Magnetic properties of two-dimensional M$_2$N$_3$ (M-metal, N=S,Se,Te) compounds

Calculated electronic and magnetic properties of the half-metallic, transition metal based Heusler compounds

In this work, results of {\it ab-initio} band structure calculations for $A_2BC$ Heusler compounds that have $A$ and $B$ sites occupied by transition metals and $C$ by a main group element are presented. This class of materials includes some interesting half-metallic and ferromagnetic properties. The calculations have been performed in order to understand the properties of the minority band gap and the peculiar magnetic behavior found in these materials. Among the interesting aspects of the electronic structure of the materials are the contributions from both $A$ and $B$ atoms to states near t…

Half-metallic properties of atomic chains of carbon-transition metal compounds

We found that magnetic ground state of one-dimensional atomic chains of carbon-transition metal compounds exhibit half-metallic properties. They are semiconductors for one spin-direction, but show metallic properties for the opposite direction. The spins are fully polarized at the Fermi level and net magnetic moment per unit cell is an integer multiple of Bohr magneton. The spin-dependent electronic structure can be engineered by changing the number of carbon and type of transition metal atoms. These chains, which are stable even at high temperature and some of which keep their spin-dependent …

Half metallic state and magnetic properties versus the lattice constant in Ti\raisebox{-.2ex}{\scriptsize 2}CoSn Heusler compound: an ab initio study

The half metallic properties of Ti\raisebox{-.2ex}{\scriptsize 2}CoSn full-Heusler compound is studied within the framework of the density functional theory with the Perdew Burke Ernzerhof generalized gradient approximation (GGA). Structural optimization was performed and the calculated equilibrium lattice constant is 6.340 A. The spin up band of compound has metallic character and spin down band is semiconducting with an indirect gap of 0.598 eV at equilibrium lattice constant. For the lattice parameter, ranging from 6.193 to 6.884 A the compound presents 100% spin polarization and a total ma…

Band structure calculations of Ti\raisebox{-.2ex}{\scriptsize 2}FeSn: a new half-metallic compound

Within the framework of density functional theory, the electronic structure and magnetic properties have been studied for the Ti\raisebox{-.2ex}{\scriptsize 2}FeSn full-Heusler compound. The ferromagnetic state is found to be energetically more favorable than paramagnetic and antiferromagnetic states. The spin-polarized results show that Ti\raisebox{-.2ex}{\scriptsize 2}FeSn compound has half-metallic ferromagnetic character with a total spin moment of $2 μ_{B}$ and a band gap in the minority spin channel of 0.489 eV, at the equilibrium lattice constant a=6.342 A.

Crossover from Metal to Insulator in Dense Lithium-Rich Compound $\textrm{CLi}_{4}$

Crystal structures of $\textrm{CLi}_4$ compounds are explored through \emph{ab} \emph{initio} evolutionary methodology. Phase transition from metal to semimetal and semiconductor, and eventually to insulator with increasing pressure are revealed under pressure. Pressure-induced evolution of anti-metallization has been described quantitatively by Fermi Surface Filling Ratio and electron energy band gap using \emph{ab} \emph{initio} \emph{GW} calculations. Anti-metallization is attributed to the hybrid valence electrons and their repulsion by core electrons into the lattice interstices. Very wea…

Charge transfer on the metallic atom pair bond, and the crystal structures adopted by intermetallic compounds

Pauling had proposed that charge is transferred from the more electronegative atom to the less electronegative one during metallic alloying. An expression has recently been derived for the energy of the unlike atom-pair bond using that concept. In this paper, Pauling's view is further supported by showing that the electronegativity and valence differences between unlike atoms in binary metallic alloys are of the required magnitude and direction so as to bring the metallic radii ratio R_A/R_B by charge transfer to a value r_A/r_B for ideal packing in an intermetallic compound. It is also demons…

Systematic understanding of half-metallicity of ternary compounds in Heusler and Inverse Heusler structures with 3$d$ and 4$d$ elements

