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K. Asokan

Publications and source records attributed to K. Asokan.

14 recordsLinked to original sources

Wind speed forecast using random forest learning method

Wind speed forecasting models and their application to wind farm operations are attaining remarkable attention in the literature because of its benefits as a clean energy source. In this paper, we suggested the time series machine learning approach called random forest regression for predicting wind speed variations. The computed values of mutual information and auto-correlation shows that wind speed values depend on the past data up to 12 hours. The random forest model was trained using ensemble from two weeks data with previous 12 hours values as input for every value. The computed root mean square error shows that model trained with two weeks data can be employed to make reliable short-term predictions up to three years ahead.

cs.LG

Enhancement of concentration of XeV and GeV centers in nanocrystalline diamond through He+ irradiation

Atomic defect centers in diamond have been widely exploited in numerous quantum applications like quantum information, sensing, quantum photonics and so on. In this context, there is always a requirement to improve and optimize the preparation procedure to generate the defect centers in controlled fashion, and to explore new defect centers which can have the potential to overcome the current technological challenges. Through this letter we report enhancing the concentration of Ge and Xe vacancy centers in nanocrystalline diamond (NCD) by means of He+ irradiation. We have demonstrated controlled growth of NCD by chemical vapor deposition (CVD) and implantation of Ge and Xe ions into the CVD-grown samples. NCDs were irradiated with He+ ions and characterized through optical spectroscopy measurements. Recorded photoluminescence results revealed a clear signature of enhancement of the Xe-related and Ge vacancies in NCDs.

cond-mat.mtrl-sci

Low temperature ferromagnetism in perovskite SrIrO$_3$ films

The 5$d$ based SrIrO$_3$ represents prototype example of nonmagnetic correlated metal which mainly originates from a combined effect of spin-orbit coupling, lattice dimensionality and crystal structure. Therefore, tuning of these parameters results in diverse physical properties in this material. Here, we study the structural, magnetic and electrical transport behavior in epitaxial SrIrO$_3$ film ($\sim$ 40 nm) grown on SrTiO$_3$ substrate. Opposed to bulk material, the SrIrO$_3$ film exhibits a ferromagnetic ordering at low temperature below $\sim$ 20 K. The electrical transport data indicate an insulating behavior where the nature of charge transport follows Mott's variable-range-hopping model. A positive magnetoresistance is recorded at 2 K which has correlation with magnetic moment. We further observe a nonlinear Hall effect at low temperature ($<$ 20 K) which arises due to an anomalous component of Hall effect. An anisotropic behavior of both magnetoresistance and Hall effect has been evidenced at low temperature which coupled with anomalous Hall effect indicate the development of ferromagnetic ordering. We believe that an enhanced (local) structural distortion caused by lattice strain at low temperatures induces ferromagnetic ordering, thus showing structural instability plays vital role to tune the physical properties in SrIrO$_3$.

cond-mat.str-el

Structural, magnetic and dielectric properties in 3$d$-5$d$ based Sr$_2$FeIrO$_6$ thin films

The structural, magnetic and dielectric properties have been investigated in 3$d$-5$d$ based double perovskite Sr$_2$FeIrO$_6$ thin films deposited by pulse laser deposition technique. To understand the effect of strain, epitaxial films are grown with varying thickness as well as on different substrates i.e., SrTiO$_3$ (100) and LaAlO$_3$ (100). The films with highest thickness are found to be more relaxed. Atomic force microscope images indicate all films are of good quality where grain sizes increase with increase in film thickness. X-ray absorption spectroscopy measurements indicate a Ir$^{5+}$ charge state in present films while providing a detailed picture of hybridization between Fe/Ir-$d$ and O-$p$ orbitals. The bulk antiferromagnetic transition is retained in films though the transition temperature shifts to higher temperature. Both dielectric constant ($ε_r$) and loss ($\tanδ$) show change around the magnetic ordering temperatures of bulk Sr$_2$FeIrO$_6$ indicating a close relation between dielectric and magnetic behaviors. A Maxwell-Wagner type relaxation is found to follow over whole frequency range down to low temperature in present film. On changing the substrate i.e., LaAlO$_3$ (100), the $ε_r(T)$ and ($\tanδ(T)$) show almost similar behavior but $ε_r$ shows a higher value which is due to an increased strain coming from high mismatch of lattice parameters.

