arXiv subjects

Aftab Alam

Publications and source records attributed to Aftab Alam.

At least 73 records · Page 4Linked to original sources

arXiv2019

Type-II Dirac states in full Heusler compounds XInPd2 (X = Ti, Zr and Hf)

We predict three full Heusler compounds XInPd2 (X = Zr, Hf and Ti) to be potential candidates for type-II Dirac semimetals. The crystal symmetry of these compounds have appropriate chemical environment with a unique interplay of inversion, time reversal and mirror symmetry. These symmetries help to give six pairs of type-II Dirac nodes on the C_4 rotation axis, closely located at/near the Fermi level. Using first principle calculations, symmetry arguments and crystal field splitting analysis, we illustrate the occurrence of such Dirac nodes in these compounds. Bulk Fermi surfaces have been studied to understand the Lorentz symmetry breaking and Lifshitz transition (LT) of Fermi surfaces. Bulk nodes are projected on the (001) and (111) surfaces which form the surface Fermi arcs, that can further be detected by probes such as angle resolved photo-emission and scanning tunneling spectroscopy. By analyzing the evolution of arcs with changing chemical potential, we prove the fragile nature and the absence of topological protection of the Dirac arcs. Our predicted compounds overcome the limitations of the previously reported PtTe2 class of compounds.

cond-mat.mtrl-sci↗

arXiv2019

High Performance Ternary Alkali Nitrides for Renewable Energy Applications

Rapid decline in fossil fuel energy necessitates the immediate need for renewable energy resources. Here, we report a previously unexplored class of nitrides AMN$_2$ keeping renewable energy applications in mind. Using a detailed structure and stability analysis using first principles simulation, we discovered twelve such compounds (few of which are already synthesized before), which are chemically, mechanically and dynamically stable. These twelve compounds were then evaluated for their suitability for three renewable energy applications, (i) photovoltaics, (ii) water splitting, and (iii) thermoelectrics. Careful analysis of electronic structure reveals high optical transition strength resulting in sharp rise in absorption. This in turn yields high short circuit current and hence excellent solar efficiency for few compounds namely CsVN$_2$ and RbVN$_2$. Along with excellent absorption quality, some compounds show favorable band edge positions compared to water redox levels and hence are promising as photoelectrodes in photo(electro)chemical water splitting devices. Mixture of flat and dispersive bands in the band structure yields both high Seebeck and electrical conductivity, thus excellent power factor for seven compounds. Simulated lattice thermal conductivity shows moderate to ultralow values and thus the possibility of achieving high thermoelectric figure of merit (ZT), even at lower temperatures. From the experimental perspective, we discuss the possible challenges that may arise while utilizing these compounds for the desired applications, and suggest possible pathways to overcome them. We believe such theoretical prediction of promising materials are extremely useful for new materials discovery and anticipate rapid response from the experimental community.

cond-mat.mtrl-sci↗

arXiv2019

A measure of authorship by publications

Measuring publication success of a researcher is a complicated task as publications are often co-authored by multiple authors, and so, require comparison of solo publications with joint publications. In this paper, like \cite{price1981multiple}, we argue for an egalitarian perspective in accomplishing this task. More specifically, we justify the need for an ethical perspective in quantifying academic author by identifying certain ethical difficulties of some popular contemporary indices used for this purpose. And then we show that for any given dataset of research papers, the unique method satisfying the ethical notions of {\it identity independence} and performance invariance must be the egaliatarian E-index proposed by \cite{bps} and \cite{price1981multiple}. In our setting, this egalitarian method divides authorship of joint projects equally among authors and sums across all publications of each author.

cs.DL↗

arXiv2019

Accurate high-throughput screening of I-II-V 8-electron Half-Heusler compounds for renewable-energy applications