Employing {\it ab initio} electronic structure calculations we extensively study ternary Heusler compounds having the chemical formula X$_2$X$^\prime$Z, where X = Mn, Fe or Co; Z = Al or Si; and X$^\prime$ changes along the row of 4$d$ transition metals. A comprehensive overview of these compounds, addressing the trends in structural, electronic, magnetic properties and Curie temperature is presented here along with the search for new materials for spintronics applications. A simple picture of hybridization of the $d$ orbitals of the neighboring atoms is used to explain the origin of the half-…

Ab-initio design of half-metallic fully-compensated ferrimagnets: the case of Cr$_2$MnZ (Z= P, As, Sb, Bi) compounds

Electronic structure calculations from first-principles are employed to design some new half-metallic fully-compensated ferrimagnets (or as they are widely known half-metallic antiferromagnets) susceptible of finding applications in spintronics. Cr$_2$MnZ (Z= P, As, Sb, Bi) compounds have 24 valence electrons per unit cell and calculations show that their total spin moment is approximately zero for a wide range of lattice constants in agreement with the Slater-Pauling behavior for ideal half-metals. Simultaneously, the spin magnetic moments of Cr and Mn atoms are antiparallel and the compounds…

Defects in CrAs and related compounds: a route to half-metallic ferrimagnetism

Half-metallic ferrimagnetism is crucial for spintronic applications with respect to ferromagnets due to the lower stray fields created by these materials. Studying the effect of defects in CrAs and related transition-metal chalcogenides and pnictides crystallizing in the zinc-blende structure, we reveal that the excess of the transition-metal atoms leads to half-metallic ferrimagnetism. The surplus of these atoms are antiferromagnetically coupled to the transition-metal atoms sitting at the perfect lattice sites. The needed condition to achieve half-metallic ferrimagnetism is to prevent the mi…

A compound figure of merit for photonic applications of metal nanocomposites

Selecting nanocomposites for photonic switching applications requires optimizing their thermal, nonlinear and two-photon absorption characteristics. We simplify this step by defining a compound figure of merit (FOM_{C}) for nanocomposites of noble metals in dielectric based on criteria that limit these structures in photonic applications, i.e. thermal heating and two-photon absorption. The device independent results predict extremely large values of FOM_{C} for a specific combination of the metal and insulator dielectric constant given by ε_{h}=\frac{ε_{1}-ε_{2}}{2}, where ε_{h} is the dielect…

Ab-initio study of stable 3d, 4d and 5d transition metal based Quaternary Heusler compounds

The realization of the stable structure of Heusler compounds and the study of different properties is an important step for their potential application in spintronics and magnetoelectronic devices. In this paper, using the plane-wave pseudopotential method within the framework of density functional theory (DFT), we investigate 25 Quaternary Heusler compounds for their electronic, magnetic, and mechanical properties. The Open Quantum Materials Database (OQMD) is used to screen a large number of compounds to narrow down the possible synthesizable materials. The convex-hull distance and elastic c…

Stability of ferromagnetism in the half-metallic pnictides and similar compounds: A first-principles study

Based on first-principles electron structure calculations and employing the frozen-magnon approximation we study the exchange interactions in a series of transition-metal binary alloys crystallizing in the zinc-blende structure and calculate the Curie temperature within both the mean-field approximation (MFA) and random-phase approximation (RPA). We study two Cr compounds, CrAs and CrSe, and four Mn compounds: MnSi, MnGe, MnAs and MnC. MnC, MnSi and MnGe are isovalent to CrAs and MnAs is isoelectronic with CrSe. Ferromagnetism is particular stable for CrAs, MnSi and MnGe: All three compounds s…

Dependence of nonthermal metallization kinetics on bond ionicity of compounds

It is known that covalently bonded materials undergo nonthermal structure transformations upon ultrafast excitation of an electronic system, whereas metals exhibit phonon hardening. Here we study how ionic bonds react to electronic excitation. Density-functional molecular dynamics predicts that ionic crystals may melt nonthermally, however, into an electronically insulating state, in contrast to covalent materials. We demonstrate that the band gap behavior during nonthermal transitions depends on a bonding type: it is harder to collapse the band gap in more ionic compounds, which is illustrate…