cond-mat.str-el

Thermoelectric Properties of GaN with Carrier Concentration Modulation: An Experimental and Theoretical Investigation

The present work investigates the less explored thermoelectric properties of n-type GaN semiconductors by combined both the experimental and computational tools. Seebeck coefficients of epitaxial thin films of GaN were experimentally measured in the wide temperature range from 77 K to 650 K in steps of ~10 K covering both low and high-temperature regimes as a function of carrier concentration 2 x1016, 2 x 1017, 4 x 1017 and 8 x 1017 cm-3. The measured Seebeck coefficient at room temperature was found to be highest, -374 microV/K, at the lowest concentration of 4 x 1016 cm-3 and decreases in magnitude monotonically (-327.6 microV/K, -295 microV/K, -246 microV/K for 2 x 1017, 4 x 1017, 8 x 1017 cm-3, respectively) as the carrier concentration of samples increases. Seebeck coefficient remains negative in the entire temperature range under study indicate that electrons are dominant carriers. To understand the temperature-dependent behavior, we have also carried out the electronic structure, and transport coefficients calculations by using Tran-Blaha modified Becke-Johnson (TB-mBJ) potential, and semiclassical Boltzmann transport theory implemented in WIEN2k and BoltzTraP code, respectively. The experimentally observed carrier concentrations were used in the calculations. The estimated results obtained under constant relaxation time approximations provide a very good agreement between theoretical and experimental data of Seebeck coefficients in the temperature range of 260 to 625 K.

cond-mat.mtrl-sci

Wide range temperature-dependent (80-630 K) study of Hall effect and the Seebeck coefficient of \b{eta}-Ga2O3 single crystals

Investigation of Seebeck coefficient in ultra-wide bandgap materials presents a challenge in measurement, nevertheless, it is essential for understanding fundamental transport mechanisms involved in electrical and thermal conduction. \b{eta}-Ga2O3 is a strategic material for high power optoelectronic applications. Present work reports Seebeck coefficient measurement for single crystal Sn doped \b{eta}-Ga2O3 in a wide temperature range (80-630 K). The non-monotonic trend with large magnitude and negative sign in the entire temperature range shows electrons are dominant carriers. The structural and Raman characterization confirms the single-phase and presence of low, mid, and high-frequency phonon modes, respectively. Temperature dependent (90-350 K) Hall effect measurement was carried out as supplementary study. Hall mobility showed T1.12 for T less than 135 K and T-0.70 for T more than 220 K. Activation energies from Seebeck coefficient and conductivity analysis revealed presence of inter band conduction due to impurity defects. The room temperature Seebeck coefficient, power factor and thermal conductivity were found as 68.57 microV/K, 0.15 microW/K2cm and 14.2 W/mK, respectively. The value of the figure-of-merit for \b{eta}-Ga2O3 was found to be aprox. 0.01 (300 K).

cond-mat.mtrl-sci

Magnetic and transport properties in pyrochlore iridates (Y$_{1-x}$Pr$_x$)$_2$Ir$_2$O$_7$: The role of $f$-$d$ exchange interaction and $d$-$p$ orbital hybridization

The $f$-$d$ magnetic exchange interaction is considered to be a key ingredient for many exotic topological phases in pyrochlore iridates. Here, we have investigated the evolution of structural, magnetic and electronic properties in doped pyrochlore iridate, (Y$_{1-x}$Pr$_x$)$_2$Ir$_2$O$_7$. Apart from geometrical frustration, pyrochlore iridates are well known for its active spin-orbit coupling effect. The substitution of Pr$^{3+}$ (4$f^2$) for the nonmagnetic Y$^{3+}$ (4$d^0$) acts as a magnetic doping, which provides an ideal platform to study $f$-$d$ exchange interaction without altering the Ir-sublattice. With Pr substitution, system retains its original cubic structural symmetry but the local structural parameters show an evolution with the doping concentration $x$. The robust magnetic-insulating state in Y$_2$Ir$_2$O$_7$ is drastically weakened, while Pr$_2$Ir$_2$O$_7$ ($x$ = 1.0) shows a paramagnetic-metallic behavior. A metal-insulator transition is observed for $x$ = 0.8 sample. This evolution of magnetic and electronic properties are believed to be induced by an exchange interaction between localized Pr-4$f$ and itinerant Ir-5$d$ electrons as well as by an increased hybridization between Ir-$t_{2g}$ and (basal) O-$p$ orbitals as observed in XAS study. The resistivity in insulating materials follows a power-law behavior with a decreasing exponent with $x$. A negative magnetoresistance is observed for present series of samples at low temperature and where the magnetoresistance shows a quadratic field dependence at higher fields.

