Aftab Alam | IIT Bombay (original) (raw)

Papers by Aftab Alam

Research paper thumbnail of Reply to Comments on "Accelerated discovery of new 8-Electron Half-Heusler Compounds as Promising Energy and Topological Quantum Materials

The Journal of Physical Chemistry C

Research paper thumbnail of Effects of Intrinsic Defects on Topological Insulator Behavior: Theory and Experiments on Ternary Tetradymite Compounds

Ternary tetradymites Bi$_{2}$Te$_{x}$Se$_{(3-x)}$ are predicted to be topological insulators via ... more Ternary tetradymites Bi$_{2}$Te$_{x}$Se$_{(3-x)}$ are predicted to be topological insulators via density functional theory (DFT) surface band structure calculations. Experimentally, we find that Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ form a continuous solid solution at the two ...

Research paper thumbnail of An augmented space approach to the study of phonons in disordered alloys : comparison between the itinerant coherent-potential approximation and the augmented space recursion

A first principles density functional based linear response theory (the so called Density Functio... more A first principles density functional based linear response theory (the so called Density Functional Perturbation theory [1]) has been combined separately with two recently developed formalism for a systematic study of the lattice dynamics in disordered binary alloys. The two formalisms are the Augmented space recursion (ASR) [2] and the Itinerant coherent potential approximation (ICPA)[3]. The two different theories (DFPT-ASR and DFPT-ICPA) systematically provides a hierarchy of improvements upon the earlier single site based theories (like CPA etc.) and includes non-local correlations in the disorder configurations. The formalisms explicitly take into account fluctuations in masses, force constants and scattering lengths. The combination of DFPT with these formulation helps in understanding the actual interplay of force constants in alloys. We illustrate the methods by applying to a fcc Fe50Pd50 alloy.

Research paper thumbnail of Phonon modes and vibrational entropy of disordered alloys with short-range order: A first-principles calculation

Phys Rev B, 2011

There has been increasing evidence about the effects of short-range order (or local chemical envi... more There has been increasing evidence about the effects of short-range order (or local chemical environment effects) on the lattice dynamics of alloys, which eventually affect the vibrational entropy difference among various phases of a compound, and hence their relative stability. In this ...

Research paper thumbnail of Electronic structure, magnetism, and antisite disorder in CoFeCrGe and CoMnCrAl quaternary Heusler alloys

Physical Review B, 2015

We present a combined theoretical and experimental study of two quaternary Heusler alloys CoFeCrG... more We present a combined theoretical and experimental study of two quaternary Heusler alloys CoFeCrGe (CFCG) and CoMnCrAl (CMCA), promising candidates for spintronics applications. Magnetization measurement shows the saturation magnetization and transition temperature to be 3 µB, 866 K and 0.9 µB, 358 K for CFCG and CMCA respectively. The magnetization values agree fairly well with our theoretical results and also obey the Slater-Pauling rule, a prerequisite for half metallicity. A striking difference between the two systems is their structure; CFCG crystallizes in fully ordered Y-type structure while CMCA has L21 disordered structure. The antisite disorder adds a somewhat unique property to the second compound, which arises due to the probabilistic mutual exchange of Al positions with Cr/Mn and such an effect is possibly expected due to comparable electronegativities of Al and Cr/Mn. Ab-initio simulation predicted a unique transition from half metallic ferromagnet to metallic antiferromagnet beyond a critical excess concentration of Al in the alloy.

Research paper thumbnail of All-electron KKR Calculations for Metallic Systems with Thousands of Atoms Per Cell via Sparse Matrix Iterative Solvers

Research paper thumbnail of Optimized basis-set representation for electronic-structure methods: Better Energetics

Research paper thumbnail of Origin of spin gapless semiconductor behavior in CoFeCrGa: Theory and Experiment