Renewable energy resources have emerged as the best alternatives to fossil fuel energy which are rapidly declining with time. Here, eight valence-electron count Half-Heusler(HH) alloys have been studied using reliable first principles calculations in the search of potential candidates for renewable energy applications like thermoelectric (TE), solar harvesting, topological insulator (TI) and transparent conductor (TC) applications. The initial screening parameters used for our study are chemical and thermal stability, band gap, nature of bandgap and band inversion strength. We have performed quasistatic G0W0 calculation starting from HSE groundstate wavefunction to predict the most accurate estimation of bandgap for these class of compounds. A total of 960 compounds were simulated. 121 out of 960 compounds were found to be thermally and chemically stable. 31 compounds with bandgap less than 1.5 eV were studied for thermoelectric application out of which 13 compounds were found to show thermoelectric figure of merit ZT > 0.7 for both p-type and n-type conduction. 30 compounds with band gap 1-1.8 eV were studied for optoelectronic application out of which 13 compounds were found to show Spectroscopic Limited Maximum Efficiency (SLME) more than 20%, comparable to existing state of the art materials. 21 compounds were found to show band inversion at ambient conditions which is a necessary condition for topological insulators. The surface band structure calculations for one of the promising candidate was done to check robustness of the topological behaviour. 29 compounds were found to have bandgap more than 2 eV which are promoted for transparent conductor applications with further band engineering. We strongly believe that our calculations will give useful insights to experimentalists for synthesizing and investigating proposed compounds for different energy applications.

cond-mat.mtrl-sci↗

arXiv2019

Half-metallic ferromagnetism and Ru-induced localization in quaternary Heusler alloy CoRuMnSi

We report a combined theoretical and experimental investigation of half-metallic ferromagnetism in equiatomic quaternary Heusler alloy CoRuMnSi. Room temperature XRD analysis reveals that the alloy crystallizes in L21 disorder instead of pristine Y-type structure due to 50% swap disorder between the tetrahedral sites, i.e., Co and Ru atoms. Magnetization measurements reveal a net magnetization of 4 $μ_B$ with Curie temperature of ~780 K. Resistivity measurement reveals the presence of localization effect below 35 K while above 100 K, a linear dependence is observed. Resistivity behavior indicates the absence of single magnon scattering, which indirectly supports the half-metallic nature. The majority spin band near the Fermi level clearly indicates the overlap of flat eg bands with sharply varying conduction bands that are responsible for the localization. In-depth analysis of the projected atomic d-orbital character of band structure reveals unusual bonding, giving rise to the flat eg bands purely arising out of Ru ions. Co-Ru swap disorder calculations indicate the robustness of half-metallic nature, even when the structure changes from Y-type to L21-type, with no major change in the net magnetization of the system. Thus, robust half-metallic nature, stable structure, and high Curie temperature make this alloy quite a promising candidate to be used as a source of highly spin-polarized currents in spintronic applications.

cond-mat.mtrl-sci↗

arXiv2019

Experimental and Theoretical Investigation on the Possible Half-metallic Behaviour of Equiatomic Quaternary Heusler Alloys: CoRuMnGe and CoRuVZ (Z = Al, Ga)

In this report, structural, electronic, magnetic and transport properties of quaternary Heusler alloys CoRuMnGe and CoRuVZ (Z = Al, Ga) are investigated. All the three alloys are found to crystallize in cubic structure. CoRuMnGe exhibits L2$_1$ structure whereas, the other two alloys have B2-type disorder. For CoRuMnGe and CoRuVGa, the experimental magnetic moments are in close agreement with the theory as well as those predicted by the Slater-Pauling rule, while for CoRuVAl, a relatively large deviation is seen. The reduction in the moment in case of CoRuVAl possibly arises due to the anti-site disorder between Co and Ru sites as well as V and Al sites. Among these alloys, CoRuMnGe has the highest T$\mathrm{_C}$ of 560 K. Resistivity variation with temperature reflects the half-metallic nature in CoRuMnGe alloy. CoRuVAl shows metallic character in both paramagnetic and ferromagnetic states, whereas the temperature dependence of resistivity for CoRuVGa is quite unusual. In the last system, $ρ$ vs. T curve shows an anomaly in the form of a maximum and a region of negative temperature coefficient of resistivity (TCR) in the magnetically ordered state. The ab initio calculations predict nearly half-metallic ferromagnetic state with high spin polarization of 91, 89 and 93 \% for CoRuMnGe, CoRuVAl and CoRuVGa respectively. To investigate the electronic properties of the experimentally observed structure, the Co-Ru swap disordered structures of CoRuMnGe alloy are also simulated and it is found that the disordered structures retain half-metallic nature, high spin polarization with almost same magnetic moment as in the ideal structure. Nearly half-metallic character, high T$\mathrm{_C}$ and high spin polarization make CoRuMnGe alloy promising for room temperature spintronic applications.