cond-mat.str-el

Apparatus for Seebeck coefficient measurement of wire, thin film and bulk materials in the wide temperature range (80-650K)

A Seebeck coefficient measurement apparatus has been designed and developed, which is very effective for accurate characterization of different type of samples in a wide temperature range (80 - 650K) simultaneously covering low as well as the high-temperature regime. Reducing the complexity of the technical design of sample holder and data collections has always been challenging to implement in a single instrument when samples are in different geometrical shape and electronic structure. Our unique design of sample holder with pressure probes covers measurements of different samples shapes (wires, thin films and pellets) as well as different resistivity ranges (metals, semiconductors and insulators). It is suitable for characterization of different samples sizes (3-12 mm). A double heater configuration powered by a dual channel source meter is employed for maintaining a desired constant temperature difference across the sample for the whole temperature range. Two K-type thermocouples are used for simultaneously reading of temperatures and Seebeck voltages by utilizing different channels of a multichannel digital multimeter. Calibration of the system has been carried out using constantan, chromel and alumel materials and recorded data is found to be very accurate and consistent with earlier reports. The Seebeck coefficients of standard samples of constantan (wire) and GaN (thin film) have been reported, which shows the measurement capability of designed setup with versatile samples.

physics.app-ph

Evolution of Microstructure and Relaxor Ferroelectric Properties in (La_zBa_(1-z)(Ti0.80Sn0.20)O3

We report a study on a series of lanthanum doped barium stannate-titanate (LBTS) ceramics towards correlating their microstructure with ferroelectric properties. The samples were prepared by solid state reaction technique and the XRD analysis indicated predominantly single phase microstructure. Scanning Electron Microscopy revealed that while a low La doping of 1 atomic % yielded inhibited grain growth in 1LBTS, higher amounts of La doping yield larger leafy grains. This peculiar grain growth and microstructure evolution could be attributed to the combination of the following two competing factors: (i) suppression of O vacancies due to La doping leading to grain growth inhibition and (ii) enhancement of mechanism leading to nucleation and growth of 2-D (leafy) grains for La doping higher than 2 atomic % due to eutectic phenomena. Dielectric permittivity measurements indicate that addition of La enhances diffused phase transition in BTS, but its frequency dependence is observed only at 2 atomic % La doping. This signature relaxor behaviour and the increase in the intensity of a particular Raman peak with La doping, as well as non-saturated PE hysteresis loops confirm the presence of polar nano-regions for 2 and 3 atomic % La doped BTS. However significant enhancement in dielectric permittivity could not be achieved due to reduced grain size.

