arXiv subjects

Tapas Ganguli

Publications and source records attributed to Tapas Ganguli.

13 recordsLinked to original sources

arXiv2018

Presence of atomic disorder and its effect on magnetic and electronic properties of NiCrGa half Heusler alloy

In this work, polycrystalline NiCrGa half Heusler alloy, which is predicted to be half-metallic ferromagnet from first principles calculations, has been synthesized by arc meting technique and its structural, magnetic as well as the electronic properties have been studied. The measured x-ray diffraction (XRD) pattern shows the signature of a disordered structure. From the magnetization measurements, there is no evidence of ferromagnetic ordering observed in this system down to the lowest temperature studied. Instead, the system shows the signature of an antiferromagnetic ordering at very low temperature. The experimentally observed structural and magnetic properties are found to be significantly different from the theoretically predicted properties of the ordered cubic C1b structure. To probe the possible disorder present in the system and its effect on the magnetic properties, we have carried out first principles calculations using the spin-polarized-relativistic Korringa-Kohn-Rostoker method (SPR-KKR). Using a combination of XRD, photoelectron spectroscopy, magnetization measurements and first principles calculations, we conclude that NiCrGa has significant amount of atomic disorder. Although, the ordered structure is energetically more stable than the disordered structures, we find that after synthesis, the system tends to stabilize in a disordered structure. With this atomic disorder present in the sample, the ferromagnetic ordering is disturbed and the calculated spin polarization is consequently reduced.

cond-mat.mtrl-sci↗

arXiv2018

Data reduction procedure for correction of geometrical factors in the analysis of specular x-ray reflectivity of small samples

For small samples, the modification of the XRR profile by the geometrical factors manifesting due to profile and size of the beam and the size of the sample is significant. Geometrical factors extend till spill over angle which is often greater than critical angle for small samples. To separate the geometrical factor, it is necessary to know the spill over angle. Since geometrical factor is a smoothly varying function and extends beyond critical angle, it is impossible to determine the spill over angle from XRR profile of small samples. We have shown by comparing the normal XRR profile of a small sample with the XRR profile taken with a surface contact knife edge on the same sample, that the spill over angle can be determined. Thus we have developed a procedure for data reduction for small samples and validated it with suitable experiments. Unlike hitherto used methods which have drawbacks, this is a self consistent method for data reduction

cond-mat.mtrl-sci↗

arXiv2018

An experimental and Ab-initio study of Electronic and Magnetic properties of FeGa3

Electronic structure of FeGa3 has been studied using experiments and ab-initio calculations. Magnetization measurements show that FeGa3 is inherently diamagnetic in nature. Our studies indicate that the previously reported magnetic moment on the Fe atoms in FeGa3 is not an intrinsic property of FeGa3, but is primarily due to the presence of disorder, defects, grain boundaries etc that break the symmetry about the Fe dimers. Analysis of the results obtained from magnetic measurements, photoelectron spectroscopy, Fe K-edge X-ray absorption near edge spectroscopy and ab-initio calculations clearly indicates that, the effects of on-site Coulomb repulsion between the Fe 3d electrons do not play any role in determining the electronic and magnetic properties of FeGa3. Detailed analysis of results of single crystal and poycrystalline FeGa3, helps to resolve the discrepancy in the electronic and magnetic properties in FeGa3 existing in the literature, consistently.

cond-mat.mtrl-sci↗

arXiv2018

Electronic structure of the Mo$_{1-x}$Re$_x$ alloys studied through resonant photoemission spectroscopy

We have studied the electronic structure of Mo rich Mo$_{1-x}$Re$_x$ alloys (0$\leq$ x $\leq$0.4) using valence band photoemission spectroscopy in the photon energy range 23-70 eV and density of states calculations. Comparison of the photoemission spectra with the density of states calculations suggests that with respect to the Fermi level $E_F$, the $d$ states lie mostly in the range 0 to -6 eV binding energy whereas $s$ states lie in the range -4 to -10 eV binding energy. We have observed two resonances in the photoemission spectra of each sample, one at about 35 eV photon energy and other at about 45 eV photon energy. Our analysis suggest that the resonance at 35 eV photon energy is related to the Mo $4p$-$5s$ transition and the resonance at 45 eV photon energy is related to the contribution from both the Mo $4p$-$4d$ transition (threshold: 42 eV) and Re $5p$-$5d$ transition (threshold: 46 eV). In the CIS plot, the resonance at 35 eV incident photon energy for binding energy features in the range of $E_F$ (B.E. = 0) to -5 eV becomes progressively less prominent with the increasing Re concentration $x$ and vanishes for $x>$ 0.2. The difference plots obtained by subtracting the valence band photoemission spectrum of Mo from that of Mo$_{1-x}$Re$_x$ alloys, measured at 47 eV photon energy, reveal that the Re $d$ like states appear near $E_F$ when Re is alloyed with Mo. These results indicate that interband $s$-$d$ interaction, which is weak in Mo, increases with the increasing $x$ and influences the nature of superconductivity in the alloys with higher $x$.

