A K Rajarajan
Homi Bhabha National Institute, Physical Sciences, Faculty Member
- I am interested in promoting science and researching in condensed matter physics especially superconductivity and magnetismedit
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We have evaluated the structural and mechanical stability, electronic structure, total spin magnetic moment, and Curie temperature of LaCoTiIn Equiatomic Quaternary Heusler Alloy (EQHA) using first-principles studies. The Generalized... more
We have evaluated the structural and mechanical stability, electronic structure, total spin magnetic moment, and Curie temperature of LaCoTiIn Equiatomic Quaternary Heusler Alloy (EQHA) using first-principles studies. The Generalized Gradient Approximation (GGA) and GGA+U schemes have been used as exchange-correlation functional for the above calculations. From the ground state calculation, LaCoTiIn EQHA with a Type-III structure in the ferromagnetic (FM) state is found to be stable. The electronic structure of LaCoTiIn EQHA depicts half-metallic behavior which has metallic overlap in the spin up ([Formula: see text] channel and a semiconductor band gap in the other channel. The spin–orbit coupling of LaCoTiIn has a great influence on the band gap of the material. The computed band gap values for the spin down ([Formula: see text] channel are 0.480 eV and 0.606 eV by using the GGA and GGA+U schemes. The total spin magnetic moment is 1 [Formula: see text], according to the Slater–Pau...
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Abstract We report the results of detailed investigations on structural and magnetic properties of polycrystalline GdFeO3 compound. Temperature-dependent magnetization studies support an antiferromagnetic transition at TN(Gd3+) ∼ 2.8 K... more
Abstract We report the results of detailed investigations on structural and magnetic properties of polycrystalline GdFeO3 compound. Temperature-dependent magnetization studies support an antiferromagnetic transition at TN(Gd3+) ∼ 2.8 K associated with Gd3+ spins. A clear spin-flop type metamagnetic transition associated with Gd3+ sub-lattice is observed in field-dependent magnetization measurements made across TN(Gd3+). The compound also shows a weak hysteresis behaviour linked to canted Fe3+ moments near-zero field region, with a remnant magnetization of ∼0.022 μB.
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The explosive detection via NQR is based on presence of 14N as all explosives usually contain nitrogen. The detected nitrogen nuclei contained in explosives and other nitrogen containing materials gives different frequencies and also... more
The explosive detection via NQR is based on presence of 14N as all explosives usually contain nitrogen. The detected nitrogen nuclei contained in explosives and other nitrogen containing materials gives different frequencies and also different explosives have their own NQR frequencies, explosives can be detected and uniquely identified by 14N NQR frequency. A Review on research and development in the field of detection of 14N using NQR has been presented in this paper.
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NQR measurements on YBa2(Cu1-xGax)3O7-delta ( delta ⋍ 0.05) at 4.2K are reported. Resonance lines become broader with the introduction of Ga-ions suggesting a distribution of the electric field gradient (EFG) at the... more
NQR measurements on YBa2(Cu1-xGax)3O7-delta ( delta ⋍ 0.05) at 4.2K are reported. Resonance lines become broader with the introduction of Ga-ions suggesting a distribution of the electric field gradient (EFG) at the Cu-sites. EFG at both the sites is affected though Ga-ions occupy only the chain sites. Spin-lattice relaxation time reduces sharply with the increase of x which suggests strongly
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The emergence of new thermoelectric materials for energy generation is crucial to satisfying the world's energy demand. The electron charge density and thermoelectric transport properties of the LaCoTiIn quaternary Heusler alloy were... more
The emergence of new thermoelectric materials for energy generation is crucial to satisfying the world's energy demand. The electron charge density and thermoelectric transport properties of the LaCoTiIn quaternary Heusler alloy were investigated in this study. We used the Generalized Gradient Approximation (GGA – PBE) exchange correlation potential to illustrate the physical properties of a material. The electronic structure of the alloy shows half-metallic ferromagnetic behavior. The minority spin channel's small band gap enhances its thermoelectric characteristics. The high electrical conductivity, small thermal conductivity, and good figure of merit all indicate that this material could be a suitable material to employ in thermoelectric energy conversion.
