Search

Generating random numbers is prerequisite to any Monte Carlo method implemented in a computer program. Therefore, identifying a good random number generator is important to guarantee the quality of the output of the Monte Carlo method. However, sequences of numbers generated by means of algorithms are not truly random, but having certain control on its randomness essentially makes them pseudo-random. What then matters the most is that the simulation of a physical variable with a probability distribution, needs to have the same distribution generated by the algorithm itself. In this perspective, considering the example of gauge block calibration given in "Evaluation of measurement data—Supplement 1 to the “Guide to the expression of uncertainty in measurement”—Propagation of distributions using a Monte Carlo method", we explore the properties and output of three commonly used random number generators, namely the linear congruential (LC) generator, Wichmann-Hill (WH) generator and the Mersenne-Twister (MT) generator. Extensive testing shows that the performance of the MT algorithm transcends that of LC and WH generators, particularly in its time of execution. Further, these generators were used to estimate the uncertainty in the measurement of the length, with input variables having different probability distributions. While, in the conventional GUM approach the output distribution appears to be Gaussian-like, we from our Monte-Carlo calculations find it to be a students' t-distribution. Applying the Welch-Satterthwaite equation to the result of the Monte Carlo simulation, we find the effective degrees of freedom to be 16. On the other hand, using a trial–error fitting method to determine the nature of the output PDF, we find that the resulting distribution is a t-distribution with 46 degrees of freedom. Extending these results to calculate the expanded uncertainty, we find that the Monte-Carlo results are consistent with the recently proposed mean/median-based unbiased estimators which takes into account the artifact of transformation distortion.

Both LaAlO3 and BiAlO3 are isostructural, isoelectronic and band insulators. Therefore, in analogy to the LaAlO3/SrTiO3 heterostructure, a quasi two dimensional electron gas (q-2DEG) could be anticipated in BiAlO3/SrTiO3 heterostructures. Our density functional theory based scalar relativistic calculations show that BiAlO3/SrTiO3 heterostructures remain insulating for a BiAlO3 film thickness up to 5 unit cells. However, with spin orbit coupling included in the crystal Hamiltonian, we find a thickness dependent insulator to metal transition for BiAlO3/SrTiO3 heterostructures. However, unlike the Ti3+/Ti4+ electronic reconstruction in LaAlO3/SrTiO3, the conductivity in BiAlO3/SrTiO3 is found to originate from the subsurface Bi 6p states. The results suggest that the properties of q-2DEG in BiAlO3/SrTiO3 can be controlled using an external electric field, leading to a wide range of solid state applications.

A systematic study, using first-principles methods, is performed to understand the nature and origin of electronic gap and magnetism in the double perovskite antiferromagnetic insulator Ba2CaIrO6. By virtue of its perfectly aligned IrO6, motifs the system may be anticipated as an ideal spin-orbit driven J eff system. However, different from the Ir + 4 and Ir + 5 iridates, our calculations reveal that the electronic gap in Ba2CaIrO6 is implicitly associated with an unconventional antiferromagnetic ordering of Ir spins, thereby classifying this Ir + 6 iridate as a Slater-type antiferromagnetic insulator. On the other hand, spin-orbit coupling enhances the Ir 5 d - O 2 p orbital hybridization, due to which both the magnitude of the electronic gap and the local Ir magnetic moment decreases in comparison to the scalar relativistic calculations. Our results not only affirm the validity of local approximations to the exchange-correlation potential in the scalar relativistic Kohn-Sham Hamiltonian, but also strongly convey that the insulating nature of iridates may be intimately linked with magnetism.

A comprehensive set of electronic structure calculations are performed to understand the origin of the insulating gap and magnetic properties of hexagonal Ca2IrO4. Although, isoelectronic with Sr2IrO4, the spin-orbit coupling driven J eff model is anticipated to be less appropriate for Ca2IrO4 following its structural considerations. We find that the local density approximation (LDA), and those including the effects of Coulomb correlations and spin-orbit coupling fail to reproduce the experimental results. Moreover, the calculations employing the modified Becke-Johnson formalism seems to provide sufficiently good results, by predicting Ca2IrO4 to be an antiferromagnetic insulator. The origin of electronic gap is attributed to the antiferromagnetic ordering of Ir spins and the effects of spin-orbit coupling is found marginal. However, due to the anisotropy in the Ir O bonding within the distorted IrO6 octahedra we deduce large magneto-anisotropic energy in Ca2IrO4. Further, our analysis shows that Ca2IrO4 is an itinerant material, suggesting that band structure effects play an important role in determining the ground state properties of iridates.

Investigation of intermediate compounds of Heusler alloys has been the prime focus to study the various magnetic properties. We have successfully synthesized non-stoichiometric full- Heusler intermediate compounds of Ni2-xMn1+x Sb (0.25≤x≤0.75) to investigate their magnetic properties. XRD analysis using Rietveld refinement reveals the presence of a single-phase of Cu 2 MnAl prototype full-Heusler phase. Transmission electron microscopy analysis also confirms the presence of a single phase of best optimized composition (Ni 1.5Mn1.5Sb) of full-Heusler alloy. Interestingly, the saturated magnetic moments obtained from the M-H curves were found to be 4.45 μ B/f.u., 3.16 μ B/f.u. and 4.65 μ B/f.u. for Ni2-xMn1+xSb (x= 0.25, 0.5, 0.75) respectively which is the highest reported value (~4.65 μ B/f.u. for Ni1.25Mn1.75Sb) so far in this family of the compounds. Further, the reversibility of zero-field cooled (ZFC) and field cooled (FC) curves indicate that samples are stabilized in a magnetically homogeneous state. The first principle calculation performed on these materials is well corroborated with the experimental results.

