Bound States of D-Branes and the Non-Abelian Born-Infeld Action

We attempt to settle the issue as to what is the correct non-abelian generalisation of the Born-Infeld action, via a consideration of the two-loop $\beta$--function for the non-abelian background gauge field in open string theory. An analysis of the bosonic theory alone shows the recent proposal of Tseytlin's to be somewhat lacking. For the superstring, however, this proposal would seem to be correct, and not just within the approximation used in \cite{tseytlin}. Since it is this latter case that is relevant to the description of D-branes we, in effect, obtain an independent verification of Tseytlin's result. Some issues involved in the concept of non-abelian T--duality are discussed; and it is shown how the interaction between separated and parallel branes, in the form of massive string states, emerges.Comment: 24 pages, 4 eps figures, LaTeX. Further comments concerning earlier work added, minor typos corrected. Version to be published in Nucl. Phys.

New Perspectives in Sinographic Language Processing Through the Use of Character Structure

Chinese characters have a complex and hierarchical graphical structure carrying both semantic and phonetic information. We use this structure to enhance the text model and obtain better results in standard NLP operations. First of all, to tackle the problem of graphical variation we define allographic classes of characters. Next, the relation of inclusion of a subcharacter in a characters, provides us with a directed graph of allographic classes. We provide this graph with two weights: semanticity (semantic relation between subcharacter and character) and phoneticity (phonetic relation) and calculate "most semantic subcharacter paths" for each character. Finally, adding the information contained in these paths to unigrams we claim to increase the efficiency of text mining methods. We evaluate our method on a text classification task on two corpora (Chinese and Japanese) of a total of 18 million characters and get an improvement of 3% on an already high baseline of 89.6% precision, obtained by a linear SVM classifier. Other possible applications and perspectives of the system are discussed.Comment: 17 pages, 5 figures, presented at CICLing 201

Hot String Soup

Above the Hagedorn energy density closed fundamental strings form a long string phase. The dynamics of weakly interacting long strings is described by a simple Boltzmann equation which can be solved explicitly for equilibrium distributions. The average total number of long strings grows logarithmically with total energy in the microcanonical ensemble. This is consistent with calculations of the free single string density of states provided the thermodynamic limit is carefully defined. If the theory contains open strings the long string phase is suppressed.Comment: 13 pages, no figures, uses LaTex, some errors in equations have been corrected, NSF-ITP-94-83, UCSBTH-94-3

Microcraters in aluminum foils exposed by Stardust

We will present preliminary results on the nature and size frequency distribution of microcraters that formed in aluminum foils during the flyby of comet Wild 2 by the Stardust spacecraft

Neutron scattering search for static magnetism in oxygen ordered YBa2Cu3O6.5

We present elastic and inelastic neutron scattering results on highly oxygen ordered YBa2Cu3O6.5 ortho-II. We find no evidence for the presence of ordered magnetic moments to a sensitivity of 0.003 Bohr magnetons, an order of magnitude smaller than has been suggested in theories of orbital or d-density-wave (DDW) currents. The absence of sharp elastic peaks, shows that the d-density-wave phase is not present, at least for the superconductor with the doping of 6.5 and the ordered ortho-II structure. We cannot exclude the possibility that a broad peak may exist with extremely short-range DDW correlations. For less ordered or more doped crystals it is possible that disorder may lead to static magnetism. We have also searched for the large normal state spin gap that is predicted to exist in an ordered DDW phase. Instead of a gap we find that the Q-correlated spin susceptibility persists to the lowest energies studied, 6 meV. Our results are compatible with the coexistence of superconductivity with orbital currents, but only if they are dynamic, and exclude a sharp phase transition to an ordered d-density-wave phase.Comment: 6 pages 4 figures RevTex Submitted to Phys Rev B January 23, 200

Superfield T-duality rules

A geometric treatment of T-duality as an operation which acts on differential forms in superspace allows us to derive the complete set of T-duality transformation rules which relate the superfield potentials of D=10 type IIA supergravity with those of type IIB supergravity including Ramond-Ramond superfield potentials and fermionic supervielbeins. We show that these rules are consistent with the superspace supergravity constraints.Comment: 24 pages, latex, no figures. V2 misprints corrected. V3. One reference ([30]) and a comment on it ('Notice added') on p. 19 adde