Physical Review B, 2015

L. Bainsla and A. I. Mallick contributed equally to this work. Despite a plethora of materials su... more L. Bainsla and A. I. Mallick contributed equally to this work. Despite a plethora of materials suggested for spintronic applications, a new class of materials has emerged, namely spin gapless semiconductors (SGS), that offers potentially more advantageous properties than existing ones. These magnetic semiconductors exhibit a finite band gap for one spin channel and a closed gap for the other. Here, supported by the firstprinciples, electronic-structure calculations, we report the first experimental evidence of SGS behavior in equiatomic quaternary CoFeCrGa, having a cubic Heusler (L2 1) structure but exhibiting chemical disorder (DO 3 structure). CoFeCrGa is found to transform from SGS to half-metallic phase under pressure, which is attributed to unique electronic-structure features. The saturation magnetization (M S) obtained at 8 K agrees with the Slater-Pauling rule and the Curie temperature (T C) is found to exceed 400 K. Carrier concentration (up to 250 K) and electrical conductivity are observed to be nearly temperature independent, prerequisites for SGS. The anomalous Hall coefficient is estimated to be 185 S/cm at 5 K. Considering the SGS properties and high T C

Research paper thumbnail of Fermi surfaces and phase stability of Ba(<span class="aps-inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mi mathvariant="normal">Fe</mi><mrow><mn>1</mn><mo>−</mo><mi>x</mi></mrow></msub><msub><mi>M</mi><mi>x</mi></msub><msub><mrow><mo>)</mo></mrow><mn>2</mn></msub><...

Physical Review B, 2014

BaFe2As2 with transition-metal doping exhibits a variety of rich phenomenon from coupling of stru... more BaFe2As2 with transition-metal doping exhibits a variety of rich phenomenon from coupling of structure, magnetism, and superconductivity. Using density functional theory, we systematically compare the Fermi surfaces (FS), formation energies (∆E f), and density of states (DOS) of electrondoped Ba(Fe1−xMx)2As2 with M={Co, Ni, Cu, Zn} in tetragonal (I4/mmm) and orthorhombic (Fmmm) structures in nonmagnetic (NM), antiferromagnetic (AFM), and paramagnetic (PM, disordered local moment) states. We explain changes to phase stability (∆E f) and Fermi surfaces (and nesting) due to chemical and magnetic disorder, and compare to observed/assessed properties and contrast alloy theory with that expected from rigid-band model. With alloying, the DOS changes from common-band (Co,Ni) to split-band (Cu,Zn), which dictates ∆E f and can overwhelm FS-nesting instabilities, as for Cu,Zn cases.

Research paper thumbnail of Green's function multiple-scattering theory with a truncated basis set: An augmented-KKR formalism

Physical Review B, 2014

Korringa-Kohn-Rostoker (KKR) Green's function, multiple-scattering theory is an efficient sitecen... more Korringa-Kohn-Rostoker (KKR) Green's function, multiple-scattering theory is an efficient sitecentered, electronic-structure technique for addressing an assembly of N scatterers. Wave-functions are expanded in a spherical-wave basis on each scattering center and indexed up to a maximum orbital and azimuthal number Lmax = (l, m)max, while scattering matrices, which determine spectral properties, are truncated at Ltr = (l, m)tr where phase shifts δ l>l tr are negligible. Historically, Lmax is set equal to Ltr, which is correct for large enough Lmax but not computationally expedient; a better procedure retains higher-order (free-electron and single-site) contributions for Lmax > Ltr with δ l>l tr set to zero [Zhang and Butler, Phys. Rev. B 46, 7433]. We present a numerically efficient and accurate augmented-KKR Green's function formalism that solves the KKR equations by exact matrix inversion [R 3 process with rank N (ltr + 1) 2 ] and includes higher-L contributions via linear algebra [R 2 process with rank N (lmax + 1) 2 ]. Augmented-KKR approach yields properly normalized wave-functions, numerically cheaper basis-set convergence, and a total charge density and electron count that agrees with Lloyd's formula. We apply our formalism to fcc Cu, bcc Fe and L10 CoPt, and present the numerical results for accuracy and for the convergence of the total energies, Fermi energies, and magnetic moments versus Lmax for a given Ltr.