Human resource is fundamental to all strategic and coherent approaches in organizational management. To be a valued asset, human resource management assigns appropriate and skilled people working collectively towards a goal. Therefore, it becomes important for the organization to facilitate the necessary requirements such as infrastructure, education and skills. In this chapter the status of human resource in metrology in India has been reviewed and associated advantages and impediments were analysed. The growths of developed economies are also briefly discussed. The role of human resources in establishing a workforce in metrology, from Indian context, so as to enhance effectiveness of Quality infrastructure and make a socioeconomic impact, is also presented. Considering that human resource development requires both theoretical as well as practical knowledge imparted at laboratory or industry, the need to introduce the essentials of measurements from secondary level education to skill-development in tertiary university levels, as a part of the curriculum were highlighted. Besides, with rapid development in science and technology, it is also important that the industry personnel are updated with the current measurement science and technology by rigorous training. Analysis of gaps, challenges has been made and solutions are proposed for tackling the problem through a revised scheme of education system which includes personnel certification, skill developments and capability enhancement coupled with availability of career progression.

CSIR-NPL is the custodian of National standards of electrical and electronic parameters. These include DC parameters such as voltage, current and resistance; low frequency and high frequency impedance related quantities such as capacitance, inductance and AC resistance; AC/DC high voltage and AC high current; AC power and energy; and quantum standard which includes quantum hall resistance (QHR), quantum current (QC) and quantum nanophotonics (QN). The metrological traceability of the electrical and electronics parameters to SI units is derived from Josephson Voltage Standard (JVS), Quantum Hall Resistance (QHR) standard and frequency (time) standards; all of them are being maintained at CSIR-NPL with metrological precision at par with international standards. The traceability of the aforementioned parameters is disseminated through an unbroken chain of apex level calibrations and testing at par with international level to the industries and strategic sectors of the country to improve the quality of life, which in turn will lead to the inclusive growth of the country and economic development. R&D efforts on the development of quantum standards is a constant endeavour and continues to be at the forefront. Specifically, CSIR-NPL focuses on the development of quantum standards related to the unit of current (ampere), the unit of resistance (ohm) and quantum nanophotonics which aims for detection of few photons (or even single photons) using the novel concept of superconducting nanowire single photon detectors (SNSPD). Among these the research on QC needs special mention, as this will lead to realisation of the SI unit of electric current (ampere), the only unit out of the seven base units of SI system. The quantum current standard (QCS) realisation is bifurcated into two approaches, (i) based on the single electron tunnelling effect (SET) observable in semiconductor quantum dot (QD) structures and (ii) the quantum phase slip phenomenon (QPS) observable in superconducting nanowires of cross-sectional area of the order of coherence length of the system. The realisation of the resistance unit (ohm) is based on the quantum hall effect observable in semiconductor 2DEG structures such as GaAs/AlGaAs systems. Recently there has been tremendous evidence emerging for the use of monolayer graphene for the use of QHR metrology. CSIR-NPL also started the growth of “epitaxial graphene anchored on SiC” and have obtained encouraging results. The nano-photonics measurement research is also taken up actively for its applications to realize quantum standards for optical radiation and device fabrication.

In search of novel oxide heterostructures that display highly mobile two-dimensional charge transport at its heterointerface, we report the synthesis of non-stoichiometric perovskite CaxTayO3+δ epitaxial films on TiO2 terminated SrTiO3 (001) substrate. The layer-by-layer growth of the films was accomplished by the pulsed laser deposition (PLD) technique monitored by the in-situ Reflection high energy electron diffraction (RHEED). The experiment shows that a quasi-two-dimensional electron gas (q-2DEG) type charge transport occurs beyond 9 unit-cell of the film-thickness, with its carrier mobility ≈102 cm2/Vs.

A quantum critical behavior is reported in equi-atomic NiCoCr alloy, where the ferromagnetic transition temperature goes to zero. We perform a systematic study of (NiCo)1−xCrx alloys; 0 ≤ x ≤ 1 3 , by means of density functional theory based Green’s function method, to understand the changes in the electronic structure and magnetic properties of this equi-atomic ternary alloy as a function of Cr at%. Chemical disorder is modeled by means of coherent-potential. approximation. To determine the propensity of spin fluctuations, we adopt to the fixed-spin moment method. A fit to the phenomenological Ginzburg Landau equation and estimation of the second derivative of the total energy with respect to magnetization shows that (NiCo)1−xCrx alloys display an increasing propensity of longitudinal spin fluctuations, proportional to the Cr at% in these alloys.

