Physics

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Inspired by the fact that relatively small values of the effective higgsino mass parameter of the Z3-symmetric Next-to-Minimal Supersymmetric Standard Model (NMSSM) could render the scenario ‘natural', we explore the plausibility of having relatively light neutralinos and charginos (the electroweakinos or the ewinos) in such a scenario with a rather light singlino-like Lightest Supersymmetric Particle (LSP), which is a Dark Matter (DM) candidate and singlet-dominated scalar excitations. By first confirming the indications in the existing literature that finding simultaneous compliance with results from the Large Hadron Collider (LHC) and those from various DM experiments with such light states is, in general, a difficult ask, we proceed to demonstrate, with the help of a few representative benchmark points, how exactly and to what extent could such a highly motivated ‘natural' setup with a singlino-like DM candidate still remains plausible.

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Journal of High Energy Physics

Revisiting singlino dark matter of the natural Z 3-symmetric NMSSM in the light of LHC

Partial Differential Equations for Mathematical Physicists is intended for graduate students, researchers of theoretical physics and applied mathematics, and professionals who want to take a course in partial differential equations. This book offers the essentials of the subject with the prerequisite being only an elementary knowledge of introductory calculus, ordinary differential equations, and certain aspects of classical mechanics. We have stressed more the methodologies of partial differential equations and how they can be implemented as tools for extracting their solutions rather than dwelling on the foundational aspects. After covering some basic material, the book proceeds to focus mostly on the three main types of second order linear equations, namely those belonging to the elliptic, hyperbolic, and parabolic classes. For such equations a detailed treatment is given of the derivation of Green's functions, and of the roles of characteristics and techniques required in handling the solutions with the expected amount of rigor. In this regard we have discussed at length the method of separation variables, application of Green's function technique, and employment of Fourier and Laplace's transforms. Also collected in the appendices are some useful results from the Dirac delta function, Fourier transform, and Laplace transform meant to be used as supplementary materials to the text. A good number of problems is worked out and an equally large number of exercises has been appended at the end of each chapter keeping in mind the needs of the students. It is expected that this book will provide a systematic and unitary coverage of the basics of partial differential equations. Key Features. An adequate and substantive exposition of the subject. Covers a wide range of important topics. Maintains mathematical rigor throughout. Organizes materials in a self-contained way with each chapter ending with a summary. Contains a large number of worked out problems. © 2019 by Taylor & Francis Group, LLC. All Rights Reserved.

Partial Differential Equations for Mathematical Physicists

Partial differential equations for mathematical physicists

Two-dimensional (2D) group-VI transition metal dichalcogenide (TMD) semiconductors, such as MoS2, MoSe2, WS2 and others manifest strong light matter coupling and exhibit direct band gaps which lie in the visible and infrared spectral regimes. These properties make them potentially interesting candidates for applications in optics and optoelectronics. The excitons found in these materials are tightly bound and dominate the optical response, even at room temperatures. Large binding energies and unique exciton fine structure make these materials an ideal platform to study exciton behaviors in two-dimensional systems. This review article mainly focuses on studies of mechanisms that control dynamics of excitons in 2D systems-an area where there remains a lack of consensus in spite of extensive research. Firstly, we focus on the kinetics of dark and bright excitons based on a rate equation model and discuss on the role of previous 'unsuspected' dark excitons in controlling valley polarization. Intrinsically, dark and bright exciton energy splitting plays a key role in modulating the dynamics. In the second part, we review the excitation energy-dependent possible characteristic relaxation pathways of photoexcited carriers in monolayer and bilayer systems. In the third part, we review the extrinsic factors, in particular the defects that are so prevalent in single layer TMDs, affecting exciton dynamics, transport and non-radiative recombination such as exciton-exciton annihilation. Lastly, the optical response due to pump-induced changes in TMD monolayers have been reviewed using femtosecond pump-probe spectroscopy which facilitates the analysis of underlying physical process just after the excitation. This journal is © The Royal Society of Chemistry.