Semi-metallicity and electron-hole liquid in two-dimensional C and BN based compounds

Insulating-metallic transition mediated by substitutional atoms is predicted in a series of two-dimensional carbon-based structures. Introducing Si atoms in selected sites of tetrahexcarbon [Carbon 137 (2018) 266] according to rational chemical rules, metallicity by trivial band inversion without band gap opening is induced. Additional substitution of remaining C atoms by BN dimers introduces no changes in the metallic properties. A series of isomorphous two-dimensional materials with isoelectronic structures derived by exchanging group IV elements exhibiting various band gaps is obtained. Dyn…

A regression-based feature selection study of the Curie temperature of transition-metal rare-earth compounds: prediction and understanding

The Curie temperature ($T_C$) of binary alloy compounds consisting of 3$d$ transition-metal and 4$f$ rare-earth elements is analyzed by a machine learning technique. We first demonstrate that nonlinear regression can accurately reproduce $T_C$ of the compounds. The prediction accuracy for $T_C$ is maximized when five to ten descriptors are selected, with the rare-earth concentration being the most relevant. We then discuss an attempt to utilize a regression-based model selection technique to learn the relation between the descriptors and the actuation mechanism of the corresponding physical ph…

Stability of Weyl points in magnetic half-metallic Heusler compounds

We employ {\it ab-initio} fully-relativistic electronic structure calculations to study the stability of the Weyl points in the momentum space within the class of the half-metallic ferromagnetic full Heusler materials, by focusing on Co$_2$TiAl as a well-established prototype compound. Here we show that both the number of the Weyl points together with their $k$-space coordinates can be controlled by the orientation of the magnetization. This alternative degree of freedom, which is absent in other topological materials (e.g. in Weyl semimetals), introduces novel functionalities, specific for th…

Seebeck coefficients of half-metallic ferromagnets

In this report the Co2 based Heusler compounds are discussed as potential materials for spin voltage generation. The compounds were synthesized by arcmelting and consequent annealing. Band structure calculations were performed and revealed the compounds to be half-metallic ferromagnets. Magnetometry was performed on the samples and the Curie temperatures and the magnetic moments were determined. The Seebeck coefficients were measured from low to ambient temperatures for all compounds. For selected compounds high temperature measurements up to 900 K were performed.

Plasmon dispersions in simple metals and Heusler compounds

We present a comprehensive study of plasmon dispersions in simple metals and Heusler compounds based on an accurate ab-initio evaluation of the momentum and frequency dependent dielectric function in the random-phase approximation. Using a momentum-dependent tetrahedron method for the computation of the dielectric function, we extract and analyze "full" and "intraband" plasmon dispersions: The "full" plasma dispersion is obtained by including all bands, the intraband plasma dispersion by including only intraband transitions. For the simple metals silver and alu- minum, we show that the intraba…

First-principles prediction of redox potentials in transition-metal compounds with LDA+U

First-principles calculations within the Local Density Approximation (LDA) or Generalized Gradient Approximation (GGA), though very successful, are known to underestimate redox potentials, such as those at which lithium intercalates in transition metal compounds. We argue that this inaccuracy is related to the lack of cancellation of electron self-interaction errors in LDA/GGA and can be improved by using the DFT+$U$ method with a self-consistent evaluation of the $U$ parameter. We show that, using this approach, the experimental lithium intercalation voltages of a number of transition metal c…

Role of the presence of transition-metal atoms at the antisites in CrAs, CrSe and VAs zinc-blende compounds

In a recent publication [Galanakis I et al 2006 \PR B \textbf{74} 140408(R)] we have shown that in the case of CrAs and related transition-metal chalcogenides and pnictides, crystallizing in the zinc-blende structure, the excess of the transition-metal atoms leads to half-metallic ferrimagnetism. The latter property is crucial for spintronic applications with respect to ferromagnets due to the lower stray fields created by these materials. We extend this study to cover the case where the transition-metal atoms sitting at antisites are not identical to the ones in the perfect sites. In Cr-based…

Inter-cluster reactivity of Metallo-aromatic and anti-aromatic Compounds and Their Applications in Molecular Electronics: A Theoretical Investigation

