arXiv subjects

Amit Verma

Publications and source records attributed to Amit Verma.

53 records · Page 3Linked to original sources

arXiv2015

Modeling of dual-metal Schottky contacts based silicon micro and nano wire solar cells

We study solar cell properties of single silicon wires connected at their ends to two dissimilar metals of different work functions. Effects of wire dimensions, the work functions of the metals, and minority carrier lifetimes on short circuit current as well as open circuit voltage are studied. The most efficient photovoltaic behavior is found to occur when one metal makes a Schottky contact with the wire, and the other makes an Ohmic contact. As wire length increases, both short circuit current and open circuit voltage increase before saturation occurs. Depending on the work function difference between the metals and the wire dimensions, the saturation length increases by approximately an order of magnitude with a two order magnitude increase in minority carrier length. However current per surface area exposed to light is found to decrease rapidly with increase in length. The use of a multi-contact interdigitated design for long wires is investigated to increase the photovoltaic response of the devices.

cond-mat.mes-hall↗

arXiv2015

Simulation Study of Schottky Contact Based Single Si Wire Solar Cell

We simulate single silicon nanowire (SiNW) solar cells with dissimilar work function metal contacts. Both short circuit current (ISC) and open circuit voltage (VOC) have been investigated. Effects of nanowire dimension, minority carrier lifetime, and contact metal work function difference are understood through simulations. Both ISC and VOC increase with nanowire length but saturate due to minority carrier recombination. The saturation length is found to be five times the diffusion length. The larger the contact work function difference, the more improved the solar cell characteristics. Large work function differences may also avoid need for any doping in axial p-i-n nanowire solar cells. Saturation in ISC as well as degradation in current density with length can be minimized by spreading the contacts along the length of the nanowire.

cond-mat.mes-hall↗

arXiv2015

A General Method for Selecting Quantum Channel for Bidirectional Controlled State Teleportation and Other Schemes of Controlled Quantum Communication

Recently, a large number of protocols for bidirectional controlled state teleportation (BCST) have been proposed using $n$-qubit entangled states ($n\in\{5,6,7\}$) as quantum channel. Here, we propose a general method of selecting multi-qubit $(n>4)$ quantum channels suitable for BCST and show that all the channels used in the existing protocols of BCST can be obtained using the proposed method. Further, it is shown that the quantum channels used in the existing protocols of BCST forms only a negligibly small subset of the set of all the quantum channels that can be constructed using the proposed method to implement BCST. It is also noted that all these quantum channels are also suitable for controlled bidirectional remote state preparation (CBRSP). Following the same logic, methods for selecting quantum channels for other controlled quantum communication tasks, such as controlled bidirectional joint remote state preparation (CJBRSP) and controlled quantum dialogue, are also provided.

quant-ph↗

arXiv2014

Anisotropic Thermal Conductivity in Single Crystal beta-Gallium Oxide

The thermal conductivities of beta-Ga2O3 single crystals along four different crystal directions were measured in the temperature range of 80-495K using the time domain thermoreflectance (TDTR) method. A large anisotropy was found. At room temperature, the [010] direction has the highest thermal conductivity of 27.0+/-2.0 W/mK, while that along the [100] direction has the lowest value of 10.9+/-1.0 W/mK. At high temperatures, the thermal conductivity follows a ~1/T relationship characteristic of Umklapp phonon scattering, indicating phonon-dominated heat transport in the \b{eta}-Ga2O3 crystal. The measured experimental thermal conductivity is supported by first-principles calculations which suggest that the anisotropy in thermal conductivity is due to the differences of the speed of sound along different crystal directions.

cond-mat.mtrl-sci↗

arXiv2014

Au-Gated SrTiO3 Field-Effect Transistors with Large Electron Concentration and Current Modulation

