Nucleation and Growth of GaN/AlN Quantum Dots

We study the nucleation of GaN islands grown by plasma-assisted molecular-beam epitaxy on AlN(0001) in a Stranski-Krastanov mode. In particular, we assess the variation of their height and density as a function of GaN coverage. We show that the GaN growth passes four stages: initially, the growth is layer-by-layer; subsequently, two-dimensional precursor islands form, which transform into genuine three-dimensional islands. During the latter stage, island height and density increase with GaN coverage until the density saturates. During further GaN growth, the density remains constant and a bimodal height distribution appears. The variation of island height and density as a function of substrate temperature is discussed in the framework of an equilibrium model for Stranski-Krastanov growth.Comment: Submitted to PRB, 10 pages, 15 figure

Neutrino masses through see-saw mechanism in 3-3-1 models

Some years ago it was shown by Ma that in the context of the electroweak standard model there are, at the tree level, only three ways to generate small neutrino masses by the see-saw mechanism via one effective dimension-five operator. Here we extend this approach to 3-3-1 chiral models showing that in this case there are several dimension-five operators and we also consider their tree level realization.Comment: RevTex, 7 pages and 4 .eps figures. Version published in Phys. Rev. D. with a change in the titl

Can the Zee Model Explain the Observed Neutrino Data?

The eigenvalues and mixing angles in the Zee model are investigated parameter-independently. When we require |\Delta m^2_{12}/\Delta m^2_{23}| \ll 1 in order to understand the solar and atmospheric data simultaneously, the only solution is one which gives bi-maximal mixing. It is pointed out that the observed values \sin^2 2\theta_{solar} \simeq 0.66 in the MSW LMA solution cannot be explained within the framework of the Zee model, because we derive a severe constraint on the value of \sin^2 2 \theta_{solar}, \sin^2 2 \theta_{solar} \geq 1 -(1/16)(\Delta m^2_{solar}/\Delta m^2_{atm})^2.Comment: Latex file, 10 pages, 1 figure, explanations and references added, typos corrected, to be published in Phys.Rev.

Escape from washing out of baryon number in a two-zero-texture general Zee model compatible with the large mixing angle MSW solution

We propose a two-zero-texture general Zee model, compatible with the large mixing angle Mikheyev-Smirnov-Wolfenstein solution. The washing out of the baryon number does not occur in this model for an adequate parameter range. We check the consistency of a model with the constraints coming from flavor changing neutral current processes, the recent cosmic microwave background observation, and the Z-burst scenario.Comment: 22 pages, 2 eps figures, Type set revtex

Constraints On Radiative Neutrino Mass Models From Oscillation Data

The three neutrino Zee model and its extension including three active and one sterile species are studied in the light of new neutrino oscillation data. We obtain analytical relations for the mixing angle in solar oscillations in terms of neutrino mass squared differences. For the four neutrino case, we obtain the result $\mathsf{sin^2 2 \theta_\odot \approx 1 - [ (\Delta m^2_{Atm})^2/(4 \Delta m^2_{LSND} \Delta m^2_\odot) ]^2}$, which can accommodate both the large and small mixing scenarios. We show that within this framework, while both the SMA-MSW and the LMA-MSW solutions can easily be accommodated, it would be difficult to reconcile the LOW-QVO solutions. We also comment on the active-sterile admixture within phenomenologically viable textures.Comment: The paper has been substantially rewritten, especially in Section IV, though the basic results are unchanged. Some new references and an appendix have been adde

Bilarge Neutrino Mixing and \mu - \tau Permutation Symmetry for Two-loop Radiative Mechanism