RSC Advances

Physics of excitons and their transport in two dimensional transition metal dichalcogenide semiconductors

The statistics of the smallest eigenvalue of Wishart–Laguerre ensemble is important from several perspectives. The smallest eigenvalue density is typically expressible in terms of determinants or Pfaffians. These results are of utmost significance in understanding the spectral behavior of Wishart–Laguerre ensembles and, among other things, unveil the underlying universality aspects in the asymptotic limits. However, obtaining exact and explicit expressions by expanding determinants or Pfaffians becomes impractical if large dimension matrices are involved. For the real matrices (β= 1) Edelman has provided an efficient recurrence scheme to work out exact and explicit results for the smallest eigenvalue density which does not involve determinants or matrices. Very recently, an analogous recurrence scheme has been obtained for the complex matrices (β= 2). In the present work we extend this to β-Wishart–Laguerre ensembles for the case when exponent α in the associated Laguerre weight function, λ α e - β λ / 2 , is a non-negative integer, while β is positive real. This also gives access to the smallest eigenvalue density of fixed trace β-Wishart–Laguerre ensemble, as well as moments for both cases. Moreover, comparison with earlier results for the smallest eigenvalue density in terms of certain hypergeometric function of matrix argument results in an effective way of evaluating these explicitly. Exact evaluations for large values of n (the matrix dimension) and α also enable us to compare with Tracy–Widom density and large deviation results of Katzav and Castillo. We also use our result to obtain the density of the largest of the proper delay times which are eigenvalues of the Wigner–Smith matrix and are relevant to the problem of quantum chaotic scattering. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.

Journal of Statistical Physics

Recursion for the Smallest Eigenvalue Density of β -Wishart–Laguerre Ensemble

The hole and electron extracting interlayers in the organic solar cells (OSCs) play an important role in high performing devices. The present work focuses on an investigation of Zinc oxide/bulk heterojunction (ZnO/BHJ) and BHJ/MoO x (Molybdenum oxide) buried planar interfaces in inverted OSC devices using the optical contrast in various layers along with the electrical measurements. The x-ray reflectivity (XRR) analysis demonstrates the formation of additional intermixing layers at the interfaces of ZnO/BHJ and BHJ/MoO x . Our results indicate infusion of PC71BM into ZnO layer up to ∼4 nm which smoothen the ZnO/BHJ interface. In contrast, thermally evaporated MoO x molecules diffuse into PTB7-Th dominant upper layers of BHJ active layer resulting in an intermixed layer at the interface of MoO x /BHJ. The high recombination resistance (∼5 kω cm 2 ) and electron lifetime (∼70 μs), obtained from the impedance spectroscopy (IS), support such vertical segregation of PTB7-Th and PC71BM in the active layer. The OSC devices, processed in ambient condition, exhibit high power conversion efficiency of 6.4\%. We consider our results have great significance to understand the structure of buried planar interfaces at interlayers and their correlation with the electrical parameters representing various interfacial mechanisms of OSCs. © 2019 IOP Publishing Ltd.

Journal of Physics Condensed Matter

Investigation of the buried planar interfaces in multi-layered inverted organic solar cells using x-ray reflectivity and impedance spectroscopy

Synthesis of stable hybrid carbon nanostructure for high-performance supercapacitor electrode with long life-cycle for electronic and energy storage devices is a real challenge. Here, we present a one-step synthesis method to produce conductive boron-doped hybrid carbon nanowalls (HCNWs), where sp2-bonded graphene has been integrated with and over a three-dimensional curved wall-like network of sp3-bonded diamond. The spectroscopic studies such as X-ray absorption, Raman, and X-ray photoelectrons clearly reveal the coexistence of diamond and graphene in these nanowalls, while the detailed transmission electron microscopy studies confirm the unique microstructure where a diamond nanowall is encased by a multilayered graphene. Interestingly, these HCNWs yield a high double layer capacitance value of 0.43 mF cm-2 and electrode retention of 98\% over 10 000 cycles of charging/discharging in 1 M Na2SO4 electrolyte. The remarkable supercapacitive performance can be attributed to the 3D interconnected network of diamond nanowalls surrounded by highly conducting graphene. © 2019 American Chemical Society.

Journal of Physical Chemistry C

3D Hierarchical Boron-Doped Diamond-Multilayered Graphene Nanowalls as an Efficient Supercapacitor Electrode

The Al-induced crystallization of amorphous (a)-Si under thermal annealing and swift heavy ion irradiation has been investigated. The c-Al (50 nm)/a-Si (150 nm) thin films have been prepared on thermally oxidized Si-substrates. A set of samples have been annealed at temperatures ranging from 100 °C to 500 °C to achieve crystallization. Another set of similar samples have been irradiated at an elevated temperature of 100 °C using 100 MeV Ni+7 ions at fluences of 1 × 1012 ions-cm−2, 5 × 1012 ions-cm−2, 1 × 1013 ions-cm−2, and 5 × 1013 ions-cm−2. The crystallization of a-Si is observed at annealing temperature of 200 °C. The crystallinity increases with increasing temperature. On the other hand, irradiation using swift heavy ion leads to crystallization of a-Si at significantly lower temperature of 100 °C. The irradiation induced crystallization is explained in terms of creation of vacancies, interstitials and mixing of Si and Al atoms at a-Si/c-metal interface due to energy deposited by swift Ni ions. © 2019 Elsevier B.V.