We report the fabrication of low-leakage rectifying Pt and Au Schottky diodes and Au-gated metal-semiconductor field effect transistors (MESFETs) on n-type SrTiO3 thin films grown by hybrid molecular beam epitaxy. In agreement with previous studies, we find that compared to Pt, Au provides a higher Schottky barrier height with SrTiO3. As a result of the large dielectric constant of SrTiO3 and the large Schottky barrier height of Au, the Au-gated MESFETs are able to modulate ~1.6 x 1014 cm-2 electron density, the highest modulation yet achieved using metal gates in any material system. These MESFETs modulate current densities up to ~68 mA/mm, ~20x times larger than the best demonstrated SrTiO3 MESFETs. We also discuss the roles of the interfacial layer, and the field-dependent dielectric constant of SrTiO3 in increasing the pinch off voltage of the MESFET.

cond-mat.mtrl-sci↗

arXiv2014

Intrinsic Mobility Limiting Mechanisms in Lanthanum-doped Strontium Titanate

The temperature dependent Hall mobility data from La-doped SrTiO3 thin films has been analyzed and modeled considering various electron scattering mechanisms. We find that a ~6 meV transverse optical phonon (TO) deformation potential scattering mechanism is necessary to explain the dependence of transport on temperature between 10-200 K. Also, we find that the low temperature electron mobility in intrinsic (nominally undoped) SrTiO3 is limited by acoustic phonon scattering. Adding the above two scattering mechanisms to longitudinal optical phonon (LO) and ionized impurity scattering mechanisms, excellent quantitative agreement between mobility measurement and model is achieved in the whole temperature range (2-300K) and carrier concentrations ranging over a few orders of magnitude (8x1017 cm-3 - 2x1020 cm-3).

cond-mat.mes-hall↗

arXiv2013

High-Voltage Field Effect Transistors with Wide-Bandgap β-Ga2O3 Nanomembranes

Nanoscale semiconductor materials have been extensively investigated as the channel materials of transistors for energy-efficient low-power logic switches to enable scaling to smaller dimensions. On the opposite end of transistor applications is power electronics for which transistors capable of switching very high voltages are necessary. Miniaturization of energy-efficient power switches can enable the integration with various electronic systems and lead to substantial boosts in energy efficiency. Nanotechnology is yet to have an impact in this arena. In this work, it is demonstrated that nanomembranes of the wide-bandgap semiconductor gallium oxide can be used as channels of transistors capable of switching high voltages, and at the same time can be integrated on any platform. The findings mark a step towards using lessons learnt in nanomaterials and nanotechnology to address a challenge that yet remains untouched by the field.

cond-mat.mes-hall↗

arXiv2013

Optoelectronic response calculations in the framework of k.p coupled to Non-equilibrium Green's functions for 1D systems in the ballistic limit

We present theory of the carrier-optical interaction in 1D systems based on the non-equilibrium Greens function formalism in the 4x4 k.p model. As a representative parameters we chose the GaAs. Although theory is presented in 4x4 k.p many subbands, results and discussion section is based on the simplified model such as 2x2 k.p model (two transverse modes). Even though 2x2 k.p model is simple enough it shows many phenomena that have not been seen before. We focus mainly on the ballistic extraction of photogenerated free carriers at the radiative limit which is described by the self-energy term derived in dipole approximation and solved in self-consistent manner with Keldysh quantum kinetic equations. Any relaxation or non-radiative recombination mechanisms as well as excitonic features are neglected. Effect of non-locality of electron-photon self energy term is considered and discussed. Spontaneous emission is also considered and shown to be small in short devices under medium bias conditions. Electron and hole spatial current oscillations are seen and discussed. It is shown that neglecting off-diagonal correlation in the band index not only produces quantitatively wrong results but it also alters the qualitative picture. All simulations are done in the full-rank approximation, with all spatial and band correlation effects are kept intact. This allows us to study not only quantitative effects but also qualitative behaviour.