cond-mat.mtrl-sci↗

arXiv2019

Broken symmetry driven topological semi-metal to gaped phase transitions in SrAgAs

We show the occurence of Dirac, Triple point, Weyl semimetal and topological insulating phase in a single ternary compound using specific symmetry preserving perturbations. Based on {\it first principle} calculations, \textbf{\textit{k.p}} model and symmetry analysis, we show that alloying induced precise symmetry breaking in SrAgAs (space group P6$_3/mmc$) leads to tune various low energy excitonic phases transforming from Dirac to topological insulating via intermediate triple point and Weyl semimetal phase. We also consider the effect of external magnetic field, causing time reversal symmetry (TRS) breaking, and analyze the effect of TRS towards the realization of Weyl state. Importantly, in this material, the Fermi level lies extremely close to the nodal point with no extra Fermi pockets which further, make this compound as an ideal platform for topological study. The multi fold band degeneracies in these topological phases are analyzed based on point group representation theory. Topological insulating phase is further confirmed by calculating Z2 index. Furthermore, the topologically protected surface states and Fermi arcs are investigated in some detail.

cond-mat.mtrl-sci↗

arXiv2019

Co-existence of spin semi-metallic and Weyl semi-metallic behavior in FeRhCrGe

In this letter, we report the discovery of a new class of spintronic materials, namely spin semi-metals (SSM), employing both theoretical and experimental tools. The band structure of this class of materials is such that one of the spin bands resembles that of a semi-metal, while the other is similar to that of an insulator/semiconductor. This report is the experimental verification of the first SSM, FeRhCrGe, a quaternary Heusler alloy with a magnetic moment 3 $μ_B$ and a Curie temperature of 550 K. The measurement below 300 K shows nearly temperature independent conductivity and a relatively moderate Hall effect. SSM behavior for FeRhCrGe is also confirmed by rigorous first principles calculations. Band structure calculations also reveal that the spin up (semi metallic) band has combined features of type II Weyl and nodal line semimetal. As such, this study opens up the possibility of a new class of material with combined spintronic and topological properties, which is important both from fundamental and applied point of view.

cond-mat.mtrl-sci↗

arXiv2019

Revealing the nature of antiferro-quadrupolar ordering in Cerium Hexaboride: CeB$_6$

Cerium-hexaboride (CeB$_6$) f-electron compound displays a rich array of low-temperature magnetic phenomena, including `magnetically hidden' order, identified as multipolar in origin via advanced x-ray scattering. From first-principles electronic-structure results, we find that the \emph{antiferro-quadrupolar} (AFQ) ordering in CeB$_{6}$ arises from crystal-field splitting and yields band structure in agreement with experiments. With interactions of $p$-electrons between Ce and B$_{6}$ being small, the electronic state of CeB$_{6}$ is suitably described as Ce(4$f^{1}$)$^{3+}$(e$^{-}$)(B$_{6}$)$^{2-}$. The AFQ state of orbital spins is caused by an exchange interaction induced through spin-orbit interaction, which also splits J=5/2 state into $Γ_{8}$ ground state and $Γ_{7}$ excited state. Within the smallest antiferromagnetic (111) configuration, an orbital-ordered AFQ state appears during charge self-consistency, and supports the appearance of `hidden' order. Hydrostatic pressure (either applied or chemically induced) stabilizes the AFM (AFQ) states over a ferromagnetic one, as observed at low temperatures.

cond-mat.mtrl-sci↗

arXiv2018

Spin Gapless Semiconducting Nature in Co-rich Co1+xFe1-xCrGa: Insight and Advancements

