Physics

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The existence of Dark Matter (DM) has been well established from various cosmological and astrophysical evidences. However, the particle properties of DM are largely undetermined and attempts to probe its interactions with the Standard Model (SM) particles have, so far, not met with any success. The stringent constraints on the DM-SM interactions, while does not exclude the standard lore of producing weakly massive interacting particle DM candidates through thermal freeze-out mechanism in its entirety, have certainly cast shadow on the same. In this work, we consider non-thermal production of DM within a simple extension of the SM including an inflaton field and a scalar DM candidate. Assuming negligible interactions between the SM particles and the DM, we study the production of the latter at the end of inflation, during the (p)reheating epoch. In this context, we explore the role of DM self-interactions and its interaction with the inflaton field and find that DM can be over produced in a significant region of the parameter space. We further demonstrate that large self-interaction of the DM can suppress its abundance during preheating and to a certain extent helps to achieve the observed relic abundance via cannibalization.

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(SoNS) School of Natural Sciences

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Journal of Cosmology and Astroparticle Physics

Non-thermal production of Dark Matter after inflation

We comment on the paper 'Cutset Bounds on the Capacity of MIMO Relay Channels' by Jeong et al. and point out that, unlike what appears from a remark and some other contents by these authors, the matrix distribution for the sum of two complex random Wishart matrices has already been derived by Kumar for the general case of arbitrary covariance matrices and not only for the special case when one of them is assumed proportional to the identity matrix. The latter assumption has been made only for deriving the corresponding eigenvalue distribution. Furthermore, we draw attention to the result that when all covariance matrices are chosen proportional to the identity matrix, then it is possible to obtain exact and closed form expressions for the sum of an arbitrary number of Wishart matrices and not only for two as considered by Jeong et al. © 2013 IEEE.

IEEE Access

Comments on "cutset Bounds on the Capacity of MIMO Relay Channels"

Vertically aligned nitrogen-doped nanocrystalline diamond nanorods are fabricated from nitrogen-doped nanocrystalline diamond films using reactive ion etching in oxygen plasma. These nanorods show enhanced thermionic electron emission (TEE) characteristics, viz., a high current density of 12.0 mA/cm2 and a work function value of 4.5 eV with an applied voltage of 3 V at 923 K. The enhanced TEE characteristics of these nanorods are ascribed to the induction of nanographitic phases at the grain boundaries and the field penetration effect through the local field enhancement from nanorods owing to a high aspect ratio and an excellent field enhancement factor. Copyright © Materials Research Society 2018.

MRS Communications

Fabrication, microstructure, and enhanced thermionic electron emission properties of vertically aligned nitrogen-doped nanocrystalline diamond nanorods

In this review article, an insight of the physics that explains the phenomenon of torques induced by currents in systems comprising ferromagnetic (FM)-non-magnetic (NM) materials has been provided with particular emphasis on experiments that concern the observation of such torques. An important requirement of systems that enables observation of such relativistic torques is that the material needs to possess large spin-orbit coupling (SOC). In addition, the FM/NM interface should be of high quality so that spin angular momentum can be transferred across the interface. Under such conditions, the magnetization of a magnetic material experiences a torque, and can be reversed, thanks to the phenomenon of the spin Hall effect in the NM layer with large SOC. A reciprocal process also occurs, in which a changing magnetization orientation can produce spin current, i.e. current that supports spin angular momentum. It is important to know how these processes occur which often tells us about the close connection between magnetization and spin transport. This paves the way to transform technologies that process information via magnetization direction, namely in magnetic recording industry. This field of physics being relatively young much remains to be understood and explored. Through this review we have attempted to provide a glimpse of existing understanding of current induced torques in ferromagnetic thin film heterostructures along with some future challenges and opportunities of this evolving area of spintronics. Specifically, we have discussed the state-of-the art demonstrations of current-induced torque devices that show great promise for enhancing the functionality of magnetic memory devices. © The Royal Society of Chemistry.

RSC Advances

Relativistic torques induced by currents in magnetic materials: physics and experiments

We study an extension of the minimal supersymmetric standard model (MSSM) with additional right-handed singlet neutrino superfields. While such an extension incorporates a mechanism for the neutrino mass, it also opens up the possibility of having the right-sneutrinos ($\nu$˜ ) as the lightest supersymmetric particle (LSP). In this work, we focus on the viability of rather small (≲500 GeV) higgsino mass parameter ($\mu$), an important ingredient for “naturalness”, in the presence of such a LSP. For simplicity, we assume that the bino and wino mass parameters are much heavier; thus we only consider (almost) pure and compressed higgsino-like states, with small O(10−2) gaugino admixture which nevertheless still affect the decay of the low-lying higgsino-like states, thus significantly affecting the proposed signatures at colliders. Considering only prompt decays of the higgino-like states, especially the lightest chargino, we discuss the importance of leptonic channels consisting of up to two leptons with large missing transverse energy to probe this scenario at the Large Hadron Collider (LHC). In addition we also comment on the dark matter predictions for the studied scenario.

Journal of High Energy Physics

Natural SUSY at LHC with right-sneutrino LSP

Temperature-pressure phase diagram of the Kitaev hyperhoneycomb iridate $\beta$-Li2IrO3 is explored using magnetization, thermal expansion, magnetostriction and muon spin rotation measurements, as well as single-crystal x-ray diffraction under pressure and ab initio calculations. The N{\'{e}}el temperature of $\beta$-Li2IrO3 increases with the slope of 0.9 K/GPa upon initial compression, but the reduction in the polarization field Hc reflects a growing instability of the incommensurate order. At 1.4 GPa, the ordered state breaks down upon a first-order transition, giving way to a new ground state marked by the coexistence of dynamically correlated and frozen spins. This partial freezing in the absence of any conspicuous structural defects may indicate the classical nature of the resulting pressure-induced spin liquid, an observation paralleled to the increase in the nearest-neighbor off-diagonal exchange $\Gamma$ under pressure.