Local reactivity descriptors such as the condensed local softness and Fukui function have been employed to investigate the inter-cluster reactivity of the metallo-aromatic (Al4Li- and Al4Na-) and anti-aromatic (Al4Li4 and Al4Na4) compounds. We use the concept of group softness and group Fukui function to study the strength of the nucleophilicity of the Al4 unit in these compounds. Our analysis shows that the trend of nucleophilicity of the Al4 unit in the above clusters is as follows; Al4Li- > Al4Na- > Al4Li4 > Al4Na 4 For the first time we have used the reactivity descriptors to show that the…

N-doped graphitic carbon materials hybridized with transition metals (compounds) for hydrogen evolution reaction: Understanding the synergistic effect from atomistic level

The hybrid nanostructures of nitrogen doped carbon materials and nonprecious transition metals are among the most promising electrocatalysts to replace noble metal catalysts for renewable energy applications. However, the fundamental principles governing the catalytic activity of such hybrid materials remain elusive. Herein, we systematically explore the electrocatalytic properties of transition metals, transition metal oxides and carbides substrates covered by nitrogen-doped graphitic sheets for hydrogen evolution reaction (HER). Our first-principles calculations show that the graphitic sheet…

Carbon cage-like materials as potential low work function metallic compounds: Case of clathrates

We present an ab-initio calculation of the electronic affinity of the hypothetical C-46 clathrate by studying its bare and hydrogenated (100) surfaces. We show that such a system shares with the diamond phase a small electronic affinity. Further, contrary to the diamond phase, the possibility of doping endohedrally these cage-like systems allows to significantly raise the position of the Fermi level, resulting in a true metal with a small work function. This is illustrated in the case of the Li8@C-46 doped compound. Such a class of materials might be of much interest for the design of electron…

First-principles investigation of half-metallic ferromagnetism of half-Heusler compounds XYZ

We investigate the electronic structure and magnetism of half-Heusler compounds XYZ (X, Y=V, Cr, Mn, Fe, Co and Ni; Z=Al, Ga, In, Si, Ge, Sn, P, As, and Sb) using the ab initio density functional theory calculations. Nine half-metals with half-Heusler structure have been predicted with the half-metallic gap of 0.07-0.67 eV. The calculations show that the formation energies for these nine half-Heusler compounds range from -1.32 to -0.12 eV/f.u., and for CoCrSi, CoCrGe, CoFeGe, CoMnSi, CoMnGe, FeMnGe and FeMnAs, the total energy differences between the half-Heusler structure and the correspondin…

Spin-polarized two-dimensional electron/hole gas at the interface of non-magnetic semiconducting half-Heusler compounds: Modified Slater-Pauling rule for half-metallicity at the interface

Half-Heusler compounds with 18 valence electrons per unit cell are well-known non-magnetic semiconductors. Employing first-principles electronic band structure calculations, we study the interface properties of the half-Heusler heterojunctions based on FeVSb, CoTiSb, CoVSn, and NiTiSn compounds, which belong to this category of materials. Our results show that several of these heterojunction interfaces become not only metallic but also magnetic. The emergence of spin-polarization is accompanied by the formation of two-dimensional electron gas (2DEG) or hole gas (2DHG) at the interface. We qual…

Influence of mixing the low-valent transition metal atoms (Y,Y$^*$=Cr,Mn,Fe) on the properties of the quaternary Co$_2$[Y$_{1-x}$Y$^*_x$]Z (Z=Al,Ga,Si,Ge,Sn) Heusler compounds

We complement our study on the doping and disorder in Co$_2$MnZ compounds [I. Galanakis \textit{et al.}, Appl. Phys. Lett. \textbf{89}, 042502 (2006) and K. Özdogan \textit{et al.}, Phys. Rev. B \textbf{74}, (2006)] to cover also the quaterarny Co$_2$[Y$_{1-x}$Y$^*_x$]Z compounds with the lower-valent transition metals Y,Y$^*$ being Cr, Mn or Fe and the sp atom Z being one of Al, Ga, Si, Ge, Sn. This study gives a global overview of the magnetic and electronic properties of these compounds since we vary both Y and Z elements. Our results suggest that for realistic applications the most appropr…

