• Sandeep Kumar

      Articles written in Pramana – Journal of Physics

    • Quasi-one dimensional electrical conductivity and thermoelectric power studies on a discotic liquid crystal

      V S K Balagurusamy S Krishna Prasad S Chandrasekhar Sandeep Kumar M Manickam C V Yelamaggad

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      We have studied the electrical conductivity of well aligned samples of hexahexylthiotriphenylene (HHTT) in the pure as well as doped states. The dopant used was a small concentration (0.62 mole %) of the electron acceptor trinitrofluorenone (TNF). In the columnar phases, doping causes the AC(1 kHz) conductivity along the columnar axis (σ) to increase by a factor of 107 or more relative to that in undoped samples; σ attains a value of 10−2S/m, which was the maximum measurable limit of our experimental set up. On the other hand, in the isotropic phase doping makes hardly any difference to the conductivity. The frequency dependence of the conductivity has been investigated. The DC conductivity of doped samples exhibits an enormous anisotropy, σ/σ ≥ 1010, which is 7 orders higher than that reported for any liquid crystalline system, and, to our knowledge, the largest observed in an organic conductor.

      We also report the first thermoelectric power studies on these ‘molecular wires’. The sign of the thermoelectric power is in conformity with the expected nature of the charge carriers, namely, holes.

    • Molecularengineering of discotic nematic liquid crystals

      Sandeep Kumar

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      Connecting two columnar phase forming discotic mesogens via a short rigid spacer leads to the formation of a π-conjugated discotic dimer snowing discotic nematic (ND) phase. Attaching branched-alkyl chains directly to the core in hexaalkynylbenzene resulted in the stabilisation of ND phase at ambient temperature. Pentalkynylbenzene derivatives possessing a combination of normal- and branched-alkoxy chains display a very broad ND, phase which is stable well below and above the room temperature.

    • A comparative study of graphene and graphite-based field effect transistor on flexible substrate


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      In the present era, there has been a great demand of cost-effective, biodegradable, flexible and wearable electronics which may open the gate to many applications like flexible displays, RFID tags, health monitoring devices, etc. Due to the versatile nature of plastic substrates, they have been extensively used in packaging, printing, etc. However, the fabrication of electronic devices requires specially prepared substrates with high quality surfaces, chemical compositions and solutions to the related fabrication issues along with its non-biodegradable nature. Therefore, in this report, a cost-effective, biodegradable cellulose paper as an alternative dielectric substrate material for the fabrication of flexible field effect transistor (FET) is presented. The graphite and liquid phase exfoliated graphene have been used as the material for the realisation of source, drain and channel on cellulose paper substrate for its comparative analysis.The mobility of fabricated FETs was calculated to be $\rm{83 cm^{2}/V s}$ (holes) and $\rm{33 cm^{2}/V s}$ (electrons) for graphite FET and $\rm{100 cm^{2}/V s$ (holes) and $\rm{52 cm^{2}/V s}$ (electrons) for graphene FET, respectively. The output characteristic of the device demonstrates the linear behaviour and a comprehensive increase in conductance as a function of gate voltages. The fabricated FETs may be used for strain sensing, health care monitoring devices, human motion detection, etc.

    • High-performance ultra-low leakage current graphene-based screen-printed field-effect transistor on paper substrate


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      Exploiting the advantages of additive patterning process over complex fabrication processes, herein we report the fabrication of field-effect transistor (FET) using the screen-printing method. The graphene conductive composite dielectric ink as the channel and the dielectric layer respectively was screen printed on cellulose paper substrate. The fabricated device shows the hole and electron mobility of $\rm{135 cm^{2}/V s}$ and $\rm{98 cm^{2}/V s}$ respectively with an ultra-low leakage current of $\sim 25 \rm{nA}$. The proposed technique can be used for large-scale roll-to-roll commercial manufacturing of disposable FET-based sensors such as temperature and IR sensors, health monitoring devices etc.

    • Self-action effects of quadruple-Gaussian laser beams in collisional plasmas and their resemblance to Kepler’s central force problem


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      This paper presents theoretical investigation on the self-action effects of intense laser beams propagating through collisional plasmas. Particularly, the phenomena of self-focussing, self-trapping and self-phase modulation of the laser beam have been investigated in detail. In order to see the effect of uniformity as well as non-uniformity of the irradiance over the cross-section of the laser beam on its propagation characteristics field distribution in the medium has been expressed in terms of quadruple Gaussian (QG) profile instead of Gaussian profile. Following the moment theory approach, the nonlinear partial differential equation for the slowly varying envelope of the laser beam has been reduced to a set of coupled ordinary differential equations for the evolution of beam width and longitudinal phase. The equations so obtained have been solved numerically to envision the effect of laser as well as medium parameters on the propagation characteristics of the laser beam.

    • THz pulses from optically excited Fe-, Pt- and Ta-based spintronic heterostructures


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      Spintronic heterostructures are considered to be the new generation terahertz (THz) sources because of their capability of producing high power and broadband THz radiation. Here, we provide a brief review on the state-of-the-art in this field. The optically excited bi- and tri-layer combinations of ferromagnetic and non-magnetic thin films have become increasingly popular. Towards optimising the THz conversion efficiency and broadband gapless spectrum from these THz emitters, various control parameters need to be taken into consideration. The inverse spin Hall effect in the heavy metal layer of the heterostructure is primarily responsible for the generation of THz pulses. A few new results on iron-, platinum- and tantalum-based heterostructures have also been reported here. It is observed that the Ta(2 nm)/Fe(2 nm)/Pt(2 nm) tri-layer heterostructure generates ~40(250)% stronger THz signal than the counterpart Fe(2 nm)/Pt(2 nm) (Fe(3 nm)/Ta(2 nm)) bi-layer heterostructure.

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