Augmented Space Recursion Research Articles

Electrical and Magnetic Properties of Rough Alloyed Interface of Fe/Co.

We investigated the electronic structure of rough interfaces in random Co/Fe  alloys, where cobalt (Co) was deposited on an iron (Fe) substrate with varying composition ratios. To model the rough surface, we utilized a coupled continuum equation as described by Huda. The electronic properties, specifically the density of states of these random alloys, were calculated using the augmented space recursion method in conjunction with the Linear Muffin-tin Orbital (LMTO) Method. Furthermore, we analyzed the magnetic and electronic properties derived from the calculated density of states.

Effect of disorder on the optical response of NiPt and Ni3Pt alloys

In this communication we present a detailed study of the effect of chemical disorder on the optical response of Ni1−xPtx (0.1⩽x⩽0.75) and Ni3(1−x)/3Ptx (0.1⩽x⩽0.3). We shall propose a formalism which will combine a Kubo-Greenwood approach with a DFT based tight-binding linear muffin-tin orbitals (TB-LMTO) basis and augmented space recursion (ASR) technique to explicitly incorporate the effect of disorder. We show that chemical disorder has a large impact on optical response of Ni-Pt systems. In ordered Ni-Pt alloys, the optical conductivity peaks are sharp. But as we switch on chemical disorder, the UV peak becomes broadened and its position as a function of composition and disorder carries the signature of a phase transition from NiPt to Ni3Pt with decreasing Pt concentration. Quantitatively this agrees well with Massalski’s Ni-Pt phase diagram (Massalski, 1990). Both ordered NiPt and Ni3Pt have an optical conductivity transition at 4.12eV. But disordered NiPt has an optical conductivity transition at 3.93eV. If we decrease the Pt content, it results a chemical phase transition from NiPt to Ni3Pt and shifts the peak position by 1.67eV to the ultraviolet range at 5.6eV. There is a significant broadening of UV peak with increasing Pt content due to enhancement of 3d(Ni)-5d(Pt) bonding. Chemical disorder enhances the optical response of NiPt alloys nearly one order of magnitude. Our study also shows the fragile magnetic effect on optical response of disordered Ni1−xPtx (0.4<x<0.6) binary alloys. Our theoretical predictions agree reasonably well with both previous experimental findings as well as theoretical investigations.

Band-gap tuning and optical response of two-dimensionalSixC1−x: A first-principles real-space study of disordered two-dimensional materials

We present a real-space formulation for calculating the electronic structure and optical conductivity of such random alloys based on the Kubo-Greenwood formalism interfaced with the augmented space recursion (ASR) [A. Mookerjee, J. Phys. C: Solid State Phys. {\bf 6}, 1340 (1973)] formulated with the Tight-binding Linear Muffin-tin Orbitals (TB-LMTO) basis with van Leeuwen-Baerends corrected exchange (vLB) [Singh et al, Phys. Rev B {\bf 93}, 085204, (2016)]. This approach has been used to quantitatively analyze the effect of chemical disorder on the configuration averaged electronic properties and optical response of 2D honeycomb siliphene Si$_{x}$C$_{1-x}$ beyond the usual Dirac-cone approximation. We predicted the quantitative effect of disorder on both the electronic-structure and optical response over a wide energy range, and the results discussed in the light of the available experimental and other theoretical data. Our proposed formalism may open up a facile way for planned band gap engineering in opto-electronic applications.

Interplay of force constants in the lattice dynamics of disordered alloys: Anab initiostudy

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 (atleast 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 a 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 further future studies in more complex alloy systems.

Electronic and magnetic properties of disordered AuCr alloys: A first-principles study

In this communication we have studied the electronic structure and magnetism in the disordered alloy AuCr, for compositions in which the Cr concentration is less than 30at%. The study is an application of a first principles density functional based augmented space recursion technique generalized to study non-collinear magnetic structures. We showed that, at low temperatures but compositions with <15at% Cr there is a spin-glass phase and the 1Q anti-ferromagnetic ordering stable above compositions of 15at% Cr. We used the Lichtenstein procedure to map the problem onto a random Ising model and studied the phase diagram using a formalism suggested by one of us earlier. Our results are in excellent agreement with existing experimental data.

Effect of disorder on the electronic properties of graphene: A theoretical approach

In order to manipulate the properties of graphene, it is very important to understand the electronic structure in the presence of disorder. We investigate, within a tight-binding description, the effects of disorder in the on-site (diagonal disorder) term in the Hamiltonian as well as in the hopping integral (off-diagonal disorder) on the electronic dispersion and density of states by the augmented space recursion method. Extrinsic off-diagonal disorder is shown to have dramatic effects on the two-dimensional (2D) Dirac cone, including asymmetries in the band structures as well as the presence of discontinuous bands (because of resonances) in certain limits. Disorder-induced broadening, related to the scattering length (or lifetime) of Bloch electrons, is modified significantly with increasing strength of disorder. We propose that our methodology is suitable for the study of the effects of disorder in other 2D materials, such as a boron nitride monolayer.

