arXiv subjects

Ajay D. Thakur

Publications and source records attributed to Ajay D. Thakur.

At least 19 recordsLinked to original sources

arXiv2021

An Enquiry on similarities between Renormalization Group and Auto-Encoders using Transfer Learning

Physicists have had a keen interest in the areas of Artificial Intelligence (AI) and Machine Learning (ML) for some time now, with a special inclination towards unravelling the mechanism at the core of the process of learning. In particular, exploring the underlying mathematical structure of a neural net (NN) is expected to not only help us in understanding the epistemological meaning of `Learning' but also has the potential to unravel the secrets behind the workings of the brain. Here, it is worthwhile to establish correspondences and draw parallels between methods developed in core areas of Physics and the techniques developed at the forefront of AI and ML. Although recent explorations indicating a mapping between the Renormalisation Group(RG) and Deep Learning(DL) have shown valuable insights, we intend to investigate the relationship between RG and Autoencoders(AE) in particular. We will use Transfer Learning(TL) to embed the procedure of coarse-graining in a NN and compare it with the underlying mechanism of encoding-decoding through a series of tests.

cond-mat.dis-nn↗

arXiv2021

Improved Thermoelectric Properties in (1-x)LaCoO3/(x)La0.7Sr0.3CoO3 Composite

A high Seebeck coefficient (S), large electrical conductivity (σ), and reduced thermal conductivity (κ) are required to achieve a high figure-of-merit (zT) in an ideal thermoelectric (TE) system, which is challenging in a single system due to the interdependence of TE parameters. Composite approach is promising to manipulate the TE parameters. In this study, TE properties of (1-x)LaCoO3/(x)La0.7Sr0.3CoO3 (0.00 \leq x \leq 0.05) composite is discussed. The structural analysis confirms individual phases in the composite, which is further supported by electron microscopy analysis. The x-ray photoelectron analysis indicates that oxygen vacancies (VO) are present in the parent LaCoO3 system and increase with the addition of La0.7Sr0.3CoO3 (LSCO) in the composite. The increase in VO raises the degenerate states of cobalt and hence improves S in the composites. Temperature variation in S and σ are consistent with the spin-state transition and shows the correlation between these two parameters. The reduction in κ and σ with the addition of ball-milled La0.7Sr0.3CoO3 in the composite is attributed to the enhanced phonon-phonon and charge carrier scattering, respectively. A synergistic effect of enhanced S and reduced κ} result in five times improvement in zT of the composite compared to the parent LaCoO3 system at 800 K. This approach also improves the operating temperature for LaCoO3 based systems.

cond-mat.mtrl-sci↗

arXiv2021

Direction dependent thermoelectric properties of layered compound In2Te5 single crystal

We analyze the anisotropic electrical and thermal transport measurements in single crystals of In2Te5 belonging to monoclinic space group C12 c1 with the temperature gradient applied parallel and perpendicular to the crystallographic c-axis of the crystals. The thermal conductivity along the c-axis thermal conductivity parallel was found to smaller by a factor of 2 compared to the thermal conductivity along the direction perpendicular to the c-axis over the entire temperature range. In contrast, the Seebeck coefficient along the c-axis parallel was found to be higher than its value along the direction perpendicular to the c-axis. At room temperature, the figure of merit ZT parallel is found to be 4 times larger as compared to the figure of merit ZT perpendicular.

cond-mat.mtrl-sci↗

arXiv2020

Colossal Seebeck coefficient in Aurivillius Phase-Perovskite Oxide Composite

We propose an inexpensive scalable approach for achieving extremely high values of Seebeck coefficient ($α$) by exploiting the natural superlattice structure in Aurivillius phase oxides. In particular, we report an $α\approx $ 319\,mV/K at 300\,K in a composite of Aurivillius phase compound SrBi$_4$Ti$_4$O$_{15}$ (as a matrix) and a perovskite phase material (e.g., La$_{0.7}$Sr$_{0.3}$MnO$_3$ or, La$_{0.7}$Sr$_{0.3}$CoO$_3$ as filler). Such a colossal value of $α$ can be attributed to contributions from the enhanced density of states due to the effective low dimensional character of Bi$_2$O$_2$ layer. The corresponding thermal conductivity ($κ$) and the electrical conductivity ($σ$) lies in the range 0.7 - 1.25 W/m-K and 10 - 100 $μ$S/m, respectively at 300\,K. Attributed to the high $α$ values, such oxide composites can be used as thermopile sensors and highly sensitive bolometric applications. We anticipate that the demonstration of colossal $α$ in oxide composites using a simple synthesis strategy also sets the stage for future material innovations for high temperature thermoelectric applications.