Research paper thumbnail of Mixed valency and site-preference chemistry for cerium and its compounds: A predictive density-functional theory study

Physical Review B, 2014

Cerium and its technologically relevant compounds are examples of anomalous mixed valency, origin... more Cerium and its technologically relevant compounds are examples of anomalous mixed valency, originating from two competing oxidation states-itinerant Ce 3+ and localized Ce 4+. Under applied stress, anomalous transitions are observed but not well understood. Here we treat mixed valency as an "alloy" involving two valences with competing and numerous site-occupancy configurations, and we use density functional theory with Hubbard U (i.e., DFT+U) to evaluate the effective valence and predict properties, including controlling valence by pseudo-ternary alloying. For Ce and its compounds, such as (Ce-La)2(Fe-Co)14B permanent magnets, we find a stable mixed-valent α-state near the spectroscopic value of νs = 3.53. Ce valency in compounds depends on its steric volume and local chemistry; for La doping, Ce-valency shifts towards γ-like Ce 3+ , as expected from steric volume; for Co doping, valency depends on local Ce-site chemistry and steric volume. Our approach captures the key origins of anomalous valency and site-preference chemistry in complex compounds.

Research paper thumbnail of Origin for the disorder-induced quantum criticality in NbFe $ _2$

Bulletin of the American Physical Society, 2011

... Aftab Alam (Division of Materials Science and Engineering, Ames Laboratory, Ames, Iowa). ... ... more ... Aftab Alam (Division of Materials Science and Engineering, Ames Laboratory, Ames, Iowa). ... uBCP) with vanishing quasi-particle velocity [2]. Moving off-stoichiometry by increasing Nb, or reducing electrons (e/a), we find the Fermi level E$_f$ increases (rather than decreases ...

Research paper thumbnail of Reply to “Comment on ‘Accurate and fast numerical solution of Poisson's equation for arbitrary, space-filling Voronoi polyhedra: Near-field corrections revisited’ ”

Research paper thumbnail of Phonons in disordered alloys: Comparison between augmented-space-based approximations for configuration averaging to integration from first principles

Physical Review B, 2007

A first principles density functional based linear response theory (the so called Density Functio... more A first principles density functional based linear response theory (the so called Density Functional Perturbation theory [1]) has been combined separately with two recently developed formalism for a systematic study of the lattice dynamics in disordered binary alloys. The two formalisms are the Augmented space recursion (ASR) [2] and the Itinerant coherent potential approximation (ICPA)[3]. The two different theories (DFPT-ASR and DFPT-ICPA) systematically provides a hierarchy of improvements upon the earlier single site based theories (like CPA etc.) and includes non-local correlations in the disorder configurations. The formalisms explicitly take into account fluctuations in masses, force constants and scattering lengths. The combination of DFPT with these formulation helps in understanding the actual interplay of force constants in alloys. We illustrate the methods by applying to a fcc Fe50Pd50 alloy.

Research paper thumbnail of Vibrational properties of phonons in random binary alloys: An augmented space recursive technique in the k representation

Physical Review B, 2004

We present here an augmented space recursive technique in the k-representation which include diag... more We present here an augmented space recursive technique in the k-representation which include diagonal, off-diagonal and the environmental disorder explicitly : an analytic , translationally invariant , multiple scattering theory for phonons in random binary alloys. We propose the augmented space recursion (ASR) as a computationally fast and accurate technique which will incorporate configuration fluctuations over a large local environment. We apply the formalism to N i55P d45 , N i88Cr12 and N i50P t50 alloys which is not a random choice. Numerical results on spectral functions, coherent structure factors, dispersion curves and disordered induced FWHM's are presented. Finally the results are compared with the recent itinerant coherent potential approximation (ICPA) and also with experiments.

Research paper thumbnail of Phonon modes and vibrational entropy of disordered alloys with short-range order: A first-principles calculation

Physical Review B, 2011

There has been increasing evidence about the effects of short-range order (or local chemical envi... more There has been increasing evidence about the effects of short-range order (or local chemical environment effects) on the lattice dynamics of alloys, which eventually affect the vibrational entropy difference among various phases of a compound, and hence their relative stability. In this ...

Research paper thumbnail of Native defects in tetradymite Bi<span class="aps-inline-formula"><math><msub><mrow></mrow><mn>2</mn></msub></math></span>(Te<span class="aps-inline-formula"><math><msub><mrow></mrow><mi>x</mi></msub></math></span>Se<span class="aps-inline-formula"><math><msub><mrow></mrow><mrow><mn>3</mn><mo>−</m...