A study of magnetism in disordered Pt–Mn, Pd–Mn and Ni–Mn alloys: an augmented space recursion approach

In this paper we shall study three binary alloy systems, one constituent of which is Mn. The other constituents are chosen from a particular column of the periodic table: Ni(3d), Pt (4d) and Pd (5d). As we go down the column, the d-bands become wider, discouraging spin-polarization. In a disordered alloy, the situation becomes more complicated, as the exchange interaction between two atoms is environment dependent. We shall compare and contrast their magnetic behaviour using robust electronic structure techniques. In all three alloy systems conjectures are made to explain experimental data. In this paper we shall examine whether there is any basis to these conjectures.

Study of disorder–order transitions in Fe xAl 1− x binary alloys using the augmented space recursion based orbital peeling technique

In this communication we propose a method for the study of disorder–order transitions in Fe x Al 1− x binary alloys. We turn to our earlier development [1] of a combination of the recursion method introduced by Haydock et al. [2] and our augmented space approach [3] with the orbital peeling technique proposed by Burke [4] to determine the small energy differences required in obtaining the pair energies which go as input to the generalized perturbation technique [5] of studying disorder–order transitions.

Magnetic behaviour of AuFe and NiMo alloys

We study the electronic structure and a mean-field phase analysis based on the pair–pair energies derived from first-principles electronic structure calculations of AuFe and NiMo alloys. We have used the tight-binding linear muffin-tin orbitals-based augmented space recursion (TB-LMTO-ASR) method to do so. We investigate different behaviours of the two alloy systems by mapping the problems onto equivalent Ising models and then discuss the magnetic phase diagrams using the calculated pair energies. All three phases: paramagnetic, random ferromagnetic and spin glass, have been studied.

Magnetic phase diagram for CuMn

To study the magnetic phases of CuMn, we carry out a generalized perturbation expansion of the band energy across the composition range. We obtain the pair energy using the orbital peeling technique of Burke, based on the tight-binding linear muffin-tin orbital and augmented space recursion methods. We analyze the phases of the system by studying the probability distribution of the local staggered occupation ’fields’ within a mean-field approach. We show that our predicted phase boundaries agree well with experimental results.

Magnetism in FeNiW disordered alloys: Experiment and theory

We report here an experimental study of magnetization of FeNiW alloys at different compositions. We have studied variation of magnetization with temperature (at low external fields) and magnetic field (at low temperatures). The alloy shows para to ferromagnetic transitions across the composition range. We do not find any indication of the spin-glass phase. We have supplemented the experimental work with theoretical analysis using the first-principles tight-binding linear muffin-tin orbitals based augmented space recursion method. Our theoretical estimates of magnetic moment and Curie temperatures agree well with experiment. Our mean-field phase analysis also does not indicate the possibility of a spin-glass phase.

Ab initioelectronic structure calculation of disorder ternary alloys: A reciprocal-space formulation

Using first-principles density-functional calculations, we present a formalism for a reciprocal-space study of the electronic structure of random ternary alloys. The formalism is based on the augmented-space recursion introduced earlier by us in conjunction with the tight-binding linearized muffin-tin orbital method. Emphasis shall be given to the configurationally averaged Bloch spectral function, which will be a key quantity of our formalism and the reflection of its nature on the density of states will be discussed. We showcase the feasibility of our formalism by applying to two different alloy systems namely: face-centered-cubic based ${\text{Cu}}_{1\ensuremath{-}x\ensuremath{-}y}{\text{Ni}}_{x}{\text{Zn}}_{y}$ alloy and the series of disordered ternary Invar alloys, ${\text{Fe}}_{1\ensuremath{-}x\ensuremath{-}y}{\text{Ni}}_{x}{X}_{y}$ $[X=\text{Co},\text{Pd},\text{Pt}]$. The effects of short-range ordering and the magnetic properties of these alloys will be discussed in some detail and it will be shown that our predicted magnetic moment agrees well with earlier results.

