Physics

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We discuss an effective way for analyzing the system on the magnetized twisted orbifolds in operator formalism, especially in the complicated cases T2/Z3, T2/Z4 and T2/Z6. We can obtain the exact and analytical results which can be applicable for any larger values of the quantized magnetic flux M, and show that the (non-diagonalized) kinetic terms are generated via our formalism and the number of the surviving physical states are calculable in a rigorous manner by simply following usual procedures in linear algebra in any case. Our approach is very powerful when we try to examine properties of the physical states on (complicated) magnetized orbifolds T2/Z3, T2/Z4, T2/Z6 (and would be in other cases on higher-dimensional torus) and could be an essential tool for actual realistic model construction based on these geometries. (Note: This article is registered under preprint number: arXiv:1409.5421.). © 2014 The Authors.

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(SoNS) School of Natural Sciences

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Nuclear Physics B

Operator analysis of physical states on magnetized T2/ZN orbifolds

We propose a three loop radiative neutrino mass scenario with an isolated doubly charged singlet scalar k±± without couplings to the charged leptons, while two other singly charged scalars h1± and h2± attach to them. In this setup, the lepton flavor violation originating from k±± exchanges is suppressed and the model is less constrained, where some couplings can take sizable values. As reported in our previous work [1], the loop suppression factor at the three loop level would be too strong and realized neutrino masses in a three loop scenario could be smaller than the observed minuscule values. The sizable couplings can help us to enhance neutrino masses without drastically large scalar trilinear couplings appearing in a neutrino mass matrix, which tends to drive the vacuum stability to become jeopardized at the one loop level. Now the doubly charged scalar k±± has less constraint via lepton flavor violation and the vacuum can be quite stable, and thus a few hundred GeV mass in k±± is possible, which is within the LHC reach and this model can be tested in the near future. Note that the other h1± and h2± should be heavy at least around a few TeV. We suitably arrange the charges of an additional global U(1) symmetry, where the decay constant of the associated Nambu-Goldstone boson can be around a TeV scale consistently. Also, this model is indirectly limited through a global analysis on results of the LHC Higgs search and issues on a dark matter candidate, the lightest Majorana neutrino. After h1± and h2± are decoupled, this particle couples to the standard model particles only through two charge parity even scalars in theory and thus information on this scalar sector is important. Consistent solutions are found, but a part of them is now on the edge. © 2015 American Physical Society.

Physical Review D - Particles, Fields, Gravitation and Cosmology

Three loop neutrino model with isolated k±± three loop neutrino model with isolated k±± Kenji Nishiwaki, Hiroshi Okada, and Yuta Orikasa

Ta and Nb substituted TiO2 are important transparent conducting oxides that have potential for applications in photovoltaics, photocatalysis, and water splitting/CO2 sequestration. In addition to donating electrons, what are the effects of Nb and Ta substitution? Here we observe strong experimental evidence that Ta and Nb substitution induces large and small polarons in anatase TiO2 epitaxial thin films. The degenerate donor electrons (from both Nb and Ta) show a high temperature T3 dependence on electrical resistivity, which confirms the presence of large polarons, along with room temperature metallic transport. This is further confirmed by the enhancement in the electron effective mass, which was estimated from thermopower measurements. Femtosecond transient absorption (fs-TA) reveals the life time of the Ti-t2g and eg levels and the separation of these levels are consistent with the X-ray absorption spectroscopy (XAS) measurement. In addition, fs-TA reveals the presence of small polarons with a life time substantially &gt;1 ns, which arises from defect levels and is a consequence of Ta and Nb substitution. X-ray photoelectron spectroscopy (XPS) provides evidence of Ti3+, which may be identified as the defects responsible for the small polarons. These long-lived small polarons may provide a way to minimize recombination dynamics in TiO2-based electrodes for photo-excited devices. © The Royal Society of Chemistry 2015.

Journal of Materials Chemistry C

Effect of Nb and Ta substitution on donor electron transport and ultrafast carrier dynamics in anatase TiO2 thin films

Al-doped ZnO (AZO) thin films of thicknesses 5,10, 15, 20, and 30nm were deposited on 500eV argon ion-beam fabricated nanoscale self-organized rippled-Si substrates at room temperature and are compared with similar films deposited on pristine-Si substrates (without ripples). It is observed that morphology of self-organized AZO films is driven by the underlying substrate morphology. For instance, for pristine-Si substrates, a granular morphology evolves for all AZO films. On the other hand, for rippled-Si substrates, morphologies having chain-like arrangement (anisotropic in nature) are observed up to a thickness of 20nm, while a granular morphology evolves (isotropic in nature) for 30nm-thick film. Photoluminescence studies reveal that excitonic peaks corresponding to 5-15nm-thick AZO films, grown on rippled-Si templates, show a blue shift of 8nm and 3nm, respectively, whereas the peak shift is negligible for 20-nm thick film (with respect to their pristine counter parts). The observed blue shifts are substantiated by diffuse reflectance study and attributed to quantum confinement effect, associated with the size of the AZO grains and their spatial arrangements driven by the anisotropic morphology of underlying rippled-Si templates. The present findings will be useful for making tunable AZO-based light-emitting devices. © 2015 AIP Publishing LLC.

