arXiv subjects
Publications and source records attributed to Ashwani Kumar.
In this paper we report successful transmission of 10 Gbit/s on-off-keying (OOK) modulated signal through the LR-DLSPPWs with almost negligible degradation of the data flow consistency
Using a new brand of commercially available carbon resistor we built a cryogenic thermometer with an extremely good thermal contact to its thermal environment. Because of its superior thermal contact the thermometer is insensitive to low levels of spurious radio frequency heating. We calibrated our thermometer down to 5mK using a quartz tuning fork He-3 viscometer and measured its thermal resistance and thermal response time.
The unoccupied electronic states of LaCoO$_3$ and PrCoO$_3$ are studied using room temperature inverse photoemission spectroscopy and \emph{ab initio} GGA+$\emph{U}$ band structure calculations. A fairly good agreement between experiment and theory is obtained. The intensity of the peak just above the Fermi-level is found to be very much sensitive to the hybridization of Co 3$d$ and O 2$p$ orbitals. Moreover, the band just above the Fermi-level is of Co 3$d$ character with little contribution from O 2$p$ states.
The experimental Mn K-edge x-ray absorption spectra of La$_{1-x}$Sr$_x$MnO$_{3}$, $x$ = 0 - 0.7 are compared with the band structure calculations using spin polarized density functional theory. It is explicitly shown that there is a correspondence between the inflection point on the absorption edge and the centre of gravity of the unoccupied Mn 4$p$-band. This correspondence has been used to separate the doping and size contributions to edge shift due to variation in number of electrons in valence band and Mn-O bond lengths, respectively when Sr is doped into LaMnO$_3$. Such separation is helpful to find the localization behaviour of charge carriers and to understand the observed transport properties of these compounds.
We investigate the spin state of LaCoO3 using state-of-the-art photoemission spectroscopy and ab initio band structure calculations. The GGA+U calculations provide a good description of the ground state for the experimentally estimated value of electron correlation strength, U. In addition to the correlation effect, spin-orbit interaction is observed to play a significant role in the case of intermediate spin and high spin configurations. The comparison of the calculated Co 3d and O 2p partial density of states with the experimental valence band spectra indicates that at room temperature, Co has dominant intermediate spin state configuration and that the high spin configuration may not be significant at this temperature. The lineshape of the La 5p and O 2s core level spectra could be reproduced well within these ab initio calculations.
Room temperature Co K- and La L-edges X-ray absorption studies have been carried out on LaCoO$_3$. Experimental near edge structures have been analysed by theoretical LDA+U density of states (DOS) and multiple scattering (MS) calculations. Use of both MS and DOS calculations yield additional information about hybridization of the states of central atom with neighbouring atoms responsible for producing the near edge structures. Absorption processes at Co K-, La L1-, L2-, and L3-edges have been attributed to electronic transitions from Co 1s to Co 4p, La 2s to La 6p, La 2p$_{1/2}$ to La 5d, and La 2p$_{3/2}$ to La 5d, respectively. All the pre-edge and post-edge features including the shape of the main absorption edge have been generated by taking the convolution of the calculated DOS, indicating that single particle approximation is sufficient to express all experimentally observed major structures. Two pre-edge structures observed in Co K-edge spectrum are attributed to Co 1s to e$_g^{\uparrow}$ and e$_g^$\downarrow}$ quadrupole transitions in contrast to earlier identification of the same to Co 1s to t$_{2g}$ and e$_g$ transitions. The influence of La 6p states on the Co 4p states is such that the inclusion of La atoms in the MS calculations is necessary to generate post-edge structures in Co K-edge spectrum. The importance of the hybridization of O 2p state with La 6p and 5d in the L-edge absorption processes has also been discussed. 10% contribution of quadrupole channel has been estimated in La L-edges.
Room temperature Mn K-edge extended x-ray absorption fine structure (EXAFS) studies were carried out on La$_{1-x}$Sr$_x$MnO$_{3+δ}$ (x = 0.1 to 0.9) compounds. It is found from the detailed EXAFS analysis that the local structure around Mn sites is different from the global structure inferred from x-ray diffraction, especially for x <= 0.4, indicating presence of local distortions in MnO$_6$ octahedra. For the rhombohedral compounds, x = 0.1 to 0.3 the distortion is maximum for x = 0.1 and two bond lengths are seen- short one in basal plane and long one in apical plane. For compounds with x = 0.4 to 0.8 two short bonds in basal plane and four long bonds- two in the basal plane and remaining two in the apical plane are seen. For the compounds up to x = 0.3 compositions long bond length decreases and short bond length increases with increase in x whereas for the compounds 0.4 <= x <= 0.8 both types of bond lengths decrease. Such behaviour of bond lengths is an indication of the changed nature of distortion from Jahn-Teller type to breathing type at x = 0.4 composition.