cond-mat.mes-hall↗

arXiv2013

Multiscale periodicities in aerosol optical depth over India

Aerosols exhibit periodic or cyclic variations depending on natural and anthropogenic sources over a region, which can get modulated by synoptic meteorological parameters such as winds, rainfall and relative humidity, and long-range transport. Information on periodicity and phase in aerosol properties assumes significance in prediction as well as to examine the radiative and climate effects of aerosols including its association with changes in cloud properties and rainfall. Periodicity in aerosol optical depth, which is a columnar measure of aerosol distribution, is determined using continuous wavelet transform over 35 locations in India. Continuous wavelet transform is used in the study because it is better suited to extract the periodic and local modulations present at various frequency ranges, as these features are invisible in conventional methods such as Fourier Transform. Monthly mean aerosol optical depths (AODs) from MODerate Resolution Imaging Spectroradiometer (MODIS) on board the Terra satellite from January 2001 to December 2012 are used. Annual and quasi-biennial oscillations (QBO) in AOD are evident in addition to the weak semi-annual and quasi-triennial. The semi-annual and annual oscillations are consistent with the seasonal and yearly cycle of variations in AODs. The 40-month periodicity indicates the presence of long term correlations in AOD. The observed periodicities in MODIS Terra AODs are also evident in the ground-based AOD measurements made over Kanpur in the Indo-Gangetic Plain. The phase of the periodicity in AOD is stable in the mid-frequency range, while local disturbances in the high-frequency range and long term changes in the atmospheric composition give rise to unstable phases in low-frequency range. That modulations in AOD over one location/region can influence the other is revealed by the presence of phase relation among different locations.

physics.ao-ph↗

arXiv2010

Higher Order Squeezing and Higher Order Subpoissonian Photon Statistics in Intermediate States

Recently simpler criteria for the Hong-Mandel higher order squeezing (HOS) and higher order subpossonian photon statistics (HOSPS) are provided by us [Phys. Lett. A 374 (2010) 1009]. Here we have used these simplified criteria to study the possibilities of observing HOSPS and HOS in different intermediate states, such as generalized binomial state, hypergeometric state, negative binomial state and photon added coherent state. It is shown that these states may satisfy the condition of HOS and HOSPS. It is also shown that the depth and region of nonclassicality can be controlled by controlling various parameters related to intermediate states. Further, we have analyzed the mutual relationship between different signatures of higher order nonclassicality with reference to these intermediate states. We have observed that the generalized binomial state may show signature of HOSPS in absence of HOS. Earlier we have shown that NLVSS shows HOS in absence of HOSPS. Consequently it is established that the HOSPS and HOS of same order are independent phenomenon.

quant-ph↗

arXiv2009

Higher order antibunching and subpossonian photon statistics in five wave mixing process

We have investigated the possibility of observing higher order antibunching (HOA) and higher order subpossonian photon statistics (HOSPS) in five wave mixing and third harmonic generation process. It had been shown that both processes satisfy the criteria of HOA and HOSPS. Further, some observations on the nature of interaction which produces HOA and HOSPS are reported.

quant-ph↗

arXiv2009

Which optical processes are suitable to make probabilistic single photon sources for quantum cryptography?

Single photon sources to be used in quantum cryptography must show higher order antibunching (HOA). HOA is reported by us in several many wave mixing processes. In the present work we have investigated the possibility of observing HOA in multiwave mixing processes in general. The generalized Hamiltonian is solved for several particular cases in Heisenberg picture and possibility of observing HOA is investigated with the help of criterion of Pathak and Garcia. Several particular cases of the generalized Hamiltonian are solved with the help of short time approximation technique and HOA is reported for pump modes of different multiwave mixing processes. It is also found that HOA can not be observed for the signal and stokes modes in of the cases studied here.