The Ir ion in Sr n + 1 Ir n O 3 n + 1 series of compounds is octahedrally coordinated. However, unlike Sr 2 IrO 4 ( n =1) and Sr 3 Ir 2 O 7 ( n =2) which are insulating due to spin-orbit induced J eff splitting of the t 2g bands, SrIrO 3 ( n = ∞ ) is conducting. To explore whether such a splitting is relevant in SrIrO 3 , and if so to what extent, we investigate the electronic structure of orthorhombic SrIrO 3 using density functional theory. Calculations reveal that the crystal field split Ir t 2 g bands in SrIrO 3 are indeed split into J eff = 3 2 and J eff = 1 2 states. However, the splitting is found to be strongly k − dependent with its magnitude determined by the Ir - O orbital hybridization. Besides, we find that the spin-orbit induced pseudo-gap, into which the Fermi energy is positioned, is composed of both light electron-like and heavy hole-like bands. These features in the band structure of SrIrO 3 suggest that variations in the carrier concentration control the electronic transport properties in SrIrO 3 , which is consistent with the experiments.

The origin of quasi-two dimensional electron gas at the interface of polar-nonpolar oxide hetero-structure, such as LaAlO3/SrTiO3, is debated over electronic reconstruction and defects/disorder models. Common to these models is the partial valence transformation of substrate Ti ions from its equilibrium 4 + state to an itinerant 3 + state. Given that the Hf ions have a lower ionization potential than Ti due to the 4 f orbital screening, one would expect a hetero-interface conductivity in the polar-nonpolar LaAlO3/SrHfO3 system as well. However, our first principles calculations show the converse. Unlike the Ti 3 + − Ti 4 + valence transition which occur at a nominal energy cost, the barrier energy associated with its isoelectronic Hf 3 + − Hf 4 + counterpart is very high, hence suppressing the formation of quasi-two dimensional electron gas at LaAlO3/SrHfO3 hetero-interface. These calculations, therefore, emphasize on the propensity of mixed valence at the interface as a necessary condition for an oxide hetero-structure to exihibit quasi two-dimensional electron gas.

La3 Ir3 O11 crystallizes in a cubic symmetry with its structure composed of electronically active IrO6 octahedral motifs, which are corner and edge shared. Empirical considerations suggest Ir to be in the +4.33 valence state in La3 Ir3 O11, thereby anticipating the material to be a good conductor primarily due to the presence of holes in the crystal field split t 2 g bands of the Ir 5d manifold. However, a comprehensive investigation of the electronic structure by means of first principles calculations find La3 Ir3 O11 to be a spin-orbit driven semi-metallic iridate, with its Fermi energy comprised of both hole and electron bands. The properties of the electron-hole asymmetry in La3 Ir3 O11 is clearly manifested in the temperature dependent variation of electrical conductivity and Seebeck coefficient, which are calculated using the Boltzmann transport theory. Besides, finding that Fermi energy is positioned in a steep valley like feature in the density of states spectra, the calculations also predict that both hole and electron doping would induce a semi-metal to metal transition in La3 Ir3 O11.

Manganese (Mn) based full-Heusler alloys have been shown to be the most promising materials due to their excellent magnetic behaviors for applications in magneto resistance, magnetic tunnel devices, spin valve and spin injection devices. Here, we report the study of crystal structure, electronic structure and magnetic properties of Mn2NiSb full-Heusler alloy. XRD investigation of Mn2NiSb followed by Rietveld refinement reveals the material to be successfully crystallized in a well-ordered Cu2MnAl prototype structure manifested by the existence of super-lattice diffraction peaks of (1 1 1) and (2 2 0). The experimental lattice parameter is estimated to be 6.01 A which is larger than the available reports. Magnetic measurement (M-H) performed by applying external magnetic field up to 1.8 Tesla reveals the saturated magnetic moment (MS) of 4.07μB/formula unit and 2.50μB/formula unit at temperature 5 K and 300 K respectively which is the highest value reported so far at 5 K for Mn2NiSb stabilized in Cu2MnAl type structure. Further, magnetization versus temperature (M-T) curve with zero-field cooled (ZFC) and field-cooled (FC) up to 300 K indicates that Mn2NiSb crystallized in a single phase. However, the square of magnetization and temperature curve was best fitted with linear fitting supporting to Stoner like ferromagnetism. In order to have a better understanding of magnetic behavior, the First principal calculation has also been performed. Interestingly, our experimental observations were found to be well corroborated with the theoretically estimated magnetic behavior.

Quasi-two-dimensional electron gas (q-2DEG) at the interface of oxide heterostructures, as an alternative to semiconductor electronics, is limited by its low carrier mobility. This is largely due to the nature of Ti 3 d t 2 g conduction electrons of the SrTiO 3 substrate. Here, we explore the interface conductivity between LaAlO 3 / Sr 2 AlNbO 6 ( 001 ) using density functional theory based first principles calculations. These heterostructures show an insulator–metal transition with increasing overlayer thickness and the carriers being Nb 4 d x y in origin. The relatively higher dispersion of these carriers yields a higher carrier mobility and, hence, is significant to electronics applications based on q-2DEG.

There exists several inconsistencies between the experimental results and theoretical band structure calculations for Ti 2 O 3 . While the local approximations describing the exchange correlation potential of the crystal Hamiltonian completely fails to describe the metal-insulator transition, the hybrid functionals and the Hartree-Fock method tends to overestimate the materials band gap. In this work, we report the physical properties of Ti 2 O 3 using the modified Becke-Johnson exchange correlation potential. The approach not only captures the metal–insulator transition as a function of increasing Ti–Ti bond distance, but also reasonably predicts the magnitude of the materials band gap. The distribution of electronic states is consistent with the Van Zandt-Honig-Goodenough model which attributes to the electronic repulsion between the bonding and antibonding states of the otherwise split t 2 g bands of Ti 3 d manifold, in a rhombohedral field. Transport properties calculated using the Boltzmann transport equation in the constant relaxation time approximation clearly reveals the semi-metallic characteristics of Ti 2 O 3 . The Seebeck coefficient, calculated as high as > 400 μV / K at ≃ T = 140 K infers that the material can be used as a potential low temperature oxide thermoelectric.

