B. K. Sahoo

COSMIC EVOLUTION IN GENERALISED BRANS–DICKE THEORY

We have studied the generalised Brans–Dicke theory and obtained exact solutions of a(t), ϕ(t) and ω(t) for different epochs of the cosmic evolution. We discuss how inflation, deceleration, cosmic acceleration can result from this solution. The time variation of G(t) is also examined.

TIME DEPENDENCE OF BRANS–DICKE PARAMETER ω FOR AN EXPANDING UNIVERSE

We have studied the time dependence of ω for an expanding universe in the generalised B-D theory and have obtained its explicit dependence on the nature of matter contained in the universe in different era. Lastly, we discuss how the observed accelerated expansion of the present universe can be accommodated in the formalism.We have studied the time dependence of w for an expanding universe in the generalised B-D theory and have obtained its explicit dependence on the nature of matter contained in the universe,in different era.Lastly we discuss how the observed accelerated expansion of the present universe can be accomodated in the formalism.

GRAVITATIONAL WAVES IN GENERALISED BRANS–DICKE THEORY

We have solved cosmological gravitational Wave(GW)equation in the frame work of Generalised Brans-Dicke(GBD) theory for all epochs of the Universe.The solutions are expressed in terms of the present value of the Brans-Dicke coupling parameter

Macroscopic Polarization Effect on Bowing Constant of Thermal Parameters of In x Ga1−x N

\omega(\phi)

THE EFFECT OF SPONTANEOUS AND PIEZOELECTRIC POLARIZATION ON THERMAL CONDUCTIVITY OF InN

.It is seen that the solutions represent travelling growing modes for negative values of

Built-in-polarization field effect on intrinsic and extrinsic thermal conductivities of AlN/GaN/AlN quantum well

\omega_{0}

Spectra of relic gravitons and Brans-Dicke theory

for all epochs of the Universe.

Built-in-polarization effect on relaxation time and mean free path of phonons in AlxGa1−xN/GaN heterostructure

In this work, we have investigated theoretically the effect of macroscopic polarization [sum of spontaneous (SP) and piezoelectric (PZ) polarization] on various physical parameters of InxGa1−xN alloy. The macroscopic polarization contributes to the effective elastic constants of InxGa1−xN alloy. This modifies the phonon group velocity, Debye temperature, and Debye frequency of the alloy. These thermal parameters are estimated as a function of In composition. Our calculation shows that these material parameters are enhanced and vary nonlinearly with In composition, i.e., show bowing. The cause of bowing is the nonlinear dependence of the spontaneous and piezoelectric polarization on composition. The bowing constant of each material parameter (with and without polarization) is also theoretically predicted by the best-fit method. The results show that the polarization mechanism not only enhances the parameters but also contributes significantly to the bowing constant. The macroscopic polarization contributes more than 25% to the bowing constant. The obtained results can be used to predict the effect of the polarization mechanism on the thermoelectric properties of InxGa1−xN alloy.

The effect of spontaneous and piezoelectric polarization on phonon relaxation rates in binary wurtzite nitrides

The spontaneous (SP) and piezoelectric (PZ) polarization present in the wurtzite III nitrides influence the optical and electrical properties of these materials. The effects of SP and PZ polarization on thermal properties of III nitrides have yet to be investigated. Here we have investigated the SP and PZ effects on thermal conductivity of InN. Inclusion of polarization property modifies the group velocity of phonons. The combined phonon scattering rates and thermal conductivity k of InN are calculated using modified phonon group velocity, Debye frequency and Debye temperature. Without SP and PZ polarization, the room temperature thermal conductivity of InN is found to be 132.55 W/m.K. Inclusion of SP and PZ polarization property enhances the room temperature thermal conductivity from 132.55 to 134.32 W/m.K. Our predicted thermal conductivity values are closer to the recent experimental value 120 W/m.K measured by Levander et al. for a high quality irradiated InN films at room temperature.

Macroscopic polarization and thermal conductivity of GaN

In this paper, the effect of built-in-polarization field on lattice thermal conductivity of AlN/GaN/AlN quantum well (QW) has been theoretically investigated. The built-in-polarization field at the hetero-interface of GaN/AlN modifies elastic constant, phonon velocity and Debye temperature of GaN QW. The relaxation time of acoustic phonons (AP) in various scattering processes in GaN with and without built-in-polarization field has been computed at room temperature. The result shows that combined relaxation time of AP is enhanced by built-in-polarization field and implies a longer mean free path. The revised intrinsic and extrinsic thermal conductivities of GaN have been estimated. The theoretical analysis shows that up to a certain temperature the polarization field acts as negative effect and reduces the thermal conductivities. However, after this temperature both thermal conductivities are significantly contributed by polarization field. This gives the idea of temperature dependence of polarization effect which signifies the pyro-electric character of GaN. The intrinsic thermal conductivity at room temperature for with and without polarization mechanism is found to be 491 Wm-1K-1 and 409 Wm-1K-1, respectively i.e., 20% enhancement. However, the extrinsic thermal conductivity at room temperature for with and without polarization mechanism is found to be 280 Wm-1K-1 and 245 Wm-1K-1, respectively i.e., 13% enhancement. The method we have developed may be taken into account during the simulation of heat transport in optoelectronic nitride devices to minimize the self-heating processes and in polarization engineering strategies to optimize the thermoelectric performance of GaN alloys.