quant-ph↗

arXiv2009

Generalized structure of higher order nonclassicality

A generalized notion of higher order nonclassicality (in terms of higher order moments) is introduced. Under this generalized framework of higher order nonclassicality, conditions of higher order squeezing and higher order subpoissonian photon statistics are derived. A simpler form of the Hong-Mandel higher order squeezing criterion is derived under this framework by using an operator ordering theorem introduced by us in [J. Phys. A. 33 (2000) 5607]. It is also generalized for multi-photon Bose operators of Brandt and Greenberg. Similarly, condition for higher order subpoissonian photon statistics is derived by normal ordering of higher powers of number operator. Further, with the help of simple density matrices, it is shown that the higher order antibunching (HOA) and higher order subpoissonian photon statistics (HOSPS) are not the manifestation of the same phenomenon and consequently it is incorrect to use the condition of HOA as a test of HOSPS. It is also shown that the HOA and HOSPS may exist even in absence of the corresponding lower order phenomenon. Binomial state, nonlinear first order excited squeezed state (NLESS) and nonlinear vacuum squeezed state (NLVSS) are used as examples of quantum state and it is shown that these states may show higher order nonclssical characteristics. It is observed that the Binomial state which is always antibunched, is not always higher order squeezed and NLVSS which shows higher order squeezing does not show HOSPS and HOA. The opposite is observed in NLESS and consequently it is established that the HOSPS and HOS are two independent signatures of higher order nonclassicality

quant-ph↗

arXiv2008

Reduction of Quantum Phase Fluctuations in Intermediate States

Recently we have shown that the reduction of the Carruthers-Nieto symmetric quantum phase fluctuation parameter (U) with respect to its coherent state value corresponds to an antibunched state, but the converse is not true. Consequently reduction of U is a stronger criterion of nonclassicality than the lowest order antibunching. Here we have studied the possibilities of reduction of $U$ in intermediate states by using the Barnett Pegg formalism. We have shown that the reduction of phase fluctuation parameter U can be seen in different intermediate states, such as binomial state, generalized binomial state, hypergeometric state, negative binomial state, and photon added coherent state. It is also shown that the depth of nonclassicality can be controlled by various parameters related to intermediate states. Further, we have provided specific examples of antibunched states, for which $U$ is greater than its poissonian state value.

quant-ph↗

arXiv2006

Characterizing and modeling cyclic behavior in non-stationary time series through multi-resolution analysis

A method based on wavelet transform and genetic programming is proposed for characterizing and modeling variations at multiple scales in non-stationary time series. The cyclic variations, extracted by wavelets and smoothened by cubic splines, are well captured by genetic programming in the form of dynamical equations. For the purpose of illustration, we analyze two different non-stationary financial time series, S&P CNX Nifty closing index of the National Stock Exchange (India) and Dow Jones industrial average closing values through Haar, Daubechies-4 and continuous Morlet wavelets for studying the character of fluctuations at different scales, before modeling the cyclic behavior through GP. Cyclic variations emerge at intermediate time scales and the corresponding dynamical equations reveal characteristic behavior at different scales.

physics.data-an↗

arXiv2006

Locally Adaptive Block Thresholding Method with Continuity Constraint

We present an algorithm that enables one to perform locally adaptive block thresholding, while maintaining image continuity. Images are divided into sub-images based some standard image attributes and thresholding technique is employed over the sub-images. The present algorithm makes use of the thresholds of neighboring sub-images to calculate a range of values. The image continuity is taken care by choosing the threshold of the sub-image under consideration to lie within the above range. After examining the average range values for various sub-image sizes of a variety of images, it was found that the range of acceptable threshold values is substantially high, justifying our assumption of exploiting the freedom of range for bringing out local details.

cs.CV↗

arXiv2006

Multilevel Thresholding for Image Segmentation through a Fast Statistical Recursive Algorithm

A novel algorithm is proposed for segmenting an image into multiple levels using its mean and variance. Starting from the extreme pixel values at both ends of the histogram plot, the algorithm is applied recursively on sub-ranges computed from the previous step, so as to find a threshold level and a new sub-range for the next step, until no significant improvement in image quality can be achieved. The method makes use of the fact that a number of distributions tend towards Dirac delta function, peaking at the mean, in the limiting condition of vanishing variance. The procedure naturally provides for variable size segmentation with bigger blocks near the extreme pixel values and finer divisions around the mean or other chosen value for better visualization. Experiments on a variety of images show that the new algorithm effectively segments the image in computationally very less time.

cs.CV↗