By means of structural refinements, magnetic susceptibility measurements and density functional theory based calculations; we study the physical properties of LaCoO3 with Al substitutions. Despite Al substitutions, which result in a reduction of unit-cell volume, increased chemical disorder, and local octahedral distortions, we find that the thermally mediated low-spin and high-spin states in LaCoO3 survive against the odds. The observed relative shift of the low-spin and high-spin representative peaks in the magnetic susceptibility data is determined to be of electronic origin via a change in the material’s chemical bonding properties. Our study reveals a strong interplay between bonding and disorder for the temperature dependent spin states of LaCoO3.

Zintl compounds are potential candidates for efficient thermoelectric materials, because typically they are small band gap semiconductors. In addition, such compounds allow fine tuning of the carrier concentration by chemical doping for the optimization of thermoelectric performance. Herein, such tunability is demonstrated in Mg3Sb2-based Zintl compounds via Zn(2+) doping at the Mg(2+) site of the anionic framework (Mg2Sb2)(2-), in the series Mg3-xZnxSb2 (0 ≤ x ≤ 0.1). The materials have been successfully synthesized using the spark plasma sintering (SPS) technique. X-ray diffraction (XRD) analysis confirms a single solid solution phase of Mg3-xZnxSb2 (0 ≤ x ≤ 0.1). The thermoelectric properties are characterized by the Seebeck coefficient, electrical conductivity, and thermal conductivity measurements from 323 K to 773 K. Isoelectronic Zn substitution at the Mg site presents the controlled variation in the carrier concentration for optimizing the high power factor and reduced thermal conductivity. These results lead to a substantial increase in ZT of 0.37 at 773 K for a composition with x = 0.10 which is ∼42% higher than undoped Mg3Sb2. The electronic transport data for the Mg3-xZnxSb2 (0 ≤ x ≤ 0.1) compound are analyzed using a single parabolic band model predicting that Mg2.9Zn0.1Sb2 exhibits a near-optimal carrier concentration for high ZT. The electronic structure of transport properties of these disordered Mg3-xZnxSb2 (0 ≤ x ≤ 0.1) is also studied using density functional theory and the results obtained are in good agreement with experimental results. The low cost, lightness and non-toxicity of the constituent elements make these materials ideal for mid-temperature thermoelectric applications.

To resolve the controversy of whether or not the origin of an electronic gap in antiferromagnetic post-perovskite (pPv) CaIrO3 is due to Coulomb repulsion or spin–orbit coupling, and/or both, we have performed comprehensive full potential density functional theory based calculations. A rather consistent electronic structure, which explains the origin and magnitude of the electronic gap, inter-band d–d transition energies, high thermopower and large magneto-crystalline anisotropy, is obtained with the use of a modified Becke–Johnson (mBJ) exchange potential. Fundamentally, mBJ calculations correctly capture the strong interplay of the crystal field and long range antiferromagnetic ordering of Ir spins as the mechanism that drives pPv-CaIrO3 to an insulating state. Based on our findings, we propose that pPv-CaIrO3 is a conventional Slater type antiferromagnetic insulator.

Mid-temperature thermoelectric applications include waste heat recovery from automobile exhausts, various industrial process and solar thermoelectrics. The current systems that have been extensively investigated for mid-temperature applications are mostly tellurides. However, the low crustal abundance of tellurium contributes significantly to their price volatility which impedes their usage in commercial technology. Looking from the perspective of material cost effectiveness in terms of crustal abundance and environmental sustainability, we investigate the thermoelectric properties of Mg9Si5 systems, the constituents of which are geo-abundant and non-toxic. By means of first principles calculations, we find Mg9Si5 to be a potential mid-temperature thermoelectric material, with an operational temperature in the range of ≃400–600 K, depending on the exact carrier concentration. We also discuss the enhanced viability of substitution in Mg9Si5, a case that seems to be restricted in Mg2Si because of its chemically balanced Zintl nature.

Transport properties of LaOBiS2 are studied using the Boltzmann theory in the constant scattering time approximation, the input to which are derived from the ab-initio calculations with spin–orbit interactions included. By virtue of the materials chemical bonding and band structure, strong anisotropy in the transport is observed, which also have non-symmetric behavior with respect to carrier type. With thermopower being in few hundreds μV/K, and LaO layers being charge inactive, it could be well anticipated that the thermal conductivity would be significantly suppressed along the crystallographic c-axis. We argue LaOBiS2 as a potential oxide thermoelectric material for high temperature applications. Further optimization of the materials properties in terms of “p” and “n” type conduction could also be accomplished via Sr and F doping, respectively, following the discovery of its isoelectronic counterpart SrFBiS2.