Band crossings in honeycomb-layered transition metal compounds

Two-dimensional electron dispersions with peculiar band crossings provide a platform for realizing topological phases of matter. Here we theoretically show that the $e_g$-orbital manifold of honeycomb-layered transition metal compounds accommodates a plethora of peculiar band crossings, such as multiple Dirac point nodes, quadratic band crossings, and line nodes. From the tight-binding analysis, we find that the band topology is systematically changed by the orbital dependent transfer integrals on the honeycomb network of edge-sharing octahedra, which can be modulated by distortions of the oct…

Tunable Hydrogen Storage in Magnesium - Transition Metal Compounds

Magnesium dihydride ($\mgh$) stores 7.7 weight % hydrogen, but it suffers from a high thermodynamic stability and slow (de)hydrogenation kinetics. Alloying Mg with lightweight transition metals (TM = Sc, Ti, V, Cr) aims at improving the thermodynamic and kinetic properties. We study the structure and stability of Mg$_x$TM$_{1-x}$H$_2$ compounds, $x=[0$-1], by first-principles calculations at the level of density functional theory. We find that the experimentally observed sharp decrease in hydrogenation rates for $x\gtrsim0.8$ correlates with a phase transition of Mg$_x$TM$_{1-x}$H$_2$ from a f…

Influence of defects on the electronic and magnetic properties of half-metallic CrAs, CrSe and CrSb zinc-blende compounds

We present an extended study of single impurity atoms and atomic swaps in half-metallic CrAs, CrSb and CrSe zinc-blende compounds. Although the perfect alloys present a rather large gap in the minority-spin band, all defects under study, with the exception of void impurities at Cr and sp sites and Cr impurities at sp sites (as long as no swap occurs), induce new states within the gap. The Fermi level can be pinned within these new minority states depending on the lattice constant used for the calculations and the electronegativity of the sp atoms. Although these impurity states are localized i…

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

Half-metallic ferromagnets like the full Heusler compounds with formula X$_2$YZ are supposed to show an integer value of the spin magnetic moment. Calculations reveal in certain cases of X = Co based compounds non-integer values, in contrast to experiments. In order to explain deviations of the magnetic moment calculated for such compounds, the dependency of the electronic structure on the lattice parameter was studied theoretically. In local density approximation (LDA), the minimum total energy of Co$_2$FeSi is found for the experimental lattice parameter, but the calculated magnetic moment i…

Ab initio study of the half-metallic full-Heusler compounds Co$_2$ZAl [Z = Sc, Ti, V, Cr, Mn, Fe]; the role of electronic correlations

We study the structural, electronic, and magnetic properties of Co$_2$ZAl compounds employing a pseudopotential electronic bandstructure method. The stability of the compounds is established through the formation and cohesive energy calculations. The effect of the lattice parameter variation on the electronic and magnetic properties of the compounds is investigated and meticulous explanation is provided for the observed behavior. The variation of the individual spin magnetic moments and the stability of the total spin magnetic moment during the expansion and contraction of the lattice paramete…

Half-metallic ferromagnetism in transition-metal doped germanium nitride: A first-principles study

The electronic and magnetic properties of transition-metal doped $β$-Ge3N4 have been studied using first-principles calculations. The results show that the substitutional transition-metal impurities tend to cluster. The V and Cr doped Ge3N4 compounds are ferromagnetic semiconductors, while the compounds with Mn and Fe doping show a half-metallic ferromagnetic character.