cond-mat.mtrl-sci↗

arXiv2018

Role of contact work function, back surface field and conduction band offset in CZTS solar cell

We employ simulation based approach for enhancing the efficiency of Cu2ZnSnS4 (CZTS) based solar cells. Initial benchmarking of simulation with the experimentally reported solar cell in literature is performed by incorporating a suitable defect model. We then explore the effects of: (a) conduction band offset (CBO) at CZTS/CdS junction, (b) back surface field (BSF) due to an additional layer with higher carrier density, and (c) high work function back contact. Efficiency is observed to improve by about 70% upon optimizing the above three parameters. We also observe that utilizing BSF in the configuration can reduce the high work function requirement of the back contact. A work function of 5.2 eV (e.g., using Ni), a BSF layer (e.g., using SnS), and a CBO of 0.1 eV (e.g., using ZnS) constitute an optimal configuration.

physics.app-ph↗

arXiv2016

Magnetism in La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Co$_x$O$_3$ ($0 \leq x \leq 1$)

We study the structural and magnetic properties of La$_{0.7}$Sr$_{0.3}$Mn$_{1-x}$Co$_x$O$_3$ ($0 \leq x \leq 1$). Rietveld refinement of X-ray Diffraction (XRD) pattern suggests phase purity of the polycrystalline samples with R$\bar{3}$c space group. Interplay of Ferromagnetic (FM) and Antiferromagnetic (AFM) interaction upon Co substitution at Mn site in La$_{0.7}$Sr$_{0.3}$MnO$_3$ is evident from magnetic measurements. There is an optimal cobalt substitution at which the coercive field is maximum.

cond-mat.mtrl-sci↗

arXiv2016

Extreme Sensitivity of Magnetic Properties on the Synthesis Routes in La$_{0.7}$Sr$_{0.3}$MnO$_3$

La$_{0.7}$Sr$_{0.3}$MnO$_3$ polycrystalline samples have been prepared using different synthesis routes. X-ray Diffraction (XRD) confirms that the samples are of single phase with R$\bar{3}$c space group. The surface morphology and particle size has been measured using Field Emission Scanning Electron Microscopy (FESEM). Magnetic measurement shows that the magnetization in the materials are affected by low crystallite size which destroys the spin ordering due to strain at grain boundaries and in turn also lead to reduction in magnetization as well as an enhanced coercivity in the material.

cond-mat.mtrl-sci↗

arXiv2016

n-type SnSe$_{1-x}$ for Thermoelectric Application

We report the synthesis of n-type SnSe$_{1-x}$ using a self-sacrificial, facile, solvo-thermal synthesis route. Electrical and thermal transport measurements suggest a low thermal conductivity and a significant thermopower in the temperature range 100 - 400\,K. We also propose the possibility of developing an all SnSe thermoelectric module.

cond-mat.mtrl-sci↗

arXiv2015

Cu2ZnSnS4 Films using an Eco-friendly Direct Liquid Coating Approach for Solar Cell Applications

Cu2ZnSnS4 (CZTS) is a promising candidate as an absorber material for thin film solar cells. The reported wet chemical synthesis approaches often have a high environmental impact due to the usage of abrasive solvents such as hydrazines, hydroxylamines, etc. We report an eco-friendly solvent based approach for making CZTS thin films with desirable absorption characteristics for solar cell applications.

cond-mat.mtrl-sci↗

arXiv2015

Analysis Of SnS2 Buffer Layer And SnS Back Surface Layer Based CZTS Solar Cells Using SCAPS