This work reports a quasi-two-dimensional electron gas (q-2DEG) system at the interface of two wideband-gap insulators, (TiO2-terminated) SrTiO3 and LaScO3, with a minimum thickness of 4-unit cell (uc). The highly crystalline and abrupt heterointerface is confirmed with high-resolution electron microscopy. The mixed Ti4+ and Ti3+ valence states (for 4 uc of LaScO3) obtained from the x-ray photoelectron spectroscopy study suggest an intrinsic electronic reconstruction at the interface, leading to a metallic nature. This origin is well supported by density functional theory calculations that reveal an emergence of 3.3 states/eV/spin at the Fermi level for 4 uc in accordance with the polar catastrophe model. The study offers one more perovskite heterostructure, like LaAlO3/SrTiO3, for unraveling the q-2DEG phenomena toward a clear mechanism and futuristic applications.This work reports a quasi-two-dimensional electron gas (q-2DEG) system at the interface of two wideband-gap insulators, (TiO2-terminated) SrTiO3 and LaScO3, with a minimum thickness of 4-unit cell (uc). The highly crystalline and abrupt heterointerface is confirmed with high-resolution electron microscopy. The mixed Ti4+ and Ti3+ valence states (for 4 uc of LaScO3) obtained from the x-ray photoelectron spectroscopy study suggest an intrinsic electronic reconstruction at the interface, leading to a metallic nature. This origin is well supported by density functional theory calculations that reveal an emergence of 3.3 states/eV/spin at the Fermi level for 4 uc in accordance with the polar catastrophe model. The study offers one more perovskite heterostructure, like LaAlO3/SrTiO3, for unraveling the q-2DEG phenomena toward a clear mechanism and futuristic applications.

The τ - phase of the binary Mn-Al system is a promising rare-earth free permanent magnet. However, the experimentally determined figure-of-merit (energy-factor) is significantly lower than the theoretical estimate. This is partly being associated with the low volume fraction of τ -MnAl during synthesis, chemical disorder driven inter-sublattice antiferromagnetism, and presence of multiple binary phases. In this work, with the help of first-principles linear response calculations, we show that the presumed long range ferromagnetic ordering in τ -MnAl is unstable. Calculation of Mn magnetic pair-exchange interaction for τ -MnAl show competing ferromagnetic and antiferromagnetic interactions. As a result, the study infers to a non-collinear magnetic ground state, which then partly explains the low magnetization as observed in experiments. Local probe experiments such as neutron diffraction, corroborating our prediction of the non-collinear state in τ -MnAl is therefore desired.

For iridates with large spatially extended 5d orbitals, it may be anticipated that distant neighbor interactions would play a crucial role in their ground state properties. From this perspective, we investigate the magnetic structure of Sr2IrO4 by including interactions beyond first and second neighbors, via supercell modeling. Adopting to first-principles scalar relativistic methods, it is found that the minimum in total energy among various magnetic structures correspond to a [Formula: see text] type antiferromagnetic ordering of the Ir ions for which the magnitude of the electronic gap, that of the Ir local moments and, the facsimile of the two-peaked structure in the optical conductivity spectra of Sr2IrO4 were found to be in good agreement with the experiments. The results unequivocally show that the origin of the electronic gap in Sr2IrO4 is due to an unconventional antiferromagnetic ordering of Ir ions, thereby classifying the system as a Slater magnet, rather than the spin-orbit coupling driven [Formula: see text] Mott insulator.

Quasi-two-dimensional electron gas has been realized at the polar-nonpolar interface of several insulating oxide heterostructures. However, its hole counterpart remains elusive. In an attempt to find a novel system that exhibits quasi-two-dimensional hole gas (q-2DHG) at the heterointerface, we adopt to materials search, first based on phenomenology followed by a comprehensive set of calculations based on first-principles density functional theory. Our studies show the epitaxial growth of cubic Ca0.5TaO3 on TiO2 terminated substrate display (q-2DHG). The hole gas emanates from the O 2p orbitals of the TiO2 layers of the substrate. On the other hand, an electron gas is formed at the (001) TaO2 top surface, thereby representing the heterostructure as a coupled quantum well system. The partial filling of the Ta 5dt2g conduction band indicates electron reconstruction, in agreement with the polar catastrophe model. Besides, a critical thickness of three monolayers is deduced from the calculations for the formation of q-2DHG in the Ca0.5TaO3/SrTiO3 heterostructure, which is consistent with the model prediction based on the modern theory of polarization. With both cubic systems, Ca0.5TaO3 and SrTiO3, having a similar underlying symmetry and minimal lattice mismatch, epitaxial growth with an abrupt interface can be well anticipated. Such a single-tier oxide heterostructure composed of separated confined hole-electron subsystems is expected to provide a platform to unravel exciting physics and also for functional devices related to oxide electronics.

Mid-temperature thermoelectric applications include waste heat recovery from automobile exhausts, various industrial process and solar thermoelectrics. The current systems that have been extensively investigated for mid-temperature applications are mostly tellurides. However, the low crustal abundance of tellurium contributes significantly to their price volatility which impedes their usage in commercial technology. Looking from the perspective of material cost effectiveness in terms of crustal abundance and environmental sustainability, we investigate the thermoelectric properties of Mg9Si5 systems, the constituents of which are geo-abundant and non-toxic. By means of first principles calculations, we find Mg9Si5 to be a potential mid-temperature thermoelectric material, with an operational temperature in the range of ≃400–600 K, depending on the exact carrier concentration. We also discuss the enhanced viability of substitution in Mg9Si5, a case that seems to be restricted in Mg2Si because of its chemically balanced Zintl nature.