Comprehensive search for buckled honeycomb binary compounds based on noble metals (Cu, Ag, and Au)

Honeycomb structure has been frequently observed in two-dimensional (2D) materials. CuAu in the buckled honeycomb (BHC) structure has been synthesized recently, which is the first case of 2D intermetallic compounds. Here, the dynamical stability of 2D $AX$ in the BHC structure, where $A=$ Cu, Ag, and Au and $X$ is a metallic element in the periodic table, is systematically studied by calculating phonon dispersions from first-principles. Among 135 $AX$, more than 50 $AX$ are identified to be dynamically stable. In addition, (i) a relationship between the dynamical stability and the formation en…

Origin of pressure-induced insulator-to-metal transition in the van der Waals compound FePS3 from first-principles calculations

Pressure-induced insulator-to-metal transition (IMT) has been studied in the van der Waals compound iron thiophosphate (FePS3) using first-principles calculations within the periodic linear combination of atomic orbitals method with hybrid Hartree-Fock-DFT B3LYP functional. Our calculations reproduce correctly the IMT at ~15 GPa, which is accompanied by a reduction of the unit cell volume and of the vdW gap. We found from the detailed analysis of the projected density of states that the 3p states of phosphorus atoms contribute significantly at the bottom of the conduction band. As a result, th…

Above-room-temperature ferromagnetism in half-metallic Heusler compounds NiCrP, NiCrSe, NiCrTe and NiVAs: A first-principles study

We study the interatomic exchange interactions and Curie temperatures in half-metallic semi Heusler compounds NiCrZ (Z=P, Se, Te) and NiVAs. The study is performed within the framework of density functional theory. The calculation of exchange parameters is based on the frozen-magnon approach. It is shown that the exchange interactions in NiCrZ vary strongly depending on the Z constituent. The Curie temperature, Tc, is calculated within the mean field and random phase approximations. The difference between two estimations is related to the properties of the exchange interactions. The predicted …

Doping of Mn$_2$VAl and Mn$_2$VSi Heusler alloys as a route to half-metallic antiferromagnetism

Half-metallic antiferromagnets are the ideal materials for spintronic applications since their zero magnetization leads to lower stray fields and thus tiny energy losses. Starting from the Mn$_2$VAl and Mn$_2$VSi alloys we substitute Co or Fe for Mn and we show by means of first-principle electronic structure calculations that the resulting compounds are ferrimagnets. When the total number of valence electrons reaches the magic number of 24 the Fe-doped compounds are semi-metals and thus non-magnetic while the Co-doped ones show the desirable half-metallic antiferromagnetic character. The comp…

掺杂诱导亚铁磁半金属Mn$_{4}$Al$_{11}$化合物的磁性与电子相变

Doping Induced Magnetic and Electronic phase Transition in Ferrimagnetic Half-metallic Mn$_{4}$Al$_{11}$ Compound

本文利用密度泛函理论研究Mn$_{4}$Al$_{11}$在应变和Ge取代下的电子与磁性能变化，发现Ge取代可诱导金属-绝缘体转变并调控半金属性和磁性，为无机固体功能材料设计提供新途径。

CuSe-based layered compound Bi$_{2}$YO$_{4}$Cu$_{2}$Se$_{2}$ as a quasi-two-dimensional metal

We have investigated the physical properties of a new layered oxyselenide Bi$_{2}$YO$_{4}$Cu$_{2}$Se$_{2}$, which crystallizes in an unusual intergrowth structure with Cu$_{2}$Se$_{2}$ and Bi$_{2}$YO$_{4}$ layers. Electric transport measurement indicates that Bi$_{2}$YO$_{4}$Cu$_{2}$Se$_{2}$ behaves metallic. Thermal transport and Hall measurements show that the type of the carriers is hole-like and it may be a potential thermoelectric material at high temperatures. First principle calculations are in agreement with experimental results and show that Bi$_{2}$YO$_{4}$Cu$_{2}$Se$_{2}$ is a quasi…

Large magnetoresistance in low temperature metallic region of manganite compounds (La0.7-2xEux)(Ca0.3Srx)MnO3 (0.05<x<0.15)

Magnetoresistance (MR) and magnetization (d.c and a.c) measurements have been carried out on the manganites, (La0.7-2xEux)(Ca0.3Srx)MnO3 (0.05<x<0.15), in the temperature range of 5K-320K. At 5K, an unusually large MR of almost 98% is observed in the x=0.15 sample, nearly up to fields of 4-5 Tesla. This large high-field MR occurs in the metallic region, far below the insulator-metal transition temperature, and does not vary linearly with applied field. The unusual magnetoresistance is explained the light of various possibilities of phase segregation and cluster spin-glass behavior. Keywords: M…

