high momentum transfer can be approximated by tree graphs for point-like nucleons supplemented by empirical form factors for each nucleon. The latter represent the internal structure of the nucleon, and incorporate nonperturbative physics, which can allow for early onset of scaling behavior. The nucleon form factors are evaluated at the net momentum transfer experienced by the given nucleon, with use of $G_E$ for a no-flip contribution and $G_M$ for a helicity-flip contribution. Results are compared with data where available. The deuteron photodisintegration asymmetry $\Sigma$ is obtained with a value of $\Sigma(90^\circ)\simeq -0.06$, which is much closer to experiment than the value of -1 originally expected. The method also provides an estimate of the momentum transfer values required for scaling onset. We find that the deuteron structure function $B$ is a good place to look, above momentum transfers of 10 GeV$^2$.Comment: 26 pages, 18 figures, REVTeX 4.

Cluster Cores, Gravitational Lensing, and Cosmology

Many multiply--imaged quasars have been found over the years, but none so far with image separation in excess of 8\arcsec. The absence of such large splittings has been used as a test of cosmological models: the standard Cold Dark Matter model has been excluded on the basis that it predicts far too many large--separation double images. These studies assume that the lensing structure has the mass profile of a singular isothermal sphere. However, such large splittings would be produced by very massive systems such as clusters of galaxies, for which other gravitational lensing data suggest less singular mass profiles. Here we analyze two cases of mass profiles for lenses: an isothermal sphere with a finite core radius (density $\rho \propto (r^2+r_{core}^2)^{-1})$, and a Hernquist profile ($\rho \propto r^{-1}(r+a)^{-3}$). We find that small core radii $r_{core} \sim 30 h^{-1}$ kpc, as suggested by the cluster data, or large a \gsim 300 h^{-1} kpc, as needed for compatibility with gravitational distortion data, would reduce the number of large--angle splittings by an order of magnitude or more. Thus, it appears that these tests are sensitive both to the cosmological model (number density of lenses) and to the inner lens structure, which is unlikely to depend sensitively on the cosmology, making it difficult to test the cosmological models by large--separation quasar lensing until we reliably know the structure of the lenses themselves.Comment: 17 pages, uuencoded compressed tarred postscript file including text and 1 figure. To appear in January 20, 1996 issue of ApJ Letter

Tuning the Kondo effect with a mechanically controllable break junction

We study electron transport through C60 molecules in the Kondo regime using a mechanically controllable break junction. By varying the electrode spacing, we are able to change both the width and height of the Kondo resonance, indicating modification of the Kondo temperature and the relative strength of coupling to the two electrodes. The linear conductance as a function of T/T_K agrees with the scaling function expected for the spin-1/2 Kondo problem. We are also able to tune finite-bias Kondo features which appear at the energy of the first C60 intracage vibrational mode.Comment: 4 pages with 4 figure

Ga-induced atom wire formation and passivation of stepped Si(112)

We present an in-depth analysis of the atomic and electronic structure of the quasi one-dimensional (1D) surface reconstruction of Ga on Si(112) based on Scanning Tunneling Microscopy and Spectroscopy (STM and STS), Rutherford Backscattering Spectrometry (RBS) and Density Functional Theory (DFT) calculations. A new structural model of the Si(112)6 x 1-Ga surface is inferred. It consists of Ga zig-zag chains that are intersected by quasi-periodic vacancy lines or misfit dislocations. The experimentally observed meandering of the vacancy lines is caused by the co-existence of competing 6 x 1 and 5 x 1 unit cells and by the orientational disorder of symmetry breaking Si-Ga dimers inside the vacancy lines. The Ga atoms are fully coordinated, and the surface is chemically passivated. STS data reveal a semiconducting surface and show excellent agreement with calculated Local Density of States (LDOS) and STS curves. The energy gain obtained by fully passivating the surface calls the idea of step-edge decoration as a viable growth method toward 1D metallic structures into question.Comment: Submitted, 13 pages, accepted in Phys. Rev. B, notational change in Fig.

Star formation in low density HI gas around the Elliptical Galaxy NGC2865

Interacting galaxies surrounded by HI tidal debris are ideal sites for the study of young clusters and tidal galaxy formation. The process that triggers star formation in the low-density environments outside galaxies is still an open question. New clusters and galaxies of tidal origin are expected to have high metallicities for their luminosities. Spectroscopy of such objects is, however, at the limit of what can be done with existing 8-10m class telescopes, which has prevented statistical studies of these objects. NGC2865 is an UV-bright merging elliptical galaxy with shells and extended HI tails. The regions observed in this work were previously detected using multi-slit imaging spectroscopy. We obtain new multislit spectroscopy of six young star-forming regions around NGC2865, to determine their redshifts and metallicities. The six emission-line regions are located 16-40 kpc from NGC2865 and they have similar redshifts. They have ages of ~10Myears and an average metallicity of 12+log(O/H) ~ 8.6, suggesting a tidal origin for the regions. It is noted that they coincide with an extended HI tail, which has projected density of N$_{HI}$ < 10$^{19}$ cm$^{-2}$, and displays a low surface brightness counterpart. These regions may represent the youngest of the three populations of star clusters already identified in NGC2865. The high, nearly-solar, oxygen abundances found for the six regions in the vicinity of NGC2865 suggest that they were formed by pre-enriched material from the parent galaxy, from gas removed during the last major merger. Given the mass and the location of the HII regions, we can speculate that these young star-forming regions are potential precursors of globular clusters that will be part of the halo of NGC2865 in the future. Our result supports the use of the multi-slit imaging spectroscopy as a useful tool for finding nearly-formed stellar systems around galaxies.Comment: 7 pages, 2 figures accepted in A&