Stoichiometric and ordered hexagonal phase of MnBi are difficult to obtain due to the peritectic reaction of the materials constituents. Thus, inter-site mixing of Mn and Bi is expected. Given that the material is a promising rare-earth free permanent magnet, we study the effects of inter-site chemical disorder on its magnetic properties, using ab initio density functional based electronic structure methods. Our calculations, focusing on inter-site substitutional disorder, show that excess of Mn and Bi ions in MnBi have detrimental effects on its magnetic properties. We find that inter-site chemical disorder not only decreases the total magnetization, but also reduces the materials׳ ferromagnetic transition temperature. The latter is deduced from the calculated magnetic pair-exchange parameters by means of linear response technique. Our calculations suggest that the inter-site disorder of Mn and Bi ions in MnBi must be minimized for its optimum performance in permanent magnet applications.

Given that energy scales associated with crystal field splitting, spin orbit coupling and coulomb correlations in iridates are comparable, hence leading to exotic properties, we investigate the physical properties of orthorhombic SrIrO3 using density functional theory. Our calculations, however, show that SrIrO 3 is a bad metal with no long range magnetic ordering, unlike its sister compounds Sr2IrO4 and Sr3Ir2O7. Moreover, despite having large band width, it appears conclusive that the larger resistivity in SrIrO 3 is due to spin orbit interactions. Besides, the effects of electron-electron correlations on its electronic structure and magnetic properties are also discussed.

Despite continuous efforts, a conceptual perception on the origin of quasi-two-dimensional electron gas (q-2DEG) in oxide heterostructures remains far from any generalization. Along this perspective, a systematic study identifying systems that exhibit heterointerface conductivity (${\mathrm{LaAlO}}_{3}{/\mathrm{SrTiO}}_{3}$) and that do not (${\mathrm{LaCrO}}_{3}{/\mathrm{SrTiO}}_{3}$) is undertaken. High quality ${\mathrm{LaAl}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{O}}_{3}$ ($0\ensuremath{\le}x\ensuremath{\le}1$) films were fabricated on ${\mathrm{TiO}}_{2}$ terminated ${\mathrm{SrTiO}}_{3}$ (001) substrates using a pulsed laser deposition technique and its growth was monitored by reflection high energy electron diffraction (RHEED). The transport and photoemission spectroscopy experiments reveal that the the origin of heterointerface q-2DEG is associated with ${\mathrm{Ti}}^{3+}$ states and that it ceases to exist with increasing Cr ion concentration in the overlayer films. Following experimental evidences of the Cr concentration dependent metal-insulator electronic phase transition at the heterointerface, explanation is sought within the realms of a polar catastrophe model. Based on our transport and spectroscopy measurements, we propose that an electronic reconstruction drives the formation of q-2DEG for pristine LAO/STO and Al rich samples due to the formation of ${\mathrm{Ti}}^{3+}{/\mathrm{Ti}}^{4+}$ mixed valent state, while for heavily doped Cr samples, the required electron count necessary to solve the polar catastrophe instability are trapped in the ${\mathrm{LaAl}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{O}}_{3}$ overlayers in the Cr $3d$ states. These trapped electrons in the overlayers are manifested in the form of ${\mathrm{Cr}}^{2+}$ ions.

Orthorhombic SrIrO3 subjected to strain show tunable transport properties. With underlying symmetry remaining invariant, these properties are associated with IrO6 octahedral tilting. Adopting to first-principles methods, the effects of crystal field, spin-orbit coupling, and Coulomb correlations, on comparable interaction length scales, are discussed. While tilting induces a t2g-eg crystal-field splitting and band narrowing, spin-orbit coupling induces a partial splitting of the Jeff bands rendering SrIrO3 a semi-metallic ground state. The SOC enhanced hybridization of Ir-O orbitals, serve as a explanation to why the critical Hubbard correlation strength increases with increasing SOC strength in SrIrO3 to induce an insulating phase.

We report the effect of $\ensuremath{\delta}$ doping at the LaAlO${}_{3}$/SrTiO${}_{3}$ interface with LaMnO${}_{3}$ monolayers on the photoconducting (PC) state. The PC is realized by exposing the samples to broadband optical radiation of a quartz lamp and 325 and 441 nm lines of a He-Cd laser. Along with the significant modification in electrical transport which drives the pure LaAlO${}_{3}$/SrTiO${}_{3}$ interface from metal-to-insulator with increasing LaMnO${}_{3}$ sub-monolayer thickness, we also observe an enhancement in the photoresponse and relaxation time constant. A possible scenario for the PC based on defect clusters, random potential fluctuations, and large lattice relaxation models, along with the role of structural phase transition in SrTiO${}_{3}$, is discussed. For pure LaAlO${}_{3}$/SrTiO${}_{3}$, the photoconductivity appears to originate from interband transitions between Ti-derived $3d$ bands which are ${e}_{g}$ in character and O 2$p$--Ti ${t}_{2g}$ hybridized bands. The band structure changes significantly when fractional layers of LaMnO${}_{3}$ are introduced. Here the Mn ${e}_{g}$ bands ($\ensuremath{\approx}$1.5 eV above the Fermi energy) within the photoconducting gap lead to a reduction in the photoexcitation energy and a gain in overall photoconductivity.