Determining electronic properties from L-edge X-ray absorption spectra of transition metal compounds with artificial neural networks

X-ray absorption spectroscopy at the L-edge probes transitions of 2p-electrons into unoccupied d-states. Applied to transition metal atoms, this experimental technique can provide valuable information about the electronic structure of d-states. However, multiplet effects, spin-orbit coupling, a large number of possible transitions can cause a rather involved nature of 2p XAS spectra, which can often complicate extracting of information directly from them. Here, artificial neural networks trained on simulated spectra of a 2p XAS model Hamiltonian are presented that can directly determine inform…

Inducing half metallicity with alloying in Heusler Compound CoFeMnSb

First principles studies were performed in order to find out the possibility of inducing half-metallicity in Heusler Compound CoFeMnSb, by means of alloying it with 3d-transition metal elements. Proper alloying element is selected through the calculations of formation energies. These calculations were tested with different concentrations of alloying elements at different atomic sites. Among the selected transition metal elements Sc and Ti are proposed to be excellent alloying elements particularly at Mn site. By using these alloying elements complete half metallic behaviour is obtained in CoFe…

Lattice Dynamics and Thermal Expansion Behavior in Metal Cyanides, MCN (M=Cu, Ag, Au): Neutron Inelastic Scattering and First Principles Calculations

We report measurement of temperature dependence of phonon spectra in quasi one-dimensional metal cyanides MCN (M=Cu, Ag and Au). Ab initio lattice dynamics calculations have been performed to interpret the phonon-spectra as well as to understand the anamolous thermal expansion behavior in these compounds. We bring out the differences in the phonon mode behavior to explain the differences in the thermal expansion behavior among the three compounds. The chain-sliding modes are found to contribute maximum to the negative thermal expansion along c axis in the Cu and Ag compounds, while the same mo…

Novel lithium-nitrogen compounds at ambient and high pressures

Using ab initio evolutionary simulations, we predict the existence of five novel stable Li-N compounds at pressures from 0 to 100 GPa (Li13N, Li5N, Li3N2, LiN2, and LiN5). Structures of these compounds contain of isolated N ions, N2 dimers, polyacetylene-like N chains and N5 rings, respectively. The structure of Li13N consists of Li atoms and Li12N icosahedra (with N atom in the center of the Li12 icosahedron) such icosahedra are notdescribed by Wade-Jemmis electron counting rules and are unique. Electronic structure of Li-N compounds is found to dramatically depend on composition and pressure…

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

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

Unexpected Xe cations and superconductivity in Y-Xe compounds under pressure

The metal-based noble gas compounds exhibit interesting behavior of electronic valence states under pressure. For example, Xe upon compression can gain electrons from the alkali metal, or lose electrons unexpectedly to Fe and Ni, toward formation of stable metal compounds. In addition, the Na2He is not even stabilized by the local chemical bonds but via the long-range Coulomb interactions. Herein, by using the first-principles calculations and the unbiased structure searching techniques, we uncover that the transition metal Y is able to react with Xe above 60 GPa within various Y-Xe stochiomet…

Benchmark Results and Theoretical Treatments for Valence-to-Core X-ray Emission Spectroscopy in Transition Metal Compounds

We report measurement of the valence-to-core (VTC) region of the K-shell x-ray emission spectra from several Zn and Fe inorganic compounds, and their critical comparison with several existing theoretical treatments. We find generally good agreement between the respective theories and experiment, and in particular find an important admixture of dipole and quadrupole character for Zn materials that is much weaker in Fe-based systems. These results on materials whose simple crystal structures should not, a prior, pose deep challenges to theory, will prove useful in guiding the further development…

Superconductivity in ferromagnetic metals and in compounds without inversion centre

The symmetry properties and the general overview of the superconductivity theory in the itinerant ferromagnets and in materials without space parity are presented. The basic notions of unconventional superconductivity are introduced in broad context of multiband superconductivity which is inherent property of ferromagnetic metals or metals without centre of inversion.