cond-mat.supr-con↗

arXiv2017

High pressure studies on properties of FeGa3: role of on-site coulomb correlation

High pressure X-ray diffraction measurements have been carried out on the intermetallic semiconductor FeGa$_3$ and the equation of state for FeGa$_3$ has been determined. First principles based DFT calculations within the GGA approximation indicate that although the unit cell volume matches well with the experimentally obtained value at ambient pressure, it is significantly underestimated at high pressures and the difference between them increases as pressure increases. GGA + U calculations with increasing values of U$_{Fe(3d)}$ (on-site Coulomb repulsion between the Fe 3d electrons) at high pressures, correct this discrepancy. Further, the GGA+U calculations also show that along with U$_{Fe(3d)}$, the Fe 3d band width also increases with pressure and around a pressure of 4 GPa, a small density of states appear at the Fermi level. High pressure resistance measurements carried out on FeGa$_3$ also clearly show a signature of an electronic transition. Beyond the pressure of 19.7 GPa, the diffraction peaks reduce in intensity and are not observable beyond $\sim$ 26 GPa, leading to an amorphous state.

cond-mat.mtrl-sci↗

arXiv2016

Structural and electronic properties of Fe(AlxGa1-x)3 system

FeGa3 is a well known d-p hybridization induced intermetallic bandgap semiconductor. In this work, we present the experimental and theoretical results on the effect of Al substitution in FeGa3, obtained by x-ray diffraction (XRD), temperature dependent resistance measurement, room temperature Mossbauer measurements and density functional theory based electronic structure calculations. It is observed that upto x = 0.178 in Fe(AlxGa1-x)3, which is the maximum range studied in this work, Al substitution reduces the lattice parameters 'a' and 'c' preserving the parent tetragonal P42/mnm crystal structure of FeGa3. The bandgap of Fe(AlxGa1-x)3 for x = 0.178 is reduced by ~ 24% as compared to FeGa3. Rietveld refinement of the XRD data shows that the Al atoms replace Ga atoms located at the 8j sites in FeGa3. A comparison of the trends of the lattice parameters and energy bandgap observed in the calculations and the experiments also confirms that Al primarily replaces the Ga atoms in the 8j site.

cond-mat.mtrl-sci↗

arXiv2016

Electronic Structure of FeAl Alloy Studied by Resonant Photoemission Spectroscopy and Ab Initio Calculations

Resonant photoemission spectroscopy has been used to investigate the character of Fe 3d states in FeAl alloy. Fe 3d states have two different character, first is of itinerant nature located very close to the Fermi level, and second, is of less itinerant (relatively localized character), located beyond 2 eV below the Fermi level. These distinct states are clearly distinguishable in the resonant photoemission data. Comparison between the results obtained from experiments and first principle based electronic structure calculation show that the origin of the itinerant character of the Fe 3d states is due to the ordered B2 structure, whereas the relatively less itinerant (localized) Fe 3d states are from the disorders present in the sample. The exchange splitting of the Fe 3s core level peak confirms the presence of local moment in this system. It is found that the itinerant electrons arise due to the hybridization between Fe 3d and Al 3s-3p states. Presence of hybridization is observed as a shift in the Al 2p core-level spectra as well as in the X-ray near edge absorption spectra towards lower binding energy. Our photoemission results are thus explained by the co-existence of ordered and disordered phases in the system.