In this report, we present structural, electronic, magnetic and transport properties of Co-rich spin gapless semiconductor CoFeCrGa using both theoretical and experimental techniques. The key advantage of Co-rich samples $\mathrm{Co_{1+x}Fe_{1-x}CrGa}$ is the high Curie temperature (T$\mathrm{_C}$) and magnetization, without compromising the SGS nature (up to x = 0.4), and hence our choice. The quaternary Heusler alloys $\mathrm{Co_{1+x}Fe_{1-x}CrGa}$ (x = 0.1 to 0.5) are found to crystallize in LiMgPdSn-type structure having space group $F\bar{4}3m$ (\# 216). The measured Curie temperature increases from 690 K (x = 0) to 870 K (x = 0.5). Observed magnetization values follow the Slater-Pauling rule. Measured electrical resistivity, in the temperature range of 5-350 K, suggests that the alloys retain the SGS behavior up to x = 0.4, beyond which it reflects metallic character. Unlike conventional semiconductors, the conductivity value ($\mathrm{σ_{xx}}$) at 300 K lies in the range of 2289 S $\mathrm{cm^{-1}}$ to 3294 S $\mathrm{cm^{-1}}$, which is close to that of other reported SGS materials. The anomalous Hall effect is comparatively low. The intrinsic contribution to the anomalous Hall conductivity increase with x, which can be correlated with the enhancement in chemical order. The anomalous Hall coefficient is found to increase from 38 S/cm for x = 0.1 to 43 S/cm for 0.3. Seebeck coefficients turn out to be vanishingly small below 300 K, another signature for being SGS. All the alloys (for different x) are found to be both chemically and thermally stable. Simulated magnetization agrees fairly with the experiment. As such Co-rich CoFeCrGa is a promising candidate for room temperature spintronic applications, with enhanced T$\mathrm{_C}$, magnetic properties and SGS nature.

cond-mat.mtrl-sci↗

arXiv2018

Quaternary Heusler Alloy: An Ideal Platform to Realize Triple Point Fermion

The existence of three fold rotational, mirror and time reversal symmetries often give rise to the triply degenerate nodal point (TP) in the band structure of a material. Based on point group symmetry analysis and first principle electronic structure, we predict, in this article, a series of quaternary Heusler alloys host an ideal platform for the occurrence of TP. We simulated, the projection of these TPs onto the (111) and (100) surfaces lead to form topological Fermi arcs, which may further be detected by scanning tunneling spectroscopy and angle resolved photoemission spectroscopy. These Fermi arcs arise due to the symmetry protected band degeneracies, which are robust and can not be avoided due to the non-trivial band topology. Interestingly the TPs, in these class of Heusler alloys are far away from the $Γ$ point along C$_3$ axes, which allow to overcome the experimental difficulties over previously studied hexagonal and HgTe-type compounds.

cond-mat.mtrl-sci↗

arXiv2018

Topological Nontrivial Phase in Hexagonal Antiperovskites A3BiB (A=Ba,Sr; B=P,N)

In this article, we predict the occurrence of topological non-trivial phase in hexagonal antiperovskite systems. By carefully investigating the evolution of band structure, we have studied the pressure induced topological phase transition in Ba$_{3}$BiP, Ba$_{3}$BiN, and Sr$_{3}$BiN compounds using the hybrid functional calculations. The non-trivial topology has been verified by computing Dirac like surface dispersion and topological invariant {\it Z$_2$} index via parity analysis. The unconventional spin texture has been analyzed which guaranteed the absence of impurity induced backscattering on boundary of the sample while respecting the time reversal symmetry. Our simulation confirms the chemical and mechanical stability of all the three compounds. The present study introduces an important new class of hexagonal antiperovskite compounds in the topological regime and are believed to capture ample of attention both from theoretical as well as experimental front.

cond-mat.mtrl-sci↗

arXiv2018

Double Perovskites overtaking the single perovskites : A set of new solar harvesting materials with much higher stability and efficiency