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We report magnetic properties and magnetocaloric effect in intermetallic compounds NdMn2−xCoxSi2 (x = 0.2, 0.4, 0.6, 0.8, and 1). dc magnetization study shows that these compounds undergo a ferromagnetic-like transition at around 45 K.... more
We report magnetic properties and magnetocaloric effect in intermetallic compounds NdMn2−xCoxSi2 (x = 0.2, 0.4, 0.6, 0.8, and 1). dc magnetization study shows that these compounds undergo a ferromagnetic-like transition at around 45 K. Neutron diffraction study at 5 K for the x = 0.2 sample shows a canted-ferromagnetic state at lower temperature (5 K), and a collinear antiferromagnetic state at higher temperature (above ∼50 K). At 5 K, the Nd moments are aligned along the crystallographic c-axis and the Mn moments are canted to the c-axis. At higher temperatures (50, 100, 200, and 300 K), Nd sublattice does not order but Mn sublattice orders antiferromagnetically. A magnetocaloric effect (MCE) is found with a magnetic entropy change of 14.4 and 12.4 J kg−1 K−1 for the x = 0.2 and 0.4 samples, respectively, at 47.5 K under a field variation of 50 kOe. Various interesting phenomena such as metamagnetic transitions and domain wall pinning have been observed, and their role in obtaining...
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The structural and superconducting properties of the 1:2:3 system YBa2-xLaxCu3O7+ delta (0 Tc=81 K) conductivity remaining metallic, while semiconductivity is exhibited for x>or=0.35. The neutron profile analysis shows that La goes... more
The structural and superconducting properties of the 1:2:3 system YBa2-xLaxCu3O7+ delta (0 Tc=81 K) conductivity remaining metallic, while semiconductivity is exhibited for x>or=0.35. The neutron profile analysis shows that La goes into the Ba sites, as expected, with an indication of a small but significant fraction of vacancies at the Y sites.
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Nuclear quadrupole resonance (NQR) spectroscopy is used to identify narcotics and explosive materials. Detection of NQR signal generated by $${14}^{{N}}$$ isotope in an open environment is a challenging task due to the presence of... more
Nuclear quadrupole resonance (NQR) spectroscopy is used to identify narcotics and explosive materials. Detection of NQR signal generated by $${14}^{{N}}$$ isotope in an open environment is a challenging task due to the presence of external random noise and RF interference. Unlike the existing wavelet-based and other frequency-domain approaches that use averaged data, the present work exploits raw data by saving the acquisition time. In this context, a novel singular spectral analysis (SSA) and non-linear least square fit (NLLSF) algorithms are proposed to denoise the NQR signal and obtain the required parameters for detection of the NQR signal. Considering signal to noise ratio (SNR), segmental SNR (SSNR), and Pink noise as the performance parameters, the proposed algorithms are tested under various noise conditions by passing synthesized NQR signal and observed 35.7 dB gain SSNR. Furthermore, tests are carried out on NQR spectroscopy data acquired from the $$\hbox {NaNO}_{2}$$ sample, and an improvement in terms of signal quality and acquisition time are noticed.
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The structural, magnetic and magneto-electric (ME) properties of polycrystalline BiFeO3 and Bi0.9–xGdxLa0.1FeO3 (x = 0.00 and 0.05) samples have been investigated. X-ray diffraction patterns confirm the formation of single phases in all... more
The structural, magnetic and magneto-electric (ME) properties of polycrystalline BiFeO3 and Bi0.9–xGdxLa0.1FeO3 (x = 0.00 and 0.05) samples have been investigated. X-ray diffraction patterns confirm the formation of single phases in all the samples with ‘R3c’ space group. Magnetic anisotropy and ferroelectric hysteresis behaviour are observed in Bi0.85Gd0.05La0.1FeO3 through magnetization and polarization measurements respectively. In addition, the same compound shows weak ferromagnetism at room temperature with a significantly enhanced remnant magnetization of ~ 23.6 emu.mol−1, concomitant with an exchange bias. ME measurements on the compound show a non-linear dependence of magnetic and electric vectors.