cond-mat.mtrl-sci↗

arXiv2014

Development of high power quantum well lasers at RRCAT

We at RRCAT have recently developed high power laser diodes in the wavelength range of 740 to 1000 nm. A typical semiconductor laser structure is consisted of about 10 epilayers with different composition, thickness and doping values. For example, a laser diode operating at 0.8 micron has either GaAs or GaAsP quantum well as an active layer. The quantum well is sandwiched between AlGaAs wider bandgap waveguide and cladding layers. The complete laser structure is grown by metal organic vapour phase epitaxy technique and devices are fabricated through standard procedure using photolithography. We recently achieved about 5.3 Watt peak power at 853 nm. These laser diodes were tested under pulsed operation at room temperature for 500 nanosecond pulse duration with a duty cycle of 1:1000. Laser diode arrays consisting of 6-10 elements were also developed and tested for operation in pulsed mode at room temperature.

physics.optics↗

arXiv2014

Realization of epitaxial ZnO layers on GaP(111) substrates by pulsed laser deposition

Epitaxy of ZnO layers on cubic GaP (111) substrates has been demonstrated using pulsed laser deposition. Out of plane and in-plane epitaxial relationship of ZnO layer with respect to GaP substrate determined using phi scans in high resolution X-ray diffraction measurements are (0001) ZnO || (111) GaP and (-1 2 -1 0) ZnO || (-1 1 0) GaP respectively. Our results of epitaxy of ZnO and its intense excitonic photoluminescence with very weak defect luminescence suggest that (111) oriented GaP can be a potential buffer layer choice for the integration of ZnO based optoelectronic devices on Si(111) substrates.

cond-mat.mtrl-sci↗

arXiv2010

Distorted wurtzite unit cells: Determination of lattice parameters of non-polar a-plane AlGaN and estimation of solid phase Al content

Unlike c-plane nitrides, ``non-polar" nitrides grown in e.g. the a-plane or m-plane orientation encounter anisotropic in-plane strain due to the anisotropy in the lattice and thermal mismatch with the substrate or buffer layer. Such anisotropic strain results in a distortion of the wurtzite unit cell and creates difficulty in accurate determination of lattice parameters and solid phase group-III content (x_solid) in ternary alloys. In this paper we show that the lattice distortion is orthorhombic, and outline a relatively simple procedure for measurement of lattice parameters of non-polar group III-nitrides epilayers from high resolution x-ray diffraction measurements. We derive an approximate expression for x_solid taking into account the anisotropic strain. We illustrate this using data for a-plane AlGaN, where we measure the lattice parameters and estimate the solid phase Al content, and also show that this method is applicable for m-plane structures as well.

cond-mat.mtrl-sci↗

arXiv2008

Non-intrinsic superconductivity in InN epilayers: role of Indium Oxide

In recent years there have been reports of anomalous electrical resistivity and the presence of superconductivity in semiconducting InN layers. By a careful correlation of the temperature dependence of resistivity and magnetic susceptibility with structural information from highresolution x-ray diffraction measurements we show that superconductivity is not intrinsic to InN and is seen only in samples that show traces of oxygen impurity. We hence believe that InN is not intrinsically a superconducting semiconductor.

cond-mat.mtrl-sci↗

arXiv2007

The role of hydrostatic stress in determining the bandgap of InN epilayers

We establish a correlation between the internal stress in InN epilayers and their optical properties such as the measured absorption band edge and photoluminescence emission wavelength. By a careful evaluation of the lattice constants of InN epilayers grown on c-plane sapphire substrates under various conditions by metalorganic vapor phase epitaxy we find that the films are under primarily hydrostatic stress. This results in a shift in the band edge to higher energy. The effect is significant, and may be responsible for some of the variations in InN bandgap reported in the literature.

cond-mat.mtrl-sci↗

arXiv2007

Influence of growth parameters on structural properties and bandgap of InN epilayers deposited in a showerhead MOVPE system

From a detailed analysis of InN epilayers deposited in a close-coupled showerhead metalorganic vapour phase epitaxy (MOVPE) system under various conditions we investigate the effect of growth parameters on the lattice constants of the InN layer. The layers are under significant internal hydrostatic stress which influences the optical properties. Samples typically fall into two broad categories of stress, with resultant luminescence emission around 0.8eV and 1.1eV. We can correlate the internal stress in the layer and the value of the optical absorption edge, and the PL emission wavelength.

cond-mat.mtrl-sci↗