We report the use of Boltzmann transport theory to investigate the electrical properties of thermoelectric Mg${}_{2}$Si, Mg${}_{2}$Sn, and a supercell model of the 50-50 alloy. The results are based on first-principles electronic structure calculations with the modified Becke-Johnson potential of Tran and Blaha, which yields band gaps in good accord with experiment. The calculated transport coefficients are discussed in relation to the thermoelectric performance of these materials. The results imply roughly symmetric behavior with respect to carrier type and the possibility of improvements in $ZT$, especially for $p$-type and lower temperatures.

The two-carrier transport model as proposed for the two-dimensional electron gas formed at the interfaces of oxide heterostructures is investigated by means of a combined perturbation of near ultra-violet radiation and electrostatic field, applied both separately and simultaneously. Comparison of the photo-response of the prototype systems such as the band insulator LaAlO3 and Mott insulator LaTiO3 films on TiO2 terminated SrTiO3 show remarkably similarities. Two types of non-equilibrium carriers are generated in each system, having a signature of a particular type of perturbation characterized by distinctly different relaxation process. While, the photo-conducting state diminishes in a stretched exponential manner, with a temperature dependent activation energy varying from few tens of meV to ~ 1 to 2 meV on lowering the temperature, and a relaxation time of several hours, the recovery from electrostatic gating occurs in milli-seconds time scale. An attempt is also made to explain the experimental observations using the ab-initio density functional calculations. The calculations show that the electronic transitions associated with near ultra-violet radiation emerge from bands located at ~ 2 eV above and below the Fermi energy, which are the Ti 3d states of the SrTiO3 substrate, and that from the AlO2 (TiO2) layers of the LaAlO3 (LaTiO3) films, respectively. The slow decay of the photo-current to the unperturbed state is explained in terms of the closely spaced Ti 3dxy states in the lower conduction band, which are manifested as flat bands (or localized states) in the band structure. Such localization leads to increased carrier life-times, through the energy-time relationship of the uncertainty principle., Comment: 10 pages, 8 figures

Using linear-response density functional theory, magnetic excitations in the striped phase of ${\mathrm{CaFe}}_{2}{\mathrm{As}}_{2}$ are studied as a function of local-moment amplitude. We find a new kind of excitation: sharp resonances of Stoner-like (itinerant) excitations at energies comparable to the N\'eel temperature, originating largely from a narrow band of Fe $d$ states near the Fermi level, and coexisting with more conventional (localized) spin waves. Both kinds of excitations can show multiple branches, highlighting the inadequacy of a description based on a localized spin model.

Neutron scattering measurements of the magnetic excitations in single crystals of antiferromagnetic CaFe2As2 reveal steeply dispersive and well-defined spin waves up to an energy of approximately 100 meV. Magnetic excitations above 100 meV and up to the maximum energy of 200 meV are however broader in energy and momentum than the experimental resolution. While the low energy modes can be fit to a Heisenberg model, the total spectrum cannot be described as arising from excitations of a local moment system. Ab initio calculations of the dynamic magnetic susceptibility suggest that the high energy behavior is dominated by the damping of spin waves by particle-hole excitations.

Inelastic neutron scattering measurements on the low energy spin waves in CaFe2As2 show that the magnetic exchange interactions in the Fe layers are exceptionally large and similar to the cuprates. However, the exchange between layers is ~10% of the coupling in the layers and the magnetism is more appropriately categorized as anisotropic three-dimensional, in contrast to the two-dimensional cuprates. Band structure calculations of the spin dynamics and magnetic exchange interactions are in good agreement with the experimental data., Comment: 17 pages, 3 figures

The viability of Si-Ge alloys in thermoelectric applications lies in its high figure-of-merit, non-toxicity and earth-abundance. However, what restricts its wider acceptance is its operation temperature (above 1000 K) which is primarily due to its electronic band gap. By means of density functional theory calculations, we propose that iso-electronic Sn substitutions in Si-Ge can not only lower its operation to mid-temperature range but also deliver a high thermoelectric performance. While calculations find a near invariance in the magnitude of thermopower, empirical models indicate that the materials thermal conductivity would also reduce, thereby substantiating that Si-Ge-Sn alloys are promising mid-temperature thermoelectrics.

Here we investigate the LaAlO3-SrTiO3 heterostructure with δ-doping of the interface by LaMnO3 at less than one monolayer. This doping strongly inhibits the formation of a mobile electron layer at the interface. This results in a giant increase of the resistance and the thermopower of the heterostructure. Several aspects of this phenomena are investigated. A model to calculate the carrier concentration is presented and the effect of doping with detailed temperature dependence is analyzed in terms of model parameters and weak-scattering theory. The large enhancement of the thermopower is attributed to the increased spin and orbital entropy originating from the LaMnO3 monolayer.

The thermoelectric properties of Pb doped Mg2Si, synthesized using reactive sintering employing spark plasma sintering, are investigated and are compared with other dopants reported in literature. While a moderate decrease in Seebeck coefficient and thermal conductivity is observed for 2 at. % of Pb doping in Mg2Si, a substantial enhancement in the material's thermoelectric figure-of-merit is observed, which is due to an enormous increase in its electrical conductivity. A brick-layer model is proposed to explain these results, wherein the inter-granular electronic conductivity is facilitated by Pb (or Mg2Pb) phases at grain boundaries, which is supported by microstructural evidences.