Journal of Non-Crystalline Solids

Aluminum induced crystallization of amorphous Si: Thermal annealing and ion irradiation process

Typical eigenstates of quantum systems, whose classical limit is chaotic, are well approximated as random states. Corresponding eigenvalue spectra are modeled through an appropriate ensemble described by random matrix theory. However, a small subset of states violates this principle and displays eigenstate localization, a counterintuitive feature known to arise due to purely quantum or semiclassical effects. In the spectrum of chaotic systems, the localized and random states interact with one another and modify the spectral statistics. In this work, a 3×3 random matrix model is used to obtain exact results for the ratio of spacing between a generic and localized state. We consider time-reversal-invariant as well as noninvariant scenarios. These results agree with the spectra computed from realistic physical systems that display localized eigenmodes. © 2018 American Physical Society.

Physical Review E

Exact distribution of spacing ratios for random and localized states in quantum chaotic systems

We investigate here the structural phase transformation and electrical resistive switching properties of TiO 2 thin films (80 nm) after their self-ion implantation with 50 keV Ti + ions at several fluences. UV-Raman, grazing incidence x-ray diffraction (GIXRD), transmission electron microscopy, x-ray photoelectron spectroscopy, and atomic force microscopy techniques have been utilized to investigate the modifications in thin films. Both, the as-grown and ion implanted, films display mixed phases of rutile (R) and anatase (A). Surprisingly, however, a phase transition from A to R is observed at a critical fluence, where some anatase content transforms into rutile. This A to R transformation increases with additional fluence. The critical fluence found by GIXRD is slightly smaller ( 1 × 10 13 ions/cm 2) than from UV-Raman ( 1 × 10 14 ions/cm 2), indicating the first initiation of phase transformation probably in bulk. All the films contain anatase in nanocrystalline form also and the phase transformation seems to take place via aggregation of anatase nanoparticles. Thin films also show the presence of oxygen vacancies (O V) Ti 3 +, whose number grows with fluence. These O V as well as thermal spikes created during Ti + ion implantation are also crucial for the A-R transition. After implantation at the highest fluence, TiO 2 thin films show bipolar resistive switching behavior. The development of conducting filaments, formed by the migration of many oxygen vacancies generated during ion implantation, can be responsible for this behavior. © 2018 Author(s).

Journal of Applied Physics

The effect of Ti+ ion implantation on the anatase-rutile phase transformation and resistive switching properties of TiO 2 thin films

In this report, an effort has been made to develop an efficient PbS quantum dot-sensitized photoanode by simple successive ionic layer adsorption and reduction technique to enhance the overall photovoltaic performance of PbS quantum dot-sensitized solar cells. Three strategies have been adopted for the improvement of the photovoltaic performance of PbS quantum dot-sensitized solar cells, i.e., (i) by incorporation of TiO2-Au nanocomposites, where Au nanoparticles of different sizes are embedded into a TiO2 matrix, and (ii) variation of temperature at which quantum dots are deposited (iii) by postdeposition annealing of QD-sensitized photoanode in Ar atmosphere. We have used electrophoretic deposition technique to develop the nanocomposite-doped photoanode. High-resolution transmission electron microscopy confirms that the Au particles dispersed in the TiO2 matrix vary from 2 to 50 nm and PbS quantum dot size ranges 3.5–6 nm. The optical absorption of PbS quantum dot-sensitized TiO2-Au-incorporated photoanode is substantially enhanced as confirmed from the UV-visible absorption spectra measurements. The current-voltage characteristics of all the plasmonic quantum dot-sensitized solar cells under illumination (100 mW/cm2, AM 1.5) show significant improvement in power conversion efficiency using the abovementioned strategies. The maximum power conversion efficiency observed in PbS quantum dot-based quantum dot-sensitized solar cells is 7.0\%. Electroimpedance spectroscopy has been utilized to understand the recombination kinetics in these solar cells. © 2018, Springer Nature B.V.