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An FPGA based NQR spectrometer has been designed for detection of N nuclei. The digital circuits required for NQR spectrometer i.e Pulse Programmer, DDS, digital receiver have been built inside FPGA. Combining FPGA chip with analog... more
An FPGA based NQR spectrometer has been designed for detection of N nuclei. The digital circuits required for NQR spectrometer i.e Pulse Programmer, DDS, digital receiver have been built inside FPGA. Combining FPGA chip with analog components, NQR spectrometer has been developed. N from NaNO2 is observed using same spectrometer. By adding a permanent magnet of uniform field NMR signal of proton as well as deuterium was also observed. Development of Low Frequency Pulsed NQR/NMR Spectrometer © 2016 Global Journals Inc. (US) G lo ba l J o ur na l of R es ea rc he s in E ng in ee ri ng ( ) V ol 13 Y e a r 20 16 F um e X V I I ss ue II V er si on I Preeti Hemnani , A. K.Rajarajan σ , Gopal Joshi, ρ & S. V. G Ravindranath Ѡ AbstractAn FPGA based NQR spectrometer has been An FPGA based NQR spectrometer has been designed for detection of N nuclei. The digital circuits required for NQR spectrometer i.e Pulse Programmer, DDS, digital receiver have been built inside FPGA. Combining FPGA chip w...
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Abstract Reduced graphene oxide (rGO) assisted two-dimensional (2D) graphitic ZnO (gZnO) have been prepared and characterized by Raman, FTIR, Cross-sectional high resolution transmission electron microscopy (HRTEM) and X-ray absorption... more
Abstract Reduced graphene oxide (rGO) assisted two-dimensional (2D) graphitic ZnO (gZnO) have been prepared and characterized by Raman, FTIR, Cross-sectional high resolution transmission electron microscopy (HRTEM) and X-ray absorption spectroscopy (XAS) techniques comprising of X-ray near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. Raman spectroscopy and FTIR measurements suggest formation of ZnO and the same has been corroborated by XANES measurements. HRTEM and polarization dependent XANES measurements with two different sample orientations confirm the formation of gZnO/rGO sandwiched layers. Density Functional Theory simulated relaxed initial structure of bilayers of gZnO sandwiched between successive rGO layers fits the EXAFS data reasonably well, supported by formation energy calculations which indicate better stability for samples having bilayers to a few layers of gZnO between rGO layers than with monolayer. These relaxed structures also help to understand the electronic structure contributing in XANES spectrum, with the help of ligand field theory.
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Abstract Y3AlC has been synthesized by arc melting and subsequent annealing. Rietveld analysis of the powder x-ray diffraction (XRD) data confirms cubic Pm-3m structure. Electrical resistivity (ρ) of Y3AlC exhibits metallic behaviour. No... more
Abstract Y3AlC has been synthesized by arc melting and subsequent annealing. Rietveld analysis of the powder x-ray diffraction (XRD) data confirms cubic Pm-3m structure. Electrical resistivity (ρ) of Y3AlC exhibits metallic behaviour. No sign of superconductivity is observed down to the lowest measurement temperatures of 4.2 K in ρ, and 2 K in magnetic susceptibility (χ) and specific heat (Cp) measurements. The value of the electronic specific heat coefficient γ is 1.36 mJ/K2 mol from which the density of states (DOS) at the Fermi energy (EF) is obtained as 0.57 states/eV.unit cell. The value of Debye temperature θD is estimated to be 315 K. Electronic band structure calculations of Y3AlC reveal a pseudo-gap in the DOS at EF leading to a small value of 0.5 states/eV unit cell which matches quite well with that obtained from γ. Non-zero value of the DOS indicates metallic behaviour as confirmed by our ρ data. Covalent and ionic bonding seem to co-exist with metallic bonding in Y3AlC as indicated by van Arkel- Ketelaar triangle for Zintl-like systems.