Physical Review B, 2013

Formation energies of native defects in Bi 2 (Te x Se 3−x), with comparison to ideal Bi 2 Te 2 S,... more Formation energies of native defects in Bi 2 (Te x Se 3−x), with comparison to ideal Bi 2 Te 2 S, are calculated in density-functional theory to assess transport properties. Bi 2 Se 3 is found to be n type for both Bi-and Se-rich growth conditions, while Bi 2 Te 3 changes fromn to p type going from Te-to Bi-rich conditions, as observed. Bi 2 Te 2 Se and Bi 2 Te 2 S are generally n type, explaining observed heavily doped n-type behavior in most samples. A (0/−) transition level at 16 meV above valence-band maximum for Bi on Te antisites in Bi 2 Te 2 Se is related to the observed thermally active transport gap causing a p-ton transition at low temperature. Bi 2 (Te x Se 3−x) with x>2 are predicted to have high bulk resistivity due to effective carrier compensation when approaching the n-to-pcrossover. Predicted behaviors are confirmed from characterization of our grown single crystals.

Research paper thumbnail of Inelastic neutron scattering in random binary alloys: An augmented space approach

Physical Review B, 2005

Combining the augmented space representation for phonons with a generalized version of Yonezawa-M... more Combining the augmented space representation for phonons with a generalized version of Yonezawa-Matsubara diagrammatic technique, we have set up a formalism to seperate the coherent and incoherent part of the total intensity of thermal neutron scattering from disordered alloys. This is done exacly without taking any recourse to mean-field like approximation (as done previously). The formalism includes disorder in masses, force constants and scattering lengths. Implementation of the formalism to realistic situations is performed by an augmented space Block recursion which calculates entire Green matrix and self energy matrix which in turn is needed to evaluate the coherent and incoherent intensities. we apply the formalism to N i55P d45 and N i50P t50 alloys. Numerical results on coherent and incoherent scattering cross sections are presented along the highest symmetry directions. Finally the incoherent intensities are compared with the CPA and also with experiments.

Research paper thumbnail of Interplay of force constants in the lattice dynamics of disordered alloys: An <i>ab initio</i> study

Physical Review B, 2014

A reliable prediction of interatomic force constants in disordered alloys is an outstanding probl... more A reliable prediction of interatomic force constants in disordered alloys is an outstanding problem. This is due to the need for a proper treatment of multisite (at least pair) correlation within a random environment. The situation becomes even more challenging for systems with large difference in atomic size and mass. We propose a systematic density functional theory (DFT) based study to predict the ab-initio force constants in random alloys. The method is based on a marriage between special quasirandom structures (SQS) and the augmented space recursion (ASR) to calculate phonon spectra, density of states (DOS) etc. bcc TaW and fcc NiPt alloys are considered as the two distinct test cases. Ta-Ta (W-W) bond distance in the alloy is predicted to be smaller (larger) than those in pure Ta (W), which, in turn, yields stiffer (softer) force constants for Ta (W). Pt-Pt force constants in the alloy, however, are predicted to be softer compared to Ni-Ni, due to large bond distance of the former. Our calculated force constants, phonon spectra and DOS are compared with experiments and other theoretical results, wherever available. Correct trend of present results for the two alloys pave a path for future studies in more complex alloy systems.

Research paper thumbnail of Structural properties and relative stability of (meta)stable ordered, partially ordered, and disordered Al-Li alloy phases

Physical Review B, 2012

We resolve issues that have plagued reliable prediction of relative phase stability for solid sol... more We resolve issues that have plagued reliable prediction of relative phase stability for solid solutions and compounds. Due to its commercially important phase diagram, we showcase the Al-Li system because historically density-functional theory (DFT) results show large scatter and limited success in predicting the structural properties and stability of solid solutions relative to ordered compounds. Using recent advances in an optimal basis-set representation of the topology of electronic charge density (and, hence, atomic size), we present DFT results that agree reasonably well with all known experimental data for the structural properties and formation energies of ordered, off-stoichiometric partially ordered, and disordered alloys, opening the way for reliable study in complex alloys.