The presence of approximate electron number conservation and \mu-\tau permutation symmetry of S_2 is shown to naturally provide bilarge neutrino mixing. First, the bimaximal neutrino mixing together with U_{e3}=0 is guaranteed to appear owing to S_2 and, then, the bilarge neutrino mixing together with |U_{e3}|<<1 arises as a result of tiny violation of S_2. The observed mass hierarchy of \Delta m^2_{\odot}<<\Delta m^2_{atm} is subject to another tiny violation of the electron number conservation. This scenario is realized in a specific model based on SU(3)_L x U(1)_N with two-loop radiative mechanism for neutrino masses. The radiative effects from heavy leptons contained in lepton triplets generate the bimaximal structure and those from charged leptons, which break S_2, generate the bilarge structure together with |U_{e3}|<<1. To suppress dangerous flavor-changing neutral current interactions due to Higgs exchanges especially for quarks, this S_2 symmetry is extended to a discrete Z_8 symmetry, which also ensures the absence of one-loop radiative mechanism.Comment: 18 pages, 7 figures, to appear in Phys. Rev.

Minimal Scalar Sector of 3-3-1 Models without Exotic Electric Charges

We study the minimal set of Higgs scalars, for models based on the local gauge group $SU(3)_c \otimes SU(3)_L \otimes U(1)_X$ which do not contain particles with exotic electric charges. We show that only two Higgs $SU(3)_L$ triplets are needed in order to properly break the symmetry. The exact tree-level scalar mass matrices resulting from symmetry breaking are calculated at the minimum of the most general scalar potential, and the gauge bosons are obtained, together with their couplings to the physical scalar fields. We show how the scalar sector introduced is enough to produce masses for fermions in a particular model which is an $E_6$ subgroup. By using experimental results we constrain the scale of new physics to be above 1.3 TeV.Comment: LaTeX, 22 pages, 1 figure include

Lepton Masses from a TeV Scale in a 3-3-1 Model

In this work, using the fact that in 3-3-1 models the same leptonic bilinear contributes to the masses of both charged leptons and neutrinos, we develop an effective operator mechanism to generate mass for all leptons. The effective operators have dimension five for the case of charged leptons and dimension seven for neutrinos. By adding extra scalar multiplets and imposing the discrete symmetry $Z_9\otimes Z_2$ we are able to generate realistic textures for the leptonic mixing matrix. This mechanism requires new physics at the TeV scale.Comment: RevTex, 13 pages. Extended version to be published in Physical Review

Generating Neutrino Mass in the 331 Model

A mechanism for generating small tree-level Majorana mass for neutrinos is implemented in the 331 Model. No additional fermions or scalars need to be added, and no mass scale greater than a few TeV is invoked.Comment: LaTex, 7 pages, no figures. Revised version to appear in Phys. Rev.

Two-loop Radiative Neutrino Mechanism in an SU(3)L×U(1)NSU(3)_L\times U(1)_N Gauge Model

By using the $L_e$ - $L_\mu$ - $L_\tau$ symmetry, we construct an $SU(3)_L\times U(1)_N$ gauge model that provides two-loop radiative neutrino masses as well as one-loop radiative neutrino masses. The generic smallness of two-loop neutrino masses leading to $\Delta m^2_\odot$ compared with one-loop neutrino masses leading to $\Delta m^2_{atm}$ successfully explains $\Delta m^2_{atm}$ $>>$ $\Delta m^2_{\odot}$ by invoking the $L_e$ - $L_\mu$ - $L_\tau$ breaking. The Higgs scalar ($h^+$) that initiates radiative mechanisms is unified into a Higgs triplet together with the standard Higgs scalar ($\phi^+$, $\phi^0$) to form ($\phi^+$, $\phi^0$, $h^+$), which calls for three families of lepton triplets: ($\nu^i_L$, $\ell^i_L$, $\omega^i_L$) (i = 1,2,3), where $\omega^i$ denote heavy neutral leptons. The two-loop radiative mechanism is found possible by introducing a singly charged scalar, which couples to $\ell^i_R\omega^j_R$ (i,j = 2,3).Comment: with 10 pages, revtex, including 2 figures, accepted for publication in Phys. Rev. D (with undefined latex citation indices removed