Donor Centers and Absorption Spectra in Quantum Dots

We have studied the electronic properties and optical absorption spectra of three different cases of donor centers, D^{0}, D^{-} and D^{2-}, which are subjected to a perpendicular magnetic field, using the exact diagonalization method. The energies of the lowest lying states are obtained as function of the applied magnetic field strength B and the distance zeta between the positive ion and the confinement xy-plane. Our calculations indicate that the positive ion induces transitions in the ground-state, which can be observed clearly in the absorption spectra, but as zeta goes to 0 the strength of the applied magnetic field needed for a transition to occur tends to infinity.Comment: 5 pages, 4 figures, REVTeX 4, gzipped tar fil

Worldsheet correlators in AdS(3)/CFT(2)

The AdS_3/CFT_2 correspondence is checked beyond the supergravity approximation by comparing correlation functions. To this end we calculate 2- and 3-point functions on the sphere of certain chiral primary operators for strings on AdS_3 x S^3 x T^4. These results are then compared with the corresponding amplitudes in the dual 2-dimensional conformal field theory. In the limit of small string coupling, where the sphere diagrams dominate the string perturbation series, beautiful agreement is found.Comment: 23 page

Tachyons in Compact Spaces

We discuss condensations of closed string tachyons localized in compact spaces. Time evolution of an on-shell condensation is naturally related to the worldsheet RG flow. Some explicit tachyonic compactifications of Type II string theory is considered, and some of them are shown to decay into supersymmetric theories known as the little string theories.Comment: 14 page

O(\alpha^2 \ln(m_\mu/m_e)) Corrections to Electron Energy Spectrum in Muon Decay

O(\alpha^2 \ln(m_\mu/m_e)) corrections to electron energy spectrum in muon decay are computed using perturbative fragmentation function approach. The magnitude of these corrections is comparable to anticipated precision of the TWIST experiment at TRIUMF where Michel parameters will be extracted from the measurement of the electron energy spectrum in muon decay.Comment: 8 pages, LaTeX, revtex4.cls, 1 PostScript figur

Three-point functions in superstring theory on AdS3xS3xT4

We consider R and NS spectral flow sectors of type IIB superstring theory on AdS(3)xS(3)xT(4) in the context of the AdS(3)/CFT(2) correspondence. We present a derivation of the vertex operators creating spectral flow images of chiral primary states previously proposed in the literature. We compute spectral flow conserving three-point functions involving these operators on the sphere. Using the bulk-to-boundary dictionary, we compare the results with the corresponding correlators in the dual conformal field theory, the symmetric product orbifold of T(4). In the limit of small string coupling, agreement is found in all the cases considered.Comment: 21 pages; v2: New results added. Conclusion on RRNS correlators revised. Version to appear in JHE

On Minisuperspace Models of S-branes

In this note we reconsider the minisuperspace toy models for rolling and bouncing tachyons. We show that the theories require to choose boundary conditions at infinity since particles in an exponentially unbounded potential fall to infinity in finite world-sheet time. Using standard techniques from operator theory, we determine the possible boundary conditions and we compute the corresponding energy spectra and minisuperspace 3-point functions. Based on this analysis we argue in particular that world-sheet models of S-branes possess a discrete spectrum of conformal weights containing both positive and negative values. Finally, some suggestions are made for possible relations with previous studies of the minisuperspace theory.Comment: 24 pages, 3 figure

Bosonic D-brane Effective Action in Linear Dilaton Background

In this paper we will study tachyon effective action for Dp-brane in bosonic string theory in the linear dilaton background. We obtain the tachyon effective Lagrangian from boundary state coeficient of Dp-brane in the linear dilaton background and compare it with tachyon effective Lagrangians that were proposed in previous papers.Comment: 16 pages, Typos correcte

Spectral Flow in AdS(3)/CFT(2)

We study the spectral flowed sectors of the H3 WZW model in the context of the holographic duality between type IIB string theory in AdS(3)x S^3 x T^4 with NSNS flux and the symmetric product orbifold of T^4. We construct explicitly the physical vertex operators in the flowed sectors that belong to short representations of the superalgebra, thus completing the bulk-to-boundary dictionary for 1/2 BPS states. We perform a partial calculation of the string three-point functions of these operators. A complete calculation would require the three-point couplings of non-extremal flowed operators in the H3 WZW model, which are at present unavailable. In the unflowed sector, perfect agreement has recently been found between the bulk and boundary three-point functions of 1/2 BPS operators. Assuming that this agreement persists in the flowed sectors, we determine certain unknown three-point couplings in the H3 WZW model in terms of three-point couplings of affine descendants in the SU(2) WZW model.Comment: 50 pages, 2 figure

Rolling Tachyon Boundary State, Conserved Charges and Two Dimensional String Theory

The boundary state associated with the rolling tachyon solution on an unstable D-brane contains a part that decays exponentially in the asymptotic past and the asymptotic future, but it also contains other parts which either remain constant or grow exponentially in the past or future. We argue that the time dependence of the latter parts is completely determined by the requirement of BRST invariance of the boundary state, and hence they contain information about certain conserved charges in the system. We also examine this in the context of the unstable D0-brane in two dimensional string theory where these conserved charges produce closed string background associated with the discrete states, and show that these charges are in one to one correspondence with the symmetry generators in the matrix model description of this theory.Comment: LaTeX file, 37 pages; v3: references added; v4: minor change