Anomalous Raman scattering from phonons and electrons of superconducting FeSe0.82

We report interesting anomalies in the temperature dependent Raman spectra of FeSe0.82 measured from 3 K to 300 K in the spectral range from 60 to 1800 cm(-1) and determine their origin using complementary first-principles density functional calculations. A phonon mode near 100 cm-1 exhibits a sharp increase by similar to 5% in the frequency below a temperature T-s (similar to 100 K) attributed to strong spin-phonon coupling and onset of short-range antiferromagnetic order. In addition, two high frequency modes are observed at 1350 cm-1 and 1600 cm-1, attributed to electronic Raman scattering from (x(2)-y(2)) to xz/yz d-orbitals of Fe. (C) 2010 Elsevier Ltd. All rights reserved.

Electronic structure and bonding of beta-rhombohedral boron using cluster fragment approach

Theoretical studies using density functional theory are carried out to understand the electronic structure and bonding and electronic properties of elemental beta-rhombohedral boron. The calculated band structure of ideal beta-rhombohedral boron (B-105) shows valence electron deficiency and depicts metallic behavior. This is in contrast to the experimental result that it is a semiconductor. To understand this ambiguity we discuss the electronic structure and bonding of this allotrope with cluster fragment approach using our recently proposed mno rule. This helps us to comprehend in greater detail the structure of B-105 and materials which are closely related to beta-rhombohedral boron. The molecular structures B12H12-2, B28H21+1, BeB27H21, LiB27H21-1, CB27H21+2, B57H36+3, Be3B54H36, and Li2CB54H36, and corresponding solids Li8Be3B102 and Li10CB102 are arrived at using these ideas and studied using first principles density functional theory calculations.

Charge Ordering Induced Ferromagnetic Insulator: K2Cr8O16

Usually metallicity accompanies ferromagnetism. K2Cr8O16 is one of the less common examples of magnetic materials, exhibiting ferromagnetism in the insulating state. Analyzing the electronic and magnetic properties within first principles electronic structure calculations, we find that the doped electrons due to K induce a charge-ordered and insulating ground state and interestingly also introduce a ferromagnetic coupling between the Cr ions. The primary considerations driving the charge ordering are found to be electrostatic ones with the charge being localized on two Cr atoms that minimize the electrostatic energy. The structural distortion that accompanies the ordering gives rise to a rare example of a charge-order driven ferromagnetic insulator.

InMnO3: A biferroic

InMnO3 which has a hexagonal structure similar to that of YMnO3 is found to show a canted antiferromagnetic behavior below 50 K (T-N) and a ferroelectric (FE) transition at 500 K accompanied by hystersis. We have determined the structure, polarization, and energetics of the FE and paraelectric (PE) phases of InMnO3 using first-principles density functional theory calculations based on pseudopotentials and a plane-wave basis, and find the polarization of the PE phase to be a half-integer quantum. The difference in polarization of the FE and PE phases calculated along a simple path is different from the absolute value of polarization of the FE phase. A weak piezoelectric response is exhibited by InMnO3 to uniaxial strain.

Raman evidence for the superconducting gap and spin-phonon coupling in the superconductor Ca( Fe0.95Co0.05)(2)As-2

Inelastic light scattering studies on a single crystal of electron-doped Ca(Fe0.95Co0.05)(2)As-2 superconductor, covering the tetragonal-to-orthorhombic structural transition as well as the magnetic transition at T-SM similar to 140 K and the superconducting transition temperature T-c similar to 23 K, reveal evidence for superconductivity-induced phonon renormalization. In particular, the phonon mode near 260 cm(-1) shows hardening below T-c, signaling its coupling with the superconducting gap. All three Raman active phonon modes show anomalous temperature dependence between room temperature and T-c, i.e. the phonon frequency decreases with lowering temperature. Further, the frequency of one of the modes shows a sudden change in temperature dependence at TSM. Using first-principles density functional theory based calculations, we show that the low temperature phase (T-c < T < T-SM) exhibits short-ranged stripe antiferromagnetic ordering, and estimate the spin-phonon couplings that are responsible for these phonon anomalies.

Symmetry-dependent phonon renormalization in monolayer MoS2 transistor

A strong electron-phonon interaction which limits the electronic mobility of semiconductors can also have significant effects on phonon frequencies. The latter is the key to the use of Raman spectroscopy for nondestructive characterization of doping in graphene-based devices. Using in situ Raman scattering from a single-layer MoS2 electrochemically top-gated field-effect transistor (FET), we show softening and broadening of the A(1g) phonon with electron doping, whereas the other Raman-active E-2g(1) mode remains essentially inert. Confirming these results with first-principles density functional theory based calculations, we use group theoretical arguments to explain why the A(1g) mode specifically exhibits a strong sensitivity to electron doping. Our work opens up the use of Raman spectroscopy in probing the level of doping in single-layer MoS2-based FETs, which have a high on-off ratio and are of technological significance.