A Copper-Zinc-Tin-Sulphide (CZTS)based solar cell with a modified ce3ll configuration of Mo/SnS/CZTS/SnS2/ZnO is simulated using SCAPS. An SnS2 buffer layer is used in simulation instead of the standard CdS layer. An additional back surface passivation layer of SnS is added in the modified cell configuration. An improvement in the solar cell efficiency compared to the standard CdS buffer based solar cell configuration Mo/CZTS/CdS/ZnO is found. The observations suggest the possibility of using SnS2 as a potential replacement of CdS. In addition, the use of a back surface passivation layer leads to improved solar cell performance.

cond-mat.mtrl-sci↗

arXiv2015

Improvement in thermoelectric properties by tailoring at In and Te site in In2Te5

We study role of site substitutions at In and Te site in In2Te5 on the thermoelectric behavior. Single crystals with compositions In2(Te1-xSex)5 (x = 0, 0.05, 0.10) and Fe0.05In1.95(Te0.90Se0.10)5 were prepared using modified Bridgman-Stockbarger technique. Electrical and thermal transport properties of these single crystals were measured in the temperature range 6 - 395 K. A substantial decrease in thermal conductivity is observed in Fe substituted samples attributed to the enhanced phonon point-defect scattering. Marked enhancement in Seebeck coefficient S along with a concomitant suppression of electrical resistivity \r{ho} is observed in Se substituted single crystals. An overall enhancement of thermoelectric figure of merit (zT) by a factor of 310 is observed in single crystals of Fe0.05In1.95(Te0.90Se0.10)5 compared to the parent In2Te5 single crystals.

cond-mat.mtrl-sci↗

arXiv2015

Growth and angular dependent resistivity of Nb2Pd0.73S5.7 superconducting single crystals fiber

We report the growth of Nb2Pd0.73S5.7 superconducting single crystal fibers via slow cooling solid state reaction method. Superconducting transition temperature (Tc ~ 6.5K) is confirmed from magnetization and transport measurements. A comparative study is performed for determination of superconducting anisotropy, Γ, via conventional method (by taking ration of two superconducting parameters) and scaling approach method. Scaling approach, defined within the framework of the Ginzburg-Landau theory is applied to the angular dependent resistivity measurements to estimate the anisotropy. The value of Γ close to Tc from scaling approach is found to be ~ 2.5 that is slight higher compare to conventional approach (~2.2). Further, variation of anisotropy with temperature suggests that it is a type of multi-band superconductor.

cond-mat.supr-con↗

arXiv2015

Effect of Sb deficiency on the thermoelectric properties of Zn4Sb3

We have investigated the effect of Sb-deficiency on the thermoelectric figure of merit (zT) of Zn4Sb3 prepared by solid state reaction route. At high temperatures, the Seebeck coefficient (S) and electrical conductivity (σ) increase with increase in Sb deficiency whereas the thermal conductivity (\k{appa}) decreases giving rise to an increase in the overall zT value. The observations suggest that creation of vacancies could be an effective route in improving the thermoelectric properties of Zn4Sb3 system. This coupled to nanostructuring strategy could lead to the ultimate maximum value of zT in this system for high temperature thermoelectric applications.

cond-mat.mtrl-sci↗

arXiv2015

Enhancement in thermoelectric properties of FeSb2 by Sb site deficiency

We report a strategy based on introduction of point defects for improving the thermoelectric properties of FeSb2, a promising candidate for low temperature applications. Introduction of Sb deficiency to the tune of 20% leads to enhancement in the values of electrical conductivity (σ) and Seebeck coefficient (S) accompanied with a concomitant suppression in lattice thermal conductivity (\k{appa}lat) values in samples prepared using conventional solid state reaction route. These observations in polycrystalline FeSb2-x provides ample motivation for a dedicated exploration of thermoelectric behavior of the corresponding single crystalline as well as hot-pressed polycrystalline counterparts.

cond-mat.mtrl-sci↗

arXiv2013

Growth, Characterization, Vortex Pinning and Vortex Flow Properties of Single Crystals of Iron Chalcogenide Superconductor FeCr$_{0.02}$Se