Toeplitz operators on symplectic manifolds

We study the Berezin-Toeplitz quantization on symplectic manifolds making use of the full off-diagonal asymptotic expansion of the Bergman kernel. We give also a characterization of Toeplitz operators in terms of their asymptotic expansion. The semi-classical limit properties of the Berezin-Toeplitz quantization for non-compact manifolds and orbifolds are also established.Comment: 40 page

Four Dimensional String/String/String Triality

In six spacetime dimensions, the heterotic string is dual to a Type $IIA$ string. On further toroidal compactification to four spacetime dimensions, the heterotic string acquires an SL(2,\BbbZ)_S strong/weak coupling duality and an SL(2,\BbbZ)_T \times SL(2,\BbbZ)_U target space duality acting on the dilaton/axion, complex Kahler form and the complex structure fields $S,T,U$ respectively. Strong/weak duality in $D=6$ interchanges the roles of $S$ and $T$ in $D=4$ yielding a Type $IIA$ string with fields $T,S,U$. This suggests the existence of a third string (whose six-dimensional interpretation is more obscure) that interchanges the roles of $S$ and $U$. It corresponds in fact to a Type $IIB$ string with fields $U,T,S$ leading to a four-dimensional string/string/string triality. Since SL(2,\BbbZ)_S is perturbative for the Type $IIB$ string, this $D=4$ triality implies $S$-duality for the heterotic string and thus fills a gap left by $D=6$ duality. For all three strings the total symmetry is SL(2,\BbbZ)_S \times O(6,22;\BbbZ)_{TU}. The O(6,22;\BbbZ) is {\it perturbative} for the heterotic string but contains the conjectured {\it non-perturbative} SL(2,\BbbZ)_X, where $X$ is the complex scalar of the $D=10$ Type $IIB$ string. Thus four-dimensional triality also provides a (post-compactification) justification for this conjecture. We interpret the $N=4$ Bogomol'nyi spectrum from all three points of view. In particular we generalize the Sen-Schwarz formula for short multiplets to include intermediate multiplets also and discuss the corresponding black hole spectrum both for the $N=4$ theory and for a truncated $S$--$T$--$U$ symmetric $N=2$ theory. Just as the first two strings are described by the four-dimensional {\it elementary} and {\it dual solitonic} solutions, so theComment: 36 pages, Latex, 2 figures, some references changed, minor changes in formulas and tables; to appear in Nucl. Phys.

Optical investigation of the charge-density-wave phase transitions in NbSe3NbSe_{3}

We have measured the optical reflectivity $R(\omega)$ of the quasi one-dimensional conductor $NbSe_{3}$ from the far infrared up to the ultraviolet between 10 and 300 $K$ using light polarized along and normal to the chain axis. We find a depletion of the optical conductivity with decreasing temperature for both polarizations in the mid to far-infrared region. This leads to a redistribution of spectral weight from low to high energies due to partial gapping of the Fermi surface below the charge-density-wave transitions at 145 K and 59 K. We deduce the bulk magnitudes of the CDW gaps and discuss the scattering of ungapped free charge carriers and the role of fluctuations effects

Thin accretion disc with a corona in a central magnetic field

We study the steady-state structure of an accretion disc with a corona surrounding a central, rotating, magnetized star. We assume that the magneto-rotational instability is the dominant mechanism of angular momentum transport inside the disc and is responsible for producing magnetic tubes above the disc. In our model, a fraction of the dissipated energy inside the disc is transported to the corona via these magnetic tubes. This energy exchange from the disc to the corona which depends on the disc physical properties is modified because of the magnetic interaction between the stellar magnetic field and the accretion disc. According to our fully analytical solutions for such a system, the existence of a corona not only increases the surface density but reduces the temperature of the accretion disc. Also, the presence of a corona enhances the ratio of gas pressure to the total pressure. Our solutions show that when the strength of the magnetic field of the central neutron star is large or the star is rotating fast enough, profiles of the physical variables of the disc significantly modify due to the existence of a corona.Comment: Accepted for publication in Astrophysics & Space Scienc