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ABSTRACT
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ABSTRACT The effect of simultaneous substitution of Ga3+ and Zn2+ in YBa2 Cu3O7−y has been studied with respect to its superconducting transition temperature and structural parameters. The result indicates the preferential substitution of... more
ABSTRACT The effect of simultaneous substitution of Ga3+ and Zn2+ in YBa2 Cu3O7−y has been studied with respect to its superconducting transition temperature and structural parameters. The result indicates the preferential substitution of Zn and Ga at plane and chain sites of Cu respectively as well as the creation of local magnetic moments at the respective sites due to slowing down of spin and charge fluctuations. The mutual coupling of the moments at the chain- and plane-sites gives rise to enhanced depression of Tc.
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We present an exact enumeration algorithm for identifying the native configuration - a maximally compact self avoiding walk configuration that is also the minimum energy configuration for a given set of contact-energy schemes; the process... more
We present an exact enumeration algorithm for identifying the native configuration - a maximally compact self avoiding walk configuration that is also the minimum energy configuration for a given set of contact-energy schemes; the process is implicitly sequence-dependent. In particular, we show that the 25-step native configuration on a diamond lattice consists of two sheet-like structures and is the same for all the contact-energy schemes, (-1,0,0);(-7,-3,0); (-7,-3,-1); (-7,-3,1); on a square lattice also, the 24-step native configuration is independent of the energy schemes considered. However, the designing sequence for the diamond lattice walk depends on the energy schemes used whereas that for the square lattice walk does not. We have calculated the temperature-dependent specific heat for these designed sequences and the four energy schemes using the exact density of states. These data show that the energy scheme (-7,-3,-1) is preferable to the other three for both diamond and...
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We present results of transport properties measurement on a single crystal of V3Si showing distinct signature of peak-effect (PE) in its vortex state. The field variation of the electrical resistance in the flux-line lattice prepared by... more
We present results of transport properties measurement on a single crystal of V3Si showing distinct signature of peak-effect (PE) in its vortex state. The field variation of the electrical resistance in the flux-line lattice prepared by different experimental path, namely zero field ...
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Nuclear quadrupole resonance (NQR) of copper in Lu 0.9 Pr 0.1 Ba 2 Cu 3 O 7 at 4.2 K has been observed. The resonances of 63 Cu(II) and 65 Cu(II) have been observed at 31.50 and 29.15 MHz, respectively. These frequencies are the same as... more
Nuclear quadrupole resonance (NQR) of copper in Lu 0.9 Pr 0.1 Ba 2 Cu 3 O 7 at 4.2 K has been observed. The resonances of 63 Cu(II) and 65 Cu(II) have been observed at 31.50 and 29.15 MHz, respectively. These frequencies are the same as those in YBa 2 Cu 3 O 7 but not in ...
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The valence state of thallium in TlBa 1-x Sr x LaCuO 5 (x=0.0, 0.4, 0.7, and 1.0) has been investigated by measuring Tl 4f core levels by x-ray photoelectron spectroscopy. It is shown that the valence state of Tl in TlBaLaCuO 5... more
The valence state of thallium in TlBa 1-x Sr x LaCuO 5 (x=0.0, 0.4, 0.7, and 1.0) has been investigated by measuring Tl 4f core levels by x-ray photoelectron spectroscopy. It is shown that the valence state of Tl in TlBaLaCuO 5 (nonsuperconducting) is 3+, whereas that in TlSrLaCuO 5 ( ...
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Superconductivity has been observed in a Tm-Ba-Cu-O system with Tc=89 K. Even though the predominant phase on this system was found to be TmBa2Cu3O9-x, this compound could not be synthesised as a single phase. Magnetisation measurements... more
Superconductivity has been observed in a Tm-Ba-Cu-O system with Tc=89 K. Even though the predominant phase on this system was found to be TmBa2Cu3O9-x, this compound could not be synthesised as a single phase. Magnetisation measurements indicated that at least 8% of the sample was superconducting. At 10 K, the lower critical field was found to be 1.5 kG.