Electronic states of PrCoO$_3$ are studied using x-ray photoemission spectroscopy. Pr 3d$_{5/2}$ core level and valence band (VB) were recorded using Mg K$_β$ source. The core level spectrum shows that the 3d$_{5/2}$ level is split into two components of multiplicity 4 and 2, respectively due to coupling of the spin states of the hole in 3d$_{5/2}$ with Pr 4f holes spin state. The observed splitting is 4.5 eV. The VB spectrum is interpreted using density of states (DOS) calculations under LDA and LDA+U. It is noted that LDA is not sufficient to explain the observed VB spectrum. Inclusion of on-site Coulomb correlation for Co 3d electrons in LDA+U calculations gives DOS which is useful in qualitative explanation of the ground state. However, it is necessary to include interactions between Pr 4f electrons to get better agreement with experimental VB spectrum. It is seen that the VB consists of Pr 4f, Co 3d and O 2p states. Pr 4f, Co 3d and O 2p bands are highly mixed indicating strong hybridization of these three states. The band near the Fermi level has about equal contributions from Pr 4f and O 2p states with somewhat smaller contribution from Co 3d states. Thus in the Zaanen, Sawatzky, and Allen scheme PrCoO$_3$ can be considered as charge transfer insulator. The charge transfer energy $Δ$ can be obtained using LDA DOS calculations and the Coulomb-exchange energy U' from LDA+U. The explicit values for PrCoO$_3$ are $Δ$ = 3.9 eV and U' = 5.5 eV; the crystal field splitting and 3d bandwidth of Co ions are also found to be 2.8 and 1.8 eV, respectively.
We demonstrate here a method to fabricate CdSe photonic crystal from a very cheap fabrication route of templated self-assembly. The hexagonal close-packed photonic crystals are formed by the electrochemical growth of CdSe through the interstitial spaces between polymer nano/micro sphere templates. The confocal voids containing photonic crystals can be made either interconnected or well separated, with high uniformity. Structural and optical characterization confirms the good quality of electrochemically grown CdSe. These cheaply fabricated 2D photonic crystals provide a wide range of opportunities for optoelectronic devices.
We show that newly found BF3-doped polyaniline, though highly conducting, remains amorphous. Magnetic studies reveal many unusual properties, while suggesting that the intrinsic conductivity of this system is significantly larger than all other known forms of conducting polyaniline, establishing it as an interesting class of highly conducting amorphous polymer.
We investigate the electronic structure of Sr$_2$FeMoO$_6$ combining photoemission spectroscopy with a wide range of photon energies and electronic structure calculations based on first-principle as well as model Hamiltonian approaches to reveal several interesting aspects. We find evidence for unusually strong Coulomb correlation effects both in the Fe 3$d$ and O 2$p$ states, with an enhanced manifestation in the majority spin channel. Additionally, O 2$p$ states exhibit a spin-splitting of nonmagnetic origin, which nevertheless is likely to have subtle influence on the stability of novel ferromagnetism of this compound.
We have investigated the electronic and magnetic structures of Sr2FeMoO6 employing site-specific direct probes, namely x-ray absorption spectroscopy with linearly and circularly polarized photons. In contrast to some previous suggestions, the results clearly establish that Fe is in the formal trivalent state in this compound. With the help of circularly polarized light, it is unambiguously shown that the moment at the Mo sites is below the limit of detection (< 0.25mu_B), resolving a previous controversy. We also show that the decrease of the observed moment in magnetization measurements from the theoretically expected value is driven by the presence of mis-site disorder between Fe and Mo sites.
We have analysed the unusual electronic structure of Sr_2FeMoO_6 combining ab-initio and model Hamiltonian approaches. Our results indicate that there are strong enhancements of the intraatomic exchange strength at the Mo site as well as the antiferromagnetic coupling strength between Fe and Mo sites. We discuss the possibility of a negative effective Coulomb correlation strength (U_{eff}) at the Mo site due to these renormalised interaction strengths.
We have prepared crystallographically ordered and disorder specimens of the double perovskite, Sr$_2$FeMoO$_6$ and investigated their magnetoresistance behaviour. The extent of ordering between the Fe and Mo sites in the two samples is determined by Rietveld analysis of powder x-ray diffraction patterns and reconfirmed by Mössbauer studies. While the ordered sample exhibits the sharp low-field response, followed by moderate changes in the magnetoresistance at higher fields, the disordered sample is characterised by the absence of the spectacular low-field response. We argue that the low field response depends crucially on the half-metallic ferromagnetism, while the high-field response follows from the overall magnetic nature of the sample, even in absence of the half-metallic state.
Average effective two-body interaction matrix elements in the s-d shell have been extracted, from data on experimentally measured isospin centroids, by combining the recently derived new sum rules equations for pick-up reactions with similar known equations for stripping reactions performed on general multishell target states. Using this combination of stripping and pick-up equations, the average effective matrix elements for the shells, 1d^2_5/2, 2s^2_1/2 and 1d^2_3/2 respectively have been obtained. A new feature of the present work is that the restriction imposed in earlier works on target states, that it be populated only by active neutrons has now been abandoned.
The present study concentrates on the average effective two-body interaction matrix elements being extracted, using sum-rule techniques, from transfer reactions on target states having single orbital as well as two orbitaloccupancy. This investigation deals with transfer reactions on f-p shell nuclei involving (i) $1f_{7/2}$ and $2p_{3/2}$ transfer on target states using $^{40}$Ca as inert core, and (ii) $2p_{3/2}$ and$1f_{5/2}$ transfer on states using $^{56}$Ni as core.
Sum Rules equations for pick-up reactions are presented for the first time for the energy centroids of states both for the isospin T_< (\equiv T_0 - 1 \over 2) and T_> (\equiv T_0 + {1 \over 2}) of the final nucleus when a nucleon is picked up from a general multishell target state with isospin T_0. These equations contain two-body correlation terms, , which, at the present moment, are difficult to handle analytically. These terms are managed by combining these equations with the known stripping reactions equations. Sample applications of these equations to experimental data are presented.