Hybrid Lead halide perovskites, despite having unique intrinsic properties with the possibility of flexible synthesis and device fabrication, still suffer from two fundamental issues, i.e. stability in external environment and toxicity due to lead. More recently, double perovskite materials have emerged as a promising choice. The main outcome from various studies on this class can essentially be summarized into two categories, (i) either they have indirect band gap or (ii) direct but large optical band gap, which are not suitable for solar devices. Here we propose a combinatorial set of stable double perovskite materials, Cs$_2$BB$^{'}$X$_6$ (for various B, B$^{'}$) (X=Cl,Br,I), which show indirect to direct band gap transition via small Pb$^{+2}$ doping. This kind of doping has helped to change the topology of band structure triggering an optically allowed transition from valence band maxima to conduction band minima. It also reduces the band gap significantly, bringing it well in the visible region. Simulation reveals comparable/higher absorption coefficient and solar efficiency with respect to the state of the art photovoltaic absorber material CH$_3$NH$_3$PbI$_3$. Our experimentally measured properties on Cs$_2$(Ag$_{0.75}$Pb$_{0.25}$)(Bi$_{0.75}$Pb$_{0.25}$)Br$_6$ agrees fairly well with the theoretical predictions. With higher stability than CH$_3$NH$_3$PbI$_3$, this material shows the potential to be a better candidate.

cond-mat.mtrl-sci↗

arXiv2018

$\mathrm{Co_2Fe_{1-x}Cr_xSi}$ Heusler Alloys : A promising material for spintronics application

In this article, we investigated the effect of Cr substitution in place of Fe on the structural, magnetic and transport properties of $\mathrm{Co_2FeSi}$ alloy. A comprehensive structural analysis is done using X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy. Quaternary Heusler compounds $\mathrm{Co_2Fe_{1-x}Cr_xSi}$ with Cr content (x = 0.1, 0.3, 0.5) were found to crystallize in cubic structure. The synchrotron based EXAFS studies reveal that the anti-site disorder increases with the increase in Cr concentration. The saturation magnetization values in all the alloys are found to be less than those expected from the Slater-Pauling rule, which may be due to the some inherent disorder. A detailed resistivity analysis in the temperature range of 5-300 K is done, taking into account different scattering mechanisms. The residual resistivity ratio is found to decrease with increasing Cr concentration. A disorder induced resistivity minimum due to weak localization effect is seen for x = 0.5. The resistivity measurements also indicate that the half-metallic character survives upto 100 K for x = 0.1, whereas the alloys with x= 0.3 and 0.5 show signature of half- metallic nature even at higher temperatures. First principles calculation done with a more robust exchange correlation functional (namely HSE-06) confirms the half metallicity in the entire concentration range. Theoretically simulated band gap and magnetic moments compliment the experimental findings and are compared wherever possible. All these properties make $\mathrm{Co_2Fe_{1-x}Cr_xSi}$ a promising material for spintronics application.

cond-mat.mtrl-sci↗

arXiv2018

Emergence of Topological insulator and Nodal line semi-metal states in XX'Bi (X=Na, K, Rb, Cs; X'=Ca, Sr)

In this letter, we predict the emergence of non-trivial band topology in the family of XX'Bi compounds having $P\overline{6}2m$ (\# 189) space group. Using first principles calculations within hybrid functional framework, we demonstrate that NaSrBi and NaCaBi are strong topological insulator under controlled band engineering. Here, we propose three different ways to engineer the band topology to get a non-trivial order: (i) hydrostatic pressure, (ii) biaxial strain (due to epitaxial mismatch), and (iii) doping. Non-triviality is confirmed by investigating bulk band inversion, topological Z$_2$ invariant, surface dispersion and spin texture. Interestingly, some of these compounds also show a three dimensional topological nodal line semi-metal (NLS) state in the absence of spin orbit coupling (SOC). In these NLS phases, the closed loop of band degeneracy in the Brillouin zone lie close to the Fermi level. Moreover, a drumhead like flat surface state is observed on projecting the bulk state on the [001] surface. The inclusion of SOC opens up a small band gap making them behave like a topological insulator.

cond-mat.mtrl-sci↗

arXiv2017

Pharmacokinetics and Molecular Docking studies of Plant-Derived Natural Compounds to Exploring Potential Anti-Alzheimer Activity