Activation of Lattice Oxygen of TiO2 by Pd2+ Ion: Correlation of Low-Temperature CO and Hydrocarbon Oxidation with Structure of Ti1-xPdxO2-x (x=0.01-0.03)

Lattice oxygen of TiO2 is activated by the substitution of Pd ion in its lattice. Ti1-xPdxO2-x (x = 0.01-0.03) have been synthesized by solution combustion method crystallizing in anatase TiO2 structure. Pd is in +2 oxidation state and Ti is in +4 oxidation state in the catalyst. Pd is more ionic in TiO2 lattice compared to Pd in PdO. Oxygen storage capacity defined by ``amount of oxygen that is used reversibly to oxidize CO'' is as high as 5100 mu mol/g of Ti0.97Pd0.03O1.97. Oxygen is extracted by CO to CO2 in absence of feed oxygen even at room temperature which is more than 20 times compared to pure TiO2. Rate of CO oxidation is 2.75 mu mol g(-1) s(-1) at 60 degrees C over Ti0.97Pd0.03O1.97 and C2H2 gets oxidized to CO2 and H2O at room temperature. Catalyst is not poisoned on long time operation of the reactor. Such high catalytic activity is due to activated lattice oxygen created by the substitution of Pd ion as seen from first-principles density functional theory (DFT) calculations with 96 atom supercells of Ti32O64, Ti31Pd1O63, Ti30Pd2O62, and Ti29Pd3O61. The compounds crystallize in anatase TiO2 structure with Pd2+ ion in nearly square planar geometry and TiO6 octahedra are distorted by the creation of weakly bound oxygens. Structural analysis of Ti31Pd1O63 which is close to 3% Pd ion substituted TiO2 shows that oxygens associated with both Ti and Pd ions in the lattice show bond valence sum of 1.87, a low value characteristic of weak oxygen in the lattice compared to oxygens with valence 2 and above in the same lattice. Exact positions of activated oxygens have been identified in the lattice from DFT calculations.

Sensory-organ-like response determines the magnetism of zigzag-edged honeycomb nanoribbons

We present an analytical effective theory for the magnetic phase diagram for zigzag-edge terminated honeycomb nanoribbons described by a Hubbard model with an interaction parameter U. We show that the edge magnetic moment varies as ln U and uncover its dependence on the width W of the ribbon. The physics of this owes its origin to the sensory-organ-like response of the nanoribbons, demonstrating that considerations beyond the usual Stoner-Landau theory are necessary to understand the magnetism of these systems. A first-order magnetic transition from an antiparallel orientation of the moments on opposite edges to a parallel orientation occurs upon doping with holes or electrons. The critical doping for this transition is shown to depend inversely on the width of the ribbon. Using variational Monte Carlo calculations, we show that magnetism is robust to fluctuations. Additionally, we show that the magnetic phase diagram is generic to zigzag-edge terminated nanostructures such as nanodots. Furthermore, we perform first-principles modeling to show how such magnetic transitions can be realized in substituted graphene nanoribbons. DOI: 10.1103/PhysRevB.87.085412

Single crystalline ultrathin gold nanowires: Promising nanoscale interconnects

Using first principles based density functional calculation we study the mechanical, electronic and transport properties of single crystalline gold nanowires. While nanowires with the diameter less than 2 nm retain hexagonal cross-section, the larger diameter wires show a structural smoothening leading to circular cross-section. These structural changes significantly affect the mechanical properties of the wires, however, strength remains comparable to the bulk. The transport calculations reveal that the conductivity of these wires are in good agreement with experiments. The combination of good mechanical, electronic and transport properties make these wires promising as interconnects for nano devices. Copyright 2013 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License. http://dx.doi.org/10.1063/1.4796188]

Thermoelectric properties of beta-FeSi2

We investigate the thermoelectric properties of beta-FeSi2 using first principles electronic structure and Boltzmann transport calculations. We report a high thermopower for both p- and n-type beta-FeSi2 over a wide range of carrier concentration and in addition find the performance for n-type to be higher than for the p-type. Our results indicate that, depending upon temperature, a doping level of 3 x 10(20) to 2 x 10(21) cm(-3) may optimize the thermoelectric performance. (C) 2013 AIP Publishing LLC.

Platinum Ion-Doped TiO2: High Catalytic Activity of Pt2+ with Oxide Ion Vacancy in Ti1-x4+Ptx2+O2-x Compared to Pt4+ without Oxide Ion Vacancy in Ti1-x4+Ptx4+O2