Research paper thumbnail of Reply to Comments on "Accelerated discovery of new 8-Electron Half-Heusler Compounds as Promising Energy and Topological Quantum Materials

The Journal of Physical Chemistry C

Research paper thumbnail of Effects of Intrinsic Defects on Topological Insulator Behavior: Theory and Experiments on Ternary Tetradymite Compounds

Ternary tetradymites Bi$_{2}$Te$_{x}$Se$_{(3-x)}$ are predicted to be topological insulators via ... more Ternary tetradymites Bi$_{2}$Te$_{x}$Se$_{(3-x)}$ are predicted to be topological insulators via density functional theory (DFT) surface band structure calculations. Experimentally, we find that Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ form a continuous solid solution at the two ...

Research paper thumbnail of An augmented space approach to the study of phonons in disordered alloys : comparison between the itinerant coherent-potential approximation and the augmented space recursion

A first principles density functional based linear response theory (the so called Density Functio... more A first principles density functional based linear response theory (the so called Density Functional Perturbation theory [1]) has been combined separately with two recently developed formalism for a systematic study of the lattice dynamics in disordered binary alloys. The two formalisms are the Augmented space recursion (ASR) [2] and the Itinerant coherent potential approximation (ICPA)[3]. The two different theories (DFPT-ASR and DFPT-ICPA) systematically provides a hierarchy of improvements upon the earlier single site based theories (like CPA etc.) and includes non-local correlations in the disorder configurations. The formalisms explicitly take into account fluctuations in masses, force constants and scattering lengths. The combination of DFPT with these formulation helps in understanding the actual interplay of force constants in alloys. We illustrate the methods by applying to a fcc Fe50Pd50 alloy.

Research paper thumbnail of Phonon modes and vibrational entropy of disordered alloys with short-range order: A first-principles calculation

Phys Rev B, 2011

There has been increasing evidence about the effects of short-range order (or local chemical envi... more There has been increasing evidence about the effects of short-range order (or local chemical environment effects) on the lattice dynamics of alloys, which eventually affect the vibrational entropy difference among various phases of a compound, and hence their relative stability. In this ...

Research paper thumbnail of Electronic structure, magnetism, and antisite disorder in CoFeCrGe and CoMnCrAl quaternary Heusler alloys

Physical Review B, 2015

We present a combined theoretical and experimental study of two quaternary Heusler alloys CoFeCrG... more We present a combined theoretical and experimental study of two quaternary Heusler alloys CoFeCrGe (CFCG) and CoMnCrAl (CMCA), promising candidates for spintronics applications. Magnetization measurement shows the saturation magnetization and transition temperature to be 3 µB, 866 K and 0.9 µB, 358 K for CFCG and CMCA respectively. The magnetization values agree fairly well with our theoretical results and also obey the Slater-Pauling rule, a prerequisite for half metallicity. A striking difference between the two systems is their structure; CFCG crystallizes in fully ordered Y-type structure while CMCA has L21 disordered structure. The antisite disorder adds a somewhat unique property to the second compound, which arises due to the probabilistic mutual exchange of Al positions with Cr/Mn and such an effect is possibly expected due to comparable electronegativities of Al and Cr/Mn. Ab-initio simulation predicted a unique transition from half metallic ferromagnet to metallic antiferromagnet beyond a critical excess concentration of Al in the alloy.

Research paper thumbnail of All-electron KKR Calculations for Metallic Systems with Thousands of Atoms Per Cell via Sparse Matrix Iterative Solvers

Research paper thumbnail of Optimized basis-set representation for electronic-structure methods: Better Energetics

Research paper thumbnail of Origin of spin gapless semiconductor behavior in CoFeCrGa: Theory and Experiment