The deployment of thermoelectric materials for deriving an enhanced figure of merit (ZT) for power generation in inexpensive, non-toxic and relatively abundant bulk homogeneous solid relies on the extent of achieving the “phonon-glass electron crystal” (PGEC) characteristics. Here, a proof of principal has been established experimentally in the present work for a Zintl compound of Mg3Sb2 and its derivative of isoelectronically Bi doped Bi; Mg3Sb2−xBix (0 ≤ x ≤ 0.4) alloys in Mg3Sb2. Single phase p-type Mg3Sb2 compounds, with Mg and Sb powders as starting materials, have been prepared directly by spark plasma sintering (SPS) in a one step process. The structural refinements of this hexagonal Zintl compound by X-ray diffraction analysis (XRD) and high resolution transmission electron microscopy (HRTEM) investigation reveal that they are single phase devoid of any oxides or Sb precipitates. Transport measurements indicate low thermoelectric figure of merit (ZT = 0.26 at 750 K) for Mg3Sb2. However, an optimum doping of 0.2 at% with iso-electronic Bi ions at the Sb site enhances the ZT to 0.6 at 750 K, which is comparable with the present day industrial materials such as Bi based tellurides and selenides which are toxic. We note that the system becomes metal with carrier density exceeding 15 × 1020/cm3 for x ≥0.25. The substantial increase in ZT in Mg3Sb2−xBix (0 ≤ x ≤ 0.4) owes to a partial decoupling of the electronic and phonon subsystem, as expected for a Zintl phase compound. While the reduction in thermal conductivity in Mg3Sb2−xBix (0 ≤ x ≤ 0.4) accounts to mass fluctuations and grain boundary scattering, the enhancement in the electronic power-factor is attributed to the presence of heavy and light bands in its valence band structure. The latter has been confirmed by means of both X-ray photo electron spectroscopy studies and first-principles density functional based calculations. These measurements established that a high figure of merit can be achieved in this class of materials with appropriate doping. Further, relative abundance of the material ingredients combined with its one step synthesis leads to a cost effective production and less toxicity makes the material an environmentally benign system for thermoelectric power generation.

We report a significant enhancement in the thermoelectric figure-of-merit of phosphorous doped nanostructured n-type Si80Ge20 alloys, which were synthesized employing high energy ball milling followed by rapid-heating using spark plasma sintering. The rapid-heating rates, used in spark plasma sintering, allow the achievement of near-theoretical density in the sintered alloys, while retaining the nanostructural features introduced by ball-milling. The nanostructured alloys display a low thermal conductivity (2.3 W/mK) and a high value of Seebeck coefficient (−290 μV/K) resulting in a significant enhancement in ZT to about 1.5 at 900 °C, which is so far the highest reported value for n-type Si80Ge20 alloys.

High energy ball milled Zr0.25Hf0.75NiSn alloys subjected to spark plasma sintering show an enhanced thermoelectric figure of merit in comparison with its normal bulk material synthesized by arc-melting process. The enhancement is due to increase in Seebeck coefficient with simultaneous decrease in thermal conductivity which follows due to increase in the cell volume. Theoretical calculations find that volume expansion facilitates band narrowing effects leading to high Seebeck coefficient and that decreasing orbital overlap which results in weak bonding leads to dampening the phonon propagation in addition to the interface scattering of phonons from phase boundaries.

The structure of the Sn1-xGex random alloys is studied using density functional theory and the coherent potential approximation. We report on the deviation of the Sn1-xGex alloys from Vegard's law, addressing their full compositional range. The findings are compared to the related Si1-xGex alloys and to experimental results. Interestingly, the deviation from Vegard's law is quantitatively and qualitatively different between the Sn1-xGex and Si1-xGex alloys. An almost linear dependence of the bulk modulus as a function of composition is found for Si1-xGex, whereas for Sn1-xGex the dependence is strongly nonlinear.

Density functional theory is used to study the electronic properties of the oxide heterointerface CaHfO3/SrTiO3. Structural relaxation is carried out with and without O vacancies. As compared to related interfaces, strongly reduced octahedral distortions are found. Stoichiometric interfaces between the wide band gap insulators CaHfO3 and SrTiO3 turn out to exhibit an insulating state. However, interface metallicity is introduced by O vacancies, in agreement with experiment. The reduced octahedral distortions and necessity of O deficiency indicate a less complicated mechanism for the creation of the interfacial electron gas.

Using linear-response density-functional theory, we obtain the magnetic interactions in several iron pnictides. The ground state has been found to be non-collinear in FeSe, with a large continuum of nearly degenerate states lying very close to the magnetic 'striped' structure. The presence of non-collinearity also seems to be a generic feature of iron pnictides when the Fe moment is small. At small RFe − Se the system is itinerant: strong frustration gives rise to an excess of spin entropy, long ranged interactions create incommensurate orderings and strong biquadratic coupling violates the applicability of the Heisenberg model. There is a smooth transition to more localized behavior as RFe − Se increases: stable magnetic orbital order develops which favors long range AFM stripe ordering with strongly anisotropic in-plane exchange couplings. The stabilization of the stripe magnetic order is accompanied by the inversion of the exchange coupling.