Journal of Applied Physics

Thickness-dependent blue shift in the excitonic peak of conformally grown ZnO:Al on ion-beam fabricated self-organized Si ripples

Abstract: We study the viability of having two relatively light top squarks (‘stops') in the framework of the Next-to-Minimal Supersymmetric Standard Model (NMSSM). Such light stops render the NMSSM rather ‘natural'. These are shown to be allowed by the relevant direct searches at the Large Hadron Collider (LHC) and to be compatible with the latest LHC results on the Higgs sector, other low energy electroweak constraints and recent constraints from the dark matter (DM) sector. We propose dedicated searches for such light stops at the LHC within a ‘simplified' scenario that may have a bino-like or a singlino-like neutralino LSP as the DM candidate and point out various final states carrying the imprint of their collective presence. Under certain circumstances, in such a scenario, presence of two light stops may give rise to final states which are not so typical in their search. Thorough studies at the detector level reveal the status of such a scenario after the 8 TeV run of the LHC and shed light on the prospects of its 13 and 14 TeV runs. In favorable regions of the NMSSM parameter space, with low-lying spectra, signals with significance ≳ 5$\sigma$ are possible with a few tens to a few hundreds of fb<sup>−1</sup> of integrated luminosity in diverse final states.

Journal of High Energy Physics

Two light stops in the NMSSM and the LHC

We consider four nontrivial ensembles involving Gaussian Wigner and Wishart matrices. These are relevant to problems ranging from multiantenna communication to random supergravity. We derive the matrix probability density, as well as the eigenvalue densities for these ensembles. In all cases the joint eigenvalue density exhibits a biorthogonal structure. A determinantal representation, based on a generalization of Andréief's integration formula, is used to compactly express the r-point correlation function of eigenvalues. This representation circumvents the complications encountered in the usual approaches, and the answer is obtained immediately by examining the joint density of eigenvalues. We validate our analytical results using Monte Carlo simulations. © 2015 American Physical Society.

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics

Random matrix ensembles involving Gaussian Wigner and Wishart matrices, and biorthogonal structure

We investigate the photoexcited state dynamics in a donor-acceptor copolymer, poly{3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]- pyrrole-1,4-dione-alt-naphthalene} (pDPP-TNT), by picosecond fluorescence and femtosecond transient absorption spectroscopies. Timeresolved fluorescence lifetime measurements of pDPP-TNT thin films reveal that the lifetime of the singlet excited state is 185 ± 5 ps and that singlet-singlet annihilation occurs at excitation photon densities above 6 × 1017 photons/cm3. From the results of singlet-singlet annihilation analysis, we estimate that the single-singlet annihilation rate constant is (6.0 ± 0.2) × 109cm3 s-1 and the singlet diffusion length is -7 nm. From the comparison of femtosecond transient absorption measurements and picosecond fluorescence measurements, it is found that the time profile of the photobleaching signal in the charge-transfer (CT) absorption band coincides with that of the fluorescence intensity and there is no indication of long-lived species, which clearly suggests that charged species, such as polaron pairs and triplet excitons, are not effectively photogenerated in the neat pDPP-TNT polymer. © 2014 The Japan Society of Applied Physics.

Japanese Journal of Applied Physics

Excited-state dynamics in diketopyrrolopyrrole-based copolymer for organic photovoltaics investigated by transient optical spectroscopy

In the minimal supersymmetric standard model (MSSM) the lightest superpartner of the left-handed neutrinos is ruled out as a candidate for dark matter because of its large elastic cross section with the nucleus mediated via the Z boson. We resurrect this candidate by extending the MSSM with two triplets with opposite hypercharge. The addition of the triplets not only plays the role of generating small Majorana masses for the left-handed active neutrinos, but it also makes the lightest sneutrino a viable candidate for dark matter. We then discuss the relevant parameter space that can give rise to the right amount of (thermal) relic abundance as well as satisfy the current direct-detection constraints from XENON100 and LUX. We find that sneutrino dark matter with mass 370-550 GeV can give rise to the correct thermal relic abundance while coannihilating with the bino-like neutralino.

Resurrecting L -type sneutrino dark matter in light of neutrino masses and LUX data

Photon harvesting by reducing reflection loss is the basis of photovoltaic devices. Here, we show the efficacy of Al-doped ZnO (AZO) overlayer on ion beam-synthesized nanofaceted silicon for suppressing reflection loss. In particular, we demonstrate thickness-dependent tunable antireflection (AR) from conformally grown AZO layer, showing a systematic shift in the reflection minima from ultraviolet to visible to near-infrared ranges with increasing thickness. Tunable AR property is understood in light of depth-dependent refractive index of nanofaceted silicon and AZO overlayer. This improved AR property significantly increases the fill factor of such textured heterostructures, which reaches its maximum for 60-nm AZO compared to the ones based on planar silicon. This thickness matches with the one that shows the maximum reduction in surface reflectance. PACS: 81.07.-b; 42.79.Wc; 81.16.Rf; 81.15.Cd © 2014, Basu et al.; licensee Springer.

Nanoscale Research Letters

Tunable antireflection from conformal Al-doped ZnO films on nanofaceted Si templates

We investigate a model on an extra dimension S1 where plenty of effective boundary points described by point interactions (zero-thickness branes) are arranged. After suitably selecting the conditions on these points for each type of five-dimensional fields, we realize the tiny active neutrino masses, the charged lepton mass hierarchy, and lepton mixings with a CP-violating phase, simultaneously. Not only the quark’s but also the lepton’s configurations are generated in a unified way with acceptable accuracy, with neither the see-saw mechanism nor symmetries in Yukawa couplings, by suitably setting the model parameters, even though their flavor structures are dissimilar each other. One remarkable point is that a complex vacuum expectation value of the five-dimensional Higgs doublet in this model becomes the common origin of the CP violation in both quark and lepton sectors. The model can be consistent with the results of the precision electroweak measurements and Large Hadron Collider experiments. © 2014, The Author(s).