Physical Review Letters

Breakdown of Magnetic Order in the Pressurized Kitaev Iridate $\beta$-Li2IrO3

Herein, we report the effect of incorporation of two types of plasmonic nanocomposites, TiO2–Au and TiO2–Ag in different ratios, in the TiO2 photoanode of dye-sensitized solar cells (DSSCs). Electrophoretic deposition technique (EPD) has been utilized for the deposition of these nanocomposite photoanodes. The high-resolution transmission electron microscopy reveals that the nanocomposites, TiO2–Au and TiO2–Ag, have a wide size distribution of Au (5–60 nm) and Ag (20–130 nm) nanoparticles embedded in the TiO2 matrix. The UV–Visible absorption spectra of these nanocomposite films reveal the enhancement in the optical density due to the plasmonic effect. The DSSC based on photoanode consists of plasmonic nanocomposite TiO2–Au:TiO2–Ag (3:1 ratio) showed power conversion efficiency (PCE) of 10.9\% which is 187\% higher than that pristine TiO2 counterpart. The enhancement in the PCE has been confirmed by the photoluminescence and electro-impedance spectroscopy indicating that both Au and Ag play an important role in enhancing the PCE of DSSCs due to the plasmonic effect. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.

Journal of Materials Science: Materials in Electronics

Fabrication of efficient dye-sensitized solar cells with photoanode containing TiO2–Au and TiO2–Ag plasmonic nanocomposites

Co-filled' and 'Fe-filled' multiwall carbon nanotubes (MWCNTs) were grown using microwave-plasma chemical vapour deposition (MPCVD) and thermal chemical vapour deposition (TCVD) methods respectively, and their structural and magnetic properties were studied for magnetic device applications. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show the average tube length ≈80-500 μm with outer (inner) diameter ≈20-50 (≈10-20) nm for MWCNTs prepared by both methods. The diffraction peaks of both x-ray diffraction patterns show the interlayer distance, d002 ≈ 3.36 Å, which is comparable to the graphite structure (d002 = 3.35 Å). The graphitic crystallite sizes (La) of MPCVD (TCVD) synthesized MWCNTs are ≈24.78 nm (≈22.13 nm) as obtained from the intensity ratio of (ID/IG) D-peak, the disordered structure of graphite and G-peak, the C−C bond in graphitic structure of Raman spectra. The magnetization of 'Fe-filled' TCVD grown MWCNTs is much higher than 'Co-filled' MPCVD grown MWCNTs due to the formation of higher content of Fe-C and/or Fe-oxides in the MWCNT structures. The higher magnetic coercivity ≈2900 Oe and formation of isolated single-domain Fe-nanoparticles in 'Fe-filled' TCVD grown MWCNTs, as found from SEM / TEM micrographs, makes the ferromagnetic MWCNTs a promising material for the high-density magnetic recording media. © 2018 IOP Publishing Ltd

Materials Research Express

Magnetic properties of microwave-plasma (thermal) chemical vapour deposited Co-filled (Fe-filled) multiwall carbon nanotubes: Comparative study for magnetic device applications

<p>The thermal stability and tribological performance of silicon- and oxygen-incorporated diamond-like carbon (DLC) and silicon doped-DLC films were investigated. The DLC and DLC:Si are deposited on various (silicon, stainless steel and aluminium) substrates within the thickness range 200–400&nbsp;nm by radio frequency plasma-enhanced chemical vapour deposition (PECVD) method. Carbon dioxide (CO2) precursor gas is used to reduce the hydrogen content and to increase the adhesion of the films to the substrate. The X-ray photoelectron spectroscopy, Raman spectroscopy, surface profilometry and nano-indentation are used to study the chemical composition, microstructure, thermal stability and mechanical properties of the films. For CO2 precursor made DLC samples, Raman parameters did not show any significant change up to temperature 500&nbsp;°C. The lowest coefficient of friction was found to be 0.298 for the DLC:Si film prepared with CO2 at room temperature and corresponded lowest wear rate of 1.77&nbsp;×&nbsp;10−&nbsp;10&nbsp;mm3/Nm. The micro-structural properties at various annealing temperature were critically analysed by monitoring graphitization behaviour and oxidation of the film surface. © 2016 Elsevier B.V.</p>

Diamond and Related Materials

A facile method for the deposition of thermally stable diamond like carbon thin films via carbon dioxide precursor gas

Oxygen vacancy (OV) controlled hydrophobicity of self-assembled TiO2 nanorods (NRs) on chemically etched Si pyramids is investigated by irradiating with 50 keV Ar+-ions at room temperature. Apparent contact angle (CA) is found to increase from 122° to 141° up to a fluence of 1 × 1015 ions/cm2, followed by a gradual reduction to 130° at 1 × 1017 ions/cm2. However, the drop in apparent CA is found to be associated with the decrease in fractional surface area via transformation of NRs to an amorphous layer above 1 × 1015 ions/cm2, though it is still higher than that of as-grown one. Detailed X-ray photoelectron spectroscopy and electron paramagnetic resonance measurements suggest that the control of hydrophobic behavior is related to the suppression of surface free energy via migration of OVs into the voids in TiOx layers. © 2016 American Chemical Society.