Physical Review B, 2015

L. Bainsla and A. I. Mallick contributed equally to this work. Despite a plethora of materials su... more L. Bainsla and A. I. Mallick contributed equally to this work. Despite a plethora of materials suggested for spintronic applications, a new class of materials has emerged, namely spin gapless semiconductors (SGS), that offers potentially more advantageous properties than existing ones. These magnetic semiconductors exhibit a finite band gap for one spin channel and a closed gap for the other. Here, supported by the firstprinciples, electronic-structure calculations, we report the first experimental evidence of SGS behavior in equiatomic quaternary CoFeCrGa, having a cubic Heusler (L2 1) structure but exhibiting chemical disorder (DO 3 structure). CoFeCrGa is found to transform from SGS to half-metallic phase under pressure, which is attributed to unique electronic-structure features. The saturation magnetization (M S) obtained at 8 K agrees with the Slater-Pauling rule and the Curie temperature (T C) is found to exceed 400 K. Carrier concentration (up to 250 K) and electrical conductivity are observed to be nearly temperature independent, prerequisites for SGS. The anomalous Hall coefficient is estimated to be 185 S/cm at 5 K. Considering the SGS properties and high T C

Research paper thumbnail of Fermi surfaces and phase stability of Ba(<span class="aps-inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mi mathvariant="normal">Fe</mi><mrow><mn>1</mn><mo>−</mo><mi>x</mi></mrow></msub><msub><mi>M</mi><mi>x</mi></msub><msub><mrow><mo>)</mo></mrow><mn>2</mn></msub><...

Physical Review B, 2014

BaFe2As2 with transition-metal doping exhibits a variety of rich phenomenon from coupling of stru... more BaFe2As2 with transition-metal doping exhibits a variety of rich phenomenon from coupling of structure, magnetism, and superconductivity. Using density functional theory, we systematically compare the Fermi surfaces (FS), formation energies (∆E f), and density of states (DOS) of electrondoped Ba(Fe1−xMx)2As2 with M={Co, Ni, Cu, Zn} in tetragonal (I4/mmm) and orthorhombic (Fmmm) structures in nonmagnetic (NM), antiferromagnetic (AFM), and paramagnetic (PM, disordered local moment) states. We explain changes to phase stability (∆E f) and Fermi surfaces (and nesting) due to chemical and magnetic disorder, and compare to observed/assessed properties and contrast alloy theory with that expected from rigid-band model. With alloying, the DOS changes from common-band (Co,Ni) to split-band (Cu,Zn), which dictates ∆E f and can overwhelm FS-nesting instabilities, as for Cu,Zn cases.

Research paper thumbnail of Green's function multiple-scattering theory with a truncated basis set: An augmented-KKR formalism

Physical Review B, 2014

Korringa-Kohn-Rostoker (KKR) Green's function, multiple-scattering theory is an efficient sitecen... more Korringa-Kohn-Rostoker (KKR) Green's function, multiple-scattering theory is an efficient sitecentered, electronic-structure technique for addressing an assembly of N scatterers. Wave-functions are expanded in a spherical-wave basis on each scattering center and indexed up to a maximum orbital and azimuthal number Lmax = (l, m)max, while scattering matrices, which determine spectral properties, are truncated at Ltr = (l, m)tr where phase shifts δ l>l tr are negligible. Historically, Lmax is set equal to Ltr, which is correct for large enough Lmax but not computationally expedient; a better procedure retains higher-order (free-electron and single-site) contributions for Lmax > Ltr with δ l>l tr set to zero [Zhang and Butler, Phys. Rev. B 46, 7433]. We present a numerically efficient and accurate augmented-KKR Green's function formalism that solves the KKR equations by exact matrix inversion [R 3 process with rank N (ltr + 1) 2 ] and includes higher-L contributions via linear algebra [R 2 process with rank N (lmax + 1) 2 ]. Augmented-KKR approach yields properly normalized wave-functions, numerically cheaper basis-set convergence, and a total charge density and electron count that agrees with Lloyd's formula. We apply our formalism to fcc Cu, bcc Fe and L10 CoPt, and present the numerical results for accuracy and for the convergence of the total energies, Fermi energies, and magnetic moments versus Lmax for a given Ltr.