Electrical property of Half Metallic Ferromagnet Pr0.95Mn0.939O3

Half metallic ferromagnetic compounds (HMF) are intensively explored for their potential in spintronics application. According to theoretical studies, HMF are those materials where the conduction mechanism is due to one type of spin state, either up or down. Band theory strongly predicts that Praseodymium (Pr) based transition metal compound (TMC) is HMF whose electrical properties are a consequence of the contribution from both the 4f and 3d orbitals of Pr and TMC ions. Hence, an investigation has been made by preparing Pr0.95Mn0.939O3 by ball milling assisted sintering process. The phase con…

Optimization of criteria for an efficient screening of new thermoelectric compounds: the TiNiSi structure type as a case-study

High-throughput calculations are a very promising tool for screening a large number of compounds in order to discover new useful materials. Ternary intermetallic are thus investigated in the present work to find new compounds potentially interesting for thermoelectric applications. The screening of the stable non-metallic compounds required for such applications is obtained by calculating their electronic structure by DFT methods. In a first part, the study of the density of states at the Fermi level of well-known chemical elements and binary compounds allows to empirically optimize the select…

自旋能隙金属半赫斯勒化合物中的巡游磁性与局域磁性：斯托纳判据与磁相互作用

Itinerant versus localized magnetism in spin gapped metallic half-Heusler compounds: Stoner criterion and magnetic interactions

本文系统研究自旋能隙金属半赫斯勒化合物的磁性起源，结合cRPA计算的斯托纳判据区分巡游与局域磁性，并对磁性基态计算交换参数、居里温度及自旋波性质。

Hypothetical FrAu: An outlier in the B2 compounds

The ordered alloys of alkali metals (Rb and Cs) and gold (Au) have the B2 (CsCl-type) structure and show a semiconducting property, irrespective to the metallic constituents. Francium (Fr) is classed as an alkali metal and is expected to form the B2 structure with Au. However, it is difficult to synthesize such a compound experimentally due to a half-life of a few ten minutes in the Fr atom. In this paper, by using the first-principles method, we study the structural and electronic properties of FrAu in the B2 structure. The FrAu has a relatively large lattice constant and a relatively small b…

Design and discovery of a novel Half-Heusler transparent hole conductor made of all-metallic heavy elements

Metallic conductors that are optically transparent represent a rare breed of generally contraindicated physical properties that are nevertheless critically needed for application where both functionalities are crucial. Such rare materials have traditionally been searched in the general chemical neighborhood of compounds containing metal oxides, expected to be wide gap insulators that might be doped to induce conductivity.Focusing on the family of 18 valence electron ABX compounds we have searched theoretically for the ability of the compound's electronic structure to simultaneously lead to opt…

Signature of an antiferromagnetic metallic ground state in heavily electron doped Sr2FeMoO6

Sr$_{2}$FeMoO$_6$ is a double perovskite compound, known for its high temperature behavior. Combining different magnetic and spectroscopic tools, we show that this compound can be driven to rare example of antiferromagnetic metallic state through heavy electron doping. Considering synthesis of Sr$_{2-x}$La$_x$FeMoO$_6$ (1.0 $\le{x}\le$ 1.5) compounds, we find compelling evidences of antiferromagnetic metallic ground state for $x\ge$1.4. The local structural study on these compounds reveal unusual atomic scale phase distribution in terms of La, Fe and Sr, Mo-rich regions driven by strong La-O c…

Prediction of half-metallicity and spin-gapless semiconducting behavior in the new series of FeCr-based quaternary Heusler alloys: an ab initio study

This paper presents a detailed investigation of FeCr-based quaternary Heusler alloys. By using ultrasoft pseudopotential, electronic and magnetic properties of the compounds are studied within the framework of Density Functional Theory (DFT) by using the Quantum Espresso package. The thermodynamic, mechanical, and dynamical stability of the compounds is established through the comprehensive study of different mechanical parameters and phonon dispersion curves. The meticulous study of elastic parameters such as bulk, Young's, shear moduli, etc. is done to understand different mechanical propert…