Ti0.97Pt0.032+O1.97 and Ti0.97Pt0.034+O2 have been synthesized by a solution combustion method using alanine and glycine as the fuels, respectively. Both crystallize in anatase TiO2 structure with 15 nm average crystallite size. X-ray photoelectron spectroscopy (XPS) confirmed Pt ions are in the 2+ state in Ti0.97Pt0.03O1.97 (alanine) and 4+ state in Ti0.97Pt0.03O2 (glycine). The rate of CO oxidation occurring over Ti0.97Pt0.032+O1.97 (0.76 mu mol.g(-1).s(-1)) is similar to 10, times more than that over Ti0.97Pt0.034+O2 at 60 degrees C (0.08 mu mol.g(-1).s(-1)). A large shift in 100% hydrocarbons conversion to lower temperature was observed for Pt2+ ion-substituted TiO2 relative 10 that for Pt4+ ion-substituted TiO2. After reoxidation of the reduced compound by H-2 as well as CO, Pt ions are stabilized in mixed valences, 2+ and 4+ states. The role of oxide ion vacancy has been demonstrated by CO oxidation and H-2 + O-2 recombination reactions in the presence and absence of O-2. We analyze the activated lattice oxygens upon substitution of Pt2+ and Pt4+ ions in TiO2, using first-principles density functional theory (DFT) calculations with supercells of Ti31Pt1O63, Ti30Pt2O62, and Ti29Pt3O61 for Pt2+ ion substitution and Ti31Pt1O64, Ti30Pt2O62, and Ti29Pt3O61 for Pt4+ ion substitution in TiO2. We find that the local structure of Pt2+ ion has a distorted square planar geometry and that of Pt4+ ion has an octahedral geometry similar to that of Ti4+ ion in pure TiO2. The change in coordination of Pt2+ ion gives rise to weakly bonded oxygens, and these oxygens are involved in high rates of catalytic reaction. Thus, the high catalytic activity results from synergistic roles of Pt2+ ion and oxide ion vacancy and weakly bonded lattice oxygen.

New insights into designing metallacarborane based room temperature hydrogen storage media

Metallacarboranes are promising towards realizing room temperature hydrogen storage media because of the presence of both transition metal and carbon atoms. In metallacarborane clusters, the transition metal adsorbs hydrogen molecules and carbon can link these clusters to form metal organic framework, which can serve as a complete storage medium. Using first principles density functional calculations, we chalk out the underlying principles of designing an efficient metallacarborane based hydrogen storage media. The storage capacity of hydrogen depends upon the number of available transition metal d-orbitals, number of carbons, and dopant atoms in the cluster. These factors control the amount of charge transfer from metal to the cluster, thereby affecting the number of adsorbed hydrogen molecules. This correlation between the charge transfer and storage capacity is general in nature, and can be applied to designing efficient hydrogen storage systems. Following this strategy, a search for the best metallacarborane was carried out in which Sc based monocarborane was found to be the most promising H-2 sorbent material with a 9 wt.% of reversible storage at ambient pressure and temperature. (C) 2013 AIP Publishing LLC.

Mechanical and electronic properties of pristine and Ni-doped Si, Ge, and Sn sheets

Silicene, a graphene analogue of silicon, has been generating immense interest due to its potential for applications in miniaturized devices. Unlike planar graphene, silicene prefers a buckled structure. Here we explore the possibility of stabilizing the planar form of silicene by Ni doping using first principles density functional theory based calculations. It is found that planar as well as buckled structure is stable for Ni-doped silicene, but the buckled sheet has slightly lower total energy. The planar silicene sheet has unstable phonon modes. A comparative study of the mechanical properties reveals that the in-plane stiffness of both the pristine and the doped planar silicene is higher compared to that of the buckled silicene. This suggests that planar silicene is mechanically more robust. Electronic structure calculations of the planar and buckled Ni-doped silicene show that the energy bands at the Dirac point transform from linear behavior to parabolic dispersion. Furthermore, we extend our study to Ge and Sn sheets that are also stable and the trends of comparable mechanical stability of the planar and buckled phases remain the same.

Low formation energy and kinetic barrier of Stone-Wales defect in infinite and finite silicene

Stone-Wales (SW) defects in materials having hexagonal lattice are the most common topological defects that affect the electronic and mechanical properties. Using first principles density functional theory based calculations, we study the formation energy and kinetic barrier of SW-defect in infinite and finite sheets of silicene. The formation energies as well as the barriers in both the cases are significantly lower than those of graphene. Furthermore, compared with the infinite sheets, the energy barriers and formation energies are lower for finite sheets. However, due to low barriers these defects are expected to heal out of the finite sheets. (C) 2013 Elsevier B.V. All rights reserved.

Universal binding energy relation for cleaved and structurally relaxed surfaces

The universal binding energy relation (UBER), derived earlier to describe the cohesion between two rigid atomic planes, does not accurately capture the cohesive properties when the cleaved surfaces are allowed to relax. We suggest a modified functional form of UBER that is analytical and at the same time accurately models the properties of surfaces relaxed during cleavage. We demonstrate the generality as well as the validity of this modified UBER through first-principles density functional theory calculations of cleavage in a number of crystal systems. Our results show that the total energies of all the relaxed surfaces lie on a single (universal) energy surface, that is given by the proposed functional form which contains an additional length-scale associated with structural relaxation. This functional form could be used in modelling the cohesive zones in crack growth simulation studies. We find that the cohesive law (stress-displacement relation) differs significantly in the case where cracked surfaces are allowed to relax, with lower peak stresses occurring at higher displacements.