Research paper thumbnail of Mixed valency and site-preference chemistry for cerium and its compounds: A predictive density-functional theory study

Physical Review B, 2014

Cerium and its technologically relevant compounds are examples of anomalous mixed valency, origin... more Cerium and its technologically relevant compounds are examples of anomalous mixed valency, originating from two competing oxidation states-itinerant Ce 3+ and localized Ce 4+. Under applied stress, anomalous transitions are observed but not well understood. Here we treat mixed valency as an "alloy" involving two valences with competing and numerous site-occupancy configurations, and we use density functional theory with Hubbard U (i.e., DFT+U) to evaluate the effective valence and predict properties, including controlling valence by pseudo-ternary alloying. For Ce and its compounds, such as (Ce-La)2(Fe-Co)14B permanent magnets, we find a stable mixed-valent α-state near the spectroscopic value of νs = 3.53. Ce valency in compounds depends on its steric volume and local chemistry; for La doping, Ce-valency shifts towards γ-like Ce 3+ , as expected from steric volume; for Co doping, valency depends on local Ce-site chemistry and steric volume. Our approach captures the key origins of anomalous valency and site-preference chemistry in complex compounds.

Research paper thumbnail of Origin for the disorder-induced quantum criticality in NbFe $ _2$

Bulletin of the American Physical Society, 2011

... Aftab Alam (Division of Materials Science and Engineering, Ames Laboratory, Ames, Iowa). ... ... more ... Aftab Alam (Division of Materials Science and Engineering, Ames Laboratory, Ames, Iowa). ... uBCP) with vanishing quasi-particle velocity [2]. Moving off-stoichiometry by increasing Nb, or reducing electrons (e/a), we find the Fermi level E$_f$ increases (rather than decreases ...

Research paper thumbnail of Reply to “Comment on ‘Accurate and fast numerical solution of Poisson's equation for arbitrary, space-filling Voronoi polyhedra: Near-field corrections revisited’ ”

Research paper thumbnail of Phonons in disordered alloys: Comparison between augmented-space-based approximations for configuration averaging to integration from first principles

Physical Review B, 2007

A first principles density functional based linear response theory (the so called Density Functio... more A first principles density functional based linear response theory (the so called Density Functional Perturbation theory [1]) has been combined separately with two recently developed formalism for a systematic study of the lattice dynamics in disordered binary alloys. The two formalisms are the Augmented space recursion (ASR) [2] and the Itinerant coherent potential approximation (ICPA)[3]. The two different theories (DFPT-ASR and DFPT-ICPA) systematically provides a hierarchy of improvements upon the earlier single site based theories (like CPA etc.) and includes non-local correlations in the disorder configurations. The formalisms explicitly take into account fluctuations in masses, force constants and scattering lengths. The combination of DFPT with these formulation helps in understanding the actual interplay of force constants in alloys. We illustrate the methods by applying to a fcc Fe50Pd50 alloy.

Research paper thumbnail of Vibrational properties of phonons in random binary alloys: An augmented space recursive technique in the k representation

Physical Review B, 2004

We present here an augmented space recursive technique in the k-representation which include diag... more We present here an augmented space recursive technique in the k-representation which include diagonal, off-diagonal and the environmental disorder explicitly : an analytic , translationally invariant , multiple scattering theory for phonons in random binary alloys. We propose the augmented space recursion (ASR) as a computationally fast and accurate technique which will incorporate configuration fluctuations over a large local environment. We apply the formalism to N i55P d45 , N i88Cr12 and N i50P t50 alloys which is not a random choice. Numerical results on spectral functions, coherent structure factors, dispersion curves and disordered induced FWHM's are presented. Finally the results are compared with the recent itinerant coherent potential approximation (ICPA) and also with experiments.

Research paper thumbnail of Phonon modes and vibrational entropy of disordered alloys with short-range order: A first-principles calculation

Physical Review B, 2011

There has been increasing evidence about the effects of short-range order (or local chemical envi... more There has been increasing evidence about the effects of short-range order (or local chemical environment effects) on the lattice dynamics of alloys, which eventually affect the vibrational entropy difference among various phases of a compound, and hence their relative stability. In this ...

Research paper thumbnail of Native defects in tetradymite Bi<span class="aps-inline-formula"><math><msub><mrow></mrow><mn>2</mn></msub></math></span>(Te<span class="aps-inline-formula"><math><msub><mrow></mrow><mi>x</mi></msub></math></span>Se<span class="aps-inline-formula"><math><msub><mrow></mrow><mrow><mn>3</mn><mo>−</m...