Ab initioaugmented space recursion to study complex multicomponent materials: Application to the pseudobinary alloyNi1−xPtxAl

Using first-principles density-functional calculations, we studied the electronic structure and phase stability of the pseudobinary alloys. The calculations have been carried out using the augmented space recursion based on the all electron, tight-binding linear muffin-tin orbital basis. This work, in particular, provides a further generalization of our earlier developed technique to the case of systems of multiple sublattices with varying degree of disorder on them. We showcase the feasibility of our formalism by applying to a face-centered tetragonal-based pseudobinary ${({\text{Ni}}_{1\ensuremath{-}x}{\text{Pt}}_{x})}_{3}\text{Al}$ alloy system. Based on the calculation of effective pair interactions and their lattice Fourier transform, our phase stability search yields two stable superordered structures, namely, $\text{L}{1}_{0}$-type ${\text{Ni}}_{2}\text{PtAl}$ and $\text{L}{1}_{0}$-type ${\text{NiPt}}_{2}\text{Al}$ of which the latter has been observed experimentally. An estimate of the minima in the effective pair potential surface $V(\mathbf{k})$ predicted the order-disorder transition temperatures of the two stable structures to be $\ensuremath{\sim}1027$ and $\ensuremath{\sim}1379\text{ }\text{K}$, respectively. The results are in agreement with the previous findings, proving the effectiveness of augmented space recursive technique in dealing with systems of multiple sublattices with varying degree of disorder on them. Our calculated additional physical quantities such as short-range order maps can be compared with future experimental studies.

AUGMENTED SPACE RECURSION CODE AND APPLICATION IN SIMPLE BINARY METALLIC ALLOY

We present here an optimized and parallelized version of the augmented space recursion code for the calculation of the electronic and magnetic properties of bulk disordered alloys, surfaces and interfaces, either flat, corrugated or rough, and random networks. Applications have been made to bulk disordered alloys to benchmark our code.

Augmented space recursion study of the effect of disorder on superconductivity

We present a real-space approach to study the effect of disorder on superconductivity. The method is based on augmented space formalism that goes beyond mean-field approximations for configuration averaging and effectively deals with the influence of configuration fluctuations of the neighborhood of an atom. In the regime of validity of Anderson's theorem our results for $s$- and $d$-wave dirty superconductors have excellent agreement with existing results. The formalism is extended and tested for random negative $U$ Hubbard model. Having verified the reliability of our method we use it to study environment-dependent inhomogeneous randomness in disordered superconducting systems.

An augmented space approach to the study of random ternary alloys: I. Electronicstructure with uncorrelated disorder and short ranged order

We present here a generalized augmented space recursive technique which includes theeffects of diagonal and environmental disorder explicitly: an analytic, lattice translationalinvariant, multiple scattering theory for the study of short range ordering in randomternary alloys. Our generalized augmented space formalism includes atomic correlationsover a finite cluster including short range order (SRO). We propose the augmentedspace recursion (ASR), a computationally fast and accurate technique whichincorporates configuration fluctuations over a large local environment. We apply theformalism to a tight-binding linear muffin-tin orbital (LMTO) study of stainless steelFe80−xNixCr20 (x = 14 and 17). We have demonstrated the effects of short range ordering by calculating theconfiguration averaged density of states with and without SRO and with different kinds ofcluster environment embedded in an averaged medium.

Study of phase stability of MnCr using the augmented space recursion based orbital peeling technique

In an earlier communication we have developed a recursion based approach to the study of phase stability and transition of binary alloys. We had combined the recursion method introduced by Haydock, Heine and Kelly and the our augmented space approach with the orbital peeling technique proposed by Burke to determine the small energy differences involved in the discussion of phase stability. We extend that methodology for the study of MnCr alloys.

Electronic and magnetic properties of disordered Fe–Cr alloys using different electronicstructure methods

In this paper we have studied the electronic structure of bccFexCr1−x alloys in the ferromagnetic phase using three different techniques: augmented spacerecursion coupled with tight-binding linearized muffin-tin orbitals (TB-LMTO-ASR), thecoherent potential approximation based on the Korringa–Kohn–Rostocker method(KKR-CPA) and the special quasi-random structure technique linked with the projectoraugmented wave (PAW-SQS). The aim was to provide a comparison between thedifferent methods and examine their strengths and weaknesses vis-à-vis one another.

Study of phase stability in a class of binary alloys using augmented space recursion based orbital peeling technique

In this communication we have developed a recursion based approach to the study of phase stability and transition of binary alloys. We have combined the recursion method introduced by Haydock et al. [J. Phys. C: Solid State Phys. 5 (1972) 2845] and the our augmented space approach [A. Mookerjee, J. Phys. C: Solid State Phys. 6 (1973) 1340] with the orbital peeling technique proposed by Burke [Surf. Sci. 58 (1976) 349] to determine the small energy differences involved in the discussion of phase stability. We have studied three alloy systems: one which segregates and the other two orders at lower temperatures.

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

A first principles density functional based linear response theory (the so called Density Functional Perturbation theory \cite{dfpt}) 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) and the Itinerant coherent potential approximation (ICPA). 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 Fe$_{50}$Pd$_{50}$ alloy.