arXiv subjects

Hem Chandra Kandpal

Publications and source records attributed to Hem Chandra Kandpal.

8 recordsLinked to original sources

arXiv2019

Experimental Observation of Invariance of Spectral Degree of Coherence with Change in Bandwidth of Light

An experimental study is conducted to show the effect of the change in bandwidth of light on the spectral degree of coherence at a pair of points in the cross-section of a beam. For this purpose a polychromatic source and a monochromator with variable entrance and exit slits were used to produce a variable bandwidth source. The classic Youngs interferometer was used to produce an interference pattern. The spectral measurements of the visibility of the interference fringes show that the spectral degree of coherence remains unaffected by the change in the frequency pass-band of the light.

physics.optics↗

arXiv2013

Polymeric Frameworks as Organic Semiconductors with Controlled Electronic Properties

The rational assembly of monomers, in principle, enables the design of a specific periodicity of polymeric frameworks, leading to a tailored set of electronic structure properties in these solid-state materials. The further development of these emerging systems requires a combination of both experimental and theoretical studies. Here, we investigated the electronic structures of two-dimensional polymeric frameworks based on triazine and benzene rings, by means of electrochemical techniques. The experimental density of states was obtained from quasi-open-circuit voltage measurements through galvanostatic intermittent titration technique, which we show to be in excellent agreement with first principles calculations performed for two and three-dimensional structures of these polymeric frameworks. These findings suggest that the electronic properties do not only depend on the number of stacked layers but also on the ratio of the different aromatic rings.

cond-mat.mtrl-sci↗

arXiv2007

Variation of the bonding interactions and magnetism in GdAuX (X = Mg, Cd, and In)

Results of first-principles electronic structure calculations for the isotypic compounds GdAuX (ZrNiAl type, X = Mg, Cd, and In) are presented. We report on a systematic examination of the electronic structure and nature of the bonding in these intermetallics. Our calculations indicate a metallic state for all of the compounds. We find that the indium in GdAuIn and magnesium in GdAuMg have significant bonding interactions with Au. We have also identified In s lone pair in GdAuIn has more localized behaviour as compared with Mg s in GdAuMg. The magnetic properties are well described within the local density approximation.

physics.comp-ph↗

arXiv2006

Electronic structure, magnetism, and disorder in the Heusler compound Co$_2$TiSn

Polycrystalline samples of the half-metallic ferromagnet Heusler compound Co$_2$TiSn have been prepared and studied using bulk techniques (X-ray diffraction and magnetization) as well as local probes ($^{119}$Sn Mössbauer spectroscopy and $^{59}$Co nuclear magnetic resonance spectroscopy) in order to determine how disorder affects half-metallic behavior and also, to establish the joint use of Mössbauer and NMR spectroscopies as a quantitative probe of local ion ordering in these compounds. Additionally, density functional electronic structure calculations on ordered and partially disordered Co$_2$TiSn compounds have been carried out at a number of different levels of theory in order to simultaneously understand how the particular choice of DFT scheme as well as disorder affect the computed magnetization. Our studies suggest that a sample which seems well-ordered by X-ray diffraction and magnetization measurements can possess up to 10% of antisite (Co/Ti) disordering. Computations similarly suggest that even 12.5% antisite Co/Ti disorder does not destroy the half-metallic character of this material. However, the use of an appropriate level of non-local DFT is crucial.

cond-mat.mtrl-sci↗

arXiv2006

Correlation in the transition metal based Heusler compounds Co$_2$MnSi and Co$_2$FeSi

Half-metallic ferromagnets like the full Heusler compounds with formula X$_2$YZ are supposed to show an integer value of the spin magnetic moment. Calculations reveal in certain cases of X = Co based compounds non-integer values, in contrast to experiments. In order to explain deviations of the magnetic moment calculated for such compounds, the dependency of the electronic structure on the lattice parameter was studied theoretically. In local density approximation (LDA), the minimum total energy of Co$_2$FeSi is found for the experimental lattice parameter, but the calculated magnetic moment is about 12% too low. Half-metallic ferromagnetism and a magnetic moment equal to the experimental value of $6μ_B$ are found, however, only after increasing the lattice parameter by more than 6%. To overcome this discrepancy, the LDA$+U$ scheme was used to respect on-site electron correlation in the calculations. Those calculations revealed for Co$_2$FeSi that an effective Coulomb-exchange interaction $U_{eff}=U-J$ in the range of about 2eV to 5eV leads to half-metallic ferromagnetism and the measured, integer magnetic moment at the measured lattice parameter. Finally, it is shown in the case of Co$_2$MnSi that correlation may also serve to destroy the half-metallic behavior if it becomes too strong (for Co$_2$MnSi above 2eV and for Co$_2$FeSi above 5eV). These findings indicate that on-site correlation may play an important role in the description of Heusler compounds with localized moments.