Physical Review B, 2013

Formation energies of native defects in Bi 2 (Te x Se 3−x), with comparison to ideal Bi 2 Te 2 S,... more Formation energies of native defects in Bi 2 (Te x Se 3−x), with comparison to ideal Bi 2 Te 2 S, are calculated in density-functional theory to assess transport properties. Bi 2 Se 3 is found to be n type for both Bi-and Se-rich growth conditions, while Bi 2 Te 3 changes fromn to p type going from Te-to Bi-rich conditions, as observed. Bi 2 Te 2 Se and Bi 2 Te 2 S are generally n type, explaining observed heavily doped n-type behavior in most samples. A (0/−) transition level at 16 meV above valence-band maximum for Bi on Te antisites in Bi 2 Te 2 Se is related to the observed thermally active transport gap causing a p-ton transition at low temperature. Bi 2 (Te x Se 3−x) with x>2 are predicted to have high bulk resistivity due to effective carrier compensation when approaching the n-to-pcrossover. Predicted behaviors are confirmed from characterization of our grown single crystals.

Research paper thumbnail of Inelastic neutron scattering in random binary alloys: An augmented space approach

Physical Review B, 2005

Combining the augmented space representation for phonons with a generalized version of Yonezawa-M... more Combining the augmented space representation for phonons with a generalized version of Yonezawa-Matsubara diagrammatic technique, we have set up a formalism to seperate the coherent and incoherent part of the total intensity of thermal neutron scattering from disordered alloys. This is done exacly without taking any recourse to mean-field like approximation (as done previously). The formalism includes disorder in masses, force constants and scattering lengths. Implementation of the formalism to realistic situations is performed by an augmented space Block recursion which calculates entire Green matrix and self energy matrix which in turn is needed to evaluate the coherent and incoherent intensities. we apply the formalism to N i55P d45 and N i50P t50 alloys. Numerical results on coherent and incoherent scattering cross sections are presented along the highest symmetry directions. Finally the incoherent intensities are compared with the CPA and also with experiments.

Research paper thumbnail of Interplay of force constants in the lattice dynamics of disordered alloys: An <i>ab initio</i> study

Physical Review B, 2014

A reliable prediction of interatomic force constants in disordered alloys is an outstanding probl... more A reliable prediction of interatomic force constants in disordered alloys is an outstanding problem. This is due to the need for a proper treatment of multisite (at least pair) correlation within a random environment. The situation becomes even more challenging for systems with large difference in atomic size and mass. We propose a systematic density functional theory (DFT) based study to predict the ab-initio force constants in random alloys. The method is based on a marriage between special quasirandom structures (SQS) and the augmented space recursion (ASR) to calculate phonon spectra, density of states (DOS) etc. bcc TaW and fcc NiPt alloys are considered as the two distinct test cases. Ta-Ta (W-W) bond distance in the alloy is predicted to be smaller (larger) than those in pure Ta (W), which, in turn, yields stiffer (softer) force constants for Ta (W). Pt-Pt force constants in the alloy, however, are predicted to be softer compared to Ni-Ni, due to large bond distance of the former. Our calculated force constants, phonon spectra and DOS are compared with experiments and other theoretical results, wherever available. Correct trend of present results for the two alloys pave a path for future studies in more complex alloy systems.

Research paper thumbnail of Structural properties and relative stability of (meta)stable ordered, partially ordered, and disordered Al-Li alloy phases

Physical Review B, 2012

We resolve issues that have plagued reliable prediction of relative phase stability for solid sol... more We resolve issues that have plagued reliable prediction of relative phase stability for solid solutions and compounds. Due to its commercially important phase diagram, we showcase the Al-Li system because historically density-functional theory (DFT) results show large scatter and limited success in predicting the structural properties and stability of solid solutions relative to ordered compounds. Using recent advances in an optimal basis-set representation of the topology of electronic charge density (and, hence, atomic size), we present DFT results that agree reasonably well with all known experimental data for the structural properties and formation energies of ordered, off-stoichiometric partially ordered, and disordered alloys, opening the way for reliable study in complex alloys.