We report the growth and characterization of single crystals of iron chalcogenide superconductor FeCr$_{0.02}$Se. There is an enhancement of the superconducting transition temperature (T$_{\rm c}$) as compared to the T$_{\rm c}$ of the single crystals of the parent compound Fe$_{1+x}$Se by about 25%. The superconducting parameters such as the critical fields, coherence length, penetration depth and the Ginzburg-Landau parameter have been estimated for these single crystals. Analysis of the critical current data suggests a fluctuation in electronic mean free path induced ($δl$) pinning mechanism in this material. Thermally activated transport across the superconducting transition in the presence of external magnetic fields suggests a crossover from a single vortex pinning regime at low fields to a collective flux creep regime at higher magnetic fields. The nature of charge carriers in the normal state estimated from the Hall effect and thermal transport measurements could provide crucial information on the mechanism of superconductivity in Fe-based materials.

cond-mat.supr-con↗

arXiv2012

Nanostructured Zinc Oxide as a Prospective Room Temperature Thermoelectric Material

Nanostructured Zinc oxide (ZnO) was synthesized via a ball milling for 10 hours using high energy planetary ball mill. Phase purity and homogeneity of all the samples have been investigated by X-ray diffraction (XRD) and Field Emission Scanning Electron Microscopy (FE-SEM). All the diffraction peaks can be indexed to the hexagonal phase ZnO with hexagonal symmetry (space group P63mc). Average crystallite size was observed to be 20 nm. There was a remarkable suppression in thermal conductivity (κ) compared to the bulk values by a factor of ~50 at room temperature. This suggests to the possibility of using nanostructured ZnO as a prospective room temperature thermoelectric material.

cond-mat.mtrl-sci↗

arXiv2011

Magnetization hysteresis and time decay measurements in FeSe$_{0.50}$Te$_{0.50}$ : Evidence for fluctuation in mean free path induced pinning

We present results of magnetic measurements relating to vortex phase diagram in a single crystal of FeSe$_{0.5}$Te$_{0.5}$ which displays second magnetization peak anomaly for $H \parallel c$. The possible role of the crystalline anisotropy on vortex pinning is explored via magnetic torque magnetometry. We present evidence in favor of pinning related to spatial variations of the charge carrier mean free path leading to small bundle vortex pinning by randomly distributed (weak) pinning centers for both $H \parallel c$ and $H \perp c$. This is further corroborated using magnetization data for $H \parallel c$ in a single crystal of FeSe$_{0.35}$Te$_{0.65}$. Dynamical response across second magnetization peak (SMP) anomaly in FeSe$_{0.5}$Te$_{0.5}$ has been compared with that across the well researched phenomenon of peak effect (PE) in a single crystal of CeRu$_2$.

cond-mat.supr-con↗

arXiv2011

Quantum Interference of Impurity Bound States in Bi$_{2}$Sr$_{2}$Ca(Cu$_{1-x}$Zn$_{x}$)$_{2}$O$_{8+δ}$ Probed by Scanning Tunneling Spectroscopy

In conventional superconductors, magnetic impurities form an impurity band due to quantum interference of the impurity bound states, leading to suppression of the superconducting transition temperature. Such quantum interference effects can also be expected in d-wave superconductors. Here, we use scanning tunneling microscopy to investigate the effect of multiple non-magnetic impurities on the local electronic structure of the high-temperature superconductor Bi$_{2}$Sr$_{2}$Ca(Cu$_{1-x}$Zn$_{x}$)$_{2}$O$_{8+δ}$. We find several fingerprints of quantum interference of the impurity bound states including: (i) a two-dimensional modulation of local density-of-states with a period of approximately 5.4 Å along the $a$- and $b$-axes, which is indicative of the d-wave superconducting nature of the cuprates; (ii) abrupt spatial variations of the impurity bound state energy; (iii)an appearance of positive energy states; (iv) a split of the impurity bound state. All of these findings provide important insight into how the impurity band in d-wave superconductors is formed.

cond-mat.supr-con↗