cond-mat.mtrl-sci↗

arXiv2005

Investigation of Co$_2$FeSi: The Heusler compound with Highest Curie Temperature and Magnetic Moment

This work reports on structural and magnetic investigations of the Heusler compound Co$_2$FeSi. X-Ray diffraction and Mößbauer spectrometry indicate an ordered $L2_1$ structure. Magnetic measurements by means of X-ray magnetic circular dichroism and magnetometry revealed that this compound is, currently, the material with the highest magnetic moment ($6 μ_B$) and Curie-temperature (1100K) in the classes of Heusler compounds as well as half-metallic ferromagnets.

cond-mat.mtrl-sci↗

arXiv2005

Design of magnetic materials: Co$_2$Cr$_{1-x}$Fe$_{x}$Al

Doped Heusler compounds Co$_2$Cr$_{1-x}$Fe$_{x}$Al with varying Cr to Fe ratio $x$ were investigated experimentally and theoretically. The electronic structure of the ordered, doped Heusler compound Co$_2$Cr$_{1-x}$Fe$_{x}$Al ($x=n/4, n=0,1,2,3,4)$ was calculated using different types of band structure calculations. The ordered compounds turned out to be ferromagnetic with small Al magnetic moment being aligned anti-parallel to the 3d transition metal moments. All compounds show a gap around the Fermi-energy in the minority bands. The pure compounds exhibit an indirect minority gap, whereas the ordered, doped compounds exhibit a direct gap. Magnetic circular dichroism (MCD) in X-ray absorption spectra was measured at the $L_{2,3}$ edges of Co, Fe, and Cr of the pure compounds and the $x=0.4$ alloy in order to determine element specific magnetic moments. Calculations and measurements show an increase of the magnetic moments with increasing iron content. The experimentally observed reduction of the magnetic moment of Cr can be explained by Co-Cr site-disorder. The presence of the gap in the minority bands of Co$_2$CrAl can be attributed to the occurrence of pure Co$_2$ and mixed CrAl (001)-planes in the $L2_1$ structure. It is retained in structures with different order of the CrAl planes but vanishes in the $X$-structure with alternating CoCr and CoAl planes.

cond-mat.mtrl-sci↗

arXiv2005

Covalent bonding and the nature of band gaps in some half-Heusler compounds

Half-Heusler compounds \textit{XYZ}, also called semi-Heusler compounds, crystallize in the MgAgAs structure, in the space group $F\bar43m$. We report a systematic examination of band gaps and the nature (covalent or ionic) of bonding in semiconducting 8- and 18- electron half-Heusler compounds through first-principles density functional calculations. We find the most appropriate description of these compounds from the viewpoint of electronic structures is one of a \textit{YZ} zinc blende lattice stuffed by the \textit{X} ion. Simple valence rules are obeyed for bonding in the 8-electron compound. For example, LiMgN can be written Li$^+$ + (MgN)$^-$, and (MgN)$^-$, which is isoelectronic with (SiSi), forms a zinc blende lattice. The 18-electron compounds can similarly be considered as obeying valence rules. A semiconductor such as TiCoSb can be written Ti$^{4+}$ + (CoSb)$^{4-}$; the latter unit is isoelectronic and isostructural with zinc-blende GaSb. For both the 8- and 18-electron compounds, when \textit{X} is fixed as some electropositive cation, the computed band gap varies approximately as the difference in Pauling electronegativities of \textit{Y} and \textit{Z}. What is particularly exciting is that this simple idea of a covalently bonded \textit{YZ} lattice can also be extended to the very important \textit{magnetic} half-Heusler phases; we describe these as valence compounds \textit{ie.} possessing a band gap at the Fermi energy albeit only in one spin direction. The \textit{local} moment in these magnetic compounds resides on the \textit{X} site.

cond-mat.mtrl-sci↗