Alzheimer disease (AD) is the leading cause of dementia, accounts for 60 to 80 percent cases. Two main factors called beta amyloid plaques and tangles are prime suspects in damaging and killing nerve cells. However, oxidative stress, the process which produces free radicals in cells, is believed to promote its progression to the extent that it may responsible for the cognitive and functional decline observed in AD. As of today there are few FDA approved drugs in the market for treatment, but their cholinergic adverse effect, potentially distressing toxicity and limited targets in AD pathology limits their use. Therefore, it is crucial to find an effective compounds to combat AD. We choose 45 plant derived natural compounds that have antioxidant properties to slow down disease progression by quenching free redicals or promoting endogenous antioxidant capacity. However, we performed molecular docking studies to investigate the binding interactions between natural compounds and 13 various anti Alzheimer drug targets. Three known Cholinesterase inhibitors (Donepezil, Galantamine and Rivastigmine) were taken as reference drugs over natural compounds for comparison and drug likeness studies. Few of these compounds showed good inhibitory activity besides anti oxidant activity. Most of these compounds followed pharmacokinetics properties that make them potentially promising drug candidates for the treatment of Alzheimer disease.

q-bio.QM↗

arXiv2017

Competing magnetic and spin gap-less semiconducting behaviour in fully compensated ferrimagnet CrVTiAl: Theory and Experiment

We report the structural, magnetic and transport properties of polycrystalline CrVTiAl alloy along with first principles calculations. It crystallizes in the LiMgPdSn type structure with lattice parameter 6.14 Å at room temperature. Absence of (111) peak along with the presence of a weak (200) peak indicates the antisite disorder of Al with Cr and V atoms. The magnetization measurements reveal a ferrimagnetic transition near 710 K and a coercive field of 100 Oe at 3 K. Very low moment and coercive field indicate fully compensated ferrimagnetism in the alloy. Temperature coefficient of resistivity is found to be negative, indicating a characteristic of semiconducting nature. Absence of exponential dependence of resistivity on temperature indicates a gapless/spin-gapless semiconducting behaviour. Electronic and magnetic properties of CrVTiAl for three possible crystallograpic configurations are studied theoretically. All the three configurations are found to be different forms of semiconductors. Ground state configuration is a fully compensated ferrimagnet with band gaps 0.58 eV and 0.30 eV for up and down spin bands respectively. The next higher energy configuration is also ferrimagnetic, but has spin-gapless semiconducting nature. The highest energy configuration corresponds to a non-magnetic gapless semiconductor. The energy differences among these configurations are quite small ($<$ 1 $\mathrm{mRy/atom}$) which hints that at finite temperatures, the alloy exists in a disordered phase, which is a mixture of the three configurations. By taking into account the theoretical and the experimental findings, we conclude that CrVTiAl is a fully compensated ferrimagnet with predominantly spin gap-less semiconductor nature.

cond-mat.mtrl-sci↗

arXiv2017

Anomalous random correlations of force constants on the lattice dynamical properties of disordered Au-Fe alloys

Au-Fe alloys are of immense interest due to their biocompatibility, anomalous hall conductivity, and applications in various medical treatment. However, irrespective of the method of preparation, they often exhibit a high-level of disorder, with properties sensitive to the thermal or magnetic annealing temperatures. We calculate lattice dynamical properties of Au$_{1-x}$Fe$_x$ alloys using density functional theory methods, where, being a multisite property, reliable interatomic force constant (IFC) calculations in disordered alloys remain a challenge. We follow a two fold approach: (1) an accurate IFC calculation in an environment with nominally zero chemical pair correlations to mimic the homogeneously disordered alloy; and (2) a configurational averaging for the desired phonon properties (e.g., dispersion, density of states, and entropy). We find an anomalous change in the IFC's and phonon dispersion (split bands) near $x$=0.19, which is attributed to the local stiffening of the Au-Au bonds when Au is in the vicinity of Fe. Other results based on mechanical and thermo-physical properties reflect a similar anomaly: Phonon entropy, e.g., becomes negative below $x$=0.19, suggesting a tendency for chemical unmixing, reflecting the onset of miscibility gap in the phase diagram. Our results match fairly well with reported data, wherever available.

cond-mat.mtrl-sci↗