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Nuclear Magnetic Resonance (NMR) is a RF technique that is able to detect any compound by sensing the excited resonance signals from atomic nuclei having non-zero spin. NQR is similar to NMR but the only difference is NMR needs a DC... more
Nuclear Magnetic Resonance (NMR) is a RF technique that is able to detect any compound by sensing the excited resonance signals from atomic nuclei having non-zero spin. NQR is similar to NMR but the only difference is NMR needs a DC magnetic field and due to this its application in the field is limited. A FPGA based NQR spectrometer is designed using a single FPGA chip to perform the digital tasks required for NQR spectrometer. Design of Probe for NMR/NQR spectrometer is researched. Parallel tuned and series tuned Probes are discussed and simulated.14N NQR from NaNO2 is observed from spectrometer designed with parallel tuned probe.
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The authors report their observation of some anomalous superconducting fluctuations in (RE)BaâCuâO/sub 7-y/ and associated systems. The resistivity behavior of Y(Ba/sub 1.6/Î/sub 0.4/)CuâO/sub 7-y/ shows a transition from semiconducting... more
The authors report their observation of some anomalous superconducting fluctuations in (RE)BaâCuâO/sub 7-y/ and associated systems. The resistivity behavior of Y(Ba/sub 1.6/Î/sub 0.4/)CuâO/sub 7-y/ shows a transition from semiconducting to superconducting on thermal cycling. In a sample of Y(Ba/sub 1.8/Pb/sub 0.2/Cuâ)/sub 7-y/, superconductivity was found to be extremely short-lived. A reproducible magnetic response was observed just above the onset of
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Superconductivity has been observed in a Tm-Ba-Cu-O system with Tc=89 K. Even though the predominant phase on this system was found to be TmBa2Cu3O9-x, this compound could not be synthesised as a single phase. Magnetisation measurements... more
Superconductivity has been observed in a Tm-Ba-Cu-O system with Tc=89 K. Even though the predominant phase on this system was found to be TmBa2Cu3O9-x, this compound could not be synthesised as a single phase. Magnetisation measurements indicated that at least 8% of the sample was superconducting. At 10 K, the lower critical field was found to be 1.5 kG.
ABSTRACT We have shown the effect of fast neutron irradiation on the magnetic phase transition and magnetocaloric effect (MCE) in a doped Ce(Fe0.96Ru0.04)2, intermettalic. We show that this leads to suppression of MCE and a to a... more
ABSTRACT We have shown the effect of fast neutron irradiation on the magnetic phase transition and magnetocaloric effect (MCE) in a doped Ce(Fe0.96Ru0.04)2, intermettalic. We show that this leads to suppression of MCE and a to a disordered ferromagnetic phase.
ABSTRACT Recently precious metals supported on oxides were reported to be very promising catalysts for the lower-temperature WGS reaction. Here we report the synthesis of platinum doped barium cerate and its use as catalysts for WGS... more
ABSTRACT Recently precious metals supported on oxides were reported to be very promising catalysts for the lower-temperature WGS reaction. Here we report the synthesis of platinum doped barium cerate and its use as catalysts for WGS reaction. It has been found that maximum CO conversion was obtained at 3500C which enhanced after the first cycle. XPS analysis shows that after the first cycle more ionic Platinum species are present on the surface of the catalyst. Neutron diffraction at room temperature shows that the oxygen vacancies are in the O2 position and increases with increasing Pt substitution.
ABSTRACT Inelastic neutron scattering measurements to determine the phonon density of states along with lattice dynamical studies based on a supercell and a simple transferable potential model have been done to confirm the two mode... more
ABSTRACT Inelastic neutron scattering measurements to determine the phonon density of states along with lattice dynamical studies based on a supercell and a simple transferable potential model have been done to confirm the two mode behavior exhibited by ZnS1-xSex. In addition, the contribution of bond-length distribution to the existence of anomalous modes found in the frequency range of the ZnS-like TO and LO modes has been emphasized.