cond-mat.mtrl-sci

Shallow acceptor state in ZnO realized by simple irradiation and annealing route

Activation of shallow acceptor state has been observed in ion irradiated and subsequently air annealed polycrystalline ZnO material. Low temperature photoluminescence (PL) spectrum of the sample exhibits clear signature of acceptor bound exciton (ABX) emission at 3.360 eV. The other two samples, pristine and ion irradiated (without annealing), however, do not show acceptor related PL emission in the nearby energy region. Electron transition from shallow donor (most probable site is interstitial zinc for undoped ZnO) to such newly formed shallow acceptor state creates new donor-acceptor pair (DAP) luminescence peak ~ 3.229 eV. ABX and DAP peak energy positions confirm that the acceptor is N related. The acceptor exciton binding energy has been estimated to be 180 +/- 15 meV which is in conformity with earlier reports. The activation of shallow acceptors without any source of atomic nitrogen can only be possible through diffusion of molecular nitrogen inside the sample during annealing. The N2 molecules get trapped at bulk defect sites incorporated by ion irradiation and subsequent annealing. X-ray diffraction (XRD) and Raman spectroscopic (RS) investigation have been employed to probe the changing defective nature of the ZnO samples. Irradiation induced increased disorder has been detected (both by XRD and RS) which is partially removed/modified by annealing at 300 oC. Simultaneous activation of molecular nitrogen acceptor in purposefully defective ZnO is the key finding of this work. Results presented here provide a simple but controlled way of producing shallow acceptor state in ZnO. If optimized through suitable choice of ion, its energy and fluence as well as the annealing temperature, this methodology can trigger further scope to fabricate devices using ZnO epitaxial thin films or nanowires.

cond-mat.mtrl-sci

Magnetic and Electronic Structure study of Fe/MgO/Fe/Co Multilayer Stack Deposited by E-Beam Evaporation

Present work investigates the magnetic and electronic structure of MgO/Fe/MgO/Fe/Co/Au multilayer stack grown on Si(100) substrates by electron beam evaporation method. X-ray diffraction study depicts polycrystalline nature of the multilayers. Results obtained from vibrating sample magnetometry (VSM) and near-edge X-ray absorption fine structure spectra (NEXAFS) at Fe & Co L- and Mg & O K-edges are applied to understand the magnetic and electronic properties of this stack and its interface properties. While the spectral features of Fe L-edge spectrum recorded by surface sensitive total electron yield (TEY) mode shows the formation of FeOx at the Fe/MgO interface, the bulk sensitive total fluorescence yield (TFY), shows Fe in metallic nature. Co L-edge spectrum reveals the presence of metallic nature of cobalt in both TEY and TFY modes. Above results are well correlated with X-ray reflectometry.

cond-mat.mtrl-sci

Electronic and magnetic structure of carbon nanotubes using x-ray absorption and magnetic circular dichroism spectroscopy

Carbon nanotubes are a fraction of the size of transistors used in today's best microchips, as it could reduce power demands and heating in next electronics revolution. Present study investigates the electronic and magnetic structure of multi walled carbon nanotubes (MWCNT) synthesized by chemical vapor deposition technique using near edge x-ray absorption spectroscopy (NEXAFS) measurement at C K-edge and x-ray magnetic circular dichroism (XMCD) at Co and Fe $L_{3,2}$ -edges. NEXAFS at C K-edge shows significant $π$-bonding, and Fe(Co) L-edge proves the presence of Co$^{2+}$ and Fe$^{2+}$ in octahedral symmetry, and embedded in C-matrix of MWCNT. Element specific hystersis loops and XMCD spectra clearly shows that these MWCNTs exhibits room temperature ferromagnetism. These measurements elucidated the electronic structure of CNTs and presence of magnetic interactions at room temperature.

cond-mat.mtrl-sci

Magnetic and humidity sensing properties of nanostructured Cu[x]Co[1-x]Fe2O4 synthesized by auto combustion technique

Magnetic nanomaterials (23-43 nm) of Cu$_x$Co$_{1-x}$Fe$_2$O$_4$\ (x = 0.0, 0.5 and 1.0) were synthesized by auto combustion method. The crystallite sizes of these materials were calculated from X-ray diffraction peaks. The band observed in Fourier transform infrared spectrum near 575 cm$^{-1}$ in these samples confirm the presence of ferrite phase. Conductivity measurement shows the thermal hysteresis and demonstrates the knee points at 475$^o$C, 525$^o$C and 500$^o$C for copper ferrite, cobalt ferrite and copper-cobalt mixed ferrite respectively. The hystersis M-H loops for these materials were traced using the Vibrating Sample Magnetometer (VSM) and indicate a significant increase in the saturation magnetization (M$_s$) and remanence (M$_r$) due to the substitution of Cu$^{2+}$ ions in cobalt ferrite, while the intrinsic coercivity (H$_c$) was decreasing. Among these ferrites, copper ferrite exhibits highest sensitivity for humidity.

cond-mat.mtrl-sci